Life-supporting pig-to-primate renal xenotransplantation using genetically modified donors

The problem of anti-pig antibodies in pig-to-primate xenografting: current and novel methods of depletion and/or suppression of production of anti-pig antibodies

The role of antibodies directed against Galalpha1-3Gal (alpha-Gal) epitopes in porcine-to-primate xenotransplantation has been widely studied during the past few years. These antibodies (anti-alpha-Gal) have been associated with both hyperacute rejection and acute vascular rejection of vascularized organs. Depletion and (temporary or permanent) suppression of production of anti-alpha-Gal seem to be essential to the long-term survival of these organs, even when the ultimate aim is accommodation or tolerance. Although more than 95% depletion of anti-alpha-Gal can be achieved by the use of immunoaffinity column technology, to date no regimen has been successful in preventing the return of anti-alpha-Gal (from continuing production). In this review, we discuss current and novel methods for achieving depletion or inhibition (i.e. extracorporeal immunoadsorption, anti-idiotypic antibodies, the intravenous infusion of immunoglobulin or oligosaccharides) and suppression of production (i.e. irradiation, pharmacologic agents, specific monoclonal antibodies, immunotoxins) of anti-alpha-Gal antibodies.

Current trends in transplant immunology

Recent advances in transplant immunology are wide ranging. These include the testing of new approaches to tolerance induction by the interruption of co-stimulatory pathways, the analysis of molecular events underlying the development of chronic rejection, efforts to increase the donor pool by consideration of the role of brain death in donor-dependent outcomes of allografting, and progress towards renal xenografting. In addition, current molecular approaches are paramount to understanding key events from ischemia/reperfusion injury and the role of apoptosis in remodelling of the host immune response to an allograft. Important papers relevant to the field from the past year are reviewed with an eye to clinically relevant new data involving renal transplantation.

Porcine kidney and heart transplantation in baboons undergoing a tolerance induction regimen and antibody adsorption

BACKGROUND

Xenotransplantation would provide a solution to the current shortage of organs for transplantation. Our group has been successful in inducing tolerance in mice and monkey models of allogeneic transplantation. The present study attempts to extend the same tolerance-inducing regimen to a pig-to-baboon organ transplantation model.

METHODS

Nine baboons underwent a conditioning regimen (consisting of nonmyeloablative or myeloablative whole body and thymic irradiation, splenectomy, antithymocyte globulin, pharmacologic immunosuppression and porcine bone marrow transplantation [BMTx]), which has previously been demonstrated to induce donor-specific allograft tolerance in monkeys. In addition, immunoadsorption of anti-alphaGal antibody (Ab) was performed. Four of the nine baboons received pig kidney transplants (KTx), and one also underwent repeat transplantation with an SLA-matched kidney. Two received heterotopic pig heart transplants (HTx). Three baboons underwent conditioning without organ transplantation for long-term studies of natural Ab kinetics.

RESULTS

In the three baboons that received the conditioning regimen without an organ transplant, immunoadsorption reduced Ab by approximately 90%, but recovery of Ab to pretreatment level or higher occurred within 7 days. In contrast, the level of Ab remained low after organ transplant. No Ab to pig antigens other than alphaGal was detected in any baboon before or after BMTx, KTx, or HTx. No graft succumbed to hyperacute rejection. KTx function began to deteriorate within 3-6 days, with oliguria and hematuria progressing to anuria, and the kidneys were excised after 3, 6, 9, 11, and 14 days, respectively. One HTx ceased functioning at 8 days; the second baboon died with a contracting HTx at 15 days. Features of coagulopathy and thrombocytopenia developed in all six transplanted baboons (high D-dimer, prolonged prothrombin time and partial thromboplastin time, and falling fibrinogen) resulting in serious bleeding complications in two baboons, one of which died on day 9. Donor organs showed progressive acute humoral rejection with deposits of IgM, IgG, and complement; a focal mononuclear cellular infiltrate was also observed. The ureter was the earliest structure of the KTx affected by rejection, with progression to necrosis.

CONCLUSIONS

This conditioning regimen prevented hyperacute rejection but was ineffective in preventing the return of Ab, which was associated with the development of acute humoral rejection with features of coagulopathy. No baboon developed anti-pig Ab other than alphaGal Ab. Further modifications of the protocol directed toward suppression of production of Ab are required to successfully induce tolerance to pig organs in baboons.

Complement regulatory proteins in glomerular diseases

Complement activation plays a critical role in the pathogenesis of many forms of glomerulonephritis. Complement activation leads to tissue injury through various mechanisms including the generation of chemotactic factors and activation of the resident glomerular cells following C5b-9 insertion. Recent advances have disclosed the mechanisms of regulation of complement activation by discovery of a number of complement regulatory proteins. Decay accelerating factor (DAF), membrane cofactor protein (MCP), and complement receptor type 1 (CR1) act by inactivating C3/C5 convertase. They belong to the gene superfamily known as the regulators of complement activation (RCA), and share a common structural motif called a short consensus repeat (SCR). In contrast, CD59 works by inhibiting formation of C5b-9. The glomerulus is particularly well endowed with these membrane-bound complement regulatory proteins. DAF, MCP, and CD59 are ubiquitously expressed by all three resident glomerular cells, while CR1 is localized exclusively in podocytes. Expression of complement regulatory proteins can be changed by many factors including complement attack itself, and their expression levels are affected in various glomerular disorders. Studies utilizing cultured glomerular cells and animal models of glomerular diseases suggest important protective roles of complement regulatory proteins against immune-mediated renal injury. Recent progress in molecular biological techniques has made new therapeutic strategy feasible. Systemic administration of soluble recombinant complement regulatory proteins and local overexpression of complement regulatory proteins are promising therapeutic approaches.