Efficacy of Type 2 PRRSV vaccine against Chinese and Vietnamese HP-PRRSV challenge in pigs

Progress and persistence of diseases of high consequence to livestock in the United States

The USDA/ARS-National Disease Center (NADC) will celebrate its 65th anniversary of existence in November 2026. NADC continues as one of the world's premier animal health research centers conducting basic and applied research on endemic diseases with economic impact on U.S. livestock and wildlife. This research center also supports a program studying important food safety pathogens such as Salmonella, E. coli and Campylobacter. NADC has contributed significantly to the elimination of a few diseases, notably hog cholera and milk fever, and made progress in reducing the impact of many other animal diseases through vaccines, therapies and managerial recommendations. Despite nearly 65 years of targeted research on these diseases and much progress, some of these continue to persist. The reasons for such persistence varies for each disease condition and they are often multifactorial involving host susceptibility, virulence and even environmental conditions. Individually and in aggregate, these disease conditions have a massive economic impact and can be devasting to animal producers, owners and individuals that become infected through zoonotic disease agents such as tuberculosis, leptospirosis and avian influenza. They also diminish the health, well-being and welfare of affected animals, which directly affects the food supply. The NADC is using all available technologies including genomic, biochemical, reverse genetics, and vaccine trials in the target host to combat these significant diseases. We review the progress and reasons for persistence of selected diseases and food safety pathogens as well as the progress and potential outcomes should research and programmatic plans to eliminate these disease conditions cease.

Field efficacy of a novel porcine reproductive and respiratory syndrome modified-live virus vaccine with an emphasis on growth performance

BACKGROUND

This field evaluation was designed to evaluate the efficacy of a new porcine reproductive and respiratory syndrome virus-2 (PRRSV-2) modified live virus vaccine at three independent pig farms.

METHODS

Three farms were selected for this study based on their respiratory disease status caused by PRRSV-2 infection in post-weaning and growing pigs. Each farm housed a total of 40, 18-day-old pigs that were randomly allocated to one of two treatment groups. Pigs were administered a 1.0 mL dose of the bivalent vaccine intramuscularly at 21 days of age in accordance with the manufacturer's recommendations, whereas unvaccinated pigs were administered a single dose of phosphate buffered saline at the same age.

RESULTS

Vaccinated groups were measured and calculated significantly (p < 0.05) higher in body weight and average daily weight gain on all three farms compared with unvaccinated groups. Vaccinated groups elicited PRRS antibodies and PRRSV-2-specific interferon-γ secreting cells, which reduced the amount of PRRSV-2 genomic copies in the blood and reduced macroscopic and microscopic lung lesions severity when compared with unvaccinated groups.

CONCLUSIONS

The field evaluation data demonstrated that a new PRRSV-2 modified live virus vaccine was efficacious in swine herds suffering from respiratory diseases caused by PRRSV-2 infection.

In vitro and in vivo antiviral effects of CLEVir-X against porcine reproductive and respiratory syndrome virus

The aim of this study was to investigate the in vitro and in vivo antiviral effects of CLEVir-X, against porcine reproductive and respiratory syndrome virus (PRRSV). CLEVir-X is a nucleoside analogue and a dialdehyde form of xanthosine. CLEVir-X demonstrated antiviral action during the in vitro portion of this experiment with its inosine monophosphate dehydrogenase (IMPDH) inhibition against PRRSV. The anti-PRRSV effect of CLEVir-X was recovered through supplementation with guanosine. This suggests that PRRSV replication may be regulated through IMPDH and its guanosine biosynthetic pathway. CLEVir-X treatment in cultures resulted in mutation frequency increase of up to 7.8-fold within the viral genomes (e.g. ORF6) compared to their parallel, untreated cultures. The incorporation of CLEVir-X into the viral genome causes lethal mutagenesis and subsequent decrease in specific infectivity. During the in vivo antiviral experiment, 21-day-old pigs began oral administration of 5 mL of phosphate buffered saline containing CLEVir-X (with purity of 68 % and dosage of 40 mg/kg body weight). This treatment was provided twice daily at 9:00AM and 5:00PM for 14 days. Pigs were simultaneously intranasally inoculated with PRRSV at the beginning of CLEVir-X treatment (21 days of age). Several beneficial effects from the oral administration of CLEVir-X were observed including reduction of body temperature, alleviation of respiratory clinical signs, decreased PRRSV load in both blood and lung tissues, and mitigation of lung interstitial pneumonia lesions. The results of the present study demonstrated that CLEVir-X has mutagenic and nonmutagenic modes of antiviral action against PRRSV based on both in vitro and in vivo antiviral experiments.

Protective evaluation of the commercialized porcine reproductive and respiratory syndrome virus vaccines in piglets challenged by NADC34-like strain

In China, the porcine reproductive and respiratory syndrome virus (PRRSV) has undergone several variations over the decades and contributed to the diversity of the clinical epidemic PRRSV strains. This has complicated the prevention and control of PRRS. In particular, the efficacy of the currently available commercial vaccines against the highly pathogenic NADC34-like strains is unclear. Therefore, the objective of this study was to evaluate the protection efficacy of three commercial PRRS modified-live virus (MLV) vaccines derived from classical PRRS VR2332 MLV and R98 MLV against challenge with a heterologous NADC34-like PRRSV strain, JS2021NADC34, which has high pathogenicity in pigs. PRRSV- and antibody-free piglets were immunized with the PRRS VR2332 MLV vaccine or either of two R98 MLV vaccines (from different manufacturers) and were challenged with the JS2021NADC34 strain 28 days after immunization. Rectal temperature, clinical symptoms, viremia and viral shedding from the nose, gross lesions in the thymus and lungs, microscopic lesions and viral distribution in the lungs, as well as the humoral immune response and mortality rates were recorded over a 14-day post-challenge period. The results showed that PRRS VR2332 MLV had better efficacy against the JS2021NADC34 challenge than PRRS R98 MLV, with vaccinated piglets in the former group showing transient and mild symptoms, mild pathological lesions in the lungs, mild thymic atrophy, and low viral levels in sera and nasal swabs, as well as better growth performance and a 100% survival rate. In contrast, two PRRS R98 MLVs exhibited limited efficacy against the JS2021NADC34 challenge, with the piglets in two R98 groups showing obvious clinical symptoms and pathological changes in the lungs and thymus; moreover, there were two deaths caused by PRRS in two R98 groups, respectively. Despite this, the mortality rate was lower than that of the unvaccinated piglets that were challenged with JS2021NADC34. The cumulative results demonstrate that PRRS VR2332 MLV was partly effective against the highly pathogenic PRRSV NADC34-like strain based on the observations over the 14-day post-challenge period. Thus, it might be a viable option among the commercially available vaccines for control of NADC34-like virus infections in swine herds.

