In Rheumatoid Arthritis (RA), regulatory T cells (Tregs) have been found to be enriched in the synovial fluid. Despite their accumulation, they are unable to suppress synovial inflammation. Recently, we showed the synovial enrichment of interleukin-9 (IL-9) producing helper T cells and its positive correlation with disease activity. Therefore, we investigated the impact of IL-9 on synovial Tregs in RA. Here, we confirmed high synovial Tregs in RA patients, however these cells were …