Pneumococcus Hiding from the Host

Septicaemia caused by Streptococcus pneumoniae (the pneumococcus) remains a large cause of morbidity and mortality, despite the availability of treatments and vaccines. A number of high profile clinical trials aimed at improving the outcome of sepsis, have failed due to a fundamental lack of understanding of the pathogenesis of bacterial disease. The premise of this work was to better understand the early host-pathogen interactions occurring prior to invasive disease, with the aim of developing and optimising therapeutic interventions.

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The paper in Nature Microbiology is here:

Our previous work has shown that mice intravenously challenged with 10^5 pneumococci, develop a bacteraemia from a single bacterial cell. Considering what we know about the 'big picture' of pneumococcal pathogenesis, we realised that there was an intrinsic problem with this observation.  

How could a single bacterial cell multiply rapidly in an immune competent host who at the same time is able to clear hundreds of thousands of identical bacterial cells from the blood stream? 

The only reasonable explanation was the existence of an extravascular hiding place for bacteria in the host. We felt identification of the immune-protected niche was crucial , as such a severe population bottleneck represents a point in which therapeutic intervention will likely be most effective. Our journey to finding bacteria capable of hiding from the host led us to the spleen. 

Through a combination of IV mouse infections, and confocal microscopy, we were able to observe a clear clustering of bacteria in an apparent intracellular niche within a subset of CD169+ splenic macrophages. To determine whether these clusters originated as single cells we conducted infections with 1:1 mixes of GFP and RFP tagged bacteria. This revealed exclusively monochrome clusters of bacteria in the spleen, indicating that a single founder bacterium enters a macrophage, and replicates intracellularly (as opposed to continuous phagocytic events). This was surprising, as the pneumococcus was thought to be exclusively an extracellular pathogen, with no reports of the bacterium surviving within phagocytes. Further experiments using both high resolution electron microscopy, and antibiotic therapy specifically geared to target intracellular bacteria confirmed this finding. Groups have previously shown that the CD169 receptor is responsible for permissiveness to viral (including HIV) replication. Our data, using a CD169-blocking antibody, suggest the CD169 is also crucial to successfully establish an invasive pneumococcal infection.

           Intracellular replication of pneumococci

Mouse spleens are anatomically distinct from those of humans, leading us to consider the translatability of our findings. Pigs represent a good model, as their spleens are immunologically very similar to humans. We therefore established a translational ex vivo pig spleen perfusion model, using abattoir-sources spleens (a lovely job for those of us who collect the spleens...). In this model, we saw the same association of pneumococci with CD169+, indicating that our data may be translatable to humans. 

We feel this is a new chapter in pneumococcal pathogenesis research, and may alter the way we think about treatment of pneumococcal disease. We show that in light of an intracellular niche, macrolides are more efficient at preventing pneumococcal disease than beta-lactams (a current frontline treatment). But how else can we improve our treatment strategies? How else can the pneumococcus deceive the host so efficiently? And is this a bacteria-wide Achilles heel in infectious disease, or a phenomenon limited to the pneumococcus?

Joe Wanford

PhD student, University of Leicester