To evaluate the role of T-bet in CD4 + T cells on the pathogenesis of arthritis, we generated T-bet conditional KO (cKO) mice by crossing CD4-Cre mice with T-betfl/fl mice (which selectively lose the T-bet gene in T cells) (Fig. S4), and compared the severity of CIA in T-betfl/fl control mice and cKO mice. Here, CD4 is linked to arthritic joint disease.