the ter huurne strain ( cncm i - 1102 ) of prrsv was isolated in 1991 ( wensvoort et al ., 1991 ). the u . s . atcc - vr2332 strain was isolated by benfield et al . ( 1992 ). strain nl1 ( netherlands , 1991 ) was isolated in our lab , strain ny2 ( england , 1991 ) was kindly provided by t . drew , strain den ( denmark , 1992 ) was kindly provided by a . botner , strain lux ( luxemburg , 1992 ) was kindly provided by losch , spa1 and spa2 ( spain , 1992 ) were kindly provided by shokouhi and espuna , respectively , and strain fra ( france , 1992 ) was kindly provided by y . leforban . prrsv and vr2332 were grown on cl2621 cells as described previously ( van nieuwstadt et al , 1996 ). the seven different european isolates were grown in porcine alveolar macrophages . macrophages were maintained as described before ( wensvoort et al , 1991 ). bhk - 21 cells were maintained in dulbecco &# 39 ; s minimal essential medium supplemented with 5 % fetal bovine serum and antibiotics . for transfection experiments , bhk - 21 cells were grown in glasgow minimal essential medium ( gibco - brl / life technologies ltd ). porcine anti - prrsv serum 21 and rabbit anti - peptide sera 698 and 700 were used in previous experiments . serum 700 is directed against amino acids 106 to 122 ( clfyasemsekgfkvif ) encoded by orf4 of prrsv and was obtained from a rabbit . the production and characterization of mabs have been described ( van nieuwstadt et al , 1996 ). the hybridomas were derived from five consecutive fusion experiments and directed against orf 4 protein ( mab 121 . 4 , 122 . 1 , 122 . 12 , 122 . 20 , 122 . 29 , 122 . 30 , 122 . 59 , 122 . 66 , 122 . 68 , 122 . 70 , 122 . 71 , 126 . 1 , 126 . 7 , 130 . 7 , 138 . 28 ) or orf7 protein ( mab 122 . 17 , 125 . 1 , 126 . 9 , 126 . 15 , 130 . 2 , 130 . 4 , 131 . 7 , 138 . 22 , wbe1 , wbe4 , wbe5 , wbe6 , sdow17 ) mabs wbe were graciously provided by dr . drew , weybridge , uk ; mab sdow 17 was graciously provided by dr . benfield , south dakota , u . s . two oligonucleotides located upstream ( prrsv 13 ) and downstream ( prrsv14 ) of orf4 have been used earlier to amplify and clone orf4 of isolate i - 1102 in pgem - 4z using the barnhi and hindiii sites introduced in the primers ( meulenberg et al . 1995 ). the resulting plasmid was named pabv209 . two oligonucleotides located at a similar position with respect to the initiation codon ( prrsv4 ) and the termination codon ( prrsv5 ) of orf4 of vr2332 were used to amplify orf4 of vr2332 by means of rt - pcr as described in previous studies . the pcr fragment was digested with bamhi and partially with hindiii since orf4 of vr2332 contains an internal hindiii site , and cloned in pgem - 4z resulting in plasmid pabv270 . recombinant dna techniques were performed essentially as described by sambrook et al . ( molecular cloning , a laboratory manual , cold spring harbor lab , cold spring harbor , n . y ., 1989 ). the nucleotide sequence of vr2332 orf4 in pabv270 , determined on an automated dna sequencer ( applied biosystems ), was identical to the published sequence ( murtaugh et al ., arch . virol . 140 ; 1451 - 1460 , 1995 ). subsequently , orf4 of i - 1102 and vr2332 were transferred to semliki forest virus expression vector psfv1 . pabv209 and pabv270 were digested with bamhi and hindiii ( partially for pabv270 ), the orf4 fragments were treated with klenow polymerase ( pharmacia ) to create blunt ends and these were ligated in the smai site of psfv1 , dephosphorylated with calf intestinal alkaline phosphatase ( pharmacia ). plasmids containing orf4 of i - 1102 ( pabv265 ) and vr2332 ( pabv271 ) in the correct orientation were further tested for expression of the gp 4 protein . in addition , four different chimeric orf4 genes of i - 1102 and vr2332 were made . the nucleotide sequence of orf4 encoding amino acids 1 through 39 of the gp 4 protein of vr2332 was amplified from plasmid pabv270 with oligonucleotides prrsv4 and prrsv6 . the obtained fragment was digested with bamhi and sacii . this fragment was ligated in pabv209 digested with bamhi and sacii to create an in frame fusion between amino acids 1 through 39 of the gp 4 protein of vr2332 and 40 through 183 of the gp 4 protein of i - 1102 in pabv306 . the nucleotide sequence of orf4 encoding amino acids 1 through 75 of gp 