PDCD1 and cancer: The KEGG enrichment analysis revealed an increase in immune-associated pathways (such as PD-L1 expression and PD-1 checkpoint pathway in cancer, Th17 cell differentiation, Th1 and Th2 cell differentiation, T cell receptor signaling pathway, and Toll-like signaling pathway) in these CD8+ T cell subpopulations (Fig. 3G).