Species-specific NLRP3 regulation and its role in CNS autoinflammatory diseases

The NLRP3 inflammasome is essential for caspase-1 activation and the release of interleukin (IL)-1β, IL-18, and gasdermin-D in myeloid cells. However, research on species-specific NLRP3's physiological impact is limited. We engineer mice with the human NLRP3 gene, driven by either the human or mouse promoter, via syntenic replacement at the mouse Nlrp3 locus. Both promoters facilitate hNLRP3 expression in myeloid cells, but the mouse promoter responds more robustly to LPS. Investigating the disease impact of differential NLRP3 regulation, we introduce the D305N gain-of-function mutation into both humanized lines. Chronic inflammation is evident with both promoters; however, CNS outcomes vary significantly. Despite poor response to LPS, the human promoter results in D305N-associated aseptic meningitis, mirroring human pathology. The mouse promoter, although leading to increased CNS expression post-LPS, does not induce meningitis in D305N mutants. Therefore, human-like NLRP3 expression may be crucial for accurate modeling of its role in disease pathogenesis.

Universal STING mimic boosts antitumour immunity via preferential activation of tumour control signalling pathways

The efficacy of STING (stimulator of interferon genes) agonists is due to various factors, primarily inefficient intracellular delivery, low/lack of endogenous STING expression in many tumours, and a complex balance between tumour control and progression. Here we report a universal STING mimic (uniSTING) based on a polymeric architecture. UniSTING activates STING signalling in a range of mouse and human cell types, independent of endogenous STING expression, and selectively stimulates tumour control IRF3/IFN-I pathways, but not tumour progression NF-κB pathways. Intratumoural or systemic injection of uniSTING-mRNA via lipid nanoparticles (LNPs) results in potent antitumour efficacy across established and advanced metastatic tumour models, including triple-negative breast cancer, lung cancer, melanoma and orthotopic/metastatic liver malignancies. Furthermore, uniSTING displays an effective antitumour response superior to 2'3'-cGAMP and ADU-S100. By favouring IRF3/IFN-I activity over the proinflammatory NF-κB signalling pathway, uniSTING promotes dendritic cell maturation and antigen-specific CD8+ T-cell responses. Extracellular vesicles released from uniSTING-treated tumour cells further sensitize dendritic cells via exosome-containing miRNAs that reduced the immunosuppressive Wnt2b, and a combination of LNP-uniSTING-mRNA with α-Wnt2b antibodies synergistically inhibits tumour growth and prolongs animal survival. Collectively, these results demonstrate the LNP-mediated delivery of uniSTING-mRNA as a strategy to overcome the current STING therapeutic barriers, particularly for the treatment of multiple cancer types in which STING is downregulated or absent.

Comparative study of acetalated-dextran microparticle fabrication methods for a clinically translatable subunit-based influenza vaccine

The most common influenza vaccines are inactivated viruses produced in chicken eggs, which is a time-consuming production method with variable efficacy due to mismatches of the vaccine strains to the dominant circulating strains. Subunit-based vaccines provide faster production times in comparison to the traditional egg-produced vaccines but often require the use of an adjuvant to elicit a highly protective immune response. However, the current FDA approved adjuvant for influenza vaccines (MF59) elicits a primarily helper T-cell type 2 (Th2)-biased humoral immune response. Adjuvants that can stimulate a Th1 cellular response are correlated to have more robust protection against influenza. The cyclic dinucleotide cGAMP has been shown to provide a potent Th1 response but requires the use of a delivery vehicle to best initiate its signalling pathway in the cytosol. Herein, acetalated dextran (Ace-DEX) was used as the polymer to fabricate microparticles (MPs) via double-emulsion, electrospray, and spray drying methods to encapsulate cGAMP. This study compared each fabrication method's ability to encapsulate and retain the hydrophilic adjuvant cGAMP. We compared their therapeutic efficacy to Addavax, an MF59-like adjuvant, and cGAMP Ace-DEX MPs provided a stronger Th1 response in vaccinated BALB/c mice. Furthermore, we compared Ace-DEX MPs to spray dried MPs composed from a commonly used polymer for drug delivery, poly(lactic-co-glycolic acid) (PLGA). We observed that all Ace-DEX MPs elicited similar humoral and cellular responses to the PLGA MPs. Overall, the results shown here indicate Ace-DEX can perform similarly to PLGA as a polymer for drug delivery and that spray drying can provide an efficient way to produce MPs to encapsulate cGAMP and stimulate the immune system.

Dynamic bidirectional regulation of NLRC3 and gammaherpesviruses during viral latency in B lymphocytes

While most NOD-like receptors (NLRs) are predominately expressed by innate immune cells, NLRC3, an inhibitory NLR of immune signaling, exhibits the highest expression in lymphocytes. The role of NLRC3 or any NLRs in B lymphocytes is completely unknown. Gammaherpesviruses, including human Epstein-Barr virus (EBV) and murine gammaherpesvirus 68 (MHV-68), establish latent infection in B lymphocytes, which requires elevated NF-κB. This study shows that during latent EBV infection of human B cells, viral-encoded latent membrane protein 1 (LMP1) decreases NLRC3 transcript. LMP1-induced-NF-κB activation suppresses the promoter activity of NLRC3 via p65 binding to the promoter. Conversely, NLRC3 inhibits NF-κB activation by promoting the degradation of LMP1 in a proteasome-dependent manner. In vivo, MHV-68 infection reduces Nlrc3 transcripts in splenocytes, and Nlrc3-deficient mice show greater viral latency than controls. These results reveal a bidirectional regulatory circuit in B lymphocytes, where viral latent protein LMP1 reduces NLRC3 expression, while NLRC3 disrupts gammaherpesvirus latency, which is an important step for tumorigenesis.

An injectable subcutaneous colon-specific immune niche for the treatment of ulcerative colitis

As a chronic autoinflammatory condition, ulcerative colitis is often managed via systemic immunosuppressants. Here we show, in three mouse models of established ulcerative colitis, that a subcutaneously injected colon-specific immunosuppressive niche consisting of colon epithelial cells, decellularized colon extracellular matrix and nanofibres functionalized with programmed death-ligand 1, CD86, a peptide mimic of transforming growth factor-beta 1, and the immunosuppressive small-molecule leflunomide, induced intestinal immunotolerance and reduced inflammation in the animals' lower gastrointestinal tract. The bioengineered colon-specific niche triggered autoreactive T cell anergy and polarized pro-inflammatory macrophages via multiple immunosuppressive pathways, and prevented the infiltration of immune cells into the colon's lamina propria, promoting the recovery of epithelial damage. The bioengineered niche also prevented colitis-associated colorectal cancer and eliminated immune-related colitis triggered by kinase inhibitors and immune checkpoint blockade.

The NLR gene family: from discovery to present day

The mammalian NLR gene family was first reported over 20 years ago, although several genes that were later grouped into the family were already known at that time. Although it is widely known that NLRs include inflammasome receptors and/or sensors that promote the maturation of caspase 1, IL-1β, IL-18 and gasdermin D to drive inflammation and cell death, the other functions of NLR family members are less well appreciated by the scientific community. Examples include MHC class II transactivator (CIITA), a master transcriptional activator of MHC class II genes, which was the first mammalian NBD-LRR-containing protein to be identified, and NLRC5, which regulates the expression of MHC class I genes. Other NLRs govern key inflammatory signalling pathways or interferon responses, and several NLR family members serve as negative regulators of innate immune responses. Multiple NLRs regulate the balance of cell death, cell survival, autophagy, mitophagy and even cellular metabolism. Perhaps the least discussed group of NLRs are those with functions in the mammalian reproductive system. The focus of this Review is to provide a synopsis of the NLR family, including both the intensively studied and the underappreciated members. We focus on the function, structure and disease relevance of NLRs and highlight issues that have received less attention in the NLR field. We hope this may serve as an impetus for future research on the conventional and non-conventional roles of NLRs within and beyond the immune system.