4 of vr2332 was amplified with oligonucleotides prrsv4 and prrsv9 ( see table 2 ). this fragment was digested with kpni and bamhi . the nucleotide sequence of orf4 encoding amino acids 80 to 183 of the i - 1102 gp 4 protein were amplified with prrsv46 and prrsv14 and the amplified fragment was digested with kpni and bamhi . both fragments were ligated together in pgem - 4z digested with bamhi and hindiii , resulting in plasmid pabv308 . in the same way a complementary construct was created in pabv314 consisting of the nucleotide sequence encoding amino acids 1 through 79 of the i - 1102 gp4 protein amplified with prrsv 13 and prrsv57 ligated to a fragment encoding amino acids 76 through 178 of vr2332 , which was amplified with prrsv10 and prrsv5 , in pgem - 4z . a fourth chimeric construct consisted of a fragment encoding amino acids 40 through 79 of the prrsv gp 4 protein fused to fragments encoding amino acids 1 through 39 and amino acids 76 through 178 of the vr2332 gp 4 protein . this was achieved by ligating the bamhi / sacii orf4 fragment of pabv270 and the sacii / hindiii orf4 fragment of pabv314 in pgem - 4z digested with bamhi and hindiii . this plasmid was designated pabv325 . plasmids pabv306 , pabv308 , pabv314 , and pabv325 were checked for the correct sequence by oligonucleotide sequencing . the chimeric orf4 genes were transferred from pabv306 , pabv308 , pabv314 , and pabv325 to psfvi , identical as described above for the orf4 genes of vr2332 and prrsv , resulting in pabv296 , pabv305 , pabv321 , and pabv326 , respectively ( fig3 ). two oligonucleotides located upstream ( lv108 ; 5 ′ ggagtggttaacctcgtcaagtatggccggtaaaaaccagagcc 3 ′) ( seq id no . 21 ) and downstream ( lv112 ; 5 ′ ccattcacctgactgtttaattaacttgcaccctga 3 ′) ( seq id no . 22 ) of orf7 were used to amplify and clone the orf7 gene in pgem - t , resulting in pabv431 . the sequences and position of these and other oligonucleotides used to amplify fragments of orf7 are listed in table 1 . in addition , four different chimeric constructs were made by pcr - directed mutagenesis . the sequences coding for amino acids 25 - 26 , 28 - 30 ( site b ; fig3 ) were substituted for the corresponding sequences of the eav n protein . this was accomplished by pcr amplification of orf7 with lv108 and lv134 ( 5 ′ tggggaatggccagccagtcaatgacctgtgccggatgtttggtgcaatgataaa gtcc 3 ′) ( seq id no . 23 ). the mutated dna fragment was introduced in pabv431 using the msci and paci site , which resulted in pabv455 . the region of orf7 encoding amino acids 51 to 67 was substituted for the corresponding region of ldv orf7 . pabv431 was digested with econi and clai and ligated to a pcr fragment produced with primers lv98 ( 5 ′ ccagcaacctaggggaggacaggccaaaaagaaaaagcagccgaagctacattt tcccatggctggtccatctgac 3 ′) ( seq id no . 24 ) and lv99 ( 5 ′ cgtctggatcgattgcaagcagagggagcgttcagtctgggtgaggacgtgccgg aggtcagatggaccagcc 3 ′) ( seq id no . 25 ), digested with the same enzymes . this plasmid was designated pabv463 . the region of orf7 encoding amino acids 80 to 90 was substituted for the corresponding region of the ldv orf7 gene . the orf7 gene of lv was mutated in a pcr with primers lv 101 ( 5 ′ gcttgcaggcgcccgtgacgcttttcaatcaaggcggaggacaggcgtcgctttc atcca3 ′) ( seq id no . 26 ) and lv 112 . the obtained fragment was digested with nari and paci and ligated to pabv43 1 digested with clai and paci . this resulted in pabv453 . finally , the region encoding the c terminal part of the n protein ( amino acids 111 - 128 ) was replaced for a sequence encoding the corresponding amino acids of the n protein of ldv . the orf7 gene was amplified with primers lv 108 and lv 102 ( 5 ′ atgtcccgggctaagcggcggaggaattagcagaagcgttaatcaggcgctgtgt agcagcaaccggcag 3 ′) ( seq id no . 