Although these in vitro concentrations are higher than the in vivo concentration of IL-17 in the synovial fluid of RA patients [15]–[17], MSCs can be exposed to much higher local concentrations of IL-17 in vivo than in the synovial fluid, since Th17 cells accumulate around MSCs at local inflammatory sites, such as synovial membranes [29], [30]. Here, IL17A is linked to rheumatoid arthritis.