Phagosomal Acidification Is Required to Kill Streptococcus pneumoniae in a Zebrafish Model

Abstract

Streptococcus pneumoniae (the pneumococcus) is a major human pathogen causing invasive disease, including community-acquired bacteraemia, and remains a leading cause of global mortality. Understanding the role of phagocytes in killing bacteria is still limited, especially in vivo. In this study, we established a zebrafish model to study the interaction between intravenously administered pneumococci and professional phagocytes such as macrophages and neutrophils, to unravel bacterial killing mechanisms employed by these immune cells. Our model confirmed the key role of polysaccharide capsule in promoting pneumococcal virulence through inhibition of phagocytosis. Conversely, we show pneumococci lacking a capsule are rapidly internalised by macrophages. Low doses of encapsulated S. pneumoniae cause near 100% mortality within 48 hours postinfection (hpi), while 50 times higher doses of unencapsulated pneumococci are easily cleared. Time course analysis of in vivo bacterial numbers reveals that while encapsulated pneumococcus proliferates to levels exceeding 105 CFU at the time of host death, unencapsulated bacteria are unable to grow and are cleared within 20 hpi. Using genetically induced macrophage depletion, we confirmed an essential role for macrophages in bacterial clearance. Additionally, we show that upon phagocytosis by macrophages, phagosomes undergo rapid acidification. Genetic and chemical inhibition of vacuolar ATPase (v-ATPase) prevents intracellular bacterial killing and induces host death indicating a key role of phagosomal acidification in immunity to invading pneumococci. We also show that our model can be used to study the efficacy of antimicrobials against pneumococci in vivo. Collectively, our data confirm that larval zebrafish can be used to dissect killing mechanisms during pneumococcal infection in vivo and highlight key roles for phagosomal acidification in macrophages for pathogen clearance.

Genome-wide association study identifies genes and networks that influence innate immune response in an age-specific manner in Drosophila melanogaster

“…For example, Short and Lazzaro looked at differences in virgin vs. mated females and identi ed many differentially expressed immune-response genes in response to infection between the two 27 . Similar ndings on age-dependent genetic effects have been observed in other organisms, such as mice [30][31][32] , zebra sh 33,34 , and even cows 15 , indicating the importance of identifying the underlying mechanisms that contribute to age-related changes, not only in immune response, but other biological processes as well. Future studies using mated ies of both sexes, and/or a different pathogen, will be necessary to gain a more complete understanding of the genes involved in regulating the age-dependent immune response.…”