IDO1 and rheumatoid arthritis: It is interesting that chemical molecules, such as quercetin, may upregulate the immunomodulatory effect of UCMSCs and improve the outcomes by increasing the functional molecules, including nitric oxide, IDO, and IL-6 via activation Toll-like receptor-3 signaling, suggesting pretreated MSCs may be a potential therapeutic for the treatment of RA in the clinic [65].