IL-2 induces TPH1 expression in CD8+ T cells by continuously activating signal transducer and activator of transcription 5 (STAT5) in the tumor, while 5-HTP in turn activates AhR in CD8+ T cells, which enables them to acquire a depletion phenotype, resulting in synergistic upregulation of inhibitory receptors. Here, CD8A is linked to neoplasm.