Immunology_Janeway

Immunology_Janeway. In mice, transplantable tumors exhibit a variable pattern of growth when injected into syngeneic recipients. Most tumors grow progressively and eventually kill the host. However, if mice are injected with irradiated tumor cells that cannot grow, they are frequently protected against subsequent injection with a normally lethal dose of viable cells of the same tumor (Fig. 16.12). There seems to be a spectrum of immunogenicity among transplantable tumors: injections of irradiated tumor cells seem to induce varying degrees of protective immunity against a challenge injection of viable tumor cells at a distant site. These protective effects are not seen in T-cell-deficient mice but can be conferred by adoptive transfer of T cells from immune mice, showing the need for T cells to mediate these effects.


Immunology_Janeway

Immunology_Janeway. These observations indicate that the tumors express antigens that can become targets of a tumor-specific T-cell response that rejects the tumor. These tumor rejection antigens are expressed by experimentally induced murine tumors (in which they are often termed tumor-specific transplantation antigens), and are usually specific for an individual tumor. Thus, immunization with irradiated tumor cells from one tumor usually protects a syngeneic mouse from challenge with live cells from that same tumor, but not from challenge with a different syngeneic tumor (see Fig. 16.12). 16-14 Tumors are ‘edited’ by the immune system as they evolve and can escape rejection in many ways.


Immunology_Janeway

Immunology_Janeway. Paul Ehrlich, who received the 1908 Nobel Prize for his work in immunology, was perhaps the first to propose that the immune system could be used to treat established tumors, suggesting that the molecules we call antibodies might be used to deliver toxins to cancer cells. In the 1950s, Frank MacFarlane Burnet, recipient of the 1960 Nobel Prize, and Lewis Thomas formulated the ‘immune surveillance’ hypothesis, according to which cells of the immune system would detect and destroy tumor cells. Since then, it has become clear that the relationship between the immune system and cancer is considerably more complex, and this hypothesis has been modified to consider three phases of tumor growth. The first is the ‘elimination phase,’ in which the immune system recognizes and destroys potential tumor cells—the phenomenon previously referred to as immune surveillance (Fig. 16.13). If elimination is not completely successful, what follows is an ‘equilibrium phase,’ in which tumor cells undergo


Immunology_Janeway

Immunology_Janeway. cells—the phenomenon previously referred to as immune surveillance (Fig. 16.13). If elimination is not completely successful, what follows is an ‘equilibrium phase,’ in which tumor cells undergo changes or mutations that aid their survival as a result of the selection pressure imposed by the immune system. During the equilibrium phase, a process known as cancer immunoediting continuously shapes the properties of the tumor cells that survive. In the final ‘escape phase,’ tumor cells that have acquired the ability to elude the attentions of the immune system and grow unimpeded become clinically detectable.


Immunology_Janeway

Immunology_Janeway. Mice with targeted gene deletions or treated with antibodies to remove specific components of innate and adaptive immunity have provided the best evidence that immune surveillance influences the development of certain types of tumors. For example, mice lacking perforin, part of the killing mechanism of NK cells and CD8 cytotoxic T cells (see Section 9-31), show an increased frequency of lymphomas—tumors of the lymphoid system. Strains of mice lacking the RAG and STAT1 proteins, thus being deficient in both adaptive and certain innate immune mechanisms, develop gut epithelial and breast tumors. Mice lacking T lymphocytes expressing γ:δ receptors show markedly increased susceptibility to skin tumors induced by the topical application of carcinogens, illustrating a role for intraepithelial γ:δ T cells (see Section 6-20) in surveying and killing abnormal epithelial cells. Both IFN-γ and IFN-α/β are important in the elimination of tumor cells, either directly or indirectly through their


Immunology_Janeway

Immunology_Janeway. γ:δ T cells (see Section 6-20) in surveying and killing abnormal epithelial cells. Both IFN-γ and IFN-α/β are important in the elimination of tumor cells, either directly or indirectly through their actions on other cells. Studies of the various effector cells of the immune system show that γ:δ T cells are a major source of IFN-γ, which may explain their importance in the removal of cancer cells.


Immunology_Janeway

Immunology_Janeway. Fig. 16.13 Malignant cells can be controlled by immune surveillance. Some types of tumor cells are recognized by a variety of immune-system cells, which can eliminate them (left panel). If the tumor cells are not completely eliminated, variants occur that eventually escape the immune system and proliferate to form a tumor.


Immunology_Janeway

Immunology_Janeway. According to the immunoediting hypothesis, those tumor cells that survive the equilibrium phase have acquired additional changes, either due to additional mutations or from selection during the equilibrium phase, that prevent their elimination by the immune system. In an immunocompetent individual, the equilibrium phase of the immune response continually removes tumor cells, delaying tumor growth; if the immune system is compromised, the equilibrium phase quickly turns into escape, as no tumor cells at all are removed. An excellent clinical example to support the presence of the equilibrium phase is the occurrence of cancer in recipients of organ transplants. One study reported the development of melanoma between 1 and 2 years after transplantation in two patients who had received kidneys from the same donor, a patient who had had successfully treated malignant melanoma 16 years before her death. Presumably, melanoma cells, which are known to spread easily to other organs, were


Immunology_Janeway

Immunology_Janeway. from the same donor, a patient who had had successfully treated malignant melanoma 16 years before her death. Presumably, melanoma cells, which are known to spread easily to other organs, were present in the donor kidneys at the time of transplantation but were in equilibrium phase with the immune system. If so, this would indicate that the melanoma cells had not been killed off completely by the immunocompetent immune system of the donor, but instead had simply been held in check. Because the recipients’ immune systems were immunosuppressed to prevent graft rejection, the melanoma cells were released from equilibrium and began to divide rapidly and spread to other parts of the body.


Immunology_Janeway

Immunology_Janeway. Another situation in which the suppression of immunity can lead to tumor development is in post-transplant lymphoproliferative disorder, which can occur when patients are immunosuppressed after, for example, solid-organ transplantation. It usually takes the form of a B-cell expansion driven by Epstein–Barr virus (EBV) in which the B cells can undergo mutations and become malignant. Here, antiviral immunity functions as a form of cancer immunosurveillance, as it normally eliminates the EBV that leads to B-cell transformation.


Immunology_Janeway

Immunology_Janeway. Tumors can avoid stimulating an immune response or can evade it when it occurs by means of numerous mechanisms, which are summarized in Fig. 16.14. Spontaneous tumors may initially lack mutations that produce new tumor-specific antigens that elicit T-cell responses (see Fig. 16.14, first panel). And even when a tumor-specific antigen is expressed and is taken up and presented by antigen-presenting cells (APCs), if co-stimulatory signals are absent the antigen-presenting cell will tend to tolerize any antigen-specific naive T cells rather than activating them (see Fig. 16.14, second panel). How long such tumors are treated as ‘self’ is unclear. Recent sequencing of entire tumor genomes reveals that mutations may generate as many as 10–15 unique antigenic peptides that could be recognized as ‘foreign’ by T cells. In addition, cellular transformation is frequently associated with induction of MHC class Ib proteins (such as MIC-A and MIC-B) that are ligands for NKG2D, thus allowing tumor


Immunology_Janeway

Immunology_Janeway. ‘foreign’ by T cells. In addition, cellular transformation is frequently associated with induction of MHC class Ib proteins (such as MIC-A and MIC-B) that are ligands for NKG2D, thus allowing tumor recognition by NK cells (see Section 6-17). But cancer cells tend to be genetically unstable, so that clones that are not recognized by an immune response may be able to escape elimination.


Immunology_Janeway

Immunology_Janeway. Some tumors, such as colon and cervical cancers, lose the expression of a particular MHC class I molecule, perhaps through immune selection by T cells specific for a peptide presented by that MHC class I molecule (see Fig. 16.14, third panel). In experimental studies, when a tumor loses expression of all MHC class I molecules (Fig. 16.15), it can no longer be recognized by cytotoxic T cells, although it might become susceptible to NK cells (Fig. 16.16). Tumors that lose only one MHC class I molecule might be able to avoid recognition by specific CD8 cytotoxic T cells while still remaining resistant to NK cells, conferring a selective advantage in vivo. Tumors also seem to be able to evade immune attack by creating a micro-environment that is generally immunosuppressive (see Fig. 16.14, fourth panel). Many tumors make immunosuppressive cytokines. Transforming growth factor-β (TGF-β) was first identified in the culture supernatant of a


Immunology_Janeway

Immunology_Janeway. Fig. 16.14 Tumors can avoid immune recognition in a variety tumors may be eliminated. The genetic instability of tumors allows of ways. First panel: tumors can have low immunogenicity. Tumors antigenic change, part of an equilibrium phase, during which tumor may lack antigens recognized by T cells, may have lost one or more cells lacking immunogenic antigens can expand. Fourth panel: tumors MHC molecules, or may express inhibitory molecules such as PD-L1 often produce molecules, such as TGF-β, IL-10, IDO, or PD-L1, that that repress T-cell function. Second panel: tumor-specific antigens suppress immune responses directly or recruit regulatory T cells that may be cross-presented by dendritic cells without co-stimulatory can secrete immunosuppressive cytokines. Fifth panel: tumor cells signals, inducing a tolerant state in T cells. Third panel: tumors can can secrete molecules such as collagen that form a physical barrier initially express antigens to which the immune system responds.


Immunology_Janeway

Immunology_Janeway. inducing a tolerant state in T cells. Third panel: tumors can can secrete molecules such as collagen that form a physical barrier initially express antigens to which the immune system responds. Such around the tumor, preventing lymphocyte access.


Immunology_Janeway

Immunology_Janeway. tumor (hence its name), and, as we have seen, it tends to suppress the inflammatory T-cell responses and cell-mediated immunity that are needed to control tumor growth. Recall that TGF-β induces the development of inducible regulatory T cells (Treg cells; see Section 9-21), which have been found in a variety of cancers and might expand specifically in response to tumor antigens. In mouse models, removal of Treg cells increases resistance to cancer, whereas their transfer into a Treg-negative recipient allows cancer cells to more greatly proliferate.


Immunology_Janeway

Immunology_Janeway. The microenvironments of some tumors also contain populations of myeloid cells, collectively called myeloid-derived suppressor cells (MDSCs), which can inhibit T-cell activation within the tumor. MDSCs may be heterogeneous, comprising both monocytic and polymorphonuclear cells, and are incompletely characterized at present. Several tumors of different tissue origins, such as melanoma, ovarian carcinoma, and B-cell lymphoma, have also been shown to produce the immunosuppressive cytokine IL-10, which can reduce dendritic cell activity and inhibit T-cell activation.


