The study was supported by funds from the University of California, Davis, Center for Companion Animal Health (CCAH 2018-30-F). The authors would like to acknowledge Dr. Kevin Woolard for usage of the fluorescence microscope.

All methods described here were performed in accordance to the guidelines of the Institutional Animal Care and Use Committee at the University of California, Davis. Necropsies and biopsies of tissues were performed with owners&#8217; consent.
1. Tissue fixation, embedding, and sectioning
<ol>
	<li>Dissect out the aortic bifurcation, including the descending aorta, femoral artery, and the common iliac arteries (Figure 1A), shortly after humane euthanasia or death. Blunt dissect out the fascia (Figure 1B) before submerging it completely in 10% neutral-buffered formalin for a minimum of 24 h and no longer than 48 h.</li>
	<li>To dehydrate the sample, first submerge in 10% neutral-buffered formalin heated to 37 &#176;C for 1 h. Then, submerge in increasing concentrations of ethanol heated to 37 &#176;C (70%, 95%, 100%) 2x for 1 h each. Finally, without rinsing, submerge 2x in 100% toluene heated to 37 &#176;C for 1 h each.</li>
	<li>Add paraffin heated to 62 &#176;C and allow the paraffin to solidify completely overnight.</li>
	<li>Section 2&#8211;3 &#181;m of the paraffin-embedded tissue using a microtome and place on positively charged glass slides. Store sectioned tissues at -80 &#176;C until further analysis.</li>
</ol>
2. Deparaffinization, rehydration, and heat-induced antigen retrieval
<ol>
	<li>To perform deparaffinization and rehydration of sections on glass slides, place glass slides in racks and process in the following order:
	<ol>
		<li>Submerge completely in 100% xylene for 3 min. Repeat this step 2x. Do not rinse in between steps.</li>
		<li>Submerge completely in decreasing concentrations of ethanol (100%, 95%, 70%) at room temperature (RT), 3x for 3 min each. Do not rinse in between steps.</li>
		<li>Submerge completely in deionized water for 2 min. Repeat.</li>
	</ol>
	</li>
	<li>Place sections into Tris-buffered saline with 0.1 % Tween (TBST, pH = 7.6) for 2&#8211;3 min.</li>
	<li>Fill the reservoir with deionized water heated to 100 &#176;C. Allow the steamer chamber to equilibrate for 20 min. 
	NOTE: Heat-induced antigen retrieval is best performed with indirect heating generated by a steamer with a preset temperature setting, such as a food steamer.</li>
	<li>Heat the commercially available antigen retrieval solution containing Tris and EDTA (pH = 9) to 95&#8211;97 &#176;C on a temperature-controlled hot plate with constant stirring. Ensure that it does not boil. 
	NOTE: The solution should turn cloudy once it is warmed.</li>
	<li>Pour the heated antigen retrieval solution into a slide container and place the container in the chamber of the steamer. Allow the antigen retrieval solution to equilibrate to the temperature of the steamer for 3&#8211;4 min. Ensure that the temperature of the chamber is ~95 &#176;C.</li>
	<li>Submerge the slides completely in the heated antigen retrieval solution and continue the application of external heating via the steamer for 20 min.</li>
	<li>Remove the slide container from the steamer and allow the slides and the antigen retrieval solution to cool to RT. Store the diluted antigen retrieval solution at 4 &#176;C and reuse up to 2x if needed.</li>
	<li>Wash the slides 3x with TBST for 5 min.</li>
</ol>
3. Immunolabeling and autofluorescence quenching
NOTE: Table 1 details the composition of the blocking buffers used in the following steps.
<ol>
	<li>Incubate sections in Blocking Buffer 1 for 2 h at RT under gentle rocking (30&#8211;50 rpm). Seal with paraffin film to avoid drying.</li>
	<li>Without washing, immediately apply 100 &#181;L of diluted rabbit polyclonal anti-human citrullinated histone H3 (citH3) antibody (0.03 mg/mL diluted in blocking buffer 1) directly onto the slide.</li>
	<li>Place a coverslip (24 mm x 40 mm x 0.13&#8211;0.17 mm) on each section to allow even distribution of the antibody mixture.</li>
