An improved system to generate recombinant canine distemper virus

BACKGROUND

Canine distemper virus (CDV) is a pathogen with the capability of cross-species transmission. It has crossed the species barrier to infect many other species, and its host range is expanding. The reverse genetic platform, a useful tool for scientific research, allows the generation of recombinant viruses from genomic cDNA clones in vitro.

METHODS

To improve the reverse genetic system of CDV, a plasmid containing three independent expression cassettes was constructed for co-expression of the N, P, and L genes and then transfected with a full-length cDNA clone of CDV into Vero cells.

RESULTS

The results indicated that the established rescue system has the advantages of being more convenient, easy to control the transfection ratio, and high rescue efficiency compared with the conventional reverse genetics system.

CONCLUSION

This method not only reduces the number of transfection plasmids, but also improves the rescue efficiency of CDV, which could provide a reference for the recovery of other morbilliviruses.

Rapid Rescue of Goose Astrovirus Genome via Red/ET Assembly

The host-specific infection of Avian Astrovirus (AAstVs) has posed significant challenges to the poultry industry, resulting in substantial economic losses. However, few reports exist on the functional consequences of genome diversity, cross-species infectivity and mechanisms governing virus replication of AAstVs, making it difficult to develop measures to control astrovirus transmission. Reverse genetics technique can be used to study the function of viruses at the molecular level, as well as investigating pathogenic mechanisms and guide vaccine development and disease treatment. Herein, the reverse genetics technique of goose astrovirus GAstV/JS2019 strain was developed based on use of a reconstructed vector including CMV promotor, hammerhead ribozyme (HamRz), hepatitis delta virus ribozyme (HdvRz), and SV40 tail, then the cloned viral genome fragments were connected using Red/ET recombineering. The recombinant rGAstV-JS2019 was readily rescued by transfected the infectious clone plasmid into LMH cells. Importantly, the rescued rGAstV/JS2019 exhibited similar growth kinetics comparable to those of the parental GAstV/JS2019 isolate in cultured cells. Our research results provide an alternative and more effective reverse genetic tool for a detailed understanding of viral replication, pathogenic mechanisms, and molecular mechanisms of evolution.

A Stable Reverse Genetics System of Zika Virus Based on a Self-Splicing Group II Intron

Zika virus (ZIKV) is a mosquito-borne flavivirus of the Flaviviridae family first isolated from a sentinel monkey in the Zika Forest, Uganda, in 1947. Since 2007, the virus has had a vast geographic expansion that extended to the Americas in 2015, leading to a series of large outbreaks. Although mainly transmitted by the bite of Aedes mosquitoes, human infection of ZIKV can also happen through unconventional routes such as sexual intercourse and, more importantly, vertical transmission. The genome of ZIKV is a single-stranded, positive-sense RNA molecule about 11 kb in length. The genome contains a single opening reading frame (ORF) flanked by highly structured 5' and 3' untranslated regions. To understand the mechanisms about ZIKV replication, transmission, and pathogenesis, reverse genetic tools are of great importance. In this chapter, a novel system is described for the generation and manipulation of a ZIKV infectious clone stabilized by a self-splicing group II intron, a mobile element with ribozyme activity. The intron can be spliced in vitro, and thus full-length vRNA can be prepared allowing virus genome manipulation required for further studies.

Bacterial Artificial Chromosome Reverse Genetics Approaches for SARS-CoV-2

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a new member of the Coronaviridae family responsible for the coronavirus disease 19 (COVID-19) pandemic. To date, SARS-CoV-2 has been accountable for over 624 million infection cases and more than 6.5 million human deaths. The development and implementation of SARS-CoV-2 reverse genetics approaches have allowed researchers to genetically engineer infectious recombinant (r)SARS-CoV-2 to answer important questions in the biology of SARS-CoV-2 infection. Reverse genetics techniques have also facilitated the generation of rSARS-CoV-2 expressing reporter genes to expedite the identification of compounds with antiviral activity in vivo and in vitro. Likewise, reverse genetics has been used to generate attenuated forms of the virus for their potential implementation as live-attenuated vaccines (LAV) for the prevention of SARS-CoV-2 infection. Here we describe the experimental procedures for the generation of rSARS-CoV-2 using a well-established and robust bacterial artificial chromosome (BAC)-based reverse genetics system. The protocol allows to produce wild-type and mutant rSARS-CoV-2 that can be used to understand the contribution of viral proteins and/or amino acid residues in viral replication and transcription, pathogenesis and transmission, and interaction with cellular host factors.

Reverse Genetics of Zika Virus Using a Bacterial Artificial Chromosome

Zika virus (ZIKV) is a mosquito-borne member of the Flaviviridae family that has become a global threat to human health. Although ZIKV has been known to circulate for decades causing mild febrile illness, the more recent ZIKV outbreaks in the Americas and the Caribbean have been associated with severe neurological disorders and congenital abnormalities. The development of ZIKV reverse genetics approaches have allowed researchers to address key questions on the biology of ZIKV by genetically engineering infectious recombinant (r)ZIKV. This has resulted in a better understanding of the biology of ZIKV infections, including viral pathogenesis, molecular mechanisms of viral replication and transcription, or the interaction of viral and host factors, among others aspects. In addition, reverse genetics systems have facilitated the identification of anti-ZIKV compounds and the development of new prophylactic approaches to combat ZIKV infections. Different reverse genetics strategies have been implemented for the recovery of rZIKV. All these reverse genetics systems have faced and overcome multiple challenges, including the viral genome size, the toxicity of viral sequences in bacteria, etc. In this chapter we describe the generation of a ZIKV full-length complementary (c)DNA infectious clone based on the use of a bacterial artificial chromosome (BAC) and the experimental procedures for the successful recovery of rZIKV. Importantly, the protocol described in this chapter provides a powerful method for the generation of infectious clones of other flaviviruses with genomes that have stability problems during bacterial propagation.

Reverse Genetics Systems for Filoviruses

Filoviruses are causative agents of severe hemorrhagic fevers with high case fatality rates in humans. For studies of virus biology and the subsequent development of countermeasures, reverse genetic systems, and especially those facilitating the generation of recombinant filoviruses, are indispensable. Here, we describe the generation of recombinant filoviruses from cDNA.

Immunogenic response of recombinant pseudorabies virus carrying B646L and B602L genes of African swine fever virus in mice

African swine fever (ASF) is an acute infectious disease that poses a high lethality risk to domestic pigs and wild boars, causing substantial economic losses to the global pig industry. The prevention and control of ASF remain challenging, necessitating the urgent development of a safe and effective vaccine. This study focused on the essential structural protein p72 of ASFV (encoded by the B646L gene) and its chaperone protein pB602L (encoded by the B602L gene) as the target antigenic proteins. Based on CRISPR/Cas9 gene-editing technology, we constructed a live attenuated recombinant pseudorabies virus vector expressing the p72 and pB602L proteins (designated as rPRVXJ-EGFP/B602L/B646L), and assessed its immunization effect in mice. The recombinant virus rPRVXJ-EGFP/B602L/B646L successfully proliferated and demonstrated stable expression of the p72 and pB602L proteins in BHK-21 cells. Moreover, it exhibited excellent safety when used in mice and induced specific humoral and cellular immune responses targeting p72 and pB602L. In addition, it provided complete protection (100%) against the virulent PRV strain (PRV-XJ). These results indicate that the recombinant virus rPRVXJ-EGFP/B602L/B646L possesses robust immunogenicity and safety in mice. In conclusion, PRV represents a promising viral vector for expressing ASFV gene, and our study serves as an essential reference for the development of viral vector vaccines against ASFV.

