Remembering immunity: Neuronal ensembles in the insular cortex encode and retrieve specific immune responses
Posted on: 7 May 2021
Preprint posted on 6 May 2021
Investigating the “memory” of the immune system: direct evidence for specific immune representations in the brain
Selected by Kristina Kuhbandner
Categories: cell biology, immunology, neuroscience
Background
Today, it is well accepted that our mental and emotional state has a considerable impact on our bodies’ health. One example from our daily life is that stressful events can weaken our immune system and we are more likely to get sick (typically in our long-awaited vacations)1. Furthermore, some gut-related inflammatory conditions are thought to be psychosomatic.
Being the major gatekeeper of our bodies’ health, the immune system can take incredibly sophisticated actions to protect us from harmful attacks. However, our understanding as to what extent and how these processes are regulated by the nervous system are surprisingly limited. One concept, supported among others by neuroimaging and behavioral studies, proposes the existence of an immune representation in the brain2,3. The goal of this study was to investigate gut-related immune representations with a focus on the insular cortex (InsCtx), a specific brain region that receives major input from visceral organs such as the gastrointestinal (GI) tract.
Approach and key findings
First, the authors tracked neurons that were activated during an immunological challenge using transgenic FOSTRAP (TRAP = targeted recombination in active populations) mice. These mice express a Tamoxifen-inducible Cre recombinase under the control of the endogenous cFos promoter, which is only active in stimulated neurons. FOSTRAP mice were crossed with a Cre-dependent tdTomato reporter line. Thus, neurons of FosTRAP2:tdTomato mice which are active in the presence of Tamoxifen experience a recombination event and turn on the tdTomato reporter (Fig. 1).
Fig. 1 Illustration of the TRAP strategy used to label active neurons. FosTRAP2:tdTomato mice harbor two transgenes: one that expresses CreERT2 from the endogenous activity-dependent cFOS promoter and one that allows the Cre-dependent expression of an effector gene, such as tdTomato. In the presence of Tamoxifen, CreERT2 recombination occurs in electrically active neurons which results in tdTomato expression (taken from Koren et al., made available by a CC BY-NC-ND 4.0 International license).
To induce an inflammatory response in the GI tract, DSS (Dextran sodium sulfate)-induced colitis, a model of inflammatory bowel disease, was employed.
In a first experiment, Cre recombination was induced by Tamoxifen injection 48 h following DSS-treatment. After 7 days, Koren et al. observed increased tdTomato expression indicating neuronal activity in several brain regions that were previously associated with peripheral immune responses, including the thalamus and the InsCtx. Similar results were obtained by specifically labeling activated neurons in the InsCtx of FOSTRAP mice by stereotactically injecting a viral vector expressing the hSyn-Cre-mCherry construct.
To actually prove that the identified set of InsCtx neurons activated in response to colitis are involved in the processing of immune-relevant information, they re-activated this population choosing a clever chemogenetic strategy. Therefore, next to the fluorescent mCherry reporter, stimulatory DREADDs (designer receptors exclusively activated by designer drugs) were co-expressed in the captured neurons of FOSTRAP mice. These genetically engineered receptors can be activated by a synthetic ligand (CNO) resulting in increased neuronal firing, thus allowing the re-activation of the captured neurons at a specific time point, in this case after recovery from DSS-induced colitis4. Then, immune activity in the colon was assessed by analyzing a broad range of immune parameters. In accordance with the proposed concept, they observed enhanced inflammatory responses in the colon of these mice, which were restricted to the GI tract. However, in some aspects this immune response differed from the immunological profile found after direct induction of DSS-colitis. Interestingly, a general activation of neurons in the InsCtx did not evoke an inflammatory response in the colon, thus indicating that the gut-specific immune information is encoded by a specific set of neurons.
Finally, the authors addressed the question whether the observed effects are limited to the gut-brain axis by testing another unrelated model of inflammation, namely Zymosan-induced peritonitis. In this setting, the immune characteristics induced by re-activation of InsCtx neurons captured during peritonitis closely resembled those of directly induced Zymosan peritonitis. In conclusion, this preprint provides direct evidence that the brain is able to build an “immunological memory” and to initiate specific immune responses.
Why I like this preprint
While our knowledge about the influence of peripheral immune responses on the central nervous system (CNS) is rapidly evolving, the effects of the CNS on (peripheral) immunity are far less well understood5. During my PhD, I investigated the effect of the gut immune system on neuroinflammatory processes in a mouse model of multiple sclerosis. Since then, I have been fascinated by the complex interactions between the CNS and peripheral immunity. The immune system by itself is hard to understand and including the CNS as a modulator adds additional levels of complexity. The approach taken by Koren et al. in this preprint is very smart and allows them to follow the processing of immune-related information in a specific brain area. Ultimately, understanding the links between the nervous and immune system can open up completely novel avenues for the effective treatment of inflammatory diseases. Of course, many more questions are waiting to be addressed, among others: What are the specific characteristics of the captured neuronal populations and how do they change following activation/re-activation? Can the brain directly communicate with effectors of the peripheral immune system? Which cells in the periphery are involved in the correspondence to the brain during immunological processes?
Questions to the authors
- A general activation of InsCtx neurons did not change the inflammatory state in the colon. Did you also look for immune responses in other organs?
- Do you think that in a state of chronic inflammation deactivation of the activated neurons could dampen the immune response in the colon?
- Initially you reported that also neuronal populations in other brain regions, such as the thalamus, are activated in response to DSS-induced colitis. Did you also investigate the role of these cells in your model?
- Your results indicate that organ-specific immune-related information is processed by specific sets of neurons. What would you expect for a systemic inflammatory response as observed for example in septic shock (activation of more neurons or neurons in different brain areas)?
References
- Morey, J. N., Boggero, I. A., Scott, A. B. & Segerstrom, S. C. Current directions in stress and human immune function. Current Opinion in Psychology 5, 13–17 (2015).
- Hadamitzky, M., Lückemann, L., Pacheco-López, G. & Schedlowski, M. Pavlovian Conditioning of Immunological and Neuroendocrine Functions. Physiol Rev 100, 357–405 (2020).
- Sergeeva, M., Rech, J., Schett, G. & Hess, A. Response to peripheral immune stimulation within the brain: magnetic resonance imaging perspective of treatment success. Arthritis Res Ther 17, 268 (2015).
- Roth, B. L. DREADDs for Neuroscientists. Neuron 89, 683–694 (2016).
- Schiller, M., Ben-Shaanan, T. L. & Rolls, A. Neuronal regulation of immunity: why, how and where? Nature Reviews Immunology 1–17 (2020) doi:10.1038/s41577-020-0387-1.
doi: https://doi.org/10.1242/prelights.28894
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