University of Veterinary Medicine Vienna Tumor Cell Plasticity in Equine Papillomavirus-Positive Versus-Negative Squamous Cell Carcinoma of the Head and Neck Squamous cell carcinoma of the head and neck (HNSCC) is a common malignant tumor in humans and animals. In humans, papillomavirus (PV)-induced HNSCCs have a better prognosis than papillomavirus-unrelated HNSCCs. The ability of tumor cells to switch from epithelial to mesenchymal, endothelial, or therapy-resistant stem-cell-like phenotypes promotes disease progression and metastasis. In equine HNSCC, PV-association and tumor cell phenotype switching are poorly understood. We screened 49 equine HNSCCs for equine PV (EcPV) type 2, 3 and 5 infection. Subsequently, PV-positive versus -negative lesions were analyzed for expression of selected epithelial (keratins, β-catenin), mesenchymal (vimentin), endothelial (COX-2), and stem-cell markers (CD271, CD44) by immunohistochemistry (IHC) and immunofluorescence (IF; keratins/vimentin, CD44/CD271 double-staining) to address tumor cell plasticity in relation to PV infection. Only EcPV2 PCR scored positive for 11/49 equine HNSCCs. IHC and IF from 11 EcPV2-positive and 11 EcPV2-negative tumors revealed epithelial-to-mesenchymal transition events, with vimentin-positive cells ranging between <10 and >50%. CD44- and CD271-staining disclosed the intralesional presence of infiltrative tumor cell fronts and double-positive tumor cell subsets independently of the PV infection status. Our findings are indicative of (partial) epithelial-mesenchymal transition events giving rise to hybrid epithelial/mesenchymal and stem-cell-like tumor cell phenotypes in equine HNSCCs and suggest CD44 and CD271 as potential malignancy markers that merit to be further explored in the horse. en TEXT article Cancer Stem-Cells; Epithelial-Mesenchymal Transition; Cadherin Down-Regulation; Beta-Catenin; Nuclear Translocation; Initiating Cells; Expression; Dna; Metastasis; Phenotype 2022 2022 Strohmayer, Carina (University of Veterinary Medicine Vienna) Brandt, Sabine (University of Veterinary Medicine Vienna) Kneissl, Sibylle (University of Veterinary Medicine Vienna) Redmer, Torben (University of Veterinary Medicine Vienna) Weissenbacher-Lang, Christiane (University of Veterinary Medicine) Jindra, Christoph (University of Veterinary Medicine Vienna) Walter, Ingrid (University of Veterinary Medicine Vienna) Klang, Andrea (University of Veterinary Medicine Vienna) Kummer, Stefan (University of Veterinary Medicine Vienna) MDPI