pulmonary vascular remodeling is a hallmark of most forms of pulmonary hypertension (PH), both primary and secondary. This remodeling takes the form of concentric medial thickening of small arterioles, neomuscularization of previously nonmuscular capillary-like vessels, and structural wall changes in larger pulmonary arteries. The pulmonary arterial muscularization is characterized by increased numbers of vascular smooth muscle cells (SMCs) as well as by hypertrophy of individual SMCs (25). In human PH, characteristic plexiform lesions develop, and there is increasing discussion of neovasculogenesis and angiogenesis during remodeling. The vascular remodeling is associated with hypoxia- or inflammation-induced production of growth factors, angiogenic factors, inflammatory mediators, and vasoconstrictors. For decades, investigators have attempted to dissect the mechanisms behind this lung-specific vascular remodeling, investigating a wide range of environmental stimuli, hemodynamic events, biochemical and molecular signaling pathways, and extracellular matrix changes.
Recently, investigators have begun looking in greater detail at the heterogeneity of cells in lung vascular walls under normal and pathological remodeling conditions (14, 26). Initially, it was recognized that even within a single region, endothelial cells differ and may have highly distinct characteristics and functions. Now there is increasing interest in and evidence for the existence of diverse cell types within the media of remodeling lung vessels. Emerging evidence suggests that endogenous or circulating inflammatory and/or progenitor cells contribute significantly to the remodeling process (9, 22). The term “circulating inflammatory/progenitor cells” characterizes a wide variety of cell populations that differ with respect to their structural characteristics, expression of marker molecules, and biological functions. These cells include macrophages and other inflammatory cells, mononuclear cells, stem cells, endothelial progenitor cells, fibrocytes, and myofibroblasts. The nature of these cells, their relative importance, and their derivation and source continue to be unclear and highly controversial. The phenotype of these cells may change during the remodeling process or vary with the specific initiating pathophysiology or type of PH. Thus, it is important to define the phenotype of cells in the vascular media and the roles of the cells in the normal and diseased pulmonary vasculature. The exact source of the cells is also important to know because it may determine cell phenotype and targets for therapy.
In a recent article, Frid et al. (10) provide further experimental evidence that cells other than resident differentiated SMCs contribute to medial remodeling in hypoxia-induced neonatal forms of PH (10). In their in vivo study, the remodeled distal pulmonary artery (dPA) of hypertensive calves comprised cells that expressed hematopoietic (CD45), leukocytic/monocytic (CD11b, CD14), progenitor (cKit), and motility-associated (S100A4, also known as mts1) cell markers, in contrast to the phenotypically uniform SMC composition of dPA media in control animals. In their in vitro study, they isolated those phenotypes from dPA media of hypertensive calves, placed the cells into culture, and confirmed that they differentiated into SMCs (as defined by α-smooth muscle actin and smooth muscle myosin expression). The authors called one of these cell types R cells, as it is morphologically rhomboidal. The R cells transiently expressed CD11b and constitutively expressed type I procollagen, cKit, CD34, and CD73, all markers used to identify mesenchymal cells. In agreement with that study, Yao et al. (31) showed that the level of circulating mesenchymal progenitor CD45-/collagen I-positive fibrocytes in thromboembolic PH correlates with a poor prognosis, suggesting that circulating vascular and mesenchymal progenitor cells play a role in the pathogenesis of pulmonary vascular diseases.
