CCL2 and rheumatoid arthritis: To clarify the molecular mechanism by which RA MSCs displayed the compromised effect on suppressing Th17 cells, we assessed the mRNA levels of several factors that have been reported to be involved in MSC-mediated Th17 cell regulation, including TGF-β1, IDO, PGE2, IL-6, and CCL2 [14, 17–19].