Enhancing heterologous protection in pigs vaccinated with chimeric porcine reproductive and respiratory syndrome virus containing the full-length sequences of shuffled structural genes of multiple heterologous strains

A chimeric porcine reproductive and respiratory syndrome virus 1 strain containing synthetic ORF2-6 genes can trigger T follicular helper cell and heterologous neutralizing antibody responses and confer enhanced cross-protection

The prevalence of porcine reproductive and respiratory syndrome virus 1 (PRRSV1) isolates has continued to increase in Chinese swine herds in recent years. However, no effective control strategy is available for PRRSV1 infection in China. In this study, we generated the first infectious cDNA clone (rHLJB1) of a Chinese PRRSV1 isolate and subsequently used it as a backbone to construct an ORF2-6 chimeric virus (ORF2-6-CON). This virus contained a synthesized consensus sequence of the PRRSV1 ORF2-6 gene encoding all the envelope proteins. The ORF2-6 consensus sequence shared > 90% nucleotide similarity with four representative strains (Amervac, BJEU06-1, HKEU16 and NMEU09-1) of PRRSV1 in China. ORF2-6-CON had replication efficacy similar to that of the backbone rHLJB1 virus in primary alveolar macrophages (PAMs) and exhibited cell tropism in Marc-145 cells. Piglet inoculation and challenge studies indicated that ORF2-6-CON is not pathogenic to piglets and can induce enhanced cross-protection against a heterologous SD1291 isolate. Notably, ORF2-6-CON inoculation induced higher levels of heterologous neutralizing antibodies (nAbs) against SD1291 than rHLJB1 inoculation, which was concurrent with a higher percentage of T follicular helper (Tfh) cells in tracheobronchial lymph nodes (TBLNs), providing the first clue that porcine Tfh cells are correlated with heterologous PRRSV nAb responses. The number of SD1291-strain-specific IFNγ-secreting cells was similar in ORF2-6-CON-inoculated and rHLJB1-inoculated pigs. Overall, our findings support that the Marc-145-adapted ORF2-6-CON can trigger Tfh cell and heterologous nAb responses to confer improved cross-protection and may serve as a candidate strain for the development of a cross-protective PRRSV1 vaccine.

A novel chimeric vaccine candidate for porcine reproductive and respiratory syndrome virus (PRRSV) I and II elicits neutralizing antibodies against both types

Porcine reproductive and respiratory syndrome virus (PRRSV) is an economically important virus within the swine industry. The virus causes respiratory disease and reproductive failure. Two species of PRRSV-I and II are co-dominant, yet no effective vaccination strategy has been developed to protect against these two types. With an aim to develop a chimeric vaccine strain to protect against both types, in this study, a chimeric porcine reproductive and respiratory syndrome virus (PRRSV) type I and II was rescued using reverse genetics for the first time. Four chimeric infectious clones were designed based on the genomic arrangement of the structural proteins. However, only the clone carrying the transcriptional regulatory sequence (TRS) and ORF6 of a PRRSV-I and ORF6 of a PRRSV-II generated a viable recombinant virus, suggesting that concurrent expression of ORF6 from both parental viruses is essential for the recovery of type I and II chimeric PRRSV. The chimeric virus showed significantly lower replication ability than its parental strains in vitro, which was improved by serial passaging. In vivo, groups of pigs were inoculated with either the chimeric virus, one of the parental strains, or PBS. The chimeric virus replicated in pig tissue and was detected in serum 7 days post-inoculation. Serum neutralization tests indicated that pigs inoculated with the chimeric virus elicited neutralizing antibodies that inhibited infection with strains of both species and with greater coverage than the parental viruses. In conclusion, the application of this technique to construct a chimeric PRRSV holds promise for the development of a highly effective modified live vaccine candidate. This is particularly significant since there are currently no approved commercial divalent vaccines available to combat PRRSV-I and II co-infections.

