Detection of ASC Speck Formation by Flow Cytometry and Chemical Cross-linking

Comparative Analysis of Canonical Inflammasome Activation by Flow Cytometry, Imaging Flow Cytometry and High-Content Imaging

Inflammasome activation occurs in various diseases, including rare diseases that require multicenter studies for investigation. Flow cytometric analysis of ASC speck+ cells in patient samples can be used to detect cell type-specific inflammasome activation. However, this requires standardized sample processing and the ability to compare data from different flow cytometers. To address this issue, we analyzed stimulated and unstimulated PBMCs from healthy donors using seven different flow cytometers. Additionally, human PBMCs were analyzed by fluorescence microscopy, imaging flow cytometry and high-content imaging (HCI). Flow cytometers differed significantly in their ability to detect ASC speck+ cells. Aria III, Astrios EQ, and Canto II performed best in separating ASC speck+ from diffuse ASC cells. Imaging flow cytometry and HCI provided additional insight into ASC speck formation based on image-based parameters. For optimal results, the ability to separate cells with diffuse ASC from ASC speck+ cells is decisive. Image-based parameters can also differentiate cells with diffuse ASC from ASC speck+ cells. For the first time, we analyzed ASC speck detection by HCI in PBMCs and demonstrated advantages of this technique, such as high-throughput, algorithm-driven image quantification and 3D-rendering. Thus, inflammasome activation by ASC speck formation can be detected by various technical methods. However, the results may vary depending on the device used.

Bystander monocytic cells drive infection-independent NLRP3 inflammasome response to SARS-CoV-2

The pathogenesis of COVID-19 is associated with a hyperinflammatory immune response. Monocytes and macrophages play a central role in this hyperinflammatory response to SARS-CoV-2. NLRP3 inflammasome activation has been observed in monocytes of patients with COVID-19, but the mechanism and consequences of inflammasome activation require further investigation. In this study, we inoculated a macrophage-like THP-1 cell line, primary differentiated human nasal epithelial cell (hNEC) cultures, and primary monocytes with SARS-CoV-2. We found that the activation of the NLRP3 inflammasome in macrophages does not rely on viral replication, receptor-mediated entry, or actin-dependent entry. SARS-CoV-2 productively infected hNEC cultures without triggering the production of inflammasome cytokines IL-18 and IL-1β. Importantly, these cytokines did not inhibit viral replication in hNEC cultures. SARS-CoV-2 inoculation of primary monocytes led to inflammasome activation and induced a macrophage phenotype in these cells. Monocytic cells from bronchoalveolar lavage (BAL) fluid, but not from peripheral blood, of patients with COVID-19, showed evidence of inflammasome activation, expressed the proinflammatory marker CD11b, and displayed oxidative burst. These findings highlight the central role of activated macrophages, as a result of direct viral sensing, in COVID-19 and support the inhibition of IL-1β and IL-18 as potential therapeutic strategies to reduce immunopathology without increasing viral replication.

IMPORTANCE

Inflammasome activation is associated with severe COVID-19. The impact of inflammasome activation on viral replication and mechanistic details of this activation are not clarified. This study advances our understanding of the role of inflammasome activation in macrophages by identifying TLR2, NLRP3, ASC, and caspase-1 as dependent factors in this activation. Further, it highlights that SARS-CoV-2 inflammasome activation is not a feature of nasal epithelial cells but rather activation of bystander macrophages in the airway. Finally, we demonstrate that two pro inflammatory cytokines produced by inflammasome activation, IL-18 and IL-1β, do not restrict viral replication and are potential targets to ameliorate pathological inflammation in severe COVID-19.

