IGLV3-21<sup>R110</sup>-directed bispecific antibodies activate T cells and promote killing in a high-risk subset of chronic lymphocytic leukemia

We previously used a disease-specific B cell receptor (BCR) point mutation (IGLV3-21R110) for selective targeting of a highrisk subset of chronic lymphocytic leukemia (CLL) with chimeric antigen receptor (CAR) T cells. Since CLL is a disease of the elderly and a significant fraction of patients is not able to physically tolerate CAR T-cell treatment, we explored bispecific antibodies as an alternative for precision targeting of this tumor mutation. Heterodimeric IgG1-based antibodies consisting of a fragment crystallizable region (Fc) attached to both an anti-IGLV3-21R110 Fab and an anti-CD3 (UCHT1) single chain variable fragment (R110-bsAb) selectively killed cell lines engineered to express high levels of the neoepitope as well as primary CLL cells using healthy donor and CLL patient-derived T cells as effectors. R110-bsAb spared polyclonal human B cells (as opposed to CD19-targeting blinatumomab) as well as CD34+ human stem cells. Yet, R110-bsAb induced lower T-cell activation than blinatumomab with primary CLL cells likely due to lower expression of target antigen. In vivo, R110- bsAb specifically killed IGLV3-21R110-expressing cell lines and CLL cells while sparing peripheral blood mononuclear cells. These findings highlight bispecific antibodies as a potential off-the-shelf immunotherapy for high-risk CLL patients, offering selective targeting while preserving healthy B cells.