Bài tập tổng hợp - C6

Two sets of oligonucleotide primers (1008PS-1009PR and 11OPLS-IOIIPLR) were designed according to the sequence of the nucleocapsid protein (N) gene of Quebec reference strain IAF-exp91 o[r]

(1)

0095-1137/94/$04.00+0

Detection of Porcine Reproductive and

Respiratory

Syndrome

Virus and Efficient Differentiation between Canadian

and

European

Strains

by

Reverse

Transcription

and

PCR

Amplification

HELMI MARDASSI, LOUISE WILSON, SAMIR MOUNIR,ANDSERGE DEA*

Centre deRechercheen Virologie, InstitutArmand-Frappier, Universiuedii Qiibec, Laval, Qubec, Canada H7N 4Z3 Received 21 January 1994/Returned formodification 6April 1994/Accepted 21 June 1994

Two setsofoligonucleotideprimers(1008PS-1009PRand11OPLS-IOIIPLR)weredesignedaccordingtothe sequence of thenucleocapsid protein (N) gene ofQuebecreference strain IAF-exp91 ofporcine reproductive andrespiratory syndromevirus (PRRSV).Theprimerswereused inreversetranscriptionandPCR(RT-PCR) experiments for detection ofviralgenomicRNAeither from infectedporcinealveolarmacrophages (PAM) or tissues from experimentally infected specific-pathogen-free pigs Considering the high degree of variation detected between the nucleotide sequences of the N genes ofIAF-exp9l and Lelystad virus (LV) strains of PRRSV,the primers 1008PS-1009PRwerereferred to asthe specific primers, sincetheywerechosen in such a mannerthattheycouldamplify onlysequencesfromIAF-exp9l RNAandnot from LV On the otherhand, the primerpair 1O1OPLS-1O11PLR was common to both strains of PRRSV When analyzed by agarose gel electrophoresis,theproductsofRT-PCR fromeach set ofprimerswereresolvedassinglebandof thepredicted size, the specificity ofamplified products beingconfirmed by Southernblotting witha specific 1AF-exp91 N gene probe No amplification was observed when RNA was extracted from uninfected PAM or from other porcineviruses Asexpected, onlythe commonprimer pairwasable toamplifyRNAfrom theQuebecreference strain and two European strains (LV and Weybridge) The resulting bands displayed differences in electrophoretic mobilities duetothe absenceof 37 nucleotides in bothEuropean strains, thusallowingtheir differentiation from the 1AF-exp91 strain Most of the tissue culture-adapted Quebec isolates weredetected with bothprimer pairs The sensitivityof the enzymatic amplificationmethod for detection ofPRRSV from lungtissues was a 50% tissue culture infective dose of 5.RT-PCRwasfound to be more sensitive than indirect immunofluorescence assay for detection of PRRSV in tissues fromexperimentallyinfectedpigsandassensitive as virus isolation in PAM, especiallywhen combined with Southern blottingwith the digoxigenin-labeled N probe and chemiluminescence detection

Since 1986-1987, many swine herds throughout the world haveexperienced important outbreaks ofadisease character-ized by severe reproductive failure in sows ofany parity and respiratory problems affecting pigs ofall ages (5, 19, 35) The disease, named porcine reproductiveandrespiratory syndrome (PRRS), is caused by a fastidious agent, the Lelystad virus (LV), whichcanbepropagatedonprimary cultures of porcine alveolar macrophages (PAM) (34) The LV is a spherical, enveloped virus, 48 to 83 nm in diameter, with a central isometric nucleocapsid approximately25 to30nmindiameter (4,33) In allcountries where the disease has been described, viruses with similarmorphological characteristicswereisolated (2, 4, 12, 26, 28) While European isolates of PRRS virus (PRRSV) appear to belong to the same serotype, antigenic variability has been demonstrated among North American isolates, andbetween NorthAmerican and European isolates (32) Molecular studies have shown that the viral genome consists of a single-stranded RNA molecule with positive polarity, approximately15 kblong,which generates in infected cells a 3'-coterminal nested set of six major subgenomic mRNAs (10, 24) The genomic organization and replication strategyof thisnewporcine virusappearsimilar to those of the

