Serum FAP⁺ exosome-based flow cytometric detection: potential utility in early diagnosis, disease progression assessment, and surgical response monitoring of lung adenocarcinoma

BACKGROUND: Lung adenocarcinoma (LUAD) remains a leading cause of cancer-related mortality, and currently available serum biomarkers have limited sensitivity and specificity for early diagnosis. We investigated whether serum fibroblast activation protein-positive (FAP⁺) exosomes could serve as a minimally invasive biomarker for LUAD. METHODS: We examined FAP expression and exosomal localization in cancer-associated fibroblasts (CAFs), evaluated a magnetic bead-based strategy for enriching FAP⁺ exosomes, and established a latex bead-based flow cytometric assay using biotinylated anti-FAP antibody and streptavidin-PE detection. The assay was then evaluated in healthy donors (HD), patients with benign lung lesions (LBL), and patients with LUAD, and its diagnostic performance was compared with conventional serum biomarkers, including CEA, NSE, and CYFRA21-1. RESULTS: Exosomes released by CAFs showed elevated FAP expression, and FAP was associated with the exosomal marker CD9, supporting a possible CD9-associated exosomal sorting mechanism. In the clinical cohort, serum FAP⁺ exosome levels were significantly higher in LUAD than in HD and LBL, and increased with disease stage. In early-stage LUAD, serum FAP⁺ exosomes showed favorable diagnostic performance, with an AUC of 0.932, 88.4% sensitivity, and 87.6% specificity. Compared with conventional serum biomarkers, including CEA, NSE, and CYFRA21-1, serum FAP⁺ exosomes showed superior diagnostic performance in this cohort, while combined biomarker models provided only modest incremental improvement over FAP⁺ exosomes alone. Serum FAP⁺ exosome levels also decreased significantly after surgery, suggesting that this marker may reflect early postoperative changes. CONCLUSION: This study provides evidence that FAP is enriched in exosomes released by CAFs and may be incorporated into exosomes through a CD9-associated mechanism. We further established a serum FAP⁺ exosome-based flow cytometric assay that may serve as a promising minimally invasive strategy for LUAD diagnosis, disease progression assessment, and early postoperative monitoring.