Generation and evaluation of a chimeric classical swine fever virus expressing a visible marker gene

Essential role of cis-encoded mature NS3 in the genome packaging of classical swine fever virus

Classical swine fever virus (CSFV) is a member of the genus Pestivirus within the family Flaviviridae. The enveloped particles contain a plus-stranded RNA genome encoding a single large polyprotein. The processing of this polyprotein undergoes dynamic changes throughout the infection cycle. The release of mature NS3 from the polyprotein is mediated and regulated by the NS2 autoprotease and a cellular co-factor, restricting efficient cleavage to the early phases of infection. NS3 is a multifunctional viral enzyme exhibiting helicase, NTPase, and protease activities pivotal for viral replication. Hence, the release of mature NS3 fuels replication, whereas unprocessed NS2-3 precursors are vital for progeny virus production in later phases of infection. Thus far, no packaging signals have been identified for pestivirus RNA. To explore the prerequisites for particle assembly, trans-packaging experiments were conducted using CSFV subgenomes and coreless CSFV strains. Intriguingly, we discovered a significant role of mature NS3 in genome packaging, effective only when the protein is encoded by the RNA molecule itself. This finding was reinforced by employing artificially engineered CSFV strains with duplicated NS3 genes, separating uncleavable NS2-3 precursors from mature NS3 molecules. The model for NS2-3/NS3 functions in genome packaging of pestiviruses appears to be much more complicated than anticipated, involving distinct functions of the mature NS3 and its precursor molecule NS2-3. Moreover, the reliance of genome packaging on cis-encoded NS3 may act as a downstream quality control mechanism, averting the packaging of defective genomes and coordinating the encapsidation of RNA molecules before membrane acquisition.

IMPORTANCE

Pestiviruses are economically significant pathogens in livestock. Although genome organization and non-structural protein functions resemble those of other Flaviviridae genera, distinct differences can be observed. Previous studies showed that coreless CSFV strains can produce coreless virions mediated by single compensatory mutations in NS3. In this study, we could show that only RNA molecules encoding these mutations in the mature NS3 are packaged in the absence of the core protein. Unlike this selectivity, a pool of structural proteins in the host cell was readily available for packaging all CSFV genomes. Similarly, the NS2-3-4A precursor molecules required for packaging could also be provided in trans. Consequently, genome packaging in pestiviruses is governed by cis-encoded mature NS3. Reliance on cis-acting proteins restricts the acceptance of defective genomes and establishes packaging specificity regardless of RNA sequence-specific packaging signals. Understanding the role of NS3 in pestiviral genome packaging might uncover new targets for antiviral therapies.

A Genetically Engineered Bivalent Vaccine Coexpressing a Molecular Adjuvant against Classical Swine Fever and Porcine Epidemic Diarrhea

Classical swine fever (CSF) and porcine epidemic diarrhea (PED) are highly contagious viral diseases that pose a significant threat to piglets and cause substantial economic losses in the global swine industry. Therefore, the development of a bivalent vaccine capable of targeting both CSF and PED simultaneously is crucial. In this study, we genetically engineered a recombinant classical swine fever virus (rCSFV) expressing the antigenic domains of the porcine epidemic diarrhea virus (PEDV) based on the modified infectious cDNA clone of the vaccine strain C-strain. The S1N and COE domains of PEDV were inserted into C-strain cDNA clone harboring the mutated 136th residue of Npro and substituted 3'UTR to generate the recombinant chimeric virus vC/SM3'UTRN-S1NCOE. To improve the efficacy of the vaccine, we introduced the tissue plasminogen activator signal (tPAs) and CARD domain of the signaling molecule VISA into vC/SM3'UTRN-S1NCOE to obtain vC/SM3'UTRN-tPAsS1NCOE and vC/SM3'UTRN-CARD/tPAsS1NCOE, respectively. We characterized three vaccine candidates in vitro and investigated their immune responses in rabbits and pigs. The NproD136N mutant exhibited normal autoprotease activity and mitigated the inhibition of IFN-β induction. The introduction of tPAs and the CARD domain led to the secretory expression of the S1NCOE protein and upregulated IFN-β induction in infected cells. Immunization with recombinant CSFVs expressing secretory S1NCOE resulted in a significantly increased in PEDV-specific antibody production, and coexpression of the CARD domain of VISA upregulated the PEDV-specific IFN-γ level in the serum of vaccinated animals. Notably, vaccination with vC/SM3'UTRN-CARD/tPAsS1NCOE conferred protection against virulent CSFV and PEDV challenge in pigs. Collectively, these findings demonstrate that the engineered vC/SM3'UTRN-CARD/tPAsS1NCOE is a promising bivalent vaccine candidate against both CSFV and PEDV infections.

Generation of infectious clone of bovine adenovirus type I expressing a visible marker gene

BACKGROUND AND OBJECTIVE

Bovine adenovirus type 3 (BAdV3) has been widely used as a vector for vaccine research and development, whereas BAdV1 biology and BAdV1-based vectored vaccine have been less frequently reported. We aimed to construct an infectious BAdV1 clone and explore the functions of BAdV1 genes.

METHODS

First, the infectious clone of pUCBAdV1 containing the full-length BAdV1 DNA and the recombinant plasmid pUCBAV1-EYFP expressing the marker gene EYFP were constructed. Then, the recombinant viruses BAdV101 and rBAdV1-EYFP were rescued. The stability of the exogenous EYFP gene was analyzed by continuous passage, PCR, and western blotting. Finally, the virus neutralization titer of the rescued viruses was evaluated.

RESULTS

The infectious clones of pUCBAdV1 and pUCBAV1-EYFP were constructed and the recombinant viruses BAdV101 and rBAdV1-EYFP were rescued successfully. Moreover, the results showed that the EYFP gene could be expressed continuously. In addition, the replication of rBAdV1-EYFP was less efficient than that of the wild-type virus wtBAdV1 in vitro, while the efficacy of BAdV101 replication was almost the same as that of wtBAdV1. Furthermore, the neutralization test showed that the neutralization titer of rBAdV1-EYFP was consistent with that of wtBAdV1.

CONCLUSION

To our knowledge, the infectious genome of pUCBAV1-EYFP expressing a visible marker gene EYFP was constructed for the first time, and the finding forms a basis for the development of BAdV1-based efficient vectored vaccine.

Applications of Replicating-Competent Reporter-Expressing Viruses in Diagnostic and Molecular Virology

Commonly used tests based on wild-type viruses, such as immunostaining, cannot meet the demands for rapid detection of viral replication, high-throughput screening for antivirals, as well as for tracking viral proteins or virus transport in real time. Notably, the development of replicating-competent reporter-expressing viruses (RCREVs) has provided an excellent option to detect directly viral replication without the use of secondary labeling, which represents a significant advance in virology. This article reviews the applications of RCREVs in diagnostic and molecular virology, including rapid neutralization tests, high-throughput screening systems, identification of viral receptors and virus-host interactions, dynamics of viral infections in vitro and in vivo, vaccination approaches and others. However, there remain various challenges associated with RCREVs, including pathogenicity alterations due to the insertion of a reporter gene, instability or loss of the reporter gene expression, or attenuation of reporter signals in vivo. Despite all these limitations, RCREVs have become powerful tools for both basic and applied virology with the development of new technologies for generating RCREVs, the inventions of novel reporters and the better understanding of regulation of viral replication.