Discovery and functional characterization of novel programmed death-ligand 1 monoclonal antibodies.

Background/aim: Targeted therapies with monoclonal antibodies provide cancer patients with better prognosis and disease-free survival. The blockade of immune checkpoints, including programmed cell death protein-1 (PD-1) and its ligand PD-L1, with monoclonal antibodies may boost immune responses against tumors and is regarded as an effective strategy in cancer immunotherapy. We describe the generation of anti-PD-L1 monoclonal antibodies with high affinity and specificity, and we assess their potential for therapeutic use in cancer. Materials and methods: Hybridomas were selected for PD-L1 specificity and cross-reactivity with other immune checkpoint proteins and PD-L1 orthologs using indirect ELISA. Immunofluorescence and Western blotting assays were conducted for further characterization of the antibodies. The affinities of the antibodies for PD-L1 were determined using surface plasmon resonance. Receptor blocking activities were examined through competitive ELISA and cell-based luciferase reporter assays. Sequences of variable regions of the selected antibodies were determined by Sanger sequencing and subjected to BLAST analysis. Results: A total of 25 PD-L1-specific monoclonal antibodies were generated. While most clones reacted with PD-L1 from cynomolgus monkeys, none of the antibodies displayed cross-reactivity with other checkpoint proteins. Immunofluorescence assays showed that the selected clones stained PD-L1-expressing cell membranes specifically, but not those of PD-L1-negative cells. Western blotting revealed that most of the clones recognized both glycosylated and nonglycosylated PD-L1, and a few reacted with the glycosylated form only. Only two clones with subnanomolar affinity for human PD-L1 were effective at blocking PD-1/PD-L1 and CD80/PD-L1 interactions. Sequence analysis of their variable regions revealed their unique specificity. Conclusion: Of the 25 monoclonal antibodies produced in this study, only one was identified as a potential therapeutic drug candidate thanks to its high capacity for checkpoint blockade and affinity, as well as its unique sequence specificity. These properties are comparable to those of anti-PD-L1 antibodies currently used in clinical practice.