Enhanced metabolism of LDL aggregates mediated by specific human monocyte IgG Fc receptors

The role of Fcγ receptors in atherosclerosis

Atherosclerosis is widely considered to be an immune-mediated process. Fcγ receptors (Fcγ Rs) contribute to the regulation of a multitude of immune and inflammatory responses and are implicated in human atherosclerotic lesions. Major cell types involved in the pathogenesis of atherosclerosis express Fcγ Rs and their proatherogenic ligands such as immune complexes and C-reactive protein, which act to activate Fcγ R signaling pathways. This review summarizes recent significant progress addressing the multifaceted roles of Fcγ Rs in atherogenesis which comes from the studies of Fcγ R-deficient animal models, clinical investigations and in vitro molecular and cellular studies. These new findings help us appreciate the emerging role of Fcγ Rs in atherosclerosis, and suggest Fcγ Rs as a potential therapeutic target for atherosclerosis.

The inhibitory FcγRIIb modulates the inflammatory response and influences atherosclerosis in male apoE(-/-) mice

BACKGROUND

Atherosclerosis is widely accepted as an inflammatory disease involving both innate and adaptive immunity. B cells and/or antibodies have previously been shown to play a protective role against atherosclerosis. Aside from their ability to bind to antigens, antibodies can influence inflammatory responses by interacting with various Fcγ receptors on the surface of antigen presenting cells. Although studies in mice have determined that stimulatory Fcγ receptors contribute to atherosclerosis, the role of the inhibitory Fcγ receptor IIb (FcγRIIb) has only recently been investigated.

METHODS AND RESULTS

To determine the importance of FcγRIIb in modulating the adaptive immune response to hyperlipidemia, we generated FcγRIIb-deficient mice on the apoE-deficient background (apoE/FcγRIIb(-/-)). We report that male apoE/FcγRIIb(-/-) mice develop exacerbated atherosclerosis that is independent of lipid levels, and is characterized by increased antibody titers to modified LDL and pro-inflammatory cytokines in the aorta.

CONCLUSIONS

These findings suggest that antibodies against atherosclerosis-associated antigens partially protect against atherosclerosis in male apoE(-/-) mice by conveying inhibitory signals through the FcγRIIb that downregulate pro-inflammatory signaling via other immune receptors. These data are the first to describe a significant in vivo effect for FcγRIIb in modulating the cytokine response in the aorta in male apoE(-/-) mice.

Fcgamma receptor deficiency confers protection against atherosclerosis in apolipoprotein E knockout mice

IgG Fc receptors (FcgammaRs) play a role in activating the immune system and in maintaining peripheral tolerance, but their role in atherosclerosis is unknown. We generated double-knockout (DKO) mice by crossing apolipoprotein E-deficient mice (apoE(-/-)) with FcgammaR gamma chain-deficient mice (gamma(-/-)). The size of atherosclerotic lesions along the aorta was approximately 50% lower in DKO compared with apoE(-/-) control mice, without differences in serum lipid levels. The macrophage and T-cell content of lesions in the DKO were reduced by 49+/-6% and 56+/-8%, respectively, compared with the content in apoE(-/-) lesions. Furthermore, the expression of monocyte chemoattractant protein-1 (MCP-1), RANTES (Regulated on Activated Normal T-cell Expressed and Secreted), and intercellular adhesion molecule-1 (ICAM-1) and the activation of nuclear factor-kappaB (NF-kappaB) were significantly reduced in aortic lesions from DKO mice. In vitro, vascular smooth muscle cells (VSMCs) from both gamma(-/-) and DKO mice failed to respond to immune complexes, as shown by impaired chemokine expression and NF-kappaB activation. ApoE(-/-) mice have higher levels of activating FcgammaRI and FcgammaRIIIA, and inhibitory FcgammaRIIB, compared with wild-type mice. The DKO mice express only the inhibitory FcgammaRIIB receptor. We conclude that FcgammaR deficiency limits development and progression of atherosclerosis. In addition to leukocytes, FcgammaR activation in VSMCs contributes to the inflammatory process, in part, by regulating chemokine expression and leukocyte invasion of the vessel wall. These results underscore the critical role of FcgammaRs in atherogenesis and support the use of immunotherapy in the treatment of this disease.

