New research utilizing a live mouse-adapted influenza virus advances earlier studies involving mice, shedding light on how maternal infections can influence fetal brain development. The findings suggest that changes in the fetal brain are more probable once the severity of the mother’s influenza infection crosses a certain threshold.
Experiencing a severe influenza infection during pregnancy can heighten the risk of fetal neurodevelopmental disorders, such as autism spectrum disorder and schizophrenia. However, it is not the virus itself that causes harm but rather the immune response of the mother.
Researchers at the University of Illinois Urbana-Champaign have improved upon previous mouse studies by using a live influenza virus that mimics seasonal flu outbreaks in humans. This new research reveals that fetal brain changes are more likely when maternal infection severity reaches a specific level.
“Our findings strongly support the idea of an infection severity threshold, similar to what we see in humans. Only certain maternal infections will be severe enough to raise concerns. Therefore, pregnant individuals should certainly receive the flu vaccine to lower their risk,” said Adrienne Antonson, the senior author of the study and assistant professor in the Department of Animal Sciences at Illinois.
This study stands out as one of only a few that have examined maternal infections in mice using live influenza virus at dosages that reflect actual seasonal human flu outbreaks. “This means our findings are more applicable to human pathological infections,” stated Ashley Otero, the lead author and a doctoral student in the Neuroscience Program at Illinois.
Antonson’s team infected pregnant mice with live influenza A virus instead of the viral mimic – an inert molecule that simulates viral activity – commonly used by research teams in recent decades. The viral mimic triggers an innate immune response, characterized by non-specific inflammation that occurs within 24 to 48 hours post-infection.
While previous studies using viral mimics have provided significant insights into the specific inflammatory proteins produced by the mother and how they interact with the fetal brain, Otero points out that viral mimics might induce slightly different immune responses in both the mother and fetus. Furthermore, they do not capture the mother’s adaptive immune response, which happens later and aids the animal in “remembering” past infections.
To explore these points, the team administered one of two doses of the virus to the mice, simulating either a moderate or severe infection at a gestation period corresponding closely to the end of the first trimester in humans. Subsequently, at two and seven days after infection, they observed the progress of the infection in the mothers’ lungs and intestines and examined how the mother’s immune response products affected the fetal brain. They also measured various physical aspects of the fetal brains, such as cortical thickness, which has been correlated with neurodevelopmental disorders in humans.
Previous significant studies using viral mimics in mice have associated an immune protein called interleukin-17—with T helper (Th)-17 cells in the mother’s intestines— with changes in the fetal brain leading to behaviors resembling neurodevelopmental disorders in neonates. However, the live virus did not stimulate the production of this protein.
“Initially, when I found that interleukin-17 levels in our mothers did not rise following influenza infection, I thought we would not observe any changes in the developing fetal brain,” Otero recalled. “Surprisingly, we did witness very similar responses in the developing neocortex, including significant reductions in upper neuronal layers in fetuses from mothers with a higher dose of infection.”
Otero further points out that human postmortem studies have revealed smaller cortical structures in individuals with schizophrenia and autism spectrum disorder. “Thus, our findings align with observations in human brains.”
Antonson also noted, “We don’t believe a single inflammatory molecule causes all these diverse effects. However, this research is the first to comprehensively examine this pathway and compare it with models that have thus far shown that interleukin-17 plays a significant role. Hence, moving past viral mimic models to genuine live virus research is essential.”
After ruling out interleukin-17—at least during the observed time points— the team examined immune cells in the fetal brain. They found microglia, which penetrate the brain and interact with developing neurons, exhibited signs of heightened inflammatory activity in fetuses of mothers infected with a high dose of flu. Additionally, fetal border-associated macrophages (BAMs), which serve as immune sentinels along the brain’s surface, were found to be more active and abundant. Both types of cells generally promote healthy brain development, but Otero and Antonson suspect that an overactive state might lead them to harm rather than support the healthy growth of neurons.
Previous studies with viral mimics have also linked overly active microglia with fetal brain deformities, while BAM activity remains vastly underexplored. Otero intends to conduct further studies to better understand the role of BAMs in the context of prenatal influenza infection.
Antonson stresses that maternal infection is just one of many factors that could contribute to neurodevelopmental disorders.
“These disorders arise from a variety of influences, including environmental factors, genetics, pharmacological exposures, and more. While we are primarily focusing on the in-utero period, both the early postnatal period and adolescence are also crucial. This is just one facet of a very complex issue.”
This research received support from the Roy J. Carver Charitable Trust (grant #23-5683), USDA NIFA Research Capacity Fund (Hatch project #ILLU-538-940), the Department of Animal Sciences, and the College of ACES.
Antonson is also associated with the Beckman Institute, the Microbial Systems Initiative, and the Carl R. Woese Institute for Genomic Biology at Illinois.