Functional characterization of African swine fever virus I329L gene by transcriptome analysis

African swine fever (ASF) is an acute, severe, and highly contagious disease caused by the African swine fever virus (ASFV), which infects domestic pigs and wild boars. The incidence and mortality rates of swine infected with virulent strains of ASFV can reach up to 100%. The large genome, its complex structure, multiple genotypes, and a lack of understanding regarding ASFV gene function are serious obstacles to the development of safe and effective vaccines. Here, ASFV I329L was identified as a relatively conserved gene that is expressed during the late stage of infection. A recombinant virus with I329L gene deletion (ASFV CN/GS/2018-ΔI329L) was produced by replacing I329L with an enhanced green fluorescent protein (EGFP) cassette. In order to explore the function of the ASFV I329L gene, transcriptome sequencing (RNA-seq) was performed on porcine alveolar macrophages (PAMs) infected with ASFV CN/GS/2018 and ASFV CN/GS/2018-ΔI329L. GO functional and KEGG pathway analyses were performed to analyze differentially expressed genes, and different alternative splicing (AS) events were also analyzed. We compared the sequencing data for each sample with the ASFV CN/GS/2018 reference sequence. Interestingly, we found 3 and 1 up-regulated genes and 12 and 19 down-regulated genes at 12 and 24 h post-infection, respectively. In addition, we verified the expression of 5 up-regulated and 5 down-regulated genes by RT-qPCR, and the results were consistent with those obtained based on RNA-seq. In summary, the results obtained in this study provide new insights for further elucidation of ASFV proteins and ASFV-host interactions. These findings will contribute to implementing a comprehensive strategy for controlling the spread of ASF.

Fusing a TurboID tag with the Andrias davidianus ranavirus 2L reduced virus adsorption efficiency

Andrias davidianus ranavirus (ADRV) is a member of the genus ranavirus (family Iridoviridae). ADRV 2L is an envelope protein that could be essential in viral infection. In the present study, the function of ADRV 2L was investigated by fusion with the biotin ligase TurboID tag. A recombinant ADRV with a V5-TurboID tag fused in the N-terminal of 2L (ADRVT-2L) and a recombinant ADRV expressing V5-TurboID (ADRVT) were constructed, respectively. Infection of the recombinant viruses and wild-type ADRV (ADRVWT) in the Chinese giant salamander thymus cell line (GSTC) showed that ADRVT-2L had reduced cytopathic effect and lower virus titers than the other two viruses, indicating the fusion of a big tag affected ADRV infection. Analysis of the temporal expression profile showed that the expression of V5-TurboID-2L was delayed than wild-type 2L. However, electron microscopy found that the virion morphogenesis was not affected in ADRVT-2L-infected cells. Furthermore, the virus binding assay revealed that the adsorption efficiency of ADRVT-2L was considerably decreased compared to the other two viruses. Therefore, these data showed that linking the TurboID tag to ADRV 2L affected virus adsorption to the cell membrane, which suggested an important role of 2L in virus entry into cells.

Spatial resolution of virus replication: RSV and cytoplasmic inclusion bodies

Respiratory Syncytial Virus (RSV) is a major cause of respiratory illness in young children, elderly and immunocompromised individuals worldwide representing a severe burden for health systems. The urgent development of vaccines or specific antivirals against RSV is impaired by the lack of knowledge regarding its replication mechanisms. RSV is a negative-sense single-stranded RNA (ssRNA) virus belonging to the Mononegavirales order (MNV) which includes other viruses pathogenic to humans as Rabies (RabV), Ebola (EBOV), or measles (MeV) viruses. Transcription and replication of viral genomes occur within cytoplasmatic virus-induced spherical inclusions, commonly referred as inclusion bodies (IBs). Recently IBs were shown to exhibit properties of membrane-less organelles (MLO) arising by liquid-liquid phase separation (LLPS). Compartmentalization of viral RNA synthesis steps in viral-induced MLO is indeed a common feature of MNV. Strikingly these key compartments still remain mysterious. Most of our current knowledge on IBs relies on the use of fluorescence microscopy. The ability to fluorescently label IBs in cells has been key to uncover their dynamics and nature. The generation of recombinant viruses expressing a fluorescently-labeled viral protein and the immunolabeling or the expression of viral fusion proteins known to be recruited in IBs are some of the tools used to visualize IBs in infected cells. In this chapter, microscope techniques and the most relevant studies that have shed light on RSV IBs fundamental aspects, including biogenesis, organization and dynamics are being discussed and brought to light with the investigations carried out on other MNV.

Pathogenicity and immunogenicity of gI/gE/TK-gene-deleted Felid herpesvirus 1 variants in cats

BACKGROUND

Felid herpesvirus 1 (FHV-1) is a major pathogenic agent of upper respiratory tract infections and eye damage in felines worldwide. Current FHV-1 vaccines offer limited protection of short duration, and therefore, do not reduce the development of clinical signs or the latency of FHV-1.

METHODS

To address these shortcomings, we constructed FHV ∆gIgE-eGFP, FHV ∆TK mCherry, and FHV ∆gIgE/TK eGFP-mCherry deletion mutants (ΔgI/gE, ΔTK, and ΔgIgE/TK, respectively) using the clustered regularly interspaced palindromic repeats (CRISPR)/CRISP-associated protein 9 (Cas9) system (CRISPR/Cas9), which showed safety and immunogenicity in vitro. We evaluated the safety and efficacy of the deletion mutants administered with intranasal (IN) and IN + subcutaneous (SC) vaccination protocols. Cats in the vaccination group were vaccinated twice at a 4-week interval, and all cats were challenged with infection 3 weeks after the last vaccination. The cats were assessed for clinical signs, nasal shedding, and virus-neutralizing antibodies (VN), and with postmortem histological testing.

RESULTS

Vaccination with the gI/gE-deleted and gI/gE/TK-deleted mutants was safe and resulted in significantly lower clinical disease scores, fewer pathological changes, and less nasal virus shedding after infection. All three mutants induced virus-neutralizing antibodies after immunization.

CONCLUSIONS

In conclusion, this study demonstrates the advantages of FHV-1 deletion mutants in preventing FHV-1 infection in cats.

Simultaneous expression of three reporter proteins from a porcine reproductive and respiratory syndrome virus-based vector

The mechanism of discontinuous transcription for the synthesis of a series of sub-genomic mRNAs to express the structural proteins of porcine reproductive and respiratory syndrome virus (PRRSV) potentially allows for the simultaneous expression of multiple foreign genes. This can occur by insertion of multiple novel independent transcription units between the ORF sequences of the PRRSV genome. Here, an expression cassette consisting of a red fluorescent protein (RFP) gene flanked at its 3' end by transcription-regulating sequences (TRS) and an expression cassette consisting of an iLOV gene flanked at its 5' end by TRS, was constructed. The resulting expression cassette containing a RFP and an iLOV gene were introduced between ORF1b and 2 as well as ORF7 and 3'UTR, respectively, in an infectious PRRSV cDNA clone. Transfection of the resulting clone (pGX-12RFP-73iLOV) into cells resulted in the recovery of a recombinant virus (rGX-12RFP-73iLOV). Simultaneous expression of RFP and iLOV was observed in MARC-145 cells infected with rGX-RFP-iLOV. To test the ability of the PRRSV genome to express all three reporter genes simultaneously, an expression cassette containing the Gluc gene and one containing the iLOV gene were also inserted in between ORF1b and 2 as well as ORF7 and 3'UTR, respectively. This was performed in a recently obtained infectious PRRSV cDNA clone carrying a RFP gene in nsp2. Transfection of the construct (pGX-R-Gluc-iLOV) carrying the three reporter genes into cells allowed the rescue of the recombinant reporter virus (rGX-R-Gluc-iLOV) which showed similar growth characteristics to the parental virus but yielded 100-fold less infectious viruses. Fluorescence microscopy of cells infected with rGX-R-Gluc-iLOV demonstrated the presence of both RFP and iLOV genes. Gluc activities in supernatants harvested at different time points from cells infected with recombinant viruses carrying Gluc showed increased levels of Gluc activity as the infection progressed. This indicated that Gluc gene as well as its activity were acceptable parameters to monitor viral propagation. Our results indicate that it is possible to introduce at least three foreign proteins simultaneously in a PRRSV-based vector and such studies will prove invaluable in our future understanding of these viruses.