Killing tumor-associated bacteria with a liposomal antibiotic generates neoantigens that induce anti-tumor immune responses

Increasing evidence implicates the tumor microbiota as a factor that can influence cancer progression. In patients with colorectal cancer (CRC), we found that pre-resection antibiotics targeting anaerobic bacteria substantially improved disease-free survival by 25.5%. For mouse studies, we designed an antibiotic silver-tinidazole complex encapsulated in liposomes (LipoAgTNZ) to eliminate tumor-associated bacteria in the primary tumor and liver metastases without causing gut microbiome dysbiosis. Mouse CRC models colonized by tumor-promoting bacteria (Fusobacterium nucleatum spp.) or probiotics (Escherichia coli Nissle spp.) responded to LipoAgTNZ therapy, which enabled more than 70% long-term survival in two F. nucleatum-infected CRC models. The antibiotic treatment generated microbial neoantigens that elicited anti-tumor CD8+ T cells. Heterologous and homologous bacterial epitopes contributed to the immunogenicity, priming T cells to recognize both infected and uninfected tumors. Our strategy targets tumor-associated bacteria to elicit anti-tumoral immunity, paving the way for microbiome-immunotherapy interventions.

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.

An epithelial-immune circuit amplifies inflammasome and IL-6 responses to SARS-CoV-2

Elevated levels of cytokines IL-1β and IL-6 are associated with severe COVID-19. Investigating the underlying mechanisms, we find that while primary human airway epithelia (HAE) have functional inflammasomes and support SARS-CoV-2 replication, they are not the source of IL-1β released upon infection. In leukocytes, the SARS-CoV-2 E protein upregulates inflammasome gene transcription via TLR2 to prime, but not activate, inflammasomes. SARS-CoV-2-infected HAE supply a second signal, which includes genomic and mitochondrial DNA, to stimulate leukocyte IL-1β release. Nuclease treatment, STING, and caspase-1 inhibition but not NLRP3 inhibition blocked leukocyte IL-1β release. After release, IL-1β stimulates IL-6 secretion from HAE. Therefore, infection alone does not increase IL-1β secretion by either cell type. Rather, bi-directional interactions between the SARS-CoV-2-infected epithelium and immune bystanders stimulates both IL-1β and IL-6, creating a pro-inflammatory cytokine circuit. Consistent with these observations, patient autopsy lungs show elevated myeloid inflammasome gene signatures in severe COVID-19.

Human ACE2 expression, a major tropism determinant for SARS-CoV-2, is regulated by upstream and intragenic elements

Angiotensin-converting enzyme 2 (ACE2), part of the renin-angiotensin system (RAS), serves as an entry point for SARS-CoV-2, leading to viral proliferation in permissive cell types. Using mouse lines in which the Ace2 locus has been humanized by syntenic replacement, we show that regulation of basal and interferon induced ACE2 expression, relative expression levels of different ACE2 transcripts, and sexual dimorphism in ACE2 expression are unique to each species, differ between tissues, and are determined by both intragenic and upstream promoter elements. Our results indicate that the higher levels of expression of ACE2 observed in the lungs of mice relative to humans may reflect the fact that the mouse promoter drives expression of ACE2 in populous airway club cells while the human promoter drives expression in alveolar type 2 (AT2) cells. In contrast to transgenic mice in which human ACE2 is expressed in ciliated cells under the control of the human FOXJ1 promoter, mice expressing ACE2 in club cells under the control of the endogenous Ace2 promoter show a robust immune response after infection with SARS-CoV-2, leading to rapid clearance of the virus. This supports a model in which differential expression of ACE2 determines which cell types in the lung are infected, and this in turn modulates the host response and outcome of COVID-19.

Viral protease cleavage of MAVS in genetically modified mice with hepatitis A virus infection

BACKGROUND & AIMS

Consistent with its relatively narrow host species range, hepatitis A virus (HAV) cannot infect C57BL/6 mice. However, in Mavs-/- mice with genetic deficiency of the innate immune signaling adaptor MAVS, HAV replicates robustly in the absence of disease. The HAV 3ABC protease cleaves MAVS in human cells, thereby disrupting virus-induced IFN responses, but it cannot cleave murine MAVS (mMAVS) due to sequence differences at the site of scission. Here, we sought to elucidate the role of 3ABC MAVS cleavage in determining HAV pathogenesis and host species range.

METHODS

Using CRISPR/Cas9 gene editing, we established two independent lineages of C57BL/6 mice with knock-in mutations altering two amino acids in mMAVS ('mMAVS-VS'), rendering it susceptible to 3ABC cleavage without loss of signaling function. We challenged homozygous Mavsvs/vs mice with HAV, and compared infection outcomes with C57BL/6 and genetically deficient Mavs-/- mice.

RESULTS

The humanized murine mMAVS-VS protein was cleaved as efficiently as human MAVS when co-expressed with 3ABC in Huh-7 cells. In embyronic fibroblasts from Mavsvs/vs mice, mMAVS-VS was cleaved by ectopically expressed 3ABC, significantly disrupting Sendai virus-induced IFN responses. However, in contrast to Mavs-/- mice with genetic MAVS deficiency, HAV failed to establish infection in Mavsvs/vs mice, even with additional genetic knockout of Trif or Irf1. Nonetheless, when crossed with permissive Ifnar1-/- mice lacking type I IFN receptors, Mavsvs/vsIfnar1-/- mice demonstrated enhanced viral replication coupled with significant reductions in serum alanine aminotransferase, hepatocellular apoptosis, and intrahepatic inflammatory cell infiltrates compared with Ifnar1-/- mice.

CONCLUSIONS

MAVS cleavage by 3ABC boosts viral replication and disrupts disease pathogenesis, but it is not by itself sufficient to break the host-species barrier to HAV infection in mice.

IMPACT AND IMPLICATIONS

The limited host range of human hepatitis viruses could be explained by species-specific viral strategies that disrupt innate immune responses. Both hepatitis A virus (HAV) and hepatitis C virus express viral proteases that cleave the innate immune adaptor protein MAVS, in human but not mouse cells. However, the impact of this immune evasion strategy has never been assessed in vivo. Here we show that HAV 3ABC protease cleavage of MAVS enhances viral replication and lessens liver inflammation in mice lacking interferon receptors, but that it is insufficient by itself to overcome the cross-species barrier to infection in mice. These results enhance our understanding of how hepatitis viruses interact with the host and their impact on innate immune responses.

Convergence of Targeted and Immune Therapies for the Treatment of Oncogene-Driven Cancers

SUMMARY

In this issue, Hattori and colleagues capitalized on targeted small-molecule covalent inhibitors of one KRAS mutant with a G12C substitution and of other oncoproteins to create drug-peptide conjugates that serve as cancer neoantigens that prompt an immune response to oncogene-mutant cancer cells. This immunotherapy strategy can serve as an effective approach to overcome the treatment-induced resistance that limits the effectiveness of essentially all small molecule-based targeted anticancer drugs. See related article by Hattori et al., p. 132 (9).

