Tetrandrine ameliorates collagen-induced arthritis in mice by restoring the balance between Th17 and Treg cells via the aryl hydrocarbon receptor

Tetrandrine is an alkaloid constituent of the root of Stephania tetrandra S. Moore. The long-term clinical uses of tetrandrine for treatments of rheumatalgia and arthralgia as well as the inhibition of rat adjuvant-induced arthritis imply that tetrandrine may have therapeutic potential in rheumatoid arthritis (RA). Here, we explored its anti-RA mechanism in collagen-induced arthritis (CIA) in relation to the balance between T helper (Th) 17 cells and regulatory T (Treg) cells. DBA/1 mice were immunized with chicken type II collagen and were orally administered tetrandrine for 14 consecutive days. Then, the mice were sacrificed, their joints were removed for histological analysis, and spleens and mesenteric lymph nodes (MLNs) were removed to examine the Th17 and Treg cells. Tetrandrine markedly alleviated the severity of arthritis, reduced the serum levels of pro-inflammatory cytokines, and restored the Th17/Treg balance, as demonstrated by the serum levels of their related cytokines (IL-17 and IL-10) and the proportion of each cell type. Tetrandrine inhibited Th17 cell differentiation and induced Treg cell differentiation in vitro . Notably, aryl hydrocarbon receptor (AhR) was proven to play a crucial role in tetrandrine-mediated T cell differentiation. The correlation between AhR activation, regulation of Th17/Treg and amelioration of arthritis by tetrandrine was verified in the CIA mice. Moreover, tetrandrine might be a ligand of AhR because it facilitated the expression of the AhR target gene cytochrome P450 1A1 (CYP1A1) and the activation of its downstream signaling pathways. Taken together, tetrandrine exerts its anti-arthritis efficacy by restoring Th17/Treg balance via AhR.