Gut microorganisms can produce metabolites, including short-chain fatty acids, that influence immune activity and other physiological processes. They can also interact with intestinal cells and help shape the development and function of immune defenses. In turn, immune signals can affect tissues beyond the digestive tract.
During pregnancy, these interactions become especially interesting because the mother's body is supporting the development of another organism. Nutrients, hormones, antibodies, inflammatory signals, and other maternal factors can influence the prenatal environment.
Researchers are therefore investigating whether differences in the maternal microbiome might change certain immune signals during pregnancy and, under particular circumstances, affect aspects of fetal brain development.
The important distinction is that a biological pathway that appears plausible in an animal model does not automatically establish a cause of autism in humans.
Why the Maternal Immune System Matters During Pregnancy
Pregnancy requires a carefully coordinated relationship between the maternal immune system and the developing fetus.
The immune system must continue protecting the mother against infections while also supporting the biological processes necessary for pregnancy. This does not mean that immunity simply switches off. Rather, immune activity changes over time and varies among tissues, cell types, and stages of pregnancy.
Inflammatory signaling can be part of a normal, healthy immune response. Certain signals help coordinate tissue repair, defend against pathogens, and regulate cellular activity. However, the timing, intensity, and duration of inflammation can matter considerably.
Researchers studying prenatal development have become particularly interested in maternal immune activation, a term used to describe immune responses triggered by factors such as infection or other inflammatory stimuli during pregnancy.
In experimental animals, maternal immune activation can influence offspring brain development and behavior. The precise effects depend on the animal model, the immune stimulus, the timing of exposure, genetic background, and other environmental conditions.
These findings have led scientists to examine specific immune molecules that might help explain how maternal inflammation communicates with the developing nervous system.
One such molecule is interleukin-17A, often abbreviated IL-17A.
What Is Interleukin-17A?
IL-17A is a signaling protein known as a cytokine. Cytokines help immune cells communicate, coordinate their responses, and regulate inflammation.
IL-17A is associated with a group of immune cells called T helper 17 cells, or Th17 cells, although other cell types can also contribute to its production.
One of its established functions is helping the body defend itself against certain extracellular bacteria and fungi. It can stimulate tissues to produce inflammatory mediators and recruit immune cells to areas where they are needed.
These protective functions are important. The immune system must be able to recognize and respond to threats, and IL-17A is one part of that defense network.
However, immune signaling can have different effects depending on where and when it occurs. In some circumstances, excessive or persistent IL-17A activity contributes to inflammatory disease. The pathway has been implicated in conditions such as psoriasis and certain forms of inflammatory arthritis.
Its potential role in prenatal brain development is a separate research question.
In some mouse studies, researchers have investigated whether maternal immune activation involving IL-17A can alter fetal brain development and produce behavioral changes in offspring.
These experiments have provided evidence that immune signaling during pregnancy can influence neurodevelopment in specific laboratory settings. They do not establish that IL-17A causes autism in humans or that blocking the molecule would prevent autism.
How the Gut Microbiome May Influence IL-17A
The relationship between the gut microbiome and the immune system is complex.
Some intestinal microorganisms help maintain the intestinal barrier, metabolize dietary components, and influence the activity of immune cells. Particular microbial communities can also affect the conditions under which Th17 cells develop or become activated.
In experimental settings, differences in the microbiome can therefore influence the strength or character of certain immune responses.
This is one reason researchers have explored whether the maternal microbiome could affect the immune environment during pregnancy.
A possible sequence of events is that a particular microbial community influences maternal immune activity, which changes the production of inflammatory signals. Those signals may then affect biological processes involved in fetal development.
However, this sequence is not universal. Different microbial communities can have different effects, and the same microorganism may behave differently depending on its surrounding environment.