Transplantation

1. Transplantation

2. Introduction • Transplantation is a widely used treatment for replacement of nonfunctioning organs and tissues with healthy organs or tissues. • Technically, transplantation is the process of taking cells, tissues, or organs, called a graft, from one individual and placing them into a (usually) different individual. • The individual who provides the graft is called the donor, and the individual who receives the graft is called recepient or host .

3. Introduction • If the graft is placed into its normal anatomic location, the procedure is called orthotopic transplantation; if the graft is placed in a different site, the procedure is called heterotopic transplantation.

4. Introduction Methods of Transplantation • May take place between: • different parts of the same organism (autografting) • different organisms of the same species (allografting) • different species (xenografting)

5. Methods of Transplantation: 1. Autografting • The transfer of self tissue from one body site to another in the same individual • Due to the genetic homology of the tissue, the immune system does not respond to it • Use: synthetic implantation • skin grafts • bone marrow transplantation • hair

6. Methods of Transplantation: 2. Allografting • Definition: The transfer of organs or tissue from human to human. • As there are more and more people every year waiting for donor organs and tissues, allografting transplantation has become quite common. • Allografting transplantation has many applications.

7. Methods of Transplantation:3. Xenografting Definition: Xenotransplantation is the transfer of tissue from one species to another • Usually refers to the implantation of animal tissue in humans • provides a new source of organs for humans • many different types of tissue can be transplanted: e.g. heart, kidney, liver or lung

8. Xenogeneic Transplantation • The use of solid organ transplantation as a clinical therapy is greatly limited by the lack of availability of donor organs. • For this reason, the possibility of transplantation of organs from other mammals, such as pigs, into human recipients has kindled great interest.

9. Xenogeneic Transplantation • A major immunologic barrier to xenogeneic transplantation is the presence of natural antibodies that cause hyperacute rejection. • More than 9 5 % of primates have natural IgM antibodies that are reactive with carbohydrate determinants expressed by cells of species that are evolutionarily distant, such as the pig. • The majority of human anti-pig natural antibodies are directed at one particular carbohydrate determinant formed by α-galactosyltransferase enzyme. • This enzyme places an a-linked galactose moiety on the same substrate that in human and other primate cells is fucosylated to form the blood group H antigen.

10. IntroductionTransplantation Terminology • A graft transplanted from one individual to the same individual is called an autologous graft. • A graft transplanted between two genetically identical or syngeneic individuals is called a syngeneic graft. • A graft transplanted between two genetically different individuals of the same species is called an allogeneic graft (or allograft). • A graft transplanted between individuals of different species is called a xenogeneic graft (or xenograft).

11. IntroductionTransplantation Terminology • The molecules that are recognized as foreign on allografts are called alloantigens, and those on xenografts are called xenoantigens. • The lymphocytes and antibodies that react with alloantigens or xenoantigens are described as being alloreactive or xenoreactive, respectively.

12. Introduction • Clinical transplantation to treat human diseases has increased steadily during the past 45 years, and transplantation of kidneys, hearts, lungs, livers, pancreata, and bone marrow is widely used today. • More than 30,000 kidney, heart, lung, liver, and pancreas transplantations are currently performed in the United States each year. In addition, transplantation of many other organs or cells, including stem cells, is now being attempted.

13. People in the United States living with functioning organ grafts, 1999-2007.

14. Transplantation: an overview • Transplantation of cells or tissues from one individual to a genetically non identical individual invariably leads to rejection of the transplant due to an adaptive immune response. • This problem was first appreciated when attempts to replace damaged skin on burn patients with skin from unrelated donors proved to be uniformly unsuccessful. • During a matter of 1 to 2 weeks, the transplanted skin would undergo necrosis and fall off.

15. Transplantation: an overview • The failure of the grafts led Peter Medawar and many other investigatorsto study skin transplantation in animal models. These experiments established that the failure of skin grafting was caused by an inflammatory reaction called rejection. • Graft rejection is the result of an adaptive immune response demonstrate that the process had characteristics of memory and specificity and was mediated by lymphocytes.