Pyrin Inflammasome Activation Defines Colchicine-Responsive SURF Patients from FMF and Other Recurrent Fevers

Syndrome of undifferentiated recurrent fever (SURF) is characterized by recurrent fevers, a lack of confirmed molecular diagnosis, and a complete or partial response to colchicine. Despite the clinical similarities to familial Mediterranean fever (FMF), the underlying inflammatory mechanisms of SURF are not yet understood. We here analyzed the in vitro activation of the pyrin inflammasome in a cohort of SURF patients compared to FMF and PFAPA patients. Peripheral blood mononuclear cells (PBMC) were collected from SURF (both colchicine-treated and untreated), FMF, PFAPA patients, and healthy donors. PBMC were stimulated ex vivo with Clostridium difficile toxin A (TcdA) and a PKC inhibitor (UCN-01), in the presence or absence of colchicine. The assembly of the pyrin inflammasome was evaluated by measuring the presence of apoptosis-associated Speck-like protein containing caspase recruitment domain (ASC) specks in monocytes using flow cytometry. IL-1β secretion was quantified using an ELISA assay. No differences in TcdA-induced activation of pyrin inflammasome were observed among FMF, PFAPA, and healthy donors. Untreated SURF patients showed a reduced response to TcdA, which was normalized after colchicine treatment. In contrast to FMF, SURF patients, similar to PFAPA patients and healthy donors, did not exhibit pyrin inflammasome activation in response to UCN-01-mediated pyrin dephosphorylation. These data demonstrate that in vitro functional analysis of pyrin inflammasome activation can differentiate SURF from FMF and PFAPA patients, suggesting the involvement of the pyrin inflammasome in the pathophysiology of SURF.

Molecular mechanisms underlying NLRP3 inflammasome activation and IL-1β production in air pollution fine particulate matter (PM2.5)-primed macrophages

Exposure to air pollution fine particulate matter (PM2.5) aggravates respiratory and cardiovascular diseases. It has been proposed that PM2.5 uptake by alveolar macrophages promotes local inflammation that ignites a systemic response, but precise underlying mechanisms remain unclear. Here, we demonstrate that PM2.5 phagocytosis leads to NLRP3 inflammasome activation and subsequent release of the pro-inflammatory master cytokine IL-1β. Inflammasome priming and assembly was time- and dose-dependent in inflammasome-reporter THP-1-ASC-GFP cells, and consistent across PM2.5 samples of variable chemical composition. While inflammasome activation was promoted by different PM2.5 surrogates, significant IL-1β release could only be observed after stimulation with transition-metal rich Residual Oil Fly Ash (ROFA) particles. This effect was confirmed in primary human monocyte-derived macrophages and murine bone marrow-derived macrophages (BMDMs), and by confocal imaging of inflammasome-reporter ASC-Citrine BMDMs. IL-1β release by ROFA was dependent on the NLRP3 inflammasome, as indicated by lack of IL-1β production in ROFA-exposed NLRP3-deficient (Nlrp3-/-) BMDMs, and by specific NLRP3 inhibition with the pharmacological compound MCC950. In addition, while ROFA promoted the upregulation of pro-inflammatory gene expression and cytokines release, MCC950 reduced TNF-α, IL-6, and CCL2 production. Furthermore, inhibition of TNF-α with a neutralizing antibody decreased IL-1β release in ROFA-exposed BMDMs. Using electron tomography, ROFA particles were observed inside intracellular vesicles and mitochondria, which showed signs of ultrastructural damage. Mechanistically, we identified lysosomal rupture, K+ efflux, and impaired mitochondrial function as important prerequisites for ROFA-mediated IL-1β release. Interestingly, specific inhibition of superoxide anion production (O2•-) from mitochondrial respiratory Complex I, but not III, blunted IL-1β release in ROFA-exposed BMDMs. Our findings unravel the mechanism by which PM2.5 promotes IL-1β release in macrophages and provide a novel link between innate immune response and exposure to air pollution PM2.5.

