Zika virus: The path to fetal infection

These findings suggest that while Zika appears able to cross through the placenta and infect the fetus, the mechanism does not involve direct infection of the trophoblasts, at least in the later stages of pregnancy. The virus must either evade the type III interferon antiviral signals generated by the trophoblasts or somehow bypass these cells to cross the placenta ( Cell Host Microbe. 2016 May 11;19[5]:705-12 ).

Interestingly, the Cell study mentioned above, in which Zika infection of mice early in pregnancy infected placental cells and the brain, also showed reduced Zika presence in the mouse mononuclear trophoblasts and syncytiotrophoblasts, in areas of the placenta analogous to the human villi.

Some experts have suggested, based the study of other viruses, that the Zika virus is better able to infect the placenta when the infection occurs early in the first trimester or the second trimester. It is indeed possible – and makes intuitive sense – that first-trimester trophoblasts confer less resistance and a lower level of protection than the mature trophoblasts we studied. At this point, however, we cannot say with certainty whether or not the placenta is more or less permissive to Zika infection at different points in pregnancy.

Interestingly, investigators who prospectively followed a small cohort of pregnant women in Brazil with suspected Zika infection identified abnormalities in fetuses of women who were infected at various points of their pregnancies, even in the third trimester. Fetuses infected in the first trimester had findings suggestive of pathologic change during embryogenesis, but central nervous system abnormalities were seen in fetuses infected as late as 27 weeks of gestation, the investigators said ( N Engl J Med. 2016 Mar 4. doi: 10.1056/NEJMoa1602412) .

The interferon-conferred resistance demonstrated in our recent study is one of two mechanisms we’ve identified by which placental trophoblasts orchestrate resistance to viral infection. In earlier research, we found that resistance can be conferred to nontrophoblast cells by the delivery of micro-RNAs. These micro-RNAs (C19MC miRNAs) are uniquely expressed in the placenta and packaged within trophoblast-derived nanovesicles called exosomes. The nanovesicles can latch onto other cells in the vicinity of the trophoblasts, attenuating viral replication in these recipient cells.

This earlier in-vitro study involved a panel of diverse and unrelated viruses, including coxsackievirus B3, poliovirus, vesicular stomatitis virus, and human cytomegalovirus ( Proc Natl Acad Sci U S A. 2013 Jul 16;110[29]:12048-53 ). It did not include the Zika virus, but our ongoing preliminary research suggests that the same mechanisms might be active against Zika.

Furthering research

Research at our institution and in other laboratories has shed light on various ways in which the fetus is protected from viruses, but we must learn more in order to understand how particular viruses, such as Zika, are able to reach the fetal compartment and cause particular birth defects.

We must further investigate the role and importance of antibody-dependent enhancement, and we must continue to study the placenta and its various cell types. Continuing efforts to better elucidate the placenta’s defense mechanisms and to identify cell types that are more or less resistant to the Zika virus – and understand their differences – may lead us to potential therapeutic strategies.