Bone sparing effect of osteoprotegerin and anti-inflammatory cytokines in collagen-induced arthritis

Rheumatoid arthritis (RA) is associated with focal and systemic bone loss involving RANKL, which is secreted by both osteoblasts and activated lymphocytes, whereas osteoprotegerin (OPG) inhibits bone resorption in osteoclast precursors. RANKL expression is increased by inflammatory cytokines. The aim of our studies is to evaluate the respective effect of OPG, on one hand, and either IL-4 or antibody, on the other, on inflammation and on bone tumor necrosis factor alpha (TNF-α) remodelling. Indeed, we hypothesised that inhibiting inflammation could be sufficient to inhibit bone loss. Moreover, we tested whether the combination of OPG with either IL-4 or TNF-α antibody had an additive effect on bone loss. IL-4 and TNF-α antibodies were tested in two independent experiments. We used a model of collagen-induced arthritis (CIA) in DBA/1 mice by immunisation (bovine type II collagen). Bone mineral density (BMD) was measured at the total body (Piximus Lunar) at baseline before immunization and at sacrifice, allowing one to measure the bone gain (ΔBMD). Deoxypyridinolin changes (ΔD-Pyr) were measured in urine. Histomorphometric parameters were measured at the femur metaphysis. OPG had no effect on clinical arthritis score in any experiment. By contrast, OPG was able to increase the ΔBMD and to decrease the ΔD-Pyr and trabecular spacing evaluated by histomorphometry. By contrast, both IL-4 and TNF-α antibody decreased the arthritis score but did not induce a change in ΔBMD and decrease in bone resorption evaluated by ΔD-Pyr and trabecular spacing. There was no additive effect of OPG and TNF-α antibody on any parameter related to bone, whereas we observed a significant additive effect of IL-4 and OPG on the inhibition of bone resorption and the increase in ΔBMD.

In conclusion, these data show that, in this model of inflammatory arthritis, systemic bone loss is prevented only by treatment directly decreasing osteoclast differentiation. Furthermore, treatment with one anti-inflammatory cytokine, IL-4, but not with TNF-α antibody had an additive bone-sparing effect with OPG.