Atherosclerosis development is promoted by chronic inflammation and the accumulation of inflammatory cells and lipids in the arterial wall leading to an atherosclerotic plaque (1–5). Leukocyte subsets such as neutrophils, monocytes and T cells infiltrate the vessel wall during various stages of atherogenesis and contribute critically to vessel pathology (1–6). Given the importance of leukocyte recruitment for atherosclerosis development, endogenous inhibitors of the leukocyte recruitment cascade could be of particular interest as targets of therapeutic intervention. Developmental endothelial locus-1 (Del-1) is an endothelial cell-derived 52-kDa glycoprotein, which consists of three N-terminal epidermal growth factor (EGF)-like repeats and two discoidin-like domains (7–9). Upon its release from endothelial cells, Del-1 can physically associate with the endothelial cell surface and/or with the extracellular matrix (9, 10). We have previously shown that Del-1 binds to the β2 integrins LFA-1 and Mac-1 and interferes with β2 integrin-dependent leukocyte recruitment (11, 12). In an animal model of periodontitis, a risk factor for atherosclerosis development (13), Del-1 was shown to inhibit neutrophil accumulation, IL-17-dependent inflammation and inflammatory bone loss (14–16). These previous studies suggest that the anti-inflammatory functions of Del-1 may represent an endogenous homeostatic mechanism that regulates inflammation to prevent disease development. Atherosclerotic plaque progression towards advanced stages is characterized by the accumulation of cell debris, derived mostly from apoptotic lipid-laden macrophages, resulting in formation of an acellular, pro-thrombotic, lipid core (17, 18). Oxidized low-density lipoprotein (oxLDL), which may be spilled from such apoptotic macrophages that are not cleared by phagocytosis, represents a major pathogenic trigger in the atherogenic process. Interestingly, Del-1 was recently shown to bind to oxLDL and suppress the oxLDL-induced pro-inflammatory gene expression; moreover, global Del-1 overexpression in mice attenuated atherosclerosis development (19). Due to the strong connection between endothelial cells and atherosclerosis, we aimed to study here the role of endothelialspecific Del-1 overexpression on atherosclerosis development. To this end, we engaged mice with endothelial-specific overexpression of Del-1 (Del-1-Tg)(20). An additional rationale for choosing this approach was that, in contrast to small vessels such as those of the lung (11), Del-1 expression in large arteries, like the aorta, is severely diminished (▶ Figure 1 A). Thus, by overexpressing Del-1 in the endothelium, we aimed to endow the endothelial cells of large arteries with the anti-adhesive/anti-inflammatory properties of Del-1. Del-1 overexpression was confirmed by qRT-PCR (RNeasy Micro kit, Qiagen; iScript cDNA synthesis kit, Bio-Rad; SsoF-ast EvaGreen Supermix, Bio-Rad). Indeed, Del-1 mRNA was increased 20-fold in the aorta of Del-1-Tg as compared to Del-1-WT mice (▶ Figure 1 A). Calculation was based on the threshold cycle (CT) method (21) and normalized to β-2 microglobulin RNA. Moreover, to confirm expression at the protein level, we performed immunofluorescence analysis for Del-1 in aorta sections of Del-1-Tg and Del-1-WT mice. We found substantial Del-1 protein expression predominantly in the intima of Del-1-Tg mice, whereas much less Del-1 staining was observed in the Del-1-WT mice (see Suppl. Figure 1, available online at www. thrombosis-online. com). Additionally, we have studied Del-1 expression at the mRNA level in sorted monocytes obtained from the blood and bone marrow of wild …