Signal requirements for T lymphocyte activation. I. Replacement of macrophage function with phorbol myristic acetate

Purified guinea pig T lymphocytes cannot be induced to proliferate by plant lectins in vitro unless they are reconstituted with either macrophages or a macrophage-derived soluble factor. The effects of phorbol myristic acetate (PMA) on the activation of this purified T lymphocyte population were studied. Although PMA by itself was nonmitogenic for guinea pig T cells, at very low concentrations (10^-5 μg/ml) it enabled purified T cells to respond to the lectins, PHA or Con A. However, it could not induce primed T cells to respond to a soluble antigen in the absence of macrophages. On the basis of several criteria, PMA appeared to be acting independently of macrophages. Its effect was unrelated to the number of macrophages in a lymphocyte preparation and it did not increase the accessory capability of additional macrophages. Analysis of the kinetics of the interaction between PMA, lectins, and T cells revealed that these compounds could be added to the T cells independently and in any order and still induce activation. However, maximal T cell proliferation required that PMA and lectin be added within the first 4 hr of culture. In addition, both agents were capable of exerting their effects after a brief (1 hr) exposure. The results of this study indicate that PMA can replace macrophages as a source of the second signal that is required to induce T cell proliferation in vitro. This agent may prove to be extremely valuable in the analysis of both the biochemical alterations involved in the process of T cell activation, and the mechanism by which macrophages induce these alterations.