27 ) and cloned in the pgem - t vector , which resulted in pabv456 . the wild type and mutated orf7 genes were excised from pabv431 , pabv453 , pabv455 , and pabv463 by digestion with paci ( blunt ended ) and hpai and from pabv456 by digestion with hpai and swai . these genes were subsequently inserted in the dephosphorylated smai site of semliki forest virus expression vector psfv1 . plasmids pabv470 , pabv460 , pabv462 , pabv518 and pabv471 containing the respective orf7 genes in the correct orientation were further tested for expression of the n protein . in vitro transcription and transfection of semliki forest virus orf7 rna was as described above for the sfv - orf4 constructs . to clone the orf4 genes of seven different european isolates macrophages were infected with nl1 , ny2 , den , fra , spa1 , spa2 , and lux , and rna was isolated as described by meulenberg et al . ( 1993 ). the orf4 genes were amplified by means of rt - pcr with oligonucleotides prrsv 13 and prrsv 14 , and cloned with bamhi and hindiii in pgem - 4z . for each strain , the nucleotide sequence of orf4 of two clones derived from two independent pcrs was determined . the protein sequences derived from the nucleotide sequence were aligned using the multiple sequence alignment program clustal of pcgene ( intelligenetics tm ). psfv1 plasmids containing different orf4 constructs were linearized by digestion with spel and transcribed in vitro . the synthesized rna was transfected to bhk - 21 cells in 15 mm wells of twenty four - well plates using lipofectin . cells were fixed with ice - cold 50 % ( v / v ) methanol / aceton and the gp4 protein expressed by the different orf4 constructs was stained with mabs in the immunoperoxidase monolayer assay ( ipma ). to analyze the orf4 expression products by immunoprecipitation , 10 7 bh k - 21 cells were transfected with 10 mg in vitro transcribed sfv - orf4 rna by electroporation . the electroporated cells were plated in three 35 mm wells of six - well plates and 18 h after transfection cells were labeled . a complete set of overlapping nonapeptides or dodecapeptides was synthesized from amino acids derived of the orf4 or orf7 sequence of prrsv , as was determined previously ( meulenberg et al ., 1993 ). the synthesis of solid - phase peptides on polyethylene rods and immunoscreening with an enzyme - linked immunosorbent assay ( elisa ) type of analysis were carried out according to established pepscan procedures ( geysen et al ., pnas , 81 , 3998 - 4002 , 1984 ). we have previously described a panel of neutralizing mabs that reacted with a 31 to 35 kda protein of prrsv , designated gp4 , and a panel of mabs reactive with the n protein , by western immunoblot analysis . gp4 was shown to be a structural glycoprotein encoded by orf4 , n was shown to be the nucleocapsid protein encoded by orf7 . in immunoprecipitation experiments with gp 4 specific mabs , the gp 4 protein derived from lysates of cells infected with prrsv , migrated as a discrete band of 28 kda together with a light smear of somewhat higher apparent molecular weight . the mabs immunoprecipitated a diffuse ( glycosylated ) gp4 protein of about 31 kda from the extracellular medium of prrsv - infected but not from the extracellular medium of mock - infected cells . we have demonstrated earlier that mabs specific for the gp 4 protein recognized i - 1102 but not the u . s . isolate vr2332 ( van nieuwstadt et al ., 1996 ). in order to identify the binding domain of the neutralizing mabs in the gp 4 protein , we made fusion proteins of the gp 4 protein of i - 1102 and vr2332 . these proteins were expressed in the semliki forest virus expression system , developed by liljeström et al . ( biotechnol , 9 , 1356 - 1362 , 1991 ). first , orf4 of i - 1102 was cloned in psfv1 resulting in plasmid pabv265 ( fig1 ). rna transcribed from pabv265 was transfected to bhk - 21 cells and 24 h after transfection cells were positively stained with the panel of fifteen neutralizing mabs . the mabs did not react with bhk - 21 cells transfected with psfv1 - rna . the recombinant gp 4 protein was immunoprecipitated with mab 126 . 1 from l -[ 35 s ]- methionine labeled bhk - 21 cells transfected with pabv265 rna . it had a similar size as the authentic gp , protein synthesized in cl2621 cells infected with i - 1102 and also contained pngasef and endoh sensitive n - glycans . the gp 4 protein of vr2332 was also cloned in psfv1 , but this protein was not recognized by the mabs upon expression in bhk - 21 cells ( fig1 ). to further localize the region in the gp 4 protein recognized by the mabs , four chimeric genes of orf4 of i - 11022 and vr2332 were constructed in psfvi ( fig1 ). rna transcribed from plasmids pabv296 , pabv305 , pabv321 , and pabv326 was transfected to bhk - 21 cells and the reactivity of the expressed proteins with the gp 4 - specific mabs