Synergistic induction of CTLA-4 expression by costimulation with TCR plus CD28 signals mediated by increased transcription and messenger ribonucleic acid stability.

T cell activation requires at least two signals transduced by the Ag-specific TCR plus a costimulatory receptor. The CD28 costimulatory molecule has been shown to promote T cell proliferation and cytokine production. CTLA-4, a cell surface molecule homologous to CD28, can function as a repressor of T cell activation. Thus, CTLA-4 and CD28 may have opposing functions during T cell activation. CTLA-4 is expressed at low levels on resting T cells and up-regulated after T cell activation. Regulation of CTLA-4 expression is critical to the normal regulation of immunity. For example, CTLA-4-deficient mice develop early onset lethal autoimmunity. We previously showed that CTLA-4 transcription is increased after T cell activation and that induction was controlled by 335 bp of CTLA-4 upstream sequence. In this work, we show that cell surface CTLA-4 expression is increased synergistically by TCR plus CD28 signals. Synergistic induction is mediated by two mechanisms: an enhanced rate of transcription and increased mRNA stability. In contrast to the regulation of IL-2 and IL-2R expression, which is inhibited by cyclosporin A-, but not rapamycin-dependent signal transduction pathways, CTLA-4 expression is inhibited by either cyclosporin A or rapamycin. Thus, synergistic induction of CTLA-4 expression requires both cyclosporin A- and rapamycin-dependent signals.