Retrieval of Conditioned Immune Response in Mice is Mediated by an Anterior-Posterior Insular Circuit

Academic Background

The bidirectional relationship between the brain and the immune system is a cornerstone of philosophical and scientific research. In recent years, researchers have identified multiple pathways through which the immune system influences brain activity, and evidence also shows that the brain modulates immune responses. The conditioned immune response (CIR) is a typical Pavlovian conditioning process where a sensory stimulus (e.g., taste) is paired with an immunomodulatory agent. Re-exposure to the taste elicits aversive behavior and an anticipatory immune response. Although the insular cortex plays a key role in CIR, the specific neural circuit mechanisms remain unclear.

This study aims to uncover the neural circuit mechanisms underlying CIR, particularly the specific roles of bidirectional connections between the anterior insular cortex (AIC) and posterior insular cortex (PIC) in CIR. By studying CIR in mice, the researchers hope to reveal how the brain regulates immune responses through neural circuits, thereby providing a theoretical foundation for novel immune therapies based on behavior and brain stimulation.

Source of the Paper

This paper was co-authored by researchers from the University of Haifa, Israel, including Haneen Kayyal, Federica Cruciani, Sailendrakumar Kolatt Chandran, and others. It was published in Nature Neuroscience, with the DOI: 10.1038/s41593-024-01864-4.

Research Process and Results

1. Induction and Behavioral Manifestation of Conditioned Immune Response

The study first induced CIR by pairing saccharin (as the conditioned stimulus, CS) with low-dose lipopolysaccharide (LPS, as the unconditioned stimulus, UCS) in mice. The specific procedure was as follows:

• Conditioning Phase: Water-restricted mice were presented with saccharin, followed by intraperitoneal injection of LPS or PBS (control group) 40 minutes later.
• Retrieval Phase: Four days later, the mice underwent a saccharin versus water choice test to assess their aversion index.

The results showed that LPS-treated mice exhibited a significantly higher aversion index toward saccharin compared to PBS-treated control mice, indicating successful conditioning.

2. Assessment of Immune Response

To evaluate whether CIR elicited an immune response similar to LPS re-exposure, the researchers divided the mice into five groups, subjected them to different treatments, and collected peritoneal lavage fluid and blood samples during the retrieval phase. Flow cytometry was used to analyze the numbers of monocytes and macrophages and their surface markers CD80 and CD86.

The results showed that the number of monocytes, especially CD80+ monocytes, significantly increased in the CIR group, resembling the LPS re-exposure group. This indicates that CIR can partially mimic the immune response triggered by LPS.

3. Activation and Functional Connectivity of Insular Neurons

To investigate the specific role of functional connectivity between the AIC and PIC in CIR, the researchers labeled AIC-to-PIC and PIC-to-AIC neurons using retrograde adeno-associated virus (AAV). After CIR retrieval, they detected the expression of the neuronal activation marker pERK through immunohistochemistry.

The results showed that after CIR retrieval, the activation of AIC-to-PIC neurons significantly increased, while no significant change was observed in PIC-to-AIC neurons. This suggests that the AIC-to-PIC pathway plays a dominant role in CIR retrieval.

4. Neuronal Excitability and Synaptic Plasticity

Through electrophysiological experiments, the researchers further analyzed the excitability and synaptic plasticity of AIC-to-PIC neurons. The results showed that after CIR retrieval, the excitability of AIC-to-PIC neurons significantly decreased, and both the frequency and amplitude of excitatory postsynaptic currents (mEPSCs) were reduced, while the frequency of inhibitory postsynaptic currents (mIPSCs) increased, leading to a decrease in the excitatory/inhibitory (E:I) ratio.

5. Chemogenetic Inhibition of Neural Circuits

To verify the necessity of the AIC-to-PIC pathway in CIR, the researchers inhibited AIC-to-PIC neuronal activity using chemogenetic methods. The results showed that inhibiting AIC-to-PIC neurons significantly reduced the mice’s aversive behavior toward saccharin but had minimal impact on the immune response. This indicates that the AIC-to-PIC pathway primarily mediates the behavioral manifestation of CIR, while immune response regulation depends on bidirectional connectivity between the AIC and PIC.

Conclusions and Implications

This study is the first to reveal the specific role of bidirectional neural circuits between the AIC and PIC in conditioned immune responses. The AIC-to-PIC pathway primarily mediates the behavioral retrieval of CIR, while bidirectional connectivity between the AIC and PIC regulates immune responses. These findings provide new insights into how the brain regulates immune responses through neural circuits and lay the groundwork for developing novel immune therapies based on behavior and brain stimulation.

Highlights of the Study

1. First Revelation of Specific Roles of AIC and PIC in CIR: Using chemogenetics, electrophysiology, and immunohistochemistry, the researchers detailed the functional connectivity between the AIC and PIC in CIR.
2. Specificity of Neural Circuits: The study revealed the specific role of the AIC-to-PIC pathway in behavioral retrieval of CIR, providing new evidence for understanding the function of neural circuits in learning and memory.
3. Neural Regulation of Immune Responses: This research uncovered how the brain regulates immune responses through neural circuits, offering a theoretical basis for developing novel immune therapies.

Other Valuable Information

The study also found that activation of the AIC-to-PIC pathway not only affects the behavioral manifestation of CIR but also participates in regulating immune responses during LPS re-exposure. This suggests that this neural pathway has broad roles in immune regulation and may play an important role in other immune-related behaviors.

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Through this study, we have gained a deeper understanding of the complex interactions between the brain and the immune system, and it provides crucial scientific evidence for future development of immune therapies based on neural circuit modulation.