Abstract A033: <i>In situ</i> vaccination with <i>Flt3l</i> gene-modified CD103+ type 1 conventional dendritic cells (cDC1) in murine models of non-small cell lung cancer (NSCLC)

Abstract A major hurdle in treatment of Non-small cell lung cancer (NSCLC) with anti-PD-1 immune checkpoint blockade (ICB) therapy is a lack of response (primary resistance) and relapse after an initial response (acquired resistance). Recent studies reveal that responses to PD-1/PD-L1 blockade are associated with high tumor mutational burden (TMB), increased CD8+ T cell infiltration, and high baseline PD-L1 expression within the tumor microenvironment (TME), while impaired tumor antigen presentation and the immunosuppressive TME have been associated with ICB resistance. Numerous studies have established that the generation of an anti-tumor immune response driven by CD8+ T cells requires type I conventional dendritic cell (cDC1) mediated cross presentation of tumor associated antigens, which can license CD8+ T cells to initiate an anti-tumor immune response. In addition, previous publications have shown that systemic administration of the FMS-like tyrosine kinase 3 ligand (FLT3L) cytokine can expand endogenous cDC1s in the TME and augment anti-tumor immune responses to ICB therapy. Considering these data, we hypothesize that a viable approach to overcome NSCLC anti-PD-1 resistance is to intratumorally vaccinate tumors with Flt3l-gene modified cDC1s. Using lenti-viral transduction, we engineered murine CD103+ cDC1s to constitutively secrete FLT3L (FLT3L_cDC1) and performed in situ vaccination studies on murine models of NSCLC with Lkb1-deficiency and elevated TMB that better represents human disease. In situ vaccination with FLT3L_cDC1 promotes anti-tumor immune responses in NSCLC tumors that are non-responders to unmodified cDC1 vaccination and synergizes with anti-PD-1 ICB to significantly inhibit tumor growth. FLT3L_cDC1 therapy induces significant activation and expansion T cells within the TME at both an early and late timepoint post vaccination. Furthermore, FLT3L_cDC1 + anti-PD-1 combination therapy significantly increases DC progenitor numbers within the tumor draining lymph node, including DC progenitors that are committed to the cDC1 lineage. NSCLC tumor bearing mice cured following FLT3L_cDC1 + anti-PD-1 therapy independently reject rechallenge with the same NSCLC tumor model, suggesting that combination therapy promotes tumor-specific immune memory responses. Our data suggests in situ vaccination with FLT3L_cDC1 represents a promising strategy to potentiate the efficacy of ICB and can improve outcomes for patients with primary resistance to PD-1/PD-L1 monotherapy. Citation Format: Jensen Abascal, Ramin Salhi-Rad, Michael S Oh, William P Crosson, Camelia Dumitras, Bin Liu, Steven M Dubinett. In situ vaccination with Flt3l gene-modified CD103+ type 1 conventional dendritic cells (cDC1) in murine models of non-small cell lung cancer (NSCLC) [abstract]. In: Proceedings of the AACR IO Conference: Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2025 Feb 23-26; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2025;13(2 Suppl):Abstract nr A033.