Targeting Tumor Immunity: The Discovery and Characterization of a Novel CD137xPD-L1 Bispecific IgG1 Antibody

CD137, a costimulatory receptor found on T and NK cells, is crucial for enhancing immune responses against tumors. Its signaling is typically initiated by receptor clustering, which can be achieved by the CD137 ligand or through the use of agonistic antibodies. However, developing CD137-targeted cancer therapies has been challenging due to toxicity concerns with agonistic antibodies and limited effectiveness with crosslinking antibodies.

A novel approach has led to the identification of a highly selective CD137xPD-L1 bispecific antibody (bAb), MCLA-145, through an extensive functional screening process. This bAb was created by combining various CD137 and PD-L1 Fab fragments, resulting in a diverse set of antibodies. The screening process involved testing these antibodies on reporter cell lines that express the respective receptors.

MCLA-145 is unique in that it requires the presence of PD-L1 on a neighboring cell to activate CD137, meaning it functions in a 'trans' manner. Flow cytometry showed that MCLA-145 can cross-react with cynomolgus monkey CD137 and PD-L1. The CD137 portion of the antibody inhibits the interaction between CD137 and its ligand, while the PD-L1 portion blocks the PD-1/PD-L1 interaction. These blocking activities were confirmed through competition assays and ELISA.

Epitope mapping using a flow-based screen and hydrogen-deuterium exchange experiments revealed the specific binding sites on CD137 and PD-L1 for MCLA-145. The CD137 binding domain was identified as the ligand binding domain (CRDII), and the PD-L1 binding site was in the PD-1 binding N-terminal V domain. The binding affinities were determined to be in the low nanomolar range for CD137 and subnanomolar for PD-L1, with surface plasma resonance (SPR) confirming the simultaneous binding to both proteins.

The distinct binding characteristics of MCLA-145 suggest a potential for a broader therapeutic application by activating CD137-expressing cells in the tumor microenvironment where PD-L1 is present, while also blocking the inhibitory PD-1/PD-L1 pathway. This could lead to an improved safety profile and enhanced antitumor activity.