Immunology_Janeway

Immunology_Janeway. 9-26 The effector functions of T cells are determined by the array of effector molecules that they produce. The effector molecules produced by effector T cells fall into two broad classes: cytotoxins, which are stored in specialized cytotoxic granules and released by CD8 cytotoxic T cells (see Fig. 9.38), and cytokines and related membrane-associated proteins, which are synthesized de novo by all effector T cells. Cytotoxins are the principal effector molecules of cytotoxic T cells and are discussed in Section 9-31. Their release in particular must be tightly regulated because they are not specific: they can penetrate the lipid bilayer and trigger apoptosis in any cell. By contrast, CD4 effector T cells act mainly through the production of cytokines and membrane-associated proteins, and their actions are largely restricted to cells bearing MHC class II molecules and expressing receptors for these proteins.


Immunology_Janeway

Immunology_Janeway. The main effector molecules of T cells are summarized in Fig. 9.39. The cytokines are a diverse group of proteins and we will review them briefly before discussing the T-cell cytokines and their actions. Secreted cytokines and membrane-associated molecules often act in concert to mediate these effects. 9-27 Cytokines can act locally or at a distance. Cytokines are small soluble proteins secreted by cells that can alter the behavior or properties of the secreting cell itself (autocrine actions) or of another cell (paracrine actions). Cytokines are produced by many cell types in addition to those of the immune system. We have already introduced the families of


Immunology_Janeway

Immunology_Janeway. Fig. 9.38 The cellular polarization of T cells during specific antigen recognition allows effector molecules to be focused on the antigen-bearing target cell. The example illustrated here is a Cd8 cytotoxic T cell. Cytotoxic T cells contain specialized lysosomes called cytotoxic granules (shown in red in the left panels), which contain cytotoxic proteins. Initial binding to a target cell through adhesion molecules does not have any effect on the location of the cytotoxic granules. Binding of the T-cell receptor causes the T cell to become polarized: reorganization within the cortical actin cytoskeleton at the site of contact aligns the microtubule-organizing center (MToC), which in turn aligns the secretory apparatus, including the Golgi apparatus (Ga), toward the target cell. proteins stored in cytotoxic granules derived from the Golgi are then directed specifically onto the target cell. The photomicrograph in panel a shows an unbound, isolated cytotoxic T cell. The microtubule


Immunology_Janeway

Immunology_Janeway. stored in cytotoxic granules derived from the Golgi are then directed specifically onto the target cell. The photomicrograph in panel a shows an unbound, isolated cytotoxic T cell. The microtubule cytoskeleton is stained in green and the cytotoxic granules in red. note how the granules are dispersed throughout the T cell. panel b depicts a cytotoxic T cell bound to a (larger) target cell. The granules are now clustered at the site of cell–cell contact in the bound T cell. The electron micrograph in panel c shows the release of granules from a cytotoxic T cell. panels a and b courtesy of G. Griffiths. panel c courtesy of e. podack.


Immunology_Janeway

Immunology_Janeway. Fig. 9.39 The different types of effector T-cell subsets produce different effector molecules. Cd8 T cells are predominantly killer T cells that recognize peptide:MhC class I complexes. They release perforin (which helps deliver granzymes into the target cell) and granzymes (which are pro-proteases that are activated intracellularly to trigger apoptosis in the target cell), and often also produce the cytokine IFn-γ. They also carry the membrane-bound effector molecule Fas ligand (Cd178). When this binds to Fas (Cd95) on a target cell it activates apoptosis in the Fas-bearing cell. The various functional subsets of Cd4 T cells recognize peptide:MhC class II complexes. Th1 cells are specialized to activate macrophages that are infected by or have ingested pathogens; they secrete IFn-γ to activate the infected cell, as well as other effector molecules. They can express membrane-bound Cd40 ligand and/or Fas ligand.


Immunology_Janeway

Immunology_Janeway. Cd40 ligand triggers activation of the target cell, whereas Fas ligand triggers the death of Fas-bearing targets, and so which molecule is expressed strongly influences Th1 function. Th2 cells are specialized for promoting immune responses to parasites and also promote allergic responses. They provide help in B-cell activation and secrete the B-cell growth factors IL-4, IL-5, IL-9, and IL-13. The principal membrane-bound effector molecule expressed by Th2 cells is Cd40 ligand, which binds to Cd40 on B cells and induces B-cell proliferation and isotype switching (see Chapter 10). Th17 cells produce members of the IL-17 family and IL-22, and promote acute inflammation by helping to recruit neutrophils to sites of infection. T cells produce inhibitory cytokines such as IL-10 and TGF-β that may act at a distance, but also exert inhibitory actions such as sequestration of B7 and IL-2, which act via cell–cell interactions.


Immunology_Janeway

Immunology_Janeway. cytokines and their receptors that are important in innate and adaptive immunity in Chapters 3 and 7 (see Sections 3-15 and 7-1). Here we are concerned with cytokines that mediate the effector functions of T cells. Many cytokines produced by T cells are given the name interleukin (IL) followed by a number. The cytokines produced by T cells are shown in Fig. 9.40, and a more comprehensive list of cytokines of immunological interest is in Appendix III. Although many cytokines can have diverse biological effects when tested in vitro, targeted disruption of the genes for cytokines and cytokine receptors in mice (see Appendix I, Section A-35) has helped to clarify their physiological roles.


Immunology_Janeway

Immunology_Janeway. Binding of the T-cell receptor orchestrates the polarized release of cytokines so that they are concentrated at the site of contact with the target cell (see Section 9-25). Furthermore, most of the soluble cytokines have local actions that synergize with those of the membrane-bound effector molecules. The effect of all these molecules is therefore combinatorial, and, because the membrane-bound effectors can bind only to receptors on an interacting cell, this is another mechanism by which selective effects of cytokines are focused on the target cell. The effects of some cytokines are further confined to target cells by tight regulation of their synthesis: the synthesis of IL-2, IL-4, and IFN-γ, for example, is controlled by mRNA instability (see Section 9-16), so that their secretion by T cells does not continue after the interaction with a target cell has ended.


Immunology_Janeway

Immunology_Janeway. Some cytokines have distant effects. IL-3 and GM-CSF (see Fig. 9.39) are released by TH1, TH2, and TH17 cells and act on bone marrow cells to stimulate the production of macrophages and granulocytes, which are important innate effector cells in both antibodyand T-cell-mediated immunity. IL-3 and GM-CSF also stimulate the production of dendritic cells from bone marrow precursors. IL-17A and IL-17F produced by TH17 cells act primarily on stromal cells, activating them to produce G-CSF, which enhances production


Immunology_Janeway

Immunology_Janeway. Fig. 9.40 The nomenclature and functions of well-defined many effects through what is called a ‘cytokine network.’ ↑ increase; T-cell cytokines. each cytokine has multiple activities on different ↓, decrease; CTL, cytotoxic lymphocyte; nK cells, natural killer cells; cell types. Major activities of effector cytokines are highlighted in CsF, colony-stimulating factor; IBd, inflammatory bowel disease; no, red. The mixture of cytokines secreted by a given cell type produces nitric oxide. of neutrophils by the bone marrow. TH2 cells produce IL-5, which stimulates bone marrow production of eosinophils. Whether a given cytokine effect is local or more distant is likely to reflect the amounts released, the degree to which this release is focused on the target cell, and the stability of the cytokine in vivo. 9-28 T cells express several TNF-family cytokines as trimeric proteins that are usually associated with the cell surface.


Immunology_Janeway

Immunology_Janeway. 9-28 T cells express several TNF-family cytokines as trimeric proteins that are usually associated with the cell surface. Most effector T cells express members of the TNF family as membrane-associated proteins on the cell surface. These include TNF-α, the lymphotoxins (LTs), Fas ligand (CD178), and CD40 ligand, the latter two always being cell-surface associated. TNF-α is made by T cells in soluble and membrane-associated forms and assembles into a homotrimer. Secreted LT-α is a homotrimer, but in its membrane-bound form, LT-α is linked to a third, transmembrane member of this family called LT-β to form heterotrimers, called simply LT-β (see Section 9-2). The receptors for TNF-α and LT-α, TNFR-I and TNFR-II, form homotrimers when bound to their ligands. The trimeric structure is characteristic of all members of the TNF family, and the ligand-induced trimerization of their receptors seems to be the critical event in initiating signaling.


Immunology_Janeway

Immunology_Janeway. Fas ligand and CD40 ligand bind respectively to the transmembrane proteins Fas (CD95) and CD40 on target cells. Fas contains a ‘death’ domain in its cytoplasmic tail, and binding of Fas by Fas ligand induces death by apoptosis in the Fas-bearing cell (see Fig. 11.22). Other TNFR-family members, including TNFR-I, are also associated with death domains and can also induce apoptosis. Thus, TNF-α and LT-α can induce apoptosis by binding to TNFR-I.


Immunology_Janeway

Immunology_Janeway. CD40 ligand is particularly important for CD4 T-cell effector function; its expression is induced on TH1, TH2, TH17, and TFH cells, and it delivers activating signals to B cells and innate immune cells through CD40. The cytoplasmic tail of CD40 lacks a death domain; instead, it is linked downstream to proteins called TRAFs (TNF-receptor-associated factors). CD40 is involved in the activation of B cells and macrophages; the ligation of CD40 on B cells promotes growth and isotype switching, whereas CD40 ligation on macrophages induces them to secrete higher amounts of pro-inflammatory cytokines (for example, TNF-α) and become receptive to much lower concentrations of IFN-γ. Deficiency in CD40 ligand expression is associated with immunodeficiency, as we will learn in Chapter 13. Summary.


Immunology_Janeway

Immunology_Janeway. Interactions between effector T cells and their targets are initiated by transient antigen-nonspecific adhesion. T-cell effector functions are elicited only when peptide:MHC complexes on the surface of the target cell are recognized by the receptor on an effector T cell. This recognition event triggers the effector T cell to adhere more strongly to the antigen-bearing target cell and to release its effector molecules directly at the target cell, leading to the activation or death of the target. The immunological consequences of antigen recognition by an effector T cell are determined largely by the set of effector molecules that the T cell produces on binding a specific target cell. CD8 cytotoxic T cells store preformed cytotoxins in specialized cytotoxic granules whose release is tightly focused at the site of contact with the infected target cell, thus killing it without killing any uninfected cells nearby. Cytokines and members of the TNF family of membrane-associated effector


Immunology_Janeway

Immunology_Janeway. focused at the site of contact with the infected target cell, thus killing it without killing any uninfected cells nearby. Cytokines and members of the TNF family of membrane-associated effector proteins are synthesized de novo by most effector T cells. Membrane-associated effector molecules can deliver signals only to an interacting cell bearing the appropriate receptor, whereas soluble cytokines can act on cytokine receptors expressed locally on the target cell, or on other cells at a distance. The actions of cytokines and membrane-associated effector molecules through their specific receptors, together with the effects of the cytotoxins released by CD8 cells, account for most of the effector functions of T cells.


Immunology_Janeway

T-cell-mediated cytotoxicity.

Immunology_Janeway. T-cell-mediated cytotoxicity.


Immunology_Janeway

Immunology_Janeway. All viruses, and some bacteria, multiply in the cytoplasm of infected cells; indeed, a virus is a highly sophisticated parasite that has no biosynthetic or metabolic apparatus of its own and, in consequence, can replicate only inside cells. Although susceptible to antibody-mediated clearance before they enter cells, once they enter cells these pathogens are not accessible to antibodies and can be eliminated only by the destruction or modification of the infected cells in which they replicate. This role in host defense is largely filled by CD8 cytotoxic T cells, although TH1 cells may also acquire cytotoxic capacities. The crucial role of cytotoxic T cells in limiting such infections is seen in the increased susceptibility of animals artificially depleted of these T cells, or of mice or humans that lack the MHC class I molecules that present antigen to CD8 T cells. The elimination of infected cells without the destruction of healthy tissue requires the cytotoxic mechanisms of CD8 T


Immunology_Janeway

Immunology_Janeway. humans that lack the MHC class I molecules that present antigen to CD8 T cells. The elimination of infected cells without the destruction of healthy tissue requires the cytotoxic mechanisms of CD8 T cells to be both powerful and accurately targeted.


