Xenotransplantation with pig organs offers the best near term hope for satisfying the limitation imposed by shortage of human organs. Although hyperacute rejection of pig organs in primate recipients has been overcome by the production of (-1,3-galactosyltransferase knockout (GalT-KO) pigs, experiments to date with GalT-KO organs strongly suggest that clinically relevant chronic immunosuppression of recipients will be insufficient to overcome rejection of xenotransplants. Tolerance induction through hematopoietic chimerism, which has recently been shown clinically to allow human allograft acceptance without chronic immunosuppression could potentially play a key role in allowing acceptance of xenogeneic organs as well. Our proposed product will consist of genetically modified pigs with enhanced xenogeneic hematopoietic chimerism potential that we will make available commercially to the research community, thereby providing a strategy for advancing solid organ xenotransplantation toward the clinic. Recent evidence suggests that a key barrier to the establishment of chimerism following pig-to-primate bone marrow transplantation is species incompatibility of the CD47 (IAP, integrin associated protein) cell surface molecule. Ubiquitously expressed, CD47 binds SIRP( receptors on macrophages and thereby inhibits phagocytosis. Expression of primate CD47 on pig cells greatly reduces their susceptibility to phagocytosis by human macrophages. The goal of Phase I of this STTR proposal is to develop and verify the tools necessary for production, via nuclear transfer, of pigs expressing primate CD47 on a GalT-KO background. This includes development of targeting/expression vectors for CD47, development of molecular assays for identifying targeted cell clones, verification of a primary cell clone isolation process and isolation of primary fetal fibroblast lines for use in the production of CD47 transgenic pigs. In Phase II, we will use these tools to produce GalT-KO pigs expressing primate CD47 via nuclear transfer, analyze CD47 expression in these pigs, evaluate the efficacy of CD47 expression using in vitro systems, and perform a limited number of proof-of-concept transplant experiments.
The demand for transplantable human organs far exceeds the current supply and the gap between demand and supply continues to grow. Transplantation of pig hearts, kidneys and other organs offers the opportunity to greatly reduce or eliminate this shortage. The overall goal of this project is to produce genetically modified pigs as a commercial product that will further the development of pig-to-human organ transplantation.