Background

Mucosal-associated invariant T (MAIT) cells are innate-like T lymphocytes restricted by MR1 that bridge innate and adaptive immunity. In systemic lupus erythematosus (SLE), peripheral MAIT cell frequencies are consistently reduced during active disease, yet their functional trajectory as a predictive biomarker for organ-specific flare remains unexplored. Simultaneously, the IL-18/IL-18BP axis—governing inflammasome-driven inflammation—is dysregulated in lupus nephritis (LN) but has not been integrated with MAIT cell functional profiling.

Hypothesis

We hypothesize that a composite MAIT Cell Functional Exhaustion Index (MFEI)—derived from MR1-tetramer+ CD161++ Vα7.2+ T cells co-expressing exhaustion markers (PD-1, TIM-3, LAG-3) and exhibiting diminished IL-17A/IFN-γ dual-production capacity upon 5-OP-RU stimulation—combined with the serum free IL-18/IL-18BP molar ratio, predicts lupus nephritis relapse 6–14 weeks before proteinuria escalation, independent of anti-dsDNA titers and complement C3/C4 levels.

Mechanistic Rationale

1. MAIT cell exhaustion precedes clinical flare: MAIT cells home to inflamed kidneys via CCR6/CXCR6 chemokine gradients. Progressive exhaustion of circulating MAIT cells reflects ongoing renal tissue sequestration and chronic MR1-dependent activation by riboflavin metabolites released from damaged tubular epithelium.

2. IL-18 axis amplification: Free IL-18 (unbound by IL-18 binding protein) drives MAIT cell terminal differentiation toward an exhausted phenotype while simultaneously activating renal resident macrophages, creating a feed-forward loop detectable in serum before proteinuria manifests.

3. Composite biomarker superiority: Neither MAIT frequency alone nor IL-18 alone achieves sufficient predictive performance due to confounding by infections (MAIT) and systemic inflammation (IL-18). Their combination captures the kidney-specific pathological circuit.

Testable Predictions

• P1: In a prospective cohort of ≥120 LN patients in remission, serial MFEI measurements (monthly MR1-tetramer flow cytometry with functional stimulation) will show a rising exhaustion trajectory ≥6 weeks before proteinuria exceeds 0.5 g/day, with AUROC >0.82 for relapse prediction.
• P2: The composite MFEI + free IL-18/IL-18BP ratio will outperform anti-dsDNA + C3/C4 (ΔAUROC ≥0.10, p<0.05) for 14-week relapse prediction.
• P3: Renal biopsy at relapse will show MR1+ tubular epithelial cells co-localizing with tissue-infiltrating MAIT cells expressing TIM-3+PD-1+ exhaustion phenotype, confirmed by spatial transcriptomics.
• P4: In vitro blockade of the IL-18/IL-18BP axis will partially rescue MAIT cell effector function (≥40% recovery of IL-17A/IFN-γ dual production), supporting the mechanistic model.

Study Design

Prospective longitudinal cohort, 120 LN patients (ISN/RPS Class III-V) in complete or partial remission (UPCR <0.5 g/day, stable eGFR). Monthly blood sampling for 18 months with MR1-tetramer flow cytometry panel (CD3, CD161, Vα7.2, PD-1, TIM-3, LAG-3, Ki-67) and functional assay (5-OP-RU stimulation, intracellular IL-17A/IFN-γ). Serum free IL-18 by ELISA with IL-18BP subtraction. Primary endpoint: time to renal relapse (UPCR >0.5 g/day confirmed on two measurements). Statistical analysis via time-dependent Cox proportional hazards with MFEI and IL-18 ratio as time-varying covariates, adjusted for baseline ISN class, maintenance immunosuppression, and ethnicity.

Limitations

• MR1-tetramer assay complexity: Requires specialized reagents and standardized protocols; inter-laboratory variability may limit multicenter validation.
• Confounding by infection: Bacterial/fungal infections alter MAIT frequencies; rigorous exclusion criteria and adjustment for intercurrent infections are necessary.
• IL-18BP isoform heterogeneity: Multiple IL-18BP splice variants with differing IL-18 binding affinities complicate free IL-18 quantification.
• Sample size: 120 patients may be underpowered for subgroup analyses by ISN/RPS class.
• Generalizability: MAIT cell frequencies vary by ethnicity and age, requiring diverse cohort enrollment.

Clinical Significance

If validated, the MFEI + IL-18 composite biomarker would provide a 6–14 week early warning system for LN relapse, enabling preemptive immunosuppression intensification before irreversible nephron loss. This is particularly valuable because current serological markers (anti-dsDNA, complement) have poor sensitivity for isolated renal flare and often lag behind histological activity. A reliable early relapse predictor could reduce protocol biopsy burden and improve long-term renal survival in SLE.

LES AI • DeSci Rheumatology