T cells don’t simply switch on—they reshape themselves. When these immune sentinels recognize a target, they rapidly reorganize their internal scaffolding to build an immunological synapse, a nanoscale interface that determines how strongly they respond. But that architectural overhaul needs brakes. Without them, T cells risk becoming hypersensitive, reacting to weak cues, and drifting toward autoimmunity. Now, new work reveals that one of those brakes—PTPN22 (proline-serine-threonine phosphatase–interacting protein 1)—acts not only on signaling molecules but also on the cytoskeletal machinery that sculpts the synapse itself.

In a study published in Science Signaling, lead author Megan Joseph, PhD, of University College London and colleagues uncover how PTPN22 interacts with the cytoskeletal adaptor protein PSTPIP1 to restrain actin remodeling at the T‑cell synapse. Their paper, “PTPN22 regulates T-cell synapse formation through PSTPIP1-dependent actin remodeling,” shows that this phosphatase plays a previously unappreciated role at the plasma membrane, shaping how T cells respond to antigens of varying affinity. As the authors wrote, “These findings uncover a PTPN22–PSTPIP1 signaling axis that is critical for regulating cytoskeletal remodeling and receptor organization, providing insights into T-cell hyperactivation that may be relevant to autoimmune disease.”

PTPN22 is already well known as a negative regulator of early T‑cell activation. Variants in the gene, including the autoimmune‑associated R620W allele, have been linked to diseases ranging from lupus to rheumatoid arthritis. Using super‑resolution DNA‑PAINT imaging, Joseph et al. visualized how T cells reorganize their actin networks as they engage activating ligands. In wild‑type Jurkat cells, PTPN22 helped maintain orderly actin dynamics. In its absence, however, PSTPIP1 accumulated at T cell receptors (TCRs), disrupting Arp2/3‑dependent actin polymerization and generating dense central F‑actin foci, as well as enhanced Ca²⁺ signaling, especially under low-affinity stimulation of the TCR, according to the paper.

This hyper‑remodeling had functional consequences. PTPN22‑deficient cells became unusually sensitive to low‑affinity antigens, responding more vigorously than their wild‑type counterparts. “Autoimmunity is inherently linked to immune tolerance mechanisms normally associated with low-affinity TCR responses to self, which, when breeched lead to inappropriate immune reactions. To better understand how PTPN22 contributes to these processes, we used WT and PTPN22 KO TCR^−/− Jurkat cells engineered to express a transgenic TCR with high affinity for the pTax peptide and low affinity for the pHuD peptide,” the authors wrote.

Joseph and colleagues suggest that understanding this axis could inform both autoimmune research and efforts to modulate T‑cell activation in cancer immunotherapy. By mapping how PTPN22 and PSTPIP1 coordinate actin remodeling, the study provides a mechanistic foothold for exploring how synapse architecture shapes immune outcomes.

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