The role of FOXM1 in regulating immune memory in triple-negative breast cancer and targeting FOXM1 to develop a therapeutic vaccine.

1119 Background: Triple-negative breast cancer (TNBC) is an aggressive malignancy with limited therapeutic options and poor long-term outcomes. Although immune checkpoint blockade has improved survival for a subset of patients, durable responses remain uncommon, underscoring the need for strategies that enhance tumor immunogenicity and immune memory. We recently identified a novel function of the transcription factor FOXM1 in promoting an immune-suppressive tumor microenvironment (TME) in TNBC by regulating stress ligands and the STING pathway. This study investigates the role of FOXM1 in shaping antitumor immune memory and leverages FOXM1 targeting to identify tumor antigens for therapeutic peptide vaccine development. Methods: FOXM1 was knocked out in TNBC cell lines using CRISPR/Cas9, and syngeneic mouse models were utilized to evaluate tumor-immune interactions. Single-cell RNA sequencing (scRNA-seq) evaluated immune landscape remodeling following FOXM1 loss. Immunopeptidomics was performed to identify tumor-associated antigens enriched following FOXM1 loss. Candidate peptides were functionally validated using dendritic cell–based antigen presentation assays and patient-derived peripheral blood mononuclear cells (PBMCs). Clinical relevance was evaluated using TNBC patient datasets and immunotherapy response cohorts. Results: scRNA-seq of syngeneic tumors revealed that FOXM1 deletion reshaped the immune microenvironment, with increased CD8+ T cell and NK cell infiltration and expansion of memory T cell populations. Tumor-intrinsic FOXM1 suppressed antitumor immunity by downregulating MHC-I and the stress ligand ULBP1, disrupting signaling pathways required for NK and CD8+ T cell–mediated cytotoxicity. FOXM1-deficient tumors conferred durable protection upon tumor rechallenge, consistent with long-term immune memory. Analysis of patient datasets demonstrated that elevated FOXM1 and DNMT1 expression, coupled with reduced STING and ULBP1 levels, correlated with poorer survival and reduced responsiveness to immunotherapy. Leveraging enhanced antigen presentation in FOXM1-depleted cells, immunopeptidomics identified 275 differentially presented tumor antigens. Dendritic cell pulsing of the top 15 enriched peptides induced robust T cell responses in patient-derived PBMCs, supporting their immunogenicity and its potential to serve as bona fide candidates for a therapeutic vaccine tailored for TNBC patients. Conclusions: These findings establish FOXM1 as a clinically relevant regulator of immune evasion and immune memory in TNBC. Importantly, the identification of immunogenic peptides provides a strong rationale for the development of a peptide-based therapeutic vaccine, either as a monotherapy or in combination with immune checkpoint blockade, to improve long-term disease control in patients with TNBC.