Original Article
Unraveling the Mesenchymal Stromal Cells' Paracrine Immunomodulatory Effects

https://doi.org/10.1016/j.tmrv.2015.11.004Get rights and content

Highlights

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    Mesenchymal stromal cells (MSCs)' major immunomodulatory factors are reviewed.

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    Studies in vitro and in vivo on MSC immunomodulatory paracrine effects are analyzed and discussed.

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    The understanding of MSC immunomodulatory effects remains incomplete and may require tools such as immunologic bioinformatics.

Abstract

In the last 10 years, the role of mesenchymal stromal cells (MSCs) in modulating inflammatory and immune responses has been characterized using both in vitro studies and in vivo models of immune disorders. Mesenchymal stromal cell immunomodulatory properties have been linked to various paracrine factors which expression varies depending on the pathologic condition to which the MSCs are exposed. These factors may directly impact key cells of the adaptive immune system, such as T cells. Indeed, coculturing MSCs with T cells in a mixed lymphocyte reaction assay inhibits T-cell proliferation through the secretion of immunomodulatory cytokines. However, in a context of inflammation, MSCs may secrete paracrine factors that influence other immune cell subpopulations such as dendritic cells and macrophages and polarize them toward a tolerogenic phenotype. In vivo, these same immunomodulatory factors are shown to be increased in the serum of animal models presenting with inflammatory diseases treated with MSC administration. In light of the results from these landmark studies, we review the main MSC secreted factors identified to play a role in modulating inflammatory immune responses either in vitro or in vivo, and we assess the impact of these factors on the therapeutic applications of MSC-based cell therapies in immune diseases.

Section snippets

Mesenchymal Stromal Cells Inhibit Immune T-Cell Proliferation - MLR Inhibition

The MLR is an in vitro method to assay for proliferation of responder CD4 + T cells, in the presence of an allogeneic T-cell population [25]. Di Nicola et al [18] first explored the in vitro mechanism of MSC immunomodulatory effect on T cells by coculturing T cells from 2 donor populations in vitro in a 1-way MLR assay and adding MSCs. In the presence of MSCs, the CD4 allo-T-cell proliferation response was diminished by more than 60%. However, to further ask whether MSC–T-cell contact was

Transforming Growth Factor β

Mesenchymal stromal cell's immunomodulatory abilities to suppress both CD4 + and CD8 + T-cell proliferation, with or without direct contact with MSCs in the MLR, could be abrogated by the addition of monoclonal antibodies that neutralized distinct cytokines produced by MSCs hereby elucidating the role of specific cytokines in MSC immunomodulation. Transforming growth factor β, a known MSC-secreted protein regulating cell proliferation and differentiation, plays a major role during immune

Hepatocyte Growth Factor

Hepatocyte growth factor is a cellular growth factor produced by MSCs that plays a major role during organogenesis and angiogenesis and has been recently shown to have regenerative properties in myelodegenerative diseases [36]. In a murine model of allogeneic hematopoietic stem cell transplantation, HGF treatment was shown to ameliorate acute GVHD, suggesting that HGF also had immunomodulatory effects [37]. Indeed, using the similar MLR in vitro model described above, HGF blocking antibodies

Prostaglandin E2

Prostaglandins have been long associated with inflammation and targeted by cyclooxygenase (COX) inhibitors to treat inflammatory diseases. However, one of these prostaglandins, prostaglandin E2 (PGE2), has been recognized as having anti-inflammatory [38] and immunomodulatory effects [39]. Interestingly, PGE2 was identified by Aggarwal and Pittenger [20] as a key cytokine secreted in the culture medium of hMSCs exposed to different populations of immune cells. In cocultures of HLA-unmatched

Interleukin 10 and Interleukin 1 Receptor Antagonist

Interleukin 10 is a cytokine primarily expressed by monocytes, Th2 lymphocytes, and Tregs. Interleukin 10 has multiple immunoregulatory and anti-inflammatory effects; indeed, it down-regulates macrophage expression of MHC class II and Th1 inflammatory cytokines. In an MLR study on MSC paracrine factors associated with inhibition of T-cell proliferation, Nasef et al [26] demonstrated increased hMSC IL-10 gene expression, which was T-cell contact dependent. The role of IL-10 in MSC-mediated

Interleukin 6

Interleukin 6 has long been recognized as an interleukin having both proinflammatory and anti-inflammatory properties depending on the source of synthesis. Interleukin 6 is constitutively expressed by hMSCs in the BM niche during hematopoiesis, but its expression is up-regulated in the presence of IL-1α[49]. Subsequently, parallel immune studies showed that mMSCs derived from IL-6 knockout (IL-6 −/−) mice could not strongly suppress allo-T-cell proliferation in an MLR assay. These data

Leukocyte Inhibitory Factor

Leukocyte inhibitory factor (LIF), similar to IL-6, is constitutively expressed by hMSCs. Leukocyte inhibitory factor expression can be further increased, up to 7-fold, in MSCs cocultured with CD3 + lymphocytes. Furthermore, in an MLR assay, LIF neutralizing antibody restored lymphocyte proliferation and decreased Foxp3 Treg ratio [53]. Whether the LIF effect is direct or indirect needs further analysis.

