Lethally irradiated rats reconstituted with syngeneic bone marrow and treated with cyclosporine (CsA) for 40 d develop a graft-vs.-host disease-like syndrome (GVHD) after CsA therapy. We attempted to assess the development of autoreactivity in these animals. Results revealed that a majority of the animals with syngeneic GVHD develop autocytotoxic T lymphocytes of the OX8 phenotype. In addition to reactivity with self, these cells were capable of lysing appropriate target cells from a variety of different rat strains. The target antigens appeared to be class II major histocompatibility antigens, because lysis could be effectively blocked by an anti-Ia monoclonal antibody. Cold target inhibition studies indicated that one effector cell was capable of lysing various target cells, and provided evidence against a polyclonal activation of multiple anti-Ia-reactive cells. These results suggested that the anti-class II autoreactive cell associated with syngeneic GVHD either recognizes a common class II determinant ("public" epitope) shared by multiple strains of rats, or was polyspecific with respect to "private" class II determinants.

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