Transplantation Immunology

United Network for Organ Sharing

In 1901, Landseiner discovered ABO blood group antigens. In 1954, Murray performed the first successful kidney transplant between identical twin brothers. In 1958, Dausset discovered the major histocompatibility complex in human, the human leukocyte antigens (HLA). In 1963, the first liver allograft transplant by starzl and the first lung transplant was performed by Hardy. In 1967 the first heart allograft transplant was performed by Barnard. In 1972, Borel discovered immunosupressive properties of Cyclosporine (isolated from fungus in Norway). In 1983, Baby Fay received the first baboon heart and survived for 21 days. In 1999, Rapamycin was approvied for clinical kindey transplantaion. These are all milestones in transplantation therapy, improving graft transplantation.

Today, a 80% 1 year survival rate for kidney, liver, heart and pancreas transplantation is obtained. However, chronic rejection of transplants remains a large problem. Another major problem is organ shortage. The other problems in translantation are infections due to overimmunosupression. The solution to all of these problem would be to make organs accepted to reduce the need to make transplantations.

Tolerance means the absence of a destructive response to the allograft in an immunocompetant host. Tolerance, although easy to acheive in small animal models, has been extremely difficult in large animal models and humans. There is also no assay to measure tolerance (no simple assay). There is also a problem of compatbility of tolerance induction strategies with conventional immunosuppression. So any new treatments have to be compatible with immunosupressive drugs people are already taking.

GVHD (graft-versus-host-disease) occurs when donor derived T cells recognize and react to histo-incompatible recipient antigens leading to a variety of host tissue injuries. GVHD is the major cause of morbidity and mortabiliy after allogeneic BM transplantation, even when siblings are matched at the human luekocyte antigen (HLA) locus. GVHD occurs in both acute and chronic forms, each with different kinetics and distinctive pathology. The skin is the organ the most affected by GVHD and clinical symptoms range form a simple rash to a dramatic epidermolysis. Other affected organs are the gut, the liver, the lung and lymphoid organs. Chronic GVHD occurs less than 100 days after transplantation and affects the same tissues, in addition to the joints and the mucosal surfaces with an incidence of 40-60% in trnasplant recipients surviving more than 100 days.

The principal stragegies to prevent GVHD center around the depletion of donor T cells. However, this may lead to the loss of GVHD (graft verus tumor effect) and to an increased risk of infections and graft failure. Recently focus has been on host DCs as key stimulators of donor T cells, inducing GVHD.

Pathways of graft Rejection

Acute/Classical pathway: This is cell mediated (predominantly CD4 and CD8 T cells) directed at donor MHC antigens. CD4 T cells recognize these classes (allosensitization) which help develop effector functions (B cells, CD8T cells, Macrophages) which destroy the graft. Current immunosupression is targeted at acute rejection.

In allorecognition the TCR is working as the antigen. This means the precursor frequency is different. 1-% of T cells will become activated rather then with nominal antigen where 1 in about 10000 T cells are active to respond. The frequency of antigens presented on a cell is much greater then in the traditional sense. In direct recognition, the T cell recognizes an allo APC presenting a foreign pathway. The direct pathway is predominant (greater than 90%). It activates both CD4 and CD8 T cells and is sufficient to cause rejection. There is also an indirect recognition (lower frequency) where peptides are presented by self (host) APC. This is probably important in chronic rejection and may have a role in tolerance induction. This pathway involves epitope spreading.

Hyperacute rejection: occurs minutes to hours after transplantation due to antibody mediated and complement dependent pathways. These are preformed antibodies specific for MHC antigens (eg.., graft before, etc). The target of the response is vacular endothelium. This can be avoided by prescreening (taking serum from patient and reacting it against donor cells).

Chronic rejection: is poorly defined that involves immune and non-immune components. It is a slow process that occurs months to years after transplantation and characterized by arteriole thickening and interstitial fibrosis. Current immunosuppressive are ineffective at treating chronic rejection.

Dendritic Cells in Alloreactivity: DCs are specialized, exptremely potent APC that stimulate both CD4 and CD8 T cells in mixed luekocyte response. 30 years ago it was noted if depleted DCs before transplantation, found greater survival rate. Both donor and host CD contribute to alloactivation (direct and indirect pathways above).

Trying to take advantage of idea that certain DCs can promote tolerogenicity, BM DCs were derived in low dose GM-CSF and shown that they are poor stimulators of T cells. Low dose gave rise to what looked like immature DCs. High does gave rise to mature and immature DC. The low dose, immature DCs transplanted at day 3 prolonged graft survival.

Role of Costimulatory molecules and T cell activation: For example, CD40 on APC and CD154 is an important interaction. Antibodies against CD154 to block this interaction has been used in animal studies. Blockage of CD28/B7 and CD40/Cd40L pathways shows synergy in prolonging graft survival.

Role of autoantigens in alloreactivity: In one report, gave rats allograft and then challenge rate with specific antigen to see whether ear swelling occurred which indicates T cell response.

Immunosupressive Drugs in Transplantation

Conventional Immunosupressive: include calcineurin inhibitors (prevents early events in TCR signalling) that block FK506 and cyclosporine. These two drugs have also been instrumental in understanding T cells activation (NFAT and IL-2 transcription). Other dugs are anti-proliferative (cell cycle progression, Rapamycin for example), block T cell molecules and are geared toward lymphocyte depletion.

Drugs in development: FTY720 effects lymphocyte trafficking (ability to leave thymus). Other drugs like deoxyspergulin affect APC fucntion. Campath 1-H deplete T cells. CP690-550 is a Jak3 inhibitor which may affect NK cells.

Mechanisms of Tolerance Induction

Clonal Deletion: The strategy here is to induce intrathymic deletion of donor reactive thymocytes by introducing donor bone marrow into pre-conditioned host in order to get mixed chimerisms. The strategy is to get clonal deletion of donor reactive and host reactive thymocytes leading to tolerant donor and host T cells.

Anergy: If you activate a T cell through TCR but do not give another required signal, one gets anergic T cells. This can be reversed by exogenous IL-2.

Immune Regulation: CD4+CD25+ Treg cells promote allograft survival.

Zenotransplantation: Human serum contains abundant amounts of naturally occuring IgM specific antiboides against the Gal α1-3 Gal sugar moiety expressed on pig tissues. These antibodies cause hyperactue rejection dependent on complement activation. A step forward was taken when a human gene encoding a complement regulatory protein, decay accelerating factor (CD55) was first introduced into pigs. Survival of vacularized organs rose from hours to weeks, but graft loss eventually occurred, probably due to activation of coagulation and inflammatory cascades as a result of antibody deposition, even in the absence of complement activation.

The filed has continued to advance and pigs have been produced in which the gene encoding the enzyme that creates the Gal α1-3 Gal carboyhydrate epitope has been inactivated using nuclear transfer or cloning.