Down-regulation of Mpl marks the transition to lymphoid-primed multipotent progenitors with gradual loss of granulocyte-monocyte potential

Evidence for a novel route of adult hematopoietic stem-cell lineage commitment through Lin(-)Sca-l(+)Kit(+)FIt3(hi) (LSKFIt3(hi)) lymphoid-primed multipotent progenitors (LMPPs) with granulocyte/monocyte (GM) and lymphoid but little or no megakaryocyte/erythroid (MkE) potential was recently challenged, as LSKFIt3(hi) cells were reported to possess MkE potential. Herein, residual (1%-2%) MkE potential segregated almost entirely with LSKFlt3(hi) cells expressing the thrombo-poietin receptor (MpI), whereas LSKFIt3(hi)MpI(-) LMPPs lacked significant MkE potential in vitro and in vivo, but sustained combined GM and lymphoid potentials, and coexpressed GM and lymphoid but not MkE transcriptional lineage programs. Gradually increased transcriptional lymphoid priming in single LMPPs from Rag1(GFP) mice was shown to occur in the presence of maintained GM lineage priming, but gradually reduced GM lineage potential. These functional and mo-lecular findings reinforce the existence of GM/lymphoid-restricted progenitors with dramatically down-regulated probability for committing toward MkE fates, and support that lineage restriction occurs through gradual rather than abrupt changes in specific lineage potentials.