To analyze the expression pattern of osteoclast differentiation factor (ODF) and its contribution to osteoclastogenesis in rheumatoid arthritis (RA).

The expression of ODF was analyzed by reverse transcriptase–polymerase chain reaction (RT‐PCR) in RA synovial fibroblasts (RASF) isolated from 7 RA patients and in normal skin fibroblasts. Using RNA probes specific for ODF, in situ hybridization was performed. Immunohistochemical double labeling for CD68 was applied to characterize the ODF‐expressing cells. ODF protein and messenger RNA (mRNA) expression by RASF with or without 1,25(OH)₂D₃ was studied by Western blot analysis and quantitative real‐time PCR. In addition, we performed coculture experiments with RASF and normal peripheral blood mononuclear cells with or without 1,25(OH)₂D₃.

By RT‐PCR, ODF mRNA expression was found in all RASF investigated, but not in normal skin fibroblasts. In situ hybridization revealed that in RA synovial tissues, ODF mRNA was expressed mainly in the lining layer and at sites where synovium was attached to bone. Immunohistochemical double labeling demonstrated ODF mRNA expression mainly in CD68− fibroblast‐like synoviocytes and CD68+ multinucleated osteoclast‐like cells. By Western blotting, all RASF expressed ODF protein. However, different levels of ODF expression were found in the RASF from different patients. Interestingly, RASF expressing higher levels of ODF induced a larger number of osteoclast‐like cells than did RASF expressing only low levels of ODF. Although 1,25(OH)₂D₃ did not alter the levels of ODF expression in RASF on either Western blot or quantitative real‐time PCR, osteoclastogenesis required the presence of 1,25(OH)₂D₃.

The present results suggest that activated RASF, by expressing ODF, play an important role in rheumatoid bone destruction. Moreover, the data provide evidence that RASF not only activate osteoclasts, but also contribute directly to osteoclastogenesis.