*Correspondingauthor Mailingaddress: Centre deRecherche en

Virologie, Institut Armand-Frappier, 531 Boulevard des Prairies, Laval, Quebec, CanadaH7N4Z3 Phone: (514) 686-5303 Fax: (514) 686-5627

arteriviruses, including equine arteritis virus, lactate dehydro-genase-elevating virus, and simian hemorrhagic fever virus (27) Recently, partial nucleotide sequence analysis of the 3'-terminalregionofthegenomeofaQuebec reference strain (IAF-exp9l) ofPRRSV revealed major differences from nu-cleocapsid proteingenes ofprototype European strains (23) Indeed, the IAF-exp91 strain showed identities of63 and59% with that of LV at the nucleotide and amino acid levels, respectively

(2)

fetuses or dyspneic piglets Two sets of oligonucleotide primers have been assessed for their ability to detect the nucleocapsid geneof PRRSV, and to differentiate between European and Quebec isolates

3' Nproteingene(Orf7)

1008PS _ 468bp _ 1009PR

MATERIALS AND METHODS

Virus stocks The Quebec reference strain of PRRSV, IAF-exp9l, was initially isolated from the lung of a specific-pathogen-free (SPF) pig which had been intranasally inocu-latedwith pooled homogenate of tissues from suckling piglets in a Quebec herd experiencing a typical acute outbreak of PRRS (11) The virus propagated in primary cultures of PAM wasshown to reproduceclinical disease and respiratory lesions in SPFpigs and reproductive disorders in sows (12)

Thirteen other Quebec isolates of PRRSV were propagated onPAM fromlung homogenates of stillborn fetuses or blood samples of dyspneic piglets obtained from pig farms where acute orchronicoutbreaks of PRRS had occurred (23) Viral identification was confirmed by indirect immunofluorescence

(IIF)in PAMwith monoclonal antibodies SDOW17 and VO17

(obtained from D A Benfield and E.Nelson, South Dakota StateUniversity)directed against thep15nucleocapsid protein of the American reference strain ATCC VR-2332 of PRRSV (4) Two European reference strains (LVand Weybridge) of PRRSV were kindly provided to us by G Wenswoort (Nation-al Veterinary Laboratory Center, Lelystad, The Netherlands) and S Edwards (Central Veterinary Laboratory, Weybridge, Surrey, United Kingdom) Stockviruseswereproduced byat least five successive passages in PAM In all cases, infectivity titersranged from

104.50

106.50

(TCID50)

Other viruses used as controls included the Purdue strain of transmissible gastroenteritis virus (ATCC VR-763), the NADL-2strain ofporcine parvovirus, the porcine enterovirus type 1,the 67N strain ofporcine hemagglutinating

encephalo-myelitis virus(ATCCVR-741),andtheQ890 strainofporcine encephalomyocarditis virus Theoriginsand cultivation proce-dures of these reference porcineviruses have been described elsewhere (13, 14) Sucrose

gradient-purified

Quebec, Canada

Clinical samples Twelve5-week-old SPFpigswere intrana-sally exposed, with 1.5 ml per nostril of the tissue

culture-adapted Quebec referencestrain IAF-exp91 of PRRSV

(105.0

TCID50/ml).

preparedinphosphate-bufferedsaline(10mM,pH7.4)witha Sorvall Omnimixer (Yvan Sorvall, Inc., La Jolla, Calif.) and clarified by centrifugation at 5,000 x g for 20 at 4°C

Specimenswere usedimmediatelyfor virus isolation in PAM orstoredat-70°C.Frozenlungsectionswerepreparedfor IIF

staining, asdescribed elsewhere (12)

Sources and extraction ofgenomic RNA PRRSV-infected PAM and MARC-145 cultureswere harvested followingtwo freeze-thawcycles, when 50to60%(6x

107.0)

433 bp J101 1PLR

Haeiin fragment probe (468 bp)

