PART IV The Immune System in Health and Disease donors and a recipient(Figure 21-4c). Lymphocytes from a potential donor that have been x-irradiated or treated witl mitomycin C serve as the stimulator cells, and lymphocytes from the recipient serve as responder cells. Proliferation of the recipient T cells, which indicates T-cell activation, is mea- greater the class ll mhc differences between the donor and E recipient cells, the more [H]thymidine uptake will be ob- served in an MLR assay. Intense proliferation of the recipient lymphocytes indicates a poor prognosis for graft survival. F The advantage of the mlr over microcytotoxicity typing is that it gives a better indication of the degree of TH-cell acti vation generated in response to the class II MHC antigens of the potential graft. The disadvantage of the mlr is that it takes several days to run the assay. If the potential donor is a cadaver, for example, it is not possible to wait for the results of the mlr. because the organ must be used soon after re Time after transplantation, months moval from the cadaver. In that case, the microcytotoxicity test, which can be performed within a few hours, must be relied on HLA mismatches(no portance of MHC matching for acceptance of all Curve no grafts is confirmed by data gathered from recipients of kid ney transplants. The data in Figure 21-5 reveal that survival of kidney grafts depends primarily on donor-recipient match- 123456 1 or 2 3 or 4 000 ing of the hLa class ll antigens. Matching or mismatching of 1 or 2 1 or 2 the class I antigens has a lesser effect on graft survival unless there also is mismatching of the class II antigens. A two-year survival rate of 90% is seen for kidney transplants in which one or two class I HLA loci are mismatched, while trans- FIGURE 21-5 The effect of HLA class I and class ll antigen match planted kidneys with differences in the class II MHC have ing on survival of kidney grafts Mismatching of one or two class I only a 70% chance of lasting for this period. Those with (HLA-A or HLA-B)antigens has little effect on graft survival. A Single greater numbers of mismatches have a very low survival rate class ll difference(line 4) has the same effect as 3 or 4 differences in at one year after transplant. As described below, HLA match- class I antigens (line 3). When both class I and class ll antigens are ing is most important for kidney and bone- marrow trans- mismatched, rejection is accelerated. /Adapted from T Moen et al plants; liver and heart transplants may survive with greater 1980, N Engl J Med. 303: 850. Current understanding of the killer-inhibitory receptors KiR)on the nK cell(see Chapter 14)suggests that absence of a class I antigen recognized by the Kir molecules could lead to killing of the foreign cell. Rejection was observed in is usually less vigorous than that induced by major histo experimental bone-marrow transplants where the class I compatibility differences. Still, reaction to these minor tissue molecule recognized by the recipient NK-inhibitory receptor differences often results in graft rejection. For this reason, is absent on donor cells. The effects of such class I mismatch- successful transplantation even between HLA-identical indi g on solid organ grafts may be less marked. viduals requires some degree of immune suppression MHC identity of donor and host is not the sole factor determining tissue acceptance. When tissue is transplanted Cell-Mediated Graft Rejection Occurs between geneticaly difterent individua. even if their MHc in Two Stages because of differences at various minor histocompatibility Graft rejection is caused principally by a cell-mediated im loci As described in Chapter 10, the major histocompatibility mune response to alloantigens(primarily, MHC molecules) antigens are recognized directly by TH and Tc cells, a phe- expressed on cells of the graft. Both delayed-type hyperser nomenon termed alloreactivity. In contrast, minor histocom- tive and cell-mediated cytotoxicity reactions have been im- patibility antigens are recognized only when they are pre- plicated. The process of graft rejection can be divided into two sented in the context of self-MHC molecules. The tissue stages: (1)a sensitization phase, in which antigen-reactive rejection induced by minor histocompatibility differences lymphocytes of the recipient proliferate in response to allo-
