Name: a high-throughput cre-lox activated viral membrane fusion assay to identify inhibitors of hiv-1 viral membrane fusion
Uploaded: 2018-08-14T14:00:00
Description: Watch this Scientific Journal Video about A High-throughput Cre-Lox Activated Viral Membrane Fusion Assay to Identify Inhibitors of HIV-1 Viral Membrane Fusion at JoVE.com

This research was supported by grants NIH/NIAID AI112423 and NIH/NIGMS GM113885 to Benjamin K. Chen and NIH/NIAID K08-AI120806 to Talia H. Swartz. We would like to thank the Icahn School of Medicine at Mount Sinai Dean&#39;s flow Cytometry CORE.

1. Generation of Target Cell Lines
NOTE: This step is optional; if using an existing RG target cell line, start at step 2.
<ol>
	<li>Cotransfect a 70% confluent 10 cm plate of 293T cells11 [in 10 mL of Dulbecco&#39;s modified Eagle&#39;s medium (DMEM) with 10% fetal bovine serum (FBS)] with pMSCV-loxp-dsRed-loxp-eGFP-Puro-WPRE10 and pCL-10A112 (packaging plasmid) in a 1:1 ratio (20 &#181;g total) using calcium-based tran...

<ol>
	<li>Esposito, A. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+high+throughput+Cre-lox+activated+viral+membrane+fusion+assay+identifies+pharmacological+inhibitors+of+HIV+entry.">A high throughput Cre-lox activated viral membrane fusion assay identifies pharmacological inhibitors of HIV entry.</a> Virology. 490, 6-16 (2016).</li><li>Cavrois, M., De Noronha, C., Greene, W. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+sensitive+and+specific+enzyme-based+assay+detecting+HIV-1+virion+fusion+in+primary+T+lymphocytes.">A sensitive and specific enzyme-based assay detecting HIV-1 virion fusion in primary T lymphocytes.</a> Nature Biotechnology. 20, 1151-1154 (2002).</li><li>Huerta, L., Lamoyi, E., Baez-Saldana, A., Larralde, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Human+immunodeficiency+virus+envelope-dependent+cell-cell+fusion:+a+quantitative+fluorescence+cytometric+assay.">Human immunodeficiency virus envelope-dependent cell-cell fusion: a quantitative fluorescence cytometric assay.</a> Cytometry. 47, 100-106 (2002).</li><li>Sakamoto, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Establishment+of+an+HIV+cell-cell+fusion+assay+by+using+two+genetically+modified+HeLa+cell+lines+and+reporter+gene.">Establishment of an HIV cell-cell fusion assay by using two genetically modified HeLa cell lines and reporter gene.</a> Journal of Virological Methods. 114, 159-166 (2003).</li><li>Herschhorn, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+inducible+cell-cell+fusion+system+with+integrated+ability+to+measure+the+efficiency+and+specificity+of+HIV-1+entry+inhibitors.">An inducible cell-cell fusion system with integrated ability to measure the efficiency and specificity of HIV-1 entry inhibitors.</a> PLoS One. 6, e26731 (2011).</li><li>Swartz, T. H., Esposito, A. M., Durham, N. D., Hartmann, B. M., Chen, B. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=P2X-selective+purinergic+antagonists+are+strong+inhibitors+of+HIV-1+fusion+during+both+cell-to-cell+and+cell-free+infection.">P2X-selective purinergic antagonists are strong inhibitors of HIV-1 fusion during both cell-to-cell and cell-free infection.</a> Journal of Virology. 88, 11504-11515 (2014).</li><li>Marin, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-Throughput+HIV-Cell+Fusion+Assay+for+Discovery+of+Virus+Entry+Inhibitors.">High-Throughput HIV-Cell Fusion Assay for Discovery of Virus Entry Inhibitors.</a> Assay and Drug Development Technologies. 13, 155-166 (2015).</li><li>Giroud, C., 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=Screening+and+Functional+Profiling+of+Small-Molecule+HIV-1+Entry+and+Fusion+Inhibitors.">Screening and Functional Profiling of Small-Molecule HIV-1 Entry and Fusion Inhibitors.</a> Assay and Drug Development Technologies. 15, 53-63 (2017).</li><li>Hubner, W., 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=Sequence+of+human+immunodeficiency+virus+type+1+(HIV-1)+Gag+localization+and+oligomerization+monitored+with+live+confocal+imaging+of+a+replication-competent,+fluorescently+tagged+HIV-1.">Sequence of human immunodeficiency virus type 1 (HIV-1) Gag localization and oligomerization monitored with live confocal imaging of a replication-competent, fluorescently tagged HIV-1.</a> Journal of Virology. 81, 12596-12607 (2007).</li><li>Koo, B. K., 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=Controlled+gene+expression+in+primary+Lgr5+organoid+cultures.">Controlled gene expression in primary Lgr5 organoid cultures.</a> Nature Methods. 9, 81-83 (2011).</li><li>Pear, W. S., 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=Production+of+high-titer+helper-free+retroviruses+by+transient+transfection.">Production of high-titer helper-free retroviruses by transient transfection.</a> Proceedings of the National Academy of Sciences. 90, 8392-8396 (1993).</li><li>Naviaux, R. K., Costanzi, E., Haas, M., Verma, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+pCL+vector+system:+Rapid+production+of+helper-free,+high+titer,+recombinant+retroviruses.">The pCL vector system: Rapid production of helper-free, high titer, recombinant retroviruses.</a> Journal of Virology. 70, 5701-5705 (1996).</li><li>Kingston, R. E., Chen, C. A., Okayama, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Calcium+Phosphate+Transfection.">Calcium Phosphate Transfection.</a> Current Protocols in Immunology. 10, 10-13 (2001).</li></ol>

