Inhibition of T Cell-Dependent and RANKL-Dependent Osteoclastogenic Processes Associated With High Levels of Bone Mass in Interleukin-15 Receptor-Deficient Mice

Objective. T cell production of RANKL, interferon-gamma (IFN), and other cytokines in inflammatory processes such as rheumatoid arthritis or secondary to conditions such as estrogen deficiency stimulates osteoclast activity, which leads to bone resorption and bone loss. The purpose of this study was to characterize the effects of interleukin-15 (IL-15), a master T cell growth factor whose role in bone remodeling remains unknown. Methods. We used mice lacking the IL-15 receptor (IL-15R alpha(-/-)) to investigate the effects of IL-15 on osteoclast development, T cell and dendritic cell activation in vitro and in vivo, bone mass, and microarchitecture in intact and ovariectomized (OVX) mice. Results. In wild-type (WT) animals, IL-15 and RANKL provided a costimulatory signal for osteoclast development. Spleens from IL-15R alpha(-/-) mice contained few c-Kit+ osteoclast precursors, and the expression of NF-ATc1 and the osteoclastogenic response to RANKL were impaired. In addition, dendritic cell-dependent and T cell-dependent mechanisms of osteoclast activation, including RANKL and IFN gamma production, were impaired in IL-15R alpha(-/-) mice. In turn, IL-15R alpha(-/-) T cells failed to stimulate WT osteoclasts, whereas WT T cells failed to stimulate IL-15R alpha(-/-) osteoclasts. Compared with WT mice, both intact and OVX IL-15R alpha(-/-) mice had significantly greater bone mineral density and microarchitecture, including a higher trabecular bone volume fraction and cortical thickness. The numbers of osteoclasts on the bone surface as well as markers of bone turnover were significantly decreased in IL-15R alpha(-/-) mice. Conclusion. In the absence of IL-15 signaling, several converging mechanisms of osteoclastogenesis are inhibited, both directly and indirectly, through T cells, which leads to a high bone mass phenotype. Targeting the IL-15 pathway may represent a novel therapeutic approach to treating primary and secondary osteoporosis.