We thank countless students in our labs (PJD and BJM) who have worked with us over the years refining these methods. A special thanks to Stan Person, under whose tutelage this methodology was first developed. This work was partially supported by the Towson University Fisher College of Science and Math Undergraduate Research Support fund and NIGMS Bridges to the Baccalaureate grant 5R25GM058264. This content is solely the authors&#39; responsibility and does not necessarily represent the official views of the National Institutes of Health&#39;s National Institute of General Medical Sciences.

All procedures with the Vero cells and live herpesviruses have been approved by the Towson University Institutional Biosafety Committee. A generalized scheme of these procedures is represented in Figure 1.
1. Seeding of the Vero cells
<ol>
	<li>The day before initiating the plaque assay, trypsinize Vero cells and resuspend them in regular Dulbecco&#39;s Modified Eagles Medium (DMEM) and supplement as per standard cell culture methodology19. Resuspend the trypsinized cells in 10 mL of DMEM per T-75 flask of the confluent Vero cells (~107 cells). 
	NOTE: DMEM is used for plaque assays instead of alpha-MEM because it slightly slows the cell growth rate and favors better plaque assay results.</li>
	<li>Count the cells by one&#39;s preferred method (e.g., a standard hemocytometer with Trypan Blue exclusion)19.</li>
	<li>Seed the cells at 4 x 106 cells/plate; for HSV-2, use 6-well plates with 2.5 mL of DMEM (with supplements) per well; and for HSV-1, use 12-well plates with 1.25 mL of DMEM (with supplements) per well. 
	NOTE: The number of cells should be constant regardless of the plate used, though the size of the wells matters concerning accuracy in the plaque assay. It is essential to get cells evenly distributed, which can be most effectively accomplished by moving the plate back and forth, not in a circular fashion.</li>
	<li>Allow the cells to grow overnight at 37 &#176;C/5% CO2 in a humidified incubator. 
	&#8203;NOTE: Humidity is maintained with a pan of distilled water containing an algicide in the bottom of the incubator.</li>
</ol>
2. Sample dilution
<ol>
	<li>Complete the sample dilution and the addition of virus to cells in one lab session. 
	CAUTION: HSV-1 and -2 are infectious for humans and must be handled under proper biosafety containment (BSL-2). Keep all materials that come into contact with the virus separate and disinfect with a quaternary agent at 1/256 dilution (see Table of Materials), iodophor, bleach, or strong ionic detergent (e.g., SDS) before removing them from the biosafety cabinet.</li>
	<li>The day&#160;after seeding of the Vero cells, serially dilute each virus sample to be plaqued in 1x PBS; typically use 10-fold dilutions for most precise tracking.</li>
	<li>Make these dilutions through 10-4 (for low concentrations of viruses) or through 10-9 (for expectations of higher titer samples) with at least 1 mL of each sample remaining for the plaque assay itself.</li>
	<li>Keep all the viral dilutions on ice (no longer than 1 h) until ready to add these diluted virus samples to the cells.</li>
</ol>
3. Addition of virus to the cells
<ol>
	<li>Remove the cell culture medium from one or two wells by pipette. 
	NOTE: Do not use an aspirator because it removes too much liquid from the cell monolayer and dries the cells out. One crucial consideration is ensuring that the cell monolayer remains hydrated throughout the entire course of the experiment. If the cells dry out, they will wash off the plate in the final step and generate unusable data. Therefore, process only one or two wells at a time to reduce this possibility.</li>
	<li>Carefully add the diluted virus sample (100-400 &#181;L and 50-200 &#181;L of virus sample are used for 6-well and 12-well plates, respectively) dropwise to each monolayer, adding the drops down the side of each well. Repeat the process for every one or two wells until the entire plate is filled with the virus samples being plaqued. 
	NOTE: Adding the drops to the center of the well may inadvertently slough cells off the substrate.</li>
	<li>Gently rock the entire plate by hand to ensure the PBS that contains the virus covers the entire monolayer of cells in each well, then place the plate in a CO2 incubator at 37 &#176;C to allow the virus to adsorb.</li>
	<li>Every 10 min within 1 h, remove the plate from the incubator, gently rock it again to spread the virus more evenly across each well, and then place it back in the incubator. 
	NOTE: Each experiment will dictate the number of replicates and which dilutions are used; the reader&#39;s preference dictates that decision.</li>
