DPP8/9 inhibitors activate the CARD8 inflammasome in resting lymphocytes

The hydrophobicity of the CARD8 N-terminus tunes inflammasome activation

Mounting evidence indicates that proteotoxic stress is a primary activator of the CARD8 inflammasome, but the complete array of signals that control this inflammasome have not yet been established. Notably, we recently discovered that several hydrophobic radical-trapping antioxidants (RTAs), including JSH-23, potentiate CARD8 inflammasome activation through an unknown mechanism. Here, we report that these RTAs directly alkylate several cysteine residues in the N-terminal disordered region of CARD8. These hydrophobic modifications destabilize the repressive CARD8 N-terminal fragment and accelerate its proteasome-mediated degradation, thereby releasing the inflammatory CARD8 C-terminal fragment from autoinhibition. Consistently, we also found that unrelated (non-RTA) hydrophobic electrophiles as well as genetic mutation of the CARD8 cysteine residues to isoleucines similarly potentiate inflammasome activation. Overall, our results not only provide further evidence that protein folding stress is a key CARD8 inflammasome-activating signal, but also indicate that the N-terminal cysteines can play key roles in tuning the response to this stress.

NLRP1 inflammasome promotes senescence and senescence-associated secretory phenotype

BACKGROUND

Senescence is a cellular aging-related process triggered by different stresses and characterized by the secretion of various inflammatory factors referred to as senescence-associated secretory phenotype (SASP), some of which are produced by the NLRP3 inflammasome. Here, we present evidence that the NLRP1 inflammasome is a DNA damage sensor and a key mediator of senescence.

METHODS

Senescence was induced in fibroblasts in vitro and in mice. Cellular senescence was assessed by Western blot analysis of several proteins, including p16, p21, p53, and SASP factors, released in the culture media or serum. Inflammasome components, including NLRP1, NLRP3 and GSDMD were knocked out or silenced using siRNAs.

RESULTS

In vitro and in vivo results suggest that the NLRP1 inflammasome promotes senescence by regulating the expression of p16, p21, p53, and SASP factors in a Gasdermin D (GSDMD)-dependent manner. Mechanistically, the NLRP1 inflammasome is activated in response to genomic damage detected by the cytosolic DNA sensor cGMP-AMP (cGAMP) synthase (cGAS).

CONCLUSION

Our findings show that NLRP1 is a cGAS-dependent DNA damage sensor during senescence and a mediator of SASP release through GSDMD. This study advances the knowledge on the biology of the NLRP1 inflammasome and highlights this pathway as a potential pharmcological target to modulate senescence.

CARD8: A Novel Inflammasome Sensor with Well-Known Anti-Inflammatory and Anti-Apoptotic Activity

Inflammasomes comprise a group of protein complexes with fundamental roles in the induction of inflammation. Upon sensing stress factors, their assembly induces the activation and release of the pro-inflammatory cytokines interleukin (IL)-1β and -18 and a lytic type of cell death, termed pyroptosis. Recently, CARD8 has joined the group of inflammasome sensors. The carboxy-terminal part of CARD8, consisting of a function-to-find-domain (FIIND) and a caspase activation and recruitment domain (CARD), resembles that of NLR family pyrin domain containing 1 (NLRP1), which is recognized as the main inflammasome sensor in human keratinocytes. The interaction with dipeptidyl peptidases 8 and 9 (DPP8/9) represents an activation checkpoint for both sensors. CARD8 and NLRP1 are activated by viral protease activity targeting their amino-terminal region. However, CARD8 also has some unique features compared to the established inflammasome sensors. Activation of CARD8 occurs independently of the inflammasome adaptor protein apoptosis-associated speck-like protein containing a CARD (ASC), leading mainly to pyroptosis rather than the activation and secretion of pro-inflammatory cytokines. CARD8 was also shown to have anti-inflammatory and anti-apoptotic activity. It interacts with, and inhibits, several proteins involved in inflammation and cell death, such as the inflammasome sensor NLRP3, CARD-containing proteins caspase-1 and -9, nucleotide-binding oligomerization domain containing 2 (NOD2), or nuclear factor kappa B (NF-κB). Single nucleotide polymorphisms (SNPs) of CARD8, some of them occurring at high frequencies, are associated with various inflammatory diseases. The molecular mechanisms underlying the different pro- and anti-inflammatory activities of CARD8 are incompletely understood. Alternative splicing leads to the generation of multiple CARD8 protein isoforms. Although the functional properties of these isoforms are poorly characterized, there is evidence that suggests isoform-specific roles. The characterization of the functions of these isoforms, together with their cell- and disease-specific expression, might be the key to a better understanding of CARD8's different roles in inflammation and inflammatory diseases.

Metabolism-inflammasome crosstalk shapes innate and adaptive immunity

Inflammasomes are a central component of innate immunity and play a vital role in regulating innate immune response. Activation of inflammasomes is also indispensable for adaptive immunity, modulating the development and response of adaptive immunity. Recently, increasing studies have shown that metabolic alterations and adaptations strongly influence and regulate the differentiation and function of the immune system. In this review, we will take a holistic view of how inflammasomes bridge innate and adaptive (especially T cell) immunity and how inflammasomes crosstalk with metabolic signals during the immune responses. And, special attention will be paid to the metabolic control of inflammasome-mediated interactions between innate and adaptive immunity in disease. Understanding the metabolic regulatory functions of inflammasomes would provide new insights into future research directions in this area and may help to identify potential targets for inflammasome-associated diseases and broaden therapeutic avenues.

Inhibition of caspase pathways limits CD4+ T cell loss and restores host anti-retroviral function in HIV-1 infected humanized mice with augmented lymphoid tissue

The study of HIV infection and pathogenicity in physical reservoirs requires a biologically relevant model. The human immune system (HIS) mouse is an established model of HIV infection, but defects in immune tissue reconstitution remain a challenge for examining pathology in tissues. We utilized exogenous injection of the human recombinant FMS-like tyrosine kinase 3 ligand (rFLT-3 L) into the hematopoietic stem cell (HSC) cord blood HIS mouse model to significantly expand the total area of lymph node (LN) and the number of circulating human T cells. The results enabled visualization and quantification of HIV infectivity, CD4 T cell depletion and other measures of pathogenesis in the secondary lymphoid tissues of the spleen and LN. Treatment with the Caspase-1/4 inhibitor VX-765 limited CD4+ T cell loss in the spleen and reduced viral load in both the spleen and axillary LN. In situ hybridization further demonstrated a decrease in viral RNA in both the spleen and LN. Transcriptomic analysis revealed that in vivo inhibition of caspase-1/4 led to an upregulation in host HIV restriction factors including SAMHD1 and APOBEC3A. These findings highlight the use of rFLT-3 L to augment human immune system characteristics in HIS mice to support investigations of HIV pathogenesis and test host directed therapies, though further refinements are needed to further augment LN architecture and cellular populations. The results further provide in vivo evidence of the potential to target inflammasome pathways as an avenue of host-directed therapy to limit immune dysfunction and virus replication in tissue compartments of HIV+ persons.

