Non-canonical inflammasome activation targets caspase-11

Abstract

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.

CASP4/caspase-11 promotes autophagosome formation in response to bacterial infection

“…Recognition of intracellular B. cenocepacia in murine macrophages is mediated through NOD-like receptors including NLRP3 and MEFV/PYRIN [19,20]. Non-canonical NLRP3-PYCARD/ASC-dependent inflammasome activation in response to Gram-negative pathogens requires CASP4 [15]. Here, we report that both CASP4 and MEFV/PYRIN are required for B. cenocepacia -induced CASP1 activation.…”

“…S1B-C) and a 6-fold increase at 6 h compared to non-infected control cells (Figure 1e and g). Because CASP4 stimulates non-canonical NLRP3 inflammasome activation in response to Gram-negative bacteria [15], we further analyzed if CASP4 contributes to B. cenocepacia -induced CASP1 activation. The processing of CASP1 in WT and casp4 −/- macrophages was compared via immunoblot.…”

“…The lack of CASP4, rather than CASP1, protects against lipopolysaccharide-induced septic shock due to reduced secretion of the pro-inflammatory cytokines IL1A and IL1B [15,40]. Yet, casp4 −/- mice are highly susceptible to infection with virulent B. pseudomallei and avirulent B. thailandensis resulting in increased mortality [18].…”

“…Activation of CASP1/caspase-1 within a molecular platform comprising cytosolic NOD-like receptors (NLRs) and various adaptor proteins drives the secretion of pro-inflammatory cytokines such as IL1B/IL-1β (interleukin 1 beta) and IL18, which can be accompanied by plasma membrane disruption and host cell death (pyroptosis). As part of the non-canonical inflammasome, CASP4/caspase-11 contributes to cytokine release and bacterial restriction either upstream or independently of CASP1 [15–18]. B. cenocepacia activates CASP1 via NLRP3 in murine macrophages [19] and via MEFV/PYRIN in human monocytes [20] in a type VI secretion system (T6SS)-dependent manner, resulting in pronounced inflammation.…”

GSDMD membrane pore formation constitutes the mechanism of pyroptotic cell death

“…Initial work has identified the pro‐inflammatory cytokine interleukin (IL)‐1β as a key substrate of caspase‐1 (Thornberry et al , 1992). Subsequently, it was found that caspase‐1, as well as caspase‐11 and its human orthologs caspase‐4 and caspase‐5, induces a novel programmed cell death pathway that is characterized by cell swelling, lysis, and the release of cytoplasmic content (Fink & Cookson, 2007; Kayagaki et al , 2011; Shi et al , 2014), presumably as a result of the formation of membrane pores (Fink et al , 2008). Since this type of cell death is morphologically distinct from apoptosis and intrinsically pro‐inflammatory, it was named pyroptosis, from the Greek pyro (fire or fever) and ptosis (to fall) (Bergsbaken et al , 2009).…”