Porcine reproductive and respiratory syndrome virus upregulates SMPDL3B to promote viral replication by modulating lipid metabolism

Porcine reproductive and respiratory syndrome virus (PRRSV) poses a severe threat to the health of pigs globally. Host factors play a critical role in PRRSV replication. Using PRRSV as a model for genome-scale CRISPR knockout (KO) screening, we identified a host factor critical to PRRSV infection: sphingomyelin phosphodiesterase acid-like 3B (SMPDL3B). Our findings show that SMPDL3B restricted PRRSV attachment, entry, replication, and secretion and that its depletion significantly inhibited PRRSV proliferation, indicating that SMPDL3B plays a positive role in PRRSV replication. Our data also show that SMPDL3B deficiency resulted in an accumulation of intracellular lipid droplets (LDs). The expression level of key genes (ACC, SCD-1, and FASN) involved in lipogenesis was increased, whereas the fundamental lipolysis gene, ATGL, was inhibited when SMPDL3B was knocked down. Overall, our findings suggest that SMPDL3B deficiency can effectively inhibit viral infection through the modulation of lipid metabolism.

Codon Pair Deoptimization (CPD)-Attenuated PRRSV-1 Vaccination Exhibit Immunity to Virulent PRRSV Challenge in Pigs

Commercially used porcine respiratory and reproductive syndrome (PRRS) modified live virus (MLV) vaccines provide limited protection with heterologous viruses, can revert back to a virulent form and they tend to recombine with circulating wild-type strains. Codon pair deoptimization (CPD) is an advanced method to attenuate a virus that overcomes the disadvantages of MLV vaccines and is effective in various virus vaccine models. The CPD vaccine against PRRSV-2 was successfully tested in our previous study. The co-existence of PRRSV-1 and -2 in the same herd demands protective immunity against both viruses. In this study, live attenuated PRRSV-1 was constructed by recoding 22 base pairs in the ORF7 gene of the E38 strain. The efficacy and safety of the CPD live attenuated vaccine E38-ORF7 CPD to protect against virulent PRRSV-1 were evaluated. Viral load, and respiratory and lung lesion scores were significantly reduced in animals vaccinated with E38-ORF7 CPD. Vaccinated animals were seropositive by 14 days post-vaccination with an increased level of interferon-γ secreting cells. In conclusion, the codon-pair-deoptimized vaccine was easily attenuated and displayed protective immunity against virulent heterologous PRRSV-1.

Genome-Wide Characterization of QYYZ-Like PRRSV During 2018–2021

In the last decade, the emergence of QYYZ-like porcine reproductive and respiratory syndrome virus (PRRSV) has attracted increasing attention due to the high incidence of PRRSV mutation and recombination. However, the endemic status and genomic characteristics of the QYYZ-like strains are unclear. From 2018 to October 2021, 24 QYYZ-like PRRSV isolates were obtained from 787 PRRSV-positive clinical samples. Only one QYYZ-like positive sample was from a northern province, and the rest were from central and southern provinces. We selected 9 samples for whole-genome sequencing, revealing genome lengths of 15,008–15,316 nt. We retrieved all the available whole-genome sequences of QYYZ-like PRRSVs isolated in China from 2010 to 2021 (n = 28) from GenBank and analyzed them together with the new whole-genome sequences (n = 9). Phylogenetic tree analysis based on the ORF5 gene showed that all QYYZ-like PRRSV strains belonged to sublineage 3.5 but were clustered into three lineages (sublineage 1.8, sublineage 3.5, and sublineage 8.7) based on whole-genome sequences. Genomic sequence alignment showed that QYYZ-like strains, have characteristic amino acids insertions or deletions in the Nsp2 region (same as NADC30, JXA1 and QYYZ) and that thirteen strains also had additional amino acid deletions, mostly between 468 and 518 aa. Moreover, QYYZ-like strains (sublineage 3.5) have seven identical characteristic amino acid mutations in ORF5. Recombination analysis revealed that almost all QYYZ-like complete genome sequences (36/37) were products of recombination and mainly provided structural protein fragments (GP2-N) for the recombinant strains. Overall, QYYZ-like strains were mainly prevalent in central and southern China from 2018 to 2021, and these strains provided recombinant fragments in the PRRSV epidemic in China.

Host Cells Actively Resist Porcine Reproductive and Respiratory Syndrome Virus Infection via the IRF8-MicroRNA-10a-SRP14 Regulatory Pathway

MicroRNAs (miRNAs) play an important role in the virus-host interaction. Our previous work has indicated that the expression level of miR-10a increased in porcine alveolar macrophages (PAMs) during porcine reproductive and respiratory syndrome virus (PRRSV) infection and further inhibited viral replication through downregulates the expression of host molecule signal-recognition particle 14 (SRP14) protein. However, the molecular mechanism of miR-10a increased after PRRSV infection remains unknown. In the present study, transcription factor interferon regulatory factor 8 (IRF8) was identified as a negative regulator of miR-10a. PRRSV infection decreases the expression level of IRF8 in PAMs, leading to upregulating miR-10a expression to play an anti-PRRSV role. Meanwhile, this work first proved that IRF8 promoted PRRSV replication in an miR-10a-dependent manner. Further, we explained that SRP14, the target gene of miR-10a, promotes the synthesis of the PRRSV genome by interacting with the viral components Nsp2, thus facilitating PRRSV replication. In conclusion, we identified a novel IRF8-miR-10a-SRP14 regulatory pathway against PRRSV infection, which provides new insights into virus-host interactions and suggests potential new antiviral strategies to control PRRSV. IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) has rapidly spread to the global pig industry and caused incalculable economic damage since first discovered in the 1980s. However, conventional vaccines do not provide satisfactory protection. Understanding the molecular mechanisms of host resistance to PRRSV infection is necessary to develop safe and effective strategies to control PRRSV. During viral infection, miRNAs play vital roles in regulating the expression of viral or host genes at the posttranscriptional level. The significance of our study is that we revealed the transcriptional regulation mechanism of the antiviral molecule miR-10a after PRRSV infection. Moreover, our research also explained the mechanism of host molecule SRP14, the target gene of miR-10a regulating PRRSV replication. Thus, we report a novel regulatory pathway of IRF8-miR-10a-SRP14 against PRRSV infection, which provides new insights into virus-host interactions and suggests potential new control measures for future PRRSV outbreaks.

