One Treg subpopulation (CD4+/CD25+/CD45RA−/CD127−) and MC03 (CD25+CD38−CD127−CD194+) were the lowest in HCs (HCs: 1.554%; RA: 2.520%; SSc: 2.520%; SLE: 2.675%). Here, CD4 is linked to rheumatoid arthritis.