The molecular basis of antibody protection against West Nile virus

Balancing functions of regulatory T cells in mosquito-borne viral infections

Mosquito-borne viral infections are on the rise worldwide and can lead to severe symptoms such as haemorrhage, encephalitis, arthritis or microcephaly. A protective immune response following mosquito-borne viral infections requires the generation of a controlled and balanced immune response leading to viral clearance without immunopathology. Here, regulatory T cells play a central role in restoring immune homeostasis. In current review, we aim to provide an overview and summary of the phenotypes of FOXP3+ Tregs in various mosquito-borne arboviral disease, their association with disease severity and their functional characteristics. Furthermore, we discuss the role of cytokines and Tregs in the immunopathogenesis of mosquito-borne infections. Lastly, we discuss possible novel lines of research which could provide additional insight into the role of Tregs in mosquito-borne viral infections in order to develop novel therapeutic approaches or vaccination strategies.

CD4 T Cell Determinants in West Nile Virus Disease and Asymptomatic Infection

West Nile (WN) virus infection of humans is frequently asymptomatic, but can also lead to WN fever or neuroinvasive disease. CD4 T cells and B cells are critical in the defense against WN virus, and neutralizing antibodies, which are directed against the viral glycoprotein E, are an accepted correlate of protection. For the efficient production of these antibodies, B cells interact directly with CD4 helper T cells that recognize peptides from E or the two other structural proteins (capsid-C and membrane-prM/M) of the virus. However, the specific protein sites yielding such helper epitopes remain unknown. Here, we explored the CD4 T cell response in humans after WN virus infection using a comprehensive library of overlapping peptides covering all three structural proteins. By measuring T cell responses in 29 individuals with either WN virus disease or asymptomatic infection, we showed that CD4 T cells focus on peptides in specific structural elements of C and at the exposed surface of the pre- and postfusion forms of the E protein. Our data indicate that these immunodominant epitopes are recognized in the context of multiple different HLA molecules. Furthermore, we observed that immunodominant antigen regions are structurally conserved and similarly targeted in other mosquito-borne flaviviruses, including dengue, yellow fever, and Zika viruses. Together, these findings indicate a strong impact of virion protein structure on epitope selection and antigenicity, which is an important issue to consider in future vaccine design.

Protection of a single dose west nile virus recombinant subviral particle vaccine against lineage 1 or 2 strains and analysis of the cross-reactivity with Usutu virus

West Nile virus (WNV) is a neurovirulent mosquito-borne flavivirus. High WNV virulence was mainly associated with lineage 1 strains, but recent outbreaks have unveiled circulation of highly virulent lineage 2 strains. Co-expression of flavivirus prM and E glycoproteins drives the assembly of recombinant subviral particles (RSPs) that share antigenic features with virions. Mouse immunization with lineage 1 WNV RSPs induced a potent humoral response against WNV with production of neutralizing antibodies. A single inoculation of RSPs formulated with Al(OH)₃ as adjuvant protected mice against a lethal challenge with WNV strains from lineage 1 or 2. The cross-reactivity of the response elicited by these RSPs was analyzed against the related flavivirus Usutu virus (USUV), which shares multiple ecological and antigenic features with WNV. Immunization with WNV-RSPs increased specific, although low, antibody titers found upon subsequent USUV infection.

Targeting host factors to treat West Nile and dengue viral infections

West Nile (WNV) and Dengue (DENV) viruses are major arboviral human pathogens belonging to the genus Flavivirus. At the current time, there are no approved prophylactics (e.g., vaccines) or specific therapeutics available to prevent or treat human infections by these pathogens. Due to their minimal genome, these viruses require many host molecules for their replication and this offers a therapeutic avenue wherein host factors can be exploited as treatment targets. Since several host factors appear to be shared by many flaviviruses the strategy may result in pan-flaviviral inhibitors and may also attenuate the rapid emergence of drug resistant mutant viruses. The scope of this strategy is greatly enhanced by the recent en masse identification of host factors impacting on WNV and DENV infection. Excellent proof-of-principle experimental demonstrations for host-targeted control of infection and infection-induced pathogenesis have been reported for both WNV and DENV. These include exploiting not only those host factors supporting infection, but also targeting host processes contributing to pathogenesis and innate immune responses. While these early studies validated the host-targeting approach, extensive future investigations spanning a range of aspects are needed for a successful deployment in humans.

Recombinant West Nile virus envelope protein E and domain III expressed in insect larvae protects mice against West Nile disease

In this study, West Nile virus (WNV) envelope (rE) protein and its domain III (rDIII) were efficiently expressed in a cost-effective system based on insect larvae as non-fermentative living biofactories. Mice immunized with the partially purified rE or rDIII elicited high antibodies titers that neutralized viral infectivity in cell culture and in suckling mice. All vaccinated animals were fully protected when challenged with neurovirulent WNV NY99. Passive transfer of protective antibodies from immunized mothers to their offspring occurred both by transplacental and lactation routes. These results indicate that the insect-derived antigens tested may constitute potential vaccine candidates to be further evaluated.

Chimeric vaccine composed of viral peptide and mammalian heat-shock protein 60 peptide protects against West Nile virus challenge

The protective efficacy and immunogenicity of a chimeric peptide against West Nile virus (WNV) was evaluated. This virus is the aetiological agent of West Nile fever, which has recently emerged in the western hemisphere. The rapid spread of WNV throughout North America, as well as the constantly changing epidemiology and transmission of the virus by blood transfusion and transplantation, have raised major public-health concerns. Currently, there are no effective treatments for WNV or vaccine for human use. We previously identified a novel, continuous B-cell epitope from domain III of the WNV envelope protein, termed Ep15. To test whether this epitope can protect against WNV infection, we synthesized a linear chimeric peptide composed of Ep15 and the heat-shock protein 60 peptide, p458. The p458 peptide is an effective carrier peptide for subunit vaccines against other infectious agents. We now report that mice immunized with the chimeric peptide, p458-Ep15, were resistant to lethal challenges with three different WNV strains. Moreover, their brains were free of viral genome and infectious virus. Mice immunized with Ep15 alone or with p431-Ep15, a control conjugate, were not protected. The chimeric p458-Ep15 peptide induced WNV-specific immunoglobulin G antibodies that neutralized the virus and induced the secretion of interferon-gammain vitro. Challenge of chimeric peptide-immunized mice considerably enhanced WNV-specific neutralizing antibodies. We conclude that this chimeric peptide can be used for formulation of a human vaccine against WNV.