Hypoxia-induced pulmonary vascular remodeling requires recruitment of circulating mesenchymal precursors of a monocyte/macrophage lineage

Vascular remodeling in chronic hypoxic pulmonary hypertension includes marked fibroproliferative changes in the pulmonary artery (PA) adventitia. Although resident PA fibroblasts have long been considered the primary contributors to these processes, we tested the hypothesis that hypoxia-induced pulmonary vascular remodeling requires recruitment of circulating mesenchymal precursors of a monocyte/macrophage lineage, termed fibrocytes. Using two neonatal animal models (rats and calves) of chronic hypoxic pulmonary hypertension, we demonstrated a dramatic perivascular accumulation of mononuclear cells of a monocyte/macrophage lineage (expressing CD45, CD11b, CD14, CD68, ED1, ED2). Many of these cells produced type I collagen, expressed alpha-smooth muscle actin, and proliferated, thus exhibiting mesenchymal cell characteristics attributed to fibrocytes. The blood-borne origin of these cells was confirmed in experiments wherein circulating monocytes/macrophages of chronically hypoxic rats were in vivo-labeled with DiI fluorochrome via liposome delivery and subsequently identified in the remodeled pulmonary, but not systemic, arterial adventitia. The DiI-labeled cells that appeared in the vessel wall expressed monocyte/macrophage markers and procollagen. Selective depletion of this monocytic cell population, using either clodronate-liposomes or gadolinium chloride, prevented pulmonary adventitial remodeling (ie, production of collagen, fibronectin, and tenascin-C and accumulation of myofibroblasts). We conclude that circulating mesenchymal precursors of a monocyte/macrophage lineage, including fibrocytes, are essential contributors to hypoxia-induced pulmonary vascular remodeling.

Hypoxia-induced pulmonary artery adventitial remodeling and neovascularization: contribution of progenitor cells

Information is rapidly emerging regarding the important role of the arterial vasa vasorum in a variety of systemic vascular diseases. In addition, increasing evidence suggests that progenitor cells of bone marrow (BM) origin may contribute to postnatal neovascularization and/or vascular wall thickening that is characteristic in some forms of systemic vascular disease. Little is known regarding postnatal vasa formation and the role of BM-derived progenitor cells in the setting of pulmonary hypertension (PH). We sought to determine the effects of chronic hypoxia on the density of vasa vasorum in the pulmonary artery and to evaluate if BM-derived progenitor cells contribute to the increased vessel wall mass in a bovine model of hypoxia-induced PH. Quantitative morphometric analyses of lung tissue from normoxic and hypoxic calves revealed that hypoxia results in a dramatic expansion of the pulmonary artery adventitial vasa vasorum. Flow cytometric analysis demonstrated that cells expressing the transmembrane tyrosine kinase receptor for stem cell factor, c-kit, are mobilized from the BM in the circulation in response to hypoxia. Immunohistochemistry revealed an increase in the expression of c-kit+ cells together with vascular endothelial growth factor, fibronectin, and thrombin in the hypoxia-induced remodeled pulmonary artery vessel wall. Circulating mononuclear cells isolated from neonatal calves exposed to hypoxia were found to differentiate into endothelial and smooth muscle cell phenotypes depending on culture conditions. From these observations, we suggest that the vasa vasorum and circulating progenitor cells could be involved in vessel wall thickening in the setting of hypoxia-induced PH.