Ssu72 phosphatase directly binds to ZAP-70, thereby providing fine-tuning of TCR signaling and preventing spontaneous inflammation

Significance The appropriate T cell receptor (TCR)–mediated activation in T cells is mandatory for efficient adaptive immunity. However, the mechanism for maintaining appropriate TCR signaling remains elusive. Moreover, Ssu72 is a phosphatase that regulates RNA polymerase II activity in the nucleus, and its functions in the fine-tuning of TCR signaling has not been investigated. This study demonstrates that Ssu72 regulates the fine-tuning of TCR signaling by binding to ZAP-70 and regulating its tyrosine phosphorylation, thereby preventing spontaneous inflammation. These results provide an immunologic insight for the fine-tuning of TCR signaling during autoimmunity. ZAP-70 is required for the initiation of T cell receptor (TCR) signaling, and Ssu72 is a phosphatase that regulates RNA polymerase II activity in the nucleus. However, the mechanism by which ZAP-70 regulates the fine-tuning of TCR signaling remains elusive. Here, we found that Ssu72 contributed to the fine-tuning of TCR signaling by acting as tyrosine phosphatase for ZAP-70. Affinity purification–mass spectrometry and an in vitro assay demonstrated specific interaction between Ssu72 and ZAP-70 in T cells. Upon TCR stimulation, Ssu72-deficient T cells increased the phosphorylation of ZAP-70 and downstream molecules and exhibited hyperresponsiveness, which was restored by reducing ZAP-70 phosphorylation. In vitro assay demonstrated that recombinant Ssu72 reduced tyrosine phosphorylation of ZAP-70 via phosphatase activity. Cd4-CreSsu72fl/fl mice showed a defect in the thymic development of invariant natural killer T cells and reductions in CD4+ and CD8+ T cell numbers in the periphery but more CD44hiCD62Llo memory T cells and fewer CD44loCD62Lhi naïve T cells, compared with wild-type mice. Furthermore, Cd4-CreSsu72fl/fl mice developed spontaneous inflammation at 6 mo. In conclusion, Ssu72 phosphatase regulates the fine-tuning of TCR signaling by binding to ZAP-70 and regulating its tyrosine phosphorylation, thereby preventing spontaneous inflammation.