was tested in ipma . the reaction pattern of these fifteen mabs was identical , and indicated that these mabs were directed to a region of 40 amino acids in the gp 4 protein ; the expression product of pabv326 , consisting of amino acids 40 through 79 derived from the gp4 protein of isolate cncm i - 1102 and surrounded by sequences derived from the vr2332 gp4 protein was still recognized by the panel of mabs . to ensure that the different gp4 proteins , especially those which were not recognized by the mabs , were properly expressed in bhk21 cells , they were immunoprecipitated from lysates of bhk - 21 cells that were transfected with rna transcribed in vitro from plasmids pabv265 , pabv271 , pabv296 , pabv305 , pabv321 , and pabv326 . immunoprecipitation was carried out with porcine anti - prrsv serum 21 , mab 126 . 1 , and anti - peptide sera 698 and 700 . serum 700 is directed against amino acids 106 - 122 of the prrsv gp4 protein of isolate cncn i - 1102 , a sequence which is identical in the gp4 protein of isolate atcc - vr2332 , apart from amino acid 121 . therefore all gp4 proteins were immunoprecipitated with serum 700 . they were indistinguishable in size , when analyzed by sds - page , except for the gp4 proteins expressed by pabv305 and pabv271 , which migrated slightly faster . this is most likely due to the deletion of 4 amino acids in the vr2332 sequence relative to the i - 1102 sequence , between amino acids 62 - 64 ( fig3 ). the complete set of gp4 - specific mabs recognized the gp4 proteins expressed from pabv265 , pabv296 , pabv321 , pabv326 , but not those expressed from pabv305 and pabv27 1 , which confirmed the results obtained by ipma ( fig3 ). serum 698 had the same reaction profile as the mabs . serum 698 is directed against amino acids 62 to 77 of gp4 of prrsv , which are located within the now identified neutralization domain of the gp4 protein . this region is highly heterogeneous in vr2332 orf4 , and therefore the expression products containing the vr2332 sequence in this region were not recognized by this serum . however , neutralizing polyclonal pig sera recognize the i - 1102 gp4 protein and the chimeric gp4 proteins and the vr2332 gp4 protein , indicating that in porcine anti - prrsv sera a variety of neutralizing antibodies that are directed against the neutralizing site formed by amino acids 40 to 79 of the gp 4 protein are present . since the fifteen mabs reactive with the orf4 protein all reacted with the gp4 protein in western blot analysis , they were expected to recognize a linear epitope in a region spanning amino acids 40 through 79 of gp4 of isolate i - 1102 . to further map the binding region of the mabs , a pepscan analysis was performed using overlapping nonapeptides oe dodecapeptides in this region . peptides were considered to represent antigenic sites if peaks in such a set reproducibly amounted to more than twice the background . mabs 122 - 29 , 122 - 30 , 122 - 66 , 122 - 71 , 130 - 7 , 138 - 28 reacted positively with one specific antigenic site consisting of amino acids 59 through 67 ( saaqekisf ) ( seq id no . 28 ) ( fig2 ). mab 122 - 12 reacted only weakly to this antigenic site , whereas the remaining 7 mabs were negative in the pepscan analysis . polyclonal pig sera also identified this site in pepscan . neutralizing serum 21 , taken at week 6 after infection of pig 21 with prrsv reacted strongly and broadly with the site and its flanking regions . in addition , neutralizing polyclonal pig sera ( va12 and va14 ), taken at 54 days after vaccination with prv - orf4 vector virus and at slaughter at 30 days after challenge at day 54 with prrsv , reacted strongly and more broadly with the neutralization site identified in pepscan . in isolate i - 1102 , the core sequence of the neutralization site comprises the aa sequence saaqekisf ( seq id no . 29 ) located from aa position 59 - 67 . in other isolates the core sequence can be found at or around the corresponding aa position , which is an amino acid sequence corresponding to a neutralization site of protein gp 4 , comprising for example sequences such as saaqeeisf ( seq id no . 30 ), or staqenisf ( seq id no . 31 ) or staqenipf ( seq id no . 32 ) or seesqsvt ( seq id no . 33 ) or saseair ( seq id no . 34 ) or saseafr ( seq id no . 35 ) or papeafr ( seq id no . 