Tyrosine kinases couple the T cell receptor (TCR) to discrete signaling cascades, each of which is capable of inducing a distinct functional outcome. Precisely how TCR signals are channeled toward specific targets remains unclear. TCR stimulation triggers 'alternative' activation of the mitogen-activated protein kinase p38, whereby the Lck and Zap70 tyrosine kinases directly activate p38. Here we report that alternatively activated p38 associated with the Dlgh1 MAGUK scaffold protein. 'Knockdown' of Dlgh1 expression blocked TCR-induced activation of p38 and the transcription factor NFAT but not of the mitogen-activated protein kinase Jnk or transcription factor NF-kappaB. A Dlgh1 mutant incapable of binding p38 failed to activate NFAT. Along with reports that the CARMA1 MAGUK scaffold protein coordinates activation of Jnk and NF-kappaB but not of p38 or NFAT, our findings identify MAGUK scaffold proteins as 'orchestrators' of TCR signal specificity.

Lipid raft membrane compartmentalization and membrane-associated guanylate kinase (MAGUK) family molecular scaffolds function in establishing cell polarity and organizing signal transducers within epithelial cell junctions and neuronal synapses. Here, we elucidate a role for the MAGUK protein, Dlgh1, in polarized T cell synapse assembly and T cell function. We find that Dlgh1 translocates to the immune synapse and lipid rafts in response to T cell receptor (TCR)/CD28 engagement and that LckSH3-mediated interactions with Dlgh1 control its membrane targeting. TCR/CD28 engagement induces the formation of endogenous Lck–Dlgh1–Zap70–Wiskott-Aldrich syndrome protein (WASp) complexes in which Dlgh1 acts to facilitate interactions of Lck with Zap70 and WASp. Using small interfering RNA and overexpression approaches, we show that Dlgh1 promotes antigen-induced actin polymerization, synaptic raft and TCR clustering, nuclear factor of activated T cell activity, and cytokine production. We propose that Dlgh1 coordinates TCR/CD28-induced actin-driven T cell synapse assembly, signal transduction, and effector function. These findings highlight common molecular strategies used to regulate cell polarity, synapse assembly, and transducer organization in diverse cellular systems.

TCR engagement triggers the polarized recruitment of membrane, actin, and transducer assemblies within the T cell–APC contact that amplify and specify signaling cascades and Teffector activity. We report that caveolin-1, a scaffold that regulates polarity and signaling in nonlymphoid cells, is required for optimal TCR-induced actin polymerization, synaptic membrane raft polarity, and function in CD8, but not CD4, T cells. In CD8+ T cells, caveolin-1 ablation selectively impaired TCR-induced NFAT-dependent NFATc1 and cytokine gene expression, whereas caveolin-1 re-expression promoted NFATc1 gene expression. Alternatively, caveolin-1 ablation did not affect TCR-induced NF-κB–dependent Iκbα expression. Cav-1−/− mice did not efficiently promote CD8 immunity to lymphocytic choriomeningitis virus, nor did cav-1−/− OT-1+ CD8+ T cells efficiently respond to Listeria mono-cytogenes-OVA after transfer into wild-type hosts. Therefore, caveolin-1 is a T cell-intrinsic orchestrator of TCR-mediated membrane polarity and signal specificity selectively employed by CD8 T cells to customize TCR responsiveness.

During thymocyte development, the Tcell receptor (TCR) can discriminate major histocompatibility complex (MHC)/ peptide ligands over a narrow range of affinities and translate subtle differences into functional fate decisions. How small differences in TCR input are translated into absolute differences in functional output is unclear. We examined the effects of galectin-1 ablation in the context of class-I-restricted thymocyte development. Galectin-1 expression opposed TCR partial agonist-driven positive selection, but promoted TCR agonist-driven negative selection of conventional CD8 ؉ T cells. Galectin-1 expression also promoted TCR agonist-driven CD8␣␣ intestinal intraepithelial lymphocytes ( IntroductionDuring T-cell development in the thymus, thymocytes are subjected to selection processes designed to ensure the generation of a diverse repertoire of functional T cells (positive selection), the removal of self-reactive thymocytes with auto-aggressive potential (negative selection), and the development of regulatory T populations that function in maintaining self-tolerance. 1 Selection is cued through T-cell receptor (TCR) interactions with specific self-peptide/major histocompatibility complexes (MHCs) expressed by thymic antigen-presenting cells. Very weak TCR-peptide-MHC interactions are insufficient to elicit signals required for thymocyte survival, whereas exceptionally strong TCRagonist peptide-MHC interactions direct thymocyte apoptosis during negative selection or promote the generation of regulatory CD8␣␣ intestinal intraepithelial lymphocytes (IEL) or T-regulatory cells. [2][3][4] Thymocytes bearing TCRs with intermediate affinity for self-peptide/ MHC complexes (partial agonists) are cued to survive and initiate programs for development into mature T-helper or cytotoxic T lymphocyte (CTL) lineages during positive selection. 4 How the TCR discriminates subtle differences in ligand binding and translates them into distinct signals and functional fates remains unresolved. Recent findings indicate that developing thymocytes convert small differences in TCR binding affinity into discrete functional outcomes by controlling the compartmentalization, duration, and intensity of mitogen-activated protein (MAP) kinase signaling cascades. 5,6 However, it remains unclear how TCR engagement differentially couples to MAP kinase activation pathways under these circumstances.Immunologists have long recognized that developing thymocytes express characteristic patterns of cell surface glycosylation. 7,8 Plant lectins were first used to define thymocyte subsets expressing particular oligosaccharide ligands. 7,8 For instance, peanut agglutinin (PNA) binds developing CD4 ϩ CD8 ϩ double-positive (DP) thymocytes, but not mature CD4 ϩ or CD8 ϩ single-positive (SP) thymocytes due to its specificity for the O-linked disaccharide Gal␤1,3GalNAc, which becomes masked on mature SP thymocytes due to sialic acid addition. 8 In contrast, Sambucus nigra lectin (SNA), a lectin that recognizes ␣-2, 6-linked sialic acid on N-glycans, b...

