CD4 and arthritic joint disease: In this study, we demonstrate that D2R expressed on CD4+ T cells is protective against the imbalance between pro-inflammatory and anti-inflammatory T cells and the symptoms of arthritis in CIA mice by means of the approaches of global D1r or D2r deficiency (D1r–/– or D2r–/–), CD4+ T cell-specific D2r deletion (D2rfl/fl/CD4Cre), D2R agonist administration, and in vitro treatments of CD4+ T cells with D2R agonist or/and antagonist.