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Neutralizing monoclonal antibodies to human rotavirus and indications of antigenic drift among strains from neonates.

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Vol.54, No. 1 JOURNALOFVIROLOGY,Apr. 1985, p. 14-20

0022-538X/85/040014-07$02.00/0

Copyright © 1985, AmericanSociety forMicrobiology

Neutralizing

Monoclonal Antibodies

to

Human

Rotavirus and

Indications of Antigenic Drift Among Strains from

Neonates

BARBARA S.

COULSON,'*

KERRY J. FOWLER,2 RUTH F. BISHOP,' ANDRICHARD G. H. COTTON2 DepartmentofGastroenterology' and Birth Defects Research Institute,2 Royal Children's Hospital, Parkville, Victoria,

3052, Australia

Received 21September1984/Accepted 14 December 1984

Cellsproducing neutralizingmonoclonalantibodies toaserotype 3 human neonatal rotavirusstrainRV-3 werederived by fusion ofhyperimmunizedmousespleen cellswithmousemyelomacells. Asascitesfluid, three

rotavirus-neutralizing monoclonal antibodies werecharacterized byhemagglutination inhibition and reacted with17cultivable mammalian rotavirusesrepresenting five virusserotypes,by fluorescent focus neutralization andenzymeimmunoassay. Twoantibodies, MabRV-3:1andMabRV-3:2, reacted withthesevenserotype3 rotaviruses only. Mab RV-3:1 was shown tobind to the outer capsid glycoprotein gp34 ofrotavirus when variantsof SA11rotaviruswereused, anditthereforeappearsto reactwiththemajorneutralizationepitope ofserotype 3 rotaviruses.TheantibodyMabRV-3:3wasspecificforanepitopeof RV-3 rotavirus notpresent on any other rotavirus of any serotype tested, including another neonatal isolate of identical RNA electropherotypeisolated from thesamewardof thesamehospitalasRV-3 3 months earlier. These two viruses were also distinguishable by fluorescent focus neutralization, using antiserum to RV-3 virus. Western blot analysisshowedbindingofMab RV-3:3 to thetrypsin cleavage productofthe outercapsidprotein p86ofRV-3. This suggests that antigenic drift may have occurred among neonatal rotaviruses in Melbourne. These monoclonalantibodieswillbeuseful inserotypingassaysofrotavirusesdirectlyinstoolsamples,and infurther analysisofantigenic variation within the serotype.

Anumber of authors have reportedproduction of

mono-clonal antibodies to mammalian rotaviruses (11, 13, 24).

Most of these antibodies were directed to the major inner

capsid

protein, which bears shared antigenicdeterminants as

wellasthoseinvolved in subgroup reactions (13). Recently, strain-specific neutralizing monoclonal antibodies directedat

surface proteinsof the serotype 3 simian rotaviruses SA 11

(27) and RRV-2 (14) have been described. Two distinct epitopes on the major surface glycoprotein ofSA 11

rota-virus whichrelate tovirusneutralizationwereidentified with theneutralizing monoclonal antibodiestoSA 11(28). How-ever, the production ofneutralizing monoclonal antibodies

tohumanrotaviruseshas not been reported.

An important application of rotavirus-neutralizing

mono-clonal antibodies not yet addressed is the serotyping of rotavirus strains directlyfrom stools, byenzyme

immunoas-say (EIA) or equivalent method. Rotavirus serotyping has

beenachieved with EIA (29) andsolid-phase immune elec-tron microscopy (12), with carefully cross-absorbed

anti-sera. However, as these antisera are difficult to produce,

most serotyping has been by plaque neutralization assays

(17, 19, 28, 31) and fluorescent focus neutralization (FFN) assays (3). These methods usually are suitable only for cultivablerotaviruses, so

serotyping

has been restricted and progress impeded in cross-protection studies and evaluation

trialsofcandidatehumanrotavirus vaccines.

Inthis paper, we describe the productionand characteri-zationof neutralizing monoclonal antibodiestothe neonatal human serotype 3 rotavirus, RV-3 (1). These antibodies werereacted with a panelofmammalian rotavirusesby EIA and FFN assay. The possible occurrence ofantigenic drift suggested by the monoclonal antibody reactions with the viruspanel wasinvestigatedwithpolyclonal antiserum, and

*

Corresponding

author. Previously known as Barbara S.

Mc-Lean.

the virus

proteins

to which the monoclonal antibodies are

directedwereidentifiedbyWesternblotanalysis and SA 11

rotavirus variants.

