CD28 signaling causes a sustained down-regulation of I kappa B alpha which can be prevented by the immunosuppressant rapamycin.

CD28, an important T cell surface molecule, mediates a costimulatory signal in the activation of T cell immune responses. CD28 signaling is resistant to the immunosuppressant cyclosporin A (CsA) but sensitive to the immunosuppressant rapamycin. CD28 costimulation induces transcription from the interleukin (IL)-2 promoter via the CD28 response element. The levels of c-Rel, a CD28 response element-binding factor, were found previously to be increased by CD28 costimulation. Therefore, we focused our present study on the mechanism(s) of c-Rel up-regulation by CD28 signaling in Jurkat T cells. In this paper, we showed that CD28 costimulation accelerated the kinetics of nuclear translocation of c-Rel. We showed that CD28 signaling, distinct from other stimuli such as phorbol 12-myristate 13-acetate, IL-1, and tumor necrosis factor-alpha, caused a sustained down-regulation of the inhibitor I kappa B alpha in Jurkat T cells. However, the levels of two other c-Rel inhibitors, namely NFKB1 (p105) and NFKB2 (p100), were not affected. Remarkably, the CD28-mediated down-regulation of I kappa B alpha was prevented by rapamycin but not by CsA. The counter-regulation of I kappa B alpha by CD28 signaling and rapamycin observed in Jurkat T cells is also reproducible in primary T cells. In contrast, the phorbol 12-myristate 13-acetate/ionomycin-mediated down-regulation of I kappa B alpha was prevented by CsA but not by rapamycin. Our data suggest that I kappa B alpha is the down-stream target of both CD28 signaling and rapamycin; a continued down-regulation of I kappa B alpha by CD28 costimulation leads to enhanced nuclear translocation of c-Rel, which in turn causes a sustained up-regulation of IL-2 gene expression.