Cytotoxins of the human pathogen Aeromonas hydrophila trigger, via the NLRP3 inflammasome, caspase‐1 activation in macrophages

Abstract

Aeromonas hydrophila is a Gram-negative pathogen that causes serious infectious disease in humans. A. hydrophila induces apoptosis in infected macrophages, but the host proinflammatory responses triggered by macrophage death are largely unknown. Here, we demonstrate that the infection of mouse macrophages with A. hydrophila triggers the activation of caspase-1 and release of IL-1b. Caspase-1 activation was abrogated in macrophages deficient in Nod-like receptor family, pyrin domain containing 3 (NLRP3) and apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), but not NLR family, CARD domain containing 4 (NLRC4). The activation of the NLRP3 inflammasome was mediated by three cytotoxins (aerolysin, hemolysin and multifunctional repeat-in-toxin) produced by A. hydrophila. Our results indicated that the NLRP3 inflammasome senses A. hydrophila infection through the action of bacterial cytotoxins.Key words: Aeromonas . Caspase-1 . Nod-like receptor Supporting Information available online IntroductionThe genus Aeromonas is responsible for a significant number of animal and human infections. Aeromonas are opportunistic human pathogens that can cause intestinal and extraintestinal, wound, soft tissue, skin, and blood infections and septicemia. The most common human disease associated with Aeromonas infection is gastroenteritis with diarrhea symptoms. Among the 21 species that have been differentiated based on DNA-DNA hybridization, A. caviae, A. veronii biotype sobria and A. hydrophila are most commonly associated with human infections and account for more than 85% of all clinical isolates [1,2].Recently, it has been reported that infection with A. hydrophila induces apoptosis in macrophages [3,4]. Although the cytotoxicity by the bacteria is believed to enable the bacteria to evade eradication and clearance by the macrophages, the host proinflammatory responses triggered by the death of the macrophages remain unknown.Induction of apoptosis is considered a virulence mechanism of pathogenic bacteria that can cause tissue damage and Here, we investigated the cell death of macrophages infected with A. hydrophila and demonstrate that the bacteria cause pyroptosis with caspase-1 activation via the NLRP3 inflammasome. Aerolysin, hemolysin (HlyA) and multifunctional repeat-intoxin (RtxA) produced by A. hydrophila are required to elicit the NLRP3 inflammasome and necrotic cell death. We identify the essential bacterial and host factors for eliciting caspase-1 activation and proinflammatory responses. SHORT COMMUNICATION Results and discussionInfection with A. hydrophila induces caspase-1 activation, IL-1b release and pyroptosis It has been reported that aerolysin produced by A. salmonicida induces caspase-1 activation in CHO cells [20,21]. However, whether whole bacterial infection by A. hydrophila leads to caspase-1 activation in macrophages has not been elucidated. We examined caspase-1 activation in infected primary mouse BMderived macrophages (BMM) and observed that BMM infected with WT strai...

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