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Mast cell tumors (MCTs) are common skin neoplasms in dogs. They are graded histologically, and dogs with high-grade MCT have poorer prognosis. Canine MCTs express mRNA for programmed cell death-1 ligand 2 (PD-L2), which regulates T cell activity and serves as a prognostic indicator for various human tumors. We hypothesized that PD-L2 contributes to immune evasion and affects the prognosis of canine MCTs. PD-L2 expression was evaluated by immunohistochemistry in 38 canine MCTs, including 15 Kiupel low-grade and 23 high-grade cases. PD-L2 was primarily detected on tumor cell surfaces, and most cells in low-grade MCTs (median percentage of PD-L2-positive tumor cells: 83.5%) expressed PD-L2. In contrast, PD-L2 expression was highly variable in high-grade MCTs (median: 25.5%), and cases with fewer PD-L2-positive tumor cells had poorer prognosis than those with more (median survival time: 148 vs. 468 days). This unexpected result led us to examine the novel hypothesis that PD-L2 on tumor cells negatively regulates behavior of tumor cells by receiving reverse signals. Consistent with this, RNA-sequencing detected altered expression of 70 genes, following adenovirus-mediated PD-L2 gene transfer in the canine MCT cell line BR and subsequent soluble PD-1 exposure. Moreover, PD-1-PD-L2 reverse signaling suppressed cell proliferation in PD-L2-expressing BR cells. Collectively, our findings demonstrate the prognostic significance of PD-L2 expression in dogs with high-grade MCTs and suggest that cell proliferation of PD-L2-positive tumor cells may be regulated by PD-L2 reverse signaling.
Research papers (academic journals)
