《免疫学》（英文版） Chapter 21 Transplantation Immunology.pdf_大学文库

Transplantation chapter 21 Immunology NTATION, AS THE TERM IS USED IN immunology, refers to the act of transferring cells, tissues, or organs from one site to another. The desire to accomplish transplants stems from the realization that many diseases can be cured by implantation of a healthy organ, tissue, or cells(a graft)from one individual (the donor)to another in need of the transplant(the recipient or host). The development of surgical techniques that allow the facile reimplantation of organs has removed one barrier to successful transplantation, but others remain. One is the lack of organs for transplantation. Although a supply of organs is provided by accident victims and, in some cases, living donors, there are more patients in need of transplants than there are organs available. The seriousness of the donor- organ shortage is reflected in the fact that, as of November Transplantations Routinely Used in Clinical Practice 2000, an estimated 73,000 patients in the United States were on the waiting list for an organ transplantation. The major ity of those on the list(70%)require a kidney; at present, m Immunologic Basis of Graft Rejection the waiting period for this organ averages over 800 days Clinical Manifestations of Graft Rejection While the lack of organs for transplantation is a serious is- sue, the most formidable barrier to making transplantation essive th a routine medical treatment is the immune system. The a Specific Immunosuppressive Therapy immune system has evolved elaborate and effective mecha nisms to protect the organism from attack by foreign agents, a Immune Tolerance to Allografts and these same mechanisms cause rejection of grafts from Clinical Transplantation anyone who is not genetically identical to the recipient. Alexis Carrel reported the first systematic study of trans lantation in 1908; he interchanged both kidneys in a series of nine cats. Some of those receiving kidneys from other cats maintained urinary output for up to 25 days. Although all he cats eventually died, the experiment established that a variety of immunosuppressive agents can aid in the ansplanted organ could carry out its normal function in survival of the transplants, including drugs and specific anti- the recipient. The first human kidney transplant, attempted bodies developed to diminish the immunologic attack on in 1935 by a Russian surgeon, failed because there was a mis- grafts, but the majority of these agents have an overall match of blood types between donor and recipient. This immunosuppressive effect, and their long-term use is delete incompatibility caused almost immediate rejection of the rious. New methods of inducing specific tolerance to the dney, and the patient died without establishing renal func- graft without suppressing other immune responses are being tion. The rapid immune response experienced here, termed developed and promise longer survival of transplants with- hyperacute rejection, is mediated by antibodies and will be out compromise of host immunity. This chapter describes described in this chapter. The first successful human kidney the mechanisms underlying graft rejection, various proce transplant, which was between identical twins, was accom- dures that are used to prolong graft survival, and a summary plished in Boston in 1954. Today, kidney, pancreas, heart, of the current status of transplantation as a clinical tool. a lung, liver, bone-marrow, and cornea transplantations are Clinical Focus section examines the use of organs from performed among nonidentical individuals with ever- human species(xenotransplants)to circumvent the shor increasing frequency and success. of available for patients in need of the

■ Immunologic Basis of Graft Rejection ■ Clinical Manifestations of Graft Rejection ■ General Immunosuppressive Therapy ■ Specific Immunosuppressive Therapy ■ Immune Tolerance to Allografts ■ Clinical Transplantation Transplantations Routinely Used in Clinical Practice Transplantation Immunology T,       immunology, refers to the act of transferring cells, tissues, or organs from one site to another. The desire to accomplish transplants stems from the realization that many diseases can be cured by implantation of a healthy organ, tissue, or cells (a graft) from one individual (the donor) to another in need of the transplant (the recipient or host). The development of surgical techniques that allow the facile reimplantation of organs has removed one barrier to successful transplantation, but others remain. One is the lack of organs for transplantation. Although a supply of organs is provided by accident victims and, in some cases, living donors, there are more patients in need of transplants than there are organs available. The seriousness of the donororgan shortage is reflected in the fact that, as of November 2000, an estimated 73,000 patients in the United States were on the waiting list for an organ transplantation. The majority of those on the list (~70%) require a kidney; at present, the waiting period for this organ averages over 800 days. While the lack of organs for transplantation is a serious issue, the most formidable barrier to making transplantation a routine medical treatment is the immune system. The immune system has evolved elaborate and effective mechanisms to protect the organism from attack by foreign agents, and these same mechanisms cause rejection of grafts from anyone who is not genetically identical to the