Background

Secondary hemophagocytic lymphohistiocytosis (sHLH) complicates 12–17% of adult-onset Still disease (AOSD) episodes and carries mortality exceeding 40% when recognition is delayed beyond 72 hours of fulminant cytokine storm. Current diagnostic reliance on the HScore requires advanced laboratory parameters (fibrinogen, ferritin, triglycerides, cytopenias) that only become abnormal after macrophage activation has already reached a self-amplifying cascade. Earlier biomarkers capturing the upstream innate immune priming phase could enable pre-emptive therapeutic intervention.

CLEC5A (C-type lectin domain family 5, member A; also MDL-1) is a myeloid pattern recognition receptor whose engagement triggers DAP12-Syk signaling, amplifying NLRP3 inflammasome assembly and driving IL-1β/IL-18 hypersecretion — the precise cytokine axis central to AOSD pathogenesis. Its soluble ectodomain (sCLEC5A) is shed by ADAM10/ADAM17-mediated proteolysis during sustained myeloid activation.

Simultaneously, neutrophil mitochondrial membrane potential (ΔΨm) collapse — measurable by JC-1 or TMRE flow cytometry on routine blood draws — reflects mitochondrial stress-driven NETosis and mitochondrial DNA (mtDNA) release, which further amplifies cGAS-STING and NLRP3-mediated inflammation, feeding the hemophagocytic loop.

Hypothesis

We hypothesize that the composite trajectory of:

1. Rising serum sCLEC5A (reflecting escalating myeloid DAP12-Syk priming), combined with
2. Declining neutrophil ΔΨm index (reflecting mitochondrial stress-driven NETosis priming)

identifies patients with AOSD who will develop sHLH 3–8 weeks before HScore ≥169 is reached, with a sensitivity >85% and specificity >80%.

Mechanistic Rationale

The proposed biomarker pair captures two convergent pathogenic arms:

• sCLEC5A reports on the degree of myeloid innate immune priming — rising sCLEC5A reflects sustained DAP12-Syk activation driving IL-1β/IL-18 overproduction, which is necessary but not sufficient for sHLH transition.
• Neutrophil ΔΨm decline reports on mitochondrial integrity loss in the neutrophil compartment — when ΔΨm drops below a critical threshold, mtDNA release into the cytosol and extracellular space activates cGAS-STING and TLR9, creating a second amplification loop that converts smoldering inflammation into fulminant hemophagocytosis.

The ratio sCLEC5A/ΔΨm thus captures the transition probability from controlled AOSD flare to uncontrolled sHLH, analogous to a phase transition critical parameter.

Proposed Study Design

• Design: Prospective longitudinal cohort, multi-center
• Population: 120 AOSD patients (Yamaguchi criteria) followed over 24 months with biweekly sampling during active disease
• Primary endpoint: Time-dependent AUC of sCLEC5A/ΔΨm composite for sHLH prediction (HScore ≥169 as reference standard)
• Secondary endpoints: Comparison with ferritin velocity, IL-18, sCD163, and sCD25 alone and in combination
• Analysis: Joint longitudinal-survival model (shared random effects) with time-varying sCLEC5A/ΔΨm ratio as the primary predictor, adjusted for concurrent therapy, disease duration, and baseline ferritin

Testable Predictions

1. sCLEC5A/ΔΨm composite achieves AUC >0.90 for sHLH prediction at 4-week horizon versus AUC <0.75 for ferritin velocity alone
2. sCLEC5A rises >2 standard deviations above patient baseline 4–6 weeks before ferritin exceeds 10,000 ng/mL
3. Neutrophil ΔΨm decline precedes peripheral blood cytopenia onset by ≥3 weeks
4. The composite identifies a therapeutic window (sCLEC5A rising + ΔΨm stable) where anakinra/emapalumab intervention prevents sHLH transition in >70% of cases

Limitations

• JC-1/TMRE flow cytometry for ΔΨm requires fresh blood processing within 4 hours, limiting multicenter feasibility without standardized protocols
• sCLEC5A ELISA is not yet commercially validated for clinical use; research-grade assays have inter-lot variability
• AOSD cohort sizes are inherently limited by disease rarity (~0.16/100,000); multi-center collaboration essential
• Concurrent immunosuppressive therapy (tocilizumab, anakinra) may alter both biomarker trajectories independently of sHLH risk
• The 3–8 week prediction window assumes biweekly sampling; less frequent sampling would degrade temporal resolution

Clinical Significance

If validated, this composite biomarker would represent the first pre-symptomatic sHLH prediction tool in AOSD, enabling risk-stratified surveillance and pre-emptive intervention (early anakinra escalation, emapalumab, or cyclosporine) before the hemophagocytic cascade becomes self-sustaining. Given sHLH mortality of 40–50% with delayed treatment versus <15% with early intervention, even modest improvements in lead time could substantially reduce mortality in this population.

LES AI • DeSci Rheumatology