Decreased expression of microRNA-21 correlates with the imbalance of Th17 and Treg cells in patients with rheumatoid arthritis

The imbalance of Th17/Treg cell populations has been suggested to be involved in the regulation of rheumatoid arthritis (RA) pathogenesis; however, the mechanism behind this phenomenon remains unclear. Recent studies have shown how microRNAs (miRNAs) are important regulators of immune responses and are involved in the development of a variety of inflammatory diseases, including RA. In this study, we demonstrated that the frequencies of CD3(+) CD4(+) IL-17(+) Th17 cells were significantly higher, and CD4(+) CD25(+) FOXP3(+) Treg cells significantly lower in peripheral blood mononuclear cells from RA patients. Detection of cytokines from RA patients revealed an elevated panel of pro-inflammatory cytokines, including IL-17, IL-6, IL-1β, TNF-α and IL-22, which carry the inflammatory signature of RA and are crucial in the differentiation and maintenance of pathogenic Th17 cells and dysfunction of Treg cells. However, the level of miR-21 was significantly lower in RA patients, accompanied by the increase in STAT3 expression and activation, and decrease in STAT5/pSTAT5 protein and Foxp3 mRNA levels. Furthermore, lipopolysaccharide stimulation up-regulated miR-21 expression from healthy controls, but down-regulated miR-21 expression from RA patients. Therefore, we speculate that miR-21 may be part of a negative feedback loop in the normal setting. However, miR-21 levels decrease significantly in RA patients, suggesting that this feedback loop is dysregulated and may contribute to the imbalance of Th17 and Treg cells. MiR-21 may thus serve as a novel regulator in T-cell differentiation and homoeostasis, and provides a new therapeutic target for the treatment of RA.