Effects of steroid hormones on phytohemagglutinin-stimulated human peripheral blood lymphocytes.

The interactions of steroid hormones and human peripheral blood lymphocytes (PBLs) have been investigated following glass-wool column separation of PBLs and incubation in a serum-free medium in the presence of phytohemagglutinin (PHA). Addition of hydrocortisone or progesterone above physiological concentrations resulted in inhibition of [14C]2-TdR incorporation. 17beta-Estradiol and 5alpha-dihydrotestosterone inhibited [14C]2-TdR incorporation only at steroid concentrations thousands of times higher than the physiological concentrations. The kinetics of hydrocortisone and progesterone inhibition appeared to be similar and suggested that events occurring early after PHA addition were most sensitive to steroid inhibition and that addition of steroid at 28 hr, at the end of the early prereplicative phase of the cell cycle, inhibited DNA synthesis only at very high concentrations. The PHA-induced increase in RNA synthesis could be prevented by hydrocortisone addition even if delayed for up to 4 hr; morphological transformation was similarly affected. These data, and other investigations showing that progesterone binds to a specific glucocorticoid receptor in PBLs suggest that the progesterone inhibition of macromolecular synthesis in human PBLs is exerted via a glucocorticoid-like mechanism, and that glucocorticoid sensitivity is greatest during the early phases of lymphocyte activation. These results also provide a rationale for the apparent in vivo immunosuppressive capability of progesterone.