Nuclear factor of activated T cells and AP-1 are insufficient for IL-2 promoter activation: requirement for CD28 up-regulation of RE/AP. 
IL-2 gene transcription in T cells requires both TCR and costimulatory signals. IL-2 promoter activation in Jurkat T cells stimulated with superantigen presented by Raji B cells requires CD28 activation. The addition of rCTLA4Ig, which blocks CD28 binding to its ligand, to the cultures decreased IL-2 promoter activation by >80%. Interestingly, CTLA4Ig did not significantly inhibit the activation of either NF of activated T cells (NFAT) or AP-1 reporters. Therefore, activation of NFAT and AP-1 is insufficient for IL-2 promoter activation. In contrast, an RE/AP reporter was blocked by CTLA4Ig by >90%. Thus, the requirement for CD28 in IL-2 promoter activation appears to be due to RE/AP and not the NFAT or AP-1 sites. In addition, these data suggest that transcriptional activation of RE/AP is not mediated by NFAT, because activation of a NFAT reporter is not affected by the addition of CTLA4Ig.