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B cells have acquired an important role in the pathogenesis of rheumatoid arthritis (PA) since B cell depletion allowed to rescue patients poorly responders to TNF alpha blockers. This study focused on the involvement of ZAP-70 as a bio-marker of B cells immune activation in RA. ZAP-70 expression in synovial fluid (SF) B cells obtained from RA patients was increased compared to SF B cells of osteoarthritis (OA) patients. Moreover we found that ZAP-70 positive/CD38 positive and ZAP-70 positive/CD5 positive B cells were enriched in SF. The analysis of B cell apoptosis in vitro showed that the percentage of ZAP-70 negative B cells spontaneously undergoing apoptosis was significantly higher than ZAP-70 positive B cells. The ZAP-70 positive B cell ratio (SF/peripheral blood (PB)) showed a positive correlation with SF autoantibody levels and with local levels of BAFF and IL6. ZAP-70 positive B cells seem to define a subset characterized by increased survival and high relationship with local inflammation and autoimmunity. (C) 2008 Elsevier Inc. All rights reserved.