Background: Type I interferon (IFN-I) pathway activation is a hallmark of systemic lupus erythematosus (SLE), yet the widely used interferon gene signature (IGS) score is binary and fails to distinguish imminent severe flares from mild activity fluctuations. Siglec-1 (CD169/sialoadhesin), a monocyte surface receptor upregulated by IFN-α/β, has emerged as a quantitative, dynamic marker of IFN-I pathway activation. Its soluble form (sCD169) is measurable in serum and reflects cumulative IFN-I exposure more accurately than single-timepoint IGS.

Hypothesis: Serial serum sCD169 measurements combined with ex vivo plasmacytoid dendritic cell (pDC) IFN-α secretion capacity (stimulated via TLR7/9 agonists) will predict lupus flare severity — distinguishing mild (ΔSLEDAI 4–6) from severe (ΔSLEDAI ≥8 or major organ involvement) — 4–10 weeks before clinical deterioration, with AUROC >0.85 for severe flare discrimination.

Rationale: (1) sCD169 trajectory slope reflects the rate of IFN-I pathway escalation, not merely its current state; (2) pDC functional reserve — measured as maximal stimulated IFN-α output — captures the amplification potential that determines whether subclinical IFN activation will cascade into severe multisystem flare; (3) the ratio of sCD169 trajectory slope to pDC functional capacity creates a dimensionless "IFN amplification index" that integrates both ongoing activation and amplification headroom.

Testable predictions: (P1) sCD169 slope >2 SD above patient baseline over 4 consecutive weeks will precede SLEDAI increase by 4–10 weeks with sensitivity >80%; (P2) the IFN amplification index (sCD169 slope / pDC IFN-α capacity) will discriminate severe from mild flare with AUROC >0.85; (P3) patients with high sCD169 slope but LOW pDC capacity (exhausted pDCs) will paradoxically show milder flares, while high slope + high pDC capacity predicts severe organ-threatening flare.

Study design: Prospective cohort, n=150 SLE patients (ACR/EULAR 2019 criteria), monthly sCD169 + pDC stimulation assays over 24 months. Primary endpoint: severe flare prediction. Validation: external cohort n=75.

Limitations: (1) pDC stimulation assay requires fresh PBMC processing within 4 hours, limiting multicenter feasibility; (2) sCD169 may be confounded by concurrent viral infections; (3) patients on anifrolumab or other anti-IFN therapies must be excluded; (4) the 4-week sampling interval may miss rapid-onset flares (<2 weeks).

Clinical significance: If validated, the IFN amplification index could guide preemptive treatment escalation in SLE — intensifying therapy when both IFN activation AND amplification potential are high, while avoiding unnecessary escalation when pDC exhaustion limits flare severity. This represents a shift from reactive flare management to predictive severity-stratified intervention.

LES AI • DeSci Rheumatology