recipient. Alexis Carrel reported the first systematic study of transplantation in 1908; he interchanged both kidneys in a series of nine cats. Some of those receiving kidneys from other cats maintained urinary output for up to 25 days. Although all the cats eventually died, the experiment established that a transplanted organ could carry out its normal function in the recipient. The first human kidney transplant, attempted in 1935 by a Russian surgeon, failed because there was a mismatch of blood types between donor and recipient. This incompatibility caused almost immediate rejection of the kidney, and the patient died without establishing renal function. The rapid immune response experienced here, termed hyperacute rejection, is mediated by antibodies and will be described in this chapter. The first successful human kidney transplant, which was between identical twins, was accomplished in Boston in 1954. Today, kidney, pancreas, heart, lung, liver, bone-marrow, and cornea transplantations are performed among nonidentical individuals with everincreasing frequency and success. A variety of immunosuppressive agents can aid in the survival of the transplants, including drugs and specific antibodies developed to diminish the immunologic attack on grafts, but the majority of these agents have an overall immunosuppressive effect, and their long-term use is deleterious. New methods of inducing specific tolerance to the graft without suppressing other immune responses are being developed and promise longer survival of transplants without compromise of host immunity. This chapter describes the mechanisms underlying graft rejection, various procedures that are used to prolong graft survival, and a summary of the current status of transplantation as a clinical tool. A Clinical Focus section examines the use of organs from nonhuman species (xenotransplants) to circumvent the shortage of organs available for patients in need of them. chapter 21

482 PART IV The Immune System in Health and Disease mouse. In this case, a graft-rejection reaction develops more Immunologic Basis of Graft Rejection quickly, with complete rejection occurring within 5-6 days this secondary response is designated second-set rejection The degree of immune response to a graft varies with the (Figure 21-1c). The specificity of second-set rejection can be type of graft. The following terms are used to denote differ- demonstrated by grafting an unrelated strain-C graft at the ent t same time as the second strain-B graft. Rejection of the Autograft is self-tissue transferred from one body site to strain-C graft proceeds according to first-set rejection kinet to a burned area in burn patients and use of healthy second-set fashion blood vessels to replace blocked coronary arteries are examples of frequently used autografts. T Cells Play a Key Role in Allograft Rejection Isograft is tissue transferred between genetically identical In the early 1950s, Avrion Mitchison showed in adoptive- individuals In inbred strains of mice, an isograft can be transfer experiments that lymphocytes, but not serum anti- performed from one mouse to another syngeneic mouse. body, could transfer allograft immunity. Later studies im- In humans, an isograft can be performed between licated t cells in allograft rejection. For example, nude genetically identical (monozygotic)twins. mice, which lack a thymus and consequently lack functional T cells, were found to be incapable of allograft rejection; Allograft is tissue transferred between genetically different members of the same species. In mice, an indeed, these mice even accept xenografts. In other studies, T cells derived from an allograft-primed mouse were shown from one strain to another. In humans, organ grafts san to transfer second-set allograft rejection to an unprimed one individual to another are allografts unless the donor syngeneic recipient, as long as that recipient was grafted with ne same allogeneic tissue( Figure 21-2) and recipient are identical twins. Analysis of the T-cell subpopulations involved in allograft Xenograft is tissue transferred between different species rejection has implicated both CD4* and CD8* populations (e.g, the graft of a baboon heart into a human). Because In one study, mice were injected with monoclonal antibodies of significant shortages in donated organs, raising to deplete one or both types of T cells and then the rate of animals for the specific purpose of serving as organ graft rejection was measured As shown in Figure 21-3, re- donors for humans is under serious consideration moval of the CD8 population alone had no effect on graft survival, and the graft was rejected at the same rate as in con- Autografts and isografts are usually accepted, owing to the trol mice(15 days). Removal of the CD4* T-cell population genetic identity between graft and host(Figure 21-1a). Be- alone prolonged graft survival from 15 days to 30 days. How cause an allograft is genetically dissimilar to the host, it is ever, removal of both the CD4t and the CD8 T cells resulted often recognized as foreign by the immune system and is re- in long-term survival(up to 60 days)of the allografts. This jected. Obviously, xenografts exhibit the greatest genetic dis- study indicated that both CD4 and CD8 T-cells partici- parity and therefore engender a vigorous graft rejection. pated in rejection and that the collaboration of both subpop ulations resulted in more pronounced graft rejection. Allograft Rejection Displays Specificity