Mesenchymal Stromal Cells Modulate Macrophages in Clinically Relevant Lung Injury Models by Extracellular Vesicle Mitochondrial Transfer.

RATIONALE: Acute respiratory distress syndrome (ARDS) remains a major cause of respiratory failure in critically ill patients. Mesenchymal stromal cells (MSCs) are a promising candidate for a cell-based therapy. However, the mechanisms of MSCs' effects in ARDS are not well understood. In this study, we focused on the paracrine effect of MSCs on macrophage polarization and the role of extracellular vesicle (EV)-mediated mitochondrial transfer. OBJECTIVES: To determine the effects of human MSCs on macrophage function in the ARDS environment and to elucidate the mechanisms of these effects. METHODS: Human monocyte-derived macrophages (MDMs) were studied in noncontact coculture with human MSCs when stimulated with LPS or bronchoalveolar lavage fluid (BALF) from patients with ARDS. Murine alveolar macrophages (AMs) were cultured ex vivo with/without human MSC-derived EVs before adoptive transfer to LPS-injured mice. MEASUREMENTS AND MAIN RESULTS: MSCs suppressed cytokine production, increased M2 macrophage marker expression, and augmented phagocytic capacity of human MDMs stimulated with LPS or ARDS BALF. These effects were partially mediated by CD44-expressing EVs. Adoptive transfer of AMs pretreated with MSC-derived EVs reduced inflammation and lung injury in LPS-injured mice. Inhibition of oxidative phosphorylation in MDMs prevented the modulatory effects of MSCs. Generating dysfunctional mitochondria in MSCs using rhodamine 6G pretreatment also abrogated these effects. CONCLUSIONS: In the ARDS environment, MSCs promote an antiinflammatory and highly phagocytic macrophage phenotype through EV-mediated mitochondrial transfer. MSC-induced changes in macrophage phenotype critically depend on enhancement of macrophage oxidative phosphorylation. AMs treated with MSC-derived EVs ameliorate lung injury in vivo.

Mitochondrial Transfer via Tunneling Nanotubes is an Important Mechanism by Which Mesenchymal Stem Cells Enhance Macrophage Phagocytosis in the In Vitro and In Vivo Models of ARDS.

Mesenchymal stromal cells (MSC) have been reported to improve bacterial clearance in preclinical models of Acute Respiratory Distress Syndrome (ARDS) and sepsis. The mechanism of this effect is not fully elucidated yet. The primary objective of this study was to investigate the hypothesis that the antimicrobial effect of MSC in vivo depends on their modulation of macrophage phagocytic activity which occurs through mitochondrial transfer. We established that selective depletion of alveolar macrophages (AM) with intranasal (IN) administration of liposomal clodronate resulted in complete abrogation of MSC antimicrobial effect in the in vivo model of Escherichia coli pneumonia. Furthermore, we showed that MSC administration was associated with enhanced AM phagocytosis in vivo. We showed that direct coculture of MSC with monocyte-derived macrophages enhanced their phagocytic capacity. By fluorescent imaging and flow cytometry we demonstrated extensive mitochondrial transfer from MSC to macrophages which occurred at least partially through tunneling nanotubes (TNT)-like structures. We also detected that lung macrophages readily acquire MSC mitochondria in vivo, and macrophages which are positive for MSC mitochondria display more pronounced phagocytic activity. Finally, partial inhibition of mitochondrial transfer through blockage of TNT formation by MSC resulted in failure to improve macrophage bioenergetics and complete abrogation of the MSC effect on macrophage phagocytosis in vitro and the antimicrobial effect of MSC in vivo. Collectively, this work for the first time demonstrates that mitochondrial transfer from MSC to innate immune cells leads to enhancement in phagocytic activity and reveals an important novel mechanism for the antimicrobial effect of MSC in ARDS. Stem Cells 2016;34:2210-2223.

SMAD inhibition attenuates epithelial to mesenchymal transition by primary keratinocytes in vitro.

Epithelial to mesenchymal transition (EMT) is a process whereby epithelial cells undergo transition to a mesenchymal phenotype and contribute directly to fibrotic disease. Recent studies support a role for EMT in cutaneous fibrotic diseases including scleroderma and hypertrophic scarring, although there is limited data on the cytokines and signalling mechanisms regulating cutaneous EMT. We investigated the ability of TGF-ß and TNF-α, both overexpressed in cutaneous scleroderma and central mediators of EMT in other epithelial cell types, to induce EMT in primary keratinocytes and studied the signalling mechanisms regulating this process. TGF-ß induced EMT in normal human epidermal keratinocytes (NHEK cells), and this process was enhanced by TNF-α. EMT was characterised by changes in morphology, proteome (down-regulation of E-cadherin and Zo-1 and up-regulation of vimentin and fibronectin), MMP secretion and COL1α1 mRNA expression. TGF-ß and TNF-α in combination activated SMAD and p38 signalling in NHEK cells. P38 inhibition with SB203580 partially attenuated EMT, whereas SMAD inhibition using SB431542 significantly inhibited EMT and also reversed established EMT. These data highlight the retained plasticity of adult keratinocytes and support further studies of EMT in clinically relevant in vivo models of cutaneous fibrosis and investigation of SMAD inhibition as a potential therapeutic intervention.

Analysis of Mitochondrial Transfer in Direct Co-cultures of Human Monocyte-derived Macrophages (MDM) and Mesenchymal Stem Cells (MSC).

Mesenchymal stem/stromal cells (MSC) are adult stem cells which have been shown to improve survival, enhance bacterial clearance and alleviate inflammation in pre-clinical models of acute respiratory distress syndrome (ARDS) and sepsis. These diseases are characterised by uncontrolled inflammation often underpinned by bacterial infection. The mechanisms of MSC immunomodulatory effects are not fully understood yet. We sought to investigate MSC cell contact-dependent communication with alveolar macrophages (AM), professional phagocytes which play an important role in the lung inflammatory responses and anti-bacterial defence. With the use of a basic direct co-culture system, confocal microscopy and flow cytometry we visualised and effectively quantified MSC mitochondrial transfer to AM through tunnelling nanotubes (TNT). To model the human AM, primary monocytes were isolated from human donor blood and differentiated into macrophages (monocyte derived macrophages, MDM) in the presence of granulocyte macrophage colony-stimulating factor (GM-CSF), thus allowing adaptation of an AM-like phenotype (de Almeida et al., 2000; Guilliams et al., 2013). Human bone-marrow derived MSC, were labelled with mitochondria-specific fluorescent stain, washed extensively, seeded into the tissue culture plate with MDMs at the ratio of 1:20 (MSC/MDM) and co-cultured for 24 h. TNT formation and mitochondrial transfer were visualised by confocal microscopy and semi-quantified by flow cytometry. By using the method we described here we established that MSC use TNTs as the means to transfer mitochondria to macrophages. Further studies demonstrated that mitochondrial transfer enhances macrophage oxidative phosphorylation and phagocytosis. When TNT formation was blocked by cytochalasin B, MSC effect on macrophage phagocytosis was completely abrogated. This is the first study to demonstrate TNT-mediated mitochondrial transfer from MSC to innate immune cells.