Development of flavivirus subviral particles with low cross-reactivity by mutations of a distinct antigenic domain

The most conserved fusion loop (FL) domain present in the flavivirus envelope protein has been reported as a dominant epitope for cross-reactive antibodies to mosquito-borne flaviviruses (MBFVs). As a result, establishing accurate serodiagnosis for MBFV infections has been difficult as anti-FL antibodies are induced by both natural infection and following vaccination. In this study, we modified the most conserved FL domain to overcome this cross-reactivity. We showed that the FL domain of lineage I insect-specific flavivirus (ISFV) has differences in antigenicity from those of MBFVs and lineage II ISFV and determined the key amino acid residues (G106, L107, or F108), which contribute to the antigenic difference. These mutations were subsequently introduced into subviral particles (SVPs) of dengue virus type 2 (DENV2), Zika virus (ZIKV), Japanese encephalitis virus (JEV), and West Nile virus (WNV). In indirect enzyme-linked immunosorbent assays (ELISAs), these SVP mutants when used as antigens reduced the binding of cross-reactive IgG and total Ig induced by infection of ZIKV, JEV, and WNV in mice and enabled the sensitive detection of virus-specific antibodies. Furthermore, immunization of ZIKV or JEV SVP mutants provoked the production of antibodies with lower cross-reactivity to heterologous MBFV antigens compared to immunization with the wild-type SVPs in mice. This study highlights the effectiveness of introducing mutations in the FL domain in MBFV SVPs with lineage I ISFV-derived amino acids to produce SVP antigens with low cross-reactivity and demonstrates an improvement in the accuracy of indirect ELISA-based serodiagnosis for MBFV infections. KEY POINTS: • The FL domain of Lineage I ISFV has a different antigenicity from that of MBFVs. • Mutated SVPs reduce the binding of cross-reactive antibodies in indirect ELISAs. • Inoculation of mutated SVPs induces antibodies with low cross-reactivity.

Long-distance spread of Tembusu virus, and its dispersal in local mosquitoes and domestic poultry in Chongming Island, China

BACKGROUND

Chongming Island in China serves as a breeding and shelter point on the East Asian-Australasian Flyway. The resting frequency of migratory birds, abundance of mosquito populations, and the popular domestic poultry industry pose a potential risk of mosquito-borne zoonotic diseases. The aim of this study is to explore the role of migratory birds in the spread of mosquito-borne pathogens and their prevalent status on the island.

METHODS

We conducted a mosquito-borne pathogen surveillance in 2021, in Chongming, Shanghai, China. Approximately 67,800 adult mosquitoes belonging to ten species were collected to investigate the presence of flaviviruses, alphaviruses, and orthobunyaviruses by RT-PCR. Genetic and phylogenetic analyses were conducted to explore the virus genotype and potential nature source. Serological survey was performed by ELISA to characterize Tembusu virus (TMUV) infection among domestic poultry.

RESULTS

Two strains of TMUV and Chaoyang virus (CHAOV) and 47 strains of Quang Binh virus (QBV) were detected in 412 mosquito pools, with the infection rate of 0.16, 0.16, and 3.92 per 1000 Culex tritaeniorhynchus, respectively. Furthermore, TMUVs viral RNA was found in serum samples of domestic chickens and faecal samples of migratory birds. Antibodies against TMUV were detected in domestic avian serum samples, generally ranging from 44.07% in pigeons to 55.71% in ducks. Phylogenetic analyses indicated that the TMUV detected in Chongming belonged to Cluster 3, Southeast Asia origin, and most closely related to the CTLN strain, which caused a TMUV outbreak in chickens in Guangdong Province in 2020, but distant from strains obtained previously in Shanghai, which were involved in the 2010 TMUV outbreak in China.

CONCLUSIONS

We speculate that the TMUV was imported to Chongming Island through long-distance spreading by migratory birds from Southeast Asia, followed by spill over and transmission in mosquitoes and domestic avian species, threatening the local domestic poultry. In addition, the expansion and prevalence of insect-specific flaviviruses and its simultaneous circulation with mosquito-borne virus are worthy of close attention and further study.

