Background: B cell-targeting chimeric antigen receptor T-cell (CAR-T) therapies have shown promising clinical activity in various autoimmune diseases, such as systemic lupus erythematosus and systemic sclerosis. However, their clinical adoption is limited by several factors, including manufacturing and logistical complexity, safety concerns, and retreatment challenges. T-cell engagers (TCEs) offer a more flexible off-the-shelf therapeutic approach. Given that autoimmunity may involve pathogenic B cells at different developmental stages with distinct functions, innovative approaches that target a broader B-cell lineage are of great interest. Here we report the development of a CD19 × BCMA × CD3 trispecific TCE designed to deplete both CD19 ⁺ B cells and BCMA ⁺ plasma cells, which may deliver more effective and durable disease control with a favorable safety profile.

Objectives: To characterize the preclinical properties of a CD19 × BCMA × CD3 trispecific antibody designed to deplete a broad spectrum of the B-cell lineage with reduced cytokine release.

Methods: A CD19×BCMA×CD3 trispecific antibody was generated with carefully selected proprietary building block antibodies with a unique format and anti-CD3 positioning. Target cell binding, T-cell activation, and PBMC-mediated cytotoxicity were evaluated. CD19-mediated B-cell depletion in peripheral blood and tissue was assessed using human CD19/CD3 transgenic mice. Depletion of BCMA ⁺ cells was separately evaluated in human PBMC-reconstituted mice bearing H929 multiple myeloma cell line. B-cell depletion and cytokine release were further analyzed in non-human primates.

Results: The trispecific antibody showed potent binding to human CD19 ⁺ and BCMA ⁺ cells. A CD3-masked design limited CD3 ⁺ cell binding and T-cell activation in the absence of target B cells. Potent T-cell-mediated killing was consistently observed against CD19 ⁺ or BCMA ⁺ cell lines and autologous B cells in human PBMCs across multiple donors. In human CD19/CD3 transgenic mice, rapid and deep B-cell clearance was achieved and sustained for up to 7 weeks, with complete depletion in lymph nodes and bone marrow by day 14. In the BCMA ⁺ multiple myeloma mouse model, the antibody induced significant tumor regression while maintaining stable body weight. Preliminary studies in cynomolgus monkeys showed deep B-cell depletion and T-cell activation, with low and transient cytokine release.

Conclusions: Dual targeting of CD19 and BCMA by a novel T-cell engager leads to deep and durable B-cell depletion in preclinical models. The unique trispecific format enables low and transient cytokine release. These data support its potential as a promising therapeutic candidate with improved efficacy and a favorable safety profile for the treatment of various autoimmune diseases.

REFERENCES: NIL.

Acknowledgments: NIL.

Disclosure of Interests: None declared.