MATERIALS AND METHODS

Viruses and cells. Human neonatal rotaviruses RV-1 and RV-3 (serotype 3) andhuman rotavirusesRV-4(serotype 1) and RV-5(serotype 2) were adapted for culture in our labo-ratoryby J. Albert(1). RV-1and RV-3 wereusedatpassage levels 11 and 13,respectively. Humanrotavirus strains YO, S2,and KU weredonatedbyT.Kutsuzawa(31). MMU18006,

rhesusrotavirus strain2(RRV-2)wasthegift ofN.Schmidt,

and simianrotavirus SAl was kindly supplied by H. Mal-herbe (San Antonio, Tex.). Canine rotavirus LSU-79C36 (K9; 10) wasprovided by G. Woode, who, with J. Bridger, isolated the UK strain of bovine rotavirus provided by T. Flewett(33). HumanrotavirusstrainsWa,DS-1,P, and ST-3 were kindly donated byR. G. Wyatt.

All viruses were propagated in MA104 cells in the pres-enceoftrypsin, aspreviously described (25).

The myeloma cell line P3-X63-Ag8-653 (clone 653) used

for fusion is an

azaguanine-resistant

derivative ofa

nonse-creting line of myeloma cell clones (18). The cells were cultured as described previously (6) in Dulbecco modified

Eagle medium (DMM) with 20% (vol/vol) heat-inactivated

fetal calfserum.

Immunization.BALB/cmice 8 weeksoldwereimmunized with intact, double-shelled RV-3 rotavirus particles, which were prepared by fluorocarbon extraction and banding in cesium chloride

gradients

as previously described (23). Throughout the virus purification procedure, Tris-buffered

saline (pH 7.2) with 10 mM calcium chloride was used to stabilize the outer capsid layer (5). Mice were immunized intraperitoneally with 0.4 ml of virus in Fruend complete adjuvant, and two micewereboosted 2months laterbythe same route with 0.1 ml of virus in Freund incomplete

14

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adjuvant,as well as with 0.2 ml of virus by the intravenous route. Three days later, one mouse (A) wassacrificedandits spleen was removed for fusion. The second mouse (B)

receiveda further intravenous boost 2 months after its first boost and wassacrificedfor splenic fusion 3 days later. Both mice showed preinoculation anti-RV-3 EIA titers of 1:50 in serum. After the first booster injection, their EIA titers rose to 1:160,000, with RRV-2 hemagglutination inhibition (HI)

titersof greater than 1:2,000 and RV-3 FFN titers of 1:50,000.

Production ofhybridoma cell lines. The methodology for

production ofhybridoma cell lines has been described else-where (6). Spleen cells from hyperimmunized mice were

fused with clone 653 myeloma cells, using polyethylene

glycol. Parental myeloma cells were killed by medium

con-taining100 ,uMhypoxanthine,4 nM aminopterin, and 16,uM

thymidine. Fused cells weredistributed into 96-well (0.2-ml) Linbro plates. The culture fluids were tested for antibody

productionbyEIA, HI, and FFN. Hybridomas that showed HI and FFN activity were subcloned at least twice by

limiting dilution and grown in syngeneic mice primed with

Pristane (ICN Pharmaceuticals, Plainview, N.J. as ascites. EIA forrotavirusantibodies. THEEIA was performed as describedpreviously (21). Hybrid culture fluids were tested at adilutionof 1 in 5 inphosphate-buffered saline (PBS) (pH 7.2)containing 1%(wt/vol) bovine serum albumin (PBS-B) in

polyvinyl microtiter plates (Dynatech Laboratories, Inc.,

Alexandria, Va.) coated with optimal dilutions of RV-3 (serotype 3), RV-5 (serotype 2), and SA 11 virus antigen and

MA104 cell control antigen. These EIA antigens were

pro-duced by fluorocarbon extraction and ultracentrifugation of

virus-infected or uninfected cell harvests as described pre-viously (5, 23). Bound antibody was detected with goat antimouse immunoglobulins conjugated to horseradish peroxidase (DAKO Immunoglobulins, Copenhagen, Den-mark) andorthophenylenediamine (SigmaChemicalCo., St.

Louis, Mo.) as substrate (32). Theabsorbance of each well at492 nm wasrecorded with a TitertekMultiskan

spectropho-tometer (FlowLaboratories, Inc., Rockville,Md.). Optimal dilutions of reagents were determined by checkerboard

titration.

Ascites fluids and affinity-purified monoclonal antibodies weretitrated byEIA,usingdoubling dilutions ofmonoclonal

antibody. Wells were coated with any of the rotaviruses as

described above, and MA104 control antigen, at optimal dilution.