CD45 is dynamically repositioned within lipid rafts and the immune synapse during T cell activation, although the molecular consequences of CD45 repositioning remain unclear. In this study we examine the role of CD45 membrane compartmentalization in regulating murine T cell activation. We find that raft-localized CD45 antagonizes IL-2 production by opposing processive TCR signals, whereas raft-excluded CD45 promotes ERK-dependent polarized synaptic lipid raft clustering and IL-2 production. We propose that these dual CD45 activities ensure that only robust TCR signals proceed, whereas signals meeting threshold requirements are potentiated. Our findings highlight membrane compartmentalization as a key regulator of CD45 function and elucidate a novel signal transduction pathway by which raft-excluded CD45 positively regulates T cell activation.

T cell burst size is regulated by the duration of TCR engagement and balanced control of Ag-induced activation, expansion, and apoptosis. We found that galectin-1-deficient CD8 T cells undergo greater cell division in response to TCR stimulation, with fewer dividing cells undergoing apoptosis. TCR-induced ERK signaling was sustained in activated galectin-1-deficient CD8 T cells and antagonized by recombinant galectin-1, indicating galectin-1 modulates TCR feed-forward/feedback loops involved in signal discrimination and procession. Furthermore, recombinant galectin-1 antagonized binding of agonist tetramers to the TCR on activated OT-1 T cells. Finally, galectin-1 produced by activated Ag-specific CD8 T cells negatively regulated burst size and TCR avidity in vivo. Therefore, galectin-1, inducibly expressed by activated CD8 T cells, functions as an autocrine negative regulator of peripheral CD8 T cell TCR binding, signal transduction, and burst size. Together with recent findings demonstrating that gal-1 promotes binding of agonist tetramers to the TCR of OT-1 thymocytes, these studies identify galectin-1 as a tuner of TCR binding, signaling, and functional fate determination that can differentially specify outcome, depending on the developmental and activation stage of the T cell.

BackgroundThe polarized reorganization of the T cell membrane and intracellular signaling molecules in response to T cell receptor (TCR) engagement has been implicated in the modulation of T cell development and effector responses. In siRNA-based studies Dlg1, a MAGUK scaffold protein and member of the Scribble polarity complex, has been shown to play a role in T cell polarity and TCR signal specificity, however the role of Dlg1 in T cell development and function in vivo remains unclear.Methodology/Principal FindingsHere we present the combined data from three independently-derived dlg1-knockout mouse models; two germline deficient knockouts and one conditional knockout. While defects were not observed in T cell development, TCR-induced early phospho-signaling, actin-mediated events, or proliferation in any of the models, the acute knockdown of Dlg1 in Jurkat T cells diminished accumulation of actin at the IS. Further, while Th1-type cytokine production appeared unaffected in T cells derived from mice with a dlg1germline-deficiency, altered production of TCR-dependent Th1 and Th2-type cytokines was observed in T cells derived from mice with a conditional loss of dlg1 expression and T cells with acute Dlg1 suppression, suggesting a differential requirement for Dlg1 activity in signaling events leading to Th1 versus Th2 cytokine induction. The observed inconsistencies between these and other knockout models and siRNA strategies suggest that 1) compensatory upregulation of alternate gene(s) may be masking a role for dlg1 in controlling TCR-mediated events in dlg1 deficient mice and 2) the developmental stage during which dlg1 ablation begins may control the degree to which compensatory events occur.Conclusions/SignificanceThese findings provide a potential explanation for the discrepancies observed in various studies using different dlg1-deficient T cell models and underscore the importance of acute dlg1 ablation to avoid the upregulation of compensatory mechanisms for future functional studies of the Dlg1 protein.

Subunit vaccines are typically poorly immunogenic when administered alone, and require adjuvants for robust responses. Triggering TLRs to boost antigen-specific adaptive immunity is an attractive approach to increase the potency and quality of vaccines. However, recent reports suggest that alterations in TLR expression are associated with the pathogenesis of inflammatory and autoimmune diseases. To compare genetic studies with adjuvant studies, we examined whether stimulation through a TLR agonist induces or increases the autoimmune phenotype of healthy or autoimmune mice. C57BL/6, MRL/lpr, and Fcγr2b-deficient mice were dosed i.p. with Poly I:C every other day for 3 weeks, and monitored for signs of autoimmunity over 3 months. A separate group of mice was vaccinated three times i.m. with rPA anthrax antigen with or without Poly I:C with 2 weeks between doses. Immunized groups exhibited robust responses to vaccine and C57BL/6 and MRL/lpr mice showed a statistically significant increase in anti-rPA IgG responses in the presence of Poly I:C. Interestingly, Fcγr2b-/- mice showed increases with the base rPA vaccine, which was not significantly increased when Poly I:C was used as an adjuvant. In the chronically dosed groups, we also observed subtle alterations in levels of total antibody and some autoantibodies. However, there were no statistically significant differences in autoimmune syndrome, as measured by proteinurea, kidney pathology, weight loss, and mortality, with Poly I:C administration in chronic or vaccination mode. Taken together, these results suggest that administration of TLR3 agonists in chronic or vaccination mode does not induce or exacerbate models of systemic lupus erythematosus.