The human respiratory tract is a barrier between the body and external environment. Inhaled bacteria and those colonizing the nasopharynx encounter many host defenses including the innate immune response. Bacteria enter the lungs and move to alveoli defended by resident macrophages. Dissecting host-pathogen interactions in the alveoli is critical for ultimately combating pulmonary diseases. This dissertation examines the human macrophage response to two diverse pulmonary bacterial pathogens, Staphylococcus aureus and Coxiella burnetii.

S. aureus is an opportunistic pathogen capable of causing necrotizing pneumonia. Conflicting literature regarding the pathogen’s ability to establish infection, survive, and replicate within macrophages is a result of differing infection models. We aim to bridge the gap between animal models and human disease by establishing a disease-relevant primary lung tissue platform using human alveolar macrophages (hAMs) to study S. aureus intracellular infection. S. aureus survives, but does not replicate, within lysosome-like compartments within hAMs that mount an inflammatory response to infection. Alterations in exotoxin production and activity demonstrate modulation of S. aureus virulence factors by the intracellular hAM environment. Overall, we defined a new disease-relevant infection platform to investigate S. aureus-hAM interactions.

In contrast to S. aureus, C. burnetii is an obligate, intracellular bacterium that causes zoonotic Q fever. C. burnetii preferentially infects hAMs, replicating within a lysosome-like parasitophorous vacuole (PV). To establish infection, C. burnetii employs a Dot/Icm Type IV Secretion System (T4SS) and multiple effectors localize to and/or disrupt the endoplasmic reticulum (ER) and secretory transport. During infection, unfolded nascent proteins may exceed the folding capacity of the ER, activating the unfolded protein response (UPR). We investigated the impact of the UPR on C. burnetii infection of human macrophages. Overall, our results show that T4SS-dependent activation of the eukaryotic initiation factor 2α pathway is necessary for efficient infection, and C. burnetii prevents UPR-related cell death.

Together, we established novel events that occur during S. aureus and C. burnetii infection of human macrophages. Each event involves bacterial virulence determinants that are critical for progression of infection, showcasing the versatility of human macrophages to respond to diverse pathogens.