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.

An Imaging Flow Cytometry Method to Measure Citrullination of H4 Histone as a Read-out for Neutrophil Extracellular Traps Formation

The formation of neutrophil extracellular traps (NETs) is thought to play a critical role in infections and propagating sterile inflammation. Histone citrullination is an essential and early step in NETs formation, detectable prior to the formation of the hallmark extracellular DNA-scaffolded strands. In addition to the classical microscopy method, new technologies are being developed for studies of NETs and their detection, both for research and clinical purposes. Classical microscopy studies of NETs are subjective, low throughput and semi-quantitative, and limited in their ability to capture the early steps. We have developed this novel Imaging Flow Cytometry (IFC) method that specifically identifies and quantifies citrullination of histone H4 as a NETs marker and its relationship with other alterations at nuclear and cellular level. These include nuclear decondensation and super-condensation, multi-lobulated nuclei versus 1-lobe nuclei and cell membrane damage. NETs markers can be quantified following variable periods of treatment with NETs inducers, prior to the formation of the specific extracellular DNA-scaffolded strands. Because these high throughput image-based cell analysis features can be performed with statistical rigor, this protocol is suited for both experimental and clinical applications as well as clinical evaluations of NETosis as a biomarker.

Detection and Quantification of Histone H4 Citrullination in Early NETosis With Image Flow Cytometry Version 4

Neutrophil extracellular traps (NETs) formation has been implicated in an increasing number of infectious and non-infectious pathologies. NETosis is a tightly regulated process; the end-stage and read-out is the formation of DNA strands extruded from the nuclei, and traditionally assessed by fluorescence microscopy. Since NETosis has emerged as a possible biomarker of the inflammatory process, there is a need for less time-consuming, consistent, and quantitative approaches to improve its application in clinical assessment of pro-inflammatory conditions. Imaging Flow Cytometry (IFC) combines features of conventional flow cytometry with qualitative power of fluorescence microscopy and has an added advantage of the capability of assessing the early processes leading up to extrusion of the DNA-scaffolded strands. We explored the optimal imaging-based tools that can be used to measure citrullination of H4 in early NETosis. IFC identified and quantified histone 4 citrullination (H4cit3) induced with several known NETosis stimuli (Ionophore, PMA, LPS, Hemin, and IL-8) following treatment periods ranging from 2 to 60 min. Its relationship with other alterations at nuclear and cellular level, such as nuclear decondensation and super-condensation, multi-lobulated nuclei vs. 1-lobe nuclei and cell membrane damage, were also quantified. We show that the early progress of the H4cit3 response in NETosis depends on the stimulus. Our method identifies fast (Ionophore and Hemin), intermediate and slow (PMA) inducers and shows that H4cit3 appears to have a limited contribution to both early LPS- and IL-8-induced NETosis. While this method is rapid and of a higher throughput compared to fluorescence microscopy, detection and quantification is limited to H4cit3-mediated nuclear events and is likely to be stimulus- and signaling pathway dependent.

Neutrophil extracellular traps are a heterogeneous feature in sickle cell disease

Sickle cell disease (SCD) is an inherited hemoglobin disorder caused by a single base change (A-T) in the beta-globin gene of hemoglobin with ensuing recurring inflammation that results in progressive end-organ damage and early mortality. Production of neutrophil extracellular traps (NETs) have been shown to be a key component of this inflammatory process in SCD pathophysiology.

We sought to determine the relevance of NETosis versus a different type of programmed cell death (apoptosis) in SCD. We used an indirect assay using healthy neutrophils exposed to paired plasma samples (n=16) from SCD patients in steady-state and crisis for 30 min to 7 hrs, and a direct approach of observing neutrophils (3 healthy and 3 SCD) themselves up to 4 hrs. Features of NETosis (histone citrullination, nuclear decondensation, string nets) and apoptosis (piknotic nuclei, c-Parp, membrane damage) were assessed by imaging flow cytometry and fluorescence microscopy. Additionally, plasma titers for 20 inflammatory analytes were determined for the 16 paired SCD samples and 4 healthy controls. High circulating IL-6 levels at crisis found in 9/16 of the paired samples correlated with an increased number of NETs at crisis compared to steady-state. NETs were also observed directly in SCD neutrophils at steady state but not consistently. When NETs or histone citrullination were absent or low, c-Parp was typically detected. Additionally, membrane damage was observed in neutrophils from all SCD donors. The data suggest that the SCD environment disrupts neutrophil homeostasis and activate multiple cell death programs. SCD NETosis appears to be heterogeneous, its contribution to the disease pathophysiology remains to be determined.

