Zika virus (ZIKV) causes microcephaly, whereas other related pathogenic flaviviruses do not. To reach the fetal brain, a virus must be transported from the maternal to the fetal circulation, which requires crossing of the placental barrier. This study demonstrates that ZIKV, but not two other globally relevant flaviviruses, efficiently infects fetal endothelial cells, a key component of the placental barrier, because only ZIKV can efficiently use the cell-surface receptor AXL. This paper also shows that AXL, a receptor tyrosine kinase, is the primary ZIKV entry cofactor on human umbilical vein endothelial cells.
AXL-dependent infection of human fetal endothelial cells distinguishes Zika virus from other pathogenic flaviviruses. PNAS doi: 10.1073/pnas.1620558114
Although a causal relationship between Zika virus (ZIKV) and microcephaly has been established, it remains unclear why ZIKV, but not other pathogenic flaviviruses, causes congenital defects. Here we show that when viruses are produced in mammalian cells, ZIKV, but not the closely related dengue virus (DENV) or West Nile virus (WNV), can efficiently infect key placental barrier cells that directly contact the fetal bloodstream. We show that AXL, a receptor tyrosine kinase, is the primary ZIKV entry cofactor on human umbilical vein endothelial cells (HUVECs), and that ZIKV uses AXL with much greater efficiency than does DENV or WNV. Consistent with this observation, only ZIKV, but not WNV or DENV, bound the AXL ligand Gas6. In comparison, when DENV and WNV were produced in insect cells, they also infected HUVECs in an AXL-dependent manner. Our data suggest that ZIKV, when produced from mammalian cells, infects fetal endothelial cells much more efficiently than other pathogenic flaviviruses because it binds Gas6 more avidly, which in turn facilitates its interaction with AXL.