Figure 2. Lack of Cre expression and activity in myeloid progenitors at steady-state and upon in vivo and in vitro differentiation.

(A) Quantitative RT-PCR analyses of Cre recombinase mRNA levels in Flk2+ (MPP^F) and Flk2- (HSC; myeloid progenitors (MyPro; Lin^-c-kit⁺Sca1^- cells), and erythroid progenitors (EP; Ter119^-Mac1^-Gr1^-B220^-CD3^-CD71⁺)) cell populations from FlkSwitch mice revealed that only MPP^F express Cre. Bar graph indicates the relative levels of Cre mRNA in cell populations sorted from individual (grey bar) or multiple (white bar; n=3) FlkSwitch mice. β-actin was used as a positive control for all populations. Error bars indicate SEM. (B-D) Tom+ CMP remain unfloxed during myeloid development. (B) Flow cytometry analysis of CMP progeny after 10 days of in vitro methylcellulose culture (n=6 in 2 independent experiments). (C) Tom and GFP analysis of donor-derived nucleated cells (total), GM, B-cells, T-cells, and Plts in PB of sublethally irradiated mice transplanted with Tomato+ or GFP+ MPP^F (800 per mouse) or CMP (10,000 per mouse) (n=5-7 in 2 independent experiments). (D) Erythroid progenitor (EP) readout in spleens of lethally irradiated mice 9 days post-transplantation of Tom+ or GFP+ MPP^F or CMP (n=10 in 2 independent experiments).