HI assay for rotavirus antibodies. Viral hemagglutinins usedfor the HI assay were prepared byfreezingand thawing

rotavirus-infected MA104cells showing complete cytopathic

effect. This virus preparation was stored in aliquots at

-70°C. Allhybridomacultures were screened for inhibition

ofRRV-2 hemagglutination (HA), using PBS-B throughout

asdiluent. Analiquot(25

p.1)

ofhybrid culture fluid diluted 1

in 5 was incubated at 37°C with an equal volume of HA

antigen-containing 4 HA units in U-bottomed microtiter trays. After1h, 50

pt1

of0.5% human group0 erythrocytes

wasadded, and results were recorded after further incuba-tion for1 hat37°C. Antigen, cell, andpositive andnegative hybridoma

supernatants

andsera wereincludedin eachtest. Rotavirus HI antibodies in ascites fluids were titrated in

duplicate, using serial twofold dilutions. The HI titers were expressed as the reciprocal of the highestdilution of

mono-clonal antibody showing complete inhibition of HA. FFNassay. The FFN assayprocedurewas based on that of Beards et al. (3) withseveralmodifications, and wasused to screen cell hybridoma cultures positive for rotavirus

anti-body production by EIA. Hybridoma supernatants 2"I LI1)

were diluted 1:1 with DMMcontaining 1

,ug

ofSigmaporcine trypsin per ml (DMM-T) and then weremixed withan equal

volume of RV-3 virus diluted in DMM-T to give ca. 100 fluorescent cell-forming units in 50 p.l. The mixtures were incubated at37°C for 1 h, and 50

IlI

per well was inoculated in duplicate onto confluent monolayers of MA104 cells in microtiter plates. After centrifugation at 1,200 x g for 30

min, the plates were incubated at 37°C in an atmosphere of 5% CO2 and95%relative humidity overnight and stained by indirect immunofluorescence.

The dilution of rotavirus RV-3 (serotype 3) giving 100 fluorescentcell-forming units was determined previously by inoculation of MA104 cell monolayers with doubling

dilu-tions of virus in DMM-T, followed by centrifugation, incu-bation, and staining as described above.

As virus controls, DMM-T and clone 653 supernatantfluid were included. Incubation of clone 653 supernatant alone withRV-3 was found to give a 10% reduction in fluorescent cell count. However, neutralizing hybridoma supernatants reduced the number of fluorescing cells at least 10-fold.

Mouse ascites fluids were titered for neutralizing antibody by using fourfold dilutions of ascites mixed with an equal volume of any of the rotaviruses described above, diluted to give ca. 100 fluorescent cell-forming units per well. The

neutralization titer of each fluid was expressed as the recip-rocal of the highest dilution giving 50% reduction in the number of fluorescing cells.

Indirectimmunofluorescence. Rotavirus-infected cells were fixed inacetone for 5minand air dried. Rabbit hyperimmune antiserum to SA 11 (50

RI)

at optimal dilution (1:500) in PBS was added to each well, and the plates were incubated for 1 h at 37°C. The plates were washed three times with PBS before the addition to each well of 25 ,u of fluorescein

isothiocyanate-labeled goat antirabbit immunoglobulin G (IgG) (TAGO Inc.) diluted 1:50 in PBS. After a further 1-h

incubation at 37°C, the plates were washed again and air dried. The inverted wells were observed with a Zeiss

epi-fluorescent UV microscope (magnification, x80). Specific

fluorescence appeared only in the cytoplasm of infected cells; uninfected cells showed no specific fluorescence.

Plaque neutralization assay. The plaque neutralization as-saymethod of Matsuno et al. (19), modified by Smith et al. (26), was adopted for use with RRV-2 virus by the addition of 50 p.g ofDEAE-dextran per ml (Pharmacia Fine Chemi-cals, Inc., Piscataway, N.J.) to the first agarose overlay.

Hybridomaculturesupernatantswere screened at a dilution of 1 in 4.

Polyacrylamide gel electrophoresis ofdouble-stranded RNA of rotavirus. Electropherotyping of viral RNA was per-formed on all stocks ofrotavirus strains studied (see Tables 4 and 5). Viral RNA was prepared by phenol-chloroform

extraction (8). Polyacrylamide gel electrophoresis of viral RNA wasperformedwith theLaemmli buffer system (26) at 15 mA for 18 h at4°C. The RNA was visualized with silver stain (8).

Western blotanalysis. Partially purified rotavirus particles, preparedby fluorocarbon extraction andultracentrifugation

ofinfected cell harvests (5, 23), were solubilized in sample buffer and electrophoresed in 9% polyacrylamide-Laemmli

gels (26) at 25 mAuntil the dye frontreached 1 cmfrom the end of the gel. To obtain fully reduced proteins, samples were boiled in sample buffer containing 5% 2-mercaptoe-thanol and2%sodium dodecyl sulfate (SDS) and were run in agelcontainingSDS. Forunreducedproteins, samples were heated to 60°C for 5 min in sample buffer without

2-mer-captoethanolandapplied to a gel prepared without SDS. All

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16 COULSON ET AL.