	<li>Incubate for 12&#8211;16 h at 4 &#176;C with gentle rocking (30&#8211;50 rpm). Seal with parafilm film to avoid drying.</li>
	<li>Wash 3x with TBST for 5 min.</li>
	<li>Apply 100 &#181;L of goat anti-rabbit antibody conjugated to Alexa Fluor 488 (diluted to a final concentration of 0.04 mg/mL or 1:50 in Blocking Buffer 1) as described in step 3.3. Incubate for 1 h at RT under gentle rocking (30&#8211;50 rpm). Protect slides from light.</li>
	<li>Wash with TBST 3x for 5 min.</li>
	<li>Incubate sections in Blocking Buffer 2 overnight at 4 &#176;C under gentle rocking (30&#8211;50 rpm). Protect from light.</li>
	<li>Wash with TBST 3x for 5 min.</li>
	<li>Block sections in Blocking Buffer 3 as described in step 3.3 at RT for 2 h under gentle rocking (30&#8211;50 rpm).</li>
	<li>Incubate sections with biotinylated polyclonal rabbit anti-human NE antibody (final concentration = 0.2 &#181;g/mL in Blocking Buffer 3) at 4 &#176;C for 12&#8211;16 h as described in steps 3.2&#8211;3.4.</li>
	<li>Wash with TBST 3x for 5 min.</li>
	<li>Incubate with Alexa Fluor 594 streptavidin conjugate (dilute to 1:100 or 0.02 mg/mL in Blocking Buffer 3) as described in steps 3.2&#8211;3.3 for 1 h at RT. Protect from light and seal with paraffin to prevent drying.</li>
	<li>Wash with TBST 1x for 5 min.</li>
	<li>Apply 100 &#181;L of autofluorescence quenching solution mixture directly onto the sections for 1 min as instructed by the manufacturer.</li>
	<li>Immediately wash the slides with TBST 6x for 10 min.</li>
	<li>Cover each slide with 100 &#181;L of 300 nM DAPI for 5 min in the dark.</li>
	<li>Wash with TBST for 3 min. Repeat this for a total of 5x.</li>
	<li>Apply a drop (~50 &#956;L) of antifade mounting medium, part of the autofluorescence quenching kit, directly onto the glass slide surrounding the section. Place a coverslip (24 mm x 40 mm x 0.13&#8211;0.17 mm) gently onto the section without creating any bubbles.</li>
	<li>Allow samples to cure overnight in the dark at 4 &#176;C until the mounting medium has hardened for microscopic analysis with immersion lenses.</li>
</ol>
4. Neutrophil extracellular trap identification
NOTE: The following protocol utilizes an inverted epifluorescence microscope with a 1,280 x 960 digital CCD camera (see Table of Materials).
<ol>
	<li>To locate thrombi, scan cranially to caudally along the length of the aorta, aortic bifurcation, and each femoral artery using phase contrast microscopy with a 10x objective. A thrombus is a conglomeration of tissue containing red blood cells, white blood cells, and platelets adjacent to the endothelium on phase contrast and bright field microscopy (Figure 2A, Figure 2B).</li>
	<li>First examine sections for NETs using the DAPI channel (excitation = 357/44 nm) with 10x and 20x objectives (Figure 2C). Note that cfDNA appears as decondensed DNA that is not within the confines of the cytoplasm of a cell when seen on phase contrast or bright field microscopy.</li>
	<li>Identify extracellular NE and citH3 on the Texas Red channels (excitation = 585/29 nm, emission = 628/32 nm) and green fluorescent protein channel (excitation = 470/22 nm, emission = 525/50 nm), respectively with 10, 20, and 40x objectives.</li>
	<li>Evaluate and analyze NETs within a thrombus using available software, such as Image J (NIH). NET formation is identified based on the colocalization of cfDNA, extracellular citH3, and NE as previously described18. Maintain consistent exposure time and gains of each channel throughout the acquisition of images to avoid saturation in pixel intensity.</li>
	<li>Map each thrombus based on its proximity to the descending aorta by dividing it into three equal zones, with Zone 1 closest to the aorta, Zone 3 furthest from the aorta, and Zone 2 between Zones 1 and 3). With the operator blinded to the medical condition of each subject, take at least ten random fields in each zone. Characterize the distribution of NETs in thrombi by averaging the numbers of fields with NETs in each zone or calculating the average NET-occupying area per zone.</li>
</ol>