History of advances in genetic engineering of viruses before COVID-19 pandemic

Background

On December 31, 2019, the World Health Organization's China Country Office was alerted to cases of pneumonia of unknown cause detected in Wuhan City, Hubei Province of China.

Methods

Due to the fact that to date, the question of the origin of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has not been resolved yet, the author analyzed the main advances in the development of genetic engineering of viruses that took place before the onset of the COVID-19 pandemic.

Results

The first artificial genetically modified viruses could appear in nature in the mid-1950s. The technique of nucleic acid hybridization was developed by the end-1960s. In the late 1970s, a method called the "reverse genetics" emerged to synthesize ribonucleic acid and deoxyribonucleic acid molecules. In the early 1980-s, it became possible to combine the genes of different viruses and insert the genes of one virus into the genome of another virus. Since that time, the production of vector vaccines began. At present, by modern technologies one can assemble any virus based on the nucleotide sequence available in the virus database or designed by a computer as a virtual model.

Conclusion

Scientists around the world are invited to answer the call of Neil Harrison and Jeffrey Sachs of Columbia University, for a thorough and independent investigation into the origin of SARS-CoV-2. Only a full understanding of the origin of the new virus can minimize the likelihood of a similar pandemic in the future.

Cloning Strategies for the Generation of Recombinant Capripoxvirus Through the Use of Screening and Selection Markers

The ability to manipulate capripoxvirus through gene knockouts and gene insertions has become an increasingly valuable research tool in elucidating the function of individual genes of capripoxvirus, as well as in the development of capripoxvirus-based recombinant vaccines. The homologous recombination technique is commonly used to generate capripoxvirus knockout viruses (KO), and is based on the targeting of a particular viral gene of interest. This technique can also be used to insert a gene of interest. A protocol for the generation of a viral gene knockout is described. This technique involves the use of a plasmid which encodes the flanking sequences of the regions where the homologous recombination will occur, and will result in the insertion of an EGFP reporter gene for visualization of recombinant virus, as well as the E. coli gpt gene as a positive selection marker. If an additional gene is to be incorporated, this can be achieved by inserting a gene of interest for expression under a poxvirus promoter into the plasmid between the flanking regions for insertion. This chapter describes a protocol for generating such recombinant capripoxviruses. An alternative step for the removal of both the EGFP and gpt cassettes and an optional selection step using CRISPR technology are also described.

Generation of Bi-Reporter-Expressing Tri-Segmented Arenavirus

Reverse genetics systems provide a powerful tool to generate recombinant arenavirus expressing reporters to facilitate the investigation of the arenavirus life cycle and also for the discovery of antiviral countermeasures. The plasmid-encoded viral ribonucleoprotein components initiate the transcription and replication of a plasmid-driven full-length viral genome, resulting in infectious virus. Thereby, this approach is ideal for the generation of recombinant arenaviruses expressing reporter genes that can be used as valid surrogates for virus replication. By splitting the small viral segment (S) into two viral segments (S1 and S2), each of them encoding a reporter gene, recombinant tri-segmented arenavirus can be rescued. Bi-reporter-expressing recombinant tri-segmented arenaviruses represent an excellent tool to study the biology of arenaviruses, including the identification and characterization of both prophylactic and therapeutic countermeasures for the treatment of arenaviral infections. In this chapter, we describe a detailed protocol on the generation and in vitro characterization of recombinant arenaviruses containing a tri-segment genome expressing two reporter genes based on the prototype member in the family, lymphocytic choriomeningitis virus (LCMV). Similar experimental approaches can be used for the generation of bi-reporter-expressing tri-segment recombinant viruses for other members in the arenavirus family.

Isolation of Porcine Bone Marrow Cells and Generation of Recombinant African Swine Fever Viruses

Genetic manipulation of ASFV has been increasingly used not only for the development of live attenuated vaccines but also as an indispensable tool to further our understanding of the virus-host interactions. Here we present methods for isolation of porcine bone marrow cells and purification of recombinant ASFV using both chromogenic and fluorescent reporters. We also describe in detail a newly developed method to purify genetically modified ASFV using fluorescence-activated cell sorting (FACS).

Exploring propriospinal neuron-mediated neural circuit plasticity using recombinant viruses after spinal cord injury

Propriospinal neurons (PSNs) play a crucial role in motor control and sensory processing and contribute to plastic reorganization of spinal circuits responsible for recovery from spinal cord injury (SCI). Due to their scattered distribution and various intersegmental projection patterns, it is challenging to dissect the function of PSNs within the neuronal network. New genetically encoded tools, particularly cell-type-specific transgene expression methods using recombinant viral vectors combined with other genetic, pharmacologic, and optogenetic approaches, have enormous potential for visualizing PSNs in the neuronal circuits and monitoring and manipulating their activity. Furthermore, recombinant viral tools have been utilized to promote the intrinsic regenerative capacities of PSNs, towards manipulating the 'hostile' microenvironment for improving functional regeneration of PSNs. Here we summarize the latest development in this fast-moving field and provide a perspective for using this technology to dissect PSN physiological role in contributing to recovery of function after SCI.

Selection of Vaccinia Virus Recombinants Using CRISPR/Cas9

The engineering of poxvirus genomes is fundamental to primary and applied virology research. Indeed, recombinant poxviruses form the basis for many novel vaccines and virotherapies but producing and purifying these viruses can be arduous. In recent years, CRISPR/Cas9 has become the favoured approach for genome manipulation due to its speed and high success rate. However, recent data suggests poxvirus genomes are not repaired well following Cas9 cleavage. As a result, CRISPR/Cas9 is inefficient as an editing tool, but very effective as a programmable selection agent. Here, we describe protocols for the generation and enrichment of recombinant vaccinia viruses using targeted Cas9 as a selection tool. This novel use of Cas9 is a simple addition to current homologous recombination-based methods that are widespread in the field, facilitating implementation in laboratories already working with poxviruses. This is also the first method that allows for isolation of new vaccinia viruses in less than a fortnight, without the need to incorporate a marker gene or manipulation of large poxvirus genomes in vitro and reactivation with helper viruses. Whilst this protocol describes applications for laboratory strains of vaccinia virus, it should be readily adaptable to other poxviruses. Graphic abstract: Pipeline for Cas9 selection of recombinant poxviruses.

An inactivated vaccine based on artificial non-pathogenic fowl adenovirus 4 protects chickens against hepatitis-hydropericardium syndrome

Hepatitis-hydropericardium syndrome (HHS) in birds is mainly caused by virulent fowl adenovirus 4 (FAdV-4). A novel genotype, hypervirulent FAdV-4, emerged in 2015 with a high mortality rate ranging from 30 % to 100 % in chickens. Vaccination is an economically feasible method to control HHS. Although there have been various reports of inactivated vaccines from virulent wild-type FAdV-4 against HHS, biosafety threats of inactivated vaccines from potential pathogenic components have been presented to the poultry industry, and safer vaccines are urgently needed. A non-pathogenic recombinant FAdV-4 strain, designated as rHN20, was generated based on the hypervirulent strain in our previous study. Here, we developed a novel inactivated oil-adjuvanted vaccine derived from rHN20 strain and evaluated its immunogenicity in specific-pathogen-free chickens. Chickens subcutaneously or intramuscularly immunized with the inactivated vaccine produced high titers of neutralizing antibodies and were protected from a lethal dose of virulent wild-type FAdV-4 challenge. Collectively, an inactivated vaccine was developed, which was capable of providing full protection for chickens against HHS, and significantly reduced the potential biosafety threats.

