Macrophages mediate crucial innate immune responses via caspase-1-dependent processing and secretion of IL-1β and IL-18. While wild type Salmonella typhimurium infection is lethal to mice, a strain that persistently expresses flagellin was cleared by the cytosolic flagellin detection pathway via NLRC4 activation of caspase-1; however, this clearance was independent of IL-1β and IL-18. Instead, caspase-1 induced pyroptotic cell death released bacteria from macrophages, exposing them to uptake and killing by reactive oxygen species in neutrophils. Similarly, caspase-1 cleared Legionella and Burkholderia by cytokine independent mechanisms. Our results show, for the first time, that caspase-1 can clear intracellular bacteria in vivo independent of IL-1β and IL-18, and establish pyroptosis as an efficient mechanism of bacterial clearance by the innate immune system.

Caspase-1-induced pyroptosis is an innate immune effector mechanism against intracellular bacteria (111.33)

The NLRP3 inflammasome is a multimeric cytosolic protein complex that assembles in response to cellular perturbations. This assembly leads to the activation of caspase-1, which promotes maturation and release of the inflammatory cytokines interleukin-1β (IL-1β) and IL-18, as well as inflammatory cell death (pyroptosis). The inflammatory cytokines contribute to the development of systemic low-grade inflammation, and aberrant NLRP3 activation can drive a chronic inflammatory state in the body to modulate the pathogenesis of inflammation-associated diseases. Therefore, targeting NLRP3 or other signaling molecules downstream, such as caspase-1, IL-1β or IL-18, has the potential for great therapeutic benefit. However, NLRP3 inflammasome-mediated inflammatory cytokines play dual roles in mediating human disease. While they are detrimental in the pathogenesis of inflammatory and metabolic diseases, they have a beneficial role in numerous infectious diseases and some cancers. Therefore, fine tuning of NLRP3 inflammasome activity is essential for maintaining proper cellular homeostasis and health. In this Review, we will cover the mechanisms of NLRP3 inflammasome activation and its divergent roles in the pathogenesis of inflammation-associated diseases such as cancer, atherosclerosis, diabetes and obesity, highlighting the therapeutic potential of targeting this pathway.

NLRP3 inflammasome in cancer and metabolic diseases.

The crosstalk between inflammation and tumorigenesis is now clearly established. However, how inflammation is elicited in the metastatic environment and the corresponding contribution of innate immunity pathways in suppressing tumor growth at secondary sites are poorly understood. Here, we show that mice deficient in Nlrp3 inflammasome components had exacerbated liver colorectal cancer metastatic growth, which was mediated by impaired interleukin-18 (IL-18) signaling. Control of tumor growth was independent of differential cancer cell colonization or proliferation, intestinal microbiota effects, or tumoricidal activity by the adaptive immune system. Instead, the inflammasome-IL-18 pathway impacted maturation of hepatic NK cells, surface expression of the death ligand FasL, and capacity to kill FasL-sensitive tumors. Our results define a regulatory signaling circuit within the innate immune system linking inflammasome activation to effective NK-cell-mediated tumor attack required to suppress colorectal cancer growth in the liver.

The Nlrp3 inflammasome suppresses colorectal cancer metastatic growth in the liver by promoting Natural killer cell tumoricidal activity

The past decade has witnessed major breakthroughs in cancer immunotherapy. This development has been largely motivated by cancer cell evasion of immunological control and consequent tumor resistance to conventional therapies. Immunogenic cell death (ICD) is considered one of the most promising ways to achieve total tumor cell elimination. It activates the T-cell adaptive immune response and results in the formation of long-term immunological memory. ICD can be triggered by many anticancer treatment modalities, including photodynamic therapy (PDT). In this review, we first discuss the role of PDT based on several classes of photosensitizers, including porphyrins and non-porphyrins, and critically evaluate their potential role in ICD induction. We emphasize the emerging trend of ICD induction by PDT in combination with nanotechnology, which represents third-generation photosensitizers and involves targeted induction of ICD by PDT. However, PDT also has some limitations, including the reduced efficiency of ICD induction in the hypoxic tumor microenvironment. Therefore, we critically evaluate strategies for overcoming this limitation, which is essential for increasing PDT efficiency. In the final part, we suggest several areas for future research for personalized cancer immunotherapy, including strategies based on oxygen-boosted PDT and nanoparticles. In conclusion, the insights from the last several years increasingly support the idea that PDT is a powerful strategy for inducing ICD in experimental cancer therapy. However, most studies have focused on mouse models, but it is necessary to validate this strategy in clinical settings, which will be a challenging research area in the future.

https://jitc.bmj.com/content/jitc/9/1/e001926.full.pdf

Targeting immunogenic cancer cell death by photodynamic therapy: past, present and future.