The Gag-iCre assay has proven to be very useful for screening drug candidates which may inhibit the fusion step of viral replication. When performing this assay, the most important steps to get a good signal are similar to most viral infection assays. The first critical step is producing high titers of good-quality virus. This step requires that the 293T cells are passaged frequently (at least 1x every 48 h) so they do not become overconfluent and clump together. Additionally, it may be worth experimenting with different...

The need for novel antiretroviral therapies has prompted the developement of&#160;high-throughput screens for&#160;inhibitors of HIV-1 entry. The Gag-iCre reporter assay is developed to identify inhibitors of viral entry at the fusion step in a cell-to-cell infection system by specifically measuring viral membrane fusion with the host cell membrane1. An assay was developed to&#160;screen for novel inhibitors that acted specifically at the early stages of HIV-1 infection up to the point of viral membrane fusion. One challenge to measuring cell-cell infection is that the initial inoculum contains infected donor cells and ininfected&#160;target cells, so measuring new infection is difficult to discriminate from the input cells. The ideal system would involve a heterologous gene marker that can be induced by the initiation of a target cell infection and is not present in the donor cell. A popular viral contents mixing assay based on a viral protein-enzyme fusion,&#160;BlaM-Vpr, can be effective, but has&#160;limitations for high throughput, cell-cell screening2. This assay involves a beta-lactamase (BlaM) reporter gene that is fused to HIV-1 Vpr, packaged into virions, and delivered into target cells upon viral membrane fusion. The substrate CCF2-AM is loaded into the cytoplasm of target cells and undergoes a fluorescence shift upon cleavage. The CCF2-AM substrate is expensive and can be prohibitive for high-throughput screens. In order to distinguish donor and target cells, it is necessary to dye-label the target populations. Finally, the protocol requires several wash and incubation steps which can be burdensome and costly when testing large numbers of compounds.
The Gag-iCre assay was developed as a solution to these issues, to develop a high-throughput screening for inhibitors of fusion in cell-to-cell transmission. This system does not require&#160;substrate to be loaded into cells. The assay can also be used for cell-free studies and is adaptable to other viral fusion studies using pseudo-typed HIV-1 Gag-iCre virus particles. HIV Env mediated cell-cell fusion assays can measure virus Env protein-mediated cell fusion, however these do not use actual virus particles as the HIV-1 Gag-iCre assay does3,4,5. This assay can be read with&#160;flow cytometry or fluorescence microscopy. It has been successfully used to screen the FDA library as well as a small library of purinergic inhibitors1,6. Other investigators have also identified purinergic inhibitors in HIV-1 fusion using an adapted and optimized high-throughput fusion assay that reports the transfer of virus-encapsulated beta-lactamase into the cytoplasm7,8.
The Gag-iCre virus was designed with a similar approach to the Gag-iGFP virus, inserting Cre recombinase between matrix and capsid, flanked by HIV-1 protease sites9. The Cre enzyme is made as a precursor inserted within the Gag polyprotein that matures when HIV-1 protease is activated within the nascent virus particle. The Cre delivery is, thus, dependent upon the delivery of the contents of a protease-activated HIV-1 particle into a target cell via a viral membrane fusion process. Two versions of the protocol are provided here. The first uses an HIV-1 infected population to infect target cells, to study direct cell-to-cell transmission of HIV. The second version uses a cell-free virus to study a cell-free viral infection. The cell-to-cell transmission assay takes 7 days to complete from the day the cells are thawed, or 5 days if the cells are already thawed and passaged. The cell-free infection assay can be performed&#160;in 5 days if the cells need to be thawed and 3 days if the cells are thawed and passaged. Instructions are also given to generate an RG (red-to-green) target cell line in the desired cell type (if a preexisting RG target cell line is not being used) using a plasmid created by the Clevers Lab10. It is recommended that appropriate biosafety precautions are taken with the virus and the virus-expressing cells in this assay. We conduct the infectious portion of this assay in a BSL2+ tissue culture facility. After the cells are fixed, they can be analyzed in standard flow cytometry and microscopy facilities.
Here we describe the application of this assay to screen for novel compounds that inhibit HIV-1 viral membrane fusion (Figure 1). Purinergic receptors are pro-inflammatory mediators. Our laboratory has demonstrated that non-selective inhibitors of purinergic receptors act as inhibitors of HIV-1 viral membrane fusion6. We report that the utilization of this assay in high throughput can identify novel HIV-1 viral membrane fusion inhibitors. We demonstrate that an inhibitor of the purinergic class of receptors represents a novel class of HIV-1 viral membrane fusion inhibitors.

