Dendritic epidermal T cells (DETCs) remain activated in vivo at steady state through their monomorphic Vgamma5-Vdelta1 TCR, forming phosphotyrosine-rich aggregates located on projections, called PALPs (Chodaczek et al. 2012). This recent discovery suggests the existence of beneficial self-reactivity on the part of the evolutionarily-preserved γδ TCR. Located at barrier-forming squamous keratinocyte junctions, and remarkably long-lived, the PALPs display multiple hallmarks of true immunological synapses, including TCR clustering and CD3-zeta and ZAP70 tyrosine phosphorylation. Using two independent super-resolution microscopy techniques: the structured illumination microscopy (SIM) and stimulated emission depletion (STED), we show that PALPs consist of several tens of peripherally distributed, tyrosine-phosphorylated TCR microclusters. Individual microclusters are uniformly sized at 150-160 nm and are associated with distinct signaling intermediates, suggesting that they represent supra-molecular signaling assemblies of a defined composition. Intravital fluorescence recovery after photobleaching and high speed time-lapse imaging showed that small intracellular cargo granules trafficked bi-directionally within T cell protrusions between large stationary granules and the PALPs. These studies reveal the PALPs as dynamically active TCR-activatory immunological synapses, and suggest that this novel form of physiological TCR self-reactivity mediates homeostatic secretory functions.
- Copyright © 2013 by The American Association of Immunologists, Inc.