Reduction of particle-induced osteolysis by interleukin-6 involves anti-inflammatory effect and inhibition of early osteoclast precursor differentiation

The goal of this study was to define the anti-osteoclastogenic and/or anti-inflammatory role of IL-6 in inflammatory bone resorption using in vivo and in vitro methods. To this end, titanium particles were placed on murine calvaria, and bone resorption and osteoclast formation quantified in wild-type and IL-6^-/- mice. In this model, calvarial bone loss and osteoclast formation were increased in titanium-treated IL-6^-/- mice. Although basal numbers of splenic osteoclast precursors (OCP) were similar, IL-6^-/- mice treated with particles in vivo had increased splenic OCP suggesting an enhanced systemic inflammatory response. In vitro osteoclastogenesis was measured using splenic (OCP) at various stages of maturation, including splenocytes from WT, IL-6^-/- and TNFα transgenic mice. ELISA was used to measure TNFα production. IL-6 inhibited osteoclastogenesis in early OCP obtained from wild-type and IL-6^-/- spleens. Pre-treatment of OCP with M-CSF for three days increased the CD11b^high/c-Fms+ cell population, resulting in an intermediate staged OCP. Osteoclastogenesis was unaffected by IL-6 in M-CSF pre-treated and TNFα transgenic derived OCP. IL-6^-/- splenocytes secreted greater concentrations of TNFα in response to titanium particles than WT; addition of exogenous IL-6 to these cultures decreased TNFα expression while anti-IL-6 antibody increased TNFα. While IL-6 lacks effects on intermediate staged precursors, the dominant in vivo effects of IL-6 appear to be related to strong suppression of early OCP differentiation and an anti-inflammatory effect targeting TNFα. Thus, the absence of IL-6 results in increased inflammatory bone loss.