E-selectin-mediated rapid NLRP3 inflammasome activation regulates S100A8/S100A9 release from neutrophils via transient gasdermin D pore formation

S100A8/S100A9 is a proinflammatory mediator released by myeloid cells during many acute and chronic inflammatory disorders. However, the precise mechanism of its release from the cytosolic compartment of neutrophils is unclear. Here, we show that E-selectin-induced rapid S100A8/S100A9 release during inflammation occurs in an NLRP3 inflammasome-dependent fashion. Mechanistically, E-selectin engagement triggers Bruton's tyrosine kinase-dependent tyrosine phosphorylation of NLRP3. Concomitant potassium efflux via the voltage-gated potassium channel KV1.3 mediates ASC oligomerization. This is followed by caspase 1 cleavage and downstream activation of pore-forming gasdermin D, enabling cytosolic release of S100A8/S100A9. Strikingly, E-selectin-mediated gasdermin D pore formation does not result in cell death but is a transient process involving activation of the ESCRT III membrane repair machinery. These data clarify molecular mechanisms of controlled S100A8/S100A9 release from neutrophils and identify the NLRP3/gasdermin D axis as a rapid and reversible activation system in neutrophils during inflammation.

Tiliroside Attenuates NLRP3 Inflammasome Activation in Macrophages and Protects against Acute Lung Injury in Mice

The Nod-like receptor family PYRIN domain containing 3 (NLRP3) inflammasome is a multiprotein signaling complex that plays a pivotal role in innate immunity, and the dysregulated NLRP3 inflammasome activation is implicated in various diseases. Tiliroside is a natural flavonoid in multiple medicinal and dietary plants with known anti-inflammatory activities. However, its role in regulating NLRP3 inflammasome activation and NLRP3-related disease has not been evaluated. Herein, it was demonstrated that tiliroside is inhibitory in activating the NLRP3 inflammasome in macrophages. Mechanistically, tiliroside promotes AMP-activated protein kinase (AMPK) activation, thereby leading to ameliorated mitochondrial damage as evidenced by the reduction of mitochondrial reactive oxygen species (ROS) production and the improvement of mitochondrial membrane potential, which is accompanied by attenuated NLRP3 inflammasome activation in macrophages. Notably, tiliroside potently attenuated lipopolysaccharide (LPS)-induced acute lung injury in mice, which has been known to be NLRP3 inflammasome dependent. For the first time, this study identified that tiliroside is an NLRP3 inflammasome inhibitor and may represent a potential therapeutic agent for managing NLRP3-mediated inflammatory disease.

Measuring NLR Oligomerization II: Detection of ASC Speck Formation by Confocal Microscopy and Immunofluorescence

Inflammasomes are crucial sentinels of the innate immune system that sense clues of infection, cellular stress, or metabolic imbalances. Upon activation, the inflammasome sensor (e.g., NLRP3) recruits the adaptor protein apoptosis-associated speck-like protein containing a CARD (ASC). ASC rapidly oligomerizes to form a micron-sized structure termed "ASC speck." These are crucial for the activation of caspase-1 and downstream inflammatory signals released following a specific form of lytic cell death called pyroptosis. Hence, due to their considerably large size, ASC specks can be easily visualized by microscopy as a simple upstream readout for inflammasome activation. Here, we provide three detailed protocols for imaging ASC specks: (1) live-cell imaging of macrophage cell lines expressing a fluorescent protein fusion form of ASC, (2) imaging of human primary cells using immunofluorescence staining of endogenous ASC, and (3) visualization and quantification of specks on a single-cell level using imaging flow cytometry.

Examining the Kinetics of Phagocytosis-Coupled Inflammasome Activation in Murine Bone Marrow-Derived Dendritic Cells

In the present chapter, we describe procedures to assess NLRP3 and NLRC4 inflammasome assembly by immunofluorescence microscopy or live cell imaging, together with inflammasome activation by biochemical and immunological techniques upon phagocytosis. We also include a step-by-step guide to automating the counting of inflammasome "specks" after imaging. While our focus resides on murine bone marrow-derived dendritic cells differentiated in the presence of granulocyte-macrophage colony-stimulating factor, which results in a cell population that resembles inflammatory dendritic cells, the strategies described herein may apply to other phagocytes as well.

