Infectivity of Immature Neurons to Zika Virus: A Link to Congenital Zika Syndrome

• The mechanism of Zika virus-mediated microcephaly is explored using infected undifferentiated, partially differentiated and terminally differentiated neuroblastoma cell lines.
• Undifferentiated, progenitor neurons, as well as partially differentiated cells representative of an early developing fetal brain, were highly permissive to Zika virus In contrast, terminally differentiated neurons, representative of an adult brain, were highly resistant to Zika virus.
• This study shows the differential susceptibilities of immature versus mature neurons and may shed light on the development of microcephaly in early prenatal fetuses and Guillain-Barré syndrome in adult humans exposed to Zika virus.Cases of babies born with small head size (known as microcephaly) in the Zika virus infected areas of Brazil have created fear at a global scale. Generally, people wonder why Zika would cause small brain size in newborns whose mothers have been infected with the virus but do not have any apparent brain illness. This study explains the reasons behind such reported observations. We found that Zika virus primarily kills, or damages, brain cells that are growing very fast and are still immature, whereas, it has no effect on mature neurons found in an adult brain.

BackgroundEpidemiological data strongly suggest that microcephaly cases in Brazil are associated with the ongoing epidemic of Zika virus (ZIKV). In order to further solidify the possible link, we investigated the infectivity of ZIKV using various neuroblastoma (NB) cell lines.MethodsSix undifferentiated, two terminally differentiated and two retinoic acid (RA) –induced, partially differentiated cell lines were exposed to ZIKV strain PRVABC59, which is genetically similar to the French Polynesia strain, with 97–100% genetic homology to the current ZIKV strain found in Brazil. All infections were confirmed by real-time PCR (RT-qPCR), immunofluorescence assay (IFA) probing with anti-flavivirus E antibody, and evaluation of cytopathic effects.FindingsZIKV infected all six undifferentiated NB cell lines. In five out of six NB cell lines, between 90 and 70% cells were positive by IFA whereas for one cell line, CCL-127, ~ 80% of cells were positive for ZIKV as determined by IFA but showed persistent infection. Two differentiated cell lines, JFEN and T-268, were highly resistant to ZIKV with < 1% of the cells being susceptible, as determined by IFA and confirmed by qRT-PCR. Two retinoic acid (RA)-induced NB partially differentiated cell lines showed no difference in permissiveness as compared to their undifferentiated mother cell lines.InterpretationThese findings strengthen the reported association between high incidences of microcephaly and ZIKV infection in newborns in Brazil. Our results suggest that the undifferentiated neurons are highly permissive to ZIKV infection, as one would expect during the early stages of neurogenesis in fetal brains; whereas differentiated neurons, representative of adult brain neurons, are relatively resistant to the virus, which explains the rare occurrence of neurological complications in adults infected with ZIKV. Our studies confirm the neurotropism of the ZIKV strain closely related to the current epidemic in Latin America.