This work was funded by grants from Monash University, the National Health and Medical Research Council, and the Monash Lung and Sleep Institute. The authors would like to thank the staff of Clinical Immunology at Monash Health, Judy Callaghan, Alex Fulcher, Kirstin Elgass, and Camden Lo of Monash Micro Imaging (MMI) for help with the confocal microscopy imaging, and the authors acknowledge the MMI facilities of Monash University. The authors acknowledge the facilities, and scientific and technical assistance of the Monash Histology Platform.

This protocol follows experiments approved in: (1) humans by the Ethics Committee of Monash Medical Centre, and (2) animals by the Ethics Committee of the University of Melbourne.
1. Bronchoalveolar Lavage (BAL) Macrophages
<ol>
	<li>Use BAL to obtain lung macrophages in: (1) human subjects by bronchoscopy6, and (2) mice by using intra-tracheal aspiration13. 
	NOTE: Acquisition of the macrophages generally causes some activation of these cells. Macrophages are obtained from patients who require a bronchoscopy to investigate a potential medical problem and not all subjects may be suitable for analysis of METs (this will need to be determined for each individual project).
	<ol>
		<li>Take the BAL solution and spin down (500 x g for 10 min) to form a pellet, wash twice with culture medium (Roswell Park Memorial Institute Medium (RPMI) with 5% fetal calf serum and 1% L-glutamine) at room temperature, and then dilute cells in 4 mL of culture medium.</li>
		<li>If necessary, request a formal differential count; most of the cells (generally &#62;80%) will be macrophages6. 
		NOTE: This will generally be performed by a clinical histology service using the morphologic appearance of the different cells types6.</li>
		<li>Perform a viable macrophage cell count on the BAL solution using Trypan blue exclusion and a hemocytometer. 
		NOTE: BAL solution is obtained from humans and mice as mentioned in step 1.1.</li>
		<li>Add 1 - 4 x 105 macrophages to 24-well plates on coverslips in 500 &#181;L of culture medium per well and leave overnight at 37&#176;C; the cells will adhere to the coverslips. For human samples, add antibiotics (e.g., penicillin and streptomycin, 104 U/mL) to prevent bacterial contamination.</li>
	</ol>
	</li>
</ol>
2. Immunofluorescence Labeling/Microscopy of BAL Macrophages
NOTE: Cells are adhered onto coverslips as mentioned above.
<ol>
	<li>Remove the culture medium and wash the cells once with phosphate buffered saline (PBS). Fix cells in 2% paraformaldehyde/periodate/lysine (PLP) fixative for 10 min.</li>
	<li>Wash cells briefly in PBS, and then permeabilize in 0.2% Tween 20 in PBS for 20 min.</li>
	<li>Block cells with 10% chicken sera in 5% bovine serum albumin (BSA) diluted in PBS for 30 min.</li>
	<li>Incubate with primary and isotype control antibodies for 1 h at room temperature at 1:100 concentrations (more specific details are listed in Table of Materials) at 37 &#176;C. 
	NOTE: For the BAL samples, a specific marker of macrophages is usually not required.</li>
	<li>After the addition of primary antibodies, wash the cells in PBS, and further incubate with the corresponding secondary antibodies for 40 min at room temperature.</li>
	<li>Wash the sections in PBS and mount with DAPI-based mounting medium for visualization using a confocal microscope (as mentioned above) at laser excitations 405, 488, 561 and 647 nm and a 40X, 1.0 NA oil objective.</li>
</ol>
3. Lung Tissue Samples
NOTE: For in vivo studies of lung tissue, study the samples after the relevant exposure (e.g., as described by O&#39;Sullivan et al.7). The exposure that is most relevant for this experiment are infectious microorganisms, particularly bacteria. Mouse strains that could be used for this method are C57BL/6 or BALB/c mice, 10 - 12 weeks old, weight 25 - 30 g, and male.
<ol>
	<li>Euthanize mice, via an intraperitoneal injection of ketamine (400 mg/kg) and xylazine (40 mg/kg). Open the thoracic cavity and expose the lungs and heart using surgical forceps and scissors.
	<ol>
		<li>Infuse warm PBS using a 21-gauge needle into the right ventricle to wash out the blood from the vessels for 30 s, then infuse 10 mL of 2% PLP fixative for 30 s (into the right ventricle).</li>
		<li>Use warm PBS (42 &#176;C) to lavage the open chest cavity. Intubate the trachea with a 21-gauge needle and a piece of 0.8 mm tubing cut to size, and inject 800 &#181;L of pre-warmed (to 42 &#176;C), low-melting point 3% agarose solution.</li>
		<li>Tie off the trachea with braided silk (4.0 USP). Place the thread around the trachea, just below the insertion point of the cannula, and secure it via a simple overhand knot. To solidify the agarose, administer ice-cold PBS around the lungs. Remove the lungs and heart as a block from the thoracic cavity using both scissors and blunt dissection. Remove each lung individually and then fix them for 16 h in 2% PLP or formalin at 4 &#176;C.</li>
		<li>Store the specimens in PBS at 4 &#176;C until tissue sectioning. These methods for obtaining lung tissue have been previously described13.</li>
	</ol>
	</li>
	<li>To prepare the lung tissue samples use the following steps.
	<ol>
		<li>Fix lung tissues (resected in step 3.1) tissue in 10% natural-buffered formalin for 24 h at room temperature. Transfer to 75% ethanol and put in a tissue processor on a 12 h cycle, (2.5 h in 75% EtOH, 3 h in absolute EtOH, 3.5 h in solvent 3B, and for 3 h in paraffin).</li>
		<li>Embed in standard paraffin to create formalin-fixed, paraffin-embedded (FFPE) blocks which are stored at room temperature. 
		NOTE: At this stage, it is generally convenient to cut the lung sections for standard slides.</li>
		<li>Cut the FFPE lung sections at 4 - 5 &#181;m thickness using a microtome and mount on charged, coated slides as previously described by O&#39;Sullivan et al.7</li>