17. Transplantation: an overview • Second set rejection is usually faster that first set rejection. • This implying that the recipient developed memory for the grafted tissue. • Individuals who have rejected a graft from one donor show accelerated rejection of another graft from the same donor but not from a different donor, demonstrating that the rejection process is immunologically specific.

18. The Immunology of Transplantation is Important for Several Reasons • First, immunologic rejection remains one of the major problems in clinical transplantation. • Second, the immune response to allogeneic molecules has been a useful model for studying the mechanisms of lymphocyte activation. • Third, many immunosuppressive therapies that have proved to be useful for a variety of immunologic and inflammatory diseases were first tested and shown to be effective for treatment of graft rejection.

19. Immune Responses to Allografts

20. 1. Recognition of Alloantigens • Recognition of transplanted cells as self or foreign is determined by polymorphic genes, called MHC genes, which differ among different members of a species. • This conclusion is based on the results of experimental transplantation between inbred strains of mice, and in some cases, the results have been confirmed in human transplantation ( as shown in the figure, next slide).

22. 1. Recognition of Alloantigens • The basic rules of transplantation immunology, which are derived from such animal experiments, are the following: • Cells or organs transplanted between genetically identical individuals (identical twins or members of the same inbred strain of animals) are never rejected. • Cells or organs transplanted between genetically non-identical people or members of two different inbred strains of a species are almost always rejected. • The offspring of a mating between two different inbred strains of animal will typically not reject grafts from either parent. In other words, an (A x B)F1 animal will not reject grafts from an A or B strain animal. (This rule is violated by bone marrow transplantation). • A graft derived from the offspring of a mating between two different inbred strains will almost always be rejected by either parent. In other words, a graft from an (A x B)F1 animal will be rejected by either an A or a B strain animal.

23. 2. Alloantigent MHC Presentation • Allogeneic MHC molecules of a graft may be presented for recognition by the T cells of the recipient in two fundamentally different ways, called direct and indirect. • When the recipient T cells recognize intact, unprocessed MHC molecules in the graft, and this is called direct presentation of alloantigens. • When the recipient T cells recognize graft MHC molecules only in the context of the recipient's MHC molecules, implying that the recipient's MHC molecules must be presenting allogenic graft MHC proteins to recipient T cells. This process is called indirect presentation, and it is essentially the same as the presentation of any foreign (e.g., microbial) protein antigen.

24. Direct Presentation of MHC Alloantigens: keynotes • In direct presentation, an intact MHC molecule is displayed by donor antigen-presenting cells (APCs) in the graft and recognized by recipient T cells without a need for host APCs.

25. Direct Presentation of MHC Alloantigens: keynotes • There are several likely explanations for this surprisingly strong recognition of foreign MHC molecules: • The structure of all T cell receptors (TCRs) is inherently biased to recognize MHC molecules, even before selection in the thymus. • TCR genes encode a protein structure that has some, probably low, intrinsic affinity for MHC molecules. • During T cell development in the thymus, positive selection results in survival of T cells with weak self MHC reactivity, and among these T cells, there may be many with strong reactivity to allogeneic MHC molecules. • Negative selection in the thymus eliminates T cells with high affinity for self MHC, but it does not necessarily eliminate T cells that bind strongly to allogeneic MHC molecules, simply because these molecules are not present in the thymus.

26. The result is that the mature repertoire has an intrinsic weak affinity for self MHC molecules and includes many T cells that bind allogeneic MHC molecules with high affinity.

27. Direct Presentation of MHC Alloantigens: keynotes 2. The structure of an allogeneic MHC molecule is similar enough to self MHC that many self MHC-restricted T cells recognize the foreign MHC molecule. • Allogeneic MHC molecule with a bound peptide can mimic the determinant formed by a self MHC molecule plus a particular foreign peptide.

