ICOS and rheumatoid arthritis: To clarify the phenotypic characteristics of Treg subsets in RA, we analyzed the expression of ten co-stimulatory and inhibitory surface molecules—including CD28, CTLA-4, PD-1, Fas, ICOS, LAG-3, TIM-3, OX40, HLA-DR, and 4-1BB—in total Tregs as well as their functional subfractions: naïve Tregs (Fraction I) and effector Tregs (Fraction II).