CD4 and rheumatoid arthritis: In humans, various CD4+ and CD8+ TRM (sub)populations were found to be enriched in inflammatory diseases that manifest at barrier tissues, such as inflammatory bowel disease (IBD) in the gut [24] and psoriasis in the skin [29], but also in disease manifested in non-barrier tissues, including diabetes type I [30], Sjogren’s syndrome [31], lupus nephritis [32], multiple sclerosis (MS) [33,34] and several arthritic diseases [35,36,37,38,39] such as Juvenile Idiopathic Arthritis (JIA) [35,36], psoriatic arthritis [37], ankylosing spondylitis [38], and rheumatoid arthritis (RA) [39].