B111-25 Flt3l-secreting Cdc1 In Situ Vaccination Induces Tertiary Lymphoid Structures And Synergizes With Pd-1 Blockade In Murine Non-small Cell Lung Cancer

Rationale

Non-small cell lung cancer (NSCLC) frequently evades immune surveillance through defective antigen presentation and a suppressive tumor microenvironment (TME), limiting the efficacy of immune checkpoint blockade (ICB). Conventional type 1 dendritic cells (cDC1s) are essential for initiating anti-tumor CD8⁺ T cell responses, yet their abundance and function are often diminished in NSCLC. FMS-like tyrosine kinase 3 ligand (FLT3L) is a critical regulator of cDC1 development and activation. We hypothesized that localized delivery of FLT3L via gene-modified cDC1s could reprogram the TME, enhance endogenous antigen presentation, and overcome resistance to ICB.

Methods

We developed an in situ vaccination (ISV) strategy using murine cDC1s genetically engineered to secrete FLT3L (FLT3L-cDC1). FLT3L-cDC1 ISV was administered intratumorally in multiple syngeneic murine NSCLC models, including LKB1-deficient tumors. Tumor growth, survival, and immune memory were assessed following monotherapy and combination treatment with anti-PD-1 antibody. Immune profiling was performed using flow cytometry, multiplex immunofluorescence, and transcriptional analyses of tumors and tumor-draining lymph nodes. Correlative analyses were conducted using The Cancer Genome Atlas (TCGA) NSCLC datasets to evaluate associations between FLT3L expression, immune cell signatures, and tertiary lymphoid structure (TLS)-associated programs.

Results

FLT3L-cDC1 ISV reshaped the immune landscape in multiple murine NSCLC models, rapidly inducing T lymphocyte infiltration, expanding cytolytic CD8⁺ T cells, and promoting tertiary lymphoid structure (TLS) formation. TCGA NSCLC analyses revealed that FLT3L expression correlated with activated DC, T cell, and B cell signatures and TLS-associated programs characterized by high endothelial venule (HEV) gene expression. When combined with PD-1 blockade, FLT3L-cDC1 ISV elicited robust local and systemic T cell activation, expanded activated CCR7⁺PD-L1⁺ cDC1s and stem-like TCF1⁺PD-1⁺ CD8⁺ progenitors, and achieved complete, durable tumor regression in an LKB1-deficient murine NSCLC model. Complete responses generated long-lasting systemic tumor-specific immune memory, consistent with effective antigen-agnostic tumor vaccination.

Conclusion

FLT3L-cDC1 ISV represents a locally delivered, cytokine-enhanced cellular immunotherapy that induces TLS formation and overcomes key mechanisms of immune resistance in NSCLC. These findings support its potential clinical translation.

This abstract is funded by: Department of Veterans Affairs Career Development Award-2 (1IK2BX006194-01)