Immune responses similar to those triggered by viral infections in pregnant mice altered brain structure in their offspring to cause behaviors associated with autism spectrum disorder (ASD). This is the finding of a study published on January 28 in Science, and led by researchers from NYU Langone Medical Center, Massachusetts Institute of Technology, University of Massachusetts Medical School and University of Colorado, Boulder.
Greater activation of one family of immune cells - helper T (Th) cells that produce the signaling chemical interleukin 17a - was found to be the mechanism by which viral infection in pregnant mothers led to mouse versions of ASD symptoms: social deficits, abnormal communication and repetitive behavior.
Equally important, the research team also determined that blocking the action of Th17 cells and IL-17a in the womb using antibodies completely restored normal behavior and brain structure.
"To our knowledge, this is the first study to identify a specific population of immune cells that may have a direct role in causing behaviors linked to autism," says immunologist and corresponding study author Dan Littman, MD, PhD, the Helen L. and Martin S. Kimmel Professor of Molecular Immunology in the Department of Pathology at NYU Langone.
"With these study results, Th17 cells, as well as specific proteins they produce, become candidate therapeutic targets as part of future efforts to prevent autism," adds Littman, who led the study with his former postdoctoral fellow, Jun Huh, PhD, now a professor at the University of Massachusetts Medical School.
Littman notes that the immune reactions in the current study linked to autism risk are the kind caused only by viral infections in the mother, and have "nothing whatsoever to do with vaccines." Some have linked childhood vaccination to autism risk despite "overwhelming scientific evidence to the contrary."
Complete Rescue of Structure and Function
The new study centers on T lymphocytes, immune cells that react to infections by expanding into a cellular army that attacks the invading microbe at hand. A subset of T cells, Th17 cells release interleukin 17 (IL-17), a signaling protein (cytokine) that amplifies normal immune responses to fend off infections. When overactive or off target, cytokines contribute to inflammatory and autoimmune diseases like asthma and inflammatory bowel disease. Th17 cells supply most of a version of IL-17 called IL-17a.
Specifically, the newly published study found that activation of Th17 cells and related IL-17a production are the core mechanisms by which viral infections in mouse mothers create behavioral abnormalities in their litters. The results in particular point to Th17 cell activation regulated by retinoid-related orphan receptor gamma t (ROR gamma t), which turns on genes that enable T cells to mature and produce IL-17a in humans and mice.
Several past human studies have suggested a link between maternal viral infection during pregnancy and risk for autism spectrum disorder, but the mechanisms behind this are unknown. For this reason, the field has invested heavily in recent years to create mouse models for the study of maternal immune activation related to autistic behavior.
Along these lines, the research team injected mice with polyinosinic:polycytidylic acid (poly I:C), which mimics a viral infection but is easier to study. Its structure includes a snippet of genetic material present in viruses like influenza, which makes it the right shape to trigger immune reactions.
In behavioral experiments, the study authors found that exposure in the womb to IL-17a, produced in higher levels in reaction to injecting the mother with poly-I:C, created autism-like symptoms. For example, mice exposed to higher IL-17a levels in the womb had trouble telling the difference between another live mouse and a toy. They interacted with both equally, whereas normal mice spent more time socially interacting with live mice.
In terms of communication differences, mouse pups are known to cry to their mothers, but recordings revealed that offspring exposed to IL-17a inflammation vocalized abnormally. Mice exposed to this kind of immune response were also more likely to bury marbles found in their cages one after the other in a compulsive, repetitive behavior.
The research team also found that Th17-driven inflammation via IL-17a interfered with interfered with normal development of certain regions of the brain. Otherwise carefully ordered nerve cell layers became chaotic in the cortex, the part of the brain that sculpts sights and sounds into thoughts. The team focused on this feature because disorganized cortical architecture had been found in past studies of human patients with autism.
Blocking the action of Th17 cells completely restored normal structure and function to the brains of the study offspring, regardless of whether this was accomplished by treatment with anti-IL17a antibodies or by blocking the expression of ROR gamma to shut down the IL-17a gene.
"What is needed next is a study of viral infection during pregnancy that follows thousands of women, tracking instances of viral infection or flare-ups in autoimmune conditions and the diagnosis of autism spectrum disorder in children over several years," says Littman, also a member of the Kimmel Center for Biology and Medicine within NYU Langone's Skirball Institute for Biomolecular Medicine, and an investigator for the Howard Hughes Medical Institute. "Only such a study could determine if there is increased incidence of autism in the children of mothers who reported an inflammatory event in a given part of gestation."
Littman and colleagues have filed a patent on their methods for blocking the IL-17a and ROR gamma t pathways during pregnancy to avert autism risk in the offspring, but it would be many years before a treatment could potentially be developed. A recent study in his lab, published in Nature in December 2015, suggested a new way to block ROR gamma t action through its partner molecules, reducing IL-17 levels only in Th17 cells.
Source: New York University