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Immunology_Janeway. Cambier, J.C., Pleiman, C.M., and Clark, M.R.: Signal transduction by the B cell antigen receptor and its coreceptors. Annu. Rev. Immunol. 1994, 12:457–486. DeFranco, A.L., Richards, J.D., Blum, J.H., Stevens, T.L., Law, D.A., Chan, V.W., Datta, S.K., Foy, S.P., Hourihane, S.L., and Gold, M.R.: Signal transduction by the B-cell antigen receptor. Ann. N.Y. Acad. Sci. 1995, 766:195–201. Harwood, N.E., and Batista, F.D.: Early events in B cell activation. Annu. Rev. Immunol. 2010, 28:185–210. Koretzky, G.A., Abtahian, F., and Silverman, M.A.: SLP76 and SLP65: complex regulation of signalling in lymphocytes and beyond. Nat. Rev. Immunol. 2006, 6:67–78. Kurosaki, T., and Hikida, M.: Tyrosine kinases and their substrates in B lymphocytes. Immunol. Rev. 2009, 228:132–148. 7-21 The cell-surface protein CD28 is a required co-stimulatory signaling receptor for naive T cell activation.


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Immunology_Janeway. 7-21 The cell-surface protein CD28 is a required co-stimulatory signaling receptor for naive T cell activation. Acuto, O., and Michel, F.: CD28-mediated co-stimulation: a quantitative support for TCR signalling. Nat. Rev. Immunol. 2003, 3:939–951. Frauwirth, K.A., Riley, J.L., Harris, M.H., Parry, R.V., Rathmell, J.C., Plas, D.R., Elstrom, R.L., June, C.H., and Thompson, C.B.: The CD28 signaling pathway regulates glucose metabolism. Immunity 2002, 16: 769–777. Sharpe, A.H.: Mechanisms of costimulation. Immunol. Rev. 2009, 229:5–11. 7-22 Maximal activation of PLC-γ, which is important for transcription factor activation, requires a co-stimulatory signal induced by CD28. Chen, L., and Flies, D.B.: Molecular mechanisms of T cell co-stimulation and co-inhibition. Nat. Rev. Immunol. 2013, 13:227–242. 7-23 TNF receptor superfamily members augment T-cell and B-cell activation.


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Immunology_Janeway. 7-23 TNF receptor superfamily members augment T-cell and B-cell activation. Chen, L., and Flies, D.B.: Molecular mechanisms of T cell co-stimulation and co-inhibition. Nat. Rev. Immunol. 2013, 13:227–242. Rickert, R.C., Jellusova, J., and Miletic, A.V.: Signaling by the tumor necrosis factor receptor superfamily in B-cell biology and disease. Immunol. Rev. 2011, 244:115–133. 7-24 Inhibitory receptors on lymphocytes downregulate immune responses by interfering with co-stimulatory signaling pathways. Qureshi, O.S., Zheng, Y., Nakamura, K., Attridge, K., Manzotti, C., Schmidt, E.M., Baker, J., Jeffery, L.E., Kaur, S., Briggs, Z., et al.: Trans-endocytosis of CD80 and CD86: a molecular basis for the cell-extrinsic function of CTLA-4. Science 2011, 332:600–603. Rudd, C.E., Taylor, A., and Schneider, H.: CD28 and CTLA-4 coreceptor expression and signal transduction. Immunol. Rev. 2009, 229:12–26.


Immunology_Janeway

Immunology_Janeway. Rudd, C.E., Taylor, A., and Schneider, H.: CD28 and CTLA-4 coreceptor expression and signal transduction. Immunol. Rev. 2009, 229:12–26. 7-25 Inhibitory receptors on lymphocytes downregulate immune responses by recruiting protein or lipid phosphatases. Acuto, O., Di Bartolo, V., and Michel, F.: Tailoring T-cell receptor signals by proximal negative feedback mechanisms. Nat. Rev. Immunol. 2008, 8:699–712. Chen, L., and Flies, D.B.: Molecular mechanisms of T cell co-stimulation and co-inhibition. Nat. Rev. Immunol. 2013, 13:227–242. The Development of B and T Lymphocytes


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Immunology_Janeway. The production of new lymphocytes, or lymphopoiesis, takes place in specialized lymphoid tissues—the central (or primary) lymphoid tissues— which are the bone marrow for most B cells and the thymus for most T cells. Precursors for both populations originate in the bone marrow, but whereas B cells complete most of their development there, the precursors of most T cells migrate to the thymus, where they develop into mature T cells. A major goal of lymphopoiesis is to generate a diverse repertoire of B-cell receptors and T-cell receptors on circulating B and T cells, respectively, thereby enabling an individual to make adaptive immune responses against the wide range of pathogens encountered during a lifetime. In the fetus and the juvenile, the central lymphoid tissues are the sources of large numbers of new lymphocytes, which migrate to populate the peripheral lymphoid tissues (also called secondary lymphoid tissues) such as lymph nodes, spleen, and mucosal lymphoid tissue. In mature


Immunology_Janeway

Immunology_Janeway. numbers of new lymphocytes, which migrate to populate the peripheral lymphoid tissues (also called secondary lymphoid tissues) such as lymph nodes, spleen, and mucosal lymphoid tissue. In mature individuals, the development of new T cells in the thymus slows down, and peripheral T-cell numbers are maintained by the division of mature T cells outside the central lymphoid organs. New B cells, in contrast, are continually produced from the bone marrow, even in adults. This chapter will focus on the development of T cells and B cells from their uncommitted progenitors, with an emphasis on the major populations of CD4+ and CD8+ T cells and B cells. The development of additional subsets of T cells and B cells, such as invariant NKT (iNKT) cells, Treg cells, γ:δ TCR+ T cells, B-1 B cells, and marginal zone B cells will be briefly discussed.


Immunology_Janeway

Immunology_Janeway. The structure of the antigen-receptor genes expressed by B cells and T cells, and the mechanisms by which a complete antigen receptor is assembled, were described in Chapters 4 and 5. Once an antigen receptor has been formed, rigorous testing is required to select lymphocytes that carry useful antigen receptors—that is, antigen receptors that can recognize a wide spectrum of pathogens and yet will not react against an individual’s own cells. Given the incredible diversity of receptors that the rearrangement process can generate, it is important that those lymphocytes that mature are likely to be useful in recognizing and responding to foreign antigens, especially as an individual can express only a small fraction of the total possible receptor repertoire in his or her lifetime. We describe how the specificity and affinity of the receptor for self ligands are tested to determine whether the immature lymphocyte will either survive and join the mature repertoire, or die. In general, it


Immunology_Janeway

Immunology_Janeway. how the specificity and affinity of the receptor for self ligands are tested to determine whether the immature lymphocyte will either survive and join the mature repertoire, or die. In general, it seems that developing lymphocytes whose receptors interact weakly with self antigens, or that bind self antigens in a particular way, receive a signal that enables them to survive. This process, known as positive selection, is particularly critical in the development of α:β T cells, which recognize composite antigens consisting of peptides bound to MHC molecules, because it ensures that an individual’s T cells will be able to respond to peptides bound to one’s own MHC molecules.


Immunology_Janeway

Immunology_Janeway. In contrast, lymphocytes with strongly self-reactive receptors must be eliminated to prevent autoimmune reactions; this process of negative selection is one of the ways in which the immune system is made self-tolerant. The default fate of developing lymphocytes, in the absence of any signal being received from the receptor, is death by apoptosis, and as we will see, the vast Development of B lymphocytes. Development of T lymphocytes. Positive and negative selection of T cells.


Immunology_Janeway

Immunology_Janeway. Fig. 8.1 B cells develop in the bone marrow and migrate to peripheral lymphoid organs, where they can be activated by antigens. In the first phase of development, progenitor B cells in the bone marrow rearrange their immunoglobulin genes. This phase is independent of antigen but is dependent on interactions with bone marrow stromal cells (first panels). It ends in an immature B cell that carries an antigen receptor in the form of cell-surface IgM (second panels), and in the second phase it can now interact with antigens in its environment. Immature B cells that are strongly stimulated by antigen at this stage either die or are inactivated in a process of negative selection, thus removing many self-reactive B cells from the repertoire. In the third phase of development, the surviving immature B cells emerge into the periphery and mature to express IgD as well as IgM. They can now be activated by encounter with their specific foreign antigen in a peripheral lymphoid organ (third


Immunology_Janeway

Immunology_Janeway. immature B cells emerge into the periphery and mature to express IgD as well as IgM. They can now be activated by encounter with their specific foreign antigen in a peripheral lymphoid organ (third panels). Activated B cells proliferate, and differentiate into antibody-secreting plasma cells and long-lived memory cells (fourth panels).


Immunology_Janeway

Immunology_Janeway. majority of developing lymphocytes die before emerging from the central lymphoid organs or before completing maturation in the peripheral lymphoid organs.


Immunology_Janeway

Immunology_Janeway. In this chapter we describe the different stages of the development of B cells and T cells in mice and humans from the uncommitted stem cell in the bone marrow up to the mature, functionally specialized lymphocyte with its unique antigen receptor ready to respond to a foreign antigen. The final stages in the life history of a mature lymphocyte, in which an encounter with its antigen activates it to become an effector or memory lymphocyte, are discussed in Chapters 9–11. We now know that the Band T-cell development that predominates during late fetal life and after birth is distinct from waves of lymphocyte development that take place earlier in fetal ontogeny. These earlier waves originate from stem cells found in the fetal liver and in even more primitive hematopoietic tissues in the developing embryo. Unlike the lymphocytes that develop from bone marrow stem cells, B and T cells that develop from these early fetal progenitors generally populate mucosal and epithelial tissues and


Immunology_Janeway

Immunology_Janeway. developing embryo. Unlike the lymphocytes that develop from bone marrow stem cells, B and T cells that develop from these early fetal progenitors generally populate mucosal and epithelial tissues and function in innate immune responses. In the adult, these subsets of lymphocytes are minority populations in secondary lymphoid tissues. This chapter will focus on B and T cells that develop from bone marrow stem cells and that comprise the cells of the adaptive immune response (see Figs 1.7 and 1.20). The chapter is divided into three parts. The first two describe B-cell and T-cell development, respectively. In the third section, we discuss the positive and negative selection of T cells in the thymus.


Immunology_Janeway

Immunology_Janeway. Development of B lymphocytes. The main phases of a B lymphocyte’s life history are shown in Fig. 8.1. The stages in both B-cell and T-cell development are defined mainly by the successive steps in the assembly and expression of functional antigen-receptor genes. Generation of B-cell receptors in the bone marrow Negative selection in the bone marrow Migration of B cells through the circulatory system to lymphoid organs and B-cell activation


Immunology_Janeway

Immunology_Janeway. Generation of B-cell receptors in the bone marrow Negative selection in the bone marrow Migration of B cells through the circulatory system to lymphoid organs and B-cell activation At each step of lymphocyte development, the progress of gene rearrangement is monitored; the major recurring theme is that successful gene rearrangement leads to the production of a protein chain that serves as a signal for the cell to progress to the next stage. We will see that a developing B cell is presented with opportunities for multiple rearrangements that increase the likelihood of expressing a functional antigen receptor, but that there are also checkpoints that reinforce the requirement that each B cell express just one receptor specificity. We will start by looking at how the earliest recognizable cells of the B-cell lineage develop from the multipotent hematopoietic stem cells in the bone marrow, and at what point the B-cell and T-cell lineages diverge.


