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HSV Growth, Preparation, and Assay
June Harland and S. Moira Brown


1. Introduction
Whether herpes simplex vn-us (HSV) is viewed as a pathogen or as a model
eukaryotic system,it 1svirtually certam that any experimental work will require
the virus to be grown and assayed. The following chapter 1stherefore seen as
the fundamental first step before embarking on more mtellectually and techni-
cally challengmg technology. Its importance should not however be underesti-
mated. It never fails to surprtze us that people who describe themselves as
vuologists have little understandmg of the basic requirements needed to attain
a contammation-free, high-titer, low particle:plaque-forming units (PFU) ratio,
genetically pure virus stock
HSV grows well m a wide variety of cell types to yield high-titer stocks. In
general, HSV type 1 (HSV-1) grows to a higher titer than type 2 (HSV-2) and
IS less cell-associated, i.e., more mfectious vn˜.˜s released into the growth
is
medium. Cell lines routinely used to grow HSV Include BHK (hamster kidney),
RK13 (rabbit kidney), Vero (monkey kidney), and CVl (monkey kidney).
When HSV infects a single cell, the surrounding cells will also become
Infected by spread of progeny virus from cell to cell. This focus of infection
normally causescell necrosis, resulting in a hole in the monolayer wtth rounded
cells at the periphery. Alternatively, certain virus strains can pass from cell to
cell and cause fusion of the infected cells, resulting in a syncitium. For either
type, these foci of mfection are called plaques and are a measure of the number
of infectious particles wtthin a virus stock. The titer of a virus stock is expressed
as the number of PFU per milliliter of virus (PFU/mL).
Spontaneous genomic mutations (point mutations, deletions, insertions)
occur relatively frequently wtthm a vn-us stock and, If nonlethal, they will be
maintained. Therefore, to achieve genomic homogeneity, it is essential that a
From Methods m Molecular MedIcme, Vol 10 Herpes &mp/ex Vvus Protocols
Edlted by S M Brown and A R MacLean Humana Press Inc , Totowa, NJ
Hat-land and Brown
2
vnus stock originates from a smgle vnus plaque (single mfectious particle) and
that subsequent passagenumbers are kept to a minimum. To ensure the purity of
the isolate from which the stock will be derived, it must be stringently plaque-
purrfled. This is done by serral dilution of the vn-us until preferably only one
plaque is present on a monolayer. This plaque is picked, the vnus titrated again,
and a single plaque picked. A mmrmum of three rounds of stringent purification
is usually required to yield a pure stock. Once a vn-us stock has been grown up
from this plaque-purified isolate, tt should be retained as an elite master stock
and used as the only source of vnus for generating working vu-us stocks.
The quality of vn-us stocks can also be adversely affected if the correct pro-
cedures are not followed when growing the virus Defective particles are gen-
erated when mcomplete virus genomes are packaged If the DNA m the
defective particle contams an origin of replication, it can be replicated m the
presence of the standard vnus, which supplies essential helper virus functions.
All virus stocks should be grown from low multiplicity of mfectton (MOI)
mocula. This optimizes amplification and packaging of complete vnus genomes
as opposed to defectives, during the several cycles of genomic replrcation
required to generate a stock. The proportion of defective particles wtthm a
stock 1s a good mdtcation of the quality of the virus. It is desirable for most
experimental procedures to use stock with as low a particle:PFU ratio as pos-
sible. Wild-type stocks of HSV-1 with a ratio of 5: 1 or less can be achieved,
and a stock with a ratio > 10: 1 should be considered poor. For HSV-2, the aver-
age ratio of a good stock is <loo* 1
2. Materials
2.7. Reagents
1 ETCiO*Glasgow modified Eagle™s medium with the addition of 10% newborn
calf serum, 100 U/mL pemcillm, 100 U/mL streptomycin and 10% tryptose phos-
phate broth (TP)
2 ETMC 10% Glasgow modified Eagle™s medium with the addition of 10% new-
born calf serum, 100 U/mL penicillin, 100 U/mL streptomycm, 10% TP, and 1%
methylcellulose Smce the methylcellulose needs to be heated to solubilize, 1OX
concentrated Eagle™s medium is used The requisite amount of low-viscosity car-
boxymethylcellulose, sodium salt IS dtssolved m water to gave a final concentra-
tion m the medium of 1% After autoclavmg, the methylcellulose solution is
substitutedfor water when making up the media
3 Phosphate-buffered saline (PBS)/calf serum PBS with the addition of 5% new-
born calf serum
4 Bram heart infusion (BHI) agar
5 Blood agar: BHI agar containing 10% horse blood.
6. Giemsa: Giemsa™s stain (Gurr)
7 V&on
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HSV Growth, Preparation, and Assay
2.2. Equipment
1 Trays for Petri dishes
2 Bijoux racks.
3 Cell monolayer scrapers
4 Vortex.
5 Sonibath.
6 Stereo zoom plate microscope.
7 Centrtfuge (2000 rpm), e g , Beckman GPR centrifuge
8 Centrifuge (12,000 rpm), e.g , Sorvall RCSC
9 CO* mcubators
Roller bottle incubators
10
11. Class II hood
12 -7O™C Freezer
13. Availabtltty of an electron microscope (for parttcle counts)