Construction of an infectious cloning system of porcine reproductive and respiratory syndrome virus and identification of glycoprotein 5 as a potential determinant of virulence and pathogenicity

Porcine reproductive and respiratory syndrome virus (PRRSV) infection of pigs causes a variety of clinical manifestations, depending on the pathogenicity and virulence of the specific strain. Identification and characterization of potential determinant(s) for the pathogenicity and virulence of these strains would be an essential step to precisely design and develop effective anti-PRRSV intervention. In this study, we report the construction of an infectious clone system based on PRRSV vaccine strain SP by homologous recombination technique, and the rescue of a chimeric rSP-HUB2 strain by replacing the GP5 and M protein-coding region from SP strain with the corresponding region from a highly pathogenic strain PRRSV-HUB2. The two recombinant viruses were shown to be genetically stable and share similar growth kinetics, with rSP-HUB2 exhibiting apparent growth and fitness advantages. Compared to in cells infected with PRRSV-rSP, infection of cells with rSP-HUB2 showed significantly more inhibition of the induction of type I interferon (IFN-β) and interferon stimulator gene 56 (ISG56), and significantly more promotion of the induction of proinflammatory cytokines IL-6, IL-8, ISG15 and ISG20. Further overexpression, deletion and mutagenesis studies demonstrated that amino acid residue F16 in the N-terminal region of the GP5 protein from HUB2 was a determinant for the phenotypic difference between the two recombinant viruses. This study provides evidence that GP5 may function as a potential determinant for the pathogenicity and virulence of highly pathogenic PRRSV.

Testable Candidate Immune Correlates of Protection for Porcine Reproductive and Respiratory Syndrome Virus Vaccination

Porcine reproductive and respiratory syndrome virus (PRRSV) is an on-going problem for the worldwide pig industry. Commercial and experimental vaccinations often demonstrate reduced pathology and improved growth performance; however, specific immune correlates of protection (CoP) for PRRSV vaccination have not been quantified or even definitively postulated: proposing CoP for evaluation during vaccination and challenge studies will benefit our collective efforts towards achieving protective immunity. Applying the breadth of work on human diseases and CoP to PRRSV research, we advocate four hypotheses for peer review and evaluation as appropriate testable CoP: (i) effective class-switching to systemic IgG and mucosal IgA neutralizing antibodies is required for protective immunity; (ii) vaccination should induce virus-specific peripheral blood CD4+ T-cell proliferation and IFN-γ production with central memory and effector memory phenotypes; cytotoxic T-lymphocytes (CTL) proliferation and IFN-γ production with a CCR7- phenotype that should migrate to the lung; (iii) nursery, finishing, and adult pigs will have different CoP; (iv) neutralizing antibodies provide protection and are rather strain specific; T cells confer disease prevention/reduction and possess greater heterologous recognition. We believe proposing these four CoP for PRRSV can direct future vaccine design and improve vaccine candidate evaluation.

Recent advances in the study of NADC34-like porcine reproductive and respiratory syndrome virus in China

Since porcine reproductive and respiratory syndrome virus (PRRSV) was first described in China in 1996, several genetically distinct strains of PRRSV have emerged with varying pathogenicity and severity, thereby making the prevention and control of PRRS more difficult in China and worldwide. Between 2017 and 2021, the detection rate of NADC34-like strain in China increased. To date, NADC34-like strains have spread to 10 Chinese provinces and have thus developed different degrees of pathogenicity and mortality. In this review, we summarize the history of NADC34-like strains in China and clarify the prevalence, genomic characteristics, restriction fragment length polymorphisms, recombination, pathogenicity, and vaccine status of this strain in China. In so doing, this study aims to provide a basis for the further development of prevention and control measures targeting the NADC34-like strain.

Systemic Homologous Neutralizing Antibodies Are Inadequate for the Evaluation of Vaccine Protective Efficacy against Coinfection by High Virulent PEDV and PRRSV

