Fig 2: Box-and-whisker plots of collagen content (A) and of FAP-positive cells (B) in areas representing either active (n = 10) or mature fibrosis (n = 10), calculated with quantitative digital analysis in CIPF lung sections. The box represents the median and interquartile range. The whiskers represent the values within 1.5 times the interquartile range. Significance level: *** indicates a p-value below 0.001. FAP, fibroblast activation protein.

Fig 3: Panel showing sequential sections in HE staining (1st row), Picro Sirus red staining (2nd row), anti-FAP immunohistochemistry staining (3rd row) and the superimposition of cell detections (blue: FAP-positive, green: FAP-negative) based on anti-FAP immunohistochemistry onto Picro Sirus red-stained collagen (4th row). Images (A–D) show an area of strongly collagenic mature fibrosis with rare FAP-positive cells (bar: 50 μm). Images (E–H) illustrate an area of active fibrosis with low collagen content and numerous FAP-positive cells (bar: 50 μm). Images (I–L) show an entire section with mixed fibrosis pattern: few FAP-positive cells within highly collagenic mature fibrosis areas, from which less collagenic, FAP-rich areas extend (bar: 500 μm). FAP, fibroblast activation protein.

Fig 4: Box-and-whisker plot of plasma levels of soluble FAP in dogs with CIPF (n = 6) and in healthy dogs (n = 9). The box represents the median and interquartile range, and the whiskers represent the values within 1.5 times the interquartile range. Significance level: ** indicates a p-value below 0.01. FAP, fibroblast activation protein; CIPF, canine idiopathic pulmonary fibrosis.

Fig 5: Comparison of FAP immunostaining in canine lung biopsies. No FAP expression in healthy lung (A) and high FAP expression in CIPF (B) and primary pulmonary adenocarcinoma, either in cancer-associated fibroblasts (C) or in cancer cells (D). Immunoperoxidase-diaminobenzidine, hematoxylin counterstain (bar: 100 μm). FAP, fibroblast activation protein; CIPF, canine idiopathic pulmonary fibrosis.