We tend to assume our thoughts and feelings are controlled almost exclusively by our brains. But peripheral systems, such as our hormonal and immune systems also influence how we think, feel and behave.
Communication between the brain and rest of the body goes both ways. Psychological stress and stress hormones alter the status of the immune system. In turn, an agitated immune system will impact brain function and structure.
Bidirectional neuro-immune pathways serve to achieve internal homeostasis or biological ‘equilibrium’. Disequilibrium, if sustained or severe, results in disease.
A team of researchers from NIH–DHHS facility in Bethesda Maryland recently explored the idea of bidirectional brain-immune communication to see if cells of the adaptive immune system retain the memory of psychosocial stress and thereby alter mood states and brain function.
To generate ‘stressed mice’, pairs of mice were housed together: a dominant aggressive mouse with an ‘intruder’ who became stressed and ‘socially defeated’. Lymphocytes (a type of immune cell) were isolated from the stressed mouse and injected into another healthy ‘non-stressed out’ host mouse.
Unexpectedly, the stressed immune cells didn’t confer anxiety and depression, instead they appeared to protect against stress. They built resilience in the host mouse.
The recipient mice showed:
- less anxiety and depression-related behaviours and increased sociability
- reduced pro-inflammatory cytokine levels in the blood
- increased cell proliferation in the hippocampus (of the like seen with anti-depressant use)
- microglia (a immune cell found exclusively in the brain) skewed toward an anti-inflammatory, neuro-protective phenotype
Remarkably, the behaviours and biochemical profiles associated with stress were reversed in the mice receiving cells from stressed donors. As the authors say “In effect, equilibrium was restored.”
These results suggests that psychological stress can modify adaptive immune cells in a long-lasting manner that may boost resilience to stress.
“…psychological stress might induce an immunological memory within the adaptive immune system that supports stress resilience.”
The team wonders if differences in the adaptive immune system may account for different susceptibilities to stress and resilience, and could potentially be useful in identifying at-risk individuals. Finally, they suggest the immune system could be a viable target for antidepressant therapies.