CD4 and rheumatoid arthritis: K2P5.1 expression and activity is up-regulated in the CD4+ T cells of patients with autoimmune diseases such as rheumatoid arthritis (RA) [7], and multiple sclerosis (MS) [8] and those from inflammatory bowel disease (IBD) model mice [9]; however, the mechanisms that underly inflammatory response-mediated K2P5.1 up-regulation in CD4+ T cells remain unclear.