Immunology_Janeway

Immunology_Janeway. Some tumors express cell-surface proteins that directly inhibit immune responses (see Fig. 16.14, fourth panel). For example, some tumors express programmed death ligand-1 (PD-L1), a B7 family member and ligand for the inhibitory receptor PD-1 expressed by activated T cells (see Section 7-24). Furthermore, tumors can produce enzymes that act to suppress local immune responses. The enzyme indoleamine 2,3-dioxygenase (IDO) catabolizes tryptophan, an essential amino acid, in order to produce the immunosuppressive metabolite kynurenine. IDO seems to function in maintaining a balance between immune responses and tolerance during infections, but can be induced during the equilibrium phase of tumor development. Finally, tumor cells can produce materials such as collagen that create a physical barrier to interaction with cells of the immune system (see Fig. 16.14, last panel). Fig. 16.15 Loss of MHC class I expression in a prostatic carcinoma.


Immunology_Janeway

Immunology_Janeway. Fig. 16.15 Loss of MHC class I expression in a prostatic carcinoma. Some tumors can evade immune surveillance by a loss of expression of MHC class I molecules, preventing their recognition by CD8 T cells. Shown is a section of a human prostate cancer that has been stained with a peroxidase-conjugated antibody against HLA class I molecules. The brown stain that represents HLA class I expression is restricted to infiltrating lymphocytes and tissue stromal cells. The tumor cells that occupy most of the section show no staining. Photograph courtesy of G. Stamp.


Immunology_Janeway

Immunology_Janeway. Fig. 16.16 Tumors that lose expression of all MHC class I molecules as a mechanism of escape from immune surveillance are more susceptible to being killed by NK cells. Regression of transplanted tumors is largely due to the actions of cytotoxic T lymphocytes (CTLs), which recognize novel peptides bound to MHC class I antigens on the surface of the cell (left panels). NK cells have inhibitory receptors that bind MHC class I molecules, so variants of the tumor that have low levels of MHC class I, although less sensitive to CD8 cytotoxic T cells, become susceptible to NK cells (center panels). Although nude mice lack T cells, they have higher than normal levels of NK cells, and so tumors that are sensitive to NK cells grow less well in nude mice than in normal mice. Transfection with MHC class I genes can restore both resistance to NK cells and susceptibility to CD8 cytotoxic T cells (right panels). The bottom panels show scanning electron micrographs of NK cells attacking leukemia


Immunology_Janeway

Immunology_Janeway. class I genes can restore both resistance to NK cells and susceptibility to CD8 cytotoxic T cells (right panels). The bottom panels show scanning electron micrographs of NK cells attacking leukemia cells. The NK cell is the smaller cell on the left in both photographs. Left panel: shortly after binding to the target cell, the NK cell has put out numerous microvillous extensions and established a broad zone of contact with the leukemia cell. Right panel: 60 minutes after mixing, long microvillous processes can be seen extending from the NK cell (bottom left) to the leukemia cell and there is extensive damage to the leukemia cell; the plasma membrane has rolled up and fragmented. Photographs reprinted from Herberman, R., and Callewaert, D: Mechanisms of Cytotoxicity by Natural Killer Cells, 1985, with permission from Elsevier.


Immunology_Janeway

Immunology_Janeway. 16-15 Tumor rejection antigens can be recognized by T cells and form the basis of immunotherapies.


Immunology_Janeway

Immunology_Janeway. The tumor rejection antigens recognized by the immune system are peptides of tumor-cell proteins that are presented to T cells on MHC molecules. These peptides become the targets of a tumor-specific T-cell response even though they can also be present on normal tissues. For instance, strategies to induce immunity to the relevant antigens in melanoma patients can induce vitiligo, an autoimmune destruction of pigmented cells in healthy skin. Several categories of tumor rejection antigens can be distinguished (Fig. 16.17). One consists strictly of tumor-specific antigens that result from point mutations or gene rearrangements that occur during oncogenesis and that affect a particular gene product. Point mutations in the gene for a particular protein may alter the epitope for T cells by altering the specific residues in a peptide that is already able to bind to MHC class I molecules, or they may allow some mutant peptides to bind de novo to MHC class I molecules. These peptides are often


Immunology_Janeway

Immunology_Janeway. the specific residues in a peptide that is already able to bind to MHC class I molecules, or they may allow some mutant peptides to bind de novo to MHC class I molecules. These peptides are often referred to as neoepitopes as they are newly immunogenic versions of normal proteins. Either change may evoke a new T-cell response against the tumor. In Band T-cell tumors, which are derived from single clones of lymphocytes,


Immunology_Janeway

Immunology_Janeway. The molecules listed here have all been shown to be recognized by cytotoxic T lymphocytes raised from patients with the tumor type listed. a special class of tumor-specific antigen comprises the unique rearranged antigen receptor expressed by the clone. However, not all mutated peptides may be properly processed or be able to associate with MHC molecules and thus ensure that they stimulate an effective response.


Immunology_Janeway

Immunology_Janeway. The second category of tumor rejection antigens is the cancer-testis antigens. These are proteins encoded by genes that are normally expressed only in male germ cells in the testis. Male germ cells do not express MHC molecules, and therefore peptides from these molecules are not normally presented to T lymphocytes. Tumor cells show widespread abnormalities of gene expression, including the activation of genes encoding cancer-testis antigens, such as the melanoma-associated antigens (MAGE) (see Fig. 16.17). When expressed by tumor cells, peptides derived from these ‘germ-cell’ proteins can now be presented to T cells by MHC class I molecules. Therefore, these proteins are effectively tumor-specific in their expression as antigens. Perhaps the cancer-testis antigen best characterized immunologically is NY-ESO-1 (New York esophageal squamous cell carcinoma-1), which is highly immunogenic and is expressed by a variety of human tumors, including melanomas.


Immunology_Janeway

Immunology_Janeway. The third category is the ‘differentiation antigens’ encoded by genes that are expressed only in particular types of tissues. Examples of these are the differentiation antigens expressed in melanocytes and melanoma cells, which include several proteins in the pathways that produce the black pigment melanin, and the CD19 antigen expressed by B cells. The fourth category consists of antigens that are strongly overexpressed in tumor cells compared with their normal counterparts. An example is HER-2/neu (also known as c-Erb-2), which is a receptor tyrosine kinase homologous to the epidermal growth factor receptor. HER-2/neu is overexpressed in many adenocarcinomas, including breast and ovarian cancers, where it is associated with a poor clinical prognosis. CD8 T lymphocytes have been found infiltrating solid tumors overexpressing HER-2/neu but are not capable of destroying such tumors in vivo. The fifth category of tumor rejection antigens consists of molecules that display abnormal


Immunology_Janeway

Immunology_Janeway. infiltrating solid tumors overexpressing HER-2/neu but are not capable of destroying such tumors in vivo. The fifth category of tumor rejection antigens consists of molecules that display abnormal post-translational modifications. An example is underglycosylated mucin, MUC-1, which is expressed by several tumors, including breast and pancreatic cancers. The sixth category consists of novel proteins that are generated when one or more introns are retained in the mRNA transcribed from a gene, which occurs in melanoma. Proteins encoded by viral oncogenes comprise the seventh category of tumor rejection antigens. These oncoviral proteins can have a critical role in the oncogenic process and, because they are foreign, they can evoke a T-cell response. Examples are the human papilloma virus type 16 proteins E6 and E7, which are expressed in cervical carcinoma (see Section 16-18).


Immunology_Janeway

Immunology_Janeway. In melanoma, tumor-specific antigens were discovered by culturing irradiated tumor cells with autologous lymphocytes, a technique known as the mixed lymphocyte–tumor cell culture. From such cultures, cytotoxic T cells were identified that were reactive against melanoma peptides and would kill tumor cells bearing the relevant tumor-specific antigen. Such studies have revealed that melanomas carry at least five different antigens that can be recognized by cytotoxic T lymphocytes. It seems that cytotoxic T lymphocytes reactive against melanoma antigens are not effective in vivo, perhaps due to deficient priming and insufficient effector function, or to downstream resistance mechanisms. However, melanoma-specific T cells can be isolated from peripheral blood, lymph nodes, or directly from lymphocytes infiltrating the tumor and propagated in vitro. These T cells do not recognize the products of mutant proto-oncogenes or tumor suppressor genes, but instead recognize antigens derived from


Immunology_Janeway

Immunology_Janeway. infiltrating the tumor and propagated in vitro. These T cells do not recognize the products of mutant proto-oncogenes or tumor suppressor genes, but instead recognize antigens derived from the protein products of other mutant genes or from normal proteins that are now displayed on tumor cells at levels detectable by T cells for the first time. Cancer-testis antigens such as the melanoma MAGE antigens discussed above probably represent early developmental antigens reexpressed in the process of tumorigenesis. Only a minority of melanoma patients have T cells reactive to the MAGE antigens, indicating that these antigens either are not expressed or are not immunogenic in most cases.


Immunology_Janeway

Immunology_Janeway. The most common melanoma antigens are peptides from the enzyme tyrosinase or from three other proteins—gp100, MART1, and gp75. These are differentiation antigens specific to the melanocyte lineage. It is likely that over-expression of these antigens in tumor cells leads to an abnormally high density of specific peptide:MHC complexes and it is this that makes them immunogenic. Although tumor rejection antigens are usually presented as peptides complexed with MHC class I molecules, the enzyme tyrosinase has also been shown to stimulate CD4 T-cell responses in some melanoma patients. This is likely because it is ingested and presented by cells expressing MHC class II molecules. Both CD4 and CD8 T cells are likely to be important in achieving immunological control of tumors. CD8 cells can kill the tumor cells directly, while CD4 T cells have a role in the activation of CD8 cytotoxic T cells and the establishment of memory. CD4 T cells may also kill tumor cells by means of the cytokines


Immunology_Janeway

Immunology_Janeway. the tumor cells directly, while CD4 T cells have a role in the activation of CD8 cytotoxic T cells and the establishment of memory. CD4 T cells may also kill tumor cells by means of the cytokines that they secrete, such as TNF-α.


