POU5F1 and pancreatic neuroendocrine tumor: Immunosuppressive miRNA transfer is exemplified by miR-21 in pancreatic neuroendocrine tumor (PNET) exosomes, which polarizes tumor-associated macrophages toward the immunosuppressive M2 phenotype by directly targeting Krüppel-like factor 3 (KLF3) and activating stemness-promoting transcription factors like Nanog and Oct4 [29,30,31].