CD209 and osteosarcoma: The removal of CSCs reduces tumor resistance and thus prevents tumor recurrence.121 TAMs can directly interact with CSCs and maintain the stem cell-like characteristics of CSCs, thereby triggering tumorigenesis and tumor progression.122 In addition, CD209-positive M2-type TAMs were found to activate CSCs and promote osteosarcoma formation, whereas all-trans retinoic acid (ATRA) inhibited in vitro osteosarcoma cell colony formation and spheroidogenic capacity as well as TAM-induced osteosarcoma formation in mice in vivo by decreasing the activity of CSCs and inhibiting M2-type TAMs.123