Neutrophils are recruited from the blood to sites of sterile inflammation, where they contribute to wound healing but may also cause tissue damage. By using spinning disk confocal intravital microscopy, we examined the kinetics and molecular mechanisms of neutrophil recruitment to sites of focal hepatic necrosis in vivo. Adenosine triphosphate released from necrotic cells activated the Nlrp3 inflammasome to generate an inflammatory microenvironment that alerted circulating neutrophils to adhere within liver sinusoids. Subsequently, generation of an intravascular chemokine gradient directed neutrophil migration through healthy tissue toward foci of damage. Lastly, formyl-peptide signals released from necrotic cells guided neutrophils through nonperfused sinusoids into the injury. Thus, dynamic in vivo imaging revealed a multistep hierarchy of directional cues that guide neutrophil localization to sites of sterile inflammation.

Inflammation significantly contributes to the progression of chronic kidney disease (CKD). Inflammasome-dependent cytokines, such as IL-1␤ and IL-18, play a role in CKD, but their regulation during renal injury is unknown. Here, we analyzed the processing of caspase-1, IL-1␤, and IL-18 after unilateral ureteral obstruction (UUO) in mice, which suggested activation of the Nlrp3 inflammasome during renal injury. Compared with wild-type mice, Nlrp3 Ϫ/Ϫ mice had less tubular injury, inflammation, and fibrosis after UUO, associated with a reduction in caspase-1 activation and maturation of IL-1␤ and IL-18; these data confirm that the Nlrp3 inflammasome upregulates these cytokines in the kidney during injury. Bone marrow chimeras revealed that Nlrp3 mediates the injurious/inflammatory processes in both hematopoietic and nonhematopoietic cellular compartments. In tissue from human renal biopsies, a wide variety of nondiabetic kidney diseases exhibited increased expression of NLRP3 mRNA, which correlated with renal function. Taken together, these results strongly support a role for NLRP3 in renal injury and identify the inflammasome as a possible therapeutic target in the treatment of patients with progressive CKD.

Inflammatory bowel diseases (IBD) are chronic relapsing and remitting conditions associated with long-term gut dysfunction resulting from alterations to the enteric nervous system and a loss of enteric neurons1,2. The mechanisms underlying inflammation-induced enteric neuron death are unknown. Here we report using in vivo models of experimental colitis that inflammation causes enteric neuron death by activating a neuronal signaling complex comprised of P2X7 receptors (P2X7Rs), pannexin–1 (Panx1) channels, Asc and caspases. Inhibiting P2X7Rs, Panx1, Asc or caspase activity prevents inflammation-induced neuron cell death. Preservation of enteric neurons by inhibiting Panx1 in vivo prevented the onset of inflammation-induced colonic motor dysfunction. Panx1 expression is reduced in Crohn’s disease but not ulcerative colitis. We conclude that activation of neuronal Panx1 underlies neuron death and subsequent development of the abnormal gut motility in IBD. Targeting Panx1 represents a novel neuroprotective strategy to ameliorate the progression of IBD–associated dysmotility.

Attenuated innate immune responses to the intestinal microbiota have been linked to the pathogenesis of Crohn’s disease (CD). Recent genetic studies have revealed that hypofunctional mutations of NLRP3, a member of the NOD-like receptor (NLR) superfamily, are associated with an increased risk of developing CD. NLRP3 is a key component of the inflammasome, an intracellular danger sensor of the innate immune system. When activated, the inflammasome triggers caspase-1-dependent processing of inflammatory mediators, such as IL-1β and IL-18. In the current study we sought to assess the role of the NLRP3 inflammasome in the maintenance of intestinal homeostasis through its regulation of innate protective processes. To investigate this role, Nlrp3−/− and wild-type (WT) mice were assessed in the DSS- and TNBS-models of experimental colitis. Nlrp3−/− mice were found to be more susceptible to experimental colitis, an observation that was associated with reduced IL-1β reduced anti-inflammatory cytokine IL-10, and reduced protective growth factor TGF-β. Macrophages isolated from Nlrp3−/− mice failed to respond to bacterial muramyl dipeptide. Furthermore, Nlrp3-deficient neutrophils exhibited reduced chemotaxis and enhanced spontaneous apoptosis, but no change in oxidative burst. Lastly, Nlrp3−/− mice displayed altered colonic β-defensin expression, reduced colonic antimicrobial secretions and a unique intestinal microbiota. Our data confirm an essential role for the NLRP3 inflammasome in the regulation of intestinal homeostasis and provide biological insight into disease mechanisms associated with increased risk of CD in individuals with NLRP3 mutations.

