A recent study, performed on gastric cancer (GC) cell lines and on patients, highlighted a new mechanism through which IL-15, secreted by GC mesenchymal stem cells in the microenvironment, upregulated the Tregs ratio and enhanced programmed cell death protein 1 (PD-1) expression in CD4+ T cells, promoting EMT [49]. The gene discussed is PDCD1; the disease is gastric cancer.