STING-induced regulatory B cells compromise NK function in cancer immunity

An immunosuppressive tumour microenvironment is a major obstacle in the control of pancreatic and other solid cancers1-3. Agonists of the stimulator of interferon genes (STING) protein trigger inflammatory innate immune responses to potentially overcome tumour immunosuppression4. Although these agonists hold promise as potential cancer therapies5, tumour resistance to STING monotherapy has emerged in clinical trials and the mechanism(s) is unclear5-7. Here we show that the administration of five distinct STING agonists, including cGAMP, results in an expansion of human and mouse interleukin (IL)-35+ regulatory B cells in pancreatic cancer. Mechanistically, cGAMP drives expression of IL-35 by B cells in an IRF3-dependent but type I interferon-independent manner. In several preclinical cancer models, the loss of STING signalling in B cells increases tumour control. Furthermore, anti-IL-35 blockade or genetic ablation of IL-35 in B cells also reduces tumour growth. Unexpectedly, the STING-IL-35 axis in B cells reduces proliferation of natural killer (NK) cells and attenuates the NK-driven anti-tumour response. These findings reveal an intrinsic barrier to systemic STING agonist monotherapy and provide a combinatorial strategy to overcome immunosuppression in tumours.

Microparticle Delivery of a STING Agonist Enables Indirect Activation of NK Cells by Antigen-Presenting Cells

Natural killer (NK) cells are an important member of the innate immune system and can participate in direct tumor cell killing in response to immunotherapies. One class of immunotherapy is stimulator of interferon gene (STING) agonists, which result in a robust type I interferon (IFN-I) response. Most mechanistic studies involving STING have focused on macrophages and T cells. Nevertheless, NK cells are also activated by IFN-I, but the effect of STING activation on NK cells remains to be adequately investigated. We show that both direct treatment with soluble STING agonist cyclic di-guanosine monophosphate-adenosine monophosphate (cGAMP) and indirect treatment with cGAMP encapsulated in microparticles (MPs) result in NK cell activation in vitro, although the former requires 100× more cGAMP than the latter. Additionally, direct activation with cGAMP leads to NK cell death. Indirect activation with cGAMP MPs does not result in NK cell death but rather cell activation and cell killing in vitro. In vivo, treatment with soluble cGAMP and cGAMP MPs both cause short-term activation, whereas only cGAMP MP treatment produces long-term changes in NK cell activation markers. Thus, this work indicates that treatment with an encapsulated STING agonist activates NK cells more efficiently than that with soluble cGAMP. In both the in vitro and in vivo systems, the MP delivery system results in more robust effects at a greatly reduced dosage. These results have potential applications in aiding the improvement of cancer immunotherapies.

The All-Encompassing Importance of Innate Immunity

In their AAI President’s Addresses reproduced in this issue, Jeremy M. Boss, Ph.D. (AAI ’94; AAI President 2019–2020) and Jenny P.-Y. Ting, Ph.D. (AAI ’97; AAI President 2020–2021) welcomed attendees to the AAI annual meeting, Virtual IMMUNOLOGY2021™. Due to the SARS-CoV-2 pandemic and the cancellation of IMMUNOLOGY2020™, Dr. Boss and Dr. Ting each presented their respective President’s Address to open the meeting.

STING agonist-containing microparticles improve seasonal influenza vaccine efficacy and durability in ferrets over standard adjuvant

The current pandemic highlights the need for effective vaccines against respiratory viruses. An ideal vaccine should induce robust and long-lasting responses with high manufacturing scalability. We use an adjuvant comprised of a Stimulator of Interferon Genes (STING) agonist incorporated in a scalable microparticle platform to achieve durable protection against the influenza virus. This formulation overcomes the challenges presented by the cytosolic localization of STING and the hydrophilicity of its agonists. We evaluated a monoaxial formulation of polymeric acetalated dextran microparticles (MPs) to deliver the STING agonist cyclic GMP-AMP (cGAMP) which achieved >10× dose-sparing effects compared to other published work. Efficacy was evaluated in ferrets, a larger animal model of choice for influenza vaccines. cGAMP MPs with recombinant hemagglutinin reduced viral shedding and improved vaccine outcomes compared to a seasonal influenza vaccine. Importantly, sustained protection against a lethal influenza infection was detected a year after a single dose of the vaccine-adjuvant.

Impact of intracellular innate immune receptors on immunometabolism

Immunometabolism, which is the metabolic reprogramming of anaerobic glycolysis, oxidative phosphorylation, and metabolite synthesis upon immune cell activation, has gained importance as a regulator of the homeostasis, activation, proliferation, and differentiation of innate and adaptive immune cell subsets that function as key factors in immunity. Metabolic changes in epithelial and other stromal cells in response to different stimulatory signals are also crucial in infection, inflammation, cancer, autoimmune diseases, and metabolic disorders. The crosstalk between the PI3K-AKT-mTOR and LKB1-AMPK signaling pathways is critical for modulating both immune and nonimmune cell metabolism. The bidirectional interaction between immune cells and metabolism is a topic of intense study. Toll-like receptors (TLRs), cytokine receptors, and T and B cell receptors have been shown to activate multiple downstream metabolic pathways. However, how intracellular innate immune sensors/receptors intersect with metabolic pathways is less well understood. The goal of this review is to examine the link between immunometabolism and the functions of several intracellular innate immune sensors or receptors, such as nucleotide-binding and leucine-rich repeat-containing receptors (NLRs, or NOD-like receptors), absent in melanoma 2 (AIM2)-like receptors (ALRs), and the cyclic dinucleotide receptor stimulator of interferon genes (STING). We will focus on recent advances and describe the impact of these intracellular innate immune receptors on multiple metabolic pathways. Whenever appropriate, this review will provide a brief contextual connection to pathogenic infections, autoimmune diseases, cancers, metabolic disorders, and/or inflammatory bowel diseases.

Move over NAIP, DDX17 diversifies the NLRC4 inflammasome

[Figure: see text].

Inflammasome Assays In Vitro and in Mouse Models

This article presents assays that allow induction and measurement of activation of different inflammasomes in mouse macrophages, human peripheral blood mononuclear cell (PBMC) cultures, and mouse peritonitis and endotoxic shock models. Basic Protocol 1 describes how to prime the inflammasome in mouse macrophages with different Toll-like receptor agonists and TNF-α; how to induce NLRP1, NLRP3, NLRC4, and AIM2 inflammasome activation by their corresponding stimuli; and how to measure inflammasome activation-mediated maturation of interleukin (IL)-1β and IL-18 and pyroptosis. Since the well-established agonists for NLRP1 are inconsistent between mice and humans, Basic Protocol 2 describes how to activate the NLRP1 inflammasome in human PBMCs. Basic Protocol 3 describes how to purify, crosslink, and detect the apoptosis-associated speck-like protein containing a CARD (ASC) pyroptosome. Formation of the ASC pyroptosome is a signature of inflammasome activation. A limitation of ASC pyroptosome detection is the requirement of a relatively large cell number. Alternate Protocol 1 is provided to stain ASC pyroptosomes using an anti-ASC antibody and to measure ASC specks by fluorescence microscopy in a single cell. Intraperitoneal injection of lipopolysaccharides (LPS) and inflammasome agonists will induce peritonitis, which is seen as an elevation of IL-1β and other proinflammatory cytokines and an infiltration of neutrophils and inflammatory monocytes. Basic Protocol 4 describes how to induce NLRP3 inflammasome activation and peritonitis by priming mice with LPS and subsequently challenging them with monosodium urate (MSU). The method for measuring cytokines in serum and through peritoneal lavage is also described. Finally, Alternate Protocol 2 describes how to induce noncanonical NLRP3 inflammasome activation by high-dose LPS challenge in a sepsis model. © 2020 Wiley Periodicals LLC. Basic Protocol 1: Priming and activation of inflammasomes in mouse macrophages Basic Protocol 2: Activation of human NLRP1 inflammasome by DPP8/9 inhibitor talabostat Basic Protocol 3: Purification and detection of ASC pyroptosome Alternate Protocol 1: Detection of ASC speck by immunofluorescence staining Basic Protocol 4: Activation of canonical NLRP3 inflammasome in mice by intraperitoneal delivery of MSU crystals Alternate Protocol 2: Activation of noncanonical NLRP3 inflammasome in mice by intraperitoneal delivery of LPS.

