CD4-positive T cells, which are helper T cells, are clearly important in RA pathogenesis based on the following findings: (1) CD4-positive T-cell-depleted mice are protected from arthritis [99]; (2) genome-wide association studies have reported CD4-positive T-cell-related genes as RA susceptibility genes [100,101]; and (3) abatacept (ABT), which is a T cell co-stimulation modulator by binding to CD80 and CD86 receptors, thereby suppressing T cell activation and B cell stimulation [102], is effective against RA [103]. Here, CD80 is linked to rheumatoid arthritis.