Pyruvate oxidase as a key determinant of pneumococcal viability during transcytosis across the blood-brain barrier endothelium

Streptococcus pneumoniae (SPN / pneumococcus), invades myriad of host tissues following efficient breaching of cellular barriers. However, strategies adopted by pneumococcus for evasion of host intracellular defences governing successful transcytosis across host cellular barriers remain elusive. In this study, using brain endothelium as a model host barrier, we observed that pneumococcus containing endocytic vacuoles (PCVs) formed following SPN internalization into brain microvascular endothelial cells (BMECs), undergo early maturation and acidification, with a major subset acquiring lysosome-like characteristics. Exploration of measures that would preserve pneumococcal viability in the lethal acidic pH of these lysosome-like vacuoles revealed a critical role of the two-component system response regulator, CiaR, which has been previously implicated in induction of acid tolerance response. Pyruvate oxidase (SpxB), a key sugar metabolizing enzyme that catalyses oxidative decarboxylation of pyruvate to acetyl phosphate, was found to contribute to acid stress tolerance, presumably via acetyl phosphate-mediated phosphorylation and activation of CiaR, independent of its cognate kinase CiaH. Hydrogen peroxide, the by-product of SpxB catalysed reaction, was also found to improve pneumococcal intracellular survival, by oxidative inactivation of lysosomal cysteine cathepsins, thus compromising the degradative capacity of the host lysosomes. Expectedly, a ΔspxB mutant was found to be significantly attenuated in its ability to survive inside the BMEC endocytic vacuoles, reflecting in its reduced transcytosis ability. Collectively, our studies establish SpxB as an important virulence determinant facilitating pneumococcal survival inside host cells, ensuring successful trafficking across host cellular barriers. AUTHOR SUMMARY Eukaryotic cells which constitute host barriers have innate immune defences to restrict microbial passage into sterile compartments. This necessitates need for pathogens to devise strategies to evade these, for successful establishment of disease. In this study, by focussing on the blood-brain barrier endothelium, we investigate the mechanisms which enable the opportunistic pathogen Streptococcus pneumoniae to traverse host barriers. Pyruvate oxidase, a pneumococcal sugar metabolizing enzyme was found to play a critical role in this key event, owing to production of acetyl phosphate and hydrogen peroxide via its enzymatic activity. On one hand, acetyl phosphate, by contributing to activation of acid tolerance stress response, enabled pneumococci to maintain viability in the lethal acidic pH of the lysosome-like vacuoles inside brain endothelium. On the other, hydrogen peroxide, was found to oxidise and inactivate a subset of degradative lysosomal enzymes. This two-pronged approach, aided by pyruvate oxidase, enabled pneumococci to evade intracellular degradation for successful transcytosis across the endothelium. Thus, pyruvate oxidase is a key determinant of pneumococcal virulence and hence can potentially serve as a viable candidate for therapeutic interventions for better management of invasive pneumococcal diseases.