Single cell PCR studies showed that hematopoietic stem cells (HSCs) express a variety of lineage-affiliated genes. However, it remains unclear whether these cells exhibiting "lineage priming" represent bona fide stem cells or a subpopulation earmarked for differentiation. Here we have used a Cre-Lox approach to follow the fate of cells expressing a lineage-affiliated marker. We crossed lysozyme Cre mice with yellow fluorescent protein (EYFP) reporter mice and found EYFP gene expression not only in myelomonocytic cells but also in a fraction of HSCs as well as B cells and T cells. Transplantation of EYFP+ HSCs into primary and secondary recipients generated mice in which all hematopoietic cells were EYFP+. In contrast, crosses between CD19 Cre and lck Cre mice with reporter mice showed no EYFP expression in HSCs or intermediate progenitors. Our results demonstrate that lysozyme expression does not mark myeloid commitment and that long-term repopulation potential is maintained in primed HSCs.
