Dexamethasone enhances CTLA-4 expression during T cell activation

T cell activation is enhanced by the costimulatory interaction of B7 on ant igen-presenting cells and CD28 on T cells, resulting in long-term T cell pr oliferation, differentiation and production of large amounts of cytokines, such as interleukin (IL)-2. CTLA-4 is a co-stimulation receptor that shares 31% homology with CD28 and binds B7 family members with higher affinity. C TLA-4 is transiently expressed intracellularly and on the cell surface foll owing activation of T cells. We have studied the kinetics of CTLA-4 express ion and the effects of dexamethasone on CTLA-4 expression during T cell act ivation in cultures of mouse spleen cells stimulated by a mixture of immobi lized anti-CD3 and anti-CD28 monoclonal antibodies (anti-CD3/CD28 mAb) or c oncanavalin A (ConA). CTLA-4 expression peaked on day 2 and returned to bac kground levels after 7 days. Dexamethasone was found to potentiate CTLA-4 e xpression in a dose-dependent manner with an EC50 effective concentration 5 0%) of about 10(-8) M. In contrast, other immunosuppressive agents, such as rapamycin or cyclosporin A had no or an inhibitory effect on CTLA-4 expres sion, respectively. Dexamethasone also stimulated CD28 expression, but inhi bited IL-2R expression during anti-CD3/CD28 mAb-induced mouse splenic T cel l activation. Western blot analyses of lysates of activated mouse T cells s howed that dexamethasone increased CTLA-4 protein levels twofold during ant i-CD3/CD28 mAb-induced activation. Dexamethasone also enhanced CTLA-4 messe nger RNA twofold as quantified by ribonuclease protection assay. The effect s of dexamethasone on CTLA-4 expression were glucocorticoid-specific and co mpletely inhibited by the glucocorticoid receptor antagonist mifepristone ( RU486), indicating that the effect of dexamethasone on CTLA-4 expression is mediated through the glucocorticoid receptor. In conclusion, the immunosup pressive agent dexamethasone actually stimulates CTLA-4 expression, which i s involved in downregulation of T cell activation.