Are mesenchymal stem cells a chance for the breakthrough in the treatment of acute respiratory distress syndrome?
Acute respiratory distress syndrome (ARDS) is the most severe form of lung injury with mortality reaching up to 40% (1). It is also one of the most common clinical syndromes treated in the intensive care units (ICU). The etiology of ARDS is very broad and associated with multiple other conditions. Clasically, two sources of ARDS are distinguished: pulmonary (i.e. pneumonia, choking, inhalatory burn) and non-pulmonary sources (sepsis, acute pancreatitis, trauma, massive blood transfusions). The most frequent cause of ARDS is sepsis (2). The diagnostic proces is based on clinical signs like: acute beginning, hypoxia (PaO2/FiO2 < 300), bilateral opacities on chest imaging not explained by other pulmonary pathology (e.g. pleural effusion, pneumothorax, or nodules) and respiratory failure not explained by heart failure or volume overload (3). Heterogenous clinical picture and very wide range of patients together with lack of specific symptoms constitute the first therapeutical problem. Although the pathomorphological changes in ARDS are relatively well defined (presence of hyalin membranes, neutrophil infiltrates, significant alveolar damage followed by a pattern of fibrosis and regeneration), the pathophysiological processes are complicated and remain not well understood. The first, acute phase of ARDS is characterized by exaggerated inflammatory response reflected by e.g. increased levels of pro-inflammatory cytokines: TNF, IL-6, IL-8 in serum and bronchoalveolar lavage fluid, activation of pulmonary macrophages and infiltration of activated neutrophils. These processes along with factors inducing ARDS (e.g. bacterial toxins, hot gases) evoke apoptosis of pneumocytes and injury of the microcirculatory endothelium of lungs. The injury of endothelial-epithelial barrier enhances further influx of neutrophils and is a cause of pulmonary edema (4, 5). Even in the early phase of the disease, simultaneously with the inflammatory process, activation of fibroblasts and regenerative mechanisms begins (6). Nowadays, patients rarely die because of the early respiratory failure. Common cause of deaths include nosocomial infections and development of Multiorgan failure. In 50% of patients who died with ARDS, lung fibrosis is observed, what suggests impaired activation of regenerative mechanisms what causes deterioration of clinical state (7).
In spite of many clinical trials and many years of intensive research, there is no effective treatment of ARDS so far. Conducted trials with glucocorticoids, exogenous surfactant, inhaled nitric oxide, prostaglandins, anticoagulants did not improve the outcome (8). Reduction of mortality was only achieved by introducing protective mechanical ventilation (airway pressure not exceeding 30 cm H2O), restrictive fluid therapy and ventilation in prone position (9-11). These procedures rather limit the iatrogenic injury than treat the disease. Due to the complex pathophysiology and harmful influence of injured lungs on other organs, effective treatment of ARDS should be multidirectional.
Originally, mesenchymal stromal cells (called also mesenchymal stem cells) have been isolated and characterized from the bone marrow by Friedenstein et al. (12). These multipotent stem cells have the capacity to multilineage differentiation into the cells of the connective tissue and therefore play an important role in the regenerative mechanisms of the adult organism. Other research groups have identified these cells in virtually every organ of the human body (lungs, heart, liver, adipose tissue, placenta, cord blood) (13). Localization among the pericytes surrounding vessels present in all tissues explain nicely the widespread presence of these cells in the body (14). Primary and still the most important method of isolation of MSCs is a culture of adherent cell colonies from a given tissue. Although, a consensus was made about the methods that confirm presence of MSCs, there is a lack of efficient method of their isolation based on one antigen. Such method is needed to obtain a homogenous population of cells. Criteria of MSCs are fulfilled by cells that are able to form adherent colonies in the in vitro culture, under special medium can differentiate into osteoblasts, adipocytes and chondrocytes and finally they express surface markers as: CD90, CD105, CD73, CD44 but not: CD45, CD34, CD31 (15).
MSCs are in the focus of interest of investigators working on the new therapies of multiple diseases (e.g. graft versus host disease, inflammatory bowel disease, infarction, multiple sclerosis) (16). MSCs seem so attractive because of their capacities to modulate the immune response and injured tissues, although their potential to differentiate into harmed tissues is also important. First reports on the utility of MSCs in the regeneration of injured lungs, suggesting high level of engraftment by transplanted cells (17), were not confirmed by other groups (18-21). In these reports, the level of engraftment in the injured lungs was below 1% of lung cells what suggests other mechanisms of action of injected MSCs. Currently, many research are aimed at investigating the immunomodulatory properties of MSCs. Numerous papers reported immunosuppressive effects of these cell on both innate and adaptive immunity (22-25). This effect is achieved by direct contact between cells and also by paracrine mediators. MSCs produce factors like: PGE2, indoleamine 2,3-dioxygenase, cytokines: TGFβ, IL-1RA, IL-10. The role of PGE2 was widely investigated and described: produced by MSCs – “re-programs” pulmonary macrophages during sepsis to produce IL-10 (26). IL-10 is a key mediator in the protective pathways utilized by transplanted MSCs in the organ injury models in sepsis (26). In the animal model of direct lung injury by endotoxin with subsequent intratracheal application of MSCs, the reduction of mortality and improvement of pathomorphological picture of lungs was correlated with reduced concentration of TNF and increased level of IL-10 in the BALF and serum (27). However, the interaction between MSCs and immunity is more complexed. These cells can also improve the anti-microbial defense what is extremely important in the sepsis induced ARDS and in the prevention of nosocomial infections in sterile ARDS. After stimulation with TNF, MSCs secrete IL-6 (28), which was shown in in vitro models to induce production of IgG by B cells (29). Also, an effect of MSCs on granulocytes is very interesting. MSCs inhibit apoptosis and degranulation of granulocytes, improving their phagocytic capacity (30). MSCs also secrete anti-bacterial peptide called LL-37 (31), what makes themselves a part of the immune system. Abovementioned interactions seem however, quite selective knowing the results of studies examining effect of MSCs on the pattern of genes expression in a murine model of sepsis. The study has revealed that systemic application of MSCs re-programs expression of hundreds of immune-related genes (32).
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