Mucosal-Associated Invariant T Cells Confer Protection in Pediatric Metabolic dysfunction-Associated Steatotic Liver Disease

Technical Summary: Mucosal-Associated Invariant T Cells Confer Protection in Pediatric Metabolic Dysfunction-Associated Steatotic Liver Disease

Problem Statement
Metabolic dysfunction-associated steatotic liver disease (MASLD) is the most prevalent chronic liver disorder in the pediatric population, closely linked to the rising rates of childhood obesity. While the role of peripheral CD3⁺ T cell subsets in adult MASLD pathogenesis has been explored, a comprehensive understanding of these dynamics in children is lacking. Ethical constraints limit liver biopsies in minors, and existing juvenile animal models often lack fidelity, leading to a reliance on adult data which may not reflect the unique biology of pediatric MASLD. Specifically, the contribution of non-classical T cell subsets, such as Mucosal-Associated Invariant T (MAIT) cells, to the pathogenesis and progression of pediatric MASLD remains poorly defined.

Methodology
The study employed a multi-modal approach combining human clinical data, single-cell genomics, metabolomics, and in vivo/in vitro animal models:

1. Human Cohort and Single-Cell RNA Sequencing (scRNA-seq): Peripheral blood mononuclear cells (PBMCs) were collected from 50 children stratified into three groups: Normal Weight (NW, n=7), Healthy Obesity (HO, n=15), and MASLD (n=28). scRNA-seq was performed to profile CD3⁺ T cell subsets. Data were processed using Cell Ranger and ScanPy, with batch effects mitigated via Harmony integration.
2. Validation and Multi-omics: Findings were validated using flow cytometry, untargeted fecal metabolomics (to assess riboflavin synthesis and MR1 ligand precursors), and multiplex cytokine assays (Luminex) for serum IL-12 and IL-18.
3. Adult Liver Reference: Publicly available adult MASLD liver scRNA-seq datasets (GSE181483, GSE136103) were analyzed to compare hepatic MAIT cell signatures and chemokine receptor expression.
4. Animal Models:
   • Juvenile MASLD Model: 3-week-old C57BL/6J mice were fed a Western diet plus fructose water for 5 weeks.
   • Intervention Groups: Mice received intraperitoneal injections of MAIT cell agonists (5-OP-RU for MR1-dependent activation; ODN-1668 for cytokine-driven activation via IL-12/IL-18) with or without a CXCR6 antagonist (ML-339).
   • Deficiency Model: MR1-knockout (MR1-KO) mice were used to assess the impact of MAIT cell absence.
   • Adoptive Transfer: DiR-labeled peripheral MAIT cells isolated from healthy children were transferred into NCG mice (immunodeficient) with juvenile MASLD to track hepatic homing.
5. Human Organoids: Lipid-laden hepatic organoids derived from human induced pluripotent stem cells (hiPSCs) were co-cultured with activated pediatric MAIT cells to assess lipid accumulation.

Key Contributions and Results

• Peripheral T Cell Dynamics: scRNA-seq revealed a stepwise expansion of inflammatory CD4⁺ effector-memory (Tem) and CD8⁺ effector (Teff) populations from NW to HO to MASLD. Notably, unlike adult studies where MAIT cells are often depleted in late-stage disease, pediatric MASLD showed a proportional increase in MAIT cells that correlated with disease severity and CD4⁺ Tem expansion.
• MAIT Cell Activation Mechanisms: Transcriptomic analysis indicated upregulation of semi-invariant TCR chains (TRAV1-2), IL-18 receptor, and IL-12 receptor subunits in MASLD. Metabolomics showed increased fecal riboflavin (a precursor for MR1 ligands), and cytokine assays confirmed elevated serum IL-12 and IL-18. This suggests MAIT cells in pediatric MASLD are activated via both MR1-restricted antigen presentation and MR1-independent IL-12/IL-18 pathways.
• Homing Mechanism (CXCR6-CXCL16 Axis): scRNA-seq identified high expression of the chemokine receptor CXCR6 on circulating MAIT cells. Correspondingly, liver sinusoidal endothelial cells (LSECs) expressed the ligand CXCL16. This CXCR6-CXCL16 axis was validated by flow cytometry and immunofluorescence. In adult liver datasets, MAIT cells also expressed CXCR6 and CXCR4, while endothelial cells expressed their respective ligands.
• Protective Role in Juvenile MASLD:
  • Activation: In juvenile mice, MAIT cell activation reduced hepatic lipid accumulation (Oil Red O staining), improved NAS scores, and normalized lipid metabolic enzymes (upregulating ACOX1, downregulating SCD1/FABP1).
  • Blockade: The protective effects of MAIT activation were abolished by CXCR6 blockade, confirming that CXCR6-mediated homing is essential for the observed hepatoprotection.
  • Deficiency: MR1-KO mice (MAIT-deficient) on a Western diet exhibited exacerbated steatosis, glucose intolerance, and liver injury compared to wild-type controls.
  • Adoptive Transfer: Transferring pediatric peripheral MAIT cells into juvenile MASLD mice reduced hepatic lipid accumulation and modulated cytokine profiles (increased Il-10 and Tnf, decreased Il-17). This homing was significantly impaired by CXCR6 blockade.
• Organoid Validation: Co-culture of lipid-challenged human liver organoids with activated pediatric MAIT cells significantly reduced intracellular lipid droplets (BODIPY staining), confirming a direct hepatoprotective effect in a human-relevant system.

Significance
The paper claims to provide the first high-resolution single-cell atlas of peripheral CD3⁺ T cells in pediatric MASLD. It challenges the prevailing adult-centric view by demonstrating that, in children, MAIT cells expand rather than deplete as the disease progresses. The study elucidates a specific mechanism where activated pediatric MAIT cells home to the liver via the CXCR6-CXCL16 axis to confer protection against steatotic injury.

The authors position MAIT cells as promising early biomarkers for pediatric MASLD and potential therapeutic targets. They highlight a distinct mechanistic divergence between pediatric and adult MASLD immunopathology, suggesting that therapeutic strategies effective in adults (which might involve depleting or modulating MAIT cells differently) may not apply to children. The work underscores the necessity of dedicated mechanistic investigations in pediatric populations due to ethical limitations on tissue sampling and biological differences in immune development.