Development of a heteroduplex mobility assay to identify field isolates of porcine reproductive and respiratory syndrome virus with nucleotide sequences closely related to those of modified live-attenuated vaccines

Transcriptome Analysis of Porcine Immune Cells Stimulated by Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) and Caesalpinia sappan Extract

The current level of knowledge on transcriptome responses triggered by Caesalpinia sappan (CS) extract in porcine peripheral blood mononuclear cells (PBMCs) after porcine reproductive and respiratory syndrome virus (PRRSV) infection is limited. Therefore, in the present study, we aimed to detect significant genes and pathways involved in CS extract supplementation responsiveness of PBMCs after PRRSV infection. RNA sequencing was conducted on PBMCs, which were isolated from six weaned piglets. The resultant transcriptional responses were examined by mRNA sequencing. Differential expression analysis identified 263 and 274 differentially expressed genes (DEGs) between the PRRSV and CTRL groups, and the PRRSV+CS and CTRL groups, respectively. Among these, ZNF646 and KAT5 emerged as the most promising candidate genes, potentially influencing the interaction between PRRSV-infected PBMCs and CS extract supplementation through the regulation of gene networks and cellular homeostasis during stress. Two pathways were detected to be associated with CS extract supplementation responsiveness: the cellular response to stress pathway and the NF-kB signaling pathway. Consequently, our study reveals a novel mechanism underlying cellular stress response and the NF-κB signaling pathway in PRRSV-infected PBMCs, and identifies a potential application of CS extract for activating the NF-κB signaling pathway. In conclusion, by supplementing CS extract in PBMC cells infected with PRRSV, we found that CS extract modulates PRRSV infection by inducing cellular stress, which is regulated by the NF-κB signaling pathway. This induced stress creates an adverse environment for PRRSV survival. This study contributes to a deeper understanding of the therapeutic targets and pathogenesis of PRRSV infection. Importantly, our results demonstrate that CS extract has the potential to be a candidate for modulating PRRSV infection.

Development of a bead-based assay for detection and differentiation of field strains and four vaccine strains of type 2 porcine reproductive and respiratory syndrome virus (PRRSV-2) in the USA

Porcine reproductive and respiratory syndrome (PRRS) remains one of the most economically devastating diseases in swine population in the United States of America. Due to high mutation rate of the PRRS virus (PRRSV) genome, it is difficult to develop an accurate diagnostic assay with high strain coverage. Differentiation of field strains from the four vaccines that have been used in the USA, namely Ingelvac PRRS MLV, Ingelvac ATP, Fostera PRRS and Prime Pac PRRS, adds an additional challenge. It is difficult to use current real-time PCR systems to detect and differentiate the field strains from the vaccine strains. Luminex xTAG technology allows us to detect more molecular targets in a single reaction with a cost similar to a single real-time PCR reaction. By analysing all available 678 type 2 PRRSV (PRRSV-2) complete genome sequences, including the 4 vaccine strains, two pairs of detection primers were designed targeting the conserved regions of ORF4-ORF7, with strain coverage of 98.8% (670/678) based on in silico analysis. The virus strains sharing ≥98% identity of the complete genomes with the vaccine strains were considered vaccine or vaccine-like strains. One pair of primers for each vaccine strain were designed targeting the nsp2 region. In silico analysis showed the assay matched 94.7% (54/57) of Ingelvac PRRS^® MLV (MLV) strain and the MLV-like strains, and 100% of the other three vaccine strains. Analytical sensitivity of the Luminex assay was one to two logs lower than that of the reverse transcription real-time PCR assay. Evaluated with 417 PRRSV-2 positive clinical samples, 95% were detected by the Luminex assay. Compared to ORF5 sequencing results, the Luminex assay detected 92.4% (73/79) of MLV strains, 78.3% (18/23) of Fostera strains and 50% (2/4) of ATP strains. None of the 472 samples were the Prime Pac strain tested by either ORF5 sequencing or the Luminex assay.

