Sputum exosomes - promising biomarkers for idiopathic pulmonary fibrosis

Background: Idiopathic pulmonary fibrosis (IPF) is a progressive fibrosing interstitial lung disease of unknown etiology which leads rapidly to death. As diagnosis of IPF is complex, the development of novel molecular biomarkers is a central challenge for the future of translational research. Consequently, we sought to characterize microRNA (miR) content of exosomes from sputum of IPF patients compared to healthy donors in order to identify novel biomarkers of the disease.

Methods: Exosomes were isolated from induced sputum samples of 14 IPF patients diagnosed following American Thoracic Society (ATS) /European Respiratory Society (ERS) recommendations and 11 healthy donors with standard ultracentrifugation protocol. Exosomal miR content was analysed by miR qPCR arrays, and diseases/biological processes associated to altered miRs were determined by bioinformatic analysis.

Results: The presence of exosomes was confirmed in sputum from both IPF patients and healthy donors. The profiling of exosomal miRs revealed 21 differentially expressed miRs in the sputum of IPF patients compared to healthy donors. Further validation of miRs presenting an aberrant expression allowed us to identify for the first time an IPF-specific miR signature from sputum exosomes, among which miR-142-3p and miR-33a-5p present an upregulation (fold change (FC)>3, p < 0.01), whereas let-7d-5p a downregulation (FC < 0.5, p < 0.01). The bioinformatic analysis revealed that altered miRs are associated to inflammatory diseases, among which IPF is the most relevant one (p = 3.78E-10). Interestingly, most of the biological processes highlighted in this analysis are in agreement with IPF etiology, which confers to our candidates an evident role as IPF biomarkers. Based on these findings, functional tests with IPF-sputum exosomes and mimics of altered miRs are underway to test their impact on IPF progression.

Summary/Conclusion: For the first time, we identified potential biomarkers for IPF from sputum exosomes. Our findings may thus lead to a better understanding about the roles of these miRs in the pathogenesis of IPF and thus open new avenues for therapeutic approaches. This study reinforced the concept that sputum exosomes might be a novel source of biomarkers for the diagnosis of pulmonary diseases.