Efficacy of the Synergy Between Live-Attenuated and Inactivated PRRSV Vaccines Against a NADC30-Like Strain of Porcine Reproductive and Respiratory Syndrome Virus in 4-Week Piglets

The NADC30-like strain of porcine reproductive and respiratory syndrome virus (PRRSV) is a novel strain responsible for substantial economic losses to swine production in China. This study evaluated the cross-protective efficacy of the synergy between live-attenuated and inactivated PRRSV vaccines compared with a single vaccination with PRRS modified-live virus (MLV) vaccine against challenge with NADC30-like strain, v2016/ZJ/09-03. A total of 45 PRRSV free pigs were randomly divided into five groups: (1) strict control (SC); (2) positive control (PC); (3) single MLV dose (M1); (4) primed intramuscularly with MLV and boosted with killed vaccine 3 weeks later (MK1); and (5) intramuscular prime MLV boosted subcutaneously with killed vaccine B 3 weeks later (MK2). Serological tests in MK groups revealed no differences in both anti-N and anti-GP protein antibodies compared with M1 group, and failed to provide further protection against clinical signs, virus shedding, and gross lesions. However, the viremic titer, gross lung lesions, and average daily weight gain were significantly improved in the MLV vaccinated groups, suggesting that MLV provides substantial cross-protection against the NADC30-like virus. Thus, as a booster, the killed vaccine confers minimal additional protection in NADC30-like infected piglets.

Immunity against a Japanese local strain of porcine reproductive and respiratory syndrome virus decreases viremia and symptoms of a highly pathogenic strain

Background

The type 2 highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) has spread throughout countries of southeast Asia, where it has caused severe economic losses. Even countries presently free of PRRSV are at high risk for infection and spread of this virus. Some of these countries, including Japan, have broad epidemics of the local type 2 PRRSV, creating chronic pathogenicity in the domestic pig population. The present study aimed to evaluate the protective efficacy of immunity by infection with a Japanese field isolate, EDRD1, against heterologous challenge with a Vietnamese HP-PRRSV field strain. To this end, four groups of PRRSV-negative crossbreed piglets were used for a challenge study. Groups 1 and 2 were inoculated with EDRD1 via the intranasal route. After 26 days, Groups 2 and 3 were inoculated with HP-PRRSV via the same route. Group 4 served as an uninfected control. Blood and oral fluid samples were taken every 3–4 days after HP-PRRSV challenge; on day 16 post-challenge, all pigs were euthanized, and examined pathologically.

Results

The nucleotide sequence analysis of nonstructural protein 2 gene of EDRD1 and comparison with Vietnamese HP-PRRSV showed that the 39 amino acid deletion sites of EDRD1 was nearly in the same region as the 29 amino acid deletion sites of HP-PRRSV. Immunity conferred by inoculation with EDRD1 dramatically reduced viral load in the sera and tissues besides viral shedding (Group 2) compared with those in pigs infected only with HP-PRRSV (Group 3). The clinical signs and rectal temperature were significantly reduced, and the average daily weight gain was significantly improved in the EDRD1-inoculated pigs (Group 2) compared with the Group 3 pigs. Notably, no viral RNA was detected in various organs of the Group 2 pigs 16 days post-infection with HP-PRRSV, except in one pig. Therefore, the immunity induced by EDRD1 and its genetically close field isolates may play a role in reducing viremia caused by HP-PRRSV.

Conclusions

The results of the present study demonstrate that pigs are highly protected against heterologous Vietnamese HP-PRRSV challenge by immunity against a Japanese local strain, EDRD1.

Porcine Reproductive and Respiratory Syndrome Modified Live Virus Vaccine: A "Leaky" Vaccine with Debatable Efficacy and Safety

Porcine reproductive and respiratory syndrome (PRRS) caused by the PRRS virus (PRRSV) is one of the most economically important diseases, that has significantly impacted the global pork industry for over three decades, since it was first recognized in the United States in the late 1980s. Attributed to the PRRSV extensive genetic and antigenic variation and rapid mutability and evolution, nearly worldwide epidemics have been sustained by a set of emerging and re-emerging virus strains. Since the first modified live virus (MLV) vaccine was commercially available, it has been widely used for more than 20 years, for preventing and controlling PRRS. On the one hand, MLV can induce a protective immune response against homologous viruses by lightening the clinical signs of pigs and reducing the virus transmission in the affected herd, as well as helping to cost-effectively increase the production performance on pig farms affected by heterologous viruses. On the other hand, MLV can still replicate in the host, inducing viremia and virus shedding, and it fails to confer sterilizing immunity against PRRSV infection, that may accelerate viral mutation or recombination to adapt the host and to escape from the immune response, raising the risk of reversion to virulence. The unsatisfied heterologous cross-protection and safety issue of MLV are two debatable characterizations, which raise the concerns that whether it is necessary or valuable to use this leaky vaccine to protect the field viruses with a high probability of being heterologous. To provide better insights into the immune protection and safety related to MLV, recent advances and opinions on PRRSV attenuation, protection efficacy, immunosuppression, recombination, and reversion to virulence are reviewed here, hoping to give a more comprehensive recognition on MLV and to motivate scientific inspiration on novel strategies and approaches of developing the next generation of PRRS vaccine.

Porcine TRIM21 RING-finger E3 ubiquitin ligase is essential for anti-PRRSV activity

Porcine reproductive and respiratory syndrome (PRRS) causes substantial economic losses to the global pig industry. Members of the tripartite motif (TRIM) family are the important effectors of the innate immune response against viral infections. We have previously characterized the entire porcine TRIM (pTRIM) family, and predicted pTRIM5, 14, 21, 25 and 38 as host restriction factors against PRRSV infection. However, little is known about whether and how pTRIMs restrict the infection of PRRSV. In this study, we firstly performed the amino acid alignments of the RING domain of pTRIM5, 21, 25 and 38, and found that pTRIM proteins contained the characteristic consensus C3HC4 type zinc-binding motif which is important for the ubiquitination function. Then we detected the mRNA changes of pTRIMs in porcine alveolar macrophages (PAMs) by transcriptome sequencing after PRRSV infection in piglets. Transcriptional profiles showed that the expression of pTRIM5, 21 and 26 was significantly (P < 0.05) up-regulated, consistent with their expression in vitro. Finally, as the most up-regulated gene after PRRSV infection both in vivo and in vitro, pTRIM21 was investigated for its anti-PRRSV activity in immortalized PAMs (iPAMs) in two aspects: knockdown and overexpression of pTRIM21. Knockdown of endogenic pTRIM21 could significantly promote PRRSV replication at 12 and 24 h post infection in iPAMs. Meanwhile, overexpression of pTRIM21 could significantly suppress PRRSV replication but not affect its attachment and endocytosis. Moreover, pTRIM21 RING-finger E3 ubiquitin ligase was essential for anti-PRRSV activity. Our data enhance our understanding of the pTRIMs against PRRSV infection, which may help us develop novel therapeutic tools to control PRRSV.