		<li>To mount slides, put 3 drops of mounting medium on 60 mm x 24 mm coverslips (size 1.5), invert the slide on the coverslip and push out excess medium. Hermetically seal with nail polish and store at room temperature.</li>
	</ol>
	</li>
</ol>
4. Preparation/Microscopy of Lung Tissue Samples
<ol>
	<li>To de-wax, rehydrate and pretreat the FFPE lung tissue sample with antigen retrieval solution.
	<ol>
		<li>Oven dry the FFPE slides for 60 min at 60 &#176;C. Then put the slides in xylene solution for 30 min and transfer to 70% ethanol solution for 5 min at room temperature. Rinse in tap water.</li>
		<li>Place in heat-proof plastic wrap and subject to HIER-antigen retrieval in a pressure cooker for 10 min in 10 m/mol/L Tris, 1 mmol/EDTA pH 9.0. Cool for 20 min. Wash twice in tap water for 5 min on a rocker, then once with PBS on rocker.</li>
		<li>Block with 10% chicken serum in 5% BSA/PBS for 30 min at room temperature.</li>
	</ol>
	</li>
	<li>Stain the tissue.
	<ol>
		<li>To define METs in macrophages, add primary antibodies (MMP-9, H3Cit, and F4/80) in 1% BSA/PBS for 16 h at 4 &#176;C at 1/100 dilution (specific details about antibody amounts are listed in Table of Materials). Wash two times with PBS for 5 min on a rocker.</li>
		<li>Achieve fluorescent detection by incubation with corresponding secondary antibodies in 1% BSA/PBS for 40 min at room temperature. Antibodies are: (1) chicken anti-mouse 488 (green), (2) chicken anti-goat 594 (red), and (3) donkey anti-mouse 647 (far red).</li>
		<li>Wash the sections in PBS and mount with DAPI-containing mounting medium for chromatin staining.</li>
	</ol>
	</li>
	<li>Obtain fluorescent images using a confocal laser scanning head attached to an inverted microscope.
	<ol>
		<li>Excite the preparation with 405, 488, 561, and 647 nm lasers. Capture single plane 512 x 512-pixel images by clicking on the line-sequential, leveling button (2 line averaging) using 20X&#160;0.1 NA air and 40X&#160;1.0 NA oil objectives.</li>
		<li>Obtain at least ten fields of view per section for analysis and data for each result (i.e., 10 high-power fields of view (HFOV) for the control, 10 for the stimulated sample, etc., for each mouse). 
		NOTE: To obtain a representative sample of the whole field, a matrix system can be used in which the field is divided into different sections and for each different sample the same matrix is used to select fields of view (e.g., the field can be divided into 9 sections, and from the center and the 4 corners, two HFOV are taken).</li>
	</ol>
	</li>
</ol>
5. Three-dimensional (3-D) Imaging of METs
NOTE: As METs are 3-D structures, by taking multiple z-stack images, which are reconstituted, may give valuable information.
<ol>
	<li>Section the lung specimens from paraffin blocks, at 200 &#181;m using a vibratome at an amplitude set at 1.8 mm and speed of 0.1-0.5 mm/s.</li>
	<li>Block lung tissue with 20% of the respective serum (10% fetal calf serum and 10% chicken serum) and then permeabilize in PBS supplemented and 3.75% Triton-X 100 for 7 h at room temperature under gentle agitation.</li>
	<li>Stain the sections for 16 h at 4 &#176;C with the relevant primary antibodies (MMP-9, H3Cit, and F4/80) in 200 &#181;L volumes in microcentrifuge tubes (antibody concentrations are listed in Table of Materials).</li>
	<li>Wash the sections in PBS, 3 times for 10 min before incubating with relevant secondary antibodies at room temperature for 1 h.</li>
	<li>Stain the lung sections for DAPI (1:5,000) in solution for 20 min, before adding the cover slips to the microscope slides in PBS.</li>
	<li>Obtain fluorescence images using an inverted confocal microscope (40x 1.3 NA) equipped with 405 nm, 440 nm, 473 nm, 543 nm, and 635 nm lasers.</li>
	<li>Record 3-D z-stacks of 0.54 &#181;m thickness with optimal z-sectioning for the 40x 1.3 NA oil objective. 
	NOTE: The software used will vary depending on the individual microscope. An example of how the software can be used for one microscope is provided in Supplementary File 1.</li>
</ol>
6. Image Analysis
NOTE: The analysis of samples requires the use of specific imaging analysis software and examples are listed in Table of Materials. While the analysis of results will depend on the specific program used, the listed points below are important.
<ol>
	<li>Analysis of BAL samples
	<ol>
		<li>Define the number of macrophages by selecting the blue channel for DAPI and assigning a minimum size for the cells (e.g., &#62; 18 &#181;m in diameter). Using the relevant software, measure the total number of macrophages per field.</li>
		<li>Count the number of cells that have the features of a MET by the presence of extracellular chromatin detected by DAPI with co-expression of other markers (e.g., MMP12, PAD2, and H3Cit). 
		NOTE: The number of markers that can be analyzed depends on the number of lasers available, but ideally in addition to DAPI, at least two other markers should be included. Other less specific parameters for MET expression (e.g., number of cells with extracellular chromatin and an enlarged nucleus) can be used.</li>
		<li>To compare the MET expression in BAL samples (between the control, smoke, and smoke/DNase treated groups), analyze a minimum of 100 BAL macrophages per sample.</li>
		<li>To measure secondary antibody and isotype control levels of fluorescence; measure a minimum of 100 cells in each sample for their fluorescence. Measure the average level of background fluorescence for each marker (e.g., H3Cit) using standard software.</li>
		<li>To define MET expression, count the cells with the typical phenotype (e.g., extracellular chromatin with co-expression of H3Cit and MMP9) and for each MET, measure the level of fluorescence of markers (e.g., H3Cit and MMP9). Exclude cells producing METs if their staining was not above background and isotype control.</li>
