Treatment of severe hypercholesterolemia in apolipoprotein E-deficient mice by bone marrow transplantation

Quantitation of apolipoprotein epsilon gene expression by competitive polymerase chain reaction in a patient with familial apolipoprotein E deficiency

A simple method of obtaining semiquantitative and reliable data on apolipoprotein (apo) sigma gene expression is described. We detected apo sigma specific sequences by reverse transcription (rT)-PCR. For quantitative measurement, an apo sigma DNA standard was produced allowing the development of a competitive PCR-method. The efficiency of RNA extraction and cDNA synthesis was controlled by quantitation of a housekeeping gene (glyceraldehyde-3-phosphatedehydrogenase, G3PDH) in separate reactions. To imitate a defined induction of apo sigma gene expression, serial twofold dilutions of total RNA were reversely transcribed and the respective cDNAs used to perform a competitive apo sigma and G3PDH PCR. The change in apo sigma cDNA and G3PDH cDNA was 1.7-2.3-fold with an expected value of 2.0-fold. Standard deviations in three independently performed experiments were within a range of < 15% of the mean, indicating low intra-assay variation and high reproducibility. To illustrate this method, apo sigma gene expression was measured in a patient with complete lack of functional active apo E in comparison to healthy controls. The method presented here might be valuable in assessment of apo sigma gene expression in human disease.

T-lymphocytes and monocytes in atherogenesis

Atherosclerosis is characterized as a chronic inflammatory-fibroproliferative disease of the vessel wall. The attachment of monocytes and T-lymphocytes to the injured endothelium followed by their migration into the intima is one of the first and most crucial steps in lesion development. The co-localization of CD4+ T-cells and macrophages in the lesion, the abundant expression of HLA Class II molecules and the co-stimulatory molecule CD40 and its ligand (CD40L) indicate a contribution of cell-mediated immunity to atherogenesis. Transgenic mouse models revealed that dependent on the model T- and B-cells may promote lesion progression, monocytes and macrophages are in contrast essential for the development of atherosclerotic lesions. Apart from the local process in the vessel wall, systemic signs of an inflammatory reaction are also associated with lesion development. Thus plasma levels of C-reactive protein and fibrinogen and the white blood cell count are positively correlated to the risk of cardiovascular disease. Recently, an inflammatory phenotype of circulating peripheral blood monocytes could be demonstrated as a specific cellular correlate to lipid and lipoprotein risk factors. Thus the pool size of LPS receptor (CD14)dim and Fc gamma IIIa receptor (CD16a)+ monocytes positively correlates to plasma cholesterol levels, to triglycerides levels and to the apolipoprotein E4 (apo E4) phenotype in contrast to a negative correlation to the high density lipoprotein (HDL) cholesterol concentration. This CD14dim CD16a+ monocytes are further characterized by a high expression of beta 1- and beta 2-integrins, suggesting a higher capacity for attachment at sites of inflammation. A proinflammatory cytokine pattern and an expansion of these cells in other inflammatory diseases are indicating that these cells promote the inflammatory process during atherogenesis. Surface expression of the activation antigen CD45RA on monocytes in correlation to plasma LDL cholesterol and Lp(a) levels further indicates an inflammatory reaction. Regarding the potential mechanisms of the phenotypic changes of peripheral blood monocytes, in a serum free in vitro differentiation model supplemented with M-CSF monocytes from probands which are homozygous for apo E4 showed a significantly higher increase of CD16a expression compared to apo E3/E3 cells indicating that a genetic polymorphism of a single apolipoprotein gene locus may affect monocyte differentiation. The further characterization of the cellular immunology of monocytes and T-lymphocytes in lesion development will provide new specific diagnostic and therapeutic targets in atherogenesis.

