Recent observations have demonstrated that one of the functions of mesenchymal stem/stromal cells (MSCs) is to serve as guardians against excessive inflammatory responses. One mode of action of the cells is that they are activated to express the interleukin (IL)-1 receptor antagonist. A second mode of action is to create a negative feedback loop in which tumor necrosis factor-α (TNF-α) and other proinflammatory cytokines from resident macrophages activate MSCs to secrete the multifunctional anti-inflammatory protein TNF-α stimulated gene/protein 6 (TSG-6). The TSG-6 then reduces nuclear factor-κB (NF-κB) signaling in the resident macrophages and thereby modulates the cascade of proinflammatory cytokines. A third mode of action is to create a second negative feedback loop whereby lipopolysaccharide, TNF-α, nitric oxide, and perhaps other damage-associated molecular patterns (DAMPs) from injured tissues and macrophages activate MSCs to secrete prostaglandin E₂ (PGE₂). The PGE₂ converts macrophages to the phenotype that secretes IL-10. There are also suggestions that MSCs may produce anti-inflammatory effects through additional modes of action including activation to express the antireactive oxygen species protein stanniocalcin-1.

Human mesenchymal stem/progenitor cells (hMSCs) repair tissues and modulate immune systems but the mechanisms are not fully understood. We demonstrated that hMSCs are activated by inflammatory signals to secrete the anti-inflammatory protein, TNF-α–stimulated gene 6 protein (TSG-6) and thereby create a negative feedback loop that reduces inflammation in zymosan-induced peritonitis. The results demonstrate for the first time that TSG-6 interacts through the CD44 receptor on resident macrophages to decrease zymosan/TLR2-mediated nuclear translocation of the NF-κB. The negative feedback loop created by MSCs through TSG-6 attenuates the inflammatory cascade that is initiated by resident macrophages and then amplified by mesothelial cells and probably other cells of the peritoneum. Because inflammation underlies many pathologic processes, including immune responses, the results may explain the beneficial effects of MSCs and TSG-6 in several disease models.

To investigate the anti-inflammatory and anti-angiogenic effects of mesenchymal stem cells (MSC) in the chemically burned corneas, we mechanically removed the corneal epithelium of rats after 100% alcohol instillation. The rats were then randomized into four groups: fresh media, conditioned media derived from the MSC culture (MSC-CM), MSC applied topically to the damaged corneas for 2 hours immediately after the injury or MSC-CM applied either once or 3 times per day for 3 consecutive days. Corneal surface was evaluated every week. After 3 weeks, the corneas were stained with the hematoxylin-eosin, and the expression of interleukin (IL)-2, interferon (IFN)-γ, IL-6, IL-10, transforming growth factor (TGF)-β1, thrombospondin-1 (TSP-1), matrix metalloproteinase-2 (MMP-2), and vascular endothelial growth factor (VEGF) were analyzed. CD4+ cells were assessed in the corneas. We found that both MSC and three-time applied MSC-CM (1) reduced corneal inflammation and neovascularization, (2) decreased IL-2 and IFN-γ, although increased IL-10 and TGF-β1 as well as IL-6, (3) reduced the infiltration of CD4+ cells, and (4) upregulated the expression of TSP-1, although downregulated that of MMP-2. Interestingly, whereas three-time application of MSC-CM was partially effective, transplantation of MSC achieved a better outcome in suppressing corneal inflammation. The results of this study suggest that the anti-inflammatory and anti-angiogenic action of MSC in the chemically burned corneas might be mediated in part through paracrine pathways involving soluble factors such as IL-10, TGF-β1, IL-6 and TSP-1. Disclosure of potential conflicts of interest is found at the end of this article.

Accumulating evidence shows that extracellular vesicles (EVs) produced by mesenchymal stem/stromal cells (MSCs) exert their therapeutic effects in several disease models. We previously demonstrated that MSCs suppress autoimmunity in models of type 1 diabetes (T1D) and experimental autoimmune uveoretinitis (EAU). Therefore, here, we investigated the therapeutic potential of MSC-derived EVs using our established mouse models for autoimmune diseases affecting the pancreas and the eye: T1D and EAU. The data demonstrate that MSC-derived EVs effectively prevent the onset of disease in both T1D and EAU. In addition, the mixed lymphocyte reaction assay with MSC-derived EVs indicated that EVs inhibit activation of antigen-presenting cells and suppress development of T helper 1 (Th1) and Th17 cells. These results raise the possibility that MSC-derived EVs may be an alternative to cell therapy for autoimmune disease prevention.

Previous reports demonstrated that the deleterious effects of chemical injury to the cornea were ameliorated by local or systemic administration of adult stem/progenitor cells from bone marrow referred to as mesenchymal stem or stromal cells (MSCs). However, the mechanisms for the beneficial effects of MSCs on the injured cornea were not clarified. Herein, we demonstrated that human MSCs (hMSCs) were effective in reducing corneal opacity and inflammation without engraftment after either intraperitoneal (i.p.) or intravenous (i.v.) administration following chemical injury to the rat cornea. A quantitative assay for human mRNA for glyceraldehyde 3-phosphate dehydrogenase (GAPDH) demonstrated that less than 10 hMSCs were present in the corneas of rats 1-day and 3 days after i.v. or i.p. administration of 1 × 107 hMSCs. In vitro experiments using a transwell coculture system demonstrated that chemical injury to corneal epithelial cells activated hMSCs to secrete the multipotent anti-inflammatory protein TNF-α stimulated gene/protein 6 (TSG-6). In vivo, the effects of i.v. injection of hMSCs were largely abrogated by knockdown of TSG-6. Also, the effects of hMSCs were essentially duplicated by either i.v. or topical administration of TSG-6. Therefore, the results demonstrated that systemically administered hMSCs reduce inflammatory damage to the cornea without engraftment and primarily by secretion of the anti-inflammatory protein TSG-6 in response to injury signals from the cornea.

Previous reports demonstrated that adult stem/progenitor cells from bone marrow (multipotent mesenchymal stem cells; MSCs) can repair injured tissues with little evidence of engraftment or differentiation. In exploring this phenomenon, our group has recently discovered that the therapeutic benefits of MSCs are in part explained by the cells being activated by signals from injured tissues to express an anti-inflammatory protein TNF-α–stimulated gene/protein 6 (TSG-6). Therefore, we elected to test the hypothesis that TSG-6 would have therapeutic effects in inflammatory but noninfectious diseases of the corneal surface. We produced a chemical and mechanical injury of the cornea in rats by brief application of 100% ethanol followed by mechanical debridement of corneal and limbal epithelium. Recombinant human TSG-6 or PBS solution was then injected into the anterior chamber of the eye. TSG-6 markedly decreased corneal opacity, neovascularization, and neutrophil infiltration. The levels of proinflammatory cytokines, chemokines, and matrix metalloproteinases were also decreased. The data indicated that TSG-6, a therapeutic protein produced by MSCs in response to injury signals, can protect the corneal surface from the excessive inflammatory response following injury.