Background— Activated macrophages contribute to the pathogenesis of inflammatory diseases such as atherosclerosis. Although Notch signaling participates in various aspects of immunity, its role in macrophage activation remains undetermined. Methods and Results— To explore the role of Notch signaling in inflammation, we examined the expression and activity of Notch pathway components in human primary macrophages in vitro and in atherosclerotic plaques. Macrophages in culture express various Notch pathway components including all 4 receptors (Notch1 to Notch4). Notch3 selectively increased during macrophage differentiation; however, silencing by RNA interference demonstrated that all receptors are functional. The ligand Delta-like 4 (Dll4) increased in macrophages exposed to proinflammatory stimuli such as lipopolysaccharide, interleukin-1β, or minimally-modified low-density lipoprotein in a Toll-like receptor 4– and nuclear factor-κB–dependent fashion. Soluble Dll4 bound to human macrophages. Coincubation of macrophages with cells that expressed Dll4 triggered Notch proteolysis and activation; increased the transcription of proinflammatory genes such as inducible nitric oxide synthase, pentraxin 3 and Id1; resulted in activation of mitogen-activated protein kinase, Akt, and nuclear factor-κB pathways; and increased the expression of Dll4 in macrophages. Notch3 knockdown during macrophage differentiation decreased the transcription of genes that promote inflammation, such as inducible nitric oxide synthase, pentraxin 3, Id1, and scavenger receptor-A. These in vitro findings correlate with results of quantitative immunohistochemistry, which demonstrated the presence of Dll4 and other Notch components within macrophages in atherosclerotic plaques. Conclusion— Dll4-triggered Notch signaling may mediate inflammatory responses in macrophages and promote inflammation.

Background— Interstitial collagen plays a crucial structural role in arteries. Although in vitro results suggest collagenase activity for membrane-bound matrix metalloproteinase type 1 (MMP-14), in vivo evidence for such a function in atherosclerosis remains scant. Methods and Results— Because Mmp14 −/− mice die by 3 weeks of age, this study used lethally irradiated low-density lipoprotein receptor–deficient mice reconstituted with syngeneic bone marrow cells of Mmp14 −/− or Mmp14 +/+ mice. In both groups, histological analyses of the aortic root revealed similar plaque size and macrophage and smooth muscle cell content after 8 or 16 weeks of atherogenic diet. By 16 weeks, however, the plaques of low-density lipoprotein receptor–deficient mice engrafted with Mmp14 −/− bone marrow (n=12) contained significantly more interstitial collagen than those receiving Mmp14 +/+ bone marrow (n=14; P <0.05). In vitro, bone marrow–derived macrophages from Mmp14 −/− mice had significantly less interstitial collagenase activity than those from Mmp14 +/+ mice both basally ( P <0.01) and on tumor necrosis factor-α stimulation ( P <0.05). Western blot analysis and gelatin zymography of aortic extracts revealed that MMP-14 deficiency yielded decreased activation of pro–MMP-13 but not of pro–MMP-2 or pro–MMP-8. Conclusion— MMP-14 from bone marrow–derived cells can influence the collagen content of mouse atheroma, a critical component of plaque stability.