Atherogenic autoantigen: oxidized LDL complexes with beta2-glycoprotein I

Beta2-Glycoprotein I (beta2-GPI) is a major antigen for antiphospholipid antibodies present in patients with antiphospholipid syndrome (APS). In 1997, we demonstrated that beta2-GPI specifically binds to Cu2+-oxidized low-density lipoprotein (oxLDL) and that the beta2-GPI-oxLDL complex is subsequently targeted by anti-beta2-GPI antibodies in vitro. Then ligands for beta2-GPI were purified from oxLDL and characterized as omega-carboxylated 7-ketocholesteryl esters, such as 7-ketocholesteryl-9-carboxynonanoate (oxLig-1) and 7-ketocholesteryl-12-carboxy (keto) dodecanoate (oxLig-2). These ligands mediate to form oxLDL-beta2-GPI complexes, and the complexes are taken up avidly by macrophages via anti-beta2-GPI autoantibody-mediated phagocytosis. We recently demonstrated that appearance of autoantibodies against a complex of beta2-GPI and oxLig-1 are highly associated with a history of arterial thrombosis. Serum oxLDL-beta2-GPI complex and their IgG immune complexes are also risk factors arterial thrombosis in APS patients. There is increasing circumstantial evidence of autoimmune mechanism involving beta2-GPI and oxLDL in the atherogenesis in APS.

Autoantibody-mediated atherosclerosis

Beta2-glycoprotein I (beta2-GPI) is a major antigen for antiphospholipid antibodies (aPL) present in patients with antiphospholipid syndrome (APS). Oxidized low-density lipoprotein (oxLDL) is subsequently targeted by beta2-GPI and anti-beta2-GPI autoantibodies. Ligands specific for beta2-GPI derived from oxLDL have been characterized as oxidized forms of cholesteryl linoleate, such as 7-ketocholesterol-9-carboxynonanoate, i.e. 9-oxo-9-(7-ketocholest-5-en-3beta-yloxy) nonanoic acid, (namely oxLig-1). The in vitro phenomenon that it is significantly increased in binding of oxLig-1 containing liposomes to macrophages via an interaction with beta2-GPI and an anti-beta2-GPI autoantibody (via the Fcgamma receptor) may propose a novel mechanism on 'autoantibody-mediated atherosclerosis'. Furthermore, autoantibodies against a complex of beta2-GPI and oxLig-1 are detected in sera of APS patients and appearance of the antibodies is associated with episodes of thrombosis, especially, arterial thrombosis. Thus, autoimmune atherogenesis linked to beta2-GPI interaction with oxLDL and autoantibodies may be present in APS.

Omega-carboxyl variants of 7-ketocholesteryl esters are ligands for beta(2)-glycoprotein I and mediate antibody-dependent uptake of oxidized LDL by macrophages

beta(2)-Glycoprotein I (beta(2)-GPI) is a major antigen for anticardiolipin antibodies (aCL, Abs) present in patients with antiphospholipid syndrome. We recently reported that beta(2)-GPI specifically binds to oxidized LDL (oxLDL) and that the beta(2)-GPI's major ligand, oxLig-1 is 7-ketocholesteryl-9-carboxynonanoate (Kobayashi, K., E. Matsuura, Q. P. Liu, J. Furukawa, K. Kaihara, J. Inagaki, T. Atsumi, N. Sakairi, T. Yasuda, D. R. Voelker, and T. Koike. 2001. A specific ligand for beta(2)-glycoprotein I mediates autoantibody-dependent uptake of oxidized low density lipoprotein by macrophages. J. Lipid Res. 42: 697-709). In the present study, we demonstrate that omega-carboxylated 7-ketocholesteryl esters are critical for beta(2)-GPI binding. A positive ion mass spectrum of a novel ligand, designated oxLig-2, showed fragmented ions at m/z 383 and 441 in the presence of acetone, which share features of oxLig-1 and 7-ketocholesterol. In the negative ion mode, ions at m/z 627, 625, and 243 were observed. oxLig-2 was most likely 7-ketocholesteryl-12-carboxy (keto) dodecanoate. These ligands were recognized by beta(2)-GPI. Liposome binding to macrophages was significantly increased depending on the ligand's concentration, in the presence of beta(2)-GPI and an anti-beta(2)-GPI Ab. Synthesized variant, 7-ketocholesteryl-13-carboxytridecanoate (13-COOH-7KC), also showed a significant interaction with beta(2)-GPI and a similar binding profile with macrophages. Methylation of the carboxyl function diminished all of the specific ligand interactions with beta(2)-GPI. Thus, omega-carboxyl variants of 7-ketocholesteryl esters can mediate anti-beta(2)-GPI Ab-dependent uptake of oxLDL by macrophages, and autoimmune atherogenesis linked to beta(2)-GPI interaction with oxLDL.