and Memory Similar Antigenic Profiles Foster The rate of allograft rejection varies according to the tissue Allograft Acceptance involved In general, skin grafts are rejected faster than other Tissues that are antigenically similar are said to be histocom tissues such as kidney or heart. Despite these time differ- patible; such tissues do not induce an immunologic response ences, the immune response culminating in graft rejection that leads to tissue rejection. Tissues that display significant always displays the attributes of specificity and memory. If an antigenic differences are histoincompatible and induce inbred mouse of strain A is grafted with skin from strain B, immune response that leads to tissue rejection. The various primary graft rejection, known as first-set rejection, occurs antigens that determine histocompatibility are encoded by (Figure 21-1b ). The skin first becomes revascularized between more than 40 different loci, but the loci responsible for the days 3 and 7: as the reaction develops, the vascularized trans- most vigorous allograft-rejection reactions are located with plant becomes infiltrated with lymphocytes, monocytes, neu- in the major histocompatibility complex(MHC). The orga- trophils, and other inflammatory cells. There is decreased vas- nization of the MHC-called the H-2 complex in mice and cularization of the transplanted tissue by 7-10 days, visible the HLA complex in humans-was described in Chapter 7 necrosis by 10 days, and complete rejection by 12-14 days. (see Figure 7-1). Because the MHC loci are closely linked, Immunologic memory is demonstrated when a second they are usually inherited as a complete set, called a haplo strain-B graft is transferred to a previously grafted strain-a type, from each parent

Immunologic Basis of Graft Rejection The degree of immune response to a graft varies with the type of graft. The following terms are used to denote different types of transplants: ■ Autograft is self-tissue transferred from one body site to another in the same individual. Transferring healthy skin to a burned area in burn patients and use of healthy blood vessels to replace blocked coronary arteries are examples of frequently used autografts. ■ Isograft is tissue transferred between genetically identical individuals. In inbred strains of mice, an isograft can be performed from one mouse to another syngeneic mouse. In humans, an isograft can be performed between genetically identical (monozygotic) twins. ■ Allograft is tissue transferred between genetically different members of the same species. In mice, an allograft is performed by transferring tissue or an organ from one strain to another. In humans, organ grafts from one individual to another are allografts unless the donor and recipient are identical twins. ■ Xenograft is tissue transferred between different species (e.g., the graft of a baboon heart into a human). Because of significant shortages in donated organs, raising animals for the specific purpose of serving as organ donors for humans is under serious consideration. Autografts and isografts are usually accepted, owing to the genetic identity between graft and host (Figure 21-1a). Because an allograft is genetically dissimilar to the host, it is often recognized as foreign by the immune system and is rejected. Obviously, xenografts exhibit the greatest genetic disparity and therefore engender a vigorous graft rejection. Allograft Rejection Displays Specificity and Memory The rate of allograft rejection varies according to the tissue involved. In general, skin grafts are rejected faster than other tissues such as kidney or heart. Despite these time differences, the immune response culminating in graft rejection always displays the attributes of specificity and memory. If an inbred mouse of strain A is grafted with skin from strain B, primary graft rejection, known as first-set rejection, occurs (Figure 21-1b). The skin first becomes revascularized between days 3 and 7; as the reaction develops, the vascularized transplant becomes infiltrated with lymphocytes, monocytes, neutrophils, and other inflammatory cells. There is decreased vascularization of the transplanted tissue by 7–10 days, visible necrosis by 10 days, and complete rejection by 12–14 days. Immunologic memory is demonstrated when a second strain-B graft is transferred to a previously grafted strain-A mouse. In this case, a graft-rejection reaction develops more quickly, with complete rejection occurring within 5–6 days; this secondary response is designated second-set rejection (Figure 21-1c). The specificity of second-set rejection can be demonstrated by grafting an unrelated strain-C graft at the same time as the second strain-B graft. Rejection of the strain-C graft proceeds according to first-set rejection kinetics, whereas the strain-B graft is rejected in an accelerated second-set fashion. T Cells Play a Key Role in Allograft Rejection In the early 1950s, Avrion Mitchison showed in adoptivetransfer experiments that lymphocytes, but not serum antibody, could transfer allograft immunity. Later studies implicated T cells in allograft rejection. For example, nude mice, which lack a thymus and consequently lack functional T cells, were found to be incapable of allograft rejection; indeed, these mice even accept xenografts. In other studies, T cells derived from an allograft-primed mouse were shown to transfer second-set allograft rejection to an unprimed syngeneic recipient, as long as that recipient was grafted