Pyroptosis Markers in Human Primary Specimens: Quantification of Intracellular ASC Specks by Imaging Flow Cytometry and Extracellular Oxidized Mitochondrial by ELISA

Pyroptosis is an immunological response to infection and cellular stresses initiated by inflammasome oligomerization resulting in the release of pro-inflammatory factors including cytokines and other immune stimuli into the extracellular matrix. In order to understand the role of inflammasome activation and subsequent pyroptosis in human infection and disease pathogenesis and to explore markers of these signaling events as potential disease or response biomarkers, we must utilize quantitative, reliable, and reproducible assays to readily investigate these pathways in primary specimens. Here, we describe two methods using imaging flow cytometry for evaluation of inflammasome ASC specks in homogeneous peripheral blood monocytes and in bulk, heterogeneous peripheral blood mononuclear cells. Both methods can be applied to assess speck formation as a biomarker for inflammasome activation in primary specimens. Additionally, we describe the methods for quantification of extracellular oxidized mitochondrial DNA from primary plasma samples, serving as a proxy for pyroptosis. Collectively, these assays may be utilized to determine pyroptotic influences on viral infection and disease development or as diagnostic aids and response biomarkers.

The phagosomal solute transporter SLC15A4 promotes inflammasome activity via mTORC1 signaling and autophagy restraint in dendritic cells

Phagocytosis is the necessary first step to sense foreign microbes or particles and enables activation of innate immune pathways such as inflammasomes. However, the molecular mechanisms underlying how phagosomes modulate inflammasome activity are not fully understood. We show that in murine dendritic cells (DCs), the lysosomal histidine/peptide solute carrier transporter SLC15A4, associated with human inflammatory disorders, is recruited to phagosomes and is required for optimal inflammasome activity after infectious or sterile stimuli. Dextran sodium sulfate-treated SLC15A4-deficient mice exhibit decreased colon inflammation, reduced IL-1β production by intestinal DCs, and increased autophagy. Similarly, SLC15A4-deficient DCs infected with Salmonella typhimurium show reduced caspase-1 cleavage and IL-1β production. This correlates with peripheral NLRC4 inflammasome assembly and increased autophagy. Overexpression of constitutively active mTORC1 rescues decreased IL-1β levels and caspase1 cleavage, and restores perinuclear inflammasome positioning. Our findings support that SLC15A4 couples phagocytosis with inflammasome perinuclear assembly and inhibition of autophagy through phagosomal content sensing. Our data also reveal the previously unappreciated importance of mTORC1 signaling pathways to promote and sustain inflammasome activity.

Nanobodies dismantle post-pyroptotic ASC specks and counteract inflammation in vivo

Inflammasomes sense intracellular clues of infection, damage, or metabolic imbalances. Activated inflammasome sensors polymerize the adaptor ASC into micron‐sized “specks” to maximize caspase‐1 activation and the maturation of IL‐1 cytokines. Caspase‐1 also drives pyroptosis, a lytic cell death characterized by leakage of intracellular content to the extracellular space. ASC specks are released among cytosolic content, and accumulate in tissues of patients with chronic inflammation. However, if extracellular ASC specks contribute to disease, or are merely inert remnants of cell death remains unknown. Here, we show that camelid‐derived nanobodies against ASC (VHH_ASC) target and disassemble post‐pyroptotic inflammasomes, neutralizing their prionoid, and inflammatory functions. Notably, pyroptosis‐driven membrane perforation and exposure of ASC specks to the extracellular environment allowed VHH_ASC to target inflammasomes while preserving pre‐pyroptotic IL‐1β release, essential to host defense. Systemically administrated mouse‐specific VHH_ASC attenuated inflammation and clinical gout, and antigen‐induced arthritis disease. Hence, VHH_ASC neutralized post‐pyroptotic inflammasomes revealing a previously unappreciated role for these complexes in disease. VHH_ASC are the first biologicals that disassemble pre‐formed inflammasomes while preserving their functions in host defense.

