Abstract Xenotransplantation has long been proposed as a therapeutic strategy to address the ongoing organ shortage in transplantation. In recent years, pig-to-primate xenotransplantation outcomes have dramatically improved following advances in the genetic engineering of pig donors and utilization of costimulation blockade-based immunosuppression, such that translation to the clinic appears within reach. Highly allosensitized patients, those who have developed anti-donor antibody as a response to foreign HLA exposure, are potential first candidates for xenotransplantation given their reduced chances of undergoing allotransplantation. However, the impact of allosensitization in the setting of xenotransplantation has not yet been fully elucidated in pig-to- primate models. Our preliminary data suggest that allosensitization promotes antibody-mediated rejection (AMR) following kidney xenotransplantation and leads to early graft failure. Use of donor kidneys from highly engineered pigs prolong xenograft survival yet do not fully alleviate AMR development. Additional therapeutic strategies are thus needed to dampen the post-transplant humoral response following xenotransplantation in sensitized recipients. This project aims to evaluate novel desensitization and immunosuppression strategies to control the post-transplant humoral response and foster long-term xenograft survival in sensitized recipients. Our overarching hypothesis is that both conditioning the host immune response ahead of xenotransplantation through desensitization and continuous targeting of B cells, plasma cells, or complements following transplantation, are necessary to control the post-transplant humoral response and alter the repopulating xenoreactive immune repertoire to favor long-term graft acceptance. To explore this, we propose 3 specific aims: Specific aim 1: We will define the effects of desensitization (costimulation blockade and proteasome inhibitor) pre-transplant in rhesus monkeys undergoing kidney xenotransplantation. Specific aim 2: We will define the impact of adjuvant therapies targeting downstream elements of the humoral response following xenotransplantation. Specific aim 3: We will identify the functional phenotype of xeno-specific T and B cell repertoires required to establish long-term AMR-free xenograft survival while preserving anti-viral/vaccinal response. These advances will ultimately position us to conduct a first-in-human xenokidney transplantation in sensitized patients testing this optimized immunosuppressive regimen. Our proposal involves many academic and industry collaborations that are ongoing as attested by letters of support. The impact of this proposal has broad implications that may benefit U.S. citizens affected by end stage renal failure or by immune-mediated illnesses or infections.