MHC and Peptide Specificity Underpin CD8+ T cell Direct Alloresponse.

The direct alloresponse, pivotal in transplant rejection, occurs when recipient T cells recognize intact allogeneic peptide-MHC complexes. Despite extensive research, our understanding of alloreactive CD8+ T cells against an individual MHC allele in humans remains limited, especially their precursor frequency, MHC specificity, and peptide specificity. By utilizing K562 cell-based artificial antigen-presenting cells (aAPCs) expressing HLA-A*01:01, HLA-A*02:01, or HLA-A*03:01, we determined that the precursor frequency of alloreactive CD8+ T cells against a single MHC allele ranges from 0.1% to 0.5%. Further, these cells exhibited MHC-specificity regarding proliferation, activation, IFN-γ secretion, and cytolytic ability, with limited cross-reactivity towards non-targeted MHC alleles. Focusing on anti-A2 alloreactive CD8+ T cells, we developed a peptide-exchangeable aAPC that displays selected peptides on HLA-A*02:01. From a set of 95 computationally curated A2-restricted peptides most abundant in renal tubular cells, we identified two immunogenic kidney peptides across multiple donors. Overall, our findings significantly enhance the understanding of direct alloresponse and provide a toolkit for future mechanistic studies and reproducible patient monitoring.