In the present study, we provided evidence that UC-MSCs can exert a profound inhibitory effect on FLSs and T cells from RA patients. They could suppress proliferation, the invasive behavior and inflammatory responses of FLSs, inhibit activation of T cells and induce the Tregs expression. Furthermore, we showed that UC-MSC mediated suppression on T cells and FLSs proliferation through several soluble factors, including IDO, PGE2, NO, IL-10 and TGF-β1, respectively. Systemic infusion of UC-MSCs significantly reduced the severity of CIA in mice. The improvement of clinical manifestation was accompanied by the decreased secretion of various inflammatory cytokines and chemokines, and the downregulated Th1/Th17 cells. Furthermore, in the UC-MSCs treated mice, the expansion of Th2/Tregs and the production of anti-inflammatory IL-10 were elevated.
MSCs have the capability of self-renewal and differentiation into various lineages of mesenchymal tissues. Moreover, MSCs have been consistently shown to exert a potent immunosuppressive effect superior in magnitude to any other immunosuppressive cell types thus far described [39
]. Compared with those from bone marrow, MSCs derived from UC have higher proliferative potency, stronger differentiation capacity, and lower risk for viral contamination. However, their therapeutic potential in the treatment of RA has not been investigated.
Recently, the FLSs have been shown to straddle both components of RA, the immune activation and tissue destruction. Therefore, targeting FLSs may abrogate the disease progression [40
]. Our data demonstrated that UC-MSCs could inhibit the proliferation of TNF-α stimulated FLSs. Notably, delayed addition of UC-MSCs maintained such inhibitory effects, suggesting that the transplantation of these cells is practicable and effective for treatment of RA. Interestingly, the invasive behavior of FLSs was inhibited by UC-MSCs, indicating that UC-MSCs might be potentially important in the inhibition of bone erosion in RA.
T cells are believed to play a critical role in orchestrating the inflammatory response in RA. Suppression of T cell responses is of great importance in RA treatment, as evidenced by the facts that allogeneic BM-MSCs and hASCs both suppress the responses of CII-reactive T cells in RA [17
]. In agreement, we observed that UC-MSCs could inhibit the PHA-stimulated-T cell proliferation and secretion of TNF-α. Similar to RA, Th1 and Th17 cell-mediated responses play an important role in the pathogenesis of CIA [41
]. Our results demonstrated that administration of human UC-MSCs could downregulate IFN-γ-producing Th1 cells and tend to decrease IL-17-producing Th17 cells, while upregulate IL-4-producing Th2 cells in mice CIA. Tregs play an important role in the prevention of autoimmunity, and it has been demonstrated that they could modulate the severity of CIA [37
]. Several studies have shown that BM-MSCs and hASCs could recruit, regulate and maintain the T-regulatory phenotype and function over time [43
]. In this study, we found UC-MSCs could also induce the Tregs, both in vitro
and in vivo
, suggesting that the immunosuppressive activity of UC-MSCs could be prolonged by the participation of Tregs. However, the observation that the DTH response to the immunizing antigen existed in UC-MSC-treated mice indicates that priming of T lymphocytes occurred. Therefore, maybe a complex mechanism existed in the suppressive effect of UC-MSCs.
To date, the molecular mechanisms responsible for the immunosuppressive effects of MSCs have not been completely understood. In BM-MSCs, there have been no agreements among different research groups. However, the main focus is on the soluble factors including IDO, NO, PGE2, IL-10 and TGF-β1 [27
]. A recent study identified TGFβ1 as a critical mediator involved in the suppressive response of human BM-MSCs on CII-activated PBMCs from RA patients [17
]. However, the TGFβ1 blockade did not significantly affect the immunosuppressive action of hASCs on T cells from RA patients [18
], suggesting that MSCs of different origins maybe mediated suppression through different cytokines. In this study, we demonstrated that TGF-β1, PGE2 and NO are potent modulators involved in UC-MSCs mediated T-cell inhibition, while IDO, TGF-β1 and IL-10 were mainly involved in the suppressive effect of UC-MSCs on FLSs.
Systemic administration of human UC-MSCs in established CIA in mice significantly ameliorated the clinical and histopathologic severity of the disease. The therapeutic effect was xenogeneic, which means that the immunosuppressive action of UC-MSCs is major histocompatibility complex unrestricted and that the infused UC-MSCs are sufficiently well immunotolerated by the host.
Direct evidence of the beneficial effect is that administration of UC-MSCs attenuated systemic inflammation in CIA in mice. UC-MSCs downregulated the production of the proinflammatory cytokines TNF-α, and IL-6 in vitro
and in vivo
. In addition, MCP-1 is a member of the CC family and could be induced by inflammatory cytokines. Several groups have detected MCP-1 in the synovial fluid of RA patients, with markedly higher concentrations than those in other rheumatic diseases, including osteoarthritis [44
]. Therefore, reduction of MCP-1 could partly explain the absence of inflammatory infiltrates in the synovium of mice treated with human UC-MSCs. Moreover, UC-MSCs increased the levels of the antiinflammatory cytokine IL-10. Aside from its role as an antiinflammatory factor [45
], IL-10 is a signature cytokine for Tregs, and plays a key role in the control of self-antigen-reactive T cells in vivo
]. The upregulation of IL-10 is in line with the induction of Tregs in vitro
and in vivo
in our study. Moreover, in our experiments we did not find UC-MSCs in the joint of the CIA mice. The short-term presence of UC-MSCs in the spleen suggests that the therapeutic effect of UC-MSCs does not rely on the capacity to engraft and survive long-term in the appropriate target organs. More likely, UC-MSCs could "educate" other cells to inhibit the pathogenic immune reaction.