PD-L1, PD-1, and CTLA-4 mRNA In Situ Expression by Canine Oral Melanoma Cells and Immune Cells of the Tumour...

Area of Science:

• Veterinary Oncology
• Immunology
• Cancer Research

Background:

• Canine oral melanoma (OM) has a poor prognosis and limited treatment options.
• Immune checkpoint inhibitors (ICIs) show promise in human melanoma, but their efficacy in dogs is hindered by unclear immunosuppressive mechanisms in OM.
• Understanding these mechanisms is crucial for developing effective canine cancer therapies.

Purpose of the Study:

• To evaluate the expression of immune checkpoints PD-1, PD-L1, and CTLA-4 in canine OM using RNAscope in situ hybridization (ISH).
• To investigate the expression patterns of these checkpoints and their potential role in melanoma progression.
• To explore the utility of ISH as a biomarker for ICI treatment selection in canine OM.

Main Methods:

• Utilized RNAscope in situ hybridization (ISH) on 24 formalin-fixed, paraffin-embedded canine OM samples.
• Performed dual ISH-immunohistochemistry with specific markers (Melanoma Triple Cocktail, CD3, CD20, Iba1) to identify cell types expressing checkpoints.
• Analyzed expression patterns and correlated them with clinical parameters like mitotic count, disease-free interval, and overall survival.

Main Results:

• PD-L1 expression was detected in 100% of tumors, primarily on tumor-associated macrophages and at the host-tumor interface.
• PD-1 and CTLA-4 were found in 54% and 75% of tumors, respectively, predominantly on tumor-infiltrating T lymphocytes.
• PD-1 expression in neoplastic cells correlated with mitotic count, but no checkpoint expression correlated with survival outcomes.

Conclusions:

• PD-L1, PD-1, and CTLA-4 likely contribute to the immunosuppressive tumor microenvironment in canine oral melanoma.
• The expression patterns suggest distinct roles for these checkpoints in immune evasion.
• Further studies are needed to determine if ISH can serve as a predictive biomarker for ICI therapy in canine OM.