Background and objectives Mesenchymal stem cells (MSC) are the stromal cells of bone marrow, but they can also be found in other tissues including the synovia. Our goal was to isolate and cultivate human synovial fluid-derived MSC-like cells (SYF-MSC) and study their role in immunity and angiogenesis with relevance to rheumatoid arthritis (RA).

Materials and methods Synovial fluid was harvested from patients with active RA (according to the Guidelines of the Helsinki Declaration). The isolated fluid/cells were cultured in vitro, and the expression of well-known MSC, hematopoietic, endothelial markers as well as high-end glycosylation products were measured by flow cytometry. In vitro differentiation assays were used to determine the stemness of SYF-MSCs. The immunosuppressive function of these cells was studied by mitogen-activated lymphocyte reaction, and their immunophenotype was investigated by cell activation with TLR ligands and pro-inflammatory cytokines; the secreted cytokines were measured by ELISA.

Results The cells isolated from synovial fluid of patients with RA grew as monolayers in vitro and could be maintained in culture for more than 10 passages. The cells expressed the most important MSC markers (CD44, CD73, CD90 and CD105) with absence of endothelial (CD31, VEGFR2) or hematopoietic cell markers (CD34, CD45, CD69, CD133), respectively. SYF-MSCs were able to differentiate into bone, fat and cartilage tissue in vitro fulfilling the ISCT criteria, and could suppress the proliferation of mitogen activated peripheral blood lymphocytes. Furthermore, SYF-MSCs secreted increased amount of IL-6 after 12 and 24 h treatment by LPS (15423.61 + 9348.61 pg/ml at 12 h; 17479.17 + 3848.63 pg/ml at 24 h time points), Poly:IC (15215.28 + 10834.72 and 24090.28 + 3626.40 pg/ml), TNFα (7437.5 + 1168.06 and 11562.5 + 845.83 pg/ml) and IL-1β (19465.28 + 2306.94 and 21229.17 + 12.5 pg/ml) compared to untreated controls (1590.28 + 1015.28 and 2187.50 + 640.28 pg/ml), respectively. IL-8 could be detected in the supernatants of SYF-MSC only upon LPS (10876.76 + 7553.24 and 12117.94 + 2912.06 pg/ml), Poly:IC (8972.35 + 8707.65 and 30567.94 + 3637.94 pg/ml) TNFα (5516.47 + 1336.47 and 110325.29 + 5645.29 pg/ml) and IL-1β (18063.53 + 3083.53 and 24970.88 + 9940.88 pg/ml) treatment, but not in the untreated controls.

Conclusions The in vitro data suggest that SYF-MSC may play a role in the inflammatory processes of RA, and their activation could lead to the release of pro-angiogenic cytokines as well.