CD4 and rheumatoid arthritis: Flow cytometric analysis demonstrated that OX40 expression on CD4+ T cells in PB samples was more frequent in patients with RA than in patients with OA or HCs (9.30 ± 3.14% vs 7.55 ± 3.26%, 6.90 ± 2.64%, respectively, P = 0.008) (Fig. 1a and b).