In search of innovation against yellow fever

Researchers of the Albert Einstein College of Medicine, in the United States, in a partnership with the Oswaldo Cruz Institute (IOC/Fiocruz) and with the University of São Paulo (USP) and other institutions, have given an important step towards the finding of a treatment for yellow fever. Considered a worldwide trend, synthetic monoclonal antibodies have been the subject of studies all over the planet. These antibodies are laboratory products and bind to a specific region of the viral particle, interrupting the infection process.

(Image: Oswaldo Cruz Institute)

Drugs of this kind were developed, for instance, for the treatment of COVID-19 and Zika. In the case of yellow fever, these drugs can be an alternative to treat infections and for prophylaxis for people who are traveling to risk areas but cannot take the yellow fever vaccine due to counterindications. The first results of the study, recently published on Cell Host & Microbe, show the mapping of variations of amino acids that alter the surface of the viral particle, making the action of some classes of antibodies more difficult.

“Monoclonal antibodies are based on natural antibodies produced by our own bodies. Our study shows that it is compulsory to select molecules against yellow fever taking into account the viruses that circulate in each different region, because some antibodies that neutralize the vaccine virus and other strains originally from Africa were not capable of neutralizing South American viruses”, says researcher Myrna Bonaldo, chief of the Laboratory of Flaviviruses Molecular Biology of the IOC and one of the authors of the paper.

Mapped variation

In the first stage of the study, researchers evaluated the capacity of vaccinated people’s serum, which contains tens of types of antibodies induced by the immunization, to neutralize different viral lineages. The study examined the lineages used to formulate yellow fever vaccines in Brazil and in the United States. Both are attenuated versions of a virus isolated in Africa in 1927 and a strain isolated in Brazil in 2017. The neutralization of the Brazilian virus was significantly lower than that of vaccine viruses. Next, scientists investigated the actuation capacity of about one hundred monoclonal antibodies, produced based on natural antibodies present in the serum of vaccinatedpeople.

“We detected that the neutralization capacity of some molecules that were very reactive against the vaccine virus plummeted to zero against the Brazilian virus. Our experiments showed that the antibodies that failed had a target in common: domain II of protein E”, says doctorate student of the post-graduation program in cellular and molecular biology of the IOC and another author of the paper, Nathalia Dias Furtado.

Protein E makes up the envelope that wraps the viral particle and presents three segments that are exposed on the surface of the virus, called domains I, II, and III. The molecule plays an important role in infection. It works like a key that binds to the host cell, promoting the opening of the cellular membrane so the virus can penetrate into the cell. Therefore, in order to block the infection, the antibodies target the different segments of protein E.

Comparing the Brazilian virus with the vaccine lineage and two other viruses of African origin, researchers have identified variations in two points in domain II of protein E. According to scientists, these alterations explain why part of the antibodies loses the ability to neutralize the virus. “Antibodies that target other domains of protein E remain capable of acting. But without the effect of the antibodies that target domain II, the overall neutralizing power of vaccinated people’s serum drops”, comments Furtado.

The final stage of the work consisted in analyzing 281 genetic sequences of the yellow fever virus, available in databases. As for the genome region related to protein E, researchers observed that the mutations in the two points of domain II were present in almost all viruses of genotype I of South America, the predominant one in Brazil. The viruses of genotype II of South America, more frequently found in Bolivia, Peru, and Ecuador, showed only one of the variation points. None of the African viruses had this type of variation in their envelope protein.

Adaptation to the New World

For the authors of the paper, these data indicate that mutations are part of the process of adaptation of the yellow fever virus to the New World. “The yellow fever virus is originally from Africa, and was brought to the Americas with the slave trade. Here it found species of mosquitoes and non-human primates that were different from those in the African continent, and this forced the virus to adapt, which probably took place centuries ago”, says Furtado.

Drawing particular attention to the cycle of yellow fever in the wild, the researchers also point out that the genetic evolution of this pathogen is motivated mainly by ecological factors; the virus is not “attempting to escape the vaccine”. “The yellow fever virus needs to replicate in two very different hosts, an insect and a mammal. Protein E must be a skeleton key. This generates selective pressure for the virus to remain stable, because if it adapts too well to a single type of host, it risks losing adaptation to the other one. Should this happen, it would be eliminated from nature”, explains Bonaldo.

The researcher also highlights the difference regarding Sars-CoV-2, which causes COVID-19 and began to spread among humans very recently, through direct transmission from one person to another and with a high mutation rate. “Escaping the vaccine can be an important evolutionary advantage for Sars-CoV-2, but is not a core factor for the yellow fever virus, as it has a cycle in the wild and has been well established in nature for centuries”, she says.

Vaccine efficacy

According to the researchers, in spite of the lower capacity of neutralization of antibodies, the efficacy of the yellow fever vaccine can remain high due to other immunological mechanisms. “In practical terms, even with the differences observed, we have an effective vaccine that has been a powerful weapon to control outbreaks in Africa as well as in South America”, states Bonaldo, reminding us that the most recent yellow fever epidemics in Brazil spread in areas with low vaccine coverage and were controlled with vaccine rollout.

“Antibodies are just one part of the immune response. Vaccination stimulates, for instance, CD8 cytotoxic T cells, which are very important to recognize and eliminate infected cells”, she adds. The authors of the paper also point out that there are many ongoing research lines to develop new yellow fever vaccines, using different technologies than the current vaccine that uses an attenuated live virus. Among the options under study are inactivated virus, viral vector, messenger RNA, and antigen.

In this context, the data obtained in the research may guide the search for formulations that can stimulate the production of antibodies against different viral lineages. “With more advanced technologies, it is possible to think of a multiple vaccine. To achieve this, studies will have to prove that the new formulations are more effective than the vaccine currently used”, ponders Bonaldo.