Effective revascularization of non-healing wounds by the human Stromal Vascular Fraction relies on direct cell integration and paracrine signals

Funding Acknowledgements

Type of funding sources: Public grant(s) – EU funding. Main funding source(s): PREFER

Introduction

With the increased prevalence of chronic diseases, non-healing wounds place a significant burden on the health system, with a prevalence of 2-5%, similar to the one of heart failure. They are persistent full-thickness skin lesions that affect patients suffering from vascular disorders, such as diabetes and peripheral artery disease. Skin implants and substitutes are currently applied to promote the closure of non-healing wounds. However, both approaches are poorly effective because of lack of appropriate vascularization.

Purpose

To promote the neo-vascularization of non-healing wounds, we use Stromal Vascular Fraction (SVF) as innovative therapeutic opportunity for wound treatment.

Here, we aim to 1) characterize and demonstrate the pro-angiogenic role of SVF cells and 2) provide pre-clinical evidence of the therapeutic efficacy of the human SVF in promoting the neo-vascularization in a new mouse model of ischemic, non-healing wound.

Methods

To assess capacity of SVF-derived cells to improve wound revascularization, we created a new model of non-healing wound generated by wounding an ischemic limb in mice. Human and mouse SVF was purified from adipose tissue and seeded on a clinical-grade skin substitute prior to its implantation on the ischemic wound of a recipient animal. The transplantation of human SVF into NSG immunodeficient mice was verified using species-specific antibodies, while the use of genetically modified mice allowed us to trace the fate of both endothelial and non-endothelial cells upon their transplantation into syngeneic recipient animals. The function of SVF-induced vessels was assessed by systemic injection of biotinylated lectin and by Single Photon Emission Tomography (SPECT) of the treated limb.

Results

At day 7 the implanted mouse SVF gives rise to a widespread vascular network composed by arteries, capillaries, veins, as well as lymphatic vessels. Similarly, human SVF-derived endothelial cells formed hybrid human-mouse vessels that were stabilized by perivascular cells. At both histological and functional analysis, these vessels were connected with the host circulatory system and determined a 2-fold increase in tissue perfusion. The comparison of the activity of human SVF from different donors allowed us to disclose its dual mechanisms of action.

Conclusions

Here we demonstrated the efficacy of the SVF in promoting neo-vascularization of a skin substitute in a mouse model of ischemic, non-healing wounds. Its therapeutic efficacy relies on dual mechanisms of action. On the one hand, SVF-derived ECs engraft and expand, directly forming new vascular units that colonize the scaffold and extend into surrounding tissues. On the other hand, the mesenchymal progenitors stimulate the expansion of the host vasculature, which extends into the scaffold, with the eventual appearance of donor-host hybrid vessels.

Collectively, these data support the use of human SVF as a powerful cell therapy to treat non-healing wounds.

PDGF-BB regulates splitting angiogenesis in skeletal muscle by limiting VEGF-induced endothelial proliferation

VEGF induces normal or aberrant angiogenesis depending on its dose in the microenvironment around each producing cell in vivo. This transition depends on the balance between VEGF-induced endothelial stimulation and PDGF-BB-mediated pericyte recruitment, and co-expression of PDGF-BB normalizes aberrant angiogenesis despite high VEGF doses. We recently found that VEGF over-expression induces angiogenesis in skeletal muscle through an initial circumferential vascular enlargement followed by longitudinal splitting, rather than sprouting. Here we investigated the cellular mechanism by which PDGF-BB co-expression normalizes VEGF-induced aberrant angiogenesis. Monoclonal populations of transduced myoblasts, expressing similarly high levels of VEGF alone or with PDGF-BB, were implanted in mouse skeletal muscles. PDGF-BB co-expression did not promote sprouting and angiogenesis that occurred through vascular enlargement and splitting. However, enlargements were significantly smaller in diameter, due to a significant reduction in endothelial proliferation, and retained pericytes, which were otherwise lost with high VEGF alone. A time-course of histological analyses and repetitive intravital imaging showed that PDGF-BB co-expression anticipated the initiation of vascular enlargement and markedly accelerated the splitting process. Interestingly, quantification during in vivo imaging suggested that a global reduction in shear stress favored the initiation of transluminal pillar formation during VEGF-induced splitting angiogenesis. Quantification of target gene expression showed that VEGF-R2 signaling output was significantly reduced by PDGF-BB co-expression compared to VEGF alone. In conclusion, PDGF-BB co-expression prevents VEGF-induced aberrant angiogenesis by modulating VEGF-R2 signaling and endothelial proliferation, thereby limiting the degree of circumferential enlargement and enabling efficient completion of vascular splitting into normal capillary networks despite high VEGF doses.