donors and a recipient (Figure 21-4c). Lymphocytes from a potential donor that have been x-irradiated or treated with mitomycin C serve as the stimulator cells, and lymphocytes from the recipient serve as responder cells. Proliferation of the recipient T cells, which indicates T-cell activation, is measured by the uptake of [3 H]thymidine into cell DNA. The greater the class II MHC differences between the donor and recipient cells, the more [3 H]thymidine uptake will be observed in an MLR assay. Intense proliferation of the recipient lymphocytes indicates a poor prognosis for graft survival. The advantage of the MLR over microcytotoxicity typing is that it gives a better indication of the degree of TH-cell activation generated in response to the class II MHC antigens of the potential graft. The disadvantage of the MLR is that it takes several days to run the assay. If the potential donor is a cadaver, for example, it is not possible to wait for the results of the MLR, because the organ must be used soon after removal from the cadaver. In that case, the microcytotoxicity test, which can be performed within a few hours, must be relied on. The importance of MHC matching for acceptance of allografts is confirmed by data gathered from recipients of kidney transplants. The data in Figure 21-5 reveal that survival of kidney grafts depends primarily on donor-recipient matching of the HLA class II antigens. Matching or mismatching of the class I antigens has a lesser effect on graft survival unless there also is mismatching of the class II antigens. A two-year survival rate of 90% is seen for kidney transplants in which one or two class I HLA loci are mismatched, while transplanted kidneys with differences in the class II MHC have only a 70% chance of lasting for this period. Those with greater numbers of mismatches have a very low survival rate at one year after transplant. As described below, HLA matching is most important for kidney and bone-marrow transplants; liver and heart transplants may survive with greater mismatching. Current understanding of the killer-inhibitory receptors (KIR) on the NK cell (see Chapter 14) suggests that absence of a class I antigen recognized by the KIR molecules could lead to killing of the foreign cell. Rejection was observed in experimental bone-marrow transplants where the class I molecule recognized by the recipient NK-inhibitory receptor is absent on donor cells. The effects of such class I mismatching on solid organ grafts may be less marked. MHC identity of donor and host is not the sole factor determining tissue acceptance. When tissue is transplanted between genetically different individuals, even if their MHC antigens are identical, the transplanted tissue can be rejected because of differences at various minor histocompatibility loci. As described in Chapter 10, the major histocompatibility antigens are recognized directly by TH and TC cells, a phenomenon termed alloreactivity. In contrast, minor histocompatibility antigens are recognized only when they are presented in the context of self-MHC molecules. The tissue rejection induced by minor histocompatibility differences is usually less vigorous than that induced by major histocompatibility differences. Still, reaction to these minor tissue differences often results in graft rejection. For this reason, successful transplantation even between HLA-identical individuals requires some degree of immune suppression. Cell-Mediated Graft Rejection Occurs in Two Stages Graft rejection is caused principally by a cell-mediated immune response to alloantigens (primarily, MHC molecules) expressed on cells of the graft. Both delayed-type hypersensitive and cell-mediated cytotoxicity reactions have been implicated. The process of graft rejection can be divided into two stages: (1) a sensitization phase, in which antigen-reactive lymphocytes of the recipient proliferate in response to allo- 486 PART IV The Immune System in Health and Disease Cumulative graft survival, % Time after transplantation, months 50 100 3 6 12 24 0 6 5 3 1 4 2 HLA mismatches (no.) Curve no. Class I Class II 1 2 3 4 5 6 0 1 or 2 3 or 4 0 1 or 2 3 or 4 0 0 0 1 or 2 1 or 2 1 or 2 FIGURE 21-5 The effect of HLA class I and class II antigen matching on survival of kidney grafts. Mismatching of one or two class I (HLA-A or HLA-B) antigens has little effect on graft survival. A single class II difference (line 4) has the same effect as 3 or 4 differences in class I antigens (line 3). When both class I and class II antigens are mismatched, rejection is accelerated. [Adapted from T. Moen et al., 1980, N. Engl. J. Med. 303:850.]