Immunology_Janeway

Immunology_Janeway. Other potential tumor rejection antigens include the products of mutated cellular oncogenes or tumor suppressors, such as Ras and p53, and also fusion proteins, such as the Bcr–Abl tyrosine kinase that results from the chromosomal translocation (t9;22) found in chronic myeloid leukemia (CML). When present on CML cells, the HLA class I molecule HLA-A*0301 can display a peptide derived from the fusion site between Bcr and Abl. This peptide was detected by a powerful technique known as ‘reverse immunogenetics,’ in which endogenous peptides were eluted from the MHC binding groove and their sequence was determined by highly sensitive mass spectrometry. This technique has identified HLA-bound peptides from other tumor antigens, such as the MART1 and gp100 tumor antigens of melanomas, as well as candidate peptide sequences for vaccination against infectious diseases.


Immunology_Janeway

Immunology_Janeway. T cells specific for the Bcr–Abl fusion peptide can be identified in peripheral blood from patients with CML by using tetramers of HLA-A*0301 carrying the peptide as specific ligands for the antigen-specific T-cell receptor (see Section 7-24). Cytotoxic T lymphocytes specific for this and other tumor antigens can be selected in vitro by using peptides derived from the mutated or fused portions of these oncogenic proteins; these cytotoxic T cells are able to recognize and kill tumor cells.


Immunology_Janeway

Immunology_Janeway. After a bone marrow transplant to treat CML, mature lymphocytes from the bone marrow donor infused into the patient can help to eliminate any residual tumor. This technique is known as donor lymphocyte infusion (DLI). At present, it is not clear to what extent the clinical response is due to a graftversus-host effect, in which the donor lymphocytes are responding to general alloantigens expressed on the leukemia cells (see Section 15-36), or whether a specific antileukemic response is important. It is encouraging that it has been possible to separate T lymphocytes in vitro that mediate either a graft-versushost effect or a graft-versus-leukemia effect. The ability to prime the donor cells against leukemia-specific peptides offers the prospect of enhancing the antileukemic effect while minimizing the risk of graft-versus-host disease. 16-16 T cells expressing chimeric antigen receptors are an effective treatment in some leukemias.


Immunology_Janeway

Immunology_Janeway. 16-16 T cells expressing chimeric antigen receptors are an effective treatment in some leukemias. Adoptive T-cell therapy involves ex vivo expansion of tumor-specific T cells to large numbers and the infusion of those T cells into patients. Cells are expanded in vitro by various methods, such as treatment with IL-2, anti-CD3 antibodies, and allogeneic antigen-presenting cells to provide a co-stimulatory signal. Adoptive T-cell therapy is made more effective when the patient is immunosuppressed before treatment and IL-2 is then administered systemically. Another approach that has excited much interest is the use of retroviral vectors to transfer tumor-specific T-cell receptor (TCR) genes into patients’ T cells before reinfusion. This can have long-lasting effects as a result of the ability of T cells to become memory cells, and there is no requirement for histocompatibility because the transfused cells are derived from the patient.


Immunology_Janeway

Immunology_Janeway. Another form of adoptive immunotherapy also uses retroviruses to introduce genes into a patient’s T cells, but involves expressing a novel type of receptor, known as a chimeric antigen receptor (CAR). CARs are fusion receptors that contain extracellular antigen-specific domains fused to intracellular domains that provide signals for activation and co-stimulation. These receptors are introduced into T cells via retroviral vectors to produce so-called CAR T cells. This approach differs from conventional adoptive T-cell therapies as the use of a CAR allows the T cell’s target specificity to be almost any molecule recognizable by an antibody rather than just peptide:MHC complexes. Recently this approach was used to target CD19 as a tumor rejection antigen in treating acute lymphocytic leukemia (ALL), an aggressive cancer of transformed B cells (Fig. 16.18). The CAR used in this case had an extracellular domain of an antibody that recognizes human CD19. The intracellular domain had three


Immunology_Janeway

Immunology_Janeway. (ALL), an aggressive cancer of transformed B cells (Fig. 16.18). The CAR used in this case had an extracellular domain of an antibody that recognizes human CD19. The intracellular domain had three ITAMs from the ζ chain of the T-cell receptor CD3 complex (see Chapter 7) fused with a co-stimulation domain from 4-1BB, a member of the TNF receptor superfamily. These CART-19 transduced T cells were expanded in vitro and transferred into a patient. The results of this case combined with others have demonstrated that CD8 T cells expressing CART-19 (see Fig. 16.18) are effective at achieving complete clinical remissions in many patients with ALL. This approach is not without its side-effects, however, as it also eliminates normal B cells in patients and they therefore require treatment with IVIG.


Immunology_Janeway

Immunology_Janeway. 16-17 Monoclonal antibodies against tumor antigens, alone or linked to toxins, can control tumor growth. Using monoclonal antibodies to destroy tumors requires that a tumor-specific antigen be expressed on the tumor’s cell surface, so that antibody can direct Fig. 16.18 Chimeric antigen receptors (CARs) expressed in T cells can confer antitumor specificity to a patient’s lymphocytes. Bottom panel: a chimeric antigen receptor, CART-19, is composed of an extracellular single-chain antibody that binds to CD19 that is fused to intracellular signaling domains from 4-1BB and the CD3ζ chain. Top row of panels: a lentivirus, a type of retrovirus, is used to express the gene encoding CART-19 in the T cells harvested from a patient diagnosed with ALL. After in vitro activation and expansion, the transfected CART-19 cells are infused into the patient and exert cytotoxic actions against CD19-expressing tumor cells as well as nontransformed B cells. T cells are harvested from the blood of a


Immunology_Janeway

Immunology_Janeway. the transfected CART-19 cells are infused into the patient and exert cytotoxic actions against CD19-expressing tumor cells as well as nontransformed B cells. T cells are harvested from the blood of a patient with a B-cell tumor A retrovirus encoding an anti-CD19 CAR infects T cells that are activated with antibodies to CD3 and CD28 Infected T cells express an anti-CD19 CAR T cells are infused into patient to mediate antitumor activity CD19 anti-CD19 Anti-CD19 CAR 4-1BB signaling domain Anti-CD19 CAR chimeric receptor tumor cell T cell T cell CD3˜ chain ITAMs VH TCR CD28 CD3 VL


Immunology_Janeway

Immunology_Janeway. Fig. 16.19 Monoclonal antibodies that recognize tumor-specific antigens have been used to help eliminate tumors. Tumor-specific antibodies of the correct isotypes can lyse tumor cells by recruiting effector cells such as NK cells and activating them via their Fc receptors (left panels). Another strategy has been to couple the antibody to a powerful toxin (center panels). When the antibody binds to the tumor cell and is endocytosed, the toxin is released from the antibody and can kill the tumor cell. If the antibody is coupled to a radioisotope (right panels), binding of the antibody to a tumor cell will deliver a dose of radiation sufficient to kill the tumor cell. In addition, nearby tumor cells could also receive a lethal radiation dose, even though they do not bind the antibody. Antibody fragments have started to replace whole antibodies for coupling to toxins or radioisotopes.


Immunology_Janeway

Immunology_Janeway. the activity of a cytotoxic cell, toxin, or even radioactive nuclide specifically to the tumor (Fig. 16.19). Some of the cell-surface molecules targeted in clinical trials are shown in Fig. 16.20, and some of these treatments have now been licensed. Striking improvements in survival have been reported for breast cancer patients treated with the monoclonal antibody trastuzumab (Herceptin), which targets the receptor HER-2/neu. This receptor is overexpressed in about one-quarter of breast cancer patients and is associated with a poor prognosis.


Immunology_Janeway

Immunology_Janeway. Herceptin is thought to act by blocking the binding of the natural ligand (so far unidentified) to this receptor, and by downregulating the level of expression of the receptor. The effects of this antibody can be enhanced when it is combined with conventional chemotherapy. Beyond blocking a growth signal for tumor cells, experiments in mice suggest that some of trastuzumab’s antitumor effects also involve innate and adaptive immune responses, such as directing ADCC or inducing antitumor T-cell responses. A monoclonal antibody that has yielded excellent results in the treatment of non-Hodgkin’s B-cell lymphoma is the anti-CD20 antibody rituximab, which triggers the apoptosis of B cells upon binding to CD20 on their surface (see Section 16-7). ADCC may be another mechanism by which rituximab acts, as its clinical efficacy has been linked to polymorphisms in activating Fc receptors.


Immunology_Janeway

Immunology_Janeway. Technical problems with monoclonal antibodies as therapeutic agents include inefficient killing of cells after binding the monoclonal antibody, inefficient penetration of the antibody into the tumor mass (which can be improved by using small antibody fragments), and soluble target antigens mopping up the antibody. The efficiency of killing can be enhanced by linking the antibody to a toxin, producing a reagent called an immunotoxin (see Fig. 16.19): two favored toxins are ricin A chain and Pseudomonas toxin. The antibody must be internalized to allow cleavage of the toxin from the antibody in the endocytic compartment, permitting the freed toxin chain to penetrate and kill the cell. Toxins coupled to native antibodies have had limited success in cancer therapy, but fragments of antibodies such as single-chain Fv molecules (see Section 4-3) show more promise. An example of a successful immunotoxin is a recombinant Fv anti-CD22 antibody fused to a fragment of Pseudomonas toxin, which


Immunology_Janeway

Immunology_Janeway. such as single-chain Fv molecules (see Section 4-3) show more promise. An example of a successful immunotoxin is a recombinant Fv anti-CD22 antibody fused to a fragment of Pseudomonas toxin, which induced complete remissions in two-thirds of a group of patients with a type of B-cell leukemia known as hairy-cell leukemia in whom the disease was resistant to conventional chemotherapy.


Immunology_Janeway

Immunology_Janeway. Monoclonal antibodies can also be conjugated to chemotherapeutic drugs or to radioisotopes. In the case of a drug-linked antibody, the binding of the antibody to a cell-surface antigen concentrates the drug to the site of the tumor. After internalization, the drug is released in the endosomes and exerts its cytostatic or cytotoxic effect. For example, the antibody trastuzumab has been linked to the cytotoxic agent mertansine, a drug that inhibits the assembly of microtubules, in a conjugate called T-DM1. Since HER-2 is overexpressed only in cancer cells, T-DM1 selectively delivers the toxin specifically to tumor cells.


