Abstract B004: CD40L and FLT3L fusion proteins in engineered T cells drive myeloid reprogramming and durable antitumor immunity

Abstract Tumor-associated macrophages and dysfunctional dendritic cells accumulate in many solid and hematologic malignancies, where they suppress cytotoxic T cell activity, support tumor growth, and limit the efficacy of checkpoint inhibitors and conventional adoptive T cell therapies. Strategies that reprogram these myeloid populations toward pro-inflammatory, antigen-presenting phenotypes while simultaneously enhancing T cell intrinsic costimulation may broaden and deepen responses to cellular immunotherapy across cancer types. We developed a modular engineering platform that fuses a ligand ectodomain to a T cell costimulatory endodomain, generating Dual Costimulatory Receptors (DCRs) that can be incorporated into tumor-targeted T cells. In this study, T cells were engineered to express either (i) a CD40L-based DCR that engages CD40 on macrophages and dendritic cells to promote their activation and pro-inflammatory polarization or (ii) a FLT3L-based DCR designed to expand and license conventional dendritic cells and enhance cross-priming of endogenous antitumor T cells. Engineered T cells were evaluated in vitro in co-cultures with human tumor cell lines and suppressive myeloid populations and in vivo in syngeneic models of solid and blood cancers using CD40L or FLT3L DCRs with different endodomains. To enhance sustained surface expression, a novel cleavage-resistant CD40L DCR was designed. CD40L DCR-T cells showed improved resistance to exhaustion, reprogrammed inhibitory macrophages, and displayed increased antitumor activity. FLT3L-GITR DCR-T cells demonstrated enhanced intratumoral accumulation, acquisition of a tissue-resident memory phenotype, increased endogenous dendritic cell activation and tumor-reactive T cells, and superior therapeutic efficacy in two solid tumor models (melanoma and pancreatic cancer). Expression of CD40L- or FLT3L-based DCRs in engineered T cells offers a myeloid-focused strategy to integrate innate and adaptive immune modulation within a single cellular product. These data support further development of multi-DCR T cell therapies as a broadly applicable platform to remodel the tumor microenvironment, augment endogenous immunity, and improve the depth and durability of responses across a wide range of advanced malignancies. AI was used to copyedit this abstract. Citation Format: Shannon Oda, Ryma Toumi, Simonne Guenette. CD40L and FLT3L fusion proteins in engineered T cells drive myeloid reprogramming and durable antitumor immunity [abstract]. In: Proceedings of the AACR Immuno-Oncology Conference (AACR IO): Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2026 Feb 18-21; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2026;14(2 Suppl):Abstract nr B004.