View Abstract

A4738 - Endothelial HIF-2α Maintains Airway Microvasculature Through Angiopoietin-1/TIE2 Signaling
Author Block: A. Tu, X. Jiang, W. Tian, M. R. Nicolls; Stanford School of Medicine, Palo Alto, CA, United States.
RATIONALE: The main limitation for long-term survival of transplanted lung allografts is chronic rejection, manifested as chronic lung allograft dysfunction (CLAD). Autopsy studies suggest that loss of functional airway microvasculature precedes the onset of CLAD. Lung allografts are relatively hypoxic as evidenced by increased expression of hypoxia associated genes and decreased oxygen saturation. Furthermore, hypoxia is associated with airway necrosis and central airway stenosis following lung transplantation. Hypoxia-inducible factors (HIFs) including HIF-1α and HIF-2α are key mediators of the adaptive response to hypoxic stress and play many roles in maintaining lung homeostasis. Airway microvascular damage and hypoxic stress appear to be important pathological components and are likely to play causative roles in the development of airway complications and chronic rejection, a better understanding of airway microvascular endothelial cell (EC) responses to hypoxia will improve our understanding of mechanisms associated with chronic rejection and uncover novel therapeutic targets.
METHODS: The orthotopic tracheal transplantation (OTT) model was performed in MHC-mismatched mice to study airway microvascular pathology following rejection. EC specific HIF-1α or HIF-2α knockout (KO)(H1ECKO, H2ECKO) or overexpression (OE)(H1ECOE, H2ECOE) were created by crossing mice expressing VE-Cadherin-CreERT2 with HIF floxed mice. Non-transplanted HIF KO or OE tracheal microvasculature was characterized by blood perfusion unit (BPU) measurement, FITC-lectin perfusion, histological and qPCR analysis. Similar analyses were performed following OTT.
RESULTS: Endothelial-specific deletion of HIF-2α, but not HIF-1α in adult mice causes airway microvascular attenuation. Tracheas of H2ECKO mice display progressive BPU decline and an increase in microvascular permeability. Pimonidazole staining also shows H2ECKO tracheas are hypoxic. Angiopoietin-1 (Ang1) overexpression prevents airway microvascular attenuation in H2ECKO mice. Ang1 also restores BPU readings and normalizes vascular permeability.
Tracheas with H2ECKO, but not H1ECKO display accelerated microvascular loss following transplantation. Conversely, H2ECOE in the donor protects microvasculature and attenuates inflammation during transplant rejection. H2ECOE transplants displayed improved microvascular perfusion, increased BPU readings and diminished pimonidazole staining. H2ECOE donor tracheas showed decreased expression of ICAM-1, and proinflammatory cytokines. Immunofluorescence staining of H2ECOE donor tracheas further shows a decrease in immune cell infiltration.
CONCLUSION: Endothelial HIF-2α maintains normal airway microvasculature, protects airway microvascular function against transplant rejection and may be used as a treatment adjunct to prevent CLAD. Our findings may be relevant in other chronically rejecting solid organ transplants characterized by loss and remodeling of the microvascular circulation. This study may also provide novel therapeutic insights into lung diseases characterized by microvascular pathology.