During pregnancy and for some time after giving birth, a woman’s immune response in the brain may be tuned down, a new study in rats suggests. This research was published in the November 2017 issue of Brain, Behavior, and Immunity.

The findings may help reveal a connection between the brain’s immune function and anxiety and mood disorders that are common during pregnancy and the postpartum period.

The changes in the brain’s immune system mirror what happens in the rest of the body during pregnancy. Previous research has shown the response of the body’s peripheral immune system – the part of our protective system that does not protect the brain -- is suppressed during pregnancy to protect the fetus from being attacked by the mother’s immune system. While for most women these changes are too subtle to notice, those with autoimmune disorders such as rheumatoid arthritis or multiple sclerosis may experience a temporary relief from their symptoms while pregnant.

The team of researchers, led by Jaclyn M. Schwarz Ph.D. of the University of Delaware and a 2016 Young Investigator grantee, sought to determine whether the immune changes seen in the body also occur in the brain during pregnancy and the postpartum period. The researchers first injected both pregnant and non-pregnant mice with a chemical that mimics a bacterial infection and elicits a strong immune response. Then they measured the amount of inflammatory molecules produced in the brain in response to the immune challenge.

The team examined the hippocampus and the medial prefrontal cortex—two brain regions implicated in depression and anxiety. The results: pregnant mice showed reduced immune response in these regions, particularly during later stages of pregnancy. Unlike the shift in the peripheral immune function of the mice, the suppression of the brain’s immune response extended into postpartum period.

The time around giving birth is associated with an increased risk of anxiety and depression in women. These disorders have been linked to alterations in hormonal and immune function in the body. The findings about similarly altered immune function in the brain may inform future investigations into reasons behind this increased risk, the researchers said.