Vascular Repair by Tissue-Resident Endothelial Progenitor Cells in Endotoxin-Induced Lung Injury

Abstract

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.

Endothelial-to-mesenchymal transition in lipopolysaccharide-induced acute lung injury drives a progenitor cell-like phenotype

“…In previous studies we have shown that pulmonary microvascular endothelial cells were enriched with progenitor cells that had vasculogenic capacity (27,38). LPS-induced EndMTPVECs occurred at the capillary level where tissue-resident EPCs are also located (27,38).…”

“…LPS-induced EndMTPVECs occurred at the capillary level where tissue-resident EPCs are also located (27,38). Although it has been reported that the expression of mesenchymal progenitor cell markers is upregulated during the process of EndMT, our findings suggested the possibility that some survived pulmonary endothelial cells underwent EndMT to dedifferentiate toward endothelial progenitor-like cells, contributing thereby to the replacement of the injured endothelium.…”

“…To do so, we used BM-chimera mice to distinguish tissue-resident from BM-derived circulating cells. Setting time as 3 mo between BM transplantation and LPS challenge enabled us to establish ϳ95% chimerism of peripheral blood cells (27), avoiding underestimation due to insufficient BM reconstitution; lung interstitial leukocytes were efficiently replaced by BM-derived cells, whereas ϳ98% of EndMT-PVECs were non-BM-derived cells before and after LPS challenge. These results indicated that the majority of EndMT-PVECs originated from tissue-resident PVECs, not from the BM.…”

“…Because several ATP-binding cassette (Abc) transporter genes, which are associated with the efflux of noxious reagents from the cells, have been used as functional markers of stem/ progenitor cells (27,47), we examined the expression of three Abc transporter genes. Figure 7E shows that the expression of ABCB1 significantly increased in EndMT-HMVEC-Ls (2.1-fold), suggesting the survival potential of EndMT-HMVEC-Ls in acute lung injury.…”

“…In addition to these, we recently reported that lung tissueresident endothelial progenitor cells (EPCs) contribute to pulmonary vascular repair after endotoxin-induced injury (27). We hypothesized that, in the process of tissue repair, LPSinduced EndMT-cells may dedifferentiate into a more progenitor cell-like phenotype.…”