With respect to methotrexate, in rheumatoid arthritis (RA) patients treated with methotrexate [RA(MTX)] or tumor necrosis factor (TNF)-α inhibitors [RA(TNFi)], the expression of ICOS by CD8+CD28− Treg was found to be significantly lower in both RA(MTX) and RA(TNFi) when compared with healthy subjects [51]. Here, ICOS is linked to rheumatoid arthritis.