		<li>For human BAL samples, analyze a minimum of 100 cells for each sample for the percentage of macrophages that make METs. For murine samples, as the cells are smaller and METs are harder to define, analyze a larger number of cells for each sample (e.g., 200 macrophages).</li>
	</ol>
	</li>
	<li>Analysis of lung tissue samples
	<ol>
		<li>To determine the presence of METs in lung tissue, use specific macrophage marker such as F4/80.</li>
		<li>Define the presence of METs in lung tissue by using the co-localization of F4/80 in cells that express extracellular chromatin with other parameters as listed in section 6.1.</li>
		<li>Determine the levels of background fluorescence in samples with secondary antibody and the isotype controls. Exclude METs from analysis that are not above background.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Brinkmann, V., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Neutrophil+extracellular+traps+kill+bacteria.">Neutrophil extracellular traps kill bacteria.</a> Science. 303 (5663), 1532-1535 (2004).</li><li>Brinkmann, V., Zychlinsky, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Neutrophil+extracellular+traps:+is+immunity+the+second+function+of+chromatin?.">Neutrophil extracellular traps: is immunity the second function of chromatin?.</a> J Cell Biol. 198 (5), 773-783 (2012).</li><li>Jorch, S. K., Kubes, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+emerging+role+for+neutrophil+extracellular+traps+in+noninfectious+disease.">An emerging role for neutrophil extracellular traps in noninfectious disease.</a> Nat Med. 23 (3), 279-287 (2017).</li><li>Cheng, O. Z., Palaniyar, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=NET+balancing:+a+problem+in+inflammatory+lung+diseases.">NET balancing: a problem in inflammatory lung diseases.</a> Frontiers immunol. 4, 1 (2013).</li><li>Boe, D. M., Curtis, B. J., Chen, M. M., Ippolito, J. A., Kovacs, E. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Extracellular+traps+and+macrophages:+new+roles+for+the+versatile+phagocyte.">Extracellular traps and macrophages: new roles for the versatile phagocyte.</a> J Leukoc Biol. 97 (6), 1023-1035 (2015).</li><li>King, P. T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nontypeable+Haemophilus+influenzae+induces+sustained+lung+oxidative+stress+and+protease+expression.">Nontypeable Haemophilus influenzae induces sustained lung oxidative stress and protease expression.</a> PloS one. 10 (3), e0120371 (2015).</li><li>O'Sullivan, K. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Renal+participation+of+myeloperoxidase+in+antineutrophil+cytoplasmic+antibody+(ANCA)-associated+glomerulonephritis.">Renal participation of myeloperoxidase in antineutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis.</a> Kidney Int. 88 (5), 1030-1046 (2015).</li><li>Fuchs, T. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Novel+cell+death+program+leads+to+neutrophil+extracellular+traps.">Novel cell death program leads to neutrophil extracellular traps.</a> J Cell Biol. 176 (2), 231-241 (2007).</li><li>Wang, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Histone+hypercitrullination+mediates+chromatin+decondensation+and+neutrophil+extracellular+trap+formation.">Histone hypercitrullination mediates chromatin decondensation and neutrophil extracellular trap formation.</a> J Cell Biol. 184 (2), 205-213 (2009).</li><li>Papayannopoulos, V., Metzler, K. D., Hakkim, A., Zychlinsky, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Neutrophil+elastase+and+myeloperoxidase+regulate+the+formation+of+neutrophil+extracellular+traps.">Neutrophil elastase and myeloperoxidase regulate the formation of neutrophil extracellular traps.</a> J Cell Biol. 191 (3), 677-691 (2010).</li><li>Rohrbach, A. S., Slade, D. J., Thompson, P. R., Mowen, K. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Activation+of+PAD4+in+NET+formation.">Activation of PAD4 in NET formation.</a> Front Immunol. 3, 360 (2012).</li><li>Lewis, H. D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Inhibition+of+PAD4+activity+is+sufficient+to+disrupt+mouse+and+human+NET+formation.">Inhibition of PAD4 activity is sufficient to disrupt mouse and human NET formation.</a> Nat Chem Biol. 11 (3), 189-191 (2015).</li><li>Ruwanpura, S. M., McLeod, L., Dousha, L. F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Therapeutic+Targeting+of+the+IL-6+Trans-Signaling/Mechanistic+Target+of+Rapamycin+Complex+1+Axis+in+Pulmonary+Emphysema.">Therapeutic Targeting of the IL-6 Trans-Signaling/Mechanistic Target of Rapamycin Complex 1 Axis in Pulmonary Emphysema.</a> Am J Respir Crit Care Med. 194 (12), 1494-1505 (2016).</li><li>Brinkmann, V., Laube, B., Abu Abed, U., Goosmann, C., Zychlinsky, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Neutrophil+extracellular+traps:+how+to+generate+and+visualize+them.">Neutrophil extracellular traps: how to generate and visualize them.</a> Journal of visualized experiments : JoVE. (36), (2010).</li><li>Chow, O. A., von Kockritz-Blickwede, M., Bright, A. T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Statins+enhance+formation+of+phagocyte+extracellular+traps.">Statins enhance formation of phagocyte extracellular traps.</a> Cell host &amp; microbe. 8 (5), 445-454 (2010).</li><li>Wong, K. W., Jacobs, W. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mycobacterium+tuberculosis+exploits+human+interferon+gamma+to+stimulate+macrophage+extracellular+trap+formation+and+necrosis.">Mycobacterium tuberculosis exploits human interferon gamma to stimulate macrophage extracellular trap formation and necrosis.</a> J Infect Dis. 208 (1), 109-119 (2013).</li></ol>