Hepatitis B core antigen-based vaccine demonstrates cross-neutralization against heterologous North American Porcine Reproductive and Respiratory Syndrome Virus (PRRSV-2) strains

The U.S. swine industry have been bearing the financial impact of Porcine Reproductive and Respiratory Syndrome (PRRS) for decades. Absent of a safe and efficacious vaccine to combat PRRS virus's genetic heterogeneity, it remains a costly disease on pig farms across the country. We have developed virus-like-particle (VLP) based vaccines that incorporate 4 PRRSV epitopes in the hepatitis B core antigen (HBcAg) backbone. Administration of the vaccines in female BALB/C mice resulted in extremely significant PRRSV epitope specific antibody response. One vaccine candidate GP3-4 was able to mount a significant viral neutralizing response against both parental PRRSV strain VR2385 and heterologous PRRSV strain NADC20, showing a promising potential for cross-protection against PRRSV.

Transcriptome of Porcine PBMCs over Two Generations Reveals Key Genes and Pathways Associated with Variable Antibody Responses post PRRSV Vaccination

Porcine reproductive and respiratory syndrome virus (PRRSV) is a virus susceptible to antibody dependent enhancement, causing reproductive failures in sows and preweaning mortality of piglets. Modified-live virus (MLV) vaccines are used to control PRRS in swine herds. However, immunized sows and piglets often generate variable antibody levels. This study aimed to detect significant genes and pathways involved in antibody responsiveness of pregnant sows and their offspring post-PRRSV vaccination. RNA sequencing was conducted on peripheral blood-mononuclear cells (PBMCs), which were isolated from pregnant sows and their piglets with high (HA), median (MA), and low (LA) PRRS antibody levels following vaccination. 401 differentially expressed genes (DEGs) were identified in three comparisons (HA versus MA, HA versus LA, and MA versus LA) of sow PBMCs. Two novel pathways (complement and coagulation cascade pathway; and epithelial cell signaling in H. pylori infection pathway) revealed by DEGs in HA versus LA and MA versus LA were involved in chemotactic and proinflammatory responses. TNF-α, CCL4, and NFKBIA genes displayed the same expression trends in subsequent generation post-PRRS-MLV vaccination. Findings of the study suggest that two pathways and TNF-α, CCL4, and NFKBIA could be considered as key pathways and potential candidate genes for PRRSV vaccine responsiveness, respectively.

Genetic diversity of the Korean field strains of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome (PRRS) is one of the most economically significant diseases in the swine industry. The PRRS virus (PRRSV) has genetically diverse populations, like other RNA viruses, and various field strains continue to be reported worldwide. The molecular epidemiological study of PRRSV can provide important data for use in controlling the disease. In this study, 50 oral fluid samples from conventional farms in Korea were taken to analyze nucleotide sequences of the open reading frame 5 of PRRSV. The viruses present in more than 80% of oral fluid samples genetically originated from the type 2 PRRSV, which is North American (NA) lineage. In addition 8.9% of samples contained both of the type 1 PRRSV, which is European (EU) lineage and the type 2 PRRSV. About 60% of farms involved in this study had more than two strains of PRRSV. In phylogenetic analysis, the Korean field strains of PRRSV detected from the oral fluid samples were divided into several subgroups: four subgroups of Korean field strains clustered with the type 1 PRRSV, and other five subgroups of Korean field strains clustered with the type 2. These results suggest that the type 2 PRRSV is more prevalent than the type 1 in Korea and heterologous strains of PRRSV can simultaneously infect a single pig farm.

