CD81 belongs to a family of cell-surface proteins (tetraspanins) known to be up-regulated in rheumatoid arthritis (RA) synoviocytes. Recently, we showed that small interfering RNA targeting CD81 (CD81 siRNA) has a therapeutic effect on RA. We also showed that CD81 siRNA decreased the expression of TNF-in SW982 cells, suggesting that TNF-stimulates the expression of synoviolin via CD81. Here, we show that a monoclonal antibody (MAb) against CD81, which was raised by immunization with recombinant CD81, decreased the proliferation of rat C6 glioma cells, showing that this antibody is functionally active and probably inhibits cell proliferation through the inhibition of CD81 function on the cell surface.

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.