The authors have no disclosures to make in relation to this work.

The method described in this review is based on that used for defining the presence of NETs14. Macrophages are by far the dominant cell type in BAL samples and this method of collection is particularly suited for studying METs. If red blood cells are present in the BAL, these cells should be lysed by using ammonium chloride. The BAL procedure will generally activate the macrophages and therefore it is expected that there will be METs present in un-stimulated samples. The BAL macrophages are usuall...

Neutrophil extracellular traps (NETs), were first described by Brinkmann et al.1 They are predominantly produced in response to infection (especially to bacteria) and have an important role in host defense1,2. They have also been described to occur in response to non-infectious disease, including vasculitis and systemic lupus erythematosus (SLE); and to the mitogen phorbol 12-myrisate 13-acetate (PMA)2,3. It has recently been recognized that other cell types may also produce extracellular traps, including macrophages. Macrophage extracellular traps (METs) are not yet a well-defined entity in the literature4,5. We have recently established methods to detect the presence of METs both in vitro and in vivo6,7. In this article, the measurement of METs using confocal microscopy will be described.
Key features of NETosis which distinguish it from other cellular pathways (such as apoptosis8) are the extrusion of chromatin in conjunction with: (1) citrullination of histones (H3Cit)9, (2) co-expression of granule proteases10, and (3) involvement of peptidyl arginine deiminase (PAD) 411,12. Macrophages also express H3Cit, granule proteases, and PAD, and these features can be used to define the presence of METs.
METs may have a particularly important role in the lung, as the macrophage is the dominant cell present in the alveoli and airways of the lung and has the initial role in directing the cellular immune response to infection/inflammation. In addition, while much of the lung is empty space (e.g., within the alveoli), the METs are potentially able to expand into the space available, in contrast to solid organs.
The most widely used method to define the presence of NETs is by confocal microscopy. There is not yet a clearly defined way to measure METs. The technique for the measurement of NETs has been adapted to measure the presence of METs in this protocol. The main requirements for this method are access to confocal microscopy and appropriate imaging software for analysis.