Immunology_Janeway

Immunology_Janeway. 9-29 Cytotoxic T cells induce target cells to undergo programmed cell death via extrinsic and intrinsic pathways of apoptosis.


Immunology_Janeway

Immunology_Janeway. To deprive cytosolic pathogens of their cellular host, cytotoxic T cells target the infected host cells for death. Cells can die in various ways. Physical or chemical injury, such as the deprivation of oxygen that occurs in heart muscle during a heart attack or membrane damage with antibody and complement, leads to cell disintegration or necrosis. This form of cell death is often accompanied by local inflammation and stimulates a wound healing response. The other form of cell death is known as programmed cell death, which can occur by apoptosis or autophagy. Apoptosis is a regulated process that is induced either by specific extracellular signals or by the lack of signals required for survival, and proceeds by a series of cellular events that include plasma membrane blebbing, changes in the distribution of membrane lipids, and enzymatic fragmentation of chromosomal DNA. A hallmark of apoptosis is the fragmentation of nuclear DNA into pieces 200 base pairs long through the activation


Immunology_Janeway

Immunology_Janeway. the distribution of membrane lipids, and enzymatic fragmentation of chromosomal DNA. A hallmark of apoptosis is the fragmentation of nuclear DNA into pieces 200 base pairs long through the activation of nucleases that cleave the DNA between nucleosomes. As described in Chapter 6, autophagy is the process of degrading senescent or abnormal proteins and organelles. In autophagic cell death, large vacuoles degrade cellular organelles before the condensation and destruction of the nucleus that is characteristic of apoptosis.


Immunology_Janeway

Immunology_Janeway. Cytotoxic T cells kill by inducing their targets to undergo apoptosis (Fig. 9.41). Two general pathways are involved in signaling apoptotic cell death. One, called the extrinsic pathway of apoptosis, is mediated by the activation of so-called death receptors by extracellular ligands. Engagement of ligand stimulates apoptosis in receptor-bearing cells. The other pathway is known as the intrinsic or mitochondrial pathway of apoptosis and is induced in response to noxious stimuli (for example, ultraviolet irradiation or chemotherapeutic drugs), or lack of the growth factors required for survival. Common to both pathways is the activation of specialized proteases called aspartic acid-specific cysteine proteases, or caspases, which were introduced in Chapter 3 for their role in processing the cytokines IL-1 and IL-18 to their mature forms.


Immunology_Janeway

Immunology_Janeway. Fig. 9.41 Cytotoxic CD8 T cells can induce apoptosis in target cells. specific recognition of peptide:MhC complexes on a target cell (top panels) by a cytotoxic Cd8 T cell (CTL) leads to the death of the target cell by apoptosis. Cytotoxic T cells can recycle to kill multiple targets. each killing requires the same series of steps, including receptor binding and the directed release of cytotoxic proteins stored in granules. The process of apoptosis is shown in the micrographs (bottom panels), where panel a shows a healthy cell with a normal nucleus. early in apoptosis (panel b) the chromatin becomes condensed (red) and, although the cell sheds membrane vesicles, the integrity of the cell membrane is retained, in contrast to the necrotic cell in the upper part of the same field. In late stages of apoptosis (panel c), the cell nucleus (middle cell) is very condensed, no mitochondria are visible, and the cell has lost much of its cytoplasm and membrane through the shedding of vesicles.


Immunology_Janeway

Immunology_Janeway. of apoptosis (panel c), the cell nucleus (middle cell) is very condensed, no mitochondria are visible, and the cell has lost much of its cytoplasm and membrane through the shedding of vesicles. photographs (×3500) courtesy of r. Windsor and e. hirst.


Immunology_Janeway

Immunology_Janeway. Like many other proteases, caspases are synthesized as inactive pro-enzymes, in this case, pro-caspases, in which the catalytic domain is inhibited by an adjacent pro-domain. Pro-caspases are activated by other caspases that cleave the protein to release the inhibitory pro-domain. There are two classes of caspases involved in the apoptotic pathway: initiator caspases promote apoptosis by cleaving and activating other caspases; effector caspases initiate the cellular changes associated with apoptosis. The extrinsic pathway uses two related initiator caspases, caspase 8 and caspase 10, whereas the intrinsic pathway uses caspase 9. Both pathways use caspases 3, 6, and 7 as effector caspases. The effector caspases cleave a variety of proteins that are critical for cellular integrity and also activate enzymes that promote the death of the cell. For example, they cleave and degrade nuclear proteins that are required for the structural integrity of the nucleus, and activate the endonucleases


Immunology_Janeway

Immunology_Janeway. enzymes that promote the death of the cell. For example, they cleave and degrade nuclear proteins that are required for the structural integrity of the nucleus, and activate the endonucleases that fragment the chromosomal DNA.


Immunology_Janeway

Immunology_Janeway. Cytotoxic T cells can induce target-cell death by either the extrinsic or the intrinsic apoptotic pathway. The extrinsic pathway is mediated by expression of FasL and TNF-α or LT-α, receptors for which (Fas, or CD95, and TNFR-I) are expressed by other cells of the immune system, as well as non-immune-system cells. Because the distribution of these receptors is somewhat restricted, cytotoxic T cells have acquired a more universal mechanism for inducing cell death in antigen-specific targets: the directional release of cytotoxic granules that activate the intrinsic pathway of apoptosis. When cytotoxic T cells are mixed with target cells and rapidly brought into contact by centrifugation, they can induce antigen-specific target cells to die within 5 minutes, although death can take hours to become fully evident. The rapidity of this response reflects the release of preformed effector molecules that are delivered to the target cell. In addition to killing the host cell, the apoptotic


Immunology_Janeway

Immunology_Janeway. to become fully evident. The rapidity of this response reflects the release of preformed effector molecules that are delivered to the target cell. In addition to killing the host cell, the apoptotic mechanism may also act directly on cytosolic pathogens. For example, the nucleases that are activated in apoptosis to destroy cellular DNA can also degrade viral DNA. This prevents the assembly of virions and the release of infectious virus, which could otherwise infect nearby cells. Other enzymes activated in the course of apoptosis may destroy nonviral cytosolic pathogens. Apoptosis is therefore preferable to necrosis as a means of killing infected cells; in cells dying by necrosis, intact pathogens are released from the dead cell, and these can continue to infect healthy cells or parasitize the macrophages that ingest them.


Immunology_Janeway

Immunology_Janeway. 9-30 The intrinsic pathway of apoptosis is mediated by the release of cytochrome c from mitochondria. Apoptosis by the intrinsic pathway is triggered by the release of cytochrome c from mitochondria, which triggers the activation of caspases. Once in the cytoplasm, cytochrome c binds to a protein called Apaf-1 (apoptotic protease activating factor-1), stimulating its oligomerization to form the apoptosome. The apoptosome then recruits an initiator caspase, pro-caspase 9, aggregation of which promotes its self-cleavage and frees its catalytic domain to activate effector caspases (Fig. 9.42). The release of cytochrome c is controlled by interactions between members of the Bcl-2 family of proteins. The Bcl-2 family of proteins is defined by the presence of one or more Bcl-2 homology (BH) domains and can be divided into two general groups: members that promote apoptosis, and members that


Immunology_Janeway

Immunology_Janeway. When programmed cell death is induced, the mitochondria swell and leak, releasing cytochrome c, which binds to and induces a conformational change in Apaf-1 caspase 3 cleaved ICAD Apaf-1:cytochrome ccomplex self assembles into an apoptosome, which recruits and activates multiple copies of pro-caspase 9, which in turn activates pro-caspase 3 Caspase 3 cleaves ICAD, releasing CAD to enter the nucleus and cleave DNA pro-caspase 3 pro-caspase 3 cleavage CAD Apaf-1


Immunology_Janeway

Immunology_Janeway. Fig. 9.42 In the intrinsic pathway, cytochrome c release from apaf-1. The resultant conformational change that ensues in apaf-1 mitochondria induces formation of the apoptosome, which induces self-assembly of the multimeric apoptosome, which recruits activates pro-caspase 9 to initiate programmed cell death. pro-caspase 9 (third panel). Clustering of pro-caspase 9 by the In normal cells, cytochrome c is confined to the mitochondria apoptosome activates it, allowing it to cleave downstream caspases, (first panel). however, during stimulation of the intrinsic pathway, such as caspase 3; this results in the activation of enzymes such as the mitochondria swell, allowing the cytochrome c to leak out ICad, which can cleave dna (fourth panel). into the cytosol (second panel), where cytochrome c is bound by Fig. 9.43 General scheme of intrinsic pathway regulation by the Bcl-2 family of proteins. extracellular apoptotic stimuli activate a group of pro-apoptotic (sentinel) proteins. sentinel


Immunology_Janeway

Immunology_Janeway. c is bound by Fig. 9.43 General scheme of intrinsic pathway regulation by the Bcl-2 family of proteins. extracellular apoptotic stimuli activate a group of pro-apoptotic (sentinel) proteins. sentinel proteins can function either to block the protection provided by pro-survival, protector proteins or to directly activate pro-apoptotic, executioner proteins. In mammalian cells, apoptosis is mediated by the executioner proteins Bax, Bak, and Bok. In normal cells, these proteins are prevented from acting by the protector proteins (Bcl-2, Bcl-XL, and Bcl-W). The release of activated executioner proteins causes the release of cytochrome c and subsequent cell death, as shown in Fig. 9.42.


Immunology_Janeway

Immunology_Janeway. Apoptotic stimuli Sentinels Bad, Bil, Bid, Bim, p53, PUMA, NOXA Death Protectors Bcl-2, Bcl-XL, Bcl-W Executioners Bax, Bak, Bok inhibit apoptosis (Fig. 9.43). Pro-apoptotic Bcl-2 family members, such as Bax, Bak, and Bok (referred to as executioners), bind to mitochondrial membranes and can directly cause cytochrome c release. How they do this is still not known, but they may form pores in the membranes.


Immunology_Janeway

Immunology_Janeway. The anti-apoptotic Bcl-2 family members are induced by stimuli that promote cell survival. The best known of the anti-apoptotic proteins is Bcl-2 itself. The Bcl2 gene was first identified as an oncogene in a B-cell lymphoma, and its overexpression in tumors makes the cells more resistant to apoptotic stimuli and thus more likely to progress to an invasive cancer. Other members of the inhibitory family include Bcl-XL and Bcl-W. Anti-apoptotic proteins function by binding to the mitochondrial membrane to block the release of cytochrome c. The precise mechanism of inhibition is not clear, but they may function by directly blocking the function of the pro-apoptotic family members.


Immunology_Janeway

Immunology_Janeway. A second family of pro-apoptotic Bcl-2 family members are termed 'sentinels' and are activated by apoptotic stimuli. Once activated, these proteins, which include Bad, Bid, and PUMA, can either act to block the activity of the antiapoptotic proteins or act directly to stimulate the activity of the executioner pro-apoptotic proteins. 9-31 Cytotoxic effector proteins that trigger apoptosis are contained in the granules of CD8 cytotoxic T cells.


Immunology_Janeway

Immunology_Janeway. The principal mechanism of cytotoxic T-cell action is the calcium-dependent release of specialized cytotoxic granules upon recognition of antigen on the surface of a target cell. Cytotoxic granules are modified lysosomes that contain at least three distinct classes of cytotoxic effector proteins that are expressed specifically in cytotoxic T cells: perforin, granzymes, and granulysin (Fig. 9.44). These proteins are stored in cytotoxic granules in an active form, but conditions within the granules prevent their actions until after release. Perforin acts by forming pores in, or perforating, the target-cell plasma membrane, which both causes direct damage to the target cell and forms a conduit through which other contents of cytotoxic granules are delivered into the cytosol of the target cell. Granzymes, of which there are 5 in humans and 10 in the mouse, activate apoptosis once delivered to the target-cell cytosol via pores formed by perforin. Granulysin, which is expressed in humans


Immunology_Janeway

Immunology_Janeway. Granzymes, of which there are 5 in humans and 10 in the mouse, activate apoptosis once delivered to the target-cell cytosol via pores formed by perforin. Granulysin, which is expressed in humans but not in mice, has antimicrobial activity and at high concentrations is also able to induce apoptosis in target cells. Cytotoxic granules also contain the proteoglycan serglycin, which acts as a scaffold, forming a complex with perforin and the granzymes.


