Studies have shown that Tregs efficiently uptake lactic acid from the tumor microenvironment through MCT1, convert lactic acid into malic acid and citric acid, and then transfer it to mitochondria to participate in the tricarboxylic acid cycle, enhancing PD-1 expression and Treg immunosuppressive function, and reducing the effectiveness of immunotherapy [100]. Here, PDCD1 is linked to neoplasm.