<ol>
	<li>Maron, B. J., Fox, P. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hypertrophic+cardiomyopathy+in+man+and+cats.">Hypertrophic cardiomyopathy in man and cats.</a> Journal of Veterinary Cardiology: The Official Journal of the European Society of Veterinary Cardiology. 17, 6-9 (2015).</li><li>Payne, J. R., 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=Prognostic+indicators+in+cats+with+hypertrophic+cardiomyopathy.">Prognostic indicators in cats with hypertrophic cardiomyopathy.</a> Journal of Veterinary Internal Medicine. 27 (6), 1427-1436 (2013).</li><li>Borgeat, K., Wright, J., Garrod, O., Payne, J. R., Fuentes, V. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Arterial+Thromboembolism+in+250+Cats+in+General+Practice:+2004-2012.">Arterial Thromboembolism in 250 Cats in General Practice: 2004-2012.</a> Journal of Veterinary Internal Medicine. 28 (1), 102-108 (2014).</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=Beneficial+suicide:+why+neutrophils+die+to+make+NETs.">Beneficial suicide: why neutrophils die to make NETs.</a> Nature Reviews. Microbiology. 5 (8), 577-582 (2007).</li><li>Goggs, R., Jeffery, U., LeVine, D. N., Li, R. H. 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D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+pathway+of+neutrophil+extracellular+traps+towards+atherosclerosis+and+thrombosis.">The pathway of neutrophil extracellular traps towards atherosclerosis and thrombosis.</a> Atherosclerosis. 288, 9-16 (2019).</li><li>Perdomo, J., 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+activation+and+NETosis+are+the+major+drivers+of+thrombosis+in+heparin-induced+thrombocytopenia.">Neutrophil activation and NETosis are the major drivers of thrombosis in heparin-induced thrombocytopenia.</a> Nature Communications. 10 (1), 1322 (2019).</li><li>Li, B., 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+enhance+procoagulant+activity+in+patients+with+oral+squamous+cell+carcinoma.">Neutrophil extracellular traps enhance procoagulant activity in patients with oral squamous cell carcinoma.</a> Journal of Cancer Research and Clinical Oncology. 145 (7), 1695-1707 (2019).</li><li>Li, R. H. L., Tablin, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+Vitro+Canine+Neutrophil+Extracellular+Trap+Formation:+Dynamic+and+Quantitative+Analysis+by+Fluorescence+Microscopy.">In Vitro Canine Neutrophil Extracellular Trap Formation: Dynamic and Quantitative Analysis by Fluorescence Microscopy.</a> Journal of Visualized Experiments. (138), e58083 (2018).</li><li>de Boer, O. J., Li, X., Goebel, H., van der Wal, A. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nuclear+smears+observed+in+H&amp;E-stained+thrombus+sections+are+neutrophil+extracellular+traps.">Nuclear smears observed in H&amp;E-stained thrombus sections are neutrophil extracellular traps.</a> Journal of Clinical Pathology. 69 (2), 181-182 (2016).</li><li>Farkas, &. #. 1. 9. 3. ;. Z., 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+in+thrombi+retrieved+during+interventional+treatment+of+ischemic+arterial+diseases.">Neutrophil extracellular traps in thrombi retrieved during interventional treatment of ischemic arterial diseases.</a> Thrombosis Research. 175, 46-52 (2019).</li><li>Qi, H., Yang, S., Zhang, L. <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+and+Endothelial+Dysfunction+in+Atherosclerosis+and+Thrombosis.">Neutrophil Extracellular Traps and Endothelial Dysfunction in Atherosclerosis and Thrombosis.</a> Frontiers in Immunology. 8, 928 (2017).</li><li>Laridan, E., 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+in+ischemic+stroke+thrombi.">Neutrophil extracellular traps in ischemic stroke thrombi.</a> Annals of Neurology. 82 (2), 223-232 (2017).</li><li>Laridan, E., Martinod, K., Meyer, S. F. D. <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+in+Arterial+and+Venous+Thrombosis.">Neutrophil Extracellular Traps in Arterial and Venous Thrombosis.</a> Seminars in Thrombosis and Hemostasis. 45 (1), 86-93 (2019).</li><li>Li, R. H. L., Johnson, L. R., Kohen, C., Tablin, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+novel+approach+to+identifying+and+quantifying+neutrophil+extracellular+trap+formation+in+septic+dogs+using+immunofluorescence+microscopy.">A novel approach to identifying and quantifying neutrophil extracellular trap formation in septic dogs using immunofluorescence microscopy.</a> BMC Veterinary Research. 14 (1), 210 (2018).</li><li>Brinkmann, V., 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=Immunodetection+of+NETs+in+Paraffin-Embedded+Tissue.">Immunodetection of NETs in Paraffin-Embedded Tissue.</a> Frontiers in Immunology. 7, 513 (2016).</li><li>Moelans, C. B., Oostenrijk, D., Moons, M. J., van Diest, P. 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The authors have nothing to disclose.

