IMMUNOLOGY - Roitt I. - Mir 2000

Chapter 27. Transplantation and Rejection

THE ROLE OF T LYMPHOCYTES IN GRAFT REJECTION

T Cells play a leading role in graft rejection

Rodents with a Congenital absence of the Thymus (athymic, or "nude", animals) lack mature T cells and fail to reject grafts. The same is true for normal mice and rats thymectomised neonatally before mature T cells populate peripheral Lymphoid Organs. A similar effect can be achieved by thymectomy in adult mice and rats (which eliminates The production of mature cells) followed by irradiation (to eliminate existing mature T cells) and Bone Marrow transplantation (to restore haematopoiesis). This Procedure yields ATx.BM recipients (derived from adult thymectomy and bone marrow), which are devoid of T cells and incapable of rejecting grafts.

In all of the aforementioned animals (athymic, neonatally thymectomised, or ATx.BM), The ability to destroy transplanted Tissues can be restored by administering T cells from normal animals of the same strain. Thus, T cells are essential for graft rejection. This does not mean that Antibodies, B lymphocytes, or other Cell types take no part in rejection. Specifically, antibodies cause damage to the transplanted tissue, while macrophages contribute to The Development of inflammatory reactions within it.

The Molecular Basis of the rejection reaction is the TCR–MHC interaction

Through their T CELL RECEPTORS (TCRs), the T lymphocytes participating in the rejection response recognise donor Peptides expressed on graft cells in association with MHC Antigens. As is well known (see Chapter 7), the T cell receptor is structured such that T cells can "see" only those antigenic peptides that are associated with MHC molecules. This MHC restriction arises As a result of positive Selection in the thymus (see Chapters 12 and 14 and Fig. 27.8). Therefore, to evaluate the role that T cells play in the rejection response, it is necessary to determine the differences between donor and recipient MHC molecules and to establish The Significance of these differences for the presentation of a broad spectrum of antigens to the receptors of recipient T lymphocytes.

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Fig. 27.8. Under normal physiological conditions, bone marrow-derived pre-T cells enter the thymus, where they undergo positive and negative selection. Positive selection occurs during the interaction of thymocytes with self MHC molecules on cortical epithelial cells. As a result of this process, pre-T cells escape programmed cell death. Negative selection (deletion of autoreactive clones) is carried out upon contact of thymocytes with dendritic cells in the cortico-medullary junction. Newly formed autoreactive cells can be inactivated (though not necessarily eliminated) outside the thymus in the periphery under METABOLISM/18.html">The Influence of antigen. Alternatively, they may become activated suppressor cells. In animals tolerant to allogeneic cells, foreign antigen may be present in the thymic medulla (carried by dendritic cells of donor origin, i.e., "passenger" leukocytes, or presented by recipient dendritic cells). This alloantigen is capable of acting as a self antigen and inducing tolerance via negative selection of T cells reacting with it (clonal deletion). In peripheral lymphoid organs, foreign antigen can induce clonal anergy (i.e., induce tolerance—unresponsiveness) or, in animals tolerant to alloantigens, simulate active suppression.

Various MHC molecules are structurally similar overall, yet differ in the architecture of their peptide-binding cleft. Structurally, different MHC molecules are almost identical. Each molecule features two α-helices resting on a β-pleated sheet atop two immunoglobulin-like domains anchored in The Cell membrane (see Chapter 7). Between the α-helices lies a deep cleft in which peptide binding occurs. The region of the MHC molecule responsible for T Cell Recognition is represented by the upward-facing surface of the α-helices, which is relatively conserved among different MHC molecules.

The amino acid residues determining important differences between MHC molecules—for example, between the allelic variants A2 and Aw68 of the HLA-A antigen—are located largely within the cleft formed by the α-helices rather than on their upper surfaces contacting the TCR (see Chapter 7). Therefore, for T cell recognition, the primary significance lies in differences in the shape and charge of The surface of the peptide-binding cleft (see Chapter 7), which determine which peptides can bind and in what orientation they will be presented for recognition by the T cell receptor (see Chapter 9).

Transplant and recipient MHC molecules present different peptides. Under normal physiological conditions, the clefts of MHC molecules contain peptides that represent fragments of normal cellular components generated as a result of intracellular protein degradation. The state of immunological tolerance induced in the thymus (clonal deletion of autoreactive cells, see Chapter 14) prevents the occurrence of autoimmune reactions that might otherwise develop as a result of T cell recognition of "self peptide–self MHC molecule" complexes. However, if cells are infected (e.g., with a virus), self peptides in the clefts of MHC molecules of "professional" APCs may be replaced by foreign ones. In this case, T cells will respond to foreign peptides associated with "self" MHC molecules.

However, transplantation of genetically foreign tissue gives rise to a third scenario. Here, a different set of peptides is presented On the surface of the graft cells, determined by differences in the shape and charge of the surface of the peptide-binding cleft of the donor MHC molecules. Furthermore, the graft may contain allelic variants of normal cellular components distinct from those of the recipient (determined by minor histocompatibility Gene loci). This significantly affects the CHARACTERISTICS OF THE peptides presented by the graft cells. Differences between the donor of the transplanted tissue and the recipient in MHC antigens (different shapes and surface charges of the peptide-binding cleft) or in minor Histocompatibility Antigens (differing antigenic peptides) cause the graft to express an extremely large number of novel foreign antigens that can be recognised by the recipient's T cells. Consequently, up to 10% of the body's T cells are capable of reacting against these allogeneic graft antigens.

