Highly Variable Frequencies of Donor Antigen-Specific Naive Cells May Dictate the Immunogenicity of Minor Histocompatibility Antigens after Hematopoietic Stem Cell Transplantation

In unrelated HLA-matched hematopoietic stem cell transplantation there are thousands of mismatches of non-synonymous SNPs between donor and patient, some of which result in polymorphic peptides presented in MHC context, known as Minor Histocompatibility Antigens (MiHAs). As these peptides are foreign to the donor immune cells they may drive allogeneic immune response, causing both graft-versus-host disease and beneficial graft-versus-tumor effect. But due to the phenomenon of immunodominance some of MiHAs are much more immunogenic than the others. We propose that antigenic dominance could be at lest partially explained by the frequencies of precursor T cells in the donor immune system and thus in the graft. As donor lymphocytes have never encountered polymorphic human peptides, before transplantation MiHA specific T cells reside exclusively in naïve T cell (T_N) subpopulation. To test the proposed hypothesis we compared the frequencies of naïve T cells, specific for highly immunogenic minor antigen HA-2 and less immunogenic ACC-1Y in individuals negative for these antigens.

We have selected a panel of healthy volunteers bearing HLA-A*02:01 and HLA-A*24:02 which can present HA-2 and ACC-1Y respectively. The panel was genotyped for SNP encoding these antigens using rapid multiplex MiHA genotyping approach developed in our laboratory. We have chosen donors having HA-2 -/- and ACC1Y -/- genotypes as sources of MiHA specific T_N cells and HA-2 +/- and ACC1Y +/- donors as negative controls. Dendritic cells were derived from donor peripheral blood monocytes, loaded with appropriate peptides and co-cultured with autologous T_N cells in extreme limiting dilution assay, which allowed us to calculate T_N frequencies.

We have determined that frequency of specific T_N was substantially higher for immunodominant HA-2 antigen than for subdominant ACC-1Y (5 x 10^-6 vs. 2 x 10^-7, p=3 x 10^-9), which correlates with immunodominance of HA-2 and falls in line with the initial hypothesis. The variation of frequencies of T_N specific to various MiHAs could be explained either by the peptide properties or by the influence of the negative selection on initial T cell repertoire. ACC-1Y has the alternative allelic variant (ACC-1C) presented in the thymus, which leads to elimination of all cross-reactive clones, whereas alternative allelic variant of HA-2 is cleaved by proteasome and is not presented to the immune system. Further studies of antigen-specific T_N frequencies would facilitate creating a complex model of MiHA immunogenicity, which could be used to predict allogeneic immune response after transplantation. Funding was provided by Russian Science Foundation grant 17-15-01512