Commercial PRRS Modified-Live Virus Vaccines

Porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) presents one of the challenging viral pathogens in the global pork industry. PRRS is characterized by two distinct clinical presentations; reproductive failure in breeding animals (gilts, sows, and boars), and respiratory disease in growing pigs. PRRSV is further divided into two species: PRRSV-1 (formerly known as the European genotype 1) and PRRSV-2 (formerly known as the North American genotype 2). A PRRSV-2 modified-live virus (MLV) vaccine was first introduced in North America in 1994, and, six years later, a PRRSV-1 MLV vaccine was also introduced in Europe. Since then, MLV vaccination is the principal strategy used to control PRRSV infection. Despite the fact that MLV vaccines have shown some efficacy, they were problematic as the efficacy of vaccine was often unpredictable and depended highly on the field virus. This paper focused on the efficacy of commercially available MLV vaccines at a global level based on respiratory disease in growing pigs, and maternal and paternal reproductive failure in breeding animals.

Genetic characterization of a novel recombined porcine reproductive and respiratory syndrome virus 2 among Nadc30-like, Jxa1-like and TJ-like strains

Porcine reproductive and respiratory syndrome (PRRS) is one of the most economically devastating viral diseases in the global pig industry, including China. Recently, we successfully isolated a porcine reproductive and respiratory syndrome virus (PRRSV) from lung tissue and peripheral blood of piglets at a farm from Dujiangyan in Sichuan, China, and named it the DJY-19 strain. The full-length genome sequence of DJY-19 shared 86.8%-94.1% nucleotide similarity with NADC30-like and NADC30 PRRSV strains. We compared the open reading frame (ORF) 5 gene of DJY-19 with 34 PRRSV strains from Genbank. Phylogenetic analysis showed that DJY-19 clustered with NADC30 strains, characterized by a predicted 131-amino-acid deletion in the nonstructural protein (NSP) 2. The results of homology analysis showed that the homology between DJY-19 and NADC30 (JN654459.1) strains was the highest (95.9%), whereas homology with other domestic strains was lower (80.9%-92.6%). Furthermore, we identified four recombination breakpoints in the DJY-19 genome; they separated the DJY-19 genome into four regions. The 8106-9128 nucleotide (nt) region of DIY-19 was highly similar to the TJ strain, and the 12106-12580 nt region of DIY-19 was highly similar to the JXA1-R strain. Our findings demonstrate that DJY-19 arose from the recombination of North America NADC30 strain and TJ strain and JXA1-R in China. The application of multiple attenuated vaccine strains has led to complex recombination of PRRSV strains in China. This study provides a theoretical basis for making a more reasonable PRRS virus control and prevention strategy.

Efficacy of two porcine reproductive and respiratory syndrome (PRRS) modified-live virus (MLV) vaccines against heterologous NADC30-like PRRS virus challenge

The emergence of novel and variant porcine reproductive and respiratory syndrome virus (PRRSV) strains has made controlling this disease a challenge in China. Several NADC30-like PRRSV outbreaks have occurred in mainland China since 2013. The objective of the present study was to evaluate the cross-protection efficacy of two commercial PRRS modified-live virus (MLV) vaccines, derived from classical PRRSV (VR2332) and highly pathogenic (HP) PRRSV (TJM-F92), against an increasingly circulating NADC30-like lineage in pigs. Thirty-five PRRSV- and antibody-free pigs were randomly divided into the following four groups: strict control (SC), negative control (NC), Boehringer control (BC), and Zoetis control (ZC) groups. The NADC30-like PRRSV used in this study caused fever, clinical respiratory signs, and gross and microscopic lung lesions in inoculated pigs in the NC group. Vaccination with the VR2332 vaccine significantly reduced the percentage of viremic pigs as well as gross lung lesions and improved average daily weight gain compared to the ZC and NC groups, suggesting that this MLV vaccine provides cross-protection against the NADC30-like virus. There were no significant differences in the efficacy of the two MLV vaccines based on clinical scores, immunological responses, or pathological outcomes. This study demonstrated that VR2332 MLV was effective against circulating NADC30-like PRRSV and could be used to control NADC30-like virus infections in the field.

Update on Streptococcus suis Research and Prevention in the Era of Antimicrobial Restriction: 4th International Workshop on S. suis

Streptococcus suis is a swine pathogen and a zoonotic agent afflicting people in close contact with infected pigs or pork meat. Sporadic cases of human infections have been reported worldwide. In addition, S. suis outbreaks emerged in Asia, making this bacterium a primary health concern in this part of the globe. In pigs, S. suis disease results in decreased performance and increased mortality, which have a significant economic impact on swine production worldwide. Facing the new regulations in preventive use of antimicrobials in livestock and lack of effective vaccines, control of S. suis infections is worrisome. Increasing and sharing of knowledge on this pathogen is of utmost importance. As such, the pathogenesis and epidemiology of the infection, antimicrobial resistance, progress on diagnosis, prevention, and control were among the topics discussed during the 4th International Workshop on Streptococcus suis (held in Montreal, Canada, June 2019). This review gathers together recent findings on this important pathogen from lectures performed by lead researchers from several countries including Australia, Canada, France, Germany, Japan, Spain, Thailand, The Netherlands, UK, and USA. Finally, policies and recommendations for the manufacture, quality control, and use of inactivated autogenous vaccines are addressed to advance this important field in veterinary medicine.

Genetic Diversity of Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) From 1996 to 2017 in China

Porcine reproductive and respiratory syndrome (PRRS) is one of the most devastating diseases of the global swine industry. The causative agent porcine reproductive and respiratory syndrome virus (PRRSV) was first isolated in China in 1996 and has evolved quickly during the last two decades. To fully understand virus diversity, epidemic situation in the field, and make future predictions, a total of 365 PRRSV strains were used for evolution and genome analysis in which 353 strains were isolated from mainland China. The results showed that high diversity was found among PRRSV isolates. Total PRRSV isolates could be divided into eight subgroups. Among these subgroups strains, Original HP-PRRSV, NADC30-like, and Intermediate PRRSV were the major epidemic PRRSV strains circling in the field and would play a major role in PRRS epidemic in the future. Deletions, insertions, and recombinations have occurred frequently in the PRRSV genome. Deletions were the main driving force of viral evolution before 2006 and may also contribute further to the virus' evolution in a relatively closed or low strain diversity circumstance. The recombinant strains could be divided into three groups: the Inner group, Extensional group, and Propagating group. The evolutionary directions of the isolates in the Extensional and Propagating groups have changed, and the routes of recombination in the Propagating group were analyzed and sorted into three types. The increases in recombinant strains and high rates of recombination in recent years indicate that recombination has played a very important role in the virus' evolution. Isolates, which incorporate the advantages of their parental strains, will influence PRRSV evolution and make adverse effects on PRRS control in the future.