TLR9- and CD40-Targeting Vaccination Promotes Human B Cell Maturation and IgG Induction via pDC-Dependent Mechanisms in Humanized Mice

Mice reconstituted with a human immune system (humanized mice) provide a robust model to study human immunology, vaccinology, and human infectious diseases. However, the development and function of B cells in humanized mice is impaired. B cells from humanized mice are immature and are impaired in IgM to IgG isotype switch in response to infection or vaccination. In the present study we report that Toll-like receptor 9 (TLR9) agonist CpG-B combined with CD40-targeting vaccination triggered human B cell immunoglobin class-switch from IgM⁺ to IgG⁺ B cells in humanized mice. Human B cells from mice vaccinated with CpG-B as adjuvant were more mature in phenotype and produced significant levels of both total IgG and antigen-specific IgG. We found that CpG-B treatment activated human pDCs (plasmacytoid dendritic cells) in vivo to induce interferon-alpha (IFN-α)expression in humanized mice. Pre-depletion of human pDC in vivo abrogated the adjuvant effect of CpG-B. Our results indicate that TLR9 and CD40-targeting vaccination triggers human B cell maturation and immunoglobulin class-switch in a pDC-dependent manner in humanized mice. The findings also shed light on induction of human IgG antibodies in humanized mouse models.

STING Agonist Mitigates Experimental Autoimmune Encephalomyelitis by Stimulating Type I IFN-Dependent and -Independent Immune-Regulatory Pathways

The cGAS-cyclic GMP-AMP (cGAMP)-stimulator of IFN genes (STING) pathway induces a powerful type I IFN (IFN-I) response and is a prime candidate for augmenting immunity in cancer immunotherapy and vaccines. IFN-I also has immune-regulatory functions manifested in several autoimmune diseases and is a first-line therapy for relapsing-remitting multiple sclerosis. However, it is only moderately effective and can induce adverse effects and neutralizing Abs in recipients. Targeting cGAMP in autoimmunity is unexplored and represents a challenge because of the intracellular location of its receptor, STING. We used microparticle (MP)-encapsulated cGAMP to increase cellular delivery, achieve dose sparing, and reduce potential toxicity. In the C57BL/6 experimental allergic encephalomyelitis (EAE) model, cGAMP encapsulated in MPs (cGAMP MPs) administered therapeutically protected mice from EAE in a STING-dependent fashion, whereas soluble cGAMP was ineffective. Protection was also observed in a relapsing-remitting model. Importantly, cGAMP MPs protected against EAE at the peak of disease and were more effective than rIFN-β. Mechanistically, cGAMP MPs showed both IFN-I-dependent and -independent immunosuppressive effects. Furthermore, it induced the immunosuppressive cytokine IL-27 without requiring IFN-I. This augmented IL-10 expression through activated ERK and CREB. IL-27 and subsequent IL-10 were the most important cytokines to mitigate autoreactivity. Critically, cGAMP MPs promoted IFN-I as well as the immunoregulatory cytokines IL-27 and IL-10 in PBMCs from relapsing-remitting multiple sclerosis patients. Collectively, this study reveals a previously unappreciated immune-regulatory effect of cGAMP that can be harnessed to restrain T cell autoreactivity.

AIM2 in regulatory T cells restrains autoimmune diseases

The inflammasome comprised of an innate immune receptor/sensor, pro-caspase-1, and a common adaptor molecule, ASC (apoptotic speck-containing protein with a CARD) initiates innate defense and inflammatory response by activating caspase-1 and pyroptotic cell death in myeloid cells. Consistent with their pro-inflammatory function, caspase-1, ASC and NLRP3 were known to exacerbate autoimmunity during experimental autoimmune encephalomyelitis (EAE) by enhancing IL-1b and IL-18 secretion in myeloid cells. Here we reveal an unexpected function of a DNA-binding inflammasome receptor, AIM2 (Absent in Melanoma 2), in T regulatory cells (Tregs) to restrain two models of autoimmune disease, including experimental autoimmune encephalomyelitis and T cell-mediated colitis, by studying whole-body and Treg-specific Aim2-deficient mice. AIM2 is highly expressed by human and mouse Tregs, with its expression induced by TGF-b and its promoter occupied by transcription factors associated with Tregs, including Runx1, Ets1, Bcl11b and CREB. Moreover, we demonstrate that AIM2 promotes the stability of Tregs during inflammation in lineage tracing mice. RNA-seq, biochemical and metabolic analyses revealed that AIM2 attenuates Akt-phosphorylation, mTOR, Myc and glycolysis, but promotes lipid oxidative phosphorylation in Tregs. Mechanistically, AIM2 interacts with the RACK1/PP2A-phosphatase complex to restrain Akt-phosphorylation. While AIM2 is generally accepted as an inflammasome effector in myeloid cells, this report unveils a T cell-intrinsic role of AIM2 in restraining autoimmunity by diminishing Akt-mTOR signaling and altering immune-metabolism to enhance Treg stability.

Multi-omics analyses reveal that HIV-1 alters CD4+ T cell immunometabolism to fuel virus replication

Individuals infected with human immunodeficiency virus type-1 (HIV-1) show metabolic alterations of CD4⁺ T cells through unclear mechanisms with undefined consequences. We analyzed the transcriptome of CD4⁺ T cells from patients with HIV-1 and revealed that the elevated oxidative phosphorylation (OXPHOS) pathway is associated with poor outcomes. Inhibition of OXPHOS by the US Food and Drug Administration-approved drug metformin, which targets mitochondrial respiratory chain complex-I, suppresses HIV-1 replication in human CD4⁺ T cells and humanized mice. In patients, HIV-1 peak viremia positively correlates with the expression of NLRX1, a mitochondrial innate immune receptor. Quantitative proteomics and metabolic analyses reveal that NLRX1 enhances OXPHOS and glycolysis during HIV-1-infection of CD4⁺ T cells to promote viral replication. At the mechanistic level, HIV infection induces the association of NLRX1 with the mitochondrial protein FASTKD5 to promote expression of mitochondrial respiratory complex components. This study uncovers the OXPHOS pathway in CD4⁺ T cells as a target for HIV-1 therapy.

DCAF1 regulates Treg senescence via the ROS axis during immunological aging

As a hallmark of immunological aging, low-grade, chronic inflammation with accumulation of effector memory T cells contributes to increased susceptibility to many aging-related diseases. While the proinflammatory state of aged T cells indicates a dysregulation of immune homeostasis, whether and how aging drives regulatory T cell (Treg) aging and alters Treg function are not fully understood owing to a lack of specific aging markers. Here, by a combination of cellular, molecular, and bioinformatic approaches, we discovered that Tregs senesce more severely than conventional T (Tconv) cells during aging. We found that Tregs from aged mice were less efficient than young Tregs in suppressing Tconv cell function in an inflammatory bowel disease model and in preventing Tconv cell aging in an irradiation-induced aging model. Furthermore, we revealed that DDB1- and CUL4-associated factor 1 (DCAF1) was downregulated in aged Tregs and was critical to restrain Treg aging via reactive oxygen species (ROS) regulated by glutathione-S-transferase P (GSTP1). Importantly, interfering with GSTP1 and ROS pathways reinvigorated the proliferation and function of aged Tregs. Therefore, our studies uncover an important role of the DCAF1/GSTP1/ROS axis in Treg senescence, which leads to uncontrolled inflammation and immunological aging.

