F1 hybrid (A X B) rats immunized with parental strain (A) T cells, or which recover from graft-versus-host (GVH) reactions caused by parental T cells, develop strong T cell-mediated immune responses against anti-major histocompatibility complex (MHC) receptor structures on donor T cells specific for host (MHCb) alloantigens. This immune response provides the basis for a profound and specific resistance to the subsequent induction in these animals of local or systemic GVH disease. Using subset-specific monoclonal antibodies and negative selection rosetting procedures, we attempted to determine which donor T cell subset possesses the immunogenic idiotypic markers, and which host T cell subpopulation mediates GVH resistance. We show here that the immunizing donor cell population belongs to the W3/25+ helper T cell (Th) subset, the same one that causes local GVH reactions, and that both the W3/25+ Th and the OX8+ killer/suppressor (Tk/s) subsets of host T cells are able to transfer GVH resistance to secondary F1 recipients.

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