Development and characterization of mouse anti-canine PD-L1 monoclonal antibodies and their expression in canine tumors by immunohistochemistry in vitro.

Immune escape is the hallmark of carcinogenesis. This widely known mechanism is the overexpression of immune checkpoint ligands, such as programmed cell death protein 1 and programmed death-ligand 1 (PD-1/PD-L1), leading to T cell anergy. Therefore, cancer immunotherapy with specific binding to these receptors has been developed to treat human cancers. Due to the lack of cross-reactivity of these antibodies in dogs, a specific canine PD-1/PD-L1 antibody is required. The aim of this study is to develop mouse anti-canine PD-L1 (cPD-L1) monoclonal antibodies and characterize their in vitro properties. Six mice were immunized with recombinant cPD-L1 with a fusion of human Fc tag. The hybridoma clones that successfully generated anti-cPD-L1 antibodies and had neutralizing activity were selected for monoclonal antibody production. Antibody properties were tested by immunosorbent assay, surface plasmon resonance, and immunohistochemistry. Four hybridomas were effectively bound and blocked to recombinant cPD-L1 and cPD-1-His-protein, respectively. Candidate mouse monoclonal antibodies worked efficiently on formalin-fixed paraffin-embedded tissues of canine cancers, including cutaneous T-cell lymphomas, mammary carcinomas, soft tissue sarcomas, squamous cell carcinomas, and malignant melanomas. However, functional assays of these anti-cPD-L1 antibodies need further investigation to prove their abilities as therapeutic drugs in dogs as well as their applications as prognostic markers.