36 ) or papeair ( seq id no . 37 ) or safetfr ( seq id no . 38 ) or stseafr ( seq id no . 39 ), but it is to be expected that other isolates of prrsv have corresponding but slightly differing core sequences of the neutralization site located at or around the aa position corresponding to aa 59 - 67 of the orf 4 amino acid sequence of the i - 1102 isolate of prrsv . also , artificial changes that maintain the antigenicity and thus functionality of the above core sequences can easily be introduced by the average expert skilled in the art of peptide design and synthesis . also , as is clearly demonstrated by the much broader reactivity in pepscan of the neutralizing polyclonal sera , aa sequences comprising aa core sequences and aa sequences flanking the core sequences of the various prrsv isolates in addition constitute the neutralization site on the orf4 protein of prrsv . especially sequences located at positions corresponding to about aa 40 to 79 constitute the neutralization site ( fig1 ). again , artificial changes that maintain the antigenicity and thus functionality of the above antigenic sites can easily be introduced by the average expert skilled in the art of peptide design and synthesis . also , considering the broad reactivity of the polyclonal neutralizing sera va12 and va14 ( fig2 ), aa sequences located at positions corresponding from about aa 52 to 75 more specifically constitute a broadly reactive neutralization site . the mabs directed against the orf7 protein reacted in four different groups in pepscan , group a ( 4 ), b ( 2 ), c ( 3 ) and d ( 1 ). group 1 ( d ) ( in which among others mabs 122 . 17 , 130 . 3 , 130 . 4 , 131 . 7 , wbe1 , wbe4 , wbe6 , sdow17 and comprising conserved and non - conserved reactive sites ) reacted with a conformational epitope not detectable in pepscan . group 2 ( b ) ( in which among others 125 . 1 , 126 . 9 , ns95 and ns99 and reactive with all isolates of prrsv tested , thus identifying a conserved antigenic site ) identifies a core sequence vnqlcqllga ( seq id no . 1 ) ( found in isolate i - 1102 from aa position 22 to about 32 ) or vnqlcqmlgk ( seq id no . 2 ). group 3 ( c ) ( in which among others mab 126 . 15 and mainly reactive with strains of prrsv isolated in europe , thus identifying a differentiating antigenic site ) identifies a core sequence prggqakkkk ( seq id no . 3 ) ( found in isolate i - 1102 from aa position 41 to about 50 ) or prggqakrkk ( seq id no . 4 ) or prggqakkrk ( seq id no . 5 ) or gpgkknkkkn ( seq id no . 6 ) or gpgkknkkkt ( seq id no . 7 ) or gpgkknrkkn ( seq id no . 8 ) or gpgkkfkkkn ( seq id no . 9 ) or gpgkkikkkn ( seq id no . 10 ) or gpgqinkkin ( seq id no . 11 ). group 4 ( a ) ( in which among others mab 138 . 22 and mainly reactive with strains of prrsv isolated in europe , thus identifying a differentiating antigenic site ) identifies a core sequence magknqsqkk ( seq id no . 12 ) ( found in isolate i - 1102 from aa position 1 to about 10 ) or mpnnngkqte ( seq id no . 13 ) or mpnnngkqpk ( seq id no . 14 ) or mpnnngkqqk ( seq id no . 15 ) or mpnnngkqqn ( seq id no . 16 ) or mpnnngkqqk ( seq id no . 17 ). also , artificial changes that maintain the antigenicity and thus functionality of the above antigenic sites in the n protein can easily be introduced by the average expert skilled in the art of peptide design and synthesis . although group 1 does not constitute linear epitopes , comparison of prrsv aa sequences with ldv sequences shows that conformational epitopes ( which vary greatly among the various isolates ) can be found at positions corresponding to those found in isolate i - 1102 from aa position 51 to about 68 ( in isolate i - 1102 aa sequence pkphfplaaeddirhhl ) ( seq id no . 18 ) or from 79 to about 90 ( in isolate i - 1102 aa sequence siqtafnqgagt ) ( seq id no . 19 ) or from 111 to 124 ( in isolate i - 1102 aa sequence htvrlirvtstsas ) ( seq id no . 