Limited Cross-Linking of 4-1BB by 4-1BB Ligand and the Agonist Monoclonal Antibody Utomilumab

Monoclonal antibodies (mAbs) targeting the co-stimulatory molecule 4-1BB are of interest for tumor immunotherapy. We determined the complex structures of human 4-1BB with 4-1BB ligand (4-1BBL) or utomilumab to elucidate the structural basis of 4-1BB activation. The 4-1BB/4-1BBL complex displays a typical TNF/TNFR family binding mode. The structure of utomilumab/4-1BB complex shows that utomilumab binds to dimeric 4-1BB with a distinct but partially overlapping binding area with 4-1BBL. Competitive binding analysis demonstrates that utomilumab blocks the 4-1BB/4-1BBL interaction, indicating the interruption of ligand-mediated signaling. The binding profiles of 4-1BBL and utomilumab to monomeric or dimeric 4-1BB indicate limited cross-linking of 4-1BB molecules. These findings provide mechanistic insight into the binding of 4-1BB with its ligand and its agonist mAb, which may facilitate the future development of anti-4-1BB biologics for tumor immunotherapy.