Identification of a Single MiHA Specificity That Induces Resistance to MHC-Matched Allogeneic HCT.

T cell populations specific for transplantation antigens have been detected in sensitized individuals following multiple blood transfusions, marrow transplants as well as in multiparous females. Resistance to allogeneic hematopoietic cell transplant (HCT) in such sensitized individuals is consistent with the presence of a host memory T cell (TM) population specific for donor cell antigens. We hypothesized that a single donor minor histocompatibility (MiHA) epitope could elicit antigen-specific CD8 TM capable of resisting MHC-matched allogeneic hematopoietic cell engraftment. To address this question, CD8 TM were generated against a single MiHA epitope to determine if such cells could mediate resistance after ablative TBI conditioning. B6 mice were sensitized 2X to the H60 immunodominant MiHA epitope utilizing bone marrow-derived dendritic cells pulsed with the H60 (LTFNYRNL) peptide. Three weeks following booster sensitization, CD8 T cells were detected by tetramer staining in peripheral blood samples. These T cells exhibited a phenotype characteristic of memory cells (CD44^hi, Ly 6C⁺). B6 (H2^b) mice containing CD8⁺ H60⁺ T cells were subsequently conditioned with 9.0 Gy TBI and transplanted with 5 × 10⁶ BALB.B (H2^b) BM-TCD. One week post-transplant, naive recipients of BALB.B (H60⁺) or B6-H60 congenic TCD-BM contained >10-fold higher levels of circulating donor cells than the B6 dendritic cell/peptide sensitized recipients. Donor progenitor cells were also found to be significantly reduced in sensitized recipients of allogeneic TCD-BM at this time. Two weeks post-HCT, recipients of syngeneic marrow exhibited >10-fold greater frequency of circulating donor cells compared to recipients of MHC-matched allogeneic marrow (< 5% donor chimerism was detected). These findings demonstrate that host T cells with specificity against a single donor MiHA determinant are sufficient to induce resistance to MHC-matched allogeneic marrow engraftment. Such observations regarding the effector response of HVG contrast those by donor T cell responses post-transplant in which single MiHA differences fail to induce GVHD. Finally, heterologous immunity to virus has been reported to generate allo-reactive TM cells. Since such TM repertoires could include specificity for MiHA immunodominant epitopes, the presence of TM populations that can mediate resistance in “naive” recipients may be more prevalent than hitherto appreciated.