NLRP inflammasomes in health and disease

NLRP inflammasomes are a group of cytosolic multiprotein oligomer pattern recognition receptors (PRRs) involved in the recognition of pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) produced by infected cells. They regulate innate immunity by triggering a protective inflammatory response. However, despite their protective role, aberrant NLPR inflammasome activation and gain-of-function mutations in NLRP sensor proteins are involved in occurrence and enhancement of non-communicating autoimmune, auto-inflammatory, and neurodegenerative diseases. In the last few years, significant advances have been achieved in the understanding of the NLRP inflammasome physiological functions and their molecular mechanisms of activation, as well as therapeutics that target NLRP inflammasome activity in inflammatory diseases. Here, we provide the latest research progress on NLRP inflammasomes, including NLRP1, CARD8, NLRP3, NLRP6, NLRP7, NLRP2, NLRP9, NLRP10, and NLRP12 regarding their structural and assembling features, signaling transduction and molecular activation mechanisms. Importantly, we highlight the mechanisms associated with NLRP inflammasome dysregulation involved in numerous human auto-inflammatory, autoimmune, and neurodegenerative diseases. Overall, we summarize the latest discoveries in NLRP biology, their forming inflammasomes, and their role in health and diseases, and provide therapeutic strategies and perspectives for future studies about NLRP inflammasomes.

Active site-directed probes targeting dipeptidyl peptidases 8 and 9

Dipeptidyl peptidases (DPP) 8 and 9 are intracellular serine proteases that play key roles in various biological processes and recent findings highlight DPP8 and DPP9 as potential therapeutic targets for hematological and inflammasome-related diseases. Despite the substantial progress, the precise biological functions of these proteases remain elusive, and the lack of selective chemical tools hampers ongoing research. In this paper, we describe the synthesis and biochemical evaluation of the first active site-directed DPP8/9 probes which are derived from DPP8/9 inhibitors developed in-house. Specifically, we synthesized fluorescent inhibitors containing nitrobenzoxadiazole (NBD), dansyl (DNS) and cyanine-3 (Cy3) reporters to visualize intracellular DPP8/9. We demonstrate that the fluorescent inhibitors have high affinity and selectivity towards DPP8/9 over related S9 family members. The NBD-labeled DPP8/9 inhibitors were nominated as the best in class compounds to visualize DPP8/9 in human cells. Furthermore, a method has been developed for selective labeling and visualization of active DPP8/9 in vitro by fluorescence microscopy. A collection of potent and selective biotinylated DPP8/9-targeting probes was also prepared by replacing the fluorescent reporter with a biotin group. The present work provides the first DPP8/9-targeting fluorescent compounds as useful chemical tools for the study of DPP8 and DPP9's biological functions.

Pyroptosis in health and disease

The field of cell death has witnessed significant advancements since the initial discovery of apoptosis in the 1970s. This review delves into the intricacies of pyroptosis, a more recently identified form of regulated, lytic cell death, and explores the roles of pyroptotic effector molecules, with a strong emphasis on their mechanisms and relevance in various diseases. Pyroptosis, characterized by its proinflammatory nature, is driven by the accumulation of large plasma membrane pores comprised of gasdermin family protein subunits. In different contexts of cellular homeostatic perturbations, infections, and tissue damage, proteases, such as caspase-1 and caspase-4/5, play pivotal roles in pyroptosis by cleaving gasdermins. Gasdermin-D (GSDMD), the most extensively studied member of the gasdermin protein family, is expressed in various immune cells and certain epithelial cells. Upon cleavage by caspases, GSDMD oligomerizes and forms transmembrane pores in the cell membrane, leading to the release of proinflammatory cytokines. GSDMD-N, the NH2-terminal fragment, displays an affinity for specific lipids, contributing to its role in pore formation in pyroptosis. While GSDMD is the primary focus, other gasdermin family members are also discussed in detail. These proteins exhibit distinct tissue-specific functions and contribute to different facets of cell death regulation. Additionally, genetic variations in some gasdermins have been linked to diseases, underscoring their clinical relevance. Furthermore, the interplay between GSDM pores and the activation of other effectors, such as ninjurin-1, is elucidated, providing insights into the complexity of pyroptosis regulation. The findings underscore the molecular mechanisms that govern pyroptosis and its implications for various physiological and pathological processes.

The CARD8 inflammasome dictates HIV/SIV pathogenesis and disease progression

While CD4+ T cell depletion is key to disease progression in people living with HIV and SIV-infected macaques, the mechanisms underlying this depletion remain incompletely understood, with most cell death involving uninfected cells. In contrast, SIV infection of "natural" hosts such as sooty mangabeys does not cause CD4+ depletion and AIDS despite high-level viremia. Here, we report that the CARD8 inflammasome is activated immediately after HIV entry by the viral protease encapsulated in incoming virions. Sensing of HIV protease activity by CARD8 leads to rapid pyroptosis of quiescent cells without productive infection, while T cell activation abolishes CARD8 function and increases permissiveness to infection. In humanized mice reconstituted with CARD8-deficient cells, CD4+ depletion is delayed despite high viremia. Finally, we discovered loss-of-function mutations in CARD8 from "natural hosts," which may explain the peculiarly non-pathogenic nature of these infections. Our study suggests that CARD8 drives CD4+ T cell depletion during pathogenic HIV/SIV infections.

Harnessing Pyroptosis for Cancer Immunotherapy

Cancer immunotherapy is a novel pillar of cancer treatment that harnesses the immune system to fight tumors and generally results in robust antitumor immunity. Although immunotherapy has achieved remarkable clinical success for some patients, many patients do not respond, underscoring the need to develop new strategies to promote antitumor immunity. Pyroptosis is an immunostimulatory type of regulated cell death that activates the innate immune system. A hallmark of pyroptosis is the release of intracellular contents such as cytokines, alarmins, and chemokines that can stimulate adaptive immune activation. Recent studies suggest that pyroptosis promotes antitumor immunity. Here, we review the mechanisms by which pyroptosis can be induced and highlight new strategies to induce pyroptosis in cancer cells for antitumor defense. We discuss how pyroptosis modulates the tumor microenvironment to stimulate adaptive immunity and promote antitumor immunity. We also suggest research areas to focus on for continued development of pyroptosis as an anticancer treatment. Pyroptosis-based anticancer therapies offer a promising new avenue for treating immunologically 'cold' tumors.

NLRP1- A CINDERELLA STORY: a perspective of recent advances in NLRP1 and the questions they raise

NLRP1, while the first inflammasome described, has only recently begun to gain significant attention in disease pathology, inflammation research, and potentially, as a therapeutic target. Recently identified human variants provide key insights into NLRP1 biology while its unique expression in barrier cells such as keratinocytes and airway epithelial cells has aligned with new, human specific agonists. This differentiates NLRP1 from other inflammasomes such as NLRP3 and identifies it as a key therapeutic target in inflammatory diseases. Indeed, recent discoveries highlight that NLRP1 may be the predominant inflammasome in human barrier cells, its primary role akin to NLRP3, to respond to cellular stress. This review focuses on recent studies identifying new human-specific NLRP1 mechanisms of activation of, gain-of-function human variants and disease, its role in responding to cellular stress, and discuss potential advances and the therapeutic potential for NLRP1.

