Gut immunity – stress-induced depression
A recent study led by Johns Hopkins Medicine researchers sheds light on a particular immune cell in the gut, called γδ T cells, and its role in brain functions associated with stress-induced manifestations such as depression and social avoidance. These findings open new doors for potential therapies targeting immune cell-driven changes in the gut to alleviate symptoms of depression. Published in Nature Immunology on March 20, 2023, the study identifies dectin-1, a protein receptor found on γδ T cells, as a promising therapeutic target for modulating stress responses.
Understanding γδ T Cells, Dectin-1, and the Gut-Brain Connection
γδ T cells are a unique subgroup of T lymphocytes concentrated in peripheral tissues, including the intestines. These cells play dual roles in adaptive immunity and an “innate-like” immune response, impacting inflammation and broader immune regulation.
Dectin-1 is a protein receptor on immune cells that identifies microbial patterns and triggers immune reactions, including responses linked to stress-induced behaviors in the gut.
Previous studies have linked gut inflammation to depressive disorders, prompting the Johns Hopkins team, led by Dr. Atsushi Kamiya, to investigate how imbalances in the gut microbiota influence psychological stress responses.
“The results of our study highlight the previously unrecognized role of intestinal gamma delta T cells (γδ T cells) in modifying psychological stress responses, and the importance of a protein receptor known as dectin-1, found on the surface of immune cells, as a potential therapeutic target for the treatment of stress-induced behaviors.”
Atsushi Kamiya, M.D., Ph.D., professor of psychiatry and behavioral sciences at the Johns Hopkins University School of Medicine and study’s senior author.
Chronic Social Defeat Stress and the Gut-Immune System
To explore this connection, the researchers used a chronic social defeat stress (CSDS) model in mice. This standard experimental setup simulates stress-induced behaviors, such as social avoidance, commonly associated with depression. Mice subjected to repeated social challenges were classified as either stress-resilient (maintaining normal social behavior) or stress-susceptible (exhibiting social avoidance).
Fecal samples from the mice underwent genetic analysis to assess gut microbiota diversity. The results revealed a marked reduction in microbial diversity among stress-susceptible mice compared to their resilient counterparts, with a notable decrease in Lactobacillus johnsonii (L. johnsonii). This probiotic strain is known to improve gut barrier function and modulate the immune system.
“Stress increased γδ T cell activity, which in turn heightened social avoidance,” explained Dr. Xiaolei Zhu, the study’s lead author.
In response to this, the stressed mice were supplemented with L. johnsonii reducing both social avoidance and γδ T cell levels, suggesting a regulatory relationship between gut bacteria and immune cells in shaping stress responses.
Dectin-1’s Role in Stress-Related Behavioral Responses
A pivotal discovery was the role of dectin-1 in driving γδ T cell activity during stress. This receptor facilitates the differentiation of γδ T cells into IL-17-producing γδ17 T cells, which accumulate in the meninges—the protective layers surrounding the brain and spinal cord—and contribute to stress susceptibility.
Blocking dectin-1 activity reduced γδ17 T cell responses and alleviated stress-induced social avoidance in mice. These findings underscore dectin-1’s potential as a therapeutic target to mitigate stress-related symptoms by modulating immune responses in the gut.
Pachyman: A Natural Modulator of Dectin-1
Seeking natural interventions, the researchers examined pachyman, a β-glucan derived from the mushroom Poria cocos. Widely used in Eastern medicine for its anti-inflammatory and mood-enhancing properties, pachyman was administered to mice exposed to CSDS.
Flow cytometry analysis confirmed that pachyman binds to dectin-1, inhibiting γδ17 T cell activity in the gut. This interaction reduced stress-induced social avoidance and other behavioral symptoms. Pachyman’s ability to balance immune responses and prevent γδ17 T cell accumulation in the meninges offers a novel mechanism for addressing stress-related mood disorders.
Translating Findings to Human Depression
To explore the clinical relevance, the researchers analyzed gut microbiota samples from 66 human participants—32 with major depressive disorder (MDD) and 34 healthy controls. The results revealed an inverse relationship between Lactobacillus levels and depression severity: participants with higher Lactobacillus abundance scored lower on depression and anxiety scales.
Although human and mouse gut microbiomes differ, this finding suggests a shared mechanism where gut bacteria influence mood regulation through stress-related immune pathways. It points to the potential for microbiota-based therapies to support mental health by restoring microbial balance.
Future Directions in Gut-Immune Therapy
The study emphasizes the need for further research into γδ T cells, dectin-1, and their roles in gut-brain communication under stress. Targeting gut receptors like dectin-1 with pharmacological agents or natural compounds, such as pachyman, could revolutionize treatments for stress-induced mood disorders.
“In addition to probiotic supplements, targeting receptors like dectin-1 in the gut immune system could yield novel approaches for preventing and treating psychiatric symptoms such as depression,” said Dr. Kamiya.
By leveraging dietary and microbial interventions to influence immune-mediated behavioral responses, researchers are paving the way for innovative therapies. These findings highlight the interconnected roles of the gut microbiota, immune cells, and the brain in managing stress and improving mental health.
Conclusion
This groundbreaking research positions γδ T cells and dectin-1 at the forefront of gut-brain axis studies, linking immune responses in the gut to stress-related behaviors. By modulating γδ17 T cell activity through dietary compounds like pachyman or restoring beneficial gut bacteria, scientists are uncovering new therapeutic strategies for mood disorders.
The microbiome’s immune landscape holds immense promise for understanding mental health pathways and enhancing resilience against stress-induced neuro-physiological challenges.
Keywords: #brain #immunity #stress #anxiety #research #mentalhealth #PTSD #depression #microbiome #gutbrainaxis #braingutaxis
Source: Nature Immunology, volume 24, pages 625–636 (2023)
Read More: Nature Immunology
Author’s Affiliations: BG Mancini is a Neurodevelopmental Researcher and Founder of the Brain & Gut Institute. She specializes in combining research with clinical experience through a lens of human connection and neuro-integrative medicine. You may contact her via her website, www.braingutinstitute.com or via linkedin.com/in/bg-mancini-62ab0410.
Cover Image Credit: The inextricable connection that exists between the Gut-Brain-Immune Process as well as the Brain-Gut-Immune Process: Hypothetical schematic overview and appearance of gut-brain axis cross-talk between the brain and the gut (intestine/colon) in addition to the brain-immune axis and the gut-immune axis have been shown. Arrowhead means stimulation. From: Gut microbiota could modulate the effects of neuro-immune responses and memory traces via the gut-brain-immune axis in schizophrenia, Explor Neuroprot Ther. 2022; 2:74–86. Open Access.
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