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Elevated AIM2-mediated pyroptosis triggered by hypercytotoxic Francisella mutant strains is attributed to increased
Kaitian Peng1, Petr Broz, Jonathan Jones
1Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford, California, USA.
Abstract:
Intracellular bacterial pathogens Francisella novicida and the Live Vaccine Strain (LVS) are recognized in the macrophage cytosol by the AIM2 inflammasome, which leads to the activation of caspase-1 and the processing and secretion of active IL-1β, IL-18 and pyroptosis. Previous studies have reported that F. novicida and LVS mutants in specific genes (e.g. FTT0584, mviN and ripA) induce elevated inflammasome activation and hypercytotoxicity in host cells, leading to the proposal that F. novicida and LVS may have proteins that actively modulate inflammasome activation. However, there has been no direct evidence of such inflammasome evasion mechanisms. Here, we demonstrate for the first time that the above mutants, along with a wide range of F. novicida hypercytotoxic mutants that are deficient for membrane-associated proteins (ΔFTT0584, ΔmviN, ΔripA, ΔfopA and ΔFTN1217) or deficient for genes involved in O-antigen or LPS biosynthesis (ΔwbtA and ΔlpxH) lyse more intracellularly, thus activating increased levels of AIM2-dependent pyroptosis and other innate immune signalling pathways. This suggests that an inflammasome-specific evasion mechanism may not be present in F. novicida and LVS. Furthermore, future studies may need to consider increased bacterial lysis as a possible cause of elevated stimulation of multiple innate immune pathways when the protein composition or surface carbohydrates of the bacterial membrane is altered.
Insights
Francisella bacteria are recognized by the AIM2 inflammasome in macrophages. Increased bacterial lysis, not active evasion, likely explains heightened immune responses observed with certain mutants.
Area of Science:
- Immunology
- Microbiology
- Cell Biology
Background:
- The AIM2 inflammasome recognizes intracellular bacteria like Francisella novicida and the Live Vaccine Strain (LVS) in the macrophage cytosol.
- Activation of AIM2 inflammasome leads to caspase-1 activation, IL-1β/IL-18 secretion, and pyroptosis.
- Previous studies suggested F. novicida/LVS mutants might possess proteins that actively evade inflammasome activation due to observed hypercytotoxicity.
Purpose of the Study:
- To investigate whether Francisella novicida and LVS actively evade inflammasome activation.
- To determine the cause of elevated inflammasome activation and hypercytotoxicity in specific bacterial mutants.
- To explore the role of bacterial lysis in stimulating innate immune signaling pathways.
Main Methods:
- Utilized a range of F. novicida mutants deficient in membrane-associated proteins (e.g., ΔFTT0584, ΔmviN, ΔripA, ΔfopA, ΔFTN1217).
- Examined mutants deficient in O-antigen or LPS biosynthesis genes (e.g., ΔwbtA, ΔlpxH).
- Assessed AIM2-dependent pyroptosis and innate immune signaling pathway activation in host cells.
Main Results:
- Mutants with altered membrane proteins or surface carbohydrates exhibited increased intracellular bacterial lysis.
- Increased bacterial lysis correlated with elevated AIM2-dependent pyroptosis and innate immune signaling.
- No direct evidence for an inflammasome-specific evasion mechanism in F. novicida and LVS was found.
Conclusions:
- Increased bacterial lysis, rather than active inflammasome evasion, is the likely cause of heightened innate immune responses in F. novicida/LVS mutants.
- Future studies should consider bacterial lysis as a factor when investigating altered innate immune stimulation due to changes in bacterial surface composition.
- The findings challenge the notion of active inflammasome evasion mechanisms in these pathogens.