	<li>To diminish the possibility of inadvertently counting unabsorbed inoculum, remove the virus sample from the wells with 1000 &#181;L pipette tips and place it in a waste beaker. 
	NOTE: Again, this procedure is conducted with only one or two wells at a time to maintain hydration of the cell monolayer.</li>
	<li>Place 2.5 mL of a methylcellulose overlay (dissolve 5% methylcellulose w/w in 100 mL of PBS, autoclave for 15 min at 121 &#176;C and 15 psi on a liquid cycle, then add 375 mL of DMEM plus 25 mL of FBS).</li>
	<li>Once an entire plate contains overlay, place the plate back in the incubator to allow growth for two days. 
	NOTE: If the virus and cells are allowed to grow for three days, even in DMEM plus supplements, a substantial loss of cells in the monolayer may result, thereby compromising the final data.</li>
	<li>Disinfect the waste beaker, typically with the addition of bleach, an iodophor, or a similar virucide, before removing it from the biosafety cabinet.</li>
</ol>
4. Staining for plaques
<ol>
	<li>After the two-day incubation, remove the methylcellulose overlay by a pipette, again one or two wells at a time, and place it in a waste beaker.</li>
	<li>Add ~2 mL of 1% crystal violet (see Table of Materials) in 50% ethanol to each well to stain the plaques. 
	NOTE: It is not essential to remove every last drop of the overlay.</li>
	<li>Incubate the plate with all wells filled with the stain for 30 min at 37 &#176;C. At this point, disinfect the waste beaker as above in step 3.8.</li>
	<li>Wash the plates vigorously with tap water until the runoff is clear. 
	NOTE: A gentle stream of water is not vigorous enough; full pressure from a lab sink fixture is required.</li>
	<li>At this point, ensure that there are approximately 10-fold differences in the number of plaques across each dilution series.</li>
	<li>Allow the plates to dry overnight upside down, after which the individual plaques may be counted.</li>
</ol>
5. Counting plaques and determining virus titer
<ol>
	<li>Regardless of the approach (see NOTE below), multiply the number of plaques by the dilution factor (e.g., if the plaques were counted in the 10-4 well, then the number of plaques would be multiplied by 104). 
	NOTE: Although counting within a range of 10-100 plaques is useful5, counting wells with 30-300 plaques is somewhat more reliable and takes into account approximately &#189;-log dilutions instead of full-log dilutions.</li>
	<li>Divide this number by the volume of inoculum (as above in step 3.2), and average all wells that have plaques within the stated range to get the most accurate titer of HSV in the original sample.</li>
	<li>Perform the calculations in steps 5.3.1 to 5.3.5, considering the use of mock data in Table 1.
	<ol>
		<li>Consider only wells with 30-300 plaques (step 5.1). Hence, in Table 1, use only the 10-5 column for replicates 1, 2, and 3.</li>
		<li>Take the number of plaques at the given dilution and multiply by the reciprocal of the dilution factor. Hence, the 10-5 column of sample 1 goes from 81 plaques at the 10-5 dilution to 81 plaques times 105.</li>
		<li>Then divide that number by the volume (in mL) of the virus dilution added to the cells in that well. Assuming the data on HSV-1 in Table 1 are from a 12-well plate, 0.2 mL would be the most appropriate measurement. Hence the calculation from step 5.3.2 becomes 81 x 105 divided by 0.2 mL, or 4.0 x 107 PFU/mL of HSV-1.</li>
		<li>Repeat this process for all useful data and average. Hence conduct the calculation in 5.3.2-5.3.3 on all samples in the 10-5 column, assuming they originated from the same virus sample and the biological replicates were conducted to obtain the most accurate titer measurement. In the case of Table 1, average 4.0 x 107, 2.1 x 107, and 2.6 x 107 PFU/mL to obtain a final average titer of 2.9 x 107 &#177; 0.98 x 107 PFU/mL for this sample.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Dimmock, N., Easton, A., Leppard, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Introduction to Moden Virology. 6th end. , (2007).</li><li>Mahy, B., Collier, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Topley and Wilson's Microbiology and Microbial Infections. 9th edn. 1, (1998).</li><li>Anderson, B., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Enumeration+of+bacteriophage+particles:+Comparative+analysis+of+the+traditional+plaque+assay+and+real-time+QPCR-+and+nanosight-based+assays.">Enumeration of bacteriophage particles: Comparative analysis of the traditional plaque assay and real-time QPCR- and nanosight-based assays.</a> Bacteriophage. 1 (2), 86-93 (2011).</li><li>Earle, W. R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Production+of+malignancy+in+vitro;+IV.