G2 porcine epidemic diarrhea virus (G2 PEDV) and highly pathogenic porcine reproductive and respiratory syndrome virus 2 (HP-PRRSV2) are two of the most prevalent swine pathogens in China's swine herds, and their coinfection occurs commonly. Several PED and PRRS vaccines have been utilized in China for decades, and systemic homologous neutralizing antibodies (shnAbs) in serum are frequently used to evaluate the protective efficacy of PED and PRRS vaccines. To develop a vaccine candidate against G2 PEDV and HP-PRRSV2 coinfection, in this study, we generated a chimeric virus (rJSTZ1712-12-S) expressing S protein of G2 PEDV using an avirulent HP-PRRSV2 rJSTZ1712-12 infectious clone as the viral vector. The rJSTZ1712-12-S strain has similar replication efficacies as the parental rJSTZ1712-12 virus. In addition, animal inoculation indicated that rJSTZ1712-12-S is not pathogenic to piglets and can induce shnAbs against both G2 PEDV and HP-PRRSV2 isolates after prime-boost immunization. However, passive transfer study in neonatal piglets deprived of sow colostrum showed that rJSTZ1712-12-S-induced shnAbs may only decrease PEDV and PRRSV viremia but cannot confer sufficient protection against dual challenge of high virulent G2 PEDV XJ1904-34 strain and HP-PRRSV2 XJ17-5 isolate. Overall, this study provides the first evidence that shnAbs confer insufficient protection against PEDV and PRRSV coinfection and are inadequate for the evaluation of protective efficacy of PED and PRRS bivalent vaccine (especially for the PED vaccine). IMPORTANCE Porcine epidemic diarrhea virus (PEDV) and porcine reproductive and respiratory syndrome virus (PRRSV) coinfection occurs commonly and can synergistically reduce feed intake and pig growth. Vaccination is an effective strategy utilized for PED and PRRS control, and systemic homologous neutralizing antibodies (shnAbs) in serum are commonly used for protective efficacy evaluation of PED and PRRS vaccines. Currently, no commercial vaccine is available against PEDV and PRRSV coinfection. This study generated a chimeric vaccine candidate against the coinfection of prevalent PEDV and PRRSV in China. The chimeric strain can induce satisfied shnAbs against both PEDV and PRRSV after prime-boost inoculation in pigs. But the shnAbs cannot confer sufficient protection against PEDV and PRRSV coinfection in neonatal piglets. To the best of our knowledge, these findings provide the first evidence that shnAbs confer insufficient protection against PEDV and PRRSV coinfection and are inadequate for evaluating PED and PRRS bivalent vaccine protective efficacy.

Minor and major envelope proteins of PRRSV play synergistic roles in inducing heterologous neutralizing antibodies and conferring cross protection

High genetic diversity of porcine reproductive and respiratory syndrome virus (PRRSV) isolates is a major obstacle for the development of effective PRRS vaccines. A chimeric highly pathogenic PRRSV2 (HP-PRRSV2) strain containing the consensus sequence of ORF2-6 genes was constructed in our previous study, which could induce broadly neutralizing antibodies (bnAbs) and confer satisfied cross protection against virulent NADC30-like isolate. To further elucidate the roles of minor and major envelope proteins encoded by ORF2-4 and ORF5-6 genes in conferring cross protection, two chimeric HP-PRRSV2 strains (rJS-ORF2-4-CON and rJS-ORF5-6-CON) containing consensus sequences of ORF2-4 or ORF5-6 were constructed and rescued in this study. The rJS-ORF5-6-CON strain has similar replication efficiency as the backbone HP-PRRSV2 rJSTZ1712-12 virus, while rJS-ORF2-4-CON has significantly lower in vitro and in vivo replication efficiency comparing to rJS-ORF5-6-CON. Animal inoculation indicated that both rJS-ORF2-4-CON and rJS-ORF5-6-CON did not cause obvious clinical signs in piglets and could induce heterologous nAbs after immunization. Challenge with a virulent heterologous NADC30-like SD17-38 isolate showed that even though both immunized groups presented lower viremia, faster virus elimination, less fever and alleviated lung gross lesions when compared with the only challenged pigs, rJS-ORF2-4-CON and rJS-ORF5-6-CON could not confer enough cross protection. Considering the bnAbs and satisfied cross protection induced by the chimeric virus containing ORF2-6 consensus sequence, our results support that minor and major envelope proteins play synergistic roles in inducing broader nAbs and conferring better cross protection.

