《免疫学》（英文版） Chapter 21 Transplantation Immunology.pdf

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