Abstract
BACKGROUND: Canine cutaneous mast cell tumors (MCTs) are a common, yet clinically challenging tumor type given their variable biological behavior. Although patients with low grade MCTs can often be effectively managed with surgery alone, most dogs with high grade MCTs succumb to their disease despite multimodal therapy. An improved understanding of the immune tumor microenvironment (TME) may help identify novel prognostic and therapeutic targets. METHODS: In this study, we interrogated the immune transcriptional profiles of the TME in low and high grade MCTs, and quantified intratumoral T cells. Twelve client-owned dogs with MCTs (6 Kiupel low grade with clinically benign behavior and 6 Kiupel high grade with clinically aggressive behavior) that underwent curative-intent surgery were selected. Tumor grade was confirmed by a single veterinary pathologist. RNA was extracted from all tumors followed by immune transcriptional profiling utilizing the NanoString Canine IO panel and analysis using the ROSALIND platform. T cell density was determined by immunohistochemical staining for CD3 and quantified using ImageScope software (Leica Biosystems) following digital slide capture. Lymphocytic infiltrate was further characterized in the TME of one high grade MCT using co-immunofluorescence. RESULTS: Immune transcriptional profiling identified 9 differentially expressed genes between low and high grade MCTs (p-adj < 0.05). Programmed cell death protein 1 (PDCD1) and inducible T-cell costimulator ligand (ICOSLG) gene expression were significantly higher in a subset of high grade MCTs. ICOSLG expression positively correlated with T cell score (r(s) = 0.6434, p = 0.0278). Although the T cell density was not significantly different between low (mean of 76.42 CD3 + /mm(2), SD 12 CD3 + /mm(2)) and high grade MCTs (mean of 129.1 CD3 + /mm(2), SD 96.06 CD3 + /mm(2)), greater variation of T cell densities was observed across high grade MCTs compared to low grade (p = 0.0059). Immunofluorescence of one high grade MCT with marked T cell infiltration revealed organized aggregates of T and B cells consistent with tertiary lymphoid structures (TLS). CONCLUSIONS: Our data revealed significant differences in the immune TME of low and high grade MCTs and provides rationale to further investigate potential prognostic and therapeutic roles of immune checkpoints in canine MCTs. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s44356-025-00020-9.