TABLE 1. Screening patterns from four fusions of hybridoma clones producing antibodytoRV-3

No.of clones EIAresult withantigen:

detected No.of HI FFN

Fusions Fusions clones RV-3 SA11 RV-5 result result

98 and 100and retained (serotype (serotype (serotype

99 101 3) 3) 2)

51 124 10 + + + -

-1 2 1 + + - -

-2 1 2 + + - + +

0 4 1 + - - - +

running buffers contained SDS. Electrophoresed proteins were transferred electrophoretically to Millipore HAHY nitrocellulose (30) overnightat30 V, usingastransfer buffer 25 mMTris-192mMglycine-20% methanol(pH 8.3). Nitro-cellulose sheets were probed with antibody by a modifica-tion of themethod of Burnette (4). The nitrocellulosesheets were soaked in 10 mM Tris-0.85% (wt/vol) NaCl-2.5% (wt/vol) Carnation skim milk powder (TNM; 16) for 2 h at 20°C with rocking. Appropriate nitrocellulose lanes were reacted with dilutions of rotavirus hyperimmune sera,

post-immune sera, and mouse ascites fluids in TNM for 2 h at 20°C. After being washed in TNM, Tris-saline with 0.05% Nonidet P-40 and Tris-saline over a 30-min period with rocking, stripswereimmersed in TNMcontaining0.1 p.Ci of 125I-labeled proteinAperlane (Amersham Corp., Arlington Heights, Ill.) for 30min. After beingwashedasbefore, strips were air dried and exposedto Kodak XAR-5 film.

Rotavirus variants analysis. Variants ofSA 11 rotavirus capable of producing plaques under an agarose overlay

containing an excess of the SA 11-neutralizing monoclonal antibody A10/N3 were selected and kindly provided by S. Sonza, Division ofAnimalHealth,Commonwealth Scientific and Industrial Research Organization, Parkville, Victoria, Australia. These variants were not neutralized by A10/N3 (see Table 5; S. Sonza, personal communication). The ability of Mab RV-3:1 and Mab RV-3:2 to neutralize these variants wasassessed by FFN assay.

RESULTS

[image:3.612.325.566.532.635.2]

Production of monoclonal antibodiestorotavirus and char-acterization by EIA, FFN, and HI. Sufficient spleen cells wereobtained from immunized mouseA(onebooster injec-tion) for two fusions (98 and 99) and from mouse B (two boosterinjections)for fourfusions, although onlytwowere performed (100and101);fusion 101wasdone with twice the usual number (2 x 108) of spleen cells. The resulting numbers ofhybridcultures andscreening resultsareshown in Table 1.

TABLE 2. Characteristics of neutralizing monoclonal antibodies

toRV-3 (serotype 3)rotavirus

Immuno- Titeragainst RV-3 HI titeragainst

Antibody globuln asmeasuredby': antigenofstrain':

designation

class' EIA FFN RRV-2 SA 11 K9

Mab RV-3:1 G2b 200,000 235,000 128,000 64,000 64,000 MabRV-3:2 G2b 200,000 235,000 512,000 1,000 16,000 MabRV-3:3 G2a 200,000 130,000 <10 10 <10

aDetermined by immunodiffusion with antimouse class-specificand

sub-class-specific antisera (Miles Laboratories,Inc.,Elkhart,Ind.).

bAs ascites fluid. Alllineswerederived fromseparateclones.

TABLE 3. FFN titers of neutralizing monoclonal antibodies to RV-3 rotavirus against a panel of serotype 3 human and

mammalian rotaviruses

Rotavirus' Reciprocalneutralization titer of monoclonal

antibodyb

Strain Origin RV-3:1 RV-3:2 RV-3:3

RV-3 Human, 235,000 235,000 130,000

neonatal

RV-1 Human, 195,000 195,000 280

neonatal

P Human 330,000 425,000 195

YO Human 170,000 2,350 <100

RRV-2 Simian 277,000 100,000 < 100

SA 11 Simian 940,000 12,000 <100

K9 Canine 100,000 2,950 < 100

" Allvirusstocks used in neutralization and EIA were determined to be of

uniqueelectropherotype by gelelectrophoresisof viral RNA. The RNA of

antigenswas shown to co-electrophorese with viral RNA extracted from first-passagevirusstocks. No extra RNA bandswerepresent.

bTiter ofmouse ascites fluid. Titers of the three monoclonal antibodies

againsthuman serotype 1 strains (Wa, KU,RV-4),human serotype 2 strains

(DS-1, S2,RV-5), the human serotype 4 strain ST-3, and the UK bovine strain were all <100. Serotyping of RV-1, RV-3, and RV-4 was performed by FFN,

using hyperimmunesera to Wa and SA 11rotavirusesprepared as previously

described(2) and Wa and SA 11 rotaviruses as controls. The serotype of RV-5 was determined by Dyall-Smith and Holmes(8).Serotyping of the remaining

strainshasbeendescribed previously (34).

