Epithelial inflammasomes

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 gas dermin 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, tumor formation, metabolic and neurodegenerative diseases (Ippagunta SK et al. 2011).

Activation of epithelial inflammasomes in response to invading pathogens has important cell-autonomous effects on the infected cell itself and on its communication with other cell types at local and global levels (Sellin ME et al. 2018). These downstream consequences of inflammasome activation can be divided into three categories (Sellin ME et al. 2018).

1. Death of the epithelial cell itself
2. Release of soluble proinflammatory molecules
3. Recruitment and activation of effector cells.

In addition, activation of the epithelial inflammasome induces contraction of the epithelial layers and prevents loss of integrity at later stages of infection (Fattinger SA et al. 2021). To maintain the integrity of the epithelial barrier, cell death and subsequent elimination of the infected cell must occur in a coordinated manner so that the gap in the epithelium can be closed by neighboring cells. Epithelial cell death can be triggered in a direct, cell-autonomous manner by inflammasome activation itself, as well as by local recruitment of other death-inducing cell types or global inflammation, leading to increased epithelial turnover that removes both infected and uninfected cells. The most important consequence of epithelial cell death is the reduction of epithelial pathogen load to maintain barrier integrity (Sellin ME et al. 2018). Inflammasome activation triggers the cleavage, activation and secretion of proinflammatory IL-1β and IL-18. This leads to the recruitment of different types of effector cells, coordinating the innate immune response. The expression of pro-IL-1β depends on signaling through Toll-like receptors. Therefore, intestinal epithelial cells themselves produce only very small amounts of IL-1β. Pro-IL-18, on the other hand, is constitutively expressed by various types of epithelial cells and is readily secreted upon inflammasome activation. IL-18 secreted by the epithelium can induce the production of IFN-γ by different cell types. Of particular importance, IL-18 induced by the inflammasome is also involved in the recruitment of natural killer (NK) cells, which play a crucial role in early stages of the innate immune response. In addition, IL-18 can also stimulate the effector functions of NK cells that have accumulated at the site of infection. The activated NK cells may then help to limit the pathogen burden and spread to systemic sites at later time points compared to epithelial autonomous induction of cell death. Finally, NK cells also secrete IFN-γ to recruit other inflammatory cell types.

In any case, the urinary bladder epithelium secretes high amounts of IL-1β during bacterial infections. This shows that IL-1beta secretion is dependent on the NLRP3 inflammasome and caspase-1 and that the secreted proinflammatory cytokine is required for the recruitment of mast cells to the site of infection. The mast cells then induce a lytic form of cell death in the epithelium. In addition to NK cells and mast cells, neutrophils are other important innate immune effector cells that infiltrate the infected tissue after pathogens have breached the epithelial barriers. Both IL-1β and IL-18, which are secreted in response to inflammasome activation, are involved in neutrophil recruitment. These are able to immobilize and eliminate the invading pathogens. In addition, they secrete inflammatory mediators such as IFN-gamma and interleukin-22.

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