Human Umbilical Cord Mesenchymal Stem Cells Reduce Fibrosis of Bleomycin-Induced Lung Injury

Acute respiratory distress syndrome is characterized by loss of lung tissue as a result of inflammation and fibrosis. Augmenting tissue repair by the use of mesenchymal stem cells may be an important advance in treating this condition. We evaluated the role of term human umbilical cord cells derived from Wharton’s jelly with a phenotype consistent with mesenchymal stem cells (uMSCs) in the treatment of a bleomycin-induced mouse model of lung injury. uMSCs were administered systemically, and lungs were harvested at 7, 14, and 28 days post-bleomycin. Injected uMSCs were located in the lung 2 weeks later only in areas of inflammation and fibrosis but not in healthy lung tissue. The administration of uMSCs reduced inflammation and inhibited the expression of transforming growth factor-β, interferon-γ, and the proinflammatory cytokines macrophage migratory inhibitory factor and tumor necrosis factor-α. Collagen concentration in the lung was significantly reduced by uMSC treatment, which may have been a consequence of the simultaneous reduction in Smad2 phosphorylation (transforming growth factor-β activity). uMSCs also increased matrix metalloproteinase-2 levels and reduced their endogenous inhibitors, tissue inhibitors of matrix metalloproteinases, favoring a pro-degradative milieu following collagen deposition. Notably, injected human lung fibroblasts did not influence either collagen or matrix metalloproteinase levels in the lung. The results of this study suggest that uMSCs have antifibrotic properties and may augment lung repair if used to treat acute respiratory distress syndrome.An enduring problem in respiratory and critical medicine is the treatment of acute respiratory distress syndrome (ARDS)/acute lung injury, a condition that is characterized by refractory hypoxemia in patients with bilateral lung infiltrates in the absence of pulmonary edema.¹ A National Institutes of Health study estimated the incidence of acute respiratory distress syndrome/acute lung injury to be 75 per 100,000 population in the United States with 40 to 60% mortality.² ARDS may be the end result of several conditions that directly injure the lung such as pneumonia, pulmonary contusion, inhalational injury, and near drowning.³ Generic injury to the lung results in damage to the epithelial and endothelial cells and a compromised alveolar-capillary barrier. There is exudation of fluid into the alveolar space followed by inflammatory cells, a process driven by cytokines such as interleukin (IL)-8, tumor necrosis factor (TNF)-α and IL-1. The progression of acute lung injury to fibrosis portends a poor prognosis and may be observed as early as 5 to 7 days after injury.⁴ Many strategies have been directed at augmenting repair of ARDS. These include improved ventilation techniques, surfactant therapy, vasodilators, and anti-inflammatory agents.¹ Notably, there has been an increasing focus on the acceleration of resolution by epithelial restitution and the consequent reduction in fibrosis of ARDS.To this end, new stem cell therapies have raised the possibility of improving lung repair. Mesenchymal stem cells (MSCs)⁵ are multipotent and differentiate into a range of cell types and are being tested for their regenerative potential, particularly in myocardial infarction and some neurodegenerative disorders.⁶ MSCs are adherent cells and a common MSC immuno-phenotype can be identified in cells from many sources including bone marrow, umbilical cord blood, and adult organs.⁷The role of MSCs in the treatment of lung injury has been the subject of several studies. Indeed, MSCs have displayed the potential to improve lung function in pulmonary disease through several mechanisms. Murine bone marrow MSCs (bmMSCs) have been shown to selectively home to sites of injury through the chemokine receptor CXCR4 and chemokine, Stromal derived factor as well as Flk surface receptors^8,9 and improved respiratory capacity in bleomycin, lipopolysaccharide, and monocrotaline-induced models of lung injury.^10,11,12 Furthermore, both in vivo and in vitro studies have shown that murine and human bmMSCs and human umbilical cord blood cells may differentiate into cells with markers of lung epithelium.^13,14 Aguilar et al supported the safety profile of human MSCs by demonstrating that murine bmMSCs but not human bmMSCs differentiated into osteosarcomas when injected into the lung.¹⁵Based on these studies, we hypothesized that MSCs derived from the Wharton’s jelly of the umbilical cord (uMSCs) would repair lung injury and prevent fibrosis. The umbilical cord is derived from the extraembryonic mesoderm and develops from the proximal epiblast during the formation of the embryonic primitive streak.¹⁶ The umbilical cord contains two arteries and a vein that are surrounded by a matrix rich in hyaluronic acid known as Wharton’s jelly (WJ). Recently, groups have cultured MSCs from the WJ of the umbilical cord and differentiated them in vitro into several tissue types.^17,18 These cells have the advantage of ready availability, do not require invasive bone marrow biopsies, and are more plentiful than umbilical cord blood-derived MSCs. In the present study, we examined the therapeutic potential of uMSCs in a bleomycin-induced model of lung injury that shares many features in common with the phenotype of ARDS in human subjects.