Zika virus: The path to fetal infection

Interestingly, other flaviviruses such as the dengue and chikungunya viruses have not been associated with microcephaly or other congenital disorders. This suggests that the Zika virus employs unique mechanisms to infect or bypass the placental barrier and, in turn, to cause neuronal-focused damage.

Placental passage

The villous trophoblasts, cells that are bathed in maternal blood, form the placenta’s first line of defense. Viruses, including the Zika virus, must cross or somehow bypass this initial barrier before crossing the placental basement membrane and endothelial cells, if they are to potentially invade the intrauterine cavity and infect the fetal brain and other tissues.

Research has demonstrated that cells of various types of tissue may express certain proteins, such as AXL, MER and TYRO3. While not yet proven, these proteins may mediate the entry of viruses such as Zika, enabling them to cross the placental trophoblast layer. These proteins are indeed expressed in trophoblasts, especially in early pregnancy, but we do not yet know if the proteins actually aid Zika’s passage through the placenta.

Another mechanism that has been postulated in the case of Zika infection is antibody-dependent enhancement, a process by which a current infection is enhanced by prior infection with another virus from the same family. Some experts believe that pre-existing immunity to the dengue virus – another member of the flavivirus family that has been endemic in Brazil – may be enhancing the spread of Zika infection as antibodies against dengue cross-react with the Zika virus.

While antibody-dependent enhancement has been shown to occur and to advance infection in various body systems, it has not been proven to affect the placenta. Until we learn more, we must simply appreciate that the presence of antibodies from another member of a family of viruses does not necessarily confer resistance. Instead, it may enable new infections to advance.

One might view pregnancy as a time of immune compromise, but we have shown in our laboratories that trophoblasts in fact have inherent resistance to a number of viruses. In a recent study, we found that trophoblasts are refractory to direct infection with the Zika virus. We isolated trophoblast cells from healthy full-term human placentas, cultured these cells for several days, and infected them with the Zika virus. We then measured viral replication and compared the infectivity of these cells with the infectivity of human brain microvascular endothelial cells – nontrophoblast cells that served as a control.

Our findings were extremely interesting to us: The trophoblast cells appeared to be significantly more resistant to the Zika virus than the nontrophoblast cells.

We learned, moreover, that this resistance was mediated by a particular interferon released by the trophoblast cells – type III interferon IFN1 – and that this type III interferon appeared to protect not only the trophoblasts but the nontrophoblast cells as well. It acted in both an autocrine and a paracrine manner to protect cells from the Zika virus. When we blocked the antiviral signaling of this interferon, resistance to the virus was attenuated.