We thank Dr. Zilong Wen for sharing zebrafish strains, Dr. Stefan Oehlers and Dr. David Tobin for sharing M. marinum related resources, Yuepeng He for assistance in figure preparation. This work was supported by the National Natural Science Foundation of China (81801977) (B.Y.), the Outstanding Youth Training Program of Shanghai Municipal Health Commission (2018YQ54) (B.Y.), Shanghai Sailing Program (18YF1420400) (B.Y.), and Open Fund of Shanghai Key Laboratory of Tuberculosis (2018KF02) (B.Y.).

Zebrafish were raised under standard conditions in compliance with laboratory animal guidelines for ethical review of animal welfare (GB/T 35823-2018). All zebrafish experiments in this study were approved (2019-A016-01) and conducted at Shanghai Public Health Clinical Center, Fudan University.
1. M. marinum Single Cell Inoculum Preparation (Figure 1)
<ol>
	<li>Thaw Cerulean-fluorescent M. marinum glycerol stock from -80 &#176;C and inoculate a 7H10 agar plate with 10% (v/v) OADC, 0.25% glycerol and 50 &#956;g/mL hygromycin. Incubate the plate at 32 &#176;C for around 10 days.</li>
	<li>Select a colony expressing positive fluorescence and inoculate 3 mL of 7H9 medium with 10% OADC, 0.5% glycerol, and 50 &#956;g/mL hygromycin.
	<ol>
		<li>Incubate the inoculation at 32 &#176;C and 100 revolutions per minute (rpm) for 4&#8211;6 days until the culture reaches logarithmic phase (OD600 = 0.6&#8211;1.0).</li>
		<li>Subculture (1:100) in 30 mL of fresh 7H9 medium with 10% OADC, 0.5% glycerol, 0.05% Tween-80, and 50 &#956;g/mL until the OD600 reaches ~1.0. 
		NOTE: For the highest culture quality, a subculture step is recommended at this point. In our experience, adding the clone directly to a large volume of medium will lead to the formation of bacterial clumps.</li>
	</ol>
	</li>
	<li>Collect M. marinum cells as described below.
	<ol>
		<li>Centrifuge at 3,000 x g for 10 min to collect the M. marinum as a pellet. Discard all but 300 &#956;L of the supernatant and use it for resuspending the pellet.</li>
		<li>Add 3 mL of 7H9 medium with 10% glycerol to further resuspend the pellet, then sonicate the suspension in a water bath at 100 W at 15 s ON, 15 s OFF for a total 2 min. 
		NOTE: The purpose of sonication is to achieve a single cell homogenate for the inoculum, which will prevent blockage of the microinjection needle.</li>
	</ol>
	</li>
	<li>Transfer the bacterial suspension to a 10 mL syringe, then pass through a 5 &#956;m filter to remove any bacterial clumps.</li>
	<li>Measure the optical density (OD) of the suspension using a spectrophotometer and dilute it to OD600 = 1.0 with 7H9 media containing 10% glycerol. Divide the suspension into 10 &#956;L aliquots and store at -80 &#176;C freezer for future use.</li>
	<li>Confirm the bacterial concentration of the inoculum (cfu/mL) by serial dilution and plating of the bacterial stock on a 7H10 agar plate containing 10% (v/v) of OADC, 0.5% of glycerol, and 50 &#956;g/mL of hygromycin.</li>
</ol>
2. Zebrafish Embryo Preparation
<ol>
	<li>One the day before spawning, set up zebrafish breeding pairs in the breeding chamber. 
	NOTE: Add only one pair to each breeding chamber.</li>
	<li>Collect embryos the next morning within 1 h post fertilization (hpf). Carefully wash the embryos with distilled water and transfer up to 100 embryos into a 100 mm Petri dish containing 30 mL of E3 medium. Incubate at 28.5 &#176;C.</li>
	<li>After 12 h, observe under a microscope and discard nonfertilized or damaged eggs.</li>
	<li>At 24 hpf, change the medium to fresh E3 medium with N-phenylthiourea (PTU, 0.2 nM final concentration) to prevent the development of pigment. Incubate the embryos at 28.5 &#176;C until the embryos are ready for microinjection.</li>
</ol>
3. Infection via Bacterial Microinjection
<ol>
	<li>Prepare borosilicate glass microcapillary injection needles as previously described in reference13.</li>
	<li>Zebrafish embryos mounting for infection
	<ol>
		<li>Microwave 100 mL of 1% (w/v) and 100 mL of 0.5% (w/v) low melting agarose in an autoclaved E3 medium until the agarose is completely melted. Divide into 1 mL aliquot tubes and store at 4 &#176;C for future use.</li>
		<li>Before use, heat the agarose in a 95 &#176;C heating block until it is completely melted. Maintain the agarose in liquid form by placing it in a 45 &#176;C heating block (Figure 2).</li>
		<li>Mounting for intramuscular infection in the trunk region
		<ol>
			<li>Create the bottom agarose layer by pouring 0.5 mL of 1% (w/v) agarose evenly onto a glass slide. Place on an ice box or cold surface for 3 min to solidify.</li>
			<li>Anesthetize the zebrafish embryos (48&#8211;72 hpf) in egg water with tricaine (200 &#181;g/mL) and PTU for 5 min prior to mounting. Place up to 60 zebrafish embryos on the bottom agarose layer and lay them out carefully into two rows (Figure 2B).</li>
			<li>Remove any remaining water on the bottom agarose layer with tissue paper before adding 0.3 mL of 0.5% (w/v) agarose to create the upper layer. Ensure that the embryos are completely embedded in the agarose. Return the glass slide to the ice box again to solidify the agarose and prevent dehydration.</li>
			<li>Keep the top layer of agarose moist by covering the surface with extra E3 egg water.</li>
		</ol>
		</li>
		<li>Mounting for midbrain infection
		<ol>
			<li>Cover the groove of a single concavity glass microscopy slide with 1% (w/v) agarose, and then transfer the 4&#8211;6 tricaine-anesthetized embryos into the agarose.</li>
			<li>Position the head of each embryo upwards carefully with a 10 G needle (Figure 2C).</li>