+The+mouse+fibroblast+cultures+and+changes+seen+in+the+living+cells.">Production of malignancy in vitro; IV. The mouse fibroblast cultures and changes seen in the living cells.</a> Journal of the National Cancer Institute. 4 (2), 165-212 (1943).</li><li>Dulbecco, R., Vogt, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Some+problems+of+animal+virology+as+studied+by+the+plaque+technique.">Some problems of animal virology as studied by the plaque technique.</a> Cold Spring Harbor Symposia on Quantitative Biology. 18, 273-279 (1953).</li><li>Enders, J. F., Weller, T. H., Robbins, F. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cultivation+of+the+lansing+strain+of+poliomyelitis+virus+in+cultures+of+various+human+embryonic+tissues.">Cultivation of the lansing strain of poliomyelitis virus in cultures of various human embryonic tissues.</a> Science. 109 (2822), 85-87 (1949).</li><li>Enders, J. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cytopathology+of+virus+infections:+particular+reference+to+tissue+culture+studies.">Cytopathology of virus infections: particular reference to tissue culture studies.</a> Annual Review of Microbiology. 8, 473-502 (1954).</li><li>Roizman, B., Knipe, D. M., Whitley, R. J., Knipe, D. 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> Fields Virology. 1, 1823-1897 (2013).</li><li>Madavaraju, K., Koganti, R., Volety, I., Yadavalli, T., Shukla, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Herpes+simplex+virus+cell+entry+mechanisms:+An+update.">Herpes simplex virus cell entry mechanisms: An update.</a> Frontiers in Cellular and Infection Microbiology. 10, 617578 (2020).</li><li>Cooper, P. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+plaque+assay+of+animal+viruses.">The plaque assay of animal viruses.</a> Advances in Virus Research. 8, 319-378 (1961).</li><li>Farnham, A. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+formation+of+microscopic+plaques+by+herpes+simplex+virus+in+HeLa+cells.">The formation of microscopic plaques by herpes simplex virus in HeLa cells.</a> Virology. 6 (2), 317-327 (1958).</li><li>Garabedian, G. A., Scott, L. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Plaque+assay+for+herpes+simplex+virus+in+L-929+(Earle)+mouse+fibroblasts.">Plaque assay for herpes simplex virus in L-929 (Earle) mouse fibroblasts.</a> Proceedings of the Society for Experimental Biology and Medicine. 126 (2), 568-571 (1967).</li><li>Lancz, G. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Herpes+simplex+viruses+types+1+and+2.+Type+and+strain+specific+characteristics+affecting+virus+plaque+formation.">Herpes simplex viruses types 1 and 2. Type and strain specific characteristics affecting virus plaque formation.</a> Arch Gesamte Virusforsch. 46 (1-2), 36-43 (1974).</li><li>Muratore, O., Tonoli, E. L., Pesce Schito, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+short-term+plaque+assay+for+antiviral+drug+research+on+herpes+simplex+virus+type+2.">A short-term plaque assay for antiviral drug research on herpes simplex virus type 2.</a> New Microbiologica. 19 (3), 257-261 (1996).</li><li>Sato, S., Kaneki, H., Shirai, J., Takahashi, K., Kawana, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Differentiation+of+herpes+simplex+virus+types+1+and+2+by+plaque+appearances+on+semicontinuous+rabbit+lens+epithelial+cells+in+the+clinical+laboratory.">Differentiation of herpes simplex virus types 1 and 2 by plaque appearances on semicontinuous rabbit lens epithelial cells in the clinical laboratory.</a> Kansenshogaku Zasshi. 67 (6), 561-573 (1993).</li><li>Yazaki, S., Taniguchi, S., Yoshino, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Improvement+of+the+plaque+assay+of+herpes+simplex+virus+in+HeLa+cells.">Improvement of the plaque assay of herpes simplex virus in HeLa cells.</a> Japanese Jouranl of Microbiology. 10 (3), 133-139 (1966).</li><li>Stanners, C. P., Eliceiri, G. L., Green, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Two+types+of+ribosome+in+mouse-hamster+hybrid+cells.">Two types of ribosome in mouse-hamster hybrid cells.</a> Nature New Biology. 230 (10), 52-54 (1971).</li><li>Giannasca, N. J., Suon, J. S., Evans, A. C., Margulies, B. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Matrix-based+controlled+release+delivery+of+acyclovir+from+poly-(ethylene+co-vinyl+acetate)+rings.">Matrix-based controlled release delivery of acyclovir from poly-(ethylene co-vinyl acetate) rings.</a> Journal of Drug Delivery Science and Technology. 55, 101391 (2020).</li><li>Freshney, R. I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Culture of Animal Cells: A Manual of Basic Technique and Specialized Applications. 7th edn. , (2016).