AIM2 in regulatory T cells restrains autoimmune diseases

The inflammasome initiates innate defense and inflammatory response by activating caspase-1 and pyroptotic cell death in myeloid cells^1,2. It is comprised of an innate immune receptor/sensor, pro-caspase-1, and a common adaptor molecule, ASC (apoptotic speck-containing protein with a CARD). Consistent with their pro-inflammatory function, caspase-1, ASC and NLRP3 exacerbate autoimmunity during experimental autoimmune encephalomyelitis (EAE) by enhancing IL-1β and IL-18 secretion in myeloid cells^3–6. Here we reveal an unexpected function of a DNA-binding inflammasome receptor, AIM2 (Absent in Melanoma 2)^7–10, in T regulatory cells (Tregs) to restrain two models of autoimmunity (experimental autoimmune encephalomyelitis and T cell-mediated colitis) by studying whole-body and Treg-specific Aim2^–/– mice. AIM2 is highly expressed by human and mouse Tregs, with its expression induced by TGF-β and its promoter occupied by transcription factors associated with Tregs, including Runx1, Ets1, Bcl11b and CREB. RNA-seq, biochemical and metabolic analyses revealed that AIM2 attenuates Akt-phosphorylation, mTOR, c-Myc and glycolysis, but promotes lipid oxidative phosphorylation in Tregs. Mechanistically, AIM2 interacts with the RACK1/PP2A-phosphatase complex to restrain Akt-phosphorylation. Lineage tracing demonstrates that AIM2 promotes the stability of Tregs during inflammation. While AIM2 is generally accepted as an inflammasome effector in myeloid cells, this report reveals a T cell-intrinsic role of AIM2 in restraining autoimmunity by diminishing Akt-mTOR signaling and altering immune-metabolism to enhance Treg stability.

Mitochondrial GSDMD Pores DAMPen Pyroptosis

Noncanoncial inflammasome activation by cytosolic lipopolysaccharide (LPS) causes pyroptotic cell death facilitated by gasdermin D (GSDMD) pore formation. In this issue of Immunity, Huang et al. describe how cytosolic LPS in endothelial cells does not cause cell death but restrains endothelial cell proliferation.

Pulmonary Delivery of Nanoparticle-Bound Toll-like Receptor 9 Agonist for the Treatment of Metastatic Lung Cancer

CpG oligodeoxynucleotides are potent toll-like receptor (TLR) 9 agonists and have shown promise as anticancer agents in preclinical studies and clinical trials. Binding of CpG to TLR9 initiates a cascade of innate and adaptive immune responses, beginning with activation of dendritic cells and resulting in a range of secondary effects that include the secretion of pro-inflammatory cytokines, activation of natural killer cells, and expansion of T cell populations. Recent literature suggests that local delivery of CpG in tumors results in superior antitumor effects as compared to systemic delivery. In this study, we utilized PRINT (particle replication in nonwetting templates) nanoparticles as a vehicle to deliver CpG into murine lungs through orotracheal instillations. In two murine orthotopic metastasis models of non-small-cell lung cancer-344SQ (lung adenocarcinoma) and KAL-LN2E1 (lung squamous carcinoma), local delivery of PRINT-CpG into the lungs effectively promoted substantial tumor regression and also limited systemic toxicities associated with soluble CpG. Furthermore, cured mice were completely resistant to tumor rechallenge. Additionally, nanodelivery showed extended retention of CpG within the lungs as well as prolonged elevation of antitumor cytokines in the lungs, but no elevated levels of proinflammatory cytokines in the serum. These results demonstrate that PRINT-CpG is a potent nanoplatform for local treatment of lung cancer that has collateral therapeutic effects on systemic disease and an encouraging toxicity profile and may have the potential to treat lung metastasis of other cancer types.

TRAF3IP3 negatively regulates cytosolic RNA induced anti-viral signaling by promoting TBK1 K48 ubiquitination

Innate immunity to nucleic acids forms the backbone for anti-viral immunity and several inflammatory diseases. Upon sensing cytosolic viral RNA, retinoic acid-inducible gene-I-like receptors (RLRs) interact with the mitochondrial antiviral signaling protein (MAVS) and activate TANK-binding kinase 1 (TBK1) to induce type I interferon (IFN-I). TRAF3-interacting protein 3 (TRAF3IP3, T3JAM) is essential for T and B cell development. It is also well-expressed by myeloid cells, where its role is unknown. Here we report that TRAF3IP3 suppresses cytosolic poly(I:C), 5'ppp-dsRNA, and vesicular stomatitis virus (VSV) triggers IFN-I expression in overexpression systems and Traf3ip3-/- primary myeloid cells. The mechanism of action is through the interaction of TRAF3IP3 with endogenous TRAF3 and TBK1. This leads to the degradative K48 ubiquitination of TBK1 via its K372 residue in a DTX4-dependent fashion. Mice with myeloid-specific gene deletion of Traf3ip3 have increased RNA virus-triggered IFN-I production and reduced susceptibility to virus. These results identify a function of TRAF3IP3 in the regulation of the host response to cytosolic viral RNA in myeloid cells.

Platelet-activating factor (PAF) mediates NLRP3-NEK7 inflammasome induction independently of PAFR

Platelet-activating factor (PAF) can drive pathophysiological inflammation, but the mechanism remains incompletely understood. Here, Deng et al. report that PAF activates the canonical NLRP3 inflammasome independently of its receptor PAFR.

The role of lipids in inflammasome activation remains underappreciated. The phospholipid, platelet-activating factor (PAF), exerts multiple physiological functions by binding to a G protein–coupled seven-transmembrane receptor (PAFR). PAF is associated with a number of inflammatory disorders, yet the molecular mechanism underlying its proinflammatory function remains to be fully elucidated. We show that multiple PAF isoforms and PAF-like lipids can activate the inflammasome, resulting in IL-1β and IL-18 maturation. This is dependent on NLRP3, ASC, caspase-1, and NEK7, but not on NLRC4, NLRP1, NLRP6, AIM2, caspase-11, or GSDMD. Inflammasome activation by PAF also requires potassium efflux and calcium influx but not lysosomal cathepsin or mitochondrial reactive oxygen species. PAF exacerbates peritonitis partly through inflammasome activation, but PAFR is dispensable for PAF-induced inflammasome activation in vivo or in vitro. These findings reveal that PAF represents a damage-associated signal that activates the canonical inflammasome independently of PAFR and provides an explanation for the ineffectiveness of PAFR antagonist in blocking PAF-mediated inflammation in the clinic.

Viral DNA Binding to NLRC3, an Inhibitory Nucleic Acid Sensor, Unleashes STING, a Cyclic Dinucleotide Receptor that Activates Type I Interferon

Immune suppression is a crucial component of immunoregulation and a subgroup of nucleotide-binding domain (NBD), leucine-rich repeat (LRR)-containing proteins (NLRs) attenuate innate immunity. How this inhibitory function is controlled is unknown. A key question is whether microbial ligands can regulate this inhibition. NLRC3 is a negative regulator that attenuates type I interferon (IFN-I) response by sequestering and attenuating stimulator of interferon genes (STING) activation. Here, we report that NLRC3 binds viral DNA and other nucleic acids through its LRR domain. DNA binding to NLRC3 increases its ATPase activity, and ATP-binding by NLRC3 diminishes its interaction with STING, thus licensing an IFN-I response. This work uncovers a mechanism wherein viral nucleic acid binding releases an inhibitory innate receptor from its target.