			<li>Once all embryos&#39; positions are fixed, transfer the glass slide to an ice box or cold surface to let the agarose solidify. 
			NOTE: Avoid dilution of low melting agarose by minimizing the volume of egg water transfer with the embryos.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Bacteria preparation for infection
	<ol>
		<li>Add 1 &#956;L of sterile-filtered phenol red (10x) to a 10 &#956;L aliquot of bacterial stock (made in step 1) and mix by vortexing briefly. 
		NOTE: The final concentration can be adjusted using sterile PBS.</li>
		<li>Sonicate the preparation using 100 W at 10 s ON, 10 s OFF for 1 min to break up any clumps that may have formed14.</li>
	</ol>
	</li>
	<li>Infection via microinjection
	<ol>
		<li>Adjust the microinjector and micromanipulator to the proper position and setting for microinjection as previously reported13.</li>
		<li>Transfer 3 &#181;L of the bacterial preparation using a micro loader into the prepared needle (see step 3.1). Pipette slowly and carefully to avoid forming air bubbles.</li>
		<li>For the trunk region infection, inject 100 cfu into the trunk region (Figure 3A). Avoid injecting bacteria into the notochord. 
		NOTE: The cfu for injection is estimated by the formula cfu = bacterial stock concentration x dilution factor x injection droplet volume. The actual cfu is confirmed by plating one drop of bacterial inoculum on a 7H10 agar plate containing 10% (v/v) of OADC, 0.5% of glycerol, and 50 &#956;g/mL of hygromycin.</li>
		<li>For the midbrain infection, inject about 500 cfu into the midbrain region (Figure 3B).</li>
		<li>After microinjection, carefully flush the zebrafish embryos into fresh egg water with a plastic pipette. 
		NOTE: Mount embryos in the glass bottom dish as soon as possible to cover the observation of the very early innate immune cell response.</li>
	</ol>
	</li>
</ol>
4. Live Imaging of the Infection
<ol>
	<li>Fish mounting for live imaging
	<ol>
		<li>Transfer up to 10 tricaine-anesthetized embryos to the middle of a glass bottom 35 mm dish. Discard extra E3 medium.</li>
		<li>Cover the dish with 1% low melting point agarose and orient the zebrafish embryos carefully using a 10 G needle. Incubate the glass bottom dish on ice for 10 s to solidify the agarose. 
		NOTE: For midbrain injection, embryos should be mounted in the agarose with the head directed upwards (Figure 4A). For the trunk region intramuscular infection, embryos should be mounted laterally in the agarose (Figure 4B).</li>
		<li>Once it has completely solidified, cover the agarose with a layer of egg water (plus 1 x tricaine and PTU).</li>
	</ol>
	</li>
	<li>Three-color high-resolution time lapse confocal microscopy 
	NOTE: The following steps are operated on a confocal microscopy equipped with a 63.0x 1.40 oil UV objective lens.
	<ol>
		<li>Set the temperature of the environmental chamber to 28.5 &#176;C. Place some wet tissue paper inside the chamber to provide humidity and prevent evaporation of the egg water (Supplemental Figure 1).</li>
		<li>Place the 35 mm glass bottom dish with the zebrafish in the environmental chamber.</li>
		<li>Open the confocal software and initialize the stage. Switch to the 63.0 x 1.40 oil UV objective, and locate the zebrafish using the bright field channel with a differential interference contrast (DIC) filter.</li>
		<li>Open the 405 Diode, Argon (20% power), and DPSS 561 nm laser. Set up the appropriate laser power and spectrum settings. 
		NOTE: The following are the spectrum settings for Cerulean (excitation = 405 nm; emission = ~456&#8211;499 nm), eGFP (excitation = 488 nm; emission = ~500&#8211;550 nm), DsRed2 (excitation = 561 nm; emission = ~575&#8211;645 nm) (Supplemental Figure 2B).</li>
		<li>Choose the &#34;XYZ&#34; &#34;Sequential Scan&#34; acquisition mode and set images format to &#34;512 x 512 pixels&#34; (Supplemental Figure 2A).</li>
		<li>Switch to &#34;Live Data Mode&#34;. Target the position of the first zebrafish and mark the &#34;Begin&#34; and &#34;End&#34; Z position. Repeat this process for each of the remaining embryos. A &#34;Pause&#34; can be added at the end of the program (Supplemental Figure 2C).</li>
		<li>Define the loop and cycle of the program.</li>
		<li>Save the file.</li>
	</ol>
	</li>
</ol>
5. Single Cell UV Irradiation to Induce Apoptosis and Live Imaging
<ol>
	<li>Mount fish as described in step 4.1.</li>
	<li>Imaging the midbrain region of 3 days post fertilization (dpf) macrophage specific transgenic Tg(mfap4-eGFP) embryo15</li>
	<li>Select the region of interest of one single fluorescently labeled macrophage and scan at 400 Hz speed and 6% UV laser power for 50 s. 
	NOTE: Scanning speed and time should be optimized based on the individual microscope. Scanning time should be optimized to cause the extensive DNA damage that will subsequently cause target cell apoptosis, but not photobleach the entire cell.</li>
	<li>Repeat the above step to irradiate more target cells.</li>
	<li>Perform time lapse imaging of the midbrain region as described in section 4.</li>
</ol>
6. Image Processing
<ol>
	<li>Perform &#34;Maximum Projection&#34; for the acquired images.</li>
	<li>Find and mark the XY position and time of the target cells under the &#34;Maximum Projection&#34; view.</li>
	<li>Go back to the standard view to find and mark the Z position of the target cells.</li>
	<li>Crop the single layer image of the target cells.</li>
	<li>Export the overlay channel and bright field as videos in AVI format.</li>
	<li>Crop the area of interest of the overlay channel and bright field in ImageJ.</li>
	<li>Combine the two videos in the last step vertically and save as one AVI format video in ImageJ.</li>
</ol>