/pdf/colitis-induced-in-mice-with-dextran-sulfate-sodium-dss-is-4hy95j7x8k.pdf

Colitis induced in mice with dextran sulfate sodium (DSS) is mediated by the NLRP3 inflammasome

Cell biology of inflammasome activation

Colorectal cancer is a multifactorial disease and a leading cause of cancer-related deaths worldwide. Inflammation is a driver across multiple stages in the development of colorectal cancer. The inflammasome is a cytosolic multiprotein complex of the innate immune system central to the regulation of inflammation, pyroptosis, and other cellular processes important for maintaining gut homeostasis. Studies using mouse models of colitis and colitis-associated colorectal cancer have highlighted diverse and sometimes contrasting roles of inflammasomes in maintaining a balance between intestinal barrier function and the gut microbiota. In addition, persistent and/or dysregulated stimulation of inflammasome sensors finetune inflammation and tumorigenesis in the intestine. This review highlights the emerging role of inflammasome signaling in colitis and colitis-associated colorectal cancer. We also review the key mechanisms by which inflammasome signaling modulate inflammation and tumor development. Finally, we speculate the importance of using more tightly regulated experimental approaches to examine the role of gut microbiota in colorectal cancer.

/pdf/inflammasomes-in-colitis-and-colorectal-cancer-mechanism-of-31zabhm3p6.pdf

Inflammasomes in Colitis and Colorectal Cancer: Mechanism of Action and Therapies.

The pore-forming gasdermin proteins mediate a lytic and proinflammatory form of cell death called pyroptosis and have been linked to the host defense against infection. Two recent studies published in Nature revealed that induction of pyroptosis in tumor cells promotes anti-tumor activity, highlighting gasdermins as potential new targets in cancer immunotherapy. Gasdermins are a family of pore-forming proteins expressed in immune and non-immune cells. Cells that have been infected by pathogens or exposed to danger-associated signals undergo pyroptosis mediated by gasdermins. In this process, inflammatory caspases, apoptotic caspases and serine proteases induce proteolytic cleavage of gasdermin proteins, releasing the active N-terminal fragment. The N-terminal cleavage product forms pores on the plasma membrane, which leads to pyroptosis and the release of intracellular contents, including pro-inflammatory cytokines. This terminal event removes infected or damaged cells and contributes to the killing and clearance of pathogens. The lytic and immunogenic nature of pyroptosis might be an ideal attribute for cancer immunotherapy, because lysis of cancer cells ensures their demise, and immunogenic elicitation of the immune system might further accelerate destruction of cancer cells. Zhang et al. and Wang et al. investigated the possibility of harnessing the activity of gasdermin proteins for use in anti-tumor immunity (Fig. 1). Mutations in gasdermins have been identified in many types of cancers, including breast cancer, gastric cancer and colorectal cancer. Zhang et al. identified that 91% (20 of 22) of the mutations in the gene encoding gasdermin E (GSDME) from cancer patients are loss of function. To investigate the role of GSDME in cancer biology, the authors genetically deleted Gsdme in two mouse cell lines, triple negative breast cancer EMT6 and colorectal cancer CT26, and implanted these cells in immunocompetent mice. Implanted Gsdme tumors derived from both cell lines grew faster compared to control tumors expressing endogenous GSDME. In addition, the authors took advantage of two other cell lines with low endogenous amounts of GSDME (melanoma B16-F10 and triple negative breast cancer 4T1E) and engineered them to express either wild-type GSDME or GSDME carrying a F2A mutation that impairs its ability to form pores. Following implantation, cancer cells expressing wild-type GSDME were markedly impaired in their ability to proliferate compared to cells expressing the mutant form of GSDME, highlighting the importance of pore-forming activity of GSDME in the control of tumor growth. The anti-tumor activity seen in GSDME might extend to other pore-forming gasdermins. Wang and colleagues used a bioorthogonal system which enables a controlled release of gasdermin proteins in tumor cells. The authors generated a nanoparticle (NP)-gasdermin A3 (GSDMA3) conjugate (NPGSDMA3), which carries a linker that can be specifically desilylated by the compound phenylalanine trifluoroborate (Phe-BF3), releasing active GSDMA3. Using this system, the authors treated mice carrying implanted 4T1 or EMT6 tumors with either NP-GSDMA3, Phe-BF3 or in combination. Indeed, only the treatment regimen of NP-GSDMA3 and Phe-BF3 in combination delivered over three rounds led to substantial tumor regression. Further, treatment with a single round of NPGSDMA3 and Phe-BF3 in combination was effective when a checkpoint inhibitor anti-PD1 antibody was administered. These data demonstrate that controlled activation of pyroptosis in tumor cells can induce anti-tumor immunity against implantable tumors, an effect that can be enhanced by checkpoint blockade. Both research groups further profiled the cellular changes within the tumor microenvironment following pyroptosis. Zhang and colleagues observed substantially more tumor-infiltrating natural killer (NK) cells and tumor-associated macrophages in tumors engineered to express GSDME compared to that in control tumors, and that more CD8 tumor-infiltrating lymphocytes in tumors engineered to express GSDME expressed granzyme B, perforin, interferon-γ (IFN-γ) and tumor necrosis factor (TNF). In agreement with this finding, Wang and colleagues observed a marked increase in infiltrating CD3 T cells (both CD4 and CD8 subsets) and a decrease in CD4 FOXP3 T regulatory cells in tumors treated with NP-GSDMA3 and Phe-BF3 in combination. To further confirm the requirement of the immune system in mediating gasdermin-dependent tumor suppression, both groups performed experiments on immunodeficient mice lacking specific immune cell subsets. Zhang and colleagues found that the antitumor effect of GSDME was compromised in NSG (non-obese diabetic, severe combined immunodeficient, interleukin-2receptor-γ null) mice deficient in B cells, T cells and functional NK cells. Similarly, Wang and colleagues found that athymic Nu/ Nu mice lacking mature T cells had an impaired ability to execute GSDMA3-mediated tumor control. Furthermore, both groups showed that antibody-mediated depletion of CD4 T cells, CD8 T cells and/or NK cells led to impaired gasdermin-dependent suppression of tumor growth in BALB/c mice. The importance of killer lymphocytes in gasdermin-dependent tumor suppression suggests that these cell subsets might trigger pyroptosis in cancer cells. Indeed, Zhang and colleagues showed that the human NK cell line YT induced cleavage of GSDME and pyroptosis in HeLa cells. Granzymes and perforin are normally secreted by cytotoxic lymphocytes, including NK cells, to be

