Mechanisms of Cigarette Smoke-Induced Inflammation and the Exacerbated Response to Bacteria in Mice

Abstract

Chronic obstructive pulmonary disease (COPD) is a leading cause of global morbidity and mortality, with the potential to afflict as many as half of the 1.1 billion smokers in the world. The inflammatory response to cigarette smoke is believed to mediate the progressive and irreversible loss of lung function that characterizes COPD. The greatest burden of the disease arises from episodes of worsened symptoms and inflammation, usually triggered by microbial infection. Currently, the mechanisms that drive cigarette smoke-induced inflammation are being elucidated but ambiguity remains about this response and the inflammatory response engaged in a smoke-exposed lung experiencing a microbial infection. This thesis sought to investigate inflammatory mediators induced by cigarette smoke and those induced by bacteria, the most common cause of infectious exacerbations of COPD, in the context of smoke exposure. In chapter two we investigated the role of Breast Regression Protein-39 (BRP-39), a gene commonly observed to be increased under inflammatory conditions, in the inflammatory response to cigarette smoke. In order to determine the mechanisms of BRP-39 induction, its expression and inflammation was assessed in IL-13, IL-18, and IL-1R1 deficient mice. BRP-39 was found to be redundant in cigarette smoke-induced inflammation, but these data confirmed that IL-1R1 was a crucial mediator of this response. After examining the inflammatory response elicited by smoke alone, we investigated the importance of IL-1 signaling in a model of bacterial exacerbation of cigarette smoke-induced inflammation. We found that the exacerbated neutrophilia that typifies the response of a smoke-exposed lung to bacteria was dependent on IL-1α-mediated production of the CXCR2 ligand CXCL5. This study identified the unique phenomenon that cigarette smoke primes alveolar macrophages to produce excessive amounts of IL-1α in response to bacterial stimuli. The purpose of the final study of the thesis was to more comprehensively characterize the extent to which cigarette smoke changes the phenotype of macrophages. Examining total gene expression by microarray found that smoke-exposed alveolar macrophages were in a proliferative state expressing a unique profile of inflammatory mediators. Further analysis revealed that this was likely the result of a pulmonary environment rich in growth factors. Taken together, these data provide detail to the understanding of the biological process of inflammation that drives the pathogenesis of COPD. These studies identify a phenomenon that predisposes smokers to experience more severe responses to bacteria and reinforces the targeting of IL-1 signaling in the treatment of COPD.