A cloned classical swine fever virus derived from the vaccine strain GPE- causes cytopathic effect in CPK-NS cells via type-I interferon-dependent necroptosis

Establishment of a superinfection exclusion method for pestivirus titration using a recombinant reporter pestiviruses

Pestiviruses are classified into two biotypes based on their cytopathogenicity. As the majority of pestivirus field isolates are noncytopathogenic, their titration requires alternative methods rather than direct observation of cytopathogenic effects, such as immunostaining using specific antibodies or interference with cytopathogenic strains. However, these methods require microscopic observation to assess virus growth, which is time- and labor-intensive, especially when handling several samples. In this study, we developed a novel luciferase-based pestivirus titration method using the superinfection exclusion phenomenon with recombinant reporter pestiviruses that possessed an 11-amino-acid subunit derived from NanoLuc luciferase (HiBiT). In this method, swine kidney cells were inoculated with classical swine fever virus (CSFV) and superinfected with the reporter CSFV vGPE-/HiBiT 5 days postinoculation. Virus titer was determined based on virus growth measured in luminescence using the culture fluid 3 days after superinfection; the resultant virus titer was comparable to that obtained by immunoperoxidase staining. Furthermore, this method has proven to be applicable for the titration of border disease virus (BDV) by superinfection with both the homologous reporter BDV and heterologous reporter CSFV, suggesting that this novel virus titration method is a simple technique for automated virus detection based on the luciferase system.

Generation and Efficacy of Two Chimeric Viruses Derived from GPE- Vaccine Strain as Classical Swine Fever Vaccine Candidates

A previous study proved that vGPE^- mainly maintains the properties of classical swine fever (CSF) virus, which is comparable to the GPE^- vaccine seed and is a potentially valuable backbone for developing a CSF marker vaccine. Chimeric viruses were constructed based on an infectious cDNA clone derived from the live attenuated GPE^- vaccine strain as novel CSF vaccine candidates that potentially meet the concept of differentiating infected from vaccinated animals (DIVA) by substituting the glycoprotein E^rns of the GPE^- vaccine strain with the corresponding region of non-CSF pestiviruses, either pronghorn antelope pestivirus (PAPeV) or Phocoena pestivirus (PhoPeV). High viral growth and genetic stability after serial passages of the chimeric viruses, namely vGPE^-/PAPeV E^rns and vGPE^-/PhoPeV E^rns, were confirmed in vitro. In vivo investigation revealed that two chimeric viruses had comparable immunogenicity and safety profiles to the vGPE^- vaccine strain. Vaccination at a dose of 10^4.0 TCID₅₀ with either vGPE^-/PAPeV E^rns or vGPE^-/PhoPeV E^rns conferred complete protection for pigs against the CSF virus challenge in the early stage of immunization. In conclusion, the characteristics of vGPE^-/PAPeV E^rns and vGPE^-/PhoPeV E^rns affirmed their properties, as the vGPE^- vaccine strain, positioning them as ideal candidates for future development of a CSF marker vaccine.

Removal of the Erns RNase Activity and of the 3' Untranslated Region Polyuridine Insertion in a Low-Virulence Classical Swine Fever Virus Triggers a Cytokine Storm and Lethal Disease

In this study, we assessed the potential synergistic effect of the E^rns RNase activity and the poly-U insertion in the 3′ untranslated region (UTR) of the low-virulence classical swine fever virus (CSFV) isolate Pinar de Rio (PdR) in innate and adaptive immunity regulation and its relationship with classical swine fever (CSF) pathogenesis in pigs. We knocked out the E^rns RNase activity of PdR and replaced the long polyuridine sequence of the 3′ UTR with 5 uridines found typically at this position, resulting in a double mutant, vPdR-H₃₀K-5U. This mutant induced severe CSF in 5-day-old piglets and 3-week-old pigs, with higher lethality in the newborn (89.5%) than in the older (33.3%) pigs. However, the viremia and viral excretion were surprisingly low, while the virus load was high in the tonsils. Only alpha interferon (IFN-α) and interleukin 12 (IL-12) were highly and consistently elevated in the two groups. Additionally, high IL-8 levels were found in the newborn but not in the older pigs. This points toward a role of these cytokines in the CSF outcome, with age-related differences. The disproportional activation of innate immunity might limit systemic viral spread from the tonsils and increase virus clearance, inducing strong cytokine-mediated symptoms. Infection with vPdR-H₃₀K-5U resulted in poor neutralizing antibody responses compared with results obtained previously with the parent and RNase knockout PdR. This study shows for the first time the synergistic effect of the 3′ UTR and the E^rns RNase function in regulating innate immunity against CSFV, favoring virus replication in target tissue and thus contributing to disease severity.

IMPORTANCE CSF is one of the most relevant viral epizootic diseases of swine, with high economic and sanitary impact. Systematic stamping out of infected herds with and without vaccination has permitted regional virus eradication. However, the causative agent, CSFV, persists in certain areas of the world, leading to disease reemergence. Nowadays, low- and moderate-virulence strains that could induce unapparent CSF forms are prevalent, posing a challenge for disease eradication. Here, we show for the first time the synergistic role of lacking the E^rns RNase activity and the 3′ UTR polyuridine insertion from a low-virulence CSFV isolate in innate immunity disproportional activation. This might limit systemic viral spread to the tonsils and increase virus clearance, inducing strong cytokine-mediated symptoms, thus contributing to disease severity. These results highlight the role played by the E^rns RNase activity and the 3′ UTR in CSFV pathogenesis, providing new perspectives for novel diagnostic tools and vaccine strategies.