Immunology_Janeway

Immunology_Janeway. Fig. 16.21 An effective vaccine against human papilloma virus (HPV) induces antibodies that protect against HPV infection. Serotype 16 of HPV (HPV-16) is highly associated with the development of cervical cancer. In a clinical trial, 755 healthy uninfected women were immunized with a vaccine generated from highly purified noninfectious ‘viruslike particles’ (VLP) consisting of the capsid protein L1 of HPV-16 and formulated with an alum adjuvant (in this case aluminum hydroxyphosphate sulfate). In comparison with the very low titers of antibody in placebo-treated uninfected women (green line), or women previously infected with HPV who received placebo (blue line), the women treated with the viruslike particle vaccine (red line) developed high titers of antibody against the L1 capsid protein. None of these immunized women subsequently became infected by HPV-16. An anti-HPV vaccine marketed as Gardasil is now available and recommended for use in girls and young women as a protection from


Immunology_Janeway

Immunology_Janeway. None of these immunized women subsequently became infected by HPV-16. An anti-HPV vaccine marketed as Gardasil is now available and recommended for use in girls and young women as a protection from cervical cancer caused by HPV serotypes 6, 11, 16, and 18. mMU, milli-Merck units.


Immunology_Janeway

Immunology_Janeway. The HPV-16 vaccine induces high titers of specifc antibody that persist long after vaccination 1000 uninfected recipients of placebo recipients previously infected with HPV-16 placebo recipients not previously infected with HPV-16 Another drug–antibody conjugate, brentuximab vedotin, links an anti-CD30 antibody with a different microtubule inhibitor and is approved for certain forms of relapsed lymphomas.


Immunology_Janeway

Immunology_Janeway. A variation on this approach is to link an antibody to an enzyme that metabolizes a nontoxic pro-drug to the active cytotoxic drug, a technique known as antibody-directed enzyme/pro-drug therapy (ADEPT). With this technique, a small amount of enzyme localized by the antibody can generate much larger amounts of active cytotoxic drug in the immediate vicinity of tumor cells. Monoclonal antibodies linked to radioisotopes (see Fig. 16.19) have been successfully used to treat refractory B-cell lymphoma, using anti-CD20 antibodies linked to yttrium-90 (ibritumomab tiuxetan). These approaches have the advantage of also killing neighboring tumor cells, because the released drug or radioactive emissions can affect cells adjacent to those that bind the antibody. Monoclonal antibodies coupled to γ-emitting radioisotopes have also been used successfully to image tumors for the purposes of diagnosis and monitoring tumor spread.


Immunology_Janeway

Immunology_Janeway. 16-18 Enhancing the immune response to tumors by vaccination holds promise for cancer prevention and therapy. In addition to CAR T cells and monoclonal antibody-based therapies, there are two other major approaches to cancer immunotherapy. Cancer vaccines are based on the idea that tumors are intrinsically poorly immunogenic, and the vaccine should act to supply the immunogenicity. A second approach, called checkpoint blockade, which we will discuss in the next section, is based on the idea that the immune system has been primed but is held in check by tolerance mechanisms, which can be blocked by therapeutic interventions.


Immunology_Janeway

Immunology_Janeway. Many cancers are associated with viral infections, and vaccines that prevent these infections can reduce cancer risk. A major breakthrough in anticancer therapy occurred in 2005 with the completion of a clinical trial involving 12,167 women that tested a vaccine against human papilloma virus (HPV). This trial showed that a recombinant vaccine against HPV was 100% effective in preventing cervical cancer caused by two key HPV strains, HPV-16 and HPV-18, which are associated with 70% of cervical cancers. The vaccine most likely prevents infection of cervical epithelium by HPV through the induction of anti-HPV antibodies (Fig. 16.21). Although this trial showed the potential of vaccines to prevent cancer, attempts to use vaccines to treat existing tumors have been less effective. In the case of HPV, certain types of vaccines that have increased immunogenicity for eliciting T-cell responses are beginning to show effectiveness in treating existing intraepithelial neoplasia caused by the


Immunology_Janeway

Immunology_Janeway. of HPV, certain types of vaccines that have increased immunogenicity for eliciting T-cell responses are beginning to show effectiveness in treating existing intraepithelial neoplasia caused by the virus. Similarly, the majority of liver cancers are associated with chronic hepatitis caused by several viruses. The vaccine against hepatitis B can reduce primary liver cancer due to this virus, although it will not protect against cancers caused by infections by other viruses such as hepatitis C.


Immunology_Janeway

Immunology_Janeway. Vaccines based on tumor antigens are, in principle, the ideal approach to T-cell-mediated cancer immunotherapy, but they are difficult to develop. For HPV, the relevant antigens are known. For most spontaneous tumors, however, relevant peptides of tumor rejection antigens may not be the same in different patients’ tumors and may be presented only by particular MHC alleles. This means that an effective tumor vaccine must include a range of tumor antigens. It is also clear that cancer vaccines for therapy should be used only when the tumor burden is low, such as after adequate surgery and chemotherapy.


Immunology_Janeway

Immunology_Janeway. The sources of antigens for cell-based cancer vaccines are the individual patients’ tumors removed at surgery. These vaccines are prepared by mixing either irradiated tumor cells or tumor extracts with killed bacteria such as Bacille Calmette–Guérin (BCG) or Corynebacterium parvum, which act as adjuvants to enhance the immunogenicity of the tumor antigens (see Appendix I, Section A-41). Vaccination using BCG adjuvants has had variable results in the past, but renewed interest is based on recent appreciation of their interaction with Toll-like receptors (TLRs). Stimulation of TLR-4 by BCG and other ligands has been tested against melanoma and other solid tumors. CpG DNA, which binds to TLR-9, has also been used to increase the immunogenicity of cancer vaccines. In cases where candidate tumor rejection antigens have been identified, for example, in melanomas, experimental vaccination strategies have included the use of whole proteins, peptide vaccines based on sequences recognized by


Immunology_Janeway

Immunology_Janeway. rejection antigens have been identified, for example, in melanomas, experimental vaccination strategies have included the use of whole proteins, peptide vaccines based on sequences recognized by cytotoxic T lymphocytes and helper T lymphocytes (either administered alone or presented by the patient’s own dendritic cells), and recombinant viruses encoding these peptide epitopes.


Immunology_Janeway

Immunology_Janeway. The potency of dendritic cells in activating T-cell responses provides the rationale for yet another antitumor vaccination strategy. The use of antigen-loaded dendritic cells to stimulate therapeutically useful cytotoxic T-cell responses to tumors has now undergone clinical trials in humans with cancer. One such vaccine, sipuleucel-T (Provenge), was recently approved for treatment of metastatic prostate cancer. In this therapy, a patient’s monocytes are extracted from peripheral blood and cultured with a fusion protein containing the antigen prostatic acid phosphatase (PAP), which is expressed by most prostate cancers, and the cytokine granulocyte–macrophage colony-stimulating factor (GM-CSF), which induces monocytes to undergo maturation to monocyte-derived dendritic cells. The resulting cells are reinfused into the patient to induce an immune response specific to the PAP antigen. This treatment reduced the risk of death by 22% and improved survival by about 4 months relative to a


Immunology_Janeway

Immunology_Janeway. cells are reinfused into the patient to induce an immune response specific to the PAP antigen. This treatment reduced the risk of death by 22% and improved survival by about 4 months relative to a placebo group. Other methods in clinical trials include loading dendritic cells ex vivo with DNA encoding the tumor antigen or with mRNA derived from tumor cells, and the use of apoptotic or necrotic tumor cells as sources of antigens.


Immunology_Janeway

Immunology_Janeway. 16-19 Checkpoint blockade can augment immune responses to existing tumors.


Immunology_Janeway

Immunology_Janeway. Other approaches to tumor immunotherapy attempt to strengthen the natural immune responses against a tumor by one of two approaches: by making the tumor itself more immunogenic, or by relieving the normal inhibitory mechanisms that regulate these responses. The first kind of approach has been explored by inducing the expression of co-stimulatory molecules, such as B7 molecules, on tumor cells and then using these cells to activate tumor-specific naive T cells. Similarly, tumor cells may be transfected with the gene encoding granulocyte–macrophage colony-stimulating factor (GM-CSF) in order to induce the maturation of tumor-proximal monocytes into monocytederived dendritic cells. Once these cells have differentiated and capture antigens from the tumor, they may migrate to the local lymph nodes and activate tumor-specific T cells. No approved therapies have yet emerged from these approaches. In mice, B7-transfected cells seem less potent than the monoctye-derived dendritic cells


Immunology_Janeway

Immunology_Janeway. lymph nodes and activate tumor-specific T cells. No approved therapies have yet emerged from these approaches. In mice, B7-transfected cells seem less potent than the monoctye-derived dendritic cells differentiated by GM-CSF in inducing antitumor responses. This may be because molecules in addition to B7 act in priming naive T cells and these molecules may be expressed only by specific types of cross-presenting dendritic cells.


Immunology_Janeway

Immunology_Janeway. Another approach to cancer immunotherapy is called checkpoint blockade, which attempts to interfere with the normal inhibitory signals that regulate lymphocytes. Immune responses are controlled by several positive and negative immunological checkpoints. A positive checkpoint for T cells is controlled by the B7 co-stimulatory receptors expressed by professional antigen-presenting cells such as dendritic cells, as discussed earlier. Negative immunological checkpoints are provided by inhibitory receptors such as CTLA-4 and PD-1.


Immunology_Janeway

Immunology_Janeway. CTLA-4 imposes a critical checkpoint for potentially autoreactive T cells by binding to B7 molecules on dendritic cells and delivering a negative signal that must be overcome by other signals before T cells can become activated. Blocking CTLA-4 with antibodies may therefore lower the threshold for T-cell activation. Some evidence also indicates that anti-CTLA-4 antibodies may augment immune responses by eliminating regulatory T cells, which express CTLA-4 on their surface. Whatever the mechanism, the absence of this checkpoint causes self-reactive T cells that are normally held in check to become activated instead and produce a multi-tissue autoimmune reaction, as seen in CTLA-4-deficient mice.


Immunology_Janeway

Immunology_Janeway. Since checkpoint blockade relies on activation of the patient’s own immune system against tumors, its effects are not immediately evident, presenting a challenge in evaluating clinical responses to such therapy. Guidelines for evaluating clinical responses were based on the immediate effects of chemotherapeutic drugs or radiation, whereas checkpoint blockade requires more time for reversing immune inhibition and expanding tumor-specific T cells that then exert their effects within the tumor. Once such issues were considered, it became possible to design clinical trials that could document the effects of checkpoint blockade used in combination with traditional anticancer therapies.