Hae III HaeIII

FIG Diagram of the 3' end of the PRRSV genome, showing the localizationof theoligonucleotide primers, the sizes of the predicted amplified products, and the N probe Orf, open reading frame

concentratedbydifferential centrifugation at 100,000xgfor hthrougha30% (wt/vol)sucrosecushion RNA was extracted

from50-,ul aliquots of the concentrated viral preparations by theguanidiniumisothiocyanate-acid phenol method of

Chom-czynsky and Sacchi (9) andresuspended in 10

[lI

volume of DEPC-treated water Inoneexperiment, 0.5 mlof each10-fold dilutionfromaninitial IAF-Klop viralsuspension

at 106

TCID5Jml

Primers.The designsofprimersfor RT-PCR amplification

were based on the 3'-terminal nucleotide sequence of the QuebecIAF-exp91strain(23)and thenucleotide sequences of openreading frames and of the LV strain of PRRSV(24)

Sequenceanalysesfor selection ofprimerswereperformedon anAppleMacIntosh computerwith the McVector 3.5

(Inter-nationalBiotechnologies) and Gene Works 2.2

(IntelliGenet-ics Inc., Mountain View, Calif.) programs Oligonucleotides

used in the RT-PCRweresynthesizedon anautomated Gene Assembler DNA synthesizer (Pharmacia LKB) The primer

localizations and sizes of thepredicted amplified productsare shown in Fig Their sequences (5' to

3')

1009PR (position 450 to 470), TAGGTGACTTAGAG

GCACA; 1010PLS (position51to69), ATGGCCAGCCAGT CAATCA;and 1011PLR (position464to

483),

Reverse transcription One microgram of total RNA was mixed with 25 pmol of one of the reverse

oligonucleotide

primers (1009PR or 1011PLR) in 10 pA of DEPC-treated distilled water, denatured at 70°C for 10 min, and then immediately cooled onice.ReversetranscriptionoftheRNA into cDNA was made in a final volume of 20 pA of reaction buffercontaining1xPCRbuffer(50mMTris-HCl

[pH 8.3],

dithiothreitol),

(each)

dATP,dCTP, dGTP,and

dTTP,

,ul,

and 200 U of RNase H- Moloney murine leukemia virus reversetranscriptase(GIBCOBRL,Burlington,

Ontario,

Can-ada).After incubation for 1hat42°C,the mixturewasheated to 94°C for and quickly chilledon ice In every set of

experiments,weincluded anegative control in which DEPC-treated distilledwaterwasusedinstead ofRNA

PCR For the PCR step, the reversetranscription reaction

(3)

mMTris-HCI [pH 9.0],50mMKCl,2.5mMMgCl2),0.2 mM

(each) dGTP, dATP,dCTP anddYTP,and 25pmolof eachof the reverseand senseprimers.After addition of2.5 UofTaq DNApolymerase (Perkin-Elmer Cetus, Norwalk, Conn.),the reaction mixtureswereoverlaid with 50

[lI

Biomedicals Ltd., Mississauga, Ontario, Canada) The tubes weresubjectedto35 amplification cycles,eachcycleconsisting

of1-mindenaturation, 1-minannealing,and2-minelongation

steps at temperatures of94, 60, and 72°C, respectively The PCRwasended withafinal elongationstepof 10 minat72°C Amplified products were detected by electrophoresing 5- to

10-p1 aliquots through 2% agarose gels (Promega, Madison, Wis.) in TAE buffer (0.04 M Tris-acetate [pH 8.5], 0.002 M

EDTA) in the presence of ethidium bromide for approxi-mately30 at 10 V/cmand photographing the gels under UV illumination

Molecular probe preparation and labeling APRRSV

nu-cleocapsid-specific

(clone 12) containing the 3' terminal 530 nucleotides of the IAF-exp91 strain genome, which encompasses most of the

nucleocapsid protein gene and the downstream noncoding

region (23) This fragment was isolated from the plasmid

vectorbyBamHI-EcoRI digestion,andtheprobe (463 bp)was

generated by cutting the resulting fragment with the HaeIII endonuclease enzyme The genomic region correspondingto the probeisrepresentedin

Fig.

