Bidirectional effects of corticosterone on splenic T-cell activation: critical role of cell density and culture time.

Glucocorticoids inhibit stimulus-induced T-cell proliferation, an early and essential parameter of cellular immunity. It was recently found however that physiological concentrations of glucocorticoids can also accelerate, not only inhibit, rat T-cell mitogenesis. We investigated mechanism(s) underlying mitogenic actions of glucocorticoids on anti-T-cell receptor (TCR)- and concanavalin A (Con A)-induced T-cell proliferation. Surprisingly, the ability of the glucocorticoid corticosterone (CORT) to either enhance or inhibit T-cell proliferation was found to depend primarily on the cell density and the timing of the cultures. At cell densities up to 1 x 10(5) cells/well (i.e. 'low' density), CORT inhibited T-cell proliferation irrespective of the culture time. In contrast, at cell densities of 2 x 10(5) cells/well and higher ('high' density), CORT potently stimulated T-cell mitogenesis during the first 2-3 culture days, but subsequently inhibited the proliferative response after 5-7 days. The glucocorticoid receptor antagonist RU486 completely abolished the effects of CORT. However, production of the main T cell growth factor interleukin (IL)-2 was inhibited by CORT at both 'low' and 'high' cell densities. In addition, irrespective of cell density, T-cell mitogenesis under either control conditions or in presence of CORT was completely blocked by an anti-IL-2-receptor-alpha-chain (IL-2Ralpha) antibody, indicating that T-cell proliferation was dependent on the IL-2 pathway. Immunofluorescence staining of IL-2Ralpha on CD4+ cells after 2-3 days in culture was increased by CORT, but only on cells cultured at 'high' density. Thus, glucocorticoids increase T-cell responsiveness to IL-2 under conditions of 'high' cell density only. We conclude that glucocorticoids may contribute to a more efficient early stage of cellular immune responses under conditions of intimate cell-to-cell contact (i.e. 'high' cell density), a situation likely to be present in vivo, for instance in lymph nodes. Thus, these findings are relevant to our understanding of the glucocorticoid control of immune function.

Glucocorticoids regulate TCR-induced elevation of CD4: functional implications.

CD4 serves as a coreceptor during Ag recognition by the TCR. This interaction results in a marked increase in the sensitivity of a T cell to Ag presented by MHC class II molecules. Here we report that activation of T cells either by plate-bound mAb (anti-TCR, anti-CD3) or soluble activators (staphylococcal enterotoxin A, Con A) is associated with an (up to 3-fold) increase in CD4 cell surface expression on CD25+ cells, which was maximal after 72-96 h. Incubation with the glucocorticoid hormone corticosterone (CORT) shifted the enhancement of CD4 expression to a point about 24 h earlier than that observed in control cultures. In parallel, the proliferative response of these CORT-treated cells was profoundly enhanced. An involvement of increased CD4 expression in this enhanced proliferative response was evidenced by the observation that T cell proliferation in CORT-treated cultures was much less sensitive to inhibition by an inhibitory, nondepleting anti-CD4 mAb than that in control cultures. TCR down-regulation was, however, not affected by CORT. Thus, based on this study and previous reports we propose that both TCR-mediated signals and glucocorticoids are important physiological regulators of CD4 expression. In addition, these findings may be of significance for the sensitivity of CD4+ cells to HIV infection upon T cell activation, as the efficacy of primary patient HIV entry depends on the level of surface CD4.

Glucocorticoids accelerate anti-T cell receptor-induced T cell growth.

To study steroid regulation of cell-mediated immunity, we used anti-TCR-stimulated rat splenic lymphocyte mitogenesis as our experimental paradigm. Surprisingly, we found that the principal glucocorticoid of the rat, corticosterone (CORT), potently enhanced anti-TCR-induced lymphocyte proliferation after 2 to 3 days in culture, followed by inhibited cell growth after 5 to 7 days. Thus, glucocorticoids appeared to accelerate anti-TCR-induced lymphocyte mitogenesis. This effect occurred at physiologic concentrations (50-1000 nM), which are known to be released in vivo after an immune challenge. Kinetic experiments showed that CORT had to be present within 60 min after the initiation of TCR activation to produce maximal enhancing effects; a delay of 2 h or more left CORT ineffective. The lymphocytes incubated with CORT may have an increased sensitivity to IL-2 because 1) CORT suppressed IL-2 production throughout the culture period, and 2) an anti-IL-2R mAb completely blocked both control and CORT-treated anti-TCR-induced lymphocyte proliferation. Although the IL-2R alpha- and beta-chain mRNA concentrations were not altered in CORT-treated splenocyte cultures, we observed by FACS analysis an increased expression of the IL-2R alpha-chain on CORT-treated TCR alpha beta + and CD4+ T cells after 48 to 72 h of culture, suggesting an increased sensitivity of these T cells to IL-2 during the phase of enhanced proliferation. These results demonstrate a clear distinction between the enhancing effects of glucocorticoids on anti-TCR-induced lymphocyte proliferation and their well known inhibitory actions. Thus, the present study expands the regulatory role of glucocorticoids in cellular immunity, adding a novel effective stimulatory component to their inhibitory properties.