We describe a protocol to identify NETs in fixed feline cardiogenic arterial thrombi using a double immunolabeling protocol and immunofluorescence microscopy. Although only cardiogenic arterial thrombi were stained, in theory this protocol could be used for other types of thrombi and in other veterinary species. Identification of NETs within feline arterial thrombi suggests that NETs may play a role in thrombosis in cats.
Detection of NETs by immunofluorescence in fixed and paraffin-embedded t...

Cats with hypertrophic cardiomyopathy are at risk of life-threatening thromboembolic complications1,2. Despite the high morbidity and mortality associated with feline cardiogenic arterial thromboembolism (CATE), the underlying pathophysiology of CATE in cats is poorly understood. There are also limited diagnostic and therapeutic tools to treat and identify cats at risk of this devastating condition3.
In addition to its role in innate immunity, neutrophils have been shown to play a role in thrombosis by releasing neutrophil extracellular traps (NETs), which are web-like networks of cell-free DNA (cfDNA) encrusted with histones and granular proteins like neutrophil elastase (NE) and myeloperoxidase. Neutrophils undergo NETs formation in response to systemic inflammation, direct encounter with pathogens, and interaction with activated platelets4,5,6,7. In dogs, neutrophil-derived DNA has been shown to inhibit clot lysis, while NET proteins accelerate clot formation. The ability of NETs to trap circulating cells and coagulation components is also key to their thrombogenic properties8,9,10,11,12.
NETs are detected by colocalization of extracellular neutrophil proteins, histones, and cfDNA. Because of this, the identification and quantification of NETs in fixed tissues by immunofluorescence of deparaffinized tissues is superior to traditional hematoxylin and eosin (H&#38;E) stain using bright field microscopy4,5. Several human studies using immunofluorescence microscopy identified NETs as structural components of coronary arterial thrombi, cerebral stroke thrombi, atherothrombosis, and venous thrombi13,14,15,16,17. To date, a standardized method to detect and quantify NETs in feline thrombi has not been described. Because the identification of NETs in feline cardiogenic arterial thrombi may facilitate future translational research in NETs and thrombosis, we describe techniques of NET identification and assessment in paraffin-embedded arterial thrombi in cats.

