The immune network in pulmonary Legionella pneumophila infection

© 2016 Dr Andrew Stephen BrownThe lungs are constantly exposed to particulate matter in the external environment, and thus are vulnerable to exposure against microorganisms transmissible via inhalation. As a result, pulmonary diseases such as pneumonia are an important source of morbidity and mortality worldwide. Legionella pneumophila is a Gram negative bacillus and one of the leading causes of community acquired pneumonia. L. pneumophila is an accidental pathogen to humans where upon inhalation, the bacteria establish a replicative niche within alveolar macrophages of the lung. This triggers a robust host immune response that operates to eradicate the bacteria. However, despite the importance of an inflammatory response being important for L. pneumophila clearance from the lung, we do not fully understand the immune mechanisms operating between the onset of infection and bacterial clearance. This Thesis aims to investigate the molecular and cellular pathways in the lung in response to L. pneumophila infection, and define a network of cellular behavior cooperating to efficiently clear bacteria from the lung. Chapter 3 investigates the role of plasmacytoid DC (pDC) in response to L. pneumophila infection. It is shown that pDC upregulate their expression of CD8 that is associated with cellular activation. However, despite the cells trafficking to the lung and displaying an activated phenotype, the role of pDCs was not important for the overall clearance mechanisms of L. pneumophila from the lung. To understand the key cell types interacting with L. pneumophila during infection, a broad appraisal of the key phagocytic cell types in the lung during infection was made. Chapter 4 outlines that alveolar macrophages, despite being a replicative niche for L. pneumophila, were rapidly depleted in the lung after infection. This occurred alongside a strong infiltration of neutrophils and monocyte-derived cells (MCs) that actively engulfed bacteria for intracellular degradation. Previous work has shown that the IL12-INFg axis is important for pulmonary clearance of L. pneumophila. Despite this, the key cell types contributing to their production, as well as a mechanism for how these cytokines function is not well understood. Chapter 5 shows that monocytic and dendritic cells are the sole contributors of IL12-related cytokines in the lung during infection. MCs were a significant contributor of IL12 that was important for acute L. pneumophila clearance. As demonstrated in Chapter 6, IL12, in turn, stimulated the production of IFNg by lung-infiltrating NK cells, as well as memory T cells in a non-cognate manner. Lymphoid cell-derived IFNg specifically stimulated the intracellular bactericidal activity of MCs to assist with clearance of L. pneumophila from the lung. This Thesis demonstrates a level of cross-talk between myeloid and lymphoid cells in the lung during acute L. pneumophila infection that actively participate to direct bacterial clearance mechanisms innately. This work furthers our overall understanding of how these cells cooperate to stimulate antibacterial pathways in the tissues. This permits future investigation into understanding the molecular and cellular basis between individuals that are either immunocompetent or immunocompromised, and how these pathways may be manipulated therapeutically to prevent severe progression towards disease