28. Molecular basis of direct recognition of allogenic MHC molecules. Direct recognition of allogenic MHC molecules may be thought of as a cross-reaction in which aT cell specific for a self MHC molecule-foreign peptide complex (A) also recognizes an allergenic MHC molecule (B, C) Non-polymorphic donor peptides, labeled "self peptide," may not contribute to allo-recognition (B) or they may (C).

29. Direct Presentation of MHC Alloantigens: keynotes 2. Many peptides may combine with a single MHC molecule and further expand the number of T cells that can recognize these combinations. • MHC molecules that are expressed on cell surfaces normally contain bound peptides, and the peptides form part of the structure recognized by the alloreactive T cell, exactly like the role of peptides in the normal recognition of foreign antigens by self MHC-restricted T cells.

30. Most of these peptides are likely to be self peptides that are the same in the donor and the recipient, but the donor peptides are displayed by allogeneic MHC molecules and therefore appear different from self peptide-self MHC complexes.

31. Direct Presentation of MHC Alloantigens: keynotes 3. All the MHC molecules on a donor APC will be foreign and will be recognized by alloreactive T cells; in contrast, in the case of an infection, less than 1 % (and perhaps as few as 0. 1 % ) of the MHC molecules on an APC normally present microbial peptides at any time and are recognized by T cells. 4. Direct allorecognition can generate both CD4+ and CD8+ T cells that recognize graft antigens and contribute to rejection.

32. Indirect Presentation of MHC Alloantigens • In the indirect pathway, donor (allogeneic) MHC molecules are captured and processed by recipient APCs that enter grafts, and peptides derived from the allogeneic MHC molecules are presented in association with self MHC molecules.

33. Indirect Presentation of MHC Alloantigens • Because allogeneic MHC molecules have amino acid sequences different from those of the host, they can generate foreign peptides associated with self MHC molecules on the surface of host APCs. • Indirect presentation may result in allorecognition by CD4+ T cells because alloantigen is acquired by host APCs primarily through the endosomal vesicular pathway (i.e., as a consequence of phagocyto· sis) and is therefore presented by class II MHC molecules. • Some antigens of phagocytosed graft cells appear to enter the classI MHC pathway of antigen presentation and are indirectly recognized by C D 8+ T cells.

34. Indirect Presentation of MHC Alloantigens • In the setting of any transplant between genetically nonidentical donor and recipient, there will be polymorphic antigens other than MHC molecules against which the recipient may mount an immune response. • These antigens typically induce weak or slower (more gradual) rejection reactions than do MHC molecules and are therefore called minor histocompatibility antigens. • Most minor histocompatibility antigens are proteins that are processed and presented to host T cells in association with self MHC molecules on host APCs (i.e., by the indirect pathway).

35. 3. Activation of Alloreactive Lymphocytes 3.1 T cell Recognition of Alloantigens: • The T cell response to an organ graft may be initiated in the lymph nodes that drain the graft. • Most organs contain resident APCs such as dendritic cells. • Transplantation of these organs into an allogeneic recipient provides APCs that express donor MHC molecules as well as costimulators. • It is believed that these donor APCs migrate to regional lymph nodes and present, on their surface, unprocessed allogeneic MHC molecules to the recipient's T cells (the direct pathway of allorecognition).

36. 3. Activation of Alloreactive Lymphocytes 3.1 T cell Recognition of Alloantigens: • Host dendritic cells from the recipient may also migrate into the graft, pick up graft alloantigens, and transport these back to the draining lymph nodes, where they are displayed (the indirect pathway). • Naive lymphocytes that normally traffic through the lymph node encounter these alloantigensand are induced to proliferate and differentiate into effector cells. This process is sometimes called sensitizationto alloantigens. Effector T cells migrate back into the graft and mediate rejection.

38. 3. Activation of Alloreactive Lymphocytes 3.1 T cell Recognition of Alloantigens: • As many as 1 % to 2 % of an individual's T cells are capable of recognizing and responding to a single foreign MHC molecule, and this high frequency of T cells reactive with allogeneic MHC molecules is one reason that allografts elicit strong immune responses. • Many of the T cells that respond to an allogeneic MHC molecule, even on first exposure, are memory T cells.