Helper T cells (Th cells) and lymphokines participate in the rejection response

The Role of Th cells in rejection. Administration of CD4⁺ T cells (Th cells) to athymic mice or ATx.BM recipients triggers an acute Skin graft rejection reaction. Naive CD8⁺ T lymphocytes (Tc cells) are incapable of mounting such a response; however, if administered together with a very small number of CD4⁺ T cells, or if CD8⁺ T cells previously sensitised to graft antigens are used (e.g., obtained from animals that have already rejected a graft), rapid destruction of the graft is observed. Structure/19.html">The Importance of Th cells in the rejection reaction is confirmed by experiments involving the administration of anti-CD4⁺ Monoclonal Antibodies to recipients (Fig. 27.9).

Fig. 27.9. Thymectomised CBA mice were administered cytotoxic monoclonal anti-CD4 or anti-CD8 antibodies to selectively deplete the Th and Tc cell populations, respectively. The animals were then grafted with skin from B10.BR mice, which are incompatible with the recipients at minor histocompatibility loci. Graft survival was monitored. Animals treated with anti-CD4 antibodies showed significantly higher graft survival than untreated recipients or mice that received anti-CD8 antibodies. These results highlight the importance of the CD4⁺ T cell population (Th cells) in graft rejection. (Data courtesy of Prof. H. Waldmann and Dr S. Cobbold).

Th cells are activated by bone marrow-derived APCs bearing class II MHC molecules. The rejection-stimulating APCs may belong to either the donor or the recipient. Donor APCs are present within the graft as passenger leukocytes (interstitial dendritic cells) and can mediate the "direct" activation of recipient Th cells. Recipient-derived APCs located in draining lymphoid tissues acquire antigen released by the graft and present it to recipient Th cells, inducing their "indirect" activation. Direct activation provides a more potent stimulus to reject the transplanted tissue compared with the indirect pathway. Thus, passenger leukocytes can exert a powerful influence on graft survival (Fig. 27.10).

Fig. 27.10. Strain A mice exposed to X-irradiation [X(A)] were injected with bone marrow cells from either strain A or strain B mice. Skin grafts from these animals successfully engrafted when transplanted to strain A recipient mice (1). Subsequently, skin from strain B mice was grafted onto these recipients. Mice carrying the first graft from X(A) Donors that had received strain A bone marrow rejected strain B skin more slowly than animals whose first graft came from X(A) donors that had received strain B bone marrow (2). This indicates that the strain B bone marrow cells contained within the first graft as "passenger" cells primed the recipient against strain B alloantigens.

The role of lymphokines in rejection. In addition to CD4⁺ Th cells, other immunological factors are involved in rejection, including lymphokines (Fig. 27.11).

Fig. 27.11. Activated by APCs, Th cells secrete cytokines, among which IL-2 and IFNγ are required for Tc cell activation; IL-2, IL-4, and IL-5 for B cells; while TNFβ (lymphotoxin) combined with IFNγ acts as a macrophage-activating factor (MAF). These cells trigger specific cell- and antibody-mediated reactions or non-specific inflammatory processes that culminate in graft rejection.

The most crucial roles in the destruction of transplanted cells are played by interleukin-2 (IL-2), which is essential for Tc cell activation, and IFNγ, which induces MHC expression, enhances APC activity, stimulates large granular lymphocytes, and—in synergy with TNFβ (lymphotoxin)—activates macrophages. [Note: previously, a mixture of IFNγ and TNFβ was known as macrophage-activating factor (MAF)].

Lymphokines (IL-4, IL-5, and IL-6) are also required to activate B cells that produce antibodies against the graft. These antibodies fix Complement and cause vascular endothelial injury, leading to hemorrhages, platelet aggregation within the graft vessels, and subsequent thrombosis; the antibodies also induce lytic damage to graft cells and the release of pro-inflammatory complement components C3a and C5a.

For graft rejection to occur, it is not necessary for immune factors to affect all of its structures. Their primary targets are the endothelium of the graft's microvasculature and specialized parenchymal Cells of the organ, such as renal tubules, pancreatic islets of Langerhans, or myocardial myocytes.

The lymphokine IFNγ can induce a high level of MHC class II molecule expression on vascular endothelial cells, as well as the appearance of class I and II molecules on parenchymal cells that under normal conditions do not express MHC antigens or express them only in small amounts. Such an increase in MHC antigen expression by graft cells increases the number of target molecules for the action of antibodies and activated cells, leading to an enhanced rejection reaction.

Под влиянием ФНОβ и ИФγ возрастает также экспрессия молекул адгезии на сосудистом эндотелии. Эти молекулы необходимы для прилипания циркулирующих в крови лейкоцитов к стенке сосудов, перед тем как они проходят через эндотелий в Ткани.