Circulating Progenitors in Lung Injury: A Novel Therapy for Acute Respiratory Distress Syndrome?
Burnham, Ellen L. M.D.
ACUTE respiratory distress syndrome (ARDS), and its less severe form, acute lung injury (ALI), are common disorders in patients requiring critical care, occurring secondary to a variety of injuries such as aspiration, sepsis, and trauma. Mortality from ARDS approximates 40%, and no specific medical therapies exist despite years of well-conducted clinical trials.1
This compels investigators to explore novel therapies aimed at treating this disorder. In this issue of Anesthesiology, Lam et al.2
present data suggesting that an infusion of autologous endothelial progenitor cells (EPCs) ameliorates experimentally induced lung injury. Their findings highlight the potential of cell-based therapies for treatment of ALI and ARDS.
Diffuse endothelial damage is a hallmark of ALI. In its earliest phases, endothelial cells become edematous and gaps develop between normally tightly adjacent cells. These changes expose the basement membrane, contribute to the development of hyaline membranes, and result in the filling of alveolar spaces with proteinaceous pulmonary edema fluid.3
Potential therapies targeted at repair or limiting of endothelial damage in ALI have not been thoroughly explored, although it seems reasonable that such therapy might benefit patients with ALI. The oleic acid lung injury model used in this study seems to be a logical surrogate to test the effect of cell-based therapy directed at endothelium damaged in ALI.
The initial observation of circulating progenitor cells with endothelial properties was met with enthusiasm because it held the promise of cell-based therapies for a myriad of vascular diseases.4
Nevertheless, isolation and characterization of EPCs has proven difficult. Their numbers in the circulation of both healthy individuals (as well as in animals) is exceedingly low.5
Also, no consensus exists on the type or types of cell surface marker(s) for EPCs. There is some agreement that the cell surface antigens CD133 and vascular-endothelial growth factor receptor 2, along with LDL uptake and lectin binding, may be used to identify these cells.6
In addition, culture in endothelial-specific medium has been shown to promote the growth of cells with endothelial progenitor properties.7
An important recent observation relevant to EPC investigations is the identification of two EPC subtypes with potentially different roles in endothelial repair.8,9
These two disparate cell types are obtained by culturing peripheral blood mononuclear cells and examining certain properties in vitro
. Peripheral blood mononuclear cells that grow into colony-forming units on fibronectin early (approximately 7 days) are termed early EPCs
. They possess cell surface antigens consistent with hematopoietic progenitors (i.e.
, CD34), along with endothelial-specific markers (i.e.
, CD31) and markers of monocytic lineage (i.e.
, CD14), among others. They have less potential to form true endothelium in tube-forming assays in three-dimensional cultures either in isolation or in coculture with human umbilical vein endothelial cells. However, early EPCs have been reported to secrete significant amounts proangiogenic growth factors.10
In contrast to this, late or outgrowth endothelial progenitors appear after 2 or more weeks in culture and do not secrete these factors to a measurable extent. Late EPCs do share some markers in common with early EPCs, such as CD31, but do not possess hematopoietic surface antigens. These cells may be more operational in replacing damaged or destroyed endothelium, because in vitro
, they will readily form endothelioid tubes and functional blood vessels in animal models. Therefore, it may be that each of these cell types plays a unique role in proper endothelial repair.11
Before oleic acid–induced lung injury, Lam et al. isolated early EPCs by culturing peripheral blood mononuclear cells for 1 week. Cultured cells were then collected and injected into lung-injured and uninjured control animals. When the animals were killed 48 h later, fluorescent-labeled EPCs were detected in pulmonary arterioles of animals injected with EPCs. Western blotting analysis performed on pulmonary arterial blood of these animals revealed that inducible nitric oxide synthase expression was suppressed in animals that had received EPCs. In addition, animals subjected to lung injury who received EPCs had a decreased wet-to-dry weight ratio, a decrease in hyaline membrane formation, decreased hemorrhage, and a lower percentage of neutrophils present within the lung. Although not endothelial specific, these findings suggest that EPC infusion had an effect on preservation of alveolar–capillary barrier integrity.
Why did early EPC infusion seem to be beneficial in lung injury? One answer may be these cells’ ability to secrete antioxidants and thereby ameliorate the highly oxidized milieu in the lung observed in ALI, reflected in the animal model as high inducible nitric oxide synthase expression.12
These authors performed additional experiments in vitro
to assess the antioxidant capacity of human early EPCs compared with the antioxidant capacity of human umbilical vein endothelial cells. Expression of the antioxidants manganese superoxide dismutase and heme oxygenase 1 were greater in early EPCs compared with the more mature human umbilical vein endothelial cells. These findings suggest a potential mechanism whereby infusion of early EPCs acts to normalize the oxidative milieu of the injured lung and potentially lay the groundwork for adequate lung repair and normalization of cellular function.
This study by Lam et al. provides proof of the concept that infusion of early EPCs may be beneficial in ARDS and ALI. However, it is not without certain limitations. First, although injected cells EPCs appeared in the pulmonary vasculature, they were not observed elsewhere in the lung, such as within the alveoli, where lung repair is most needed. Also, the degree of EPC engraftment (or at least association) was not quantitated, although this has proven difficult for other investigators to assess as well. Therefore, it is unclear exactly what role EPCs have in preserving or restoring lung architecture, although lungs of animals given EPCs appeared significantly more normal after injury. Given the degree of lung damage elicited by oleic acid, the relatively small number of EPCs infused, and the short time period between EPC infusion and animal death, it is less likely that EPCs themselves played a significant structural role in repair of damaged endothelium. However, a paracrine role for these cells seems more plausible. Another issue raised by the authors is the relevance of an autologous cell infusion model for ALI, where it would be impossible to collect cells from a patient before disease onset to use later on in illness. A more feasible possibility might be to use therapies such as granulocyte–macrophage colony-stimulating factor that enhance endogenous EPC release and could be used after lung injury has occurred. Further clarification of the role of these therapies could be expected to move to clinical trials more quickly than cell-based therapies, because the latter will likely be subject to intense regulatory scrutiny.
Endothelial progenitor cells have proven enigmatic to study. However, despite our lack of clearly understanding their purpose and function, animal models of lung injury such as that detailed in this study and others indicate beneficial effects of EPCs in terms of preservation of lung architecture after injury.13
Nonetheless, additional investigations are clearly required to determine whether benefits of this type of therapy are clinically significant and sustained and to ensure that there are no unanticipated adverse effects. Although cell-based therapies for lung injury are still a faraway goal, studies such as this by Lam et al.
provide important information that will undoubtedly aid in the development of novel therapies for ALI and ARDS.
Ellen L. Burnham, M.D.
Division of Pulmonary Sciences and Critical Care Medicine, University of Colorado Denver and Health Sciences Center, Denver, Colorado. email@example.com
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