Discovery and characterization of a monoclonal antibody targeting a conformational epitope of IL-6/IL-6Rα to inhibit IL-6/ IL-6Rα/gp130 hexameric signaling complex formation

The functional interleukin 6 (IL-6) signaling complex is a hexameric structure composed of IL-6, IL-6Rα, and the signaling receptor gp130. There are three different modes of IL-6 signaling, classic signaling, trans-signaling, and trans-presentation, which are not functionally redundant and mediate pleiotropic effects on both physiological and pathophysiological states. Monoclonal antibodies against IL-6 or IL-6Rα have been successfully developed for clinical application. However, designing therapeutic interventions that block specific modes of IL-6 signaling in a pathologically relevant manner remains a great challenge. Here, we constructed a fusion protein Hyper-IL-6 (HyIL-6) composed of human IL-6 and IL-6Rα to develop specific blocking antibodies against the IL-6/IL-6Rα complex. We successfully screened the monoclonal antibody C14mab, which can bind to HyIL-6 with the binding constant 2.86 × 10-10 and significantly inhibit IL-6/IL-6Rα/gp130 complex formation. In vitro, C14mab effectively inhibited HyIL-6-stimulated signal transducer and activator of transcription 3 (STAT3) activation and related vascular endothelial growth factor (VEGF) induction. Moreover, C14mab efficaciously suppressed HyIL-6-induced acute phase response in vivo. Our data from hydrogen-deuterium exchange mass spectrometry demonstrate that C14mab mainly binds to site IIIa of IL-6 and blocks the final step in the interaction between gp130 and IL-6/IL-6Rα complex. Additionally, data from enzyme-linked immunosorbent assays and kinetics assays indicate that C14mab interacts simultaneously with IL-6 and IL-6Rα, while it does not interact with IL-6Rα alone. The unique features of C14mab may offer a novel alternative for IL-6 blockade and illuminate a better therapeutic intervention targeting IL-6.