Evaluation of the Cross-Protective Efficacy of a Chimeric PRRSV Vaccine against Two Genetically Diverse PRRSV2 Field Strains in a Reproductive Model

Despite the routine use of porcine reproductive and respiratory syndrome (PRRS)-modified live vaccines, serious concerns are currently being raised due to their quick reversion to virulence and limited cross-protection against divergent PRRS virus (PRRSV) strains circulating in the field. Therefore, a PRRS chimeric vaccine (JB1) was produced using a DNA-launched infectious clone by replacing open reading frames (ORFs) 3–6 with those from a mixture of two genetically different PRRSV2 strains (K07–2273 and K08–1054) and ORF1a with that from a mutation-resistant PRRSV strain (RVRp22) exhibiting an attenuated phenotype. To evaluate the safety and cross-protective efficacy of JB1 in a reproductive model, eight PRRS-negative pregnant sows were purchased and divided into four groups. Four sows in two of the groups were vaccinated with JB1, and the other 4 sows were untreated at gestational day 60. At gestational day 93, one vaccinated group and one nonvaccinated group each were challenged with either K07–2273 or K08–1054. All of the sows aborted or delivered until gestation day 115 (24 days post challenge), and the newborn piglets were observed up to the 28th day after birth, which was the end of the experiment. Overall, pregnant sows of the JB1-vaccinated groups showed no meaningful viremia after vaccination and significant reductions in viremia with K07–2273 and K08–1054, exhibiting significantly higher levels of serum virus-neutralizing antibodies than non-vaccinated sows. Moreover, the JB1-vaccinated groups did not exhibit any abortion due to vaccination and showed improved piglet viability and birth weight. The piglets from JB1-vaccinated sows displayed lower viral concentrations in serum and fewer lung lesions compared with those of the piglets from the nonvaccinated sows. Therefore, JB1 is a safe and effective vaccine candidate that confers simultaneous protection against two genetically different PRRSV strains.

Chimeric HP-PRRSV2 containing an ORF2-6 consensus sequence induces antibodies with broadly neutralizing activity and confers cross protection against virulent NADC30-like isolate

Due to the substantial genetic diversity of porcine reproductive and respiratory syndrome virus (PRRSV), commercial PRRS vaccines fail to provide sufficient cross protection. Previous studies have confirmed the existence of PRRSV broadly neutralizing antibodies (bnAbs). However, bnAbs are rarely induced by either natural infection or vaccination. In this study, we designed and synthesized a consensus sequence of PRRSV2 ORF2-6 genes (ORF2-6-CON) encoding all envelope proteins based on 30 representative Chinese PRRSV isolates. The ORF2-6-CON sequence shared > 90% nucleotide identities to all four lineages of PRRSV2 isolates in China. A chimeric virus (rJS-ORF2-6-CON) containing the ORF2-6-CON was generated using the avirulent HP-PRRSV2 JSTZ1712-12 infectious clone as a backbone. The rJS-ORF2-6-CON has similar replication efficiency as the backbone virus in vitro. Furthermore, pig inoculation and challenge studies showed that rJS-ORF2-6-CON is not pathogenic to piglets and confers better cross protection against the virulent NADC30-like isolate than a commercial HP-PRRS modified live virus (MLV) vaccine. Noticeably, the rJS-ORF2-6-CON strain could induce bnAbs while the MLV strain only induced homologous nAbs. In addition, the lineages of VDJ repertoires potentially associated with distinct nAbs were also characterized. Overall, our results demonstrate that rJS-ORF2-6-CON is a promising candidate for the development of a PRRS genetic engineered vaccine conferring cross protection.

Porcine Reproductive and Respiratory Syndrome Virus: Immune Escape and Application of Reverse Genetics in Attenuated Live Vaccine Development

Porcine reproductive and respiratory syndrome virus (PRRSV), an RNA virus widely prevalent in pigs, results in significant economic losses worldwide. PRRSV can escape from the host immune response in several processes. Vaccines, including modified live vaccines and inactivated vaccines, are the best available countermeasures against PRRSV infection. However, challenges still exist as the vaccines are not able to induce broad protection. The reason lies in several facts, mainly the variability of PRRSV and the complexity of the interaction between PRRSV and host immune responses, and overcoming these obstacles will require more exploration. Many novel strategies have been proposed to construct more effective vaccines against this evolving and smart virus. In this review, we will describe the mechanisms of how PRRSV induces weak and delayed immune responses, the current vaccines of PRRSV, and the strategies to develop modified live vaccines using reverse genetics systems.

