IL-17RC signaling connects intestinal microbiota and neuroimmune interactions in atherosclerosis

This study reveals that intestinal IL-17RC signaling protects against atherosclerosis by maintaining gut barrier integrity and microbiota homeostasis, thereby preventing microbiota-driven $\gamma\delta$ T cell activation that promotes perivascular neuronal outgrowth and subsequent neuroimmune inflammation in the aorta.

The Gist

The Big Picture: A Chain Reaction from Gut to Heart

Imagine your body is a massive, bustling city. The intestines are the city's "port" where food and bacteria enter, and the heart (specifically the arteries) is the central power plant.

Usually, we think of heart disease (atherosclerosis) as being caused by clogged pipes from bad cholesterol or a bad diet. But this study discovered a hidden, long-distance telephone line connecting the gut port directly to the heart power plant.

The researchers found that when the "security guards" in the gut (a specific protein called IL-17RC) are missing, the city's defense system breaks down. This allows bad bacteria to sneak out of the gut, travel through the bloodstream, and accidentally trigger a construction crew in the heart that builds too many nerve fibers. These extra nerves then scream "ALARM!" to the immune system, causing inflammation and speeding up heart disease.

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The Story in Four Acts

Act 1: The Broken Gate (The Gut)

Think of the lining of your intestine as a high-security fence. Its job is to keep the "wild animals" (bacteria) inside the zoo (your gut) and let only the good stuff through.

• The Problem: In this study, the researchers removed a specific security guard protein (IL-17RC) from the fence.
• The Result: The fence got holes in it. The "wild animals" (bacteria) started escaping, and the "zoo keepers" (immune cells) got confused and started fighting. The gut became inflamed and messy.

Act 2: The Messy Messenger (The Microbiome)

Because the fence was broken, the mix of bacteria in the gut changed. It became a "bad neighborhood" full of aggressive microbes.

• The Analogy: Imagine the gut is a factory. When the security guard is gone, the factory starts producing "toxic smoke" (metabolites and bad bacteria) instead of clean energy.
• The Travel: This toxic smoke travels through the bloodstream (the city's highway) all the way to the heart.

Act 3: The Overactive Construction Crew (The Nerves)

Here is the surprising part. When the "toxic smoke" from the gut arrives at the heart arteries, it doesn't just cause a fire; it calls in a construction crew of nerves.

• The Mechanism: The bad bacteria trigger a special type of immune cell (called a Gamma-Delta T-cell) to shout a signal (IL-17A).
• The Effect: This signal tells the nerves in the heart artery to grow like crazy. It's like someone telling a city planner to build 100 new telephone poles in a small park.
• The Result: The heart arteries become "over-wired" with sympathetic nerves (the nerves that control stress responses).

Act 4: The Heart Attack (The Inflammation)

Now that the heart artery is over-wired with nerves, these nerves start pumping out stress chemicals (like norepinephrine).

• The Chain Reaction: These stress chemicals wake up the "trash collectors" (macrophages) in the artery. Instead of cleaning up, the trash collectors get angry, eat too much fat, and turn into "foam cells."
• The Outcome: This creates a massive, inflamed plaque that blocks the artery. The heart disease gets much worse, much faster.

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The "Aha!" Moments (How they proved it)

The scientists didn't just guess; they ran some clever experiments to prove this chain reaction:

1. The "Roommate" Test: They put a mouse with a broken gut fence next to a healthy mouse. The healthy mouse "caught" the bad bacteria from the roommate and suddenly developed heart disease, too. This proved the problem is transferable via bacteria.
2. The "Antibiotic" Test: They gave the mice antibiotics to wipe out the bacteria. When the bacteria were gone, the heart disease stopped getting worse, even in the mice with the broken gut fence. This proved the bacteria are the fuel for the fire.
3. The "Silence the Nerves" Test: They used a chemical to cut the nerve connections in the heart. When the nerves were silenced, the heart disease improved dramatically. This proved the nerves are the spark that lights the fire.
4. The "Human Check": They looked at human heart tissue from patients and found the same pattern: lots of bad bacteria markers, extra nerves, and those specific immune cells (Gamma-Delta T-cells) hanging out together.