</li><li>Dulbecco, R. <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+plaques+in+monolayer+tissue+cultures+by+single+particles+of+an+animal+virus.">Production of plaques in monolayer tissue cultures by single particles of an animal virus.</a> Proceedings of the National Academy of Science, USA. 38 (8), 747-752 (1952).</li><li>Baer, A., Kehn-Hall, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Viral+concentration+determination+through+plaque+assays:+using+traditional+and+novel+overlay+systems.">Viral concentration determination through plaque assays: using traditional and novel overlay systems.</a> Journal of Visualized Experiments. (93), e52065 (2014).</li><li>Matrosovich, M., Matrosovich, T., Garten, W., Klenk, H. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=New+low-viscosity+overlay+medium+for+viral+plaque+assays.">New low-viscosity overlay medium for viral plaque assays.</a> Virology Journal. 3, 63 (2006).</li><li>Culley, S., Towers, G. J., Selwood, D. L., Henriques, R., Grove, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Infection+counter:+Automated+quantification+of+in+vitro+virus+replication+by+fluorescence+microscopy.">Infection counter: Automated quantification of in vitro virus replication by fluorescence microscopy.</a> Viruses. 8 (7), 201 (2016).</li><li>Hudu, S. 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=Quantitative+Hepatitis+B+e+antigen:+A+better+predictor+of+Hepatitis+B+virus+DNA+than+quantitative+Hepatitis+B+surface+antigen.">Quantitative Hepatitis B e antigen: A better predictor of Hepatitis B virus DNA than quantitative Hepatitis B surface antigen.</a> Clinical Laboratory. 64 (4), 443-449 (2018).</li><li>Baltimore, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=RNA-dependent+DNA+polymerase+in+virions+of+RNA+tumour+viruses.">RNA-dependent DNA polymerase in virions of RNA tumour viruses.</a> Nature. 226 (5252), 1209-1211 (1970).</li><li>Scolnick, E. M., Aaronson, S. A., Todaro, G. J., Parks, W. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=RNA+dependent+DNA+polymerase+activity+in+mammalian+cells.">RNA dependent DNA polymerase activity in mammalian cells.</a> Nature. 229 (5283), 318-321 (1971).</li><li>Strick, L. B., Wald, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Diagnostics+for+herpes+simplex+virus:+is+PCR+the+new+gold+standard.">Diagnostics for herpes simplex virus: is PCR the new gold standard.</a> Molecular Diagnosis and Therapy. 10 (1), 17-28 (2006).</li><li>Ramakrishnan, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+50%+endpoint+titer+using+a+simple+formula.">Determination of 50% endpoint titer using a simple formula.</a> World Jouranl of Virology. 5 (2), 85-86 (2016).</li><li>K&#228;rber, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Beitrag+zue+kollektiven+Behandlung+pharmakologischer+pharmakologischer+Reihenversuche.">Beitrag zue kollektiven Behandlung pharmakologischer pharmakologischer Reihenversuche.</a> Archiv for Experimentelle Pathologie und Pharmakologie. 162 (4), 480-487 (1931).</li><li>Reed, L. J., Muench, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+simple+method+of+estimating+fifty+percent+endpoints.">A simple method of estimating fifty percent endpoints.</a> American Journal of Hygiene. 27 (3), 493-497 (1938).</li><li>Spearman, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Method+of+'Right+and+Wrong+Cases'+(Constant+Stimuli)+without+Gauss's+formula.">The Method of 'Right and Wrong Cases' (Constant Stimuli) without Gauss's formula.</a> British Journal of Psychology. 2 (3), 227-242 (1908).</li><li>Ryu, W. -. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Molecular Virology of Human Pathogenic Viruses. 1st edn. , (2017).</li><li>Burrill, C. P., Strings, V. R., Andino, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Poliovirus:+generation,+quantification,+propagation,+purification,+and+storage.">Poliovirus: generation, quantification, propagation, purification, and storage.</a> Current Protocols in Microbiology. , (2013).</li><li>Karakus, U., Crameri, M., Lanz, C., Yanguez, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Propagation+and+titration+of+influenza+viruses.">Propagation and titration of influenza viruses.</a> Methods in Molecular Biology. 1836, 59-88 (2018).</li><li>Baz, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Zika+virus+isolation,+purification,+and+titration.">Zika virus isolation, purification, and titration.</a> Methods in Molecular Biology. 2142, 9-22 (2020).</li><li>Coleman, C. M., Frieman, M. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Growth+and+quantification+of+MERS-CoV+infection.">Growth and quantification of MERS-CoV infection.</a> Current Protocols in Microbiology. 37 (1), 1-9 (2015).</li></ol>

