Background The 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 Findings Here 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/Significance These 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.

Asymmetric cell division is a potential means by which cell fate choices during an immune response are orchestrated. Defining the molecular mechanisms that underlie asymmetric division of T cells is paramount for determining the role of this process in the generation of effector and memory T cell subsets. In other cell types, asymmetric cell division is regulated by conserved polarity protein complexes that control the localization of cell fate determinants and spindle orientation during division. We have developed a tractable, in vitro model of naive CD8+ T cells undergoing initial division while attached to dendritic cells during Ag presentation to investigate whether similar mechanisms might regulate asymmetric division of T cells. Using this system, we show that direct interactions with APCs provide the cue for polarization of T cells. Interestingly, the immunological synapse disseminates before division even though the T cells retain contact with the APC. The cue from the APC is translated into polarization of cell fate determinants via the polarity network of the Par3 and Scribble complexes, and orientation of the mitotic spindle during division is orchestrated by the partner of inscuteable/G protein complex. These findings suggest that T cells have selectively adapted a number of evolutionarily conserved mechanisms to generate diversity through asymmetric cell division.

We have previously shown that T cells utilize an evolutionarily conserved network of polarity proteins to orchestrate cell shape and polarity, and that these proteins are required for migration and immunological synapse formation in T cells (1). We describe here in vitro evidence using the OT-1 model system that T cells utilize this polarity network to conduct a physiological process not previously ascribed to lymphocytes, that of asymmetric cell division. We demonstrate that naive T cells remain attached to antigen presenting cells (dendritic cells pulsed with ovalbumin peptide) throughout cell division, and utilize this attachment to orient their axis of cell division. By maintaining the asymmetry originally associated with immunological synapse formation, the daughters of the T cell division inherit different molecular characteristics, which provide the capacity to dictate different subsequent fates. A network of PDZ-containing proteins regulates T cell polarity and morphology in motility and immunological synapse formation

CD46 is a ubiquitously expressed human cell surface protein that acts as a receptor for complements for various pathogens including the measles virus. We demonstrated that ligation of the immunoregulatory cell surface receptor, CD46, altered T cell polarity and impaired activation and effector function in response to TCR or NK cell receptor signalling. However the molecular mechanisms by which T cell function is inhibited are not known. We have previously shown that CD46 binds to the polarity protein, Discs large (Dlg), and that this interaction is important for the polarized localization of CD46. Specifically, CD46 localizes to the uropod of T cells, and to the distal pole of T cells undergoing antigen presentation. Polarization of CD46 is partially reduced by mutation of the Dlg-binding site, and is also partially reduced by mutation of the Cysteine residue in the transmembrane domain that is palmitoylated to allow association of CD46 with lipid rafts. However, comparison of CD46 mutants suggests that other determinants are also important in CD46 polarization. We hypothesize that the ERM proteins interacts with CD46, regulates its polarization in T cells, and perhaps play a role in CD46 signal transduction. To assess this we have generated mutations in the ERM-binding consensus sequences of CD46 and expressed these mutants in a uropod-containing T cell line for functional characterization. These studies will help to elucidate the molecular mechanisms by which CD46 exerts its immunoregulatory effects.