Immunology_Janeway

Immunology_Janeway. Checkpoint blockade based on the anti-CTLA-4 antibody ipilimumab has now been shown to be effective in treating metastatic melanoma and recently received FDA approval for this indication. Patients with metastatic melanoma who were treated with ipilimumab showed an increase in the number and activity of T cells recognizing NY-ESO-1, a cancer-testis antigen expressed by melanoma. Overall only about 15% of patients exhibited a response to ipilimumab, but the treatment appeared to induce long-term remission in responding patients. One side-effect of ipilimumab in these patients seemed to be an increased risk of autoimmune phenomena, in agreement with the role of CTLA-4 in maintaining tolerance of self-reactive T cells.


Immunology_Janeway

Immunology_Janeway. Another checkpoint involves the inhibitory receptor PD-1 and its ligands PD-L1 and PD-L2. PD-L1 is expressed on a wide variety of human tumors; in renal cell carcinoma, PD-L1 expression is associated with a poor prognosis. In mice, transfection of the gene encoding PD-L1 into tumor cells increased their growth in vivo and reduced their susceptibility to lysis by cytotoxic T cells. These effects were reversed by an antibody against PD-L1. In humans, the anti-PD-1 antibody pembrolizumab has been shown to be effective in previously treated melanoma patients, giving a nearly 30% response rate. It is FDA-approved for use following treatment with ipilimumab, or in patients with a BRAF mutation after treatment with ipilimumab and a B-raf inhibitor. Another anti-PD-1 antibody, nivolumab, is also approved for treatment of metastatic melanoma and is being considered for use in treatment of Hodgkin’s lymphoma. Ongoing clinical trials are evaluating checkpoint blockade using antibodies to PD-L1


Immunology_Janeway

Immunology_Janeway. approved for treatment of metastatic melanoma and is being considered for use in treatment of Hodgkin’s lymphoma. Ongoing clinical trials are evaluating checkpoint blockade using antibodies to PD-L1 and PD-L2.


Immunology_Janeway

Summary.

Immunology_Janeway. Summary.


Immunology_Janeway

Immunology_Janeway. Some tumors elicit specific immune responses that suppress or modify their growth. Tumors evade or suppress these responses in several ways, passing through various stages of a process known as immunological editing. Understanding how the immune system promotes and prevents cancer growth has led to new therapies now deployed in the clinic. The possibility of eradicating cervical cancer, for example, has been brought closer by the development of an effective vaccine against specific strains of cancer-causing human papilloma virus. Monoclonal antibodies have also been successfully developed for tumor immunotherapy in several cases, such as an anti-CD20 antibody used to treat B-cell lymphoma. Attempts are also being made to develop vaccines incorporating peptides designed to generate effective cytotoxic and helper T-cell responses. CAR T cells engineered to recognize CD19 expressed on B cells can be an effective treatment for acute lymphocytic leukemia. Checkpoint blockade strategies for


Immunology_Janeway

Immunology_Janeway. cytotoxic and helper T-cell responses. CAR T cells engineered to recognize CD19 expressed on B cells can be an effective treatment for acute lymphocytic leukemia. Checkpoint blockade strategies for CTLA-4 and PD-1 have been approved for treating melanoma, and related strategies are being developed for other biologic targets to stimulate antitumor immune responses or block inhibitory mechanisms that suppress such responses. One vaccine using dendritic cells that present tumor antigens has been approved for treating prostate cancer. A current trend in cancer therapy has been to incorporate immunotherapy with other traditional anticancer treatments to take advantage of the specificity and power of the immune system.


Immunology_Janeway

Immunology_Janeway. Fighting infectious diseases with vaccination.


Immunology_Janeway

Immunology_Janeway. The two most important contributions to public health in the past 100 years—sanitation and vaccination—have markedly decreased deaths from infectious disease, and yet infectious diseases remain the leading cause of death worldwide. Modern immunology itself grew from the success of Edward Jenner’s and Louis Pasteur’s vaccines against smallpox and chicken cholera, respectively, and its greatest triumph has been the global eradication of smallpox, announced by the World Health Organization in 1979. A global campaign to eradicate polio is now well under way. With the past decade’s tremendous progress in basic immunology, particularly in understanding innate immunity, there is now great hope that vaccines for other major infectious diseases, including malaria, tuberculosis, and HIV, are within reach. The vision of the current generation of vaccine scientists is to elevate their art to the level of modern drug design; to move it from an empiric practice to a true ‘pharmacology of the immune


Immunology_Janeway

Immunology_Janeway. reach. The vision of the current generation of vaccine scientists is to elevate their art to the level of modern drug design; to move it from an empiric practice to a true ‘pharmacology of the immune system.’


Immunology_Janeway

Immunology_Janeway. The goal of vaccination is the generation of long-lasting and protective immunity. Throughout this book, we have illustrated how the innate and the adaptive immune systems collaborate in the face of infection to eliminate pathogens and generate protective immunity with immunological memory. Indeed, a single infection is often (but not always) sufficient to generate protective immunity to a pathogen. Recognition of this important relationship was recorded more than 2000 years ago in accounts of the Peloponnesian War, during which two successive outbreaks of plague struck Athens. The Greek historian Thucydides noted that people who had survived infection during the first outbreak were not susceptible to infection during the second.


Immunology_Janeway

Immunology_Janeway. The recognition of this type of relationship perhaps prompted the practice of variolation against smallpox, in which an inoculation of a small amount of dried material from a smallpox pustule was used to produce a mild infection that was then followed by long-lasting protection against reinfection. Smallpox itself has been recognized in medical literature for more than 1000 years; variolation seems to have been practiced in India and China many centuries before its introduction into the West (some time in the 1400s–1500s), and it was familiar to Jenner. However, infection after variolation was not always mild: fatal smallpox ensued in about 3% of cases, which would not meet modern criteria of drug safety. It seems there was some recognition that milkmaids exposed to a bovine virus similar to smallpox—cowpox—seemed protected from smallpox infection, and there is even one historical account suggesting that cowpox inoculation had been tried before Jenner. However, Jenner’s achievement


Immunology_Janeway

Immunology_Janeway. to smallpox—cowpox—seemed protected from smallpox infection, and there is even one historical account suggesting that cowpox inoculation had been tried before Jenner. However, Jenner’s achievement was not only the realization that infection with cowpox would provide protective immunity against smallpox in humans without the risk of significant disease, but its experimental proof by the intentional variolation of people whom he had previously vaccinated. He named the process vaccination (from vacca, Latin for cow), and Pasteur, in his honor, extended the term


Immunology_Janeway

Immunology_Janeway. Fig. 16.22 Diseases for which effective vaccines are still needed. *Current measles vaccines are effective but heat-sensitive, which makes their use difficult in tropical countries; heat stability is being improved. Mortality data are the most recent estimated figures available (2014). Global Health Estimates 2000–2012. World Health Organization, June 2014. to the stimulation of protection against other infectious agents. Humans are not a natural host of cowpox, which establishes only a brief and limited subcutaneous infection. But the cowpox virus contains antigens that stimulate an immune response that cross-reacts with smallpox antigens and thereby confers protection against the human disease. Since the early 20th century, the virus used to vaccinate against smallpox has been vaccinia virus, which is related to both cowpox and smallpox, but whose origin is obscure.


Immunology_Janeway

Immunology_Janeway. As we will see, many current vaccines offer protection by inducing the formation of neutralizing antibodies. However, that statement contains a hidden tautology; pathogens for which current vaccines are effective may also be pathogens for which antibodies are sufficient for protection. For several major pathogens—malaria, tuberculosis, and HIV—even a robust antibody response is not fully protective. The elimination of these pathogens requires additional effector activities, such as the generation of strong and durable cell-mediated immunity, which are not efficiently generated by current vaccine technologies. These are the issues that face modern vaccine scientists. 16-20 Vaccines can be based on attenuated pathogens or material from killed organisms.


Immunology_Janeway

Immunology_Janeway. 16-20 Vaccines can be based on attenuated pathogens or material from killed organisms. Vaccine development in the early part of the 20th century followed two empirical approaches. The first was the search for attenuated organisms with reduced pathogenicity, which would stimulate protective immunity but not cause disease. This approach continues into the present with the design of genetically attenuated pathogens in which desirable mutations are introduced into the organism by recombinant DNA technologies. This idea is being applied to important pathogens, such as malaria, for which vaccines are currently unavailable, and may be important in the future for designing vaccines for influenza and HIV.


Immunology_Janeway

Immunology_Janeway. The second approach was the development of vaccines based on killed organisms and, subsequently, on purified components of organisms that would be as effective as live whole organisms. Killed vaccines were desirable because any live vaccine, including vaccinia, can cause lethal systemic infection in immunosuppressed people. Evolving from this approach were vaccines based on the conjugation of purified antigens, as described for Haemophilus influenzae (see Section 16-27). This approach continues with the addition of ‘reverse immunogenetics’ (see Section 16-15) to identify candidate peptide antigens for T cells and with strategies to use ligands that activate TLRs or other innate sensors as adjuvants to enhance responses to simple antigens.


Immunology_Janeway

Immunology_Janeway. Immunization with such approaches is now considered so safe and so important that most states in the United States require all children to be immunized against several potentially deadly diseases. These include the the viral diseases measles, mumps, and polio, for which live-attenuated vaccines are used, as well as against tetanus (caused by Clostridium tetani), diphtheria (caused by Corynebacterium diphtheriae), and whooping cough (caused by Bordetella pertussis), for which vaccines composed of inactivated toxins or toxoids prepared from the respective bacteria are used. More recently, a vaccine has become available against H. influenzae type b (HiB), one of the causative agents of meningitis, as well as two vaccines for childhood diarrhea caused by rotaviruses, and, as described in Section 16-18, a vaccine for preventing HPV infection for protection against cervical cancer. Most vaccines are given to children within the first year of life. The vaccines against measles, mumps, and


Immunology_Janeway

Immunology_Janeway. 16-18, a vaccine for preventing HPV infection for protection against cervical cancer. Most vaccines are given to children within the first year of life. The vaccines against measles, mumps, and rubella (MMR), against chickenpox (varicella), and against influenza, when recommended, are usually given between the ages of 1 and 2 years.