Inhibition of porcine reproductive and respiratory syndrome virus by Cecropin D in vitro

Porcine reproductive and respiratory syndrome virus (PRRSV) continues to cause substantial economic losses to the pig industry worldwide. Although vaccines are commercially available for the control of PRRSV infection, no vaccination regimen has been proved sustained success in terms of generating a protective immune response. Therefore, the development of novel antivirals is urgently needed. Antimicrobial peptides display broad-spectrum antimicrobial activities against bacteria, fungi, and viruses and play an important role in host innate immune response. Here, we tested whether Cecropin D (CD) could inhibit PRRSV infection and replication in vitro. The inhibitory effect of CD occurred during viral attachment and the early period of viral entry into Marc-145 cells. CD also attenuated virus-induced apoptosis during the late phase of PRRSV infection and suppressed virus release in Marc-145 cells, which might contribute to the inhibition of PRRSV infection. Similar inhibitory effects on PRRSV infection were also found with CD treatment in porcine alveolar macrophages, the major target cell type of PRRSV infection in pigs in vivo. These findings suggest that CD has the potential to develop a new therapeutic agent against PRRSV infection.

Boosting in planta production of antigens derived from the porcine reproductive and respiratory syndrome virus (PRRSV) and subsequent evaluation of their immunogenicity

Porcine reproductive and respiratory syndrome (PRRS) is a disease of swine, caused by an arterivirus, the PRRS virus (PRRSV). This virus infects pigs worldwide and causes huge economic losses. Due to genetic drift, current vaccines are losing their power. Adaptable vaccines could provide a solution to this problem. This study aims at producing in planta a set of antigens derived from the PRRSV glycoproteins (GPs) to be included in a subunit vaccine. We selected the GP3, GP4 and GP5 and optimized these for production in an Arabidopsis seed platform by removing transmembrane domains (Tm) and/or adding stabilizing protein domains, such as the green fluorescent protein (GFP) and immunoglobulin (IgG) ‘Fragment crystallizable’ (Fc) chains. Accumulation of the GPs with and without Tm was low, reaching no more than 0.10% of total soluble protein (TSP) in homozygous seed. However, addition of stabilizing domains boosted accumulation up to a maximum of 2.74% of TSP when GFP was used, and albeit less effectively, also the Fc chains of the porcine IgG3 and murine IgG2a increased antigen accumulation, to 0.96% and 1.81% of TSP respectively, while the murine IgG3 Fc chain did not. Antigens with Tm were less susceptible to these manipulations to increase yield. All antigens were produced in the endoplasmic reticulum and accordingly, they carried high-mannose N-glycans. The immunogenicity of several of those antigens was assessed and we show that vaccination with purified antigens did elicit the production of antibodies with virus neutralizing activity in mice but not in pigs.

Molecular epidemiology of PRRSV in South China from 2007 to 2011 based on the genetic analysis of ORF5

Porcine reproductive and respiratory syndrome virus (PRRSV) has proven to be highly genetically variable; however, comprehensive information regarding the virus's genetic diversity in South China is limited. In this study, a total of 3199 clinical samples were collected from 267 pig farms suspected of PRRSV infection between 2007 and 2011. The ORF5 genes of 51 PRRSV-positive samples were sequenced and analyzed. The 51 study strains were divided into three primary subgenotypes. Fourty-five of the strains belonged to subgenotype I and were closely related to the highly pathogenic PRRSV (HP-PRRSV) strains. The subgenotype I strains were generally clustered into genetically similar groups by year. Only one of the strains belonged to subgenotype II, clustering with the classical North American type, VR2332. Five of the strains were grouped into subgenotype III, which occupied a separate branch and was closely related to the recently isolated novel field strains, QYYZ and GM2. The 5 subgenotype III strains shared an amino acid identity with the remaining 46 study strains ranging from 79.6%-83.6%. Amino acid analysis showed extensive mutations in subgenotype III; the diverse genetic mutations of these novel strains are of great concern.

Impact of genetic variation and geographic distribution of porcine reproductive and respiratory syndrome virus on infectivity and pig growth

Background

The porcine reproductive and respiratory syndrome (PRRS) is a devastating disease for the pig industry. In this study, we analysed the genetic variability of PRRS virus (PRRSV) as well as the relationship between the genetic variability, the geographical and temporal distribution of the PRRSV strains. Moreover, we investigated the association between the glycosylation patterns in PRRSV sequences and pigs growth.

