Targeting SHMT2 reverses lactate-driven CAF-mediated T-cell exclusion and sensitizes HNSCC to anti-PD-1 immunotherapy

Abstract Resistance to PD-1/PD-L1 blockade in head and neck squamous cell carcinoma (HNSCC) is frequently attributed to an “immune-excluded” tumor microenvironment (TME); however, the tumor-intrinsic metabolic drivers orchestrating this spatial remodeling remain poorly defined. Here, we identify serine hydroxymethyltransferase 2 (SHMT2) as a critical determinant of the immunosuppressive stromal niche. SHMT2 is aberrantly overexpressed in malignant cells, and its high expression correlates with a cancer-associated fibroblast (CAF)-rich, T-cell-excluded phenotype in HNSCC cohorts. While SHMT2 deficiency does not alter tumor growth in immunodeficient mice, it markedly suppresses tumor progression in immunocompetent hosts, suggesting an immune-dependent therapeutic effect. Integrated single-cell and spatial profiling reveal that SHMT2 loss reduces CAF accumulation and disrupts the CAF-mediated physical barrier, facilitating robust infiltration of CD4⁺ and CD8⁺ T cells into the tumor parenchyma, without inducing a general reduction in other immunosuppressive immune populations. Mechanistically, SHMT2-driven one-carbon metabolism maintains the S-adenosylmethionine (SAM) pool, which promotes H3K4me3 enrichment at the LDHA promoter to epigenetically activate its transcription. The resulting SHMT2–SAM–LDHA axis elevates lactate secretion, which acts as a paracrine signal to trigger CAF activation and spatial accumulation. Furthermore, pharmacological targeting of the SHMT2-dependent axis using SHMT-IN-2 effectively suppresses lactate-associated CAF remodeling and synergizes with anti-PD-1 therapy in vivo. These findings suggest that the SHMT2–lactate–CAF axis contributes to the metabolic–stromal regulation of T-cell exclusion and may represent a potential therapeutic target for improving the response of immune-excluded HNSCC to PD-1 blockade.