T helper 9 (TH9) cells express the transcription factor PU.1 and produce IL-9, and have been implicated in numerous autoimmune diseases, including rheumatoid arthritis (RA). However, the specific role of these cells in the pathogenesis of RA remains unclear.

Wei Wei and colleagues provide some insights into the role of TH9 cells in RA. In the blood of individuals with RA, IL-9 correlated with disease biomarkers and disease activity scores; co-expression of PU.1 and IL-9 was higher in the synovium of individuals with RA compared with individuals with osteoarthritis and healthy control individuals. Additionally, in a mouse model of arthritis, IL-9 correlated with disease progression.