Immunology_Janeway

Immunology_Janeway. Both perforin and granzymes are required for effective target-cell killing. In cytotoxic cells that lack granzymes, the presence of perforin alone can kill target cells, but large numbers of cytotoxic cells are needed because the killing is very inefficient. In contrast, cytotoxic T cells from mice lacking perforin are unable to kill other cells, due to the lack of a mechanism to deliver granzymes into the target cell.


Immunology_Janeway

Immunology_Janeway. Granzymes trigger apoptosis in the target cell both by directly activating caspases and by damaging mitochondria, which also activates caspases. The two most abundant granzymes are granzymes A and B. Granzyme A triggers cell death by caspase-independent mitochondrial damage, through mechanisms that are not completely understood. Granzyme B, like the caspases, cleaves proteins after aspartic acid residues and activates caspase 3, thereby activating a caspase proteolytic cascade, which eventually activates the caspase-activated deoxyribonuclease (CAD) by cleaving an inhibitory protein (ICAD) that binds to and inactivates CAD. This nuclease is believed to be the enzyme that degrades DNA in target cells (Fig. 9.45). Granzyme B also targets mitochondria to activate the intrinsic apoptotic pathway; it cleaves the protein BID (for BH3-interacting domain death agonist protein), either directly recognition of its antigen on a virus-infected cell by a cytotoxic Cd8 T cell induces the T cell to


Immunology_Janeway

Immunology_Janeway. it cleaves the protein BID (for BH3-interacting domain death agonist protein), either directly recognition of its antigen on a virus-infected cell by a cytotoxic Cd8 T cell induces the T cell to release the contents of its cytotoxic granules in a directed fashion. perforin and granzymes, in a complex with the proteoglycan serglycin, are delivered to the membrane of the target cell (top panel). By an unknown mechanism, perforin directs the entry of the granule contents into the cytosol of the target cell without apparent pore formation, and the introduced granzymes then act on specific intracellular targets such as the proteins BId and pro-caspase 3 (second panel). either directly or indirectly, the granzymes cause the cleavage of BId into truncated BId (tBId) and the cleavage of pro-caspase 3 into an active caspase (third panel). tBId acts on mitochondria to release cytochrome c into the cytosol. This promotes apoptosis by inducing the formation of the apoptosome that activates


Immunology_Janeway

Immunology_Janeway. 3 into an active caspase (third panel). tBId acts on mitochondria to release cytochrome c into the cytosol. This promotes apoptosis by inducing the formation of the apoptosome that activates procaspase 9, which in turn further amplifies caspase 3 activation. activated caspase 3 targets ICad to release caspase-activated dnase (Cad), which fragments the dna (bottom panel).


Immunology_Janeway

Immunology_Janeway. or indirectly by activated caspase 3, causing disruption of the mitochondrial outer membrane and the release from the mitochondrial intermembrane space of pro-apoptotic molecules such as cytochrome c. As discussed above (Section 9-30), cytochrome c is central to amplification of the intrinsic apoptotic cascade, as it initiates assembly of the apoptosome with Apaf-1, which in turn activates the initiator caspase 9. Thus, granzyme B acts directly to activate the effector caspase 3, and indirectly to activate the initiator caspase 9.


Immunology_Janeway

Immunology_Janeway. Cells undergoing programmed cell death are rapidly ingested by phagocytic cells, which recognize a change in the cell membrane: phosphatidylserine, which is normally found only in the inner leaflet of the membrane, replaces phosphatidylcholine as the predominant phospholipid in the outer leaflet. The ingested cell is broken down and completely digested by the phagocyte without the induction of co-stimulatory proteins. Thus, apoptosis is normally an immunologically ‘quiet’ process; that is, apoptotic cells do not normally contribute to or stimulate immune responses. 9-32 Cytotoxic T cells are selective serial killers of targets expressing a specific antigen.


Immunology_Janeway

Immunology_Janeway. When cytotoxic T cells are offered a mixture of equal amounts of two target cells, one bearing a specific antigen and the other not, they kill only the target cell bearing the specific antigen. The ‘innocent bystander’ cells and the cytotoxic T cells themselves are not killed. The cytotoxic T cells are probably not killed because release of the cytotoxic effector molecules is highly polarized. As we saw in Fig. 9.38, cytotoxic T cells orient their Golgi apparatus and microtubule-organizing center to focus secretion on the point of contact with a target cell. Granule movement toward the point of contact is shown in Fig. 9.46. Cytotoxic T cells attached to several different target cells reorient their secretory apparatus toward each cell in turn and kill them one by one, strongly suggesting that the mechanism whereby cytotoxic mediators are released allows attack at only one point of contact at any one time. The narrowly focused action of CD8 cytotoxic T cells allows them to kill single


Immunology_Janeway

Immunology_Janeway. that the mechanism whereby cytotoxic mediators are released allows attack at only one point of contact at any one time. The narrowly focused action of CD8 cytotoxic T cells allows them to kill single infected cells in a tissue without creating widespread tissue damage (Fig. 9.47) and is of crucial importance in tissues where cell regeneration does not occur, as with the neurons of the central nervous system, or is very limited, as in the pancreatic islets.


Immunology_Janeway

Immunology_Janeway. Cytotoxic T cells can kill their targets rapidly because they store preformed cytotoxic proteins in forms that are inactive in the environment of the cytotoxic granule. Cytotoxic proteins are synthesized and loaded into the granules soon after the first encounter of a naive cytotoxic precursor T cell with its specific antigen. Ligation of the T-cell receptor similarly induces de novo synthesis of perforin and granzymes in effector CD8 T cells, so that the supply of cytotoxic granules is replenished. This makes it possible for a single CD8 T cell to kill a series of targets in succession. Fig. 9.45 Perforin, granzymes, and serglycin are released from cytotoxic granules and deliver granzymes into the cytosol of target cells to induce apoptosis.


Immunology_Janeway

Immunology_Janeway. Fig. 9.45 Perforin, granzymes, and serglycin are released from cytotoxic granules and deliver granzymes into the cytosol of target cells to induce apoptosis. Release of cytochrome cinto cytosol activates apoptosis, and CAD induces DNA fragmentation Truncated BID (tBID) disrupts mitochondrial outer membrane, and activated caspase 3 cleaves ICAD, releasing caspase-activated DNase (CAD) Granzyme B is delivered into the cytosol of the infected cell via pores formed by perforin, and targets BID and pro-caspase 3 pro-caspase 3BID BAX BAD Engagement of TCR by peptide:MHC complex causes directed release of perforin and granzymes complexed with serglycin cytotoxic T cell virus-infected cell perforin cytotoxic granulegranzymeserglycin MHC TCR caspase 3 tBID cytochrome cCAD cleaved ICAD DNA


Immunology_Janeway

Immunology_Janeway. Fig. 9.46 Effector molecules are released from T-cell granules in a highly polar fashion. The granules of cytotoxic T cells can be labeled with fluorescent dyes, allowing the granules to be seen under the microscope and their movements to be followed by time-lapse photography. here we show a series of pictures taken during the interaction of a cytotoxic T cell with a target cell, which is eventually killed. In the top panel, at time 0, the T cell (upper right) has just made contact with a target cell (diagonally below). at this time, the granules of the T cell, labeled with a red fluorescent dye, are distant from the point of contact. In the second panel, after 1 minute has elapsed, the granules have begun to move toward the target cell, a move that has essentially been completed in the third panel, after 4 minutes. after 40 minutes, in the last panel, the granule contents have been released into the space between the T cell and the target, which has begun to undergo apoptosis (note


Immunology_Janeway

Immunology_Janeway. third panel, after 4 minutes. after 40 minutes, in the last panel, the granule contents have been released into the space between the T cell and the target, which has begun to undergo apoptosis (note the fragmented nucleus). The T cell will now disengage from the target cell, whereupon it can go on to recognize and kill other targets. photographs courtesy of G. Griffiths.


Immunology_Janeway

Immunology_Janeway. 9-33 Cytotoxic T cells also act by releasing cytokines.


Immunology_Janeway

Immunology_Janeway. Inducing apoptosis in target cells is the main way in which CD8 cytotoxic T cells eliminate infection. However, most CD8 cytotoxic T cells also release the cytokines IFN-γ, TNF-α, and LT-α, which contribute to host defense in other ways. IFN-γ inhibits viral replication directly, and induces the increased expression of MHC class I molecules and of other proteins that are involved in peptide loading of these newly synthesized MHC class I molecules in infected cells. This increases the chance that infected cells will be recognized as target cells for cytotoxic attack. IFN-γ also activates macrophages, recruiting them to sites of infection, where they serve both as effector cells and as antigen-presenting cells. TNF-α and LT-α can synergize with IFN-γ in macrophage activation via TNFR-II, and can kill some target cells through their interaction with TNFR-I, which can induce apoptosis (see Sections 9-28 and 9-29). Thus, effector CD8 cytotoxic T cells act in a variety of ways to limit the


Immunology_Janeway

Immunology_Janeway. can kill some target cells through their interaction with TNFR-I, which can induce apoptosis (see Sections 9-28 and 9-29). Thus, effector CD8 cytotoxic T cells act in a variety of ways to limit the spread of cytosolic pathogens.


Immunology_Janeway

Summary.

Immunology_Janeway. Summary.


Immunology_Janeway

Immunology_Janeway. Effector CD8 cytotoxic T cells are essential in host defense against pathogens that reside in the cytosol: most commonly these will be viruses. These cytotoxic T cells can kill any cell harboring such pathogens by recognizing foreign peptides that are transported to the cell surface bound to MHC class I molecules. CD8 cytotoxic T cells perform their killing function by releasing three types of preformed cytotoxic proteins: granzymes, which use multiple mechanisms to induce apoptosis in any type of target cell; perforin, which acts in the delivery of granzymes into the target cell; and granulysin, which has antimicrobial activity and is pro-apoptotic. These properties allow the cytotoxic T cell to attack and destroy virtually any cell infected with a cytosolic pathogen. The membrane-bound Fas ligand, expressed by CD8 and some CD4 T cells, may also induce apoptosis by binding to Fas, which is expressed on some target cells. However, this pathway is less important in most infections than


Immunology_Janeway

Immunology_Janeway. ligand, expressed by CD8 and some CD4 T cells, may also induce apoptosis by binding to Fas, which is expressed on some target cells. However, this pathway is less important in most infections than that mediated by cytotoxic granules. CD8 cytotoxic T cells also produce IFN-γ, which inhibits viral replication and is an important inducer of MHC class I molecule expression and macrophage activation. Cytotoxic T cells kill infected targets with great precision, sparing adjacent normal cells. This precision is crucial in minimizing tissue damage while allowing the eradication of infected cells.


Immunology_Janeway

Summary to Chapter 9.

Immunology_Janeway. Summary to Chapter 9.


