Glycogen Synthase Kinase-3 Modulates Cbl-b and Constrains T Cell Activation.

The decision between T cell activation and tolerance is governed by the spatial and temporal integration of diverse molecular signals and events occurring downstream of TCR and costimulatory or coinhibitory receptor engagement. The PI3K-protein kinase B (PKB; also known as Akt) signaling pathway is a central axis in mediating proximal signaling events of TCR and CD28 engagement in T cells. Perturbation of the PI3K-PKB pathway, or the loss of negative regulators of T cell activation, such as the E3 ubiquitin ligase Cbl-b, have been reported to lead to increased susceptibility to autoimmunity. In this study, we further examined the molecular pathway linking PKB and Cbl-b in murine models. Our data show that the protein kinase GSK-3, one of the first targets identified for PKB, catalyzes two previously unreported phosphorylation events at Ser476 and Ser480 of Cbl-b. GSK-3 inactivation by PKB abrogates phosphorylation of Cbl-b at these two sites and results in reduced Cbl-b protein levels. We further show that constitutive activation of PKB in vivo results in a loss of tolerance that is mediated through the downregulation of Cbl-b. Altogether, these data indicate that the PI3K-PKB-GSK-3 pathway is a novel regulatory axis that is important for controlling the decision between T cell activation and tolerance via Cbl-b.

CD4+ and CD8+ T cell survival is regulated differentially by protein kinase Ctheta, c-Rel, and protein kinase B.

An effective immune response requires the expansion and survival of a large number of activated T cells. This study compared the role of protein kinase C (PKC)theta and associated signaling molecules in the survival of activated primary CD4+ vs CD8+ murine T cells. We demonstrate that the absence of PKCtheta resulted in a moderate survival defect in CD4+ T cells and a striking survival defect of CD8+ T lymphocytes. CD8+ T cells lacking the c-Rel, but not the NF-kappaB1/p50, member of the NF-kappaB family of transcription factors displayed a similar impairment in cell survival as PKCtheta(-/-) CD8(+) T lymphocytes. This implicates c-Rel as a key target of PKCtheta-mediated survival signals in CD8+ T cells. In addition, both c-Rel(-/-) and PKCtheta(-/-) T cells also displayed impaired expression of the antiapoptotic Bcl-x(L) protein upon activation. Changes in Bcl-x(L) expression, however, did not correlate with the survival of CD4+ or CD8+ lymphocytes. The addition of protein kinase B-mediated survival signals could restore partially CD4+ T cell viability, but did not dramatically influence CD8+ survival. Active protein kinase B was also unable to restore proliferative responses in CD8+ PKCtheta(-/-) T cells. The survival of CD4+ and CD8+ T cells deficient in either PKCtheta or c-Rel, however, was promoted by the addition of IL-2. Collectively, these data demonstrate that CD4+ and CD8+ T cell survival signals are differentially programmed.