Immunology_Janeway

Immunology_Janeway. Impressive as these accomplishments are, there are still many diseases for which we lack effective vaccines (Fig. 16.22). For many pathogens, natural infection does not seem to generate protective immunity, and infections become chronic or recurrent. In many infections of this type, such as malaria, tuberculosis, and HIV, antibodies are insufficient to prevent reinfection and to eliminate the pathogen, and cell-mediated immunity instead seems to be more important in limiting the pathogen, but alone is still insufficient to provide full immunity. It is not the absence of an immune response to the pathogen that is the problem, but rather that this response does not clear the pathogen, eliminate pathogenesis, or prevent reinfection.


Immunology_Janeway

Immunology_Janeway. Another obstacle is that even when a vaccine such as that against measles can be used effectively in developed countries, technical and economic problems can prevent its widespread use in developing countries, where mortality from these diseases is still high. For example, simple costs of storage and deployment can be significant barriers to the use of existing vaccines in poorer countries. Therefore, the development of vaccines therefore remains an important goal of immunology, and the latter half of the 20th century saw a shift to a more rational approach based on a detailed molecular understanding of microbial pathogenicity, analysis of the protective host response to pathogenic organisms, and an understanding of the regulation of the immune system to generate effective Tand B-lymphocyte responses. 16-21 Most effective vaccines generate antibodies that prevent the damage caused by toxins or that neutralize the pathogen and stop infection.


Immunology_Janeway

Immunology_Janeway. 16-21 Most effective vaccines generate antibodies that prevent the damage caused by toxins or that neutralize the pathogen and stop infection. Although the requirements for generating protective immunity vary with the nature of the infecting organism, many effective vaccines currently work by inducing antibodies against the pathogen. For many pathogens, including extracellular organisms and viruses, antibodies can provide protective immunity. This is not the case for all pathogens, unfortunately; some may require additional cell-mediated immune responses such as those mediated by CD8 T cells.


Immunology_Janeway

Immunology_Janeway. Effective protective immunity against some microorganisms requires the presence of preexisting antibody at the time of infection, either to prevent the damage caused by the pathogen or to prevent reinfection by the pathogen altogether. The first case is illustrated by vaccines against tetanus and diphtheria, whose clinical manifestations of infection are due to the effects of extremely powerful exotoxins (see Fig. 10.31). Preexisting antibody against the exotoxin is necessary to provide a defense against these diseases. Indeed, the tetanus exotoxin is so powerful that the tiny amount that can cause disease may be insufficient to lead to a protective immune response; consequently, even survivors of tetanus require vaccination to be protected against the risk of subsequent attack.


Immunology_Janeway

Immunology_Janeway. The second way in which antibodies can protect is by preventing infection a second time by the same pathogen, as in the case of certain viral infections. While CD8 T cells are able to kill already virally infected cells during an infection, antibodies are able to prevent infection of cells by the virus in the first place. This action is called neutralization. The ability of an antibody to neutralize a pathogen may depend on its affinity, its isotype subclass, complement, and the activity of phagocytic cells. For example, preexisting antibodies are required to protect against the polio virus, which infects critical host cells within a short period after entering the body and is not easily controlled by T lymphocytes once intracellular infection has been established. Vaccines against seasonal influenza virus provide protection in this same manner, by inducing antibodies that reduce the chance of a second infection by the same strain of influenza. For many viruses, antibodies produced by


Immunology_Janeway

Immunology_Janeway. influenza virus provide protection in this same manner, by inducing antibodies that reduce the chance of a second infection by the same strain of influenza. For many viruses, antibodies produced by an infection or by vaccination can neutralize the virus and prevent further spread of infection, but this is not always the case. In HIV infection, despite the generation of antibodies that can bind to surface viral epitopes, most of these antibodies fail to neutralize the virus. In addition, HIV has many different strains, or clades, and most vaccines based on HIV proteins do not induce antibodies that neutralize all clades, presenting a challenge for effective vaccine design. However, a recent clinical trial suggests that boosting previously vaccinated subjects with protein 5–7 years after immunization may induce some antibodies with crossclade activity.


Immunology_Janeway

Immunology_Janeway. Immune responses to infectious agents usually involve antibodies directed at multiple epitopes, and only some of these antibodies, if any, confer protection. The particular T-cell epitopes recognized can also affect the nature of the response. In Section 10-2, we described linked recognition, in which antigen-specific B cells and T cells provide mutually activating signals, leading to affinity maturation and isotype switching that may be required for neutralization. This process requires that an appropriate peptide epitope for T cells be presented by the B cells, and typically that the T-cell epitope be contained within the region of protein epitope recognized by the B cell, a fact that must be considered in modern vaccine design. Indeed, the predominant epitope recognized by T cells after vaccination with respiratory syncytial virus induces a vigorous inflammatory response but fails to elicit neutralizing antibodies and thus causes pathology without protection.


Immunology_Janeway

Immunology_Janeway. 16-22 Effective vaccines must induce long-lasting protection while being safe and inexpensive.


Immunology_Janeway

Immunology_Janeway. A successful vaccine must possess several features in addition to its ability to provoke a protective immune response (Fig. 16.23). First, it must be safe. Vaccines must be given to huge numbers of people, relatively few of whom are likely to die of, or sometimes even catch, the disease that the vaccine is designed to prevent. This means that even a low level of toxicity is unacceptable. Second, the vaccine must be able to produce protective immunity in a very high proportion of the people to whom it is given. Third, particularly in poorer countries where it is impracticable to give regular ‘booster’ vaccinations to dispersed rural populations, a successful vaccine must generate long-lived immunological memory. This means that the vaccine must prime both B and T lymphocytes. Fourth, vaccines must be very cheap if they are to be administered to large populations. Vaccines are one of the most cost-effective measures in health care, but this benefit is eroded as the cost per dose rises.


Immunology_Janeway

Immunology_Janeway. Another benefit of an effective vaccination program is the ‘herd immunity’ that it confers on the general population. By lowering the number of susceptible members of a population, vaccination decreases the natural reservoir of infected individuals in that population and so reduces the probability of transmission of infection. Thus, even unvaccinated members will be protected because their individual chance of encountering the pathogen is decreased. However, the herd immunity effect is seen only at relatively high levels of vaccination within a population; for mumps, the necessary level is estimated to be around 80%, and below this level sporadic epidemics can occur. This is illustrated by a marked increase in mumps in the United Kingdom in 2004-2005 in young adults as a result of the variable use in the mid-1990s of a measles/ rubella vaccine rather than the combined MMR, as the combined vaccine was in short supply at that time.


Immunology_Janeway

Immunology_Janeway. 16-23 Live-attenuated viral vaccines are usually more potent than ‘killed’ vaccines and can be made safer by the use of recombinant DNA technology.


Immunology_Janeway

Immunology_Janeway. Most antiviral vaccines currently in use consist of either live attenuated or inactivated viruses. Inactivated, or ‘killed,’ viral vaccines consist of viruses treated so that they are unable to replicate. Inactivated viruses therefore cannot produce proteins in the cytosol of infected cells, so peptides from the viral antigens are not presented by MHC class I molecules. Thus, CD8 T cells are neither efficiently generated nor needed with killed virus vaccines. Live-attenuated viral vaccines are generally far more potent: they elicit a greater number of effector mechanisms, including the activation of CD4 T cells and cytotoxic CD8 T cells. CD4 T cells help in shaping the antibody response, which is important for a vaccine’s subsequent protective effect. Cytotoxic CD8 T cells provide protection while infection by the virus itself is under way, and, if maintained, may contribute to protective memory. Attenuated viral vaccines include the routine childhood vaccines in use for polio,


Immunology_Janeway

Immunology_Janeway. while infection by the virus itself is under way, and, if maintained, may contribute to protective memory. Attenuated viral vaccines include the routine childhood vaccines in use for polio, measles, mumps, rubella, and varicella. Other attenuated live viral vaccines that are licensed for special circumstances or for use in high-risk populations include influenza, poxvirus (vaccinia), and yellow fever virus.


Immunology_Janeway

Immunology_Janeway. Traditionally, attenuation is achieved by growing the virus in cultured cells. Viruses are usually selected for preferential growth in nonhuman cells and, in the course of selection, become less able to grow in human cells (Fig. 16.24). Because these attenuated strains replicate poorly in human hosts, they induce immunity but not disease. Although attenuated virus strains contain multiple mutations in genes encoding several of their proteins, it might be possible for a pathogenic virus strain to reemerge by a further series of mutations. For example, the type 3 Sabin polio vaccine strain differs from a wild-type progenitor strain at only 10 of 7429 nucleotides. On extremely rare occasions, reversion of the vaccine to a neurovirulent strain can occur, causing paralytic disease in the unfortunate recipient.


Immunology_Janeway

Immunology_Janeway. Attenuated viral vaccines can also pose particular risks to immunodeficient recipients, in whom they often behave as virulent opportunistic infections. Immunodeficient infants who are vaccinated with live attenuated polio virus before their inherited immunoglobulin deficiencies have been diagnosed are at risk because they cannot clear the virus from their gut, and there is therefore an increased chance that mutations associated with the continuing uncontrolled replication of the virus in the gut will revert the virus to a virulent form and lead to fatal paralytic disease. An empirical approach to attenuation is still in use but might be superseded by two new approaches that use recombinant DNA technology. One is the isolation and in vitro mutagenesis of specific viral genes. The mutated genes are used to replace the wild-type genes in a reconstituted virus genome, and this


Immunology_Janeway

Immunology_Janeway. Fig. 16.24 Viruses are traditionally attenuated by selecting in cells of a different species, until it grows only poorly in human for growth in nonhuman cells. To produce an attenuated virus, cells. The adaptation is a result of mutation, usually a combination the virus must first be isolated by growing it in cultured human cells. of several point mutations. It is usually difficult to tell which of the The adaptation to growth in cultured human cells can cause some mutations in the genome of an attenuated viral stock are critical to attenuation in itself; the rubella vaccine, for example, was made in attenuation. An attenuated virus will grow poorly in the human host this way. In general, however, the virus is then adapted to growth and will therefore produce immunity but not disease.


Immunology_Janeway

Immunology_Janeway. Resulting virus is viable, immunogenic but avirulent. It can be used as a vaccine Isolate virulence gene Isolate pathogenic virus Mutate virulence gene Delete virulence gene receptor-binding protein virulence core proteins Fig. 16.25 Attenuation can be achieved more rapidly and reliably with recombinant DNA techniques. If a gene in the virus that is required for virulence but not for growth or immunogenicity can be identified, this gene can be either multiply mutated (left lower panel) or deleted from the genome (right lower panel) by using recombinant DNA techniques. This procedure creates an avirulent (nonpathogenic) virus that can be used as a vaccine. The mutations in the virulence gene are usually large, so that it is very difficult for the virus to revert to the wild type. deliberately attenuated virus can then be used as a vaccine (Fig. 16.25). The advantage of this approach is that mutations can be engineered so that reversion to wild type is virtually impossible.


