CTLA‐4Ig blocks the development and progression of citrullinated fibrinogen–induced arthritis in DR4‐transgenic mice

Abstract Objective To assess the role of T cells in the mouse model of citrullinated human fibrinogen–induced rheumatoid arthritis (RA) using CTLA‐4Ig, an agent that blocks T cell costimulation, which is required for T cell activation. Methods Humanized HLA–DRβ1*0401–transgenic (DR4‐Tg) mice were immunized with Cit–human fibrinogen to induce arthritis. Prior to, and at the onset or peak of, arthritis, the DR4‐Tg mice were treated with CTLA‐4Ig or control human IgG1 or were left untreated. Arthritis development and progression were monitored by measuring ankle swelling with calipers and by assessing histopathologic changes. The immune responses to the citrullinated antigens and the corresponding unmodified antigens, as well as the arthritogenicity of lymphocytes from these mice, were examined. The latter was performed using lymphocyte transfers from CTLA‐4Ig–treated or control mice via intraperitoneal injection into naive DR4‐Tg mice. Recipient mice also received an intraarticular injection of Cit–human fibrinogen, unmodified human fibrinogen, or vehicle. Results CTLA‐4Ig–treated, but not human IgG1‐treated, arthritic mice had significantly reduced ankle swelling and pathologic joint damage. Treatment with CTLA‐4Ig, but not human IgG1, suppressed Cit–human fibrinogen–induced T cell activation, including citrulline‐specific T cell activation, when given prior to disease onset. Transfer of splenic lymphocytes from untreated or human IgG1–treated arthritic mice caused arthritis in recipients, and this occurred when Cit–human fibrinogen, but not unmodified fibrinogen, was deposited into the joint. Splenocytes from CTLA‐4Ig–treated mice were unable to transfer arthritis. Conclusion Activated citrulline‐specific T cells play a direct role in the development and progression of arthritis in this model of Cit–human fibrinogen–induced RA.