Human fetal aorta contains vascular progenitor cells capable of inducing vasculogenesis, angiogenesis, and myogenesis in vitro and in a murine model of peripheral ischemia

Abstract

Vasculogenesis, the formation of blood vessels in embryonic or fetal tissue mediated by immature vascular cells (ie, angioblasts), is poorly understood. We report the identification of a population of vascular progenitor cells (hVPCs) in the human fetal aorta composed of undifferentiated mesenchymal cells that coexpress endothelial and myogenic markers. Under culture conditions that promoted cell differentiation, hVPCs gave rise to a mixed population of mature endothelial and mural cells when progenitor cells were stimulated with vascular endothelial growth factor-A or platelet-derived growth factor-betabeta. hVPCs grew as nonadherent cells and, when embedded in a three-dimensional collagen gel, reorganized into cohesive cellular cords that resembled mature vascular structures. hVPC-conditioned medium contained angiogenic substances (vascular endothelial growth factor-A and angiopoietin-2) and strongly stimulated the proliferation of endothelial cells. We also demonstrate the therapeutic efficacy of a small number of hVPCs transplanted into ischemic limb muscle of immunodeficient mice. hVPCs markedly improved neovascularization and inhibited the loss of endogenous endothelial cells and myocytes, thus ameliorating the clinical outcome from ischemia. We conclude that fetal aorta represents an important source for the investigation of the phenotypic and functional features of human vascular progenitor cells.

Figures

Figure 1

Coexpression of CD133, CD34, and KDR in human fetal aorta was analyzed on freshly isolated cells after collagenase dissociation of the tissues. Coexpression of markers was measured by activated cell sorting analysis. a: Live freshly digested CD133+ cells. b: Percentages of triple-positive cells are reported in the top right panel. Representative panels are of at least three independent experiments. c: Time-course live culture of immunoselected CD133+CD34+KDR+ cells derived from hFA. Cells in HM-GM grown as floating cluster of cells forming spheroids. Magnification, ×10.

Figure 2

CD133+CD34+KDR+ cells derived from hFA differentiate into vascular endothelial and mural cells. a and b: Freshly immunoselected CD133+CD34+KDR+ cells seeded on collagen type I-plus fibronectin-coated dishes were cultured under differentiated culture medium EBM-GM. a: A few hours after seeding, cells started to adhere. At 7 days of culture, the formation of mature EC colonies was observed, and at confluence, 10 to 14 days after seeding, mature ECs could be separated from the culture by using immunomagnetic beads coated with anti-CD31. The newly formed mature ECs could be grown in EBM-GM to obtain a typical endothelial monolayer in b. Magnification, ×10. The formation of EC colonies on confluent culture was also confirmed by immunoperoxidase staining with anti-CD31 antibody (c), whereas the presence of mural cells was shown by their immunoreactivity with anti-α-SMA antibody (d). e and f: Addition of PDGF-ββ or VEGF-A to EBM-GM enhanced the formation of mural and endothelial cells, respectively. Quantification is shown in e. **P < 0.01 versus control medium, ++P < 0.01 versus PDGF-ββ addition. f: Immunostaining for CD31, vWF, Tie-2, desmin, smooth muscle 22α, and NG2 of CD133+CD34+KDR+ cells 10 days on stimulation with PDGF-ββ or VEGF-A.

Figure 3

Multilineage potential of hVPCs: coexpression of myogenic and endothelial markers. a–d: Expression of desmin (green) and binding to UEA-1- (red). Cells are stained for desmin+ (a) or UEA-1+ cells (b). Nuclei are shown in c. Cells expressing desmin and UEA-1-binding are shown in d. It is possible to recognize the presence of cells that stained individually for desmin or UEA-1 and cells positive for both markers. e–h: Coexpression of Tie-2 (red) and desmin (green). Nuclei are shown in e, single Tie-2-positive cells are shown in f, desmin staining is shown in g, and cells coexpressing Tie-2 and desmin are shown in h. Note that cells coexpressing Tie-2 and desmin are colored in yellow: in this case, all of the desmin-positive cells are also Tie-2 positive, but not vice versa. i and j: Clone of hVPCs expressing both myogenic and endothelial markers. i: Limiting dilution was used to test self-renewal of hVPCs; the position of a single cell/well was identified by the use of a marker; the cell was routinely observed during the time under inverted microscopy. j: After 28 days, the clones were aspirated and transferred into larger wells and analyzed by immunocytochemistry for the expression of endothelial (Tie-2, in red) and myogenic (desmin, in green) markers. Note that most of the cells coexpressed both markers.