Inflammasomes cross-talk with lymphocytes to connect the innate and adaptive immune response

BACKGROUND

Innate and adaptive immunity are two different parts of the immune system that have different characteristics and work together to provide immune protection. Inflammasomes are a major part of the innate immune system that are expressed widely in myeloid cells and are responsible for inflammatory responses. Recent studies have shown that inflammasomes are also expressed and activated in lymphocytes, especially in T and B cells, to regulate the adaptive immune response. Activation of inflammasomes is also under the control of lymphocytes. Therefore, we propose that inflammasomes act as a bridge and they provide crosstalk between the innate and adaptive immune systems to obtain a fine balance in immune responses.

AIM OF REVIEW

This review systematially summarizes the interaction between inflammasomes and lymphocytes and describes the crosstalk between the innate and adaptive immune systems induced by inflammasomes, with the aim of providing new directions and important areas for further research.

KEY SCIENTIFIC CONCEPTS OF REVIEW

When considering the novel function of inflammasomes in various lymphocytes, attention should be given to the activity of specific inflammasomes in studies of lymphocyte function. Moreover, research on the function of various inflammasomes in lymphocytes will help advance knowledge on the mechanisms and treatment of various diseases, including autoimmune diseases and tumors. In addition, when studying inflammatory responses, inflammasomes in both lymphocytes and myeloid cells need to be considered.

Small molecule modulators of immune pattern recognition receptors

Pattern recognition receptors (PRRs) represent a re-emerging class of therapeutic targets for vaccine adjuvants, inflammatory diseases and cancer. In this review article, we summarize exciting developments in discovery and characterization of small molecule PRR modulators, focusing on Toll-like receptors (TLRs), NOD-like receptors (NLRs) and the cGAS-STING pathway. We also highlight PRRs that are currently lacking small molecule modulators and opportunities for chemical biology and therapeutic discovery.

Hemophagocytic lymphohistiocytosis-like hyperinflammation due to a de novo mutation in DPP9

BACKGROUND

Genetic defects in components of inflammasomes can cause autoinflammation. Biallelic loss-of-function mutations in dipeptidyl peptidase 9 (DPP9), a negative regulator of the NLRP1 and CARD8 inflammasomes, have recently been shown to cause an inborn error of immunity characterized by pancytopenia, skin manifestations, and increased susceptibility to infections.

OBJECTIVE

We sought to study the molecular basis of autoinflammation in a patient with severe infancy-onset hyperinflammation associated with signs of fulminant hemophagocytic lymphohistiocytosis.

METHODS

Using heterologous cell models as well as patient cells, we performed genetic, immunologic, and molecular investigations to identify the genetic cause and to assess the impact of the identified mutation on inflammasome activation.

RESULTS

The patient exhibited pancytopenia with decreased neutrophils and T, B, and natural killer cells, and markedly elevated levels of lactate dehydrogenase, ferritin, soluble IL-2 receptor, and triglycerides. In addition, serum levels of IL-1β and IL-18 were massively increased, consistent with inflammasome activation. Genetic analysis revealed a previously undescribed de novo mutation in DPP9 (c.755G>C, p.Arg252Pro) affecting a highly conserved amino acid residue. The mutation led to destabilization of the DPP9 protein as shown in transiently transfected HEK293T cells and in patient-derived induced pluripotent stem cells. Using functional inflammasome assays in HEK293T cells, we demonstrated that mutant DPP9 failed to restrain the NLRP1 and CARD8 inflammasomes, resulting in constitutive inflammasome activation. These findings suggest that the Arg252Pro DPP9 mutation acts in a dominant-negative manner.

CONCLUSIONS

A de novo mutation in DPP9 leads to severe infancy-onset autoinflammation because of unleashed inflammasome activation.

The implication of pyroptosis in cancer immunology: Current advances and prospects

Pyroptosis is a regulated cell death pathway involved in numerous human diseases, especially malignant tumors. Recent studies have identified multiple pyroptosis-associated signaling molecules, like caspases, gasdermin family and inflammasomes. In addition, increasing in vitro and in vivo studies have shown the significant linkage between pyroptosis and immune regulation of cancers. Pyroptosis-associated biomarkers regulate the infiltration of tumor immune cells, such as CD4+ and CD8+ T cells, thus strengthening the sensitivity to therapeutic strategies. In this review, we explained the relationship between pyroptosis and cancer immunology and focused on the significance of pyroptosis in immune regulation. We also proposed the future application of pyroptosis-associated biomarkers in basic research and clinical practices to address malignant behaviors. Exploration of the underlying mechanisms and biological functions of pyroptosis is critical for immune response and cancer immunotherapy.

Highly Selective Inhibitors of Dipeptidyl Peptidase 9 (DPP9) Derived from the Clinically Used DPP4-Inhibitor Vildagliptin

Dipeptidyl peptidase 9 (DPP9) is a proline-selective serine protease that plays a key role in NLRP1- and CARD8-mediated inflammatory cell death (pyroptosis). No selective inhibitors have hitherto been reported for the enzyme: all published molecules have grossly comparable affinities for DPP8 and 9 because of the highly similar architecture of these enzymes' active sites. Selective DPP9 inhibitors would be highly instrumental to address unanswered research questions on the enzyme's role in pyroptosis, and they could also be investigated as therapeutics for acute myeloid leukemias. Compounds presented in this manuscript (42 and 47) combine low nanomolar DPP9 affinities with unprecedented DPP9-to-DPP8 selectivity indices up to 175 and selectivity indices >1000 toward all other proline-selective proteases. To rationalize experimentally obtained data, a molecular dynamics study was performed. We also provide in vivo pharmacokinetics data for compound 42.

Inflammasome activation occurs in CD4+ and CD8+ T cells during graft-versus-host disease

A severe complication of hematopoietic stem cell transplantation is graft-versus-host disease (GvHD), a reaction that occurs following the transfer of donor immune cells (the graft) into an allogeneic host. Transplanted cells recognize host alloantigens as foreign, resulting in the activation of donor T cells and migration of these pathological cells into host tissues. In this study, we found that caspase-1 is activated in alloreactive murine and human CD4+ and CD8+ T cells early during acute GvHD (aGvHD). The presence of inflammasome-bound active caspase-1 (p33) and ASC-speck formation confirmed inflammasome activation in these cells. We further measured gasdermin D (GSDMD) cleavage and IL-18 secretion from alloreactive T cells ex vivo. Isolated T cells with high levels of active caspase-1 had a strong inflammatory transcriptional signature and a metabolic phenotype similar to inflammatory myeloid cells, including the upregulation of proinflammatory cytokines and metabolic switch from oxidative phosphorylation to aerobic glycolysis. We also observed oxidative stress, mitochondrial dysfunction, and cell death phenotypes consistent with inflammatory cell death in alloreactive T cells. For the first time, this study characterizes caspase-1 activation in transplanted T cells during aGvHD, using mouse and human models, adding to a body of literature supporting inflammasome function in cells of the adaptive immune system.

