Caspase-1 activation by inflammasome scaffolds comprised of intracellular nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) and the adaptor ASC is believed to be essential for production of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18 during the innate immune response. Here we show, with C57BL/6 Casp11 gene-targeted mice, that caspase-11 (also known as caspase-4) is critical for caspase-1 activation and IL-1β production in macrophages infected with Escherichia coli, Citrobacter rodentium or Vibrio cholerae. Strain 129 mice, like Casp11(-/-) mice, exhibited defects in IL-1β production and harboured a mutation in the Casp11 locus that attenuated caspase-11 expression. This finding is important because published targeting of the Casp1 gene was done using strain 129 embryonic stem cells. Casp1 and Casp11 are too close in the genome to be segregated by recombination; consequently, the published Casp1(-/-) mice lack both caspase-11 and caspase-1. Interestingly, Casp11(-/-) macrophages secreted IL-1β normally in response to ATP and monosodium urate, indicating that caspase-11 is engaged by a non-canonical inflammasome. Casp1(-/-)Casp11(129mt/129mt) macrophages expressing caspase-11 from a C57BL/6 bacterial artificial chromosome transgene failed to secrete IL-1β regardless of stimulus, confirming an essential role for caspase-1 in IL-1β production. Caspase-11 rather than caspase-1, however, was required for non-canonical inflammasome-triggered macrophage cell death, indicating that caspase-11 orchestrates both caspase-1-dependent and -independent outputs. Caspase-1 activation by non-canonical stimuli required NLRP3 and ASC, but caspase-11 processing and cell death did not, implying that there is a distinct activator of caspase-11. Lastly, loss of caspase-11 rather than caspase-1 protected mice from a lethal dose of lipopolysaccharide. These data highlight a unique pro-inflammatory role for caspase-11 in the innate immune response to clinically significant bacterial infections.

Recent studies have offered a glimpse into the sophisticated mechanisms by which inflammasomes respond to danger and promote secretion of interleukin (IL)-1β and IL-18. Activation of caspases 1 and 11 in canonical and noncanonical inflammasomes, respectively, also protects against infection by triggering pyroptosis, a proinflammatory and lytic mode of cell death. The therapeutic potential of inhibiting these proinflammatory caspases in infectious and autoimmune diseases is raised by the successful deployment of anti-IL-1 therapies to control autoinflammatory diseases associated with aberrant inflammasome signaling. This Review summarizes recent insights into inflammasome biology and discusses the questions that remain in the field.

SUMMARY Crohn’s disease and ulcerative colitis are inflammatory bowel diseases with high prevalence in humans. Nlrp3 interacts with the adaptor protein ASC to activate caspase-1 in inflammasomes, protein complexes responsible for the maturation and secretion of IL-1β and IL-18. Decreased expression of the NOD-like receptor (NLR) Nlrp3 was recently associated with susceptibility to Crohn’s disease. However, the role of Nlrp3 in colitis has not been characterized. Here, we show that mice deficient for Nlrp3 or the inflammasome effectors ASC and caspase-1 are highly susceptible to dextran sodium sulfate (DSS)-induced colitis. Defective inflammasome activation leads to loss of epithelial integrity, resulting in systemic dispersion of commensal bacteria, massive leukocyte infiltration and increased chemokine production in the colon. As a consequence, significantly higher mortality rates were noted for mice lacking components of the Nlrp3 inflammasome. Therefore, the Nlrp3 inflammasome is critically involved in the maintenance of intestinal homeostasis and protection against colitis.

The innate immune system comprises several classes of pattern recognition receptors, including Toll-like receptors (TLRs), NOD-like receptors (NLRs), and RIG-1-like receptors (RLRs). TLRs recognize microbes on the cell surface and in endosomes, whereas NLRs and RLRs detect microbial components in the cytosol. Here we discuss the recent understanding in NLRs. Two NLRs, NOD1 and NOD2, sense the cytosolic presence of the peptidoglycan fragments meso-DAP and muramyl dipeptide, respectively, and drive the activation of mitogen-activated protein kinase (MAPK) and the transcription factor NF-kappaB. A different set of NLRs induces caspase-1 activation through the assembly of large protein complexes named inflammasomes. Genetic variations in several NLR members are associated with the development of inflammatory disorders. Further understanding of NLRs should provide new insights into the mechanisms of host defense and the pathogenesis of inflammatory diseases.

SUMMARY Virus-induced IL-1β and IL-18 production in macrophages is mediated via a caspase-1 pathway. Multiple microbial components, including viral RNA, are thought to trigger assembly of the cryopyrin inflammasome and consequent caspase-1 activation. Here we demonstrate that cryopyrin−/− and caspase-1−/− mice are more susceptible than wildtype controls following infection with a pathogenic influenza A virus. This profile of enhanced morbidity correlates with decreased neutrophil and monocyte recruitment and reduced cytokine and chemokine production. Despite the effect on innate immunity, cryopyrin-deficiency was not associated with any obvious defect in virus control or on the later emergence of the adaptive response. Early epithelial necrosis was, however, more severe in the infected mutants, with extensive collagen deposition leading to later respiratory compromise. These findings reveal a novel function of the cryopyrin inflammasome in healing responses. Cryopyrin and caspase-1 are clearly central to both innate immunity and to moderating lung pathology in influenza pneumonia.