<p>Diffuse large B-cell lymphoma (DLBCL) is the most common subtype of non-Hodgkin lymphoma (NHL). Targeting fatty acid metabolism pathway represents a promising therapeutic strategy, particularly for patients with refractory or relapsed DLBCL. In this study, fatty-acid metabolism index (FMI) signature including 20 genes was constructed by univariate Cox and LASSO-Cox regression analysis using transcriptomic data from Gene Expression Omnibus database to predict overall survival of DLBCL patients. The FMI signature exhibited a negative association with anti-tumor immune response, with FMI-high patients had decreased immune cell infiltration and downregulated immune-associated signaling pathways. Based on the FMI signature, FASN was identified as the most essential gene that negatively regulated the tumor microenvironment of DLBCL. High FASN expression was associated with reduced anti-tumor immune activity, including decreased immune score, lower T-cell inflamed score, and downregulated expression of IFN molecules. Infiltration of immune cells, including CD4⁺T cells, CD8⁺T cells, dendritic cells, and macrophages were also significantly decreased in FASN-high than FASN-low patients. Accordingly, key signal molecules and chemokines for immune cells exhibited negative correlations with FASN expression. Through FASN knockdown in DB cell line, we further validated that FASN significantly suppressed chemokine secretion and promoted DLBCL proliferation in vitro. The FMI signature can effectively distinguish the prognostic stratification of DLBCL patients, further suggesting that FA metabolism imbalance may play an important role in DLBCL heterogeneity and treatment resistance. FASN is a potential negative regulator of immune microenvironment, providing novel insights into the metabolic-immune crosstalk in DLBCL treatment.</p>