*Purpose: Acute vascular rejection of allografts is associated with endothelial apoptosis which in turn contributes to the development of transplant vasculopathy. We previously showed that apoptotic endothelial cells (EC) release exosome-like vesicles (ApoExo) that enhance vascular rejection in murine models of transplantation. However, the impact of ApoExo on vascular and EC functions remains to be characterized.

*Methods: Serum-starved primary human EC were exposed to purified ApoExo in vitro. Endothelial gene expression analysis was defined by RNA-Seq. Wound closure and angiogenic activity were monitored by scratch and tube formation assays respectively. Expression of endothelial markers was measured by flow cytometry and confocal microscopy. In vivo expression of the endothelial marker CD31 was measured by immunohistochemistry in a murine model of aortic transplantation between MHC-incompatible strains. Intimal and medial areas were outlined and quantified using ImageJ.

*Results: RNA-Seq of ApoExo-treated EC compared to non-treated EC identified differentially regulated genes involved in the control of cell death, inflammation, differentiation, and cell movement. Differentially expressed genes were notably modulated by the transcription factor NFkB. Exposure to ApoExo led to increased NFkB phosphorylation, nuclear translocation and activity. EC exposed to ApoExo also showed reduced expression of the endothelial marker CD31, inhibition of apoptosis and improved wound closure along with reduced angiogenic activity suggesting endothelial dedifferentiation. NFkB knockdown impaired wound closure, restored angiogenic activity and CD31 expression suggesting that NFkB is essential to the phenotypic changes induced by ApoExo in vitro. In allografted mice, injection of ApoExo after transplantation significantly reduced CD31 expression in the graft and heightened vascular remodeling with significantly increased neointima formation.

*Conclusions: Collectively, these results suggest that ApoExo induce a pro-migratory and anti-angiogenic endothelial phenotype consistent with endothelial dedifferentiation of potential importance in vascular remodeling associated with transplant vasculopathy.

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