Uncoupled pyroptosis and IL-1β secretion downstream of inflammasome signaling

Abstract

Inflammasomes are supramolecular platforms that organize in response to various damage-associated molecular patterns and pathogen-associated molecular patterns. Upon activation, inflammasome sensors (with or without the help of ASC) activate caspase-1 and other inflammatory caspases that cleave gasdermin D and pro-IL-1β/pro-IL-18, leading to pyroptosis and mature cytokine secretion. Pyroptosis enables intracellular pathogen niche disruption and intracellular content release at the cost of cell death, inducing pro-inflammatory responses in the neighboring cells. IL-1β is a potent pro-inflammatory regulator for neutrophil recruitment, macrophage activation, and T-cell expansion. Thus, pyroptosis and cytokine secretion are the two main mechanisms that occur downstream of inflammasome signaling; they maintain homeostasis, drive the innate immune response, and shape adaptive immunity. This review aims to discuss the possible mechanisms, timing, consequences, and significance of the two uncoupling preferences downstream of inflammasome signaling. While pyroptosis and cytokine secretion may be usually coupled, pyroptosis-predominant and cytokine-predominant uncoupling are also observed in a stimulus-, cell type-, or context-dependent manner, contributing to the pathogenesis and development of numerous pathological conditions such as cryopyrin-associated periodic syndromes, LPS-induced sepsis, and Salmonella enterica serovar Typhimurium infection. Hyperactive cells consistently release IL-1β without LDH leakage and pyroptotic death, thereby leading to prolonged inflammation, expanding the lifespans of pyroptosis-resistant neutrophils, and hyperactivating stimuli-challenged macrophages, dendritic cells, monocytes, and specific nonimmune cells. Death inflammasome activation also induces GSDMD-mediated pyroptosis with no IL-1β secretion, which may increase lethality in vivo. The sublytic GSDMD pore formation associated with lower expressions of pyroptotic components, GSDMD-mediated extracellular vesicles, or other GSDMD-independent pathways that involve unconventional secretion could contribute to the cytokine-predominant uncoupling; the regulation of caspase-1 dynamics, which may generate various active species with different activities in terms of GSDMD or pro-IL-1β, could lead to pyroptosis-predominant uncoupling. These uncoupling preferences enable precise reactions to different stimuli of different intensities under specific conditions at the single-cell level, promoting cooperative cell and host fate decisions and participating in the pathogen "game". Appropriate decisions in terms of coupling and uncoupling are required to heal tissues and eliminate threats, and further studies exploring the inflammasome tilt toward pyroptosis or cytokine secretion may be helpful.

Keywords: ASC; caspase-1; death inflammasome; hyperactivation; inflammasome; inflammation; interleukin-1β; pyroptosis.

Figure 1

Pyroptotic cell death occurs before cell lysis. The separation of GSDMD pore formation and cell membrane rupture are evidenced by the differential release mechanisms of intracellular contents: IL-1β is secreted through GSDMD pores and ruptured cell membranes, and HMGB1 and LDH leak (if not always) via cell lysis. Cell swelling and plasma membrane rupture could be active events mediated by calpain and NINJ1, although unbalanced osmotically permeability induced by GSDMD pore formation may also contribute to passive lysis.

Figure 2

Cytokine processing downstream inflammasome signaling in human cells. IL-1β and IL-18 precursors are inactive and require cleavage by caspase-1 into their mature forms; IL-1α and IL-37 precursors are active, with mature forms that increase the biological activity and receptor affinity. IL-1α and IL-37 could also shuttle to the nucleus to exert their pro-/anti-inflammatory functions.

Figure 3

Schematic of the ASC speck formation. (A) The PYDs in sensors (e.g., NLRP3) oligomerize as a seed to recruit the PYDs of ASC. (B) This homotypic PYD-PYD interaction creates a unidirectional (A-end to B-end), right-handed, three-start helix as a linear ASC filamentous backbone structure with CARDs exposed and self-interacting outside the filaments, resembling buds on the stem surface. (C) CARD-CARD interaction between ASC and caspase-1 allows caspase-1 recruitment as a left-handed one-start helix, thus increasing the local concentration and promoting activation.

Figure 4

Schematic showing caspase-1 activation and deactivation. (A) Structure of pro-caspase-1 and caspase-1 species generated by dimerization and auto-processing during inflammasome activation. (B) Upon NLRP3 or NLRC4 inflammasome activation in cells expressing ASC, specks recruit pro-caspase-1 molecules, leading to proximity-induced dimerization, generation of p46 species (minor), and later auto-proteolysis; this process first occurs at the IDL to generate p33/p10 species (major), and subsequently at the CDL to create and dislocate the inactive p20/10 species. The p46 species cleaves GSDMD into GSDMD-NT, while p33/p10 species cleaves both GSDMD and IL-1β. In contrast, the p20/p10 species is incompetent for processing GSDMD and IL-1β. Within the ASC speck, caspase-1 activation and deactivation is rapidly promoted with a high turnover. (C) Upon NLRC4 inflammasome activation in ASC deficient cells, the turnover of p46 into p33/p10, and especially p33/p10 into p20/p10 (illustrated with a dotted arrow), slows down; this leads to prolonged caspase-1 activity, with p46 and p33/p10 as the major and minor species, respectively, which may contribute to the pyroptosis-predominant uncoupling without massive IL-1β release. This inflammasome complex is called the death complex.

Figure 5

Schematic showing coupling and uncoupling events downstream of inflammasome signaling. Pyroptotic death and IL-1β secretion are commonly coupled as simultaneous consequences of inflammasome activation. Cytokine-predominant uncoupling is observed in hyperactivating stimuli challenges with mechanisms that depend on GSDMD pores or GSDMD molecules but not the pore formation on the plasma membrane; SARM deficiency may be involved in unconventional IL-1β secretion without pyroptosis in certain cell types. In contrast, death complex exhibits a pyroptosis-predominant uncoupling preference, which may be due to the differential behavior of active caspase-1 species during inflammasome activation.