Labeling

[32P]dCTP

commer-cial

oligolabeling

(Pharmacia

Southern blot analysis

Capillary

SSC is 0.15 MNaCl and 0.015 Msodium

citrate,

7.0)

products

gel

(15

Que-bec,

Canada).

twice in 2x SSC,baked for 30 minat80°C,andprehybridized

for4h at42°Cin 10 mlofprehybridizationsolution(6X SSC,

5x Denhardt's reagent, 0.5% sodium dodecyl sulfate, 100 pig

of salmon sperm DNAperml,50%

formamide).

[32P]dCTP-labeled

prehybridization

was performed using the DIG Luminescent Detection kit in accordance with the manufacturer's

(Boehringer

instructions

RESULTS

Selection ofoligonucleotide primers Previous attempts to amplify genomic fragments correspondingtothe 3' endofthe

IAF-exp91 genome, using sets of oligonucleotide primers chosenon thebasis of the publishedLVnucleotide sequence, were unsuccessful (23, 24).Thesepreliminaryresults ledus to sequence the 3'-end

genomic

IAF-exp9l strain,

and a downstream noncoding region (23) Computer analysis

of the published nucleotide sequences of both the LV and

Quebec reference strains of PRRSV revealed ahigh levelof variations in the open reading frame nucleotide sequences

resulting from a number of substitutions, insertions or

dele-tions;thesevariations were more frequentin thefirst halfof the N gene (23) Such variability allowed us to select two

oligonucleotide

sequence (Fig 1) The first primer pair, designated 1008PS-1009PR, was chosen in such a manner that it could amplify

exclusively the 3' genomic region of the Quebec reference

Kb 1.636 -1.018

-0.506

-2 10 12 14 16 18 X) 22 24 26 28 MA I A a I I 11 ; 17 1C 2A )4 ?; 77 k

A

B

FIG PCR amplification of the nucleocapsid protein gene of Quebec and European isolates of PRRSV using specific (even-num-bered) and common (odd-numbered) oligonucleotide primers (A) PCR products of IAF-Kiop, IAF-183, Weybridge, LV, IAF-BAJ, IAF-CHA, LHVA-3, IAF-SBC, IAF-NUT, IAF-BUT, IAF-PVFA, and IAF-exp91 isolates(lanes and2,3 and4,7and8,9 and10, 11 and 12, 13 and 14, 15 and 16, 17 and18,21 and22,23and24,25and 26,and 27and28,respectively).Asnegative controls,RNAextracted from mock-infected PAM (lanes5 and 6) and amplificationwith no

RNA(lanes 19 and 20) are included The molecular sizes of three fragmentsof the 1-kb DNA ladder(lanesM)areindicated in the left margin (B) Corresponding autoradiogram of theamplified products asrevealedbythe 32P-labeledNprobe

strain In contrast, the second primer pair, designated 1O1OPLS-101 1PLR, corresponds to regions of absolute se-quenceconservation within the 3'genomic regions ofthe LV and Quebec strains; since this primer pair was designed to

amplify the N gene of both Quebec and European strains of PRRSV, itwasreferred as thecommon primer pair

Enzymatic amplification ofPRRSV genomic RNA by RT-PCR As illustrated in Fig 2A, specific primer pair 1008PS-1009PRyieldeda PCR product of the expected size (468 bp) for the IAF-exp9l strain and 13 other tissue culture-adapted

Quebecisolates tested.In contrast,nogenomic fragment could beenzymatically amplified by the PCR procedure for eitherof the European isolates (LV and Weybridge strains) with the

specific primer pair, even after propagation of these isolates more than three times in PAM to increase infectivity titers

(>105"-

TCID5,Jml)

hand,commonprimer pair 11OPLS-1011PLRallowedN gene

amplificationofbothEuropean strains (Fig 2A, lanes7and9) andmostoftheQuebecisolates(Fig.2A, lanes 1, 3, 11, 15, 17,

23,and27) Interestingly,comparisonanalysis of the published nucleotidesequencesof reference PRRSV strains revealed the absence in theLV genomeofatotal of35nucleotides confined to the 3' end of the N gene and the beginning of the downstream noncoding region (23) Consistent with that ob-servationwasthe difference in electrophoretic mobilityof the