Results

The data highlight that PRRSV strains evolve rapidly on individual farms, and temporal evolution of PRRSV is an important factor of genetic variability. Analysis of glycosylation sites in the glycoprotein 5 (GP5) ectodomain revealed that PRRSV isolates had seven combinations of putative N-linked glycosylation sites of which the N37/46/53 sites was found in 79% of the sequences. No significant relationship was found between the genetic variation of the PRRSV strains and the geographic distance. A significant relationship was found between the genetic variation and time of sampling when farm was considered as a factor in the analysis. Furthermore, the commercial semen from artificial insemination centres was not a source of PRRS transmission.

The PRRSV having the glycosylation site at position N46 (N46+) were observed to have higher burden on pigs and accordingly the corresponding infected pigs had lower average daily gain (ADG) compared with those infected with PRRSV lacking the glycosylation at N46 (N46-) position site. This study showed that the number of piglets by litter infected by PRRSV was lower for the Landrace breed than for the other studied breeds (Large White, Duroc and Pietrain).

Conclusions

The PRRSV genetic variability which is determined by a local and temporal evolution at the farm level could be considered in a perspective of prevention. Moreover, the association between the PRRSV glycosylation patterns and its virulence could be of interest for vaccine development. The differences of resistance to PRRSV infections among pig breeds might open new horizons for the genetic selection of robustness against PRRSV infection.

Porcine reproductive and respiratory syndrome virus vaccines: current status and strategies to a universal vaccine

Porcine reproductive and respiratory syndrome virus (PRRSV) is the causative agent of PRRS, the most significant infectious disease currently affecting swine industry worldwide. In the United States alone, the economic losses caused by PRRS amount to more than 560 million US dollars every year. Due to immune evasion strategies and the antigenic heterogeneity of the virus, current commercial PRRSV vaccines (killed-virus and modified-live vaccines) are of unsatisfactory efficacy, especially against heterologous infection. Continuous efforts have been devoted to develop better PRRSV vaccines. Experimental PRRSV vaccines, including live attenuated vaccines, recombinant vectors expressing PRRSV viral proteins, DNA vaccines and plant-made subunit vaccines, have been developed. However, the genetic and antigenic heterogeneity of the virus limits the value of almost all of the PRRSV vaccines tested. Developing a universal vaccine that can provide broad protection against circulating PRRSV strains has become a major challenge for current vaccine development. This paper reviews current status of PRRSV vaccine development and discusses strategies to develop a universal PRRSV vaccine.

Mucosal vaccines to prevent porcine reproductive and respiratory syndrome: a new perspective

Porcine reproductive and respiratory syndrome (PRRS) is an economically important infectious disease of swine. Constant emergence of variant strains of PRRS virus (PPRSV) and virus-mediated immune evasion followed by viral persistence result in increased incidence and recurrence of PRRS in swine herds. Current live and killed PRRSV vaccines administered by a parenteral route are ineffective in inducing complete protection. Thus, new approaches in design and delivery of PRRSV vaccines are needed to reduce the disease burden of the swine industry. Induction of an effective mucosal immunity to several respiratory pathogens by direct delivery of a vaccine to mucosal sites has proven to be effective in a mouse model. However, there are challenges in eliciting mucosal immunity to PRRS due to our limited understanding of safe and potent mucosal adjuvants, which could potentiate the mucosal immune response to PRRSV. The purpose of this review is to discuss methods for induction of protective mucosal immune responses in the respiratory tract of pigs. The manuscript also discusses how PRRSV modulates innate, adaptive and immunoregulatory responses at both mucosal and systemic sites of infected and/or vaccinated pigs. This information may help in the design of innovative mucosal vaccines to elicit superior cross-protective immunity against divergent field strains of PRRSV.