Intradermal co-inoculation of codon pair deoptimization (CPD)-attenuated chimeric porcine reproductive and respiratory syndrome virus (PRRSV) with Toll like receptor (TLR) agonists enhanced the protective effects in pigs against heterologous challenge

The objective of this study was to assess protective efficacy of vaccination using CPD-attenuated chimeric PRRSV and Toll like receptor (TLR) agonists (HSP70 c-terminal domain and HSPX) as adjuvants through different inoculation routes. In this study, a chimeric PRRSV composed of two field isolates was synthesized and attenuated by CPD in NSP1 as described in the previous study. The infection of the CPD-attenuated chimeric PRRSV to pigs of 3 weeks-old showed no clinical signs without pathological lesions in necropsy, while it induced improved cross immunity between its parent strains. The TLR agonists were expressed in E. coli and purified to be used. In challenge experiment, pigs of 3 weeks-old were vaccinated using the CPD-attenuated chimeric virus with the prepared TLR agonists through intramuscular or intradermal route, following heterologous challenge after 4 weeks of vaccination. In results, intramuscular or intradermal inoculation of the CPD-attenuated chimeric virus demonstrated excellent protective efficacy against heterologous challenges. Importantly, intradermal inoculation with the TLR agonists enhanced protective effects as shown in the significantly increased level of PRRSV-specific IFN-γ-SCs and cytokines in sera, and the significant reduction of pathological lesion and viral load in lung. This study suggested that the intradermal inoculation of CPD-attenuated chimeric PRRSV plus TLR agonists should be more effective for protection of pigs against diverse PRRS field viruses.

Construction and efficacy evaluation of novel swine leukocyte antigen (SLA) class I and class II allele-specific poly-T cell epitope vaccines against porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) causes an economically important global swine disease. Here we report the development of subunit PRRSV-2 vaccines by expressing swine leucocyte antigen (SLA) class I and class II allele-specific epitope antigens in a robust adenovirus vector. SLA I-specific CD8 and SLA II-specific CD4 T cell epitopes of PRRSV-2 NADC20 were predicted in silico. Stable murine leukaemia cell lines (RMA-S), which are TAP-deficient and lacking endogenous class I epitope loading, were established to express different SLA I alleles. The binding stability of PRRSV T cell epitope peptides with SLA I alleles expressed on RMA-S cells was characterized. Two PRRSV poly-T cell epitope peptides were designed. NADC20-PP1 included 39 class I epitopes, consisting of 8 top-ranked epitopes specific to each of 5 SLA I alleles, and fused to 5 class II epitopes specific to SLA II alleles. NADC20-PP2, a subset of PP1, included two top-ranked class I epitopes specific to each of the five SLA I alleles. Two vaccine candidates, Ad-NADC20-PP1 and Ad-NADC20-PP2, were constructed by expressing the polytope peptides in a replication-incompetent human adenovirus 5 vector. A vaccination and challenge study in 30 piglets showed that animals vaccinated with the vaccines had numerically lower gross and histopathology lung lesions, and numerically lower PRRSV RNA loads in lung and serum after challenge compared to the controls, although there was no statistical significance. The results suggested that the Ad-NADC20-PP1 and Ad-NADC20-PP2 vaccines provided little or no protection, further highlighting the tremendous challenges faced in developing an effective subunit PRRSV-2 vaccine.

Future perspectives on swine viral vaccines: where are we headed?

Deliberate infection of humans with smallpox, also known as variolation, was a common practice in Asia and dates back to the fifteenth century. The world's first human vaccination was administered in 1796 by Edward Jenner, a British physician. One of the first pig vaccines, which targeted the bacterium Erysipelothrix rhusiopathiae, was introduced in 1883 in France by Louis Pasteur. Since then vaccination has become an essential part of pig production, and viral vaccines in particular are essential tools for pig producers and veterinarians to manage pig herd health. Traditionally, viral vaccines for pigs are either based on attenuated-live virus strains or inactivated viral antigens. With the advent of genomic sequencing and molecular engineering, novel vaccine strategies and tools, including subunit and nucleic acid vaccines, became available and are being increasingly used in pigs. This review aims to summarize recent trends and technologies available for the production and use of vaccines targeting pig viruses.