Role of gasdermin family proteins in cancers (Review)

The gasdermin (GSDM) family comprises six proteins, including GSDMA‑GSDME and Pejvakin. Most of these proteins have a crucial role in inducing pyroptosis; in particular, GSDMD and GSDME are the most extensively studied proteins as the executioners of the pyroptosis process. Pyroptosis is a highly pro‑inflammatory form of programmed cell death and is closely associated with the incidence, development and prognosis of multiple cancer types. The present review focused on the current knowledge of the molecular mechanism of GSDM‑mediated pyroptosis, its intricate role in cancer and the potential therapeutic value of its anti‑tumor effects.

Insights Regarding the Role of Inflammasomes in Leukemia: What Do We Know?

Inflammation is a physiological mechanism of the immune response and has an important role in maintaining the hematopoietic cell niche in the bone marrow. During this process, the participation of molecules produced by innate immunity cells in response to a variety of pathogen-associated molecular patterns and damage-associated molecular patterns is observed. However, chronic inflammation is intrinsically associated with leukemogenesis, as it induces DNA damage in hematopoietic stem cells and contributes to the creation of the preleukemic clone. Several factors influence the malignant transformation within the hematopoietic microenvironment, with inflammasomes having a crucial role in this process, in addition to acting in the regulation of hematopoiesis and its homeostasis. Inflammasomes are intracellular multimeric complexes responsible for the maturation and secretion of the proinflammatory cytokines interleukin-1β and interleukin-18 and the cell death process via pyroptosis. Therefore, dysregulation of the activation of these complexes may be a factor in triggering several diseases, including leukemias, and this has been the subject of several studies in the area. In this review, we summarized the current knowledge on the relationship between inflammation and leukemogenesis, in particular, the role of inflammasomes in different types of leukemias, and we describe the potential therapeutic targets directed at inflammasomes in the leukemic context.

Tripping the wire: sensing of viral protease activity by CARD8 and NLRP1 inflammasomes

Host innate immune sensors are vital for the initial detection of pathogen infection. Such sensors thus need to constantly adapt in escalating evolutionary arms races with pathogens. Recently, two inflammasome-forming proteins, CARD8 and NLRP1, have emerged as innate immune sensors for the enzymatic activity of virus-encoded proteases. When cleaved within a rapidly evolving 'tripwire' region, CARD8 and NLRP1 assemble into inflammasomes that initiate pyroptotic cell death and pro-inflammatory cytokine release as a form of effector-triggered immunity. Short motifs in the CARD8 and NLRP1 tripwires mimic the protease-specific cleavage sites of picornaviruses, coronaviruses, and HIV-1, providing virus-specific sensing that can rapidly change between closely related hosts and within the human population. Recent work highlights the evolutionary arms races between viral proteases and NLRP1 and CARD8, including insights into the mechanisms of inflammasome activation, host diversity of viral sensing, and means that viruses have evolved to avoid tripping the wire.

A human-specific motif facilitates CARD8 inflammasome activation after HIV-1 infection

Inflammasomes are cytosolic innate immune complexes that assemble upon detection of diverse pathogen-associated cues and play a critical role in host defense and inflammatory pathogenesis. Here, we find that the human inflammasome-forming sensor CARD8 senses HIV-1 infection via site-specific cleavage of the CARD8 N-terminus by the HIV protease (HIV-1PR). HIV-1PR cleavage of CARD8 induces pyroptotic cell death and the release of pro-inflammatory cytokines from infected cells, processes regulated by Toll-like receptor stimulation prior to viral infection. In acutely infected cells, CARD8 senses the activity of both de novo translated HIV-1PR and packaged HIV-1PR that is released from the incoming virion. Moreover, our evolutionary analyses reveal that the HIV-1PR cleavage site in human CARD8 arose after the divergence of chimpanzees and humans. Although chimpanzee CARD8 does not recognize proteases from HIV or simian immunodeficiency viruses from chimpanzees (SIVcpz), SIVcpz does cleave human CARD8, suggesting that SIVcpz was poised to activate the human CARD8 inflammasome prior to its cross-species transmission into humans. Our findings suggest a unique role for CARD8 inflammasome activation in response to lentiviral infection of humans.

Host-specific sensing of coronaviruses and picornaviruses by the CARD8 inflammasome

Hosts have evolved diverse strategies to respond to microbial infections, including the detection of pathogen-encoded proteases by inflammasome-forming sensors such as NLRP1 and CARD8. Here, we find that the 3CL protease (3CLpro) encoded by diverse coronaviruses, including Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), cleaves a rapidly evolving region of human CARD8 and activates a robust inflammasome response. CARD8 is required for cell death and the release of pro-inflammatory cytokines during SARS-CoV-2 infection. We further find that natural variation alters CARD8 sensing of 3CLpro, including 3CLpro-mediated antagonism rather than activation of megabat CARD8. Likewise, we find that a single nucleotide polymorphism (SNP) in humans reduces CARD8's ability to sense coronavirus 3CLpros and, instead, enables sensing of 3C proteases (3Cpro) from select picornaviruses. Our findings demonstrate that CARD8 is a broad sensor of viral protease activities and suggests that CARD8 diversity contributes to inter- and intraspecies variation in inflammasome-mediated viral sensing and immunopathology.

A 360° view of the inflammasome: Mechanisms of activation, cell death, and diseases

Inflammasomes are critical sentinels of the innate immune system that respond to threats to the host through recognition of distinct molecules, known as pathogen- or damage-associated molecular patterns (PAMPs/DAMPs), or disruptions of cellular homeostasis, referred to as homeostasis-altering molecular processes (HAMPs) or effector-triggered immunity (ETI). Several distinct proteins nucleate inflammasomes, including NLRP1, CARD8, NLRP3, NLRP6, NLRC4/NAIP, AIM2, pyrin, and caspases-4/-5/-11. This diverse array of sensors strengthens the inflammasome response through redundancy and plasticity. Here, we present an overview of these pathways, outlining the mechanisms of inflammasome formation, subcellular regulation, and pyroptosis, and discuss the wide-reaching effects of inflammasomes in human disease.

Chemical inhibition of DPP9 sensitizes the CARD8 inflammasome in HIV-1-infected cells

Non-nucleoside reverse transcriptase inhibitors (NNRTIs) induce pyroptosis of HIV-1-infected CD4+ T cells through induction of intracellular HIV-1 protease activity, which activates the CARD8 inflammasome. Because high concentrations of NNRTIs are required for efficient elimination of HIV-1-infected cells, it is important to elucidate ways to sensitize the CARD8 inflammasome to NNRTI-induced activation. We show that this sensitization can be achieved through chemical inhibition of the CARD8 negative regulator DPP9. The DPP9 inhibitor Val-boroPro (VbP) can kill HIV-1-infected cells without the presence of NNRTIs and act synergistically with NNRTIs to promote clearance of HIV-1-infected cells in vitro and in humanized mice. More importantly, VbP is able to enhance clearance of residual HIV-1 in CD4+ T cells isolated from people living with HIV (PLWH). We also show that VbP can partially overcome NNRTI resistance. This offers a promising strategy for enhancing NNRTI efficacy in the elimination of HIV-1 reservoirs in PLWH.

