Abbreviations: EOG-EPC= early outgrowth endothelial progenitor cell; EPC = endothelial progenitor cell; LOG-EPC = late outgrowth endothelial progenitor cell; MSC = mesenchymal stem cell; PDGFR = platelet-derived growth factor receptor; PH =pulmonary hypertension; VEGF = vascular endothelial growth factor
In recent years it has been discovered that in addition to the well-characterized hematopoietic stem cells, adult bone marrow contains nonhematopoietic stem cells, such as endothelial progenitor cells (EPCs), fibrocytes, and mesenchymal stem cells (MSCs). It is believed that under homeostatic conditions these cells are released from the bone marrow, circulate in the blood, and contribute to the repair of tissues in response to general wear and tear.
Endothelial Progenitor Cells
EPCs were first described in 1997 when Asahara et al. identified a population of mononuclear cells in the blood capable of differentiating into endothelial cells in vitro. It was proposed that these circulating progenitor cells may contribute to angiogenesis and vasculogenesis (formation of new blood vessels). Currently there are no specific markers to identify EPCs in mice or humans, but as EPCs proliferate clonally and produce colonies in vitro, specific colony assays can be generally performed to quantify numbers of EPCs in blood and tissues.
In these assays, mononuclear cells are plated onto fibronectin or gelatin and grown in the presence of endothelial growth factors (including vascular endothelial growth factor [VEGF]). It is now clear that there are two distinct subsets of EPCs: the first derived from hematopoietic lineage (also known as early outgrowth EPCs [EOG-EPCs]) and the second from endothelial lineage (late outgrowth EPCs [LOG-EPCs]).
EOG-EPCs are not believed to directly form new vessels, but they have been shown to secrete key proangiogenic factors, and they promote angiogenesis/vasculogenesis through a paracrine mechanism.
The other subset of EPCs is the LOG-EPCs that are observed in colony assays after 21 days in culture and exhibit cobblestone morphology that is a characteristic of mature endothelial cells. In contrast to the EOG-EPCs, LOG-EPCs possess the ability to form vessels in vitro and in vivo in lung and heart models.
It is well documented that the ELR 1 CXC chemokines can stimulate angiogenesis in vivo, and it is, therefore, of interest that EPCs have been reported to express the chemokine receptor CXCR2.
CD34 is a cell surface marker expressed by hematopoietic progenitor cells and EPCs. In patients with COPD, levels of circulating CD34 cells are reduced compared with normal control subjects, and it has been suggested, that this reflect the increased trafficking of progenitor cells into the lungs. Levels of circulating CD34 cells appear to increase during exacerbations of COPD, which has been correlated with an elevation in the plasma levels of VEGF-A from these patients.
Finally, two small clinical trials carried out by a group in China have reported positive results examining the efficacy of EPC transplantation in patients with idiopathic pulmonary arterial hypertension, when given in addition to conventional therapy.
There is considerable evidence that bone marrow derived fibroblast progenitor cells, called fibrocytes, may also contribute to this process. It is believed that fibrocytes are recruited to sites of inflammation to stimulate repair by producing extracellular matrix molecules such as collagen I and III, vimentin, and fibronectin. These data suggest that blocking fibrocyte recruitment to the lungs may represent a potential therapeutic target to reduce lung fibrosis.
Mesenchymal Stem Cells (MCSs)
MSCs are multipotent stromal cells that can be isolated from numerous tissues, including the bone marrow, skeletal muscle, amniotic fluid, and adipose tissue. They are plastic adherent cells that exhibit trilineage differentiation into adipocytes, chondrocytes, and osteoblasts. Some studies have shown that these stem cells also have the ability to differentiate into neurons, myocytes, and skeletal muscle.
A unique feature of MSCs is their ability to produce a potent immunosuppressive effect both in vitro and in vivo. Mechanistically it has been suggested that this may be due to their ability to secrete a range of immunomodulators,
MSCs can be readily harvested from bone marrow or adipose tissue and expanded ex vivo for use as a cell therapy. Impressive immunosuppressive effects of these cells have been reported in a wide range of models of disease, including acute graft-vs-host disease and multiple sclerosis. With respect to respiratory disease, there are now a number of publications that report a reduction in disease pathology.
MSCs are currently being evaluated in several clinical trials for a variety of diseases, including Crohn’s disease, multiple sclerosis, diabetes mellitus, and acute graft-vs-host disease, with promising results reported from some of these trials.
Epithelial Progenitor Cells
Although there are endogenous epithelial progenitor cells in the adult lung. These studies suggest that bone marrow-derived progenitor cells may incorporate and differentiate into lung epithelium and thereby promote tissue repair.
Pharmacologic activation of endogenous stem cells represents an alternative approach to stem cell therapies. One strategy is to stimulate the mobilization of stem cells from the bone marrow in order to induce regeneration or immune modulation during disease development. We have recently shown that at a molecular level the mobilization of progenitor cell subsets (hematopoietic progenitor cells, vs MSCs and EPCs) is differentially regulated, suggesting that pharmacologic therapies could be developed to selectively mobilize a specific subset of stem cells from the bone marrow.
In our Hospital we harvest and concentrated the stem cells from bone marrow and peripheral blood and after these procedures they are injected into the lung in order to regenerate the lung tissue.