Semester

Fall

Date of Graduation

2020

Document Type

Dissertation

Degree Type

PhD

College

School of Medicine

Department

Physiology, Pharmacology & Neuroscience

Committee Chair

Robert Brock

Committee Member

Anna Shvedova

Committee Member

Vincent Castranova

Committee Member

Valery Khramtsov

Committee Member

Max Sokolov

Abstract

Inhalation exposure to asbestiform elongate mineral particles (EMPs) may cause pulmonary fibrosis, lung and pleural cancer. At the same time, epidemiological evidence for non-asbestiform EMP pathogenicity is scarce, with little if any risk of cancer, but consistent findings of pneumoconiosis and increased mortality. Globally, conditions caused by asbestos exposures contribute most to the occupational cancer burden and are responsible for the increasing mortality for occupational respiratory diseases. Immunological components of the fiber-induced lesions are extremely understudied, partly due to the lack of comparable test articles. In this study I utilized respirable preparations of asbestiform and non-asbestiform riebeckite and tremolite to assess pulmonary immune responses, following exposures to studied EMPs in the alveolar macrophage (AM) cell culture model and a A/J mice. First, respirable samples of asbestiform fibers and respective non-asbestiform cleavage fragments (CF) were harvested, and detailed information on the length-width ratios, variability in metal ion concentrations and chemical composition of the studied EMPs were obtained. In the in vitro study alveolar macrophage-like cells were treated with mass-, surface area- (s.a.), and particle number- (p.n.) equivalent concentrations of the test articles for 24 hours. Equal mass-based dosing resulted in markedly different responses between asbestiform and non-asbestiform EMPs; at equal “critical fiber” surface area-based doses asbestos and corresponding CF had similar cytotoxic effects, but distinct transcriptional reprogramming patterns and DNA damage response, also specific for the mineral type. For the in vivo study I used A/J mouse strain that is prone to lung tumors, originating from atypical hyperplastic lesions in the periphery of the lung, similar to humans. Mice were exposed via repeated pharyngeal aspiration to the mass- and s.a.-based doses of the studied EMPs. Neoplastic manifestations at 1 year were much more frequent in asbestos-treated animals. Histopathology also revealed hyperplasia, mild fibrosis and immune cell infiltration, but no extensive lung fibrosis in asbestiform EMP groups. CF provoked very little histological changes. DNA damage response was evoked by asbestiform EMPs with the foci of γH2AX staining found in the vicinity of deposited fibers, but not CF. Overall, the immune response to asbestos particles was characterized as leaning towards TH2- or TH17 type. There was relative depletion of resident AMs by asbestiform EMPs and increase in the mean fluorescence intensity of inflammatory CD11b surface marker on AM surface in all treatment groups compared to controls at 12 months. Exposure to asbestiform and non-asbestiform EMPs led to discrete pulmonary immune responses at 3- and 12-months post-exposure, leading to specific long-term fibroproliferative and carcinogenic outcomes. The principal findings from this work confirm that alveolar macrophages play a major role in responding to EMPs; the preservation of resident “homeostatic” pool of AMs in the lungs might led to the lack of fibrotic and neoplastic outcomes in CF-treated animals. In conclusion, this work supplements the notion that in addition to habit and biodurability, chemical composition, unique for different minerals, plays major role in the immunological responses and respective long-term pulmonary outcomes. The work presented here calls for further investigations of the pulmonary and systemic immune cell population dynamics in the development of long-term carcinogenic and non-carcinogenic outcomes following exposures to well-characterized reference materials.

Embargo Reason

Publication Pending

Available for download on Saturday, December 04, 2021

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