**4. Conclusions**

In our study, SSEA-3 positive cells were detected in RA synovial tissue even under pathological conditions such as RA. Although the synovial tissue we used was collected from various RA disease stages and surgical sites, SSEA-3 positive cells were detected with values of approximately 0.5–1% in all cultured SFMSCs. Collected SSEA-3 positive cells had higher gene expression level and differentiation ability in vitro compared with SSEA-3 negative cells that were occupying most of mesenchymal stem cells. Wakao S., et al., reported that Muse and non-Muse cells had differentiation ability of osteocytes, chondrocytes, and adipocytes, while differentiation ability in non-Muse cells was lower [31]. We think that SSEA-3 positive cells in this study had a similar nature as Muse cells, considering also the results that SSEA-3 positive cells strongly expressed CD105 in FACS analysis. SSEA-3 positive cells can be systemically administered by intravenous administration like Muse cells and have possibility of differentiation into osteoblasts, adipocytes, and chondrocyte. These suggest the possibility of repairing degenerative cartilage and destroyed joints in RA. In the CAIA mice experiment, SSEA-3 positive cells that were systemically administered had an inhibitory effect on arthritis. In the transplanted group consisting of mice transplanted with SSEA-3 positive cells, arthritis score quickly decreased after the onset of arthritis compared with SSEA-3 negative cells group. There were some previous studies on immunosuppressive effect of BMMSCs [38–40] and SFMSCs [24] as mentioned earlier. Especially, SFMSCs extracted from healthy subjects are able to inhibit T-cell proliferation [25]. However, immunomodulatory function of SFMSCs and SSEA-3 positive cells may be disturbed and cause an inefficacy of SFMSCs and SSEA-3 positive cells in inflammatory environment like uncontrolled RA [41]. In RA synovial tissue, fibroblast-like synoviocytes (FLS) are key players in the perpetuation of joint inflammation and destruction. The link between FLS and SFMSCs plays an important role in controlling the inflammation and immune hemostasis in RA. In our mice study, autologous SSEA-3 positive cells proliferated in vitro might have altered the balance of immune regulation with FLS.

Our study suggests the possibility of inhibiting arthritis and joint destruction by SSEA-3 positive cells derived from synovial tissue in RA. Further study of SSEA-3 positive cells for clinical application in humans will lead to future development as a new treatment in RA.