Rescue of recombinant canine distemper virus that expresses S1 subunit of SARS-CoV-2 spike protein in vitro

The coronavirus disease 2019 (COVID-19), as an unprecedented pandemic, has rapidly spread around the globe. Its etiological agent, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), belongs to the genus Betacoronavirus in the family Coronaviridae. The viral S1 subunit has been demonstrated to have a powerful potential in inducing protective immune responses in vivo. Since April 2020, farmed minks were frequently reported to be infected with the SARS-CoV-2 in different countries. Unfortunately, there has been no available veterinary vaccine as yet. In this study, we used reverse genetics to rescue a recombinant canine distemper virus (CDV) that could express the SARS-CoV-2 S1 subunit in vitro. The S1 subunit sequence was demonstrated to be relatively stable in the genome of recombinant CDV during twenty serial viral passages in cells. However, due to introduction of the S1 subunit sequence into CDV genome, this recombinant CDV grew more slowly than the wild-type strain did. The genomic backbone of recombinant CDV was derived from a virulence-attenuating strain (QN strain). Therefore, if able to induce immune protections in minks from canine distemper and COVID-19 infections, this recombinant would be a potential vaccine candidate for veterinary use.

Construction of an infectious clone of Zika virus stably expressing an EGFP marker in a eukaryotic expression system

Background

Zika virus is becoming one of the most widely transmitted arboviruses in the world. Development of antiviral inhibitor and vaccine requires an experimental system that allows rapid monitoring of the virus infection. This is achievable with a reverse genetic system. In this study, we constructed an infectious clone for Zika virus that stably expressing EGFP.

Methods

A PCR-mediated recombination approach was used to assemble the full-length Zika virus genome containing the CMV promoter, intron, EGFP, hepatitis delta virus ribozyme, and SV40 terminator sequence for cloning into the pBAC11 vector to produce recombinant pBAC-ZIKA-EGFP. ZIKA-EGFP virus was rescued by transfection of pBAC-ZIKA-EGFP into 293T cells. The characterization of ZIKA-EGFP virus was determined by qPCR, plaque assay, CCK-8, and Western blot.

Results

Rescued ZIKA-EGFP virus exhibited stable replication for at least five generations in tissue culture. ZIKA-EGFP can effectively infect C6/36, SH-SY5Y and Vero cells, and cause cytopathic effects on SH-SY5Y and Vero cells. The inhibition of ZIKA-EGFP by NF-κB inhibitor, caffeic acid phenethyl ester was observed by fluorescence microscopy.

Conclusion

Our results suggested that Zika virus infectious clone with an EGFP marker retained it infectivity as wide-type Zika virus which could be used for drugs screening.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12985-021-01622-z.

Recombinant pseudorabies virus with gI/gE deletion generated by overlapping polymerase chain reaction and homologous recombination technology induces protection against the PRV variant PRV-GD2013

BACKGROUND

Since 2011, numerous highly virulent and antigenic variant viral strains have been reported in pigs that were vaccinated against the swine pseudorabies virus. These infections have led to substantial economic losses in the Chinese swine industry.

RESULTS

This study, constructed a novel recombinant vaccine strain with gI/gE deletion (PRV-GD2013-ΔgI/gE) by overlapping PCR and homologous recombination technology. The growth curves and plaque morphology of the recombinant virus were similar to those of the parental strain. However, PRV-GD2013-ΔgI/gE infection was significantly attenuated in mice compared with that of PRV-GD2013. Two-week-old piglets had normal rectal temperatures and displayed no clinical symptoms after being inoculated with 105 TCID50 PRV-GD2013-ΔgI/gE, indicating that the recombinant virus was avirulent in piglets. Piglets were immunized with different doses of PRV-GD2013-ΔgI/gE, or a single dose of Bartha-K61 or DMEM, and infected with PRV-GD2013 at 14 days post-vaccination. Piglets given high doses of PRV-GD2013-ΔgI/gE showed no obvious clinical symptoms, and their antibody levels were higher than those of other groups, indicating that the piglets were completely protected from PRV-GD2013.

CONCLUSIONS

The PRV-GD2013-ΔgI/gE vaccine strain could be effective for immunizing Chinese swine herds against the pseudorabies virus (PRV) strain.

Orf virus ORFV112, ORFV117 and ORFV127 contribute to ORFV IA82 virulence in sheep

The parapoxvirus orf virus (ORFV) encodes several immunomodulatory proteins (IMPs) that modulate host innate and pro-inflammatory responses to infection. Using the ORFV IA82 strain as the parental virus, recombinant viruses with individual deletions in the genes encoding the IMPs chemokine binding protein (CBP; ORFV112), inhibitor of granulocyte-monocyte colony-stimulating factor and IL-2 (GIF, ORFV117) and interleukin 10 homologue (vIL-10; ORFV127) were generated and characterized in vitro and in vivo. The replication properties of the individual gene deletion viruses in cell culture was not affected comparing with the parental virus. To investigate the effect of the individual gene deletions in ORFV infection and pathogenesis, groups of four lambs were inoculated with each virus and were monitored thereafter. Lambs inoculated with either recombinant or with the parental ORFV developed characteristic lesions of contagious ecthyma. The onset, nature and severity of the lesions in the oral commissure were similar in all inoculated groups from the onset (3 days post-inoculation [pi]) to the peak of clinical lesions (days 11-13 pi). Nonetheless, from days 11-13 pi onwards, the oral lesions in lambs inoculated with the recombinant viruses regressed faster than the lesions produced by the parental virus. Similarly, the amount of virus shed in the lesions were equivalent among lambs of all groups up to day 15 pi, yet they were significantly higher in the parental virus group from day 16-21 pi. In conclusion, individual deletion of these IMP genes from the ORFV genome resulted in slight reduction in virulence in vivo, as evidenced by a reduction in the duration of the clinical disease and virus shedding.

3H-31, A Non-structural Protein of Heliothis virescens ascovirus 3h, Inhibits the Host Larval Cathepsin and Chitinase Activities

3h-31 of Heliothis virescens ascovirus 3h (HvAV-3h) is a highly conserved gene of ascoviruses. As an early gene of HvAV-3h, 3h-31 codes for a non-structural protein (3H-31) of HvAV-3h. In the study, 3h-31 was initially transcribed and expressed at 3 h post-infection (hpi) in the infected Spodoptera exigua fat body cells (SeFB). 3h-31 was further inserted into the bacmid of Autographa californica nucleopolyhedrovirus (AcMNPV) to generate an infectious baculovirus (AcMNPV-31). In vivo experiments showed that budded virus production and viral DNA replication decreased with the expression of 3H-31, and lucent tubular structures were found around the virogenic stroma in the AcMNPV-31-infected SeFB cells. In vivo, both LD₅₀ and LD₉₀ values of AcMNPV-31 were significantly higher than those of the wild-type AcMNPV (AcMNPV-wt) in third instar S. exigua larvae. An interesting finding was that the liquefaction of the larvae killed by the infection of AcMNPV-31 was delayed. Chitinase and cathepsin activities of AcMNPV-31-infected larvae were significantly lower than those of AcMNPV-wt-infected larvae. The possible regulatory function of the chitinase and cathepsin for 3H-31 was further confirmed by RNAi, which showed that larval cathepsin activity was significantly upregulated, but chitinase activity was not significantly changed due to the RNAi of 3h-31. Based on the obtained results, we assumed that the function of 3H-31 was associated with the inhibition of host larval chitinase and cathepsin activities, so as to restrain the hosts in their larval stages.

