Title: Ikaros and Mi-2beta mediated hematopoietic lineage determination
Description: The mechanisms underlying hematopoiesis, the process by which hematopoietic stem cells (HSCs) differentiate through a series of developmental intermediates with an increasingly restricted set of cell fates to form at least eight distinct blood lineages, are incompletely understood. A lymphoid primed multipotent progenitor (LMPP) that maintained lymphoid and myeloid, but not erythroid potential, was isolated and described using an Ikaros-GFP reporter. Ikaros-null mice developed LMPPs and decreased numbers of myeloid restricted granulocyte macrophage progenitors (GMPs), but maintained wild-type frequencies of myeloid progenitor activity, indicating that myelopoiesis as well as lymphopoiesis proceeds through LMPPs in an intact animal model. Gene array and single-cell RT-PCR analysis of gene expression in HSCs, LMPPs, GMPs, and megakaryocyte erythrocyte progenitors (MEPs) showed that lymphoid lineage priming was initiated in the HSC and that lymphoid and myeloid lineage priming were co-incident in LMPPs and GMPs but absent in MEPs. Additionally, it was determined that GMPs maintain a latent T-lymphoid potential both in vitro and in vivo.

Ikaros-null LMPPs fail to express lymphoid and have increased myeloid specific gene expression programs without fully down-regulated expression of HSC and MEP programs. Functional characterization of Ikaros-null HSCs and LMPPs indicated that lymphoid potential was severely reduced in LMPPs but not HSCs, and loss of Ikaros function in LMPPs reduced lymphoid and increased myeloid potential. Shortly after conditional inactivation of Mi-2β, there is an increase in cycling and a decrease in quiescence in an HSC-enriched bone marrow population. These cycling mutant cells differentiate into the erythroid lineage but not into the myeloid and lymphoid lineages. These effects result in initial expansions of mutant HSC and erythroid progenitors that are later depleted. Examination of gene expression in the mutant HSC reveals changes in the expression of genes associated with self-renewal and lineage priming. Cumulatively, this work demonstrates that Ikaros and Mi-2β regulate the multi-lineage priming that underlies cell fate determination in the HSC and its early progeny. Ikaros directs lymphoid and inactivates HSC and alternative lineage priming in LMPPs, while Mi-2β is associated with the priming of all lineages and expression of self-renewal programs in HSCs.