Construction of an infectious cDNA clone of Enterovirus 71: insights into the factors ensuring experimental success

The Application of the Gibson Assembly Method in the Production of Two pKLS3 Vector-Derived Infectious Clones of Foot-and-Mouth Disease Virus

The construction of a full-length infectious clone, essential for molecular virological study and vaccine development, is quite a challenge for viruses with long genomes or possessing complex nucleotide sequence structures. Herein, we have constructed infectious clones of foot-and-mouth disease virus (FMDV) types O and A by joining each viral coding region with our pKLS3 vector in a single isothermal reaction using Gibson Assembly (GA). pKLS3 is a 4.3-kb FMDV minigenome. To achieve optimal conditions for the DNA joining, each FMDV coding sequence was divided into two overlapping fragments of approximately 3.8 and 3.2 kb, respectively. Both DNA fragments contain the introduced linker sequences for assembly with the linearized pKLS3 vector. FMDV infectious clones were produced upon directly transfecting the GA reaction into baby hamster kidney-21 (BHK-21) cells. After passing in BHK-21 cells, both rescued FMDVs (rO189 and rNP05) demonstrated growth kinetics and antigenicity similar to their parental viruses. Thus far, this is the first report on GA-derived, full-length infectious FMDV cDNA clones. This simple DNA assembly method and the FMDV minigenome would facilitate the construction of FMDV infectious clones and enable genetic manipulation for FMDV research and custom-made FMDV vaccine production.

Novel strategies for the development of hand, foot, and mouth disease vaccines and antiviral therapies

INTRODUCTION

Hand, foot, and mouth disease (HFMD) poses a great threat to young children in the Asia-Pacific region. HFMD is usually caused by enterovirus A, and infection with enterovirus A71 (EV-A71) is particularly associated with severe complications. However, coxsackievirus CV-A16, CV-A6, and CV-A10 pandemics have been observed in recent HFMD outbreaks. Inactivated monovalent EV-A71 vaccines are available to prevent EV-A71 infection; however, they cannot prevent infections by non-EV-A71 enteroviruses. Anti-enteroviral drugs are still in the developmental stage. Application of novel strategies will facilitate the development of new therapies against these emerging HFMD-associated enteroviruses.

AREAS COVERED

The authors highlight the current approaches for anti-enterovirus therapeutic development and discuss the application of these novel strategies for the discovery of vaccines and antiviral drugs for enteroviruses.

EXPERT OPINION

The maturation of DNA/RNA vaccine technology could be applied for rapid and robust development of multivalent enterovirus vaccines. Structure biology and neutralization antibody studies decipher the immunodominant sites of enteroviruses for vaccine design. Nucleotide aptamer library screening is a novel, fast, and cost-effective strategy for the development of antiviral agents. Animal models carrying viral receptors and attachment factors are required for enterovirus study and vaccine/antiviral development. Currently developed antivirals require effectiveness evaluation in clinical trials.

Construction and characterization of an infectious cDNA clone of enterovirus 71: a rapid method for rescuing infectious virus based on stable cells expressing T7 polymerase

Enterovirus 71 (EV71) is a causative agent of hand, foot and, mouth disease (HFMD) in young children. It is valuable for virologists to develop a fast method to rescue infectious virus from a viral cDNA clone. Here, we report a method for rapid rescue of infectious EV71 by using cells expressing T7 polymerase. The full-length EV71 genome was amplified in one step by long-distance PCR with a T7 promoter at the 5' end, and the T7 polymerase gene was cloned into a lentivirus vector for construction of a stable cell line expressing T7 polymerase. The infectious virus was rapidly and efficiently rescued by single transfection of cells with the infectious cDNA clone. Further experiments showed that the rescued virus had characteristics similar to those of the parental virus. This method circumvented the difficulty in performing in vitro transcription of a long linear DNA to obtain high-quality RNA. The construction of the viral cDNA clone and the fast rescue of the infectious virus will greatly benefit future investigations.

Construction and characterization of an infectious cDNA clone of coxsackievirus A 10

Background

Coxsackievirus A10 (CA10) constitutes one of the four major pathogens causing hand, foot and mouth disease in infants. Infectious clones are of great importance for studying viral gene functions and pathogenic mechanism. However, there is no report on the construction of CA10 infectious clones.

Methods

The whole genome of CA10 derived from a clinical isolate was amplified into two fragments and ligated into a linearized plasmid vector in one step by In-Fusion Cloning. The obtained CA10 cDNA clones and plasmids encoding T7 RNA polymerase were co-transfected into 293 T cells to rescue CA10 virus. The rescued virus was identified by SDS-PAGE, Western blotting and transmission electron microscopic. One-day-old ICR mice were intracerebrally inoculated with the CA10 virus and clinical symptoms were observed. Multiple tissues of moribund mice were harvested for analysis of pathogenic changes and viral distribution by using H&E staining, real-time PCR and immunohistochemical staining.

