CD81 belongs to a family of cell-surface proteins (tetraspanins) that have four transmembrane domains and two outer-membrane loops. CD81 has been shown to be up-regulated in rheumatoid arthritis (RA) synoviocytes and is involved in the progression of RA through the promotion of synoviolin expression. Recently, we showed that small interfering RNA targeting CD81 (CD81 siRNA) ameliorates paw swelling in collageninduced arthritic (CIA) rats. CD81 siRNA also decreases the expression of TNF-in SW982 cells. Previously, we established hybridoma cell lines producing monoclonal antibodies (MAbs) against CD81, which decrease the proliferation of rat C6 glioma cells. Here, we established a sandwich enzyme-linked immunosorbent assay (ELISA) system for the quantitation of CD81 using these MAbs.

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