The authors declare no conflict of interest.

While plaque assays are almost as old as mammalian cell culture itself, it seems that each lab has its own set of protocols to execute this basic assay5,6,10,11,12,13,14,15,16,20. Althoug...

The beginnings of virus plaque assays go back to the first discoveries of viruses in the 1890s1. Tobacco mosaic virus was first isolated and passed on tobacco leaves, where individual spots of infection could be recognized and quantified as originating from a single, live virus entity2, later identified as a virion2. Later seminal studies with bacteria and bacteriophages perfected the techniques used to plaque these viruses, including bacteria at the mid-log phase of growth, serial dilution of bacteriophage samples, and top agar with subsequent visualization of literal holes (named plaques) in the bacterial lawn3.
Plaquing of animal viruses lagged the exciting research being conducted with bacteriophages, mainly because the methods required for growing mammalian cells in culture were not developed until the 1940s4. However, the advent of growing murine cells in the absence of the entire host organism4 spawned a new era in the ability to culture and count viruses. Such work was extended for the propagation and quantitation of Western Equine Encephalomyelitis virus in chicken cells and poliovirus in human cells5,6. As the realm of culturable mammalian cells expanded, the bevy of different host cells for various viral infections gave the world a cornucopia of possibilities to study all manner of viruses7. This included the propagation and quantification of human herpesviruses, particularly herpes simplex virus-1 (HSV-1) and -2 (HSV-2), which cause mucocutaneous lesions8. Importantly, all plaque assays are dependent on the existence of live virions, which can enter host cells in a receptor-mediated fashion in a sample9. Regardless of the ubiquity and multitude of publications on the execution of plaque assays5,10,11,12,13,14,15,16, these methods for HSV-1/-2 are a mixture of both art and science; one cannot conduct the assay without proper attention to every detail in the protocol, nor can one execute a successful assay without a crticial eye for subtlety in the process. This manuscript depicts one of the most consistently reproducible methods for HSV-1/-2 plaque assays, with precise details towards the art of the assay that are seldom discussed.
This current protocol obtains live plaque-forming units (PFU) counts for HSV-1 and -2 reliably. Best results are obtained using Vero cells (transformed African green monkey kidney epithelial cells) at low passage (below passage number 155) and routinely grown in alpha-MEM17 supplemented with 10% fetal calf serum (FCS), L-alanyl-L-glutamine, and an antibiotic/antimycotic mixture18. Vero cells are standardly propagated in this medium two to three times per week at a 1/5 dilution each time.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>12-well plates</td>
			<td>Corning</td>
			<td>3512</td>
			<td></td>
		</tr>
		<tr>
			<td>6-well plates</td>
			<td>Corning</td>
			<td>3516</td>
			<td></td>
		</tr>
		<tr>
			<td>Alpha-MEM</td>
			<td>Lonza</td>
			<td>12169F</td>
			<td></td>
		</tr>
		<tr>
			<td>Antibiotic/antimycotic</td>
			<td>Gibco</td>
			<td>15240096</td>
			<td></td>
		</tr>
		<tr>
			<td>Crystal violet</td>
			<td>Alfa Aesar</td>
			<td>B2193214</td>
			<td></td>
		</tr>
		<tr>
			<td>DMEM</td>
			<td>Gibco</td>
			<td>11965092</td>
			<td></td>
		</tr>
		<tr>
			<td>Dulbecco&#39;s PBS (no Mg++ or Ca++)</td>
			<td>Gibco</td>
			<td>14190144</td>
			<td></td>
		</tr>
		<tr>
			<td>Fetal calf serum</td>
			<td>Millipore-Sigma</td>
			<td>TMS-013-B</td>
			<td></td>
		</tr>
		<tr>
			<td>L-alanyl-L-glutamine (Glutamax)</td>
			<td>Gibco</td>
			<td>GS07F161BA</td>
			<td></td>
		</tr>
		<tr>
			<td>Hemacytometer</td>
			<td>Thermo Fisher</td>
			<td>02-671-54</td>
			<td></td>
		</tr>
		<tr>
			<td>Methylcellulose</td>
			<td>Millipore-Sigma</td>
			<td>27-441-0</td>
			<td></td>
		</tr>
		<tr>
			<td>Quaternary agent (Lysol I.C.)</td>
			<td>Thermo Fisher</td>
			<td>NC9645698</td>
			<td></td>
		</tr>
		<tr>
			<td>Trypan Blue</td>
			<td>Corning</td>
			<td>25900CI</td>
			<td></td>
		</tr>
		<tr>
			<td>Trypsin</td>
			<td>Cytiva</td>
			<td>SH30042.01</td>
			<td></td>
		</tr>
		<tr>
			<td>Vero cells</td>
			<td>ATCC</td>
			<td>CCL-81</td>
			<td></td>
		</tr>
	</tbody>
</table>

plaquing of herpes simplex viruses

There are numerous published protocols for plaquing viruses, including references within primary literature for methodology. However, plaquing viruses can be difficult to perform, requiring focus on its specifications and refinement. It is an incredibly challenging method for new students to master, mainly because it requires meticulous attention to the most minute details. This demonstration of plaquing herpes simplex viruses should help those who have struggled with visualizing the method, especially its nuances, over the years. While this manuscript is based on the same principles of standard plaquing methodology, it differs in that it contains a detailed description of (1) how best to handle host cells to avoid disruption during the process, (2) a more useful viscous medium than agarose to limit the diffusion of virions, and (3) a simple fixation and staining procedure that produces reliably reproducible results. Furthermore, the accompanying video helps demonstrate the finer distinctions in the process, which are frequently missed when instructing others on conducting plaque assays.

Table 1 shows an experiment that has optimal results. All 10-fold dilutions follow an approximately 10-fold decrease in plaque counts. These kinds of data can also be seen in Figure 2, an actual plaque assay where the countable number of plaques fell in the 10-4 range for all three replicates. The same can be seen in Figure 3, the top row, where the countable number of plaques was in the 10-3 dilution.
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Watch this Scientific Journal Video about Plaquing of Herpes Simplex Viruses at JoVE.com

Plaquing of Herpes Simplex Viruses

Plaquing is a routine method used to quantify live viruses in a population. Though plaquing is frequently taught in various microbiology curricula with bacteria and bacteriophages, plaquing of mammalian viruses is more complex and time-consuming. This protocol describes the procedures that function reliably for regular work with herpes simplex viruses.

There are numerous published protocols for plaquing viruses, including references within primary literature for methodology. However, plaquing viruses can ...