Apolipoprotein AI and HDL(3) inhibit spreading of primary human monocytes through a mechanism that involves cholesterol depletion and regulation of CDC42

The objective of the current study was to characterize the influence of high density lipoproteins (HDL) on processes related to the vascular recruitment of human monocytes, which may contribute to the anti-atherogenic properties of these lipoproteins. We show that HDL(3) and apo AI inhibit the following processes in primary human monocytes: (1) M-CSF induced cell spreading; (2) M-CSF stimulated expression of surface molecules involved in adhesion, migration, and scavenging; (3) fMLP induced chemotaxis. These processes are obviously modulated by the regulation of cellular cholesterol pools as indicated by the following findings. In Tangier monocytes with defective apo AI induced cholesterol efflux, apo AI had no influence on the spreading response. In control cells, stimulation of cholesterol efflux by p-cyclodextrin mimicked the effect of apo AI and HDL(3) on spreading and chemotaxis, whereas cholesterol loading with enzymatically modified LDL (E-LDL) showed the opposite effect. Finally, a similar inverse regulation by E-LDL and apo AI/HDL(3) was also observed in regard to the surface expression of beta(1)- and beta(2)-integrins as well as the hemoglobin/haptoglobin scavenger receptor CD163 and the Fcgamma-IIIaR CD16. CDC42 was identified as a potential downstream target linking changes in cellular cholesterol content to monocyte spreading and chemotaxis. Thus, CDC42 antisense markedly reduced spreading and, in parallel with their influence on monocyte spreading, HDL(3), apo AI and p-cyclodextrin down-regulated CDC42 expression while E-LDL had the inverse effect. The apo AI induced decrease of CDC42 protein expression was paralleled by the reduction of active GTP-bound CDC42. In summary, we provide evidence that HDL(3) and apo AI are able to inhibit processes in primary human monocytes, which are related to the recruitment of monocytes into the vessel wall and probably involve regulation of cellular cholesterol pools and CDC42 function.

Immunocytochemical detection of Fcgamma receptors in human atherosclerotic lesions

Much evidence indicates that atherosclerotic lesions are largely of an inflammatory nature. Activated macrophages and macrophage-derived foam cells laden with cholesterol esters are a major constituent of these lesions and can influence lesion formation via several potential mechanisms. One such mechanism is Fcgamma receptor activation and/or Fcgamma receptor-mediated clearance of immune complexes containing cholesterol, such as lipoprotein immune complexes. That this mechanism contributes to lesion formation would be further supported if Fcgamma receptor expression in arterial lesions were demonstrated. We therefore used monoclonal antibodies and immunocytochemical methods to analyze frozen sections of human arterial lesions for expression of each of the three primary classes of mononuclear phagocyte Fcgamma receptors. Approximately 800 sections of aorta, carotid, and coronary arteries obtained from five elderly donors were analyzed. The presence of macrophages was determined by assaying reactivity of a monoclonal antibody specific to CD163, which is expressed only on cells of the human mononuclear phagocyte lineage. Results indicate that highly cellular preatheromatous lesions contained numerous macrophages in the zone of proliferation that expressed each class of Fcgamma receptor (FcgammaRIA, FcgammaRIIA, and FcgammaRIIIA). Fcgamma receptor-positive cells were also present in medial and adventitial areas. Fcgamma receptor staining was both punctate and diffuse, the latter suggesting that soluble receptors were present in the extracellular matrix. These data further support that Fcgamma receptor-mediated clearance of immune complexes can occur in arterial lesions during atherogenesis. Expression of both the high affinity (FcgammaRIA) and lower affinity (FcgammaRIIA/FcgammaRIIIA) receptors indicates that mono- and multivalent IgG-containing immune complexes could engage Fcgamma receptors and influence lesion formation through several different inflammatory mechanisms triggered by receptor activation.

