Donor minor H antigen-specific CD8 T cells in long-term allograft acceptance

The HLA-A2-restricted peptide HA-1H (VLHDDLLEA) is an immunodominant minor H antigen expressed only by cells of hematopoetic origin. It is known to be important in graft-versus-host disease (GVHD) and graft versus leukemia (GVL) effects following bone marrow transplantation between HLA-A2+ individuals (1) (2), but its role in solid organ allograft outcome is unknown. We report that mismatched donor HA-1H peptide triggered not only effector (defined by IFN-γ production, DTH and CTL responses) but also regulatory T cell responses (defined by TGF-β production and by inhibition of IFN-γ production and DTH responses) in recipients of HLA-A2+ renal transplant from sibling donors. HA-1H specific regulation was correlated with rare donor leukocytes in patient peripheral blood (microchimerism). Effector and regulatory T cells specific to HA1-1H could be readily distinguished based on binding to HA-1H/A2 (A2H) tetramers: (1) HA-1H-specific CD8+ effector T cells bound strongly to A2H tetramers (A2H-high), produced IFN-γ in response to H-peptide in vitro and mediated strong donor-specific DTH responses. These T cells also gave rise to CTL in vitro with high cytolytic activity against donor leukocyte targets. (2) Low tetramer-binding HA-1H-specific CD8+ regulatory T cells (A2H-low) completely blocked the DTH and IFN-γ responses in vitro mediated by A2 H-high CD8 effector T cells, and the progeny from these cell in vitro had low cytolytic activity against donor targets. Regulation of donor-specific DTH response in vivo was cytokine-dependent; while inhibition of IFNγ response by T effectors cells in vitro was largely cytokine-independent, but CTLA-4-dependent. These findings establish the principle of immune regulation of donor antigen specific high avidity effector T cells by low avidity regulatory T cells in peripheral tolerance, and point to a possible linkage with microchimerism in the dendritic cell subset.