IOIOPLS-1011PLR fragments amplified from European

strains; these fragments migrated more rapidly (433-bp ex-pected product)than did thecorresponding productsobtained from the Quebec isolates (Fig 2A, compare lanes 7and to lanes 1,3, 11, 15, 17, 23,and 27) Thus,onthe basis of such a

(4)

difference in the electrophoretic mobility of the amplified products, the common primer pair appeared to represent an efficient tool for differentiation between European and Que-becois PRRSV isolates

Specificity and sensitivity of RT-PCR for detection of PRRSV Thespecificityofthe PCRproducts wasconfirmed by Southern blot analyses using the radiolabeled nucleocapsid probedescribed above(Fig 2B) As expected, no amplification product was obtained when RNAs or DNAs from several heterologous porcine viruses were used as templates (data not shown), as well as with RNA prepared from mock-infected cells or distilled water (Fig 2A, lanes and or 19 and 20, respectively) With other porcine viruses, no hybridization signal could be observed by Southern blot even after a 12-h exposure (datanot shown)

Strong hybridization signals were obtained with the two primer pair-amplified products of the IAF-exp91 strain (Fig 2B, lanes 27 and 28) aswell as with the amplified products of IAF-Klop,IAF-183, IAF-BUT (Fig 2B, lanes and 2, and 4, and 23 and24, respectively), LHVA-1, LHVA-2, and IAF-CM strains (data not shown), indicating that the amplified DNA bands contained sequences encoding the N protein Surpris-ingly, the hybridization pattern observed for IAF-BAJ, LHVA-3, and IAF-SBC(Fig 2B, lanes 11 and 12, 15 and 16, and 17 and18) after a 4-h exposure did not correlate with their efficient enzymatic amplification Indeed, the amplified DNA bands of these isolates were as prominent as those of IAF-Klop, IAF-183, and IAF-BUT and therefore the amount of DNA could not account for the lack of hybridization It is noteworthythat this inefficient hybridization, especially in the case of theLHVA-3 isolate, was not due to the absence of the N gene inthe amplified products, since a hybridization signal could be readily observed if the exposure period was extended to 12 h (data not shown) Southern blot analyses further confirmed that in the cases of the two European strains, no amplification had occurred when the specific primer pairwas used(Fig 2B, lanes and10) Moreover,amplified products of the common primer pair were not revealed by the IAF-exp91 N probe after a 4-h exposure, and only twofaint bands could be observed upon an exposure of 12 h (data not shown) Occasionally, with some of the PRRSV isolates tested, an additional discrete bandwhich migrated faster than the pre-dicted RT-PCR products for both primer pairs could be revealed by Southern blot analyses (Fig 2B and 3B)

The RT-PCR was found very sensitive in detecting viral RNA, especially when the specific primer pair was used; this pair still gave an apparent amplification band when RNA extractedfrom a dilution of the concentrated viralpreparation

corresponding to 10

TCID5o

32P-labeled Nprobe, thesensitivity of RT-PCRwas increased to a detection limit of

TCID50

3B)

In order to assess the sensitivity of the RT-PCR method when used on PRRSV-infected tissue homogenates as op-posed to partly purified virus and to examine whether the efficiency ofenzymaticamplificationof the viralgenome would be compromisedbyexcessiveamountsof cellular nucleicacids,

various dilutions of a tissue culture-adapted PRRSV strain (IAF-Klop) were injected within 1-g samples of lung tissue from an SPF piglet, as described in Materials and Methods After 40amplification cycles withprimerpair1008PS-1009PR, it was possible to visualize by ethidium bromide staining an

10 104 103 10 10 10 1CTD5O Kb M 10 11 12 13 14

0.506

-FIG Sensitivity of the RT-PCR procedure in detecting viral specific RNA extracted from 10-fold dilutions of an initial

concen-trated IAF-exp9l viral preparation corresponding to 106 TCID50 RT-PCR products were analyzedbyelectrophoresis on a2%(wt/vol) agarose gel (A) and then probed, after Southern blotting, with the 32P-labeled HaeIII fragment (B) The amplification products of the 1008PS-1009PRprimer pair are shown in lanes 1, 3, 5, 7, 9, 11, and 13, andthose oftheprimer pair11OPLS-1011PLRare shownin lanes 2, 4, 6, 8, 10, 12, and 14 Lane M, 1-kb DNA ladder

amplified product at the dilution 10-5 of the injected viral suspension, corresponding toabout5