Identification of an ASC oligomerization inhibitor for the treatment of inflammatory diseases

The ASC (apoptosis-associated speck-like protein containing a caspase recruitment domain (CARD)) protein is an scaffold component of different inflammasomes, intracellular multiprotein platforms of the innate immune system that are activated in response to pathogens or intracellular damage. The formation of ASC specks, initiated by different inflammasome receptors, promotes the recruitment and activation of procaspase-1, thereby triggering pyroptotic inflammatory cell death and pro-inflammatory cytokine release. Here we describe MM01 as the first-in-class small-molecule inhibitor of ASC that interferes with ASC speck formation. MM01 inhibition of ASC oligomerization prevents activation of procaspase-1 in vitro and inhibits the activation of different ASC-dependent inflammasomes in cell lines and primary cultures. Furthermore, MM01 inhibits inflammation in vivo in a mouse model of inflammasome-induced peritonitis. Overall, we highlight MM01 as a novel broad-spectrum inflammasome inhibitor for the potential treatment of multifactorial diseases involving the dysregulation of multiple inflammasomes.

Microscopic Detection of ASC Inflammasomes in Bone Marrow Derived Macrophages Post Stimulation

An inflammasome is an intracellular multiprotein complex that plays important roles in host defense and inflammatory responses. Inflammasomes are typically composed of the adaptor protein apoptosis-associated speck-like protein containing a CARD (ASC), cytoplasmic sensor protein, and the effector protein pro-caspase-1. ASC assembly into a protein complex termed ASC speck is a readout for inflammasome activation. Here, we provide a step-by-step protocol for the detection of ASC speck by confocal microscopy in Bone marrow derived macrophages (BMBDs) triggered by chemical stimuli and bacterial pathogens. We also describe the detailed procedure for the generation of BMDMs, stimulating conditions for inflammasome activation, immunofluorescence cell staining of ASC protein, and microscopic examination. Thus far, this method is a simple and reliable manner to visualize and quantify the intracellular localization of ASC speck. Graphic abstract: Figure 1. Confocal microscopy detection of ASC speck formation in untreated WT BMDMs and WT BMDMs stimulated with LPS and ATP, transfected with dsDNA, and infected with F. novicida or Salmonella as indicated. Arrow indicates the ASC speck. Scale bars: 10 μm.

Pan-caspase inhibition as a potential host-directed immunotherapy against MRSA and other bacterial skin infections

Staphylococcus aureus causes most skin infections in humans, and the emergence of methicillin-resistant S. aureus (MRSA) strains is a serious public health threat. There is an urgent clinical need for nonantibiotic immunotherapies to treat MRSA infections and prevent the spread of antibiotic resistance. Here, we investigated the pan-caspase inhibitor quinoline-valine-aspartic acid-difluorophenoxymethyl ketone (Q-VD-OPH) for efficacy against MRSA skin infection in mice. A single systemic dose of Q-VD-OPH decreased skin lesion sizes and reduced bacterial burden compared with vehicle-treated or untreated mice. Although Q-VD-OPH inhibited inflammasome-dependent apoptosis-associated speck-like protein containing caspase activation and recruitment domain (ASC) speck formation and caspase-1-mediated interleukin-1β (IL-1β) production, Q-VD-OPH maintained efficacy in mice deficient in IL-1β, ASC, caspase-1, caspase-11, or gasdermin D. Thus, Q-VD-OPH efficacy was independent of inflammasome-mediated pyroptosis. Rather, Q-VD-OPH reduced apoptosis of monocytes and neutrophils. Moreover, Q-VD-OPH enhanced necroptosis of macrophages with concomitant increases in serum TNF and TNF-producing neutrophils, monocytes/macrophages, and neutrophils in the infected skin. Consistent with this, Q-VD-OPH lacked efficacy in mice deficient in TNF (with associated reduced neutrophil influx and necroptosis), in mice deficient in TNF/IL-1R and anti-TNF antibody-treated WT mice. In vitro studies revealed that combined caspase-3, caspase-8, and caspase-9 inhibition reduced apoptosis, and combined caspase-1, caspase-8, and caspase-11 inhibition increased TNF, suggesting a mechanism for Q-VD-OPH efficacy in vivo. Last, Q-VD-OPH also had a therapeutic effect against Streptococcus pyogenes and Pseudomonas aeruginosa skin infections in mice. Collectively, pan-caspase inhibition represents a potential host-directed immunotherapy against MRSA and other bacterial skin infections.

