TSG101 prevents chronic T cell receptor signaling in the immunological synapse

In this dissertation, I focus on the role of TSG101 in regulating T cell receptor signaling through the immunological synapse.;Recognition of cognate MHCp by surface-expressed T cell receptors (TCR) on CD4+ T cells results not only in signal transduction and mobilization of critical transcription factors driving T cell proliferation, differentiation, and cytokine production, but also downregulation of surface TCR expression. This downregulation is critical to appropriate limitation of T cell responses. Chronic TCR signaling in the absence of TCR downregulation is associated with systemic autoimmunity, T cell exhaustion, and activation-induced cell death, while premature termination of TCR signaling is associated with T cell anergy. The processes of TCR signal initiation and signal termination are regulated through the patterning of receptor-ligand interactions within the T cell-APC contact surface known as the immunological synapse (IS). In this dissertation, I show that the critical ESCRT-I complex member TSG101 induces signal termination and degradation of engaged TCR within the IS. TSG101 is rapidly recruited to the IS following activation, where it interacts with ubiquitinated TCR complexes and specifically sorts them into centrally located multi-vesicular bodies for degradation while allowing non-ubiquitinated co-stimulatory ligands to continue signaling. Suppression of TSG101 results in chronic TCR signaling and elevated IL-2 production. TSG101 executes these sorting processes primarily at the T cell plasma membrane and in a manner that is independent of the putative upstream ESCRT-0 complex. The critical ESCRT-III component Vps24 acts downstream of TSG101 to induce TCR degradation but has no effect on TCR signaling. I also show that certain low avidity ligands are capable of inducing intermediate T cell signaling while not inducing TCR downregulation by avoiding recruitment of TSG101 to ligand-engaged TCR MCs. These ligands are dependent on CD28 for recognition, making them likely to induce tolerance breaking and autoimmunity. Indeed, I observe similar phenotypic results in autoreactive T cells interacting with autoimmune peptide-MHC complexes. I also demonstrate that only hyperstable TCR clusters appear to be downregulated during antigen recognition, and therefore offer a novel mechanism by which TCR-pMHC avidity correlates with actin-independent TCR cluster formation leading to ubiquitination and downregulation in the actin-depleted IS center. Finally, I investigate the patterning of a co-receptor, CD2, which occupies a similar intermembrane distance as TCR-pMHC interactions when engaged by its ligand, CD48. CD2-CD48 interactions are initiated in the IS periphery and extinguished in the IS center in a manner that is highly reminiscent of TCR-pMHC interactions. Thus, the IS globally regulates TCR and co-receptor signaling through construction of unique, spatially segregated domains for signal initiation and termination.