Early infection of Streptococcus suis serotype 2 increases the virulence of highly pathogenic porcine reproductive and respiratory syndrome MLV-like virus in pigs

Modified-live virus (MLV) vaccines derived from highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) were wildly used in China, which resulted in the emergence of MLV-like strains in pigs. Previous studies demonstrated that secondary bacterial infection could enhance HP-PRRSV infection-mediated inflammatory responses, but it is unknown whether early bacterial infection could enhance the HP-PRRSV MLV-like infection-mediated pathological reaction. In this paper, to gain the evidence for infection of pigs with MLV-like strains in China, we firstly analyzed the genetic characterization of the HP-PRRSV MLV-like isolate (TJxq1701) and further evaluated whether the early Streptococcus suis infection synergizes HP-PRRSV MLV-like infection-mediated pathological reaction. Our results showed that the whole genome of TJxq1701 shared the highest homology with JXA1-P80 and a total of 16 amino acids residues unique to JXA1-P80 in ORF1a, ORF1b, GP2, GP3, GP4, and GP5 were found in the corresponding locations. The results of infection experiments in pigs revealed that TJxq1701 caused transitional fever, moderate respiratory clinical sign and microscopic lung lesions in piglets, but early infection with low virulence Streptococcus suis serotype 2 (SS2) exhibited seriously clinical signs, including high fever, anorexia, and respiratory distress, leading to 60% mortality within four weeks in comparison with alone infected group. Taken together, our findings reveal that early bacterial infection could enhance the HP-PRRSV MLV-like infection-mediated pathological reaction, which provide an important clue for understanding that streptococcus infection increases the pathogenicity of MLV-like virus and a new thought for prevention and control of PRRSV.

Porcine Reproductive and Respiratory Syndrome Virus Activates Lipophagy To Facilitate Viral Replication through Downregulation of NDRG1 Expression

Autophagy maintains cellular homeostasis by degrading organelles, proteins, and lipids in lysosomes. Autophagy is involved in the innate and adaptive immune responses to a variety of pathogens. Some viruses can hijack host autophagy to enhance their replication. However, the role of autophagy in porcine reproductive and respiratory syndrome virus (PRRSV) infection is unclear. Here, we show that N-Myc downstream-regulated gene 1 (NDRG1) deficiency induced autophagy, which facilitated PRRSV replication by regulating lipid metabolism. NDRG1 mRNA is expressed ubiquitously in most porcine tissues and most strongly in white adipose tissue. PRRSV infection downregulated the expression of NDRG1 mRNA and protein, while NDRG1 deficiency contributed to PRRSV RNA replication and progeny virus assembly. NDRG1 deficiency reduced the number of intracellular lipid droplets (LDs), but the expression levels of key genes in lipogenesis and lipolysis were not altered. Our results also show that NDRG1 deficiency promoted autophagy and increased the subsequent yields of hydrolyzed free fatty acids (FFAs). The reduced LD numbers, increased FFA levels, and enhanced PRRSV replication were abrogated in the presence of an autophagy inhibitor. Overall, our findings suggest that NDRG1 plays a negative role in PRRSV replication by suppressing autophagy and LD degradation.IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV), an enveloped single-positive-stranded RNA virus, causes acute respiratory distress in piglets and reproductive failure in sows. It has led to tremendous economic losses in the swine industry worldwide since it was first documented in the late 1980s. Vaccination is currently the major strategy used to control the disease. However, conventional vaccines and other strategies do not provide satisfactory or sustainable prevention. Therefore, safe and effective strategies to control PRRSV are urgently required. The significance of our research is that we demonstrate a previously unreported relationship between PRRSV, NDRG1, and lipophagy in the context of viral infection. Furthermore, our data point to a new role for NDRG1 in autophagy and lipid metabolism. Thus, NDRG1 and lipophagy will have significant implications for understanding PRRSV pathogenesis for developing new therapeutics.

Efficacy of Type 2 PRRSV vaccine against challenge with the Chinese lineage 1 (NADC30-like) PRRSVs in pigs

The objective of the present study was to determine the cross-protection of Ingelvac PRRS MLV against challenge with the new lineage 1 PRRSV emerged in China in pigs. Two lineage 1 PRRSV strains (FJZ03 and FJWQ16 originated from recombination event between NADC30 and JXA1-like strain). We found that pigs vaccinated with the vaccine were protected against challenge with the FJZ03 as shown by fewer days of clinical fever, reduced lung pathology scores, lower PRRS virus load in the blood and developed broadly neutralizing antibodies with high titers to FJZ03. In contrast, vaccine provided limited protection against challenge with FJWQ16 with higher fever, lower antibody titers, lower neutralizing antibodies and higher viral loads in blood. These results demonstrate PRRSV-MLV provides incomplete protection against new lineage 1 PRRSVs.

Efficacy evaluation of three modified-live PRRS vaccines against a local strain of highly pathogenic porcine reproductive and respiratory syndrome virus

Highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) is characterized by high fever and high mortality in pigs of all ages and has severely affected the pork industry of China in the last decades. This study evaluated the differences in protection conferred by of three MLV PRRS vaccines derived from classical PRRSV (C-PRRSV, VR2332) and HP-PRRSV (TJM-F92 and JXA1-R) against the field challenge of HP-PRRSV TP strain (JXA1-R like). Compared to the experiment pigs in control group which were vaccinated with normal saline, the MLV PRRS vaccinated pigs had milder clinical symptoms, fewer pathological changes in the lung, and higher body weight gain at the end of the study. However, piglets vaccinated with VR2332 had higher body temperature, higher viral loads and lower body weight gain when compared with piglets vaccinated with TJM-F92 or JXA1-R vaccines at the end of the study. The results demonstrated that VR2332 vaccine provided a limited cross-protection against the HP-PRRSV TP strain infection, while in contrast the TJM-F92 and JXA1-R vaccines provided more efficacious protection. The findings of this study could serve as a valuable reference guide for the pig producers and veterinarians when considering the choice of which type of MLV PRRS vaccines to protect their pig herds against field challenge by HP-PRRSV TP strain.

Comparative evaluation of the efficacy of commercial and prototype PRRS subunit vaccines against an HP-PRRSV challenge

The objective of this study was to compare the efficacy of a commercial porcine reproductive and respiratory syndrome (PRRS) subunit vaccine and a prototype PRRS II subunit vaccine against a highly pathogenic PRRS virus (HP-PRRSV) in pigs. Both vaccines were administered intramuscularly in 2 doses at 21 and 42 days of age, and the pigs were challenged intranasally with HP-PRRSV at 63 days of age. Pigs vaccinated with the prototype PRRS II subunit vaccine had significantly higher anti-PRRSV antibody titers, a greater number of interferon-γ-secreting cells, and a greater reduction in lung lesion scores compared to pigs vaccinated with the commercial PRRS subunit vaccine. Therefore, the commercial PRRS subunit and prototype PRRS II subunit vaccines are efficacious against HP-PRRSV.