<table>
	<tbody>
		<tr>
			<td>4,6-Diamidino-2-phenylin (DAPI)</td>
			<td>Life Technologies Corporation</td>
			<td>D1306</td>
			<td></td>
		</tr>
		<tr>
			<td>Alexa Fluor 594 Streptavidin conjugate</td>
			<td>ThermoFisher Scientific</td>
			<td>Catalog # S11227</td>
			<td></td>
		</tr>
		<tr>
			<td>Anti-citrullinated histone H3 antibody</td>
			<td>Abcam</td>
			<td>Ab5103</td>
			<td></td>
		</tr>
		<tr>
			<td>EVOS FL Cell Imaging System</td>
			<td>ThermoFisher Scientific</td>
			<td>AMEFC4300</td>
			<td></td>
		</tr>
		<tr>
			<td>EVOS Imaging System Objective 10x</td>
			<td>ThermoFisher Scientific</td>
			<td>AMEP4681</td>
			<td>NA 0.25, WD 6.9/7.45 mm</td>
		</tr>
		<tr>
			<td>EVOS Imaging System Objective 20x</td>
			<td>ThermoFisher Scientific</td>
			<td>AMEP4682</td>
			<td>NA 0.40, WD 6.8 mm</td>
		</tr>
		<tr>
			<td>EVOS Imaging System Objective 40x</td>
			<td>ThermoFisher Scientific</td>
			<td>AMEP4699</td>
			<td>NA 0.75, WD 0.72 mm</td>
		</tr>
		<tr>
			<td>Goat anti-rabbit Alexa Fluor 488 antibody</td>
			<td>ThermoFisher Scientific</td>
			<td>Catalog # A32723</td>
			<td></td>
		</tr>
		<tr>
			<td>Goat serum</td>
			<td>Jackson Immuno Research Labs</td>
			<td>Catalog # NC9660079. Manufacturer Part # 005-000-121</td>
			<td></td>
		</tr>
		<tr>
			<td>Neutrophil elastase antibody</td>
			<td>Bioss Antibodies</td>
			<td>Bs-6982R-Biotin</td>
			<td>Rabbit polyclonal Antibody, Biotin conjugated</td>
		</tr>
		<tr>
			<td>NP40</td>
			<td>Pierce</td>
			<td>Product # 28324. Lot # EJ64292</td>
			<td></td>
		</tr>
		<tr>
			<td>Positive charged microscope slides</td>
			<td>Thomas Scientific</td>
			<td>Manufacturer No. 1354W-72</td>
			<td></td>
		</tr>
		<tr>
			<td>Rabbit serum</td>
			<td>Life Technology</td>
			<td>Catalog # 10510</td>
			<td></td>
		</tr>
		<tr>
			<td>Target Retrieval Solution</td>
			<td>Agilent Dako</td>
			<td>S2367</td>
			<td>TRIS/EDTA, pH 9 (10x)</td>
		</tr>
		<tr>
			<td>TrueVIEW Autofluorescence Quenching Kit</td>
			<td>Vector Laboratories</td>
			<td>SP-8400</td>
			<td></td>
		</tr>
	</tbody>
</table>

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.

Using this protocol for deparaffinization, heat-induced antigen retrieval, and double immunolabeling of paraffin-embedded thrombi, we identified NETs in feline CATE for the first time. Thrombi within the aortic bifurcation were located by fluorescence microscopy and bright field microscopy using standard H&#38;E staining and phase contrast microscopy. On bright field microscopy, feline arterial thrombi consisted of red blood cells, leukocytes, fibrin, and platelets (Figure 3A). Although H&#3...

Watch this Scientific Journal Video about Identification of Neutrophil Extracellular Traps in Paraffin-Embedded Feline Arterial Thrombi using Immunofluorescence Microscopy at JoVE.com

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

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.

Neutrophil extracellular traps (NETs), composed of cell-free DNA (cfDNA) and proteins like histones and neutrophil elastase (NE), are released by ...

identification-neutrophil-extracellular-traps-paraffin-embedded

Research

JoVE Journal

Immunology and Infection

8.1K Views.  University of California, Davis. 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.