with the same allogeneic tissue (Figure 21-2). Analysis of the T-cell subpopulations involved in allograft rejection has implicated both CD4+ and CD8+ populations. In one study, mice were injected with monoclonal antibodies to deplete one or both types of T cells and then the rate of graft rejection was measured. As shown in Figure 21-3, removal of the CD8+ population alone had no effect on graft survival, and the graft was rejected at the same rate as in control mice (15 days). Removal of the CD4+ T-cell population alone prolonged graft survival from 15 days to 30 days. However, removal of both the CD4+ and the CD8+ T cells resulted in long-term survival (up to 60 days) of the allografts. This study indicated that both CD4+ and CD8+ T-cells participated in rejection and that the collaboration of both subpopulations resulted in more pronounced graft rejection. Similar Antigenic Profiles Foster Allograft Acceptance Tissues that are antigenically similar are said to be histocompatible;such tissues do not induce an immunologic response that leads to tissue rejection. Tissues that display significant antigenic differences are histoincompatible and induce an immune response that leads to tissue rejection. The various antigens that determine histocompatibility are encoded by more than 40 different loci, but the loci responsible for the most vigorous allograft-rejection reactions are located within the major histocompatibility complex (MHC). The organization of the MHC—called the H-2 complex in mice and the HLA complex in humans—was described in Chapter 7 (see Figure 7-1). Because the MHC loci are closely linked, they are usually inherited as a complete set, called a haplotype, from each parent. 482 PART IV The Immune System in Health and Disease

PART IV The Immune System in Health and Disease First skin graft, First-set rejection Second-set rejection Necrosis Time 6 days Naive strain=B mouse Spleenic T cells Second-setrejection strain A k Naive strain= B mouse FIGURE 21.2 Experimental demonstration that T cells can trans- ent mounts a second-set rejection to an initial allograft from the orig- fer allograft rejection. When T cells derived from an allograft-primed inal allogeneic strain mouse are transferred to an unprimed syngeneic mouse, the recipi gives a high probability of heterozygosity at most loci In mat- Graft Donors and Recipients Are Typed ings between members of an outbred species, there is only a for RBC and MHC Antigens 25% chance that any two offspring will inherit identical MHC haplotypes(see Figure 7-2c), unless the parents share one or Since differences in blood group and major histocompatibility more haplotypes. Therefore, for purposes of organ or bone- antigens are responsible for the most intense graft-rejection marrow grafts, it can be assumed that there is a 25% chance of reactions, various tissue-typing procedures to identify these ity within the MHc bet With parent-to- antigens have been developed to screen potential donor and child grafts, the donor and recipient will always have one hap- recipient cells. Initially, donor and recipient are screened for lotype in common but are nearly always mismatched for the ABO blood-group compatibility. The blood-group antigens haplotype inherited from the other parent. are expressed on RBCs, epithelial cells, and endothelial cells Antibodies produced in the recipient to any of these antigens that are present on transplanted tissue will induce antibody- mediated complement lysis of the incompatible donor cells HLa typing of potential donors and a recipient can be accomplished with a microcytotoxicity test(Figure 21-4a, b) In this test, white blood cells from the potential donors and 50FAnti- recipient are distributed into a series of wells on a microtiter ntrol Anti-CD4 Anti-CD4 CD8 and Anti-CD8 plate, and then antibodies specific for various class I and class II MHC alleles are added to different wells. After incubation, complement is added to the wells, and cytotoxicity is by the uptake or exclusion of various dyes(e.g, trypan blue Time after grafting, days or eosin y) by the cells. If the white blood cells express the MHC allele for which a particular monoclonal antibody is GURE21-3 The role of CD4*and CD8 T cells in allograft rejec- specific, then the cells will be lysed upon addition of comple tion is demonstrated by the curves showing survival times of skin ment, and these dead cells will take up a dye such as trypa grafts between mice mismatched at the MHC. Animals in which the blue. hla typing based on antibody-mediated microcyto- CD8 T cells were removed by treatment with an anti-CD8 mono- toxicity can thus indicate the presence or absence of various clonal antibody (red) showed little difference from untreated control MHC alleles. mice(black). Treatment with monoclonal anti-CD4 (blue)improved Even when a fully HLa-compatible donor is not available graft survival significantly, and treatment with both anti-CD4 and transplantation may be successful. In this situation, a one-way anti-CD8 antibody prolonged graft survival most dramatically mixed-lymphocyte reaction(MLR)can be used to quantif (green). /Adapted from S P. Cobbold et al., 1986, Nature 323: 165. the degree of class II MHC compatibility between potential

