Macrophages as a Replicative Niche During Systemic Bacterial Infection

Bloodborne bacterial pathogens are exposed to multiple macrophage-mediated clearance mechanisms in organs including the liver and spleen. Some pathogens – termed intracellular pathogens – are known to resist intracellular killing and persist within cells during the pathogenesis of infection. Extracellular pathogens are not widely considered to survive and replicate within macrophages. In this thesis, I report that two typically extracellular pathogens Streptococcus pneumoniae and Klebsiella pneumoniae have key phases of infection within tissue macrophages. For S. pneumoniae, I demonstrate that following infection of mice, bacteria can replicate within CD169-positive metallophilic macrophages and red pulp macrophages, but are efficiently cleared by SIGN-R1-positive marginal zone macrophages in the spleen. CD169+ macrophages were shown to be a critical safe haven for pneumococci prior to invasive disease, as blocking these cells with a monoclonal antibody prevented disease. Replicative foci within CD169-positive macrophages were shown to be hidden from neutrophil surveillance which may facilitate pneumococcal immune evasion in the early hours of infection. Instead, for K. pneumoniae, I demonstrate that hypervirulent strains (hvKp) – characterised by their hypermucoid capsules – replicated within splenic macrophages and Kupffer cells in the liver, while non-hv strains did not. Replication of hvKp within Kupffer cells formed a focal point for resistance to neutrophil-mediated killing, which led to the formation of tissue abscesses comparable to that which is observed in human disease. I developed a model of ex vivo human spleen perfusion, and porcine spleen-liver co-perfusion which allowed the translation of our murine findings for both pathogens to the human host. Together, this thesis identifies the within-macrophage niche as a safe haven for two bacterial species traditionally considered to be extracellular during the pathogenesis of infection. This work will open new research opportunities in the short term and facilitate the development of novel treatment strategies in the future.