Overcoming inhibitory constraints to TCR signaling by targeting the pH-sensitive phosphatase suppressor of T-cell receptor signaling 1 (STS1)

Abstract T cells are critically important for adaptive immune responses. Therefore, maintaining their proper homeostasis an d controlling the magnitude of their responses is critical. To control T cell responses, T cell activity is negatively regulated by inhibitory molecules. The Cbl protein family are ubiquitin E3 ligases that suppress TCR signaling by mediating the degradation of TCR signaling molecules. However, studies and our results suggest that Cbl proteins down-regulate phospho-proteins without modulating the total level of target proteins, but the mechanisms are unclear. Here, we show that Cbl-b serves as an adapter to recruit STS1, an unconventional tyrosine phosphatase, to dephosphorylate target proteins and inhibit TCR signaling. STS1 binds to a Cbl-b PPVPP motif via its SH3 domain, and the interaction requires Cbl-b phosphorylation by ZAP70. Elimination of the PPVPP motif compromises Cbl-b suppressive activity, and deletion of STS1 leads to T cell hyper responsiveness to TCR activation which phenocopies Cbl-b deficiency. Furthermore, STS1 uniquely relies on histidine to catalyze dephosphorylation, making it hyperactive in acidic environment. As a result, STS1 and Cbl-b deficient T cells remain hyper responsive and insensitive to low pH whereas WT cells are largely suppressed. Animals with STS1 or Cbl-b deletion show delayed tumor growth and improved survival. Together, these results demonstrate Cbl-b partners with STS1 whose activity is responsive to pH changes to modulate TCR signaling. Moreover, they suggest the pH sensitivity of STS1 is uniquely suited to suppress T cell responses that occur in vivo, such those that occur within inflammatory settings and within tumors, where lower pH conditions may be encountered. Supported by fellowship from Cancer Research Institute (A133417)