<table>
	<tbody>
		<tr>
			<td>Human samples: Primary antibodies</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Rabbit anti-human H3Cit (Citrulline R26)</td>
			<td>Abcam</td>
			<td>AB5103</td>
			<td>10 ug/ml</td>
		</tr>
		<tr>
			<td>Rabbit anti-human MMP12</td>
			<td>Novus Biological</td>
			<td>NB110-57214</td>
			<td>1:100 concentration not quantifiable</td>
		</tr>
		<tr>
			<td>Mouse anti-human MMP9</td>
			<td>Novus Biological</td>
			<td>AB119906</td>
			<td>1:200 ascites, no concentration given</td>
		</tr>
		<tr>
			<td>Rabbit anti-human PADI2/PAD2</td>
			<td>Abcam</td>
			<td>AB16478</td>
			<td>7 ug/ml</td>
		</tr>
		<tr>
			<td>Mouse anti-human PADI4/PAD4</td>
			<td>Abcam</td>
			<td>AB128086</td>
			<td>10.1 ug/ml</td>
		</tr>
		<tr>
			<td>Sheep anti-human NE</td>
			<td>LifeSpan Bioscience</td>
			<td>LS-B4244</td>
			<td>25 ug/ml</td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Mouse samples: Primary antibodies</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Goat anti-mouse MMP9</td>
			<td>Abcam</td>
			<td>AF909</td>
			<td>10 ug/ml</td>
		</tr>
		<tr>
			<td>Rabbit anti-mouse H3Cit (Citrulline R26)</td>
			<td>Abcam</td>
			<td>AB5103</td>
			<td>10 ug/ml</td>
		</tr>
		<tr>
			<td>Rat anti-mouse F4/80</td>
			<td>In-house (hybridoma)</td>
			<td>In house</td>
			<td>20 ug/ml</td>
		</tr>
		<tr>
			<td>Sheep anti-human Anti-HNE /NE</td>
			<td>Sapphire Bioscience</td>
			<td>LS-B4244</td>
			<td>25 ug/ml</td>
		</tr>
		<tr>
			<td>Mouse anti-human &#173;&#173;&#173;&#173;PADI4/PAD4</td>
			<td>Abcam</td>
			<td>AB128086</td>
			<td>10.1 ug/ml</td>
		</tr>
		<tr>
			<td>Super-frost plus slides</td>
			<td>Menzel</td>
			<td>S41104A</td>
			<td></td>
		</tr>
		<tr>
			<td>Dapi-prolong gold</td>
			<td>Molecular probes</td>
			<td>P36931</td>
			<td></td>
		</tr>
		<tr>
			<td>Triton-X 100</td>
			<td>Sigma-Aldrich</td>
			<td>85111</td>
			<td></td>
		</tr>
		<tr>
			<td>Ammonium chloride</td>
			<td>Sigma-Aldrich</td>
			<td>E9434</td>
			<td></td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Secondary antibodies</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Chicken anti-rabbit AF 488/</td>
			<td>Life technologies</td>
			<td>A-21441</td>
			<td></td>
		</tr>
		<tr>
			<td>Chicken anti-rabbit AF 594</td>
			<td>Life technologies</td>
			<td>A-21442</td>
			<td></td>
		</tr>
		<tr>
			<td>Chicken anti-goat AF 594</td>
			<td>Life technologies</td>
			<td>a-21468</td>
			<td></td>
		</tr>
		<tr>
			<td>Chicken anti-mouse AF488</td>
			<td>Life technologies</td>
			<td>A-21200</td>
			<td></td>
		</tr>
		<tr>
			<td>Donkey anti-sheep AF 594</td>
			<td>Life technologies</td>
			<td>A-11018</td>
			<td></td>
		</tr>
		<tr>
			<td>Chicken anti-mouse AF 647</td>
			<td>Life technologies</td>
			<td>A-21463</td>
			<td></td>
		</tr>
		<tr>
			<td>Donkey anti-sheep AF 594</td>
			<td>Life technologies</td>
			<td>A-11016</td>
			<td></td>
		</tr>
		<tr>
			<td>Isotype control</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Rabbit IgG</td>
			<td>In house</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Rabbit IgG</td>
			<td>In house</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Mouse IgG1</td>
			<td>BD Bioscience</td>
			<td>550878</td>
			<td></td>
		</tr>
		<tr>
			<td>Rabbit IgG</td>
			<td>In house</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Mouse IgG2a</td>
			<td>BioLegend</td>
			<td>400201</td>
			<td></td>
		</tr>
		<tr>
			<td>Sheep IgG</td>
			<td>In house</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Software Programs</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Imaris</td>
			<td>Bitplane</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Image J</td>
			<td>NIH</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>To average intensity of fluorphores a standard office application like Microsoft Excel can be used</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Microscopes</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>C1 Confocal scanning microscope</td>
			<td>Nikon</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>FV1200 Confocal scanning microscope</td>
			<td>Olympus</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Tissue sources</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Human BAL samples from the bronchoscopy suite at Monash Medical Centre</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Mouse BAL samples and lung tissue from the Department of Pharmacology, University of Melbourne.</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Media</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>RPMI</td>
			<td>Sigma-Aldrich</td>
			<td>r8758</td>
			<td></td>
		</tr>
		<tr>
			<td>Fetal calf serum</td>
			<td>Sigma-Aldrich</td>
			<td>F0926</td>
			<td></td>
		</tr>
		<tr>
			<td>L-glutamine</td>
			<td>Sigma-Aldrich</td>
			<td>G7513</td>
			<td></td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Other reagents</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Sudan black</td>
			<td>Sigma-Aldrich</td>
			<td>199664</td>
			<td></td>
		</tr>
		<tr>
			<td>paraformaldehyde/periodate/lysine (PLP) fixative</td>
			<td>Sigma-Aldrich</td>
			<td>27387</td>
			<td></td>
		</tr>
		<tr>
			<td>Xylene</td>
			<td>Sigma-Aldrich</td>
			<td>214736</td>
			<td></td>
		</tr>
		<tr>
			<td>Ketamine</td>
			<td>Sigma-Aldrich</td>
			<td>K2753</td>
			<td></td>
		</tr>
		<tr>
			<td>Natural formalin</td>
			<td>Sigma-Aldrich</td>
			<td>42904</td>
			<td></td>
		</tr>
		<tr>
			<td>Paraffin</td>
			<td>Sigma-Aldrich</td>
			<td>327204</td>
			<td></td>
		</tr>
		<tr>
			<td>Agarose</td>
			<td>Sigma-Aldrich</td>
			<td>A2576</td>
			<td></td>
		</tr>
		<tr>
			<td>Solvent 3B</td>
			<td>Hi-Chem</td>
			<td>2026</td>
			<td></td>
		</tr>
		<tr>
			<td>Coverslips 12 ml</td>
			<td>Sigma-Aldrich</td>
			<td>S1815</td>
			<td></td>
		</tr>
		<tr>
			<td>Coverslips 60 x 24 ml</td>
			<td>Sigma-Aldrich</td>
			<td>C6875</td>
			<td></td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Mice</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>c57 black 6</td>
			<td>Monash animal research platform (MARP)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>BALB/c</td>
			<td>MARP</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

visualizing macrophage extracellular traps using confocal microscopy

A primary method used to define the presence of neutrophil extracellular traps (NETs) is confocal microscopy. We have modified established confocal microscopy methods to visualize macrophage extracellular traps (METs). These extracellular traps are defined by the presence of extracellular chromatin with co-expression of other components such as granule proteases, citrullinated histones, and peptidyl arginase deiminase (PAD). The expression of METs is generally measured after exposure to a stimulus and compared to un-stimulated samples. Samples are also included for background and isotype control. Cells are analyzed using well-defined image analysis software. Confocal microscopy may be used to define the presence of METs both in vitro and in vivo in lung tissue.

METs may be visualized from BAL samples, in lung tissue and in thicker lung sections with 3-D images. An example of METs visualized in a BAL sample is shown in Figure 1. The morphology of the METs will vary according to their stage of maturation. The first detectable feature on microscopy is the movement of the nucleus to the edge of the cell. This is followed by extracellular chromatin with other co-expressed mediators, such as H3Cit and granule proteases. I...

Watch this Scientific Journal Video about Visualizing Macrophage Extracellular Traps Using Confocal Microscopy at JoVE.com

Visualizing Macrophage Extracellular Traps Using Confocal Microscopy

Macrophage extracellular traps are a newly described entity. This article will concentrate on confocal microscopy methods and how they are visualized in vitro and in vivo from lung samples.