<table>
	<tbody>
		<tr>
			<td>Dulbecco&#39;s Modified Eagle&#39;s Medium (DMEM)</td>
			<td>Sigma Aldrich</td>
			<td>D5546</td>
			<td>Media for 293 Cells</td>
		</tr>
		<tr>
			<td>RPMI-1640</td>
			<td>Sigma Aldrich</td>
			<td>R0883</td>
			<td>Media for Jurkats</td>
		</tr>
		<tr>
			<td>Fetal Bovine Serum Albumin</td>
			<td>Gibco</td>
			<td>16140-071</td>
			<td>Serum for 293 Cells</td>
		</tr>
		<tr>
			<td>Cosmic Calf Serum</td>
			<td>Hyclone</td>
			<td>SH30087.03</td>
			<td>Serum for Jurkat Cells</td>
		</tr>
		<tr>
			<td>Hyclone Pennecillin Streptomycin solution</td>
			<td>GE Healthcare Life sciences</td>
			<td>SV30010</td>
			<td>Penn/Strep used in both media</td>
		</tr>
		<tr>
			<td>T75 Flasks</td>
			<td>Corning</td>
			<td>3073</td>
			<td>Used for Culture of Jurkat Cells</td>
		</tr>
		<tr>
			<td>10cm Tissue Culture Plates</td>
			<td>Corning</td>
			<td>430167</td>
			<td>Used for Culture of 293 Cells</td>
		</tr>
		<tr>
			<td>96 Well Plates (tissue culture Treated)</td>
			<td>Corning</td>
			<td>3595</td>
			<td>Used for fusion assay</td>
		</tr>
		<tr>
			<td>Polyjet Transfection Reagent</td>
			<td>Signagen</td>
			<td>SL100688</td>
			<td>Used to transfect 293 Cells</td>
		</tr>
		<tr>
			<td>Dulbecco&#39;s Phosphate Buffered Saline</td>
			<td>Sigma Aldrich</td>
			<td>D8537-100ML</td>
			<td>Used in wash steps</td>
		</tr>
		<tr>
			<td>Hyclone Trypsin Protease</td>
			<td>GE Healthcare Life sciences</td>
			<td>SH30042.01</td>
			<td>For Trypsinization of 293 cells</td>
		</tr>
		<tr>
			<td>Amaxa Cell Line Nucleofector Kit V</td>
			<td>Lonza</td>
			<td>VACA-1003</td>
			<td>For Nucleofection of Jurkats</td>
		</tr>
		<tr>
			<td>Ficoll-Paque plus</td>
			<td>GE Healthcare Life sciences</td>
			<td>17144002</td>
			<td>For Nucleofection of Jurkats</td>
		</tr>
		<tr>
			<td>Serological Pipettes</td>
			<td>Fisher Brand</td>
			<td>13-678-11E</td>
			<td>For all tissue culture</td>
		</tr>
		<tr>
			<td>Pipettor Tips</td>
			<td>Denville Scientific</td>
			<td>P3020-CPS</td>
			<td>For all tissue culture and liquid handling steps</td>
		</tr>
		<tr>
			<td>Millex Syringe Filter (0.45 micron)</td>
			<td>Millipore</td>
			<td>SLHA033</td>
			<td>For filtration of virus</td>
		</tr>
		<tr>
			<td>BD Slip Tip Sterile syringes</td>
			<td>BD Diagnostics</td>
			<td>309656</td>
			<td>For filtration of virus</td>
		</tr>
		<tr>
			<td>Amaxa Nucleofector</td>
			<td>Lonza</td>
			<td>2b</td>
			<td>for Nucleofection of Jurkats (various models available)</td>
		</tr>
		<tr>
			<td>BD Fortessa Flow Cytometer</td>
			<td>BD Biosciences</td>
			<td></td>
			<td>for flow cytometry analyss of samples</td>
		</tr>
		<tr>
			<td>Tissue Culture Hood</td>
			<td>Various models</td>
			<td>Fortessa 2</td>
			<td></td>
		</tr>
		<tr>
			<td>pMSCV-loxp-dsRed-loxp-eGFP-Puro-W PRE</td>
			<td>Addgene</td>
			<td>32702</td>
			<td>Koo BK et al. Controlled gene expression in primary Lgr5 organoid cultures. Nature Methods 9:81-83 (2011).</td>
		</tr>
		<tr>
			<td>pCL10a1</td>
			<td>Novus Bio</td>
			<td>NBP2-29542</td>
			<td>Naviaux, RK, Costanzi, E, Haas, M and Verma, I. The pCL vector system: Rapid production of helper-free, high titer, recombinant retroviruses. Journal of Virology 70: 5701-5705 (1996)</td>
		</tr>
		<tr>
			<td>Gag-iCre</td>
			<td>Benjamin Chen Lab</td>
			<td></td>
			<td>Esposito AM et al. A high throughput Cre-lox activated viral membrane fusion assay identifies pharmacological inhibitors of HIV entry. Virology 490:6-16 (2016).</td>
		</tr>
	</tbody>
</table>

a high-throughput cre-lox activated viral membrane fusion assay to identify inhibitors of hiv-1 viral membrane fusion

