PD-L1 expression and tumor-infiltrating T lymphocytes as prognostic factors in canine mammary tumors

The PD-1 protein is an immune checkpoint present on T cells and, when bound to PD-L1, it inhibits the immune response. Tumor cells can exploit this mechanism to escape immune surveillance. In this study, we characterized the expression of PD-L1 and the infiltration of cytotoxic T lymphocytes (CTLs) and regulatory T lymphocytes (RTLs) in 92 mammary tumors of 92 female dogs to assess their prognostic value. Tumor samples were subjected to immunohistochemistry for PD-L1, FOXP3, and CD8. FOXP3- and CD8-positive and negative lymphocytes were counted to obtain the percentage of positive cells. PD-L1 expression was evaluated for protein localization (nuclear and/or cytoplasmic), percentage of positive cells, and labeling intensity. The majority of the tumors were positive for PD-L1 (72%). Dogs with PD-L1-negative tumors had shorter post-surgical survival (<i>P</i> = .0328; hazard ratio = 2.35). PD-L1-positive tumors had higher percentages of CTLs and were significantly associated with clinical stage I (<i>P</i> = .0046) and absence of lymph node metastasis (<i>P</i> = .0006). An increased percentage of RTLs was an indicator of shorter survival (<i>P</i> = .0084). Our results suggest that PD-L1 positivity indicates a better prognosis for dogs with mammary carcinomas, whereas the presence of intratumoral RTLs is an indicator of poor prognosis. These findings highlight prognostic biomarkers that may support personalized treatment approaches in veterinary oncology.