A novel fiber-2-edited live attenuated vaccine candidate against the highly pathogenic serotype 4 fowl adenovirus

Recently, the outbreaks of hydropericardium-hepatitis syndrome (HHS) caused by the highly pathogenic fowl adenovirus serotype 4 (FAdV-4) have resulted in huge economic losses to the poultry industry globally. Although several inactivated or subunit vaccines have been developed against FAdV-4, live-attenuated vaccines for FAdV-4 are rarely reported. In this study, a recombinant virus FA4-EGFP expressing EGFP-Fiber-2 fusion protein was generated by the CRISPR/Cas9 technique. Although FA4-EGFP shows slightly lower replication ability than the wild type (WT) FAdV-4, FA4-EGFP was significantly attenuated in vivo compared with the WT FAdV-4. Chickens infected with FA4-EGFP did not show any clinical signs, and all survived to 14 day post-infection (dpi), whereas those infected with FAdV-4 showed severe clinical signs with HHS and all died at 4 dpi. Besides, the inoculation of FA4-EGFP in chickens provided efficient protection against lethal challenge with FAdV-4. Compared with an inactivated vaccine, FA4-EGFP induced neutralizing antibodies with higher titers earlier. All these data not only provide a live-attenuated vaccine candidate against the highly pathogenic FAdV-4 but also give a potential insertion site for developing FAdV-4-based vaccine vectors for delivering foreign antigens.

Better immune efficacy triggered by the inactivated gI/gE-deleted pseudorabies virus with the additional insertion of gC gene in mice and weaned pigs

A new variant of pseudorabies virus (PRV) with high pathogenicity has been prevalent in many swineherds vaccinated with Bartha-K61 in China since 2011. Several gene-deleted vaccine candidates have been developed based on new emerging PRV variants. PRV-AH, a new emerging PRV strain from Anhui Province, was isolated in our laboratory in 2013. In the present study, rPRV-AH-gI^-/gE^- and rPRV-AH-gI^-/gE^-/gC⁺ were generated based on PRV-AH by homologous recombination. The growth kinetics of rPRV-AH-gI^-/gE^- and rPRV-AH-gI^-/gE^-/gC⁺ were similar to their parental strains. Compared with the commercial inactivated vaccine of Ea strain, the immune efficacy of the inactivated vaccine based on recombinant viruses was evaluated in mice and weaned pigs. The result showed that the level of neutralizing antibody in mice immunized with rPRV-AH-gI^-/gE^-/gC⁺ was higher compared with those immunized with rPRV-AH-gI^-/gE- at a dose of 10⁶ TCID₅₀ at 8 weeks post initial immunization (p < 0.0001). Among the groups immunized at a dose of 10⁵ TCID₅₀, the rPRV-AH-gI^-/gE^- group showed a survival rate of 37.5 %, while the rPRV-AH-gI^-/gE^-/gC⁺ group showed a protection rate of 87.5 % against the PRV-AH challenge. Besides, the rPRV-AH-gI^-/gE^- and rPRV-AH-gI^-/gE^-/gC⁺ group immunized at a dose of 10⁶ TCID₅₀ showed a survival rate of 100 %. Interestingly, compared with the commercial vaccine group, the group of 10⁵ TCID₅₀ rPRV-AH-gI^-/gE^-/gC⁺ showed a lower level of neutralizing antibodies (p < 0.0001) but the same protection rate in mice. Moreover, in the pig experiment, the level of neutralizing antibodies in the group vaccinated with inactivated rPRV-AH-gI^-/gE^-/gC⁺ was higher than any other groups at 8 weeks post initial immunization (p < 0.05). More importantly, the milder symptoms and pathological lesions occurred in pigs vaccinated with rPRV-AH-gI^-/gE^-/gC⁺ after challenge with 10⁶ TCID₅₀ PRV-AH, revealing that additional insertion of gC gene could enhance the protective efficacy in PRV gI/gE-deleted vaccine in pigs. Collectively, these above-mentioned findings suggested that the inactivated vaccine of rPRV-AH-gI^-/gE^-/gC⁺ had a better immune efficacy, which could be regarded as a promising inactivated vaccine candidate for PRV control.

Construction and purification of ANK gene deleted recombinant goatpox virus

Sheeppox virus (SPPV) and goatpox virus (GTPV) are two pathogens of host specificity. Previous studies have hypothesized that ankyrin (ANK) family may play an important role in determining host range of SPPV and GTPV. In order to verify the function of ANK proteins, it is critical to generate and purify the ANK gene deleted GTPV. In this study, the GFP gene as a reporter gene was connected with two homologous arms of ANK gene by fusion PCR. The ANK gene deleted transfer vectors were generated by inserting the PCR products into PET42b, and were transfected into testicular primary cells which were infected by GTPV. The rGTPV were identified as green fluorescence positive and properly purified. The results showed that GFP gene and two homologous arms of ANK gene were connected. The sequence was inserted in PET42b to form ANK deleted transfer vector. ANK deleted rGTPV was generated successfully by transferring vector and GTPV in cells. The ANK deleted rGTPV was purified and identified in this study. The study successfully generated the ANK deleted rGTPV. It overcomes the technical barrier for future studies about the function of ANK genes.

Methods for the Construction of Recombinant Oncolytic Myxoma Viruses

Myxoma virus (MYXV) has proven to be an effective candidate for oncolytic virotherapy in many preclinical cancer models. As a nonhuman pathogen, MYXV does not need to overcome any preexisting antiviral immunity, and its DNA cannot integrate into the host genome, making it an extremely safe vector. Moreover, the large dsDNA genome of MYXV allows the insertion of multiple transgenes and the design of engineered recombinant oncolytic viruses (OVs) with enhanced immunostimulatory or other desired properties. In this chapter, we describe detailed protocols for the generation and characterization of transgene-armed recombinant MYXV vectors.

Recombinant subgroup B avian leukosis virus combined with the subgroup J env gene significantly increases its pathogenicity

The differences among different sub-groups of the avian leukosis virus (ALV) genome are mainly concentrated in the env gene, which binds to cell-specific receptors and determines the characteristics of viral tropism and pathogenicity. In this study, two rescued viruses rGX15MM6-2 (ALV of subgroup J, ALV-J) and rGX14FF03 (ALV of subgroup B, ALV-B) and a recombinant virus rALV-B-Jenv (ALV-B's backbone with ALV-J's env) were generated and tested utilizing both in vitro and in vivo experiments. The results showed that the replication ability of the viruses released in DF-1 cell cultures was listed in order as rGX15MM6-2 > rALV-B-Jenv > rGX14FF03. rGX15MM6-2 caused the most serious suppression of body weight gain, exhibited a significant negative effect on the development of immune organs (P < 0.05) and lower antibody responses to vaccinations with the commercial oil-emulsion vaccines (OEVs) (P<0.05) in the challenged chickens. The viral detection showed that the positive rate in blood from the birds infected with rALV-B-Jenv were respectively higher than those from the birds infected with rGX14FF03 (P < 0.05). At 25 wpi, similar tumors were found in the abdominal cavity of the birds in rGX15MM6-2 and rALV-B-Jenv groups. The results demonstrated that the ALV-J env gene significantly increases the pathogenicity of the recombinant ALV-B. With the increasing incidence of co-infections of different subgroups of ALV in the field, the possibility of viral recombination is increasing and demands further study.

