Inflammasome activation patterns and virulence factors underlie differential species-specific Listeria pathogenicity

Abstract

Listeria monocytogenes and Listeria ivanovii are two pathogenic Listeria species that share similar infection mechanisms but exhibit differential pathogenicity in humans and animals. L. monocytogenes and L. ivanovii are intracellular pathogens that undergo phagocytic cell entry, endosomal membrane rupture, cytoplasmic replication and cell- to-cell spread in the host. L. monocytogenes infection causes gastroenteritis, meningitis, abortion and sepsis in humans and animals, whereas L. ivanovii infection causes these diseases except meningitis in animals, and only occasionally in humans. Based on these properties, L. monocytogenes and L. ivanovii offers a unique model to investigate differential pathogenicity induced by closely-related pathogens.

Previous research and my study have identified L. monocytogenes and L. ivanovii are the only two among the 20 Listeria species that activates the inflammasome in macrophages. The inflammasome is an innate immune signalling complex that leads to inflammation and killing of pathogens. In this study, I compared inflammasome activation induced by L. monocytogenes and L. ivanovii in macrophages and in mice. Bone marrow-derived macrophages were generated from wild-type (WT) mice and mice lacking inflammasome components and infected with Listeria strains. The levels of inflammasome activation were assessed based on analysis of multiple inflammasome hallmarks, including caspase-1 and gasdermin D cleavage, interleukin (IL)-1beta and IL-18 secretion, and pyroptosis. To investigate the role inflammasome activation in systemic infection, WT mice and mice lacking inflammasome components were intraperitoneally infected with Listeria strains. The disease severity induced by L. monocytogenes and L. ivanovii were assessed by quantifying mouse body weightchange, and bacterial burden in the liver and spleen. The levels of inflammasome activation were assessed by quantifying systemic IL-18 secretion.

I have found that in macrophages, L. monocytogenes is sensed predominantly by the inflammasome sensor NLRP3, with a partial involvement of AIM2, whereas L. ivanovii is sensed by AIM2. L. monocytogenes activates the inflammasome with a delayed kinetics compared to L. ivanovii. The kinetics of inflammasome activation induced by Listeria is determined by their transcriptional activity of the virulence regulator PrfA. The transcriptional activity of L. monocytogenes PrfA is tightly-controlled, whereas the transcriptional activity of L. ivanovii PrfA (i-PrfA) is constitutively-active. Switching the tightly-controlled L. monocytogenes PrfA to the constitutively-active PrfA*G145S carrying a missense mutation accelerates inflammasome activation induced by L. monocytogenes, to a magnitude similar to that of L. ivanovii. In mice, L. monocytogenes induces PrfA-dependent inflammasome activation that underpins severe disease. Despite carrying a constitutively-active i-PrfA, L. ivanovii inefficiently triggers an inflammasome response and has an impaired ability to cause disease in mice.

My study indicates that, in mice, Listeria infection is exacerbated by inflammasome activity and this difference in the inflammasome response contributes to the differential pathogenicity between L. monocytogenes and L. ivanovii. Identifying the interactions between microbial factors and the innate immune responses that promote or attenuate infection may lead to novel therapeutic approaches to control infectious disease outcome. Show more