Itaconate confers tolerance to late NLRP3 inflammasome activation

Itaconate is a unique regulatory metabolite that is induced upon Toll-like receptor (TLR) stimulation in myeloid cells. Here, we demonstrate major inflammatory tolerance and cell death phenotypes associated with itaconate production in activated macrophages. We show that endogenous itaconate is a key regulator of the signal 2 of NLR family pyrin domain containing 3 (NLRP3) inflammasome activation after long lipopolysaccharide (LPS) priming, which establishes tolerance to late NLRP3 inflammasome activation. We show that itaconate acts synergistically with inducible nitric oxide synthase (iNOS) and that the ability of various TLR ligands to establish NLRP3 inflammasome tolerance depends on the pattern of co-expression of IRG1 and iNOS. Mechanistically, itaconate accumulation upon prolonged inflammatory stimulation prevents full caspase-1 activation and processing of gasdermin D, which we demonstrate to be post-translationally modified by endogenous itaconate. Altogether, our data demonstrate that metabolic rewiring in inflammatory macrophages establishes tolerance to NLRP3 inflammasome activation that, if uncontrolled, can result in pyroptotic cell death and tissue damage.

Bambouskova et al. determine the in vitro phenotype of Irg1^−/− macrophages and define itaconate as a key regulator of tolerance to late NLRP3 inflammasome activation.

Detection of In Vivo Inflammasome Activation for Predicting Sepsis Mortality

Sepsis is a severe life-threatening syndrome caused by dysregulated host responses to infection. Biomarkers that allow for monitoring the patient's immune status are needed. Recently, a flow cytometry-based detection of in vivo inflammasome activation by formation of cytoplasmic aggregates of ASC (apoptosis-associated speck-like protein containing a caspase recruitment domain) has been proposed. Here we report on the frequency of ASC-speck⁺ leukocytes correlating with the survival of sepsis. 25 patients with sepsis were sampled consecutively for 7 days. Blood, serum samples and patient data were collected according to the guidelines of the PredARRT-Sep-Trial. Flow cytometric analysis was performed on fresh whole blood samples to investigate the formation of ASC-specks in leukocyte subsets. Serum samples were analyzed for production of IL-1ß, IL-18 and additional inflammatory markers. ASC-speck formation was found to be increased in leukocytes from sepsis patients compared to healthy donor controls. The absolute number of ASC-speck⁺ neutrophils peaked on day 1. For monocytes, the highest percentage and maximum absolute number of ASC-speck⁺ cells were detected on day 6 and day 7. Inflammatory cytokines were elevated on day 1 and declined thereafter, with exception of IL-18. Survival analysis showed that patients with lower absolute numbers of ASC-speck⁺ monocytes (<1,650 cells/ml) on day 6 had a lower probability to survive, with a hazard ratio (HR) of 10.178. Thus, the frequency of ASC-speck⁺ monocytes on day 6 after onset of sepsis may serve to identify patients at risk of death from sepsis.

Human NLRP1 is a sensor for double-stranded RNA

Inflammasomes function as intracellular sensors of pathogen infection or cellular perturbation and thereby play a central role in numerous diseases. Given the high abundance of NLRP1 in epithelial barrier tissues, we screened a diverse panel of viruses for inflammasome activation in keratinocytes. We identified Semliki Forest virus (SFV), a positive-strand RNA virus, as a potent activator of human but not murine NLRP1B. SFV replication and the associated formation of double-stranded (ds) RNA was required to engage the NLRP1 inflammasome. Moreover, delivery of long dsRNA was sufficient to trigger activation. Biochemical studies revealed that NLRP1 binds dsRNA through its leucine-rich repeat domain, resulting in its NACHT domain gaining adenosine triphosphatase activity. Altogether, these results establish human NLRP1 as a direct sensor for dsRNA and thus RNA virus infection.