Genetic diversity of the ORF5 gene of porcine reproductive and respiratory syndrome virus isolates in southwest China from 2007 to 2009

To gain insight into the molecular epidemiology and possible mechanisms of genetic variation of porcine reproductive and respiratory syndrome (PRRS) in Yunnan Province of China, the ORF5 gene of 32 PRRSV isolates from clinical samples collected from 2007 to 2009 were sequenced and analyzed. Nucleotide and amino acid analyses were carried out on 32 isolates and representative strains of the North American genotype, European genotype and two representative Chinese isolates. Results revealed that these isolates share 86.9–99.0% nucleotide and 87.5–98.0% amino acid identity with VR-2332 the prototypical North American PRRSV, 61.7–62.9% and 54.3–57.8% with Lelystad virus (LV) the representative strain of European genotype, 91.2–95.4% and 90.0–94.5% with CH-1a that was isolated in mainland China in 1996, 88.1–99.3% and 85.5–99.0% with JX-A1 the representative strain of High pathogenic PRRSV in China, and 86.2–99.8% and 85.5–100.0% between isolated strains of different years, respectively. Phylogenetic analysis revealed that all 32 PRRSV isolates belonged to the North American genotype and were further divided into two different subgenotypes. Subgenotype 1 comprised twenty two Yunnan isolates which divided into two branches. Subgenotype 2 comprised ten isolates which closely related to the RespPRRS vaccine and its parent strain VR-2332. The functional domains of GP5 such as the signal peptide, ectodomain, transmembrane regions and endodomain were identified and some motifs in GP5 with known functions, such as primary neutralizing epitope (PNE) and decoy epitope were also further analyzed. Our study shown the great genetic diversity of PRRSV in southwest China, rendering the guide for control and prevention of this disease.

Immunogenicity study of plant-made oral subunit vaccine against porcine reproductive and respiratory syndrome virus (PRRSV)

Currently, killed-virus and modified-live PRRSV vaccines are used to control porcine reproductive and respiratory syndrome disease (PRRS). However, very limited efficacy of killed-virus vaccines and serious safety concerns for modified-live virus vaccines demand the development of novel PRRSV vaccines. In this report, we investigated the possibility of using transgenic plants as a cost-effective and scalable system for production and delivery of a viral protein as an oral subunit vaccine against PRRSV. Corn calli were genetically engineered to produce PRRSV viral envelope-associated M protein. Both serum and intestine mucosal antigen-specific antibodies were induced by oral administration of the transgenic plant tissues to mice. In addition, serum and mucosal antibodies showed virus neutralization activity. The neutralization antibody titers after the final boost reached 6.7 in serum and 3.7 in fecal extracts, respectively. A PRRSV-specific IFN-γ response was also detected in splenocytes of vaccinated animals. These results demonstrate that transgenic corn plants are an efficient subunit vaccine production and oral delivery system for generation of both systemic and mucosal immune responses against PRRSV.

Novel strategies and approaches to develop the next generation of vaccines against porcine reproductive and respiratory syndrome virus (PRRSV)

Porcine reproductive and respiratory syndrome virus (PRRSV) is an economically important swine pathogen. Since its discovery in the early 1990s, tremendous progresses have been made in understanding the molecular biology and pathogenesis of PRRSV. Although modified live-attenuated vaccines (MLVs) and inactivated vaccines against PRRSV have been available for more than a decade, the disease remains difficult to control. The efficacies of these vaccines especially against heterologous strains remain questionable: the MLVs were generally effective against homologous strains but variable in success against heterologous strains, and the outcomes of inactivated vaccines in the field are not very promising. With the development of PRRSV reverse genetics systems and the acquisition of new understanding on anti-PRRSV immunity, rational design of the next generation of PRRSV vaccines can now be explored. In this review, we discussed the recent advances in anti-PRRSV immunity and vaccinology, the recent progresses in PRRSV vaccine development particularly the reverse genetics system-based vaccine development, and provided a perspective on potential novel strategies and approaches that may be applicable to the development of the next generation of PRRSV vaccines.

