Although the etiology of early events in rheumatoid arthritis (RA) remains undefined, an anomaly in T cell homeostasis hyperproliferation of synovial lining cells are involved in the disease process. Since it has been reported that ephrin/Eph receptor system plays important signaling roles in inflammation processes, we attempted to examine ephrinB molecules in T cells and synovial cells derived from RA in this study. The expression level of ephrinB1 is significantly high in synovial fibroblasts and CD3-positive exudate lymphocytes in synovial tissues derived from patients with RA as compared with those in osteoarthritis (OA). The protein and mRNA levels of ephrinB1 are also higher in peripheral blood lymphocytes (PBLs) prepared from patients with RA than those from normal controls. Similar results were obtained from an animal model of human RA, CAIA mice. Moreover, a recombinant ephrinB1/Fc fusion protein stimulates normal PBLs to exhibit enhanced migration and production of TNF-. EphrinB1/Fc also activates synovial cells established from patients with RA to produce IL-6. Tyrosine phosphorylation of EphB1 is induced in these cells by ephrinB1/Fc. The CpG islands in the 5' upstream regulatory region of the ephrinB1 gene are hypomethylated in RA patients as compared with that of normal donors. These results suggest that ephrinB1 and its EphB1 receptors play an important role in the inflammatory states of RA, especially by affecting the population and function of T cells. Inhibition of ephrinB/EphB system might be a novel target to the treatment for RA.