Live attenuated Ebola virus mutant immunizes nonhuman primates

The injection of an Ebola virus carrying mutations into a protein called VP35 does not cause disease and helps protect monkeys, researchers said Sept. 17 in the newspaper Cell reports. The results suggest that the immune evasion function of VP35 is a potential target of the drug and that it might be possible to develop a live attenuated vaccine against Ebola virus if precautions are taken to avoid the return to the virulence.

"This is not the next Ebola vaccine, but the study shows that it is possible to generate Ebola viruses that can grow in cell culture and induce a strong immune response but that pose a reduced risk to scientists, "says Christopher, co-author of the study. University of Basler of Georgia State. "Such mutant viruses may be useful for safely studying many of the functions of the Ebola virus."

Ebola virus disease is a rare but serious – often fatal – disease in humans. Until late 2013, the Ebola virus was causing sporadic and isolated epidemics, mainly in Central Africa. But the 2013-2016 epidemic that affected more than 28,000 people resulted in more than 11,000 deaths and the spread of cases from West Africa to Europe and the United States. -United. Subsequent epidemics in the Democratic Republic of Congo also emphasize the importance of understanding the factors that contribute to the virulence of the Ebola virus.

At present, there is no authorized treatment to neutralize the virus, but various treatments are under development. Ebola vaccines are also under development and have been used to control the spread of epidemics in Guinea and the Democratic Republic of Congo. "Most current vaccine candidates against the Ebola virus express only a single viral protein, the glycoprotein, but patients who survive the infection develop a robust T-cell response to it." 39, other viral proteins, "Basler said. "This suggests that a candidate vaccine expressing all viral proteins could trigger a broader, potentially more effective immune response."

In the new study, Basler is associated with Thomas Geisbert of the Medical Branch of the University of Texas, Ilhem Messaoudi of the University of California at Irvine and Hideki Ebihara of the Mayo Clinic in Rochester, in Minnesota, to develop a mutant virus that genes the Ebola virus and has the structure of a real particle of the Ebola virus. They generated a virus with three mutations in the VP35 protein, which plays an important role in eliminating host immune responses.

Specifically, VP35 is known to suppress the production of immune system proteins called type I interferons and inhibit the maturation of dendritic cells, which exhibit T-cell antigens and serve as messengers between innate and adaptive immune systems. "We wanted to determine to what extent these VP35 functions were affecting Ebola virus disease and host immune responses in the animal model that best fit the human disease," Basler said.

An earlier study had shown that infection with a mutant VP35 Ebola virus protected guinea pigs from further attack by an Ebola virus adapted to guinea pigs. But in general, rodent models are more easily protected by antiviral or vaccine approaches than nonhuman primate models, which are susceptible to the same strains of the Ebola virus that cause diseases in humans. "Previous studies have also not carefully evaluated immune responses to mutant VP35 Ebola virus, as we did in our study," Basler said.

To check if their mutant virus would protect nonhuman primates against Ebola infection, the researchers used it to inoculate three cynomolgus macaques, the animal model serving as a reference standard for lambs. evaluation of the virulence of the Ebola virus. Infection with the presentation of the mutant virus-activated antigen and the innate immunity pathways resulted in an increase in the frequency of memory T cells and proliferating B cells as well as the production of antibodies.

After 28 days, the researchers challenged the monkeys with a lethal dose of unmodified Ebola virus. Two monkeys survived without signs of illness, but one died nine days later. To induce a more protective response, the researchers inoculated five macaques with a higher dose of the mutant virus. Vaccination caused mild illness in three of the monkeys. This time, all animals survived without detectable clinical signs after being exposed to a dose 300 times higher than the standard 100% lethal challenge dose of unmodified Ebola virus. But most monkeys have had temporary haematological changes such as low platelet counts or low white blood cell counts.

"These data demonstrate that VP35 is an essential immune evasion factor for the Ebola virus and provides information on the immune mechanisms of Ebola control," Basler said. "We show that the suppression of interferon responses by VP35 is an essential condition for the Ebola virus to cause disease, and that the function of VP35 has an impact not only on innate antiviral defenses, but also on adaptive immune responses a potential drug target. "

Although the mutant Ebola virus is highly attenuated and can immunize non-human primates, it has surprisingly robust replication in cell culture. "This combination of features makes the mutant Ebola virus potentially suitable for research without the harsh constraints imposed by biosafety level 4 containment, which is used for the most dangerous microbes," says Basler.

The authors say that the mutant virus is not yet suitable as a vaccine because regulatory hurdles should be overcome first and that several different attenuating mutations prevent the virus from inverting and becoming virulent. In later studies, researchers plan to determine if a similar approach could be used to mitigate associated viruses. They are also interested in identifying additional mutations that would still allow the virus to replicate while preventing it from causing the disease.