Molecular detection and identification of Culex flavivirus in mosquito species from Jeju, Republic of Korea

Background

Mosquito-borne flaviviruses are prime pathogens and have been a major hazard to humans and animals. They comprise several arthropod-borne viruses, including dengue virus, yellow fever virus, Japanese encephalitis virus, and West Nile virus. Culex flavivirus (CxFV) is a member of the insect-specific flavivirus (ISF) group belonging to the genus Flavivirus, which is widely distributed in a variety of mosquito populations.

Methods

Viral nucleic acid was extracted from adult mosquito pools and subjected to reverse transcriptase nested polymerase chain reaction (PCR) using target-specific primers for detecting CxFV nonstructural protein 5 (NS5). The PCR-positive samples were then sequenced, and a phylogenetic tree was constructed, including reference sequences obtained from GenBank.

Results

21 pools, belonging to Culex pipiens pallens (Cx. p. pallens) were found to be positive for the CxFV RNA sequence, with a minimum infection rate of 14.5/1000 mosquitoes. The phylogenetic analysis of the NS5 protein sequences indicated that the detected sequences were closely related to strains identified in China, with 95–98% sequence similarities.

Conclusion

Our findings highlight the presence of CxFV in Cx. p. pallens mosquito species in Jeju province, Republic of Korea. This is the first study reporting the prevalence of CxFV in Culex Pipiens (Cx. pipiens) host in the Jeju province, which can create possible interaction with other flaviviruses causing human and animal diseases. Although, mosquito-borne disease causing viruses were not identified properly, more detailed surveillance and investigation of both the host and viruses are essential to understand the prevalence, evolutionary relationship and genetic characteristic with other species.

Isolation of a novel insect-specific flavivirus with immunomodulatory effects in vertebrate systems

We describe the isolation and characterization of a novel insect-specific flavivirus (ISFV), tentatively named Aripo virus (ARPV), that was isolated from Psorophora albipes mosquitoes collected in Trinidad. The ARPV genome was determined and phylogenetic analyses showed that it is a dual host associated ISFV, and clusters with the main mosquito-borne flaviviruses. ARPV antigen was significantly cross-reactive with Japanese encephalitis virus serogroup antisera, with significant cross-reactivity to Ilheus and West Nile virus (WNV). Results suggest that ARPV replication is limited to mosquitoes, as it did not replicate in the sandfly, culicoides or vertebrate cell lines tested. We also demonstrated that ARPV is endocytosed into vertebrate cells and is highly immunomodulatory, producing a robust innate immune response despite its inability to replicate in vertebrate systems. We show that prior infection or coinfection with ARPV limits WNV-induced disease in mouse models, likely the result of a robust ARPV-induced type I interferon response.

Molecular epidemiology of mosquito-borne viruses at the China-Myanmar border: discovery of a potential epidemic focus of Japanese encephalitis

BACKGROUND

Mosquito-based arbovirus surveillance can serve as an early warning in evaluating the status of mosquito-borne virus prevalence and thus prevent local outbreaks. Although Tengchong County in Yunnan Province-which borders Myanmar-is abundant and diverse in mosquitoes, very few mosquito-based arbovirus investigations have been conducted in the recent decade. Herein, this study aims to evaluate the presence and the diffusion of mosquito-borne pathogens, currently prevalent in this region.

METHODS

We collected 9486 mosquitoes, representing eight species, with Culex tritaeniorhynchus and Anopheles sinensis as the dominant species, during high mosquito activity seasons (July-October) in Tengchong, in 2018. Samples collected from 342 pools were tested using reverse-transcription PCR to determine the species, distribution, and infection rates of virus and parasite, and further analyze their genotypes, phylogenetic relationships, infection rate, and potential pathogenicity.