The Takeaway

This study changes how we might think about treating heart disease.

• Old Way: Just lower the cholesterol or take blood pressure meds.
• New Way: We might need to fix the gut barrier first. If we can repair the "fence" in the gut or calm down the specific nerves in the heart, we might be able to stop heart disease before it starts.

In short: A broken gut can send a distress signal to the heart, causing the heart to build too many nerves, which then triggers a heart attack. It's a perfect example of how what happens in your stomach can literally change your heart's wiring.

Technical Summary

1. Problem Statement

Atherosclerosis is a multifactorial inflammatory disease driven by diet, genetics, and systemic inflammation. While the roles of dysbiosis (microbiota imbalance) and neuroimmune crosstalk in cardiovascular disease are increasingly recognized, the specific mechanistic circuits linking intestinal barrier dysfunction, microbiota alterations, and perivascular neuroinflammation in the aorta remain undefined. Previous studies on Interleukin-17 (IL-17) signaling in atherosclerosis have yielded conflicting results (both pro- and atheroprotective), likely due to a lack of cell-type specificity in understanding where and how IL-17 receptors function. This study aims to elucidate the role of intestinal epithelial cell (IEC)-specific IL-17RC signaling in regulating distant aortic inflammation and neuronal outgrowth.

2. Methodology

The study employed a multi-disciplinary approach combining genetic mouse models, high-throughput sequencing, spatial transcriptomics, and pharmacological interventions:

• Genetic Models:
  • IEC-specific Knockout: Generated Ldlr-/-Il17rc^DIEC mice (intestinal epithelial cell-specific ablation of Il17rc) on an atherosclerosis-prone Ldlr-/- background.
  • Neuron-specific Knockout: Generated Il17rc^THCre mice (sympathetic neuron-specific ablation of Il17rc) and induced atherosclerosis via LDLR-CRISPR AAV.
• Dietary Induction: Mice were fed a Western Diet (WD) for 16 weeks to induce atherosclerosis.
• Microbiota Manipulation:
  • Co-housing: To equalize microbiota between genotypes.
  • Antibiotic Treatment: Broad-spectrum antibiotics to deplete microbiota.
  • Fecal Microbiota Transplantation (FMT): Transferring microbiota from Il17rc^DIEC donors to Il17rc^THCre or wild-type recipients.
• Pharmacological Interventions:
  • 6-OHDA: Chemical denervation of sympathetic neurons.
  • Anti-TCR$\gamma\delta$ Antibody: Blockade of $\gamma\delta$ T cells.
• Omics and Imaging:
  • Bulk RNA-seq: Aortic and intestinal tissue transcriptomics.
  • scRNA-seq: Single-cell analysis of aortic CD45+ cells and intestinal lymphocytes (IEL/LPL).
  • Spatial Transcriptomics (Xenium): To map cell-cell interactions (specifically $\gamma\delta$ T cells and neurons) in the aortic wall.
  • Metagenomics & Metabolomics: Shotgun sequencing of gut microbiota and serum metabolite profiling.
  • Imaging: iDISCO tissue clearing with light-sheet microscopy for 3D visualization of neuronal axons; Immunofluorescence for protein localization.

3. Key Contributions

• Discovery of a Gut-Aorta Axis: The study identifies a novel long-range signaling axis where intestinal IL-17RC signaling controls perivascular innervation and atherosclerosis progression.
• Cell-Type Specific Duality of IL-17: It resolves previous contradictions in IL-17 literature by demonstrating that IL-17RC signaling has opposing roles depending on the cell type:
  • Protective in the Gut: IEC-specific IL-17RC maintains barrier integrity and suppresses dysbiosis.
  • Pathogenic in the Aorta: Neuronal IL-17RC signaling drives neuroinflammation and plaque growth.
• Mechanistic Link to Neurons: It establishes that $\gamma\delta$ T cells in the aorta, driven by dysbiotic metabolites, produce IL-17A which acts directly on sympathetic neurons to promote axonal outgrowth and activation.