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The authors have nothing to disclose.

This protocol describes the visualization of macrophage death during mycobacterial infection. Based on factors such as the integrity of the cell membrane, infection driven cell death can be divided into apoptosis and lytic cell death24,25. Lytic cell death is more stressful for the organism than apoptosis, because it triggers a strong inflammatory response 24,25. Observation of lytic cell death in vivo is...

Mycobacterial infection has been demonstrated to cause host immune cell death1. For example, an attenuated strain will trigger apoptosis in macrophages and contain the infection. However, a virulent strain will trigger lytic cell death, causing bacterial dissemination1,2. Considering the impact these different types of cell death have on host anti-mycobacterial response, a detailed observation of macrophage cell death during mycobacterial infection in vivo is needed.
The conventional methods for measuring cell death are to use dead cell stains, such as Annnexin V, TUNEL, or acridine orange/propidium iodide staining3,4,5. However, these methods are unable to shed light on the dynamic process of cell death in vivo. The observation of cell death in vitro has already been facilitated by live imaging6. However, whether the results accurately mimic physiological conditions remains unclear.
Zebrafish have been an excellent model for studying host anti-mycobacterium responses. It has a highly conserved immune system similar to that of humans, an easily manipulated genome, and the early embryos are transparent, which allows for live imaging7,8,9. After infection with M. marinum, adult zebrafish form typical mature granuloma structures, and embryonic zebrafish form early granuloma like structures9,10. The dynamic process of innate immune cell-bacteria interaction has been explored previously in the zebrafish M. marinum infection model11,12. However, due to high spatial-temporal resolution requirement, the details surrounding the death of the innate immune cells remain largely undefined.
Here we describe how to visualize the process of macrophage lytic cell death triggered by mycobacterial infection in vivo. This protocol may also be applied to visualizing cellular behavior in vivo during development and inflammation.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>0.05% Tween-80</td>
			<td>Sigma</td>
			<td>P1379</td>
			<td></td>
		</tr>
		<tr>
			<td>10 mL syringe</td>
			<td>Solarbio</td>
			<td>YA0552</td>
			<td></td>
		</tr>
		<tr>
			<td>10% OADC</td>
			<td>BD</td>
			<td>211886</td>
			<td></td>
		</tr>
		<tr>
			<td>3-aminobenzoic acid</td>
			<td>Sigma</td>
			<td>E10521</td>
			<td></td>
		</tr>
		<tr>
			<td>5 &#956;m filter</td>
			<td>Mille X</td>
			<td>SLSV025LS</td>
			<td></td>
		</tr>
		<tr>
			<td>50 &#956;l/ml hygromycin</td>
			<td>Sangon Biotech</td>
			<td>A600230</td>
			<td></td>
		</tr>
		<tr>
			<td>7H10</td>
			<td>BD</td>
			<td>262710</td>
			<td></td>
		</tr>
		<tr>
			<td>7H9</td>
			<td>BD</td>
			<td>262310</td>
			<td></td>
		</tr>
		<tr>
			<td>A glass bottom 35 mm dish</td>
			<td>In Vitro Scientific</td>
			<td>D35-10-0-N</td>
			<td></td>
		</tr>
		<tr>
			<td>Agarose</td>
			<td>Sangon Biotech</td>
			<td>A60015</td>
			<td></td>
		</tr>
		<tr>
			<td>Confocal microscope</td>
			<td>Leica</td>
			<td>TCS SP5 II</td>
