IL-27 suppresses RANKL expression in CD4+ T cells in part through STAT3

The receptor activator of NF-κB ligand (RANKL), which is expressed by not only osteoblasts but also activated T cells, plays an important role in bone-destructive diseases such as rheumatoid arthritis. IL-27, a member of the IL-6/IL-12 family cytokines, activates STAT1 and STAT3, promotes early helper T (Th)1 differentiation and generation of IL-10-producing type 1 regulatory T (Tr1) cells, and suppresses the production of inflammatory cytokines and inhibits Th2 differentiation. In addition, IL-27 was recently demonstrated to not only inhibit Th17 differentiation but also directly act on osteoclast precursor cells and suppress RANKL-mediated osteoclastogenesis through STAT1-dependent inhibition of c-Fos, leading to amelioration of the inflammatory bone destruction. In the present study, we investigated the effect of IL-27 on the expression of RANKL in CD4+ T cells. We found that IL-27 greatly inhibits cell surface expression of RANKL on naive CD4+ T cells activated by T cell receptor ligation and secretion of its soluble RANKL as well. The inhibitory effect was mediated in part by STAT3 but not by STAT1 or IL-10. In contrast, in differentiated Th17 cells, IL-27 much less efficiently inhibited the RANKL expression after restimulation. Taken together, these results indicate that IL-27 greatly inhibits primary RANKL expression in CD4+ T cells, which could contribute to the suppressive effects of IL-27 on the inflammatory bone destruction.