Porcine Reproductive and Respiratory Syndrome Virus Infection Induces both eIF2α Phosphorylation-Dependent and -Independent Host Translation Shutoff

Porcine reproductive and respiratory syndrome virus (PRRSV) is an Arterivirus that has caused tremendous economic losses in the global swine industry since it was discovered in the late 1980s. Inducing host translation shutoff is a strategy used by many viruses to optimize their replication and spread. Here, we demonstrate that PRRSV infection causes host translation suppression, which is strongly dependent on viral replication. By screening PRRSV-encoded nonstructural proteins (nsps), we found that nsp2 participates in the induction of host translation shutoff and that its transmembrane (TM) domain is required for this process. nsp2-induced translation suppression is independent of protein degradation pathways and the phosphorylation of eukaryotic initiation factor 2α (eIF2α). However, the overexpression of nsp2 or its TM domain significantly attenuated the mammalian target of rapamycin (mTOR) signaling pathway, an alternative pathway for modulating host gene expression. PRRSV infection also attenuated the mTOR signaling pathway, and PRRSV-induced host translation shutoff could be partly reversed when the attenuated mTOR phosphorylation was reactivated by an activator of the mTOR pathway. PRRSV infection still negatively regulated the host translation when the effects of eIF2α phosphorylation were completely reversed. Taken together, our results demonstrate that PRRSV infection induces host translation shutoff and that nsp2 is associated with this process. Both eIF2α phosphorylation and the attenuation of the mTOR signaling pathway contribute to PRRSV-induced host translation arrest.IMPORTANCE Viruses are obligate parasites, and the production of progeny viruses relies strictly on the host translation machinery. Therefore, the efficient modulation of host mRNA translation benefits viral replication, spread, and evolution. In this study, we provide evidence that porcine reproductive and respiratory syndrome virus (PRRSV) infection induces host translation shutoff and that the viral nonstructural protein nsp2 is associated with this process. Many viruses induce host translation shutoff by phosphorylating eukaryotic initiation factor 2α (eIF2α). However, PRRSV nsp2 does not induce eIF2α phosphorylation but attenuates the mTOR signaling pathway, another pathway regulating the host cell translational machinery. We also found that PRRSV-induced host translation shutoff was partly reversed by eliminating the effects of eIF2α phosphorylation or reactivating the mTOR pathway, indicating that PRRSV infection induces both eIF2α phosphorylation-dependent and -independent host translation shutoff.

Efficacy of a type 2 PRRSV modified live vaccine (PrimePac™ PRRS) against a Thai HP-PRRSV challenge

The Chinese highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) has caused a severe threat to the pig population in Southeast Asian countries. The purpose of this study was to investigate the efficacy of a type 2 PRRSV modified live vaccine (PrimePac™ PRRS, lineage 7) against a Thai HP-PRRSV (10PL01, lineage 8). Three-week-old PRRSV-free pigs were randomly assigned into three groups. Vaccinated challenged group (group 1, n = 16) was immunized with PrimePac™ PRRS vaccine at 3 weeks old. The unvaccinated challenged group (group 2, n = 16) was injected with PBS at 3 weeks old, and unvaccinated unchallenged group (group 3, n = 10) was served as a negative control. At 9 weeks old, all groups, except the negative control group, were challenged with the Thai HP-PRRSV. All pigs were monitored daily during 10 days post-infection (dpi) and were necropsied at 10 and 17 dpi. The results revealed that vaccinated challenged pigs showed significantly lower (p < 0.05) mean rectal temperatures, clinical respiratory scores, lung lesion scores, and levels of virus load in serum and lung tissue compared with the unvaccinated challenged pigs. Moreover, vaccinated challenged pigs exhibited PRRSV-specific serum neutralizing antibodies at the end of the experiment. Our findings indicated that the studied type 2 PRRSV vaccine provided partial protection against the Thai HP-PRRSV infection based on the body temperature, levels of viremia, and the severity of lung lesions. These results demonstrated that partial protection of PrimePac™ PRRS vaccine might be useful for controlling HP-PRRSV infection in the endemic area.

Efficacy of live attenuated porcine reproductive and respiratory syndrome virus 2 strains to protect pigs from challenge with a heterologous Vietnamese PRRSV 2 field strain

Background

Effective vaccines against porcine reproductive and respiratory syndrome virus (PRRSV), especially against highly pathogenic (HP) PRRSV are still missing. The objective of this study was to evaluate the protective efficacy of an experimental live attenuated PRRSV 2 vaccine, composed of two strains, against heterologous challenge with a Vietnamese HP PRRSV 2 field strain. For this reason, 20 PRRSV negative piglets were divided into two groups. The pigs of group 1 were vaccinated with the experimental vaccine, group 2 remained unvaccinated. All study piglets received an intranasal challenge of the HP PRRSV 2 on day 0 of the study (42 days after vaccination). Blood samples were taken on days 7 and 21 after vaccination and on several days after challenge. On day 28 after challenge, all piglets were euthanized and pathologically examined.

Results

On days 7 and 21 after vaccination, a PRRSV 2 viraemia was seen in all piglets of group 1 which remained detectable in seven piglets up to 42 days after vaccination. On day 3 after challenge, all piglets from both groups were positive in PRRSV 2 RT-qPCR. From day 7 onwards, viral load and number of PRRSV 2 positive pigs were lower in group 1 than in group 2. All pigs of group 1 seroconverted after PRRSV 2 vaccination. PRRSV antibodies were detected in serum of all study pigs from both groups from day 14 after challenge onwards. In group 2, moderate respiratory symptoms with occasional coughing were seen following the challenge with HP PRRSV 2. Pigs of group 1 remained clinically unaffected. Interstitial pneumonia was found in four piglets of group 1 and in all ten piglets of group 2. Histopathological findings were more severe in group 2.

Conclusions

It was thus concluded that the used PRRSV 2 live experimental vaccine provided protection from clinical disease and marked reduction of histopathological findings and viral load in pigs challenged with a Vietnamese HP PRRSV 2 field strain.

