ALDH1L1 reverses CD8(+) T cell exhaustion in the oral squamous cell carcinoma microenvironment by reprogramming L-glutamate metabolism.

BACKGROUND: Oral squamous cell carcinoma (OSCC) induces CD8⁺ T-cell exhaustion within the tumor microenvironment (TME) through metabolic reprogramming, contributing to the limited efficacy of immunotherapy. Targeting tumor metabolism is a pivotal strategy. Whether Aldehyde dehydrogenase 1 family member L1 (ALDH1L1), a key enzyme in folate metabolism, can modulate the function of CD8⁺ T cell to enhance immunotherapy efficacy remains unclear. This research aims to elucidate the specific mechanism by which ALDH1L1 regulates metabolic reprogramming in OSCC and influences CD8⁺ T-cell immunotherapy. METHODS: The impact of ALDH1L1 on CD8⁺ T cell was assessed using patient samples, engineered OSCC cell lines, and C3H mouse models. Integrated transcriptomics and metabolomics revealed its role in L-glutamate metabolism, further investigated via molecular docking and co-immunoprecipitation. In vitro, the direct effect of L-glutamate on CD8⁺ T-cell exhaustion was probed via transcriptomic sequencing, mitochondrial functional assays, and immunofluorescence. An ALDH1L1-targeting compound from virtual screening was evaluated in vivo to enhance anti-PD-1 therapy. RESULTS: Low expression of ALDH1L1 in OSCC correlates with decreased CD8⁺ T-cell infiltration and increased exhaustion. In vivo and in vitro models demonstrated that ALDH1L1 regulates IL-15 expression to influence CD8⁺ T-cell proliferation. Multi-omics analysis revealed that ALDH1L1 downregulation enriched the L-glutamate metabolic pathway. Mechanistically, ALDH1L1 directly interacts with GLUL, leading to L-glutamate accumulation in the TME. Subsequent analyses demonstrated that L-glutamate suppresses the PI3K/Akt/FoxO1 signaling axis in CD8⁺ T cell, impairing mitochondrial function and inhibiting oxidative phosphorylation (OXPHOS). Stevioside, identified as an ALDH1L1-targeting compound, significantly enhanced the efficacy of anti-PD-1 therapy, leading to reduced tumor growth in mouse models. CONCLUSIONS: Downregulation of ALDH1L1 in OSCC drives CD8⁺ T-cell exhaustion via a GLUL-mediated increase in L-glutamate, which suppresses mitochondrial OXPHOS. Pharmacological modulation of ALDH1L1 with stevioside represents a promising strategy to enhance anti-PD-1 immunotherapy efficacy, providing a novel combination therapeutic strategy for OSCC. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12967-026-07812-z.