Immunology_Janeway

Immunology_Janeway. An adaptive immune response is initiated when naive T cells encounter specific antigen on the surface of an antigen-presenting cell in T-cell zones of secondary lymphoid tissues. In most cases, the antigen-presenting cells responsible for activating naive T cells, and inducing their clonal expansion, are conventional dendritic cells that express the co-stimulatory molecules B7.1 and B7.2. Conventional dendritic cells not only reside in lymphoid tissues, but they also survey the periphery, where they encounter pathogens, take up antigen at sites of infection, become activated through innate recognition, and migrate to local lymphoid tissue. The dendritic cell may become a potent direct activator of naive T cells, or it may transfer antigen to dendritic cells resident in secondary lymphoid organs for cross-presentation to naive CD8 T cells. Plasmacytoid dendritic cells contribute to rapid responses against viruses by producing type I interferons. Activated T cells produce IL-2, which is


Immunology_Janeway

Immunology_Janeway. for cross-presentation to naive CD8 T cells. Plasmacytoid dendritic cells contribute to rapid responses against viruses by producing type I interferons. Activated T cells produce IL-2, which is important in modulating early proliferation and differentiation of T cells; various other signals drive the differentiation of several types of effector T cells, which primarily act by releasing mediators directly onto their target cells. This triggering of effector T cells by peptide:MHC complexes occurs independently of co-stimulation, so that any infected target cell can be activated or destroyed by an effector T cell. CD8 cytotoxic T cells kill target cells infected with cytosolic pathogens, thus removing sites of pathogen replication. CD4 T cells can become specialized effectors that in turn promote distinct arms of the immune response by targeting different innate and adaptive immune cells for enhanced effector function: macrophages (TH1); eosinophils, basophils, and mast cells (TH2);


Immunology_Janeway

Immunology_Janeway. promote distinct arms of the immune response by targeting different innate and adaptive immune cells for enhanced effector function: macrophages (TH1); eosinophils, basophils, and mast cells (TH2); neutrophils (TH17); or B cells (TFH). Thus, effector T cells control virtually all known effector mechanisms of the adaptive and innate immune response. In addition, subsets of CD4 regulatory T cells are produced that help control and limit immune responses by suppressing T-cell activity.


Immunology_Janeway

Immunology_Janeway. Fig. 9.47 Cytotoxic T cells kill target cells bearing specific antigen while sparing neighboring uninfected cells. all the cells in a tissue are susceptible to killing by the cytotoxic proteins of armed effector Cd8 T cells, but only infected cells are killed. specific recognition by the T-cell receptor identifies which target cell to kill, and the polarized release of the cytotoxic granules (not shown) ensures that neighboring cells are spared. T cell recognizes infected cell Infected cell is programmed for death Neighboring uninfected cells are not killed Questions. 9.1 Multiple Choice: Which of the following statements is true? A. development of the arterial and venous system is regulated by the homeobox transcription factor prox1. B. arteries deliver lymphotoxin to the non-hematopoietic stromal LTi cell to induce lymph-node development. C. Lymphotoxin-α3 signaling represses nFκB to induce chemokines such as CXCL13.


Immunology_Janeway

Immunology_Janeway. B. arteries deliver lymphotoxin to the non-hematopoietic stromal LTi cell to induce lymph-node development. C. Lymphotoxin-α3 signaling represses nFκB to induce chemokines such as CXCL13. D. Lymphotoxin-α3 binds TnFr-I and supports development of the cervical and mesenteric lymph nodes.


Immunology_Janeway

Immunology_Janeway. C. Lymphotoxin-α3 signaling represses nFκB to induce chemokines such as CXCL13. D. Lymphotoxin-α3 binds TnFr-I and supports development of the cervical and mesenteric lymph nodes. 9.2 Fill-in-the-Blanks: T and B cells are distributed to the secondary lymphoid organs through the blood. These are then directed to their respective compartments as instructed by chemokines. For example, CCL21 is secreted by _________ of the T-cell zone in the spleen and displayed by the _______in the lymph nodes. signaling of this chemokine as well as _______through CCr7 directs the T cells into the respective T-cell zone. In contrast, _______ is the ligand for CXCr5, which is secreted by __________ and attracts B cells to the ______. T cells can also respond to CXCL13 as a subset of T cells expresses _______, which allows them to enter the B-cell follicle and participate in the formation of the germinal center.


Immunology_Janeway

Immunology_Janeway. 9.3 Multiple Choice: Which of the following correctly describes events necessary for naive T-cell entry into the lymph node? A. CCr7 signaling induces Gαi, which results in lowered affinity for integrin binding. B. upregulation of s1p receptor on naive T cells promotes migration into the lymph node. C. rolling in the hev exposes the T cell to CCL21, which activates LFa-1 and promotes migration. D. MadCaM-1 expression on the hev interacts with Cd62L on the T cell and promotes migration into the lymph node. 9.4 Short Answer: In some cases, hsv or influenza viruses infect antigen-presenting cells from peripheral tissues that do not present the viral antigens to naive T cells. how is the immune system able to develop an adaptive immune response to such pathogens? 9.5 True or False: TLr stimulation induces CCr7 expression in the dendritic cells, which promotes migration to the lymph node through the bloodstream.


Immunology_Janeway

Immunology_Janeway. 9.5 True or False: TLr stimulation induces CCr7 expression in the dendritic cells, which promotes migration to the lymph node through the bloodstream. 9.6 Matching: Classify each of the following activation signatures as a conventional dendritic cell (cdC) or plasmacytoid dendritic cell (pdC) pathogen response. ___1. production of CCL18 ___2. Continuous MhC recycling upon activation ___3. expression of dC-sIGn ___4. expression of Cd80 and Cd86 ___5. Cd40L expression upon TLr-9 stimulation 9.7 Short Answer: how does the process of antigen presentation differ among B cells, dendritic cells, and macrophages in the context of an immune response? 9.8 Multiple Choice: Which of the following is a common consequence of TCr and CCr7 signaling? A. Integrin activation B. positive selection C. Th1 induction D. Th2 induction 9.9 Multiple Choice: Which of the following describes a mechanism by which Cd28 signaling can increase IL-2 production?


Immunology_Janeway

Immunology_Janeway. B. positive selection C. Th1 induction D. Th2 induction 9.9 Multiple Choice: Which of the following describes a mechanism by which Cd28 signaling can increase IL-2 production? A. Cd28 signaling induces the expression of proteins that stabilize the IL-2 mrna sequence. B. pI 3-kinase inhibits akt, supporting IL-2 production by cell cycle arrest. C. pI 3-kinase suppresses the production of ap-1 and nFκB, thereby increasing IL-2 production. 9.10 True or False: In the majority of viral infections, Cd8 T-cell activation requires Cd4 T-cell help. 9.11 Matching: Match each Cd4 T cell subset-specific secreted cytokine with its respective effector function. A. IL-17 i. eradication of intracellular infections B. IL-4 ii. response to extracellular bacteria C. IFn-γ iii. Control of extracellular parasites


Immunology_Janeway

Immunology_Janeway. A. IL-17 i. eradication of intracellular infections B. IL-4 ii. response to extracellular bacteria C. IFn-γ iii. Control of extracellular parasites D. IL-10 iv. suppression of T-cell responses 9.12 Matching: The following cytokines drive Cd4 Th subset effector differentiation. Match each with its respective subset-specific transcription factor. A. IFn-γ i. rorγt B. IL-4 ii. Foxp3 C. IL-6 and TGF-β iii. T-bet D. TGF-β iv. GaTa3 9.13 Multiple Choice: Which of the following statements is false? A. TCr signaling is strongest at the csMaC. B. Cb1, an e3 ligase, mediates degradation of TCrs in the csMaC. C. Cytoskeletal reorganization directs effector molecule release at the immunological synapse. D. Integrins such as LFa-1 associate in the sMaC. 9.14 Fill-in-the-Blanks: For each of the following sentences, fill in the blanks with the best word selected from the list below. not all words will be used; each word should be used only once.


Immunology_Janeway

Immunology_Janeway. 9.14 Fill-in-the-Blanks: For each of the following sentences, fill in the blanks with the best word selected from the list below. not all words will be used; each word should be used only once. Cd8 T cells can specifically mediate the destruction of infected or malignant cells. In order to do this, Cd8 T cells induce _______ cell death, which can be induced in two different ways. First, Cd8 T cells possess ligands such as _______, _______, or _______ that can induce the _______ apoptosis pathway. In contrast, cell death can also be induced through an intrinsic pathway. To initiate this mechanism, _______ are released, which allow the entrance of granzymes into the cell. once the granzymes have gained access to the cell’s cytoplasm, these can cleave and activate _______, which in turn cleaves _______, allowing _______ to degrade dna. Granzyme B also cleaves _______, and as a consequence disrupts the mitochondrial membrane, allowing for release of _______ and formation of the _______.


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Immunology_Janeway. General references. Coffman, R.L.: Origins of the TH1-TH2 model: a personal perspective. Nat. Immunol. 2006, 7:539–541. Griffith, J.W., Sokol, C.L., and Luster, A.D.: Chemokines and chemokine receptors: positioning cells for host defense and immunity. Annu. Rev. Immunol. 2014, 32:659–702. Heath, W.R., and Carbone, F.R.: Dendritic cell subsets in primary and secondary T cell responses at body surfaces. Nat. Immunol. 2009, 10:1237–1244. Jenkins, M.K., Chu, H.H., McLachlan, J.B., and Moon, J.J.: On the composition of the preimmune repertoire of T cells specific for peptide-major histocompatibility complex ligands. Annu. Rev. Immunol. 2010, 28:275–294. Springer, T.A.: Traffic signals for lymphocyte recirculation and leukocyte emigration: the multistep paradigm. Cell 1994, 76:301–314. Zhu, J., Yamane, H., and Paul, W.E.: Differentiation of effector CD4 T cell populations. Annu. Rev. Immunol. 2010 28:445–489. Section references.


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Immunology_Janeway. Zhu, J., Yamane, H., and Paul, W.E.: Differentiation of effector CD4 T cell populations. Annu. Rev. Immunol. 2010 28:445–489. Section references. 9-1 T and B lymphocytes are found in distinct locations in secondary lymphoid tissues. Liu, Y.J.: Sites of B lymphocyte selection, activation, and tolerance in spleen. J. Exp. Med. 1997, 186:625–629. Loder, F., Mutschler, B., Ray, R.J., Paige, C.J., Sideras, P., Torres, R., Lamers, M.C., and Carsetti, R.: B cell development in the spleen takes place in discrete steps and is determined by the quality of B cell receptor-derived signals. J. Exp. Med. 1999, 190:75–89. Mebius, R.E.: Organogenesis of lymphoid tissues. Nat. Rev. Immunol. 2003, 3:292–303. 9-2 The development of secondary lymphoid tissues is controlled by lymphoid tissue inducer cells and proteins of the tumor necrosis factor family.


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Immunology_Janeway. 9-2 The development of secondary lymphoid tissues is controlled by lymphoid tissue inducer cells and proteins of the tumor necrosis factor family. Douni, E., Akassoglou, K., Alexopoulou, L., Georgopoulos, S., Haralambous, S., Hill, S., Kassiotis, G., Kontoyiannis, D., Pasparakis, M., Plows, D., et al.: Transgenic and knockout analysis of the role of TNF in immune regulation and disease pathogenesis. J. Inflamm. 1996, 47:27–38. Fu, Y.X., and Chaplin, D.D.: Development and maturation of secondary lymphoid tissues. Annu. Rev. Immunol. 1999, 17:399–433. Mariathasan, S., Matsumoto, M., Baranyay, F., Nahm, M.H., Kanagawa, O., and Chaplin, D.D.: Absence of lymph nodes in lymphotoxin-α (LTα)-deficient mice is due to abnormal organ development, not defective lymphocyte migration. J. Inflamm. 1995, 45:72–78. Mebius, R.E., and Kraal, G.: Structure and function of the spleen. Nat. Rev. Immunol. 2005, 5: 606–616.


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Immunology_Janeway. Mebius, R.E., and Kraal, G.: Structure and function of the spleen. Nat. Rev. Immunol. 2005, 5: 606–616. Mebius, R.E., Rennert, P., and Weissman, I.L.: Developing lymph nodes collect CD4+CD3– LTβ+ cells that can differentiate to APC, NK cells, and follicular cells but not T or B cells. Immunity 1997, 7:493–504. Roozendaal, R., and Mebius, R.E.: Stromal-immune cell interactions. Annu. Rev. Immunol. 2011, 29:23–43. Wigle, J.T., and Oliver, G.: Prox1 function is required for the development of the murine lymphatic system. Cell 1999, 98:769–778. 9-3 T and B cells are partitioned into distinct regions of secondary lymphoid tissues by the actions of chemokines. Ansel, K.M., and Cyster, J.G.: Chemokines in lymphopoiesis and lymphoid organ development. Curr. Opin. Immunol. 2001, 13:172–179. Cyster, J.G.: Chemokines and cell migration in secondary lymphoid organs. Science 1999, 286:2098–2102. Cyster, J.G.: Leukocyte migration: scent of the T zone. Curr. Biol. 2000, 10:R30–R33.


