Some monocytes normally take up residence in tissues as sessile macrophages, but others differentiate into migratory cells resembling dendritic cells that emigrate to lymph nodes. In an in vitro model of a vessel wall, lipid mediators lysophosphatidic acid and platelet-activating factor, whose signals are implicated in promoting atherosclerosis, blocked conversion of monocytes into migratory cells and favored their retention in the subendothelium. In vivo studies revealed trafficking of monocyte-derived cells from atherosclerotic plaques during lesion regression, but little emigration was detected from progressive plaques. Thus, progression of atherosclerotic plaques may result not only from robust monocyte recruitment into arterial walls but also from reduced emigration of these cells from lesions.

Mouse aortic smooth muscle cells (SMCs) were loaded for 72 h with cholesterol by using cholesterol:methyl-␤-cyclodextrin complexes, leading to Ϸ2-fold and Ϸ10-fold increases in the contents of total cholesterol and cholesteryl ester, respectively. Foam-cell formation was demonstrated by accumulation of intracellular, Oil Red O-stained lipid droplets. Immunostaining showed decreased protein levels of smooth muscle ␣-actin and ␣-tropomyosin and increased levels of macrophage markers CD68 and Mac-2 antigen. Quantitative real-time RT-PCR revealed that after cholesterol loading, the expression of SMC-related genes ␣-actin, ␣-tropomyosin, myosin heavy chain, and calponin H1 decreased (to 11.5 ؎ 0.5%, 29.3 ؎ 1.4%, 23.8 ؎ 1.4%, and 3.8 ؎ 0.5% of unloaded cells, respectively; P < 0.05 for all), whereas expression of macrophage-related genes CD68, Mac-2, and ABCA1 mRNA increased (to 709 ؎ 84%, 330 ؎ 11%, and 207 ؎ 13% of unloaded cells, respectively; P < 0.05 for all), thereby demonstrating that the protein changes were regulated at the mRNA level.

To develop a murine model system to test the role of monocyte-derived macrophages in atherosclerosis, the osteopetrotic (op) mutation in the macrophage colonystimulating factor gene was bred onto the apolipoprotein E (apoE)-deficient background. The doubly mutant (op/apoEdeficient) mice fed a low-fat chow diet had significantly smaller proximal aortic lesions at an earlier stage of progression than their apoE-deficient control littermates. These lesions in the doubly mutant mice were composed of macrophage foam cells. The op/apoE-deficient mice also had decreased body weights, decreased blood monocyte differentials, and increased mean cholesterol levels of 41300 mg/dl. Statistical analysis determined that atherosclerosis lesion area was significantly affected by the op genotype and gender. The confounding variables of body weight, plasma cholesterol, and monocyte differential, which were all affected by op genotype, had no significant additional effect on lesion area once they were adjusted for the effects of op genotype and gender.

Atherosclerosis regression is an important clinical goal. In previous studies of regression in mice, the rapid loss of plaque foam cells was explained by emigration to lymph nodes, a process reminiscent of dendritic cells. In the present study, plaque-containing arterial segments from apoE ؊/؊ mice were transplanted into WT recipient normolipidemic mice or apoE ؊/؊ mice. Three days after transplant, in the WT regression environment, plaque size decreased by Ϸ40%, and foam cell content by Ϸ75%. In contrast, both parameters increased in apoE ؊/؊ recipients. Foam cells were isolated by laser capture microdissection. In WT recipients, there were 3-to 6-fold increases in foam cells of mRNA for liver X receptor ␣ and cholesterol efflux factors ABCA1 and SR-BI. Although liver X receptor ␣ was induced, there was no detectable expression of its putative activator, peroxisome proliferator-activated receptor ␥. Expression levels of VCAM or MCP-1 were reduced to 25% of levels in pretransplant or apoE ؊/؊ recipient samples, but there was induction at the mRNA and protein levels of chemokine receptor CCR7, an essential factor for dendritic cell migration. Remarkably, when CCR7 function was abrogated in vivo by treatment of WT recipients with antibodies to CCR7 ligands CCL19 and CCL21, lesion size and foam cell content were substantially preserved. In summary, in foam cells during atherosclerosis regression, there is induction of CCR7 and a requirement for its function. Taken with the other gene expression data, these results in vivo point to complex relationships among the immune system, nuclear hormone receptors, and inflammation during regression.cholesterol efflux ͉ dendritic cell ͉ macrophage ͉ monocyte ͉ nuclear hormone receptor M ouse models of atherosclerosis regression are relatively few.However, similarities between atherosclerosis in humans and in mice deficient in apolipoprotein E (apoE Ϫ/Ϫ ) or the LDL receptor (e.g., see ref. 1) suggest that molecular mechanisms underlying regression in mouse models could be relevant to the reduction of the large plaque burden in the middle-aged and older human population.We have previously described a mouse model of regression. First, plaques were allowed to develop in apoE Ϫ/Ϫ mice, then a segment of either thoracic aortic (2) or aortic arch (3, 4) was transplanted into the abdominal aorta of a WT recipient, quickly changing the plasma lipid environment from hyper to normolipidemia. As a control, an aortic segment was transplanted into an apoE Ϫ/Ϫ mouse. By 1 month, in the regression environment (WT recipient), essentially all of the foam cells disappeared from plaques (4), with many no longer visible after 3 days (5). In contrast, in the progression environment (apoE Ϫ/Ϫ recipient), plaque size and foam cell content increased over time.In a recent study, we showed that the rapid depletion of foam cells in the regression environment was correlated with a substantial number of these cells emigrating to lymph nodes (5). Interestingly, the emigrating cells expressed markers of den...