A primary method used to define the presence of neutrophil extracellular traps (NETs) is confocal microscopy. We have modified established confocal ...

visualizing-macrophage-extracellular-traps-using-confocal-microscopy

Research

JoVE Journal

Immunology and Infection

10.1K Views.  Monash Medical Centre. Macrophage extracellular traps are a newly described entity. This article will concentrate on confocal microscopy methods and how they are visualized in vitro and in vivo from lung samples.

Video: Visualizing Macrophage Extracellular Traps Using Confocal Microscopy

Neutrophil Extracellular Traps: How to Generate and Visualize Them

Neutrophil granulocytes are the most abundant group of leukocytes in the peripheral blood. As professional phagocytes, they engulf bacteria and kill them intracellularly when their antimicrobial granules fuse with the phagosome. We found that neutrophils have an additional way of killing microorganisms: upon activation, they release granule proteins and chromatin that together form extracellular fibers that bind pathogens. These novel structures, or Neutrophil Extracellular Traps (NETs), degrade virulence factors and kill bacteria1, fungi2 and parasites3. The structural backbone of NETs is DNA, and they are quickly degraded in the presence of DNases. Thus, bacteria expressing DNases are more virulent4. Using correlative microscopy combining TEM, SEM, immunofluorescence and live cell imaging techniques, we could show that upon stimulation, the nuclei of neutrophils lose their shape and the eu- and heterochromatin homogenize. Later, the nuclear envelope and the granule membranes disintegrate allowing the mixing of NET components. Finally, the NETs are released as the cell membrane breaks. This cell death program (NETosis) is distinct from apoptosis and necrosis and depends on the generation of Reactive Oxygen Species by NADPH oxidase5. 
Neutrophil extracellular traps are abundant at sites of acute inflammation. NETs appear to be a form of innate immune response that bind microorganisms, prevent them from spreading, and ensure a high local concentration of antimicrobial agents to degrade virulence factors and kill pathogens thus allowing neutrophils to fulfill their antimicrobial function even beyond their life span. There is increasing evidence, however, that NETs are also involved in diseases that range from auto-immune syndromes to infertility6.
We describe methods to isolate Neutrophil Granulocytes from peripheral human blood7 and stimulate them to form NETs. Also we include protocols to visualize the NETs in light and electron microscopy.

Neutrophil Extracellular Traps (NETs) are an important innate immune mechanism to fight pathogenic bacteria, fungi and parasites. Here we describe methods to isolate neutrophil granulocytes from human blood and to activate them to form NETs. We present preparation techniques to visualize NETs in light and electron microscopy.

Neutrophil granulocytes are the most abundant group of leukocytes in the peripheral blood. As professional phagocytes, they engulf bacteria and kill them ...

Visualizing Cell-to-cell Transfer of HIV using Fluorescent Clones of HIV and Live Confocal Microscopy

By fusing the green fluorescent protein to their favorite proteins, biologists now have the ability to study living complex cellular processes using fluorescence video microscopy. To track the movements of the human immunodeficiency virus core protein during cell-to-cell transmission of human immunodeficiency virus, we have GFP-tagged the Gag protein in the context of an infectious molecular clone of HIV, called HIV Gag-iGFP. We study this viral clone using video confocal microscopy. In the following visualized experiment, we transfect a human T cell line with HIV Gag-iGFP, and we use fluorescently labeled uninfected CD4+ T cells to serve as target cells for the virus. Using the different fluorescent labels we can readily follow viral production and transport across intercellular structures called virological synapses. Simple gas permeable imaging chambers allow us to observe synapses with live confocal microscopy from minutes to days. These approaches can be used to track viral proteins as they move in from one cell to the next.

This visualized experiment is a guide for utilizing a fluorescent molecular clone of HIV for live confocal imaging experiments.

By fusing the green fluorescent protein to their favorite proteins, biologists now have the ability to study living complex cellular processes using ...

Intravital Microscopy of the Spleen: Quantitative Analysis of Parasite Mobility and Blood Flow

The advent of intravital microscopy in experimental rodent malaria models has allowed major advances to the knowledge of parasite-host interactions 1,2. Thus, in vivo imaging of malaria parasites during pre-erythrocytic stages have revealed the active entrance of parasites into skin lymph nodes 3, the complete development of the parasite in the skin 4, and the formation of a hepatocyte-derived merosome to assure migration and release of merozoites into the blood stream 5. Moreover, the development of individual parasites in erythrocytes has been recently documented using 4D imaging and challenged our current view on protein export in malaria 6. Thus, intravital imaging has radically changed our view on key events in Plasmodium development. Unfortunately, studies of the dynamic passage of malaria parasites through the spleen, a major lymphoid organ exquisitely adapted to clear infected red blood cells are lacking due to technical constraints.
Using the murine model of malaria Plasmodium yoelii in Balb/c mice, we have implemented intravital imaging of the spleen and reported a differential remodeling of it and adherence of parasitized red blood cells (pRBCs) to barrier cells of fibroblastic origin in the red pulp during infection with the non-lethal parasite line P.yoelii 17X as opposed to infections with the P.yoelii 17XL lethal parasite line 7. To reach these conclusions, a specific methodology using ImageJ free software was developed to enable characterization of the fast three-dimensional movement of single-pRBCs. Results obtained with this protocol allow determining velocity, directionality and residence time of parasites in the spleen, all parameters addressing adherence in vivo. In addition, we report the methodology for blood flow quantification using intravital microscopy and the use of different colouring agents to gain insight into the complex microcirculatory structure of the spleen. 
Ethics statement
All the animal studies were performed at the animal facilities of University of Barcelona in accordance with guidelines and protocols approved by the Ethics Committee for Animal Experimentation of the University of Barcelona CEEA-UB (Protocol No DMAH: 5429). Female Balb/c mice of 6-8 weeks of age were obtained from Charles River Laboratories.

