Comprehensive Analysis of Fibroblast Activation Protein Expression in Interstitial Lung Diseases

Abstract

Rationale Sustained activation of lung fibroblasts and the resulting oversynthesis of the extracellular matrix are detrimental events for patients with interstitial lung diseases (ILDs). Lung biopsy is a primary evaluation technique for the fibrotic status of ILDs, and is also a major risk factor for triggering acute deterioration. Fibroblast activation protein (FAP) is a long-known surface biomarker of activated fibroblasts, but its expression pattern and diagnostic implications in ILDs are poorly defined. Objectives The present study aims to comprehensively investigate whether the expression intensity of FAP could be used as a potential readout to estimate or measure the amounts of activated fibroblasts in ILD lungs quantitatively. Methods FAP expression in human primary lung fibroblasts as well as in clinical lung specimens was first tested using multiple experimental methods, including real-time quantitative PCR (qPCR), Western blot, immunofluorescence staining, deep learning measurement of whole slide immunohistochemistry, as well as single-cell sequencing. In addition, FAP-targeted positron emission tomography/computed tomography imaging PET/CT was applied to various types of patients with ILD, and the correlation between the uptake of FAP tracer and pulmonary function parameters was analyzed. Measurements and Main Results Here, it was revealed, for the first time, FAP expression was upregulated significantly in the early phase of lung fibroblast activation event in response to a low dose of profibrotic cytokine. Single-cell sequencing data further indicate that nearly all FAP-positive cells in ILD lungs were collagen-producing fibroblasts. Immunohistochemical analysis validated that FAP expression level was closely correlated with the abundance of fibroblastic foci on human lung biopsy sections from patients with ILDs. We found that the total standard uptake value (SUV) of FAP inhibitor (FAPI) PET (SUVtotal) was significantly related to lung function decline in patients with ILD. Conclusions Our results strongly support that in vitro and in vivo detection of FAP can assess the profibrotic activity of ILDs, which may aid in early diagnosis and the selection of an appropriate therapeutic window.

Indocyanine-enhanced mouse model of bleomycin-induced lung fibrosis with hallmarks of progressive emphysema

“…Thus, even though at day 21 fibroblasts were still activated in BLM + ICG mice, as revealed by FAP-1α quantification, the levels of Collagen I were not significantly increased. Importantly, we showed that in vivo detection of FAP-1α can assess the fibrotic activity of BLM + ICG-mediated injury, as was recently reported for the BLM model ( 41 ). Because of the close temporal association between the development of high-grade fibrosis and the appearance of emphysema-like features, we hypothesize that mechanical traction on lung parenchyma, exerted by the accumulated fibrotic tissue, might be responsible for (or at least contribute to) the observed airspace enlargements and increase of hyperinflated areas.…”

Fibroblast Activating Protein: Skimming the Surface of Molecular Imaging to Assess Fibrotic Disease Activity

“…68 Ga-FAPI-04 PET/computed tomography has been performed in systemic sclerosis–associated ILD, and the degree of probe uptake was associated with disease progression ( 14 ). This study by Yang and colleagues ( 1 ) illustrates several important applications of molecular imaging to ILD. First, it can be used to obtain molecular information otherwise not available and thus advance our understanding of molecular heterogeneity within individuals and ILD subtypes.…”

“…In this issue of the Journal , Yang and colleagues (pp. 160–172 ) evaluate “fibroblast activating protein” (FAP) as a molecular marker of disease activity with the potential to be leveraged for prognostic purposes ( 1 ).…”

“…Here Yang and colleagues ( 1 ) evaluate FAP expression using human lung fibroblasts, a murine model of pulmonary fibrosis, single-cell sequencing, and a radiolabeled FAP inhibitor administered to patients with ILD. This study has several major findings.…”

FAPI PET/CT in infectious, inflammatory, and rheumatological diseases: “watch it like a hawk” or “one swallow does not make a summer”?

“…FAPI PET/CT correlated well with endoscopic, computed tomography enterography, and disease biomarkers. It was highly sensitive in the detection of different classes of lesions in all intestinal segments Treutlein et al [ 8 ] 2023 Germany Systemic sclerosis 14 [ 68 Ga]Ga-FAPI PET/CT may be a diagnostic option to monitor cardiac fibroblast activity being able to visualize myocardial fibrosis related to systemic sclerosis Yang et al [ 9 ] 2023 China Interstitial lung diseases 83 [ 68 Ga]Ga-FAPI PET/CT can assess the profibrotic activity of interstitial lung diseases. Radiolabelled FAPI uptake was significantly related to lung function decline in patients with interstitial lung diseases Röhrich et al [ 10 ] 2022 Germany Interstitial lung diseases 15 [ 68 Ga]Ga-FAPI PET/CT is a promising method for detecting fibrotic interstitial lung diseases and associated lung cancer Wang et al [ 11 ] 2022 China Light-chain amyloidosis 30 [ 68 Ga]Ga-FAPI PET/CT is feasible in detecting myocardial fibroblast activation in patients with light-chain cardiac amyloidosis in correlation with myocardial remodeling.…”