Immunology_Janeway

Immunology_Janeway. 8-1 Lymphocytes derive from hematopoietic stem cells in the bone marrow. The cells of the lymphoid lineage—B cells, T cells, and innate lymphoid cells (ILCs)—are all derived from common lymphoid progenitor cells, which themselves derive from the multipotent hematopoietic stem cells (HSCs) that give rise to all blood cells (see Fig. 1.3). Development from the precursor stem cell into cells that are committed to becoming B cells or T cells follows the basic principles of cell differentiation. Properties that are essential for the function of the mature cell are gradually acquired, along with the loss of properties that are more characteristic of the immature cell. In the case of lymphocyte development, cells become committed first to the lymphoid lineage, as opposed to the myeloid, and then to either the B-cell or the T-cell lineage (Fig. 8.2).


Immunology_Janeway

Immunology_Janeway. The specialized microenvironment of the bone marrow provides signals both for the development of lymphocyte progenitors from hematopoietic stem cells and for the subsequent differentiation of B cells. Such signals act on the developing lymphocytes to switch on key genes that direct the developmental program and are produced by the network of specialized nonlymphoid connective tissue stromal cells that are in intimate contact with the developing lymphocytes (Fig. 8.3). The contribution of the stromal cells is twofold. First, they form specific adhesive contacts with the developing lymphocytes by interactions between cell-adhesion molecules and their ligands. Second, they provide soluble and membrane-bound cytokines and chemokines that control lymphocyte differentiation and proliferation.


Immunology_Janeway

Immunology_Janeway. The hematopoietic stem cells first differentiate into multipotent progenitor cells (MPPs), which can produce both lymphoid and myeloid cells but are no longer self-renewing stem cells. Multipotent progenitors express a cell-surface receptor tyrosine kinase known as FLT3 that binds the membrane-bound FLT3 ligand on stromal cells. Additionally, MPPs express transcription factors and


Immunology_Janeway

Immunology_Janeway. Fig. 8.2 A multipotent hematopoietic stem cell generates all the cells of the immune system. In the bone marrow or other hematopoietic sites, the multipotent stem cell gives rise to cells with progressively more limited potential. A simplified progression is shown here. The multipotent progenitor (MPP), for example, has lost its stem-cell properties. The first branch leads to cells with myeloid and erythroid potential, on the one hand (CMPs and MEPs), and, on the other, to the common lymphoid progenitors (CLPs), with lymphoid potential. The former give rise to all nonlymphoid cellular blood elements, including circulating monocytes and granulocytes, as well as the macrophages and dendritic cells that reside in tissues and peripheral lymphoid organs (not shown). The CLP population is heterogeneous and single cells can give rise to NK cells, T cells, or B cells through successive stages of differentiation in either the bone marrow or thymus. There may be considerable plasticity in these


Immunology_Janeway

Immunology_Janeway. and single cells can give rise to NK cells, T cells, or B cells through successive stages of differentiation in either the bone marrow or thymus. There may be considerable plasticity in these pathways, in that in certain circumstances progenitor cells may switch their commitment. For example, a progenitor cell may give rise to either B cells or macrophages; however, for simplicity these alternative pathways are not shown. Some dendritic cells are also thought to be derived from the lymphoid progenitor.


Immunology_Janeway

Immunology_Janeway. Fig. 8.3 The early stages of B-cell development are dependent on bone marrow stromal cells. Interaction of B-cell progenitors with bone marrow stromal cells is required for development to the immature B-cell stage. The designations pro-B cell and pre-B cell refer to defined phases of B-cell development, as described in Fig. 8.4. Multipotent progenitor cells express the receptor tyrosine kinase FLT3, which binds to its ligand on stromal cells. Signaling through FLT3 is required for differentiation to the next stage, the common lymphoid progenitor. The chemokine CXCL12 (SDF-1) acts to retain stem cells and lymphoid progenitors at appropriate stromal cells in the bone marrow. The receptor for interleukin-7 (IL-7) is present at this stage, and IL-7 produced by stromal cells is required for the development of B-lineage cells. Progenitor cells bind to the adhesion molecule VCAM-1 on stromal cells through the integrin VLA-4 and also interact through other cell-adhesion molecules (CAMs). The


Immunology_Janeway

Immunology_Janeway. development of B-lineage cells. Progenitor cells bind to the adhesion molecule VCAM-1 on stromal cells through the integrin VLA-4 and also interact through other cell-adhesion molecules (CAMs). The adhesive interactions promote the binding of the receptor tyrosine kinase Kit (CD117) on the surface of the pro-B cell to stem-cell factor (SCF) on the stromal cell, which activates the kinase and induces the proliferation of B-cell progenitors. The actions of the listed transcription factors in B-cell development are discussed in the text. The pink horizontal bands denote the expression of particular proteins at the indicated stages of development.


Immunology_Janeway

Immunology_Janeway. receptors that are required for the development of multiple hematopoietic lineages, such as the transcription factor PU.1 and the receptor c-kit. In the next stage, MPPs produce two subsets of progenitor cells that give rise to all the lymphocyte lineages. One progenitor cell, as yet unnamed, produces the ILC subsets, ILC1, ILC2, and ILC3 cells. A second progenitor cell arising from the MPP is known as the common lymphoid progenitor (CLP). Differentiation of MPPs into CLPs requires signaling through the FLT3 receptor expressed on MPPs. Progenitor cell transfer and lineage repopulation experiments have shown that the CLP population is actually heterogeneous and represents a continuum of cells with decreasing multipotent potential. A subset of CLP cells with the broadest potential is able to generate B cells, T cells, and NK cells. A second subset of CLPs is able to generate only B cells and T cells, and a third subset of CLPs is committed exclusively to the B-cell lineage.


Immunology_Janeway

Immunology_Janeway. is able to generate B cells, T cells, and NK cells. A second subset of CLPs is able to generate only B cells and T cells, and a third subset of CLPs is committed exclusively to the B-cell lineage. B-cell-committed CLPs give rise to pro-B cells (see Fig. 8.3).


Immunology_Janeway

Immunology_Janeway. The production of lymphocyte progenitors from the multipotent progenitor cell is accompanied by expression of the receptor for interleukin-7 (IL-7), which is induced by FLT3 signaling together with the activity of PU.1. The cytokine IL-7, secreted by bone marrow stromal cells, is essential for the growth and survival of developing B cells in mice (but possibly not in humans). The IL-7 receptor is composed of two polypeptides, the IL-7 receptor α chain and the common cytokine receptor γ chain (γ-c), so called because it is also a subunit of five additional cytokine receptors. This family of cytokine receptors includes the receptors for IL-2, IL-4, IL-9, IL-15, and IL-21, in addition to IL-7. These receptors also share the tyrosine kinase Jak3, a signaling protein that binds exclusively to γ-c and is required for productive signaling by each of the receptors.


Immunology_Janeway

Immunology_Janeway. Due to the importance of IL-7 for murine B-cell development, mice with a genetic deficiency in IL-7, IL-7 receptor α, γ-c, or Jak3 all exhibit a severe block in B-cell development.


Immunology_Janeway

Immunology_Janeway. Another essential factor for B-cell development is stem-cell factor (SCF), a membrane-bound cytokine present on bone marrow stromal cells that stimulates the growth of hematopoietic stem cells and the earliest B-lineage progenitors. SCF interacts with the receptor tyrosine kinase Kit on the precursor cells (see Fig. 8.3). The chemokine CXCL12 (stromal cell-derived factor 1, SDF-1) is also essential for the early stages of B-cell development. It is produced constitutively by bone marrow stromal cells, and one of its roles may be to retain developing B-cell precursors in the marrow microenvironment. Thymic stroma-derived lymphopoietin (TSLP) resembles IL-7 and binds a receptor that includes the IL-7 receptor αchain, but not γ-c. Despite its name, TSLP may promote B-cell development in the embryonic liver and, in the perinatal period at least, in the mouse bone marrow.


Immunology_Janeway

Immunology_Janeway. A definitive B-cell stage, the pro-B cell, is specified by induction of the B-lineage-specific transcription factor E2A. It is not clear what initiates the expression of E2A in some progenitors, but it is known that the transcription factors PU.1 and Ikaros are required for E2A expression. E2A then induces the expression of the early B-cell factor (EBF). IL-7 signaling promotes the survival of these committed progenitors, while E2A and EBF act together to drive the expression of proteins that determine the pro-B-cell state.


Immunology_Janeway

Immunology_Janeway. As B-lineage cells mature, they migrate within the marrow, remaining in contact with the stromal cells. The earliest stem cells lie in a region called the endosteum, which lines the inner cavity of the long bones such as the femur and tibia. Developing B-lineage cells make contact with reticular stromal cells in the trabecular spaces, and as they mature they move toward the central sinus of the marrow cavity. The final stages of development of immature B cells into mature B cells occur in peripheral lymphoid organs such as the spleen, which we describe in Sections 8-7 and 8-8 of this chapter. 8-2 B-cell development begins by rearrangement of the heavy-chain locus.


