Dissemination of T Lymphocytes and Potential Immune-Escape Mechanisms of Bovine Skin Squamous Cell Carcinoma Regarding MHC-1 and PD-L1 Expression.

Abstract

Background: Bovine skin squamous cell carcinoma (SCC) is a malignant neoplasm in cattle triggered by sunlight exposure. Immune escape drives tumour survival and progression. Previous findings in humans have emphasised the prognostic value of immune checkpoint marker (PDL-1), major histocompatibility complex (MHC-1) and cytotoxic CD8 T cells in the context of cancer immune escape.

Objectives: We examined the distribution of CD8⁺ T cells and the possible immune-escape mechanisms of SCC in relation to PD-L1 and MHC-1 expression and correlated marker expression with tumour grade.

Animals: Twenty samples of bovine skin SCC were collected between 2025 and 2026.

Materials and methods: Histopathological evaluation, including grading, was performed on SCC samples. Immunohistochemical examination was performed for all three proteins in relation to histological grade, substantiated by the mRNA levels of MHC-1 and PD-L1.

Results: We observed a significant relationship between PD-L1 expression and grade, increasing from 33% in well-differentiated SCC to 100% in moderately and poorly differentiated SCC. PD-L1 mRNA levels in well- and moderately differentiated SCC remained comparable, whereas poorly differentiated SCC exhibited a significant upregulation. MHC-1 showed a significant grade-dependent decline, decreasing from 83% in well differentiated to complete loss in poorly differentiated. This progressive protein loss was corroborated at the transcriptional level. High CD8 T-cell levels were observed in 83% of well-differentiated SCC, with complete absence in poorly differentiated SCC.

Conclusions and clinical relevance: This study emphasises the importance of PD-L1, MHC-1 and CD8 T-cell biomarkers as prognostic indicators in SCC. High PD-L1, low MHC-1 and CD8 represent potential immune-escape mechanisms for bovine SCC.