A bio-assay for effectors of osteoclast differentiation in serum from patients with bone disease

Osteoclast differentiation and activity, and hence bone loss, depend on two opposing cytokines. Receptor activator of NF-κB ligand (RANKL) produced by osteoblasts and T-cells stimulates, while osteoprotegerin inhibits. Both of these cytokines are found in serum. Our aim was to develop a functional assay for any factors present in human serum that can affect osteoclast differentiation and to assess whether any such factors vary in diseases in which bone loss occurs. Using a culture model of osteoclast differentiation in the presence of macrophage colony stimulating factor and soluble RANKL, we have measured the effects of different human sera on osteoclast differentiation. The production of a marker enzyme for the osteoclast, tartrate-resistant acid phosphatase (TRAP), was used to follow osteoclast differentiation. In general, human serum stimulates osteoclast differentiation as indicated by TRAP activity, but in patients with low bone density this stimulation was attenuated. Sera from 40 female subjects with low bone mineral density showed significantly lower TRAP cell differentiation activity than sera from the healthy female controls. We describe a functional bio-assay for factors in human serum which can affect osteoclast differentiation. This assay may have application in monitoring the effects of therapy in bone disease.