T Cell Receptor Internalization from the Immunological Synapse is Mediated by TC21 and RhoG GTPase-Dependent Phagocytosis

The immunological synapse (IS) serves a dual role for sustained T cell receptor (TCR) signaling and for TCR downregulation. TC21 (Rras2) is a RRas subfamily GTPase that constitutively associates with the TCR and is implicated in tonic TCR signaling by activating phosphatidylinositol 3-kinase. In this study, we demonstrate that TC21 both co-translocates with the TCR to the IS and is necessary for TCR internalization from the IS through a mechanism dependent on RhoG, a small GTPase previously been associated with phagocytosis. Indeed, we found that the TCR triggers T cells to phagocytose 1-6 μm beads through a TC21- and RhoG-dependent pathway. We further show that TC21 and RhoG are necessary for the TCR-promoted uptake of major histocompatibility complex (MHC) from antigen presenting cells. Therefore, TC21- and RhoG-dependence underlie the existence of a common phagocytic mechanism that drives TCR internalization from the IS together with its peptide-MHC ligand.