This assay is designed to specifically report on HIV-1 fusion via the expression of green fluorescent protein (GFP) detectable by flow cytometry or fluorescence microscopy. An HIV-1 reporter virus (HIV-1 Gag-iCre) is generated by inserting Cre recombinase into the HIV-1 genome between the matrix and the capsid proteins of the Gag polyprotein. This results in a packaging of Cre recombinase into virus particles, which is then released into a target cell line stably expressing a Cre recombinase-activated red fluorescent protein (RFP) to GFP switch cassette. In the basal state, this cassette expresses RFP only. Following the delivery of Cre recombinase into the target cell, the RFP, flanked by loxP sites, excises, resulting in GFP expression. This assay can be used to test any inhibitors of viral entry (specifically at the fusion step) in cell-free and cell-to-cell infection systems and has been used to identify a class of purinergic receptor antagonists as novel inhibitors of HIV-1 viral membrane fusion.

Uninfected RG Jurkat Cells exhibit a low level of background GFP signal (0.3%) with a very strong RFP signal (Figure 2A, uninfected column). The infection with Gag-iCre causes an increase in GFP signal (24.9%,) while the presence of the HIV-1 fusion inhibitor AMD3100 (20 &#181;M) inhibits the development of this signal, bringing it down to uninfected background levels (0.34%). When an inhibitor of a post-fusion event such as the reverse transcription inhibito...

Watch this Scientific Journal Video about A High-throughput Cre-Lox Activated Viral Membrane Fusion Assay to Identify Inhibitors of HIV-1 Viral Membrane Fusion at JoVE.com

A High-throughput Cre-Lox Activated Viral Membrane Fusion Assay to Identify Inhibitors of HIV-1 Viral Membrane Fusion

We describe a cell-based assay to report on HIV-1 fusion via the expression of green fluorescent protein detectable by flow cytometry or fluorescence microscopy. It can be used to test inhibitors of viral entry (specifically at the fusion step) in cell-free and cell-to-cell infection systems.

This assay is designed to specifically report on HIV-1 fusion via the expression of green fluorescent protein (GFP) detectable by flow cytometry or ...

This method can help answer key questions in the HIV entry field such as drug discovery and identification of factors required for HIV fusion. The main advantage of this technique is that it can be scaled for high throughput screening. This technique can be used to screen large libraries of compounds to identify drugs that target early stages of the HIV-1 life cycle.This method can also be used to study HIV fusion inhibitors in animal models such as humanized mice. Researchers should follow biosafety precautions with infectious HIV-1. This is a recombinant full-length viral clone that does not replicate in multiple rounds, but must be approved by your biosafety office.To begin this procedure, thaw one 500 microliter vial of Jurkat RG reporter cells by placing it into a 37 degree Celsius water bath. Pipette the cells from the vial into 10 milliliters of RPMI complete medium and then centrifuge the mixture at 800 times gravity for five minutes at 23 degrees Celsius. Resuspend the pellet in 20 milliliters of RPMI complete medium and transfer the cell suspension to a T75 flask.Incubate the flask at 37 degrees Celsius overnight. On the following day, add 0.5 micrograms per milliliter of Puromycin to the cells in the flask. Culture the cells maintaining a density of 200, 000 to 800, 000 cells per milliliter.On the day prior to setting up the transfer assay, split the cells down to 200, 000 to 400, 000 cells per milliliter in fresh RPMI medium containing 0.5 micrograms per milliliter of Puromycin. Grow the cells overnight. To prepare the donor cells for cell-to-cell virus transmission, thaw untransduced Jurkat cells and culture them in a T75 flask with 20 milliliters of RPMI complete medium keeping the density of 200, 000 to 800, 000 cells per milliliter.Centrifuge 7, 500, 000 cells at 800 times gravity for five minutes. Discard the supernatant and resuspend the cells in 120 microliters of Nucleofector Solution V with supplement. Transfer the cells to an electroporation cuvette and add 4.5 micrograms of Gag-iCre DNA.Electroporate the cells using an appropriate program and then immediately transfer the cells to three milliliters of RPMI medium with 10%FBS. Allow the cells to recover by incubating them at 37 degrees Celsius with 5%CO2 overnight. On the following morning, pipette two milliliters of Ficoll into a 15 milliliter conical tube.Slowly pipette the three milliliters of cells from the six-well plate on top of the Ficoll. Centrifuge the cells at 800 times gravity for five minutes at 23 degrees Celsius and transfer the cells from the interface Ficoll media interface to three milliliters of RPMI complete medium in a six-well plate. Allow the cells to recover at 37 degrees Celsius for two hours before proceeding with the assay setup.Start this procedure by using a hemocytometer to count both the donor and target cells. Then spin down 50, 000 cells per well to be assayed of both donor and target cells. Resuspend one times 10 to the sixth cells per milliliter in RPMI complete medium without Puromycin.In a 96-well plate, add 25 microliters of donor cell suspension to each well. In another 96-well plate, add 25 microliters of target cell suspension per well. To each well, add 25 microliters of the test compound diluted in RPMI complete medium at the appropriate concentration.Incubate the donor and target cells with the compound for 30 minutes. After the 30-minute pretreatment, combine the donor cells with the target cells by pipetting the contents of one plate into the other plate and incubating the cells in a tissue culture incubator for 40 hours. To fix the cells, add paraformaldehyde to each well to a final concentration of 2%Subsequently, analyze the cells on a flow cytometer with mCherry and FITC channels.In addition, visualize the GFP signal via fluorescence microscopy at 40X magnification on GFP-specific channels. Uninfected RG Jurkat cells exhibit a low level of background GFP signal with a very strong RFP signal. Cell-free infection with Gag-iCre causes an increase in GFP signal.The presence of the HIV-1 fusion inhibitor AMD3100 inhibits the development of GFP signal bringing it down to uninfected background levels. When an inhibitor of a post-fusion event such as the reverse transcription inhibitor AZT is used, the signal is not affected significantly indicating that the GFP signal produced by the assay is specific to the HIV-1 fusion. In a cell-to-cell infection assay, RG Jurkat cells were mixed with Jurkat cells transfected with HIV-1 Gag-iCre.In addition to AMD3100, a nonselective purinergic inhibitor PPADS was tested for its ability to block viral membrane fusion at 100 micromolar. The results indicate a dose-dependent inhibition of Gag-iCre fusion with PPADS. While attempting this procedure, it's important to keep the cells within exponential growth range for both virus production and infection.Using overgrown cells will result in a dramatic decrease in signal from this assay. Don't forget that working with HIV can be hazardous. BSL two practices as indicated by your institution should be always taken while performing this procedure.