			<td></td>
		</tr>
		<tr>
			<td>Enviromental Chamber</td>
			<td>Pecon</td>
			<td>temp control 37-2 digital</td>
			<td></td>
		</tr>
		<tr>
			<td>Eppendorf microloader</td>
			<td>Eppendorf</td>
			<td>No.5242956003</td>
			<td></td>
		</tr>
		<tr>
			<td>Glass microscope slide</td>
			<td>Bioland Scientific LLC</td>
			<td>7105P</td>
			<td></td>
		</tr>
		<tr>
			<td>Glycerol</td>
			<td>Sangon Biotech</td>
			<td>A100854</td>
			<td></td>
		</tr>
		<tr>
			<td>Incubator</td>
			<td>Keelrein</td>
			<td>PH-140(A)</td>
			<td></td>
		</tr>
		<tr>
			<td>M.marinum</td>
			<td></td>
			<td></td>
			<td>ATCC BAA-535</td>
		</tr>
		<tr>
			<td>Microinjection needle</td>
			<td>World Precision Instruments</td>
			<td>IB100F-4</td>
			<td></td>
		</tr>
		<tr>
			<td>Microinjector</td>
			<td>Eppendorf</td>
			<td>Femtojet</td>
			<td></td>
		</tr>
		<tr>
			<td>Micromanipulator</td>
			<td>NARISHIGE</td>
			<td>MN-151</td>
			<td></td>
		</tr>
		<tr>
			<td>msp12:cerulean</td>
			<td></td>
			<td></td>
			<td>Ref.: PMID 25470057; 27760340</td>
		</tr>
		<tr>
			<td>Phenol red</td>
			<td>Sigma</td>
			<td>P3532</td>
			<td></td>
		</tr>
		<tr>
			<td>PTU</td>
			<td>Sigma</td>
			<td>P7629</td>
			<td></td>
		</tr>
		<tr>
			<td>Single concavity glass microscope slide</td>
			<td>Sail Brand</td>
			<td>7103</td>
			<td></td>
		</tr>
		<tr>
			<td>Sonicator</td>
			<td>SCICNTZ</td>
			<td>JY92-IIDN</td>
			<td></td>
		</tr>
		<tr>
			<td>Spectrophotometer (OD600)</td>
			<td>Eppendorf AG</td>
			<td>22331 Hamburg</td>
			<td></td>
		</tr>
		<tr>
			<td>Stereo Microscope</td>
			<td>OLYMPUS</td>
			<td>SZX10</td>
			<td></td>
		</tr>
		<tr>
			<td>Tg(mfap4:eGFP)</td>
			<td></td>
			<td></td>
			<td>Ref.: PMID 30742890</td>
		</tr>
		<tr>
			<td>Tg(coro1a:eGFP;lyzDsRed2)</td>
			<td></td>
			<td></td>
			<td>Ref.: PMID 31278008</td>
		</tr>
		<tr>
			<td>Tg(mpeg1:LRLG;lyz:eGFP)</td>
			<td></td>
			<td></td>
			<td>Ref.: PMID 27424497; 17477879</td>
		</tr>
	</tbody>
</table>

visualization of macrophage lytic cell death during mycobacterial infection in zebrafish embryos via intravital microscopy

Zebrafish is an excellent model organism for studying innate immune cell behavior due to its transparent nature and reliance solely on its innate immune system during early development. The Zebrafish Mycobacterium marinum (M. marinum) infection model has been well-established in studying host immune response against mycobacterial infection. It has been suggested that different macrophage cell death types will lead to the diverse outcomes of mycobacterial infection. Here we describe a protocol using intravital microscopy to observe macrophage cell death in zebrafish embryos following M. marinum infection. Zebrafish transgenic lines that specifically label macrophages and neutrophils are infected via intramuscular microinjection of fluorescently labeled M. marinum in either the midbrain or the trunk. Infected zebrafish embryos are subsequently mounted on low melting agarose and observed by confocal microscopy in X-Y-Z-T dimensions. Because long-term live imaging requires using low laser power to avoid photobleaching and phototoxicity, a strongly expressing transgenic is highly recommended. This protocol facilitates the visualization of the dynamic processes in vivo, including immune cell migration, host pathogen interaction, and cell death.

