Caspase-11 non-canonical inflammasome: a critical sensor of intracellular lipopolysaccharide in macrophage-mediated inflammatory responses

Abstract

Inflammatory responses mediated by macrophages are part of the innate immune system, whose role is to protect against invading pathogens. Lipopolysaccharide (LPS) found in the outer membrane of Gram-negative bacteria stimulates an inflammatory response by macrophages. During the inflammatory response, extracellular LPS is recognized by Toll-like receptor 4, one of the pattern recognition receptors that activates inflammatory signalling pathways and leads to the production of inflammatory mediators. The innate immune response is also triggered by intracellular inflammasomes, and inflammasome activation induces pyroptosis and the secretion of pro-inflammatory cytokines such as interleukin-1β (IL-1β) and IL-18 by macrophages. Cysteine-aspartic protease (caspase)-11 and the human orthologues caspase-4/caspase-5 were recently identified as components of the 'non-canonical inflammasome' that senses intracellular LPS derived from Gram-negative bacteria during macrophage-mediated inflammatory responses. Direct recognition of intracellular LPS facilitates the rapid oligomerization of caspase-11/4/5, which results in pyroptosis and the secretion of IL-1β and IL-18. LPS is released into the cytoplasm from Gram-negative bacterium-containing vacuoles by small interferon-inducible guanylate-binding proteins encoded on chromosome 3 (GBP^chr3 )-mediated lysis of the vacuoles. In vivo studies have clearly shown that caspase-11^-/- mice are more resistant to endotoxic septic shock by excessive LPS challenge. Given the evidence, activation of caspase-11 non-canonical inflammasomes by intracellular LPS is distinct from canonical inflammasome activation and provides a new paradigm in macrophage-mediated inflammatory responses.

Keywords: Gram-negative bacteria; inflammation; intracellular lipopolysaccharide; macrophage; non-canonical inflammasome.

Figure 1

Structures of canonical inflammasomes. (a–c) Three different NLRs, and (d) AIM2 assembly of canonical inflammasomes through binding with pro‐caspase‐1. (a) NLRC4 and (b) NLRP1 (lack of [PYD] in mouse NLRP1 isoforms) assemble canonical inflammasomes by interacting directly with inactive pro‐caspase‐1 through the CARD motif, whereas (c) NLRP3 and (d) AIM2 assemble canonical inflammasomes by interacting indirectly with pro‐caspase‐1 via the bipartite PYD–CARD adaptor protein ASC. After assembly of canonical inflammasomes, pro‐caspase‐1 is cleaved and matures to an active form. Abbreviations used: LRR, leucine‐rich repeat; NRL, nucleotide‐binding oligomerization domain (NOD)‐like receptor; caspase, cysteine‐aspartic protease; CARD, caspase recruit domain; NACHT, nucleotide‐binding and oligomerization domain; FIIND, function to find domain; AIM2, absent in melanoma 2; PYD, pyrin domain; HIN, haematopoietic interferon‐inducible nuclear proteins. *Autocatalytic cleavage.

Figure 2

Structures of non‐canonical inflammasomes. Non‐canonical inflammasomes are assembled by direct interaction of the lipid A moiety of lipopolysaccharide (LPS) with the CARD motif of (a) pro‐caspase‐11 (mouse) or (b) pro‐caspase‐4/5 (human). (c) Activation of non‐canonical inflammasomes by oligomerization of caspase‐11 (mouse) or caspase‐4/5 (human) through their CARD motifs. Unlike canonical inflammasomes, non‐canonical inflammasomes form complexes in the absence of a bipartite adaptor protein, such as ASC; instead, the lipid A moiety of LPS directly interacts with the CARD motif of the caspase. Caspase, cysteine‐aspartic protease; CARD, caspase recruit domain.

Figure 3

Intracellular LPS‐mediated caspase‐4/5/11 non‐canonical inflammasome activation. (a) LPS‐containing OMVs derived from Gram‐negative bacteria is internalized into a macrophage by endocytosis, and (b) LPS of the OMVs is released into the cytoplasm from the Gram‐negative bacterium‐containing vacuole by GBP ^chr3‐mediated lysis. (c) The released intracellular LPS binds directly to pro‐caspase‐11 and to pro‐caspase‐4/5/11 CARD motifs through the lipid A moiety, (d) leading to oligomerization of pro‐caspase‐4/5/11 to induce non‐canonical inflammasome activation. (e) Active caspase‐4/5/11 non‐canonical inflammasome activates the NLRP3 canonical inflammasome via an unknown mechanism, (f) inducing the proteolysis and maturation of pro‐caspase‐1. (g) Cleaved active caspase‐1, in turn, induces the proteolysis and maturation of the pro‐inflammatory cytokines IL‐1β and IL‐18. (h) Active caspase‐4/5/11 non‐canonical inflammasomes also cleave the linker loop of intact GSDMD to generate N‐ and C‐terminal fragments of GSDMD. (i) The cleaved N‐terminal fragments of GSDMD are localized to the cell membrane, bind to phosphoinositides of the cell membrane, and oligomerize to form membrane pores, resulting in cell swelling and lysis, known as pyroptosis. (j) Proteolysed mature IL‐1β and IL‐18 are secreted through GSDMD pores. Abbreviations used:OMV, outer membrane vesicle; Caspase, cysteine‐aspartic protease; GBP, guanylate‐binding protein; immunity‐related GTPase family member b10, IRGB10; LPS, lipopolysaccharide; GSDMD, gasdermin D; IL, interleukin; N, N‐terminal fragment of GSDMD; C, C‐terminal fragment of GSDMD; CARD, caspase recruit domain; NACHT, nucleotide‐binding and oligomerization domain; PYD, pyrin domain.