Perivascular Matrix Densification Dysregulates Angiogenesis and Activates Pro-Inflammatory Endothelial Cells

The potential for fibrosis across most organ systems may stem from connections to wound healing and the widespread presence of vascular endothelium. Endothelial cells (ECs) and angiogenesis have been heavily implicated in many organ-specific fibrotic conditions, but little has been established in terms of how EC phenotype governs tissue healing vs. fibrosis. Here, we examined a murine lung injury model enabling EC lineage tracing and observed the invasion of aberrant ECs from the bronchial microvasculature following injury, along with concurrent densification of surrounding extracellular matrix fibers. To investigate mechanisms governing their appearance, we established a microphysiological system of human microvessels embedded within a tunable stromal matrix and found that heightened fiber density drives endothelial to mesenchymal transition to promote aberrant tip EC (ATEC) invasion into the matrix. ATECs remained adherent to fibrotic matrix and possessed a pro-inflammatory phenotype that secretes TGF-β2. Mechanistically, we identify ATEC formation was gated by destabilization of EC adherens junctions upon adhesion to fibrous matrix and associated regulation of TGF-β signaling through a novel VE-cadherin - TGF-βR2 interaction. Altogether, this work identifies how enhanced fiber density associated with fibrogenesis regulates EC phenotype to generate pro-inflammatory ATECs and suggests new contributions of ECs to fibrotic progression.