Phenotype-dependent differences in apolipoprotein E metabolism and in cholesterol homeostasis in human monocyte-derived macrophages

In this study, we investigated the impact of the common apoE polymorphism on apoE metabolism and cholesterol homeostasis in monocyte-derived macrophages isolated from E2/2, E3/3, and E4/4 subjects. Unloaded cells of all genotypes contained similar amounts of free cholesterol, cholesteryl ester, and apoE mRNA. E3/3 cells secreted 77 and 30% more apoE than E2/2 or E4/4 cells, respectively. Pulse-chase studies confirmed that the apoE secretion rate was greatest in E3/3 and least in E2/2 cells and showed that a portion of apoE2, but not apoE3 or apoE4, was degraded intracellularly. Surface binding of apoE was greatest in E4/4 cells, as revealed by heparinase treatment. On cholesterol loading with acetylated LDL, apoE mRNA levels and protein secretion rose most in E4/4 and least in E2/2 cells. Cholesterol and cholesteryl ester content, however, rose most in E2/2 and least in E3/3 cells. Incubations with 3H-cholesterol-labeled acetylated LDL revealed that E2/2 cells were most efficient at secreting cholesterol. The greatest reuptake of 3H-cholesterol-rich particles was from E4/4 macrophage- conditioned media. Thus, E2/2 macrophages, despite a low apoE secretion rate, are protected from cholesterol storage by apoE-mediated cholesterol efflux. In E3/3 macrophages, cholesterol accumulation is lessened by a high basal apoE secretion rate. E4/4 macrophages secrete the most apoE but lack effective net cholesterol efflux due to enhanced surface binding and reuptake of cholesterol-rich particles.

Hepatic apo E expression is required for remnant lipoprotein clearance in the absence of the low density lipoprotein receptor

According to the secretion-capture model of remnant lipoprotein clearance, apo E secreted by hepatocytes into the space of Disse serves to enrich the remnants with a ligand for receptor-mediated lipoprotein endocytosis. Current evidence supports a two-receptor model of lipoprotein removal, in which apo E-containing remnants bind either the low density lipoprotein receptor (LDLR) or the LDLR-related protein (LRP). Recently, we demonstrated that reconstitution of apo E(-/-) mice with apo E(+/+) marrow results in normalization of plasma lipoprotein levels, indicating that hepatic expression of apo E is not required for remnant clearance and calling into question the relevance of the secretion-capture mechanism. To dissect the relative contributions of LDLR and LRP to the cellular catabolism of remnant lipoproteins by the hepatocyte, bone marrow transplantation (BMT) was used to reconstitute macrophage expression of apo E in mice that were null for expression of both apo E and the LDLR. Reconstitution of macrophage apo E in apo E(-/-)/LDLR(-/-) mice had no effect on serum lipid and lipoprotein concentrations, although it produced plasma apo E levels up to 16-fold higher than in C57BL/6 controls. Immunocytochemistry of hepatic sections revealed abundant staining for apo E in the space of Disse, but no evidence of receptor-mediated endocytosis of remnant lipoproteins. Transient expression of human LDLR in the livers of apo E(+/+)--> apo E(-/-)/LDLR(-/-) mice by adenoviral gene transfer resulted in normalization of serum lipid levels and in the clearance of apo E-containing lipoproteins from the space of Disse. We conclude that whereas the LDLR efficiently clears remnant lipoproteins irrespective of the site of origin of apo E, endocytosis by the chylomicron remnant receptor (LRP) is absolutely dependent on hepatic expression of apo E. These data demonstrate in vivo the physiologic relevance of the apo E secretion-capture mechanism in the liver.

Transgenic mouse models to study the role of APOE in hyperlipidemia and atherosclerosis

Transgenic technologies have provided a series of very useful mouse models to study hyperlipidemia and atherosclerosis. Normally, mice carry cholesterol mainly in the high density lipoprotein (HDL) sized lipoproteins, and have low density lipoprotein (LDL) and very low density lipoprotein (VLDL) cholesterol levels. These low LDL and VLDL levels are due to the very rapid metabolism of remnant clearance in mice, which hamper metabolic studies. In addition, due to the lack of atherogenic lipoproteins, mice will not readily develop atherosclerosis. This situation has changed completely, because to date, most known genes in lipoprotein metabolism have been used in transgenesis to obtain mice in which genes have been silenced or overexpressed. These experiments have yielded many mouse strains with high plasma lipid levels and a greater susceptibility for developing atherosclerosis. One of the most widely used strains are knock-out mice deficient for apoE, which is one of the central players in VLDL metabolism. Subsequently, a wide variety of other transgenic studies involving APOE have been performed elucidating the role of apoE and apoE mutants in lipolysis, remnant clearance, cellular cholesterol efflux and atherogenesis. In addition, the APOE mouse models are excellent tools for the development of gene therapy for hyperlipidemias.

