Macrophages and dendritic cells (DCs) are key components of cellular immunity and are thought to originate and renew from hematopoietic stem cells (HSCs). However, some macrophages develop in the embryo before the appearance of definitive HSCs. We thus reinvestigated macrophage development. We found that the transcription factor Myb was required for development of HSCs and all CD11bhighmonocytes and macrophages, but was dispensable for yolk sac (YS) macrophages and for the development of YS-derived F4/80brightmacrophages in several tissues, such as liver Kupffer cells, epidermal Langerhans cells, and microglia—cell populations that all can persist in adult mice independently of HSCs. These results define a lineage of tissue macrophages that derive from the YS and are genetically distinct from HSC progeny.

The transcription factor GATA-1 is expressed in early hematopoietic progenitors and specifically down-regulated in myelomonocytic cells during lineage determination. Our earlier observation that the differentiation of Myb-Ets-transformed chicken hematopoietic progenitors into myeloblasts likewise involves a GATA-I down-regulation, whereas expression is maintained in erythroid, thrombocytic, and eosinophilic derivatives, prompted us to study the effect of forced GATA-I expression in Myb--Ets-transformed myeloblasts. We found that the factor rapidly suppresses myelomonocytic markers and induces a reprogramming of myeloblasts into cells resembling either transformed eosinophils or thromboblasts. In addition, we observed a correlation between the level of GATA-1 expression and the phenotype of the cell, intermediate levels of the factor being expressed by eosinophils and high levels by thromboblasts, suggesting a dosage effect of the factor. GATA-1 can also induce the formation of erythroblasts when expressed in a myelomonocytic cell line transformed with a Myb-Ets mutant containing a lesion in Ets. These cells mature into erythrocytes following temperature-inactivation of the Ets protein. Finally, the factor can reprogram a v-Myc-transformed macrophage cell line into myeloblasts, eosinophils, and erythroblasts, showing that the effects of GATA-1 are not limited to Myb-Ets-transformed myeloblasts. Our results suggest that GATA-1 is a lineage-determining transcription factor in transformed hematopoietic cells, which not only activates lineage-specific genetic programs but also suppresses myelomonocytic differentiation. They also point to a high degree of plasticity of transformed hematopoietic cells.

The accumulation of smooth muscle and endothelial cells is essential for remodeling and repair of injured blood vessel walls. Bone marrow–derived progenitor cells have been implicated in vascular repair and remodeling; however, the mechanisms underlying their recruitment to the site of injury remain elusive. Here, using real-time in vivo fluorescence microscopy, we show that platelets provide the critical signal that recruits CD34+ bone marrow cells and c-Kit+ Sca-1+ Lin− bone marrow–derived progenitor cells to sites of vascular injury. Correspondingly, specific inhibition of platelet adhesion virtually abrogated the accumulation of both CD34+ and c-Kit+ Sca-1+ Lin− bone marrow–derived progenitor cells at sites of endothelial disruption. Binding of bone marrow cells to platelets involves both P-selectin and GPIIb integrin on platelets. Unexpectedly, we found that activated platelets secrete the chemokine SDF-1α, thereby supporting further primary adhesion and migration of progenitor cells. These findings establish the platelet as a major player in the initiation of vascular remodeling, a process of fundamental importance for vascular repair and pathological remodeling after vascular injury.

A CD45-negative population of pre-HSCs develops into definitive HSCs in the AGM region of the embryo.

The c‐Myb transcription factor is expressed in immature haemopoietic cells and at key stages during differentiation. Loss of the c‐myb gene results in embryonic lethality because mature blood cells fail to develop, although commitment to definitive haemopoiesis occurs. We have generated a knockdown allele of c‐myb, expressing low levels of the protein, which has enabled us to investigate further the involvement of c‐Myb in haemopoiesis. Low levels of c‐Myb are sufficient to allow progenitor expansion but, importantly, we show that progression of progenitors towards terminal differentiation is significantly altered. Suboptimal levels of c‐Myb favour differentiation of macrophage and megakaryocytes, while higher levels seem to be particularly important in the control of erythropoiesis and lymphopoiesis. We provide evidence that the transition from the CFU‐E to erythroblasts is critically dependent on c‐Myb levels. During thymopoiesis, c‐Myb appears to regulate immature cell numbers and differentiation prior to expression of CD4 and CD8. Overall, our results point to a complex involvement of c‐Myb in the regulation of proliferation and commitment within the haemopoietic hierarchy.