SUMMARY The microbiota stimulate inflammation, but the signaling pathways and the members of the microbiota involved remain poorly understood. We found that the microbiota induce interleukin-1β (IL-1β) release upon intestinal injury and this is mediated via the NLRP3 inflammasome. Enterobacteriaceae and in particular the pathobiont Proteus mirabilis, induced robust IL-1β release that was comparable to that induced by the pathogen Salmonella. Upon epithelial injury, production of IL-1β in the intestine was largely mediated by intestinal Ly6Chigh monocytes, required chemokine receptor CCR2 and was abolished by deletion of IL-1β in CCR2+ blood monocytes. Furthermore, colonization with P. mirabilis promoted intestinal inflammation upon intestinal injury via the production of hemolysin which required NLRP3 and IL-1 receptor signaling in vivo. Thus, upon intestinal injury, selective members of the microbiota stimulate newly recruited monocytes to induce NLRP3-dependent IL-1β release which promotes inflammation in the intestine.

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Bacterial pathogens utilize pore-forming toxins or sophisticated secretion systems to establish infection in hosts. Recognition of these toxins or secretion system by nucleotide-binding oligomerization domain leucine-rich repeat proteins (NLRs) triggers the assembly of inflammasomes, the multiprotein complexes necessary for caspase-1 activation and the maturation of inflammatory cytokines such as IL-1β or IL-18. Here we demonstrate that both the NLRP3 and NLRC4 inflammasomes are activated by thermostable direct hemolysins (TDHs) and type III secretion system 1 (T3SS1) in response to V. parahaemolyticus infection. Furthermore, we identify T3SS1 secreted effector proteins, VopQ and VopS, which induce autophagy and the inactivation of Cdc42, respectively, to prevent mainly NLRC4 inflammasome activation. VopQ and VopS interfere with the assembly of specks in infected macrophages. These data suggest that bacterial effectors interfere with inflammasome activation and contribute to bacterial evasion from the host inflammatory responses.

Porphyromonas gingivalis has been shown to accelerate atherosclerotic lesion development in hyperlipidemic animals. We assessed the potential of a nasal vaccine against P. gingivalis infection for the prevention of atherosclerosis. Apolipoprotein E-deficient spontaneously hyperlipidemic (Apoe shl ) mice were nasally immunized with the 40-kDa outer membrane protein (OMP) of P. gingivalis plus cholera toxin (CT) as adjuvant and then challenged intravenously with P. gingivalis strain 381. The animals were euthanized 11 or 14 weeks later. Atheromatous lesions in the proximal aorta of each animal were analyzed histomorphometrically, and the serum concentrations of 40-kDa OMP-specific antibodies and cytokines were determined. The areas of the aortic sinus that were covered with atherosclerotic plaque and the serum levels of inflammatory cytokines and chemokines were increased in Apoe shl mice challenged with P. gingivalis compared to nonchallenged mice. In comparison, nasal immunization with 40-kDa OMP plus CT significantly reduced atherosclerotic plaque accumulation in the aortic sinus and lowered the serum levels of cytokines and chemokines compared to nonimmunized animals. Nasal immunization also induced 40-kDa OMP-specific serum immunoglobulin G (IgG) and saliva IgA antibody responses. These findings suggest that systemic infection with P. gingivalis accelerates atherosclerosis in Apoe shl mice, and 40-kDa OMP plus CT may be an effective nasal vaccine for the reduction of atherosclerosis accelerated by P. gingivalis in the hyperlipidemic mouse model.

Aeromonas spp. are Gram-negative bacteria that cause serious infectious disease in humans. Such bacteria have been shown to induce apoptosis in infected macrophages, yet the host responses triggered by macrophage death are largely unknown. In this study, we demonstrate that the infection of mouse bone marrow-derived macrophages with Aeromonas veronii biotype sobria triggers activation of caspase-1 with the ensuing release of IL-1β and pyroptosis. Caspase-1 activation in response to A. veronii infection requires the adaptor apoptosis-associated speck-like protein containing a caspase recruitment domain and both the NLRP3 and NLRC4 inflammasomes. Furthermore, caspase-1 activation requires aerolysin and a functional type III secretion system in A. veronii. Aerolysin-inducing caspase-1 activation is mediated through the NLRP3 inflammasome, with aerolysin-mediated cell death being largely dependent on the NLRP3 inflammasome. In contrast, the type III secretion system activates both the NLRP3 and NLRC4 inflammasomes. Inflammasome-mediated caspase-1 activation is also involved in host defenses against systemic A veronii infection in mice. Our results indicated that multiple factors from both the bacteria and the host play a role in eliciting caspase-1 activation during A. veronii infection.