A leukocyte homologue of the IL-8 receptor CXCR-2 mediates the accumulation of macrophages in atherosclerotic lesions of LDL receptor-deficient mice

Chronic macrophage-mediated inflammation is central to atherosclerosis. A role of the monocyte chemotactic and activating C-C chemokine JE/monocyte chemotactic protein-1 has been proposed. However, the human C-X-C chemokines growth-regulated oncogene (GROalpha) and IL-8, and their shared receptor, CXCR-2, also can be expressed at sites of chronic inflammation. Because we detected CXCR-2 in the intima of human atherosclerotic lesions, we examined the role of leukocyte CXCR-2 expression in affecting lesion cellularity. Atherosclerosis-susceptible LDL receptor-deficient mice were irradiated, successfully repopulated with bone marrow cells that either lacked or expressed mIL-8RH (the homologue of CXCR-2), and fed an atherogenic diet for 16 wk. In recipients of mIL-8RH+/+ marrow, mIL-8RH colocalized with densely accumulated intimal MOMA-2 positive macrophages. In contrast, lesions in recipients of mIL-8RH-/- marrow lacked mIL-8RH, had little intimal MOMA-2 staining, and were less extensive. The mIL-8RH ligand KC/GROalpha was detected in the intima of all aortic atherosclerotic lesions. Thus, the capacity of leukocytes to express mIL-8RH, and associated intralesional expression of its ligands such as KC/GROalpha, mediated the intimal accumulation of macrophages in atherosclerotic lesions of LDL receptor-deficient mice.

An apolipoprotein E synthetic peptide targets to lipoproteins in plasma and mediates both cellular lipoprotein interactions in vitro and acute clearance of cholesterol-rich lipoproteins in vivo

Apolipoprotein (apo) E mediates lipoprotein binding to cellular lipoprotein receptors. Previously we reported that a synthetic peptide representing a linear dimeric repeat of amino acids 141-155 binds cellular LDL receptors. To prepare an apoE peptide that bound to both cholesterol-rich lipoproteins and lipoprotein receptors, an NH2-terminal acetylated apoE dimer peptide was synthesized. This acetylated peptide preferentially associated with lipoproteins in plasma, whereas nonacylated peptides were poor lipid binders. Acetylated peptide/LDL complexes (molar ratios of 4-5:1) enhanced the interaction of LDL with cultured human fibroblasts by 7-12-fold. Participation by both receptors and cell surface heparin sulfate proteoglycans was observed. When a preformed peptide/125I-LDL complex was injected intravenously into C57BL/6J apoE-deficient mice, its rate of removal was threefold higher than that of 125I-LDL alone. The liver and the spleen were major tissue distribution sites. Intravenous administration of free acetylated peptide resulted in a 30% reduction in total plasma cholesterol within 3-30 min, which reflected a 40-50% and 20-26% reduction in very low density lipoproteins and intermediate density lipoproteins, respectively. Therefore, this peptide selectively associated with cholesterol-rich lipoproteins and mediated their acute clearance in vivo.

Degradation of macrophage ApoE in a nonlysosomal compartment. Regulation by sterols

