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The mixed leukocyte culture (MLC) and the cell mediated lympholysis (CML) assays are used as in vitro models of the afferent, or recognitive, and efferent, or destructive, phases of the homograft reaction. Activity in both of these tests has been related to differences at the major histocompatibility complex, HL-A in man and H-2 in mouse. Recent evidence suggests that the presumed cell surface differences which lead to cell proliferation in MLC are different from those which act as a target for CML. Data are presented providing further support for this hypothesis; in addition separate cell populations may respond to the differences which activate cells in MLC and to the differences which serve as targets for CML. There thus appears to be a dichotomy both for genetic control of, and cell populations involved in, the recognitive and destructive phases of cell mediated immunity.

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Purified human T cells respond proliferatively to allogenic peripheral blood mononuclear (PBMC) stimulating cells but show no response to murine splenic stimulating cells. Two possible explanations for the lack of xenogeneic response are that human T cells, educated in a human thymus, cannot directly recognize a molecule as disparate as mouse antigen encoded by H-2 and/or that a cytokine(s) produced by the APCs is needed to allow a proliferative response and that the cytokine(s) produced by murine APC do not provide an adequate stimulus to the human T cells under these conditions. We show here that highly purified human T cells can respond directly in an antigen-specific manner to murine stimulating cells if human rIL-1 or rIL-2 or a T cell growth factor (TCGF) preparation are present in the culture. These findings demonstrate that human T cells can recognize murine antigens and that a highly significant response can be obtained if a human cytokine is present to permit that response.