The generation of induced pluripotent stem cells (iPSCs) has revolutionised the stem cell field, opening up avenues for both basic and translational research. However, there is still much to understand about the mechanisms underlying reprogramming to the iPSC state, particularly in human. On p. , Takashi Tada and colleagues report the isolation of stable ‘intermediately reprogrammed stem cells’ (iRSCs) that are paused in their progression to pluripotency. These cells, generated by transient expression of the reprogramming factors Oct4, Klf4, Sox2 and c-Myc, express some pluripotency markers, such as endogenous SOX2 and NANOG, but have not yet undergone mesenchymal-to-epithelial transition (MET) or upregulated endogenous OCT4. The iRSC lines are stable over multiple generations, but can easily and efficiently be induced to continue reprogramming to an iPSC-like state by culture at high density. The authors use these iRSC lines to characterise the order of events during reprogramming, finding that in human, unlike in mice, induction of endogenous OCT4 expression precedes MET. Importantly, however, this expression is initially unstable, and some cells revert to an OCT4− state and show signs of lineage commitment. These cells lines should provide a valuable tool for further investigation of the mechanisms underlying reprogramming to pluripotency of human cells.
