PDCD1 and rheumatoid arthritis: Therefore, in the current study, we isolated CD4+CD25low/−CD127hiTh/cTfh/Tph cells and CD4+CD25highCD127lowTreg/cTfr cells from the peripheral blood of RA patients and HCs and compared the activation state of the Th cells by monitoring PD-1 expression and IL-21 production.