TCID50

DetectionofPRRSVinclinicalsamples fromexperimentally infected pigs To be more practical, we next attempted to detect PRRSV nucleocapsid gene in crude clinical samples

from SPF pigs that had been experimentally infected with Quebec IAF-exp91 strain AtPID2to5, mostof the infected

pigs were feverish (rectal temperature ranging from 40 to

41.5°C) and subsequently developed mild clinical signs of

respiratory disease Total RNA was extracted by the same method used for concentrated virus extraction, except that lung homogenates wereextensively disrupted with the

guani-diniumisothiocyanate solutionby20to30 passagesthrougha 26 Gl 1/2 needle RT-PCR experiments using the specific

Kb M

1.636- 1.018-

0.506-FIG RT-PCR products resulting fromenzymatic amplification

of PRRSV (IAF-Klop strain) genomic RNAextracted from infected

porcine lungtissue Lanes to9correspondto virus doses of 0.5x106,

0.5 x 105,0.5 x 104,0.5 x 103,0.5 x 102,0.5 x 101, and 0.5TCID50,

respectively Aspositive and negative controls, RT-PCR using RNA

extracted from 107 PRRSV-infected MARC-145 cells (lane 1) and total RNA from lungtissue ofan SPFpiglet (lane 2)wasperformed

Lane M, 1-kb DNA ladder

(5)

6368-Kb M 10 11 12 13

1.636 -1.018 -0.506

-I

FIG Detection of PRRSV in lung tissues of experimentally

infected pigs by RT-PCR, Southern blotting, and virus isolation in

PAMcultures.(A)RT-PCRproducts resultingfromenzymatic

ampli-fication of genomic RNA extracted from lung homogenates of

PRRSV-infectedSPFpigseuthanized at PID4(lanes9and11),PID

10(lanes10 and12),PID 14(lane 1),PID21(lanes 2, 4,and5),and

PID 42(lanes 6, 7,and8).Lane3,PCRproductobtainedfrom lung homogenates containing0.5 x 106TCID50of virus(IAF-Klop strain)

Lane13, negativecontrol PCRproductswereelectrophoresedon2%

agarosegeland revealedbyethidium bromidestaining (B)The DNA

fragmentsfrom theagarosegelweretransferred toanylonmembrane

forhybridizationwith thedigoxigenin-labeledNprobeand revealedby

thechemiluminescent detectionsystem (C)Results of virusisolation

inPAM culturesandfinalidentificationbyIIFwith anti-Nmonoclonal

antibody.LaneM,1-kb DNA ladder

primer pair were conducted As shown in Fig 5, in pigs euthanized at PID to 10, detection of PRRSVnucleocapsid geneinlungswasobserved in twoof fourcasesafterethidium bromide staining (Fig 5A, lanes and 11) When combined

with Southern blotting,with the digoxigenin-labeled Nprobe and thechemiluminescent detectionsystem,threepigletswere found to bepositive (Fig 5B, lanes to 11) In comparison,

none of the lungs tested was found positive for PRRSV antigens by IIF assay, whereas virus isolation in PAM was

successfully achieved in these fourcases (Fig SC, lanes 9to 12) PRRSV could not be detected in crude clinical samples from control piglets by either RT-PCR or virus isolation in

PAM (Fig 5, lane13) Bythe threemethods, negativeresults

were obtained with crude clinical samples of experimentally infected pigsafter PID 14 (Fig 5, lanes 1, 2,and4 to8)

DISCUSSION

Reversetranscription and cDNAamplificationisbecoming

acommonlyused approachforthe detection of several RNA

viruses Diagnostic tests based onthis relativelynew technol-ogy have been successfully applied for human immunodefi-ciencyvirus (17), coronaviruses(1, 31), arterivirus(7),

flavivi-ruses(15), measlesvirus(29), respiratory syncytialvirus (25), andpicornaviruses (18) Resultsobtained in thepresentstudy suggested that RT-PCR may be asuitable diagnostic proce-durenotonlyfordetection butalso fordifferentiationbetween