Macrophage-derived apoE has been shown to play an important role in the susceptibility of the vessel wall to atherosclerosis. Previous studies have shown that macrophage sterol content modulates apoE synthesis and secretion, associated with a large transcriptional response of the apoE gene. The current studies were undertaken to evaluate the existence of additional post-transcriptional regulatory loci for the effect of sterols on apoE synthesis and secretion. Using a macrophage cell line transfected to constitutively express an apoE cDNA to facilitate detection of a post-transcriptional regulatory locus, we demonstrated that preincubations in 25-hydroxycholesterol and cholesterol lead to increased apoE secretion in pulse/chase experiments. Examination of cell lysates in these experiments showed that apoE not secreted by control cells was degraded and not detectable, suggesting that the preincubation in sterols increased secretion by decreasing degradation of newly synthesized apoE. The measurement of total protein and apoE degradation in cell fractions revealed an intermediate density fraction that degraded significant amounts of newly synthesized total protein and newly synthesized apoE. In this fraction, degradation of total protein and apoE was unaffected by chloroquine but was substantially reduced by N-acetyl-Leu-Leu-norleucinal plus N-acetyl-Leu-Leu-methioninal or by lactacystin, suggesting the involvement of proteasomes. Preincubation in sterol/oxysterol or acetylated low density lipoprotein did not modify total protein degradation by this fraction but inhibited apoE degradation. Similar results were obtained using intermediate density fractions isolated from human monocyte-derived macrophages. The results of our studies indicate that newly synthesized apoE in the macrophage can be degraded in an intermediate density nonlysosomal cellular compartment, which is sensitive to proteasomal inhibitors. Alteration of cellular lipid homeostasis by preincubation in sterol/oxysterol or acetylated low density lipoprotein inhibits apoE, but not total protein, degradation in this fraction. Inhibition of the degradation of apoE in this fraction likely contributes to the increased apoE secretion observed in sterol-enriched cells.

Impaired secretion of very low density lipoprotein-triglycerides by apolipoprotein E- deficient mouse hepatocytes

To explore mechanisms underlying triglyceride (TG) accumulation in livers of chow-fed apo E-deficient mice (Kuipers, F., J.M. van Ree, M.H. Hofker, H. Wolters, G. In't Veld, R.J. Vonk, H.M.G. Princen, and L.M. Havekes. 1996. Hepatology. 24:241-247), we investigated the effects of apo E deficiency on secretion of VLDL-associated TG (a) in vivo in mice, (b) in isolated perfused mouse livers, and (c) in cultured mouse hepatocytes. (a) Hepatic VLDL-TG production rate in vivo, determined after Triton WR1339 injection, was reduced by 46% in apo E-deficient mice compared with controls. To eliminate the possibility that impaired VLDL secretion is caused by aspecific changes in hepatic function due to hypercholesterolemia, VLDL-TG production rates were also measured in apo E-deficient mice after transplantation of wild-type mouse bone marrow. Bone marrow- transplanted apo E-deficient mice, which do not express apo E in hepatocytes, showed normalized plasma cholesterol levels, but VLDL-TG production was reduced by 59%. (b) VLDL-TG production by isolated perfused livers from apo E-deficient mice was 50% lower than production by livers from control mice. Lipid composition of nascent VLDL particles isolated from the perfusate was similar for both groups. (c) Mass VLDL-TG secretion by cultured apo E-deficient hepatocytes was reduced by 23% compared with control values in serum-free medium, and by 61% in the presence of oleate in medium (0. 75 mM) to stimulate lipogenesis. Electron microscopic evaluation revealed a smaller average size for VLDL particles produced by apo E-deficient cells compared with control cells in the presence of oleate (38 and 49 nm, respectively). In short-term labeling studies, apo E-deficient and control cells showed a similar time-dependent accumulation of [3H]TG formed from [3H]glycerol, yet secretion of newly synthesized VLDL-associated [3H]TG by apo E-deficient cells was reduced by 60 and 73% in the absence and presence of oleate, respectively. We conclude that apo E, in addition to its role in lipoprotein clearance, has a physiological function in the VLDL assembly-secretion cascade.

Influence of macrophage-derived apolipoprotein E on plasma lipoprotein distribution of apolipoprotein A-I in apolipoprotein E-deficient mice

High density lipoprotein (HDL) cholesterol in apolipoprotein (apo) E-deficient mice is decreased. It has been suggested that apoA-I is lost from HDL in these mice because it must substitute for apoE as a structural protein for the abnormal cholesterol-rich lipoproteins. Therefore, we examined in vivo the influence of selective apoE expression on plasma HDL cholesterol in apoE-deficient mice. Bone marrow transplantation was used to establish macrophage-specific expression of apoE. Bone marrow transplantation normalized plasma triglycerides and significantly reduced total plasma cholesterol, but it did not increase hepatic apoA-I mRNA levels or total plasma apoA-I. Although total plasma apoA-I was not increased, HDL cholesterol measured following chromatographic separation was elevated twofold. Furthermore, plasma apoA-I was recovered from this HDL in animals expressing macrophage apoE. Compared to HDL of wildtype mice, this HDL had a similar chromatographic size distribution, but it lacked apoE and was more negatively charged. These studies indicated that plasma apoA-I distribution and HDL composition are influenced by apoE and that the abnormal apoA-I lipoprotein distribution of apoE-deficient mice can be altered in vivo by macrophage-derived apoE.

