Neuroblastoma cell-adapted yellow fever virus: mutagenesis of the E protein locus involved in persistent infection and its effects on virus penetration and spread

An Overview of Current Approaches Toward the Treatment and Prevention of West Nile Virus Infection

The persistence of West Nile virus (WNV) infections throughout the USA since its inception in 1999 and its continuous spread throughout the globe calls for an urgent need of effective treatments and prevention measures. Although the licensing of several WNV vaccines for veterinary use provides a proof of concept, similar efforts on the development of an effective vaccine for humans remain still unsuccessful. Increased understanding of biology and pathogenesis of WNV together with recent technological advancements have raised hope that an effective WNV vaccine may be available in the near future. In addition, rapid progress in the structural and functional characterization of WNV and other flaviviral proteins have provided a solid base for the design and development of several classes of inhibitors as potential WNV therapeutics. Moreover, the therapeutic monoclonal antibodies demonstrate an excellent efficacy against WNV in animal models and represent a promising class of WNV therapeutics. However, there are some challenges as to the design and development of a safe and efficient WNV vaccine or therapeutic. In this chapter, we discuss the current approaches, progress, and challenges toward the development of WNV vaccines, therapeutic antibodies, and antiviral drugs.

Structures and Functions of the Envelope Glycoprotein in Flavivirus Infections

Flaviviruses are enveloped, single-stranded RNA viruses that widely infect many animal species. The envelope protein, a structural protein of flavivirus, plays an important role in host cell viral infections. It is composed of three separate structural envelope domains I, II, and III (EDI, EDII, and EDIII). EDI is a structurally central domain of the envelope protein which stabilizes the overall orientation of the protein, and the glycosylation sites in EDI are related to virus production, pH sensitivity, and neuroinvasiveness. EDII plays an important role in membrane fusion because of the immunodominance of the fusion loop epitope and the envelope dimer epitope. Additionally, EDIII is the major target of neutralization antibodies. The envelope protein is an important target for research to develop vaccine candidates and antiviral therapeutics. This review summarizes the structures and functions of ED I/II/III, and provides practical applications for the three domains, with the ultimate goal of implementing strategies to utilize the envelope protein against flavivirus infections, thus achieving better diagnostics and developing potential flavivirus therapeutics and vaccines.

Relating plaque morphology to respiratory syncytial virus subgroup, viral load, and disease severity in children

BACKGROUND

Viral culture plaque morphology in human cell lines are markers for growth capability and cytopathic effect, and have been used to assess viral fitness and select preattenuation candidates for live viral vaccines. We classified respiratory syncytial virus (RSV) plaque morphology and analyzed the relationship between plaque morphology as compared to subgroup, viral load and clinical severity of infection in infants and children.

METHODS

We obtained respiratory secretions from 149 RSV-infected children. Plaque morphology and viral load was assessed within the first culture passage in HEp-2 cells. Viral load was measured by polymerase chain reaction (PCR), as was RSV subgroup. Disease severity was determined by hospitalization, length of stay, intensive care requirement, and respiratory failure.

RESULTS

Plaque morphology varied between individual subjects; however, similar results were observed among viruses collected from upper and lower respiratory tracts of the same subject. Significant differences in plaque morphology were observed between RSV subgroups. No correlations were found among plaque morphology and viral load. Plaque morphology did not correlate with disease severity.

CONCLUSION

Plaque morphology measures parameters that are viral-specific and independent of the human host. Morphologies vary between patients and are related to RSV subgroup. In HEp-2 cells, RSV plaque morphology appears unrelated to disease severity in RSV-infected children.

Integrative effect of defective interfering RNA accumulation and helper virus attenuation is responsible for the persistent infection of Japanese encephalitis virus in BHK-21 cells

Persistence of RNA viruses is often, but not always, associated with the production of defective interfering (DI) particles. To investigate possible roles of DI particles and helper viruses in RNA virus persistence, persistent infection with Japanese encephalitis virus (JEV) was established in baby hamster kidney (BHK-21) cells. At the 6th and 7th serial undiluted passages of JEV on BHK-21 cells, viral persistence was established spontaneously with DI RNA generation. Seven cell clones exhibiting persistent infection were obtained from the initial BHK-21 cell batches exhibiting JEV persistence, and maintained for over 400 days. Most cell clones produced infectious particles (10(1) -10(5)  PFU/ml) continuously, expressed viral proteins, and resisted homologous superinfection. Two helper viruses, chvBS6-3 and chvBS7-1, were isolated from two of the seven cell clones, and characterized to investigate their roles in JEV persistence. While chvBS6-3 was restored to its full cytopathicity in the absence of DI RNA, chvBS7-1 exhibited almost no cytopathicity, regardless of DI RNA co-replication. Attenuation of chvBS7-1 did not appear to be due to inadequate adsorption or genome replication, but due to inefficient egress of the assembled progeny virions, suggesting altered helper virus emergence during JEV persistence in BHK-21 cells. These observations suggest that at least two mechanisms are involved in JEV persistence; a DI RNA-dependent mechanism, where DI RNA co-replication nullifies the helper virus's cytopathicity, or a DI RNA-independent mechanism, where the helper virus is self-attenuated. This study provides a useful in vitro tool for understanding the mechanisms underlying RNA virus persistent infections.