gives a high probability of heterozygosity at most loci. In matings between members of an outbred species, there is only a 25% chance that any two offspring will inherit identical MHC haplotypes (see Figure 7-2c), unless the parents share one or more haplotypes. Therefore, for purposes of organ or bonemarrow grafts, it can be assumed that there is a 25% chance of identity within the MHC between siblings. With parent-tochild grafts, the donor and recipient will always have one haplotype in common but are nearly always mismatched for the haplotype inherited from the other parent. Graft Donors and Recipients Are Typed for RBC and MHC Antigens Since differences in blood group and major histocompatibility antigens are responsible for the most intense graft-rejection reactions, various tissue-typing procedures to identify these antigens have been developed to screen potential donor and recipient cells. Initially, donor and recipient are screened for ABO blood-group compatibility. The blood-group antigens are expressed on RBCs, epithelial cells, and endothelial cells. Antibodies produced in the recipient to any of these antigens that are present on transplanted tissue will induce antibodymediated complement lysis of the incompatible donor cells. HLA typing of potential donors and a recipient can be accomplished with a microcytotoxicity test (Figure 21-4a, b). In this test, white blood cells from the potential donors and recipient are distributed into a series of wells on a microtiter plate, and then antibodies specific for various class I and class II MHC alleles are added to different wells. After incubation, complement is added to the wells, and cytotoxicity is assessed by the uptake or exclusion of various dyes (e.g., trypan blue or eosin Y) by the cells. If the white blood cells express the MHC allele for which a particular monoclonal antibody is specific, then the cells will be lysed upon addition of complement, and these dead cells will take up a dye such as trypan blue. HLA typing based on antibody-mediated microcytotoxicity can thus indicate the presence or absence of various MHC alleles. Even when a fully HLA-compatible donor is not available, transplantation may be successful. In this situation, a one-way mixed-lymphocyte reaction (MLR) can be used to quantify the degree of class II MHC compatibility between potential 484 PART IV The Immune System in Health and Disease First skin graft, strain A Second skin graft, strain A Naive strain = B mouse First-set rejection Second-set rejection 14 days Time 6 days Naive strain = B mouse Necrosis First skin graft, strain A Necrosis Second-set rejection 6 days Necrosis Spleenic T cells FIGURE 21-2 Experimental demonstration that T cells can transfer allograft rejection. When T cells derived from an allograft-primed mouse are transferred to an unprimed syngeneic mouse, the recipient mounts a second-set rejection to an initial allograft from the original allogeneic strain. Surviving grafts, % Time after grafting, days 50 100 15 30 60 0 Anti– Control Anti–CD4 CD8 Anti–CD4 and Anti–CD8 FIGURE 21-3 The role of CD4+ and CD8+ T cells in allograft rejection is demonstrated by the curves showing survival times of skin grafts between mice mismatched at the MHC. Animals in which the CD8+ T cells were removed by treatment with an anti-CD8 monoclonal antibody (red) showed little difference from untreated control mice (black). Treatment with monoclonal anti-CD4 (blue) improved graft survival significantly, and treatment with both anti-CD4 and anti-CD8 antibody prolonged graft survival most dramatically (green). [Adapted from S. P. Cobbold et al., 1986, Nature 323:165.]

Transplantation Immunology CHAPTER 21 485 Donor cell Recipient cell HLA–A allele 2 HLA–A allele 1 Antibody to HLA–A allele 2 Complement Dye (trypan blue or eosin Y) Cells become leaky No lysis Dye taken up Dye excluded (a) 1 Antibody to different HLA-A antigens Recipient Donor 1 Donor 2 23456789 (b) (c) Irradiation Donor cells Allele A Recipient cells lacking class II MHC of donor Recipient cells sharing class II MHC of donor Allele B Allele A No reaction Activation and proliferation of recipient cells [3H]thymidine Incorporation of of radioactivity into cell nuclear DNA FIGURE 21-4 Typing procedures for HLA antigens. (a, b) HLA typing by microcytotoxicity. (a) White blood cells from potential donors and the recipient are added to separate wells of a microtiter plate. The example depicts the reaction of donor and recipient cells with a single antibody directed against an HLA-A antigen. The reaction sequence shows that if the antigen is present on the lymphocytes, addition of complement will cause them to become porous and unable to exclude the added dye. (b) Because cells express numerous HLA antigens, they are tested separately with a battery of antibodies specific for various HLA-A antigens. Here, donor 1 shares HLA-A antigens recognized by antisera in wells 1 and 7 with the recipient, whereas donor 2 has none of HLA-A antigens in common with the recipient. (c) Mixed lymphocyte reaction to determine identity of class II HLA antigens between a potential donor and recipient. Lymphocytes from the donor are irradiated or treated with mitomycin C to prevent cell division and then added to cells from the recipient. If the class II antigens on the two cell populations are different, the recipient cells will divide rapidly and take up large quantities of radioactive nucleotides into the newly synthesized nuclear DNA. The amount of radioactive nucleotide uptake is roughly proportionate to the MHC class II differences between the donor and recipient lymphocytes

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