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Research

JoVE Journal

Immunology and Infection

Using a Pan-Viral Microarray Assay (Virochip) to Screen Clinical Samples for Viral Pathogens

The diagnosis of viral causes of many infectious diseases is difficult due to the inherent sequence diversity of viruses as well as the ongoing emergence of novel viral pathogens, such as SARS coronavirus and 2009 pandemic H1N1 influenza virus, that are not detectable by traditional methods. To address these challenges, we have previously developed and validated a pan-viral microarray platform called the Virochip with the capacity to detect all known viruses as well as novel variants on the basis of conserved sequence homology1. Using the Virochip, we have identified the full spectrum of viruses associated with respiratory infections, including cases of unexplained critical illness in hospitalized patients, with a sensitivity equivalent to or superior to conventional clinical testing2-5. The Virochip has also been used to identify novel viruses, including the SARS coronavirus6,7, a novel rhinovirus clade5, XMRV (a retrovirus linked to prostate cancer)8, avian bornavirus (the cause of a wasting disease in parrots)9, and a novel cardiovirus in children with respiratory and diarrheal illness10. The current version of the Virochip has been ported to an Agilent microarray platform and consists of ~36,000 probes derived from over ~1,500 viruses in GenBank as of December of 2009. Here we demonstrate the steps involved in processing a Virochip assay from start to finish (~24 hour turnaround time), including sample nucleic acid extraction, PCR amplification using random primers, fluorescent dye incorporation, and microarray hybridization, scanning, and analysis.

Using a Pan-Viral Microarray Assay Virochip to Screen Clinical Samples for Viral Pathogens

The Virochip is a pan-viral microarray designed to simultaneously detect all known viruses as well as novel viruses on the basis of conserved sequence homology. Here we demonstrate how to run a Virochip assay to analyze clinical samples for the presence of both known and unknown viruses.

The diagnosis of viral causes of many infectious diseases is difficult due to the inherent sequence diversity of viruses as well as the ongoing emergence ...

