Gut-lung immunometabolic crosstalk in sepsis

Gut-lung immunometabolic crosstalk in sepsis is a critical concept that highlights the bidirectional interactions between the gut and lungs, which play a pivotal role in the progression of sepsis and its associated complications, such as acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). This crosstalk is mediated by immune signaling, metabolites, and systemic circulation, creating a vicious cycle of immune-metabolic dysregulation that exacerbates inflammation, organ dysfunction, and respiratory failure. Below is a detailed explanation of the mechanisms and factors involved in gut-lung immunometabolic crosstalk during sepsis:

### 1. **Gut Microbiota Dysbiosis and Intestinal Barrier Dysfunction**

• **Sepsis-induced microbiota dysbiosis:** Sepsis disrupts the gut microbiota composition, reducing beneficial microbes and increasing pathogenic species. This imbalance leads to the production of harmful metabolites and endotoxins, such as lipopolysaccharides (LPS).
• **Intestinal barrier damage:** Sepsis compromises the intestinal epithelial barrier, allowing the translocation of endotoxins like LPS and microbial products into the systemic circulation. This leakage triggers systemic inflammation and immune activation, contributing to lung injury.

### 2. **Short-Chain Fatty Acids (SCFAs) and Gut Integrity**

• SCFAs, produced by gut microbiota during fiber fermentation, play a crucial role in maintaining gut integrity and immune balance. In sepsis, reduced SCFA levels impair gut barrier function and exacerbate systemic inflammation, worsening the gut-lung axis imbalance.

### 3. **Inflammatory Amplification and Lung Injury**

• **Endotoxin leakage:** LPS and other gut-derived toxins activate alveolar macrophages and neutrophils in the lungs, leading to excessive release of pro-inflammatory cytokines (e.g., IL-1β, IL-6, TNF-α). This inflammatory cascade damages lung tissue and increases vascular permeability.
• **Neutrophil extracellular traps (NETs):** Excessive NET formation in the lungs damages alveolar structures, impairs gas exchange, and increases the risk of respiratory failure.

### 4. **Metabolic Reprogramming in Immune Cells**

• During early sepsis, immune cells undergo metabolic reprogramming, shifting from oxidative phosphorylation to glycolysis. This glycolytic shift, driven by hypoxia-inducible factor-1α (HIF-1α), fuels hyperinflammation and promotes the release of pro-inflammatory cytokines.
• In later stages, mitochondrial dysfunction leads to energy exhaustion and immunosuppression, impairing the immune response and increasing susceptibility to secondary infections.

### 5. **Mitochondrial Dysfunction and Reactive Oxygen Species (ROS)**

• Damaged mitochondria release reactive oxygen species (ROS) and mitochondrial DNA (mtDNA), which act as damage-associated molecular patterns (DAMPs). These molecules amplify inflammation and contribute to multi-organ failure, including lung injury.

### 6. **Gut-Derived Metabolites and Lung Inflammation**

• **Bile Acids and Tryptophan Metabolites:** Bile acids and tryptophan-derived metabolites, such as indole derivatives, regulate lung inflammation through nuclear receptors like FXR, TGR5, and AhR. Dysregulated levels of these metabolites in sepsis worsen lung injury.
• **Succinate and TMAO Toxicity:** Accumulation of gut-derived metabolites like succinate and trimethylamine N-oxide (TMAO) enhances lung inflammation and pyroptosis, a form of programmed cell death.

### 7. **Macrophage Polarization Imbalance**

• Gut dysbiosis skews macrophage polarization toward a pro-inflammatory M1 phenotype, while reducing the anti-inflammatory M2 phenotype. This imbalance worsens lung inflammation and contributes to ALI/ARDS.

### 8. **Vagus Nerve and Anti-inflammatory Pathway**

• The vagus nerve connects the gut and lungs via the cholinergic anti-inflammatory pathway. Activation of α7 nicotinic acetylcholine receptors (α7nAChR) reduces cytokine release and protects lung tissues. Dysfunction of this pathway in sepsis exacerbates inflammation.

### 9. **Therapeutic Interventions**

• **Probiotics and Prebiotics:** Probiotic therapy has shown potential in modulating gut microbiota and reducing inflammation. However, its use in ICU patients must be approached cautiously due to the risk of exacerbating sepsis.
• **Fecal Microbiota Transplantation (FMT):** FMT can restore gut microbiota balance and reduce systemic inflammation in animal models. Human studies are limited but show promise.
• **Precision Medicine:** Patient-specific immune-metabolic profiling is essential for tailoring therapies targeting the gut-lung axis.
• **Multi-omics Approach:** Integrating metagenomics, metabolomics, and immunophenotyping can help identify dynamic biomarkers and develop targeted therapies.

### 10. **Clinical Vision**

• Current sepsis management focuses on general immune suppression, but emerging research advocates for “immune-metabolic editing.” This approach aims to restore gut-lung axis balance, prevent organ failure, and improve outcomes by addressing the underlying immune-metabolic dysregulation.

### Conclusion

Gut-lung immunometabolic crosstalk in sepsis is a complex interplay of disrupted gut microbiota, immune signaling, and metabolic dysfunction. Understanding and targeting this axis opens new avenues for precision medicine, with the potential to mitigate inflammation, prevent organ failure, and improve survival in sepsis patients.