NLRP1 inflammasome modulates senescence and senescence-associated secretory phenotype

Senescence is a cellular aging-related process triggered by different stresses and characterized by the secretion of various inflammatory factors referred to as the senescence-associated secretory phenotype (SASP). Here, we present evidence that the inflammasome sensor, NLRP1, is a key mediator of senescence induced by irradiation both in vitro and in vivo. The NLRP1 inflammasome promotes senescence by regulating the expression of p16, p21, p53, and SASP in Gasdermin D (GSDMD)-dependent manner as these responses are reduced in conditions of NLRP1 insufficiency or GSDMD inhibition. Mechanistically, the NLRP1 inflammasome is activated downstream of the cytosolic DNA sensor cGMP-AMP (cGAMP) synthase (cGAS) in response to genomic damage. These findings provide a rationale for inhibiting the NLRP1 inflammasome-GSDMD axis to treat senescence-driven disorders.

Prognosis Risk Model Based on Pyroptosis-Related lncRNAs for Gastric Cancer

Gastric cancer (GC) is a malignant tumor with a low survival rate, high recurrence rate, and poor prognosis. With respect to this, pyroptosis is a type of programmed cell death that can affect the occurrence and development of tumors. Indeed, long non-coding RNAs (lncRNAs) were broadly applied for the purposes of early diagnosis, treatment, and prognostic analysis in regard to cancer. Based on the association of these three purposes, we developed a novel prognosis risk model based on pyroptosis-related lncRNAs (PRlncRNAs) for GC. The PRlncRNAs were obtained via univariate and multivariate Cox regression in order to build the predictive signatures. The Kaplan–Meier and gene set enrichment analysis (GSEA) methods were used to evaluate the overall survival (OS) and functional differences between the high- and low-risk groups. Moreover, the correlation of the signatures with immune cell infiltration was determined through single-sample gene set enrichment analysis (ssGSEA). Finally, we analyzed this correlation with the treatment responses in the GC patients; then, we performed quantitative reverse transcription polymerase chain reactions (qRT-PCRs) in order to verify the risk model. The high-risk group received a worse performance in terms of prognosis and OS when compared to the low-risk group. With respect to this, the area under the receiver operating characteristic curve (ROC) was found to be 0.808. Through conducting the GSEA, it was found that the high-risk groups possessed a significant enrichment in terms of tumor–immunity pathways. Furthermore, the ssGSEA revealed that the predictive features possessed strong associations with immune cell infiltration in regard to GC. In addition, we highlighted that anti-immune checkpoint therapy, combined with conventional chemotherapy drugs, may be more suitable for high-risk patients. The expression levels of LINC01315, AP003392.1, AP000695.2, and HAGLR were significantly different between the GC cell lines and the normal cell lines. As such, the six PRlncRNAs could be regarded as important prognostic biomarkers for the purposes of subsequent diagnoses, treatments, prognostic predictions, and the mechanism research of GC.

Optimized M24B Aminopeptidase Inhibitors for CARD8 Inflammasome Activation

Inflammasomes are innate immune signaling platforms that trigger pyroptotic cell death. NLRP1 and CARD8 are related human inflammasomes that detect similar danger signals, but NLRP1 has a higher activation threshold and triggers a more inflammatory form of pyroptosis. Both sense the accumulation of intracellular peptides with Xaa-Pro N-termini, but Xaa-Pro peptides on their own without a second danger signal only activate the CARD8 inflammasome. We recently reported that a dual inhibitor of the Xaa-Pro-cleaving M24B aminopeptidases PEPD and XPNPEP1 called CQ31 selectively activates the CARD8 inflammasome by inducing the build-up of Xaa-Pro peptides. Here, we performed structure-activity relationship studies on CQ31 to develop the optimized dual PEPD/XPNPEP1 inhibitor CQ80 that more effectively induces CARD8 inflammasome activation. We anticipate that CQ80 will become a valuable tool to study the basic biology and therapeutic potential of selective CARD8 inflammasome activation.

Protein folding stress potentiates NLRP1 and CARD8 inflammasome activation

NLRP1 and CARD8 are related pattern-recognition receptors (PRRs) that detect intracellular danger signals and form inflammasomes. Both undergo autoproteolysis, generating N-terminal (NT) and C-terminal (CT) fragments. The proteasome-mediated degradation of the NT releases the CT from autoinhibition, but the stimuli that trigger NT degradation have not been fully elucidated. Here, we show that several distinct agents that interfere with protein folding, including aminopeptidase inhibitors, chaperone inhibitors, and inducers of the unfolded protein response, accelerate NT degradation. However, these agents alone do not trigger inflammasome formation because the released CT fragments are physically sequestered by the serine dipeptidase DPP9. We show that DPP9-binding ligands must also be present to disrupt these complexes and allow the CT fragments to oligomerize into inflammasomes. Overall, these results indicate that NLRP1 and CARD8 detect a specific perturbation that induces both protein folding stress and DPP9 ligand accumulation.

The NLRP1 and CARD8 inflammasomes detect reductive stress

The danger signals that activate the related nucleotide-binding domain leucine-rich repeat pyrin domain-containing 1 (NLRP1) and caspase activation and recruitment domain-containing 8 (CARD8) inflammasomes have not been fully established. We recently reported that the oxidized form of TRX1 binds to NLRP1 and represses inflammasome activation. These findings suggested that intracellular reductive stress, which would reduce oxidized TRX1 and thereby abrogate the NLRP1-TRX1 interaction, is an NLRP1 inflammasome-activating danger signal. However, no agents that induce reductive stress were known to test this premise. Here, we identify and characterize several radical-trapping antioxidants, including JSH-23, that induce reductive stress. We show that these compounds accelerate the proteasome-mediated degradation of the repressive N-terminal fragments of both NLRP1 and CARD8, releasing the inflammasome-forming C-terminal fragments from autoinhibition. Overall, this work validates chemical probes that induce reductive stress and establishes reductive stress as a danger signal sensed by both the NLRP1 and CARD8 inflammasomes.

CARD8 Inflammasome Activation by HIV-1 Protease

The pattern recognition receptor CARD8 is an inflammasome sensor for intracellular HIV-1 protease activity. Previously, the only method for studying the CARD8 inflammasome has been through utilizing DPP8/DPP9 inhibitors including Val-boroPro (VbP) to modestly and nonspecifically activate the CARD8 inflammasome. The identification of HIV-1 protease as a target for sensing by CARD8 has opened the door for a new method of studying the underlying mechanism of CARD8 inflammasome activation. Additionally, triggering the CARD8 inflammasome offers a promising strategy for reducing HIV-1 latent reservoirs. Here we describe the methods to study CARD8 sensing of HIV-1 protease activity through non-nucleoside reverse transcriptase inhibitor (NNRTI)-mediated pyroptosis of HIV-1-infected immune cells and through an HIV and CARD8 co-transfection model.

