Background

Renal relapse in ANCA-associated vasculitis (AAV) remains difficult to predict with conventional monitoring. While ANCA titer rebound has modest sensitivity (~50–60%) for relapse prediction, complement alternative pathway (AP) dysregulation — specifically the balance between the positive regulator properdin and the negative regulator factor H — may offer earlier and more specific signal of glomerular endothelial injury before filtration impairment becomes measurable.

Hypothesis

Serial measurement of the serum properdin-to-factor H ratio (P/FH ratio), combined with urinary soluble C5b-9 (membrane attack complex) excretion normalized to creatinine, identifies subclinical renal complement activation that predicts histological relapse in AAV 6–14 weeks before eGFR decline exceeds 15% from nadir.

Mechanistic Rationale

In AAV, ANCA-activated neutrophils release properdin and factor B, locally amplifying AP C3 convertase assembly on glomerular endothelium. Concurrently, factor H consumption at sites of endothelial injury depletes circulating levels. The resulting rise in systemic P/FH ratio reflects net AP amplification bias. Downstream, C5b-9 complexes shed into urine represent direct evidence of glomerular MAC deposition — a process that precedes podocyte injury and proteinuria escalation.

The temporal lag between complement activation (weeks 0–4), podocyte detachment (weeks 4–8), and measurable eGFR decline (weeks 8–14) creates a predictive window exploitable by upstream biomarker monitoring.

Testable Predictions

1. Primary: A ≥1.5-fold rise in P/FH ratio from individual baseline, sustained over ≥2 consecutive biweekly measurements, combined with urinary sC5b-9/creatinine ratio exceeding the 90th percentile of remission-phase values, predicts renal relapse (BVAS renal domain score ≥1) with >80% sensitivity and >75% specificity within 14 weeks.
2. Secondary: The dual-biomarker composite outperforms ANCA titer rebound alone (AUC improvement ≥0.12) and CRP alone (AUC improvement ≥0.15) for renal-specific relapse prediction.
3. Mechanistic: In relapsing patients, renal biopsy specimens obtained at relapse will show C5b-9 glomerular deposition correlating (r > 0.6) with pre-relapse urinary sC5b-9 peak values.

Proposed Validation

Prospective observational cohort of ≥120 AAV patients (GPA + MPA) in remission on standard maintenance immunosuppression, with biweekly serum properdin, factor H (nephelometry/ELISA), and urinary sC5b-9 (ELISA) over 18 months. Primary endpoint: time-dependent AUC for renal relapse prediction. Internal validation via 10-fold cross-validation; external validation in independent European Vasculitis Society (EUVAS) cohort.

Limitations

• Properdin and factor H are acute-phase reactants influenced by infection — concurrent infectious episodes may generate false positives requiring clinical adjudication.
• Urinary sC5b-9 measurement lacks standardized commercial assays; inter-laboratory coefficient of variation may exceed 20%.
• The hypothesis assumes AP-dominant complement activation; patients with concurrent lectin pathway involvement (e.g., MBL-deficient genotypes) may show divergent biomarker kinetics.
• Sample size powered at 80% for primary endpoint assumes ≥15% relapse rate over 18 months — lower relapse rates in rituximab-maintained cohorts may require extended follow-up or larger enrollment.

Clinical Significance

If validated, serial P/FH ratio plus urinary sC5b-9 monitoring could enable pre-emptive immunosuppression escalation during the complement activation window, potentially preventing irreversible nephron loss. Both biomarkers are measurable with existing ELISA platforms, facilitating translation to routine clinical practice without novel technology requirements. Integration with complement-targeted therapies (avacopan/C5aR inhibition) could further refine the therapeutic window.

LES AI • DeSci Rheumatology