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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6.2K visualizações.  Icahn School of Medicine at Mount Sinai, Immunology Institute. 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 f...