Immunology_Janeway

Immunology_Janeway. Such an approach might be useful in developing live influenza vaccines. As described in Chapter 13, the influenza virus can reinfect the same host several times, because it undergoes antigenic shift and thus predominantly escapes the original immune response. A weak protection conferred by previous infections with a different subtype of influenza is observed in adults, but not in children, and is called heterosubtypic immunity. The current approach to vaccination against influenza is to use a killed virus vaccine that is reformulated annually on the basis of the prevalent strains of virus. The vaccine is moderately effective, reducing mortality in elderly people and illness in healthy adults. The ideal influenza vaccine would be an attenuated live organism that matched the prevalent virus strain. This could be created by first introducing a series of attenuating mutations into the gene encoding a viral polymerase protein, PB2. The mutated gene segment from the attenuated virus could


Immunology_Janeway

Immunology_Janeway. strain. This could be created by first introducing a series of attenuating mutations into the gene encoding a viral polymerase protein, PB2. The mutated gene segment from the attenuated virus could then be substituted for the wild-type gene in a virus carrying the relevant hemagglutinin and neuraminidase antigen variants of the current epidemic or pandemic strain. Alternatively, broadly neutralizing antibodies that block the receptor-binding domain of the hemagglutinin can be generated in humans and could be used as a universal vaccine. Public attention has recently been directed toward the possibility of a flu pandemic caused by the H5N1 avian flu strain. This strain can be passed between birds and humans and is associated with a high mortality rate; however, a pandemic would occur only if human-to-human transmission could occur. A live-attenuated vaccine would be used only if a pandemic occurred, because to give it beforehand would introduce new influenza virus genes that might


Immunology_Janeway

Immunology_Janeway. if human-to-human transmission could occur. A live-attenuated vaccine would be used only if a pandemic occurred, because to give it beforehand would introduce new influenza virus genes that might recombine with existing influenza viruses.


Immunology_Janeway

Immunology_Janeway. genetically attenuated parasites (GAPs). Similar approaches have been used for bacterial vaccine development. The most important example of an attenuated vaccine is that of BCG, which is quite effective at protecting against serious disseminated tuberculosis in children, but is not protective against adult pulmonary tuberculosis. The current BCG vaccine, which remains the most widely used vaccine in the world, was obtained from a pathogenic isolate of Mycobacterium bovis and passaged in a laboratory at the beginning of the 20th century. Since then, several genetically diverse strains of BCG have evolved. The level of protection afforded by the BCG vaccine is extremely variable, ranging from none in some countries, such as Malawi, to 50–80% in the UK.


Immunology_Janeway

Immunology_Janeway. Considering that tuberculosis remains one of the biggest killers worldwide, there is an urgent need for a new vaccine. Two recombinant BCG (rBCG) vaccines intended to prevent infection in unexposed individuals recently passed Phase I clinical trials. One was engineered to overexpress an immunodominant antigen of M. tuberculosis, to engender greater specificity toward the human pathogen. The second expresses the pore-forming protein listeriolysin from L. monocytogenes to induce the passage of BCG antigens from phagosomes into the cytoplasm and allow cross-presentation (see Section 6-5) on MHC class I molecules, thereby stimulating BCG-specific cytotoxic T cells. A similar approach is being used to generate new vaccines for malaria. Analysis of different stages of Plasmodium falciparum, the major cause of fatal malaria, identified genes that are selectively expressed in sporozoites within the mosquito’s salivary gland, where they first become infectious for human


Immunology_Janeway

Immunology_Janeway. Fig. 16.26 Genetically attenuated parasites can be engineered sporozoites with targeted disruption of key genes [for example, p52 as live vaccines to provide protective immunity. Top panel: and p36 (p52–/p36–), or uis3 (uis3–)], the sporozoites circulate in the wild-type Plasmodium sporozoites transmitted through the bite of bloodstream and mimic an early infection but cannot establish a an infected mosquito enter the bloodstream and are carried to the productive infection in the liver. The mice do, however, produce an liver, where they infect hepatocytes. Each sporozoite multiplies in the immune response against the sporozoites and are protected against liver, killing the infected cell and releasing thousands of merozoites, a subsequent infection by wild-type sporozoites. the next stage in infection. Bottom panels: in mice immunized with hepatocytes. Deletion of two such genes from the P. falciparum genome rendered sporozoites incapable of establishing a blood-stage infection in


Immunology_Janeway

Immunology_Janeway. in infection. Bottom panels: in mice immunized with hepatocytes. Deletion of two such genes from the P. falciparum genome rendered sporozoites incapable of establishing a blood-stage infection in mice, yet capable of inducing an immune response that protected mice from subsequent infection by wild-type P. falciparum. This protection was dependent on CD8 T cells, and to some extent on IFN-γ, indicating that cell-mediated immunity is important for protection against this parasite (Fig. 16.26). This highlights once again the importance of being able to generate vaccines that are capable of inducing strong cell-mediated immunity.


Immunology_Janeway

Immunology_Janeway. 16-25 The route of vaccination is an important determinant of success. The ideal vaccination induces host defense at the point of entry of the infectious agent. Stimulation of mucosal immunity is therefore an important goal for vaccination against the many organisms that enter through mucosal surfaces. Still, most vaccines are given by injection. This route has several disadvantages. Injections are painful and unpopular, reducing vaccine uptake, and they are expensive, requiring needles, syringes, and a trained injector. Mass vaccination by injection is laborious. There is also the immunological drawback that injection may not be the most effective way of stimulating an appropriate immune response because it does not mimic the usual route of entry of the majority of pathogens against which vaccination is directed. Many important pathogens infect mucosal surfaces or enter the body through mucosal surfaces. Examples include respiratory microorganisms such as


Immunology_Janeway

Immunology_Janeway. Many important pathogens infect mucosal surfaces or enter the body through mucosal surfaces. Examples include respiratory microorganisms such as B. pertussis, rhinoviruses, and influenza viruses, and enteric microorganisms such as Vibrio cholerae, Salmonella typhi, enteropathogenic Escherichia coli, and Shigella. Intranasally administered live-attenuated vaccine against influenza virus induces mucosal antibodies, which are more effective than systemic antibodies in the control of upper respiratory tract infection. However, the systemic antibodies induced by injection are effective in controlling lower respiratory tract disease, which is responsible for the severe morbidity and mortality due to this disease. Thus, a realistic goal of any pandemic influenza vaccine is to prevent the lower respiratory tract disease but accept the fact that mild illness will not be prevented.


Immunology_Janeway

Immunology_Janeway. The power of the mucosal approach is illustrated by the effectiveness of live-attenuated polio vaccines. The Sabin oral polio vaccine consists of three attenuated polio virus strains and is highly immunogenic. Moreover, just as polio itself can be transmitted by fecal contamination of public swimming pools and other failures of hygiene, the vaccine can be transmitted from one individual to another by the fecal–oral route. Infection with Salmonella likewise stimulates a powerful mucosal and systemic immune response.


Immunology_Janeway

Immunology_Janeway. Presentation of soluble protein antigens by the oral route often results in tolerance, which is important given the enormous load of food-borne and airborne antigens presented to the gut and respiratory tract (see Chapter 12). Nonetheless, the mucosal immune system responds to and eliminates mucosal infections that enter by the oral route, such as pertussis, cholera, and polio. The proteins from these microorganisms that stimulate immune responses are therefore of special interest. One group of powerfully immunogenic proteins at mucosal surfaces is a group of bacterial toxins that have the property of binding to eukaryotic cells and being resistant to proteases. A recent finding of potential practical importance is that certain of these proteins, such as the


Immunology_Janeway

Immunology_Janeway. E. coli heat-labile toxin and pertussis toxin, have adjuvant properties that are retained even when the parent molecule has been engineered to eliminate its toxic properties. These molecules can be used as adjuvants for oral or nasal vaccines. In mice, nasal insufflation of either of these mutant toxins together with tetanus toxoid resulted in the development of protection against lethal challenge with tetanus toxin. 16-26 Bordetella pertussis vaccination illustrates the importance of the perceived safety of a vaccine.


Immunology_Janeway

Immunology_Janeway. 16-26 Bordetella pertussis vaccination illustrates the importance of the perceived safety of a vaccine. The history of vaccination against the bacterium that causes whooping cough, Bordetella pertussis, illustrates the challenges of developing and disseminating an effective vaccine, as well as the public appeal of acellular conjugate vaccines over attenuated live organisms. At the beginning of the 20th century, whooping cough killed about 0.5% of American children under the age of 5 years. In the early 1930s, a trial of a killed, whole bacterial cell vaccine on the Faroe Islands provided evidence of a protective effect. In the United States, systematic use of a whole-cell vaccine in combination with diphtheria and tetanus toxoids (the DTP vaccine) during the 1940s resulted in a decline in the annual infection rate from 200 to fewer than 2 cases per 100,000 of the population. First vaccination with DTP was typically given at the age of 3 months.


Immunology_Janeway

Immunology_Janeway. Whole-cell pertussis vaccine causes side-effects, typically redness, pain, and swelling at the site of the injection; less commonly, vaccination is followed by high temperature and persistent crying. Very rarely, fits and a short-lived sleepiness or a floppy unresponsive state ensue. During the 1970s, widespread concern developed after several anecdotal observations that encephalitis leading to irreversible brain damage might very rarely follow pertussis vaccination. In Japan, in 1972, about 85% of children were given the pertussis vaccine, and fewer than 300 cases of whooping cough and no deaths were reported. As a result of two deaths after vaccination in Japan in 1975, the use of DTP was temporarily suspended and then reintroduced with the first vaccination at 2 years of age rather than at 3 months. In 1979, there were about 13,000 cases of whooping cough and 41 deaths. The possibility that pertussis vaccine very rarely causes severe brain damage has been studied extensively, and


Immunology_Janeway

Immunology_Janeway. at 3 months. In 1979, there were about 13,000 cases of whooping cough and 41 deaths. The possibility that pertussis vaccine very rarely causes severe brain damage has been studied extensively, and expert consensus is that pertussis vaccine is not a primary cause of brain injury. There is no doubt that there is greater morbidity from whooping cough than from the vaccine.