Comparative infection efficiency of Porcine reproductive and respiratory syndrome virus field isolates on MA104 cells and porcine alveolar macrophages

Isolation of Porcine reproductive and respiratory syndrome virus (PRRSV) on MA104 or MARC-145 cells is frequently used in PRRS diagnosis. However, the ability of recent field isolates to grow on these established simian cell lines has not been determined. The aim of this study was to characterize the growth of PRRSV field isolates on primary porcine alveolar macrophages (PAMs) and MA104 cells in comparison with the growth of the laboratory-adapted strain VR-2332. A cytopathic effect was observed in 70% of serum samples after 1 passage on PAMs and was verified by immunofluorescent staining or reverse transcriptase-polymerase chain reaction. Field isolate growth was observed on MA104 cells for only 1 of 50 serum samples after 14 d. Strain VR-2332 grew readily in MA104 cells [maximum titer, 10(7) TCID(50) (median tissue culture infective dose) per milliliter at 30 h] but not in PAMs (10(2) TCID(50)/mL at 72 h). These results show that PAMs are superior to simian cells for diagnostic isolation of current field PRRSV strains.

Different biological characteristics of wild-type porcine reproductive and respiratory syndrome viruses and vaccine viruses and identification of the corresponding genetic determinants

Two attenuated vaccines, Ingelvac PRRS MLV and Ingelvac PRRS ATP, derived from VR2332 and JA142, respectively, have been used to control porcine reproductive and respiratory syndrome (PRRS) virus. However, there have been several field reports concerning the reversion of the vaccine virus to virulence. Furthermore, viruses genetically indistinguishable from the vaccines and wild-type parental viruses have been detected in clinical PRRS cases, raising the need for a better differential tool. As the vaccine viruses replicated better and produced bigger plaques in MARC-145 cells than did the wild-type parental strains, the following study was conducted to determine if the growth difference in MARC-145 cells can be utilized to differentiate a vaccine-like virus (VLV) from a wild-type virus and to identify genetic markers corresponding to such phenotype of the vaccine viruses. The relatedness of 83 field isolates collected between 1996 and 2005 to VR2332 and JA142 was classified genetically and antigenically. Thirteen of 25 VR2332-related viruses and 9 of 10 JA142-related viruses were determined as VLVs, since those viruses produced plaques similar to those by the vaccine viruses. Four unique amino acids each were identified throughout structural genes for MLV and ATP. Among those, F(10) in open reading frame 2 (ORF2) of MLV and E(85) and Y(165) in ORF3 of ATP were stable during pig passages. When the sequences unique for MLV were incorporated into an infectious clone constructed based on VR2332, the virus growth and resultant plaque size in MARC-145 cells were increased, suggesting that these sequences can be used as genetic markers for VLVs.

Direct inoculation of RNA transcripts from an infectious cDNA clone of porcine reproductive and respiratory syndrome virus (PRRSV) into the lymph nodes and tonsils of pigs initiates PRRSV infection in vivo

The recent construction of PRRSV infectious cDNA clones affords the opportunity for structural and functional studies of PRRSV genes. However, the inherent instability of the PRRSV genome, the requirement of cell culture propagation, and poor virus recovery have limited the usefulness of the PRRSV reverse genetics system for in vivo studies. Here, we report a unique strategy of infecting pigs by bypassing the traditional in vitro cell culture step required for in vivo studies. We demonstrate that inoculation of RNA transcripts of a PRRSV infectious cDNA clone directly into the lymph nodes and tonsils of pigs produces active PRRSV infection. The information from this study will have significant implications for the study of the molecular mechanism of PRRSV pathogenesis using the reverse genetics system.

