Cellular and Molecular Mediators of Bronchiolitis Obliterans-like Pathological Changes in a Murine Model of Chlorine Gas Inhalation
Bronchiolitis Obliterans (BO) is a major cause of chronic airway dysfunction after toxic chemical inhalation. The pathophysiology of BO is not well understood, but epithelial cell injury has been closely associated with the development of fibrotic lesions in human studies and in animal models of both toxin- and transplant-induced BO. However, while almost all cases and models of BO include epithelial injury, not all instances of epithelial injury result in BO, suggesting that epithelial damage per se is not the critical event leading to the development of BO. In this dissertation, we describe a model of chlorine (Cl2)-induced BO in which mice develop tracheal and large airway obliterative lesions within 10 days of exposure to high (350 ppm), but not low (200 ppm), concentrations of Cl2 gas. Lesions develop in a series of well-demarcated pathological changes that include epithelial denudation, inflammatory cell infiltration by day 2 after exposure, fibroblast infiltration and collagen deposition by day 5, and in-growth of blood vessels by day 7, ultimately leading to lethal airway obstruction by days 9-12. Using this model, we were able to test our hypothesis that loss of epithelial progenitor cells is a critical factor leading to the development of obliterative airway lesions after chemical inhalation. Indeed, these lesions arise only under conditions and in areas in which basal cells, the resident progenitor cells for large airway epithelium, are eliminated by Cl2 exposure.
The molecular pathways contributing to BO development are not well understood. Mechanisms of epithelial injury differ across BO models, but we hypothesized that after the inciting injury, BO models share common pathways. We compared microarray analysis from day 5 non-BO- and BO-inducing chemical injuries and subsequently identified biological pathways that may contribute to BO pathogenesis. Our findings add support to pathways previously implicated in BO development and more importantly, suggest potential new pathways and molecular mediators of BO. Furthermore, we evaluated the efficacy of therapeutic inhibition of neovascularization or inflammation to prevent Cl2-induced BO. To date, our therapeutic interventions were ineffective. Nonetheless, our findings suggest that in the context of Cl2-induced BO, vascular endothelial growth factor receptor 2 (a mediator of neovascularization) and inducible nitric oxide synthase (a mediator of inflammation) are not critical in BO pathogenesis.
In sum, our work introduces and characterizes a novel Cl2-induced murine model of BO. Using this model we demonstrated that in the absence of basal cells, epithelial regeneration does not occur and regions of epithelial denudation persist from which an aberrant repair process is initiated, leading to obliterative airway lesions. Our findings suggest that, irrespective of the cause, loss of epithelial progenitor cells may be a critical factor leading to the development of BO. Furthermore, our gene expression analysis implicates novel mediators and signaling pathways in the development of BO. Our analysis lays the foundation for more rigorous exploration of these targets in the pathogenesis of BO.
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