Results

CA10 viruses were rescued from the constructed cDNA clone and reached a maximum titer of 10^8.125TCID₅₀/mL after one generation in RD cells. The virus exhibited similar physical and chemical properties to those of the parental virus. It also showed high virulence and the ability to induce death of neonatal ICR mice. Severe necrotizing myositis, intestinal villus interstitial edema and severe alveolar shrinkage were observed in infected mice. The viral antigen and the maximum amount of viral RNA were detected in limb skeletal muscles, which suggested that the limb skeletal muscles were the most likely site of viral replication.

Conclusion

Infectious clones of CA10 were successfully constructed for the first time, which will facilitate the establishment of standardized neonatal mouse models infected with CA10 for the evaluation of vaccines and antiviral drugs, as well as preservation and sharing of model strains.

Enterovirus A71 DNA-Launched Infectious Clone as a Robust Reverse Genetic Tool

Enterovirus A71 (EV-A71) causes major outbreaks of hand, foot and mouth disease, and is occasionally associated with neurological complications and death in children. Reverse genetics is widely used in the field of virology for functional study of viral genes. For EV-A71, such tools are limited to clones that are transcriptionally controlled by T7/SP6 bacteriophage promoter. This is often time-consuming and expensive. Here, we describe the development of infectious plasmid DNA-based EV-A71 clones, for which EV-A71 genome expression is under transcriptional control by the CMV-intermediate early promoter and SV40 transcriptional-termination signal. Transfection of this EV-A71 infectious DNA produces good virus yield similar to in vitro-transcribed EV-A71 infectious RNA, 6.4 and 5.8 log₁₀PFU/ml, respectively. Infectious plasmid with enhanced green fluorescence protein and Nano luciferase reporter genes also produced good virus titers, with 4.3 and 5.0 log₁₀ PFU/ml, respectively. Another infectious plasmid with both CMV and T7 promoters was also developed for easy manipulation of in vitro transcription or direct plasmid transfection. Transfection with either dual-promoter infectious plasmid DNA or infectious RNA derived from this dual-promoter clone produced infectious viral particles. Incorporation of hepatitis delta virus ribozyme, which yields precise 3’ ends of the DNA-launched EV-A71 genomic transcripts, increased infectious viral production. In contrast, the incorporation of hammerhead ribozyme in the DNA-launched EV-A71 resulted in lower virus yield, but improved the virus titers for T7 promoter-derived infectious RNA. This study describes rapid and robust reverse genetic tools for EV-A71.

Particle infectivity of HIV-1 full-length genome infectious molecular clones in a subtype C heterosexual transmission pair following high fidelity amplification and unbiased cloning

The high genetic diversity of HIV-1 impedes high throughput, large-scale sequencing and full-length genome cloning by common restriction enzyme based methods. Applying novel methods that employ a high-fidelity polymerase for amplification and an unbiased fusion-based cloning strategy, we have generated several HIV-1 full-length genome infectious molecular clones from an epidemiologically linked transmission pair. These clones represent the transmitted/founder virus and phylogenetically diverse non-transmitted variants from the chronically infected individual׳s diverse quasispecies near the time of transmission. We demonstrate that, using this approach, PCR-induced mutations in full-length clones derived from their cognate single genome amplicons are rare. Furthermore, all eight non-transmitted genomes tested produced functional virus with a range of infectivities, belying the previous assumption that a majority of circulating viruses in chronic HIV-1 infection are defective. Thus, these methods provide important tools to update protocols in molecular biology that can be universally applied to the study of human viral pathogens.

Improved replication efficiency of echovirus 5 after transfection of colon cancer cells using an authentic 5' RNA genome end methodology

Oncolytic virotherapy is a promising novel form of cancer treatment, but the therapeutic efficiency needs improvement. A potential strategy to enhance the therapeutic effect of oncolytic viruses is to use infectious nucleic acid as therapeutic agent to initiate an oncolytic infection, without administrating infectious viral particles. Here we demonstrate improved viral replication activation efficiency when transfecting cells with 5' end authentic in vitro transcribed enterovirus RNA as compared to genomic RNA with additional non-genomic 5' nucleotides generated by conventional cloning methods. We used echovirus 5 (E5) as an oncolytoc model virus due to its ability to replicate in and completely destroy five out of six colon cancer cell lines and kill artificial colon cancer tumors (HT29 spheroids), as shown here. An E5 infectious cDNA clone including a hammerhead ribozyme sequence was used to generate in vitro transcripts with native 5' genome ends. In HT29 cells, activation of virus replication is approximately 20-fold more efficient for virus genome transcripts with native 5' genome ends compared to E5 transcripts generated from a standard cDNA clone. This replication advantage remains when viral progeny release starts by cellular lysis 22 h post transfection. Hence, a native 5' genomic end improves infection activation efficacy of infectious nucleic acid, potentially enhancing its therapeutic effect when used for cancer treatment. The clone design with a hammerhead ribozyme is likely to be applicable to a variety of oncolytic positive sense RNA viruses for the purpose of improving the efficacy of oncolytic virotherapy.