Clinicopathologic and immunophenotypic characterization of canine cutaneous mast cell tumors: Associations with tumor grade and the tumor–immune microenvironment

Canine cutaneous mast cell tumors (MCTs) exhibit heterogeneous biological behaviors, with increasing evidence implicating the tumor immune microenvironment (TIME) in disease progression. This study evaluated biopsy-confirmed cases of canine MCTs to investigate associations between tumor grade and host factors, and to characterize grade-related differences in immune marker expression. Tumors were histologically graded by the Kiupel system and analyzed using immunohistochemistry for CD4, CD8, CD25, CD57, MHC I, MHC II, PD-1, PD-L1, and mast cell tryptase. Quantitative assessments included positive cell counts, percentages, H-scores, total stained areas, and inter-marker correlations. Demographic analysis revealed significant associations between tumor grade and both age and sex, whereas breed and anatomical location showed no statistical correlation. Low grade (LG) tumors demonstrated greater infiltration of CD4⁺, CD8⁺, CD57⁺ T cells and higher MHC I/II expression, consistent with enhanced antigen presentation and immune activity. In contrast, high grade (HG) tumors exhibited increased CD25⁺, PD-1⁺, and PD-L1⁺ expression, suggesting a more immunosuppressive phenotype. Correlation analysis highlighted coordinated immune marker expression in LG tumors and disrupted immune architecture in HG tumors. These findings reveal distinct immunophenotypic and epidemiologic features across MCT grades and underscore the potential of immune profiling for prognostication and therapeutic planning in canine oncology.