Video: Identification of Neutrophil Extracellular Traps in Paraffin-Embedded Feline Arterial Thrombi using Immunofluorescence 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 ...

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 ...

In Situ Detection of Bacteria within Paraffin-embedded Tissues Using a Digoxin-labeled DNA Probe Targeting 16S rRNA

The presence of bacteria within the pocket epithelium and underlying connective tissue in gingival biopsies from patients with periodontitis has been reported using various methods, including electron microscopy, immunohistochemistry or immunofluorescence using bacteria-specific antibodies, and fluorescent in situ hybridization (FISH) using a fluorescence-labeled oligonucleotide probe. Nevertheless, these methods are not widely used due to technical limitation or difficulties. Here a method to localize bacteria within paraffin-embedded tissues using DIG-labeled DNA probes has been introduced. The paraffin-embedded tissues are the most common form of biopsy tissues available from pathology banks. Bacteria can be detected either in a species-specific or universal manner. Bacterial signals are detected as either discrete forms (coccus, rod, fusiform, and hairy form) of bacteria or dispersed forms. The technique allows other histological information to be obtained: the epithelia, connective tissue, inflammatory infiltrates, and blood vessels are well distinguished. This method can be used to study the role of bacteria in various diseases, such as periodontitis, cancers, and inflammatory immune diseases.

In Situ Detection of Bacteria within Paraffin-embedded Tissues Using a Digoxin-labeled DNA Probe Targeting 16S rRNA

Here a method to localize bacteria within paraffin-embedded tissues using DIG-labeled 16S rRNA-targeting DNA probes has been described. This protocol can be applied to study the role of bacteria in various diseases such as periodontitis, cancers, and inflammatory immune diseases.

The presence of bacteria within the pocket epithelium and underlying connective tissue in gingival biopsies from patients with periodontitis has been ...

Methods to Study Lipid Alterations in Neutrophils and the Subsequent Formation of Neutrophil Extracellular Traps

Lipid analysis performed by high performance thin layer chromatography (HPTLC) is a relatively simple, cost-effective method of analyzing a broad range of lipids. The function of lipids (e.g., in host-pathogen interactions or host entry) has been reported to play a crucial role in cellular processes. Here, we show a method to determine lipid composition, with a focus on the cholesterol level of primary blood-derived neutrophils, by HPTLC in comparison to high performance liquid chromatography (HPLC). The aim was to investigate the role of lipid/cholesterol alterations in the formation of neutrophil extracellular traps (NETs). NET release is known as a host defense mechanism to prevent pathogens from spreading within the host. Therefore, blood-derived human neutrophils were treated with methyl-&#946;-cyclodextrin (M&#946;CD) to induce lipid alterations in the cells. Using HPTLC and HPLC, we have shown that M&#946;CD treatment of the cells leads to lipid alterations associated with a significant reduction in the cholesterol content of the cell. At the same time, M&#946;CD treatment of the neutrophils led to the formation of NETs, as shown by immunofluorescence microscopy. In summary, here we present a detailed method to study lipid alterations in neutrophils and the formation of NETs.

Lipids are known to play an important role in cellular functions. Here, we describe a method to determine the lipid composition of neutrophils, with emphasis on the cholesterol level, by using both HPTLC and HPLC to gain a better understanding of the underlying mechanisms of neutrophil extracellular trap formation.

Lipid analysis performed by high performance thin layer chromatography (HPTLC) is a relatively simple, cost-effective method of analyzing a broad range of ...

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.

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 ...

