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 T-lymphocyte-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.

To Cite This Article: Hacıoğlu O, Karabulut TSFT and Sönmez K, 2026. 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. Pak Vet J, 2026, 46(10): 2531-2539. http://dx.doi.org/10.29261/pakvetj/2026.229