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Immunology_Janeway. Cyster, J.G.: Chemokines and cell migration in secondary lymphoid organs. Science 1999, 286:2098–2102. Cyster, J.G.: Leukocyte migration: scent of the T zone. Curr. Biol. 2000, 10:R30–R33. Cyster, J.G., Ansel, K.M., Reif, K., Ekland, E.H., Hyman, P.L., Tang, H.L., Luther, S.A., and Ngo, V.N.: Follicular stromal cells and lymphocyte homing to follicles. Immunol. Rev. 2000, 176:181–193. 9-4 Naive T cells migrate through secondary lymphoid tissues, sampling peptide:MHC complexes on dendritic cells. Caux, C., Ait-Yahia, S., Chemin, K., de Bouteiller, O., Dieu-Nosjean, M.C., Homey, B., Massacrier, C., Vanbervliet, B., Zlotnik, A., and Vicari, A.: Dendritic cell biology and regulation of dendritic cell trafficking by chemokines. Springer Semin. Immunopathol. 2000, 22:345–369. Itano, A.A., and Jenkins, M.K.: Antigen presentation to naïve CD4 T cells in the lymph node. Nat. Immunol. 2003, 4:733–739.


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Immunology_Janeway. Itano, A.A., and Jenkins, M.K.: Antigen presentation to naïve CD4 T cells in the lymph node. Nat. Immunol. 2003, 4:733–739. Mackay, C.R., Kimpton, W.G., Brandon, M.R., and Cahill, R.N.: Lymphocyte subsets show marked differences in their distribution between blood and the afferent and efferent lymph of secondary lymph nodes. J. Exp. Med. 1988, 167:1755–1765. Picker, L.J., and Butcher, E.C.: Physiological and molecular mechanisms of lymphocyte homing. Annu. Rev. Immunol. 1993, 10:561–591. Steptoe, R.J., Li, W., Fu, F., O’Connell, P.J., and Thomson, A.W.: Trafficking of APC from liver allografts of Flt3L-treated donors: augmentation of potent allostimulatory cells in recipient lymphoid tissue is associated with a switch from tolerance to rejection. Transpl. Immunol. 1999, 7:51–57. Yoshino, M., Yamazaki, H., Nakano, H., Kakiuchi, T., Ryoke, K., Kunisada, T., and Hayashi, S.: Distinct antigen trafficking from skin in the steady and active states. Int. Immunol. 2003, 15:773–779.


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Immunology_Janeway. Yoshino, M., Yamazaki, H., Nakano, H., Kakiuchi, T., Ryoke, K., Kunisada, T., and Hayashi, S.: Distinct antigen trafficking from skin in the steady and active states. Int. Immunol. 2003, 15:773–779. 9-5 Lymphocyte entry into lymphoid tissues depends on chemokines and adhesion molecules. Hogg, N., Henderson, R., Leitinger, B., McDowall, A., Porter, J., and Stanley, P.: Mechanisms contributing to the activity of integrins on leukocytes. Immunol. Rev. 2002, 186:164–171. Kunkel, E.J., Campbell, D.J., and Butcher, E.C.: Chemokines in lymphocyte trafficking and intestinal immunity. Microcirculation 2003, 10:313–323. Madri, J.A., and Graesser, D.: Cell migration in the immune system: the evolving interrelated roles of adhesion molecules and proteinases. Dev. Immunol. 2000, 7:103–116.


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Immunology_Janeway. Madri, J.A., and Graesser, D.: Cell migration in the immune system: the evolving interrelated roles of adhesion molecules and proteinases. Dev. Immunol. 2000, 7:103–116. Rasmussen, L.K., Johnsen, L.B., Petersen, T.E., and Sørensen, E.S.: Human GlyCAM-1 mRNA is expressed in the mammary gland as splicing variants and encodes various aberrant truncated proteins. Immunol. Lett. 2002, 83:73–75. Rosen, S.D.: Ligands for L-selectin: homing, inflammation, and beyond. Annu. Rev. Immunol. 2004, 22:129–156. von Andrian, U.H., and Mempel, T.R.: Homing and cellular traffic in lymph nodes. Nat. Rev. Immunol. 2003, 3:867–878. 9-6 Activation of integrins by chemokines is responsible for the entry of naive T cells into lymph nodes. Cyster, J.G.: Chemokines, sphingosine-1-phosphate, and cell migration in secondary lymphoid organs. Annu. Rev. Immunol. 2005, 23:127–159.


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Immunology_Janeway. Cyster, J.G.: Chemokines, sphingosine-1-phosphate, and cell migration in secondary lymphoid organs. Annu. Rev. Immunol. 2005, 23:127–159. Laudanna, C., Kim, J.Y., Constantin, G., and Butcher, E.: Rapid leukocyte integrin activation by chemokines. Immunol. Rev. 2002, 186:37–46. Lo, C.G., Lu, T.T., and Cyster, J.G.: Integrin-dependence of lymphocyte entry into the splenic white pulp. J. Exp. Med. 2003, 197:353–361. Luo, B.H., Carman, C.V., and Springer, T.A.: Structural basis of integrin regulation and signaling. Annu. Rev. Immunol. 2007, 25:619–647. Rosen, H., and Goetzl, E.J.: Sphingosine 1-phosphate and its receptors: an autocrine and paracrine network. Nat. Rev. Immunol. 2005, 5:560–570. 9-7 The exit of T cells from lymph nodes is controlled by a chemotactic lipid. Cyster, J.G., and Schwab, S.R.: Sphingosine-1-phosphate and lymphocyte egress from lymphoid organs. Annu. Rev. Immunol. 2012, 30:69-94.


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Immunology_Janeway. Cyster, J.G., and Schwab, S.R.: Sphingosine-1-phosphate and lymphocyte egress from lymphoid organs. Annu. Rev. Immunol. 2012, 30:69-94. 9-8 T-cell responses are initiated in secondary lymphoid organs by activated dendritic cells. Germain, R.N., Miller, M.J., Dustin, M.L., and Nussenzweig, M.C.: Dynamic imaging of the immune system: progress, pitfalls and promise. Nat. Rev. Immunol. 2006, 6:497–507. Miller, M.J., Wei, S.H., Cahalan, M.D., and Parker, I.: Autonomous T cell trafficking examined in vivo with intravital two-photon microscopy. Proc. Natl Acad. Sci. USA 2003, 100:2604–2609. Schlienger, K., Craighead, N., Lee, K.P., Levine, B.L., and June, C.H.: Efficient priming of protein antigen-specific human CD4+ T cells by monocyte-derived dendritic cells. Blood 2000, 96:3490–3498. Thery, C., and Amigorena, S.: The cell biology of antigen presentation in dendritic cells. Curr. Opin. Immunol. 2001, 13:45–51. 9-9 Dendritic cells process antigens from a wide array of pathogens.


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Immunology_Janeway. Thery, C., and Amigorena, S.: The cell biology of antigen presentation in dendritic cells. Curr. Opin. Immunol. 2001, 13:45–51. 9-9 Dendritic cells process antigens from a wide array of pathogens. Belz, G.T., Carbone, F.R., and Heath, W.R.: Cross-presentation of antigens by dendritic cells. Crit. Rev. Immunol. 2002, 22:439–448. Guermonprez, P., Valladeau, J., Zitvogel, L., Thery, C., and Amigorena, S.: Antigen presentation and T cell stimulation by dendritic cells. Annu. Rev. Immunol. 2002, 20:621–667. Mildner, A., and Jung, S.: Development and function of dendritic cells. Immunity 2014, 40:642–656. Satpathy, A.T., Wu, X., Albring, J.C., and Murphy, K.M.: Re(de)fining the dendritic cell lineage. Nat. Immunol. 2012, 13:1145–1154. Shortman, K., and Heath, W.R.: The CD8+ dendritic cell subset. Immunol. Rev. 2010, 234:18–31. Shortman, K., and Naik, S.H.: Steady-state and inflammatory dendritic-cell development. Nat. Rev. Immunol. 2007, 7:19–30.


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Immunology_Janeway. Shortman, K., and Naik, S.H.: Steady-state and inflammatory dendritic-cell development. Nat. Rev. Immunol. 2007, 7:19–30. induces their migration to lymphoid organs and enhances antigen processing. Allan, R.S., Waithman, J., Bedoui, S., Jones, C.M., Villadangos, J.A., Zhan, Y., Lew, A.M., Shortman, K., Heath, W.R., and Carbone, F.R.: Migratory dendritic cells transfer antigen to a lymph node-resident dendritic cell population for efficient CTL priming. Immunity 2006, 25:153–162. Bachman, M.F., Kopf, M., and Marsland, B.J.: Chemokines: more than just road signs. Nat. Rev. Immunol. 2006, 6:159–164. Blander, J.M., and Medzhitov, R.: Toll-dependent selection of microbial antigens for presentation by dendritic cells. Nature 2006, 440:808–812. Iwasaki, A., and Medzhitov, R.: Toll-like receptor control of adaptive immune responses. Nat. Immunol. 2004, 10:988–995. Reis e Sousa, C.: Toll-like receptors and dendritic cells: for whom the bug tolls. Semin. Immunol. 2004, 16:27–34.


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Immunology_Janeway. Reis e Sousa, C.: Toll-like receptors and dendritic cells: for whom the bug tolls. Semin. Immunol. 2004, 16:27–34. and may act as helper cells for antigen presentation by conventional dendritic cells. Asselin-Paturel, C., and Trinchieri, G.: Production of type I interferons: plasmacytoid dendritic cells and beyond. J. Exp. Med. 2005, 202:461–465. Krug, A., Veeraswamy, R., Pekosz, A., Kanagawa, O., Unanue, E.R., Colonna, M., and Cella, M.: Interferon-producing cells fail to induce proliferation of naïve T cells but can promote expansion and T helper 1 differentiation of antigen-experienced unpolarized T cells. J. Exp. Med. 2003, 197:899–906. Kuwajima, S., Sato, T., Ishida, K., Tada, H., Tezuka, H., and Ohteki, T.: Interleukin 15-dependent crosstalk between conventional and plasmacytoid dendritic cells is essential for CpG-induced immune activation. Nat. Immunol. 2006, 7:740–746.


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Immunology_Janeway. Swiecki, M., and Colonna, M.: Unraveling the functions of plasmacytoid dendritic cells during viral infections, autoimmunity, and tolerance. Immunol. Rev. 2010, 234:142–162. 9-12 Macrophages are scavenger cells that can be induced by pathogens to present foreign antigens to naive T cells. Barker, R.N., Erwig, L.P., Hill, K.S., Devine, A., Pearce, W.P., and Rees, A.J.: Antigen presentation by macrophages is enhanced by the uptake of necrotic, but not apoptotic, cells. Clin. Exp. Immunol. 2002, 127:220–225. Underhill, D.M., Bassetti, M., Rudensky, A., and Aderem, A.: Dynamic interactions of macrophages with T cells during antigen presentation. J. Exp. Med. 1999, 190:1909–1914. Zhu, F.G., Reich, C.F., and Pisetsky, D.S.: The role of the macrophage scavenger receptor in immune stimulation by bacterial DNA and synthetic oligonucleotides. Immunology 2001, 103:226–234. 9-13 B cells are highly efficient at presenting antigens that bind to their surface immunoglobulin.


