The expression of ICOS on CD4+ T cells in PB samples was significantly higher in patients with RA (18.90 ± 8.17%) than that in patients with OA (14.08 ± 5.83%, P = 0.0134) or HCs (14.42 ± 7.86%, P = 0.0151; Figures 1A, B). Here, CD4 is linked to rheumatoid arthritis.