Posttreatment of Maresin1 Inhibits NLRP3 inflammasome activation via promotion of NLRP3 ubiquitination

Maresin1 is a potent lipid mediator exhibiting potential anti-inflammatory activity in a variety of inflammatory diseases, however, the underlying mechanisms remain poorly understood. Excessive activation of NLRP3 inflammasome has been established in multiple inflammatory diseases. Here, we show that Maresin1 dose-dependently inhibited the NLRP3 inflammasome activation and subsequent caspase-1 activation and IL-1β secretion. This inhibitory effect could be reversed by KH7 and H89, the inhibitors of the cAMP-PKA signaling pathway. Activation of PKA kinase induced by Maresin1 led to the K63-linked ubiquitination of NLRP3 in macrophages. Maresin1 attenuated serum IL-1β secretion through inhibition of NLRP3 inflammasome in vivo using Nlrp3-deficient mouse models of lipopolysaccharide (LPS)-induced sepsis. Maresin1 also repressed MSU-induced peritonitis. This study suggests that Maresin1 is an inhibitor of NLRP3 inflammasome activation and can be used clinically in the treatment of NLRP3 inflammasome-driven inflammatory diseases.

Interferon γ and α Have Differential Effects on SAMHD1, a Potent Antiviral Protein, in Feline Lymphocytes

Sterile alpha motif and histidine/aspartic domain-containing protein 1 (SAMHD1) is a protein with anti-viral, anti-neoplastic, and anti-inflammatory properties. By degrading cellular dNTPs to constituent deoxynucleoside and free triphosphate, SAMHD1 limits viral DNA synthesis and prevents replication of HIV-1 and some DNA viruses such as HBV, vaccinia, and HSV-1. Recent findings suggest SAMHD1 is broadly active against retroviruses in addition to HIV-1, such as HIV-2, FIV, BIV, and EIAV. Interferons are cytokines produced by lymphocytes and other cells that induce a wide array of antiviral proteins, including some with activity again lentiviruses. Here we evaluated the role of IFNs on SAMHD1 gene expression, transcription, and post-translational modification in a feline CD4⁺ T cell line (FeTJ) and in primary feline CD4⁺ T lymphocytes. SAMHD1 mRNA in FetJ cells increased in a dose-related manner in response to IFNγ treatment concurrent with increased nuclear localization and phosphorylation. IFNα treatment induced SAMHD1 mRNA but did not significantly alter SAMHD1 protein detection, phosphorylation, or nuclear translocation. In purified primary feline CD4⁺ lymphocytes, IL2 supplementation increased SAMHD1 expression, but the addition of IFNγ did not further alter SAMHD1 protein expression or nuclear localization. Thus, the effect of IFNγ on SAMHD1 expression is cell-type dependent, with increased translocation to the nucleus and phosphorylation in FeTJ but not primary CD4⁺ lymphocytes. These findings imply that while SAMH1 is inducible by IFNγ, overall activity is cell type and compartment specific, which is likely relevant to the establishment of lentiviral reservoirs in quiescent lymphocyte populations.

Evaluation of Canonical Inflammasome Activation in Human Monocytes by Imaging Flow Cytometry

Canonical inflammasome activation is a tightly regulated process that has been implicated in a broad spectrum of inflammatory disorders. Inflammasome formation requires assembly of a cytosolic sensor protein with the adapter, ASC (apoptosis-associated speck-like protein containing a caspase activating and recruitment domain). Once formed, this multimeric protein structure allows for the activation of caspase-1, responsible for IL-1ß/IL-18 release. During this process, cytoplasmic dispersed ASC molecules cluster in one condensed micrometric-sized complex named ASC “speck,” which is traditionally assessed by fluorescence microscopy and widely accepted as a readout for canonical inflammasome activation. However, equally reliable but less time-consuming quantitative methods have emerged as a significant need in order to improve clinical assessment of inflammasome-related conditions. Multispectral imaging flow cytometry (MIFC) combines the qualitative power of fluorescence microscopy with high throughput capabilities and multiplexing potential of flow cytometry into one single system. Here we explored the optimal imaging-based tools to measure ASC speck formation via imaging flow cytometry by using peripheral blood mononuclear cells (PBMCs) stimulated with the NLRP3 agonist Nigericin, as a positive control. We demonstrate that this technique is also able to detect the distribution of active caspase-1 within the ASC aggregates by incubating cells with FAM-FLICA^TM, a fluorochrome inhibitor of caspase-1. By applying these tools in PBMCs from patients with distinct inflammatory disorders we demonstrate that MIFC is able to assess canonical inflammasome activation in a quantitative and statistically robust manner in clinically relevant samples. Therefore, we propose that accurate assessment of specks by MIFC could help guide preventive or therapeutic strategies in an array of human inflammatory diseases in which inflammasomes play an important role.