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Immunology_Janeway. 9-13 B cells are highly efficient at presenting antigens that bind to their surface immunoglobulin. Guermonprez, P., England, P., Bedouelle, H., and Leclerc, C.: The rate of dissociation between antibody and antigen determines the efficiency of antibody-mediated antigen presentation to T cells. J. Immunol. 1998, 161:4542–4548. Shirota, H., Sano, K., Hirasawa, N., Terui, T., Ohuchi, K., Hattori, T., and Tamura, G.: B cells capturing antigen conjugated with CpG oligodeoxynucleotides induce Th1 cells by elaborating IL-12. J. Immunol. 2002, 169:787–794. Zaliauskiene, L., Kang, S., Sparks, K., Zinn, K.R., Schwiebert, L.M., Weaver, C.T., and Collawn, J.F.: Enhancement of MHC class II-restricted responses by receptor-mediated uptake of peptide antigens. J. Immunol. 2002, 169:2337–2345. 9-14 Cell-adhesion molecules mediate the initial interaction of naive T cells with antigen-presenting cells.


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Immunology_Janeway. 9-14 Cell-adhesion molecules mediate the initial interaction of naive T cells with antigen-presenting cells. Dustin, M.L.: T-cell activation through immunological synapses and kinapses. Immunol. Rev. 2008, 221:77–89. Friedl, P., and Brocker, E.B.: TCR triggering on the move: diversity of T-cell interactions with antigen-presenting cells. Immunol. Rev. 2002, 186:83–89. Gunzer, M., Schafer, A., Borgmann, S., Grabbe, S., Zanker, K.S., Brocker, E.B., Kampgen, E., and Friedl, P.: Antigen presentation in extracellular matrix: interactions of T cells with dendritic cells are dynamic, short lived, and sequential. Immunity 2000, 13:323–332. Montoya, M.C., Sancho, D., Vicente-Manzanares, M., and Sanchez-Madrid, F.: Cell adhesion and polarity during immune interactions. Immunol. Rev. 2002, 186:68–82. Wang, J., and Eck, M.J.: Assembling atomic resolution views of the immunological synapse. Curr. Opin. Immunol. 2003, 15:286–293.


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Immunology_Janeway. Wang, J., and Eck, M.J.: Assembling atomic resolution views of the immunological synapse. Curr. Opin. Immunol. 2003, 15:286–293. 9-15 Antigen-presenting cells deliver multiple signals for the clonal expansion and differentiation of naive T cells. Bour-Jordan, H., and Bluestone, J.A.: CD28 function: a balance of costimulatory and regulatory signals. J. Clin. Immunol. 2002, 22:1–7. Chen, L., and Flies, D.B.: Molecular mechanisms of T cell co-stimulation and co-inhibition. Nat. Rev. Immunol. 2013, 13:227–242. Gonzalo, J.A., Delaney, T., Corcoran, J., Goodearl, A., Gutierrez-Ramos, J.C., and Coyle, A.J.: Cutting edge: the related molecules CD28 and inducible costimulator deliver both unique and complementary signals required for optimal T-cell activation. J. Immunol. 2001, 166:1–5. Greenwald, R.J., Freeman, G.J., and Sharpe, A.H.: The B7 family revisited. Annu. Rev. Immunol. 2005, 23:515–548.


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Immunology_Janeway. Greenwald, R.J., Freeman, G.J., and Sharpe, A.H.: The B7 family revisited. Annu. Rev. Immunol. 2005, 23:515–548. Kapsenberg, M.L.: Dendritic-cell control of pathogen-driven T-cell polarization. Nat. Rev. Immunol. 2003, 3:984–993. Wang, S., Zhu, G., Chapoval, A.I., Dong, H., Tamada, K., Ni, J., and Chen, L.: Costimulation of T cells by B7-H2, a B7-like molecule that binds ICOS. Blood 2000, 96:2808–2813. 9-16 CD28-dependent co-stimulation of activated T cells induces expression of interleukin-2 and the high-affinity IL-2 receptor. Acuto, O., and Michel, F.: CD28-mediated co-stimulation: a quantitative support for TCR signalling. Nat. Rev. Immunol. 2003, 3:939–951. Gaffen, S.L.: Signaling domains of the interleukin 2 receptor. Cytokine 2001, 14:63–77. Seko, Y., Cole, S., Kasprzak, W., Shapiro, B.A., and Ragheb, J.A.: The role of cytokine mRNA stability in the pathogenesis of autoimmune disease. Autoimmun. Rev. 2006, 5:299–305.


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Immunology_Janeway. Seko, Y., Cole, S., Kasprzak, W., Shapiro, B.A., and Ragheb, J.A.: The role of cytokine mRNA stability in the pathogenesis of autoimmune disease. Autoimmun. Rev. 2006, 5:299–305. Zhou, X.Y., Yashiro-Ohtani, Y., Nakahira, M., Park, W.R., Abe, R., Hamaoka, T., Naramura, M., Gu, H., and Fujiwara, H.: Molecular mechanisms underlying differential contribution of CD28 versus non-CD28 costimulatory molecules to IL-2 promoter activation. J. Immunol. 2002, 168:3847–3854. 9-17 Additional co-stimulatory pathways involved in T-cell activation. Croft, M.: The role of TNF superfamily members in T-cell function and diseases. Nat. Rev. Immunol. 2009, 9:271–285. Greenwald, R.J., Freeman, G.J., and Sharpe, A.H.: The B7 family revisited. Annu. Rev. Immunol. 2005, 23:515–548. Watts, T.H.: TNF/TNFR family members in costimulation of T cell responses. Annu. Rev. Immunol. 2005, 23:23–68. 9-18 Proliferating T cells differentiate into effector T cells that do not require co-stimulation to act.


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Immunology_Janeway. 9-18 Proliferating T cells differentiate into effector T cells that do not require co-stimulation to act. Gudmundsdottir, H., Wells, A.D., and Turka, L.A.: Dynamics and requirements of T cell clonal expansion in vivo at the single-cell level: effector function is linked to proliferative capacity. J. Immunol. 1999, 162:5212–5223. London, C.A., Lodge, M.P., and Abbas, A.K.: Functional responses and costimulator dependence of memory CD4+ T cells. J. Immunol. 2000, 164:265–272. Schweitzer, A.N., and Sharpe, A.H.: Studies using antigen-presenting cells lacking expression of both B7-1 (CD80) and B7-2 (CD86) show distinct requirements for B7 molecules during priming versus restimulation of Th2 but not Th1 cytokine production. J. Immunol. 1998, 161:2762–2771. 9-19 CD8 T cells can be activated in different ways to become cytotoxic effector cells.


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Immunology_Janeway. 9-19 CD8 T cells can be activated in different ways to become cytotoxic effector cells. Andreasen, S.O., Christensen, J.E., Marker, O., and Thomsen, A.R.: Role of CD40 ligand and CD28 in induction and maintenance of antiviral CD8+ effector T cell responses. J. Immunol. 2000, 164:3689–3697. Blazevic, V., Trubey, C.M., and Shearer, G.M.: Analysis of the costimulatory requirements for generating human virus-specific in vitro T helper and effector responses. J. Clin. Immunol. 2001, 21:293–302. Liang, L., and Sha, W.C.: The right place at the right time: novel B7 family members regulate effector T cell responses. Curr. Opin. Immunol. 2002, 14:384–390. Seder, R.A., and Ahmed, R.: Similarities and differences in CD4+ and CD8+ effector and memory T cell generation. Nat. Immunol. 2003, 4:835–842. Weninger, W., Manjunath, N., and von Andrian, U.H.: Migration and differentiation of CD8+ T cells. Immunol. Rev. 2002, 186:221–233.


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Immunology_Janeway. Weninger, W., Manjunath, N., and von Andrian, U.H.: Migration and differentiation of CD8+ T cells. Immunol. Rev. 2002, 186:221–233. 9-20 CD4 T cells differentiate into several subsets of functionally different effector cells. Basu, R., Hatton, R.D., and Weaver, C.T.: The Th17 family: flexibility follows function. Immunol. Rev. 2013, 252:89–103. Bluestone, J.A., and Abbas, A.K.: Natural versus adaptive regulatory T cells. Nat. Rev. Immunol. 2003, 3:253–257. Crotty, S.: Follicular helper T cells (TFH). Annu. Rev. Immunol. 2011, 29:621–663. King, C.: New insights into the differentiation and function of T follicular helper cells. Nat. Rev. Immunol. 2009, 9:757–766. Littman, D.R., and Rudensky, A.Y.: Th17 and regulatory T cells in mediating and restraining inflammation. Cell 2010, 140:845–858. Murphy, K.M., and Reiner, S.L.: The lineage decisions of helper T cells. Nat. Rev. Immunol. 2002, 2:933–944.


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Immunology_Janeway. Murphy, K.M., and Reiner, S.L.: The lineage decisions of helper T cells. Nat. Rev. Immunol. 2002, 2:933–944. Nurieva, R.I., and Chung, Y.: Understanding the development and function of T follicular helper cells. Cell Mol. Immunol. 2010, 7:190–197. 9-21 Cytokines induce the differentiation of naive CD4 T cells down distinct effector pathways. Nath, I., Vemuri, N., Reddi, A.L., Jain, S., Brooks, P., Colston, M.J., Misra, R.S., and Ramesh, V.: The effect of antigen presenting cells on the cytokine profiles of stable and reactional lepromatous leprosy patients. Immunol. Lett. 2000, 75:69–76. O’Shea, J.J., and Paul, W.E.: Mechanisms underlying lineage commitment and plasticity of helper CD4+ T cells. Science 2010, 327:1098–1102. Reese, T.A., Liang, H.E., Tager, A.M., Luster, A.D., Van Rooijen, N., Voehringer, D., and Locksley, R.M.: Chitin induces the accumulation in tissue of innate immune cells associated with allergy. Nature 2007, 447:92–96.


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Immunology_Janeway. Szabo, S.J., Sullivan, B.M., Peng, S.L., and Glimcher, L.H.: Molecular mechanisms regulating Th1 immune responses. Annu. Rev. Immunol. 2003, 21:713–758. Weaver, C.T., Harrington, L.E., Mangan, P.R., Gavrieli, M., and Murphy, K.M.: Th17: an effector CD4 lineage with regulatory T cell ties. Immunity 2006, 24:677–688. 9-22 CD4 T-cell subsets can cross-regulate each other’s differentiation through the cytokines they produce. Croft, M., Carter, L., Swain, S.L., and Dutton, R.W.: Generation of polarized antigen-specific CD8 effector populations: reciprocal action of interleukin-4 and IL-12 in promoting type 2 versus type 1 cytokine profiles. J. Exp. Med. 1994, 180:1715–1728. Grakoui, A., Donermeyer, D.L., Kanagawa, O., Murphy, K.M., and Allen, P.M.: TCR-independent pathways mediate the effects of antigen dose and altered peptide ligands on Th cell polarization. J. Immunol. 1999, 162:1923–1930.


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Immunology_Janeway. Harrington, L.E., Hatton, R.D., Mangan, P.R., Turner, H., Murphy, T.L., Murphy, K.M., and Weaver, C.T.: Interleukin 17-producing CD4+ effector T cells develop via a lineage distinct from the T helper type 1 and 2 lineages. Nat. Immunol. 2005, 6:1123–1132. Julia, V., McSorley, S.S., Malherbe, L., Breittmayer, J.P., Girard-Pipau, F., Beck, A., and Glaichenhaus, N.: Priming by microbial antigens from the intestinal flora determines the ability of CD4+ T cells to rapidly secrete IL-4 in BALB/c mice infected with Leishmania major. J. Immunol. 2000, 165:5637–5645. Martin-Fontecha, A., Thomsen, L.L., Brett, S., Gerard, C., Lipp, M., Lanzavecchia, A., and Sallusto, F.: Induced recruitment of NK cells to lymph nodes provides IFN-γ for TH1 priming. Nat. Immunol. 2004, 5:1260–1265. Nakamura, T., Kamogawa, Y., Bottomly, K., and Flavell, R.A.: Polarization of IL-4and IFN-γ-producing CD4+ T cells following activation of naïve CD4+ T cells. J. Immunol. 1997, 158:1085–1094.


