Protein tyrosine phosphatase PTPN22 has dual roles in promoting pathogen versus homeostatic-driven CD8 T-cell responses.

PTPN22 (protein tyrosine phosphatase non receptor 22) encodes a tyrosine phosphatase that functions as a key regulator of immune homeostasis. In particular, PTPN22 inhibits T-cell receptor signaling and selectively promotes type I interferon responses in myeloid cells. To date, there is little information on the CD8 T-cell-intrinsic role of PTPN22 in response to a viral pathogen. We unexpectedly found that PTPN22-deficient virus-specific CD8 T cells failed to accumulate in wild-type hosts after lymphocytic choriomeningitis virus infection. Lack of PTPN22 expression altered CD8 T-cell activation and antiviral cytokine production, but did not significantly affect the composition of effector and memory cell precursors. Most significantly, in vivo, PTPN22-deficient CD8 T cells showed a profound defect in upregulating STAT-1 after lymphocytic choriomeningitis virus infection and considerably less phosphorylation of STAT-1 in response to IFN-α treatment in vitro compared with their wild-type counterparts. In stark contrast, following transfer into lymphopenic mice, CD8 T-cell expansion and central-like phenotype, was considerably increased in the absence of PTPN22. Collectively, our results suggest that PTPN22 has dual roles in T-cell clonal expansion and effector function; whereas it promotes antigen-driven responses during acute infection by positively regulating interferon signaling in T cells, PTPN22 inhibits homeostatic-driven proliferation.