Why are only humans susceptible to Flaviviruses like Zika ?
Flavivirus is a genus of viruses in the family Flaviviridae. This genus includes the West Nile virus, dengue virus, tick-borne encephalitis virus, yellow fever virus, Zika virus and several other viruses which may cause encephalitis. Flaviviruses are named from the yellow fever virus, the type virus for the family; the word flavus means “yellow” in Latin.
These viruses have emerged as human (and other primate) pathogens because of their ability to specifically overcome our anti-viral defenses.
In the case of Zika, researchers at the Icahn School of Medicine at Mount Sinai report that one of the virus’s seven non-structural proteins (NS5) is singularly responsible for blocking the action of interferons (proteins that stop viral replication) in human cells, while mouse cells are unaffected.
Infectious disease specialist Adolfo García-Sastre and his lab members had been studying the strategies that flaviviruses use to suppress host immune responses when the Zika outbreak began. The 70+ known flaviviruses use diverse mechanisms to overcome interferons produced by host cells, and so the researchers were curious to find out which family member Zika most took after.
“To our surprise, Zika’s ability to inhibit interferon signalling in human cells is most similar to dengue virus and not its closer relative, Spondweni virus,” says García-Sastre, senior author of the paper. “Both dengue and Zika affect the same target for degradation to prevent the action of interferon, but they use different host machinery.”
The shared target is STAT2, a protein that tells the cell to start making a range of antiviral genes induced by interferons. While both dengue and Zika viruses use their non-structural protein, NS5, to bind to and shut down STAT2 activity, the mechanism of degradation of STAT2 is unique for Zika virus among the flaviviruses. How Zika NS5 has this effect is being worked out; however, there is evidence that Zika is the first known flavivirus with an NS5 protein that accumulates in specific nuclear dots, which might relate to its ability to shut down host antiviral defences.
The work, done in collaboration with Sonja Best’s lab at the National Institute of Allergy and Infectious Diseases, is a starting point for new therapeutic approaches for Zika. By blocking NS5 activity, it can make the virus more susceptible to an infected cell’s innate immune response. This approach could also be useful for vaccine development, although it will be important to learn how quickly NS5 could mutate, as viral resistance is always a possible issue.
“We’re also interested in making the virus grow in mice by replicating the human innate immune response,” says García-Sastre. “By introducing human STAT2 in a mouse, we might develop an immune-competent small animal model for testing vaccines and therapeutics.”