We show the method for performing intravital microscopy of the spleen using GFP transgenic malaria parasites and the quantification of parasite mobility and blood flow within this organ.

The advent of intravital microscopy in experimental rodent malaria models has allowed major advances to the knowledge of parasite-host interactions 1,2. ...

Assessing Anti-fungal Activity of Isolated Alveolar Macrophages by Confocal Microscopy

The lung is an interface where host cells are routinely exposed to microbes and microbial products. Alveolar macrophages are the first-line phagocytic cells that encounter inhaled fungi and other microbes. Macrophages and other immune cells recognize Aspergillus motifs by pathogen recognition receptors and initiate downstream inflammatory responses. The phagocyte NADPH oxidase generates reactive oxygen intermediates (ROIs) and is critical for host defense. Although NADPH oxidase is critical for neutrophil-mediated host defense1-3, the importance of NADPH oxidase in macrophages is not well defined. The goal of this study was to delineate the specific role of NADPH oxidase in macrophages in mediating host defense against A. fumigatus. We found that NADPH oxidase in alveolar macrophages controls the growth of phagocytosed A. fumigatus spores4. Here, we describe a method for assessing the ability of mouse alveolar macrophages (AMs) to control the growth of phagocytosed Aspergillus spores (conidia). Alveolar macrophages are stained in vivo and ten days later isolated from mice by bronchoalveolar lavage (BAL). Macrophages are plated onto glass coverslips, then seeded with green fluorescent protein (GFP)-expressing A. fumigatus spores. At specified times, cells are fixed and the number of intact macrophages with phagocytosed spores is assessed by confocal microscopy.

A method to evaluate the ability of isolated mouse alveolar macrophages to control the growth of phagocytosed Aspergillus spores by confocal microscopy.

The lung is an interface where host cells are routinely exposed to microbes and microbial products. Alveolar macrophages are the first-line phagocytic ...

Visualizing Non-lytic Exocytosis of Cryptococcus neoformans from Macrophages Using Digital Light Microscopy

Many aspects of the infection of macrophages by Cryptococcus neoformans have been extensively studied and well defined. However, one particular interaction that is not clearly understood is non-lytic exocytosis. In this process, yeast cells are released into the extracellular space by a poorly understood mechanism that leaves both the macrophage and Cn viable. Here, we describe how to follow a large number of individually infected macrophages for a 24 hr infection period by time-lapsed microscopy. Infected macrophages are housed in a heating chamber with a CO2 atmosphere attached to a microscope that provides the same conditions as a cell-culture incubator. Live digital microscopy can provide information about the dynamic interactions between a host and pathogen that is not available from static images. Being able to visualize each infected cell can provide clues as to how macrophages handle fungal infections, and vice versa. This technique is a powerful tool in studying the dynamics that are behind a complex phenomenon.

Visualizing Non-lytic Exocytosis of Cryptococcus neoformans from Macrophages Using Digital Light Microscopy

We describe how to visualize macrophage-C. neoformans (Cn) interactions in real time, with specific emphasis on the process of non-lytic exocytosis using digital light microscopy. Using this technique individually infected macrophages can be studied to ascertain various aspects of this phenomenon.

Many aspects of the infection of macrophages by Cryptococcus neoformans have been extensively studied and well defined. However, one particular ...

Simplified Human Neutrophil Extracellular Traps (NETs) Isolation and Handling

Neutrophil Extracellular Traps (NETs) have been recently identified as part of the neutrophil&#8217;s antimicrobial armamentarium. Apart from their role in fighting infections, recent research has demonstrated that they may be involved in many other disease processes, including cancer progression. Isolating purified NETs is a crucial element to allow the study of these functions.
In this video, we demonstrate a simplified method of cell free NET isolation from human whole blood using readily available reagents. Isolated NETs can then be used for immunofluorescence staining, blotting or various functional assays. This enables an assessment of their biologic properties in the absence of the potential confounding effects of neutrophils themselves.
A density gradient separation technique is employed to isolate neutrophils from healthy donor whole blood. Isolated neutrophils are then stimulated by phorbol 12-myristate 13-acetate (PMA) to induce NETosis. Activated neutrophils are then discarded, and a cell-free NET stock is obtained.
We then demonstrate how isolated NETs can be used in an adhesion assay with A549 human lung cancer cells. The NET stock is used to coat the wells of a 96 well cell culture plate O/N, and after ensuring an adequate NET monolayer formation on the bottom of the wells, CFSE labeled A549 cells are added. Adherent cells are quantified using a Nikon TE300 fluorescent microscope. In some wells, 1000U DNAse1 is added 10 min before counting to degrade NETs

Simplified Human Neutrophil Extracellular Traps NETs Isolation and Handling

In the following protocol, we describe a very simple way to isolate Neutrophil Extracellular traps (NETs) from human whole blood using readily available reagents. We then demonstrate how the isolated NETs can be used in an in vitro adhesion assay with cancer cells.

Neutrophil Extracellular Traps (NETs) have been recently identified as part of the neutrophil&#8217;s antimicrobial armamentarium. Apart from their role ...