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Immunology_Janeway. Nakamura, T., Kamogawa, Y., Bottomly, K., and Flavell, R.A.: Polarization of IL-4and IFN-γ-producing CD4+ T cells following activation of naïve CD4+ T cells. J. Immunol. 1997, 158:1085–1094. Seder, R.A., and Paul, W.E.: Acquisition of lymphokine producing phenotype by CD4+ T cells. Annu. Rev. Immunol. 1994, 12:635–673. 9-23 Regulatory CD4 T cells are involved in controlling adaptive immune responses. Fontenot, J.D., and Rudensky, A.Y.: A well adapted regulatory contrivance: regulatory T cell development and the forkhead family transcription factor Foxp3. Nat. Immunol. 2005, 6:331–337. Roncarolo, M.G., Bacchetta, R., Bordignon, C., Narula, S., and Levings, M.K.: Type 1 T regulatory cells. Immunol. Rev. 2001, 182:68–79. Sakaguchi, S.: Naturally arising Foxp3-expressing CD25+CD4+ regulatory T cells in immunological tolerance to self and non-self. Nat. Immunol. 2005, 6:345–352.


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Immunology_Janeway. Sakaguchi, S.: Naturally arising Foxp3-expressing CD25+CD4+ regulatory T cells in immunological tolerance to self and non-self. Nat. Immunol. 2005, 6:345–352. Sakaguchi, S., Yamaguchi, T., Nomura, T., and Ono, M.: Regulatory T cells and immune tolerance. Cell 2008, 133:775–787. Saraiva, M., and O’Garra, A.: The regulation of IL-10 production by immune cells. Nat. Rev. Immunol. 2010, 10:170–181. 9-24 Effector T-cell interactions with target cells are initiated by antigen-nonspecific cell-adhesion molecules. Dustin, M.L.: T-cell activation through immunological synapses and kinases. Immunol. Rev. 2008, 221:77–89. van der Merwe, P.A., and Davis, S.J.: Molecular interactions mediating T cell antigen recognition. Annu. Rev. Immunol. 2003, 21:659–684. targets to regulate signaling and to direct the release of effector molecules.


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Immunology_Janeway. targets to regulate signaling and to direct the release of effector molecules. Bossi, G., Trambas, C., Booth, S., Clark, R., Stinchcombe, J., and Griffiths, G.M.: The secretory synapse: the secrets of a serial killer. Immunol. Rev. 2002, 189:152–160. Montoya, M.C., Sancho, D., Vicente-Manzanares, M., and Sanchez-Madrid, F.: Cell adhesion and polarity during immune interactions. Immunol. Rev. 2002, 186:68–82. Trambas, C.M., and Griffiths, G.M.: Delivering the kiss of death. Nat. Immunol. 2003, 4:399–403. 9-26 The effector functions of T cells are determined by the array of effector molecules that they produce. & 9-27 Cytokines can act locally or at a distance. Basler, C.F., and Garcia-Sastre, A.: Viruses and the type I interferon antiviral system: induction and evasion. Int. Rev. Immunol. 2002, 21:305–337. Boulay, J.L., O’Shea, J.J., and Paul, W.E.: Molecular phylogeny within type I cytokines and their cognate receptors. Immunity 2003, 19:159–163.


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Immunology_Janeway. Boulay, J.L., O’Shea, J.J., and Paul, W.E.: Molecular phylogeny within type I cytokines and their cognate receptors. Immunity 2003, 19:159–163. Guidotti, L.G., and Chisari, F.V.: Cytokine-mediated control of viral infections. Virology 2000, 273:221–227. Harty, J.T., Tvinnereim, A.R., and White, D.W.: CD8+ T cell effector mechanisms in resistance to infection. Annu. Rev. Immunol. 2000, 18:275–308. Proudfoot, A.E.: Chemokine receptors: multifaceted therapeutic targets. Nat. Rev. Immunol. 2002, 2:106–115. 9-28 T cells express several TNF-family cytokines as trimeric proteins that are usually associated with the cell surface. Bekker, L.G., Freeman, S., Murray, P.J., Ryffel, B., and Kaplan, G.: TNF-alpha controls intracellular mycobacterial growth by both inducible nitric oxide synthase-dependent and inducible nitric oxide synthase-independent pathways. J. Immunol. 2001, 166:6728–6734. Hehlgans, T., and Mannel, D.N.: The TNF–TNF receptor system. Biol. Chem. 2002, 383:1581–1585.


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Immunology_Janeway. Hehlgans, T., and Mannel, D.N.: The TNF–TNF receptor system. Biol. Chem. 2002, 383:1581–1585. Ware, C.F.: Network communications: lymphotoxins, LIGHT, and TNF. Annu. Rev. Immunol. 2005, 23:787–819. 9-29 Cytotoxic T cells induce target cells to undergo programmed cell death via extrinsic and intrinsic pathways of apoptosis. Aggarwal, B.B.: Signalling pathways of the TNF superfamily: a double-edged sword. Nat. Rev. Immunol. 2003, 3:745–756. Ashton-Rickardt, P.G.: The granule pathway of programmed cell death. Crit. Rev. Immunol. 2005, 25:161–182. Bishop, G.A.: The multifaceted roles of TRAFs in the regulation of B-cell function. Nat. Rev. Immunol. 2004, 4:775–786. Green, D.R., Droin, N., and Pinkoski, M.: Activation-induced cell death in T cells. Immunol. Rev. 2003, 193:70–81. Russell, J.H., and Ley, T.J.: Lymphocyte-mediated cytotoxicity. Annu. Rev. Immunol. 2002, 20:323–370.


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Immunology_Janeway. Russell, J.H., and Ley, T.J.: Lymphocyte-mediated cytotoxicity. Annu. Rev. Immunol. 2002, 20:323–370. Siegel, R.M.: Caspases at the crossroads of immune-cell life and death. Nat. Rev. Immunol. 2006, 6:308–317. Wallin, R.P., Screpanti, V., Michaelsson, J., Grandien, A., and Ljunggren, H.G.: Regulation of perforin-independent NK cell-mediated cytotoxicity. Eur. J. Immunol. 2003, 33:2727–2735. 9-30 The intrinsic pathway of apoptosis is mediated by the release of cytochrome c from mitochondria. Borner, C.: The Bcl-2 protein family: sensors and checkpoints for life-ordeath decisions. Mol. Immunol. 2003, 39:615–647. Bratton, S.B., and Salvesen, G.S.: Regulation of the Apaf-1-caspase-9 apoptosome. J. Cell Sci. 2010, 123:3209–3214. Chowdhury, D., and Lieberman, J.: Death by a thousand cuts: granzyme pathways of programmed cell death. Annu. Rev. Immunol. 2008, 26:389–420.


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Immunology_Janeway. Chowdhury, D., and Lieberman, J.: Death by a thousand cuts: granzyme pathways of programmed cell death. Annu. Rev. Immunol. 2008, 26:389–420. Hildeman, D.A., Zhu, Y., Mitchell, T.C., Kappler, J., and Marrack, P.: Molecular mechanisms of activated T cell death in vivo. Curr. Opin. Immunol. 2002, 14:354–359. Strasser, A.: The role of BH3-only proteins in the immune system. Nat. Rev. Immunol. 2005, 5:189–200. 9-31 Cytotoxic effector proteins that trigger apoptosis are contained in the granules of CD8 cytotoxic T cells. Barry, M., Heibein, J.A., Pinkoski, M.J., Lee, S.F., Moyer, R.W., Green, D.R., and Bleackley, R.C.: Granzyme B short-circuits the need for caspase 8 activity during granule-mediated cytotoxic T-lymphocyte killing by directly cleaving Bid. Mol. Cell Biol. 2000, 20:3781–3794. Grossman, W.J., Revell, P.A., Lu, Z.H., Johnson, H., Bredemeyer, A.J., and Ley, T.J.: The orphan granzymes of humans and mice. Curr. Opin. Immunol. 2003, 15:544–552.


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Immunology_Janeway. Grossman, W.J., Revell, P.A., Lu, Z.H., Johnson, H., Bredemeyer, A.J., and Ley, T.J.: The orphan granzymes of humans and mice. Curr. Opin. Immunol. 2003, 15:544–552. Lieberman, J.: The ABCs of granule-mediated cytotoxicity: new weapons in the arsenal. Nat. Rev. Immunol. 2003, 3:361–370. Pipkin, M.E., and Lieberman, J.: Delivering the kiss of death: progress on understanding how perforin works. Curr. Opin. Immunol. 2007, 19:301–308. Yasukawa, M., Ohminami, H., Arai, J., Kasahara, Y., Ishida, Y., and Fujita, S.: Granule exocytosis, and not the Fas/Fas ligand system, is the main pathway of cytotoxicity mediated by alloantigen-specific CD4+ as well as CD8+ cytotoxic T lymphocytes in humans. Blood 2000, 95:2352–2355. 9-32 Cytotoxic T cells are selective and serial killers of targets expressing a specific antigen. Stinchcombe, J.C., and Griffiths, G.M.: Secretory mechanisms in cell-mediated cytotoxicity. Annu. Rev. Cell Dev. Biol. 2007, 23:495–517.


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Immunology_Janeway. Stinchcombe, J.C., and Griffiths, G.M.: Secretory mechanisms in cell-mediated cytotoxicity. Annu. Rev. Cell Dev. Biol. 2007, 23:495–517. Veugelers, K., Motyka, B., Frantz, C., Shostak, I., Sawchuk, T., and Bleackley, R.C.: The granzyme B-serglycin complex from cytotoxic granules requires dynamin for endocytosis. Blood 2004, 103:3845–3853. 9-33 Cytotoxic T cells also act by releasing cytokines. Amel-Kashipaz, M.R., Huggins, M.L., Lanyon, P., Robins, A., Todd, I., and Powell, R.J.: Quantitative and qualitative analysis of the balance between type 1 and type 2 cytokine-producing CD8– and CD8+ T cells in systemic lupus erythematosus. J. Autoimmun. 2001, 17:155–163. Dobrzanski, M.J., Reome, J.B., Hollenbaugh, J.A., and Dutton, R.W.: Tc1 and Tc2 effector cell therapy elicit long-term tumor immunity by contrasting mechanisms that result in complementary endogenous type 1 antitumor responses. J. Immunol. 2004, 172:1380–1390.


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Immunology_Janeway. Prezzi, C., Casciaro, M.A., Francavilla, V., Schiaffella, E., Finocchi, L., Chircu, L.V., Bruno, G., Sette, A., Abrignani, S., and Barnaba, V.: Virus-specific CD8+ T cells with type 1 or type 2 cytokine profile are related to different disease activity in chronic hepatitis C virus infection. Eur. J. Immunol. 2001, 31:894–906. The Humoral Immune Response


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Immunology_Janeway. Many pathogens multiply in the body’s extracellular spaces, and even intracellular pathogens can spread by moving through the extracellular fluids. The extracellular spaces are protected by the humoral immune response, in which antibodies produced by B cells act to destroy extracellular microorganisms and their products, and prevent the spread of intracellular infections. As we introduced in Section 1-20, antibodies contribute to immunity in three main ways: neutralization, opsonization, and complement activation (Fig. 10.1). Antibodies can bind to pathogens and prevent their ability to enter and infect cells, and therefore are thus said to neutralize the pathogen; antibodies may also bind bacterial toxins, preventing their action or ability to enter cells. Antibodies also facilitate opsonization, the uptake of the pathogens by phagocytes, by binding to Fc receptors through their constant regions (C regions). Finally, antibodies bound to pathogens can activate proteins of the


Immunology_Janeway

Immunology_Janeway. opsonization, the uptake of the pathogens by phagocytes, by binding to Fc receptors through their constant regions (C regions). Finally, antibodies bound to pathogens can activate proteins of the classical pathway of the complement system, as we described in Chapter 2. This can increase opsonization by placing other complement proteins onto the pathogen’s surface, help recruit phagocytic cells to the site of infection, and activate the membrane-attack complex, which can directly lyse certain microorganisms by forming pores in their membranes. The choice of which effector mechanisms are used is influenced by the heavy-chain isotype of the antibodies produced, which determines their class (see Section 5-12).