Detection of Foot and Mouth Disease and Porcine Reproductive and Respiratory Syndrome Viral Genes Using Microarray Chip

Two viral pathogens, namely, porcine reproductive and respiratory syndrome virus (PRRSV) and foot and mouth disease virus (FMDV), were selected as models for multiple pathogen detection in a cDNA microarray. Two signature regions selected from ORF2 (around 500 bp) and ORF5 (around 600 bp) of PRRVS (America serotype), and one signature region from structural genes VP1 (around 500 bp) of FMDV type O were designed and spotted on a nylon membrane. For PCR sensitivity study, the cloned FMDV-VP1 template could be diluted to near one copy and its PCR product was still detectable in gel electrophoresis. In the microarray detection, the labelling FMDV probes (3 mg/ml) could be diluted 320 times and still maintained a visible colour when hybridized with the chip. Using the mixing primers, the microarray chip demonstrated rapid and accurate detection of the specific genes. To our knowledge, this preliminary study is the first example reported applying the long signature sequences to the multiple pathogen detection in cDNA microarray.

Instability of the restriction fragment length polymorphism pattern of open reading frame 5 of porcine reproductive and respiratory syndrome virus during sequential pig-to-pig passages

Restriction fragment length polymorphism (RFLP) analysis is one of the tools commonly used to study the molecular epidemiology of porcine reproductive and respiratory syndrome viruses (PRRSVs). As PRRSVs are genetically variable, the stability of the RFLP pattern of a PRRSV during in vivo replication was evaluated by carrying out 13 sequential pig-to-pig passages (P1 to P13) of PRRSV ATCC VR-2332 in three independent pig lines for a total of 727 days. During P1 the pigs were inoculated with a homogeneous inoculum (CC-01) prepared through a series of plaque purifications, and during P2 to P13 the pigs were inoculated with a tissue filtrate from the corresponding pig in the previous passage. Fifteen viral plaque clones were directly isolated from CC-01 and the day 7 serum of each pig of each passage, open reading frame 5 of the clones was sequenced, and the clones were compared to CC-01 to assess the mutation rates and RFLP patterns (obtained by digestion with MluI, HincII, and SacII) over time. Among the 495 viral clones recovered during the passages, 398 clones, including CC-01, had pattern 2-5-2 (MluI-HincII-SacII); however, the remaining 97 viral clones showed different patterns (2-6-2 [P2], 1-5-2 [P3], 2-5-4 [P7], and 2-1-2 [P10]). Importantly, the MluI site that was reported to be present in only one of the PRRS modified live virus vaccine strains (Ingelvac) and its parental strain (ATCC VR-2332) can disappear during in vivo replication. Furthermore, sequence homology between CC-01 and clones with pattern 2-5-2 or clones with other patterns differed by 0.05 to 1.58% and 0.5 to 1.45%, respectively, suggesting that RFLP analysis cannot accurately predict genetic relatedness between PRRSVs. Collectively, precaution should be taken when the molecular epidemiology of PRRSVs is evaluated by RFLP analysis.

A highly pathogenic porcine reproductive and respiratory syndrome virus generated from an infectious cDNA clone retains the in vivo virulence and transmissibility properties of the parental virus

The nucleotide sequence of a highly pathogenic porcine reproductive and respiratory syndrome virus (PRRSV) was determined. Transfection of MARC-145 cells with capped in vitro transcripts derived from a full-length cDNA clone of the viral genome resulted in infectious PRRSV with growth characteristics similar to that of the parental virus. Primer extension analysis revealed that during replication, the viral polymerase corrected the two nonviral guanosine residues present at the 5′ terminus of the transfected transcripts. Animal studies showed that the cloned virus induced hyperthermia, persistent viremia, and antibody response, similar to that observed with the parental virus. Contact transmission occurred rapidly within 3 days of introduction of naïve pigs into the group of clone virus-inoculated pigs. These results suggest that the cloned virus retains the in vivo virulence and contagion properties of the parental virus, thus, providing the background for reverse genetics manipulation in systematic examination of attenuation and virulence phenotypes.