Bone marrow transplantation in apolipoprotein E-deficient mice. Effect of ApoE gene dosage on serum lipid concentrations, (beta)VLDL catabolism, and atherosclerosis

Apolipoprotein E (apoE), a high-affinity ligand for lipoprotein receptors, is synthesized by the liver and extrahepatic tissues, including cells of the monocyte/macrophage lineage. Inactivation of the apoE gene in mice leads to a prominent increase in serum cholesterol and triglyceride levels and the development of premature atherosclerosis. In this study, the role of monocyte/macrophage-derived apoE in lipoprotein remnant metabolism and atherogenesis was assessed. The influence of apoE gene dosage on serum lipid concentrations was determined by transplantation of homozygous apoE-deficient (apoE-/-), heterozygous apoE-deficient (apoE+/-), and wild-type (apoE+/+) bone marrow in homozygous apoE-deficient mice. The concentration of apoE detected in serum was found to be gene dosage dependent, being 3.52 +/- 0.30%, 1.87 +/- 0.17%, and 0% of normal in transplanted mice receiving either apoE+/+, apoE+/-, or apoE-/- bone marrow, respectively. These low concentrations of apoE nevertheless dramatically reduced serum cholesterol levels owing to a reduction of VLDL and, to a lesser extent, LDL, while HDL levels were slightly raised. After 4 months on a "Western-type" diet, atherosclerosis was evidently reduced in mice transplanted with apoE+/+ bone marrow, compared with control transplanted mice. To study the mechanism of the lipoprotein changes on bone marrow transplantation, the in vivo turnover of autologous serum (beta)VLDL was studied. The serum half-life of (beta)VLDL in transplanted mice, compared with control apoE-deficient mice, was shortened mainly as a consequence of an increased recognition and uptake by the liver. Analysis of the relative contribution of the liver parenchymal cells, endothelial cells, and Kupffer cells (liver tissue macrophages) indicated an increased uptake by parenchymal cells, while the relative contribution to Kupffer cells was decreased. In conclusion, macrophage-derived apoE can dose-dependently reduce hypercholesterolemia in apoE-deficient mice owing to increased recognition and uptake of (beta)VLDL by parenchymal liver cells, leading to a decreased susceptibility to atherosclerosis.

Effects of genotype and diet on cholesterol efflux into plasma and lipoproteins of normal, apolipoprotein A-I-, and apolipoprotein E-deficient mice

We investigated the contribution of apoE to cholesterol efflux into plasmas of normal, apoA-I-, and apoE-deficient mice, which were fed with chow- and cholesterol-rich diets. Plasmas of normal and apoA-I-deficient mice contain apoE in pre-beta-migrating VLDL as well as in HDL-like lipoproteins, which have either electrophoretic alpha- or gamma-mobilities. The latter particle resembled gamma-LpE in human plasma also by its mobility on nondenaturing two-dimensional electrophoresis. No apoE-containing lipoproteins were found in plasmas of apoE-deficient mice. When apoA-I- and apoE-deficient mice received both chow- and fat-rich diets, their plasmas released significantly less 3H-cholesterol from radiolabeled fibroblasts than did plasma of normal mice. Removal of apoE from plasmas of normal and apoA-I-deficient mice by anti-apoE immunoaffinity chromatography decreased their cholesterol efflux capacities (per 1 minute/per 1 hour) by 26%/40% (P = 0.0092/0.0007) and 30%/26% (P = 0.0092/0.0003), respectively. Net cholesterol efflux from fibroblasts into apoA-I-deficient plasma was 45% lower compared with plasma of normal mice. Incubation of fibroblasts with apoE-deficient plasma caused net influx of cholesterol. Prior addition of human apoE to or removal of apoB-containing lipoproteins from apoE-deficient plasma restored its ability to cause net cholesterol efflux to 50% of normal plasma. Some of the differences between cholesterol efflux into normal and apoE-deficient plasmas were attributable to the failure of apoE-deficient plasmas to take up cell-derived 3H-cholesterol into gamma-LpE. Compared with normal plasma, both apoA-I-deficient and apoE-deficient plasmas were significantly decreased in their activity to esterify cell-derived 3H-cholesterol. Anti-apoE chromatography decreased significantly cholesterol esterification in normal plasma and apoA-I-deficient plasma but not in apoE-deficient plasma. Taken together, the data provide evidence that apoE is an important contributor to reverse cholesterol transport, partially because of initial uptake of cell-derived cholesterol by gamma-LpE and partially because of the contribution of apoE-containing lipoproteins to esterification of cholesterol in plasma.