Research progress in the development of porcine reproductive and respiratory syndrome virus as a viral vector for foreign gene expression and delivery

INTRODUCTION

Porcine reproductive and respiratory syndrome (PRRS) is an infectious disease of swine characterized by respiratory disorders in growing and finishing pigs and reproductive failure in pregnant sows. PRRSV has been recognized as one of the most economically significant pathogens affecting the global pig industry.

AREAS COVERED

Currently, commercially available vaccines, including traditional killed virus (KV) vaccines and modified live virus (MLV) vaccines, are the cardinal approaches to prevent and control porcine reproductive and respiratory syndrome virus (PRRSV) infection. However, the protective efficacy of these vaccines is not satisfactory, resulting in the continuous evolution and recurrent appearance of the virus as well as the emergence of new variants. A safe and effective vaccine against PRRSV is in dire need. Here, we review the research progress in recent years in the development and use of PRRSV as a viral vector to express foreign genes, and their potential application in gene delivery and vaccine development.

EXPERT OPINION

The potential of using PRRSV-based vectors to express multiple antigens would be particularly instrumental for the development of a new generation of multivalent vaccines against PRRSV and other porcine viruses.

Cytotoxic T lymphocyte epitopes identified from a contemporary strain of porcine reproductive and respiratory syndrome virus enhance CD4+CD8+ T, CD8+ T, and γδ T cell responses

Immuno-stimulatory class I-restricted cytotoxic T lymphocytes (CTL) epitopes of porcine reproductive and respiratory syndrome virus (PRRSV) are important for vaccine development. In this study we first determined the expression frequency of swine leukocyte antigen (SLA) class I alleles in commercial pigs in the United States. The SLA genotyping result allowed us to predict potential CTL epitopes from a contemporary strain of PRRSV (RFLP 1-7-4) by using bioinformatic tools. The predicted epitopes were then evaluated in an ex vivo stimulation assay with peripheral blood mononuclear cells isolated from pigs experimentally-infected with PRRSV. Using flow-cytometry analysis, we identified a number of immuno-stimulatory CTL epitopes, including two peptides from GP3 and two from Nsp9 that significantly improved both degranulation marker CD107a and IFN-γ production in cytotoxic CD4+CD8+ T cells, CD8+ T cells, and γδ T cells, and two peptides that inhibited IFN-γ production. These CTL epitopes will aid future vaccine development against PRRSV.

Nsp2 and GP5-M of Porcine Reproductive and Respiratory Syndrome Virus Contribute to Targets for Neutralizing Antibodies

Porcine reproductive and respiratory syndrome virus (PRRSV) is characterized by its genetic variation and limited cross protection among heterologous strains. Even though several viral structural proteins have been regarded as inducers of neutralizing antibodies (NAs) against PRRSV, the mechanism underlying limited cross-neutralization among heterologous strains is still controversial. In the present study, examinations of NA cross reaction between a highly pathogenic PRRSV (HP-PRRSV) strain, JXwn06, and a low pathogenic PRRSV (LP-PRRSV) strain, HB-1/3.9, were conducted with viral neutralization assays in MARC-145 cells. None of the JXwn06-hyperimmuned pigs' sera could neutralize HB-1/3.9 in vitro and vice versa. To address the genetic variation between these two viruses that are associated with limited cross-neutralization, chimeric viruses with coding regions swapped between these two strains were constructed. Viral neutralization assays indicated that variations in nonstructural protein 2 (nsp2) and structural proteins together contribute to weak cross-neutralization activity between JXwn06 and HB-1/3.9. Furthermore, we substituted the nsp2-, glycoprotein2 (GP2)-, GP3-, and GP4-coding regions together, or nsp2-, GP5-, and membrane (M) protein-coding regions simultaneously between these two viruses to construct chimeric viruses to test cross-neutralization reactivity with hyperimmunized sera induced by their parental viruses. The results indicated that the swapped nsp2 and GP5-M viruses increased the neutralization reactivity with the donor strain antisera in MARC-145 cells. Taken together, these results show that variations in nsp2 and GP5-M correlate with the limited neutralization reactivity between the heterologous strains HP-PRRSV JXwn06 and LP-PRRSV HB-1/3.9.

