Expression of osteoclast differentiation factor at sites of bone erosion in collagen-induced arthritis

Objective. To investigate the cellular mechanism of bone destruction in col lagen-induced arthritis (CIA). Methods. After induction of CIA in DA rats, a histologic study of the advan ced arthritic lesion was carried out on whole, decalcified joints from the hindpaws of affected animals. To conclusively identify osteoclasts, joint t issue sections were stained for tartrate-resistant acid phosphatase (TRAP) enzyme activity, and calcitonin receptors (CTR) were identified using a spe cific rabbit polyclonal antibody. The expression of messenger RNA (mRNA) fo r the osteoclast differentiation factor (also known as receptor activator o f nuclear factor kappa B ligand [RANKL]) was investigated using in situ hyb ridization with a specific riboprobe. Results. TRAP-positive and CTR-positive multinucleated cells were invariabl y detected in arthritic lesions that were characterized by bone destruction . Osteoclasts were identified at the pannus-bone and pannus-subchondral bon e junctions of arthritic joints, where they formed erosive pits in the bone . TRAP-positive multinucleated cells were detected within synovium and at t he bone erosive front; however, CTR-positive multinucleated cells were pres ent only at sites adjacent to bone. RANKL mRNA was highly expressed in the synovial cell infiltrate in arthritic-joints, as well as by osteoclasts at sites of bone erosion. Conclusion. Focal bone erosion in CIA is attributed to cells expressing def initive features of osteoclasts, including CTR, The expression of RANKL by cells within inflamed synovium suggests a mechanism for osteoclast differen tiation and activation at sites of bone erosion. Inhibitors of RANKL may re present a novel approach to treatment of bone loss in rheumatoid arthritis.