Effect of bone marrow transplantation on lipoprotein metabolism and atherosclerosis in LDL receptor-knockout mice

The LDL receptor (LDLR) plays an important role in the removal of LDL and its precursors, the intermediate and very low density lipoproteins, from the blood circulation. The receptor is expressed on various cell types. In this study the relative importance of the LDLR on macrophages for lipoprotein metabolism and atherogenesis was assessed. For this purpose, irradiated LDLR-knockout (-/-) mice were transplanted with bone marrow of normal C57BL/6J mice. DNA analysis showed that the transplanted mice were chimeric. The transplantation resulted in a slight decrease of total serum cholesterol when compared with LDLR-/- mice that were transplanted with LDLR-/- bone marrow. This modest decrease, however, did not reach statistical significance at all time points examined. This decrease can be almost completely attributed to a decrease in LDL cholesterol. The specific lowering of LDL cholesterol could clearly be observed at 4 weeks after transplantation, but the decrease was less at 12 weeks after transplantation. Quantification of atherosclerotic lesions of mice fed a 1% cholesterol diet for 6 months revealed that there were no differences in mean lesion area between mice transplanted with wild-type bone marrow or LDLR-/- bone marrow. We anticipate that in LDLR-/- mice transplanted with wild-type bone marrow, the LDLR is downregulated by the relatively high concentrations of circulating cholesterol. In vitro incubations of peritoneal macrophages with 125I-LDL indicated that the LDLR of these cells could be downregulated by 25-hydroxycholesterol. Peritoneal macrophages isolated from LDLR-/- mice transplanted with wild-type bone marrow, in contrast to those transplanted with LDLR-/- bone marrow, were able to degrade 125I-LDL, indicating that the capacity to express functional LDLR was achieved. In conclusion, introduction of the LDLR into LDLR -/- mice via bone marrow transplantation resulted in only a relatively modest decrease of LDL cholesterol that became less pronounced at later time points, possibly due to downregulation of the LDLR. To utilize the LDLR in macrophages for effective cholesterol lowering, either the sterol-regulatory elements have to be "silenced" or a high-expression LDLR construct has to be introduced into macrophages, eg, via transplantation of in vitro transfected hematopoietic stem cells.

Increased atherosclerosis in mice reconstituted with apolipoprotein E null macrophages

Macrophage-derived foam cells express apolipoprotein E (apoE) abundantly in atherosclerotic lesions. To examine the physiologic role of apoE secretion by the macrophage in atherogenesis, bone marrow transplantation was used to reconstitute C57BL/6 mice with macrophages that were either null or wild type for the apoE gene. After 13 weeks on an atherogenic diet, C57BL/6 mice reconstituted with apoE null marrow developed 10-fold more atherosclerosis than controls in the absence of significant differences in serum cholesterol levels or lipoprotein profiles. ApoE expression was absent in the macrophage-derived foam cells of C57BL/6 mice reconstituted with apoE null marrow. Thus, lack of apoE expression by the macrophage promotes foam cell formation. These data support a protective role for apoE expression by the macrophage in early atherogenesis.

