BACKGROUND: 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 antitumor CD8(+) T-cell responses; however, their abundance and function are often diminished in NSCLC, contributing to poor outcomes and resistance to immunotherapy. We hypothesized that in situ vaccination (ISV) using gene-modified cDC1s engineered to secrete FMS-like tyrosine kinase 3 ligand (FLT3L) would enhance cDC1 function within the TME, promote antitumor immunity, and improve responses to ICB. METHODS: Syngeneic murine models of NSCLC (Kras(G12D)/P53(-/-)/Lkb1(-/-); Kras(G12D)/P53(-/-) ; and Kras(G12D) ) with varying tumor mutational burden, along with the MC38 model, were used to assess the therapeutic efficacy of FLT3L-cDC1 ISV. Flow cytometry and multiplex immunofluorescence were used to evaluate immune mechanisms of response. To assess translational relevance, immune and tertiary lymphoid structure (TLS) signatures were analyzed in The Cancer Genome Atlas (TCGA) NSCLC datasets, with TLS signatures refined using a retrained xCell2 framework incorporating curated TLS and high endothelial venule (HEV) microdissection datasets. RESULTS: FLT3L-cDC1 ISV remodeled the TME across multiple NSCLC models, inducing T lymphocyte infiltration and expanding cytolytic CD8(+) T cells. FLT3L-cDC1 ISV was associated with increased formation of immature TLS with primary follicle-like features within the TME. TCGA analyses revealed that FLT3L expression correlates with activated DC, T cell, and B cell signatures, as well as HEV-enriched TLS-associated programs. Combination with PD-1 blockade further enhanced the antitumor immunity of FLT3L-cDC1 ISV, resulting in robust local and systemic T-cell activation and the expansion of activated CCR7(+)PD-L1(+) cDC1s and stem-like TCF1(+)PD-1(+) CD8(+) progenitors within the TME. In an LKB1-deficient NSCLC model, FLT3L-cDC1 ISV plus PD-1 blockade induced complete and durable regression in 85% of tumors, leading to long-lasting systemic tumor-specific immune memory, consistent with effective tumor vaccination. CONCLUSIONS: FLT3L-cDC1 ISV represents a rational cytokine-enhanced cellular immunotherapy designed to overcome immunosuppression and restore DC function within the TME, thereby promoting tumor-specific adaptive immune responses and enhancing responsiveness to ICB.