An immunogenic peptide in the A-box of HMGB1 protein reverses apoptosis-induced tolerance through RAGE receptor.

Apoptotic cells trigger immune tolerance in engulfing phagocytes. This poorly understood process is believed to contribute to the severe immunosuppression and increased susceptibility to nosocomial infections observed in critically ill sepsis patients. Extracellular high mobility group box 1 (HMGB1) is an important mediator of both sepsis lethality and the induction of immune tolerance by apoptotic cells. We have found that HMGB1 is sensitive to processing by caspase-1, resulting in the production of a fragment within its N-terminal DNA-binding domain (the A-box) that signals through the receptor for advanced glycation end products (RAGE) to reverse apoptosis-induced tolerance. In a two-hit mouse model of sepsis, we show that tolerance to a secondary infection and its associated mortality were effectively reversed by active immunization with dendritic cells treated with HMGB1 or the A-box fragment, but not a noncleavable form of HMGB1. These findings represent a novel link between caspase-1 and HMGB1, with potential therapeutic implications in infectious and inflammatory diseases.

Cellular inhibitors of apoptosis proteins cIAP1 and cIAP2 are required for efficient caspase-1 activation by the inflammasome.

Pathogen and danger recognition by the inflammasome activates inflammatory caspases that mediate inflammation and cell death. The cellular inhibitor of apoptosis proteins (cIAPs) function in apoptosis and innate immunity, but their role in modulating the inflammasome and the inflammatory caspases is unknown. Here we report that the cIAPs are critical effectors of the inflammasome and are required for efficient caspase-1 activation. cIAP1, cIAP2, and the adaptor protein TRAF2 interacted with caspase-1-containing complexes and mediated the activating nondegradative K63-linked polyubiquitination of caspase-1. Deficiency in cIAP1 (encoded by Birc2) or cIAP2 (Birc3) impaired caspase-1 activation after spontaneous or agonist-induced inflammasome assembly, and Birc2(-/-) or Birc3(-/-) mice or mice administered with an IAP antagonist had a dampened response to inflammasome agonists and were resistant to peritonitis. Our results describe a role for the cIAPs in innate immunity and further demonstrate the evolutionary conservation between cell death and inflammation mechanisms.

Caspase-12 controls West Nile virus infection via the viral RNA receptor RIG-I.

Caspase-12 has been shown to negatively modulate inflammasome signaling during bacterial infection. Its function in viral immunity, however, has not been characterized. We now report an important role for caspase-12 in controlling viral infection via the pattern-recognition receptor RIG-I. After challenge with West Nile virus (WNV), caspase-12-deficient mice had greater mortality, higher viral burden and defective type I interferon response compared with those of challenged wild-type mice. In vitro studies of primary neurons and mouse embryonic fibroblasts showed that caspase-12 positively modulated the production of type I interferon by regulating E3 ubiquitin ligase TRIM25-mediated ubiquitination of RIG-I, a critical signaling event for the type I interferon response to WNV and other important viral pathogens.

Control of intestinal homeostasis, colitis, and colitis-associated colorectal cancer by the inflammatory caspases.

Inflammatory caspases are essential effectors of inflammation and cell death. Here, we investigated their roles in colitis and colorectal cancer and report a bimodal regulation of intestinal homeostasis, inflammation and tumorigenesis by caspases-1 and -12. Casp1(-/-) mice exhibited defects in mucosal tissue repair and succumbed rapidly after dextran sulfate sodium administration. This phenotype was rescued by administration of exogenous interleukin-18 and was partially reproduced in mice deficient in the inflammasome adaptor ASC. Casp12(-/-) mice, in which the inflammasome is derepressed, were resistant to acute colitis and showed signs of enhanced repair. Together with their increased inflammatory response, the enhanced repair response of Casp12(-/-) mice rendered them more susceptible to colorectal cancer induced by azoxymethane (AOM)+DSS. Taken together, our results indicate that the inflammatory caspases are critical in the induction of inflammation in the gut after injury, which is necessary for tissue repair and maintenance of immune tolerance.

Caspase-12 modulates NOD signaling and regulates antimicrobial peptide production and mucosal immunity.

Bacterial sensing by intracellular Nod proteins and other Nod-like receptors (NLRs) activates signaling pathways that mediate inflammation and pathogen clearance. Nod1 and Nod2 associate with the kinase Rip2 to stimulate NF-kappaB signaling. Other cytosolic NLRs assemble caspase-1-activating multiprotein complexes termed inflammasomes. Caspase-12 modulates the caspase-1 inflammasome, but unlike other NLRs, Nod1 and Nod2 have not been linked to caspases, and mechanisms regulating the Nod-Rip2 complex are less clear. We report that caspase-12 dampens mucosal immunity to bacterial infection independent of its effects on caspase-1. Caspase-12 deficiency enhances production of antimicrobial peptides, cytokines, and chemokines to entric pathogens, an effect dependent on bacterial type III secretion and the Nod pathway. Mechanistically, caspase-12 binds to Rip2, displacing Traf6 from the signaling complex, inhibiting its ubiquitin ligase activity, and blunting NF-kappaB activation. Nod activation and resulting antimicrobial peptide production constitute an early innate defense mechanism, and caspase-12 inhibits this mucosal antimicrobial response.

Alteration of soil rhizosphere communities following genetic transformation of white spruce.

The application of plant genetic manipulations to agriculture and forestry with the aim of alleviating insect damage through Bacillus thuringiensis transformation could lead to a significant reduction in the release of pesticides into the environment. However, many groups have come forward with very valid and important questions related to potentially adverse effects, and it is crucial to assess and better understand the impact that this technology might have on ecosystems. In this study, we analyzed rhizosphere soil samples collected from the first B. thuringiensis-transformed trees [with insertion of the CryIA(b) toxin-encoding gene] grown in Canada (Val-Cartier, QC, Canada) as part of an ecological impact assessment project. Using a robust amplified rRNA gene restriction analysis approach coupled with 16S rRNA gene sequencing, the rhizosphere-inhabiting microbial communities of white spruce (Picea glauca) genetically modified by biolistic insertion of the cryIA(b), uidA (beta-glucuronidase), and nptII genes were compared with the microbial communities associated with non-genetically modified counterparts and with trees in which only the genetic marker genes uidA and nptII have been inserted. Analysis of 1,728 rhizosphere bacterial clones (576 clones per treatment) using a Cramér-von Mises statistic analysis combined with a Monte Carlo comparison clearly indicated that there was a statistically significant difference (P < 0.05) between the microbial communities inhabiting the rhizospheres of trees carrying the cryIA(b), uidA, and nptII transgenes, trees carrying only the uidA and nptII transgenes, and control trees. Clear rhizosphere microbial community alterations due to B. thuringiensis tree genetic modification have to our knowledge never been described previously and open the door to interesting questions related to B. thuringiensis genetic transformation and also to the impact of commonly used uidA and nptII genetic marker genes.