The CARD8 inflammasome in HIV infection

The biggest challenge to immune control of HIV infection is the rapid within-host viral evolution, which allows selection of viral variants that escape from T cell and antibody recognition. Thus, it is impossible to clear HIV infection without targeting "immutable" components of the virus. Unlike the adaptive immune system that recognizes cognate epitopes, the CARD8 inflammasome senses the essential enzymatic activity of the HIV-1 protease, which is immutable for the virus. Hence, all subtypes of HIV clinical isolates can be recognized by CARD8. In HIV-infected cells, the viral protease is expressed as a subunit of the viral Gag-Pol polyprotein and remains functionally inactive prior to viral budding. A class of anti-HIV drugs, the non-nucleoside reverse transcriptase inhibitors (NNRTIs), can promote Gag-pol dimerization and subsequent premature intracellular activation of the viral protease. NNRTI treatment triggers CARD8 inflammasome activation, which leads to pyroptosis of HIV-infected CD4⁺ T cells and macrophages. Targeting the CARD8 inflammasome can be a potent and broadly effective strategy for HIV eradication.

The human inflammasomes

Two decades of inflammasome research has led to a vast body of knowledge on the complex regulatory mechanisms and pathological roles of canonical and non-canonical inflammasome activation in a plethora of research models of primarily rodent origin. More recently, the field has made notable progress in characterizing human-specific inflammasomes and their regulation mechanisms, including an expansion of inflammasome biology to adaptive immune cells. These exciting developments in basic research have been accompanied by potentially transformative results from large clinical trials and translational efforts to develop inflammasome-targeted small molecule inhibitors for therapeutic use. Here, we will discuss recent findings in the field with a specific emphasis on activation mechanisms of human inflammasomes and their potential role in auto-inflammatory, metabolic and neoplastic diseases.

CARD8 makes coxsackievirus more "heartbreaking"

In this issue of JEM, Nadkarni et al. (2022. J. Exp. Med.https://doi.org/10.1084/jem.20212117) identify CARD8 as an innate sensor triggered by coxsackievirus B3 proteases to drive pyroptosis of aortic endothelial cells and cardiac myocytes, fueling viral replication and heart inflammation.

Viral proteases activate the CARD8 inflammasome in the human cardiovascular system

Nucleotide-binding oligomerization domain (NBD), leucine-rich repeat (LRR) containing protein family (NLRs) are intracellular pattern recognition receptors that mediate innate immunity against infections. The endothelium is the first line of defense against blood-borne pathogens, but it is unclear which NLRs control endothelial cell (EC) intrinsic immunity. Here, we demonstrate that human ECs simultaneously activate NLRP1 and CARD8 inflammasomes in response to DPP8/9 inhibitor Val-boro-Pro (VbP). Enterovirus Coxsackie virus B3 (CVB3)-the most common cause of viral myocarditis-predominantly activates CARD8 in ECs in a manner that requires viral 2A and 3C protease cleavage at CARD8 p.G38 and proteasome function. Genetic deletion of CARD8 in ECs and human embryonic stem cell-derived cardiomyocytes (HCMs) attenuates CVB3-induced pyroptosis, inflammation, and viral propagation. Furthermore, using a stratified endothelial-cardiomyocyte co-culture system, we demonstrate that deleting CARD8 in ECs reduces CVB3 infection of the underlying cardiomyocytes. Our study uncovers the unique role of CARD8 inflammasome in endothelium-intrinsic anti-viral immunity.

Pyroptosis and pyroptosis-inducing cancer drugs

Pyroptosis, an inflammatory form of lytic cell death, is a type of cell death mediated by the gasdermin (GSDM) protein family. Upon recognizing exogenous or endogenous signals, cells undergo inflammasome assembly, GSDM cleavage, the release of proinflammatory cytokines and other cellular contents, eventually leading to inflammatory cell death. In this review, we discuss the roles of the GSDM family for anti-cancer functions and various antitumor drugs that could activate the pyroptosis pathways.

Inflammasomes in Human Immunodeficiency Virus Type 1 Infection

Innate immune responses are the host's first line of defense against human immunodeficiency virus type 1 (HIV-1) infection, with pattern recognition receptors detecting viral specific pathogen-associated molecular patterns and initiating antiviral responses. In response to HIV-1 nucleic acids or proteins, some pattern recognition receptors have the ability to assemble a large multiprotein complex called the inflammasome, which triggers pro-inflammatory cytokine release and a form of lytic programmed cell death called pyroptosis. Here, we review our current understanding of the mechanism of the inflammasome in sensing HIV-1 infection. Furthermore, we discuss the contribution of inflammasome activation in HIV-1 pathogenesis as well as potential strategies of targeting inflammasome activation for the treatment of HIV-1 infection.

Host specific sensing of coronaviruses and picornaviruses by the CARD8 inflammasome

Hosts have evolved diverse strategies to respond to microbial infections, including the detection of pathogen-encoded proteases by inflammasome-forming sensors such as NLRP1 and CARD8. Here, we find that the 3CL protease (3CL pro ) encoded by diverse coronaviruses, including SARS-CoV-2, cleaves a rapidly evolving region of human CARD8 and activates a robust inflammasome response. CARD8 is required for cell death and the release of pro-inflammatory cytokines during SARS-CoV-2 infection. We further find that natural variation alters CARD8 sensing of 3CL pro , including 3CL pro -mediated antagonism rather than activation of megabat CARD8. Likewise, we find that a single nucleotide polymorphism (SNP) in humans reduces CARD8’s ability to sense coronavirus 3CL pros , and instead enables sensing of 3C proteases (3C pro ) from select picornaviruses. Our findings demonstrate that CARD8 is a broad sensor of viral protease activities and suggests that CARD8 diversity contributes to inter- and intra-species variation in inflammasome-mediated viral sensing and immunopathology.

DPP9 deficiency: An inflammasomopathy that can be rescued by lowering NLRP1/IL-1 signaling

Dipeptidyl peptidase 9 (DPP9) is a direct inhibitor of NLRP1, but how it affects inflammasome regulation in vivo is not yet established. Here, we report three families with immune-associated defects, poor growth, pancytopenia, and skin pigmentation abnormalities that segregate with biallelic DPP9 rare variants. Using patient-derived primary cells and biochemical assays, these variants were shown to behave as hypomorphic or knockout alleles that failed to repress NLRP1. The removal of a single copy of Nlrp1a/b/c, Asc, Gsdmd, or Il-1r, but not Il-18, was sufficient to rescue the lethality of Dpp9 mutant neonates in mice. Similarly, dpp9 deficiency was partially rescued by the inactivation of asc, an obligate downstream adapter of the NLRP1 inflammasome, in zebrafish. These experiments suggest that the deleterious consequences of DPP9 deficiency were mostly driven by the aberrant activation of the canonical NLRP1 inflammasome and IL-1β signaling. Collectively, our results delineate a Mendelian disorder of DPP9 deficiency driven by increased NLRP1 activity as demonstrated in patient cells and in two animal models of the disease.

