PROJECT SUMMARY/ABSTRACT B cells are immunopathogenic drivers of systemic lupus erythematosus (SLE), an autoimmune disease characterized by a large repertoire of autoantibodies targeting self-protein and nucleic acids. While B cell- directed T-cell therapies have curative potential in the treatment of B-cell cancers and can achieve complete disease remission in refractory lupus, therapies that deplete all B cells are associated with excess morbidity and mortality from infection, precluding their use beyond life-threatening disease. Precision therapies that selectively target autoreactive B cells that drive SLE, while preserving normal B cell populations, are therefore critically needed, but targeting the plethora of autoreactive B cells in SLE poses practical challenges. The autoreactive B cell compartment in SLE is uniquely characterized by the expansion B cells bearing B-cell receptors (BCRs) using the variable heavy-chain allele VH4-34, which encodes the idiotype 9G4 (9G4id B cells). 9G4id B cells contribute 10–45% of total IgG in patients with active SLE, including antibodies against canonical lupus autoantigens. 9G4id B cells are therefore promising targets for the selective depletion of the autoreactive B cell pool, opening opportunities to treat lupus without increasing the risk of infection. Here, we hypothesize that autologous T cells can be redirected to selectively bind and kill 9G4id B cells expanded in SLE. To achieve this, we will develop 9G4id-targeted bispecific T cell-engaging antibodies designed to cross-link 9G4id B cells with any T cell, thereby inducing killing of target B cells. In this proposal, we will graft single chain variable fragments of an anti-9G4id-specific antibody and those of well-characterized CD3-specific antibodies (i.e., OKT3, UCHT1v9) into different therapeutic bispecific formats (i.e., BiTE and scDb). The efficacy and specificity of anti-9G4 bispecific antibodies to selectively bind and deplete 9G4id B cells will be established in fully-controlled in vitro model systems using engineered human B cell lines expressing 9G4id autoreactive BCRs cloned from patients with SLE, human B cell lines expressing monoclonal non-9G4id BCRs, and using autologous B cells and T cells isolated from the peripheral blood of patients with SLE. The final goal of this work is to lay the foundation for evaluating these precision immunotherapies as a novel therapeutic strategy in SLE.