Antiviral Effects of 5-Aminolevulinic Acid Phosphate against Classical Swine Fever Virus: In Vitro and In Vivo Evaluation

The inhibitory effects of 5-aminolevulinic acid phosphate (5-ALA), an important amino acid for energy production in the host, against viral infections were previously reported. Here, the antiviral effects of 5-ALA against classical swine fever virus (CSFV) belonging to the genus Pestivirus in the Flaviviridae family and its possible mechanisms were investigated. CSFV replication was suppressed in swine cells supplemented with 5-ALA or its metabolite, protoporphyrin IX (PPIX). The infectivity titer of CSFV was decreased after mixing with PPIX extracellularly. In addition, the activities of the replication cycle were decreased in the presence of PPIX based on the CSFV replicon assay. These results showed that PPIX exerted antiviral effects by inactivating virus particles and inhibiting the replication cycle. To evaluate the in vivo efficacy of 5-ALA, pigs were supplemented daily with 5-ALA for 1 week before virus inoculation and then inoculated with a virulent CSFV strain at the 10^7.0 50% tissue culture infectious dose. The clinical scores of the supplemented group were significantly lower than those of the nonsupplemented group, whereas the virus growth was not. Taken together, 5-ALA showed antiviral effects against CSFV in vitro, and PPIX played a key role by inactivating virus particles extracellularly and inhibiting the replication cycle intracellularly.

Characteristics of Classical Swine Fever Virus Variants Derived from Live Attenuated GPE- Vaccine Seed

The GPE^- strain is a live attenuated vaccine for classical swine fever (CSF) developed in Japan. In the context of increasing attention for the differentiating infected from vaccinated animals (DIVA) concept, the achievement of CSF eradication with the GPE^- proposes it as a preferable backbone for a recombinant CSF marker vaccine. While its infectious cDNA clone, vGPE^-, is well characterized, 10 amino acid substitutions were recognized in the genome, compared to the original GPE^- vaccine seed. To clarify the GPE^- seed availability, this study aimed to generate and characterize a clone possessing the identical amino acid sequence to the GPE^- seed. The attempt resulted in the loss of the infectious GPE^- seed clone production due to the impaired replication by an amino acid substitution in the viral polymerase NS5B. Accordingly, replication-competent GPE^- seed variant clones were produced. Although they were mostly restricted to propagate in the tonsils of pigs, similarly to vGPE^-, their type I interferon-inducing capacity was significantly lower than that of vGPE^-. Taken together, vGPE^- mainly retains ideal properties for the CSF vaccine, compared with the seed variants, and is probably useful in the development of a CSF marker vaccine.

Abrogation of the RNase activity of Erns in a low virulence classical swine fever virus enhances the humoral immune response and reduces virulence, transmissibility, and persistence in pigs

The prevalence of low virulence classical swine fever virus (CSFV) strains makes viral eradication difficult in endemic countries. However, the determinants for natural CSFV attenuation and persistence in the field remain unidentified. The aim of the present study was to assess the role of the RNase activity of CSFV E^rns in pathogenesis, immune response, persistent infection, and viral transmission in pigs. To this end, a functional cDNA clone pPdR-H₃₀K-36U with an E^rns lacking RNase activity was constructed based on the low virulence CSFV field isolate Pinar de Rio (PdR). Eighteen 5-day-old piglets were infected with vPdR-H₃₀K-36U. Nine piglets were introduced as contacts. The vPdR-H₃₀K-36U virus was attenuated in piglets compared to the parental vPdR-36U. Only RNA traces were detected in sera and body secretions and no virus was isolated from tonsils, showing that RNase inactivation may reduce CSFV persistence and transmissibility. The vPdR-H₃₀K-36U mutant strongly activated the interferon-α (IFN-α) production in plasmacytoid dendritic cells, while in vivo, the IFN-α response was variable, from moderate to undetectable depending on the animal. This suggests a role of the CSFV E^rns RNase activity in the regulation of innate immune responses. Infection with vPdR-H₃₀K-36U resulted in higher antibody levels against the E2 and E^rns glycoproteins and in enhanced neutralizing antibody responses when compared with vPdR-36U. These results pave the way toward a better understanding of viral attenuation mechanisms of CSFV in pigs. In addition, they provide novel insights relevant for the development of DIVA vaccines in combination with diagnostic assays for efficient CSF control.

Classical swine fever virus: the past, present and future

Classical swine fever (CSF) is among the most relevant viral epizootic diseases of swine. Due to its severe economic impact, CSF is notifiable to the world organisation for animal health. Strict control policies, including systematic stamping out of infected herds with and without vaccination, have permitted regional virus eradication. Nevertheless, CSF virus (CSFV) persists in certain areas of the world and has re-emerged regularly. This review summarizes the basic established knowledge in the field and provides a comprehensive and updated overview of the recent advances in fundamental CSFV research, diagnostics and vaccine development. It covers the latest discoveries on the genetic diversity of pestiviruses, with implications for taxonomy, the progress in understanding disease pathogenesis, immunity against acute and persistent infections, and the recent findings in virus-host interactions and virulence determinants. We also review the progress and pitfalls in the improvement of diagnostic tools and the challenges in the development of modern and efficacious marker vaccines compatible with serological tests for disease surveillance. Finally, we highlight the gaps that require research efforts in the future.