STAT5 phosphorylation in CD4 T cells from patients with SLE is related to changes in their subsets and follow-up disease severity.

Activation of the STAT5 signaling pathway up-regulates antiapoptotic protein Bcl2 and drives proliferation of autoreactive conventional CD4 T cells (Tcons ). In systemic lupus erythematosus (SLE), an increased T cell Bcl2 content and perturbed homeostasis of CD45RA- FOXP3hi activated regulatory T cells (aTregs ) were described. We assessed Tcon /Treg subsets and phosphorylation of STAT5 (pSTAT5) in blood T cells from patients with SLE by using conventional and imaging flow cytometry. Forty-one patients with SLE, 33 healthy controls, and 29 patients with rheumatoid arthritis were included. Long-term monitoring was performed in 39 patients with SLE, which were followed longitudinally for up to 1000 d. Significantly increased Bcl2 protein content in T cells from patients with SLE was associated with IL-7-dependent STAT5 activation, expressed as increased basal levels and nuclear localization of pSTAT5. pSTAT5 levels were significantly increased in the FOXP3 low-expressing CD4+ T cell subsets but not in the aTreg subset, which was significantly decreased in patients with SLE. In contrast to aTreg , SLE Tcon displayed significantly increased pSTAT5 and Bcl2 levels. Moreover, the percentage of Tcon -expressing proliferation marker Ki-67 was significantly increased in patients with SLE and was positively correlated with CD4 T cell pSTAT5 levels. Finally, a subgroup of patients characterized by an increased Tcon -pSTAT5/aTreg -pSTAT5 ratio experienced a more aggressive-relapsing disease course and displayed higher time-adjusted cumulative CD4 T cell pSTAT5 levels during follow-up, which were positively correlated with time-adjusted cumulative disease activity. Our results indicate that imbalanced STAT5 phosphorylation, which is related to Bcl2 and Ki-67 expression, may confer survival and proliferative advantage to Tcon over aTreg and could represent a possible marker of SLE disease severity.