Research

JoVE Journal

Immunology and Infection

4.8K Views.  Towson University. Plaquing is a routine method used to quantify live viruses in a population. Though plaquing is frequently taught in various microbiology curricula with bacteria and bacteriophages, plaquing of mammalian viruses is more complex and time-consuming. This protocol describes the procedures that function reliably for regular work with herpes simplex viruses.

Video: Plaquing of Herpes Simplex Viruses

Isolation of Fidelity Variants of RNA Viruses and Characterization of Virus Mutation Frequency

RNA viruses use RNA dependent RNA polymerases to replicate their genomes. The intrinsically high error rate of these enzymes is a large contributor to the generation of extreme population diversity that facilitates virus adaptation and evolution. Increasing evidence shows that the intrinsic error rates, and the resulting mutation frequencies, of RNA viruses can be modulated by subtle amino acid changes to the viral polymerase. Although biochemical assays exist for some viral RNA polymerases that permit quantitative measure of incorporation fidelity, here we describe a simple method of measuring mutation frequencies of RNA viruses that has proven to be as accurate as biochemical approaches in identifying fidelity altering mutations. The approach uses conventional virological and sequencing techniques that can be performed in most biology laboratories. Based on our experience with a number of different viruses, we have identified the key steps that must be optimized to increase the likelihood of isolating fidelity variants and generating data of statistical significance. The isolation and characterization of fidelity altering mutations can provide new insights into polymerase structure and function1-3. Furthermore, these fidelity variants can be useful tools in characterizing mechanisms of virus adaptation and evolution4-7.

The present article describes the steps required to isolate and characterize RNA polymerase fidelity variants of RNA viruses and how to use mutation frequency data to confirm fidelity changes in tissue culture.

RNA viruses use RNA dependent RNA polymerases to replicate their genomes. The intrinsically high error rate of these enzymes is a large contributor to the ...

A Primary Neuron Culture System for the Study of Herpes Simplex Virus Latency and Reactivation

Herpes simplex virus type-1 (HSV-1) establishes a life-long latent infection in peripheral neurons. This latent reservoir is the source of recurrent reactivation events that ensure transmission and contribute to clinical disease. Current antivirals do not impact the latent reservoir and there are no vaccines. While the molecular details of lytic replication are well-characterized, mechanisms controlling latency in neurons remain elusive. Our present understanding of latency is derived from in vivo studies using small animal models, which have been indispensable for defining viral gene requirements and the role of immune responses. However, it is impossible to distinguish specific effects on the virus-neuron relationship from more general consequences of infection mediated by immune or non-neuronal support cells in live animals. In addition, animal experimentation is costly, time-consuming, and limited in terms of available options for manipulating host processes. To overcome these limitations, a neuron-only system is desperately needed that reproduces the in vivo characteristics of latency and reactivation but offers the benefits of tissue culture in terms of homogeneity and accessibility.
Here we present an in vitro model utilizing cultured primary sympathetic neurons from rat superior cervical ganglia (SCG) (Figure 1) to study HSV-1 latency and reactivation that fits most if not all of the desired criteria. After eliminating non-neuronal cells, near-homogeneous TrkA+ neuron cultures are infected with HSV-1 in the presence of acyclovir (ACV) to suppress lytic replication. Following ACV removal, non-productive HSV-1 infections that faithfully exhibit accepted hallmarks of latency are efficiently established. Notably, lytic mRNAs, proteins, and infectious virus become undetectable, even in the absence of selection, but latency-associated transcript (LAT) expression persists in neuronal nuclei. Viral genomes are maintained at an average copy number of 25 per neuron and can be induced to productively replicate by interfering with PI3-Kinase / Akt signaling or the simple withdrawal of nerve growth factor1. A recombinant HSV-1 encoding EGFP fused to the viral lytic protein Us11 provides a functional, real-time marker for replication resulting from reactivation that is readily quantified. In addition to chemical treatments, genetic methodologies such as RNA-interference or gene delivery via lentiviral vectors can be successfully applied to the system permitting mechanistic studies that are very difficult, if not impossible, in animals. In summary, the SCG-based HSV-1 latency / reactivation system provides a powerful, necessary tool to unravel the molecular mechanisms controlling HSV1 latency and reactivation in neurons, a long standing puzzle in virology whose solution may offer fresh insights into developing new therapies that target the latent herpesvirus reservoir.

The protocol describes an efficient and reproducible model system to study herpes simplex virus type 1 (HSV-1) latency and reactivation. The assay employs homogenous sympathetic neuron cultures and allows for the molecular dissection of virus-neuron interactions using a variety of tools including RNA interference and expression of recombinant proteins.

Herpes simplex virus type-1 (HSV-1) establishes a life-long latent infection in peripheral neurons. This latent reservoir is the source of recurrent ...

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 ...