In vitro microfluidic model for the study of vaso-occlusive processes

Vaso-occlusion, responsible for much of the morbidity of sickle-cell disease, is a complex multicellular process, apparently triggered by leukocyte adhesion to the vessel wall. The microcirculation represents a major site of leukocyte-endothelial interactions and vaso-occlusive processes. We have developed a biochip with subdividing interconnecting microchannels that decrease in size (40 μm to 10 μm in width), for use in conjunction with a precise microfluidic device, to mimic cell flow and adhesion through channels of sizes that approach those of the microcirculation. The biochips were utilized to observe the dynamics of the passage of neutrophils and red blood cells, isolated from healthy and sickle-cell anemia (SCA) individuals, through laminin or endothelial adhesion molecule-coated microchannels at physiologically relevant rates of flow and shear stress. Obstruction of E-selectin/intercellular adhesion molecule 1-coated biochip microchannels by SCA neutrophils was significantly greater than that observed for healthy neutrophils, particularly in the microchannels of 40-15 μm in width. Whereas SCA red blood cells alone did not significantly adhere to, or obstruct, microchannels, mixed suspensions of SCA neutrophils and red blood cells significantly adhered to and obstructed laminin-coated channels. Results from this in vitro microfluidic model support a primary role for leukocytes in the initiation of SCA occlusive processes in the microcirculation. This assay represents an easy-to-use and reproducible in vitro technique for understanding molecular mechanisms and cellular interactions occurring in subdividing microchannels of widths approaching those observed in the microvasculature. The assay could hold potential for testing drugs developed to inhibit occlusive mechanisms such as those observed in SCA and thrombotic diseases.

Neutrophils of rheumatoid arthritis patients on anti-TNF-α therapy and in disease remission present reduced adhesive functions in association with decreased circulating neutrophil-attractant chemokine levels

Neutrophils participate in the initiation and progression of rheumatoid arthritis (RA) although the exact mechanisms responsible for neutrophil accumulation in rheumatoid joints are not understood. This study compared the adhesive and chemotactic functions of neutrophils from RA patients in activity (DAS28 > 3.2) and not in activity (DAS28 < 2.6) and observed the effects of different treatment approaches on these functions. Neutrophils were isolated from healthy controls (CON), and patients with active or inactive RA in use of therapy not specific for RA (NSAIDs), in use of DMARDs and in use of anti-TNF-α therapy. Adhesive and chemotactic properties were evaluated using in vitro assays; adhesion molecule expression was assessed by flow cytometry and real-time PCR and circulating chemokines were determined by ELISA. No significant alterations in the adhesive and chemotactic properties of neutrophils from active RA were observed when compared to CON neutrophils, independently of treatment regimen. In contrast, neutrophils from RA patients in disease remission presented reduced adhesive properties and a lower spontaneous chemotactic capacity, in association with decreased adhesion molecule expression, although profiles of alterations differed for those patients on DMARDs and those on anti-TNF-α therapy. Circulating levels of the major neutrophilic chemokines, IL-8 and epithelial neutrophil activating peptide-78, were also significantly decreased in those patients demonstrating a clinical response. Remission of RA appears to be associated with ameliorations in aspects important for neutrophil adhesion and chemotaxis; whether these alterations contribute to decrease neutrophil migration to the synovial fluid, with consequent improvements in the clinical manifestations of RA, remains to be determined.

Inhibition of phosphodiesterase 9A reduces cytokine-stimulated in vitro adhesion of neutrophils from sickle cell anemia individuals

OBJECTIVE

Leukocyte adhesion to vessel walls may initiate vaso-occlusion in sickle cell anemia (SCA); however, the extent to which inflammation participates in this mechanism is not understood. This in vitro study investigated whether inflammatory molecules, commonly augmented in SCA, can affect neutrophil adhesive properties and whether cyclic guanosine monophosphate (cGMP)-elevating agents can inhibit such adhesion.

SUBJECTS AND METHODS

Effects of Interleukin 8 (IL-8), tumor necrosis factor-α (TNF-α), granulocyte macrophage-colony stimulating factor (GM-CSF) cytokines, BAY 73-6691 [phosphodiesterase (PDE)-9A-inhibitor], and BAY 41-2271 (guanylate-cylase stimulator) on the adhesive properties of neutrophils from healthy control (CON) and steady-state SCA individuals were determined using static-adhesion assays.

RESULTS

SCA neutrophils demonstrated increased adhesive properties, compared to CON neutrophils; IL-8, TNF-α and GM-CSF increased CON neutrophil adhesion and further increased SCA neutrophil adhesion to fibronectin (FN). The PDE9A inhibitor, BAY-73-6691, significantly reduced basal CON neutrophil and SCA neutrophil adhesion; this was accompanied by decreased SCA neutrophil surface expressions of the L-selectin and CD11b adhesion molecules. BAY-73-6691 also significantly reduced cytokine-stimulated CON neutrophil and SCA neutrophil adhesion to FN; however, this was not accompanied by alterations in adhesion-molecule presentation.

CONCLUSIONS

The chronic inflammatory nature of SCA may contribute to leukocyte adhesive functions in SCA. Furthermore, elevation of leukocyte cGMP may be an interesting approach for inhibition of leukocyte adhesion to the vessel wall, even in the presence of inflammatory stimuli.