PTPN22 Regulates T Cell Synapse Formation through PSTPIP1- Dependent Actin Remodeling

Protein tyrosine phosphatase non-receptor type 22 (PTPN22) is a critical regulator of T cell signaling, working in concert with C-terminal SRC kinase (Csk) to dephosphorylate key signaling proteins and suppress lymphocyte activation. The R620W variant of PTPN22, one of the most prevalent mutations associated with autoimmune diseases, has been implicated in altered T cell responses, although its broader effects on T cell activation are not fully understood. Recent studies have uncovered a novel interaction between PTPN22 and proline-serine-threonine phosphatase interacting protein 1 (PSTPIP-1), a cytoskeletal adaptor protein involved in F-actin remodeling. PSTPIP-1 recruits Wiskott-Aldrich Syndrome Protein (WASp) to facilitate actin foci formation, a process integral to stabilizing TCR microclusters and amplifying downstream signaling. Given that mutations in PSTPIP-1 impair actin remodeling and localize within the PTPN22 binding domain, we hypothesized that PTPN22 modulates actin dynamics through its interaction with PSTPIP-1. Using live and fixed multi-color super-resolution fluorescence imaging, we demonstrate that PTPN22 deficiency or inhibition of its phosphatase activity leads to aberrant Arp2/3-dependent actin remodeling and exaggerated calcium signaling, particularly under low- affinity TCR stimulation. Single-protein resolution imaging via DNA-PAINT further revealed disrupted nanoscale clustering of PSTPIP-1 and TCR in PTPN22-deficient T cells, uncovering a previously unrecognized PSTPIP-1–TCR interaction in the absence of PTPN22. These findings highlight a novel PTPN22–PSTPIP-1 signaling axis, offering new insights into the molecular mechanisms that may contribute to autoimmune disease susceptibility.