In Vitro Canine Neutrophil Extracellular Trap Formation: Dynamic and Quantitative Analysis by Fluorescence Microscopy

In response to invading pathogens, neutrophils release neutrophil extracellular traps (NETs), which are extracellular networks of DNA decorated with histones and antimicrobial proteins. Excessive NET formation (NETosis) and citH3 release during sepsis is associated with multiple organ dysfunction and mortality in mice and humans but its implications in dogs are unknown. Herein, we describe a technique to isolate canine neutrophils from whole blood for observation and quantification of NETosis. Leukocyte-rich plasma, generated by dextran sedimentation, is separated by commercially available density gradient separation media and granulocytes collected for cell count and viability testing. To observe real-time NETosis in live neutrophils, cell permeant and cell impermeant fluorescent nucleic acid stains are added to neutrophils activated either by lipopolysaccharide (LPS) or phorbol 12-myristate 13-acetate (PMA). Changes in nuclear morphology and NET formation are observed over time by fluorescence microscopy. In vitro NETosis is further characterized by co-colocalization of cell-free DNA (cfDNA), myeloperoxidase (MPO) and citrullinated histone H3 (citH3) using a modified double-immunolabelling protocol. To objectively quantify NET formation and citH3 expression using fluorescence microscopy, NETs and citH3-positive cells are quantified in a blinded manner using available software. This technique is a specific assay to evaluate the in vitro capacity of canine neutrophils to undergo NETosis.

In Vitro Canine Neutrophil Extracellular Trap Formation: Dynamic and Quantitative Analysis by Fluorescence Microscopy

We describe methods to isolate canine neutrophils from whole blood and visualize NET formation in live neutrophils using fluorescence microscopy. Also described are protocols to quantify NET formation and citrullinated histone H3 (citH3) expression using immunofluorescence microscopy.

In response to invading pathogens, neutrophils release neutrophil extracellular traps (NETs), which are extracellular networks of DNA decorated with ...

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. ...

Morphological and Compositional Analysis of Neutrophil Extracellular Traps Induced by Microbial and Chemical Stimuli

Neutrophils function as the first line of cellular defense in an innate immune response by employing diverse mechanisms, such as the formation of neutrophil extracellular traps (NETs). This study analyzes the morphological and compositional changes in NETs induced by microbial and chemical stimuli using standardized in vitro methodologies for NET induction and characterization with human cells. The procedures described here allow the analysis of NET morphology (lytic or non-lytic) and composition (DNA-protein structures and enzymatic activity), and the effect of soluble factors or cellular contact on such characteristics. Additionally, the techniques described here could be modified to evaluate the effect of exogenous soluble factors or cellular contact on NET composition.
The applied techniques include the purification of polymorphonuclear cells from human peripheral blood using a double density gradient (1.079-1.098 g/mL), guaranteeing optimal purity and viability (&#8805; 95%) as demonstrated by Wright's staining, trypan blue exclusion, and flow cytometry, including FSC versus SSC analysis and 7AAD staining. NET formation is induced with microbial (Pseudomonas aeruginosa, Staphylococcus aureus, and Candida albicans) and chemical (phorbol myristate acetate, HOCl) stimuli, and the NETs are characterized by DNA-DAPI staining, immunostaining for the antimicrobial peptide cathelicidin (LL37), and quantification of enzymatic activity (neutrophil elastase, cathepsin G, and myeloperoxidase). The images are acquired through fluorescence microscopy and analyzed with ImageJ.

Presented here is a protocol for the induction and analysis of in vitro neutrophil extracellular traps (NETs). Quantification of DNA, cathelicidin (LL37), and enzyme activity yielded data that show the variability in the composition and morphology of NETs induced by microbial and chemical stimuli under similar controlled conditions.

Neutrophils function as the first line of cellular defense in an innate immune response by employing diverse mechanisms, such as the formation of ...