3. Methods
Many tissue-culture lines can be used for the growth of HSV, but for the pur-
pose of this chapter we wtll concentrate on BHK 2l/Cl3 cells, whrch are rou-
tinely used in Glasgow and whtch gtve htgh yields of infectious vu-us. BHK 2 l/
Cl3 cells are grown in ETCte at 37°C m an atmosphere contatnmg 5% COZ.
For the preparation of large stocks of vnus, 10 roller bottles of BHK cells
(approx 3 x lo8 cells/bottle) are used, whtch should yteld 5-10 mL of stock at
approx 10g-lO1o PFU/mL. Vu-us production on this scale requires an incubator
capable of accomodatmg roller bottles If a suitable incubator IS not available,
It will be necessary to scale down the method approprtately.
Wild-type HSV-1 will grow over a large range of temperatures, between 3 1
and 39°C wrth little dtscernable effect on mfecttous vu-us yield. However, tt is
preferable to grow virus stocks at 31”C, since fewer defective particles are
generated than at 37°C If roller bottle space at both 37°C (for growth of cells
prior to virus inoculatton) and 31“C (for virus growth) IS not avarlable, the
vtrus can usually be grown at 37°C with only a margmal impatrment in quality.
3.1. Growth of HSV Stocks
Good mtcrobiologtcal practice and sterile techniques need to be used
throughout the procedure.
1. Seed each of 10 roller bottles with 3 x 10™ BHK cells m 100 mL of ETC,, medmm
and add 5% CO, either from a central CO, lme or from a cylinder In practice,
this is done by attachmg a stertle Pasteur ptpet to the lme, msertmg the pipet mto
the bottle, and countmg to 51
2. Grow the cells at 37°C for 3 d until they form almost confluent monolayers.
3. Pour off the growth medium, and mfect with virus at an MO1 of 1 m 300 Assum-
4 Harland and Brown
mg 3 x 1OScells/bottle, add lo6 PFU m 20 mL of fresh ETC,,. There is no need to
add more CO,
4. Incubate the Infected cells at 3 1°C Cytopathic effect (CPE) should be apparent
after l-2 d, and the virus will be ready to harvest m 3-5 d when the cells have
rounded up and are startmg to detach from the plastic
5. The roller bottles should be shaken (unopened) until all the cells are m the medium
If this proves difficult, sterile glass beads (approx 2-mm diameter) may be added
and swirled around to detach the adherent cells
6 The medium contammg the detatched cells should be poured mto a sterile 200-mL
centrifuge bottle (the glass beads tf used will remam m the roller bottle) and spun
at 2000 rpm for 10 mm to pellet the cells Both the cell pellet and the supernatant
should be kept
7 The supernatant should be poured mto a sterile 250-mL centrifuge bottle and
spun at 12,000 rpm, e g , m a Sorvall GSA rotor for 2 h The resultant pellet will
consist of cell-releasedlsupernatant vnus (SV) and should be resuspended m 1 mL
ETC, droller bottle
8. To harvest the cell-associated (CA) virus, the cell pellet from step 6, should be
resuspended in a small volume (2-5 mL) of ETC,, This should be transferred to
a suitable contamer (glass umversal bottle) and somcated thoroughly m a sombath
to disrupt the cells The somcate should be spun at 2000 rpm for 10 mm and the
supernatant kept as fraction (1) of the CA vu-us To re-extract, a further 2-5 mL
of fresh ETC,c should be added to the pellet, the solution somcated, and the cell
debris spun out again at 2000 rpm for 10 mm This CA fraction 2 should be added
to fraction 1
9 The CA and SV vu-us preparations may be kept separate or combmed If they are
to be kept separate, the virus pellet from step 7 should be resuspended m 5-10
mL of fresh ETClo and somcated briefly ma sombath to disrupt the pellet If they
are to be combmed, then the pellet from step 7 can be resuspended directly by
somcation m the CA fraction, smce the overall resultant volume will be smaller
Usually for HSV-1, SV and CA titers are similar For HSV-2, the CA titer IS
usually 10 times higher than SV
3.2. StetWty Checks
1. Sterihty checks should be carried out on a new virus stock to ensure that tt is free
from bacterial or fungal contamination before stormg at -70°C. This is done by
streakmg an moculum of the vnus on a blood agar plate usmg a sterile platinum
loop and incubatmg the plate at 37™C for several days To test for fungal mfec-
tions, the vnus stock can be similarly streaked on a BHT agar plate and the plate
incubated at room temperature for up to a week If the stock IS contammated with
either bacteria and/or ftmgt, obvious colonies and/or hyphae wrll be seen on the
plates Usually, a distinct smell will be obvious!
2. It IS usual for contaminated stocks to be discarded, but if the virus IS “ureplace-
able,” it can be filter-steriltzed to remove bacterial or fungal contamination
Unfortunately, this results m a large drop m titer and loss of volume, so it 1sonly
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HS V Growth, Preparation, and Assay
worthwhtle if the vtrus IS very important. Clearance of contamination IS achteved
by passing the vnus through a 0 2-p pore size filter. It may be easier tf the stock
is first passed through a 0 4-p filter.
Note: It is important always to wear safety goggles when carrying out this
procedure, since there is a risk of the syringe detatchmg from the filter and spray-
mg virus mto the face
3. Mycoplasma contammatton of vnus stocks is hard to detect, although myco-
plasma usually cause blood agar plates to dtscolor. If the cells used to grow virus
test posmve for mycoplasma, the virus stock and the cells should be tmmedtately
discarded. If the vu-us 1s“irreplaceable,” it IS possible to extract vtral DNA, which
can be used to transfect clean cells to obtain a mycoplasma-free, vu-us stock
3.3. Viability
To reduce the number of freeze-thaw cycles, vn-us stocks should be altquoted
maxrmally into 1-mL amounts and stored at -70°C.
Note: HSV should never be stored at -20°C, stnce infectivrty will be lost
very rapidly. Ahquoted vtals should be frozen quickly, and when bemg thawed,
they should be warmed rapidly and kept at 0-4”C unttl use. The amount of
time the vxus 1s at 0-4”C should be kept to a mmtmum, but tt can remam at
4°C for 24 h without a stgntficant drop in titer.