Amplifying and Quantifying HIV-1 RNA in HIV Infected Individuals with Viral Loads Below the Limit of Detection by Standard Clinical Assays

Amplifying viral genes and quantifying HIV-1 RNA in HIV-1 infected individuals with viral loads below the limit of detection by standard assays (below 50-75 copies/ml) is necessary to gain insight to viral dynamics and virus host interactions in patients who naturally control the infection and those who are on combination antiretroviral therapy (cART). 
Here we describe how to amplify viral genomes by single genome sequencing (the SGS protocol) 13, 19 and how to accurately quantify HIV-1 RNA in patients with low viral loads (the single-copy assay (SCA) protocol) 12, 20. 
The single-copy assay is a real-time PCR assay with sensitivity depending on the volume of plasma being assayed. If a single virus genome is detected in 7 ml of plasma, then the RNA copy number is reported to be 0.3 copies/ml. The assay has an internal control testing for the efficiency of RNA extraction, and controls for possible amplification from DNA or contamination. Patient samples are measured in triplicate.
The single-genome sequencing assay (SGS), now widely used and considered to be non-labor intensive 3, 7, 12, 14, 15,is a limiting dilution assay, in which endpoint diluted cDNA product is spread over a 96-well plate. According to a Poisson distribution, when less than 1/3 of the wells give product, there is an 80% chance of the PCR product being resultant of amplification from a single cDNA molecule. SGS has the advantage over cloning of not being subjected to resampling and not being biased by PCR-introduced recombination 19. However, the amplification success of SCA and SGS depend on primer design. Both assays were developed for HIV-1 subtype B, but can be adapted for other subtypes and other regions of the genome by changing primers, probes, and standards.

Quantifying levels of HIV-1 RNA in plasma and sequencing single HIV-1 genomes from individuals with viral loads below the limit of detection (50-75 copies/ml) is difficult. Here we describe how to extract and quantify plasma viral RNA using a real time PCR assay that reliably measures HIV-1 RNA down to 0.3 copies/ml and how to amplify viral genomes by single genome sequencing, from samples with very low viral loads.

Amplifying viral genes and quantifying HIV-1 RNA in HIV-1 infected individuals with viral loads below the limit of detection by standard assays (below ...

Establishing a Liquid-covered Culture of Polarized Human Airway Epithelial Calu-3 Cells to Study Host Cell Response to Respiratory Pathogens In vitro

The apical and basolateral surfaces of airway epithelial cells demonstrate directional responses to pathogen exposure in vivo. Thus, ideal in vitro models for examining cellular responses to respiratory pathogens polarize, forming apical and basolateral surfaces. One such model is differentiated normal human bronchial epithelial cells (NHBE). However, this system requires lung tissue samples, expertise isolating and culturing epithelial cells from tissue, and time to generate an air-liquid interface culture.
Calu-3 cells, derived from a human bronchial adenocarcinoma, are an alternative model for examining the response of proximal airway epithelial cells to respiratory insult1, pharmacological compounds2-6, and bacterial7-9 and viral pathogens, including influenza virus, rhinovirus and severe acute respiratory syndrome - associated coronavirus10-14. Recently, we demonstrated that Calu-3 cells are susceptible to respiratory syncytial virus (RSV) infection in a manner consistent with NHBE15,16 . Here, we detail the establishment of a polarized, liquid-covered culture (LCC) of Calu-3 cells, focusing on the technical details of growing and culturing Calu-3 cells, maintaining cells that have been cultured into LCC, and we present the method for performing respiratory virus infection of polarized Calu-3 cells.
To consistently obtain polarized Calu-3 LCC, Calu-3 cells must be carefully subcultured before culturing in Transwell inserts. Calu-3 monolayer cultures should remain below 90% confluence, should be subcultured fewer than 10 times from frozen stock, and should regularly be supplied with fresh medium. Once cultured in Transwells, Calu-3 LCC must be handled with care. Irregular media changes and mechanical or physical disruption of the cell layers or plates negatively impact polarization for several hours or days. Polarization is monitored by evaluating trans-epithelial electrical resistance (TEER) and is verified by evaluating the passive equilibration of sodium fluorescein between the apical and basolateral compartments17,18 . Once TEER plateaus at or above 1,000 &Omega;&times;cm2, Calu-3 LCC are ready to use to examine cellular responses to respiratory pathogens.

Establishing a Liquid-covered Culture of Polarized Human Airway Epithelial Calu-3 Cells to Study Host Cell Response to Respiratory Pathogens In vitro

The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the Centers for Disease Control and Prevention.

The  apical and basolateral surfaces of airway epithelial cells demonstrate  directional responses to pathogen exposure in  vivo. Thus, ideal in vitro ...

High-throughput Screening for Broad-spectrum Chemical Inhibitors of RNA Viruses

RNA viruses are responsible for major human diseases such as flu, bronchitis, dengue, Hepatitis C or measles. They also represent an emerging threat because of increased worldwide exchanges and human populations penetrating more and more natural ecosystems. A good example of such an emerging situation is chikungunya virus epidemics of 2005-2006 in the Indian Ocean. Recent progresses in our understanding of cellular pathways controlling viral replication suggest that compounds targeting host cell functions, rather than the virus itself, could inhibit a large panel of RNA viruses. Some broad-spectrum antiviral compounds have been identified with host target-oriented assays. However, measuring the inhibition of viral replication in cell cultures using reduction of cytopathic effects as a readout still represents a paramount screening strategy. Such functional screens have been greatly improved by the development of recombinant viruses expressing reporter enzymes capable of bioluminescence such as luciferase. In the present report, we detail a high-throughput screening pipeline, which combines recombinant measles and chikungunya viruses with cellular viability assays, to identify compounds with a broad-spectrum antiviral profile.

