Cinnamtannin D1 Attenuates Autoimmune Arthritis by Regulating the Balance of Th17 and Treg Cells through Inhibition of Aryl Hydrocarbon Receptor Expression.

The suppression of the abnormal systemic immune response constitutes a primary strategy for treatment of rheumatoid arthritis (RA); toward this end, the identification of natural compounds with immunosuppressive activity represents a promising strategy for RA drug discovery. Cinnamtannin D1 (CTD-1), a polyphenolic compound isolated from Cinnamomum tamala, was previously reported to possess good immunosuppressive activity. However, the beneficial effect of CTD-1 on RA is currently unknown. The aim of this study was to evaluate the anti-arthritic effect of CTD-1 in collagen-induced arthritis (CIA) mice and clarify the underlying mechanisms. CTD-1 treatment significantly alleviated the severity of CIA mice, affording reduced clinical scores and paw swelling, along with reduced inflammatory cell infiltration and cartilage damage in the joints; in addition, the serum levels of IL-17, IL-6, and IL-1β were decreased whereas those of TGF-β and IL-10 were increased. CTD-1-treated mice exhibited lower frequency of Th17 cells and higher frequency of Treg cells compared to those in untreated mice, indicating that the balance of Th17/Treg cells may serve as the target for CTD-1. Consistent with this, in ex vivo assays, CTD-1 inhibited Th17 cell differentiation through the downregulation of phospho-STAT3/RORγt, whereas it promoted Treg differentiation by upregulating phospho-STAT5/Foxp3 in response to the stimulation of collagen type II. Moreover, in an in vitro naïve CD4⁺ T cell differentiation assay, CTD-1 directly inhibited Th17 cell differentiation and promoted Treg differentiation, suggesting that CTD-1 regulated the balance of Th17 and Treg cells to inhibit excessive immune response. Furthermore, the regulation effect of CTD-1 on Th17 and Treg cells was dependent on Ahr expression, as this effect was abolished when Ahr was knocked down and was impaired when Ahr was overexpressed. Together, our results indicated that CTD-1 treatment benefits CIA mice by regulating Th17 and Treg differentiation through the inhibition of AHR expression, and suggested a potential application of CTD-1 toward RA treatment.