Transplantation Immunology CHAPTER 21 antigens on the graft, and( 2)an effector stage, in which im- Langerhans cells and endothelial cells lining the blood ves- mune destruction of the graft takes place sels. Both of these cell types express class I and class II MHC antigens. SENSITIZATION STAGE Recognition of the alloantigens expressed on the cells of During the sensitization phase, CD4+ and CD8+ T cells rec- a graft induces vigorous T-cell proliferation in the host. ognize alloantigens expressed on cells of the foreign graft This proliferation can be demonstrated in vitro in a and proliferate in response. Both major and minor histo- lymphocyte reaction(see Figure 21-4c). Both dendrit compatibility alloantigens can be recognized. In general, the and vascular endothelial cells from an allogeneic graft response to minor histocompatibility antigens is weak, al- host T-cell proliferation. The major proliferating cell is the though the combined response to several minor differences CD4* Tcell, which recognizes class Il alloantigens ectly can sometimes be quite vigorous. The response to major histo- alloantigen peptides presented by host antigen-presenting compatibility antigens involves recognition of both the donor cells. This amplified population of activated TH cells is MHC molecule and an associated peptide ligand in the cleft of thought to play a central role in inducing the various effector the MHc molecule. The peptides present in the groove of mechanisms of allograft rejection allogeneic class I MHC molecules are derived from proteins synthesized within the allogeneic cell. The peptides present EFFECTOR STAGE in the groove of allogeneic class II MHC molecules are gener- A variety of effector mechanisms participate in allograft re- ally proteins taken up and processed through the endocytic jection(Figure 21-6). The most common are cell-mediated athway of the allogeneic antigen-presenting cell reactions involving delayed-type hypersensitivity and ctl A host TH cell becomes activated when it interacts with an mediated cytotoxicity: less common mechanisms are antibody antigen-presenting cell(APC)that both expresses an appro- plus-complement lysis and destruction by antibody-dependent priate antigenic ligand-MHC molecule complex and pro cell-mediated cytotoxicity (ADCC). The hallmark of graft ides the requisite co-stimulatory signal. Depending on the rejection involving cell-mediated reactions is an influx of tissue, different populations of cells within a graft may func- T cells and macrophages into the graft. Histologically, the in- tion as APCs. Because dendritic cells are found in most tis- filtration in many cases resembles that seen during a delayed- sues and because they constitutively express high levels of type hypersensitive response, in which cytokines produced class II MHC molecules, dendritic cells generally serve as the by DTh cells promote macrophage infiltration(see Figure major APC in grafts. APCs of host origin can also migrate 14-15). Recognition of foreign class I alloantigens on the into a graft and endocytose the foreign alloantigens(both graft by host CD8 cells can lead to CTL-mediated killing(see major and minor histocompatibility molecules) and present Figure 14-4). In some cases, CD4 Tcells that function as class them as processed peptides together with self-MHC mole- II MHC-restricted cytotoxic cells mediate graft rejection. In each of these effector mechanisms, cytokines secreted In some organ and tissue grafts( e. g, grafts of kidney, thy. by TH cells play a central role(see Figure 21-6). For example, mus, and pancreatic islets), a population of donor APCs IL-2, IFN-Y, and TNF-s have each been shown to be impor- called passenger leukocytes has been shown to migrate from tant mediators of graft rejection. IL-2 promotes T-cell pro- the graft to the regional lymph nodes. These passenger leuko- liferation and generally is necessary for the generation of cytes are dendritic cells, which express high levels of class II effector CTls(see Figure 14-1). IFN-y is central to the devel MHC molecules( together with normal levels of class I MHc opment of a DTH response, promoting the influx of macro- molecules)and are widespread in mammalian tissues, with phages into the graft and their subsequent activation into the chief exception of the brain. Because passenger leuko- more destructive cells. TNF-B has been shown to have a di- cytes express the allogeneic MHC antigens of the donor graft, rect cytotoxic effect on the cells of a graft. A number of cyto- they are recognized as foreign and therefore can stimulate kines promote graft rejection by inducing expression of class immune activation of T lymphocytes in the lymph node. In Ior class II MHC molecules on graft cells. The interferons(a, some experimental situations, the passenger cells have been B, and Y), TNF-o and TNF-B all increase class I MHC ex- shown to induce tolerance to their surface antigens by dele- pression, and IFN-y increases class II MHC expression as tion of thymic T-cell populations with receptors specific for well. During a rejection episode the levels of these cytokines them. Consistent with the notion that exposure to donor increase, inducing a variety of cell types within the graft to cells can induce tolerance are data showing that blood tran- express class I or class II MHC molecules. In rat cardiac allo- fusions from the donor prior to tra grafts, for example, dendritic cells are initially the only cells ceptance of the graft that express class II MHC molecules. However, as an allograft Passenger leukocytes are not the only cells involved in im- reaction begins, localized production of IFN-y in the graft mune stimulation. For example, they do not seem to play any induces vascular endothelial cells and myocytes to express role in skin grafts. Other cell types that have been implicated class II MHC molecules as well, making these cells targets for in alloantigen presentation to the immune system include CTl attack
