Rhesus macaques (RMs) are commonly used in transplantation research and serve as a relevant model for studying MHC or ABO mismatched organ transplantation. While MHC (MAMU) typing provides definitive information on MHC disparities between donors and recipients, genetic analysis of glycosyltransferases does not reliably determine ABO blood type. The lack of native ABH antigen expression on red blood cells also makes conventional hemagglutination testing unsuitable in nonhuman primate models. This study aims to genetically and serologically confirm the ABO typing (particularly AB) of donor RMs showing variable outcomes of ABO-incompatible (ABOi; AB-to-B) kidney transplantation. All RMs were genotyped for MAMU and ABO using MiSeq-based amplicon sequencing. Three (3) type B RMs received maximally MAMU mismatched kidney transplantation from AB donors. All recipients received conventional triple immunosuppression (Tacrolimus/MMF/Steroid). Sera from three Type B rhesus recipients (n=3) and their Type AB donors were analyzed using the Bio-Rad ID Gel Card Indirect Antiglobulin Test. Agglutination strength was analyzed with the titer defined as the highest dilution exhibiting a positive reaction. Both donor and recipient sera were tested against A1 and B RBCs to define anti-A and anti-B profiles. Even with ABOi without desensitization, one animal showed long-term allograft survival. All type B recipients showed the expected anti-A antibody hemagglutination pattern, consistent with the recipient ABO typing result. However, in the case of long-term survival, AB donor showed serologic profiles of the type B which is showing positivity observed in type A cards with anti-A antibody with not anti-B antibody hemagglutination suggested serologic type B expression. The discrepancies between genomic and serologic profiles were consistent in the allograft survival. The results demonstrated that ABO genotyping based on the glycosyltransferase gene does not fully predict the serologic ABO phenotype, likely due to weakened or absent antigen expression, which may influence outcomes in ABO-incompatible kidney transplantation in NHP models. Incorporating serologic testing alongside genetic typing provides a more accurate assessment of ABO status for studies involving ABO- incompatible transplantation in nonhuman primates. Symposium Presenter: Keshav Jha Establishing Energy Thresholds for Targeted Cherenkov Light

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