Mycobacterium infection can trigger different host responses based on the routes of infection. In this protocol, zebrafish embryos are infected by intramuscular microinjection of fluorescently labeled bacteria into the midbrain or trunk (Figure 3) and observed by confocal live imaging. Infection via these two routes will locally restrict the infection causing innate immune cell recruitment and subsequent cell death.
Visualizing the details of innate immune cell de...

Watch this Scientific Journal Video about Visualization of Macrophage Lytic Cell Death During Mycobacterial Infection in Zebrafish Embryos via Intravital Microscopy at JoVE.com

Visualization of Macrophage Lytic Cell Death During Mycobacterial Infection in Zebrafish Embryos via Intravital Microscopy

This protocol describes a technique for visualizing macrophage behavior and death in embryonic zebrafish during Mycobacterium marinum infection. Steps for the preparation of bacteria, infection of the embryos, and intravital microscopy are included. This technique may be applied to the observation of cellular behavior and death in similar scenarios involving infection or sterile inflammation.

Zebrafish is an excellent model organism for studying innate immune cell behavior due to its transparent nature and reliance solely on its innate immune ...

visualization-macrophage-lytic-cell-death-during-mycobacterial

Research

JoVE Journal

Immunology and Infection

6.1K Views.  Fudan University. This protocol describes a technique for visualizing macrophage behavior and death in embryonic zebrafish during Mycobacterium marinum infection. Steps for the preparation of bacteria, infection of the embryos, and intravital microscopy are included. This technique may be applied to the observation of cellular behavior and death in similar scenarios involving infection or sterile inflammation.

Video: Visualization of Macrophage Lytic Cell Death During Mycobacterial Infection in Zebrafish Embryos via Intravital Microscopy

Intravital Microscopy of the Inguinal Lymph Node

Lymph nodes (LN's), located throughout the body, are an integral component of the immune system. They serve as a site for induction of adaptive immune response and therefore, the development of effector cells. As such, LNs are key to fighting invading pathogens and maintaining health. The choice of LN to study is dictated by accessibility and the desired model; the inguinal lymph node is well situated and easily supports studies of biologically relevant models of skin and genital mucosal infection.
The inguinal LN, like all LNs, has an extensive microvascular network supplying it with blood. In general, this microvascular network includes the main feed arteriole of the LN that subsequently branches and feeds high endothelial venules (HEVs). HEVs are specialized for facilitating the trafficking of immune cells into the LN during both homeostasis and infection. How HEVs regulate trafficking into the LN under both of these circumstances is an area of intense exploration. The LN feed arteriole, has direct upstream influence on the HEVs and is the main supply of nutrients and cell rich blood into the LN. Furthermore, changes in the feed arteriole are implicated in facilitating induction of adaptive immune response. The LN microvasculature has obvious importance in maintaining an optimal blood supply to the LN and regulating immune cell influx into the LN, which are crucial elements in proper LN function and subsequently immune response. 
The ability to study the LN microvasculature in vivo is key to elucidating how the immune system and the microvasculature interact and influence one another within the LN. Here, we present a method for in vivo imaging of the inguinal lymph node. We focus on imaging of the microvasculature of the LN, paying particular attention to methods that ensure the study of healthy vessels, the ability to maintain imaging of viable vessels over a number of hours, and quantification of vessel magnitude. Methods for perfusion of the microvasculature with vasoactive drugs as well as the potential to trace and quantify cellular traffic are also presented. 
Intravital microscopy of the inguinal LN allows direct evaluation of microvascular functionality and real-time interface of the direct interface between immune cells, the LN, and the microcirculation. This technique potential to be combined with many immunological techniques and fluorescent cell labelling as well as manipulated to study vasculature of other LNs.

A technique for performing intravital microscopy of the inguinal lymph node (LN) is outlined. Such technique allows for real-time, in vivo study of the lymph node microvasculature and structure both during homeostasis and infection. This technique can be adapted to cell trafficking studies and to other lymph node sites.

Lymph nodes (LN's), located throughout the body, are an integral component of the immune system. They serve as a site for induction of adaptive immune ...

