Cytosolic innate immune pathway activation by Gram-positive bacteria during sepsis
in: Infection (2019)
Organ damage and mortality in sepsis have been attributed to deleterious and inappropriate host response during infection. Over the course of sepsis a systemic hyper inflammatory response as well as innate immunosuppression may occur. Activation of the innate immune system is a key host defense strategy for clearance of the bacteria. The innate immune system recognizes pathogen associated molecular patterns (PAMPs) and danger associated molecular patterns (DAMPs) through a set of germline-encoded proteins called pattern recognition receptors (PRR). Apart from surface PRRs, a wide array of microbial structures like lipids and nucleic acids also activate the cytosolic inflammasome signaling cascade. Depending on the ligand, inflammasome activation can occur through either the canonical and non-canonical inflammasome activation pathways. Gram-positive bacteria such as Staphylococcus aureus (SA) and Group B Streptococcus (GBS) are the main causal organisms of adult and neonatal sepsis. Molecular mechanisms which activate inflammasome pathways during Gram-positive bacterial infection and sepsis is poorly understood. Objectives: In the pathogenesis of sepsis, how Gram-positive bacterial PAMPs activate the immune system is a major question. Having recently discovered outer membrane vesicles as mediators of inflammasome activation, we are currently analyzing pathways activated by extracellular vesicles derived from SA and GBS. Methods: Re-addressing the formation and activation of the inflammasome pathway induced by bacterial extracellular vesicles. We used an array of interdisciplinary techniques including electron microscopy combined with state-of-the-art CRISPR/Cas genome editing technique and label free vibrational spectroscopic methods to study Grampositive bacterial recognition by human macrophages. Results: We carried out a comprehensive structural and biochemical analysis of extracellular vesicles isolated from Gram-positive bacteria. We found that a subset of extracellular vesicles enriched with lipids encoded by a pathogenicity island were potent inducers of canonical inflammasome pathway. Further characterization of the different biomolecules of these extracellular vesicles has allowed us to decipher a novel pathway by which SA and GBS stimulate human macrophages. Conclusions: We demonstrate that PAMPs present in bacterial extracellular vesicles are potent inducers of cytosolic inflammasome pathways and their presence coincides with Gram-positive infection. Acknowledgement: This work was supported by the Federal Ministry of Education and Research, Germany (FKZ: 01EO1002).