A chimeric recombinant infectious hematopoietic necrosis virus induces protective immune responses against infectious hematopoietic necrosis and infectious pancreatic necrosis in rainbow trout

Infectious pancreatic necrosis virus (IPNV) and infectious hematopoietic necrosis virus (IHNV) are two common viral pathogens that cause severe economic losses in all salmonid species in culture, but especially in rainbow trout. Although vaccines against both diseases have been commercialized in some countries, no such vaccines are available for them in China. In this study, a recombinant virus was constructed using the IHNV U genogroup Blk94 virus as a backbone vector to express the antigenic gene, VP2, from IPNV via the reverse genetics system. The resulting recombinant virus (rBlk94-VP2) showed stable biological characteristics as confirmed by virus growth kinetic analyses, pathogenicity analyses, indirect immunofluorescence assays and western blotting. Rainbow trout were immunized with rBlk94-VP2 and then challenged with the IPNV ChRtm213 strain and the IHNV Sn1203 strain on day 45 post-vaccination. A significantly higher survival rate against IHNV was obtained in the rBlk94-VP2 group on day 45 post-vaccination (86%) compared with the PBS mock immunized group (2%). Additionally, IPNV loads decreased significantly in the rBlk94-VP2 immunized group in the liver (28.6-fold to 36.5-fold), anterior kidney (21.7-fold to 44.2-fold), and spleen (14.9-fold to 22.7-fold), as compared with the PBS mock control group. The mRNA transcripts for several innate and adaptive immune-related proteins (IFN-γ, IFN-1, Mx-1, CD4, CD8, IgM, and IgT) were also significantly upregulated after rBlk94-VP2 vaccination, and neutralizing antibodies against both IHNV and IPNV were induced on day 45 post-vaccination. Collectively, our results suggest that this recombinant virus could be developed as a vaccine vector to protect rainbow trout against two or more diseases, and our approach lays the foundations for developing live vaccines for rainbow trout.

Pathogenicity of a QX-like strain of infectious bronchitis virus and effects of accessory proteins 3a and 3b in chickens

QX-like genotype infectious bronchitis virus (IBV) has become prevalent in recent years. Few studies have reported the effects of accessory proteins 3a and 3b on pathogenicity in vivo. We developed a reverse genetics system to manipulate the genome of a QX-like IBV strain IBYZ. Recombinant viruses rIBYZ-ScAUG3a, rIBYZ-ScAUG3b and rIBYZ-ScAUG3ab were generated. These viruses do not express the accessory proteins 3a, 3b, or 3ab due to a mutation in the AUG start codons. In SPF embryonated eggs, the recombinant viruses grew to the same viral load as parental strain rIBYZ. The pathogenicity of rIBYZ and recombinant viruses was examined in 1-day-old SPF chickens. In SPF chickens, rIBYZ-ScAUG3a had a lower mortality than rIBYZ. The clinical signs, gross lesions and histopathological changes of rIBYZ-ScAUG3a group were comparable to those of rIBYZ group. However, viral distribution and viral shedding showed that the viral loads of rIBYZ-ScAUG3a were lower than those of rIBYZ in tissue samples and swab specimens. The rIBYZ-ScAUG3b and rIBYZ-ScAUG3ab strains showed attenuated pathogenicity compared to rIBYZ, as no chickens died and all the parameters tested were considerably low. This study indicates that the absence of accessory proteins 3a and 3b in IBV lead to attenuated pathogenicity in chickens. Protein 3b has a greater effect on pathogenicity than protein 3a. These findings may be used in vaccination trials for the development of a new live-attenuated vaccine.

A recombination efficiently increases the pathogenesis of the novel K subgroup of avian leukosis virus

In this study, a recombinant ALV with ALV-K env and ALV-J backbone was generated (designated ALV-K-env-J) and tested in vitro and in vivo. The growth curve in DF1 cells showed that the recombinant virus replicated more efficiently in comparison with the ALV-J and ALV-K. Although all the infected chickens showed growth retardation compared with the non-infected chickens, the viral and serological detection showed that the positive rate and virus load detected in blood and cloaca, and the positive rate and titer of antibody against p27 from the chickens infected with ALV-K-env-J were higher than those from the chickens infected with the ALV-K, but less than those from the chickens infected with the ALV-J. All these data clearly demonstrated that the recombination event in this study increased the pathogenesis of ALV-K, and the potential recombination between different ALV subgroups should be worried when the clinical co-infections occur.

Rana grylio virus 43R encodes an envelope protein involved in virus entry

Rana grylio virus (RGV), a member of genus Ranavirus in the family Iridoviridae, is a viral pathogen infecting aquatic animal. RGV 43R has homologues only in Ranavirus and contains a transmembrane (TM) domain, but its role in RGV infection is unknown. In this study, 43R was determined to be associated with virion membrane. The transcripts encoding 43R and the protein itself appeared late in RGV-infected EPC cells and its expression was blocked by viral DNA replication inhibitor, indicating that 43R is a late expressed protein. Subcellular localization showed that 43R-EGFP fusion protein distributed in cytoplasm of EPC cells and that TM domain is essential for its distribution in cytoplasm. 43R-EGFP fusion protein colocalized with viral factories in RGV-infected cells. A recombinant RGV deleting 43R (Δ43R-RGV) was constructed by homologous recombination to investigate its role in virus infection. Compared with wild type RGV, the ability of Δ43R-RGV to induce the cytopathic effect and its virus titers were significantly reduced. Furthermore, it is revealed that 43R deletion significantly inhibited viral entry but did not influence viral DNA replication by measuring and comparing the DNA levels of RGV and Δ43R-RGV in the infected cells at the early stage of infection. RGV neutralization with anti-43R serum reduced the virus titer. Therefore, these data showed that RGV 43R is a late gene that encodes an envelope protein involved in RGV entry.

Rescue and characterization of recombinant cedar virus, a non-pathogenic Henipavirus species

BACKGROUND

Hendra virus and Nipah virus are zoonotic viruses that have caused severe to fatal disease in livestock and human populations. The isolation of Cedar virus, a non-pathogenic virus species in the genus Henipavirus, closely-related to the highly pathogenic Hendra virus and Nipah virus offers an opportunity to investigate differences in pathogenesis and receptor tropism among these viruses.

METHODS

We constructed full-length cDNA clones of Cedar virus from synthetic oligonucleotides and rescued two replication-competent, recombinant Cedar virus variants: a recombinant wild-type Cedar virus and a recombinant Cedar virus that expresses a green fluorescent protein from an open reading frame inserted between the phosphoprotein and matrix genes. Replication kinetics of both viruses and stimulation of the interferon pathway were characterized in vitro. Cellular tropism for ephrin-B type ligands was qualitatively investigated by microscopy and quantitatively by a split-luciferase fusion assay.

RESULTS

Successful rescue of recombinant Cedar virus expressing a green fluorescent protein did not significantly affect virus replication compared to the recombinant wild-type Cedar virus. We demonstrated that recombinant Cedar virus stimulated the interferon pathway and utilized the established Hendra virus and Nipah virus receptor, ephrin-B2, but not ephrin-B3 to mediate virus entry. We further characterized virus-mediated membrane fusion kinetics of Cedar virus with the known henipavirus receptors ephrin-B2 and ephrin-B3.

CONCLUSIONS

The recombinant Cedar virus platform may be utilized to characterize the determinants of pathogenesis across the henipaviruses, investigate their receptor tropisms, and identify novel pan-henipavirus antivirals. Moreover, these experiments can be conducted safely under BSL-2 conditions.

