GSDME-Dependent Incomplete Pyroptosis Permits Selective IL-1α Release under Caspase-1 Inhibition

Abstract

Pyroptosis is a form of regulated cell death that is characterized by gasdermin processing and increased membrane permeability. Caspase-1 and caspase-11 have been considered to be essential for gasdermin D processing associated with inflammasome activation. In the present study, we found that NLRP3 inflammasome activation induces delayed necrotic cell death via ASC in caspase-1/11-deficient macrophages. Furthermore, ASC-mediated caspase-8 activation and subsequent gasdermin E processing are necessary for caspase-1-independent necrotic cell death. We define this necrotic cell death as incomplete pyroptosis because IL-1b release, a key feature of pyroptosis, is absent, whereas IL-1a release is induced. Notably, unprocessed pro-IL-1b forms a molecular complex to be retained inside pyroptotic cells. Moreover, incomplete pyroptosis accompanied by IL-1a release is observed under the pharmacological inhibition of caspase-1 with VX765. These findings suggest that caspase-1 inhibition during NLRP3 inflammasome activation modulates forms of cell death and permits the release of IL-1a from dying cells.

Lytic Cell Death Mechanisms in Human Respiratory Syncytial Virus-Infected Macrophages: Roles of Pyroptosis and Necroptosis

“…Thus, inhibition of lytic cell death with ASC-and NLRP3-deficient cells was relatively higher compared to loss of lytic cell death following inhibition of RIPK3-mediated necroptotic pathway. In light of a recent study showing RIPK3-MLKL necroptotic pathway induction by ASC-NLRP3 inflammasome independent of caspase-1 [45], our result suggests a possibility of ASC-NLRP3 inflammasome not only inducing pyroptosis during RSV infection but partially contributing to necroptosis induction during infection via caspase-1 independent mechanism. It is important to note that blocking pyroptosis by using caspase-1 inhibitor reduced lytic cell death by 46% (Figure 4a), which may infer that 46% inhibition in lytic cell death observed with ASC-and NLRP3-deficent cells could be due to loss of pyroptosis, while the rest 26-29% could be due to loss of necroptosis.…”

“…In the absence of scavenger cells to complete phagocytosis of apoptosis-initiated cells, cells may undergo secondary necroptosis with similar morphological and chemical changes as found in primary necroptosis: cell membrane permeability, lysosomal rupture, and cellular swelling [41][42][43][44]. Caspase-8 activation can induce caspase-3 dependent apoptosis but can also induce caspase-3 cleavage of Gasdermin E similar to that seen in caspase-1 dependent pyroptosis through the cleavage of Gasdermin D [45]. This form of pyroptosis, termed incomplete pyroptosis, may account for the loss of LDH release when caspase-3 is inhibited.…”

“…Furthermore, drastic reduction in lytic cell death in RSV-infected ASC-and NLRP3-deficent macrophages suggests that the ASC-NLRP3 inflammasome may also play a partial role in inducing necroptosis. This possibility exists since the ASC-NLRP3 inflammasome activated the RIPK3-MLKL necroptotic pathway [45]. This occurred in caspase-1 deficient or caspase-1 inhibited cells [45].…”

“…This possibility exists since the ASC-NLRP3 inflammasome activated the RIPK3-MLKL necroptotic pathway [45]. This occurred in caspase-1 deficient or caspase-1 inhibited cells [45]. Viruses like influenza A virus and vaccinia virus encode proteins (e.g., NS1 of influenza A, serpins of vaccinia virus) that can inactivate caspase-1 [73].…”

“…Thus, it is plausible that the ASC-NLRP3 inflammasome may also partially contribute (by 20-25%) to activation of the necroptosis pathway during RSV infection. Indeed, a role of ASC-NLRP3 inflammasome in inducing RIPK3-MLKL mediated necroptosis has been recently reported [45].…”