Levels of expression of Fcgamma receptor IIA (CD32) are decreased on peripheral blood monocytes in patients with severe atherosclerosis

To obtain information in vivo concerning the role of Fcgamma receptors (FcgammaR) in atherosclerosis, we used quantitative flow cytometry to measure the levels of expression of FcgammaRI and FcgammaRIIA on peripheral monocytes in patients with severe atherosclerosis. Expression of several other markers was also measured. We found that differences in the levels of expression of FcgammaRI were not statistically significant when compared between patients and control subjects. For FcgammaRIIA, levels of expression were decreased in the patient group, a difference that was statistically significant. Levels of expression of CD14 and CD36 were also significantly decreased in the patient group. The decrease in expression of FcgammaRIIA was statistically significant when the effects of current cigarette smoking status or medication use, including statins, were taken into account. There was also a positive and statistically significant correlation between high-density lipoprotein-cholesterol and levels of expression of FcgammaRIIA for all subjects. In contrast, decreased levels of expression of CD14 and CD36 were strongly associated with current smoking status or statin use. In summary, levels of expression of FcgammaRIIA on peripheral blood monocytes were significantly decreased in patients with clinical atherosclerosis. Additional studies are warranted to determine if levels of expression of FcgammaRIIA have utility as a phenotypic marker for assessing relative risk of atherosclerotic disease.

A soluble mannose receptor immunoadhesin enhances phagocytosis of Pneumocystis carinii by human polymorphonuclear leukocytes in vitro

Pneumocystis carinii is an opportunistic pathogen that causes pneumonia in immunocompromised hosts. In the normal host, P. carinii is susceptible to an array of first line host defense mechanisms that are operative in the lung. Alveolar macrophages play a central role in the clearance of inhaled organisms. The macrophage mannose receptor (MR) appears to be sufficient for P. carinii phagocytosis. In individuals infected with the human immunodeficiency virus, MR expression on alveolar macrophages and P. carinii phagocytosis are decreased, however, Fc-receptor mediated phagocytosis remains intact. In this study, we demonstrate that a recombinant soluble MR immunoadhesin, consisting of the essential carbohydrate binding MR ectodomain and the Fc-region of human immunoglobulin (Ig)G1, binds P. carinii and leads to an 8.2-fold increased uptake of P. carinii by phagocytic cells. Our results suggest that the soluble MR immunoadhesin may have therapeutic potential in the treatment of P. carinii infections.

Inverse relationship between circulating oxidized low density lipoprotein (oxLDL) and anti-oxLDL antibody levels in healthy subjects

Oxidized low density lipoprotein (oxLDL) has been implicated in the pathogenesis of atherosclerosis. Recent studies have shown that immunization of animals with oxLDL results in suppression of atherogenesis. Antibody against oxLDL (oxLDL Ab) is detectable in human sera, although its biological significance is not well established. We examined the relationship between oxLDL Ab titer and circulating oxLDL level in 130 healthy Japanese subjects. OxLDL was measured as apolipoprotein (apo) B-containing lipoproteins carrying oxidized phosphatidylcholines by a sensitive ELISA. IgG class oxLDL Ab titer was measured by ELISA. Plasma oxLDL concentration was very low and it corresponded on average to one to two out of 1000 apoB-containing lipoproteins in plasma. Plasma oxLDL correlated positively with LDL cholesterol and inversely with oxLDL Ab titer. These associations remained significant and independent in multiple regression analysis including age, gender, smoking, and high-density lipoprotein cholesterol. These data indicate that healthy subjects have a very low concentration of oxLDL in the circulation, and that oxLDL Ab titer is in an inverse relationship with plasma oxLDL concentration in this population. Although these results suggest that oxLDL Ab may play a role in maintaining the low level of plasma oxLDL, its role in atherogenesis awaits further studies.

Expression of ACAT-1 protein in human atherosclerotic lesions and cultured human monocytes-macrophages

The acyl coenzyme A:cholesterol acyltransferase (ACAT) gene was first cloned in 1993 (Chang et al, J Biol Chem. 1993;268:20747-20755; designated ACAT-1). Using affinity-purified antibodies raised against the N-terminal portion of human ACAT-1 protein, we performed immunohistochemical localization studies and showed that the ACAT-1 protein was highly expressed in atherosclerotic lesions of the human aorta. We also performed cell-specific localization studies using double immunostaining and showed that ACAT-1 was predominantly expressed in macrophages but not in smooth muscle cells. We then used a cell culture system in vitro to monitor the ACAT-1 expression in differentiating monocytes-macrophages. The ACAT-1 protein content increased by up to 10-fold when monocytes spontaneously differentiated into macrophages. This increase occurred within the first 2 days of culturing the monocytes and reached a plateau level within 4 days of culturing, indicating that the increase in ACAT-1 protein content is an early event during the monocyte differentiation process. The ACAT-1 protein expressed in the differentiating monocytes-macrophages was shown to be active by enzyme assay in vitro. The high levels of ACAT-1 present in macrophages maintained in culture can explain the high ACAT-1 contents found in atherosclerotic lesions. Our results thus support the idea that ACAT-1 plays an important role in differentiating monocytes and in forming macrophage foam cells during the development of human atherosclerosis.