Immunology_Janeway

Immunology_Janeway. The stages of B-cell development are, in the order they occur, early pro-B cell, late pro-B cell, large pre-B cell, small pre-B cell, immature B cell, and mature B cell (Fig. 8.4). Rearrangement of the heavy-chain locus is initiated in the pro-B cell when E2A and EBF induce the expression of several key proteins that enable gene rearrangement to occur, including the RAG-1 and RAG-2 components of the V(D)J recombinase (see Chapter 5). Only one gene locus is rearranged at a time, in a fixed sequence. The first rearrangement to take place is the joining of a D gene segment to a J segment at the immunoglobulin heavy-chain (IgH) locus. D to JH rearrangement takes place mostly in the early pro-B-cell stage, but can be seen as early as the common lymphoid progenitor. In the absence of E2A or EBF this initial rearrangement event fails to occur. Another key protein induced by E2A and EBF is the transcription factor Pax5, one isoform of which is known as the B-cell activator protein (BSAP) (see


Immunology_Janeway

Immunology_Janeway. this initial rearrangement event fails to occur. Another key protein induced by E2A and EBF is the transcription factor Pax5, one isoform of which is known as the B-cell activator protein (BSAP) (see Fig. 8.3). Among the targets of Pax5 are the gene for the B-cell co-receptor component CD19 and the gene for Igα, a signaling component of both the pre-B-cell receptor and the B-cell receptor (see Section 7-7). In the absence of Pax5, pro-B cells fail to develop further down the B-cell pathway but can be induced to give rise to T cells and myeloid cell types, indicating that Pax5 is required for commitment of the pro-B cell to the B-cell lineage. Pax5 also induces the expression of the B-cell linker protein (BLNK), an SH2-containing scaffold protein that is required for further development of the pro-B cell and for signaling from the mature B-cell antigen receptor lgD Stem cell Early pro-B cell Late pro-B cell Immature B cell Mature B cell Small pre-B cell Large pre-B cell L-chain genes


Immunology_Janeway

Immunology_Janeway. the pro-B cell and for signaling from the mature B-cell antigen receptor lgD Stem cell Early pro-B cell Late pro-B cell Immature B cell Mature B cell Small pre-B cell Large pre-B cell L-chain genes Surface Ig Absent Absent Absent Intracellular ˜ chain IgM expressed on cell surface Germline Germline GermlineGermline VJ rearranged VJ rearrangedV–J rearranging H-chain genes Germline D–J rearranging V–DJ rearranging VDJ rearranged VDJ rearranged VDJ rearranged VDJ rearranged lgM lgMpre-B receptor BP-1 Amino-peptidase CD19 Unknown CD43 CD24 Growth factor receptorIL-7R Kit TdT ˛5 Ig˝ VpreB Ig˙ CD45R Btk RAG-1 RAG-2 N-nucleotide addition Protein Function Surrogate light-chain components Lymphoid-speciÿc recombinase Signal transduction ˜ chain transiently at surface as part of pre-B-cell receptor. Mainly intracellular IgD and IgM made from alternatively spliced H-chain transcripts


Immunology_Janeway

Immunology_Janeway. Fig. 8.4 The development of a B-lineage cell proceeds through several stages marked by the rearrangement and expression of the immunoglobulin genes. The stem cell has not yet begun to rearrange its immunoglobulin (Ig) gene segments; they are in the germline configuration found in all nonlymphoid cells. The heavy-chain (H-chain) locus rearranges first. Rearrangement of a D gene segment to a JH gene segment starts in the common lymphoid progenitor and occurs mostly in early pro-B cells, generating late pro-B cells in which VH to DJH rearrangement occurs. A successful VDJH rearrangement leads to the expression of a complete immunoglobulin heavy chain as part of the pre-B-cell receptor, which signals via Igα, Igβ, and Btk (see Fig 7.27). Once this occurs, the cell is stimulated to become a large pre-B cell, which proliferates to become small resting pre-B cells; at this point the cells cease expression of the surrogate light chains (λ5 and VpreB) and express the μ heavy chain alone in the


Immunology_Janeway

Immunology_Janeway. pre-B cell, which proliferates to become small resting pre-B cells; at this point the cells cease expression of the surrogate light chains (λ5 and VpreB) and express the μ heavy chain alone in the cytoplasm. Small pre-B cells reexpress the RAG proteins and start to rearrange the light-chain (L-chain) genes. Upon successfully assembling a light-chain gene, a cell becomes an immature B cell that expresses a complete IgM molecule at the cell surface, which also signals via Igα and Igβ. Mature B cells produce a δ heavy chain as well as a μ heavy chain, by a mechanism of alternative mRNA splicing (see Fig. 5.17), and are marked by the additional appearance of IgD on the cell surface. All stages through the development of immature B cells takes place in the bone marrow; the final maturation to IgM+IgD+ mature B cells occurs in the spleen. The earliest B-lineage surface markers are CD19 and CD45R (B220 in the mouse), which are expressed throughout B-cell development. A pro-B cell is also


Immunology_Janeway

Immunology_Janeway. to IgM+IgD+ mature B cells occurs in the spleen. The earliest B-lineage surface markers are CD19 and CD45R (B220 in the mouse), which are expressed throughout B-cell development. A pro-B cell is also distinguished by the expression of CD43 (a marker of unknown function), Kit (CD117), and the IL-7 receptor. A late pro-B cell starts to express CD24 (a marker of unknown function). A pre-B cell is phenotypically distinguished by the expression of the enzyme BP-1, whereas Kit is no longer expressed.


Immunology_Janeway

Immunology_Janeway. (see Section 7-20). The temporal expressions of some of the transcription factors, surface proteins, and receptors required for B-cell development are listed in Fig. 8.3 and Fig. 8.4. Although the V(D)J recombinase system operates in both Band T-lineage cells and uses the same core enzymes, rearrangements of T-cell receptor genes do not occur in B-lineage cells, nor do complete rearrangements of immunoglobulin genes occur in T cells. The ordered rearrangement events that do occur are associated with lineage-specific low-level transcription of the gene segments about to be joined.


Immunology_Janeway

Immunology_Janeway. The initial D to JH rearrangements in the immunoglobulin heavy-chain locus (Fig. 8.5) typically occur on both alleles, at which point the cell becomes a late pro-B cell. Most D to JH joins in humans are potentially useful, because most human D gene segments can be translated in all three reading frames without encountering a stop codon. Thus, there is no need for a special mechanism to distinguish successful D to JH joints, and at this early stage there is also no need to ensure that only one allele undergoes rearrangement. Indeed, given the likely rate of failure at later stages, starting off with two successfully rearranged D–JH sequences may be an advantage.


Immunology_Janeway

Immunology_Janeway. Fig. 8.5 A productively rearranged immunoglobulin gene is immediately expressed as a protein by the developing B cell. In early pro-B cells, heavy-chain gene rearrangement is initiated with D to JH rearrangements. As shown in the top panels, no functional μ protein is expressed, although transcription occurs (red arrow). In late pro-B cells, VH to DJH rearrangement occurs on one chromosome first. If no functional H-chain is produced, VH to DJH rearrangement occurs on the second chromosome. As soon as a productive heavy-chain gene rearrangement takes place, μ chains are expressed by the cell in a complex with two other chains, λ5 and VpreB, which together make up a surrogate light chain. The whole immunoglobulin-like complex is known as the pre-B-cell receptor (center panels). It is associated with two other protein chains, Igα and Igβ, which signal the B cell to halt heavy-chain gene rearrangement; this drives the transition to the large pre-B-cell stage by inducing proliferation.


Immunology_Janeway

Immunology_Janeway. with two other protein chains, Igα and Igβ, which signal the B cell to halt heavy-chain gene rearrangement; this drives the transition to the large pre-B-cell stage by inducing proliferation. Failure to produce a functional H-chain leading to a pre-B-cell receptor signal leads to cell death. The progeny of large pre-B cells stop dividing and become small pre-B cells, in which light-chain gene rearrangements commence. Vκ–Jκ rearrangement (see Section 5-2) occurs first, and if unsuccessful, Vλ to Jλ rearrangement occurs next. Successful light-chain gene rearrangement results in the production of a light chain that binds the μ chain to form a complete IgM molecule, which is expressed together with Igα and Igβ at the cell surface, as shown in the bottom panels. Signaling via this surface receptor complex is thought to trigger the cessation of light-chain gene rearrangement. Failure to produce a functional L chain leads to cell death.


Immunology_Janeway

Immunology_Janeway. To produce a complete immunoglobulin heavy chain, the late pro-B cell now proceeds with a rearrangement of a VH gene segment to a DJH sequence. In contrast to D to JH rearrangement, VH to DJH rearrangement occurs first on only one chromosome. A successful rearrangement leads to the production of intact μheavy chains, after which VH to DJH rearrangement ceases and the cell becomes a pre-B cell. Pro-B cells that do not produce a μ chain are eliminated, as they fail to receive an important survival signal mediated by the pre-B-cell receptor (see Section 8-3). At least 45% of pro-B cells are lost at this stage. In at least two out of three cases, the first VH to DJH rearrangement is nonproductive as each amino acid is encoded by a triplet of nucleotides. When this initial rearrangement is out of frame, rearrangement then occurs on the other chromosome, again with a theoretical two in three chance of failure. A rough estimate of the chance of generating a pre-B cell is thus 55% [1/3 + (2/3


Immunology_Janeway

Immunology_Janeway. of frame, rearrangement then occurs on the other chromosome, again with a theoretical two in three chance of failure. A rough estimate of the chance of generating a pre-B cell is thus 55% [1/3 + (2/3 × 1/3) = 0.55]. The actual frequency is somewhat lower, because the V gene segment repertoire contains pseudogenes that can rearrange yet have major defects that prevent the expression of a functional protein. An initial nonproductive rearrangement does not automatically lead to pro-B cell elimination, as it is possible for most loci to undergo successive rearrangements on the same chromosome, and where that fails, the locus on the other chromosome will rearrange.


Immunology_Janeway

Immunology_Janeway. The diversity of the B-cell antigen-receptor repertoire is enhanced at this stage by the enzyme terminal deoxynucleotidyl transferase (TdT). TdT is expressed by the pro-B cell and adds nontemplated nucleotides (N-nucleotides) at the joints between rearranged gene segments (see Section 5-8). In adult humans, it is expressed in pro-B cells during heavy-chain gene rearrangement, but its expression declines at the pre-B-cell stage during light-chain gene rearrangement. This explains why N-nucleotides are found in the V–D and D–J joints of nearly all heavy-chain genes but only in about a quarter of human light-chain joints. N-nucleotides are rarely found in mouse light-chain V–J joints, showing that TdT is switched off slightly earlier in the development of mouse B cells. In fetal development, when the peripheral immune system is first being supplied with T and B lymphocytes, TdT is expressed only at low levels, if at all.


Immunology_Janeway

Immunology_Janeway. 8-3 The pre-B-cell receptor tests for successful production of a complete heavy chain and signals for the transition from the pro-B cell to the pre-B cell stage.


Immunology_Janeway

Immunology_Janeway. The imprecise nature of V(D)J recombination is a double-edged sword. Although it produces increased diversity in the antibody repertoire, it also results in many unsuccessful rearrangements. Pro-B cells therefore need a way of testing whether a potentially functional heavy chain has been produced. They do this by incorporating a functional heavy chain into a receptor that can signal its successful production. This test takes place in the absence of light chains, whose loci have not yet rearranged. Instead, pro-B cells make two invariant ‘surrogate’ proteins that together have a structural resemblance to the light chain and can pair with the μ chain to form the pre-B-cell receptor (pre-BCR) (see Fig. 8.5). The assembly of a pre-B-cell receptor signals to the B cell that a productive rearrangement has been made, and the cell is then considered a pre-B cell.


Immunology_Janeway

Immunology_Janeway. The surrogate chains are encoded by nonrearranging genes separate from the antigen-receptor loci, and their expression is induced by E2A and EBF (see Fig. 8.4). One is called λ5 because of its close resemblance to the C domain of the λ light chain; the other, called VpreB, resembles a light-chain V domain but has an extra region at the amino-terminal end. Pro-B cells and pre-B cells also express the invariant proteins Igα and Igβ, introduced in Chapter 7 as the signaling components of the B-cell receptor complex on mature B cells. As components of the pre-B-cell receptor, Igα and Igβ transduce signals by interacting with intracellular tyrosine kinases through their cytoplasmic tails, just Amino-terminal tails on VpreB and ˜5 in adjacent pre-B-cell receptor molecules bind each other and cross-link the receptors, inducing clustering and signaling as they function to transduce signals through the antigen receptor on mature B cells (see Section 7-7).


