Exosomal KL-6 surpasses total plasma KL-6 in the diagnosis of Interstitial Lung Disease (ILD)

Abstract Background Krebs Von den Lungen-6 (KL-6) is an established biomarker for interstitial lung diseases (ILDs), but the utility of exosomal KL-6 versus plasma KL-6 in connective tissue disease-associated ILD (CTD-ILD) remains unclear. Methods We integrated single-cell transcriptomic re-analysis (GSE135893) with a multi-center cohort of 223 subjects, including 167 ILD, 22 CTD without ILD[CTD (Non-ILD)], 34 healthy controls(HCs). Plasma and exosomal KL-6 were measured, and diagnostic performance was evaluated via Receiver operating characteristic(ROC) analysis. Ten progressive ILD patients were monitored longitudinally. Results Single-cell sequencing revealed a significantly stronger correlation between Mucin 1 (MUC1)and exosomal markers (CD63/CD81) in alveolar type II cells from ILD patients compared to HCs, suggesting a potential vesicular association of KL-6. Clinical validation demonstrated that both plasma and exosomal KL-6 levels were significantly elevated in the ILD group. Crucially, for distinguishing CTD-ILD patients with negative plasma KL-6 results [CTD-ILD (plasma KL-6-)] from HCs, exosomal KL-6 [Area under the curve(AUC) = 0.900] outperformed plasma KL-6 (AUC = 0.879). More importantly, in differentiating CTD-ILD (plasma KL-6-) from CTD (Non-ILD), exosomal KL-6 again showed superior diagnostic accuracy (AUC = 0.782, sensitivity = 90.0%, specificity = 77.3%) compared to plasma KL-6 (AUC = 0.705, sensitivity = 65.0%, specificity = 81.8%). Plasma and exosomal KL-6 concentrations were highly correlated (rs = 0.968, p < 0.001). In longitudinal monitoring, both biomarkers increased in patients with radiographic disease progression. Conclusions Exosomal KL-6 is superior to plasma KL-6 for diagnosing CTD-ILD, especially in plasma KL-6-negative patients, and shows potential for monitoring disease progression.