Role of leukocyte-specific LDL receptors on plasma lipoprotein cholesterol and atherosclerosis in mice

Bone marrow-derived macrophages and lymphocytes express LDL receptors (LDL-R), which allow these cells to take up cholesterol-rich lipoproteins. Although these cells are ubiquitously distributed in the body, it is not known whether they influence plasma cholesterol. Macrophages and T lymphocytes also are found in atherosclerotic lesions, but it is not known whether their LDL-R expression plays a role in atherosclerosis. To address these questions, we subjected LDL-R -/-mice to total body irradiation to eliminate their endogenous bone marrow-derived cells and repopulated them with either LDL-R-expressing wild-type bone marrow (treated mice) or LDL-R -/- bone marrow (control mice). Thus, the only difference between the two groups of mice was the ability of the bone marrow-derived cells to express the LDL-R in the treated mice. Plasma cholesterol levels were similar in the two groups of mice at 8 and 16 weeks after transplantation. Chromatographic separation of the lipoproteins revealed similar lipoprotein cholesterol distributions. Although the extent of lesion area in the aortic valves of the high-fat-diet-fed mice was more severe than that in the chow-fed mice, lesions appeared similar between control and treated mice given either chow or high-fat diet. Abundant LDL-R expression was detected in the lesions of treated mice, whereas the lesions of control mice showed no LDL-R expression, indicating that donor-derived leukocytes had migrated into the lesions of the recipient mice. Thus, bone marrow transplantation can be used as a tool to replace the endogenous bone marrow-derived cells in the artery wall with those of the donor origin.

Peripheral blood mononuclear phagocyte subpopulations as cellular markers in hypercholesterolemia

Mononuclear phagocytes play a major role in the development of vascular lesions in atherogenesis. The goal of our study was to characterize circulating blood monocyte subpopulations as potential cellular markers of systemic immunological abnormalities in hypercholesterolemia. In normal subjects, three-parameter immunophenotyping of whole blood revealed that 61.3 +/- 6.0% of monocytes showed "bright" expression of the lipopolysaccharide receptor (LPSR: CD14) and Fc gamma receptor I (RI: CD64) without expression of Fc gamma-RIII (CD16). Other monocyte subsets (populations 2, 3, 4, and 5) were characterized by the simultaneous expression of both Fc gamma-R's (25.6 +/- 5.0%), isolated expression of Fc gamma-RIII (9.4 +/- 1.7%), or high expression of CD33 (3.7 +/- 1.1%) with only dim expression of CD14, respectively. The smallest subset of monocytes (population 5: 2.1 +/- 0.8%) differed from the predominant population of CD14brightCD64+CD16- monocytes by additional expression of neural cell adhesion molecule (N-CAM: CD56). In a group of hypercholesterolemic patients (n = 19), high density lipoprotein cholesterol levels were negatively correlated to the population size of CD64-CD16+ monocytes. In both healthy subjects (n = 55) and hypercholesterolemic patients, the rare apolipoprotein E3/E4 and E4/E4 phenotypes were associated with a tendency toward a larger population of CD64-CD16+ monocytes. Expression of the variant activation antigen CD45RA by peripheral blood mononuclear phagocytes showed a positive correlation to plasma levels of the atherogenic lipoproteins low density lipoprotein and lipoprotein(a). These data suggest that systemic abnormalities in mononuclear phagocyte subpopulations may play a role in the pathogenesis of atherosclerosis.

Mouse models of atherosclerosis

As a species the mouse is highly resistant to atherosclerosis. However, through induced mutations it has been possible to develop lines of mice that are susceptible to this disease. For example, mice that are deficient in apolipoprotein E, a ligand important in lipoprotein clearance, develop atherosclerotic lesions resembling those observed in humans. These lesions are exacerbated when the mice are fed a high-cholesterol, high-fat, Western-type diet. Other promising models are mice that are deficient in the low density lipoprotein receptor and transgenic mice that express human apolipoprotein B and transdominant mutant forms of apolipoprotein E. These models are now being used to study the pathogenesis of atherosclerotic lesions, as well as the influence of genetics, environment, hormones, and drugs on lesion development.