The exosomal protein biomarkers auxiliary in diagnosis of interstitial lung disease.

Background: Exosome liquid biopsies might be a good supplement for early diagnosis of interstitial lung disease (ILD), especially those challenging cases such as connective tissue disease-ILD (CTD-ILD).

Methods: We developed a circulating exosomal proteomic signature to identify novel biomarkers of ILDs combined with high-resolution CT (HRCT) examination and a new method that makes exosome testing clinically feasible. Blood-derived exosomes were extracted and characterized using a centrifugal microfluidic disc system (Exo-CMDS)-based chemiluminescence immunoassay before being subjected to proteomic analysis by mass spectrometry. Significantly differentially expressed proteins (DEPs) were identified and validated in > 600 clinical samples (collected at three hospitals) by comparing the ILD and disease/healthy control groups. Multivariable logistic regression (LR) analysis was implemented to test the diagnostic performance of the selected biomarkers either alone or in combination.

Results: Candidate biomarkers KL-6, CAPN2, SP-B were selected from the top DEPs. An LR model that combined exosomal KL-6/CAPN2/SP-B levels performed well in both the discovery (AUC = 0.987, 95%CI = 0.975-0.998) and validation (AUC = 0.936, 95%CI = 0.911-0.960) sets. The LR model based on the three biomarkers exhibited markedly better diagnostic performance (AUC = 0.880, 95%CI = 0.834-0.925) in serum-KL-6-negative ILD, than the conventional serum-KL-6-based method and could also accurately diagnose connective tissue disease associated-ILD (CTD-ILD) in the context of CTD.

Conclusion: The circulating exosomal protein detection system used in this study represents a valuable tool for identifying promising exosomal biomarkers for ILD and holds promise for improving the diagnosis and prognosis of patients with ILD in the future.