HLA-G

HLA-G is a nonclassical HLA class I molecule, which has been identified in both membrane bound and soluble forms [54]. HLA-G was originally identified at the feto-maternal interface but is more recently shown to be a key immunomodulatory factor playing a role in both innate and adaptive immune responses [54], [55], [56]. Nasef et al [26] first identified messenger RNA expression of HLA-G in hMSCs cocultured in an MLR, resulting in a significant inhibitory effect on lymphocyte proliferation. In

Indoleamine 2,3-Dioxygenase

Indoleamine 2,3-dioxygenase (IDO) is an enzyme that catalyzes the degradation of the essential amino acid l-tryptophan. Indoleamine 2,3-dioxygenase expression by DCs is induced by IFN-γ and results in increased tryptophan catabolism, which is an immunosuppressive effector pathway that inhibits T-cell responses to alloantigens in vivo [58], [59]. Based on these findings, Meisel et al [60] investigated whether hMSCs exhibited IFN-γ–inducible IDO activity and whether this mechanism contributes to

Nitric Oxide

Nitric oxide synthase (NOS) catalyzes the production of nitric oxide, which has been recognized as a key player in the mechanism of MSC-mediated immunosuppression [61]. The gene of interest coding for inducible NOS (iNOS) and is inducible in both MSCs and macrophages, playing a major role in immune regulation. In the MLR in vitro assay, mMSC–T cell coculture was associated with NOS production by mMSCs but not by T cells, and the addition of a specific inhibitor of NOS reversed T-cell

Hemeoxygenase, an Enzyme Involved in Heme Catabolism

There are 3 isoforms of hemeoxygenase enzyme. Hemeoxygenase 1 (HO-1) is an inducible isoform in response to oxidative stress and inflammatory cytokines, and its expression may be protective against oxidative damage. Rat MSCs that have been shown to inhibit T-cell proliferation in vitro express both HO-1 and iNOS [63], and inhibition of both HO-1 and iNOS was required to mitigate the suppressive abilities of rat MSCs on rat T-cell proliferation. Interestingly, for hMSCs, only the inhibition of

Galectins

Galectins are lectins in the β-galactoside–binding family expressed on T-cell surfaces, mediating T-cell suppression and apoptosis. Galectin 1 (Gal-1) is highly expressed both at the gene and protein levels in hMSCs cocultured in an MLR system. Furthermore, Gal-1 knockdown restores IFN-γ production as well as proliferation of alloreactive T cells [65]. The inflammatory cytokines IFN-γ, TNF-α, IL-2, and IL-12 in the MLR coculture supernatant were decreased in the presence of hMSCs, suppressing a

Tumor Necrosis Factor α–Stimulated Gene 6

Tumor necrosis factor α–stimulated gene 6 (TSG-6) is a multifunctional protein involved in inflammation, extracellular matrix formation, cell migration, and development. Tumor necrosis factor α–stimulated gene 6 is known for having strong anti-inflammatory effects by competing for CD44 receptor on leukocyte binding to hyaluronan during neutrophil migration into tissues [67]. Tumor necrosis factor α–stimulated gene 6 was originally found to be secreted by dermal fibroblasts incubated with TNF-α

Extracellular Vesicles—Exosomes and Microvesicles

The paracrine factors derived from MSCs may be delivered to target cells by extracellular vesicles (EVs), which are now recognized as relevant means of intercellular communication that MSC may use to impact immune cell function [71], [72], [73]. Extracellular vesicles can be either classified as exosomes, which are 50 to 100 nm in diameter containing cytoplasmic content and are secreted through vesicular fusion with the plasma membrane [74]. The other type of EV is a microvesicle, which is 0.1

Conclusion

Upon review of the main MSC-derived paracrine factors with immunoregulatory properties, we should emphasize that only a few of these are constitutively expressed such as LIF and IL-6 (Table). Indeed, secretion of most of these MSC paracrine factors is influenced by host environmental factors, and MSCs mostly acquire their immunomodulatory properties upon exposure to inflammation [80], [81]. Mesenchymal stromal cells may have direct paracrine immunomodulatory effects on the adaptive immune

Conflict of Interest

The authors declare to hold no conflict of interest with the publication of the results included in this manuscript.

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