PTPN22 phosphorylation acts as a molecular rheostat for the inhibition of TCR signaling

The hematopoietic-specific protein tyrosine phosphatase nonreceptor type 22 (PTPN22) is encoded by a major autoimmunity risk gene. PTPN22 inhibits T cell activation by dephosphorylating substrates involved in proximal T cell receptor (TCR) signaling. Here, we found by mass spectrometry that PTPN22 was phosphorylated at Ser(751) by PKC alpha in Jurkat and primary human T cells activated with phorbol ester/ionomycin or antibodies against CD3/CD28. The phosphorylation of PTPN22 at Ser(751) prolonged its half-life by inhibiting K48-linked ubiquitination and impairing recruitment of the phosphatase to the plasma membrane, which is necessary to inhibit proximal TCR signaling. Additionally, the phosphorylation of PTPN22 at Ser(751) enhanced the interaction of PTPN22 with the carboxyl-terminal Src kinase (CSK), an interaction that is impaired by the PTPN22 R620W variant associated with autoimmune disease. The phosphorylation of Ser(751) did not affect the recruitment of PTPN22 R620W to the plasma membrane but protected this mutant from degradation. Together, out data indicate that phosphorylation at Ser(751) mediates a reciprocal regulation of PTPN22 stability versus translocation to TCR signaling complexes by CSK-dependent and CSK-independent mechanisms.