Interdomain spacing and spatial configuration drive the potency of IgG-[L]-scFv T cell bispecific antibodies

T cell-bispecific antibodies (BsAbs) couple cytotoxic T lymphocytes to tumor cells, inducing their destruction. Although there are more than 60 classes of BsAbs in development, the relative importance of parameters such as interdomain spacing or spatial configuration is largely unknown. Here, we dissected a symmetric dual bivalent BsAb platform (IgG-[L]-scFv: antitumor IgG with anti-CD3 scFv fused to the light chains) to explore the importance of valency and spatial configuration for BsAb-induced T cell cytotoxicity. Our results revealed that placing tumor and T cell binding domains on the same side of a BsAb (cis-configuration) elicited substantially stronger antitumor activity, in vitro and in vivo, compared to positioning them on opposite sides (trans-configuration). Moreover, using two cis-modules in the same BsAb further improved cytotoxicity (up to 2000-fold). In addition, separating antigen-binding components with a single Ig domain (C_L) markedly enhanced cytokine release and in vivo tumor responses compared to smaller (G₄S₁) or larger (C_H1-C_H2-C_H3) spacers. These findings provide guidelines for improving BsAb function and highlight the importance of spatial configuration and dual bivalency as development parameters.