CD27 and rheumatoid arthritis: Two populations (MC18 and MC19) were significantly lower in RA patients with a common lack of CD98, CD28, and CD27, and MC18 (CD27−CD28dimCD98dimCD127−CD197−) was the lowest in RA patients (HCs: 2.718%; RA: 0.749%; SSc: 1.278%; SLE: 2.514%).