PTPN22 and systemic lupus erythematosus: Given that PTPN22 phospho-Ser449 functionally regulates PTPN22 to promote T cell responses and PTPN22 has a strong genetic association with SLE, we next sought to collect data to support a role of Ser449 phosphorylation in SLE pathogenesis by subjecting Ptpn22S452A mice to a T cell–dependent inducible model of SLE (45, 46).