Investigation of Estrogen Receptor beta (ER-β), Progesterone Receptor (PR), Human Epidermal Growth Factor Receptor 2 (HER2), Programmed Death-Ligand 1 (PD-L1) and Cytotoxic T-Lymphocyte Associated Protein 4 (CTLA-4) Expression Across Histopathological Subtypes of Canine Mammary Masses

Immunohistochemical findings in canine mammary masses (n=62) for estrogen receptor beta (ER-β), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), programmed death-ligand 1 (PD-L1), and cytotoxic Tlymphocyte-associated protein 4 (CTLA-4) are presented in this study.Expression of CTLA-4 differed between malignant and non-malignant masses (P<0.05), while HER2 expression differed between Grade II and Grade III tumors (P<0.05).Among the malignant mammary tumors, triple-negative mammary tumors (TNBC) accounted for 8.2%.Their PD-L1 expression scores were +2.5 in three cases and absent in one case.An association was found between PD-L1 expression and neutrophil-to-lymphocyte ratio (P<0.001).Our results revealed that HER2 expression may have prognostic value in high-grade tumors.Further investigation of immune checkpoint pathways in canine TNBC is needed for future treatment plans in female dogs with mammary tumor.