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SARS-CoV-2 shows maternal-fetal transmission and tropism for different fetal brain cells (animal study)

Numerous viral pathogens, including Zika virus, HSV, HIV, and others possess the capability to cross the placenta and infect fetal tissues, resulting in mild to severe neurological complications. Recent findings have shown that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has the potential to infect the placenta and fetal tissue. In this study, the authors from the United States investigated the possibility of maternal-fetal transmission of SARS-CoV-2 in mice and the effects of viral transmission on the developing fetal brain. 

Previous studies reported numerous complications in mothers infected with SARS-Cov-2 and their newborns. The complications observed in mothers include preeclampsia, preterm birth, and miscarriages, whereas complications observed in newborns exposed to maternal COVID-19 in utero include neurodevelopmental delay, motor deficits, seizures, and microcephaly. A recent study presented cases of two infants born in the third trimester to mothers who had COVID-19 infection during pregnancy. The results confirmed that second-trimester maternal SARS-CoV-2 infection with placentitis triggered an inflammatory response and oxidative stress injury to the fetoplacental unit, which in turn affected the fetal brain, resulting in progressive neurodevelopmental sequelae in two neonates. Immunofluorescence detected the S1 subunit of the SARS-CoV-2 spike (S) protein and nucleocapsid (N) protein in both placentas and throughout the brain of the deceased infant.


About the study

To investigate the vertical transmission of SARS-CoV-2 from mother to fetus and the offspring pathology, the authors used heterozygous female hACE2-KI mice to generate litters containing both hACE2+ (SARS-CoV-2-susceptible) and hACE2- (SARS-CoV-2-resistant) fetal genotypes. The 12-week-old nonpregnant heterozygous hACE2-KI female mice were intranasally infected with mock or 103 or 105 focus forming units (FFU) of delta variant SARS-CoV-2. After viral infection, mice were monitored for four days for morbidity (body weight loss) and mortality (survival). On the fourth day after the infection, the animals were euthanized, and their lungs, spleen, and brain were harvested for analysis of viral load by quantitative reverse transcription polymerase chain reaction (RT-qPCR). 

The heterozygous female hACE2-KI mice were crossed to either hemizygous hACE-2 KI males or WT males to generate desired fetal genotypes, hACE2+, and hACE2- fetuses. The first time point was the mid-gestational stage of E14.5, before the astrogliogenesis. This time point approximately matches the end of the first trimester and the start of the second trimester in humans. The second time point of E18.5 was at the late gestational stage after the astrogliogenesis. This time point approximately matches the beginning of the third trimester in humans. Both time points occur after placental formation. Body weights of pregnant mice, increased fur ruffling, and hunched posture were used as clinical signs of the SARS-CoV-2 infection. 



The mice with the mock infection and mice infected with 10 or 103 FFU of delta variant SARS-CoV-2 did not display any weight loss or signs of disease. The mice infected with 105 FFU of delta variant SARS-CoV-2 lost 20% of their pre-infection body weight within 48 hours after the infection and exhibited signs of disease. However, their body weight recovered to 100% 72 hours after the infection.

Pre-term birth was observed in approximately 45% of infected litters.

The pregnant mice experienced more severe SARS-CoV-2 infection and greater weight loss. Four days after the infection, RT-qPCR showed lung and spleen infection in mothers infected with 103 or 105 FFU. Viral transcription (an indicator of viral replication), detected by subgenomic mRNA analysis, was found in the lungs of animals infected with 105 FFU. Viral infection without replication was detected in the brains of pregnant mice at both time points.

22% of hACE2+ fetuses had a positive viral load in abdominal, thoracic, and heart tissues at E14.5, and 60% had a positive viral load in the same tissues at a later time point of E18.5. Only 5% of hACE2+ fetuses had a positive viral load in their brains at E14.5, but this percentage increased to 58% at E18.5. The virus was not detected in any tissue of the hACE2- fetuses. These findings show maternal-fetal transmission of SARS-CoV-2 in later stages of mice pregnancy, and that hACE2 expression was necessary for SARS-CoV-2 infection in this model.

The viral load was significantly higher in male fetuses than in female fetuses. These results are consistent with a recent human study that demonstrated that the risk of neurodevelopmental sequelae in fetuses exposed to maternal SARS-CoV-2 infection was gender-specific. At the 12-month and 18-month follow-ups, an increased risk for neurodevelopmental disorders was found in male offspring, but not in female offspring exposed to maternal COVID-19 in utero.

12–27% of brain cells in hACE2+ fetuses were positive for the SARS-CoV-2 S protein. The brains of hACE2- fetuses were negative for the S protein, showing that hACE2 facilitates SARS-CoV-2 infection in this model system. The S protein was found within brain blood vessels in cortical and hippocampal regions of fetal brains. Co-localization of the S protein and CD31, the marker for endothelial cells, confirmed the blood vessel infection. According to the authors, these results indicate that SARS-CoV-2 entered the fetal brain via the circulatory system, suggesting the same transmission route in humans.

About 19% of the cells positive for the S protein were also positive for the immature neuron marker, 13% for the mature neuron marker, 31% for the choroid plexus cell marker, 28% for the astrocyte marker, and 27% for the microglia marker. These results confirm that SARS-CoV-2 infection of these cells in the fetal brain. The S protein was not detected in the fetal brain of the mock-infected group. The rate of brain cell death in fetuses infected with SARS-CoV-2 did not differ from the mock-infected group.

Seven days after SARS-CoV-2 infection, the researchers observed a significant increase in cortical and hippocampal gliosis in the brains of infected pups. The astrocytes expressed glial fibrillary acidic protein (GFAP) typical of astrogliosis, while microglia had a “bushy” morphology with larger cell bodies, typical of a reactive state. At that time, the S protein was not detected, indicating that the SARS-CoV-2 infection was cleared.



This study has shown maternal-fetal transmission of SARS-CoV-2 in later stages of mice pregnancy, at time points matching the second and third trimesters of human pregnancy. The level of infection was much higher at a time point that corresponds to the third trimester in humans. Virus demonstrated tropism for various fetal brain cells, like endothelial cells of blood vessels, barrier cells of the choroid plexus, neurons, and glial cells. A concerning degree of gliosis was observed in the brains of infected fetuses, but there was no increase in cell death. Notably, male fetuses showed higher rates and levels of viral infection compared to female fetuses. In this model, hACE2 expression was necessary for SARS-CoV-2 infection.

The authors concluded that the observed deleterious effects of maternal-fetal transmission on fetal neurodevelopment may be more severe in humans, which are more susceptible to SARS-CoV-2 infection than mice. These findings, therefore, have important implications for pregnancy and fetal complications in humans following prenatal exposure to SARS-CoV-2. 

This article was published in Brain Behavior Immunology.

Journal Reference

McMahon CL, Castro J, Silvas J, et al. Fetal brain vulnerability to SARS-CoV-2 infection. Brain Behav Immun. 2023 Aug; 112: 188–205. (Open Access)



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