Immunology_Janeway

Immunology_Janeway. The public and medical perception that whole-cell pertussis vaccination might be unsafe provided a powerful incentive to develop safer pertussis vaccines. Study of the natural immune response to B. pertussis showed that infection induced antibodies against four components of the bacterium— pertussis toxin, filamentous hemagglutinin, pertactin, and fimbrial antigens. Immunization of mice with these antigens in purified form protected them against challenge with pertussis. This has led to the development of acellular pertussis vaccines, all of which contain purified pertussis toxoid—that is, toxin inactivated by chemical treatment, for example with hydrogen peroxide or formaldehyde, or more recently by genetic engineering of the toxin. Some pertussis vaccines also contain filamentous hemagglutinin, pertactin, and/or fimbrial antigens, either alone or in any combination of the three. Current evidence shows that these vaccines are probably as effective as whole-cell pertussis vaccine


Immunology_Janeway

Immunology_Janeway. pertactin, and/or fimbrial antigens, either alone or in any combination of the three. Current evidence shows that these vaccines are probably as effective as whole-cell pertussis vaccine while lacking its common minor side-effects. The acellular vaccine is more expensive, however, thus restricting its use in poorer countries.


Immunology_Janeway

Immunology_Janeway. The history of pertussis vaccination illustrates that, first and foremost, vaccines must be extremely safe and free of side-effects; second, the public and medical profession must perceive the vaccine to be safe; and third, careful study of the nature of the protective immune response can lead to acellular vaccines that are safer than whole-cell vaccines but still as effective. Still, public concerns about vaccination remain high. Unwarranted fears of a link between the combined live-attenuated MMR vaccine and autism saw the uptake of MMR vaccine in England fall from a peak of 92% of children in 1995–1996 to 84% in 2001–2002. Small clustered outbreaks of measles and mumps in London since 2002 illustrate the importance of maintaining high uptake of vaccine to maintain herd immunity. 16-27 Conjugate vaccines have been developed as a result of linked recognition between T and B cells.


Immunology_Janeway

Immunology_Janeway. 16-27 Conjugate vaccines have been developed as a result of linked recognition between T and B cells. Many bacteria, including Neisseria meningitidis (meningococcus), Streptococcus pneumoniae (pneumococcus), and H. influenzae, have an outer capsule composed of polysaccharides that are speciesand type-specific for particular strains of the bacterium. The most effective defense against these microorganisms is opsonization of the polysaccharide coat with antibody. The aim of vaccination for these organisms is therefore to elicit antibodies against the polysaccharide capsules of the bacteria. However, effective acellular vaccines cannot be made from a single isolated constituent of a microorganism, since generation of an effective antibody response requires the participation of several types of cells, and this fact has led to the development of conjugate vaccines (Fig. 16.27).


Immunology_Janeway

Immunology_Janeway. Capsular polysaccharides can be harvested from bacterial growth medium and, because they are T-cell-independent antigens (see Section 10-1), they can be used on their own as vaccines. However, young children under the age Fig. 16.27 Conjugate vaccines take advantage of linked recognition to boost B-cell responses against polysaccharide antigens. The Hib vaccine against Haemophilus influenzae type b is a conjugate of bacterial polysaccharide and the tetanus toxoid protein. The B cell recognizes and binds the polysaccharide, internalizes and degrades the whole conjugate, and then displays toxoid-derived peptides on surface MHC class II molecules. Helper T cells generated in response to earlier vaccination against the toxoid recognize the complex on the B-cell surface and activate the B cell to produce anti-polysaccharide antibody. This antibody can then protect against infection with H. influenzae type b. Chapter 16: Manipulation of the Immune Response


Immunology_Janeway

Immunology_Janeway. Fig. 16.28 The effect of vaccination against group C Neisseria meningitidis (meningococcus) on the number of cases of group B and group C meningococcal disease in England and Wales. Meningococcal infection affects roughly 5 in 100,000 people a year in the UK, with groups B and C meningococci accounting for almost all the cases. Before the introduction of the meningitis C vaccine, group C disease was the second most common cause of meningococcal disease, accounting for about 40% of cases. Group C disease now accounts for less than 10% of cases, with group B disease accounting for more than 80% of cases. After the introduction of the vaccine, there was a significant decrease in the number of laboratory-confirmed cases of group C disease in all age groups. The impact was greatest in the immunized groups, with reductions of more than 90% in these groups. An impact has also been seen in the unimmunized age groups, with a reduction of about 70%, suggesting that this vaccine has had a herd


Immunology_Janeway

Immunology_Janeway. groups, with reductions of more than 90% in these groups. An impact has also been seen in the unimmunized age groups, with a reduction of about 70%, suggesting that this vaccine has had a herd immunity effect.


Immunology_Janeway

Number of cases

Immunology_Janeway. Number of cases


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Immunology_Janeway. Number of cases of 2 years cannot make good T-cell-independent antibody responses and cannot be vaccinated effectively with polysaccharide (PS) vaccines. An efficient way of overcoming this problem is to conjugate bacterial polysaccharides chemically to protein carriers (see Fig. 16.27). This carrier protein provides peptides that can be recognized by antigen-specific T cells, thus converting a T-cell-independent response into a T-cell-dependent antipolysaccharide antibody response. Using this approach, various conjugate vaccines have been developed against H. influenzae type b, an important cause of serious childhood chest infections and meningitis, and against N. meningitidis serogroup C, an important cause of meningitis, and these are now widely applied. The success of the latter vaccine in the United Kingdom is illustrated in Fig. 16.28, which shows that the incidence of meningitis C has been markedly reduced in comparison with meningitis B, against which there is currently no


Immunology_Janeway

Immunology_Janeway. in the United Kingdom is illustrated in Fig. 16.28, which shows that the incidence of meningitis C has been markedly reduced in comparison with meningitis B, against which there is currently no vaccine. Endemic meningitis B is due to diverse serogroup B strains, so an ideal vaccine would target the group B capsular polysaccharide. Unfortunately, group B polysaccharide is identical to some polysialyl polysaccharides on human cells, and is poorly immunogenic due to tolerance of these self antigens. Some strategies to chemically modify the group B polysaccharide for use in a conjugate vaccine have been considered, but a major focus in group B meningococcal vaccine development has been to instead direct immunity against noncapsular antigens, which will be generally effective against endemic disease.


Immunology_Janeway

Immunology_Janeway. 16-28 Peptide-based vaccines can elicit protective immunity, but they require adjuvants and must be targeted to the appropriate cells and cell compartment to be effective.


Immunology_Janeway

Immunology_Janeway. Another vaccine-development strategy that does not require the whole organism, whether killed or attenuated, identifies the T-cell peptide epitopes that stimulate protective immunity. Candidate peptides can be identified in two ways: in one, overlapping peptides from immunogenic proteins are systematically synthesized and their ability to stimulate protective immunity is tested; alternatively, a reverse immunogenetic approach (see Section 16-15) can be used to predict potential peptide epitopes from a genome sequence. The latter approach has been applied to malaria by using the complete sequence of the Plasmodium falciparum genome. The starting point was the association between the human MHC class I molecule HLA-B53 and resistance to cerebral malaria, a relatively infrequent, but usually fatal, complication of infection by P. falciparum. It was thought that HLA-B53 might protect against cerebral malaria because it could present peptides that are particularly good at activating naive


Immunology_Janeway

Immunology_Janeway. fatal, complication of infection by P. falciparum. It was thought that HLA-B53 might protect against cerebral malaria because it could present peptides that are particularly good at activating naive cytotoxic T lymphocytes. Peptides eluted from HLA-B53 frequently contain a proline as the second of their nine amino acids. On the basis of this information, reverse genetic analysis identified candidate protective peptides from four proteins of P. falciparum expressed in the early phase of hepatocyte infection—an important phase of infection to target in an effective immune response. One of the candidate peptides, from liver stage antigen-1, has been shown to be recognized by cytotoxic T cells when bound to HLA-B53 and may be a useful peptide for use in vaccination.


Immunology_Janeway

Immunology_Janeway. Peptide-based vaccines, although promising, have several drawbacks. First, a particular peptide may not bind to all the MHC molecules present in the population. Because humans are highly polymorphic in the MHC, a large panel of protective peptides would be needed for coverage of most individuals. Second, some direct exchange of short peptides on MHC molecules can occur without physiological antigen processing. If the required antigenic peptides load directly onto MHC molecules on cells other than dendritic cells, this may induce tolerance in T cells rather than stimulating immunity. Third, exogenous proteins and peptides delivered by a synthetic vaccine are efficiently processed for presentation by MHC class II molecules, but require ‘cross-presentation’ in specific types of dendritic cells to be loaded onto MHC class I molecules (see Section 6-5). Directing peptide-based vaccines to such cells may enhance vaccine efficacy.


Immunology_Janeway

Immunology_Janeway. Recent peptide-based vaccines have already shown promise in human clinical trials. Patients with established vulvar intraepithelial neoplasia, an early form of vulvar cancer caused by human papilloma virus (HPV), were treated with a vaccine consisting of long peptides covering the entire length of two oncoproteins of HPV-16—E6 and E7—and delivered in an oil-in-water emulsion as adjuvant. By using very long peptides, around 100 amino acids in length, multiple candidate peptide epitopes can be delivered that may also be presented by different MHC alleles. These peptides seem to be too long for direct exchange with peptides on cell surfaces and require processing by dendritic cells in order to be loaded onto MHC class I molecules. This vaccine induced complete clinical remission in one-quarter of the patients, and about half of the treated patients showed significant clinical responses that correlated with in vitro evidence of enhanced cell-mediated immunity.


Immunology_Janeway

Immunology_Janeway. 16-29 Adjuvants are important for enhancing the immunogenicity of vaccines, but few are approved for use in humans. Vaccines based on peptides or purified proteins require additional components to mimic how real infections activate immunity. Such components of a vaccine are known as adjuvants, which are defined as substances that enhance the immunogenicity of antigens (see Appendix I, Section A-41). For example, tetanus toxoid is not immunogenic in the absence of adjuvants, and so tetanus toxoid vaccines contain inorganic aluminum salts (alum) in the form of noncrystalline gels that bind polyvalently to the toxoid by ionic interactions. Pertussis toxin has adjuvant properties in its own right and, when given mixed as a toxoid with tetanus and diphtheria toxoids, not only protects against whooping cough but also acts as an additional adjuvant for the other two toxoids. This mixture makes up the DTP triple vaccine given to infants in the first year of life.