Costimulatory capacity of CD137 mAbs on T cells depends on elaborate CRD structures but not on blocking ligand‒receptor binding

CD137 is mainly a costimulatory receptor of CD8+ T cells. Two representative CD137 antibodies, utomilumab, and urelumab, show different costimulatory capacities in clinical trials. Balancing the antitumor effect and systemic toxicity of T cells activated by CD137 signaling is a challenge that requires clinical consideration. In this study, a panel of specific anti-human CD137 monoclonal antibodies (mAbs) were prepared and their affinities, isotypes, CD137-CRD (cysteine-rich domain) binding regions, cross-reactivity to mouse and rhesus CD137, inhibition of ligand‒receptor binding and costimulatory activities were analyzed. The results showed that anti-human CD137 mAbs had high cross-reactivity with rhesus CD137. MAbs fell into three clusters according to their different binding regions of the CD137 extracellular domain. They bound to CRDI+CRDII, CRDIII or CRDIV+STP. CRDIII-binding mAbs had the strongest blocking activity. Highly costimulatory CD137 mAbs showed stronger abilities to promote CD8+ T-cell proliferation. However, the costimulatory capacity of mAbs on T cells was not closely related to their ability to block CD137L-CD137 binding and may be controlled by more elaborate CRD conformational structures. This study provides additional information for the development of next-generation CD137 mAbs to meet clinical needs.