A DNA Prime Immuno-Potentiates a Modified Live Vaccine against the Porcine Reproductive and Respiratory Syndrome Virus but Does Not Improve Heterologous Protection

The porcine reproductive and respiratory syndrome virus (PRRSV), an RNA virus inducing abortion in sows and respiratory disease in young pigs, is a leading infectious cause of economic losses in the swine industry. Modified live vaccines (MLVs) help in controlling the disease, but their efficacy is often compromised by the high genetic diversity of circulating viruses, leading to vaccine escape variants in the field. In this study, we hypothesized that a DNA prime with naked plasmids encoding PRRSV antigens containing conserved T-cell epitopes may improve the protection of MLV against a heterologous challenge. Plasmids were delivered with surface electroporation or needle-free jet injection and European strain-derived PRRSV antigens were targeted or not to the dendritic cell receptor XCR1. Compared to MLV-alone, the DNA-MLV prime- boost regimen slightly improved the IFNγ T-cell response, and substantially increased the antibody response against envelope motives and the nucleoprotein N. The XCR1-targeting of N significantly improved the anti-N specific antibody response. Despite this immuno-potentiation, the DNA-MLV regimen did not further decrease the serum viral load or the nasal viral shedding of the challenge strain over MLV-alone. Finally, the heterologous protection, achieved in absence of detectable effective neutralizing antibodies, was not correlated to the measured antibody or to the IFNγ T-cell response. Therefore, immune correlates of protection remain to be identified and represent an important gap of knowledge in PRRSV vaccinology. This study importantly shows that a naked DNA prime immuno-potentiates an MLV, more on the B than on the IFNγ T-cell response side, and has to be further improved to reach cross-protection.

High reversion potential of a cell-adapted vaccine candidate against highly pathogenic porcine reproductive and respiratory syndrome

Modified live vaccine (MLV) based on highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) is prone to quick reversion of virulence upon circulating in host animals. The objective of this study was to evaluate the virulence reversion potential of HP-PRRSV MLV and to identify elements within the HP-PRRSV genome contributing to this phenomenon. A blind passage, cell-adaptation strategy was attempted to attenuate a HP-PRRSV strain JX143, which was isolated during the atypical PRRS outbreak in 2006. Two attenuated candidates passage 87 (JXM87) and passage 105 (JXM105) used as MLVs showed the best balance of safety and efficacy in 4 week-old piglets (unpublished data). Two studies were performed during which the candidates were assessed for reversion to virulence through five back passages in susceptible piglets (21 ± 3 days of age). Both study results showed increase in clinical signs, pyrexia and lung lesions as well as decreased average daily weight gain as of passage 3 in susceptible pigs clearly, and it indicated that both candidates regained virulence, irrespective of the passage level. Increase in respective parameters was accompanied by increase in viremia in piglets: JXM87 virus titer increased from Passage 1 (P1) 4.40 Lg TCID₅₀/mL to P4 5.75 Lg TCID₅₀/mL, and JXM105 virus titer increased from P1 3.78 Lg TCID₅₀/mL to P4 6.42 Lg TCID₅₀/mL. Next generation sequencing (NGS) was performed on clinical samples (serum, lung tissue) from P4 animals. Sequence analysis comparing P4 materials with their parental strains revealed 10 amino acid mutations in 4 proteins for JXM87 and 14 amino acid mutations in 9 proteins for JXM105, respectively. Interestingly, five amino acid mutations were identical for the two candidates, which were located in nsp1β, GP5a and nsp10 coding regions, suggesting nsp1β, GP5a and nsp10 could contribute to virulence in HP-PRRSV.

