| On average, 21 people in the United States die every day while waiting for a transplantable organ. Moreover, at the end of 2014, nearly 130,000 additional individuals were still on the transplant waiting list. Xenotransplantation, the transplantation of cells and/or organs from a member of another species, is both a viable supplement for, and bridge to, allotransplantation. Pigs are the favored organ donor because they: 1) breed and grow rapidly, 2) can be genetically modified, and 3) present few ethical concerns. However, due in part to their phylogenetic distance, several barriers must be successfully overcome before clinical xenotransplantation can be fully realized. Antibody deposition is presently a barrier to successful cardiac, renal, and non-encapsulated islet xenotransplantation; although all xenografts are likely susceptible to antibody-mediated damage. The purpose of this dissertation is twofold: 1) to provide further molecular characterization of the elicited antibodies which mediate rejection of porcine xenografts, and 2) to identify clinically applicable small molecules capable of selectively inhibiting these antibodies. We have found that vascular pig xenografts, and isolated porcine pancreatic islets, both elicit antibody responses encoded by structurally related antibody gene progenitors. These structurally related antibody gene progenitors are both known to encode human antibodies capable of inhibiting clotting factor VIII (FVIII). Therefore, we subsequently identified FVIII as a novel xenoantigen, and provided a preliminary molecular analysis of the interaction between FVIII and the antibodies elicited after xenotransplantation. Additionally, our analyses strongly suggested that it was feasible for a single reagent to inhibit the majority of the antibodies elicited against transplanted xenografts. Consequently, we screened for, and identified, a single clinically relevant small molecule drug which, when tested in vitro, inhibited elicited antibody from multiple xenotransplant settings. However, it was only when utilized in combination with a previously identified experimental reagent that it was possible to reduce the post-transplant antibody to levels below, or comparable to, those present before transplantation. These results challenge long-standing presumptions with regard to the nature of xenoantigens and their role in antibody-mediated xenograft rejection. Furthermore, identification of a clinically relevant selectively inhibitory small molecule should expedite transition into large animal work. |
