CCR9 and rheumatoid arthritis: First, as this study was a single-center study, the demographic and geographic homogeneity limits generalizability of the findings to diverse racial/ethnic groups, and multi-center study with other populations will be conducted to avoid the limitation; second, mechanisms of CCR9+CD4+ T cells influence on disease activity in patients with RA is unclear, and additional functional studies of patient CCR9+CD4+ T cells will be explored in future studies; third, sample collection is challenging in the clinical setting, so a large enough sample of SF were difficult to obtain.