North American and European strains of PRRSV, a newly

recognized porcine pathogen causing major economic prob-lems forthe swineindustry

Efforts to control PRRSV infection have been largely un-successfulbecause of the lack of aconvenient diagnostictest,

the airborne spread of the virus, the existence of subclinical infections, andthepersistenceof thevirus ininfected

popula-tions(16) The restrictive tropism of this new porcine virus for alveolar macrophages still represents a major problem for large-scale propagation of the virus; consequently, this has hampered the preparation of specific immunological probes and thedevelopment of rapid and accurate diagnostic tools for detection of the virus or viral antigens from clinicalspecimens

Although PAM cells have been shown to be highlypermissive to thevirus, routine isolation from clinical specimens can be doneonly in afew laboratorieshaving access to a continuous sourceof SPF piglets, with facilities for harvesting and

main-taining the cells Morerecently, propagation of North Amer-ican PRRSV isolateswasachieved in the continuous cell lines ATCC VR-2332 andMARC-145, highly permissive cell clones of PRRSV derived from the MA-104 cell line (3, 4, 20)

Nevertheless, amongisolates which could be readily cultured in PAM,several wereunabletogrow in either of the contin-uous cell lines, and the reciprocal was also true (3, 20)

Consequently, the use of a single-cell system may not be adequate for isolation of PRRSV from field cases For isola-tion of PRRSV from serum samples, however, PAM were more sensitive probably because antibody enhances PRRSV

replicationin PAM (8) Moreover, replication of PRRSV in

susceptiblecells didnotnecessarily inducea cytopathiceffect

(3, 22)and finalidentification neededtobefurther confirmed

by IIF or immunoperoxidase tests using specific antisera

Finally,inmostcases,it takesmorethan 2weekstoculture a virus isolate from clinicalspecimens

Besides being specific, a suitable test for the control of PRRS should also be highly sensitive in order to permit identification ofasymptomatic carriers,whichcertainly repre-sent a major source of the virus between acute outbreaks

Indeed,it has beendemonstrated that the viruscanbe isolated from serumofPRRSV-inoculatedpigs up toPID 41 despite the presence of high titers of antibodies (IIF titers of 1:

1,280) The virus could also be isolated from lung, serum,

plasma,andbuffycoatcells for 6to8weeks after infection (30,

37).

TCID50

sensitivity

TCID50

detected by hybridization with a specific N probe (Fig 2B) The techniquewas also proven tobe specific, since other pig virusesresponsible forreproductive andrespiratory problems whichmightbe confused with PRRS failedtoyield an ampli-fiedproduct bythePCR method The results indicated that the 13 of the Quebec isolates propagated in PAM could be

amplified

couldbe usedto

identify

Quebec

titers when propagated in PAM (22) No amplification was observed for either of the European strains, thus confirming

the

specificity

Quebec

be-tweenQuebecandEuropeanisolates of PRRSV These results wereconfirmedbySouthern blotanalysis When the common

primer pair1O1OPLS-1011PLRwasused, both European and

Quebecisolateswereamplifiedasexpected from their design after

aligning

European strains showed a faster electrophoretic mobility,

confirming

Interestingly, all the Quebec isolates yielded a

(6)

mo-bility.This findingsuggests that all the Quebec isolates being studied here could be efficiently distinguished from the two prototype European isolates by using the common primer pair The high specificity displayed by the two primer pairs as demonstrated by Southern blotting is a point in favor of their use as a diagnostic tool, especially since the three Quebec isolates IAF-Klop, IAF-CHA, and IAF-NUT which were devoid of any cytopathic effect on PAM (22) couldbe ampli-fied