A Phenotypic Screen Identifies Potent DPP9 Inhibitors Capable of Killing HIV-1 Infected Cells

Although current antiretroviral therapy can control HIV-1 replication and prevent disease progression, it is not curative. Identifying mechanisms that can lead to eradication of persistent viral reservoirs in people living with HIV-1 (PLWH) remains an outstanding challenge to achieving cure. Utilizing a phenotypic screen, we identified a novel chemical class capable of killing HIV-1 infected peripheral blood mononuclear cells. Tool compounds ICeD-1 and ICeD-2 ("inducer of cell death-1 and 2"), optimized for potency and selectivity from screening hits, were used to deconvolute the mechanism of action using a combination of chemoproteomic, biochemical, pharmacological, and genetic approaches. We determined that these compounds function by modulating dipeptidyl peptidase 9 (DPP9) and activating the caspase recruitment domain family member 8 (CARD8) inflammasome. Efficacy of ICeD-1 and ICeD-2 was dependent on HIV-1 protease activity and synergistic with efavirenz, which promotes premature activation of HIV-1 protease at high concentrations in infected cells. This in vitro synergy lowers the efficacious cell kill concentration of efavirenz to a clinically relevant dose at concentrations of ICeD-1 or ICeD-2 that do not result in complete DPP9 inhibition. These results suggest engagement of the pyroptotic pathway as a potential approach to eliminate HIV-1 infected cells.

Pyroptosis in development, inflammation and disease

In the early 2000s, caspase-1, an important molecule that has been shown to be involved in the regulation of inflammation, cell survival and diseases, was given a new function: regulating a new mode of cell death that was later defined as pyroptosis. Since then, the inflammasome, the inflammatory caspases (caspase-4/5/11) and their substrate gasdermins (gasdermin A, B, C, D, E and DFNB59) has also been reported to be involved in the pyroptotic pathway, and this pathway is closely related to the development of various diseases. In addition, important apoptotic effectors caspase-3/8 and granzymes have also been reported to b involved in the induction of pyroptosis. In our article, we summarize findings that help define the roles of inflammasomes, inflammatory caspases, gasdermins, and other mediators of pyroptosis, and how they determine cell fate and regulate disease progression.

New insights into the role of dipeptidyl peptidase 8 and dipeptidyl peptidase 9 and their inhibitors

Dipeptidyl peptidase 8 (DPP8) and 9 (DPP9) are widely expressed in mammals including humans, mainly locate in the cytoplasm. The DPP8 and DPP9 (DPP8/9) belong to serine proteolytic enzymes, they can recognize and cleave N-terminal dipeptides of specific substrates if proline is at the penultimate position. Because the localization of DPP8/9 is different from that of DPP4 and the substrates for DPP8/9 are not yet completely clear, their physiological and pathological roles are still being further explored. In this article, we will review the recent research advances focusing on the expression, regulation, and functions of DPP8/9 in physiology and pathology status. Emerging research results have shown that DPP8/9 is involved in various biological processes such as cell behavior, energy metabolism, and immune regulation, which plays an essential role in maintaining normal development and physiological functions of the body. DPP8/9 is also involved in pathological processes such as tumorigenesis, inflammation, and organ fibrosis. In recent years, related research on immune cell pyroptosis has made DPP8/9 a new potential target for the treatment of hematological diseases. In addition, DPP8/9 inhibitors also have great potential in the treatment of tumors and chronic kidney disease.

Uncovering signals of positive selection in Peruvian populations from three ecological regions

Perú hosts extremely diverse ecosystems which can be broadly classified into three major ecoregions: the Pacific desert coast, the Andean highlands, and the Amazon rainforest. Since its initial peopling approximately 12,000 years ago, the populations inhabiting such ecoregions might have differentially adapted to their contrasting environmental pressures. Previous studies have described several candidate genes underlying adaptation to hypobaric hypoxia among Andean highlanders. However, the adaptive genetic diversity of coastal and rainforest populations has been less studied. Here, we gathered genome-wide SNP-array data from 286 Peruvians living across the three ecoregions and analysed signals of recent positive selection through population differentiation and haplotype-based selection scans. Among highland populations, we identify candidate genes related to cardiovascular function (TLL1, DUSP27, TBX5, PLXNA4, SGCD), to the Hypoxia-Inducible Factor pathway (TGFA, APIP), to skin pigmentation (MITF), as well as to glucose (GLIS3) and glycogen metabolism (PPP1R3C, GANC). In contrast, most signatures of adaptation in coastal and rainforest populations comprise candidate genes related to the immune system (including SIGLEC8, TRIM21, CD44 and ICAM1 in the coast; CBLB and PRDM1 in rainforest and the BRD2- HLA-DOA- HLA-DPA1 region in both), possibly as a result of strong pathogen-driven selection. This study identifies candidate genes related to human adaptation to the diverse environments of South America.

Eliciting pyroptosis to fuel cancer immunotherapy: mechanisms and strategies

Immune checkpoint blockade (ICB) therapy has recently shown promise in treating several malignancies. However, only a limited number of patients respond to this treatment, partially because of the "immune cold" condition of the tumor immune microenvironment. Pyroptosis is a type of gasdermin-mediated programmed cell death that often leads to inflammation and immune responses. Many studies on the mechanism and function of pyroptosis have led to increasing recognition of the role of pyroptosis in malignant progression and immune therapy. Pyroptosis has the potential to alter the tumor immune microenvironment by releasing tumor-associated antigens, damage-associated molecular patterns, and proinflammatory cytokines, thus leading to intratumoral inflammatory responses, stimulation of tumor-specific cytotoxic T cell infiltration, conversion of "cold" to "hot" tumors, and ultimately improving the efficacy of ICB therapy. Some cancer treatments have been shown to restore anticancer immunosurveillance through the induction of pyroptosis. Therapy promoting pyroptosis and ICB therapy may have synergistic effects in cancer treatment. This review summarizes the mechanisms and roles of pyroptosis in the tumor microenvironment and combination treatment strategies. An improved understanding of the roles of pyroptosis in tumorigenesis, immune evasion, and treatment would aid in the development of therapeutic strategies for malignancies.

The Regulation and Modification of GSDMD Signaling in Diseases

Gasdermin D (GSDMD) serves as a key executor to trigger pyroptosis and is emerging as an attractive checkpoint in host defense, inflammatory, autoimmune diseases, and many other systemic diseases. Although canonical and non-canonical inflammasome-mediated classic GSDMD cleavage, GSDMD-NT migration to cell membrane, GSDMD-NT oligomerization, and pore forming have been well recognized, a few unique features of GSDMD in specific condition beyond its classic function, including non-lytic function of GSDMD, the modification and regulating mechanism of GSDMD signaling have also come to great attention and played a crucial role in biological processes and diseases. In the current review, we emphasized the GSDMD protein expression, stabilization, modification, activation, pore formation, and repair during pyroptosis, especially the regulation and modification of GSDMD signaling, such as GSDMD complex in polyubiquitination and non-pyroptosis release of IL-1β, ADP-riboxanation, NINJ1 in pore forming, GSDMD binding protein TRIM21, GSDMD succination, and Regulator-Rag-mTOR-ROS regulation of GSDMD. We also discussed the novel therapeutic strategies of targeting GSDMD and summarized recently identified inhibitors with great prospect.