Furthermore, Frid et al. (10) have demonstrated that these R cells possess highly augmented proliferative, migratory, invasive, and promitogenic capabilities (via production of PDGFs, SDF-1/CVCL12, and S100A4) and express high levels of mRNA for inflammatory mediators IL-6 and MCP-1, which are elevated in PH patients and experimental PH models (13, 17, 18, 23). Chemokines are known to induce recruitment of circulating cells, including monocytes and progenitors, from the blood stream into tissues. Several chemokines, in particular MCP-1 [also known as chemokine (CC motif) ligand 2 (CCL2)], specifically attract and activate monocytes and have been heavily implicated in monocyte recruitment in tumors and other inflammatory diseases (20, 30). On the other hand, the chemokine SDF-1 mobilizes stem cells in tissue, including the lung, by binding to its receptor CXCR4 (CXC chemokine receptor 4) (12, 24); hypoxia inducible factor-1 (HIF-1) regulates SDF-1 expression under hypoxic conditions (5). A very recent study has provided direct evidence that inhibition of the SDF-1-CXCR4 axis prevents the development of hypoxia-induced pulmonary vascular remodeling and decreases pulmonary arterial pressure in neonatal mice (32). In a hypoxia-induced PH model, hypoxia-induced mitogenic factor (HIMF, also known as FIZZ1 and RELMα) expression increases in the lung and has been shown to significantly upregulate expression of SDF-1 in lung resident cells (30), including endothelial cells, epithelial cells, and fibroblasts. Moreover, circulating stem and progenitor cells have been shown to increase their expression of CXCR4 under hypoxic conditions (24). HIMF/FIZZ1 is upregulated in the remodeling lung vasculature during hypoxia and inflammatory-induced PH in animal models and has been shown to have chemokine-like properties, such as inducing bone marrow cell migration (3, 6, 8, 27, 28, 30). Preliminary work in which bone marrow was transplanted from mice that express green fluorescent protein (GFP) suggests that upregulation of HIMF/FIZZ1 in the lung by chronic hypoxia or Th2 stimuli can recruit mesenchymal-like cells to the remodeling vascular wall; many of the recruited cells take on myofibroblast-like characteristics (2).
The work of Frid et al. (10) offers additional evidence that cells (R cells) with phenotypic and functional characteristics markedly distinct from those of resident dPA cells contribute to the remodeling process. These cells are likely recruited to the arterial adventitia and stimulate vascular remodeling by producing promitogenic factors and chemokines. Recent evidence suggests that several progenitors, including resident vascular cells (1), circulating bone marrow-derived endothelial progenitors (9), and mesenchymal progenitors (2, 11), possess high proliferative capacity and induce de novo vessel formation under a variety of conditions. The origin and identity of pulmonary vascular progenitors are not well understood. It has been suggested that vascular progenitors can arise from a pool of resident vessel wall-derived progenitor cells or from circulating bone marrow-derived phenotypes (4, 15). In the lung, vascular progenitor cells are hypothesized to be recruited from the bone marrow; they have also been found within the tunica intima of blood vessels and capillaries (1), within the tunica adventitia of larger vessels (33), and within the tissue parenchyma (16). Bone marrow-derived progenitors have been shown to incorporate into the pulmonary vasculature, as demonstrated by GFP or gender mismatch bone marrow transplant studies or by identification of selective progenitor markers (19). The use of selective markers for bone marrow-derived progenitor cells is controversial because these markers can be expressed by different phenotypes, and their expression will change over time as cells differentiate. Uncertainty also exists regarding the therapeutic and pathogenic impact of bone marrow-derived cells in PH. In a mouse PH model, bone marrow injection attenuated monocrotaline-induced PH but aggravated chronic hypoxic PH (21, 29). In general, it is accepted that circulating endothelial progenitors and fibrocytes play important roles in angiogenesis and vascular remodeling (29). Circulating bone marrow-derived fibrocytes (expressing myeloid and fibroblast markers) have been suggested to be recruited from the bloodstream to promote tissue remodeling during organ and vascular fibrosis (7, 12). These cells are thought to play a key role in healing and remodeling by producing inflammatory cytokines and extracellular matrix proteins in response to injury. Yet, little information is available on the origin and genesis of these cells, and the current evidence is controversial. The study by Frid et al. (10) offers a new insight that cells (R cells) capable of differentiating into mesenchymal/myofibroblast-like cells are recruited into the pulmonary artery wall during the vascular remodeling process. In addition, the study provides new information regarding the phenotypic definition of these cells and their biological functions relevant to vascular remodeling.
The paper by Frid et al. (10) contributes to the expanding body of evidence that mesenchymal-derived cells exhibiting myofibroblast-like characteristics are the most common type in the remodeling pulmonary vasculature that characterizes all forms of PH. Definitive studies are needed to determine the specific cell phenotypes involved in remodeling, their origin, the extent to which they differentiate within the tissue or before recruitment, and their ultimate physiological and structural role in the lung. It is equally important that we gain a better understanding of the mechanisms that stimulate and regulate progenitor cell recruitment and differentiation.
No conflicts of interest are declared by the author(s).
- Copyright © 2009 the American Physiological Society