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New findings r What is the central question of this study?Heart failure is associated with persistent sterile inflammation that worsens disease severity; however, the molecular mechanisms behind cytokine recruitment and their relevance in the diseased myocardium remain unknown. r What is the main finding and its importance?We show that interleukin-1β is activated downstream of the Nlrp3 inflammasome in calcineurin-transgene-induced structural heart disease. Genetic deletion of Nlrp3 abrogated inflammasome signalling and interleukin-1β release, improving function. The role of Nlrp3 in non-ischaemic cardiomyopathy and the utility of inflammasome antagonism have not yet been explored, revealing potential for translational application.Heart failure is associated with a low-grade and chronic cardiac inflammation that impairs function; however, the mechanisms by which this sterile inflammation occurs in structural heart disease remain poorly defined. Cardiac-specific heterozygous overexpression of the calcineurin transgene (CNTg) in mice results in cardiac hypertrophy, inflammation, apoptosis and ventricular dilatation. We hypothesized that activation of the Nlrp3 inflammasome, an intracellular danger-sensing pathway required for processing the pro-inflammatory cytokine interleukin-1β (IL-1β), may contribute to myocardial dysfunction and disease progression. Here we report that Nlrp3 mRNA was increased in CNTg mice compared with wild-type. Consistent with inflammasome activation, CNTg animals had increased conversion of procaspase-1 to cleaved and activated forms, as well as markedly increased serum IL-1β. Blockade of IL-1β signalling via chronic IL-1 receptor antagonist therapy reduced cardiac inflammation and myocyte pathology in CNTg mice, resulting in improved systolic performance. Furthermore, genetic ablation of Nlrp3 in CNTg mice reduced pro-inflammatory cytokine maturation and cardiac inflammation, as well as improving systolic performance. These findings indicate that activation of the Nlrp3 inflammasome in CNTg mice promotes myocardial inflammation and systolic dysfunction through the production of pro-inflammatory IL-1β. Blockade of IL-1β signalling with the IL-1 receptor antagonist reverses these phenotypes and offers a possible therapeutic approach in the management of heart failure.

1 In phenylephrine (PHE) (1 mm)-precontracted superior mesenteric arteries from adult rats, low concentration of hydrogen peroxide (H 2 O 2 , 10-100 mm) caused only contraction, while high concentration of H 2 O 2 (0.3 -1 mm) caused a biphasic response: a transient contraction followed by a relaxation response. 2 Endothelium removal did not affect the biphasic response. 7,7-Dimethyl-(5Z,8Z)-eicosadienoic acid, diclofenac, furegrelate, or SQ 29548 greatly inhibited the contraction but did not affect the relaxation. 17-Octadecynoic acid, eicosatriynoic acid, ICI 198615, SQ 22536, or ODQ did not inhibit the biphasic response. 3 KCl at 40 mm inhibited the relaxation response to H 2 O 2 by 98724%. 4-Aminopyridine (4-AP) inhibited while tetraethylammonium chloride (TEA), charybdotoxin, or glibenclamide attenuated the relaxation response. A combination of 4-AP, TEA and glibenclamide mimicked the effects of 40 mm KCl. Iberiotoxin, apamin, or barium chloride did not inhibit the relaxation response. 4 H 2 O 2 at 1 mm hyperpolarized membrane potential and reversibly augmented K + current in smooth muscle cells of mesenteric artery. These effects of H 2 O 2 were attenuated significantly by 4-AP. 5 In summary, in PHE-precontracted rat mesenteric artery: (1) the response to H 2 O 2 shifted qualitatively from contraction to a biphasic response as H 2 O 2 increased to 0.3 mm or higher; (2) the relaxation response is caused by the activation of K + channels, with voltage-dependent K + channels playing a primary role; and the contraction is likely to be mediated by thromboxane A 2 ; (3) the K + channel activation by H 2 O 2 is independent of phospholipase A 2 , cyclooxygenase, lipoxygenase, cytochrome P450 monooxygenase, adenylate or guanylate cyclase.