Mechanisms of human T cell response to mitogens: IL 2 induces IL 2 receptor expression and proliferation but not IL 2 synthesis in PHA-stimulated T cells.

Human peripheral blood T cells were purified by a four-step procedure which included depletion of plastic-adherent cells, rosetting with sheep red blood cells, nylon wool passage, and treatment with mouse monoclonal antibodies to human Ia antigens plus complement. The purified T cells completely failed to proliferate to phytohemagglutinin (PHA). Bacterially derived recombinant human interleukin 2 (IL 2) reconstituted the proliferative response of resting T cells to PHA. The optimal concentration of IL 2 required was 100 to 200 U/ml. IL 2 alone caused no T cell proliferation. Both PHA and IL 2 needed to be present together for the proliferation of T cells to occur. Incubation of T cells with either PHA or IL 2 alone for up to 18 hr, followed by washing, then by the addition of the reciprocal reagent, resulted in no T cell proliferation. Expression of IL 2 receptors and of Ia antigens, as assessed by indirect immunofluorescent staining, revealed that both PHA and IL 2 needed to be present for Tac and Ia antigen expression by T cells. T cells incubated with PHA and IL 2 for 18 to 42 hr acquired responsiveness to IL 2. These T cells remained absolutely dependent on IL 2 for proliferation to occur. In contrast to T cells stimulated with PHA in the presence of monocytes, T cells stimulated with PHA and IL 2 released no detectable IL 2. The failure of IL 2 secretion was not caused by down-regulation of IL 2 production by IL 2 itself, because the addition of IL 2 to cultures of T cells stimulated with PHA in the presence of monocytes did not interfere with IL 2 production. These results indicate that IL 2 is a sufficient signal to induce the expression of its receptor in PHA-stimulated T cells and subsequent proliferation but is not sufficient to cause endogenous IL 2 release.