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Extracellular vesicles enhance remodeling of cell-free silk vascular scaffolds in rat aortae.

Vascular tissue engineering (VTE) is aimed at developing regenerative vascular grafts to restore tissue function by bypassing or replacing defective arterial segments with tubular biodegradable scaffolds. Scaffolds are often combined with stem/progenitor cells to prevent acute thrombosis and initiate scaffold remodeling. However, there are limitations to cell-based technologies regarding safety and clinical translation. Extracellular vesicles (EVs) are nano-sized particles released by most cell types, including stem/progenitor cells, that serve to transmit protein/RNA cargo to target cells throughout the body. EVs have been shown to replicate the therapeutic effect of their parent cell, therefore EVs derived from stem/progenitor cells may serve as a more translatable, cell-free, therapeutic base for VTE. Our study aims to determine if EV incorporation provides a positive effect on graft patency and remodeling in vivo. We first assessed the effect of human adipose derived mesenchymal stem cell (hADMSC) EVs on vascular cells using in vitro bioassays. We then developed an EV-functionalized vascular graft by vacuum-seeding EVs into porous silk-based tubular scaffolds. These constructs were implanted as aortic interposition grafts in Lewis rats and their remodeling capacity was compared to that observed for hADMSC-seeded and blank (non-seeded) controls. The EV group demonstrated improved patency (100%) compared to the hADMSC (56%) and blank controls (82%) following 8 weeks in vivo. The EV group also produced significantly more elastin (126.46%) and collagen (44.59%) compared to the blank group, while the hADMSC group failed to produce significantly more elastin (57.64%) or collagen (11.21%) compared to the blank group. Qualitative staining of the explanted neo-tissue revealed improved endothelium formation, increased smooth muscle cell infiltration, and reduced macrophage numbers in the EV group compared to the controls which aids in explaining this group's favorable preclinical outcomes.