4. Key Results

A. Intestinal IL-17RC Deficiency Exacerbates Atherosclerosis

• Ldlr-/-Il17rc^DIEC mice developed significantly larger atherosclerotic lesions compared to controls, despite no differences in body weight or systemic lipids.
• Aortic transcriptomics revealed upregulation of inflammatory pathways and, notably, neuronal outgrowth signatures (e.g., Th, Npy, Dbh, Gap43).
• Histology and 3D imaging confirmed a massive expansion of sympathetic (TH+) and general (NF200+) neuronal axons in the aortic roots of Il17rc^DIEC mice.

B. Mechanism: Barrier Dysfunction and Microbiota

• Loss of IL-17RC in IECs compromised the intestinal barrier (reduced Muc2, Claudin-1, and Goblet cells), leading to increased permeability.
• This resulted in dysbiosis (altered luminal and adhesive microbiota) and elevated serum metabolites, specifically oleic acid, uric acid, and bile acids.
• Microbiota Dependence: Co-housing Il17rc^DIEC mice with controls transferred the "atherogenic" phenotype (increased lesions and neuronal expansion) to the controls. Conversely, antibiotic treatment abolished the difference between genotypes, proving the phenotype is microbiota-driven.

C. The $\gamma\delta$ T Cell - IL-17A - Neuron Axis

• scRNA-seq of aortas showed a specific accumulation of IL-17A-producing $\gamma\delta$ T cells in Il17rc^DIEC mice. These cells expressed Gpr65 (an oleic acid receptor) and Itgae (CD103).
• Spatial proximity: Xenium analysis confirmed $\gamma\delta$ T cells co-localize with sympathetic neurons in the aortic adventitia.
• Functional Blockade: Blocking $\gamma\delta$ T cells (anti-TCR$\gamma\delta$) reduced atherosclerotic lesions, neuronal gene expression, and inflammation in Il17rc^DIEC mice but not in controls.
• Direct Neuronal Stimulation: Ex vivo stimulation of neurons with IL-17A induced pro-inflammatory and neurogenic gene expression.

D. Neuronal IL-17RC Signaling is Pro-Atherogenic

• In Il17rc^THCre mice (lacking IL-17RC in sympathetic neurons), the transfer of atherogenic microbiota (via FMT from Il17rc^DIEC donors) failed to induce significant atherosclerosis or neuronal expansion compared to controls.
• Chemical Denervation: Treatment with 6-OHDA (sympathetic denervation) significantly reduced lesion size in Il17rc^DIEC mice, confirming that neuronal activity is required for disease progression.
• Macrophage Activation: Expanded sympathetic neurons released norepinephrine, which activated macrophages (increased CD36 expression and lipid uptake), driving foam cell formation.

5. Significance

This study fundamentally shifts the understanding of atherosclerosis by integrating the gut-microbiota-immune-neuron axis:

1. Clinical Implications: It suggests that therapeutic strategies targeting IL-17 (e.g., for psoriasis or arthritis) must be carefully considered, as systemic inhibition might disrupt protective gut barrier functions while failing to address (or potentially worsening) specific neuro-immune drivers in the vessel wall.
2. Novel Pathway: It uncovers a "gut-brain-vessel" circuit where diet-induced barrier defects lead to microbiota-driven $\gamma\delta$ T cell activation, which in turn stimulates sympathetic innervation of the aorta to accelerate plaque growth.
3. Therapeutic Targets: The study highlights potential targets including intestinal barrier repair, specific microbiota modulation, $\gamma\delta$ T cell blockade, or sympathetic neuron inhibition to treat atherosclerosis, particularly in patients with comorbid inflammatory bowel disease or dysbiosis.

In summary, the paper demonstrates that intestinal IL-17RC signaling is a critical gatekeeper that prevents dysbiosis-induced neuroinflammation and atherosclerosis, revealing a complex interplay where the gut microbiome dictates the neuroimmune landscape of the cardiovascular system.