Why don't macrophages leave atherosclerotic lesions?

Arecent study proposes a novel role for inhibitory guidance cues in regulating macrophage trafficking during atherosclerosis.1 The study authors demonstrate that Netrin-1, a laminin-related protein with a previously established role in axon migration and tumorigenesis, contributes to atherosclerosis by preventing the emigration of macrophages from plaque. Monocyte recruitment to areas of subendothelial lipid deposition has long been identified as a critical early step in the inflammatory pathophysiology of atherosclerosis.2,3 Less is known, however, about the signals that mediate both the retention and efflux of macrophages from atherosclerotic lesions. Evidence from prior studies has suggested that lipid accumulation promotes the retention of macrophages in atherosclerotic lesions by disrupting CCR7-dependent chemotaxis, which under normal conditions enhances macrophage efflux from sites of inflammation.4–6 Other studies have shown that inhibitory guidance cues mediated by the laminin-related protein Netrin-1 regulate leukocyte migration into infected or ischemic tissues.7–9 Netrin has an established role in neural development and axonal migration according to the expression of activating (eg, neogenin) or inhibitory (eg, UNC5b) receptors on target cells.7,10,11 van Gils et al hypothesized that Netrin may also be involved in blocking egress of macrophages from atherosclerotic plaques, thereby contributing to the chronic inflammatory events that promote lesion growth and destabilization. The authors found expression of Netrin-1 and the inhibitory receptor UNC5b in lesional macrophages in histological sections of human coronary arteries and also showed upregulation of Netrin-1 and UNC5b mRNA in the aortas of Ldlr / and Apoe / mice fed a high-fat diet. Peritoneal macrophage “foam cells” isolated from these mice also expressed higher levels of Netrin-1 and UNC5b, as did macrophages that had been loaded with ox-LDL (but not LDL) in vitro. Furthermore, the authors demonstrated that both recombinant Netrin-1 and Netrin-1 secreted by ox-LDL treated macrophages inhibited the in vitro chemotaxis of macrophages in response to CCL2 and the CCR7-binding chemokines CCL19 and CCL21, which are known to promote macrophage efflux from atherosclerotic plaques.4–6 These effects of Netrin were dependent on macrophage UNC5b expression and are consistent with prior findings demonstrating a broad inhibitory effect of Netrin-1 on monocyte, granulocyte, and lymphocyte chemotaxis.7 By contrast, the authors demonstrated that Netrin-1 promoted the migration of human coronary artery smooth muscle cells, which was consistent with the preferential expression of the activating receptor neogenin but not UNC5b by these cells in vitro and in histological sections of human coronary arteries. To test the effect of hematopoietic deficiency of Netrin-1 on atherosclerotic disease the authors created chimeric mice by transplanting Ntn1 / or Ntn1 / bone-marrow into irradiated Ldlr / recipients (wild-type 3 Ldlr / and Ntn1 / 3 Ldlr / ). Remarkably, after 12 weeks of high-fat diet, Ntn1 / 3 Ldlr / mice had a 50% to 60% reduction in both aortic root and en face lesion area relative to wild-type 3 Ldlr / mice. Consistent with the in vitro findings above, immunohistochemical analysis of plaques from these animals revealed fewer macrophages and infiltrating smooth muscle cells in Ntn1 / 3 Ldlr / animals, which also corresponded to reductions in the extent of lesional apoptosis and necrotic area. To demonstrate that the attenuated phenotype observed in the Ntn1 / 3 Ldlr / mice was associated with an effect on macrophage efflux, the authors used a previously described technique that involves the in vivo labeling of blood monocytes with undegradable fluorescently labeled beads which can be enumerated in histological sections of atherosclerotic lesions taken at various time points after the labeling.12 Although peak numbers of bead-labeled monocytes in plaques were similar between the 2 groups at 3 days following injection of the fluorescent beads, by 14 days the labeled cells were retained in lesions of wild-type3 Ldlr / mice but reduced in Ntn1 / 3 Ldlr / mice. These findings suggested that the reduction in lesional macrophages observed in Ntn1 / 3 Ldlr / mice resulted from an increase in macrophage efflux from the atherosclerotic plaque, presumably due to the missing inhibitory effects of Netrin-1 on macrophage chemotaxis. The paper by van Gils et al provides convincing evidence that hematopoietic Netrin-1 expression plays an important role in atherogenesis, potentially by preventing the efflux of macrophage foam cells from atherosclerotic lesions. Although the plausibility of this mechanism for Netrin-1 action during atherosclerosis is certainly supported by the in vitro The opinions expressed in this Commentary are not necessarily those of the editors or of the American Heart Association. Commentaries serve as a forum in which experts highlight and discuss articles (published elsewhere) that the editors of Circulation Research feel are of particular significance to cardiovascular medicine. Commentaries are edited by Aruni Bhatnagar and Ali J. Marian. From the Duke University School of Medicine (G.K.G.), Durham, NC; Vascular Research Division (A.H.L.), Department of Pathology, Brigham and Women’s Hospital, Boston, MA; Harvard Medical School (A.H.L.), Boston, MA. Correspondence to Andrew H. Lichtman, Brigham and Women’s Hospital, 77 Avenue Louis Pasteur, NRB-752P, Boston, MA 02115. E-mail [email protected] (Circ Res. 2012;110:1273-1275.) © 2012 American Heart Association, Inc.