Infection of Zebrafish Embryos with Intracellular Bacterial Pathogens

Zebrafish (Danio rerio) embryos are increasingly used as a model for studying the function of the vertebrate innate immune system in host-pathogen interactions 1. The major cell types of the innate immune system, macrophages and neutrophils, develop during the first days of embryogenesis prior to the maturation of lymphocytes that are required for adaptive immune responses. The ease of obtaining large numbers of embryos, their accessibility due to external development, the optical transparency of embryonic and larval stages, a wide range of genetic tools, extensive mutant resources and collections of transgenic reporter lines, all add to the versatility of the zebrafish model. Salmonella enterica serovar Typhimurium (S. typhimurium) and Mycobacterium marinum can reside intracellularly in macrophages and are frequently used to study host-pathogen interactions in zebrafish embryos. The infection processes of these two bacterial pathogens are interesting to compare because S. typhimurium infection is acute and lethal within one day, whereas M. marinum infection is chronic and can be imaged up to the larval stage 2, 3. The site of micro-injection of bacteria into the embryo (Figure 1) determines whether the infection will rapidly become systemic or will initially remain localized. A rapid systemic infection can be established by micro-injecting bacteria directly into the blood circulation via the caudal vein at the posterior blood island or via the Duct of Cuvier, a wide circulation channel on the yolk sac connecting the heart to the trunk vasculature. At 1 dpf, when embryos at this stage have phagocytically active macrophages but neutrophils have not yet matured, injecting into the blood island is preferred. For injections at 2-3 dpf, when embryos also have developed functional (myeloperoxidase-producing) neutrophils, the Duct of Cuvier is preferred as the injection site. To study directed migration of myeloid cells towards local infections, bacteria can be injected into the tail muscle, otic vesicle, or hindbrain ventricle 4-6. In addition, the notochord, a structure that appears to be normally inaccessible to myeloid cells, is highly susceptible to local infection 7. A useful alternative for high-throughput applications is the injection of bacteria into the yolk of embryos within the first hours after fertilization 8. Combining fluorescent bacteria and transgenic zebrafish lines with fluorescent macrophages or neutrophils creates ideal circumstances for multi-color imaging of host-pathogen interactions. This video article will describe detailed protocols for intravenous and local infection of zebrafish embryos with S. typhimurium or M. marinum bacteria and for subsequent fluorescence imaging of the interaction with cells of the innate immune system.

Transparent zebrafish embryos have proved useful model hosts to visualize and functionally study interactions between innate immune cells and intracellular bacterial pathogens, such as Salmonella typhimurium and Mycobacterium marinum. Micro-injection of bacteria and multi-color fluorescence imaging are essential techniques involved in the application of zebrafish embryo infection models.

Zebrafish (Danio rerio) embryos are increasingly used as a model for studying the function of the vertebrate innate immune system in host-pathogen ...

Visualization of Bacterial Toxin Induced Responses Using Live Cell Fluorescence Microscopy

Bacterial toxins bind to cholesterol in membranes, forming pores that allow for leakage of cellular contents and influx of materials from the external environment. The cell can either recover from this insult, which requires active membrane repair processes, or else die depending on the amount of toxin exposure and cell type1. In addition, these toxins induce strong inflammatory responses in infected hosts through activation of immune cells, including macrophages, which produce an array of pro-inflammatory cytokines2. Many Gram positive bacteria produce cholesterol binding toxins which have been shown to contribute to their virulence through largely uncharacterized mechanisms.
Morphologic changes in the plasma membrane of cells exposed to these toxins include their sequestration into cholesterol-enriched surface protrusions, which can be shed into the extracellular space, suggesting an intrinsic cellular defense mechanism3,4. This process occurs on all cells in the absence of metabolic activity, and can be visualized using EM after chemical fixation4. In immune cells such as macrophages that mediate inflammation in response to toxin exposure, induced membrane vesicles are suggested to contain cytokines of the IL-1 family and may be responsible both for shedding toxin and disseminating these pro-inflammatory cytokines5,6,7. A link between IL-1&beta; release and a specific type of cell death, termed pyroptosis has been suggested, as both are caspase-1 dependent processes8. To sort out the complexities of this macrophage response, which includes toxin binding, shedding of membrane vesicles, cytokine release, and potentially cell death, we have developed labeling techniques and fluorescence microscopy methods that allow for real time visualization of toxin-cell interactions, including measurements of dysfunction and death (Figure 1). Use of live cell imaging is necessary due to limitations in other techniques. Biochemical approaches cannot resolve effects occurring in individual cells, while flow cytometry does not offer high resolution, real-time visualization of individual cells. The methods described here can be applied to kinetic analysis of responses induced by other stimuli involving complex phenotypic changes in cells.

