Advancing FCRL5 directed immunotherapy for improved treatment of B cell lymphoma

Despite significant advances in B-cell lymphoma therapies including monoclonal antibodies, bispecific T-cell engagers (BiTEs), and CD19/CD20-directed chimeric antigen receptor (CAR) T-cell therapies disease relapse and treatment resistance remain major clinical challenges. The principal driver of therapeutic failure is antigen escape, characterized by the downregulation or loss of CD19 and/or CD20 on malignant B cells 1 , rendering current immunotherapies ineffective. These limitations underscore the urgent need for alternative target antigens and next-generation immunotherapeutic strategies capable of circumventing antigen loss. Fc receptor-like 5 (FcRL5), a transmembrane immunoglobulin superfamily protein selectively expressed on malignant B cells and plasma cells with minimal expression in normal tissues², represents a compelling tumor-associated antigen. We generated and functionally characterized FcRL5-targeted CAR T cells using nanobody-based single-domain antibodies (VHHs) derived from an immunized llama library. Anti-FcRL5 VHHs were incorporated into second-generation CAR constructs incorporating 4-1BB-CD3ζ signaling domains. CAR candidates were first screened in Jurkat NFAT-GFP reporter cells for surface expression, antigen-specific activation, and basal tonic signaling to identify top performers. Selected CAR constructs were further evaluated in primary human CD3+ T cells for FcRL5-dependent activation, cytotoxicity, cytokine secretion, and memory differentiation following co-culture with FcRL5+ diffuse large B-cell lymphoma (DLBCL) cell lines (Su-DHL-6, Toledo) and FcRL5 - controls. These analyses demonstrated robust and selective activation, potent effector function, and strong cytotoxicity against FcRL5-expressing lymphoma cells, highlighting their therapeutic potential. Collectively, these findings establish FcRL5-targeted nanobody-based CAR T cells as a promising next-generation immunotherapy capable of overcoming antigen escape in B-cell lymphomas. Evaluated alone or in combination with FDA-approved targets, FcRL5 CAR T cells hold strong potential for translation into first-in-human clinical trials for patients with relapsed or refractory disease. Funding: Supported by the SACF Drug Discovery Research Fund References: 1.Spiegel JY, et al. Nat Med 2021;27:1419-31.2.Capone M, et al. J Clin Cell Immunol 2016;7:427.