As others havefound (11, 13, 24, 27), the majority (95%) of rotavirus antibodies appeared to be cross-reactive be-tween rotavirus strains. A further 1.6% ofantibodies were

specific for the serotype 3 rotaviruses tested (RV-3 and SA 11) but were nonneutralizing. The remaining 3.4% of

antibodies neutralized RV-3. With increased spleen cell

numbers, fusion 101 produced an increased yield of both

EIA-positive and neutralizing antibody-producing hybrids.

However, fewer than 50% of the latter hybrids survived beyondtwosubculture steps. The three

surviving

neutraliz-ing clones fellintotwogroups: twoappearedtobe specific for serotype 3 viruses and inhibited HA; one reacted with

homologous virus only and did not inhibit HA. These monoclones form the basis of the studies reported here.

Their detailed characterization by EIA, FFN, and HI is

TABLE 4. EIAtitersofneutralizing monoclonal antibodiesto RV-3against serotype3mammalian rotaviruses

Rotavirus' Reciprocal

EIA

titer of monoclonal antibody:

Strainb Origin RV-3:1' RV-3:2' RV-3:3d

RV-3 Human, neonatal 200,000 200,000 130,000 RV-1 Human, neonatal 200,000 200,000 800

P Human 300,000 300,00 6,400

YO Human 64,000 1,000 400

RRV-2 Simian 650,000 650,000 4,000

SA11 Simian 650,000 8,000 3,200

K9 Canine 500,000 300,000 3,200

See footnotea toTable3.

Titersofthe threemonoclonal antibodiesagainsthuman serotype 1strains (Wa, KU, RV-4),human serotype 2 strains (DS-1, S2, RV-5), the human serotype 4 strain ST-3, and the UK bovine strain were all s400. The

serotypingof theserotaviruseswasdescribedpreviously. See footnotebto

Table3.

'As mouseascites fluid.

dAs affinity-purified mouse IgG prepared from ascites fluid, using a

Staphylococcus aureus protein A-Sepharose 4B column (Pharmacia) as

described previously (6) andadjusted toequivalent dilutiontothe mouse

ascitesstarting material.

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shown in Tables2, 3, and4. Allwereofthe

immunoglobulin

(IgG)

G class and reacted

by

EIA with

homologous

virusto

asimilartiter

(1:200,000) (Table

2). However, Mab RV-3:3

showedalower neutralization titer with RV-3 than did Mab RV-3:1andMab RV-3:2. Each ofthese lattertwoantibodies had a

unique

HI reaction pattern,

indicating

that although

they

are ofidentical

IgG

subclass, they aredirectedagainst different

antigenic

determinants ofrotavirus.

For

testing

ofthe serotype specificity oftheantibodies,a

panel

of mammalian rotaviruses of known serotype was

assembled,

and the FFN titers were determined for each

monoclonal

antibody (Table

3).The Mab RV-3:1neutralized

all seven serotype 3 strainsto

high

reciprocal titer (100,000

to

940,000)

and did not neutralizerotaviruses ofserotype 1

(Wa, KU, RV-4),

serotype 2

(DS-1, S2,

RV-5), serotype 4

(ST-3),

or the UK bovine strain.

Similarly,

Mab RV-3:2

neutralized

only

the seven serotype 3

viruses,

but three of thesewereneutralizedtolowertiterthan withMabRV-3: 1. These two monoclonal antibodies therefore appear to be serotype 3

specific

but not identical.

Interestingly,

Mab RV-3:3 neutralized RV-3 (serotype 3) alone to

high

titer.

Very

much lower neutralization was evident withthe other human strain RV-1 (serotype 3) and with rotavirus P, but noneofthe

remaining

serotype3

strains,

or strainsofother serotypes, were neutralized

by

Mab RV-3:3. These three

monoclonal antibodies also neutralized RRV-2 by plaque neutralization assay.

TheEIAtitersofthe threemonoclonal antibodies

against

the virus

panel

are shown in Table4.

Especially

with Mab RV-3:1 and Mab RV-3:2, the EIA titers generally closely

paralleled

the neutralization titers fora

given

rotavirus.The

one

exception

was the reaction of Mab RV-3:2 with K9

rotavirus,

which was

high

titer

by

EIA but lower titer by

FFN.

Although

the Mab RV-3:3 EIA titer was at least

20-foldgreatertoRV-3(serotype

3)

thantoanyother

strain,

EIA titers of 1:400 to 1:6,400 were obtained with other serotype 3

rotaviruses,

even when

affinity-purified

mono-clonal

antibody

was used.

Hence,

itappearsthatalow level

of

binding

of Mab

RV-3:3,

notrelatedto

neutralization,

may

occurtoserotype 3 rotaviruses.

