Hypothesis

A composite biomarker integrating low-density granulocyte (LDG) NET formation capacity — quantified by ex vivo PMA-stimulated NET area per cell via live-cell imaging — with the serum HMGB1 acetylated/total ratio measured by immunoprecipitation-mass spectrometry predicts irreversible organ damage accrual (SDI ≥1 point increase) in systemic lupus erythematosus 12–24 months before clinical detection, independent of concurrent disease activity (SLEDAI-2K) and cumulative glucocorticoid exposure.

Rationale

LDGs are an expanded, pro-inflammatory neutrophil subset in SLE with enhanced capacity for spontaneous and stimulated NETosis. NETs externalize oxidized mitochondrial DNA, LL-37, and citrullinated histones that perpetuate type I interferon signaling and directly damage vascular endothelium, renal glomeruli, and serosal surfaces. Critically, NET-mediated tissue injury may accumulate subclinically before manifesting as measurable organ damage on the SLICC/ACR Damage Index (SDI).

HMGB1 is a nuclear alarmin released during NETosis and necrotic cell death. Its post-translational acetylation state determines receptor binding specificity: hyperacetylated HMGB1 preferentially signals through TLR4/RAGE, amplifying inflammasome activation, while hypo-acetylated forms remain predominantly nuclear. A rising acetylated/total HMGB1 ratio therefore reflects ongoing extracellular alarmin-driven inflammation that standard serological markers (C3, anti-dsDNA) may not capture.

The hypothesis posits that combining a functional measure of NET-forming capacity (LDG NETosis index) with a molecular marker of alarmin-driven tissue injury (HMGB1 acetylation ratio) captures complementary pathogenic axes: the effector mechanism (NETosis) and the downstream damage signal (alarmin amplification). This dual-axis composite should outperform either component alone and conventional serological monitoring in predicting irreversible damage accrual.

Testable Predictions

1. Prospective cohort validation: In a cohort of ≥200 SLE patients followed for 36 months with quarterly sampling, the composite LDG-NET/HMGB1-Ac index at baseline will predict SDI increment with AUC >0.80, superior to SLEDAI-2K alone (expected AUC 0.55–0.65) and cumulative prednisone dose (AUC 0.60–0.70).
2. Temporal precedence: Sustained elevation of the composite index (≥2 consecutive quarterly measurements above the 75th percentile) will precede SDI increment by 12–24 months in ≥70% of damage-accruing patients.
3. Organ specificity: Subgroup analysis will reveal that LDG-NET predominance predicts renal and vascular damage, while HMGB1-Ac predominance predicts serosal and pulmonary damage, reflecting distinct tissue tropisms of each pathogenic axis.
4. Therapeutic responsiveness: Patients initiated on belimumab or anifrolumab who show ≥30% reduction in the composite index by month 6 will have significantly lower 24-month SDI increment rates compared to biomarker non-responders.

Limitations

• LDG NET formation assays require fresh blood processing within 4 hours and standardized live-cell imaging protocols, limiting scalability to centers with flow cytometry and high-content imaging capability.
• HMGB1 acetylation quantification by IP-MS is technically demanding; ELISA-based approximations may introduce measurement error.
• Confounders including infections (which also elevate NETs and HMGB1), hydroxychloroquine use (which inhibits NETosis), and ethnic/genetic variation in NETosis capacity (e.g., PADI4 haplotypes) must be carefully adjusted.
• The SDI is an insensitive instrument for subtle damage; subclinical damage detected only by imaging may be missed.

Clinical Significance

Current SLE management focuses on controlling disease activity to prevent damage, but no validated biomarker directly predicts irreversible organ damage accrual with sufficient lead time for preventive intervention. If validated, this composite index could identify a high-risk window 12–24 months before damage crystallizes, enabling targeted escalation (e.g., early biologic initiation, aggressive nephroprotection) in patients who appear clinically stable by conventional metrics. This would shift the treatment paradigm from reactive damage management to proactive damage prevention, with direct implications for treat-to-target strategies in lupus.

LES AI • DeSci Rheumatology