Immunology_Janeway

Immunology_Janeway. Formation of the pre-B-cell receptor and signaling through this receptor provide an important checkpoint that mediates the transition between the pro-B cell and the pre-B cell. In mice that either lack λ5 or have mutant heavy-chain genes that cannot produce the transmembrane domain, the pre-B-cell receptor cannot be formed and B-cell development is blocked after heavy-chain gene rearrangement. In normal B-cell development, the pre-B-cell receptor complex is expressed transiently, perhaps because the production of λ5 mRNA stops as soon as pre-B-cell receptors begin to be formed. Although present at only low levels on the cell surface, the pre-B-cell receptor generates signals required for the transition from pro-B cell to pre-B cell. No antigen or other external ligand seems to be involved in signaling by the receptor. Instead, pre-B-cell receptors are thought to interact with each other, forming dimers or oligomers that generate signals as described in Section 7-16. Dimerization


Immunology_Janeway

Immunology_Janeway. in signaling by the receptor. Instead, pre-B-cell receptors are thought to interact with each other, forming dimers or oligomers that generate signals as described in Section 7-16. Dimerization involves ‘unique’ regions in the amino termini of λ5 and VpreB proteins that are not present in other immunoglobulin-like domains and which mediate the cross-linking of adjacent pre-B-cell receptors on the cell surface (Fig. 8.6). PreB-cell receptor signaling requires the scaffold protein BLNK and Bruton’s tyrosine kinase (Btk), an intracellular Tec-family tyrosine kinase (see Section 7-20). In humans and mice, deficiency of BLNK leads to a block in B-cell development at the pro-B-cell stage. In humans, mutations in the BTK gene cause a profound B-lineage-specific immune deficiency, Bruton’s X-linked agammaglobulinemia (XLA), in which no mature B cells are produced. The block in B-cell development caused by mutations in BTK is almost total, interrupting the transition from pre-B cell to


Immunology_Janeway

Immunology_Janeway. X-linked agammaglobulinemia (XLA), in which no mature B cells are produced. The block in B-cell development caused by mutations in BTK is almost total, interrupting the transition from pre-B cell to immature B cell. A similar, but less severe, defect called X-linked immunodeficiency, or xid, arises from mutations in the Btk gene in mice.


Immunology_Janeway

Immunology_Janeway. 8-4 Pre-B-cell receptor signaling inhibits further heavy-chain locus rearrangement and enforces allelic exclusion. The signaling generated by pre-B-cell receptor clustering halts further rearrangement of the heavy-chain locus and allows the pro-B cell to become sensitive to IL-7. This induces cell proliferation, initiating the transition to the large


Immunology_Janeway

Immunology_Janeway. Fig. 8.6 The pre-B-cell receptor initiates signaling through spontaneous dimerization induced by the unique regions of VpreB and λ5. Two surrogate protein chains, VpreB (orange) and λ5 (green), substitute for a light chain and bind to a heavy chain, thus allowing its surface expression. VpreB substitutes for the light-chain V region in this surrogate interaction, while λ5 takes the part of the light-chain constant region. Both VpreB and λ5 contain ‘unique’ amino-terminal regions that are not present in other immunoglobulin-like domains, shown here as unstructured tails extending out from the globular domains. These amino-terminal regions associated with one pre-B-cell receptor can interact with the corresponding regions on the adjacent pre-B-cell receptor, promoting the spontaneous formation of pre-Bcell receptor dimers on the cell surface. Dimerization generates signaling from the pre-B-cell receptor that is dependent on the presence of the ITAM-containing signaling chains Igα and


Immunology_Janeway

Immunology_Janeway. of pre-Bcell receptor dimers on the cell surface. Dimerization generates signaling from the pre-B-cell receptor that is dependent on the presence of the ITAM-containing signaling chains Igα and Igβ. The signals cause the inhibition of RAG-1 and RAG-2 expression and the proliferation of the large pre-B cell. Courtesy of Chris Garcia.


Immunology_Janeway

Immunology_Janeway. Fig. 8.7 Allelic exclusion in individual B cells. Most species have genetic polymorphisms of the constant regions of their immunoglobulin heavy-chain and light-chain genes; these polymorphisms lead to amino acid differences between the encoded proteins. These variants of heavy-chain or light-chain proteins expressed by different individuals in a species are known as allotypes. In rabbits, for example, all of the B cells in an individual homozygous for the a allele of the immunoglobulin heavy-chain locus (Igha/a) will express immunoglobulin of allotype a, whereas in an individual homozygous for the b allele (Ighb/b) all the B cells make immunoglobulin of allotype b. In a heterozygous animal (Igha/b), which carries the a allele at one of the Igh loci and the b allele at the other, individual B cells can be shown to express surface immunoglobulin of either the a-allotype or the b-allotype, but not both (bottom panel). This allelic exclusion reflects the productive rearrangement of only


Immunology_Janeway

Immunology_Janeway. B cells can be shown to express surface immunoglobulin of either the a-allotype or the b-allotype, but not both (bottom panel). This allelic exclusion reflects the productive rearrangement of only one of the two Igh alleles in the B cell, because the production of a successfully rearranged immunoglobulin heavy chain forms a pre-Bcell receptor, which signals the cessation of further heavy-chain gene rearrangement.


Immunology_Janeway

Immunology_Janeway. pre-B cell. Successful rearrangements at both heavy-chain alleles could result in a B cell producing two receptors of different antigen specificities. To prevent this, signaling by the pre-B-cell receptor enforces allelic exclusion, the state in which only one of the two alleles of a gene is expressed in a diploid cell. Allelic exclusion, which occurs at both the heavy-chain locus and the light-chain loci, was discovered nearly 50 years ago and provided one of the original pieces of experimental support for the theory that one lymphocyte expresses one type of antigen receptor (Fig. 8.7).


Immunology_Janeway

Immunology_Janeway. Signaling from the pre-B-cell receptor promotes heavy-chain allelic exclusion in three ways. First, it reduces V(D)J recombinase activity by directly reducing the expression of the RAG-1 and RAG-2 genes. Second, it further reduces levels of RAG-2 by indirectly causing this protein to be targeted for degradation, which occurs when RAG-2 is phosphorylated in response to the entry of the pro-B cell into S phase (the DNA synthesis phase) of the cell cycle. Finally, preB-cell receptor signaling reduces access of the heavy-chain locus to the recombinase machinery, although the precise details of this are not clear. At a later stage of B-cell development, RAG proteins will again be expressed in order to carry out light-chain locus rearrangement, but at that point the heavy-chain locus does not undergo further rearrangement. In the absence of pre-B-cell receptor signaling, allelic exclusion of the heavy-chain locus does not occur. Since a second important role of pre-B-cell receptor signaling


Immunology_Janeway

Immunology_Janeway. further rearrangement. In the absence of pre-B-cell receptor signaling, allelic exclusion of the heavy-chain locus does not occur. Since a second important role of pre-B-cell receptor signaling is to stimulate proliferative expansion of B-cell precursors with a successful heavy-chain rearrangement, a deficiency in this signal causes a profound reduction in the numbers of pre-B cells and mature B cells that develop.


Immunology_Janeway

Immunology_Janeway. 8-5 Pre-B cells rearrange the light-chain locus and express cell-surface immunoglobulin. The transition from the pro-B-cell to the large pre-B-cell stage is accompanied by several rounds of cell division, expanding the population of cells with successful in-frame joins by about 30to 60-fold before they become resting small pre-B cells. A large pre-B cell with a particular rearranged heavy-chain gene therefore gives rise to numerous small pre-B cells. RAG proteins are produced again in the small pre-B cells, and rearrangement of the light-chain locus begins. Each of these cells can make a different rearranged light-chain gene, and so cells with many different antigen specificities are generated from a single pre-B cell, which makes an important contribution to overall B-cell receptor diversity.


Immunology_Janeway

Immunology_Janeway. Light-chain rearrangement also exhibits allelic exclusion. Rearrangements at the light-chain locus generally take place at only one allele at a time, a process regulated by a mechanism not currently understood. The light-chain loci lack D segments, and rearrangement occurs by V to J joining; and if a particular VJ rearrangement fails to produce a functional light chain, repeated rearrangements of unused V and J gene segments at the same allele can occur (Fig. 8.8). Several attempts at productive rearrangement of a light-chain gene can therefore be made on one chromosome before initiating any rearrangements on the second chromosome. This greatly increases the chances of eventually generating an intact light chain, especially as there are two different light-chain loci. As a result, many cells that reach the preB-cell stage succeed in generating progeny that bear intact IgM molecules and can be classified as immature B cells. Figure 8.4 lists some of the proteins involved in V(D)J


Immunology_Janeway

Immunology_Janeway. cells that reach the preB-cell stage succeed in generating progeny that bear intact IgM molecules and can be classified as immature B cells. Figure 8.4 lists some of the proteins involved in V(D)J recombination and shows how their expression is regulated throughout B-cell development. Figure 8.5 summarizes the stages of B-cell development up to the point of assembly of a complete surface immunoglobulin. Developing B cells that fail to assemble a complete surface immunoglobulin undergo apoptosis in the bone marrow, and are eliminated from the B-cell pool.


Immunology_Janeway

Immunology_Janeway. Fig. 8.8 Nonproductive light-chain gene rearrangements can be rescued by further rearrangement. The organization of the light-chain loci in mice and humans offers many opportunities for the rescue of pre-B cells that initially make an out-offrame rearrangement. Light-chain rescue is illustrated here at the human κ locus. If the first rearrangement is nonproductive, a 5’ Vκ gene segment can recombine with a 3’ Jκ gene segment to remove the out-of-frame join located between them and to replace it with a new rearrangement. In principle, this can happen up to five times on each chromosome, because there are five functional Jκ gene segments in humans. If all rearrangements of κ-chain genes fail to yield a productive light-chain join, λ-chain gene rearrangement may succeed (not shown).