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Immunology_Janeway. In the first part of this chapter, we describe the interactions of naive B cells with antigen and with helper T cells that lead to the activation of B cells and antibody production. Some microbial antigens can provoke antibody production without T-cell help, but activation of naive B cells by antigens usually involves help from T follicular helper (TFH) cells (see Section 9-20). Activated B cells then differentiate into antibody-secreting plasma cells and memory B cells. Most antibody responses undergo a process called affinity maturation, in which antibodies of greater affinity for their target antigen are produced by the somatic hypermutation of antibody variable-region (V-region) genes. We examine the molecular mechanism of somatic hypermutation and its immunological consequences, as well as class switching—a process that generates the different classes of antibodies that confer functional diversity on the antibody response. Both affinity maturation and class switching occur only


Immunology_Janeway

Immunology_Janeway. well as class switching—a process that generates the different classes of antibodies that confer functional diversity on the antibody response. Both affinity maturation and class switching occur only in B cells and require T-cell help. In the second part of the chapter, we introduce the distributions and functions of various classes of antibody, in particular those secreted into mucosal sites. In the third part of the chapter, we discuss in detail how the Fc region of the antibody engages various effector mechanisms to contain and eliminate infections. Like the T-cell response, the humoral immune response produces immunological memory, and this is discussed in Chapter 11.


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Immunology_Janeway. Fig. 10.1 Antibodies mediate the humoral immune response through neutralization, opsonization, and complement activation. After being secreted by plasma cells, antibodies protect the host from infection in three main ways. They can inhibit the toxic effects or infectivity of pathogens or their products by binding to them, a process called neutralization (top panel). When bound to pathogens, the antibody’s Fc region can bind to Fc receptors on accessory cells, such as macrophages and neutrophils, helping these cells to ingest and kill the pathogen. This process is called opsonization (middle panel). Antibodies can trigger complement by activating C1, the first step in the classical complement pathway. Deposition of complement proteins enhances opsonization and can also directly kill certain bacterial cells by activating the membrane-attack complex (bottom panel). B-cell activation by antigen and helper T cells. The distributions and functions of immunoglobulin classes.


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Immunology_Janeway. B-cell activation by antigen and helper T cells. The distributions and functions of immunoglobulin classes. The destruction of antibody-coated pathogens via Fc receptors.


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Immunology_Janeway. Fig. 10.2 A second signal is required for B-cell activation by either thymus-dependent or thymus-independent antigens. The first signal (indicated as 1) required for B-cell activation is delivered through its antigen receptor (BCR) and activates several pathways as described in Chapter 7. Signaling by the BCR is enhanced by the co-receptors CD21 and CD19, which interact with C3b on opsonized microbial surfaces. For thymus-dependent antigens (first panel), a second signal (indicated as 2) is delivered by a helper T cell (TFH) that recognizes degraded fragments of the antigen as peptides bound to MHC class II molecules on the B-cell surface. CD40L on the TFH cell binds to CD40 on the B cell, activating the non-canonical NFκB signaling pathway via NFκB-inducing kinase (NIK). This induces expression of pro-survival genes such as Bcl-2 (see Section 7-17). For thymus-independent antigens (second panel), a second signal can be delivered through Toll-like receptors that recognize


Immunology_Janeway

Immunology_Janeway. induces expression of pro-survival genes such as Bcl-2 (see Section 7-17). For thymus-independent antigens (second panel), a second signal can be delivered through Toll-like receptors that recognize antigen-associated TLR ligands, such as bacterial lipopolysaccharide (LPS) or bacterial DNA, as described in Chapter 3.


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Immunology_Janeway. B-cell activation by antigen and helper T cells.


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Immunology_Janeway. The surface immunoglobulin that serves as the B-cell receptor (BCR) plays two roles in B-cell activation in response to pathogens. Like the antigen receptor on T cells, the BCR initiates a signaling cascade upon binding antigens derived from the microbe. In addition, the BCR can deliver the antigen to intracellular sites for antigen processing, so that antigenic peptides bound to MHC class II molecules can be returned to the B-cell surface. These peptide:MHC class II complexes are recognized by antigen-specific helper T cells that have already differentiated in response to the same pathogen. The effector T cells express surface molecules and cytokines that help the B cell to proliferate and to differentiate into antibody-secreting cells and into memory B cells, and a structure called the germinal center (see Section 10-6) is formed during an intermediate phase of the antibody response, before the emergence of longterm plasma cells that generate antibody or of memory B cells. Some


Immunology_Janeway

Immunology_Janeway. the germinal center (see Section 10-6) is formed during an intermediate phase of the antibody response, before the emergence of longterm plasma cells that generate antibody or of memory B cells. Some microbial antigens can activate B cells directly in the absence of T-cell help, and the ability of B cells to respond directly to these antigens provides a rapid response to many important pathogens. However, the fine tuning of antibody responses to increase the affinity of the antibody for the antigen and the switching to most immunoglobulin classes other than IgM depend on the interaction of antigen-stimulated B cells with helper T cells and other cells in the peripheral lymphoid organs. Thus, antibodies induced by microbial antigens alone tend to have lower affinity and to be less functionally versatile than those induced with T-cell help.


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Immunology_Janeway. 10-1 Activation of B cells by antigen involves signals from the B-cell receptor and either TFH cells or microbial antigens. As we learned in Chapter 8, activation of naive T cells requires signals derived from the T-cell receptor as well as co-stimulatory signals provided by professional antigen-presenting cells. Similarly, in addition to signals derived from the B-cell receptor, naive B cells also require accessory signals that can arise either from a helper T cell or, in some cases, directly from microbial constituents (Fig. 10.2). B-cell activation by antigen and helper T cells.


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Immunology_Janeway. Protein antigens alone are unable to induce antibody responses in animals or humans who lack T cells, and they are therefore known as thymus-dependent or TD antigens, and typically involve antigen-specific T-cell help. The T cells involved are TFH cells that reside in the lymphoid tissues and are not fully differentiated TH1, TH2, or TH17 effector cells. To receive T-cell help, the B cell must display antigen on its surface in a form that a T cell can recognize. This occurs when antigen bound by surface immunoglobulin on the B cell is internalized and degraded within the B cell and peptides derived from it are returned to the cell surface in a complex with MHC class II molecules (see Fig. 10.2, first panel). When the TFH cell recognizes these peptide:MHC complexes, it provides the B cell with signals that favor survival and induce proliferation. These signals include the activation of CD40 on B cells by TFH expression of its ligand, CD40L (CD154), and production of various cytokines


Immunology_Janeway

Immunology_Janeway. with signals that favor survival and induce proliferation. These signals include the activation of CD40 on B cells by TFH expression of its ligand, CD40L (CD154), and production of various cytokines by TFH cells, including IL-21 (Fig. 10.3). CD40 signaling activates the non-canonical NFκB pathway (see Section 7-23) and enhances B-cell survival by inducing the expression of anti-apoptotic molecules such as Bcl-2. IL-21 signaling activates STAT3 and enhances cellular proliferation and differentiation into plasma cells and memory B cells. Other cytokines produced by TFH cells include IL-6, TGF-β, IFN-γ, and IL-4, which provide signals that can regulate the type of antibody produced, as we will see in Section 10-12. These cytokines are also made by other differentiated effector subsets (described in Chapter 9), but TFH cells are distinct from these. For example, TFH cells transcribe the IL-4 gene using regulatory elements that are independent of the transcription factors GATA-3 and STAT6,


Immunology_Janeway

Immunology_Janeway. in Chapter 9), but TFH cells are distinct from these. For example, TFH cells transcribe the IL-4 gene using regulatory elements that are independent of the transcription factors GATA-3 and STAT6, which are responsible for IL-4 production by TH2 cells.


Immunology_Janeway

Immunology_Janeway. While B-cell responses to protein antigens rely on help from T cells, some microbial constituents can induce antibody production in the absence of helper T cells. These microbial antigens are known as thymus-independent or TI antigens because they can induce antibody responses in individuals who have no T lymphocytes. Such antigens are typically highly repetitive molecules, such as the polysaccharides of bacterial cell walls, and can cross-link the BCR on B cells. In such cases, a second signal can be derived from direct recognition of a common microbial constituent such as LPS that can activate TLR signaling in the B cell (see Fig. 10.2, second panel), activating the NFκB pathway, as described in Chapter 3. Thymus-independent antibody responses provide some protection against extracellular bacteria, and we will return to them later.


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Immunology_Janeway. Further differentiation can proceed to the germinal center, resting memory cells, or antibody-secreting plasma cells B-cell proliferation generates plasmablasts that form the primary focus TFH B cell AP-1 NFAT NF˜B Bcl-2 STAT3 germinal center B cell plasmacell memoryB cell various targets Antigen recognition by TFH cells induces signals that activate B cells IL-21 IL-6 TGF-°LFN-˛IL-4 STAT3 CD40 CD40L MHCII TCR


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Immunology_Janeway. Fig. 10.3 TFH cells provide several signals that activate B cells which activates the transcription factor STAT3 to enhance B-cell and control their subsequent differentiation. After antigen proliferation and survival. TFH cells can also produce cytokines that binding to the B-cell receptor delivers the first signal for B-cell will regulate isotype switching (see Section 10-12). After receiving activation (not shown), the TFH cell delivers additional signals these signals, activated B cells begin to proliferate (second panel), when it recognizes a peptide:MHC class II complex on the B-cell enter the germinal center, and eventually become plasma cells or surface (first panel). Besides expression of CD40 ligand, the TFH cell memory B cells (third panel). secretes several important cytokines. Included among them is IL-21, 10-2 Linked recognition of antigen by T cells and B cells promotes robust antibody responses.


Immunology_Janeway

Immunology_Janeway. B-cell activation by antigens on microbial surfaces can be greatly stimulated by the concurrent deposition of complement on these pathogens. The B-cell co-receptor complex contains the cell-surface proteins CD19, CD21, and CD81 (see Fig. 7.27). When CD21, or complement receptor 2 (CR2), binds to the complement fragments C3d and C3dg that are deposited on microbial surfaces (see Section 2-13), it is brought near to the activated B-cell receptor bound to the same surface. CD21 and CD19 are associated with each other, and CD19 becomes phosphorylated by the activated B-cell receptor. This recruits PI 3-kinase, which then stimulates several downstream pathways, enhancing proliferation, differentiation, and antibody production (see Fig. 10.2, arrow 1). This effect is shown dramatically when mice are immunized with the experimental antigen hen egg-white lysozyme that is coupled to three linked molecules of C3dg. In this case the dose of modified lysozyme needed to induce antibody in the


Immunology_Janeway

Immunology_Janeway. mice are immunized with the experimental antigen hen egg-white lysozyme that is coupled to three linked molecules of C3dg. In this case the dose of modified lysozyme needed to induce antibody in the absence of added adjuvant is as little as 1/10,000 of that needed with the unmodified lysozyme.


Immunology_Janeway

Immunology_Janeway. For T-dependent antibody responses, the T cells involved are activated by the same antigen as is recognized by the B cells; this is called linked recognition. However, the peptide recognized by the TFH cell is likely to differ from the protein epitope recognized by the B cell’s antigen receptor. Natural antigens, such as viruses and bacteria, contain multiple proteins and carry both protein and carbohydrate epitopes. For linked recognition to occur, the peptide recognized by the T cell must be physically associated with the antigen recognized by the B cell’s receptor, so that the B cell can take up and present the appropriate peptide to the T cell. For example, a B cell that recognizes an epitope on a viral coat protein will internalize the complete virus particle. The B cell can degrade multiple viral proteins into peptides for display on MHC class II molecules on the B-cell surface. CD4 T cells specific for such viral peptides may have been activated by dendritic cells earlier in


Immunology_Janeway

Immunology_Janeway. multiple viral proteins into peptides for display on MHC class II molecules on the B-cell surface. CD4 T cells specific for such viral peptides may have been activated by dendritic cells earlier in the infection, and some will have differentiated into TFH cells. When these TFH cells are activated by B cells presenting their peptide, they are stimulated to provide specific signals that help B cells to generate antibodies against the viral coat protein (Fig. 10.4).