In vitro assays to measure virus replication have been greatly improved by the development of recombinant RNA viruses expressing luciferase or other enzymes capable of bioluminescence. Here we detail a high-throughput screening pipeline that combines such recombinant strains of measles and chikungunya viruses to isolate broad-spectrum antivirals from chemical libraries.

RNA viruses are responsible for major human diseases such as flu, bronchitis, dengue, Hepatitis C or measles. They also represent an emerging threat ...

Conformational Evaluation of HIV-1 Trimeric Envelope Glycoproteins Using a Cell-based ELISA Assay

HIV-1 envelope glycoproteins (Env) mediate viral entry into target cells and are essential to the infectious cycle. Understanding how those glycoproteins are able to fuel the fusion process through their conformational changes could lead to the design of better, more effective immunogens for vaccine strategies. Here we describe a cell-based ELISA assay that allows studying the recognition of trimeric HIV-1 Env by monoclonal antibodies. Following expression of HIV-1 trimeric Env at the surface of transfected cells, conformation specific anti-Env antibodies are incubated with the cells. A horseradish peroxidase-conjugated secondary antibody and a simple chemiluminescence reaction are then used to detect bound antibodies. This system is highly flexible and can detect Env conformational changes induced by soluble CD4 or cellular proteins. It requires minimal amount of material and no highly-specialized equipment or know-how. Thus, this technique can be established for medium to high throughput screening of antigens and antibodies, such as newly-isolated antibodies.

Understanding viral surface antigens conformations is required to evaluate antibody neutralization and guide the design of effective vaccine immunogens. Here we describe a cell-based ELISA assay that allows the study of the recognition of trimeric HIV-1 Env expressed at the surface of transfected cells by specific anti-Env antibodies.

HIV-1 envelope glycoproteins (Env) mediate viral entry into target cells and are essential to the infectious cycle. Understanding how those glycoproteins ...

Nucleocapsid Annealing-Mediated Electrophoresis (NAME) Assay Allows the Rapid Identification of HIV-1 Nucleocapsid Inhibitors

RNA or DNA folded in stable tridimensional folding are interesting targets in the development of antitumor or antiviral drugs. In the case of HIV-1, viral proteins involved in the regulation of the virus activity recognize several nucleic acids. The nucleocapsid protein NCp7 (NC) is a key protein regulating several processes during virus replication. NC is in fact a chaperone destabilizing the secondary structures of RNA and DNA and facilitating their annealing. The inactivation of NC is a new approach and an interesting target for anti-HIV therapy. The Nucleocapsid Annealing-Mediated Electrophoresis (NAME) assay was developed to identify molecules able to inhibit the melting and annealing of RNA and DNA folded in thermodynamically stable tridimensional conformations, such as hairpin structures of TAR and cTAR elements of HIV, by the nucleocapsid protein of HIV-1. The new assay employs either the recombinant or the synthetic protein, and oligonucleotides without the need of their previous labeling. The analysis of the results is achieved by standard polyacrylamide gel electrophoresis (PAGE) followed by conventional nucleic acid staining. The protocol reported in this work describes how to perform the NAME assay with the full-length protein or its truncated version lacking the basic N-terminal domain, both competent as nucleic acids chaperones, and how to assess the inhibition of NC chaperone activity by a threading intercalator. Moreover, NAME can be performed in two different modes, useful to obtain indications on the putative mechanism of action of the identified NC inhibitors.

Nucleocapsid Annealing-Mediated Electrophoresis NAME Assay Allows the Rapid Identification of HIV-1 Nucleocapsid Inhibitors

This protocol describes NAME, an assay that allows the rapid identification of molecules able to inhibit in vitro the chaperone activities of HIV-1 nucleocapsid protein.

RNA or DNA folded in stable tridimensional folding are interesting targets in the development of antitumor or antiviral drugs. In the case of HIV-1, viral ...

Phage Phenomics: Physiological Approaches to Characterize Novel Viral Proteins

Current investigations into phage-host interactions are dependent on extrapolating knowledge from (meta)genomes. Interestingly, 60 - 95% of all phage sequences share no homology to current annotated proteins. As a result, a large proportion of phage genes are annotated as hypothetical. This reality heavily affects the annotation of both structural and auxiliary metabolic genes. Here we present phenomic methods designed to capture the physiological response(s) of a selected host during expression of one of these unknown phage genes. Multi-phenotype Assay Plates (MAPs) are used to monitor the diversity of host substrate utilization and subsequent biomass formation, while metabolomics provides bi-product analysis by monitoring metabolite abundance and diversity. Both tools are used simultaneously to provide a phenotypic profile associated with expression of a single putative phage open reading frame (ORF). Representative results for both methods are compared, highlighting the phenotypic profile differences of a host carrying either putative structural or metabolic phage genes. In addition, the visualization techniques and high throughput computational pipelines that facilitated experimental analysis are presented.

