Discovery and characterization of potent IL-21 neutralizing antibodies via a novel alternating antigen immunization and humanization strategy.

Interleukin-21 (IL-21), a member of the common cytokine receptor γ chain (γc) family, is secreted by CD4+ T cells and natural killer T cells and induces effector function through interactions with the IL-21 receptor (IL-21R)/γc complex expressed on both immune and non-immune cells. Numerous studies suggest that IL-21 plays a significant role in autoimmune disorders. Therapeutic intervention to disrupt the IL-21/IL-21R/γc interaction and inhibit subsequent downstream signal transduction could offer a treatment paradigm for these diseases. Potent neutralizing antibodies reported in the literature were generated after extensive immunizations with human IL-21 alone and in combination with various adjuvants. To circumvent the laborious method of antibody generation while targeting a conserved functional epitope, we designed a novel alternating-antigen immunization strategy utilizing both human and cynomolgus monkey (cyno) IL-21. Despite the high degree of homology between human and cyno IL-21, our alternating-immunization strategy elicited higher antibody titers and more potent neutralizing hybridomas in mice than did the immunization with human IL-21 antigen alone. The lead hybridoma clone was humanized by grafting the murine complementarity-determining regions onto human germline framework templates, using a unique rational design. The final humanized and engineered antibody, MEDI7169, encodes only one murine residue at the variable heavy/light-chain interface, retains the sub-picomolar affinity for IL-21, specifically inhibits IL-21/IL-21R-mediated signaling events and is currently under clinical development as a potential therapeutic agent for autoimmune diseases. This study provides experimental evidence of the immune system's potential to recognize and respond to shared epitopes of antigens from distinct species, and presents a generally applicable, novel method for the rapid generation of exceptional therapeutic antibodies using the hybridoma platform.