3.4. Titration of Virus Stocks
To quantttate the amount of mfecttous vu-us wrthm a stock, It IS necessary to
titrate the stock on cell monolayers, and count the number of plaques on plates
that have been fixed and stained to make the plaques easrly vtsrble under a
microscope. The titer is expressed as PFU/mL of virus.
1. Seed 60-mm plastic Petri dishes with 3 x lo6 BHK cells m 5 mL of ETC,,
2. Incubate the plates overnight in a 37°C mcubator in an atmosphere with 5% CO1
The cells should form Just subconfluent monolayers.
3. Serial dilutions of virus are made m PBS/calf serum, which is aliquoted in 0.9-mL
amounts into the calculated number of bijoux bottles
4 Dilute the vm.ts (l/10) by adding 100 pL of virus to a 0.9-mL aliquot of PBS/calf
serum (gtvmg a IO-™ dtlutton). Recap the bottle, and vortex to mix. Using a fresh
tip, take 100 PL of the 10-t stock and transfer into another 0.9-mL ahquot of
PBS/calf serum gtvmg a IO-* dilution Vortex, and so on Continue with this serial
dtlutton procedure until the appropriate range of dilutions has been achieved. For
a large-scale virus preparation, which may yield up to 1O™O PFU/mL, It IS neces-
sary to tttrate out to a dtlutton of 1Oe7or 1Oe8
Note: The ttp should be touched against the side of the bottle and not mto the
llqutd, since droplets on the outsrde of the tip can be carried over, mtroducing
inaccuracies.
5. Pour the growth medmm off the 60-mm plates.
6 Harland and Brown