Immunology_Janeway

Immunology_Janeway. As well as allelic exclusion, light chains also display isotypic exclusion, that is, the expression of only one type of light chain—κ or λ—by an individual B cell. Again, the mechanism regulating this process is not known. In mice and humans, the κ light-chain locus tends to rearrange before the λ locus. This was first deduced from the observation that myeloma cells secreting λ light chains generally have both their κ and λ light-chain genes rearranged, whereas in myelomas secreting κ light chains, generally only the κ genes are rearranged. This order is occasionally reversed, however, and λ gene rearrangement does not absolutely require the previous rearrangement of the κ genes. The ratios of κ-expressing versus λ-expressing mature B cells vary from one extreme to the other in different species. In mice and rats it is 95% κ to 5% λ, in humans it is typically 65%:35%, and in cats it is 5%:95%, the opposite of that in mice. These ratios correlate most strongly with the number of


Immunology_Janeway

Immunology_Janeway. species. In mice and rats it is 95% κ to 5% λ, in humans it is typically 65%:35%, and in cats it is 5%:95%, the opposite of that in mice. These ratios correlate most strongly with the number of functional Vκ and Vλ gene segments in the genome of the species. They also reflect the kinetics and efficiency of gene segment rearrangements. The κ:λ ratio in the mature lymphocyte population is useful in clinical diagnostics, because an aberrant κ:λ ratio indicates the dominance of one clone and the presence of a lymphoproliferative disorder.


Immunology_Janeway

Immunology_Janeway. 8-6 Immature B cells are tested for autoreactivity before they leave the bone marrow. Once a rearranged light chain has paired with a μ chain, IgM can be expressed on the cell surface (as a surface IgM, or sIgM) and the pre-B cell becomes an immature B cell. At this stage, the antigen receptor is first tested for reactivity to self antigens, or autoreactivity. The elimination or inactivation of autoreactive B cells ensures that the B-cell population as a whole will be tolerant of self antigens. The tolerance produced at this stage of B-cell development is known as central tolerance because it arises in a central lymphoid organ, the bone marrow. However, B cells leaving the bone marrow are not fully mature and require additional maturation steps that take place in peripheral lymphoid organs (see Section 8-8). As we shall see later in the chapter and in Chapter 15, self-reactive B cells that escape central tolerance may still be removed from


Immunology_Janeway

Immunology_Janeway. Fig. 8.9 Binding to self molecules in the bone marrow can lead to the death or inactivation of immature B cells.


Immunology_Janeway

Immunology_Janeway. First panels: immature B cells that do not encounter antigen mature normally; they migrate from the bone marrow to the peripheral lymphoid tissues, where they may become mature recirculating B cells bearing both IgM and IgD on their surface. Second panels: when developing B cells express receptors that recognize multivalent ligands, for example, ubiquitous cell-surface self molecules such as those of the MHC, these receptors are deleted from the repertoire. The B cells either undergo receptor editing (see Fig. 8.10), thereby eliminating the self-reactive receptor, or the cells themselves undergo programmed cell death (apoptosis), resulting in clonal deletion. Third panels: immature B cells that bind soluble self antigens able to cross-link the B-cell receptor are rendered unresponsive to the antigen (anergic) and bear little surface IgM. They migrate to the periphery, where they express IgD but remain anergic; if in competition with other B cells in the periphery, anergic B cells fail


Immunology_Janeway

Immunology_Janeway. antigen (anergic) and bear little surface IgM. They migrate to the periphery, where they express IgD but remain anergic; if in competition with other B cells in the periphery, anergic B cells fail to receive survival signals and die. Fourth panels: immature B cells whose antigen is inaccessible to them, or which bind monovalent or soluble self antigens with low affinity, do not receive any signal and mature normally. Such cells are potentially self-reactive, however, and are said to be clonally ignorant because their ligand is present but is unable to activate them.


Immunology_Janeway

Immunology_Janeway. the repertoire after they have left the bone marrow, a process that takes place during the final peripheral stages of B-cell maturation, and is referred to as peripheral tolerance, described in Section 8-7.


Immunology_Janeway

Immunology_Janeway. sIgM associates with Igα and Igβ to form a functional B-cell receptor complex, and the fate of an immature B cell in the bone marrow depends on signals delivered from this receptor complex when it interacts with ligands in the environment. Igα signaling is particularly important in dictating the emigration of B cells from the bone marrow and/or their survival in the periphery: mice that express Igα with a truncated cytoplasmic domain that cannot signal show a fourfold reduction in the number of immature B cells in the marrow, and a hundredfold reduction in the number of peripheral B cells. The release of immature B cells from the bone marrow into the circulation is also dependent on their expression of S1PR1, a G-protein-coupled receptor that binds to the lipid ligand S1P and promotes cell migration towards the high concentrations of S1P that exist in the blood (see Section 8-27).


Immunology_Janeway

Immunology_Janeway. Immature B cells that have no strong reactivity to self antigens continue to mature (Fig. 8.9, first panel). They leave the marrow via sinusoids that enter the central sinus, enter the circulation, and are carried by the venous blood supply to the spleen. If, however, the newly expressed receptor encounters a strongly cross-linking antigen in the bone marrow—that is, if the B cell is strongly self-reactive—development is arrested at this stage.


Immunology_Janeway

Immunology_Janeway. Experiments using genetically modified mice that enforce the expression of self-reactive B-cell receptors have shown that there are four possible fates for self-reactive immature B cells (see Fig. 8.9, last three panels). These fates are the production of a new receptor by a process known as receptor editing; cell death by apoptosis, resulting in clonal deletion; the induction of a permanent state of unresponsiveness to antigen, or anergy; and a state of immunological ignorance in which antigen concentrations are too low to stimulate B-cell receptor signaling. The outcome for each self-reactive B cell is dependent on the interaction of the B-cell receptor with the self antigen.


Immunology_Janeway

Immunology_Janeway. Fig. 8.10 Replacement of light chains by receptor editing can rescue some self-reactive B cells by changing their antigen specificity. When a developing B cell expresses antigen receptors that are strongly cross-linked by multivalent self antigens such as MHC molecules on cell surfaces (top panel), its development is arrested. The cell decreases surface expression of IgM and does not turn off the RAG genes (second panel). Continued synthesis of RAG proteins allows the cell to continue light-chain gene rearrangement. This usually leads to a new productive rearrangement and the expression of a new light chain, which combines with the previous heavy chain to form a new receptor; the process is called receptor editing (third panel). If this new receptor is not self-reactive, the cell is ‘rescued’ and continues normal development, much like a cell that had never reacted with self antigen (bottom right panel). If the cell remains self-reactive, it may be rescued by another cycle of


Immunology_Janeway

Immunology_Janeway. is ‘rescued’ and continues normal development, much like a cell that had never reacted with self antigen (bottom right panel). If the cell remains self-reactive, it may be rescued by another cycle of rearrangement; however, if it continues to react strongly with self antigen, it will undergo apoptosis, resulting in clonal deletion from the repertoire of B cells (bottom left panel).


Immunology_Janeway

Immunology_Janeway. Immature B cells that express an autoreactive receptor recognizing a multivalent self antigen can be rescued by further gene rearrangements that replace the autoreactive receptor with a new receptor that is not self-reactive. This mechanism is termed receptor editing (Fig. 8.10). When an immature B cell first produces sIgM, RAG proteins are still being made. If the receptor is not self-reactive, the absence of sIgM cross-linking allows gene rearrangement to cease and B-cell development continues, with RAG proteins eventually disappearing. For an autoreactive receptor, however, an encounter with the self antigen results in strong cross-linking of sIgM; RAG expression continues, and light-chain gene rearrangement can continue, as described in Fig. 8.8. These secondary rearrangements can rescue immature self-reactive B cells by deleting the self-reactive light-chain gene and replacing it with another sequence. If the new light chain is not autoreactive, the B cell continues normal


Immunology_Janeway

Immunology_Janeway. rescue immature self-reactive B cells by deleting the self-reactive light-chain gene and replacing it with another sequence. If the new light chain is not autoreactive, the B cell continues normal development. If the receptor remains autoreactive, rearrangement continues until a non-autoreactive receptor is produced or until no additional light-chain V and J gene segments are available for recombination. The importance of receptor editing as a mechanism of tolerance is well established, as defects in this process contribute to the human autoimmune diseases systemic lupus erythematosus and rheumatoid arthritis, two diseases characterized by high levels of autoreactive antibodies (see Chapter 15).


Immunology_Janeway

Immunology_Janeway. It was originally thought that the successful production of a heavy chain and a light chain caused the almost instantaneous shutdown of light-chain locus rearrangement and that this ensured both allelic and isotypic exclusion. The unexpected ability of self-reactive B cells to continue to rearrange their light-chain genes, even after having made a productive rearrangement, suggests an alternative mechanism of allelic exclusion, where the fall in the level of RAG proteins that follows a successful non-autoreactive rearrangement could be the principal means by which light-chain rearrangement is terminated. It is now apparent that allelic exclusion is not absolute, as there are rare B cells that express two different light chains.


Immunology_Janeway

Immunology_Janeway. Cells that remain autoreactive when receptor editing efforts fail to generate a non-autoreactive receptor undergo a process known as clonal deletion, in which they are subjected to cell death by apoptosis to eliminate their specific autoreactivity from the repertoire. Early experiments using transgenic mice expressing both chains of an immunoglobulin specific for H-2Kb MHC class I molecules, in which nearly all developing B cells expressed the anti-MHC immunoglobulin as sIgM, suggested that clonal deletion was a predominant mechanism of B-cell tolerance. These studies found that transgenic mice not expressing H-2Kb had normal numbers of B cells, all bearing the transgene-encoded anti-H-2Kb receptors. However, in mice expressing both H-2Kb and the immunoglobulin transgenes, B-cell development was blocked. Normal numbers of pre-B cells and immature B cells were found, but B cells expressing the anti-H-2Kb immunoglobulin as sIgM never matured to populate the spleen and lymph nodes;


Immunology_Janeway

Immunology_Janeway. was blocked. Normal numbers of pre-B cells and immature B cells were found, but B cells expressing the anti-H-2Kb immunoglobulin as sIgM never matured to populate the spleen and lymph nodes; instead, most of these immature B cells died in the bone marrow by apoptosis. However, more recent studies, using


Immunology_Janeway

Immunology_Janeway. Strong ligation of IgM by self antigen Arrest of B-cell development and continued light-chain rearrangement: low cell-surface IgM If the new receptor is still self-reactive, the B cell undergoes apoptosis If the new receptor is no longer self-reactive, the immature B cell migrates to the periphery and matures A new receptor speciÿcity is now expressed IgM mice bearing transgenes for autoantibody heavy and light chains that have been placed within the immunoglobulin loci by homologous recombination (see Appendix I, Section A-35, for details of this method), indicate that receptor editing, rather than clonal deletion, is the more likely outcome for immature autoreactive B cells.


