Investigation of Bartonella quintana Host Immune Evasion

Bartonella is a genus of gram-negative alphaproteobacteria that infect mammals, causing both acute and chronic disease. Bartonella are re-emerging infectious pathogens that cause a variety of clinical syndromes in humans worldwide, including cat scratch disease, trench fever, bacillary angiomatosis, and endocarditis. Bartonella spp. are spread by biting arthropods such as the sand fly, cat flea, and body louse, and have been isolated from almost all mammalian species tested. Bartonella are a re-emerging concern as the number of confirmed Bartonella diagnoses are increasing, primarily in immunocompromised groups, homeless populations, refugee camps, and in veterinary workers. The three primary human disease-causing Bartonella spp. are B. henselae, B. quintana, and B. bacilliformis. Bartonella are known to subvert the host immune system and persist within the host, often causing bacteraemia which is difficult to effectively diagnose and treat. B. quintana infects humans; after introduction to the skin the bacteria implement numerous immune evasion mechanisms to enter the bloodstream and invade erythrocytes. The mechanisms by which B. quintana modulates and evades the immune system during early infection are almost entirely unknown. Following exposure to B. quintana, the bacteria encounter host immune cells but survive, evading these cells and disseminating into the lymphatic system and eventually bloodstream. This thesis project aimed to dissect the interactions between B. quintana and the human innate immune system to better understand the early stages of infection. A gentamicin protection assay was developed to investigate the ability of THP-1 macrophages, representing human macrophages present in the skin, to internalise B. quintana. These data revealed THP-1 cells were unable to effectively internalise B. quintana, although the mechanism responsible was not determined. Subsequent experiments investigated the role of the B. quintana Type IV secreted effector protein BepA1 in the inhibition of internalisation. Bacterial effector proteins often pathogenically modulate host cell signalling to benefit the bacteria, i.e., altering the actin cytoskeleton to inhibit phagocytosis or supressing immune responses. It was hypothesised BepA1 could play a role in inhibiting phagocytosis; therefore, the host cell target of BepA1 was investigated with a yeast two-hybrid system assay. The human protein Myozap was uncovered as a potential protein that interacts with BepA1. Myozap is expressed in cardiac and lung tissue as well as epithelial and endothelial cells, where it modulates Rho-dependent actin signalling, potentially affecting the actin cytoskeleton and the transcription factor MRTF-A, which influences immune reaction through modulation of NF-κB. To investigate the functional effects of BepA1 activity in host cells, HeLa cells were transfected with BepA1; cell migration and cytokine secretion were assessed, revealing a decrease in pro-inflammatory cytokines in BepA1-transfected cells in response to TNF-a stimulation. These data suggest BepA1 may be deployed by B. quintana during infection to suppress the host immune response and avoid clearance from the site of infection. This research addressed a major gap in our understanding of B. quintana infections. Improving our understanding of the interactions between Bartonella and the host immune system is an essential first step in the development of improved diagnostic techniques and treatments.