Methods for purifying the cholesterol binding toxin streptolysin O from recombinant E. coli and visualization of toxin binding to live eukaryotic cells are described. Localized delivery of toxin induces rapid and complex changes in targeted cells revealing novel aspects of toxin biology.

Bacterial toxins bind to cholesterol in membranes, forming pores that allow for leakage of cellular contents and influx of materials from the external ...

Highly Resolved Intravital Striped-illumination Microscopy of Germinal Centers

Monitoring cellular communication by intravital deep-tissue multi-photon microscopy is the key for understanding the fate of immune cells within thick tissue samples and organs in health and disease. By controlling the scanning pattern in multi-photon microscopy and applying appropriate numerical algorithms, we developed a striped-illumination approach, which enabled us to achieve 3-fold better axial resolution and improved signal-to-noise ratio, i.e. contrast, in more than 100 &#181;m tissue depth within highly scattering tissue of lymphoid organs as compared to standard multi-photon microscopy. The acquisition speed as well as photobleaching and photodamage effects were similar to standard photo-multiplier-based technique, whereas the imaging depth was slightly lower due to the use of field detectors. By using the striped-illumination approach, we are able to observe the dynamics of immune complex deposits on secondary follicular dendritic cells &#8211; on the level of a few protein molecules in germinal centers.

High-resolution intravital imaging with enhanced contrast up to 120 &#181;m depth in lymph nodes of adult mice is achieved by spatially modulating the excitation pattern of a multi-focal two-photon microscope. In 100 &#181;m depth we measured resolutions of 487 nm (lateral) and 551 nm (axial), thus circumventing scattering and diffraction limits.

Monitoring cellular communication by intravital deep-tissue multi-photon microscopy is the key for understanding the fate of immune cells within thick ...

Study of Phagolysosome Biogenesis in Live Macrophages

Phagocytic cells play a major role in the innate immune system by removing and eliminating invading microorganisms in their phagosomes. Phagosome maturation is the complex and tightly regulated process during which a nascent phagosome undergoes drastic transformation through well-orchestrated interactions with various cellular organelles and compartments in the cytoplasm. This process, which is essential for the physiological function of phagocytic cells by endowing phagosomes with their lytic and bactericidal properties, culminates in fusion of phagosomes with lysosomes and biogenesis of phagolysosomes which is considered to be the last and critical stage of maturation for phagosomes. In this report, we describe a live cell imaging based method for qualitative and quantitative analysis of the dynamic process of lysosome to phagosome content delivery, which is a hallmark of phagolysosome biogenesis. This approach uses IgG-coated microbeads as a model for phagocytosis and fluorophore-conjugated dextran molecules as a luminal lysosomal cargo probe, in order to follow the dynamic delivery of lysosmal content to the phagosomes in real time in live macrophages using time-lapse imaging and confocal laser scanning microscopy. Here we describe in detail the background, the preparation steps and the step-by-step experimental setup to enable easy and precise deployment of this method in other labs. Our described method is simple, robust, and most importantly, can be easily adapted to study phagosomal interactions and maturation in different systems and under various experimental settings such as use of various phagocytic cells types, loss-of-function experiments, different probes, and phagocytic particles.

Phagocytic cells play a major role in the innate immune system by removing and eliminating invading microorganisms in their phagosomes. Phagosome ...

Intravital Microscopy Imaging of the Liver following Leishmania Infection: An Assessment of Hepatic Hemodynamics

Intravital microscopy (IVM) is a powerful optical imaging technique that has made possible the visualization, monitoring and quantification of various biological events in real time and in live animals. This technology has greatly advanced our understanding of physiological processes and pathogen-mediated phenomena in specific organs.
In this study, IVM is applied to the mouse liver and protocols are designed to image in vivo the circulatory system of the liver and measure red blood cell (RBC) velocity in individual hepatic vessels. To visualize the different vessel subtypes that characterize the hepatic organ and perform blood flow speed measurements, C57Bl/6 mice are intravenously injected with a fluorescent plasma reagent that labels the liver-associated vasculature. IVM enables in vivo, real time, measurement of RBC velocity in a specific vessel of interest. Establishing this methodology will make it possible to investigate liver hemodynamics under physiological and pathological conditions. Ultimately, this imaging-based methodology will be important for studying the influence of L. donovani infection&#160;on hepatic hemodynamics.
This method can be applied to other infectious models and mouse organs and might be further extended to pre-clinical testing of a drug&#8217;s effect on inflammation by quantifying its effect on blood flow.

Intravital Microscopy Imaging of the Liver following Leishmania Infection: An Assessment of Hepatic Hemodynamics

This article reports on a detailed method for the dynamic measurement and quantification of blood flow velocity within individual blood vessels of the mouse liver vasculature using intravital microscopy imaging in combination with a specific methodology for image acquisition and analysis.

Intravital microscopy (IVM) is a powerful optical imaging technique that has made possible the visualization, monitoring and quantification of various ...