Genomic variations among Quebec PRRSV isolates could be deduced from their reactivities with theIAF-exp9lN probe Indeed, the hybridization signal obtained for isolates IAF-BAJ, LHVA-3, and IAF-SBC did not correlate with the abundant amount of the amplified fragments as was the case for the other isolates One can hypothesize that the efficiencies of transfer to the membrane are not equal for each site; however, similar results have been obtained from anindependent exper-iment where the amplifiedproducts of these isolates were run at distinct positions (data not shown) Another piece of evidence which points towards a genomicvariation is the fact that the amplified product of the LV, which shared only 59% identity withIAF-exp9l,was not revealed bythe Nprobe and that the signal obtained for the IAF-exp91 strain, from which the probe was derived,was verystrong TheWeybridge strain, for which sequencing data are not yet available, yielded the same result as the LV Nonetheless, a hybridization signal could be observed for the two European strains after a long-term exposure

Large amounts of RNA extracted from lung tissues are of cellular origin It has been previously shown (6) that excess amounts ofcellular RNAcould inhibitenzymaticamplification of viral genes byinterferingmainly in thereversetranscription step In our study, the sensitivity of the RT-PCR method did not seem to be compromised when PRRSV-infected lung tissues asopposed topartiallypurified viruswere used; in fact, PRRSV was still detectable in lungtissues by RT-PCR when virus concentration was

TCID5(Jml

Although, it has beenpreviously reported that PRRSV can be isolatedin cell cultures from serum and plasma up to to weeks after infection (3, 30, 37), our results should not be interpreted as showing a lack ofsensitivity of RT-PCR Indeed, in natural conditions, pigs are constantly in contact with the virus and reinfections are consequently frequent This con-trasts with the experimental conditions used in the present study Moreover, PRRSV does not seem to persist for long periods in lungs from naturally infected animals (16) None-theless, the use ofRT-PCR can be justified in many situations (i)Severallaboratories are unable to support continuous PAM preparations; moreover, it appears that the reliability of virus isolation depends on the macrophage batch, asdifferent levels

of sensitivity have been reported even in macrophages from individual animals of the same herd (data not shown) (ii) Amplification of virus-specific sequences doesnotrequire the presence of infectious virus (iii)Some isolates not display a cytopathic effect (iv) RT-PCR could be used for both detection and strain differentiation (v) Detectionof thevirus by this method canbe applied in pathogenesis studies

In summary, we haveshownthatRT-PCR is avaluable tool for the detection of PRRSV from infected PAM, even in the case of isolates which did not produce acytopathic effect By selecting strain-specificoligonucleotide primers, we were able todetect all theQuebecisolatesand todifferentiatethem from two Europeanstrains Such a result hasimportantimplications for subsequent epidemiological studies Preliminary results suggestedthat RT-PCRwas potentiallyatoolwhichcould be applied for the direct detection of PRRSV inclinical samples Nonetheless, it appeared from this preliminary study that optimization of themethodis still needed in order toachieve the objective of eliminating theuseof PAMcultures.Inview of the results reported here, it is advisable to use RT-PCR in combination withSouthern blotting to assure thereliability of the diagnosis forclinical specimensobtained from acute cases oralternatively tomake afirst passage inPAM cultures of the suspected pathological samples to enhance the subsequent RT-PCR assay

ACKNOWLEDGMENTS

We gratefully acknowledge Robert Bilodeau for providing speci-mens from theexperimentally infected SPF pigs and for his technical assistance in the preparation of PAM cultures The technical assistance of Joanne Roger and Nicole Sawyer was greatlyappreciated We also thank the following researchers for providing reference strains of PRRSV used in this study: Ronald Magar, Laboratoire d'Hygiene Vt6rinaire et Alimentaire (LHVA), Agriculture Canada, St-Hya-cinthe, Quebec, Canada; G Wensvoort,National Veterinary Labora-tory Center; and S Edwards, Central Veterinary Laboratory The collaboration of D A Benfield and E Nelson, South Dakota State University, in providing the monoclonal antibodies to the ATCC VR-2332 strain of PRRSV was greatly appreciated

This work was partly supported by the Conseil des Recherches en P&cheetAgro-alimentaire duQuebec (grant3765), la Federation des Producteurs de Porcs du Qu6bec, and Vetrepharm Research Inc., London, Ontario, Canada

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