Although mesenchymal stromal cells (MSCs) possess the capacity to modulate immune responses, little is known about the mechanisms that underpin these processes. In this study, we show that immunosupression is mediated by activation of nuclear factor kappa B (NF-κB) in human MSCs. This pathway is activated by TNF-α that is generated following TCR stimulation of T cells. Inhibition of NF-κB through silencing of IκB kinase β or the TNF-α receptor abolishes the immunosuppressive capacity of MSCs. Our data also indicate that MSC-associated NF-κB activation primarily leads to inhibition of T-cell proliferation with little effect on expression of the activation markers CD69 and CD25. Thus, our data support the hypothesis that the TNF-α/NF-κB signalling pathway is required for the initial priming of immunosuppressive function in human MSCs. Interestingly, drugs that interfere with NF-κB activation significantly antagonise the immunoregulatory effect of MSCs, which could have important implications for immunosuppression regimens in the clinic. Keywords:Immunoregulation r MSC r NF-κB r TNF-α See accompanying Commentary by Pistoia and RaffaghelloAdditional supporting information may be found in the online version of this article at the publisher's web-site IntroductionMesenchymal stromal cells (MSCs) are multipotent progenitor cells that have the capacity to differentiate into multiple lineages. These cells are found in a variety of tissues during development, of which BM represents the most common source for research purposes. From a clinical perspective, MSCs are considered to Correspondence: Dr. César Trigueros e-mail: ctrigueros@inbiomed.org have a potential use in tissue repair for bone, cartilage and tendon. However, due to their immunomodulatory properties and their inclusion as a stromal component of the marrow microenvironment, MSCs are currently utilised in other therapeutic scenarios, such as those encountered in hematopoietic stem cell transplantation, GVH disease or chronic inflammatory diseases [1,2]. These characteristics, together with their low immunogenicity [1,2], have opened up promising new avenues of research for the use of MSCs not only in autologous but also in allogeneic settings.C 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu Eur. J. Immunol. 2014. 44: 480-488 Immunomodulation 481 The immunomodulatory activity of MSCs, directed against a wide range of effector cells of both the innate and adaptive immune system, has been described. Communication between MSCs and immune cells, through cell-to-cell contact-dependent and/or contact-independent mechanisms, has been shown to lead to increased production of soluble immunomodulatory factors such as indoleamine 2,3-deoxigenase [3,4], prostaglandin E2 [5][6][7], iNOS [8], transforming growth factor β (TGF-β), hepatocyte growth factor [9], human lymphocytes Ag molecule 5 and IL-10 [10]. Thus, the picture is complex, as it is likely that multiple regulatory mechanisms exist without an obvious hierarchy of importance.The inflammatory e...

Interleukin (IL)-6 is a pleiotropic cytokine involved in the regulation of hematological and immune responses. IL-6 is secreted chiefly by stromal cells, but little is known about its precise role in the homeostasis of human mesenchymal stromal cells (hMSCs) and the role it may play in hMSC-mediated immunoregulation. We studied the role of IL-6 in the biology of bone marrow derived hMSC in vitro by silencing its expression using short hairpin RNA targeting. Our results show that IL-6 is involved in immunosuppression triggered by hMSCs. Cells silenced for IL-6 showed a reduced capacity to suppress activated T-cell proliferation. Moreover, silencing of IL-6 significantly blocked the capacity of hMSCs to proliferate. Notably, increasing the intracellular level of IL-6 but not recovering the extracellular level could restore the proliferative impairment observed in IL-6-silenced hMSC. Our data indicate that IL-6 signals in hMSCs by a previously undescribed intracellular mechanism.

Mesenchymal stromal cells (MSCs) are multipotent cells found in connective tissues that can differentiate into bone, cartilage, and adipose tissue. Interestingly, they can regulate immune responses in a paracrine way and allogeneic MSCs do not elicit immune response. These properties have encouraged a number of clinical trials in a broad range of regenerative therapies. Although these trials were first focused on their differentiation properties, in the last years, the immunosuppressive features have gained most of the attention. In this review, we will summarize the up-to-date knowledge about the immunosuppressive mechanisms of MSCs in vivo and in vitro and the most promising approaches in clinical investigation.

Mesenchymal stem cells (MSCs), together with hematopoietic stem cells (HSCs), are the most frequently used cell type for cell-based therapeutics. As for other cell types intended for research and translational use, it is important to establish correctly typed cell lines from human tissue donations. Here, we describe methods for isolating, culturing, and identifying MSCs from various tissues obtained through human tissue donation. The methods have been used in the context of a biobank, prepared as standard operating procedures (SOPs), ensuring traceability and reproducibility of cell production.

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