Recombinant Porcine Reproductive and Respiratory Syndrome Virus Expressing Membrane-Bound Interleukin-15 as an Immunomodulatory Adjuvant Enhances NK and γδ T Cell Responses and Confers Heterologous Protection

Cytokines are often used as adjuvants to improve vaccine immunogenicity, since they are important in initiating and shaping the immune response. The available commercial modified live-attenuated vaccines (MLVs) against porcine reproductive and respiratory syndrome virus (PRRSV) are unable to mount sufficient heterologous protection, as they typically induce weak innate and inadequate T cell responses. In this study, we investigated the immunogenicity and vaccine efficacy of recombinant PRRSV MLVs incorporated with the porcine cytokine interleukin-15 (IL-15) or IL-18 gene fused to a glycosylphosphatidylinositol (GPI) modification signal that can anchor the cytokines to the cell membrane. We demonstrated that both cytokines were successfully expressed on the cell membrane of porcine alveolar macrophages after infection with recombinant MLVs. Pigs vaccinated with recombinant MLVs or the parental Suvaxyn MLV had significantly reduced lung lesions and viral RNA loads in the lungs after heterologous challenge with the PRRSV NADC20 strain. The recombinant MLVs SUV-IL-15 and SUV-IL-18 recovered the inhibition of the NK cell response seen with Suvaxyn MLV. The recombinant MLV SUV-IL-15 significantly increased the numbers of gamma interferon (IFN-γ)-producing cells in circulation at 49 days postvaccination (dpv), especially for IFN-γ-producing CD4^- CD8⁺ T cells and γδ T cells, compared to the Suvaxyn MLV and SUV-IL-18. Additionally, MLV SUV-IL-15-vaccinated pigs also had elevated levels of γδ T cell responses observed at 7 dpv, 49 dpv, and 7 days postchallenge. These data demonstrate that the recombinant MLV expressing membrane-bound IL-15 enhances NK and T cell immune responses after vaccination and confers improved heterologous protection, although this was not statistically significant compared to the parental MLV.IMPORTANCE Porcine reproductive and respiratory syndrome (PRRS) has arguably been the most economically important global swine disease, causing immense economic losses worldwide. The available commercial modified live-attenuated vaccines (MLVs) against PRRS virus (PRRSV) are generally effective against only homologous or closely related virus strains but are ineffective against heterologous strains, partially due to the insufficient immune response induced by the vaccine virus. To improve the immunogenicity of MLVs, in this study, we present a novel approach of using porcine IL-15 or IL-18 as an adjuvant by directly incorporating its encoding gene into a PRRSV MLV and expressing it as an adjuvant. Importantly, we directed the expression of the incorporated cytokines to the cell membrane surface by fusing the genes with a membrane-targeting signal from CD59. The recombinant MLV virus expressing the membrane-bound IL-15 cytokine greatly enhanced NK cell and γδ T cell responses and also conferred improved protection against heterologous challenge with the PRRSV NADC20 strain.

Improved Vaccine against PRRSV: Current Progress and Future Perspective

Porcine reproductive and respiratory syndrome virus (PRRSV), one of the most economically significant pathogens worldwide, has caused numerous outbreaks during the past 30 years. PRRSV infection causes reproductive failure in sows and respiratory disease in growing and finishing pigs, leading to huge economic losses for the swine industry. This impact has become even more significant with the recent emergence of highly pathogenic PRRSV strains from China, further exacerbating global food security. Since new PRRSV variants are constantly emerging from outbreaks, current strategies for controlling PRRSV have been largely inadequate, even though our understanding of PRRSV virology, evolution and host immune response has been rapidly expanding. Meanwhile, practical experience has revealed numerous safety and efficacy concerns for currently licensed vaccines, such as shedding of modified live virus (MLV), reversion to virulence, recombination between field strains and MLV and failure to elicit protective immunity against heterogeneous virus. Therefore, an effective vaccine against PRRSV infection is urgently needed. Here, we systematically review recent advances in PRRSV vaccine development. Antigenic variations resulting from PRRSV evolution, identification of neutralizing epitopes for heterogeneous isolates, broad neutralizing antibodies against PRRSV, chimeric virus generated by reverse genetics, and novel PRRSV strains with interferon-inducing phenotype will be discussed in detail. Moreover, techniques that could potentially transform current MLV vaccines into a superior vaccine will receive special emphasis, as will new insights for future PRRSV vaccine development. Ultimately, improved PRRSV vaccines may overcome the disadvantages of current vaccines and minimize the PRRS impact to the swine industry.