CD4 and rheumatoid arthritis: Flow cytometry analyses of PBMC samples from 20 RA patients demonstrated the upregulated PD-1 expression on CD4+CD25+T cells (26.44 ± 2.43 versus 16.3 ± 1.54, P = 0.0014), CD4+CD69+T cells (22.95 ± 2.68 versus 15.10 ± 1.432, P = 0.0185), and CD4+HLA-DR +T cells (51.26 ± 3.31 versus 26.46 ± 2.469, P < 0.0001) compared with healthy controls (Figures 2(a) and 2(b)).