Studies on T cell clonal expansion. I. Suppression of killer T cell production in vivo

Adult C57BL/6 mice immunized i.p. with an allogeneic mastocytoma cell line (P815 of the DBA/2 strain) develop cytolytically active T cells. Activity peaks at 10 to 11 days and falls rapidly thereafter. We have observed an anomalous in vitro behavior of effector cell populations harvested during this decline. The lytic activity of spleen cells harvested 12 to 18 days after alloantigen was markedly augmented after culture for 24 hr at 37 degrees C. The augmented lytic activity was caused by T cells and specificity exhibited was identical to the pre-culture population. No augmentation of lytic activity occurred when cells were cultured at 4 degrees C or when they were treated with a protein synthetic inhibitor (pactamycin) at concentrations which did not compromise cytotoxic expression by the starting population. Augmentation of cytolysis was independent of DNA synthesis and did not require the presence of added antigen. When immune lymphoid cell populations were depleted of effector cells by adsorption onto allogeneic monolayers and then cultured, new killer cells differentiated within a 24-hr period. This process was not augmented by the addition of antigen to the nonadherent cell population. We interpret these demonstrations to show: i) that there is a pre-killer T cell whose differentiation into an effector cell requires de novo protein synthesis but not cell proliferation, and ii) that the in vivo fall in lytic activity during the primary immune response is at least partially caused by suppression of the differentiation of this pre-killer cell. Furthermore, we conclude that the pre-killer cell is distinct from a memory T cell because: i) its conversion to an effector cell is antigen-independent and ii) because, unlike the memory cell, pre-killers do not bind avidly to allogeneic cell monolayers.