AbstractWe here investigate the crosstalk of PKC and PKA signaling during primary CD3+ T-lymphocyte activation using pharmacologic inhibitors and activators in combination with our established panel of PKC isotype–deficient mouse T cells in vitro. PKCθ and PKA inversely affect the CD3/CD28-induced IL-2 expression, whereas other PKC isotypes are dispensable in this signaling pathway. Gene ablation of PKCθ selectively results in a profound reduction of IL-2 production; however, complete abrogation of IL-2 production in these PKCθ–/– T cells was achieved only by simultaneous coactivation of the cAMP/PKA pathway in CD3+ T cells. Conversely, the reduced IL-2 production in PKC inhibitor–treated T cells can be rescued by inhibition of the cAMP/PKA pathway in wild-type but not in PKCθ–/– T cells. Mechanistically, the cAMP/PKA and PKCθ pathways converge at the level of NF-AT, as shown by DNA binding analysis. The combined increase in PKA and decrease in PKCθ activity leads to an enhanced inhibition of nuclear NF-AT translocation. This PKCθ/PKA crosstalk significantly affects neither the NF-κB, the AP-1, nor the CREB pathways. Taken together, this opposite effect between the positive PKCθ and the negative cAMP/PKA signaling pathways appears rate limiting for NF-AT transactivation and IL-2 secretion responses of CD3+ T lymphocytes.