Vascular endothelial growth factor (VEGF) is essential for developmental and pathological angiogenesis. Here we show that in the absence of any pathological insult, autocrine VEGF is required for the homeostasis of blood vessels in the adult. Genetic deletion of vegf specifically in the endothelial lineage leads to progressive endothelial degeneration and sudden death in 55% of mutant mice by 25 weeks of age. The phenotype is manifested without detectable changes in the total levels of VEGF mRNA or protein, indicating that paracrine VEGF could not compensate for the absence of endothelial VEGF. Furthermore, wild-type, but not VEGF null, endothelial cells showed phosphorylation of VEGFR2 in the absence of exogenous VEGF. Activation of the receptor in wild-type cells was suppressed by small molecule antagonists but not by extracellular blockade of VEGF. These results reveal a cell-autonomous VEGF signaling pathway that holds significance for vascular homeostasis but is dispensable for the angiogenic cascade.

Endothelial progenitor cell (EPC) nomenclature remains ambiguous and there is a general lack of concordance in the stem cell field with many distinct cell sub-types continually grouped under the term “EPC”. It would be highly advantageous to agree standards to confirm an endothelial progenitor phenotype and this should include detailed immunophenotyping, potency assays, and clear separation from haematopoietic angiogenic cells which are not endothelial progenitors. In this review, we seek to discourage the indiscriminate use of ‘EPCs’, and instead propose precise terminology based on defining cellular phenotype and function. Endothelial colony forming cells (ECFCs) and myeloid angiogenic cells (MACs) are examples of two distinct and well-defined cell types that have been considered ‘EPCs’ because they both promote vascular repair, albeit by completely different mechanisms of action. It is acknowledged that scientific nomenclature should be a dynamic process driven by technological and conceptual advances; ergo the ongoing ‘EPC’ nomenclature ought not to be permanent and should become more precise in the light of strong scientific evidence. This is especially important as these cells become recognised for their role in vascular repair in health and disease; and, in some cases, progress towards use in cell therapy.

B-1 B cells have been proposed to be preferentially generated from fetal progenitors, but this view is challenged by studies concluding that B-1 production is sustained throughout adult life. To address this controversy, we compared the efficiency with which hematopoietic stem cells (HSCs) and common lymphoid progenitors (CLPs) from neonates and adults generated B-1 cells in vivo and developed a clonal in vitro assay to quantify B-1 progenitor production from CLPs. Adult HSCs and CLPs generated fewer B-1 cells in vivo compared with their neonatal counterparts, a finding corroborated by the clonal studies that showed that the CLP compartment includes B-1-and B-2-specified subpopulations and that the former cells decrease in number after birth. Together, these data indicate that B-1 lymphopoiesis is not sustained at constant levels throughout life and define a heretofore unappreciated developmental heterogeneity within the CLP compartment.bone marrow | clonal analysis | hematopoiesis I mmune system development has been hypothesized to be a layered process in which lymphoid populations of increased complexity are produced in successive waves in the fetus, neonate, and adult. The initial wave of fetal lymphopoiesis has been proposed to generate lymphocytes involved in innate immunity, whereas waves appearing later produce cells involved in adaptive immune responses (1). A corollary of the layered immune system hypothesis is that, after peaking in the fetus/neonate, the initial wave(s) of lymphopoiesis wanes as the adult wave establishes.The layered immune system hypothesis arose from studies aimed at defining the origin of two types of B lymphocytes, termed B-1 and B-2 cells (2). B-1 cells are innate effectors distinguished by their preferential localization in serous cavities and an unusual sIgM high CD11b + CD5 + B-1a and sIgM high CD11b + CD5 − B-1b phenotype (3, 4). B-1a cells spontaneously secrete IgM natural antibodies, whereas production of Ig by B-1b cells can be induced by antigen exposure, and the latter cells exhibit immunologic memory (5, 6). Human B-1 cells with properties similar to those described in mice have recently been described (7). In contrast, conventional B cells, referred to as B-2 cells, are mediators of adaptive immune responses, predominate in the spleen and lymph nodes, and undergo somatic hypermutation after antigen encounter (8, 9).Studies showing that cells from fetal liver are more efficient than cells from adult bone marrow at reconstituting B-1 cells in irradiated recipients initially suggested that innate B cells arise from progenitors that appear during a fetal wave of development (10, 11). CD5 + B-1a cells, in particular, were preferentially generated from fetal sources (2, 12, 13). These early findings are supported by more recent studies showing the existence of lineage negative (Lin − ) CD45R −/low CD19 + B-1-specified progenitors that arise in the embryonic yolk sac, peak in number in the fetal liver, and then decline in the adult (14-17). Taken together with data showing th...

SUMMARY B cell development is often depicted as a linear process initiating in the fetus and continuing postnatally. Using a PU.1 hypomorphic mouse model, we found that B-1 and B-2 lymphopoiesis occurred in distinct fetal and adult waves differentially dependent on the Sfpi1 14 kB upstream regulatory element. The initial wave of fetal B-1 development was absent in PU.1 hypomorphic mice, while subsequent fetal and adult waves emerged. In contrast, B-2 lymphopoiesis occurred in distinct fetal and adult waves. Whole transcriptome profiling of fetal and adult B cell progenitors supported the existence of three waves of B-1 and two waves of B-2 development, and revealed that the network of transcription factors governing B lineage specification and commitment was highly divergent between B-1 and B-2 progenitors. These findings support the view that the B-1 and B-2 lineages are distinct and provide a genetic basis for layering of immune system development.

ABSTRACT:The concept of an Endothelial Progenitor Cell (EPC) that participates in adult angiogenesis is less than a decade old, yet it has received a great deal of attention due to its potential for cellbased clinical therapies in many pathologies. However, controversy remains as to the identity of this bone marrow-derived cell type and its ability to give rise to new endothelium in the adult. Reports on the contribution of EPCs to new vessels in ischemic tissue or tumors vary widely, ranging from 80 -90% to negligible. As researchers hone their ability to identify, isolate, and expand these cells by their markers and functionality, mounting evidence suggests that they might constitute multiple, but related cell types. At least two general phenotypes have emerged from studies of bone marrow-derived cells contributing to angiogenesis: one that incorporates into the endothelial wall directly contributing to vascular expansion and another that is able to home to neovessels, but it locates behind the endothelial wall. Nonetheless, experimental evidence indicates that this second cell type supports the viability of newly formed vessels and thus it is equally relevant to neovascular growth. As our understanding of neovascularization in pathologic states expands, a more clear definition of the multiple cellular components required for the process will shed light into new models of therapeutic intervention. The identification of a cell type that could be isolated, expanded and infused into a patient would be very useful for promoting angiogenesis in ischemia, myocardial infarct and other pathologies.