Zika virus: The path to fetal infection

The question of how viruses can enter the intrauterine compartment and infect the fetus has long been a focus of research. It is of particular urgency today as the Zika virus spreads and causes perinatal infection that threatens the developing fetus with serious adverse outcomes such microcephaly and other brain anomalies, placental insufficiency, and fetal growth restriction.

We know that viruses can take a variety of routes to the fetal compartment, but we have also learned that the placenta has a robust level of inherent resistance to viruses. This resistance likely explains why we don’t see more viral infections in pregnancy.

Dr. Yoel Sadovsky

Recent studies performed at our institution suggest that placental trophoblasts – the placenta’s primary line of defense – have inherent resistance to viruses such as Zika. It appears, therefore, that the Zika virus invades the intrauterine cavity by crossing the trophoblasts, perhaps earlier in pregnancy and prior to the development of full trophoblast resistance, by entering through breaks in this outer layer, or by utilizing alternative pathways to access the fetal compartment.

Further study of the placenta and its various cell types and mechanisms of viral defense will be critical for designing therapeutic strategies for preventing perinatal infections.

Various routes and affinities

Viruses have long been known to affect mothers and their unborn children. The rubella virus, for instance, posed a significant threat to the fetus until a vaccine program was introduced almost 50 years ago. Cytomegalovirus (CMV), on the other hand, continues be passed from mothers to their unborn children. While not as threatening as rubella once was, it can in some cases cause severe defects.

One might expect viruses to infect the placenta and then secondarily infect the fetus. While this may indeed occur, direct placental infection is not the only route by which viruses may enter the intrauterine compartment. Some viruses may be carried by macrophages or other immune cells through the placenta and into the fetal compartment, while others colonize the uterine cavity prior to conception, ready to proliferate during pregnancy.

Carolyn Coyne, Ph.D.

In still other cases, viruses may be inadvertently introduced during medical procedures such as amniocentesis or transmitted through transvaginal ascending infection, most likely after rupture of the membranes. Viruses may also be transported through infected sperm (this appears to be one of the Zika virus’s modes of transportation), and as is the case with HIV and herpes simplex viruses, transmission sometimes occurs during delivery.

When we investigate whether or not the fetus is protected against particular viruses, we must therefore think about the multifaceted mechanisms by which viruses may be transmitted. With respect to the placenta specifically, we seek to understand how viruses enter the placenta, and how the placenta resists the propagation of some viruses while allowing other viruses to gain entry to the intrauterine compartment.

An additional consideration – one that is of utmost importance in the case of Zika – is whether viruses have any special affinity for particular fetal tissues. Some viruses, like CMV, infect multiple types of fetal tissue. The Zika virus, on the other hand, appears to target neuronal tissue in the fetus. In May, investigators of two studies reported that a strain of the Zika virus efficiently infected human cortical neural progenitor cells ( Cell Stem Cell. 2016 May 5;18[5]:587-90 ), and that Zika infection of mice early in pregnancy resulted in infection of the placenta and of the fetal brain ( Cell. 2016 May 19;165[5]:1081-91 ).