Summary: New research overturns a long-held belief by showing that T cells—key components of the adaptive immune system—are present in healthy brains of both mice and humans. Previously thought to appear only during disease, these T cells are most concentrated in the subfornical organ, a brain region that monitors hunger and thirst.
The study indicates that many of these T cells originate in the gut and may travel to the brain, carrying up-to-date information about the body’s internal state. This discovery reveals a novel pathway in the gut–brain axis, suggesting immune cells act as messengers that can influence behaviour and physiological balance.
Key facts:
- Unexpected residency: T cells are present in healthy brains, notably concentrated in the subfornical organ (SFO).
- Gut–brain immune connection: Brain T cells share features with gut and adipose tissue T cells and appear to be shaped by the gut microbiome.
- Behavioral effects: Removing these brain T cells in mice alters food-seeking behaviour after short fasting periods.
Source: Yale
The brain’s sanctuary and its immune inhabitants
The brain is protected from much of the body by the blood–brain barrier, a selective interface that limits exposure to pathogens and circulating molecules. For decades, neuroscientists assumed this separation extended to immune traffic: microglia were held to be the brain’s resident immune cells, while other immune cells only entered during injury or disease.
Researchers at Yale School of Medicine now report that this view is incomplete. Their work demonstrates that αβ T cells inhabit the healthy brain in both mice and humans, with many of these cells trafficked from peripheral tissues such as the gut and white adipose tissue. The findings, published May 28 in Nature, mark the first clear demonstration of T cells occupying the brain during steady-state, non-diseased conditions.
Pathologists had occasionally observed T cells in brain tissue, but those observations were generally interpreted as evidence of past or ongoing inflammation. This new study shows that T cells can be a normal component of brain physiology. “We think of T cells as something that fights infection or causes autoimmune disease,” said David Hafler, MD, William S. and Lois Stiles Edgerly Professor of Neurology. “This paper shows T cells also have a normal, previously unrecognized role in the brain.”
The researchers located the highest density of these T cells in the subfornical organ, a small, deep-seated brain nucleus involved in thirst and hunger regulation. The SFO has a comparatively porous blood–brain barrier, which allows it to sample circulating signals related to hydration and energy balance. Finding immune cells concentrated there raises the possibility that T cells contribute direct, cell-mediated signals about bodily state to neural circuits that control feeding and drinking.
Relaying signals from the gut to the brain
T cells encompass diverse subtypes characterized by distinct surface proteins and functions. By comparing the molecular profiles of brain T cells with those in surrounding meninges and peripheral tissues, the Yale team found that parenchymal brain T cells are transcriptionally distinct: they robustly secrete interferon-gamma (IFNγ) and express tissue-residency markers such as CXCR6, which appear important for retaining them in brain tissue and supporting normal adaptive behaviour.
Evidence indicates many of these brain T cells are primed in the gut, with migration from the gastrointestinal tract and from white adipose tissue to the SFO. Experimental manipulation of the gut microbiome affected T cell trafficking: when young mice transitioned from milk to solid food, the change in gut microbes coincided with T cells moving from gut to brain. Mice raised germ-free lacked these brain T cells entirely, and animals depleted of SFO T cells displayed altered food-seeking after brief fasting.
These observations suggest an immune-mediated channel of gut–brain communication that complements known routes such as the vagus nerve and blood-borne metabolites. According to Andrew Wang, MD, PhD, the diffusion of small molecules through blood seems an inefficient signaling strategy compared with dispatching an immune cell that has sampled the gut environment and can carry a discrete report to the brain.
The investigators propose that gut-derived T cells may inform the brain about nutritional status, microbiome composition, or other aspects of systemic health. They also hypothesize that T cells may pause in adipose tissue en route to the brain, possibly serving as a quality-control step, though that idea remains to be tested.
Moving forward, the research team plans to explore the molecular cues that direct T cell migration from gut to brain and to examine how these cells behave in neurological disorders such as multiple sclerosis and Parkinson’s disease. As first author Tomomi Yoshida, a Yale doctoral student, noted: “This study raises more questions than it answers, but they’re all interesting and potentially important.”
Funding: This work was supported by the National Institutes of Health (multiple awards) and Yale University, with additional support from foundations and organizations including the Smith Family Foundation, the Colton Center for Autoimmunity at Yale, the Food and Allergy Science Initiative, PEW Charitable Trusts, Mathers Family Foundation, Ludwig Family Foundation, Knights of Columbus, Race to Erase MS, the Chan Zuckerberg Initiative and the National MS Society. The content is the responsibility of the authors and does not necessarily represent the official views of the NIH.
About this neuroscience research news
Author: Rachel Tompa (Yale)
Source: Yale School of Medicine
Contact: Rachel Tompa – Yale
Image credit: Neuroscience News
Original research: Closed access. “The subfornical organ is a nucleus for gut-derived T cells that regulate behaviour” by David Hafler et al., Nature.
Abstract
The subfornical organ is a nucleus for gut-derived T cells that regulate behaviour
Specialized immune cells that reside in tissues orchestrate diverse biological functions by communicating with parenchymal cells. While the innate immune compartment in the meninges and central nervous system is well described, the presence and role of adaptive immune cells in the healthy brain has been unclear. This study demonstrates that the subfornical organ (SFO) houses parenchymal αβ T cells in steady-state brains of mice and humans. These extravascular T cells are transcriptionally distinct from meningeal T cells: they secrete IFNγ, express tissue-residence markers such as CXCR6, and require these signals for retention and for supporting normal adaptive behaviour. Primed by the microbiome in peripheral tissues, these T cells traffic from white adipose and gastrointestinal tissues to the brain, and their abundance can be modulated by changes in gut microbiota or adipose composition. In sum, CD4 T cells reside in the brain at steady state, concentrate in the SFO, differ from meningeal T cells, and use IFNγ to support CNS homeostasis through fat–brain and gut–brain axes.