Plate out 100 pL of the sertally diluted vu-us stock onto the BHK monolayers,
6.
takmg care not to dislodge the cells from the plates when dehvermg the moculum
through an Eppendorf tip Starting with the highest drlutlon and workmg back to
the most concentrated, it IS not necessary to change tips, smce any carryover ˜111
be msrgmficant. Rock the trays of plates back and forth gently to ensure even
coverage of virus
7 Put mto a 37°C mcubator for 1 h to allow absorption of the virus onto the mono-
layers
8 Add 5 mL of ETMC 10% to each plate The methylcellulose stops progeny vu-us
from the plaques formed from the moculum from spreading through the medium
and resulting m trailmg plaques or secondary satellite plaques
9 Place the titration plates m a CO, incubator at the appropriate temperature Wild-
type vu-us can be titrated at 31 or 37°C Temperature sensitive vuus IS usually
titrated at the permissive (e g , 3 1™C) and nonpermissive (e g ,38.5”C) tempera-
ture Incubate plates for 2 d at 37°C or 38 5°C and 3 d at 3 1“C
10 The vrscoslty of the methylcellulose makes rt difficult for stam to permeate
through to the cell monolayers, and it is therefore preferable to pour off the over-
lay medium prior to the addition of 2-3 mL of Gtemsa™s stain The decanted
medium will contam vuus, and should be autoclaved or treated with an appropri-
ate vmcidal agent (e g , Virkon)
The stain should be left on the plates for 2-24 h at room temperature Stammg
fixes the cells, and any virus remammg on the plates will be macttvated The
stain can be washed off directly under runnmg tap water.
11 Using a plate microscope, count the plaques on the monolayers by mvertmg the
dish, and with a water-soluble pen, mark off each plaque as it is counted. It IS best
to count the dilutions with 20-200 plaques/plate, since too many or too few
plaques give less accurate counts Ideally, duplicates of each dilution should be
counted and the average count used In practice, it IS usually sufficient to count
the number of plaques from two plates with serial drlutions, e.g , 10m5and 10”.
The accuracy of the titration can be measured m this way
Note: Plaques should always be counted using a microscope Although some
may be visible to the naked eye, the size of plaques can vary considerably, and
many ˜111 be missed if a microscope 1s not used
12 The titer should be calculated as follows
20 plaques on the 10m7plate and 200 on the 1o-6 plate = (1)
2 x 1OS In the 100FL moculum
PFU
The titer is therefore 2 x lo9 PFU/mL

3.5. Particle Counts
Vu-us suspensions are mlxed with equal volumes of a 1% solution of sodium
silicotungstate and a suspension of latex beads of known concentration We
use a solution of 1.43 x 10™ ™ particles/ml. A droplet of this suspension is placed
on an electron microscope grad and, after 5 mm (when the particles have
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HSV Growth, Preparation, and Assay
settled), the excess suspension 1sremoved and the particles are counted. The
latex beads are of course used as the reference count
A wild-type stock of HSV-1 should ideally have a partlcle:PFU ratio of
<lO:l, and for HSV-2 this figure should be <loo.

3.6. Single and Multicycle Growth Experiments
To assay the m vitro growth phenotype of a particular virus stock, It may be
necessary to determine Its growth kinetics over one or more replication cycles
compared with a known standard. This 1sachieved by mfectmg multiple plates
of cells with virus, under the same conditions, but harvesting at different time-
points postmfectlon. The progeny virus from the different time-pomts IS titrated
to monitor progression of the infection.

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