Plant synthetic GP4 and GP5 proteins from porcine reproductive and respiratory syndrome virus elicit immune responses in pigs

We demonstrated successful overexpression of porcine reproductive and respiratory syndrome virus (PRRSV)-derived GP4D and GP5D antigenic proteins in Arabidopsis. Pigs immunized with transgenic plants expressing GP4D and GP5D proteins generated both humoral and cellular immune responses to PRRSV. Porcine reproductive and respiratory syndrome virus (PRRSV) causes PRRS, the most economically significant disease affecting the swine industry worldwide. However, current commercial PRRSV vaccines (killed virus or modified live vaccines) show poor efficacy and safety due to concerns such as reversion of virus to wild type and lack of cross protection. To overcome these problems, plants are considered a promising alternative to conventional platforms and as a vehicle for large-scale production of recombinant proteins. Here, we demonstrate successful production of recombinant protein vaccine by expressing codon-optimized and transmembrane-deleted recombinant glycoproteins (GP4D and GP5D) from PRRSV in planta. We generated transgenic Arabidopsis plants expressing GP4D and GP5D proteins as candidate antigens. To examine immunogenicity, pigs were fed transgenic Arabidopsis leaves expressing the GP4D and GP5D antigens (three times at 2-week intervals) and then challenged with PRRSV at 6-week post-initial treatment. Immunized pigs showed significantly lower lung lesion scores and reduced viremia and viral loads in the lung than pigs fed Arabidopsis leaves expressing mYFP (control). Immunized pigs also had higher titers of PRRSV-specific antibodies and significantly higher levels of pro-inflammatory cytokines (TNF-α and IL-12). Furthermore, the numbers of IFN-γ⁺-producing cells were higher, and those of regulatory T cells were lower, in GP4D and GP5D immunized pigs than in control pigs. Thus, plant-derived GP4D and GP5D proteins provide an alternative platform for producing an effective subunit vaccine against PRRSV.

An Attenuated Highly Pathogenic Chinese PRRS Viral Vaccine Confers Cross Protection to Pigs against Challenge with the Emerging PRRSV NADC30-Like Strain

A novel PRRSV strain was isolated in China that was genetically similar to the NADC30 strain which is reported to have spread throughout China. The objective of the present study was to evaluate the cross-protective efficacy of the live vaccine TJM-F92 in young pigs against challenge with a NADC30-like strain, HN201605. Twenty-five PRRSV- and antibody-free pigs were randomly divided into the following five groups: Vac/ChA, Unvac/ChA, Vac/ChB, Unvac/ChB and the mock. The pigs in groups Vac/ChA and Vac/ChB were inoculated intramuscularly with 1 mL TJM-F92 (10^5.0 TCID₅₀/mL). At 28 days post vaccination (0 days post challenge), groups Vac/ChA and Unvac/ChA were inoculated intranasally with 10^4.5 TCID₅₀/mL PRRSV strain TJ F3 (2 mL/pig), while groups Vac/ChB and Unvac/ChB were inoculated, using the same route, with the same dose of the NADC30-like strain HN201605 F3. Protective effects of the PRRSV strain were observed in all pigs in the Vac/ChA and Vac/ChB groups. Neither high fever nor signs of clinical disease were observed through the experiment in these groups, whereas pigs in Unvac/ChA group exhibited serious clinical symptoms, pathological lesions, and weight loss. In Unvac/ChB group, pigs developed milder clinical symptoms, which demonstrated that the NADC30-like strain HN201605 had moderate pathogenicity. The results suggest that the MLV vaccine strain TJM-F92 is an effective and safe vaccine candidate for use in China.

Efficacy evaluation of three modified-live virus vaccines against a strain of porcine reproductive and respiratory syndrome virus NADC30-like

Porcine reproductive and respiratory syndrome reproductive virus is a devastating pathogen causing tremendous economic losses to swine production worldwide. Emergence of novel and variant PRRSV strains always leads to variable protection efficacy of modified-live virus (MLV) vaccines. Prevalence of PRRSV NADC30-like recently emerging in China has brought about clinical outbreaks of the disease. In the present study, the pathogenicity of a NADC30-like strain CHsx1401 for piglets was analyzed, and the potential cross-protective efficacy of three MLV vaccines including two commercial MLV vaccines and an attenuated low pathogenic PRRSV against this virus was further evaluated in piglets. The NADC30-like CHsx1401 was shown to cause fever, respiratory clinical signs, and lung gross and microscopic lesions of the inoculated piglets, suggesting that this virus is moderate virulent for piglets. Vaccination of piglets with the MLV vaccines could not reduce the clinical signs and lung lesions, and was partially efficacious in the reduction of viral loads in sera upon NADC30-like CHsx1401 challenge, indicating that these three MLV vaccines provide extremely limited cross-protection efficacy against the NADC30-like virus infection. Additionally, Ingelvac PRRS MLV appeared to exert some beneficial efficiency in shortening the period of clinical fever and in improving the growth performance of the challenged pigs. Our findings give valuable guidance for the choice and use of PRRSV MLV vaccines to control NADC30-like virus infection in the field.

Pathogenesis and control of the Chinese highly pathogenic porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) has remained a major threat to the worldwide swine industry ever since its first discovery in the early 1990s. Under the selective pressures in the field, this positive-stranded RNA virus undergoes rapid genetic evolution that eventually leads to emergence in 2006 of the devastating Chinese highly pathogenic PRRSV (HP-PRRSV). The atypical nature of HP-PRRSV has caused colossal economic losses to the swine producers in China and the surrounding countries. In this review, we summarize the recent advances in our understanding of the pathogenesis, evolution and ongoing field practices on the control of this troubling virus in China.

Efficacy of Type 2 porcine reproductive and respiratory syndrome virus (PRRSV) vaccine against the 2010 isolate of Vietnamese highly pathogenic PRRSV challenge in pigs

The efficacy of a commercial attenuated live type 2 porcine reproductive and respiratory syndrome (PRRS) vaccine was tested under experimental infection with a highly virulent Vietnamese virus isolated from a diseased pig affected with highly pathogenic PRRS (HP-PRRS) using specific pathogen-free (SPF) pigs. Twenty-five 4-week-old SPF pigs were divided into three groups as follows: pigs vaccinated with a single dose of the vaccine (Group 1, n=10), unvaccinated pigs (Group 2, n=10) and unvaccinated and non-infectious control pigs (Group 3, n=5). Four weeks later, Groups 1 and 2 were challenged with a 1 ml inoculum containing 1 × 10^5.5 50% tissue culture infectious dose (TCID₅₀)/ml of a Vietnamese HP-PRRS virus isolated in 2010 via the intranasal route. Animals were monitored during the subsequent two-week period post-challenge and necropsied for virological and pathological assays. Results showed a significant reduction in viral replication and shedding in vaccinated pigs compared to unvaccinated pigs. The non-vaccinated pigs showed severe pyrogenic and respiratory illness with marked systematic lesions including interstitial pneumonia and thymic atrophy. In contrast, vaccinated pigs recovered quickly from fever with only mild pathological manifestations. Therefore, although viral shedding was still noted, immunization with the live PRRS vaccine did indeed reduce viral replication and disease severity, suggesting its utility in minimizing outbreaks of HP-PRRS.