NLRX1 Facilitates Histoplasma capsulatum-Induced LC3-Associated Phagocytosis for Cytokine Production in Macrophages

LC3-associated phagocytosis (LAP) is an emerging non-canonical autophagy process that bridges signaling from pattern-recognition receptors (PRRs) to autophagic machinery. LAP formation results in incorporation of lipidated LC3 into phagosomal membrane (termed LAPosome). Increasing evidence reveals that LAP functions as an innate defense mechanism against fungal pathogens. However, the molecular mechanism involved and the consequence of LAP in regulating anti-fungal immune response remain largely unexplored. Here we show that Histoplasma capsulatum is taken into LAPosome upon phagocytosis by macrophages. Interaction of H. capsulatum with Dectin-1 activates Syk and triggers subsequent NADPH oxidase-mediated reactive oxygen species (ROS) response that is involved in LAP induction. Inhibiting LAP induction by silencing LC3α/β or treatment with ROS inhibitor impairs the activation of MAPKs-AP-1 pathway, thereby reduces macrophage proinflammatory cytokine response to H. capsulatum. Additionally, we unravel the importance of NLRX1 in fungus-induced LAP. NLRX1 facilitates LAP by interacting with TUFM which associates with autophagic proteins ATG5-ATG12 for LAPosome formation. Macrophages from Nlrx1 ^-/- mice or TUFM-silenced cells exhibit reduced LAP induction and LAP-mediated MAPKs-AP-1 activation for cytokine response to H. capsulatum. Furthermore, inhibiting ROS production in Nlrx1 ^-/- macrophages almost completely abolishes H. capsulatum-induced LC3 conversion, indicating that both Dectin-1/Syk/ROS-dependent pathway and NLRX1-TUFM complex-dependent pathway collaboratively contribute to LAP induction. Our findings reveal new pathways underlying LAP induction by H. capsulatum for macrophage cytokine response.

A nanoparticle-incorporated STING activator enhances antitumor immunity in PD-L1-insensitive models of triple-negative breast cancer

Triple-negative breast cancer (TNBC) has few therapeutic options, and alternative approaches are urgently needed. Stimulator of IFN genes (STING) is becoming an exciting target for therapeutic adjuvants. However, STING resides inside the cell, and the intracellular delivery of CDNs, such as cGAMP, is required for the optimal activation of STING. We show that liposomal nanoparticle-delivered cGAMP (cGAMP-NP) activates STING more effectively than soluble cGAMP. These particles induce innate and adaptive host immune responses to preexisting tumors in both orthotopic and genetically engineered models of basal-like TNBC. cGAMP-NPs also reduce melanoma tumor load, with limited responsivity to anti-PD-L1. Within the tumor microenvironment, cGAMP-NPs direct both mouse and human macrophages (M), reprograming from protumorigenic M2-like phenotype toward M1-like phenotype; enhance MHC and costimulatory molecule expression; reduce M2 biomarkers; increase IFN-γ-producing T cells; augment tumor apoptosis; and increase CD4+ and CD8+ T cell infiltration. Activated T cells are required for tumor suppression, as their depletion reduces antitumor activity. Importantly, cGAMP-NPs prevent the formation of secondary tumors, and a single dose is sufficient to inhibit TNBC. These data suggest that a minimal system comprised of cGAMP-NP alone is sufficient to modulate the tumor microenvironment to effectively control PD-L1-insensitive TNBC.

Acetalated Dextran Microparticles for Codelivery of STING and TLR7/8 Agonists

Vaccines are the most effective tool for preventing infectious diseases; however, subunit vaccines, considered the safest type, suffer from poor immunogenicity and require adjuvants to create a strong and sustained immune response. As adjuvants, pathogen-associated molecular patterns (PAMPs) offer potent immunostimulatory properties and defined mechanisms of action through their cognate pattern recognition receptors (PRRs). Their activity can be further enhanced through combining two or more PAMPs, particularly those that activate multiple immune signaling pathways. However, the cytosolic localization of many PRRs requires intracellular delivery of PAMPs for optimal biological activity, which is particularly true of the stimulator of interferon genes (STING) PRR. Using acetalated dextran (Ace-DEX) microparticles (MPs) encapsulating STING agonist 3'3'-cyclic GMP-AMP (cGAMP) combined with soluble PAMPS, we screened the effect of codelivery of adjuvants using primary mouse bone marrow derived dendritic cells (BMDCs). We identified that codelivery of cGAMP MPs and soluble Toll-like receptor 7/8 (TLR7/8) agonist resiquimod (R848) elicited the broadest cytokine response. cGAMP and R848 were then coencapsulated within Ace-DEX MPs via electrospray. Using the model antigen ovalbumin, we observed that Ace-DEX MPs coencapsulating cGAMP and R848 (cGAMP/R848 Ace-DEX MPs) induced antigen-specific cellular immunity, and a balanced Th1/Th2 humoral response that was greater than cGAMP Ace-DEX MPs alone and PAMPs delivered in separate MPs. These data indicate that polymeric Ace-DEX MPs loaded with STING and TLR7/8 agonists represent a potent cellular and humoral vaccine adjuvant.

Investigation of tunable acetalated dextran microparticle platform to optimize M2e-based influenza vaccine efficacy

Influenza places a significant health and economic burden on society. Efficacy of seasonal influenza vaccines can be suboptimal due to poor matching between vaccine and circulating viral strains. An influenza vaccine that is broadly protective against multiple virus strains would significantly improve vaccine efficacy. The highly conserved ectodomain of matrix protein 2 (M2e) and 3'3' cyclic GMP-AMP (cGAMP) were selected as the antigen and adjuvant, respectively, to develop the basis for a potential universal influenza vaccine. The magnitude and kinetics of adaptive immune responses can have great impact on vaccine efficacy. M2e and cGAMP were therefore formulated within acetalated dextran (Ace-DEX) microparticles (MPs) of varying degradation profiles to examine the effect of differential vaccine delivery on humoral, cellular, and protective immunity. All Ace-DEX MP vaccines containing M2e and cGAMP elicited potent humoral and cellular responses in vivo and offered substantial protection against a lethal influenza challenge, suggesting significant vaccine efficacy. Serum antibodies from Ace-DEX MP vaccinated mice also demonstrated cross reactivity against M2e sequences of various viral strains, which indicates the potential for broadly protective immunity. Of all the formulations tested, the slowest-degrading M2e or cGAMP MPs elicited the greatest antibody production, cellular response, and protection against a viral challenge. This indicated the importance of flexible control over antigen and adjuvant delivery. Overall, robust immune responses, cross reactivity against multiple viral strains, and tunable delivery profiles make the Ace-DEX MP platform a powerful subunit vaccine delivery system.

The Inhibitory Innate Immune Sensor NLRP12 Maintains a Threshold against Obesity by Regulating Gut Microbiota Homeostasis

In addition to high-fat diet (HFD) and inactivity, inflammation and microbiota composition contribute to obesity. Inhibitory immune receptors, such as NLRP12, dampen inflammation and are important for resolving inflammation, but their role in obesity is unknown. We show that obesity in humans correlates with reduced expression of adipose tissue NLRP12. Similarly, Nlrp12-/- mice show increased weight gain, adipose deposition, blood glucose, NF-κB/MAPK activation, and M1-macrophage polarization. Additionally, NLRP12 is required to mitigate HFD-induced inflammasome activation. Co-housing with wild-type animals, antibiotic treatment, or germ-free condition was sufficient to restrain inflammation, obesity, and insulin tolerance in Nlrp12-/- mice, implicating the microbiota. HFD-fed Nlrp12-/- mice display dysbiosis marked by increased obesity-associated Erysipelotrichaceae, but reduced Lachnospiraceae family and the associated enzymes required for short-chain fatty acid (SCFA) synthesis. Lachnospiraceae or SCFA administration attenuates obesity, inflammation, and dysbiosis. These findings reveal that Nlrp12 reduces HFD-induced obesity by maintaining beneficial microbiota.

