The differentiation of cytotoxic T cells in vitro. I. Amplifying factor(s) in the primary response is Lyt 1 + cell dependent.

Within 15 hr of establishment of a murine mixed lymphocyte culture, a soluble mediator was produced that was capable of augmenting primary cytotoxic responses to alloantigen. The factor did not induce responsiveness in the absence of antigen, since the amplified response seen in its presence was specific for the stimulating alloantigen. The factor did not therefore appear to function by polyclonal activation of cytotoxic precursor cells. Production of the amplifying factor(s) was induced by unfractionated spleen cells, but not by cells subjected to UV irradiation or to sonication, making it likely that this deficiency is the basis of the well-documented failure of these stimulator cells to induce primary cytotoxic responses. The amplifying effects of the factor were distinctive from, but synergistic with, those of 2-mercaptoethanol. Production of the amplifying mediator did not require cell division but was dependent upon the presence of Lyt 1 + cells. On the other hand, Lyt 2 + cells were not needed for mediator production, but served as target cell population on which the factor exerted its action. These findings are compatible with the hypothesis that direct T-T cell collaboration can amplify the differentiation of cytotoxic cells.