The

finding

that Mab RV-3:3 neutralizes and reacts by EIA to a

significant

extent with RV-3

(serotype 3)

but not

with RV-1 (serotype 3) assumed

importance

when the

his-toryofthese two isolateswas considered. StrainRV-1was

isolated from a

5-day-old

neonate in ward 21 at

Royal

Women's

Hospital,

Melbourne,

in June 1977. Three months

later,

RV-3 was obtained from an infantofthesame agein the same ward ofthat

hospital.

Viral RNA extracted from thetwo viruseswas shown to

coelectrophorese

both before

and after

adaptation

to cell culture

(1).

Rotaviral RNA extracted from RV-1 and RV-3 virus stocks used in FFN assaysandEIA wasfound stillto

coelectrophorese (Fig.

1).

These isolates have been described

previously

as

electro-pherotype

R

(22),

which was present over 6 years in the

Royal

Women's

Hospital

butnotin the outside

community.

The detection of a neutralization

epitope

on RV-3 not presentonRV-1rotavirus with MabRV-3:3 suggests thanan

immunologically significant

difference may exist between these two viruses. To test

this,

the

ability

of rabbit

hyper-immuneantiserum

against

double-shelled RV-3 virions(2)to neutralize RV-1 and RV-3 rotaviruses was assessed. The

antiserum neutralized RV-3 to atiter of1:134,000andRV-1

to the

significantly

lower titer of 1:83,000

(Fig.

2). This

experiment

was

repeated

twice,

and each

point

represents the mean oftwo

replicates.

Attempts to repeat this

experi-mentwith the

original

fecalmaterial

containing

these viruses

failed due to insufficient numbers of infectious rotavirus particles.

Polypeptidespecificity of the RV-3(serotype3)neutralizing

monoclonal antibodies. In Western blotanalysis,noneof the RV-3 neutralizing monoclonal antibodies bound to any re-ducedprotein of RV-3orSA 11rotaviruses.However, Mab RV-3:3 bound to asingle protein (molecular weight 62,000) of RV-3 when reacted with separated unreduced rotavirus proteins (Fig. 3). No binding of Mab RV-3:3 to unreduced SA 11 virus proteins was observed. The locations of the unreducedRV-3 virus proteins corresponding to gp34 of SA 11 (the neutralization antigen) and the major group antigen p42 (2, 14) are shown in lanes F and G of Fig. 3 with monoclonal antibodies C4/B3 and 4C8, which have been shownto immunoprecipitategp34 and p42 of SA 11 virus, respectively (28). The Mab RV-3:3 therefore reacted with a

proteinunrelated tothese rotaviralantigens. Comparison of the reduced RV-3 proteins detected by the use of anti-RV-3 serum (Fig. 3, lane A) with molecular weight standards allowed identification of the RV-3 virusproteins, by analogy with SA 11 rotavirus(2).Polypeptides111,98, 94, and 86 of RV-3 correspondto polypeptides 113, 96, 91, and 84 of SA 11. The molecular weights of p42 and gp34 did not vary between the two rotaviruses when reduced proteins were studied. The presenceof the RV-3 virusproteinp62,

corre-spondingtop62of SA 11virus,thetrypsin cleavageproduct of the outercapsid protein p84 (2)orVP3 (9), wasnotable. This protein, of the same molecular weight, was also de-tected by antisera in unreduced RV-3 and SA 11 virus

A

B

C_

ABC

FIG. 1. Polyacrylamide gel electrophoresis of RV-1 and RV-3

rotavirus double-stranded RNAsegments.Lanes:A,RV-1RNA; B, RV-1 + RV-3 RNA; C, RV-3 RNA.

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18 COULSON ET AL.

a)

a)

a

U)

CD

c u

U

0

L.

0

0

z

0 8 16 24 32 40 48

[image:5.612.151.486.68.338.2]

Dilution of Antiserum to RV-3 ( x 10 )

FIG. 2. FFN of RV-1 and RV-3 rotaviruses by rabbit hyperimmune serum toRV-3 rotavirus. Symbols: 0, RV-1 rotavirus; *, RV-3

rotavirus.

preparations and corresponds to that detected by Mab RV-3:3.

Thehigh-molecular-weight products detected by 4C8are likely to represent p42 aggregates. The disappearance of

A

B

C

D

E

F G H

pill p98 p94

p86 mm III

p62 .o'

n

MIi

A.,,..

FIG. 3. Western blotanalysisofbindingof monoclonal antibod-ies andhyperimmune sera toRV-3 and SA 11 rotavirus proteins. Lanes:A,reducedRV-3proteins + anti-RV-3; B,unreduced(UR) RV-3proteins+anti-RV-3; C, UR RV-3 proteins+MabRV-3:3; D, UR SA 11proteins +MabRV-3:3; E,UR SA 11proteins+anti-SA 11; F,URRV-3proteins+ C4/B3; G, UR RV-3 proteins+4C8; H, UR RV-3proteins+negativecontrolmouseascitesfluid;laneI,UR

SA 11proteins + negativecontrol mouseascites fluid.

these aggregates in favorof thep42 band inthe presence of 2-mercaptoethanol suggests that the polymeric forms of p42 arelinked via disulfidebonds, as previously postulated (2).