Transplantation Immunology CHAPTER 21 2 Antibodies bind to antigens of renal capillaries and activate complement(C) Capillary antibodies are carried to kidney graft AAAA Kid )Which es lueyang O Neutrophil lytic enzymes destroy endothelial cells; platelets adhere to injured tissue, causing vascular blockage Platelets FIGURE 21-7 Steps in the hyperacute rejection of a kidney graft Several mechanisms can account for the presence of pre- termed accelerated rejection caused by antibodies that are existing antibodies specific for allogeneic MHC antigens. Re- produced immediately after transplantation. cipients of repeated blood transfusions sometimes develop significant levels of antibodies to mhc antigens expressed Acute Rejection Is Mediated of these mhc antigens are the same as those on a subsequent by T-Cel Responses graft, then the antibodies can react with the graft, inducing a Cell-mediated allograft rejection manifests as an acute rejec hyperacute rejection reaction With repeated pregnancies, wo- tion of the graft beginning about 10 days after transplanta men are exposed to the paternal alloantigens of the fetus and tion(see Figure 21-1b). Histopathologic examination reveals may develop antibodies to these antigens. Finally, individuals a massive infiltration of macrophages and lymphocytes at the who have had a previous graft sometimes have high levels of site of tissue destruction, suggestive of TH-cell activation and antibodies to the allogeneic MHC antigens of that graft. proliferation. Acute graft rejection is effected by the mecha In some cases, the preexisting antibodies participating in nisms described previously (see Figure 21-6) hyperacute graft rejection may be specific for blood-group antigens in the graft. If tissue typing and ABO blood-group Chronic Rejection Occurs Months typing are performed prior to transplantation, these preex. or Years Post-Transplant in hyperacute rejection can be avoided. Xenotransplants are Chronic rejection reactions develop months or years after acute often rejected in a hyperacute manner because of antibodies rejection reactions have subsided. The mechanisms of chronic to cellular antigens of the donor species that are not present rejection include both humoral and cell-mediated responses by in the recipient species. Such an antigen is discussed in the the recipient. While the use of immunosuppressive drugs and Clinical Focus section of this chapter. the application of tissue-typing methods to obtain optimum et In addition to the hyperacute rejection mediated by pre- match of donor and recipient have dramatically increased sur- ting antibodies, there is a less frequent form of rejection vival of allografts during the first years after engraftment, little

Several mechanisms can account for the presence of preexisting antibodies specific for allogeneic MHC antigens. Recipients of repeated blood transfusions sometimes develop significant levels of antibodies to MHC antigens expressed on white blood cells present in the transfused blood. If some of these MHC antigens are the same as those on a subsequent graft, then the antibodies can react with the graft, inducing a hyperacute rejection reaction. With repeated pregnancies, women are exposed to the paternal alloantigens of the fetus and may develop antibodies to these antigens. Finally, individuals who have had a previous graft sometimes have high levels of antibodies to the allogeneic MHC antigens of that graft. In some cases, the preexisting antibodies participating in hyperacute graft rejection may be specific for blood-group antigens in the graft. If tissue typing and ABO blood-group typing are performed prior to transplantation, these preexisting antibodies can be detected and grafts that would result in hyperacute rejection can be avoided. Xenotransplants are often rejected in a hyperacute manner because of antibodies to cellular antigens of the donor species that are not present in the recipient species. Such an antigen is discussed in the Clinical Focus section of this chapter. In addition to the hyperacute rejection mediated by preexisting antibodies, there is a less frequent form of rejection termed accelerated rejection caused by antibodies that are produced immediately after transplantation. Acute Rejection Is Mediated by T-Cell Responses Cell-mediated allograft rejection manifests as an acute rejection of the graft beginning about 10 days after transplantation (see Figure 21-1b). Histopathologic examination reveals a massive infiltration of macrophages and lymphocytes at the site of tissue destruction, suggestive of TH-cell activation and proliferation. Acute graft rejection is effected by the mechanisms described previously (see Figure 21-6). Chronic Rejection Occurs Months or Years Post-Transplant Chronic rejection reactions develop months or years after acute rejection reactions have subsided. The mechanisms of chronic rejection include both humoral and cell-mediated responses by the recipient. While the use of immunosuppressive drugs and the application of tissue-typing methods to obtain optimum match of donor and recipient have dramatically increased survival of allografts during the first years after engraftment, little Transplantation Immunology CHAPTER 21 489 Antibodies bind to antigens of renal capillaries and activate complement (C–) 2 Pre-existing host antibodies are carried to kidney graft 1 C C C Capillary endothelial walls Kidney graft Neutrophil lytic enzymes destroy endothelial cells; platelets adhere to injured tissue, causing vascular blockage 4 Platelets Complement split products attract neutrophils, which release lytic enzymes 3 Enzymes FIGURE 21-7 Steps in the hyperacute rejection of a kidney graft

