The specific CD4+ T-cell subpopulations and their secreted cytokines play distinct and essential roles in the pathogenesis of RA and experimentally induced arthritis.19 Pathogenic Th17 cells secreting proinflammatory cytokines, such as interleukin (IL)-17A, IL-17F, and IL-22, are regarded as the predominant positive regulators of the immune response in RA.5 Attention has also been increasingly focused on the imbalance of Th17 cells/Tregs. Here, IL17F is linked to rheumatoid arthritis.