I-KAPPA-B KINASES SERVE AS A TARGET OF CD28 SIGNALING

Optimal T cell. activation and interleukin-2 production requires a sec ond signal in addition to antigen-mediated T cell receptor (TCR) signa ling. The CD28 molecule has been demonstrated to act as an effective c ostimulatory molecule upon binding by B7.1 or B7.2 present on antigen- presenting cells, The CD28 signal acts in concert with the TCR signal to significantly augment activation of the NF-kappa B family of transc ription factors. The interleukin-a gene is regulated by NF-kappa B amo ng other transcription factors, in part, via a CD28 responsive element (CD28RE) present in the IL-2 promoter. Enhanced activation of NF-kapp a B by CD28 is mediated by rapid phosphorylation and proteasome-mediat ed degradation of the NF-kappa B inhibitory proteins I kappa B alpha a nd I kappa B beta, which allows for accelerated nuclear expression of the liberated NF-kappa B, Herein, we provide evidence that the catalyt ic activities of two recently identified I kappa B kinases, IKK alpha and IKK beta, are significantly elevated when T cells are stimulated t hrough CD28 in addition to mitogen treatment. Catalytically inactive f orms of IKKs are able to block the in vivo phosphorylation of I kappa B alpha induced by mitogen and CD28, Furthermore, CD28-mediated report er gene transactivation of the CD28RE/AP-1 composite element is consis tently attenuated by the IKK mutants. These findings suggest that cell ular signaling pathways initiated at the TCR and CD28 converge at or u pstream of IKK, resulting in more robust kinase activity and enhanced and prolonged NF-kappa B activation.