Tumor necrosis factor-alpha: Alternative role as an inhibitor of osteoclast formation in vitro

TNFα is known to stimulate the development and activity of osteoclasts and of bone resorption. The cytokine was found to mediate bone loss in conjunction with inflammatory diseases such as rheumatoid arthritis or chronic aseptic inflammation induced by wear particles from implants and was suggested to be a prerequisite for the loss of bone mass under estrogen deficiency. In the present study, the regulation of osteoclastogenesis by TNFα was investigated in co-cultures of osteoblasts and bone marrow or spleen cells and in cultures of bone marrow and spleen cells grown with CSF-1 and RANKL. Low concentrations of TNFα (1 ng/ml) caused a >90% decrease in the number of osteoclasts in co-cultures, but did not affect the development of osteoclasts from bone marrow cells. In cultures with p55TNFR−/− osteoblasts and wt BMC, the inhibitory effect was abrogated and TNFα induced an increase in the number of osteoclasts in a dose-dependent manner. Osteoblasts were found to release the inhibitory factor(s) into the culture supernatant after simultaneous treatment with 1,25(OH)2D3 and TNFα, this activity, but not its release, being resistant to treatment with anti-TNFα antibodies. Dexamethasone blocked the secretion of the TNFα-dependent inhibitor by osteoblasts, while stimulating the development of osteoclasts. The data suggest that the effects of TNFα on the differentiation of osteoclast lineage cells and on bone metabolism may be more complex than hitherto assumed and that these effects may play a role in vivo during therapies for inflammatory diseases.