Figure 4

Angiogenic potential of hFA-derived hVPCs. a: The capacity of hVPCs to organize into vascular-like structures was studied by embedding the cells in collagen gel. b: Total RNAs were extracted from hVPCs and differentiated hVPCs; ANG2 and VEGF-A expression were determined by RT-PCR, and transcript levels were unequal. c–e: HVPCs or differentiated hVPCs were cultured in HM-GM to detect the release of angiogenic factors in the culture supernatant (c); undifferentiated hVPCs produced a significant amount of VEGF-A, whereas hVPCs stopped producing VEGF-A on their differentiation (**P < 0.01 versus differentiated hVPCs at the same time of collection. ND, not detected by the assay). d: Ang-2 was rapidly produced by undifferentiated hVPCs, but its secretion by these cells declined over time; in contrast, Ang-2 was continuously released by hVPCs on their differentiation (**P < 0.01 versus undifferentiated hVPCs at the same time of collection). e: SDF-1 was released by hVPCs only after their maturation (**P < 0.01 versus differentiated hVPCs). f: CCM from hVPCs enhanced HUVEC and hFAEC proliferation. **P < 0.01 versus CCM from differentiated hVPCs, ++P < 0.01 versus control medium. Note that the addition of anti-VEGF significantly reduced (+P < 0.05 versus CCM, *P < 0.05 versus control medium) EC proliferation but did not completely block CCM stimulation.

Figure 5

In vivo studies: histological analysis. Panels a through d are related to the confocal microscopy analysis and immunohistochemistry performed on adductor muscles harvested at 14 days from induction of ischemia and the local injection of 2 × 104 hVPCs (left panels) or control culture medium (right panels) (original magnification, ×400). In a, endothelial cells were detected by fluorescence lectin (green) and transplanted human cells by their positivity to human nuclear antigen (hNAg+ in red). Nuclei were counterstained by Hoechst (blue). Arrow in left panel indicates an area containing a hNAg+ cells belonging to a blood capillary vessel. The same area is shown at ×1000 in the upper right corner. A microvessel negative to hNAg is shown for reference in the right panel. b: Sections stained for α-SMA (red) and hNAg (green). In the left panel, hNAg+ cells are incorporated into the arteriolar wall of mouse adductor muscles injected with hVPCs. As expected, the CM-injected muscle (negative control) does not show the presence of human cells. Nuclei are counterstained by Hoechst (blue). c: Sections stained for laminin (a basal lamina marker, red) and hNAg (green). Transplanted hNAg+ cells are incorporated in myofibers (left panel). The CM-injected muscle does not show hNAg+ cells. Nuclei are counterstained by Hoechst (blue). These figures are representative of three independent experiments. d: Immunohistochemical staining on 3-μm sections using an anti-CD31 antibody (brown, peroxidase). The arrow in the left panel points to a cell associated with muscle microvessels within skeletal muscle, which stained positively to human-specific anti-CD31. As expected, the CM-injected muscle (negative control) does not show the presence of human cells. e: Images of the hindlimb region of mice that had been submitted to left limb ischemia and injected with hVPCs or CM 14 days in advance. f: Real-time PCR was performed with primers specific for MyoD and MYF5 using total RNA from adductor muscles injected with hVPCs, differentiated hVPCs, or control. MYF5 is approximately 80-fold up-regulated in the hVPC-transplanted muscles with respect to differentiated hVPC-injected muscles. Error bars represent SDs calculated on three different replicas. g: Distribution of necrotic toes in the left foot of mice that were exposed to ipsilateral ischemia. Zero means absence of necrotic toes, and 5 indicates all toes being necrotic or absent. One point was scored for every necrotic toe. Each circle is representative of a single mouse. Colored transverse lines represent the median for each group. Blue, CM-injected mice; green, EPCs; yellow, differentiated hVPCs; red, hVPCs.

Figure 6

In vivo studies: clinical outcome. Graphs in a and b show the capillary density and arteriole numerical density, respectively, expressed as number of capillaries or the number of arterioles per square millimeter of transverse muscular section. Values of normoperfused (no ischemia, open column) and ischemic adductor muscles injected with CM or transplanted with 2 × 104 hVPCs are shown. Values are mean ± SD. **P < 0.01 versus no ischemia; §P < 0.05 versus CM, EPCs, and differentiated hVPCs; *P < 0.05 versus no ischemia. c: The percentage of sectional area occupied by myocytes was determined by computer-assisted morphometric evaluation of muscular sections. hVPC transplantation significantly reduced the loss of myocytes. Values are mean ± SD. §P < 0.05 versus CM, EPCs, and differentiated hVPCs; #P < 0.05 hVPCs versus no ischemia; *P < 0.05 versus no ischemia. d: hVPCs improved the perfusion of adductor muscles subjected to left limb surgical ischemia. BF of left (ischemic) and right (contralateral normoperfused control) adductors was measured by the OxyLite/OxyFlo probe at 14 days postischemia. BF recovery was calculated as the ratio of left to right BF multiplied per 100. Mice transplanted with hVPCs showed improved perfusion recovery versus all of the control groups. Values are mean ± SD. §P < 0.05 versus CM, EPCs, and differentiated hVPCs.