Vaccinia Reporter Viruses for Quantifying Viral Function at All Stages of Gene Expression

Poxviruses are a family of double stranded DNA viruses that include active human pathogens such as monkeypox, molluscum contagiousum, and Contagalo virus. The family also includes the smallpox virus, Variola. Due to the complexity of poxvirus replication, many questions still remain regarding their gene expression strategy. In this article we describe the conceptualization and usage of recombinant vaccinia viruses that enable real-time measurement of single and multiple stages of viral gene expression in a high-throughput format. This is enabled through the use of spectrally distinct fluorescent proteins as reporters for each of three stages of viral replication. These viruses provide a high signal-to-noise ratio while retaining stage specific expression patterns, enabling plate-based assays and microscopic observations of virus propagation and replication. These tools have uses for antiviral discovery, studies of the virus-host interaction, and evolutionary biology.

We describe the usage of a fluorescent reporter vaccinia virus that enables real-time measurement of viral infectivity and gene expression through the stage-specific expression of spectrally distinct reporter fluorophores. We detail a plate-based method for accurately identifying the stage at which virus replication is affected in response to small molecule inhibition.

Poxviruses are a family of double stranded DNA viruses that include active human pathogens such as monkeypox, molluscum contagiousum, and Contagalo virus. ...

Pairwise Growth Competition Assay for Determining the Replication Fitness of Human Immunodeficiency Viruses

In vitro fitness assays are essential tools for determining viral replication fitness for viruses such as HIV-1. Various measurements have been used to extrapolate viral replication fitness, ranging from the number of viral particles per infectious unit, growth rate in cell culture, and relative fitness derived from multiple-cycle growth competition assays. Growth competition assays provide a particularly sensitive measurement of fitness since the viruses are competing for cellular targets under identical growth conditions. There are several experimental factors to consider when conducting growth competition assays, including the multiplicity of infection (MOI), sampling times, and viral detection and fitness calculation methods. Each factor can affect the end result and hence must be considered carefully during the experimental design. The protocol presented here includes steps from constructing a new recombinant HIV-1 clone to performing growth competition assays and analyzing the experimental results. This protocol utilizes experimental parameter values previously shown to yield consistent and robust results. Alternatives are discussed, as some parameters need to be adjusted according to the cell type and viruses being studied. The protocol contains two alternative viral detection methods to provide flexibility as the availability of instruments, reagents and expertise varies between laboratories.

Growth competition between nearly isogenic viruses provides a sensitive measurement for determining relative replication fitness. The protocols described here include the construction of recombinant HIV-1 clones, virus propagation and growth competition and analysis methods optimized to yield sensitive and consistent results.

In vitro fitness assays are essential tools for determining viral replication fitness for viruses such as HIV-1. Various measurements have been used to ...

Intranasal Administration of Recombinant Influenza Vaccines in Chimeric Mouse Models to Study Mucosal Immunity

Vaccines are one of the greatest achievements of mankind, and have saved millions of lives over the last century. Paradoxically, little is known about the physiological mechanisms that mediate immune responses to vaccines perhaps due to the overall success of vaccination, which has reduced interest into the molecular and physiological mechanisms of vaccine immunity. However, several important human pathogens including influenza virus still pose a challenge for vaccination, and may benefit from immune-based strategies. 
Although influenza reverse genetics has been successfully applied to the generation of live-attenuated influenza vaccines (LAIVs), the addition of molecular tools in vaccine preparations such as tracer components to follow up the kinetics of vaccination in vivo, has not been addressed. In addition, the recent generation of mouse models that allow specific depletion of leukocytes during kinetic studies has opened a window of opportunity to understand the basic immune mechanisms underlying vaccine-elicited protection. Here, we describe how the combination of reverse genetics and chimeric mouse models may help to provide new insights into how vaccines work at physiological and molecular levels, using as example a recombinant, cold-adapted, live-attenuated influenza vaccine (LAIV). We utilized laboratory-generated LAIVs harboring cell tracers as well as competitive bone marrow chimeras (BMCs) to determine the early kinetics of vaccine immunity and the main physiological mechanisms responsible for the initiation of vaccine-specific adaptive immunity. In addition, we show how this technique may facilitate gene function studies in single animals during immune responses to vaccines. We propose that this technique can be applied to improve current prophylactic strategies against pathogens for which urgent medical countermeasures are needed, for example influenza, HIV, Plasmodium, and hemorrhagic fever viruses such as Ebola virus.

There is an overall lack of knowledge about how vaccines work. Here we propose the combined use of reverse genetics and bone marrow chimeric mice to gain insight into the early host immune responses to vaccines with a special focus on dendritic cells and T cell immunity.

Vaccines are one of the greatest achievements of mankind, and have saved millions of lives over the last century. Paradoxically, little is known about the ...

