Critical Roles of ASC Inflammasomes in Caspase-1 Activation and Host Innate Resistance to Streptococcus pneumoniae Infection

Abstract

Streptococcus pneumoniae is a Gram-positive, extracellular bacterium that is responsible for significant mortality and morbidity worldwide. Pneumolysin (PLY), a cytolysin produced by all clinical isolates of the pneumococcus, is one of the most important virulence factors of this pathogen. We have previously reported that PLY is an essential factor for activation of caspase-1 and consequent secretion of IL-1β and IL-18 in macrophages infected with S. pneumoniae. However, the host molecular factors involved in caspase-1 activation are still unclear. To further elucidate the mechanism of caspase-1 activation in macrophages infected with S. pneumoniae, we examined the involvement of inflammasomes in inducing this cellular response. Our study revealed that apoptosis-associated speck like protein containing a caspase recruitment domain (ASC), an adaptor protein for inflammasome receptors such as NLR family, pyrin domain containing 3 (NLRP3) and absent in melanoma 2 (AIM2), is essentially required for the induction of caspase-1 activation by S. pneumoniae. Caspase-1 activation was partially impaired in NLRP3−/− macrophages, while knockdown and knockout of AIM2 resulted in a clear decrease in caspase-1 activation in response to S. pneumoniae. These results suggest that ASC inflammasomes, including AIM2 and NLRP3, are critical for caspase-1 activation induced by S. pneumoniae. Furthermore, ASC−/− mice were more susceptible than wild-type mice to S. pneumoniae, with impaired secretion of IL-1β and IL-18 into the bronchoalveolar lavage after intranasal infection, suggesting that ASC inflammasomes contribute to the protection of host from infection with PLY-producing S. pneumoniae.

Pneumolysin-Dependent Calpain Activation and Interleukin-1α Secretion in Macrophages Infected with Streptococcus pneumoniae

“…3D). Our previous report demonstrated that S. pneumoniae induction of inflammasome activation requires the phagocytosis of bacterial cells by host macrophages (11). In this study, macrophages were pretreated with cytochalasin B, which abrogates phagocytosis by blocking actin polymerization, and mature IL-1␣ and IL-1␤ secreted into culture supernatant were detected by an ELISA (Fig.…”

“…It was reported previously that several inflammasome activators induce the maturation and secretion of IL-1␣ in a manner dependent on inflammasome components, including ASC (apoptosis-associated speck-like protein containing a CARD [caspase activation and recruitment domain]) and caspase-1 (30)(31)(32)(33). We previously reported that in S. pneumoniae-infected macrophages, the AIM2 and NLRP3 inflammasomes play a critical role in caspase-1 activation and the subsequent processing and secretion of IL-18 and IL-1␤ (11). Based on these assumptions, we hypothesized that S. pneumoniae induces IL-1␣ production through inflammasome activation.…”

“…Several reports have shown that the Δply strain, an S. pneumoniae mutant deficient for the ply gene, is incapable of inducing the production of IL-1␣, IL-1␤, and IL-18 (8)(9)(10)(11)(12). IL-1 family cytokines, including IL-1␣, IL-1␤, and IL-18, have been reported to contribute to host defense and tissue damage in pneumococcal infection models in mice (13)(14)(15)(16)(17)(18).…”

“…Therefore, the PLY-dependent secretion of these cytokines could contribute to both host protection and pathogenesis during S. pneumoniae infection. To understand the mechanism of how these cytokine responses are induced in a PLY-dependent manner, we and others previously reported that in S. pneumoniae-infected macrophages, the maturation and secretion of IL-1␤ and IL-18 are dependent on the activation of caspase-1, which is induced by the assembly of inflammasomes, including the absent in melanoma 2 (AIM2) and NLR family pyrin domain-containing 3 (NLRP3) inflammasomes (9)(10)(11)19).…”

The Duality of AIM2 Inflammasome: A Focus on its Role in Autoimmunity and Skin Diseases

“…AIM2 has been shown to provide immune-surveillance to several pathogenic bacteria like Listeria monocytogenes, Streptococcus pneumonia, Staphilococcus aureus and many others (Rathinam et al, 2010;Fernandes-Alnemri et al, 2010;Kim et al, 2010;Warren et al, 2010;Hanamsagar et al, 2014;Tsuchiya et al, 2010;Man et al, 2016). These pathogens activate AIM2 via a 'non canonical' pathway owing to its requirement for type I IFN, analogous to the non-canonical NLRP3 inflammasome pathway (Man et al, 2016) and they must escape the vacuole and undergo bacteriolysis in order to induce the activation of the AIM2 inflammasome (Fang et al, 2011;Kim et al, 2010;Tsuchiya et al, 2010). Some bacteria have evolved virulence determinants to prevent release of DNA to avoid cytoplasmic and clearance by inflammasomes (Crane et al, 2014;Dotson et al, 2013), however there is limited evidence to support the existence of mechanisms used by bacteria to evade or inhibit activation of AIM2, which is overall an extraordinary antimicrobial machinery (Man et al, 2016).…”