In experiments with mice and humans, a team led by Johns Hopkins Medicine researchers says it has identified a particular intestinal immune cell that impacts the gut microbiome, which in turn may affect brain functions linked to stress-induced disorders such as depression. Targeting changes mediated by these immune cells in the gut, with drugs or other therapies, could potentially bring about new ways to treat depression. The results of the research 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. The findings of the study were published this month in the journal Nature Immunology.
Dectin-1 binds to certain antigens, or proteins, to signal immune cells to activate in specific ways. This receptor, the researchers say, may be involved in the microbiome alteration and immune-inflammatory responses in the colon of mice, which suggests that it may be involved in stress responses via γδ T cells in the intestinal immune system. On the basis of previous studies suggesting that immune inflammatory responses in the gut are related to depression, Dr. Kamiya and his team designed experiments to focus on understanding stress-induced behaviors produced by an imbalance in the gut microbiota. To this end, the team examined the effects of chronic social defeat stress (CSDS) on the gut microbiota in mice. CSDS is a standard rodent test to study stress-induced disorders such as depression.
In a series of experiments, the researchers simulated potential stress inducing environments that could mimic similar responses in human environments. After each exposure, the mice were assessed and classified as stress-resilient (stress did not diminish social interactions) or stress-susceptible (stress increased social avoidance). Fecal samples were then collected and put through genetic analysis to identify the diversity of bacteria in the gut microbiota. The analysis showed that the intestinal organisms were less diverse in stress-susceptible mice than in stress-resilient mice. It specifically revealed that there were less Lactobacillus johnsonii (L. johnsonii), a type of probiotic, or “good” bacteria, in stress-susceptible mice compared to stress-resilient mice.
Scientists found that stress increased the γδ T cells, which in turn increased social avoidance. However, when the stressed mice were given L. johnsonii, social avoidance decreased and the γδ T cells went to normal levels, suggesting that CSDS-induced social avoidance behavior may be the result of lower levels of the bacteria and γδ T cell changes. Looking for potential natural approaches for prevention of depression rooted somehow in the gut, the researchers explored how changes in dectin-1 on CSDS-induced elevation of γδ T cells responded to pachyman. A compound extracted from wild mushrooms, pachyman is used as a natural anti-inflammatory agent and for treating depression in Eastern medicine. For this experiment, mice were fed a dose of pachyman, which was shown in previous research to affect immune function.
Data from flow cytometry analysis provided evidence that Dectin-1 binds to pachyman, inhibiting CSDS-induced γδ17 T cell activity and easing social avoidance behavior. To gain insight into how the alterations in the gut microbiota could impact the human brain, the researchers investigated the makeup of gut organisms in people with major depressive disorder (MDD) compared to people without MDD. 66 participants, ages 20 or older, were recruited in three hospitals around Tokyo, in Japan. Of the study participants, 32 had MDD (17 women and 15 men). The other 34 participants (18 women and 16 men) who did not have MDD formed the control group. Stool samples were collected from all study participants, who had comprehensive psychiatric history and screening for depression and anxiety.
The relative abundance of Lactobacillus was inversely related to higher depression and anxiety scores in the MDD group, meaning that the more Lactobacillus found in the gut, the lower the potential for depression and anxiety, according to what was found. Despite the differences of intestinal microbiota between mice and humans, the results of our study indicate that the amount of Lactobacillus in the gut may potentially influence stress responses and the onset of depression and anxiety. The investigators say more research is needed to further understand how γδ T cells in the intestinal immune system may impact the neurological functions in the brain and the role of Dectin-1 in other cell types along the gut-brain connection under stress conditions.
- Edited by Dr. Gianfrancesco Cormaci, PhD; specialist in Clinical Biochemistry.
Scientific references
Zhu X et al. Nature Immunology 2023 Mar 20 in press.
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