Altered phenotypic and metabolic characteristics of FOXP3+CD3+CD56+ natural killer T (NKT)-like cells in human malignant pleural effusion

Malignant pleural effusion (MPE) is a functional 'cold' tumor microenvironment in which the antitumor activity of CD8⁺ T cells and natural killer T (NKT)-like cells is suppressed and the function of regulatory T (T_reg) cells is enhanced. Using flow cytometry and immunofluorescence staining, we detected a distinct subset of NKT-like cells expressing FOXP3 in MPE. Through single-cell RNA Sequencing (scRNA-seq) analysis, we found that the glycolysis pathway and pyruvate metabolism were highly activated in FOXP3⁺ NKT-like cells. Similar to T_reg cells, FOXP3⁺ NKT-like cells highly expressed Monocarboxylate Transporter 1 (MCT1) and Lactate Dehydrogenase B to uptake and utilize lactate, thereby maintaining their immunosuppressive function and hyperlactylation in MPE. Furthermore, we found that MCT1 small molecule inhibitor 7ACC2 significantly reduced FOXP3 expression and histone lactylation levels in NKT-like cells in vitro. In conclusion, we reveal for the first time the altered phenotypic and metabolic features of FOXP3⁺ NKT-like cells in human MPE.

FOXP3; MCT1; Malignant pleural effusion; NKT-like cell; lactylation; scRNA-seq.