Here, we present phenomic approaches for the functional characterization of putative phage genes. Techniques include a developed assay capable of monitoring host anabolic metabolism, the Multi-phenotype Assay Plates (MAPs), in addition to the established method of metabolomics, capable of measuring effects to catabolic metabolism.

Current investigations into phage-host interactions are dependent on extrapolating knowledge from (meta)genomes. Interestingly, 60 - 95% of all phage ...

The Development of Lyophilized Loop-mediated Isothermal Amplification Reagents for the Detection of Coxiella burnetii

Coxiella burnetii, the agent causing Q fever, is an obligate intracellular bacterium. PCR based diagnostic assays have been developed for detecting C. burnetii DNA in cell cultures and clinical samples. PCR requires specialized equipment and extensive end user training, and therefore, it is not suitable for routine work especially in a resource-constrained area. We have developed a loop-mediated isothermal amplification (LAMP) assay to detect the presence of C. burnetii in patient samples. This method is performed at a single temperature around 60 &#176;C in a water bath or heating block. The sensitivity of this LAMP assay is very similar to PCR with a detection limit of about 25 copies per reaction. This report describes the preparation of the reaction using lyophilized reagents and visualization of results using hydroxynaphthol blue (HNB) or a UV lamp with fluorescent intercalating dye in the reaction. The LAMP reagents were lyophilized and stored at room temperature (RT) for one month without loss of detection sensitivity. This LAMP assay is particularly robust because the reaction mixture preparation does not involve complex steps. This method is ideal for use in resource-limited settings where Q fever is endemic.

The Development of Lyophilized Loop-mediated Isothermal Amplification Reagents for the Detection of Coxiella burnetii

We described here a simple loop-mediated isothermal amplification (LAMP) method using lyophilized reagents for the detection of C. burnetii in patient samples.

Coxiella burnetii, the agent causing Q fever, is an obligate intracellular bacterium. PCR based diagnostic assays have been developed for detecting C. ...

Measuring Influenza Neuraminidase Inhibition Antibody Titers by Enzyme-linked Lectin Assay

Antibodies to neuraminidase (NA), the second most abundant surface protein on influenza virus, contribute toward protection against influenza. Traditional methods to measure NA inhibiting (NI) antibody titers are not practical for routine serology. This protocol describes the enzyme-linked lectin assay (ELLA), a practical alternative method to measure NI titers that is performed in 96 well plates coated with a large glycoprotein substrate, fetuin. NA cleaves terminal sialic acids from fetuin, exposing the penultimate sugar, galactose. Peanut agglutinin (PNA) is a lectin with specificity for galactose and therefore the extent of desialylation can be quantified using a PNA-horseradish peroxidase conjugate, followed by addition of a chromogenic peroxidase substrate. The optical density that is measured is proportional to NA activity. To measure NI antibody titers, serial dilutions of sera are incubated at 37 &#176;C O/N on fetuin-coated plates with a fixed amount of NA. The reciprocal of the highest serum dilution that results in &#8805;50% inhibition of NA activity is designated as the NI antibody titer. The ELLA provides a practical format for routine evaluation of human antibody responses following influenza infection or vaccination.

We describe the enzyme-linked lectin assay (ELLA) for measuring influenza neuraminidase (NA)-inhibition antibody titers in sera. The assay uses peanut agglutinin to quantify galactose residues that become accessible when NA removes sialic acid from fetuin-coated, 96-well plates.

Antibodies to neuraminidase (NA), the second most abundant surface protein on influenza virus, contribute toward protection against influenza. Traditional ...

Swab Sampling Method for the Detection of Human Norovirus on Surfaces

Human noroviruses are a leading cause of epidemic and sporadic gastroenteritis worldwide. Because most infections are either spread directly via the person-to-person route or indirectly through environmental surfaces or food, contaminated fomites and inanimate surfaces are important vehicles for the spread of the virus during norovirus outbreaks.
We developed and evaluated a protocol using macrofoam swabs for the detection and typing of human noroviruses from hard surfaces. Compared with fiber-tipped swabs or antistatic wipes, macrofoam swabs allow virus recovery (range 1.2-33.6%) from toilet seat surfaces of up to 700 cm2. The protocol includes steps for the extraction of the virus from the swabs and further concentration of the viral RNA using spin columns. In total, 127 (58.5%) of 217 swab samples that had been collected from surfaces in cruise ships and long-term care facilities where norovirus gastroenteritis had been reported tested positive for GII norovirus by RT-qPCR. Of these 29 (22.8%) could be successfully genotyped. In conclusion, detection of norovirus on environmental surfaces using the protocol we developed may assist in determining the level of environmental contamination during outbreaks as well as detection of virus when clinical samples are not available; it may also facilitate monitoring of effectiveness of remediation strategies.

A macrofoam based sampling methodology was developed and evaluated for the detection and quantification of norovirus on environmental hard surfaces.

Human noroviruses are a leading cause of epidemic and sporadic gastroenteritis worldwide. Because most infections are either spread directly via the ...