IL9 and rheumatoid arthritis: For example, the observation that spontaneous resolution of synovitis in a mouse model is dependent on IL-9 raised the intriguing possibility that this cytokine could also regulate human RA persistence: an enrichment of IL-9-producing type 2 innate lymphoid cells (ILC-2s) was indeed present in the circulation and synovium of patients with active rheumatoid arthritis compared with those on effective treatment and controls [32▪▪].