An IDO activity assay showed that, after treatment with inflammatory cytokines, the production of L-kynurenine, which is one of the metabolites in the IDO pathway, increased and was higher in H-SF-MSCs than in RA-SF-MSCs (fold change mean ± SD: 105.933 ± 4.244 μM and 5.507 ± 6.935 μM, respectively; Figure 5B). Here, IDO1 is linked to rheumatoid arthritis.