Types of graft rejection Hyperacute -extremely rapid rejection (hours) occlusion of graft vessels Acute - days to week

2. Types of graft rejection Hyperacute -extremely rapid rejection (hours) occlusion of graft vessels Acute - days to weeks after transplant cell-mediated immune response Chronic -months to years after transplant graft vascular disease

3. Hyperacute rejection is mediated by pre-existing antibodies against donor antigens

4. Acute rejection is based on development of an adaptive immune response

5. “Chronic” rejection is typified by graft vascular disease resulting from inflammatory injury Heart transplant, chronic rejection. Concentric fibrosis of an artery with the later stages of graft vascular disease

6. Graft rejection is an allogeneic response Graft rejection is the result of the recipient immune system recognizing “alloantigens” in the graft as foreign and mounting an immune response. Alloantigens are endogenous (self) antigens whose structure can vary between individuals in the population.

7. Erythrocytes express the A, B, O blood group antigens. These alloantigens are glycolipids with distinct carbohydrate structures

8. Hyperacute rejection occurs within hours of transplantation as a result of pre-existing antibodies against alloantigens, e.g. blood group antigens

9. Acute rejection is based on an adaptive immune response to alloantigens: genetic mapping studies identified the MHC locus as a major determinant of acute rejection

10. HLA/MHC genes are highly polymorphic Numbers of different HLA gene alleles found in the human population

11. The TCR and co-receptors bind to MHC -peptide complexes

12. HLA polymorphisms can effect peptide binding and T cell receptor binding (but not CD4/CD8 co-receptor binding)

13. MHC alloreactivity mechanisms Indirect recognition: -Processed and presented as a foreign antigen by recipient APCs Direct recognition: -Recognized by pre-existing antibody, -Recognized directly by T cells since the repertoire was not negatively selected with MHC alleles present in the graft Presentation of a distinct peptide spectrum: -“New” endogenous peptides may be recognized by T cells as foreign antigens

14. T cell alloresponse may be mediated by recognition of either alloMHC determinants, or peptides presented by the alloMHC

15. The Mixed-Lymphocyte Response (MLR) assay can be used to assess alloreactivity

16. Graft alloantigens can be recognized in two distinct ways: either directly presented by donor APCs, or indirectly, following processing and presentation on host APCs.

17. Figure 13-36 Allelic differences at non-MHC loci can also lead to graft rejection, although the alloresponse is generally weaker. These antigens are known as minor histocompatibility antigens.

18. Activation of alloreactive T cells mediates acute graft rejection

19. Chronic rejection Mechanism remains uncertain, but appears to be an alloantigen driven response which starts a cycle of damage and inflammatory response in the graft vasculature. Associated with the presence of anti-HLA I antibodies which may be produced by anti-allotypic B cells with help from T cells stimulated through indirect alloantigen recognition.

20. Improving engraftment HLA matching Immunosuppression non-specific: corticosteriods, cytotoxic drugs specific: cyclosporin A, FK506, rapamycin anti-T cell antibodies Tolerance induction

21. Corticosteriods Non-specific immunosuppressants. Modulate expression of a relatively large set of genes, Block production of inflammatory cytokines. • Prednisone – hydrocortisone derivative, is a pro-drug; converted to its active form, prednisolone, in vivo

22. Cytotoxic drugs: kill dividing cells by inhibiting DNA replication Azathioprine: blocks purine metabolism Cyclophosphamide: alkylates DNA Methotrexate: inhibits thymidine synthesis

23. Specific immunosuppressive agents target T cell activation. The most common agents control regulation of the NFAT transcription factor.

24. Cyclosporin A and tacrolimus block T cell activation by inhibiting calcineurin and NFAT function CsA and tacrolimus bind target proteins in the cytosol The drug- protein complex associates with calcineurin preventing the dephosphorylation of NFAT NFAT is unable to translocate into the nucleus to activate IL-2 gene expression

25. Inducing tolerance to a transplant would avoid the problems associated with generalized immunosuppresion. Blocking co-stimulation during T cell activation can induce tolerance in naïve T cells

26. Figure 9-5 CD40-CD40 ligand interactions are important for both B cell maturation (shown below) and APC activation.

27. Blocking co-stimulation through CD28 or CD40 can and suppress immune responses and enhance graft survival

28. Figure 13-44 Pregnancy: An allograft protected by immunosuppression and ignorance Ignorance: Placenta forms a tissue barrier which limits access of T cells to fetus. Immunosuppression: Trophoblast cells express IDO, (indoleamine dioxygenase), causing local depletion of Trp and suppression of T cell activity. In addition, TGFß/IL4/IL10 secretion by trophoblasts suppresses Th1 cell differentiation.

29. Bone Marrow Transplantation Reconstitution of hematopoietic system through transfer of stem cells. Restoration of immune function requires shared MHC alleles so that T cell repertoire selected during development can function with bone marrow derived APCs. Transfer of T cells in bone marrow presents reciprocal problem of graft rejection - graft vs. host disease.

30. Bone marrow transplantation can restore non-functional hematopoietic systems generated by inherited immunodeficiencies or therapies for malignancies

31. Reconstitution of a functional immune system requires sharing of some HLA haplotypes: T cell selection in the host thymus determines recognition of donor APCs

32. Mature T cells in bone marrow can mediate graft vs. host disease: systemic “autoimmune” disease

33. Graft-versus-Host Disease is associated with pre-treatment tissue damage 1. Pretreatment to ablate host immune system, or reduce malignancy, causes tissue damage. 2. APCs recognize damage and activate allo-responsive donor T cells. 3. Activated T cells and other immune cells induce tissue damage.

34. Depletion of T cells from bone marrow prior to transplant reduces graft-versus-host disease. (However this depletion also reduces “graft vs. leukemia” effect).

36. Xenotransplantation

37. Xenotransplantation • Why pigs? • non-primates with organs of similar size to humans. • can make transgenic pigs to address some cross-species problems • Problems • Hyperacute rejection by pre-existing xenoantibodies. Complement inhibitory proteins are unable to function across • a large species gap which increases hyperacute rejection. • A potential risk of transferring cross-species pathogens under immunosuppressive conditions