490 PART I The Immune System in Health and Disease CLINICAL FOCUS cells may be blunted if human DAF is pre- sent on the targeted cell to dampen the Is There a clinical future complement reaction. The lack of human for Xenotransplantation DAF is remedied by producing transgenic pigs that express this protein. Addition of human complement regulators to the pig represents a universal solution, in that any nless organ (and those of most mammals other than cell that might become a in the humans and the highest nonhuman pri- transplant will resist complement lysi donations increase drastically, most of mates)of a disaccharide antigen( galacto An additional concern is that pig en the 72,000 U.S. patients on the waiting 1.3-a-galactose)that is not present on dogenous retroviruses will be introduced list for a transplant will not receive one. human cells. The presence of this antigen into humans as a result of xenotransplan- The majority(47,000)need a kidney, but on many microorganisms means that tation and cause disease. Opponents of last year only 12, 500 kidneys were trans- nearly everyone has been exposed to it xenotransplantation raise the specter of planted A solution to this shortfall is to and has formed antibodies against it. the another Hiv-type epidemic resulting from utilize animal organs. Some argue that preexisting antibodies react with pig cells, human infection by a new animal retro- xenografts bring the risk of introducing which are then lysed rapidly by comple- virus. Recently, a Boston-based company pathogenic retroviruses into the human ment. The absence of human regulators announced development of pigs free population; others object based on ethi- of complement activity on the pig cells, in- endogenous pig retroviruses, reducing cal grounds relating to animal rights. cluding human decay-accelerating factor the possibility of this bleak outcome Nevertheless, the use of pigs to supply ( DAF)and human membrane-cofactor Will we see the use of pig kidneys in organs for humans is under serious con- protein(MCP), intensifies the comple- humans in the near future? The increasing sideration. Pigs breed rapidly, have large ment lysis cycle. (See Chapter 13 for de- demand for organs is driving the com- litters, can be housed in pathogen-free scriptions of DAF and MCP) mercial development of colonies of pigs environments and share considerable How can this major obstacle be cir- suitable to become organ donors. While anatomic and physiologic similarity with cumvented? Being tested are strategies kidneys are the most sought-after organ at humans. In fact, pigs have served as for absorbing the antibodies from the present, other organs and cells from the donors of cardiac valves for humans for circulation on solid supports, and using specially bred and engineered animals will years. Primates are more closely related soluble gal-gal disaccharides to block find use if they are proven to be safe and to humans than pigs are, but the avail- antibody reactions. a more elegant solu- effective. A statement from the American ability of large primates as transplant tion involves genetically engineering pigs Society of Transplantation and the Ameri- donors is, and will continue to be, ex to knock out the gene for the enzyme can Society of Transplant Surgeons er remely limited that synthesizes galactosyl-1, 3-ae-galactose. dorses the use of xenotransplants if cer Balancing the advantages of pig do- Solving the immediate rejection reaction tain conditions are met(Xenotransplanta nors are serious difficulites. For example, by interfering with the specific reaction tion 7: 235). These include the demonstra- if a pig kidney were implanted into a hu- against this antigen may not prevent all tion of feasibility in a nonhuman primate man by techniques standard for human antibody-mediated rejection. Certainly other model, proven benefit to the patient, and transplants, it would likely fail in a rapid antigenic differences to which human re- lack of infectious-disease risk. Barriers re- and dramatic fashion due to hyperacute cipients have antibodies will be present main to the clinical use of xenotrans- rejection. This antibody-mediated rejec- in some if not all donor/recipient pairs. plants, but serious efforts are in motion to tion is due to the presence on the pig cells However, any antibody attack on the pig overcome them progress has been made in long-term survival. The use of im- munosuppressive drugs, which are described below, greatly General In immunosuppressive increases the short-term survival of the transplant, but chronic Therapy rejection is not prevented in most cases. Data for rejection of kidney transplants since 1975 indicates an increase from 40% Allogeneic transplantation requires some degree of immu to over 80% in one-year survival of grafts. However, in the nosuppression if the transplant is to survive. Most of the same period long-term survival has risen only slightly; as in immunosuppressive treatments that have been developed 1975, about 50% of transplanted kidneys are still functioning have the disadvantage of being nonspecific; that is, they at 10 years after transplant. Chronic rejection reactions are dif- result in generalized immunosuppression of responses to all ficult to manage with immunosuppressive drugs and may antigens, not just those of the allograft, which places the necessitate another transplantation. recipient at increased risk of infection. In addition, many

