This work was supported by a grant from the Swiss National Science Foundation (31003A_135278) to SSt. MOP is the beneficiary of a doctoral grant from the AXA Research Fund. We thank Patricia Nigg for help with the design of Figure 1.

Note before start: Use laminar flow hood and appropriate biocontainment when working with live virus. Here, we describe growth conditions suitable to culture influenza virus strain A/WSN/33. Multiplicity of infection (MOI) and incubation times may vary depending on virus strain used.
1. Preparation of Biotinylated A/WSN/33 Virus
<ol>
	<li>Seed Madin-Darby canine kindey (MDCK) cells into a T175 cm2 flask using Dulbecco&#8217;s Modified Eagle Medium (DMEM) supplemented with 10% FBS ...

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The authors have nothing to disclose.

Our protocol describes an easy way of measuring virus attachment and internalization by flow cytometry. It allows the use of labeled wildtype virus which mimics more closely virus infections compared to the use of virus-like particles (VLP). While our protocol has been optimized to measure attachment and internalization of IAV it can easily be adapted for other viruses. In addition, as flow cytometry is used for readout, co-stainings can be easily added to the protocol e.g. to test expression levels following kn...

The entry of influenza A virus (IAV) is a multi-step process that starts with the binding of the virus to receptors on the plasma membrane of target cells 1. The receptor for IAV is sialic acid which is present on a large variety of glycoproteins and glycolipids. The hemagglutinin (HA) protein of IAV which is present in the viral envelope binds to sialic acid and thereby mediates attachment of the viral particles to the plasma membrane of target cells 2. The virus enters the cells via clathrin-mediated endocytosis but also alternative entry pathways, such as macropinocytosis, have been described 3-6. The interaction between HA and sialic acid appears to be sufficient for mediating both, attachment and triggering internalization of viral particles 7. Nevertheless, alternative entry receptors have been proposed and their roles in IAV entry are currently being investigated 1,8,9.
Currently, efforts are made to identify host factors that are involved in the viral life cycle with the aim of developing urgently needed novel antiviral therapies 10-12. Virus entry would be a favorable step for targeting IAV growth in order to block viral infection at its earliest point. Measuring different stages of IAV entry experimentally is challenging as usually large amounts of virus are required to detect incoming virus. In addition, the detection range for changes in virus entry is only linear due to the lack of viral replication. This underscores the need for assays with high sensitivity.
The assay we present here allows the detection of attached virus at the cell surface as well as detection of internalized virus in relation to the total amount of cell-associated virus. The use of biotinylated wildtype virus enables convenient measurement through staining with STV-Cy3 and readout by flow cytometry. Biotinylated virus is cold-bound to cells to allow attachment but prevent internalization of viral particles. Cells can be fixed, permeabilized and stained with STV-Cy3 to measure attached virus. The signal from extracellular, attached virions can be abrogated if a blocking step is applied before fixation in which the cells are incubated with non-labeled streptavidin (STV). In a next step, following attachment of biotinylated IAV, temperature is shifted to 37 &#176;C and internalization of viral particles is allowed to take place. Internalized particles are protected from binding of STV thereby allowing the discrimination between extra- and intracellular viruses.
Per experimental condition, four samples are required: &#8216;0 min&#8217;: The first sample, labeled &#8216;0 min&#8217;, is to measure cold-bound virus at the cell surface. &#8216;0 min + STV&#8217;: The second sample, labeled &#8216;0 min + STV&#8217;, gives the baseline signal intensity of the experiment. Attached virus is blocked with STV and the signal following staining with STV-Cy3 should be much lower compared to the &#8216;0 min&#8217; sample. &#8217;30 min&#8217;: The third sample, &#8217;30 min&#8217; contains attached and internalized virus due to the temperature shift to 37 &#176;C for 30 min. &#8217;30 min + STV&#8217;: The fourth sample, &#8217;30 min + STV&#8217; measures the intracellular fraction of viruses. The STV blocking step is applied after the 30 min incubation period. As a result, viral particles at the cell surface are bound by STV leaving only internalized viruses available for staining with STV-Cy3. The relative amount of internalized virus can be calculated as the ratio of internalized virus (measured in the &#8217;30 min + STV&#8217; sample) divided by the total amount of virus (described by the &#8217;30 min&#8217; sample).
As controls we suggest to include mock-infected cells. The signal following STV-Cy3 staining of mock-infected cells gives the background resulting from the staining protocol. A control for IAV attachment is the pre-treatment of cells with bacterial neuraminidase (NA). NA cleaves off sialic acid from cellular glycoproteins, thereby removing attachment receptors from the cell surface. Sodium azide (SA) is a potent metabolic inhibitor and thereby blocks endocytosis 13. Cells treated with sodium azide should be positive for virus binding but negative for internalization.

