Expression of Fibroblast Activation Protein in lungs of dogs with idiopathic pulmonary fibrosis and dogs with lung cancer

Canine Idiopathic Pulmonary Fibrosis (CIPF) is a progressive fibrotic interstitial lung disease of unknown aetiology, affecting predominantly the West Highland White Terrier (WHWT) breed. Currently, there is no curative treatment option available. Fibroblast Activation Protein (FAP) is a cell surface protease usually absent from normal tissue but specifically expressed in areas of active tissue remodelling such as in fibroblast foci in human idiopathic pulmonary fibrosis. In humans, it is also upregulated in various types of cancers, either in cancer-associated fibroblasts (CAFs), in cancer cells or in both, depending on the tumour type. The aim of this study was to assess the expression and localization of FAP in the lungs of WHWTs affected with CIPF, in comparison with WHWTs with healthy lungs and dogs with lung cancer.

Post-mortem formalin-fixed lung biopsies prepared from WHWTs with CIPF (n=17, age from 10 to 15y), control WHWTs exempt from lung disease (n=4, age from 11 to 15y) and dogs from various breeds with lung cancer (n=8, age from 8 to 14y) were retrospectively used. Included lung neoplasia were adenocarcinomas (n=6), histiocytic sarcoma (n=1) and metastasized mammary adenocarcinoma (n=1). Immunohistochemistry (IHC) was performed using a rabbit anti-human FAP monoclonal antibody (#ab207178). An IHC staining index (absent, low, moderate or high) was attributed according to the percentage of positive cells combined with the staining intensity.

FAP was identified in the lungs of 16 out of 17 (94%) WHWTs with CIPF (IHC index high, moderate, or low in respectively 10, 4 and 2 dogs), 2 out of 4 (50%) WHWTs with healthy lungs (1 of each moderate and low), and 7 out of 8 (88%) dogs with lung cancer (high and moderate in respectively 6 and 1 dogs). FAP was expressed by fibroblasts in areas of active fibrosis in CIPF and by CAFs (all types of cancer) and cancer cells (adenocarcinomas only, n=5) in lung tumours.

Results of this study showed that FAP is moderately to markedly expressed by fibroblasts in most dogs affected with either CIPF or lung cancer. Accordingly, FAP should be considered as an interesting potential therapeutic target for both diseases and should encourage further studies in the future. The expression of FAP in healthy lungs of WHWTs should be further investigated, particularly in comparison with FAP expression in dogs from other breeds, as it might serve as an indicator of early fibrosis.