Immunology_Janeway

Immunology_Janeway. We have so far discussed the fate of newly formed B cells that undergo multivalent cross-linking of their sIgM. Immature B cells that encounter more weakly cross-linking self antigens of low valence, such as small soluble proteins, respond differently. In this situation, some self-reactive B cells are inactivated and enter a state of permanent unresponsiveness, or anergy, but do not immediately die (see Fig. 8.9). Anergic B cells cannot be activated by their specific antigen even with help from antigen-specific T cells. Again, this phenomenon was elucidated using transgenic mice. Hen egg-white lysozyme (HEL) was expressed in soluble form from a transgene in mice that were also transgenic for high-affinity anti-HEL immunoglobulin. The HEL-specific B cells matured and emigrated from the bone marrow, but could not respond to antigen. Furthermore, the migration of anergic B cells is impaired, as the cells are detained in the T-cell areas of peripheral lymphoid tissues and are excluded


Immunology_Janeway

Immunology_Janeway. bone marrow, but could not respond to antigen. Furthermore, the migration of anergic B cells is impaired, as the cells are detained in the T-cell areas of peripheral lymphoid tissues and are excluded from lymphoid follicles, thereby reducing their life-span and their ability to compete with immunocompetent B cells (described further in Section 8-8). Under normal circumstances, where few self-reactive anergic B cells successfully mature, these cells die relatively quickly. This mechanism ensures that the long-lived pool of peripheral B cells is purged of potentially self-reactive cells.


Immunology_Janeway

Immunology_Janeway. The fourth potential fate of self-reactive immature B cells is that nothing happens to them; they remain in a state of immunological ignorance of their self antigen (see Fig. 8.9). Immunologically ignorant cells have affinity for a self antigen but for various reasons do not sense and respond to it. The antigen may not be accessible to developing B cells in the bone marrow or spleen, or may be in low concentration, or may bind so weakly to the B-cell receptor that it does not generate an activating signal. Because some ignorant cells can be (and in fact are) activated under certain conditions such as inflammation or when the self antigen becomes available or reaches an unusually high concentration, they should not be considered inert, and they are fundamentally different from cells with non-autoreactive receptors that could never be activated by self antigens.


Immunology_Janeway

Immunology_Janeway. The fact that central tolerance is not perfect and some self-reactive B cells are allowed to mature reflects the balance that the immune system strikes between purging all self-reactivity and maintaining the ability to respond to pathogens. If the elimination of self-reactive cells were too efficient, the receptor repertoire might become too limited and thus unable to recognize a wide variety of pathogens. Some autoimmune disease is the price of this balance: we shall see in Chapter 15 that ignorant self-reactive lymphocytes can be activated and cause disease under certain circumstances. Normally, however, ignorant B cells are held in check by a lack of T-cell help, the continued inaccessibility of the self antigen, or the tolerance that can be induced in mature B cells following their emigration from the bone marrow, which is described below. 8-7 Lymphocytes that encounter sufficient quantities of self antigens for the first time in the periphery are eliminated or inactivated.


Immunology_Janeway

Immunology_Janeway. While large numbers of autoreactive B cells are purged from the population of new lymphocytes in the bone marrow, only lymphocytes specific for autoantigens that are expressed in or can reach this organ are affected. Some antigens, like the thyroid product thyroglobulin, are highly tissue specific, or are compartmentalized so that little if any is available in the circulation. Therefore, newly emigrated self-reactive B cells that encounter their specific autoantigen for the first time in the periphery must be eliminated or inactivated also. This tolerance mechanism, which acts on newly emigrated B cells that are still immature, is known as peripheral tolerance. Like self-reactive lymphocytes in the central lymphoid organs, lymphocytes that encounter self antigens de novo in the periphery can have several fates: deletion, anergy, or survival (Fig. 8.11).


Immunology_Janeway

Immunology_Janeway. In the absence of an infection, newly emigrated B cells that encounter a strongly cross-linking antigen in the periphery will undergo clonal deletion. This was elegantly shown in studies of B cells expressing B-cell receptors specific for H-2Kb MHC class I molecules. These B cells are deleted even when, in transgenic animals, the expression of the H-2Kb molecule is restricted to the liver by the use of a liver-specific gene promoter. There is no receptor editing: B cells that encounter strongly cross-linking antigens in the periphery undergo apoptosis directly, unlike their counterparts in the bone marrow, which attempt further receptor rearrangements. This difference may be due to the fact that the B cells in the periphery are somewhat more mature and can no longer rearrange their light-chain loci.


Immunology_Janeway

Immunology_Janeway. As with immature B cells in the bone marrow, newly developed peripheral B cells that encounter and bind an abundant soluble antigen become unresponsive. This was demonstrated in mice by placing the HEL transgene under the control of an inducible promoter that can be regulated by changes in the diet. It is thus possible to induce the production of lysozyme at any time and thereby study its effects on HEL-specific B cells at different stages of maturation. These experiments have shown that both peripheral and immature bone marrow B cells are inactivated when they are chronically exposed to soluble antigen. 8-8 Immature B cells arriving in the spleen turn over rapidly and require cytokines and positive signals through the B-cell receptor for maturation and long-term survival.


Immunology_Janeway

Immunology_Janeway. Fig. 8.11 Transitional B cells that recognize self antigens undergo peripheral tolerance. After emigrating from the bone marrow and entering the circulation, immature B cells are known as transitional B cells. Not yet fully mature, these cells are still subject to tolerance in the spleen following engagement of their sIgM receptor by a self antigen. Transitional B cells that encounter a multivalent self antigen receive a strong B-cell receptor signal and undergo cell death. Transitional B cells with sIgM that binds to a soluble self molecule are rendered anergic, and ultimately die within a few days due to being excluded from the B-cell follicles in the spleen (see Fig. 8.12). Transitional B cells that bind with low affinity to a soluble self molecule remain clonally ignorant of the self antigen and continue their maturation. Transitional B cells with no self reaction also continue their maturation into mature B cells. The final stages of B-cell maturation lead to upregulation of


Immunology_Janeway

Immunology_Janeway. antigen and continue their maturation. Transitional B cells with no self reaction also continue their maturation into mature B cells. The final stages of B-cell maturation lead to upregulation of sIgD, and take place in the B-cell follicles in the spleen.


Immunology_Janeway

Immunology_Janeway. When B cells emerge from the bone marrow into the periphery, they are still functionally immature. As discussed above, their final maturation in the periphery Transitional B cells that enter the follicle receive maturation and survival signals and differentiate into follicular B cells or marginal zone B cells provides an opportunity for the immature B cells to encounter peripheral self antigens and to undergo tolerance. Immature B cells express high levels of sIgM but little sIgD, whereas mature B cells express low levels of IgM and high levels of IgD; while the changes in expression of sIgM and sIgD as B cells mature is well documented, the function of sIgD on mature B cells is not known.


Immunology_Janeway

Immunology_Janeway. Most immature B cells leaving the bone marrow will not survive to become fully mature B cells. Figure 8.12 shows the possible fates of newly produced B cells that enter the periphery. The daily output of new B cells from the bone marrow is roughly 5–10% of the total B-lymphocyte population in the steady-state peripheral pool. In unimmunized animals, the size of this pool seems to remain constant, due to homeostasis, which means that the stream of new B cells needs to be balanced by the removal of an equal number of peripheral B cells. However, the majority of peripheral mature B cells are long-lived, and only 1–2% of these die each day. Thus, most of the B cells that die are in the rapidly turning-over immature peripheral B-cell population, of which more than 50% die every 3 days. The failure of most newly formed B cells to survive for more than a few days in the periphery seems to be due to competition between peripheral B cells for access to the follicles in the spleen. If newly


Immunology_Janeway

Immunology_Janeway. failure of most newly formed B cells to survive for more than a few days in the periphery seems to be due to competition between peripheral B cells for access to the follicles in the spleen. If newly produced immature B cells do not enter a follicle, their passage through the periphery is halted and they eventually die. The limited number of lymphoid follicles cannot accommodate all of the B cells generated each day, and so there is continual competition for entry.


Immunology_Janeway

Immunology_Janeway. The follicle provides signals necessary for B-cell survival. In particular, the TNF-family member BAFF (for B-cell activating factor belonging to the TNF family) is made by several cell types, but is produced abundantly by the follicular dendritic cells (FDCs). FDCs are non-hematopoietic cells resident in the B-cell follicles that are specialized to capture antigens for recognition by B-cell antigen receptors (see Section 9-1). B cells express three different receptors for BAFF, namely BAFF-R, BCMA, and TACI. The BAFF-R is the most important for follicular B-cell survival, because mutant mice lacking BAFF-R have mainly immature B cells and few long-lived peripheral B cells. BCMA and TACI also bind the related TNF family cytokine APRIL, which is not required for the survival of immature B cells but is important for IgA antibody production, as we shall see in Chapter 10.


Immunology_Janeway

Immunology_Janeway. Fig. 8.12 Transitional B cells complete their maturation in B-cell follicles in the spleen. The micrograph in the top panel shows a cross-sectional view of a mouse spleen indicating the distribution of B cells (anti-B220, brown) and T cells (anti-CD3, blue), comprising the white pulp. Surrounding the B-cell-rich follicles (intense brown staining) are the marginal zones (also brown due to presence of B220+ B cells). The white pulp cords sit within the red pulp, which is rich in myeloid cells (mostly macrophages), plasma cells, and passing red blood cells. Transitional B cells that have left the bone marrow must complete their maturation in the B-cell follicles of the spleen, where they receive necessary maturation and survival signals (middle panel). One essential component is a low level of signaling through the B-cell receptor. A second essential factor is the expression of BAFF, a TNF-family member, on follicular dendritic cells (FDCs). BAFF stimulates the BAFF-R on transitional


Immunology_Janeway

Immunology_Janeway. of signaling through the B-cell receptor. A second essential factor is the expression of BAFF, a TNF-family member, on follicular dendritic cells (FDCs). BAFF stimulates the BAFF-R on transitional B cells, promoting B-cell survival. Newly emigrated transitional B cells (T1) exhibit high levels of surface IgM, little IgD, and the BAFF-R. In the B-cell follicles, these cells upregulate CD21 to become transitional stage 2 B cells (T2). Finally, the cells upregulate surface IgD, and become long-lived mature B cells. The majority of long-lived B cells are recirculating B cells, known as follicular B cells. A second, less numerous subset is the marginal zone B-cell population. Marginal zone B cells are thought to be weakly self-reactive and express very high levels of the complement receptor CD21. These cells migrate to the marginal zones of the splenic white pulp, an area at the white pulp/red pulp junctions. In this location, marginal zone B cells are poised to make rapid responses to


Immunology_Janeway

Immunology_Janeway. CD21. These cells migrate to the marginal zones of the splenic white pulp, an area at the white pulp/red pulp junctions. In this location, marginal zone B cells are poised to make rapid responses to blood-borne antigens or pathogens. Transitional T1 B cells that are excluded from the follicles fail to receive maturation and survival signals and will die within 2–3 days of leaving the bone marrow (bottom panel). Self-reactive anergic B cells are also excluded from the follicles and undergo cell death. Micrograph courtesy of Xiaoming Wang and Jason Cyster. Howard Hughes Medical Institute and Department of Microbiology and Immunology, UCSF.


