Am J Respir Cell Mol Biol. 2015 Oct;53(4):500-12.
Vascular Repair by Tissue-Resident Endothelial Progenitor Cells in Endotoxin-Induced Lung Injury.
- 1Department of Respirology, Graduate School of Medicine, Chiba University, Chiba, Japan.
Vascular disruption is one of the pathological hallmarks in acute respiratory distress syndrome. Bone marrow (BM)-derived circulating endothelial progenitor cells (EPCs) and lung tissue-resident EPCs have been considered to play a pivotal role in pulmonary vascular repair; however, which population is predominant in local pulmonary vasculogenesis remains to be clarified. We therefore examined the origin of EPCs participating in the regenerative process of pulmonary vascular endothelial cells (PVECs) in experimental acute respiratory distress syndrome. Lung samples from mice administered LPS intratracheally were investigated for cell dynamics and EPC functions. Quantitative flow cytometric analysis demonstrated that the number of PVECs decreased by roughly 20% on Day 1 and then recovered on Day 7 of LPS challenge. Bromodeoxyuridine-incorporation assays and immunofluorescence microscopy demonstrated that proliferating PVECs preferentially located in the capillary vessels. Experiments using BM chimera mice revealed that most of the regenerating PVECs were tissue-resident cells, and BM-derived cells hardly engrafted as PVECs. The population of circulating putative phenotypical EPCs decreased during the first week after LPS challenge. The regenerating PVECs were characterized by high colony-forming and vasculogenic capacities, intracellular reactive oxygen species scavenging and aldehyde dehydrogenase activites, and enhanced gene expression of Abcb1b (a drug-resistant gene), suggesting that the population of PVECs included tissue-resident EPCs activated during regenerative process of PVECs. The proliferating PVECs expressed CD34, Flk-1/KDR, and c-kit more strongly and Prom1/CD133 less strongly on the surface than nonproliferating PVECs. Our findings indicated that lung tissue-resident EPCs predominantly contribute to pulmonary vascular repair after endotoxin-induced injury.
KEYWORDS: LPS; acute respiratory distress syndrome; endothelial progenitor cells; endothelial repair; vascular regeneration
Acute respiratory distress syndrome (ARDS) is a severe intractable clinical condition with a high mortality rate in spite of intensive care, therefore, new therapeutic strategies for ARDS have been desired.
Vascular disruption is one of the hallmarks in ARDS(1), thus we have focused on the vascular repair mechanisms which could be related to control of the disease. There are many kinds of cells comprising lung vasculature, and pulmonary vascular endothelial cells (PVECs) are main component of the lung microvasculature, dysfunction of which should lead to the pathogenesis of ARDS. Therefore, we have been interested in how to control the regenerative process of PVECs for patients suffering from ARDS to be cured.
Stem/progenitor cell biology is now developing around the world and desired to be applied to clinical sites of many areas as soon as possible. Endothelial progenitor cells (EPCs) could be defined as cells being able to proliferate, differentiate into new endothelial cells and replace the injured endothelial cells. Since EPCs were shown to exist in circulation of human in 1997 for the first time(2), research on EPC biology has been expanded dramatically. Bone marrow (BM) have been considered as a major source of EPCs, however, non-BM derived EPCs and tissue-resident EPCs have also been indicated to exist in circulation(3, 4) and systemic vessels, respectively. Therefore, it remains to be elucidated how the EPCs should be defined clearly and participate in the homeostasis for vasculature, and what mechanisms modulate the EPCs under various clinical condition.
Regarding EPCs for lung vessel, previous reports have indicated that BM-derived and lung tissue-resident EPCs exist and play a pivotal role for vascular endothelial repair(5, 6). However, it remains to be clarified which of the EPC populations is predominant in the repair process.
To determine the major source of EPCs participating in the repair of endothelium in ARDS, we employed mouse model of ARDS, and used BM chimera mice, which let us distinguish BM derived cells and non BM-derived cells. We defined EPCs as proliferative cells expressing markers of proliferation and endothelial cell marker (CD31) but leukocyte marker (CD45). As a result, we demonstrated at first BM derived cells rarely participate in regenerative process of PVECs. Then, we hypothesized lung tissue resident EPCs should be activated and play a major role in the process. To confirm the hypothesis, we examined in vitro and in vivo if such EPCs certainly exist and activated after lung injury. We defined PVECs as CD31+CD45+ lung cells, sorted the cells using magneto-beads method for culture, and confirmed if the cultured cells show functions as EPCs, i.e. colony forming and tube formation capacities in vitro, and vessel forming capacity in vivo. As a result, we showed PVECs contain cells with the EPC functions and the EPCs are activated after lung injury by LPS, indicating lung tissue resident EPCs are activated and participate in the endothelial repair.
As a conclusion, we demonstrated that lung tissue-resident EPCs rather than BM-derived EPCs play a major role for vascular endothelial repair in LPS-induced lung injury. The significance of this report is to strengthen our understanding of vascular biology in the light of EPCs participation in pathological mechanism of ARDS. We believe that further developing research as for pulmonary endothelial regeneration will lead to new strategies for ARDS based on biological mechanisms.
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