T lymphocyte activation signals regulate the expression and transactivation function of retinoid X receptor (RXR) α through an interplay of complex signaling cascades that are not yet fully understood. We show that cellular Ser/Thr protein phosphatases (PPs) play an important role in mediating these processes. Inhibitors specific for PP1 and PP2A decreased basal expression of RXRα RNA and protein in T lymphocyte leukemia Jurkat cells and prevented activation-induced RXRα accumulation in these cells. In addition, these inhibitors attenuated the RXR responsive element (RXRE)-dependent transcriptional activation in transient transfection assays. Inhibitors of calcineurin (CN), by contrast, did not have any effect on the basal RXRα expression and even augmented activation-induced RXRα expression. Expression of a dominant-active (DA) mutant of CN together with a DA mutant of protein kinase C (PKC)θ, a novel PKC isoform, significantly increased RXRE-dependent transcription. Expression of catalytically inactive PKCθ or a dominant-negative mutant of PKCθ failed to synergize with CN and did not increase RXRE-dependent transcription. Expression of a DA mutant of PKCα or treatment with PMA was found to attenuate PKCθ and CN synergism. We conclude that PP1, PP2A, and CN regulate levels and transcriptional activation function of RXRα in T cells. In addition, CN synergizes with PKCθ to induce RXRE-dependent activation, a cooperative function that is antagonized by the activation of the conventional PKCα isoform. Thus, PKCθ and PKCα may function as positive and negative modulators, respectively, of CN-regulated RXRE-dependent transcription during 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 Ser and Ser 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.
Inflammation following tissue damage promotes lymphocyte recruitment, tissue remodelling, and wound healing while maintaining self tolerance. Endogenous signals associated with tissue damage and cell death have been proposed to initiate and instruct immune responses following injury. Here we have examined the effects of elevated levels of a candidate endogenous danger signal, heat shock cognate protein 70 (Hsc70), on stimulation of inflammation and autoimmunity following cell damage. We find that damage to pancreatic β-cells expressing additional cytosolic Hsc70 leads to an increased incidence of diabetes in a transgenic mouse model. Steady-state levels of activated APC and T cell populations in the draining lymph node were enhanced, which further increased following streptozotocin-induced β-cell death. In addition, pro-inflammatory serum cytokines, and lymphocyte recruitment were increased in Hsc70 transgenic mice. Islet-antigen-specific T cells underwent a greater extent of proliferation in the lymph nodes of mice expressing Hsc70 following β-cell damage, suggesting elevated antigen presentation following release of antigen in the presence of Hsc70. These findings suggest that an elevated content of Hsc70 in cells undergoing necrotic or apoptotic cell death can increase the extent of sterile inflammation and increase the susceptibility to autoimmunity.
CCR5 is one of the primary coreceptors for Env-mediated fusion between cells and human immunodeficiency virus type 1 (HIV-1). Analyses of CCR5 variants in cohorts of HIV-1 high-risk individuals led to the identification of multiple single amino-acid substitutions, which may have functional consequences. This study focused on eight naturally occurring allelic variants located between amino-acid residues 60 and 334 of CCR5. All studied allelic variants were highly expressed on the cell surface of HEK-293 cells and permissive for HIV-1 infection. Variant G301V showed 3.5-fold increase in 50% effective concentration (EC 50 ) for CCL4 (MIP 1beta) in a competitive binding assay. There was also a significant reduction in CCL5 (RANTES) EC 50 for the R223Q, A335V and Y339F variants. The most unexpected functional abnormality was exhibited by the R60S variant that exhibited a loss of ligand-induced desensitization in chemotaxis assays, but showed normal CCL4 and CCL5 binding avidity. This mutation is located in the first intracellular loop, a domain that has not previously been shown to be involved in receptor desensitization. In conclusion, our results support earlier studies showing that these naturally occurring point mutations do not limit HIV-1 infection, and indicated that single amino-acid changes can have unexpected functional consequences.
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