CD4 and rheumatoid arthritis: The physiological role of the two-pore domain K+ channel K2P5.1 was recently clarified in lymphocytes (Nam et al., 2011; Cid et al., 2013; Shin et al., 2014), and several studies have shown the pathophysiological impact of the upregulation of the two-pore domain K+ channel K2P5.1 in CD4+ T cells on the pathogenesis of autoimmune diseases such as rheumatoid arthritis and multiple sclerosis (Bittner et al., 2010a, 2011).