<table border="0" cellpadding="0" cellspacing="0">
	<tbody>
		<tr>
			<td>DMEM</td>
			<td>Life Technologies</td>
			<td>41966-052</td>
			<td></td>
		</tr>
		<tr>
			<td>FBS</td>
			<td>Life Technologies</td>
			<td>10270-106</td>
			<td></td>
		</tr>
		<tr>
			<td>Penicillin-Streptomycin</td>
			<td>Life Technologies</td>
			<td>15140-163</td>
			<td></td>
		</tr>
		<tr>
			<td>PBS</td>
			<td>Life Technologies</td>
			<td>14190-169&#160;&#160;</td>
			<td></td>
		</tr>
		<tr>
			<td>BSA</td>
			<td>VWR Calbiochem</td>
			<td>126579</td>
			<td></td>
		</tr>
		<tr>
			<td>HEPES</td>
			<td>Life Technologies</td>
			<td>15630-100</td>
			<td></td>
		</tr>
		<tr>
			<td>D-Sucrose</td>
			<td>Fluka</td>
			<td>84100</td>
			<td></td>
		</tr>
		<tr>
			<td>TRIS</td>
			<td>Biosolve BV</td>
			<td>20092391</td>
			<td></td>
		</tr>
		<tr>
			<td>EDTA</td>
			<td>Sigma-Aldrich</td>
			<td>3680</td>
			<td></td>
		</tr>
		<tr>
			<td>EZ-LINK NHS-SS-BIOTIN kit</td>
			<td>Fisher Scientific</td>
			<td><a href="https://extranet.fisher.co.uk/insight2_ch/getProduct.do?productCode=W9971E">W9971E&#160;</a></td>
			<td></td>
		</tr>
		<tr>
			<td>Bio Rad Protein Bio Assay</td>
			<td>Bio Rad</td>
			<td>500-0006</td>
			<td></td>
		</tr>
		<tr>
			<td>deepwell tubes (1.2 ml microtubes)</td>
			<td>Milian</td>
			<td>82 00 001</td>
			<td></td>
		</tr>
		<tr>
			<td>PFA</td>
			<td>&#160;Lucerna chem&#160; Electron microscopy sciences</td>
			<td>15710</td>
			<td></td>
		</tr>
		<tr>
			<td>ultracentrifuge tubes</td>
			<td>Hemotec HmbH</td>
			<td>253070</td>
			<td></td>
		</tr>
		<tr>
			<td>Triton X-100</td>
			<td>Fluka</td>
			<td>93420</td>
			<td></td>
		</tr>
		<tr>
			<td>STV-Cy3</td>
			<td>Life Technologies</td>
			<td>43-4315</td>
			<td></td>
		</tr>
		<tr>
			<td>STV</td>
			<td>Life Technologies</td>
			<td>43-4302</td>
			<td></td>
		</tr>
		<tr>
			<td>sodium azide</td>
			<td>Fluka</td>
			<td>71290</td>
			<td></td>
		</tr>
		<tr>
			<td>bacterial neuraminidase/sialidase</td>
			<td>Sigma-Aldrich</td>
			<td>N6514-1UN</td>
			<td></td>
		</tr>
	</tbody>
</table>

measuring attachment and internalization of influenza a virus in a549 cells by flow cytometry

Attachment to target cells followed by internalization are the very first steps of the life cycle of influenza A virus (IAV). We provide here a detailed protocol for measuring relative changes in the amount of viral particles that attach to A549 cells, a human lung epithelial cell line, as well as in the amount of particles that are internalized into the cell. We use biotinylated virus which can be easily detected following staining with Cy3-labeled streptavidin (STV-Cy3). We describe the growth, purification and biotinylation of A/WSN/33, a widely used IAV laboratory strain. Cold-bound biotinylated IAV particles on A549 cells are stained with STV-Cy3 and measured using flow cytometry. To investigate uptake of viral particles, cold-bound virus is allowed to internalize at 37 &#176;C. In order to differentiate between external and internalized viral particles, a blocking step is applied: Free binding spots on the biotin of attached virus on the cell surface are bound by unlabeled streptavidin (STV). Subsequent cell permeabilization and staining with STV-Cy3 then enables detection of internalized viral particles. We present a calculation to determine the relative amount of internalized virus. This assay is suitable to measure effects of drug-treatments or other manipulations on attachment or internalization of IAV.

A cartoon describing the four different experimental conditions is shown in Figure 1. Results of a representative experiment are presented in Figures 2-5. In the &#8220;0 min&#8221; sample, biotinylated virus is cold-bound to target cells which can be visualized by STV-Cy3 staining (Figures 1 and 2). When a blocking step (in the &#8220;0 min + STV&#8221; sample) is applied, virus at the cell surface can no longer be detected by STV-Cy3 s...

Watch this Scientific Journal Video about Measuring Attachment and Internalization of Influenza A Virus in A549 Cells by Flow Cytometry at JoVE.com

Measuring Attachment and Internalization of Influenza A Virus in A549 Cells by Flow Cytometry

We present a protocol describing a semi-quantitative method for measuring both, the attachment of influenza A virus to A549 cells, as well as the internalization of virus particles into the target cells by flow cytometry.

Attachment to target cells followed by internalization are the very first steps of the life cycle of influenza A virus (IAV). We provide here a detailed ...

measuring-attachment-internalization-influenza-virus-a549-cells-flow

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Immunology and Infection

9.9K visualizações.  University of Zurich. We present a protocol describing a semi-quantitative method for measuring both, the attachment of influenza A virus to A549 cells, as well as the internalization of virus particles into the target cells by flow cytometry. 