20 ). also , artificial changes that maintain the antigenicity and thus functionality of the above conformative epitope sites in the n protein can easily be introduced by the average expert skilled in the art of peptide design and synthesis , especially with information gathered by sequence comparison of prrsv isolates , and by comparison with n protein sequences of other arteriviridae . this was determined in expressing chimeric ldv / prrsv orf7 proteins in the sfv expression system ( done as above for orf4 ) and determining their reactivity with mabs from group 1 . domain d was further mapped with constructs of orf7 expressing chimeric n proteins . since 6 out of 10 mabs directed against domain d recognized both european and north american isolates of prrsv , the regions which were most conserved between the n protein of lv and the north american prototype vr2332 ( fig4 ) were mutated . the nucleotide sequence coding for amino acids 51 to 67 , 80 to 90 , and 111 to 128 was substituted for a sequence that codes for the corresponding amino acids of ldv ( fig4 ). for completion , site b ( amino acids 25 - 30 ) that is also conserved in european and north american isolates , was mutated . since the amino acid sequence of the lv n protein was very similar to that of the ldv n protein in site b , this region of the lv n protein was substituted for a region encoding the corresponding amino acids of the eav n protein ( fig4 ). when the mutated and wild type n proteins were expressed in bhk - 21 cells using the semliki forest virus expression system , and they were tested with the n - specific mabs in ipma , the d - specific mabs reacted identical ( table 1 ). their binding was disrupted by mutations between amino acids 51 - 67 and 80 - 90 , but not by mutations between amino acids 111 - 128 or amino acids 25 - 30 ( site b ). as was expected , the n proteins with ldv sequences between amino acids 51 - 67 and 80 - 90 were still stained by mabs directed against sites a , b , and c . however , the number of cells that were stained and the brightness of this staining was less than that observed for the wild type n protein and the n proteins mutated in amino acids 25 - 30 ( site b ) or in amino acids 111 - 128 ( table 1 ). this was most likely due to a lower expression of the n proteins containing mutations between amino acids 51 - 67 or 80 - 90 , since a lower yield of these mutant n proteins compared to the other n proteins was also obtained when equal amounts of transcripts were translated in vitro ( data not shown ). as was expected , the n protein that contained eav sequences in site b was not recognized by mabs mapped to site b ( by pepscan analysis ), but was still recognized by mabs that mapped to sites a , c , or domain d . these data indicate that the epitopes mapped to domain d are conformation - dependent and consist ( partially ) of amino acids 51 - 67 and 80 - 90 . sequence analysis of the gp 4 protein of different prrsv strains . to analyze whether the major antigenic neutralization site , recognized by the gp 4 - specific antibodies , was conserved among different prrsv isolates , the reactivity and neutralizing activity of the mabs was further tested on seven different european strains . the results indicated that these mabs recognized and neutralized another dutch strain nl1 and an english strain ny , but not danish isolate den , two spanish strains spa1 and spa2 , a french isolate fra , and lux isolated in luxembourg . therefore we were interested in the amino acid sequence , in the region of the neutralization site of the gp 4 protein of these isolates . the orf4 genes were cloned by means of rt - pcr using primers derived from the prrsv sequence and the nucleotide sequence was determined . the amino acid sequence of the gp 4 protein of the different isolates derived from this nucleotide sequence were 86 to 97 % identical with that of i - 1102 . the alignment of these amino acid sequences showed that the neutralization site ( amino acids 40 through 79 ) is much more divergent than the remaining part of the protein . in this region , especially the amino acid sequences of strains dan , spa1 , spa2 and fra are different . this is in line with the finding that these strains are not neutralized by the i - 1102 specific mabs and further confirms that this site is not highly conserved among european isolates . another region of higher heterogeneity was observed in the n - terminal part of the gp 4 protein . comparison of the amino acid sequence of the prrsv gp 4 protein and that of vr2332 and other north american strains shows that the latter are also heterogenous in the neutralization site of the protein . alignment of the amino acid sequences