Programmed death-1 (PD-1) is a receptor with immunosuppressive function, and is upregulated on lymphocytes upon persistent antigen stimulation. A ligand for PD-1, PD-Ligand 1 (PD-L1), is expressed in various types of cancer cells, indicating that the PD-1/PD-L1 axis is a common mechanism for immune evasion of cancers. Interestingly, the blockade of this pathway enhances anti-tumor immune responses in humans and mice. Thus, immune checkpoint inhibitors, such as anti-PD-1 or PD-L1 antibodies, have been used for the treatment of human cancer, and considered promising immunotherapy for several types of malignancies. However, there are few reports on the involvement of the PD-1/PD-L1 pathway in anti-cancer immunity of dogs.
Molecular characterization and expression analysis of canine PD-1/PD-L1 were performed in order to determine whether this strategy can be applied to the treatment of canine cancers. Recombinant canine PD-1 was shown to bind to PD-L1-expressing cells, and its binding was blocked by an anti-PD-L1 antibody. Immunohistochemical analysis revealed that canine oral melanoma (36/40), osteosarcoma (7/10), hemangiosarcoma (6/10), mast cell tumor (3/5) and some other cancers express PD-L1, and higher PD-1 expression on tumor-infiltrating lymphocytes was observed in oral melanoma samples (n = 6, p < 0.05), determined by flow cytometer. Importantly, the treatment with an anti-PD-L1 antibody enhanced the production of interferon-gamma from tumor-infiltrating cells, suggesting a possible application of PD-1/PD-L1 inhibitors for the treatment of canine cancers. Therefore, therapeutic effect of anti-PD-L1 antibody should be further investigated in vivo.