ENTPD1 and rheumatoid arthritis: Importantly, Tregs obtained from rheumatoid arthritis patients unresponsive to methotrexate express low levels of CD39 and, consequently, generate less adenosine and display reduced suppressor ability compared with Tregs isolated from methotrexate responders (Peres et al., 2015); this suggests a link between methotrexate unresponsiveness and low CD39 levels.