Inactivated Wild-Type West Nile Virus (L2, 1986)

Flaviviruses are RNA viruses that pose a worrisome threat to human and animal health worldwide. Initially reported to cause only mild disease, Zika virus (ZIKV) has recently spread across the Americas, infecting millions of people. Zika virus infection is associated with severe neurological disorders and birth defects, particularly Guillain-Barré syndrome (GBS) and microcephaly. Here, researchers evaluated the humoral responses of immunocompetent mice to infection with three strains of Zika virus from different geographical origins (Africa, Asia, and the Americas). None of the infected mice showed any signs of disease or died following infection. However, all infected mice produced specific neutralizing antibodies. Given the rapid spread of Zika virus across the American continent and its co-circulation with other medically relevant flaviviruses, such as West Nile virus (WNV), researchers analyzed the potential for induction of protective immunity between ZIKV and WNV. Notably, protection was still observed in mice previously infected with ZIKV after challenge with WNV, with significantly higher survival rates than control mice. Furthermore, prior infection with ZIKV enhanced the humoral immune response of mice to WNV. These findings may be relevant in geographical areas where both ZIKV and WNV co-circulate, as well as for the future development of broad-spectrum flavivirus vaccines.

Here, the researchers utilized an in-house developed ELISA based on heat-inactivated West Nile Nile or Zika virus antigens generated from infected cell cultures. The ELISA was validated using a Zika virus-specific monoclonal antibody that showed good reactivity with all three Zika virus antigens, but was more reactive with the African strain (Figure 2A) because the monoclonal antibody was generated against the African MR766 strain. Notably, sera from mice infected with the Asian and American strains recognized the three Zika virus antigens in a similar manner, while sera from mice infected with the African strain mainly recognized its own specific antigen, which was the case with sera from mice infected with West Nile virus (Figure 1A).

Blood samples were collected on days after infection, and anti-Zika or anti-WNV IgG was detected by indirect ELISA using plates coated with heat-inactivated Zika or heat-inactivated WNV. Some Zika virus-infected mice developed specific antibodies 5 days after infection, and all mice had detectable antibodies 13 days after infection, and the antibody levels increased slightly after WNV challenge (Figure 1B). Notably, Zika virus infection enhanced the production of specific WNV antibodies, because 5 days after WNV challenge (19 days after Zika virus infection), specific anti-WNV antibodies were only observed in mice that had been previously infected with Zika virus, but not in mice that had been infected with WNV alone (Figure 1C). In addition, anti-WNV antibodies continued to increase until the end of the experiment, and the antibody levels of mice that had been previously infected with Zika virus were significantly higher than those of control mice that had been infected with WNV alone.

Figure 1. Induction of anti-ZIKV and anti-WNV IgGs in mice. (Vázquez-Calvo Á, et al., 2017)