39. 3. Activation of Alloreactive Lymphocytes 3.2 Role of Costmiulationin T Cell Responses to Alloantigens • In addition to recognition of alloantigen, costimulationof T cells primarily by B7 molecules on APCs is important for activating alloreactive T cells.

40. 3. Activation of Alloreactive Lymphocytes 3.3 The Mixed Lymphocyte Reaction • The response of alloreactive T cells to foreign MHC molecules can be analyzed in an in vitro reaction called the mixed lymphocyte reaction (MLR). • The MLR is used as a predictive test of T cell-mediated graft rejection. • Studies of the MLR were among the first to establish the role of class I and class II MHC molecules in activating distinct populations of T cells (CDS+ and CD4+, respectively).

41. 3. Activation of Alloreactive Lymphocytes 3.3 The Mixed Lymphocyte Reaction • The MLR is induced by culturing mononuclear leukocytes (which include T cells, B cells, natural killer [NK] cells, mononuclear phagocytes, and dendritic cells) from one individual with mononuclear leukocytes derived from another individual. • In clinical practice, these cells are typically isolated from peripheral blood; in mouse or rat experiments, mononuclear leukocytes are usually purified from the spleen or lymph nodes.

42. 3. Activation of Alloreactive Lymphocytes 3.3 The Mixed Lymphocyte Reaction • If the two individuals have differences in the alleles of the MHC genes, a large proportion of the mononuclear cells will proliferate during a period of 4 to 7 days. • This proliferative response is called the allogeneic MLR • If cells from two MHC-disparate individuals are mixed, each can react against the other and both will proliferate, thus resulting in a two-way MLR.

44. Mixed Lymphocyte Reaction: Strong Proliferation--->High incompatibility Weak proliferation--->Low incompatibility No proliferation---> 100% compatibility Helps to identify any antigenic differences between donor and recipient Recipient Donor + (Irradiate) Cell Proliferation

45. 4. Effector Functions of Alloreactive T Cells • Effector Functions of Alloreactive T Cells Alloreactive CD4+ and CD8+ T cells that are activated by graft alloantigens cause rejection by distinct mechanisms. • The CD4+ helper T cells differentiate into cytokine producing effector cells that damage grafts in a way similar to delayed type hypersensitivity. While alloreactive CD8+ cells differentiate into cytotoxic T lymphocytes, which kill graft cells that possess MHCI. • CTLs also secrete inflammatory cytokines, which can contribute to graft damage.

46. 4. Effector Functions of Alloreactive T Cells • Only CTLs that are generated by direct allogeneic MHC recognition can kill graft cells, whereas CTLs or helper T cells generated by either direct or indirect alloantigen recognition can cause cytokine-mediated damage to grafts.

47. Activation of Alloreactive B Cells and Production of Alloantibodies • Most high-affinity alloantibodies are produced by helper T cell-dependent activation of alloreactive B cells, much like antibodies against other protein antigens. • The antigens most frequently recognized by alloantibodies in graft rejection are donor HLA molecules, including both class I and class II MHC proteins.

48. Activation of Alloreactive B Cells and Production of Alloantibodies • The likely sequence of events leading to the generation of these alloantibody-producing cells is that: • Naive B lymphocytes recognize foreign MHC molecules, • Internalize and process these proteins, • and present peptides derived from them to helper T cells that were previously • Thus activation of alloreactive B cells is an example of indirect presentation of alloantigens. • Anti-HLA antibodies contribute significantly to allograft rejection.

49. Patterns and Mechanisms of Allograft Rejection • Hyperacute Rejection • Hyperacute rejection is characterized by thrombotic occlusion of the graft vasculature that begins within minutes to hours after host blood vessels are anastomosed to graft vessels and is mediated by preexisting antibodies in the host circulation that bind to donor endothelial antigens.

50. The bound antibodies to endothelial layer activate complements leading to endothelial cell injury and exposing of the subendothelial basement membrane that activate plts. This result in thrombosis and necrosis of the vessel wall.