NLRC4 Inflammasome

Inflammasomes are instruments of the innate immune system that are responsible for the activation of inflammatory reactions. Inflammasomes are cytosolic (intracellular) multi-protein complexes (Martinon F et al. 2002), which are predominantly found in immune cells, such as dendritic cells and macrophages, but also in epithelia of the skin and mucous membranes (intestinal and urinary bladder epithelia). Activation of the inflammasome promotes the proteolytic cleavage, maturation and secretion of the pro-inflammatory cytokines interleukin 1β (IL-1β) and interleukin 18 (IL-18) as well as the cleavage of gasdermin D. The N-terminal fragment resulting from gasdermin D cleavage, triggers a pro-inflammatory form of programmed cell death of pyroptosis and is responsible for the secretion of mature cytokines, presumably through the formation of pores in the plasma membrane (Broz P et al. 2016). In addition, inflammasomes can also trigger a special form of programmed cell death, PANOptosis, which has essential features of apoptosis, pyroptosis and necroptosis (Zhuang L et al.2023).

Inflammasome activation is triggered by different types of cytosolic pattern recognition receptors (PRRs), either microbe-derived (pathogen-associated molecular patterns/PAMPs) or host cell-derived (damage-associated molecular patterns/DAMPs). The pattern recognition receptors involved in inflammasomes include so-called NLRs (nucleotide-binding oligomerization domain and leucine-rich repeat-containing receptors) as well as AIM2 (absent in melanoma 2), IFI16 (IFN-inducible protein 16) and pyrin (Broz P et al. 2016).

The inflammasome receptors interact with the adaptor protein ASC via their caspase activation and recruitment domain (CARD) or via the pyrin domain (PYD), which then activates caspase-1 via its CARD domain by proteolytic cleavage (Broz P et al. 2016). Finally, the activated caspase-1 cleaves the immature pro-inflammatory cytokines pro-IL-1beta and pro-IL-18 as well as gas dermin D into their active end stages. IL 1beta and IL 18 in particular are responsible for inflammatory signaling and pyroptotic cell death.

In addition to these so-called canonical inflammasomes, non-canonical inflammasome complexes have also been described that act independently of caspase-1. In animal experiments, non-canonical inflammasomes can be activated by direct recognition of cytosolic bacterial lipopolysaccharide (LPS). In human cells, the corresponding caspases of the non-canonical inflammasome are caspase 4 and caspase 5 (Broz P et al. 2016).

So far, inflammasomes have mainly been detected in professional immune cells of the innate immune system, such as macrophages and neutrophils. However, it is now known that inflammasome components are expressed in epithelial barrier tissues (Winsor N et al. 2019). In the case of dysregulation of inflammasome activation, this can lead to significant disorders of innate immunity, chronic inflammatory states, tumorformation, metabolic and neurodegenerative diseases (Ippagunta SK et al. 2011).

NLRC4 is the subgroup of the NLRC family that forms an inflammasome and, in addition to NOD and LRR, contains only one CARD domain, which it uses to directly recruit the adapter protein ASC or pro-caspase-1. The NAIP/NLRC4 inflammasome is involved in host defense (Liu L et al. 2014). In humans, NAIPs are activated by binding to the bacterial PAMPs in the cytosol, which are provided by the needle components (NAIP1) of the bacterial type 3 secretion system of flagella. Upon ligand binding, NAIPs interact with NLRC4 to initiate assembly of the NAIP/NLRC4 inflammasome, which then recruits and activates pro-caspase-1 via its CARD domain. The NAIP/NLRC4 inflammasome is the best described epithelial inflammasome and plays an important role in restricting intraepithelial bacterial populations in early stages of enterobacterial infection, e.g. Salmonella and Citrobacter rodentium (Sellin ME et al. 2015: Sellin ME et al. 2018). Intracellular bacteria trigger activation of the inflammasome, leading to targeted expulsion of infected epithelial cells from the epithelium to reduce the bacterial load.

1. Broz P et al. (2016) Inflammasomes: mechanism of assembly, regulation and signaling. Nature Reviews. Immunology 16: 407-420.
2. Duncan JA et al. (2009) Neisseria gonorrhoeae activates the proteinase cathepsin B to mediate the signaling activities of the NLRP3 and ASC-containing inflammasome. Journal of Immunology 182: 6460-6469.
3. Fattinger SA et al. (2021) Epithelium-autonomous NAIP/NLRC4 prevents TNF-driven inflammatory destruction of the gut epithelial barrier in Salmonella-infected mice. Mucosal Immunology 14: 615-629.
4. Ippagunta SK et al. (2011) The inflammasome adaptor ASC regulates the function of adaptive immune cells by controlling Dock2-mediated Rac activation and actin polymerization. Nature Immunology 12: 1010-1016
5. Liu L et al. (2014) IPAF inflammasome is involved in interleukin-1β production from astrocytes, induced by palmitate; implications for Alzheimer's Disease". Neurobiology of Aging 35: 309-321.
6. Martinon F et al. (2002) The inflammasome: a molecular platform triggering activation of inflammatory caspases and processing of proIL-beta. Molecular Cell 10: 417-426.
7. Martinon F et al. (2006) Gout-associated uric acid crystals activate the NALP3 inflammasome. Nature 440: 237-241.
8. Sellin ME et al. (2015) Inflammasomes of the intestinal epithelium. Trends in Immunology 36: 442-450.
9. Sellin ME et al. (2018) Consequences of Epithelial Inflammasome Activation by Bacterial Pathogens. Journal of Molecular Biology. Mechanisms of Inflammasome Activation 430: 193-206.
10. Stutz A et al. (2009) Inflammasomes: too big to miss. The Journal of Clinical Investigation 119: 3502-3511.
11. Winsor N et al. (2019) Canonical and noncanonical inflammasomes in intestinal epithelial cells. Cellular Microbiology 21: e13079.
12. Yazdi AS et al.(2010) Nanoparticles activate the NLR pyrin domain containing 3 (Nlrp3) inflammasome and cause pulmonary inflammation through release of IL-1α
13. Zhuang L et al.(2023) A comprehensive analysis of PANoptosome to prognosis and immunotherapy response in pan-cancer. Sci Rep 13:3877.