Immunology_Janeway

Immunology_Janeway. Immature B cells in the spleen proceed through two distinct transitional stages, called T1 and T2, defined by the absence or presence of the B-cell co-receptor component CD21 (complement receptor 2) (see Section 2-13 and Section 7-20). In mice lacking BAFF, immature B cells progress to the T1 stage in the spleen but fail to express CD21, and the mice lack mature B cells. Signaling through the B-cell receptor is also required for immature B cells in the spleen to progress through the T1 and T2 stages and enter the long-lived peripheral B-cell pool. In this case, the B-cell receptor signals do not arise from high-affinity interactions between the sIgM of the B-cell receptor with an antigen, which would induce strong signals; instead, these B-cell receptor signals are thought to be weak, constitutive signals that are developmentally programmed into the maturing B cells, although the mechanism responsible for this constitutive signaling is not known. These weak B-cell receptor signals


Immunology_Janeway

Immunology_Janeway. constitutive signals that are developmentally programmed into the maturing B cells, although the mechanism responsible for this constitutive signaling is not known. These weak B-cell receptor signals together with the BAFF-R signals are essential to promote the final stages of B-cell maturation in the periphery. Disregulation of the appropriate balance between B-cell receptor and BAFF-R signaling occurs in individuals who overexpress BAFF, and has been linked to the development of autoimmune diseases, such as Sjögren’s syndrome, that result from a failure to purge auto-reactive B cells.


Immunology_Janeway

Immunology_Janeway. The majority of peripheral B cells that reside in the spleen and other secondary lymphoid organs are known as follicular B cells, often referred to as B-2 B cells. A second, minor population of B cells found in the spleen consists of marginal zone B cells, named for their predominance at the marginal zones that lie at the white pulp/red pulp junctions (see Fig. 8.12). Follicular and marginal zone B cells both derive from a common lineage that develops in the bone marrow and bifurcates during the final stage of B-cell maturation in the splenic follicles. Experiments reconstituting the signals promoting peripheral B-cell maturation in cell culture, starting from immature B-cell precursors, indicate that follicular versus marginal zone B-cell lineages diverge at the transitional T2 stage, as cells transition to the fully mature stage. Like follicular B cells, marginal zone B cells are dependent on BAFF signals, and are missing in mice lacking BAFF expression. Marginal zone B cells can be


Immunology_Janeway

Immunology_Janeway. cells transition to the fully mature stage. Like follicular B cells, marginal zone B cells are dependent on BAFF signals, and are missing in mice lacking BAFF expression. Marginal zone B cells can be identified by their expression of very high levels of the complement receptor CD21. Studies using rearranged immunoglobulin gene knock-in mice that express a single B-cell receptor specificity on all developing B cells have demonstrated that some B-cell receptors predominantly generate follicular B cells whereas others generate marginal zone B cells. These findings indicate that the specificity of the B-cell receptor is a major factor in determining the final commitment of transitional B cells to the follicular versus the marginal zone lineage; however, the details of this process are still not fully understood. Due to their location, marginal zone B cells are poised to make rapid responses to antigens or pathogens filtered from the blood. Therefore, it is thought that marginal zone B


Immunology_Janeway

Immunology_Janeway. not fully understood. Due to their location, marginal zone B cells are poised to make rapid responses to antigens or pathogens filtered from the blood. Therefore, it is thought that marginal zone B cells represent an early line of defense for blood-borne pathogens.


Immunology_Janeway

Immunology_Janeway. Peripheral B cells also include memory B cells, which are generated in addition to antibody-producing plasma cells from mature B cells after their first encounter with antigen; we will return to B-cell memory in Chapter 11. Competition for follicular entry favors mature B cells that are already established in the relatively long-lived and stable peripheral B-cell pool. Mature B cells have undergone phenotypic changes that might make their access to the follicles easier; for example, they express CXCR5, the receptor for CXCL13, which is expressed by FDCs (see Section 10-3). They also have increased expression of CD21 compared with immature newly developed B cells, which enhances the signaling capacity of the B cell.


Immunology_Janeway

Immunology_Janeway. The B-cell receptor plays a positive role in the maturation and continued recirculation of peripheral B cells. Mice that lack the tyrosine kinase Syk, which is involved in signaling from the B-cell receptor (see Section 7-20), have immature B cells but fail to develop mature B cells. Thus, a Syk-transduced signal may be required for final B-cell maturation or for the survival of mature B cells. Furthermore, continuous expression of the B-cell receptor is required for B-cell survival, as is evident from the loss of all B cells in mice whose BCR is conditionally deleted specifically in mature B cells. Although each B-cell receptor has a unique specificity, antigen-specific interactions may not induce the signals used for final B-cell maturation and survival; the receptor could, for example, be responsible for ‘tonic’ signaling, in which a weak but important signal is generated by the assembly of the receptor complex and infrequently triggers some or all of the downstream signaling


Immunology_Janeway

Immunology_Janeway. be responsible for ‘tonic’ signaling, in which a weak but important signal is generated by the assembly of the receptor complex and infrequently triggers some or all of the downstream signaling events.


Immunology_Janeway

Immunology_Janeway. 8-9 B-1 B cells are an innate lymphocyte subset that arises early in development. Thus far, this chapter has focused on the development of the majority populations of B cells that reside in secondary lymphoid organs, such as follicular (B-2) B cells and marginal zone B cells. These two populations comprise the B-cell arm of the adaptive immune response. A third important subset of B cells, called B-1 B cells, is part of the innate immune system. These cells are present only in low numbers in secondary lymphoid organs, and are found in large numbers in the peritoneal and pleural cavities instead. B-1 B cells are the major source of ‘natural’ antibodies, which are constitutively produced circulating antibodies that are secreted by these B cells prior to any infections. Most antibodies made by B-1 B cells recognize capsular polysaccharide antigens, and B-1 B cells are important in controlling infections of pathogenic viruses and bacteria.


Immunology_Janeway

Immunology_Janeway. One important feature of B-1 B cells is that they can produce antibodies of the IgM class without ‘help’ from T cells. Although this response can be enhanced by T-cell cooperation, the antibodies first appear within 48 hours of exposure to antigen, when T cells cannot be involved. The lack of an antigen-specific interaction with helper T cells might explain why immunological memory is not generated as a result of B-1 cell responses: repeated exposures to the same antigen elicit similar, or decreased, responses with each exposure. While the precise functions of B-1 B cells are still not clear, mice deficient in B-1 cells are more susceptible to infection with Streptococcus pneumoniae because they fail to produce an anti-phosphocholine antibody that provides protection against this bacterium. Since a significant fraction of the B-1 cells can make antibodies of this specificity, and because no antigen-specific T-cell help is required, a potent response can be produced early in infection


Immunology_Janeway

Immunology_Janeway. Since a significant fraction of the B-1 cells can make antibodies of this specificity, and because no antigen-specific T-cell help is required, a potent response can be produced early in infection with this pathogen. Whether human B-1 cells have the same role is not certain.


Immunology_Janeway

Immunology_Janeway. Unlike follicular and marginal zone B cells that develop from bone marrow stem cells, the majority of B-1 B cells are generated from progenitor cells found in the fetal liver (Fig. 8.13). During late fetal and early neonatal stages in mice, B-1 B cells are produced in large numbers. After birth, the development of follicular and marginal zone B cells predominates, and few B-1 B cells are made. Current evidence indicates that the progenitor cells giving rise to B-1 B cells are committed to this lineage, and are distinct from those producing B-2 B cells. Whereas B-2 B cells are absent in mice lacking BAFF or the BAFF-R, these deficiencies have no effect on the development or survival of B-1 B cells. Additionally, the weak B-cell receptor signals that promote the final stages of B-2 B-cell maturation in the spleen require the non-canonical NF-κB activation pathway (see Section 7-23), a signaling pathway that is dispensable for B-1 B-cell development. Cytokine requirements also differ


Immunology_Janeway

Immunology_Janeway. maturation in the spleen require the non-canonical NF-κB activation pathway (see Section 7-23), a signaling pathway that is dispensable for B-1 B-cell development. Cytokine requirements also differ between these developmental pathways. B-1 B cells develop normally in mice lacking IL-7 or IL-7R signaling components, defects that prevent the development of B-2 B cells. B-2 B-cell development also requires the transcription factor PU.1, which is not needed for the development of B-1 B cells.


Immunology_Janeway

Summary.

Immunology_Janeway. Summary.


Immunology_Janeway

Immunology_Janeway. In this section, we have followed B-cell development from the earliest progenitors in the bone marrow to the long-lived mature peripheral B-cell pool (Fig. 8.14). The heavy-chain locus is rearranged first and, if this is successful, a μ heavy chain is produced that combines with surrogate light chains to form the pre-B-cell receptor; this is the first checkpoint in B-cell development. Production of the pre-B-cell receptor signals successful heavy-chain gene rearrangement and causes cessation of this rearrangement, thus enforcing allelic exclusion. It also initiates pre-B-cell proliferation, generating numerous progeny in which subsequent light-chain rearrangement can be attempted. If the initial light-chain gene rearrangement is productive, a complete immunoglobulin B-cell receptor is formed, gene rearrangement again ceases, and the B cell continues its development. If the first light-chain gene rearrangement is unsuccessful, rearrangement continues until either a productive


Immunology_Janeway

Immunology_Janeway. is formed, gene rearrangement again ceases, and the B cell continues its development. If the first light-chain gene rearrangement is unsuccessful, rearrangement continues until either a productive rearrangement is made or all available J regions are used up. If no productive rearrangement is made, the developing B cell dies. Once a complete immunoglobulin receptor is expressed on the surface of the cell, immature B cells undergo tolerance to self antigens. This process begins in the bone marrow and continues for a short time after immature B cells emigrate to the periphery. For the majority population of B cells, the final stages of their maturation occur in the B-cell follicles of the spleen, and require the TNF family member BAFF as well as signals through the B-cell receptor.


Immunology_Janeway

Immunology_Janeway. Fig. 8.13 A comparison of the properties of B-1 cells, follicular B cells (B-2 cells), and marginal zone B cells. In addition to developing in the liver, B-1 cells can develop in unusual sites in the fetus, such as the omentum. B-1 cells predominate in the young animal, although they probably can be produced throughout life. Being produced mainly during fetal and neonatal life, their rearranged variable-region sequences contain few N-nucleotides. In contrast, marginal zone B cells accumulate after birth and do not reach peak levels in the mouse until 8 weeks of age. Follicular B-2 cells and marginal zone B cells share a common precursor population, the transitional T2 B cells in the spleen; as a consequence, both subsets are dependent on IL-7 and BAFF signals for their development. In contrast, B-1 cell development does not require IL-7 or BAFF. B-1 cells are best thought of as a partly activated self-renewing pool of lymphocytes that are selected by ubiquitous self and foreign


Immunology_Janeway

Immunology_Janeway. In contrast, B-1 cell development does not require IL-7 or BAFF. B-1 cells are best thought of as a partly activated self-renewing pool of lymphocytes that are selected by ubiquitous self and foreign antigens. Because of this selection, and possibly because the cells are produced early in life, the B-1 cells have a restricted repertoire of variable regions and antigen-binding specificities. Marginal zone B cells also have a restricted repertoire of V-region specificities that may be selected by a set of antigens similar to those that select B-1 cells. B-1 cells seem to be the major population of B cells in certain body cavities, most probably because of exposure at these sites to antigens that drive B-1 cell proliferation. Marginal zone B cells remain in the marginal zone of the spleen and are not thought to recirculate. Partial activation of B-1 cells leads to the secretion of mainly IgM antibody; B-1 cells contribute much of the IgM that circulates in the blood. The limited


Immunology_Janeway

Immunology_Janeway. and are not thought to recirculate. Partial activation of B-1 cells leads to the secretion of mainly IgM antibody; B-1 cells contribute much of the IgM that circulates in the blood. The limited diversity of both the B-1 and marginal zone B-cell repertoire and the propensity of these cells to react with common bacterial carbohydrate antigens suggest that they carry out a more primitive, less adaptive immune response than follicular B cells (B-2 cells). In this regard they are comparable to γ:δ T cells.


