Pneumolysin and the bacterial capsule of Streptococcus pneumoniae cooperatively inhibit taxis and motility of microglia

Sabrina Hupp,Asparouh I Iliev,Denis Grandgirard,Stephen L Leib,Lucy J Hathaway,Timothy J Mitchell

doi:10.1186/s12974-019-1491-7

Sabrina Hupp, Asparouh I Iliev + Show 4 more

Open Access

https://doi.org/10.1186/s12974-019-1491-7

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Abstract

BackgroundStreptococcus pneumoniae is the cause of a highly lethal form of meningitis in humans. Microglial cells in the brain represent the first line of defense against pathogens, and they participate in the inflammatory response. The cholesterol-dependent cytolysin pneumolysin and the bacterial capsule are key pathogenic factors, known to exacerbate the course of pneumococcal meningitis.MethodsWe utilized live imaging and immunostaining of glial cells in dissociated and acute brain slice cultures to study the effect of pneumococcal factors, including the cholesterol-dependent cytolysin pneumolysin and the pneumococcal capsule, on microglial motility and taxis.ResultsIn brain tissue, primary microglia cells showed an enhanced response towards lysates from bacteria lacking capsules and pneumolysin as they moved rapidly to areas with an abundance of bacterial factors. The presence of bacterial capsules and pneumolysin cumulatively inhibited microglial taxis. In mixed cultures of astrocytes and microglia, the motility of microglia was inhibited by capsular components within minutes after exposure. The reduced motility was partially reversed by mannan, a mannose receptor inhibitor. The effects on microglia were not mediated by astrocytes because pure microglial cells responded to various pneumococcal lysates similarly with distinct cell shape changes as seen in mixed cultures.ConclusionsOur data indicate that microglia possess the capacity for a very agile response towards bacterial pathogens, but key pathogenic factors, such as pneumococcal capsules and pneumolysin, inhibited this response shortly after a bacterial challenge. Furthermore, we demonstrate for the first time that the bacterial capsule affects cellular behaviors such as motility and taxis.

Highlights

Streptococcus pneumoniae is the cause of a highly lethal form of meningitis in humans
Similar to meningitis, where cortical necrosis occurs, the cutting interface contains multiple damaged cells that initiate a chemotactical response from microglia through the release of Adenosine triphosphate (ATP) and glutamate (Fig. 1a, Additional file 2: Figure S2) [23, 24]
Staining with Iba1 demonstrated that microglia rapidly clustered along the brain slice cut interface after sensing bacterial lytic products (Fig. 1b; 3D reconstruction is shown in Additional file 3: Movie M1 and Additional file 4: Movie M2)

Summary

Introduction

Streptococcus pneumoniae is the cause of a highly lethal form of meningitis in humans. The cholesterol-dependent cytolysin pneumolysin and the bacterial capsule are key pathogenic factors, known to exacerbate the course of pneumococcal meningitis. Bacterial meningitis is a disease with high lethality and substantial disability of surviving patients [1]. Experiments in meningitis animal models infected with S. pneumoniae indicates that the cholesterol-dependent cytolysin, pneumolysin (PLY), and the bacterial cell capsule play critical roles in the pathogenesis of the disease, as bacteria deficient in either of these factors are not as pathogenic as the wild-type strains [4,5,6,7]. Some differences between PLY-deficient and bacterial capsule-deficient mutants have been observed; for example, capsule mutants are rapidly eliminated, while the PLY-deficient strains still cause meningitis but with a greatly reduced lethality and a milder disease course.

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