Role of Polarity and Polarized Secretion in Dendritic Cells Functions

A timely and spatially well-organized utilization of polarity determinants appears to be an absolute functional prerequisite of all cells, including leukocytes. Numerous in vitro and in vivo studies performed in the field of DCs biology have strongly stressed the idea that DCs ascertain a correct immune response by finely tuning signal transmission within IS and by temporally regulating contact duration with T cells. In this study we elucidate another important aspect of DCs biology as we demonstrate that TLR-stimulated DCs preferentially secrete cytokines (IL-12) within the IS. We provide further insights into the mechanism of IL-12 delivery at the synapse area as we show that it strictly relies on microtubule organizing center (MTOC) reorientation toward antigen-specific synapse. On the molecular level, we find that the phenomenon of MTOC polarization in DCs is regulated by Cdc42 (a Rho GTPase) and Wiskott-Aldrich protein (WASp). Interfering with MTOC polarization had drastic impact on IL-12-dependent events occurring very early following synapse formation: pSTAT4 phosphorilation and IFN-γ production. At later time points, the absence of MTOC reorientation reflected in the decreased survival of T cells and strong reduction in their IFN-γ-producing capacity. Polarity proteins belonging to three main families (Par, Crb and Scrib), have been shown essential in ensuring polarized state of various cells. We find that in immature DCs, disc large protein (Dlg) is preferentially clustered underneath the membrane area whereas it undergoes relocation and hyperphosporilation following TLR stimulation. During DCs-T cell contact, we observed that Dlg clusters at DC-T cell interface in antigen-specific manner. Dlg depletion in DCs markedly reduced early, synapse-specific IFN-γ mRNA production by T cells; that reflected in a decreased IFN-γ production by differentiated T cells at alter time-points. Collectively, our results revealed that polarized cytokine secretion and Dlg clustering at IS are features acquired by DCs through their maturation program and provide a proof of principle that T cells fate is highly controlled by DCs.