/pdf/gasdermins-deliver-a-deadly-punch-to-cancer-4q9ln27s44.pdf

Gasdermins deliver a deadly punch to cancer

Abstract Inflammasomes are cytosolic signaling complexes capable of sensing microbial ligands to trigger inflammation and cell death responses. Here, we show that guanylate-binding proteins (GBPs) mediate pathogen-selective inflammasome activation. We show that mouse GBP1 and GBP3 are specifically required for inflammasome activation during infection with the cytosolic bacterium Francisella novicida . We show that the selectivity of mouse GBP1 and GBP3 derives from a region within the N-terminal domain containing charged and hydrophobic amino acids, which binds to and facilitates direct killing of F. novicida and Neisseria meningitidis , but not other bacteria or mammalian cells. This pathogen-selective recognition by this region of mouse GBP1 and GBP3 leads to pathogen membrane rupture and release of intracellular content for inflammasome sensing. Our results imply that GBPs discriminate between pathogens, confer activation of innate immunity, and provide a host-inspired roadmap for the design of synthetic antimicrobial peptides that may be of use against emerging and re-emerging pathogens. 

/pdf/pathogen-selective-killing-by-guanylate-binding-proteins-as-3fjsvupp.pdf

Pathogen-selective killing by guanylate-binding proteins as a molecular mechanism leading to inflammasome signaling

Clostridium species are a group of Gram-positive bacteria that cause diseases in humans, such as food poisoning, botulism, and tetanus. Here, we analyzed 10 different Clostridium species and identified that Clostridium septicum, a pathogen that causes sepsis and gas gangrene, activates the mammalian cytosolic inflammasome complex in mice and humans. Mechanistically, we demonstrate that α-toxin secreted by C. septicum binds to glycosylphosphatidylinositol (GPI)–anchored proteins on the host plasma membrane, oligomerizing and forming a membrane pore that is permissive to efflux of magnesium and potassium ions. Efflux of these cytosolic ions triggers the activation of the innate immune sensor NLRP3, inducing activation of caspase-1 and gasdermin D, secretion of the proinflammatory cytokines interleukin-1β and interleukin-18, pyroptosis, and plasma membrane rupture via ninjurin-1. Furthermore, α-toxin of C. septicum induces rapid inflammasome-mediated lethality in mice and pharmacological inhibition of the NLRP3 inflammasome using MCC950 prevents C. septicum–induced lethality. Overall, our results reveal that cytosolic innate sensing of α-toxin is central to the recognition of C. septicum infection and that therapeutic blockade of the inflammasome pathway may prevent sepsis and death caused by toxin-producing pathogens. Description Clostridium septicum toxin-induced lethality in mice can be prevented by inhibition of the NLRP3 inflammasome. Toxin-triggered tribulations Lurking within the resident human intestinal microbiota are Clostridium bacteria with the capacity to cause rare but serious deep tissue infections like gas gangrene after hematogenous dissemination. Jing et al. analyzed the downstream cellular signaling pathways triggered when Clostridium septicum α-toxin bound to the plasma membrane surface, triggering pore formation followed by potassium and magnesium ion efflux. The α-toxin–induced ion fluxes activated NLRP3 inflammasomes, leading to gasdermin D–dependent pore formation and cell death by pyroptosis that could be blocked by the NLRP3-specific inhibitor MCC950. These findings indicate that pharmacological inhibition of the NLRP3 inflammasome may provide a new option for the treatment of life-threatening C. septicum infections.

Cheng Shen

Papers

Cell biology of inflammasome activation

Inflammasomes in Colitis and Colorectal Cancer: Mechanism of Action and Therapies.

Gasdermins deliver a deadly punch to cancer

Pathogen-selective killing by guanylate-binding proteins as a molecular mechanism leading to inflammasome signaling

Clostridium septicum α-toxin activates the NLRP3 inflammasome by engaging GPI-anchored proteins