The interactions of invasive bacteria with tissue-resident macrophages

Invasive bacteria are likely to encounter macrophages during their course of pathogenesis, especially if infection progresses to disease. The study of these interactions is crucial for our understanding of pathogenesis and the identification of therapeutic strategies to combat infection. In this thesis, I utilise a number of models to study and gain insight into these interactions. I demonstrate that hypervirulent strains of Klesbsiella pneumoniae (Kp) replicate within hepatic Kupffer cells (KCs) and confirm that replication is intracellular by utilising a modified gentamicin-protection assay on isolated murine KCs. I show that this replication precedes an influx of neutrophils which subsequently aggregate to form clusters that could develop into pyogenic liver abscesses characteristic of these Kp strains. Utilising an in vivo mouse model of invasive pneumococcal disease with infection occurring during the active or rest phase, I demonstrate that differential susceptibility of mice to Streptococcus pneumoniae infection is dependent on a diurnal phenotype within splenic marginal zone macrophages (MMMs). MMMs harbour a differential permissiveness to intracellular pneumococcal replication, with increased permissiveness during the active phase resulting in higher splenic bacterial burden, earlier and more pronounced release of intracellular bacteria into the systemic circulation, and higher levels of systemic pro-inflammatory cytokines. Finally, I utilise an ex vivo human spleen perfusion model to study the localisation of S. pneumoniae with human tissue-resident macrophages. I find that the majority of bacteria are associated with red pulp macrophages (RPMs), with an extremely low number localising with capillary sheath-associated macrophages (CSAMs). Interestingly, I also record an increase in the abundance of pneumococci within RPMs and perifollicular CSAMs, indicating potential replication within human splenic macrophages as observed within murine splenic macrophages. Taken together, my results emphasise the importance of the study of bacteria-macrophage interactions, especially for the identification of novel therapeutic interventions.