PTPN22 and systemic lupus erythematosus: Because PTPN22 is a major SLE genetic risk factor (21) and disease-activity dependent increase in free cytosolic PKA Cs has been observed in lupus T cells (28), we decided to evaluate the phosphorylation of PTPN22 Ser449 in PBMCs from patients with SLE with active disease (SLE disease activity index, SLEDAI >2) (29) and healthy controls (see patient and control data in table S1).