Therapeutic Effects of Human Mesenchymal Stem Cells in Ex Vivo Human Lungs Injured with Live Bacteria

Abstract

Rationale: Mesenchymal stem cells secrete paracrine factors that can regulate lung permeability and decrease inflammation, making it a potentially attractive therapy for acute lung injury. However, concerns exist whether mesenchymal stem cells' immunomodulatory properties may have detrimental effects if targeted toward infectious causes of lung injury.Objectives: Therefore, we tested the effect of mesenchymal stem cells on lung fluid balance, acute inflammation, and bacterial clearance. Methods: We developed an Escherichia coli pneumonia model in our ex vivo perfused human lung to test the therapeutic effects of mesenchymal stem cells on bacterial-induced acute lung injury. Measurements and Main Results: Clinical-grade human mesenchymal stem cells restored alveolar fluid clearance to a normal level, decreased inflammation, and were associated with increased bacterial killing and reduced bacteremia, in part through increased alveolar macrophage phagocytosis and secretion of antimicrobial factors. Keratinocyte growth factor, a soluble factor secreted by mesenchymal stem cells, duplicated most of the antimicrobial effects. In subsequent in vitro studies, we discovered that human monocytes expressed the keratinocyte growth factor receptor, and that keratinocyte growth factor decreased apoptosis of human monocytes through AKT phosphorylation, an effect that increased bacterial clearance. Inhibition of keratinocyte growth factor by a neutralizing antibody reduced the antimicrobial effects of mesenchymal stem cells in the ex vivo perfused human lung and monocytes grown in vitro injured with E. coli bacteria. Conclusions: In E. coli-injured human lungs, mesenchymal stem cells restored alveolar fluid clearance, reduced inflammation, and exerted antimicrobial activity, in part through keratinocyte growth factor secretion.Keywords: acute lung injury; bacterial pneumonia; cell-based therapy; keratinocyte growth factor Despite extensive research into the pathogenesis of acute lung injury (ALI) and the acute respiratory distress syndrome, mortality remains high (1-3). Current treatment is supportive with lung protective ventilation and a fluid conservative strategy (4, 5). Innovative therapies are needed. Recent studies have suggested that mesenchymal stem or stromal cells (MSC) may have therapeutic applications in multiple models of lung disease (6-17). Despite initial interest in their multipotent properties, engraftment in the lung does not seem to play a major therapeutic role. The beneficial effect of MSC derives from their capacity to secrete paracrine factors that modulate immune responses and alter the responses of the endothelium or epithelium and inflammatory cells to injury (17-28). Because of the immunosuppressive properties of MSC, one safety concern is a potential deleterious effect on host defense against bacterial infection. Bacterial pneumonia and sepsis from a nonpulmonary cause are the two most common etiologies of ALI and acute respiratory distress syndrome (2). However, recent studies have provided eviden...

Mesenchymal stem cells: mechanisms of potential therapeutic benefit in ARDS and sepsis

“…Production of PGE2 by MSCs with induction of an anti-infl ammatory phenotype in host immune cells has also been reported in vitro. 57 41 MSCs decreased neutrophil infl ux and almost completely restored normal lung histology. Similar eff ects were reported when the model was extended to 10 h after injury and a higher bacterial load was used.…”

“…44 FGF7-depleted media had a negligable eff ect on alveolar fl uid clearance, whereas the addition of recombinant FGF7 to the media restored its therapeutic benefi t. Similar improvements in alveolar fl uid clearance with MSCs were noted when ex-vivo lungs were directly injured with live bacteria. 41 Finally, in a 2014 study 47 with perfused lungs that were rejected for transplant, intravenous administration of MSCs normalised alveolar fl uid clearance. Pretreatment of the perfused lung with an FGF7-blocking antibody statistically signifi cantly reduced this eff ect.…”

“…37,38,[48][49][50][51] This antibacterial eff ect is partly mediated by improved phagocytic activity of host immune cells such as macrophages, 38,41 monocytes, 48 neutrophils, 49 and ITGAMpositive cells (monocytes, macrophages, and neutrophils). 38 Studies using ex-vivo human lungs have reported similar fi ndings.…”

“…MSCs reduced alveolar bacterial counts and improved alveolar macrophage phagocytosis after direct bacterial injury. 41 This eff ect seems to be partly mediated by FGF7, because the use of an FGF7-neutralising antibody nullifi ed the antimicrobial eff ects of MSCs both in vitro and ex vivo. 41 Alveolar fl uid from lungs treated with MSCs was noted to have increased antimicrobial activity against E coli in vitro, suggesting the presence of a secreted antimicrobial factor.…”

“…52 MSC production of FGF7 has also been postulated to inhibit apoptosis of monocytes, leading to increased bacterial killing. 41 Future research will hopefully illuminate the extent and signifi cance of this pathway.…”