Bone marrow-derived mesenchymal stromal cells (BMSCs) mitigate inflammation in mouse models of acute lung injury. However, specific mechanisms of BMSC actions on CD4 T lymphocyte-mediated inflammation in vivo remain poorly understood. Limited data suggests promotion of Th2 phenotype in models of Th1-mediated diseases. However whether this might alleviate or worsen Th2-mediated diseases such as allergic asthma is unknown. To ascertain the effects of systemic administration of BMSCs in a mouse model of Th2-mediated allergic airways inflammation, ovalbumin-induced allergic airways inflammation was induced in wild type C57BL/6 and BALB/c mice as well as in IFNγ receptor null mice. Effects of systemic administration during antigen sensitization of either syngeneic or allogeneic BMSC on airways hyper-reactivity, lung inflammation, antigen-specific CD4 T lymphocytes, and serum immunoglobulins were assessed. Both syngeneic and allogeneic BMSCs inhibited airways hyper-reactivity and lung inflammation through a mechanism partly dependent on IFNγ. However, contrary to existing data, BMSCs did not affect antigen-specific CD4 T lymphocyte proliferation but rather promoted Th1 phenotype in vivo as assessed by both ova-specific CD4 T lymphocyte cytokine production and ova-specific circulating immunoglobulins. BMSCs treated to prevent release of soluble mediators and a control cell population of primary dermal skin fibroblasts only partly mimicked the BMSC effects and in some cases worsened inflammation. In conclusion, BMSCs inhibit Th2-mediated allergic airways inflammation by influencing antigen-specific CD4 T lymphocyte differentiation. Promotion of a Th1 phenotype in antigen-specific CD4 T lymphocytes by BMSCs is sufficient to inhibit Th2-mediated allergic airways inflammation through an IFNγ-dependent process.

There is a growing experience demonstrating benefit of mesenchymal stromal cell (MSC)-based cell therapies in preclinical models of asthma. In the current study, conditioned media (CM) and, in particular, the extracellular vesicle fraction obtained from the CM were as potent as the MSCs themselves in mitigating Th2/Th17-mediated allergic airway inflammation in a mouse model of severe refractory clinical asthma. Moreover, human MSC CM and extracellular vesicles were effective in this immunocompetent mouse model. These data add to a growing scientific basis for initiating clinical trials of MSCs or extracellular vesicles derived from MSCs in severe refractory asthma and provide further insight into the mechanisms by which the MSCs may ameliorate the asthma.

Systemic administration of mesenchymal stromal cells (MSCs) suppresses airway inflammation and methacholine-induced airway hyper-responsiveness (AHR) in mouse models of T helper cell (Th) type 2-mediated eosinophilic allergic airway inflammation (AAI); however, the efficacy of MSCs in mouse models of severe Th17-mediated neutrophilic AAI has not yet been demonstrated. We assessed MSC effects in a mouse model of mixed Th2/Th17 AAI produced by mucosal exposure to Aspergillus fumigatus hyphal extract (AHE). Following sensitization produced by oropharyngeal AHE administration, systemic (tail vein) administration of syngeneic MSCs on the first day of challenge significantly reduced acute AHR predominantly through reduction of Th17-mediated airway inflammation. In parallel experiments, MSCs also mitigated AHR when administered during recurrent challenge 10 weeks after initial sensitization and challenge through reduction in systemic Th17-mediated inflammation. Investigation into potential mechanistic actions of MSCs in this model demonstrated that although T regulatory cells were increased in all AHE-treated mice, MSC administration did not alter T regulatory cell numbers in either the acute or recurrent model. Differential induction of interleukin-17a secretion was observed in ex vivo restimulation of mediastinal lymph node mixed-cell cytokine analyses. Although the mechanisms by which MSCs act to decrease inflammation and AHR in this model are not yet fully elucidated, decrease in Th17-mediated airway inflammation appears to play a significant role. These results provide a basis for further investigations of MSC administration as a potential therapeutic approach for severe refractory neutrophilic asthma. STEM CELLS TRANSLATIONAL MEDICINE 2014;3:194-205

Recent data suggest that freshly thawed previously frozen mesenchymal stromal cells (MSCs) may not have the same effectiveness or breadth of anti-inflammatory activities as do continuously cultured MSCs. This has significant implications for clinical use, in which many infusion schemes use frozen cells thawed at the bedside for administration. The available data, however, predominantly evaluate in vitro MSC properties, and so far there has been limited in vivo analysis. To further assess this issue, we compared freshly thawed (thawed) versus continuously cultured (fresh) human bone marrowderived MSC (hMSC) administration in a mouse model of mixed Th2/Th17 allergic airway inflammation induced by Aspergillus hyphal extract (AHE) exposures in immunocompetent C57Bl/6 mice. Control cell populations included fresh versus thawed murine bone marrow-derived MSCs (mMSCs) and human lung fibroblasts (HLFs). Systemic administration of both thawed and fresh hMSCs and mMSCs, but not HLFs, at the onset of antigen challenge in previously sensitized mice significantly ameliorated the AHE-provoked increases in airway hyper-reactivity, lung inflammation, and antigenspecific CD4 T-cell Th2 and Th17 phenotype. Notably, there was no difference in effects of fresh versus thawed hMSCs or mMSCs on any outcome measured except for some variability in the effects on the bronchoalveolar lavage fluid composition. These results demonstrated potent xenogeneic effects of human MSCs in an immunocompetent mouse model of allergic airways inflammation and that thawed MSCs are as effective as fresh MSCs. The question of fresh versus thawed MSC effectiveness needs to be investigated carefully and may differ in different in vivo disease-specific models. STEM CELLS TRANSLATIONAL MEDICINE 2015;4:615-624 SIGNIFICANCEThis study addressed whether freshly thawed mesenchymal stromal cells (MSCs) are as effective in in vivo settings as those that have been continuously cultured. It also provided further data demonstrating that xenogeneic use of MSCs in immunocompetent mice is as effective as murine MSCs. This information provides further support and direction for potential clinical use of MSCs in patients with severe asthma.

A number of novel approaches for repair and regeneration of injured lung have developed over the past several years. These include a better understanding of endogenous stem and progenitor cells in the lung that can function in reparative capacity as well as extensive exploration of the potential efficacy of administering exogenous stem or progenitor cells to function in lung repair. Recent advances in ex vivo lung engineering have also been increasingly applied to the lung. The current status of these approaches as well as initial clinical trials of cell therapies for lung diseases are reviewed below.