Visualizing Intracellular SNARE Trafficking by Fluorescence Lifetime Imaging Microscopy

Soluble N-ethylmaleimide sensitive fusion protein (NSF) attachment protein receptor (SNARE) proteins are key for membrane trafficking, as they catalyze membrane fusion within eukaryotic cells. The SNARE protein family consists of about 36 different members. Specific intracellular transport routes are catalyzed by specific sets of 3 or 4 SNARE proteins that thereby contribute to the specificity and fidelity of membrane trafficking. However, studying the precise function of SNARE proteins is technically challenging, because SNAREs are highly abundant and functionally redundant, with most SNAREs having multiple and overlapping functions. In this protocol, a new method for the visualization of SNARE complex formation in live cells is described. This method is based on expressing SNARE proteins C-terminally fused to fluorescent proteins and measuring their interaction by F&#246;rster resonance energy transfer (FRET) employing fluorescence lifetime imaging microscopy (FLIM). By fitting the fluorescence lifetime histograms with a multicomponent decay model, FRET-FLIM allows (semi-)quantitative estimation of the fraction of the SNARE complex formation at different vesicles. This protocol has been successfully applied to visualize SNARE complex formation at the plasma membrane and at endosomal compartments in mammalian cell lines and primary immune cells, and can be readily extended to study SNARE functions at other organelles in animal, plant, and fungal cells.

This protocol describes a new method allowing for the quantitative visualization of complex formation of SNARE proteins, based on F&#246;rster resonance energy transfer, and fluorescence lifetime imaging microscopy.

Soluble N-ethylmaleimide sensitive fusion protein (NSF) attachment protein receptor (SNARE) proteins are key for membrane trafficking, as they catalyze ...

Automated Image-Based Quantification of Neutrophil Extracellular Traps Using NETQUANT

Neutrophil extracellular traps (NETs) are web-like antimicrobial structures consisting of DNA and granule derived antimicrobial proteins. Immunofluorescence microscopy and image-based quantification methods remain important tools to quantitate neutrophil extracellular trap formation. However, there are key limitations to the immunofluorescence-based methods that are currently available for quantifying NETs. Manual methods of image-based NET quantification are often subjective, prone to error and tedious for users, especially non-experienced users. Also, presently available software options for quantification are either semi-automatic or require training prior to operation. Here, we demonstrate the implementation of an automated immunofluorescence-based image quantification method to evaluate NET formation called NETQUANT. The software is easy to use and has a user-friendly graphical user interface (GUI). It considers biologically relevant parameters such as an increase in the surface area and DNA:NET marker protein ratio, and nuclear deformation to define NET formation. Furthermore, this tool is built as a freely available app, and allows for single-cell resolution quantification and analysis.

Here, we present a protocol for generating neutrophil extracellular traps (NETs) and operating NETQUANT, a fully automatic software option for quantification of NETs in immunofluorescence images.

Neutrophil extracellular traps (NETs) are web-like antimicrobial structures consisting of DNA and granule derived antimicrobial proteins. ...

Monitoring Extracellular pH in Cross-Kingdom Biofilms using Confocal Microscopy

Cross-kingdom biofilms consisting of both fungal and bacterial cells are involved in a variety of oral diseases, such as endodontic infections, periodontitis, mucosal infections and, most notably, early childhood caries. In all of these conditions, the pH in the biofilm matrix impacts microbe-host interactions and thus the disease progression. The present protocol describes a confocal microscopy-based method to monitor pH dynamics inside cross-kingdom biofilms comprising Candida albicans and Streptococcus mutans. The pH-dependent dual-emission spectrum and the staining properties of the ratiometric probe C-SNARF-4 are exploited to determine drops in pH in extracellular areas of the biofilms. Use of pH ratiometry with the probe requires a meticulous choice of imaging parameters, a thorough calibration of the dye, and careful, threshold-based post-processing of the image data. When used correctly, the technique allows for the rapid assessment of extracellular pH in different areas of a biofilm and thus the monitoring of both horizontal and vertical pH gradients over time. While the use of confocal microscopy limits Z-profiling to thin biofilms of 75 &#181;m or less, the use of pH ratiometry is ideally suited for the noninvasive study of an important virulence factor in cross-kingdom biofilms.

The protocol describes the cultivation of cross-kingdom biofilms consisting of Candida albicans and Streptococcus mutans and presents a confocal microscopy-based method for the monitoring of extracellular pH inside these biofilms.

Cross-kingdom biofilms consisting of both fungal and bacterial cells are involved in a variety of oral diseases, such as endodontic infections, ...

Identification of Neutrophil Extracellular Traps in Paraffin-Embedded Feline Arterial Thrombi using Immunofluorescence Microscopy

Neutrophil extracellular traps (NETs), composed of cell-free DNA (cfDNA) and proteins like histones and neutrophil elastase (NE), are released by neutrophils in response to systemic inflammation or pathogens. Although NETs have previously been shown to augment clot formation and inhibit fibrinolysis in humans and dogs, the role of NETs in cats with cardiogenic arterial thromboembolism (CATE), a life-threatening complication secondary to hypertrophic cardiomyopathy, is unknown. A standardized method to identify and quantify NETs in cardiogenic arterial thrombi in cats will advance our understanding of their pathological role in CATE. Here, we describe a technique to identify NETs in formaldehyde-fixed and paraffin-embedded thrombi within the aortic bifurcation, extracted during necropsy. Following deparaffinization with xylene, aortic sections underwent indirect heat-induced antigen retrieval. Sections were then blocked, permeabilized, and ex vivo NETs were identified by colocalization of cell-free DNA (cfDNA), citrullinated histone H3 (citH3), and neutrophil elastase (NE) using immunofluorescence microscopy. To optimize the immunodetection of NETs in thrombi, autofluorescence of tissue elements was limited by using an autofluorescence quenching process prior to microscopy. This technique could be a useful tool to study NETs and thrombosis in other species and offers new insights into the pathophysiology of this complex condition.

We describe a method to identify neutrophil extracellular traps (NETs) in formaldehyde-fixed and paraffin-embedded feline cardiogenic arterial thrombi using heat-induced antigen retrieval and a double immunolabeling protocol.