Explained: How Zika virus infects placenta

Zika virus can infect and replicate in immune cells from the placenta, without killing them, researchers, including one of Indian-origin, have found.

The finding may explain how the virus can pass through the placenta of a pregnant woman, on its way to infect developing brain cells in her foetus, they said. "Our results substantiate the limited evidence from pathology case reports," said Mehul Suthar from Emory University in the US. "It was known that the virus was getting into the placenta. But little was known about where the virus was replicating and in what cell type," said Suthar.

Suthar and Rana Chakraborty from Emory University found that Zika virus could infect placental macrophages, called Hofbauer cells, in cell culture. They also found that the virus could infect another type of placental cell, called cytotrophoblasts, but only after a couple of days delay and not as readily.

According to previous research, syncytiotrophoblasts, a more differentiated type of placental cell than cytotrophoblasts, are resistant to Zika infection. The cells for the experiments were derived from full-term placentae, obtained from healthy volunteers who delivered by Cesarean section, researchers said.

The level of viral replication varied markedly from donor to donor, which hints that some women's placentae may be more susceptible to viral infection than others, they said. "Not every pregnant woman who is infected by Zika transmits the virus to her foetus. Host genetics and non-viral factors, including nutrition and microbiota, as well as timing may be influencing infectivity," said Suthar. "A better understanding of these factors could allow the design of preventive measures, and eventually antiviral therapies," he said.

The viral strain used for the experiments was from Puerto Rico, closely related to the currently circulating in Brazil. For viruses related to Zika (flaviviruses) such as dengue virus, West Nile virus and yellow fever virus, it is rare for the infection to be transmitted from mother to foetus, researchers said.

This is thought to be because of the protective role of the placenta, which separates the circulatory systems of the mother and foetus, they said. "Zika may be unique in its ability to infect placental cells and cross the placental barrier, in comparison with other flaviviruses," said Suthar. "It is still possible that disruption of the placental barrier - as seen in a recent case report - or antibody-mediated mechanisms are contributing to placental infection by Zika virus," he said.

The findings were published in the journal Cell Host & Microbe.