Comparison of Asian porcine high fever disease isolates of porcine reproductive and respiratory syndrome virus to United States isolates for their ability to cause disease and secondary bacterial infection in swine

Epidemiologic data from Asian outbreaks of highly-pathogenic (HP) porcine reproductive and respiratory syndrome virus (PRRSV) suggest that disease severity was associated with both the virulence of the PRRSV isolates and secondary bacterial infections. Previous reports have indicated that U.S. isolates of PRRSV predispose to secondary bacterial infections as well, but the severity of disease that occurred in Asia in pigs infected with these HP-PRRSV strains has not been reported in the U.S. The objectives of this research were to compare the pathogenesis of Asian and U.S. PRRSV isolates with regard to their ability to cause disease and predispose to secondary bacterial infections in swine. To address these objectives groups of pigs were infected with 1 of 2 Asian HP-PRRSV strains (rJXwn06 or rSRV07) or 1 of 2 U.S. PRRSV strains (SDSU73 or VR-2332) alone or in combination with Streptococcus suis, Haemophilus parasuis, and Actinobacillus suis. Pigs infected with rJXwn06 exhibited the most severe clinical disease while the pigs infected with rSRV07 and SDSU73 exhibited moderate clinical disease, and pigs infected with VR-2332 exhibited minimal clinical signs. The frequency of secondary bacterial pneumonia was associated with the clinical severity induced by the PRRSV strains evaluated. The levels of proinflammatory cytokines in the serum were often lower for pigs coinfected with virus and bacteria compared to pigs infected with PRRSV alone indicating an alteration in the immune response in coinfected pigs. Combined our results demonstrate that severity of disease appears to be dependent on virulence of the PRRSV strain, and development of secondary bacterial infection.

Positive immunomodulatory effects of heterologous DNA vaccine- modified live vaccine, prime-boost immunization, against the highly-pathogenic PRRSV infection

Porcine reproductive and respiratory syndrome virus (PRRSV) infection is one of the most important swine pathogens, and causes a major economic impact worldwide. Recently, a new variant type 2 PRRSV, highly pathogenic PRRSV (HP-PRRSV) has emerged and continued to circulate in Southeast Asia region. Currently, commercially available PRRSV vaccines, modified live PRRS vaccines (MLV) are not able to provide complete protection against HP-PRRSV and been reported to induce negative immunomodulatory effects. Interestingly, a novel DNA vaccine was developed and successfully used to improve PRRSV-specific immune responses following MLV vaccination. To investigate the efficacy of a heterologous DNA-MLV prime-boost immunization against the HP-PRRSV infection, an experimental vaccinated-challenged study was conducted. Two-week-old, PRRSV-seronegative, crossbred pigs (5-8 pigs/group) were allocated into 5 groups. At day -14 (D-14), the treatment group (DNA-MLV) was immunized with a DNA vaccine encoding PRRSV-truncated nucleocapsid protein (pORF7t), followed by a commercial modified live type 2 PRRS vaccine (MLV) at D0. The other groups included the group that received PBS at D-14 followed by MLV at D0 (MLV), pORF7t at D-14 (DNA), PBS at D0 (PBS) and the negative control group. At D42, all groups, except the negative control group, were challenged with HP-PRRSV (strain 10PL1). The results demonstrated that pigs that received MLV, regardless of the DNA priming, exhibited less clinical signs and faster viral clearance. Following HP-PRRSV challenge, the DNA-MLV group exhibited improved PRRSV-specific immunity, as observed by increased neutralizing antibody titers and PRRSV-specific IFN-γ production, and reduced IL-10 and PRRSV-specific Treg productions. However, neither the prime-boost immunization nor the MLV was able to induce complete clinical protection against HP-PRRSV infection. In conclusion, improved immunological responses, but not clinical protection, were achieved by DNA-MLV prime-boost immunization. This study highlights the potential use of heterologous prime-boost vaccination regimen, where DNA can be incorporated with other vaccine candidates, for improving anti-PRRSV immunity that may eventually lead induction of complete PRRSV protection.

Immune responses to modified live virus vaccines developed from classical or highly pathogenic PRRSV following challenge with a highly pathogenic PRRSV strain

Modified live virus vaccines (MLVs) are used on swine farms to control porcine reproductive and respiratory syndrome virus (PRRSV). MLVs from classical PRRSV (C-PRRSV) provide some protection against emergent highly pathogenic PRRSV (HP-PRRSV). This study characterized the protective efficacy and immune response to MLVs from C-PRRSV (CH-1R) or HP-PRRSV (HuN4-F112) in a challenge using HP-PRRSV (HuN4). The outcomes were clinical signs of disease, pathological changes in the thymus and lungs, viremia, and humoral and cellular immune responses. CH-1R provided some protection against challenge with HuN4, while HuN4-F112 was protective in the HuN4 challenge. Compared to unvaccinated piglets, the vaccinated piglets had milder symptoms and fewer pathological changes in the lung and thymus. Piglets vaccinated with HuN4-F112 had higher antibody titers and lower viral loads than piglets vaccinated with CH-1R post challenge. The differences in outcome between the MLVs suggested that underlying differences in the immune responses might warrant further study.

Pigs immunized with Chinese highly pathogenic PRRS virus modified live vaccine are protected from challenge with North American PRRSV strain NADC-20

Modified live virus (MLV) vaccines developed to protect against PRRSV circulating in North America (NA) offer limited protection to highly pathogenic (HP) PRRSV strains that are emerging in Asia. MLV vaccines specific to HP-PRRSV strains commercially available in China provide protection to HP-PRRSV; however, the efficacy of these HP-PRRSV vaccines to current circulating NA PRRS viruses has not been reported. The aim of this study is to investigate whether pigs vaccinated with attenuated Chinese HP-PRRSV vaccine (JXA1-R) are protected from infection by NA PRRSV strain NADC-20. We found that pigs vaccinated with JXA1-R were protected from challenges with HV-PRRSV or NADC-20 as shown by fewer days of clinical fever, reduced lung pathology scores, and lower PRRS virus load in the blood. PRRSV-specific antibodies, as measured by IDEXX ELISA, appeared one week after vaccination and virus neutralizing antibodies were detected four weeks post vaccination. Pigs vaccinated with JXA1-R developed broadly neutralizing antibodies with high titers to NADC-20, JXA1-R, and HV-PRRSV. In addition, we also found that IFN-α and IFN-β occurred at higher levels in the lungs of pigs vaccinated with JXA1-R. Taken together, our studies provide the first evidence that JXA1-R can confer protection in pigs against the heterologous NA PRRSV strain NADC-20.