NLRX1 suppresses tumorigenesis and attenuates histiocytic sarcoma through the negative regulation of NF-κB signaling

Histiocytic sarcoma is an uncommon malignancy in both humans and veterinary species. Research exploring the pathogenesis of this disease is scarce; thus, diagnostic and therapeutic options for patients are limited. Recent publications have suggested a role for the NLR, NLRX1, in acting as a tumor suppressor. Based on these prior findings, we hypothesized that NLRX1 would function to inhibit tumorigenesis and thus the development of histiocytic sarcoma. To test this, we utilized Nlrx1-/- mice and a model of urethane-induced tumorigenesis. Nlrx1-/- mice exposed to urethane developed splenic histiocytic sarcoma that was associated with significant up-regulation of the NF-κB signaling pathway. Additionally, development of these tumors was also significantly associated with the increased regulation of genes associated with AKT signaling, cell death and autophagy. Together, these data show that NLRX1 suppresses tumorigenesis and reveals new genetic pathways involved in the pathobiology of histiocytic sarcoma.

A robust microparticle platform for a STING-targeted adjuvant that enhances both humoral and cellular immunity during vaccination

Most FDA-approved adjuvants for infectious agents boost humoral but not cellular immunity, and have poorly-understood mechanisms. Stimulator of interferon genes (STING, also known as MITA, MPYS, or ERIS) is an exciting adjuvant target due to its role in cyclic dinucleotide (CDN)-driven anti-viral immunity; however, a major hindrance is STING's cytosolic localization which requires intracellular delivery of its agonists. As a result, STING agonists administered in a soluble form have elicited suboptimal immune responses. Delivery of STING agonists via particle platforms has proven a more successful strategy, but the opportunity for improved formulations and bioactivity remains. In this study we evaluated the adjuvant activity of the potent STING agonist, CDN 3'3'-cGAMP (cGAMP), encapsulated in acid-sensitive acetalated dextran (Ace-DEX) polymeric microparticles (MPs) which passively target antigen-presenting cells for intracellular release. This formulation was superior to all particle delivery systems evaluated and maintained its bioactivity following a sterilizing dose of gamma irradiation. Compared to soluble cGAMP, the Ace-DEX cGAMP MPs enhanced type-I interferon responses nearly 1000-fold in vitro and 50-fold in vivo, caused up to a 104-fold boost in antibody titers, increased Th1-associated responses, and expanded germinal center B cells and memory T cells. Furthermore, the encapsulated cGAMP elicited no observable toxicity in animals and achieved protective immunity against a lethal influenza challenge seven months post-immunization when using CDN adjuvant doses up to 100-fold lower than previous reports. For these reasons, Ace-DEX MP-encapsulated cGAMP represents a potent vaccine adjuvant of humoral and cellular immunity.

The Toll-Like Receptor 2/6 Agonist, FSL-1 Lipopeptide, Therapeutically Mitigates Acute Radiation Syndrome

Risks of radiation exposure from nuclear incidents and cancer radiotherapy are undeniable realities. These dangers urgently compel the development of agents for ameliorating radiation–induced injuries. Biologic pathways mediated by myeloid differentiation primary response gene 88 (MyD88), the common adaptor for toll–like receptor (TLR) and Interleukin–1 receptor signaling, are critical for radioprotection. Treating with agonists prior to radiation enhances survival by activating TLR signaling, whereas radiomitigating TLR–activating therapeutics given after exposure are less defined. We examine the radiomitigation capability of TLR agonists and identify one that is superior for its efficacy and reduced toxic consequences compared to other tested agonists. We demonstrate that the synthetic TLR2/6 ligand Fibroblast–stimulating lipopeptide (FSL–1) substantially prolongs survival in both male and female mice when administered 24 hours after radiation and shows MyD88–dependent function. FSL–1 treatment results in accelerated hematopoiesis in bone marrow, spleen and periphery, and augments systemic levels of hematopoiesis–stimulating factors. The ability of FSL–1 to stimulate hematopoiesis is critical, as hematopoietic dysfunction results from a range of ionizing radiation doses. The efficacy of a single FSL–1 dose for alleviating radiation injury while protecting against adverse effects reveals a viable radiation countermeasures agent.

An NLRP3 Mutation Causes Arthropathy and Osteoporosis in Humanized Mice

The NLRP3 inflammasome plays a critical role in host defense by facilitating caspase I activation and maturation of IL-1β and IL-18, whereas dysregulation of inflammasome activity results in autoinflammatory disease. Factors regulating human NLRP3 activity that contribute to the phenotypic heterogeneity of NLRP3-related diseases have largely been inferred from the study of Nlrp3 mutant mice. By generating a mouse line in which the NLRP3 locus is humanized by syntenic replacement, we show the functioning of the human NLRP3 proteins in vivo, demonstrating the ability of the human inflammasome to orchestrate immune reactions in response to innate stimuli. Humanized mice expressing disease-associated mutations develop normally but display acute sensitivity to endotoxin and develop progressive and debilitating arthritis characterized by granulocytic infiltrates, elevated cytokines, erosion of bones, and osteoporosis. This NLRP3-dependent arthritis model provides a platform for testing therapeutic reagents targeting the human inflammasome.

A noncanonical function of cGAMP in inflammasome priming and activation

IFN-I signaling and inflammasome activation are two innate pathways important for combatting a variety of pathogens. Swanson et al. show that cGAMP activates the inflammasome in addition to IFN-I, and that the activation of both is needed to control infection by a DNA virus.

Recognition of pathogen-associated molecular patterns and danger-associated molecular patterns by host cells is an important step in innate immune activation. The DNA sensor cyclic guanosine monophosphate–adenosine monophosphate (cGAMP) synthase (cGAS) binds to DNA and produces cGAMP, which in turn binds to stimulator of interferon genes (STING) to activate IFN-I. Here we show that cGAMP has a noncanonical function in inflammasome activation in human and mouse cells. Inflammasome activation requires two signals, both of which are activated by cGAMP. cGAMP alone enhances expression of inflammasome components through IFN-I, providing the priming signal. Additionally, when combined with a priming signal, cGAMP activates the inflammasome through an AIM2, NLRP3, ASC, and caspase-1 dependent process. These two cGAMP-mediated functions, priming and activation, have differential requirements for STING. Temporally, cGAMP induction of IFN-I precedes inflammasome activation, which then occurs when IFN-I is waning. In mice, cGAS/cGAMP amplify both inflammasome and IFN-I to control murine cytomegalovirus. Thus, cGAMP activates the inflammasome in addition to IFN-I, and activation of both is needed to control infection by a DNA virus.

NLRX1 promotes immediate IRF1-directed antiviral responses by limiting dsRNA-activated translational inhibition mediated by PKR

NLRX1 is unique among nucleotide-binding domain and leucine-rich repeat (NLR) proteins in its mitochondrial localization and capacity to negatively regulate MAVS- and STING-dependent antiviral innate immunity. However, some studies suggest a positive regulatory role for NLRX1 in inducing antiviral responses. We show that NLRX1 exerts opposing regulatory effects on virus activation of the transcription factors IRF1 and IRF3, potentially explaining these contradictory results. Whereas NLRX1 suppresses MAVS-mediated IRF3 activation, NLRX1 conversely facilitates virus-induced increases in IRF1 expression, thereby enhancing control of virus infection. NLRX1 has a minimal effect on NF-κB-mediated IRF1 transcription, and regulates IRF1 abundance post-transcriptionally by preventing translational shutdown mediated by the dsRNA-activated protein kinase PKR, thereby allowing virus-induced increases in IRF1 protein abundance.