Immunology_Janeway

Immunology_Janeway. Fig. 8.14 A summary of the development of human conventional B-lineage cells. The state of the immunoglobulin genes, the expression of some essential intracellular proteins, and the expression of some cell-surface molecules are shown for successive stages of conventional B-2 B-cell development. During antigen-driven B-cell differentiation, the immunoglobulin genes undergo further changes, such as class switching and somatic hypermutation (see Chapter 5), which are evident in the immunoglobulins produced by memory cells and plasma cells. These antigen-dependent stages are described in more detail in Chapter 9.


Immunology_Janeway

Immunology_Janeway. lgGPlasmablast and plasma cell lgG Memory B cell CD135 Plasma cell antigen-1 CD38 Somatic hypermutation VJ rearranged Isotype switch to C˜, C°, or C˛. Somatic hypermutation Alternative splicing yields both membrane and secreted Ig lgM Lympho-blast lgMlgD Mature naive B cell CD45R MHC class II IgM, IgD CD19, CD20 CD21, CD40 CD45R MHC class II IgG, IgA CD19, CD20 CD21, CD40 CD45R MHC class II CD19, CD20 CD21, CD40 VDJ rearranged. ˝chain produced in membrane form. Alternative splicing yields ˝+ˆmRNA Alternative splicing yields secreted ˝chains Ig IgANTIGENDEPENDENTTERMINALDIFFERENTIATIONCD45R AA4.1, IL-7R MHC class II pre-B-R CD19, CD38 CD20, CD40 CD45R AA4.1 MHC class II CD19, CD38 CD20, CD40 Germline pre-B-cell receptor cytoplasmic ˝˝RAG-1 RAG-2 Large pre-B cell Small pre-B cell Late pro-B cell Germline TdT ˇ5, VpreB ˇ5, VpreB CD45R AA4.1, IL-7R MHC class II CD10, CD19 CD38, CD20 CD40 D–J rearranged V–DJ rearranged VDJ rearranged VDJ rearranged V–J rearrangement Early pro-B cell Stem


Immunology_Janeway

Immunology_Janeway. cell Germline TdT ˇ5, VpreB ˇ5, VpreB CD45R AA4.1, IL-7R MHC class II CD10, CD19 CD38, CD20 CD40 D–J rearranged V–DJ rearranged VDJ rearranged VDJ rearranged V–J rearrangement Early pro-B cell Stem cell Germline Germline Germline RAG-1 RAG-2 TdT ˇ5, VpreB CD34 CD45 AA4.1 CD34 CD45R AA4.1, IL-7R MHC class II CD10, CD19 CD38 Heavy-chain genesB cells Light-chain genes Intra-cellular proteins Surface marker proteinsANTIGENINDEPENDENT rearranged.


Immunology_Janeway

Immunology_Janeway. AA4.1 CD19, CD20 Development of T lymphocytes.


Immunology_Janeway

Immunology_Janeway. Like B cells, T lymphocytes derive from the multipotent hematopoietic stem cells in the bone marrow. However, their progenitor cells migrate from the bone marrow via the blood to the thymus, where they mature (Fig. 8.15); this is the reason for the name thymus-dependent (T) lymphocytes, or T cells. T-cell development parallels that of B cells in many ways, including the orderly and stepwise rearrangement of antigen-receptor genes, the sequential testing for successful gene rearrangement, and the eventual assembly of a heterodimeric antigen receptor. Nevertheless, T-cell development in the thymus has some features not seen for B cells, such as the generation of two distinct lineages of T cells expressing antigen receptors encoded by distinct genes, the γ:δ lineage and the α:β lineage. Developing T cells, which are known generally as thymocytes, also undergo rigorous selection that depends on interactions with thymic cells and that shapes the mature repertoire of T cells to ensure self


Immunology_Janeway

Immunology_Janeway. T cells, which are known generally as thymocytes, also undergo rigorous selection that depends on interactions with thymic cells and that shapes the mature repertoire of T cells to ensure self MHC restriction as well as self-tolerance. We begin with a general overview of the stages of thymocyte development and its relationship to thymic anatomy before considering gene rearrangement and the mechanisms of selection.


Immunology_Janeway

Immunology_Janeway. 8-10 T-cell progenitors originate in the bone marrow, but all the important events in their development occur in the thymus. The thymus is situated in the upper anterior thorax, just above the heart. It consists of numerous lobules, each clearly differentiated into an outer cortical region—the thymic cortex—and an inner medulla (Fig. 8.16). In young individuals, the thymus contains large numbers of developing T-cell precursors embedded in a network of epithelia known as the thymic stroma. This provides a unique microenvironment for T-cell development analogous to that provided for B cells by the stromal cells of the bone marrow.


Immunology_Janeway

Immunology_Janeway. Fig. 8.15 T cells undergo development in the thymus and migrate to the peripheral lymphoid organs, where they are activated by foreign antigens. T-cell precursors migrate from the bone marrow to the thymus, where they commit to the T-cell lineage following Notch receptor signaling. In the thymus, T-cell receptor genes are rearranged (top first panel); α:β T-cell receptors that are compatible with self MHC molecules transmit a survival signal on interacting with thymic epithelium, leading to positive selection of the cells that bear them. Self-reactive receptors transmit a signal that leads to cell death, and cells bearing them are removed from the repertoire in a process of negative selection (top second panel). T cells that survive selection mature and leave the thymus to circulate in the periphery; they repeatedly leave the blood to migrate through the peripheral lymphoid organs, where they may encounter their specific foreign antigen and become activated (top third panel).


Immunology_Janeway

Immunology_Janeway. in the periphery; they repeatedly leave the blood to migrate through the peripheral lymphoid organs, where they may encounter their specific foreign antigen and become activated (top third panel). Activation leads to clonal expansion and differentiation into effector T cells. Some of these are attracted to sites of infection, where they can kill infected cells or activate macrophages (top fourth panel); others are attracted into B-cell areas, where they help to activate an antibody response (not shown).


Immunology_Janeway

Immunology_Janeway. Precursors commit to the T-cell lineage following Notch signaling and initiate T-cell receptor gene rearrangements Activated T cells proliferate and eliminate infection Mature T cells encounter foreign antigens in the peripheral lymphoid organs and are activated Immature T cells that recognize self MHC receive signals for survival. Those that interact strongly with self antigen are removed from the repertoire T-cell precursor mature T cell dendritic cell macrophage Notch thymic stromal cell thymus lymph node spleen self MHC activates kills gastro-intestinal tract T-cell progenitors develop in the bone marrow and migrate to the thymus where the cells complete their development by rearranging their antigenreceptor genes and undergoing repertoire selection Mature T cells migrate to the peripheral lymphoid organs Activated T cells migrate to sites of infection


Immunology_Janeway

Immunology_Janeway. Fig. 8.16The cellular organization of the human thymus. The thymus, which lies in the midline of the body, above the heart, is made up of several lobules, each of which contains discrete cortical (outer) and medullary (central) regions. As shown in the diagram on the left, the cortex consists of immature thymocytes (dark blue); branched cortical epithelial cells (pale blue), with which the immature cortical thymocytes are closely associated; and scattered macrophages (yellow), which are involved in clearing apoptotic thymocytes. The medulla consists of mature thymocytes (dark blue) and medullary epithelial cells (orange), along with macrophages (yellow) and dendritic cells (yellow) of bone marrow origin. Hassall’s corpuscles are probably also sites of cell degradation. The thymocytes in the outer cortical cell layer are proliferating immature cells, whereas the deeper cortical thymocytes are mainly immature T cells undergoing thymic selection. The photograph shows the equivalent


Immunology_Janeway

Immunology_Janeway. in the outer cortical cell layer are proliferating immature cells, whereas the deeper cortical thymocytes are mainly immature T cells undergoing thymic selection. The photograph shows the equivalent section of a human thymus, stained with hematoxylin and eosin. The cortex is darkly stained, whereas the medulla is lightly stained. The large body in the medulla is a Hassall’s corpuscle. Photograph courtesy of C.J. Howe.


Immunology_Janeway

Immunology_Janeway. The thymic epithelium arises early in embryonic development from endodermderived structures known as the third pharyngeal pouches. These epithelial tissues form a rudimentary thymus, or thymic anlage. This is colonized by cells of hematopoietic origin that give rise to large numbers of thymocytes, which are committed to the T-cell lineage, and to intrathymic dendritic cells. Thymocytes are not simply passengers within the thymus: they influence the arrangement of the thymic epithelial cells on which they depend for survival, inducing the formation of a reticular epithelial structure that surrounds the developing thymocytes (Fig. 8.17).


Immunology_Janeway

Immunology_Janeway. The cellular architecture of the human thymus is illustrated in Fig. 8.16. Bone marrow-derived cells are differentially distributed between the thymic cortex and medulla. The cortex contains only immature thymocytes and scattered macrophages, whereas more mature thymocytes, along with dendritic cells, macrophages, and some B cells, are found in the medulla. As will be discussed below, this organization reflects the different developmental events that occur in these two compartments.


Immunology_Janeway

Immunology_Janeway. The importance of the thymus in immunity was first discovered through experiments on mice; indeed, most of our knowledge of T-cell development in the thymus comes from the mouse. It was found that surgical removal of the thymus (thymectomy) at birth resulted in immunodeficient mice, focusing interest on this organ at a time when the difference between T and B lymphocytes in mammals had not yet been defined. Much evidence, including observations in immunodeficient children, has since confirmed the importance of the thymus in T-cell development. In DiGeorge syndrome in humans and in mice with the nude mutation, the thymus does not form and the affected individual produces B lymphocytes but few T lymphocytes. DiGeorge syndrome is a complex combination of cardiac, facial, endocrine, and immune defects associated with deletions of chromosome 22q11. The nude mutation in mice is due to a defect in the gene for Foxn1, a transcription factor required for terminal epithelial cell