RESULTS

Fifteen Japanese encephalitis virus (JEV) strains from Cx. tritaeniorhynchus pools were detected. Seven strains of insect-specific flaviviruses (ISFVs), including two Aedes flavivirus (AeFV) and Yunnan Culex flavivirus strains each, one Culex theileri flavivirus, Yamadai flavivirus (YDFV) and Anopheles-associated flavivirus (AAFV) strains each were detected in Aedes albopictus, Cx. tritaeniorhynchus, Cx. vagans, Cx. pseudovihnui, and An. sinensis pools, respectively. The whole-genome was successfully amplified in one strain of JEV and AeFV each. Phylogenetic analysis using the E gene placed all the newly detected JEV strains into the GI-b genotype. They showed highly nucleotide identities, and were most closely related to the strain detected in Tengchong in 2010. The comparison of the E protein of JEV strains and vaccine-derived strain, showed six amino residue differences. The bias-corrected maximum likelihood estimation values (and 95% confidence interval) for JEV in Cx. tritaeniorhynchus collected in Tengchong in 2018 were 2.4 (1.4-3.9).

CONCLUSIONS

A potential Japanese encephalitis epidemic focus with the abundance of host mosquitoes and high JEV infection rate was observed in Tengchong. In addition, at least five species of ISFVs co-circulate in this area. This study highlights the importance of widespread and sustained mosquito-based arbovirus surveillance in local areas to prevent the transmission of JEV, and other emerging/re-emerging mosquito-borne pathogens.

The insect-specific Palm Creek virus modulates West Nile virus infection in and transmission by Australian mosquitoes

Background

Insect-specific viruses do not replicate in vertebrate cells, but persist in mosquito populations and are highly prevalent in nature. These viruses may naturally regulate the transmission of pathogenic vertebrate-infecting arboviruses in co-infected mosquitoes. Following the isolation of the first Australian insect-specific flavivirus (ISF), Palm Creek virus (PCV), we investigated routes of infection and transmission of this virus in key Australian arbovirus vectors and its impact on replication and transmission of West Nile virus (WNV).

Methods

Culex annulirostris, Aedes aegypti and Aedes vigilax were exposed to PCV, and infection, replication and transmission rates in individual mosquitoes determined. To test whether the virus could be transmitted vertically, progeny reared from eggs oviposited by PCV-inoculated Cx. annulirostris were analysed for the presence of PCV. To assess whether prior infection of mosquitoes with PCV could also suppress the transmission of pathogenic flaviviruses, PCV positive or negative Cx. annulirostris were subsequently exposed to WNV.

Results

No PCV-infected Cx. annulirostris were detected 16 days after feeding on an infectious blood meal. However, when intrathoracically inoculated with PCV, Cx. annulirostris infection rates were 100 %. Similar rates of infection were observed in Ae. aegypti (100 %) and Ae. vigilax (95 %). Notably, PCV was not detected in any saliva expectorates collected from any of these species. PCV was not detected in 1038 progeny reared from 59 PCV-infected Cx. annulirostris. After feeding on a blood meal containing 10⁷ infectious units of WNV, significantly fewer PCV-infected Cx. annulirostris were infected or transmitted WNV compared to PCV negative mosquitoes. Immunohistochemistry revealed that PCV localized in the midgut epithelial cells, which are the first site of infection with WNV.

Conclusions

Our results indicate that PCV cannot infect Cx. annulirostris via the oral route, nor be transmitted in saliva or vertically to progeny. We also provide further evidence that prior infection with insect-specific viruses can regulate the infection and transmission of pathogenic arboviruses.

A novel insect-specific flavivirus replicates only in Aedes-derived cells and persists at high prevalence in wild Aedes vigilax populations in Sydney, Australia

To date, insect-specific flaviviruses (ISFs) have only been isolated from mosquitoes and increasing evidence suggests that ISFs may affect the transmission of pathogenic flaviviruses. To investigate the diversity and prevalence of ISFs in Australian mosquitoes, samples from various regions were screened for flaviviruses by ELISA and RT-PCR. Thirty-eight pools of Aedes vigilax from Sydney in 2007 yielded isolates of a novel flavivirus, named Parramatta River virus (PaRV). Sequencing of the viral RNA genome revealed it was closely related to Hanko virus with 62.3% nucleotide identity over the open reading frame. PaRV failed to grow in vertebrate cells, with only Aedes-derived mosquito cell lines permissive to replication, suggesting a narrow host range. 2014 collections revealed that PaRV had persisted in A. vigilax populations in Sydney, with 88% of pools positive. Further investigations into its mode of transmission and potential to influence vector competence of A. vigilax for pathogenic viruses are warranted.

