A model system has been developed to study extrathymic T cell differentiation. Mice have been thymectomized, lethally irradiated, and reconstituted with bone marrow cells depleted of Thy-1-positive cells. After 8 wk, the spleen cells of these 5athymic, bone marrow-reconstituted chimeras contain Thy-1-positive pre-cytolytic T lymphocytes (CTL) that are able to respond to antigen only when exogenous interleukin 2 is added to culture.. The phenotype of these pre-CTL is similar to that of thymocytes, suggesting that they may be an immature T cell. Initial evaluation of the CTL repertoire of these athymic mice demonstrates that the CTL generated to trinitrophenyl-modified syngeneic cells are H-2 restricted and that the CTL generated to alloantigens have many of the cross-reactivities observed in normal but not in nude mice. The discrepancies observed in the CTL repertoire between these thymectomized chimeras and nude mice are discussed.

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