The dynamics of norovirus genotypes and genetic analysis of a novel recombinant GII.P12-GII.3 among infants and children in Bangkok, Thailand between 2014 and 2016

Norovirus (NoV) is the leading cause of viral acute gastroenteritis among all age groups in the world. We performed a molecular epidemiological study of the NoVs prevalent in Bangkok between November 2014 and July 2016 to investigate the emergence of new NoV variants in Thailand. A total of 332 stool specimens were collected from hospitalized pediatric patients with acute gastroenteritis in Bangkok, Thailand. NoVs were detected by real-time PCR. The genome of the N-terminal/shell domain was amplified, the nucleotide sequence was determined, and phylogenetic analyses were performed. GII NoV was detected in 58 (17.5%) of the 332 specimens. GII.17, a genotype strain prevalent from 2014 to mid-2015, was hardly detected and replaced by the GII.3 genotype strain. Entire genome sequencing followed by phylogenetic analysis of the GII.3 genotype strains indicated that they are new recombinant viruses, because the genome encoding ORF1 is derived from a GII.12 genotype strain, whereas that encoding ORF2-3 is from a GII.3 genotype strain. The putative recombination breakpoints with the highest statistical significance were located around the border of 3D^pol and ORF2. The change in the prevalent strain of NoV seems to be linked to the emergence of new forms of recombinant viruses. These findings suggested that the swapping of the structural and non-structural proteins of NoV is a common mechanism by which new epidemic variants are generated in nature.

Large-scale HFMD epidemics caused by Coxsackievirus A16 in Bangalore, India during 2013 and 2015

Hand foot and mouth disease (HFMD) is a relatively unreported disease in India. This study was undertaken to characterize the enterovirus type/s associated with two unexpectedly-massive epidemics that occurred in Bangalore, India in 2013 and 2015. Stool samples of 229 children with HFMD living in Northern and Southern areas of Bangalore were tested by RT-PCR; 189 (82.5%) were enterovirus positive. The Indian CV-A16 strains exhibited 98-99% sequence identity with those reported in France and China in the 5' untranslated region. BLAST and phylogenetic analyses of complete genomes of representative Indian isolates revealed that the 2015 epidemic was predominated by an inter-species recombinant between CV-A16 and coxsackievirus B5. The 2013 epidemic was primarily caused by nonrecombinant strains. The CV-A16 strains circulated in India since 2007 and phylogeographic analyses indicated imported cases in France and China. In conclusion, CV-A16-associated HFMD epidemics should be recognized as an emerging public health problem in India.

Recombinant Gallid herpesvirus 2 with interrupted meq genes confers safe and efficacious protection against virulent field strains

Gallid herpesvirus 2 (GaHV-2) continuously evolves, which reduces the effectiveness of existing vaccines. To construct new GaHV-2 candidate vaccines, LMS, which is a virulent GaHV-2 field strain isolated from diseased chicken flocks in Southwest China in 2007, was modified such that both copies of its meq oncogene were partially deleted. The resulting virus, i.e., rMSΔmeq, was characterized using PCR and sequencing. To evaluate the safety and protective efficacy of rMSΔmeq, specific pathogen-free (SPF) chickens were inoculated with 2000 plaque forming units (pfu) and 20,000pfu of rMSΔmeq immediately after hatching. All birds grew well during the experimental period, and none of the challenged chickens developed Marek's disease-associated lymphoma. In addition, the rMSΔmeq- and CVI988/Rispens-vaccinated SPF chickens were challenged with 1000 pfu and 5000 pfu of the representative virulent GaHV-2 Md5 strain and 1000 pfu of the variant GaHV-2 strains LCC or LTS. The results showed that the rMSΔmeq strain provided complete protection, which was similar to that provided by the CVI988/Rispens vaccine (protective index (PI) of 95.5) when challenged with a conventional dose of the Md5 strain. However, rMSΔmeq provided a PI of 90.9 when challenged with 5000 pfu of the Md5 strain, which was significantly higher than that provided by the CVI988/Rispens vaccine (54.5). rMSΔmeq provided a PI of 86.4 against LCC, which was equal to that provided by the CVI988/Rispens vaccine (81.8). In addition, rMSΔmeq provided a PI of 100 against LTS, which was significantly higher than that provided by the CVI988/Rispens vaccine (68.2). Altogether, the rMSΔmeq virus provided efficient protection against representative and variant GaHV-2 strains. In conclusion, the rMSΔmeq virus is a safe and effective vaccine candidate for the prevention of Marek's disease and is effective against the Chinese variant GaHV-2 strains.

Reverse Genetics for Mammalian Orthoreovirus

Reverse genetics allows introduction of specific alterations into a viral genome. Studies performed with mutant viruses generated using reverse genetics approaches have contributed immeasurably to our understanding of viral replication and pathogenesis, and also have led to development of novel vaccines and virus-based vectors. Here, we describe the reverse genetics system that allows for production and recovery of mammalian orthoreovirus, a double-stranded (ds) RNA virus, from plasmids that encode the viral genome.

Selection of differently temporally regulated African swine fever virus promoters with variable expression activities and their application for transient and recombinant virus mediated gene expression

African swine fever virus threatens pig production worldwide due to the lack of vaccines, for which generation of both deletion and insertion mutants is considered. For development of the latter, operational ASFV promoters of different temporal regulation and strengths are desirable. We therefore compared the capacities of putative promoter sequences from p72, CD2v, p30, viral DNA polymerase and U104L genes to mediate expression of luciferase from transfected plasmids after activation in trans, or p30-, DNA polymerase- and U104L promoters in cis, using respective ASFV recombinants. We identified sequences with promoter activities upstream the viral ORFs, and showed that they differ in both their expression intensity regulating properties and in their temporal regulation. In summary, p30 and DNA polymerase promoters are recommended for high level early regulated transgene expression. For late expression, the p72, CD2v and U104L promoter are suitable. The latter however, only if low level transgene expression is aimed.

Generation of Recombinant Ebola Viruses Using Reverse Genetics

Reverse genetics systems encompass a wide array of tools aimed at recapitulating some or all of the virus life cycle. In their most complete form, full-length clone systems allow us to use plasmid-encoded versions of the ribonucleoprotein (RNP) components to initiate the transcription and replication of a plasmid-encoded version of the complete viral genome, thereby initiating the complete virus life cycle and resulting in infectious virus. As such this approach is ideal for the generation of tailor-made recombinant filoviruses, which can be used to study virus biology. In addition, the generation of tagged and particularly fluorescent or luminescent viruses can be applied as tools for both diagnostic applications and for screening to identify novel countermeasures. Here we describe the generation and basic characterization of recombinant Ebola viruses rescued from cloned cDNA using a T7-driven system.

Generation of Newcastle Disease Virus (NDV) Recombinants Expressing the Infectious Laryngotracheitis Virus (ILTV) Glycoprotein gB or gD as Dual Vaccines

Infectious laryngotracheitis (ILT) is a highly contagious acute respiratory disease of chickens caused by infection with infectious laryngotracheitis virus (ILTV), a member of the family Herpesviridae. The current commercial ILT vaccines are either unsafe or ineffective. Therefore, there is a pressing need to develop safer and more efficacious vaccines. Newcastle disease (ND), caused by infection with Newcastle disease virus (NDV), a member of the family Paramyxoviridae, is one of the most serious infectious diseases of poultry. The NDV LaSota strain, a naturally occurring low-virulence NDV strain, has been routinely used as a live vaccine throughout the world. This chapter describes the generation of Newcastle disease virus (NDV) LaSota vaccine strain-based recombinant viruses expressing glycoprotein B (gB) or glycoprotein D (gD) of ILTV as dual vaccines against ND and ILT using reverse genetics technology.

A Rapid Method for Engineering Recombinant Polioviruses or Other Enteroviruses

The cloning of large enterovirus RNA sequences is labor-intensive because of the frequent instability in bacteria of plasmidic vectors containing the corresponding cDNAs. In order to circumvent this issue we have developed a PCR-based method that allows the generation of highly modified or chimeric full-length enterovirus genomes. This method relies on fusion PCR which enables the concatenation of several overlapping cDNA amplicons produced separately. A T7 promoter sequence added upstream the fusion PCR products allows its transcription into infectious genomic RNAs directly in transfected cells constitutively expressing the phage T7 RNA polymerase. This method permits the rapid recovery of modified viruses that can be subsequently amplified on adequate cell-lines.

