Stromal MTHFD2 Links Fibrotic Remodeling to Immune Evasion via Redox-Dependent Inflammatory Signaling in Lung Adenocarcinoma

Fibroblast activation and extracellular matrix stiffening are hallmarks of the fibrotic tumor microenvironment. Fibrotic microenvironmental remodeling is also linked to immune exclusion and evasion, suggesting that common microenvironmental signals might coordinate both fibrotic and inflammatory outputs. Here, we identified methylenetetrahydrofolate dehydrogenase 2 (MTHFD2) as a redox-sensitive metabolic regulator linking TGF-β-driven fibroblast activation to inflammatory signaling in lung adenocarcinoma (LUAD). MTHFD2 was elevated in cancer-associated fibroblasts across multiple scRNA-seq datasets, and TGF-β induced MTHFD2 even under compliant matrix conditions, with matrix stiffening further enhancing its expression. Notably, while MTHFD2 knockdown suppressed both myofibroblastic and inflammatory programs, enforced expression was sufficient to drive inflammatory cytokine production even in the absence of TGF-β, and this response was further amplified by concomitant TGF-β signaling. These MTHFD2-mediated responses depended on redox perturbation rather than biosynthetic one-carbon metabolism, as they resisted formate supplementation but responded to pro-oxidants. Mechanistically, MTHFD2-driven redox imbalance promoted mitochondrial stress, cytosolic mitochondrial DNA accumulation, and activation of the cGAS-STING-NF-κB pathway, resulting in IL-6 and IL-8 production. Fibroblast-specific MTHFD2 knockdown suppressed paracrine PD-L1 induction in tumor cells and inhibited tumor growth in vivo in an immune-dependent manner. Clinically, high stromal MTHFD2 correlated with epithelial PD-L1 expression and poor LUAD prognosis. Together, these findings identify stromal MTHFD2 as a redox-sensitive metabolic checkpoint linking fibrotic remodeling to immune evasion.