Assessment of ASC specks as a putative biomarker of pyroptosis in myelodysplastic syndromes: an observational cohort study

BACKGROUND

NLRP3 inflammasome-directed pyroptotic cell death drives ineffective haemopoiesis in myelodysplastic syndromes. During inflammasome assembly, the apoptosis-associated speck-like protein containing a CARD (PYCARD, commonly known as ASC) adaptor protein polymerises into large, filamentous clusters termed ASC specks that are released upon cytolysis. Specks are resistant to proteolytic degradation because of their prion-like structure, and therefore might serve as a biomarker for pyroptotic cell death in myelodysplastic syndromes.

METHODS

This observational cohort study was done at the H Lee Moffitt Cancer Center (Tampa, FL, USA). Patients with myelodysplastic syndromes, healthy controls, and patients with non-myelodysplastic syndrome haematological cancers or type 2 diabetes were recruited. We used confocal and electron microscopy to visualise, and flow cytometry to quantify, ASC specks in peripheral blood and bone marrow plasma samples. Speck percentages were compared by t test or ANOVA, correlations were assessed by Spearman's rank correlation coefficient, and biomarker efficiency was assessed by receiver operating characteristics and area under the curve (AUC) analysis.

FINDINGS

Between Jan 1, 2005, and Jan 12, 2017, we obtained samples from 177 patients with myelodysplastic syndromes and 29 healthy controls for the discovery cohort, and 113 patients with myelodysplastic syndromes and 31 healthy controls for the validation cohort. We also obtained samples from 22 patients with del(5q) myelodysplastic syndromes, 230 patients with non-myelodysplastic syndrome haematological cancers and 23 patients with type 2 diabetes. After adjustment for glucose concentration, the log₁₀-transformed mean percentage of peripheral blood plasma-derived ASC specks was significantly higher in the 177 patients with myelodysplastic syndromes versus the 29 age-matched, healthy donors (-0·41 [SD 0·49] vs -0·67 [0·59], p=0·034). The percentages of ASC specks in samples from patients with myelodysplastic syndromes were significantly greater than those in samples from individuals with every other haematological cancer studied (all p<0·05) except myelofibrosis (p=0·19). The findings were confirmed in the independent validation cohort (p<0·0001). Peripheral blood plasma danger-associated molecular pattern protein S100-A8 and protein S100-A9 concentrations from 144 patients with myelodysplastic syndromes from the discovery cohort directly correlated with ASC speck percentage (r=0·4, p<0·0001 for S100-A8 and r=0·2, p=0·017 for S100-A9). Patients with at least two somatic gene mutations had a significantly greater mean percentage of peripheral blood plasma ASC specks than patients with one or no mutation (-0·22 [SD 0·63] vs -0·53 [0·44], p=0·008). The percentage of plasma ASC specks was a robust marker for pyroptosis in myelodysplastic syndromes (AUC=0·888), in which a cutoff of 0·80 maximised sensitivity at 0·84 (95% CI 0·65-0·91) and specificity at 0·87 (0·58-0·97).

INTERPRETATION

Our results underscore the pathobiological relevance of ASC specks and suggest that ASC specks are a sensitive and specific candidate plasma biomarker that provides an index of medullary pyroptotic cell death and ineffective haemopoiesis in patients with myelodysplastic syndromes.

FUNDING

T32 Training Grant (NIH/NCI 5T32 CA115308-08), Edward P Evans Foundation, The Taub Foundation Grants Program, the Flow Cytometry, Analytic Microscopy, and Tissue Core Facilities at the H Lee Moffitt Cancer Center and Research Institute, a National Cancer Institute-designated Comprehensive Cancer Center (P30-CA076292).