Expression patterns of fibroblast activation protein and extra-domain B fibronectin in canine malignant tumors

Introduction: Fibroblast activation protein (FAP) is involved in the extracellular matrix (ECM) remodeling and wound healing. Absent in most adult tissues, it is overexpressed by neoplastic cells and/or cancer-associated fibroblasts (CAFs) in several human malignancies. The extra Domain-B of fibronectin (EDB+FN) is a splice variant of fibronectin involved in angiogenesis and tissue remodeling, overexpressed by CAFs and cancer-associated vessels (CAVs) in many aggressive human tumors. This study aims to investigate FAP and EDB+FN expressions in canine tumors and assess their potential as druggable targets in animal patients. Methods: FAP and EDB+FN expression was assessed by immunohistochemistry on 88 canine tumors, including Soft Tissue Sarcomas [STS], Osteosarcomas [OSA], Hemangiosarcomas [HSA], Apocrine Gland Anal Sac Adenocarcinomas [AGASAC], Mast Cell Tumors [MCT], Lymphomas, and Melanomas, using polyclonal and monoclonal anti-FAP and the L19 anti-EDB antibodies. Expression distribution and intensity were semi-quantitatively scored in neoplastic cells, CAFs, CAVs, and stroma. Results: FAP was variably expressed in neoplastic cells (79/88), CAFs (79/88), and CAVs (82/88) across all tumor types, but mostly in AGASACs, STSs, and MCTs. The monoclonal antibody presented greater specificity. EDB+FN expression was less present across tumor types, mostly with a vascular staining pattern. Labelling was most intense and consistent in the neoplastic cells, CAFs, and CAVs of melanomas, and to a lesser extent in AGASAC and STS. Discussion: STS, AGASAC, and MCT could be candidates for FAP-targeted strategies; melanomas are the most promising for EDB+FN-directed therapies. These results support FAP and EDB+FN as targets worth investigating for clinical applications in animal patients.