Prion-like Aggregation of Mitochondrial Antiviral Signaling Protein in Lupus Patients Is Associated With Increased Levels of Type I Interferon

OBJECTIVE

Increased levels of type I interferon (IFN) and type I IFN-regulated genes are found in patients with systemic lupus erythematosus (SLE) and may be central to its pathogenesis. Mitochondrial antiviral signaling protein (MAVS) is a key regulator of type I IFN that undergoes a dramatic prion-like aggregation and self propagates the activation signal from viral RNA to amplify downstream IFN production. We undertook this study to determine whether such MAVS aggregates might play a role in the sustained increased production of type I IFN in SLE.

METHODS

Peripheral blood mononuclear cells were isolated and mitochondrial extracts were prepared. MAVS aggregation was detected by semidenatured agarose gel electrophoresis and confirmed by immunofluorescence staining. MAVS-associated signaling proteins were analyzed by Western blotting. MAVS aggregation-associated gene expression signature was analyzed by microarray.

RESULTS

In blood cells from 22 of 67 SLE patients, essentially all MAVS was in a high molecular weight aggregated form. None of 6 rheumatoid arthritis patients and only 3 of 33 healthy controls had abnormal MAVS. Compared to MAVS aggregate-negative patients, MAVS aggregate-positive SLE patients had significantly higher serum levels of IFNβ and significantly increased levels of autoantibodies against Sm and U1 RNP. Gene array data revealed a characteristic gene expression pattern in these patients, with altered expression of genes involved in IFN signaling and membrane trafficking.

CONCLUSION

Persistent MAVS aggregates may lead to increased type I IFN production and result in unmitigated signals leading to autoimmunity.

A robust microparticle platform for a STING-targeted adjuvant that enhances both humoral and cellular immunity during vaccination

Most FDA-approved adjuvants for infectious agents boost humoral but not cellular immunity, and have poorly-understood mechanisms. Stimulator of interferon genes (STING) is an exciting adjuvant target due to its role in anti-viral immunity; however, a major hindrance is STING’s cytosolic localization which requires intracellular delivery of its agonists. As a result, STING agonists administered in a soluble form have elicited suboptimal immune responses. Delivery of STING agonists via particle platforms has proven a more successful strategy, but has only been assessed at doses that are likely cost-prohibitive to clinical translation for vaccines against infectious diseases. Hence, alternative platforms for the intracellular delivery of STING-agonists are needed. We evaluated the adjuvant activity of a potent STING agonist, encapsulated in acid-sensitive acetalated dextran (Ace- DEX) polymeric microparticles (MPs) which target antigen-presenting cells for intracellular release. This formulation was superior to all particle delivery systems tested, achieved protective immunity in vivo at doses of STING agonist 50-fold lower than previous reports, and elicited no observable toxicity in animals. Compared to soluble agonist, the STING agonist Ace-DEX MPs enhanced type-I interferon responses up to 1000-fold in vitro and 50-fold in vivo, caused up to 104-fold increases in antibody titers, enhanced Th1-associated responses, and expanded germinal center B cells and memory T cells. It also provided protection against a lethal influenza challenge. Thus, Ace-DEX MP-encapsulated STING agonist represents a novel and feasible vaccine adjuvant of humoral and cellular immunity.

NLRX1 negatively modulates type I IFN to facilitate KSHV reactivation from latency

Kaposi's sarcoma-associated herpesvirus (KSHV) is a herpesvirus that is linked to Kaposi's sarcoma (KS), primary effusion lymphoma (PEL) and multicentric Castleman's disease (MCD). KSHV establishes persistent latent infection in the human host. KSHV undergoes periods of spontaneous reactivation where it can enter the lytic replication phase of its lifecycle. During KSHV reactivation, host innate immune responses are activated to restrict viral replication. Here, we report that NLRX1, a negative regulator of the type I interferon response, is important for optimal KSHV reactivation from latency. Depletion of NLRX1 in either iSLK.219 or BCBL-1 cells significantly suppressed global viral transcription levels compared to the control group. Concomitantly, fewer viral particles were present in either cells or supernatant from NLRX1 depleted cells. Further analysis revealed that upon NLRX1 depletion, higher IFNβ transcription levels were observed, which was also associated with a transcriptional upregulation of JAK/STAT pathway related genes in both cell lines. To investigate whether IFNβ contributes to NLRX1's role in KSHV reactivation, we treated control and NLRX1 depleted cells with a TBK1 inhibitor (BX795) or TBK1 siRNA to block IFNβ production. Upon BX795 or TBK1 siRNA treatment, NLRX1 depletion exhibited less inhibitory effects on reactivation and infectious virion production, suggesting that NLRX1 facilitates KSHV lytic replication by negatively regulating IFNβ responses. Our data suggests that NLRX1 plays a positive role in KSHV lytic replication by suppressing the IFNβ response during the process of KSHV reactivation, which might serve as a potential target for restricting KSHV replication and transmission.

NLR members NLRC4 and NLRP3 mediate sterile inflammasome activation in microglia and astrocytes

Lysophosphatidylcholine is associated with neurodegeneration and demyelination. Freeman et al. demonstrate that lysophosphatidylcholine triggers NLRP3- and NLRC4-dependent inflammasome activation, and in a synergistic fashion, NLRP3 and NLRC4 contribute to a cuprizone-induced demyelination model in vivo.

Inflammation in the brain accompanies several high-impact neurological diseases including multiple sclerosis (MS), stroke, and Alzheimer’s disease. Neuroinflammation is sterile, as damage-associated molecular patterns rather than microbial pathogens elicit the response. The inflammasome, which leads to caspase-1 activation, is implicated in neuroinflammation. In this study, we reveal that lysophosphatidylcholine (LPC), a molecule associated with neurodegeneration and demyelination, elicits NLRP3 and NLRC4 inflammasome activation in microglia and astrocytes, which are central players in neuroinflammation. LPC-activated inflammasome also requires ASC (apoptotic speck containing protein with a CARD), caspase-1, cathepsin-mediated degradation, calcium mobilization, and potassium efflux but not caspase-11. To study the physiological relevance, Nlrc4^−/− and Nlrp3^−/− mice are studied in the cuprizone model of neuroinflammation and demyelination. Mice lacking both genes show the most pronounced reduction in astrogliosis and microglial accumulation accompanied by decreased expression of the LPC receptor G2A, whereas MS patient samples show increased G2A. These results reveal that NLRC4 and NLRP3, which normally form distinct inflammasomes, activate an LPC-induced inflammasome and are important in astrogliosis and microgliosis.

NLRX1 Sequesters STING to Negatively Regulate the Interferon Response, Thereby Facilitating the Replication of HIV-1 and DNA Viruses

Understanding the negative regulators of antiviral immune responses will be critical for advancing immune-modulated antiviral strategies. NLRX1, an NLR protein that negatively regulates innate immunity, was previously identified in an unbiased siRNA screen as required for HIV infection. We find that NLRX1 depletion results in impaired nuclear import of HIV-1 DNA in human monocytic cells. Additionally, NLRX1 was observed to reduce type-I interferon (IFN-I) and cytokines in response to HIV-1 reverse-transcribed DNA. NLRX1 sequesters the DNA-sensing adaptor STING from interaction with TANK-binding kinase 1 (TBK1), which is a requisite for IFN-1 induction in response to DNA. NLRX1-deficient cells generate an amplified STING-dependent host response to cytosolic DNA, c-di-GMP, cGAMP, HIV-1, and DNA viruses. Accordingly, Nlrx1(-/-) mice infected with DNA viruses exhibit enhanced innate immunity and reduced viral load. Thus, NLRX1 is a negative regulator of the host innate immune response to HIV-1 and DNA viruses.