Small size of membrane-tethered ligands regulates kinase-phosphatase segregation in γδ TCR triggering.

Abstract

Abstract Ligand recognition by the T cell antigen receptor (TCR) is central to effective immunity. Unlike αβ T cells, γδ T cells do not undergo canonical thymic selection, and consequently are not restricted by pMHC antigens or the co-receptors CD4/8. Some of these unconventional T cells recognize invariant membrane-tethered ligands, and their rapid activation and recruitment to sites of inflammation serves as a ‘first line of defense’ against incipient infection. The substantial structural and stoichiometric similarities between αβ and γδ TCRs prompted us to investigate whether γδ TCR ligand-recognition and triggering shared basic physicochemical constraints with conventional αβ TCRs. We chose as a model system the stress-inducible murine surface protein T22 that is recognized by the G8 γδ TCR - for which detailed biophysical and structural information is available. Since T22 is similar in size to pMHC molecules, we tested whether ‘size-based’ kinase-phosphatase segregation, following G8 engagement by T22, controls initiation of TCR signaling. In our ongoing studies, we have established that the small axial size of T22 is critical for G8 triggering. Elongation of the T22 ectodomain leads to failure of TCR phosphorylation and IL-2 responses. Using optical and electron microscopy we show that elongated T22/G8 complexes increases access of inhibitory phosphatases to sites of TCR engagement at cell-cell contacts. We are testing whether truncation of the ectodomain of the large cell surface phosphatases CD45/CD148, which are normally sterically segregated from engaged TCR, affects triggering. Finally, we are adapting these strategies, to investigate mechanisms of triggering by γδ T cells that recognize small-molecule organic compounds.