The endothelial-cell-derived secreted factor Egfl7 regulates vascular tube formation.

Vascular development is a complex but orderly process that is tightly regulated. A number of secreted factors produced by surrounding cells regulate endothelial cell (EC) differentiation, proliferation, migration and coalescence into cord-like structures. Vascular cords then undergo tubulogenesis to form vessels with a central lumen. But little is known about how tubulogenesis is regulated in vivo. Here we report the identification and characterization of a new EC-derived secreted factor, EGF-like domain 7 (Egfl7). Egfl7 is expressed at high levels in the vasculature associated with tissue proliferation, and is downregulated in most of the mature vessels in normal adult tissues. Loss of Egfl7 function in zebrafish embryos specifically blocks vascular tubulogenesis. We uncover a dynamic process during which gradual separation and proper spatial arrangement of the angioblasts allow subsequent assembly of vascular tubes. This process fails to take place in Egfl7 knockdown embryos, leading to the failure of vascular tube formation. Our study defines a regulator that controls a specific and important step in vasculogenesis.

Tissue distribution and receptor-mediated clearance of anti-CD11a antibody in mice.

Efalizumab (Raptiva) is a humanized monoclonal antibody specific for CD11a, the alpha-chain component of the lymphocyte function-associated antigen 1. In humans, the rate of efalizumab elimination from serum was related to the level of CD11a cell surface expression. These data suggested a role for the CD11a receptor, itself, in efalizumab clearance. Recently, we conducted a series of in vitro studies that suggested a role for CD11a-expressing T cells in efalizumab clearance as mediated by cellular internalization and lysosome-mediated degradation (Coffey et al., 2004). To further study the mechanism of anti-CD11a clearance in vivo, we assessed the tissue distribution, cellular internalization, and subcellular localization of a rat anti-mouse CD11a monoclonal antibody in various tissues in mice. Anti-CD11a antibody primarily distributed to leukocytes and macrophages in the peripheral blood, spleen, and liver, with uptake in the lymph nodes and bone marrow after 72 h. At least a portion of the antibody was internalized and cleared by peripheral blood mononuclear cells, lymphocytes, and splenocytes in a time-dependent manner in vivo. Internalized antibody costained with LysoTracker Red, suggesting that it was transported to lysosomes for degradation. Together, these data suggest that one clearance mechanism for anti-CD11a antibody in vivo is via receptor-mediated internalization and lysosomal degradation by CD11a-expressing cells and tissues.