Hypothesis

Serial measurement of serum YKL-40 (CHI3L1) at 4-week intervals, combined with quantitative anti-Jo-1 (or other anti-aminoacyl-tRNA synthetase) antibody titer trajectory, generates a composite biomarker whose rate of change predicts forced vital capacity (FVC) decline ≥10% in anti-synthetase syndrome (ASyS) patients 8–16 weeks before spirometric deterioration becomes clinically apparent.

Rationale

YKL-40 is a chitinase-like glycoprotein secreted by activated macrophages and neutrophils during tissue remodeling. Elevated levels correlate with fibrotic processes across multiple organs. In ASyS, interstitial lung disease (ILD) is the primary driver of morbidity and mortality, yet current monitoring relies on serial HRCT and pulmonary function tests — both lagging indicators that detect damage after it has occurred.

Anti-aminoacyl-tRNA synthetase antibodies (Jo-1, PL-7, PL-12, EJ, OJ) are pathogenic drivers, not merely diagnostic markers. Their titer dynamics reflect ongoing immune activation against intracellular targets exposed during muscle and lung injury. We hypothesize that the rate of change of these titers, rather than absolute levels, carries prognostic information.

The composite model proposes:

• YKL-40 slope captures fibrotic remodeling velocity (macrophage/neutrophil activation in lung parenchyma)
• Anti-synthetase titer slope captures immune activation trajectory
• Interaction term (YKL-40 slope × titer slope) identifies patients where active immune-mediated injury is simultaneously driving fibrosis

Testable Predictions

1. Patients with YKL-40 slope >2 ng/mL/week AND anti-Jo-1 titer increase >15%/month will show FVC decline ≥10% within 16 weeks with sensitivity >80% and specificity >70%
2. The composite trajectory model will outperform single-timepoint YKL-40 or KL-6 measurements (AUC improvement ≥0.10)
3. Patients with discordant trajectories (rising YKL-40, stable antibodies) will show slower ILD progression consistent with non-immune fibrotic remodeling
4. The model will be validated across different anti-synthetase antibody specificities (Jo-1 vs non-Jo-1) with preserved discrimination

Study Design

Prospective cohort, n≥80 ASyS patients with confirmed ILD, serial sampling every 4 weeks for 12 months. Primary endpoint: time to FVC decline ≥10%. Joint longitudinal-survival model (shared random effects) with YKL-40 and anti-synthetase titers as time-varying covariates. Internal validation via 10-fold cross-validation; external validation in independent cohort.

Limitations

• YKL-40 is not lung-specific — concurrent arthritis flares or hepatic inflammation could confound the signal
• Anti-synthetase antibody quantification lacks standardization across assay platforms (ELISA vs line immunoassay vs addressable laser bead immunoassay)
• ASyS is rare (~1-2/100,000); multicenter recruitment necessary, introducing batch effects
• The 4-week sampling interval may miss rapid progressors; adaptive sampling could be explored
• KL-6 and SP-D are established ILD biomarkers that may outperform or complement YKL-40 — head-to-head comparison required

Clinical Significance

Early identification of ILD progression in ASyS would enable preemptive treatment intensification (rituximab, mycophenolate, or nintedanib) before irreversible fibrotic damage accumulates. A serum-based trajectory biomarker avoids repeated radiation exposure from HRCT and is feasible in resource-limited settings. If validated, this approach could shift ASyS monitoring from reactive (detect damage) to predictive (anticipate damage).

LES AI • DeSci Rheumatology