P028 Immunogenicity of minor histocompatibility antigens may be dependent on the frequencies of naive precursor cells in the donor repertoire

Aim Out of 60+ minor histocompatibility antigens (MiHA) discovered so far some are immunodominant, i.e. almost universally cause allogeneic response when mismatched, while others are much less immunogenic. Before transplantation MiHA specific T cells reside in naive T cell (T N ) subpopulation of the donor. One potential explanation for unequal MiHA immunogenicity is the variation in frequencies of T N specific to different antigens in the donor T cell repertoires. To test this hypothesis we compared the frequencies of naive T cells, specific for highly immunogenic minor antigen HA-2 and less immunogenic ACC-1Y in individuals negative for these antigens. Methods We genotyped a panel of healthy volunteers for HA-2 and ACC-1Y polymorphisms and selected donors negative for these minor antigens, but bearing restricting HLA alleles (HLA-A∗02:01 and HLA-A∗24:02 respectively). Donor naive T cells were magnetically purified and stimulated by autologous monocyte derived dendritic cells, loaded with appropriate peptides. Expanding MiHA-specific lymphocytes were detected using MHC-multimers and flow cytometry. Frequencies of HA-2 and ACC-1Y-specific T N cells were calculated by extreme limiting dilution analysis. Results Frequency of specific T N was found to be substantially higher for HA-2 than for ACC-1Y (5 × 10 −6 vs. 2 × 10 −7 , p  = 3 × 10 −9 ), which correlates with HA-2 immunodominance and confirms the initial hypothesis. Conclusions Further studies are needed to elucidate the reason for major differences in T N frequencies between various MiHA. It is possible that the variation could be explained by the peptide properties or by the influence of the negative selection on initial T cell repertoire. In contrast with HA-2, ACC-1Y has alternative allelic variant (ACC-1C) presented in the thymus, which leads to elimination of all cross-reactive clones. Creating a model of MiHA immunogenicity would be very useful for predicting allogeneic immune response after transplantation. Funding: Russian Science Foundation grant 17-15-01512.