Lymphokine regulation of activated (G1) lymphocytes. II. Glucocorticoid and anti-Tac-induced inhibition of human T lymphocyte proliferation.

The regulation of the first cell cycle of human, activated (G1) PBL was analyzed by flow cytometry and [3H]thymidine incorporation. Endogenous IL 2 production was blocked in situ by pharmacologic concentration of DEX (100 to 1000 nM), resulting in an 80 to 90% reduction of thymidine uptake. Although T lymphocyte activation (G0-G1a transition) by PHA was unaltered, cells remained in the G1a phase of the cell cycle due to insufficient RNA synthesis for proliferation. The addition of IL 2-containing supernatants reversed this inhibitory effect of DEX by allowing the cells to synthesize more RNA (G1a-G1b transition). Such cells could enter the S phase and proliferate. Similar studies were performed on cells treated with a monoclonal antibody (anti-Tac) against the IL 2 receptor. In these studies, IL 2-induced RNA synthesis, and subsequent proliferation of DEX-treated and PHA-stimulated cells was inhibited by anti-Tac. Anti-Tac did not, however, inhibit the effect of endogenous IL 2 (PHA-stimulated PBL without DEX treatment), although it did bind equally well to such cells. Thus, IL 2 directly or indirectly regulates human T cell proliferation at the level of RNA synthesis. Furthermore, anti-Tac can inhibit the mitogenic signal given by endogenous IL 2, but not by in situ produced IL 2, an observation of importance to further investigations of the mechanisms by which IL 2 interacts with specific receptors to elicit proliferation.