Japanese encephalitis virus non-coding RNA inhibits activation of interferon by blocking nuclear translocation of interferon regulatory factor 3

Noncoding RNA (ncRNA) plays a critical role in modulating a broad range of diseases. All arthropod-borne flaviviruses produce short fragment ncRNA (sfRNA) collinear with highly conserved regions of the 3'-untranslated region (UTR) in the viral genome. We show that the molar ratio of sfRNA to genomic RNA in Japanese encephalitis virus (JEV) persistently infected cells is greater than that in acutely infected cells, indicating an sfRNA role in establishing persistent infection. Transfecting excess quantities of sfRNA into JEV-infected cells reduced interferon-β (IFN-β) promoter activity by 57% and IFN-β mRNA levels by 52%, compared to mock-transfected cells. Transfection of sfRNA into JEV-infected cells also reduced phosphorylation of interferon regulatory factor-3 (IRF-3), the IFN-β upstream regulator, and blocked roughly 30% of IRF-3 nuclear localization. Furthermore, JEV-infected sfRNA transfected cells produced 23% less IFN-β-stimulated apoptosis than mock-transfected groups did. Taken together, these results suggest that sfRNA plays a role against host-cell antiviral responses, prevents cells from undergoing apoptosis, and thus contributes to viral persistence.

Genetic determinants of Sindbis virus mosquito infection are associated with a highly conserved alphavirus and flavivirus envelope sequence

Wild-type Sindbis virus (SINV) strain MRE16 efficiently infects Aedes aegypti midgut epithelial cells (MEC), but laboratory-derived neurovirulent SINV strain TE/5'2J infects MEC poorly. SINV determinants for MEC infection have been localized to the E2 glycoprotein. The E2 amino acid sequences of MRE16 and TE/5'2J differ at 60 residue sites. To identify the genetic determinants of MEC infection of MRE16, the TE/5'2J virus genome was altered to contain either domain chimeras or more focused nucleotide substitutions of MRE16. The growth patterns of derived viruses in cell culture were determined, as were the midgut infection rates (MIR) in A. aegypti mosquitoes. The results showed that substitutions of MRE16 E2 aa 95 to 96 and 116 to 119 into the TE/5'2J virus increased MIR both independently and in combination with each other. In addition, a unique PPF/.GDS amino acid motif was located between these two sites that was found to be a highly conserved sequence among alphaviruses and flaviviruses but not other arboviruses.

Characterization of a small plaque variant of West Nile virus isolated in New York in 2000

A small-plaque variant (SP) of West Nile virus (WNV) was isolated in Vero cell culture from kidney tissue of an American crow collected in New York in 2000. The in vitro growth of the SP and parental (WT) strains was characterized in mammalian (Vero), avian (DF-1 and PDE), and mosquito (C6/36) cells. The SP variant replicated less efficiently than did the WT in Vero cells. In avian cells, SP growth was severely restricted at high temperatures, suggesting that the variant is temperature sensitive. In mosquito cells, growth of SP and WT was similar, but in vivo in Culex pipiens (L.) there were substantial differences. Relative to WT, SP exhibited reduced replication following intrathoracic inoculation and lower infection, dissemination, and transmission rates following oral infection. Analysis of the full length sequence of the SP variant identified sequence differences which led to only two amino acid substitutions relative to WT, prM P54S and NS2A V61A.

Defective interfering RNAs of Japanese encephalitis virus found in mosquito cells and correlation with persistent infection

Defective interfering (DI) RNAs are deletion mutants of viral genomes that are known in many cases to contribute to persistent infection and modification of viral pathogenesis. Cell type also plays a critical role in the establishment of viral persistence. In this study we have identified for the first time the generation of DI RNAs of Japanese encephalitis virus in C6/36 mosquito cells. A persistent infection was established by replacing growth medium on surviving cells and continued cell passaging. Persistent infection was demonstrated by a continual release of infectious virus, fluorescent antibody staining, and Northern analysis. A population of DI RNAs of approximately 8.2-9.7 kb, not detectable in acutely infected cells, became apparent in the persistently infected cells by 25 days postinfection. Sequence analyses revealed a population of DI RNAs that contained in-frame deletions of 1.3-2.8 kb covering the region of the E gene and some flanking C or prM and NS1 gene sequences. Transcripts from one cDNA clone of a DI RNA replicated in uninfected mosquito cells as demonstrated by RT-PCR. DI RNA-containing virions in supernatant fluids from persistently infected mosquito cells could be used to establish persistent infection in BHK-21 cells. The correlation of DI RNA presence with cell survival suggests that DI RNAs are contributing mechanistically to the establishment of persistent infection in both the mosquito and mammalian cells.