antigens on the graft, and (2) an effector stage, in which immune destruction of the graft takes place. SENSITIZATION STAGE During the sensitization phase, CD4+ and CD8+ T cells recognize alloantigens expressed on cells of the foreign graft and proliferate in response. Both major and minor histocompatibility alloantigens can be recognized. In general, the response to minor histocompatibility antigens is weak, although the combined response to several minor differences can sometimes be quite vigorous. The response to major histocompatibility antigens involves recognition of both the donor MHC molecule and an associated peptide ligand in the cleft of the MHC molecule. The peptides present in the groove of allogeneic class I MHC molecules are derived from proteins synthesized within the allogeneic cell. The peptides present in the groove of allogeneic class II MHC molecules are generally proteins taken up and processed through the endocytic pathway of the allogeneic antigen-presenting cell. A host TH cell becomes activated when it interacts with an antigen-presenting cell (APC) that both expresses an appropriate antigenic ligand–MHC molecule complex and provides the requisite co-stimulatory signal. Depending on the tissue, different populations of cells within a graft may function as APCs. Because dendritic cells are found in most tissues and because they constitutively express high levels of class II MHC molecules, dendritic cells generally serve as the major APC in grafts. APCs of host origin can also migrate into a graft and endocytose the foreign alloantigens (both major and minor histocompatibility molecules) and present them as processed peptides together with self-MHC molecules. In some organ and tissue grafts (e.g., grafts of kidney, thymus, and pancreatic islets), a population of donor APCs called passenger leukocytes has been shown to migrate from the graft to the regional lymph nodes. These passenger leukocytes are dendritic cells, which express high levels of class II MHC molecules (together with normal levels of class I MHC molecules) and are widespread in mammalian tissues, with the chief exception of the brain. Because passenger leukocytes express the allogeneic MHC antigens of the donor graft, they are recognized as foreign and therefore can stimulate immune activation of T lymphocytes in the lymph node. In some experimental situations, the passenger cells have been shown to induce tolerance to their surface antigens by deletion of thymic T-cell populations with receptors specific for them. Consistent with the notion that exposure to donor cells can induce tolerance are data showing that blood tranfusions from the donor prior to transplantation can aid acceptance of the graft. Passenger leukocytes are not the only cells involved in immune stimulation. For example, they do not seem to play any role in skin grafts. Other cell types that have been implicated in alloantigen presentation to the immune system include Langerhans cells and endothelial cells lining the blood vessels. Both of these cell types express class I and class II MHC antigens. Recognition of the alloantigens expressed on the cells of a graft induces vigorous T-cell proliferation in the host. This proliferation can be demonstrated in vitro in a mixedlymphocyte reaction (see Figure 21-4c). Both dendritic cells and vascular endothelial cells from an allogeneic graft induce host T-cell proliferation. The major proliferating cell is the CD4+ T cell, which recognizes class II alloantigens directly or alloantigen peptides presented by host antigen-presenting cells. This amplified population of activated TH cells is thought to play a central role in inducing the various effector mechanisms of allograft rejection. EFFECTOR STAGE A variety of effector mechanisms participate in allograft rejection (Figure 21-6). The most common are cell-mediated reactions involving delayed-type hypersensitivity and CTLmediated cytotoxicity; less common mechanisms are antibodyplus-complement lysis and destruction by antibody-dependent cell-mediated cytotoxicity (ADCC). The hallmark of graft rejection involving cell-mediated reactions is an influx of T cells and macrophages into the graft. Histologically, the infiltration in many cases resembles that seen during a delayedtype hypersensitive response, in which cytokines produced by TDTH cells promote macrophage infiltration (see Figure 14-15). Recognition of foreign class I alloantigens on the graft by host CD8+ cells can lead to CTL-mediated killing (see Figure 14-4). In some cases, CD4+ T cells that function as class II MHC–restricted cytotoxic cells mediate graft rejection. In each of these effector mechanisms, cytokines secreted by TH cells play a central role (see Figure 21-6). For example, IL-2, IFN-, and TNF- have each been shown to be important mediators of graft rejection. IL-2 promotes T-cell proliferation and generally is necessary for the generation of effector CTLs (see Figure 14-1). IFN- is central to the development of a DTH response, promoting the influx of macrophages into the graft and their subsequent activation into more destructive cells. TNF- has been shown to have a direct cytotoxic effect on the cells of a graft. A number of cytokines promote graft rejection by inducing expression of class I or class II MHC molecules on graft cells. The interferons (, , and ), TNF-, and TNF- all increase class I MHC expression, and IFN- increases class II MHC expression as well. During a rejection episode, the levels of these cytokines increase, inducing a variety of cell types within the graft to express class I or class II MHC molecules. In rat cardiac allografts, for example, dendritic cells are initially the only cells that express class II MHC molecules. However, as an allograft reaction begins, localized production of IFN- in the graft induces vascular endothelial cells and myocytes to express class II MHC molecules as well, making these cells targets for CTL attack. Transplantation Immunology CHAPTER 21 487