Bindingof Mab RV-3:1 and Mab RV-3:2 under reducing or

nonreducing conditions to RV-3 or SA 11 rotaviruses in Westernblots couldnotbe demonstrated, sovariant analy-sis was attempted. Variants of wild-type (wt) SA 11 rota-virus, selected with the SA 11-neutralizing monoclonal anti-body A1OIN3, which immunoprecipitates gp34 (28), were testedfor theirabilityto be neutralizedby Mab RV-3:1 and MabRV-3:2. The FFN titersareshown in Table5, together

with titers to wt SA 11 rotavirus and A10/N3 (S. Sonza,

personalcommunication). The MabRV-3:1 showed a four-log reductionin neutralization titerwith the SA 11variants, which was comparable to that of AION3. Therefore, the

changeingp34elicited byA1O/N3 hasprevented bindingof Mab RV-3:1 to the SA 11 rotavirus variants, and so Mab RV-3:1reactswithanantigenongp34. The Mab RV-3:2did notshowsignificantlydifferentneutralization of thevariants thanwtSA11

rotavirus, suggesting

thatif it bindstogp34,it is to a site unaffected by the change in gp34 elicited by A1ON3.

TABLE 5. FFNtiters ofRV-3-neutralizing and SA

11-neutralizing monoclonal antibodies withwtSA11and SA11

variants

ReciprocalFFNtiter of monoclonalantibodya Virus

A1O/N3 RV-3:1 RV-3:2

A10/314 100 400 8,000

A10/414 100 400 8,000

A10/514 100 200 8,000

SA 11wt >106 940,000 12,000

a As mouseascitesfluid.

J. VIROL.

I

--did

p42

gP34

IV

.jmm".

n

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(6)

DISCUSSION

Monoclonal antibodies directedtorotavirus antigens have beenproduced by a number ofresearchers (11, 13, 14, 24, 27, 28). However,we believethisto be thefirst description of the derivation of neutralizing monoclonal antibodiesto a humanrotavirus. One problemweencounteredwasthat less

than 4% of rotavirus antibody-secreting hybridomas were producing neutralizing antibodies torotavirus. The instabil-ity (5) and inefficient production (25) of intact human rota-virus particles in cell culture may contribute to the low numbers ofneutralizing monoclonal antibodies we derived to RV-3 (serotype 3) rotavirus, compared with neutralizing antibody numbersderivedtoanimal viruses whichgrowwell

in cell culture, such as SA 11 (28) and RRV-2 (14). Our results indicated that repeated booster immunizations with RV-3 virus and doubling the number of spleen cells per

fusion increased the number of neutralizing monoclonal antibodies detected.

The antibodies Mab RV-3:1 and Mab RV-3:2 proved to react specifically with serotype 3 rotaviruses, whether of human origin orfrom other mammals. Neither monoclonal antibody reacted with 10 cultivable rotaviruses ofserotypes 1, 2, 4, and UK bovine. High titerstoserotype3 rotaviruses wereshown by Mab RV-3:1, with goodcorrelation between FFN and EIA results. This monoclonal antibody has now

fulfilled its promise as a sensitive and specific serotype reagent, in an EIA developed for serotyping human rota-viruses. We have successfully serotyped anumber of rota-virus strains with monoclonal antibodiesto serotypes 2 and 3, and have produced serotype 4-specific monoclonal anti-bodies (unpublished data). Consistent with its serotype 3 specificity, Mab RV-3:1 was shown by FFN assay with variants of SA 11 rotavirus tobindto the serotype-specific

component of RV-3 rotavirus equivalent to gp34 of SA 11 rotavirus. The binding site of Mab RV-3:1 on gp34 was related tothat of the monoclonal antibody A10/N3 usedto select thevariantsof SA 11;in competition studies this site is theapparentlydominant neutralizing epitope of gp34 (28). This epitope is sensitive toreducing and denaturing agents, asitcould notbedetectedby Mab RV-3:1 inWestern blots evenundernonreducing conditions. Radioimmunoprecipita-tion experiments with RV-3 or SA 11-infected cell lysates

were also unsuccessful with Mab RV-3:1 and Mab RV-3:2

(S. Sonza and B. S. Coulson, unpublished data). This epi-tope is therefore likelyto be exposed on thesurface of the

outercapsid (24) orto requirethe nativeprotein conforma-tion foractivity.