Quantifying Myeloperoxidase-DNA and Neutrophil Elastase-DNA Complexes from Neutrophil Extracellular Traps by Using a Modified Sandwich ELISA

Certain stimuli, such as microorganisms, cause neutrophils to release neutrophil extracellular traps (NETs), which are basically web-like structures composed of DNA with granule proteins, such as myeloperoxidase (MPO) and neutrophil elastase (NE), and cytoplasmic and cytoskeletal proteins. Although interest in NETs has increased recently, no sensitive, reliable assay method is available for measuring NETs in clinical settings. This article describes a modified sandwich enzyme-linked immunosorbent assay to quantitatively measure two components of circulating NETs, MPO-DNA and NE-DNA complexes, which are specific components of NETs and are released into the extracellular space as breakdown products of NETs. The assay uses specific monoclonal antibodies for MPO or NE as the capture antibodies and a DNA-specific detection antibody. MPO or NE binds to one site of the capture antibody during the initial incubation of samples containing MPO-DNA or NE-DNA complexes. This assay shows good linearity and high inter-assay and intra-assay precision. We used it in 16 patients with COVID-19 with accompanying acute respiratory distress syndrome and found that the plasma concentrations of MPO-DNA and NE-DNA were significantly higher than in the plasma obtained from healthy controls. This detection assay is a reliable, highly sensitive, and useful method for investigating the characteristics of NETs in human plasma and culture supernatants.

We present a protocol for a modified sandwich enzyme-linked immunosorbent assay technique to quantitatively measure two components of neutrophil extracellular trap remnants, myeloperoxidase conjugated-DNA and neutrophil elastase conjugated-DNA complexes,derived from activated neutrophils.

Certain stimuli, such as microorganisms, cause neutrophils to release neutrophil extracellular traps (NETs), which are basically web-like structures ...

Staining and Imaging to Visualize Neutrophil Extracellular Traps

Immunofluorescence Imaging of Neutrophil Extracellular Traps in Human and Mouse Tissues

Neutrophil extracellular traps (NETs) are released by neutrophils as a response to bacterial infection or traumatic tissue damage but also play a role in autoimmune diseases and sterile inflammation. They are web-like structures composed of double-stranded DNA filaments, histones, and antimicrobial proteins. Once released, NETs can trap and kill extracellular pathogens in blood and tissue. Furthermore, NETs participate in homeostatic regulation by stimulating platelet adhesion and coagulation. However, the dysregulated production of NETs has also been associated with various diseases, including sepsis or autoimmune disorders, which makes them a promising target for therapeutic intervention. Apart from electron microscopy, visualizing NETs using immunofluorescence imaging is currently one of the only known methods to demonstrate NET interactions in tissue. Therefore, various staining methods to visualize NETs have been utilized. In the literature, different staining protocols are described, and we identified four key components showing high variability between protocols: (1) the types of antibodies used, (2) the usage of autofluorescence-reducing agents, (3) antigen retrieval methods, and (4) permeabilization. Therefore, in vitro immunofluorescence staining protocols were systemically adapted and improved in this work to make them applicable for different species (mouse, human) and tissues (skin, intestine, lung, liver, heart, spinal disc). After fixation and paraffin-embedding, 3 &#181;m thick sections were mounted onto slides. These samples were stained with primary antibodies for myeloperoxidase (MPO), citrullinated histone H3 (H3cit), and neutrophil elastase (NE) according to a modified staining protocol. The slides were stained with secondary antibodies and examined using a widefield fluorescence microscope. The results were analyzed according to an evaluation sheet, and differences were recorded semi-quantitatively.
Here, we present an optimized NET staining protocol suitable for different tissues. We used a novel primary antibody to stain for H3cit and reduced non-specific staining with an autofluorescence-reducing agent. Furthermore, we demonstrated that NET staining requires a constant high temperature and careful handling of samples.

Author Spotlight: Neutrophil Extracellular Traps Imaging in Human and Mouse Tissues 

Neutrophil extracellular traps (NETs) are associated with various diseases, and immunofluorescence is often used for their visualization. However, there are various staining protocols, and, in many cases, only one type of tissue is examined. Here, we establish a generally applicable protocol for staining NETs in mouse and human tissue.

Neutrophil extracellular traps (NETs) are released by neutrophils as a response to bacterial infection or traumatic tissue damage but also play a role in ...