Since the 2015 Zika outbreak, scientists around the world have been focused on learning as much as they can about the devastating, previously obscure virus. They've learned a lot about how it's transmitted, how long it can stay in a host's system and that it can lead to devastating birth defects. But, there are still some major blind spots.
"Zika virus has been very well studied for congenital disease, but we still do not know exactly what happens right after [the] mosquito bite," said Dr. Jae Jung, professor of molecular microbiology and immunology at USC and lead author of a new study in Nature Microbiology that aims to solve the mystery.
It's been unclear what happens immediately after the virus enters the bloodstream, particularly of pregnant women, that allows it to propagate before infecting an unborn baby. As it turns out, the mechanism of how the virus spreads is eerily similar to that of HIV.
"We found that Zika virus specifically targets the white blood cells," said Dr. Jung.
Once the virus enters the bloodstream of a pregnant woman, it tricks the immune system, suppresses it and spreads quickly.
When a healthy, non-pregnant person is infected with a virus, the immune system kicks into high gear. White blood cells don their pith helmets and turn into so-called " M1 macrophages" that act like little soldiers, catching the virus and killing it. Separate white blood cells (M2 macrophages) then come along to calm their M1 cousins to return the immune system to its neutral mode.
However, when the Zika virus enters the body of a pregnant woman it takes advantage of her unique biology. The immune systems of pregnant women are already compromised. Their bodies have been flooded with the chill M2 macrophages, which tell the body's immune system to relax. This immunity suppression allows the unborn baby to survive.
But, the Zika virus is sneaky. Since a pregnant woman's body is already predisposed to creating the chill cells, it convinces her body to create even more. So, rather than attack the Zika virus, the compromised immune system allows it to propagate. The virus then spreads, eventually crosses the placental barrier and infects the fetus. As a result, babies can be born with a host of neurological birth defects, including microcephaly.
Dr. Jung's team studied both the African and Asian strains of the Zika virus, but found that the Asian strain, which is spreading across the Americas and Southeast Asia, had a more profound impact on the immune systems of pregnant women particularly during the first and second trimester. During the third, the impact wasn't nearly as pronounced.
"It is very important to understand how the virus behaves in order to develop treatments and also ways to prevent this from happening," said Dr. Karin Nielsen from UCLA and one of the authors of the study.
Dr. Jung's lab previously identified the proteins in the Zika virus that can cause microcephaly in infants.
There's still a lot left to figure out. For instance, Dr. Jung said that it's possible that Zika vaccines, some of which have been proven effective on non-pregnant people, might not be as effective for pregnant women as ethical limitations have prevented their participation in vaccine trials.
"The Zika virus research has just began," Dr. Jung said. "We've only studied for two years so far. HIV has been studied over 30 years."