Constitutive photomorphogenesis 9 (COP9) signalosome 5 (CSN5), an isopeptidase that removes neural precursor cell-expressed, developmentally down-regulated 8 (NEDD8) moieties from cullins (thus termed "deNEDDylase") and a subunit of the cullin-RING E3 ligase-regulating COP9 signalosome complex, attenuates proinflammatory NF-κB signaling. We previously showed that CSN5 is up-regulated in human atherosclerotic arteries. Here, we investigated the role of CSN5 in atherogenesis in vivo by using mice with myeloidspecific Csn5 deletion. Genetic deletion of Csn5 in Apoe −/− mice markedly exacerbated atherosclerotic lesion formation. This was broadly observed in aortic root, arch, and total aorta of male mice, whereas the effect was less pronounced and site-specific in females. Mechanistically, Csn5 KO potentiated NF-κB signaling and proinflammatory cytokine expression in macrophages, whereas HIF-1α levels were reduced. Inversely, inhibition of NEDDylation by MLN4924 blocked proinflammatory gene expression and NF-κB activation while enhancing HIF-1α levels and the expression of M2 marker Arginase 1 in inflammatory-elicited macrophages. MLN4924 further attenuated the expression of chemokines and adhesion molecules in endothelial cells and reduced NF-κB activation and monocyte arrest on activated endothelium in vitro. In vivo, MLN4924 reduced LPS-induced inflammation, favored an antiinflammatory macrophage phenotype, and decreased the progression of early atherosclerotic lesions in mice. On the contrary, MLN4924 treatment increased neutrophil and monocyte counts in blood and had no net effect on the progression of more advanced lesions. Our data show that CSN5 is atheroprotective. We conclude that MLN4924 may be useful in preventing early atherogenesis, whereas selectively promoting CSN5-mediated deNEDDylation may be beneficial in all stages of atherosclerosis.A therosclerosis is the primary cause of cardiovascular diseases. As a chronic inflammatory condition of the vessel wall, atherosclerosis is characterized by endothelial cell activation resulting in the secretion of chemoattractant proteins such as CCL2 and macrophage migration inhibitory factor (MIF) and by increased expression of adhesion molecules such as intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1). These molecules synergize to sequentially recruit inflammatory cells such as monocytes and T lymphocytes into the vessel wall (1-3).The transcription factor NF-κB plays a crucial role in vascular inflammation and atherogenesis, e.g., by controlling the expression of inflammatory cytokines, chemokines, and adhesion molecules that orchestrate the recruitment and adhesion of leukocytes. Also, numerous genes that regulate differentiation, survival, and proliferation of vascular and immune cells involved in the inflammatory response are targets of NF-κB (4). In resting cells, the NF-κB dimer, p65 and p50, is inactivated by binding to the inhibitor of κB (IκB)-α protein. Inflammatory challenges such as LPS or TNF-α exposure trigg...