Like Mab RV-3:1, Mab RV-3:2 showed serotype speci-ficity and hasproven effectiveinrotavirus EIA systemsfor serotyping human strains(unpublished data). Variant studies didnotestablish thebinding site of Mab RV-3:2, although its abilityto serotype suggests it is specific fora gp34 epitope

notoverlapping with the Mab RV-3:1 binding site. Compe-tition studies and further variant analysis may decide this point.

MabRV-3:3appearstobedirectedatatryptic product of the largest outer capsid protein (p86). However, this anti-body onlyreacts by EIA with serotype 3 viruses. It is thus possible that antigenson p86 and gp34 tendtocosegregate. Furtherstudiesare neededto confirm this observation.

Thefinding that Mab RV-3:3 has substantial RV-3 (sero-type3) neutralizing activity andappearstobedirectedtothe larger trypsin cleavage product of the largest outer capsid protein (p86) is of interest. Neutralizing monoclonal antibod-ies binding to the intact form of the larger outer capsid

protein (p82) ofRRV-2(the viral hemagglutinin) have been elicited (14), but these antibodies also inhibited HA. As

monoclonal antibodies directedtogp38, theserotype-related glycoprotein of RRV-2, also neutralized RRV-2 and

inhib-ited its HA, it was not possible to conclude whether the

neutralizing

activity

oftheviral

hemagglutinin-precipitating

antibodies was related to

epitopes

on the viral

hemaggluti-nin,or tosteric hindrance of gp38epitopes.

Similarly,

SA11

virus-neutralizing monoclonal antibodies have been shown

also toinhibitHA(28). Our studies with Mab RV-3:3, which didnotinhibitHAby RRV-2, SA 11,orK9,suggestthat

p86

does contain at least one

potential

neutralization

epitope.

This site may be absent on the uncleaved protein, but it is

present onthelargertrypsin cleavageproduct (p62) ofRV-3

rotavirus and somay be involved in determination ofother

functions of the protein such as host cell

tropism

and trypsin-enhanced plaque formation (17),rather thanin viral

HA.

Thevirtual absence of reaction ofMab RV-3:3withRV-1 (serotype 3) rotavirus, isolated from the same ward of the

same

hospital

as RV-3 (serotype 3) virus and of identical electropherotype,was

surprising.

Theonly difference

previ-ouslynoted between RV-1andRV-3rotaviruses isthat RV-1

does not

produce plaques

in cell

culture,

whereas RV-3-infected cell cultures show thickened areas

(1),

similarto a

bovine

temperature-sensitive

mutant found

previously.

Again, this suggests that theviral epitope detected by Mab RV-3:3 may relate to

trypsin-enhanced

plaque formation, andalso that this site is subject to aform of

antigenic

drift.

This latterconclusion is reinforcedby the

higher

FFN titer of

polyclonal

anti-RV-3

hyperimmune

serum found with RV-3 rotavirus than with RV-1

rotavirus,

which suggests that an

immunologically

significant difference,

not

necessar-ily restricted to the difference detected with Mab RV-3:3, exists between RV-1 and RV-3 rotaviruses. This variation

may prove analagous to the

antigenic

drift ofthe influenza

virushemagglutinin (35),or tothevariation within

poliovirus

serotypes

generated by

selective pressure of

antibody (7).

The antigenicity (measured by virus

neutralization)

of bo-vinerotavirus strain Reims 18/77wasshown

recently

toalter drastically after15passages incell culture inthepresenceof

specific antibody (15).

Virus

adapted

to culture without antibody showed unaltered neutralization kinetics. Thismay represent

experimentally

the

endemicity

ofrotavirus strains suchasRV-1 and RV-3innurseries of

maternity

hospitals

in

Melbourne,

whereforanumberofyearsneonates

receiving

rotavirus antibody either in breast milk or across the

pla-centa orbothhavebeeninfected with mildor nosymptoms

(20). We

plan

further

analysis

of neonatal rotavirus

strains,

using

Mab RV-3:3 and other monoclonal antibodies to

investigate

this phenomenon.

ACKNOWLEDGMENTS

WearegratefultoS. Sonza for hishelpfuldiscussion and for the gift of variants, virus, and monoclonal antibodies. We thank L. Unicomb, W. McAdam, S. Millar,and J. Albert for theircapable assistance withthisresearch,and J. Lee and A. Peacefortypingthe

manuscript.

This work was supported by the Royal Children's Hospital Research Foundation and the National Health and Medical Re-searchCouncil of Australia.

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Figure

TABLE 2. Characteristics of neutralizing monoclonal antibodiesto RV-3 (serotype 3) rotavirus
FIG.1.rotavirusRV-1 Polyacrylamide gel electrophoresis of RV-1 and RV-3 double-stranded RNA segments
TABLE 5.neutralizing FFN titers of RV-3-neutralizing and SA 11- monoclonal antibodies with wt SA 11 and SA 11variants

References

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