Enhancement of rat splenic lymphocyte mitogenesis after short term preexposure to corticosteroids in vitro.

Numerous previous studies investigating the effects of corticosteroids on immune proliferation reported almost unanimously inhibitory effects. However, in many of these studies, high concentrations of corticosteroids (micromolar range) and long incubation times (days) were used. We have investigated whether corticosteroid hormones at low physiological (nanomolar) vs. high (micromolar) concentrations have distinct effects on immune cells after short term (minutes) as opposed to long term preexposure (hours). When rat splenocytes were preincubated with high concentrations (0.1-1 microM) of corticosterone (CORT) or with the specific glucocorticoid agonist RU 28362 (0.1 microM) for 1-6 h, washed, and then exposed to the T-cell mitogen Concanavalin-A, a time- and dose-dependent inhibition of lymphocyte mitogenesis over the next 3 days of culture was found. Preincubation with the glucocorticoid antagonist RU 486 completely blocked this inhibitory effect of CORT and RU 28362. Preexposure of splenocytes to CORT for 10-60 min did not alter mitogenesis. No differences were observed between intact and adrenalectomized (ADX) rats. However, if splenocytes from ADX rats were used in the presence of the glucocorticoid antagonist RU 486, the proliferative response over a 3-day period was stimulated by short term preexposure (10-30 min) to low concentrations (3-30 nM) of CORT. Under these incubation conditions, the mineralocorticoid aldosterone, in the presence of RU 486; also produced a stimulatory effect on ADX splenic lymphocyte mitogenesis, whereas RU 28362 was not effective. Corticosteroid receptor binding studies revealed the presence of mineralocorticoid (MR) as well as glucocorticoid (GR) receptors (45 and 600 fmol/mg protein, respectively) in the spleen. In conclusion, low physiological concentrations of CORT and aldosterone have novel stimulatory properties on mitogenesis of splenic lymphocytes from ADX rats. These effects become evident in the presence of GR antagonist and persist after short corticosteroid preexposure times. In contrast, after prolonged preincubation with high concentrations of corticosteroids, the well known immunosuppressive action is observed. We postulate that distinct MR- and GR-mediated effects may underlie these differential steroid actions on splenic lymphocyte proliferation.