The discovery and the clinical significance of autoantibodies against fibroblast activation proteins in interstitial lung disease

Interstitial lung disease (ILD) was a chronic, progressive, and irreversible fibrotic condition associated with high mortality. Current standard diagnosis relied primarily on invasive modalities such as high-resolution computed tomography (HRCT) and biopsy, highlighting the urgent need for a noninvasive and accurate screening approach. Fibroblast activation protein (FAP) was highly expressed in activated fibroblasts but minimally detected in quiescent normal fibroblasts; however, the presence of FAP-specific autoantibodies in serum and their potential association with ILD pathogenesis remained unreported. This study aimed to describe the first identification of FAP autoantibodies in the serum of ILD patients and evaluate their potential as novel blood-based biomarker for ILDs. Multiple complementary assays were used to detect FAP autoantibodies, including enzyme-linked immunosorbent assay (ELISA), western blotting, immunofluorescence staining, and flow cytometry. Chemiluminescence immunoassay was employed to measure the epithelial injury marker Krebs von den Lungen-6 (KL-6). Additionally, 18F-FAPI-04 positron emission tomography/computed tomography (PET/CT) imaging was performed to assess FAP-related signal intensity in ILD lesions, and the clinical trial registered in the Chinese Clinical Trial Registry (http://www.chictr.org.cn/, ChiCTR-IPR-2100045352) at 13 April 2021. We identified FAP autoantibodies in the serum of ILD patients and COVID-19 patients for the first time, with autoantibody levels correlating with disease progression. Furthermore, these autoantibodies, which belong predominantly to the IgG4 subclass, were detected in 25.8% of ILD patients with low epithelial injury markers (KL-6 < 500 U/mL) and in 75% of ILD patients without significant FAP-PET/CT signal (SUVtotal < 300), often regarded as normal or a mild form of the disease. FAP autoantibodies might serve as an early auxiliary diagnostic biomarker for pulmonary fibrosis, offering new possibilities for the diagnosis and management of ILDs.