Bacterial septicaemia is a major cause of mortality, but its pathogenesis remains poorly understood. In experimental pneumococcal murine intravenous infection, an initial reduction of bacteria in the blood is followed hours later by a fatal septicaemia. These events represent a population bottleneck driven by efficient clearance of pneumococci by splenic macrophages and neutrophils, but as we show in this study, accompanied by occasional intracellular replication of bacteria that are taken up by a subset of CD169+ splenic macrophages. In this model, proliferation of these sequestered bacteria provides a reservoir for dissemination of pneumococci into the bloodstream, as demonstrated by its prevention using an anti-CD169 monoclonal antibody treatment. Intracellular replication of pneumococci within CD169+ splenic macrophages was also observed in an ex vivo porcine spleen, where the microanatomy is comparable with humans. We also showed that macrolides, which effectively penetrate macrophages, prevented septicaemia, whereas beta-lactams, with inefficient intracellular penetration, failed to prevent dissemination to the blood. Our findings define a shift in our understanding of the pneumococcus from an exclusively extracellular pathogen to one with an intracellular phase. These findings open the door to the development of treatments that target this early, previously unrecognized intracellular phase of bacterial sepsis.
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G.E. was funded through an academic collaboration agreement between the University of Oxford and University of Leicester and in part by MRC grant MR/M003078/1. The authors thank J.W. Veening for providing GFP and RFP pneumococci, F. Focarelli for construction of the non-encapsulated GFP-expressing strain, M. De Ste Croix for mutant construction, S. Glenn for help with the infection experiments, the Electron Microscopy Facility, the University of Leicester for technical support and R. Kumar and J. Isherwood for help with the perfusion of the porcine organs at explant, the staff of Joseph Morris Butchers Ltd, Michael F Wood Butchers and the staff of Leicester Preclinical Research Facility for support.
The authors declare no competing interests.
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Ercoli, G., Fernandes, V.E., Chung, W.Y. et al. Intracellular replication of Streptococcus pneumoniae inside splenic macrophages serves as a reservoir for septicaemia. Nat Microbiol 3, 600–610 (2018). https://doi.org/10.1038/s41564-018-0147-1
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