results in the introduction of a gap in the neutralization site of the north american isolates ( fig3 ), which is in agreement with the observation that none of these isolates are recognized by the mabs . overall , a higher diversity was observed among the sequences of the american isolates than among the sequences of the european isolates . this is in line with the features characteristic for typical viral envelope , identified e . g . in the amino acid sequence of gp 4 . the potential of the neutralization site for vaccine development is of great importance in view of the heterogenicity of the neutralization site . comparison of the amino acid sequence of the gp 4 proteins of different european strains indicated that the neutralization site was much more variable than other parts of the protein , suggesting that this site is susceptible to immunoselection . comparison of the neutralization site sequences of european and north american strains displayed a gap of 4 amino acids in the north american sequences with respect to the european , further illustrating the large amino acid variability of the now identified neutralization site of prrsv . the neutralization site in the gp 4 protein described here , is the first site identified for lelystad virus . for two other arteriviruses , eav and ldv , the neutralizing mabs that were isolated , were all directed against the g 1 / vp3 protein encoded by orf5 ( deregt et al , 1994 ; glaser et al , 1995 ; balasuriya et al , 1995 ; harty and plagemann , 1988 ). using neutralization - escape mutants , the neutralization site of eav was mapped to specific amino acids in the ectodomain of g 1 . similar sequence comparisons were done for the orf7 protein of prrsv ( fig4 ) further illustrating the large amino acid variability of the now identified antigenically conserved site and non - conserved sites of prrsv . in this work we have identified four distinct antigenic sites in the n protein of prrsv . three sites , designated a , b , and c contain linear epitopes and these were mapped between amino acids 2 - 12 , 25 - 30 , and 40 - 46 , respectively . in contrast , the fourth site , designated domain d , contains conformation - dependent epitopes that are ( partially ) composed of amino acids 51 - 67 and 80 - 90 . sites a and c contain epitopes that are conserved in european but not in north american isolates of prrsv , site b contains epitopes that are conserved in european and north american isolates of prrsv , whereas site d contains both epitopes that are conserved and not conserved in european and north american isolates of prrsv . the conserved sites in the n protein described here , are of great importance in the development of diagnostic tests aimed at unequivocal diagnosis of prrsv infections , these tests should avoid employing non - conserved sites thereby avoiding false - negative results . in addition , knowledge about the various non - conserved sites is highly valuable in the development of differentiating tests that can e . g . discriminate vaccinated pigs from pigs infected with wild type isolates of prrsv . table 2 . sequence of primers used in pcr to clone the orf4 genes of lv and vr2332 and chimeric orf - 4 genes in plasmid vectors pgem - 4z and psfv1 5 ′ ggcaatt gg a tcc atttgga 3 ′ ( seq id no . 40 ) 5 ′ gccgtc ggtac c cc t cagtacat 3 ′ ( seq id no . 42 ) 5 ′ atgtactg a gg g gtacc gacggc 3 ′ ( seq id no . 43 ) 5 ′ ggcaatt gg a t c c acctagaatggc 3 ′ ( seq id no . 44 ) 5 ′ gcgatagg t ac c cc t gtgtatgttaccat 3 ′ ( seq id no . 48 ) a the underlined nucleotides in these primers are mutated with respect to the original sequence to create restriction sites or overhanging sequences or to avoid long stretches of one particular nucleotide . the restriction sites in the primers are shown in italics .
the invention provides antigenic sites of prrsv isolates . the antigenic sites are neutralizing , conserved , non - conserved and conformational , can elicit antibodies and are found on protein gp4 and n encoded orf4 and orf7 of prrsv . the peptide sequences identified by the sites can be incorporated in vaccines directed against prrs and in diagnostic tests for prrs . also , discriminating tests can be developed that can be used next to marker vaccines in programs designed to eradicate prrs from pig herds .