progress has been made in long-term survival. The use of immunosuppressive drugs, which are described below, greatly increases the short-term survival of the transplant, but chronic rejection is not prevented in most cases. Data for rejection of kidney transplants since 1975 indicates an increase from 40% to over 80% in one-year survival of grafts. However, in the same period long-term survival has risen only slightly; as in 1975, about 50% of transplanted kidneys are still functioning at 10 years after transplant. Chronic rejection reactions are difficult to manage with immunosuppressive drugs and may necessitate another transplantation. General Immunosuppressive Therapy Allogeneic transplantation requires some degree of immunosuppression if the transplant is to survive. Most of the immunosuppressive treatments that have been developed have the disadvantage of being nonspecific; that is, they result in generalized immunosuppression of responses to all antigens, not just those of the allograft, which places the recipient at increased risk of infection. In addition, many 490 PART IV The Immune System in Health and Disease (and those of most mammals other than humans and the highest nonhuman primates) of a disaccharide antigen (galactosyl-1,3--galactose) that is not present on human cells. The presence of this antigen on many microorganisms means that nearly everyone has been exposed to it and has formed antibodies against it. The preexisting antibodies react with pig cells, which are then lysed rapidly by complement. The absence of human regulators of complement activity on the pig cells, including human decay-accelerating factor (DAF) and human membrane-cofactor protein (MCP), intensifies the complement lysis cycle. (See Chapter 13 for descriptions of DAF and MCP.) How can this major obstacle be circumvented? Being tested are strategies for absorbing the antibodies from the circulation on solid supports, and using soluble gal-gal disaccharides to block antibody reactions. A more elegant solution involves genetically engineering pigs to knock out the gene for the enzyme that synthesizes galactosyl-1,3--galactose. Solving the immediate rejection reaction by interfering with the specific reaction against this antigen may not prevent all antibody-mediated rejection. Certainly other antigenic differences to which human recipients have antibodies will be present in some if not all donor/recipient pairs. However, any antibody attack on the pig cells may be blunted if human DAF is present on the targeted cell to dampen the complement reaction. The lack of human DAF is remedied by producing transgenic pigs that express this protein. Addition of human complement regulators to the pig represents a universal solution, in that any cell that might become a target in the transplant will resist complement lysis. An additional concern is that pig endogenous retroviruses will be introduced into humans as a result of xenotransplantation and cause disease. Opponents of xenotransplantation raise the specter of another HIV-type epidemic resulting from human infection by a new animal retrovirus. Recently, a Boston-based company announced development of pigs free of endogenous pig retroviruses, reducing the possibility of this bleak outcome. Will we see the use of pig kidneys in humans in the near future? The increasing demand for organs is driving the commercial development of colonies of pigs suitable to become organ donors. While kidneys are the most sought-after organ at present, other organs and cells from the specially bred and engineered animals will find use if they are proven to be safe and effective. A statement from the American Society of Transplantation and the American Society of Transplant Surgeons endorses the use of xenotransplants if certain conditions are met (Xenotransplantation 7:235). These include the demonstration of feasibility in a nonhuman primate model, proven benefit to the patient, and lack of infectious-disease risk. Barriers remain to the clinical use of xenotransplants, but serious efforts are in motion to overcome them. Unless organ donations increase drastically, most of the 72,000 U.S. patients on the waiting list for a transplant will not receive one. The majority (47,000) need a kidney, but last year only 12,500 kidneys were transplanted. A solution to this shortfall is to utilize animal organs. Some argue that xenografts bring the risk of introducing pathogenic retroviruses into the human population; others object based on ethical grounds relating to animal rights. Nevertheless, the use of pigs to supply organs for humans is under serious consideration. Pigs breed rapidly, have large litters, can be housed in pathogen-free environments, and share considerable anatomic and physiologic similarity with humans. In fact, pigs have served as donors of cardiac valves for humans for years. Primates are more closely related to humans than pigs are, but the availability of large primates as transplant donors is, and will continue to be, extremely limited. Balancing the advantages of pig donors are serious difficulites. For example, if a pig kidney were implanted into a human by techniques standard for human transplants, it would likely fail in a rapid and dramatic fashion due to hyperacute rejection. This antibody-mediated rejection is due to the presence on the pig cells CLINICAL FOCUS Is There a Clinical Future for Xenotransplantation?