NNMT inhibition in cancer-associated fibroblasts restores antitumor immunity.

Cancer-associated fibroblasts (CAFs) play a pivotal cancer-supportive role, yet CAF-targeted therapies remain elusive. Through spatial transcriptomics and single-cell RNA sequencing, we investigated the role of nicotinamide N-methyltransferase (NNMT) in high-grade serous ovarian cancer (HGSOC). Mechanistically, NNMT-induced H3K27me3 hypomethylation drives complement secretion from CAFs, attracting immunosuppressive myeloid-derived suppressor cells (MDSCs) to the tumor. NNMT knockout in immunocompetent mice impaired tumor growth in syngeneic ovarian, breast, and colon tumor models through enhanced CD8+ T cell activation. Using high-throughput screening, we developed a potent and specific NNMT inhibitor that reduces tumor burden and metastasis in multiple murine cancer models and restores immune checkpoint blockade efficacy by decreasing CAF-mediated MDSC recruitment and reinvigorating CD8⁺ T cell activation. Our findings establish NNMT as a central CAF regulator and promising therapeutic target to mitigate immunosuppression in the tumor microenvironment. We performed spatial transcriptomics on a tissue microarray including non-tumorous cancer-adjacent stroma (n = 10), ovarian tumours (n = 29), and omental metastases (n = 16), obtained from 30 patients with high-grade serous ovarian cancer. mRNA expression in formalin-fixed paraffin-embedded (FFPE) tissue was measured using the GeoMx Human Whole Transcriptome Atlas (NanoString, GMX-RNA-NGS HuWTA-4), which targets 18,000 protein-coding genes. Probe hybridisation and immunofluorescent staining were used to define the following areas of interest (AOIs): (1) pan-cytokeratin-positive (panCK⁺) cancer epithelium, (2) CD163-positive monocytes/macrophages, (3) CD45-positive CD163-negative immune cells, and (4) stroma (panCK⁻ CD163⁻ CD45⁻).