Understanding Inflammatory Mechanisms during Interactions between Pseudomonas aeruginosa and Host Cells in the Context of Cystic Fibrosis

Abstract

Cystic fibrosis (CF) is one of the most common genetic diseases in Europe and North America. Chronic bacterial infections with Pseudomonas aeruginosa (P. aeruginosa) are common among CF patients and are associated with increased disease progression among patients. While inflammation is considered to be a key driver of lung function decline, the precise mechanisms at play have remained unclear. The objective of this thesis was to evaluate the role of inflammatory signalling components that result in host cell death during respiratory infections observed in CF. First, I investigated the differences in inflammatory mechanisms and cytokine expression induced by P. aeruginosa isolated from early versus chronic infections in CF. I found that early respiratory isolates of P. aeruginosa from CF patients induced inflammasome signalling, cell death, and IL-1β expression by THP-1 macrophages, yet little expression of other proinflammatory cytokines. However, P. aeruginosa isolates from chronic infections induced relatively less THP-1 macrophage inflammasome signalling, cell death, and IL-1β expression but greater production of other cytokines. Using laboratory reference strains and various mutants of P. aeruginosa, I validated how due to their inability to induce early and extensive host cell death, isolates from chronic infections are able to induce sustained levels of proinflammatory cytokines, which may contribute to the pathogenesis observed in CF. I then investigated one specific virulence factor identified among clinical P. aeruginosa isolates, the effector protein ExoU. ExoU is known to induce rapid host cell death and has previously been described to be an inhibitor of caspase-1, limiting IL-1β secretion in immune cells. Using relevant laboratory reference strains, I have shown that ExoU is able to induce IL-1β expression at lower multiplicities of infection or at earlier time points than described in previous reports when infecting THP-1 macrophages and NuLi-1 bronchial epithelial cells. Through immunoblotting and the use of relevant inhibitors, it was found that this observed difference could be partially dependent on the activation of various caspases, including ones that induced canonical and non-canonical inflammasome activation. Overall, this described work adds to our understanding of respiratory infections observed among CF patients and could shed light on possible therapeutic options to reduce disease progression.