Recombinant canine adenovirus type-2 expressing TgROP16 provides partial protection against acute Toxoplasma gondii infection in mice

We previously demonstrated that the survival time of BALB/c mice challenged with Toxoplasma gondii RH strain was prolonged by immunising the mice with a eukaryotic vector expressing the protein ROP16 of T. gondii. Building upon previous findings, we are exploring improved vaccination strategies to enhance protection. In this work, a novel recombinant canine adenovirus type 2 expressing ROP16 (CAV-2-ROP16) of T. gondii was constructed and identified to express ROP16 in Madin-Darby canine kidney cells (MDCK) cells by western blot (WB) and indirect immunofluorescence (IFA) assays. Intramuscular immunisation of BALB/c mice with CAV-2-ROP16 was performed to evaluate the humoral and cellular immune responses. This vaccination triggered significant humoral and cellular responses, including ROP16-stimulated lymphoproliferation (P<0.05). Compared to control groups, the CAV-2-ROP16 immunised mice had high production of IFN-γ, IL-2 and IL-12 (P<0.05), with a predominance of IgG2a production, but not IL-10 (P>0.05), revealing that a predominant Th1-type response had developed. The cell-mediated cytotoxic activity with high levels of IFN-γ and TNF-α was significantly increased in both CD4⁺ and CD8⁺ T-cell compartments in the mice immunised with CAV-2-ROP16 (P<0.05), compared to three control groups. In addition, when immunised mice were challenged with the RH strain of T. gondii, they showed a significantly increased survival rate (25%) 80days post infection compared with control mice that all died within seven days (P<0.05). The 25% protection rate elicited by the recombinant virus CAV-2-ROP16 has not been achieved in the field of anti-T. gondii vaccination until now. Our work presents the successful use of recombinant virus CAV-2-ROP16 in vaccination protocols to protect against intraperitoneal challenge with the virulent RH strain of T. gondii. This system was shown to be extremely efficient in eliciting humoral and cellular immune responses that led to a significant improvement in survival time in mice.

A robust method for the rapid generation of recombinant Zika virus expressing the GFP reporter gene

Zika virus (ZIKV) infection is a major public health problem with severe human congenital and neurological anomalies. The screening of anti-ZIKV compounds and neutralizing antibodies needs reliable and rapid virus-based assays. Here, we described a convenient method leading to the rapid production of molecular clones of ZIKV. To generate a molecular clone of ZIKV strain MR766(NIID), the viral genome was directly assembled into Vero cells after introduction of four overlapping synthetic fragments that cover the full-length genomic RNA sequence. Such strategy has allowed the production of a recombinant ZIKV expressing the GFP reporter gene that is stable over two culturing rounds on Vero cells. Our data demonstrate that the ZIKV reporter virus is a very reliable GFP-based tool for analyzing viral growth and measuring the neutralizing antibody as well as rapid screening of antiviral effect of different classes of inhibitors.

Generation of Recombinant Capripoxvirus Vectors for Vaccines and Gene Knockout Function Studies

The ability to manipulate capripoxvirus through gene knockouts and gene insertions has become an increasingly valuable research tool in elucidating the function of individual genes of capripoxvirus, as well as in the development of capripoxvirus-based recombinant vaccines. The homologous recombination technique is used to generate capripoxvirus knockout viruses (KO), and is based on the targeting a particular viral gene of interest. This technique can also be used to insert a gene of interest. A protocol for the generation of a viral gene knockout is described. This technique involves the use of a plasmid which encodes the flanking sequences of the regions where the homologous recombination will occur, and will result in the insertion of an EGFP reporter gene for visualization of recombinant virus, as well as the E. coli gpt gene as a positive selection marker. If an additional gene is to be incorporated, this can be achieved by inserting a gene of interest for expression under a poxvirus promoter into the plasmid between the flanking regions for insertion. This chapter describes a protocol for generating such recombinant capripoxviruses.

Virus fitness differences observed between two naturally occurring isolates of Ebola virus Makona variant using a reverse genetics approach

During the large outbreak of Ebola virus disease that occurred in Western Africa from late 2013 to early 2016, several hundred Ebola virus (EBOV) genomes have been sequenced and the virus genetic drift analyzed. In a previous report, we described an efficient reverse genetics system designed to generate recombinant EBOV based on a Makona variant isolate obtained in 2014. Using this system, we characterized the replication and fitness of 2 isolates of the Makona variant. These virus isolates are nearly identical at the genetic level, but have single amino acid differences in the VP30 and L proteins. The potential effects of these differences were tested using minigenomes and recombinant viruses. The results obtained with this approach are consistent with the role of VP30 and L as components of the EBOV RNA replication machinery. Moreover, the 2 isolates exhibited clear fitness differences in competitive growth assays.

Growth properties and vaccine efficacy of recombinant pseudorabies virus defective in glycoprotein E and thymidine kinase genes

Pseudorabies virus (PRV) is an alphaherpesvirus that causes pseudorabies (PR), an economically important viral disease of pigs. Marker vaccines were widely used in PR prevention and eradication programs. The purpose of this study was to construct a novel recombinant virus with deletions at defined regions in the glycoprotein E (gE) and thymine kinase (TK) genes by homologous recombination. This study also evaluated the safety and efficacy of the virus for a live attenuated marker vaccine. No significant difference was observed in virus replication between gE gene-deleted (gE(-)), gE/TK double gene-deleted (gE(-)TK(-)), and wild-type PRV by growth curve analysis. However, gE(-)TK(-) PRV was completely attenuated in mice. To evaluate the immunogenicity of gE(-)TK(-) PRV, four 12-week-old specific-pathogen-free pigs per group were immunized intramuscularly with viral titers of 1×10(4), 1×10(5), or 1×10(6) TCID50, followed by intranasal challenge infection with virulent PRV (1×10(8) TCID50) at 3 weeks post vaccination. The gE(-)TK(-) PRV-vaccinated pigs displayed no general adverse effects after immunization and had protective immune responses after PRV challenge. Thus, gE(-)TK(-) PRV was safe and efficacious and might be a potential candidate for a live attenuated marker vaccine against PRV.

Live Cell Reporter Systems for Positive-Sense Single Strand RNA Viruses

Cell-based reporter systems have facilitated studies of viral replication and pathogenesis, virus detection, and drug susceptibility testing. There are three types of cell-based reporter systems that express certain reporter protein for positive-sense single strand RNA virus infections. The first type is classical reporter system, which relies on recombinant virus, reporter virus particle, or subgenomic replicon. During infection with the recombinant virus or reporter virus particle, the reporter protein is expressed and can be detected in real time in a dose-dependent manner. Using subgenomic replicon, which are genetically engineered viral RNA molecules that are capable of replication but incapable of producing virions, the translation and replication of the replicon could be tracked by the accumulation of reporter protein. The second type of reporter system involves genetically engineered cells bearing virus-specific protease cleavage sequences, which can sense the incoming viral protease. The third type is based on viral replicase, which can report the specific virus infection via detection of the incoming viral replicase. This review specifically focuses on the major technical breakthroughs in the design of cell-based reporter systems and the application of these systems to the further understanding and control of viruses over the past few decades.

Recombinant measles AIK-C vaccine strain expressing heterologous virus antigens

Further attenuated measles vaccines were developed more than 50 years ago and have been used throughout the world. Recombinant measles vaccine candidates have been developed and express several heterologous virus protective antigens. Immunogenicity and protective actions were confirmed using experimental animals: transgenic mice, cotton rats, and primates. The recent development of measles vaccine-based vectored vaccine candidates has been reviewed and some information on recombinant measles vaccines expressing respiratory syncytial virus proteins has been shown and discussed.