The Oct4-GFP+ cells demonstrated a characteristic small cell size with little cytoplasm and also showed a distinct fine structure of the nucleus compared with that of parental CD45+ lymphocytes (Fig. 1g). The Oct4-GFP+ cells on day 7 were smaller than non-treated CD45+ cells (Fig. 1g, h and Extended Data Fig. 2c) and embryonic stem (ES) cells (Fig. 1h), both of which are generally considered to be small in size. The diameter of low-pH-treated CD45+ cells became reduced during the first 2 days, even before they started Oct4-GFP expression (Fig. 1f), whereas the onset of GFP expression was not accompanied by cell divisions. Consistent with this, no substantial 5-ethynyl-2′-deoxyuridine (EdU) uptake was observed in the Oct4-GFP+ cells after the stressor (Extended Data Fig. 2d).
The lack of substantial proliferation argues against the possibility that CD45− cells, contaminating as a very minor population in the FACS-sorted CD45+ cells, quickly grew and formed a substantial Oct4-GFP+ population over the first few days after the low-pH treatment. In addition, genomic rearrangements of Tcrb (T-cell receptor gene) were observed in Oct4-GFP+ cells derived from FACS-purified CD45+ cells and CD90+CD45+ T cells (Fig. 1i, lanes 4, 5, and Extended Data Fig. 2e–g), indicating at least some contribution from lineage-committed T cells. Thus, Oct4-GFP+ cells were generated de novo from low-pH-treated CD45+ haematopoietic cells by reprogramming, rather than by simple selection of stress-enduring cells.
expressed pluripotency-related marker proteins (Oct4, SSEA1, Nanog and E-cadherin; Fig. 2a) and marker genes (Oct4, Nanog, Sox2, Ecat1 (also called Khdc3), Esg1 (Dppa5a), Dax1 (Nrob1) and Rex1 (Zfp42); Fig. 2b and Extended Data Fig. 3a) in a manner comparable to those seen in ES cells. Moderate levels of expression of these pluripotency marker genes were observed on day 3 (Fig. 2b and Extended Data Fig. 3b). Notably, the Oct4-GFP+ cells on day 3, but not on day 7, expressed early haematopoietic marker genes such as Flk1 (also called Kdr) and Tal1 (Extended Data Fig. 3c), indicating that Oct4-GFP+ cells on day 3, as judged by their expression pattern at the population level, were still in a dynamic process of conversion.
On day 7, unlike CD45+ cells and like ES cells, low-pH-induced Oct4-GFP+ cells displayed extensive demethylation at the Oct4 and Nanog promoter areas (Fig. 2c), indicating that these cells underwent a substantial reprogramming of epigenetic status in these key genes for pluripotency.
In vitro differentiation assays demonstrated that low-pH-induced Oct4-GFP+ cells gave rise to three-germ-layer derivatives (Fig. 2d) as well as visceral endoderm-like epithelium (Extended Data Fig. 3d). When grafted into mice, low-pH-induced Oct4-GFP+ cell clusters formed teratomas (40%, n = 20) (Fig. 2e and Extended Data Fig. 4a–c) but no teratocarcinomas that persistently contained Oct4-GFP+ cells (n = 50). Because some cellular variation was observed in the signal levels of Oct4-GFP within the clusters, we sorted GFP-strong cells (a major population) and GFP-dim cells (a minor population) by FACS on day 7 and separately injected them into mice. In this case, only GFP-strong cells formed teratomas (Extended Data Fig. 4d).
