Effects of Ruxolitinib on Murine Regulatory T Cells Are Immune Context Dependent.

Aplastic anemia is a bone marrow failure (BMF) disorder characterized by pancytopenia and hypocellular marrow from an immune-mediated etiology. Regulatory T cells (Tregs) prevent autoimmunity by suppressing autoreactive T cells. We recently demonstrated the efficacy of ruxolitinib, a JAK 1/2 inhibitor, in attenuating murine BMF. Herein, we investigated the changes of Tregs in the context of ruxolitinib treatment for murine BMF. Tregs are conventionally identified by surface expression of CD4 and CD25, in addition to intracellular transcription factor FoxP3. Ruxolitinib promoted expansion of Tregs in BMF mice defined by increased expression of FoxP3 in CD4 T cells, but suppressed expression of activation marker CD25 in CD4 and CD8 T cells. In this context, CD25 is no longer a reliable surface marker for Tregs. We observed strong co-expression of FoxP3 with surface marker GITR instead of CD25 in ruxolitinib-treated BMF mice. FACS-sorted CD4GITR cells showed high FoxP3 expression and intact suppressive function in vitro, suggesting GITR to be a surrogate marker for Tregs. In contrast to its expansive effect on Tregs in BMF condition, ruxolitinib suppressed Tregs in normal and sublethal irradiation conditions, indicating the effects of ruxolitinib on Tregs are immune-context dependent.