Project Summary/Abstract Disrupting the CD40-CD154 costimulatory pathway using an anti-CD154-based regimen has markedly improved gene-edited pig organ xenograft survival in nonhuman primate (NHP) recipients. However, the life-threatening thromboembolic complications associated with use of anti-CD154 have precluded its approval for human use. Attempts to avoid this complication by blocking CD40, the classic CD154 receptor, paradoxically impeded rather than promoted tolerance induction in NHP recipients. A recent study showed that blocking a second CD154 receptor, integrin CD11b, significantly increased efficacy of anti-CD40 in prolonging allograft survival in mice. Although mouse models are important for developing proof-of-concept, it remains to be determined if combined blockade of CD40 and CD11b can be extended to prolonging survival of xenografts in NHP recipients, studies that will be critical for guiding clinical translation. In preliminary studies, we show that an anti-CD11b mAb with a unique mechanism of action not only prevents multiligand binding to CD11b but also blocks direct and indirect NHP T cell alloresponses and prolongs survival of kidney allografts in NHP recipients. The main objective of this exploratory R21 is to test the hypothesis that combining this mAb with anti-CD40 is safer and equivalent to or even more effective than anti-CD154 in prolonging kidney xenograft survival in our established gene-edited pig-to-NHP xenotransplantation model. A second objective of this study is to evaluate the effect of this combined blockade on enumeration and function of T cell subsets in treated NHP. If successful, these studies may provide a basis for prolonging kidney xenograft survival, thus addressing a pressing yet unmet medical need.