Objective. Osteopontin (OPN) is expressed by fibroblast-like synoviocytes (FLS) in rheumatoid arthriIn patients with rheumatoid arthritis (RA), the synovium of the inflamed joints is the site of a chronic inflammatory reaction, in which leukocytes and macrophages infiltrate, synoviocytes proliferate, and several proinflammatory cytokines (especially, and tumor necrosis factor ␣), autoantibodies, and immune complexes are produced (1). In this situation, infiltrating leukocytes and proliferating synoviocytes are thought to interact with each other. Accordingly, several in vitro studies have focused on the interaction between fibroblast-like synoviocytes (FLS) and leukocytes by studying the findings in cocultures of these cells (2-7). Coculture of FLS obtained from RA synovium with B lymphocytes causes adhesion of B lymphocytes to FLS, and consequently this interaction supports the survival of B lymphocytes and also enhances the production of cytokines and immunoglobulin (8)(9)(10)(11)(12). The former mechanism is dependent on vascular cell adhesion mol-
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