CTLA4 and rheumatoid arthritis: CTLA-4-Ig (a fusion protein composed of the Fc region of immunoglobulin IgG1 fused to the extracellular domain of CTLA-4) also known as Abatacept, is an immunosuppressive agent effective in the treatment of RA that blocks the B7/CD28 co-stimulatory interaction and inhibits effector T-cell proliferation.