Beyond Inhibition: A Novel Strategy of Targeting HIV-1 Protease to Eliminate Viral Reservoirs

HIV-1 protease (PR) is a viral enzyme that cleaves the Gag and Gag-Pol polyprotein precursors to convert them into their functional forms, a process which is essential to generate infectious viral particles. Due to its broad substrate specificity, HIV-1 PR can also cleave certain host cell proteins. Several studies have identified host cell substrates of HIV-1 PR and described the potential impact of their cleavage on HIV-1-infected cells. Of particular interest is the interaction between PR and the caspase recruitment domain-containing protein 8 (CARD8) inflammasome. A recent study demonstrated that CARD8 can sense HIV-1 PR activity and induce cell death. While PR typically has low levels of intracellular activity prior to viral budding, premature PR activation can be achieved using certain non-nucleoside reverse transcriptase inhibitors (NNRTIs), resulting in CARD8 cleavage and downstream pyroptosis. Used together with latency reversal agents, the induction of premature PR activation to trigger CARD8-mediated cell killing may help eliminate latent reservoirs in people living with HIV. This represents a novel strategy of utilizing PR as an antiviral target through premature activation rather than inhibition. In this review, we discuss the viral and host substrates of HIV-1 protease and highlight potential applications and advantages of targeting CARD8 sensing of HIV-1 PR.

A ubiquitin-independent proteasome pathway controls activation of the CARD8 inflammasome

CARD8 is a pattern-recognition receptor that forms a caspase-1-activating inflammasome. CARD8 undergoes constitutive autoproteolysis, generating an N-terminal (NT) fragment with a disordered region and a ZU5 domain and a C-terminal (CT) fragment with UPA and CARD domains. Dipeptidyl peptidase 8 and dipeptidyl peptidase 9 inhibitors, including Val-boroPro, accelerate the degradation of the NT fragment via a poorly characterized proteasome-mediated pathway, thereby releasing the inflammatory CT fragment from autoinhibition. Here, we show that the core 20S proteasome, which degrades disordered and misfolded proteins independent of ubiquitin modification, controls activation of the CARD8 inflammasome. In unstressed cells, we discovered that the 20S proteasome degrades just the NT disordered region, leaving behind the folded ZU5, UPA, and CARD domains to act as an inhibitor of inflammasome assembly. However, in Val-boroPro–stressed cells, we show the 20S proteasome degrades the entire NT fragment, perhaps due to ZU5 domain unfolding, freeing the CT fragment from autoinhibition. Taken together, these results show that the susceptibility of the CARD8 NT domain to 20S proteasome-mediated degradation controls inflammasome activation.

Cardiac Remodeling in Heart Failure: Role of Pyroptosis and Its Therapeutic Implications

Pyroptosis is a kind of programmed cell death closely related to inflammation. The pathways that mediate pyroptosis can be divided into the Caspase-1-dependent canonical pathway and the Caspase4/5/11-dependent non-canonical pathway. The most significant difference from other cell death is that pyroptosis rapidly causes rupture of the plasma membrane, cell expansion, dissolution and rupture of the cell membrane, the release of cell contents and a large number of inflammatory factors, and send pro-inflammatory signals to adjacent cells, recruit inflammatory cells and induce inflammatory responses. Cardiac remodeling is the basic mechanism of heart failure (HF) and the core of pathophysiological research on the underlying mechanism. A large number of studies have shown that pyroptosis can cause cardiac fibrosis, cardiac hypertrophy, cardiomyocytes death, myocardial dysfunction, excessive inflammation, and cardiac remodeling. Therefore, targeting pyroptosis has a good prospect in improving cardiac remodeling in HF. In this review, the basic molecular mechanism of pyroptosis is summarized, the relationship between pyroptosis and cardiac remodeling in HF is analyzed in-depth, and the potential therapy of targeting pyroptosis to improve adverse cardiac remodeling in HF is discussed, providing some ideas for improving the study of adverse cardiac remodeling in HF.

Role of inflammasomes in HIV-1 infection and treatment

Although combined antiretroviral therapy (cART) is effective in inhibiting human immunodeficiency virus type 1 (HIV-1) replication, it does not eradicate the virus because small amounts of latent HIV-1 provirus persist in quiescent memory CD4⁺ T cells. Therefore, strategies for eradicating latent HIV-1 are urgently needed. Recently, several studies have reported that the inflammatory response and lymphocyte death induced by HIV-1 depend on inflammasomes and pyroptosis, suggesting that inflammasomes and pyroptosis have a vital role in HIV-1 infection and contribute to the eradication of latent HIV-1. In this review, we summarize current knowledge of the role of inflammasomes, including NLR family pyrin domain-containing protein 3 (NLRP3), caspase recruitment domain-containing protein 8 (CARD8), interferon-inducible protein 16 (IFI16), NLRP1, NLR family CARD domain-containing 4 (NLRC4), and absent in melanoma 2 (AIM2), in HIV-1 infection and discuss promising therapeutic strategies for HIV-1-associated diseases by targeting inflammasomes.

The NLRP1 Inflammasome Induces Pyroptosis in Human Corneal Epithelial Cells

PURPOSE

Inflammasomes are multiprotein complexes that detect danger-associated signals and trigger an immunostimulatory form of cell death called pyroptosis. NLRP1 is an innate immune receptor that assembles into an inflammasome, but the primary cell types in which NLRP1 is functional have not yet been fully established. Mutations in NLRP1 are associated with diseases of barrier epithelial tissues, including skin lesions and corneal intraepithelial dyskeratosis, suggesting that NLRP1 functions within the eye. Here, we investigated the expression and activity of the NLRP1 inflammasome in primary human corneal epithelial (pHCE) cells.

METHODS

The small molecule Val-boroPro (VbP) activates the NLRP1 inflammasome. Proteasome (bortezomib, MG132) and caspase-1 (VX-765, Z-VAD-FMK) inhibitors block NLRP1 activation and downstream pyroptosis, respectively. Here, we treated pHCE cells with VbP alone or in combination proteasome inhibitors and caspase-1 inhibitors. We assessed NLRP1 expression and hallmarks of pyroptosis, including lytic cell rupture, cytokine processing and release, and gasdermin D (GSDMD) processing.

RESULTS

VbP triggered pyroptosis in pHCE cells, as determined by cytokine secretion, GSDMD processing, and lactate dehydrogenase (LDH) release. Proteasome and caspase-1 inhibitors completely blocked this pyroptotic cell death. In contrast, other primary ocular epithelial cells did not undergo NLRP1-dependent pyroptosis.

CONCLUSIONS

Our findings demonstrate that NLRP1 forms a functional inflammasome in pHCE cells. Importantly, these data reveal that NLRP1 is a key innate immune sensor of the corneal epithelium, and moreover indicate how aberrant inflammasome activation causes corneal damage. Blockade of NLRP1 signaling may benefit patients with hyperactive NLRP1 mutations and warrants further investigation.