Cooperative regulation of miR-221 and miR-155 during human monocyte differentiation into DCs. In nonproliferating human monocytes, miR-221 and miR-155 are expressed at a relatively low level, accompanied by relatively high levels of p27^kip1. When monocytes differentiate into imDCs by GM-CSF and IL-4, miR-221 is up-regulated, resulting in down-regulation of p27^kip1, which leads to imDC development. During DC maturation, miR-155 is up-regulated, which promotes IL-12p70 production and DC terminal differentiation by inhibiting the expression of SOCS-1 and CD115, respectively. miR-155 also targets KPC1, which promotes p27^kip1 degradation. Accompanied by miR-155 up-regulation, miR-221 is down-regulated, resulting in p27^kip1 accumulation and eventual DC apoptosis.