Ex Vivo Infection of Murine Epidermis with Herpes Simplex Virus Type 1

To enter its human host, herpes simplex virus type 1 (HSV-1) must overcome the barrier of mucosal surfaces, skin, or cornea. HSV-1 targets keratinocytes during initial entry and establishes a primary infection in the epithelium, which is followed by latent infection of neurons. After reactivation, viruses can become evident at mucocutaneous sites that appear as skin vesicles or mucosal ulcers. How HSV-1 invades skin or mucosa and reaches its receptors is poorly understood. To investigate the invasion route of HSV-1 into epidermal tissue at the cellular level, we established an ex vivo infection model of murine epidermis, which represents the site of primary and recurrent infection in skin. The assay includes the preparation of murine skin. The epidermis is separated from the dermis by dispase II treatment. After floating the epidermal sheets on virus-containing medium, the tissue is fixed and infection can be visualized at various times postinfection by staining infected cells with an antibody against the HSV-1 immediate early protein ICP0. ICP0-expressing cells can be observed in the basal keratinocyte layer already at 1.5 hr postinfection. With longer infection times, infected cells are detected in suprabasal layers, indicating that infection is not restricted to the basal keratinocytes, but the virus spreads to other layers in the tissue. Using epidermal sheets of various mouse models, the infection protocol allows determining the involvement of cellular components that contribute to HSV-1 invasion into tissue. In addition, the assay is suitable to test inhibitors in tissue that interfere with the initial entry steps, cell-to-cell spread and virus production. Here, we describe the ex vivo infection protocol in detail and present our results using nectin-1- or HVEM-deficient mice.

Ex Vivo Infection of Murine Epidermis with Herpes Simplex Virus Type 1

The skin is one target tissue of the human pathogen herpes simplex virus type 1 (HSV-1). To explore the invasion route of HSV-1 into tissue, we established an ex vivo infection model of murine epidermal sheets which represent the outermost layer of skin.

To enter its human host, herpes simplex virus type 1 (HSV-1) must overcome the barrier of mucosal surfaces, skin, or cornea. HSV-1 targets keratinocytes ...

Rapid Molecular Detection and Differentiation of Influenza Viruses A and B

Influenza is a contagious respiratory illness caused by influenza viruses A and B in humans and causes a significant amount of morbidity and mortality every year. The Influenza A and B assay was the first CLIA-waived molecular rapid flu test available. The Influenza A and B test works by employing isothermal amplification with influenza-specific primers followed by target detection with molecular beacon probes. Here, the performance of the Influenza A and B assay on frozen, archived nasopharyngeal swab (NPS) specimens stored in viral transport medium (VTM) were compared to a respiratory panel assay.
The performance of the Influenza A and B assay was evaluated by comparing the results to the respiratory panel reference method. The sensitivity for total influenza virus A was 67.5% (95% CI (CI), 56.6-78.5) and the specificity was 86.9% (CI, 71.0-100). For influenza virus B testing, the sensitivity and specificity were 90.2% (CI, 68.5-100) and 98.8% (CI, 68.5-100), respectively.
This system has the advantage of a significantly shorter test time than any other currently available molecular assay and the simple, pipette-free procedure runs on a fully integrated, closed, small-footprint system. Overall, the Influenza A and B assay evaluated in this study has the potential to serve as a point-of-care rapid influenza diagnostic test.

We describe a rapid, molecular-based Influenza A and B assay. The Influenza assay detects each target within 15 min by employing isothermal amplification with influenza-specific primers followed by target detection with molecular beacon probes. The Influenza A and B assay is user-friendly and required minimal hands-on time to perform.

Influenza is a contagious respiratory illness caused by influenza viruses A and B in humans and causes a significant amount of morbidity and mortality ...

Temporal Analysis of the Nuclear-to-cytoplasmic Translocation of a Herpes Simplex Virus 1 Protein by Immunofluorescent Confocal Microscopy

Infected cell protein 0 (ICP0) of herpes simplex virus 1 (HSV-1) is an immediate early protein containing a RING-type E3 ubiquitin ligase. It is responsible for the proteasomal degradation of host restrictive factors and the subsequent viral gene activation. ICP0 contains a canonical nuclear localization sequence (NLS). It enters the nucleus immediately after de novo synthesis and executes its anti-host defense functions mainly in the nucleus. However, later in infection, ICP0 is found solely in the cytoplasm, suggesting the occurrence of a nuclear-to-cytoplasmic translocation during HSV-1 infection. Presumably ICP0 translocation enables ICP0 to modulate its functions according to its subcellular locations at different infection phases. In order to delineate the biological function and regulatory mechanism of ICP0 nuclear-to-cytoplasmic translocation, we modified an immunofluorescent microscopy method to monitor ICP0 trafficking during HSV-1 infection. This protocol involves immunofluorescent staining, confocal microscope imaging, and nuclear vs. cytoplasmic distribution analysis. The goal of this protocol is to adapt the steady state confocal images taken in a time course into a quantitative documentation of ICP0 movement throughout the lytic infection. We propose that this method can be generalized to quantitatively analyze nuclear vs. cytoplasmic localization of other viral or cellular proteins without involving live imaging technology.

ICP0 undergoes nuclear-to-cytoplasmic translocation during HSV-1 infection. The molecular mechanism of this event is not known. Here we describe the use of confocal microscope as a tool to quantify ICP0 movement in HSV-1 infection, which lays the groundwork for quantitatively analyzing ICP0 translocation in future mechanistic studies.