Insect-specific flaviviruses, a worldwide widespread group of viruses only detected in insects

Several flaviviruses are important pathogens for humans and animals (Dengue viruses, Japanese encephalitis virus, Yellow-fever virus, Tick-borne encephalitis virus, West Nile virus). In recent years, numerous novel and related flaviviruses without known pathogenic capacity have been isolated worldwide in the natural mosquito population. However, phylogenetic studies have shown that genomic sequences of these viruses diverge from other flaviviruses. Moreover, these viruses seem to be exclusive of insects (they do not seem to grow on vertebrate cell lines), and were already defined as mosquito-only flaviviruses or insect-specific flaviviruses. At least eleven of these viruses were isolated worldwide, and sequences ascribable to other eleven putative viruses were detected in several mosquito species. A large part of the cycle of these viruses is not well known, and their persistence in the environment is poorly understood. These viruses are detected in a wide variety of distinct mosquito species and also in sandflies and chironomids worldwide; a single virus, or the genetic material ascribable to a virus, was detected in several mosquito species in different countries, often in different continents. Furthermore, some of these viruses are carried by invasive mosquitoes, and do not seem to have a depressive action on their fitness. The global distribution and the continuous detection of new viruses in this group point out the likely underestimation of their number, and raise interesting issues about their possible interactions with the pathogenic flaviviruses, and their influence on the bionomics of arthropod hosts. Some enigmatic features, as their integration in the mosquito genome, the recognition of their genetic material in DNA forms in field-collected mosquitoes, or the detection of the same virus in both mosquitoes and sandflies, indicate that the cycle of these viruses has unknown characteristics that could be of use to reach a deeper understanding of the cycle of related pathogenic flaviviruses.

Detection of Quang Binh virus from mosquitoes in China

Flaviviruses present a wide range of genetic diversity and exhibit diverse host relationships. Mosquito-borne flaviviruses have recently been isolated and characterized worldwide. Yunnan Province of China is one of the richest areas of species diversity and is the center of multi-species evolution in mainland Asia, which supports the circulation of numerous arthropod-borne viruses (arboviruses). In a screening program of arboviruses, mosquitoes were collected during the mosquito activity season in the Yunnan Province from 2007 to 2010. Eleven flavivirus strains, named Yunnan Culex flaviviruses (YNCxFVs), were obtained from Culex tritaeniorhynchus and Anopheles sinensis specimens. Sequence analyses based on partial nonstructural protein (NS) 5 gene indicated that the YNCxFVs shared 92.8-99.6% nucleotide identity with each other and were similar to the Culex-related flaviviruses. The complete genome of one representative isolate, LSFlaviV-A20-09, was sequenced. The genome was 10,865 nucleotides long and contained a single, long open reading frame (ORF) of 10,080 nucleotides that encoded a 3360-aa polyprotein. This genome was most closely related to the Quang Binh virus (QBV) VN180 strain, an insect-specific flavivirus isolated from Culex mosquitoes in Vietnam, but only had 83.0% nucleotide and 93.8% amino acid identities for the ORF sequence. The genome has approximately 66.3%-68.5% nucleotide sequence and 69.3-73.3% amino acid sequence identities to other Culex flaviviruses, and only has 47.9-57.9% nucleotide sequence and 38.7-55.1% amino acid sequence identities to Coquillettidia-related, Mansonia-related and Aedes-related flaviviruses. Phylogenetic analyses revealed that the LSFlaviV-A20-09 fell into the Culex-related flavivirus clade. Our discoveries provide more information regarding the heterogeneity of viruses that infect mosquitoes.