Flow Cytometric Analysis of Natural Killer Cell Lytic Activity in Human Whole Blood

NK cell cytotoxicity is a widely used measure to determine the effect of outside intervention on NK cell function. However, the accuracy and reproducibility of this assay can be considered unstable, either because of user&#39;s errors or because of the sensitivity of NK cells to experimental manipulation. To eliminate these issues, a workflow that reduces them to a minimum was established and is presented here. To illustrate, we obtained blood samples, at various time points, from runners (n = 4) that were submitted to an intense bout of exercise. First, NK cells are simultaneously identified and isolated through CD56 tagging and magnetic-based sorting, directly from whole blood and from as little as one milliliter. The sorted NK cells are removed of any reagent or capping antibodies. They can be counted in order to establish an accurate NK cell count per milliliter of blood. Secondly, the sorted NK cells (effectors cells or E) can be mixed with 3,3&#39;-Diotadecyloxacarbocyanine Perchlorate (DiO) tagged K562 cells (target cells or T) at an assay-optimal 1:5 T:E ratio, and analyzed using an imaging flow-cytometer that allows for the visualization of each event and the elimination of any false positive or false negatives (such as doublets or effector cells). This workflow can be completed in about 4 h, and allows for very stable results even when working with human samples. When available, research teams can test several experimental interventions in human subjects, and compare measurements across several time points without compromising the data&#39;s integrity.

This work describes an advanced workflow for the accurate and fast determination of NK (Natural Killer) cell count and NK cell cytotoxicity in human blood samples.

NK cell cytotoxicity is a widely used measure to determine the effect of outside intervention on NK cell function. However, the accuracy and ...

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 ...

Saturated Fatty Acids Induce Ceramide-associated Macrophage Cell Death

Macrophages highly express epidermal fatty acid-binding protein and adipose fatty acid-binding protein. They actively uptake saturated and unsaturated fatty acids, which might play a critical role in regulating their immune functions. Numerous studies have shown that various fatty acids, saturated or unsaturated, may possess different impacts on cell growth and function. However, the approaches used for fatty acid preparation vary, which may lead to non-physiological results. Serum albumin, a natural carrier for fatty acids in mammalian peripheral blood, is recommended for forming a conjugate complex with the sodium salt of fatty acids to study fatty acid function in mammalian cells, thus minimizing the toxicity of fatty acid soap. Thus, a simple, relatively quick heating and sonicating method is developed and presented here for BSA-fatty acid conjugate formation. We describe a protocol using saturated fatty acids, especially stearic acids to induce severe cell death in mouse bone-marrow derived macrophages. We further demonstrate that saturated fatty acid-induced cell death is positively associated with accumulated cellular ceramide levels. This method can be extended for studies of the impact of fatty acid on other mammalian cells.

We illustrate a straight-forward method to derive murine primary macrophages from bone marrow cells and a simple method to prepare BSA-fatty acid conjugates. Then we demonstrate that saturated fatty acids can induce macrophage cell death, and such cell death is positively associated with cellular accumulation of ceramide levels.

Macrophages highly express epidermal fatty acid-binding protein and adipose fatty acid-binding protein. They actively uptake saturated and unsaturated ...

Imaging Cell Interaction in Tracheal Mucosa During Influenza Virus Infection Using Two-photon Intravital Microscopy

The analysis of cell-cell or cell-pathogen interaction in vivo is an important tool to understand the dynamics of the immune response to infection. Two-photon intravital microscopy (2P-IVM) allows the observation of cell interactions in deep tissue in living animals, while minimizing the photobleaching generated during image acquisition. To date, different models for 2P-IVM of lymphoid and non-lymphoid organs have been described. However, imaging of respiratory organs remains a challenge due to the movement associated with the breathing cycle of the animal.
Here, we describe a protocol to visualize in vivo immune cell interactions in the trachea of mice infected with influenza virus using 2P-IVM. To this purpose, we developed a custom imaging platform, which included the surgical exposure and intubation of the trachea, followed by the acquisition of dynamic images of neutrophils and dendritic cells (DC) in the mucosal epithelium. Additionally, we detailed the steps needed to perform influenza intranasal infection and flow cytometric analysis of immune cells in the trachea. Finally, we analyzed neutrophil and DC motility as well as their interactions during the course of a movie. This protocol allows for the generation of stable and bright 4D images necessary for the assessment of cell-cell interactions in the trachea.

In this study, we present a protocol to perform two-photon intravital imaging and cell interaction analysis in the murine tracheal mucosa after infection with influenza virus. This protocol will be relevant for researchers studying immune cell dynamics during respiratory infections.

The analysis of cell-cell or cell-pathogen interaction in vivo is an important tool to understand the dynamics of the immune response to infection. ...