Expression of lipoprotein receptors in atherosclerotic lesions

Atherosclerosis is characterized by the presence of lipid-loaded cells which are derived from macrophages and smooth muscle cells. Several lipoprotein receptors may be involved in cellular lipid uptake. These receptors include: scavenger receptor(s); LDL receptor-related protein/alpha2-macroglobulin receptor (LRP); LDL receptor; and VLDL receptor. With the exception of the LDL receptor, all of these receptors are expressed in atherosclerotic lesions. While scavenger receptors are mostly expressed in macrophages, the LRP and VLDL receptor may play an important role in mediating lipid uptake in smooth muscle cells. It is evident that no single receptor pathway is solely responsible for the increased lipid uptake in lesion cells but several redundant mechanisms may contribute to the uptake and degradation of lipoproteins in atherosclerotic lesions.

Aggregated low density lipoprotein induces and enters surface-connected compartments of human monocyte-macrophages. Uptake occurs independently of the low density lipoprotein receptor

Aggregation of low density lipoprotein (LDL) stimulates its uptake by macrophages. We have now shown by electron microscopic and chemical experiments that aggregated LDL (produced by vortexing (VxLDL) or treatment with phospholipase C) induced and became sequestered in large amounts within surface-connected compartments (SCC) of human monocyte-derived macrophages. This occurred through a process different from phagocytosis. Formation of SCC and accumulation of aggregated LDL in SCC are cell-mediated processes that were temperature-dependent (10 x greater cell association at 37 degrees C than at 4 degrees C) and blocked by cytochalasin D but not by nocodazole. Because of the surface connections of SCC, trypsin could release aggregated LDL from SCC. Degradation of 125I-VxLDL through the SCC pathway showed delayed and a lower rate of degradation (10-55%) compared with nonaggregated 125I-acetylated LDL that did not enter SCC. However, similar to 125I-acetylated LDL degradation, 125I-VxLDL degradation occurred through a chloroquine-sensitive pathway. Uptake of VxLDL into SCC was not mediated by the LDL receptor. Methylation of LDL prevents its binding to the LDL receptor. However, methylated LDL still entered SCC after it was aggregated by vortexing. On the other hand, degradation of 125I-VxLDL was substantially decreased by methylation of LDL and by cholesterol enrichment of macrophages, which decreases macrophage LDL receptor expression. The results suggest that whereas uptake of aggregated LDL into SCC occurs independently of the LDL receptor, movement of aggregated LDL from SCC to lysosomes may depend in part on LDL receptor function. Sequestration into SCC is a novel endocytosis pathway for uptake of aggregated LDL that allows the macrophage to store large amounts of this lipoprotein before it is further processed.

The uptake of LDL-IC by human macrophages: predominant involvement of the Fc gamma RI receptor

The incubation of human macrophages with antigen antibody complexes prepared with rabbit anti-LDL and human LDL (LDL-IC) is followed by ingestion of those immune complexes (IC), massive cholesterol ester accumulation, cytokine release and overexpression of the LDL receptor. The massive accumulation of cholesterol esters and overexpression of the native LDL receptor are specifically induced by immune complexes containing native or modified LDL, but not by any other type of IC. We report the results of a series of experiments aimed at defining the receptor preferentially involved in LDL-IC uptake. Flow cytometry studies using CD16, CD32 and CD64 monoclonal antibodies showed a sharp reduction on the expression of CD64 (Fc gamma RI) both by human monocyte-derived macrophages and THP-1 cells after incubation with LDL-IC, suggesting preferential engagement of this type of Fc receptor. Blocking experiments with aggregate-free IgG1 and CD32 monoclonal antibody confirmed that blocking Fc gamma RI prevented both LDL-IC uptake and the upregulation of LDL receptors on THP-1 cells. In contrast, blocking Fc gamma RII did not affect either the uptake of LDL-IC or the expression of LDL receptors on the same cells. The preferential engagement of Fc gamma R-I by LDL-IC suggests a biological difference of LDL-IC relative to other types of IC and opsonized particles. The precise molecular mechanism(s) responsible for the paradoxical upregulation of LDL receptor after the uptake of LDL-IC remain to be elucidated.

Evidence that human Fc gamma receptor IIA (CD32) subtypes are not receptors for oxidized LDL

Several lines of evidence suggest that clearance of oxidized LDL (oxLDL) immune complexes by macrophage IgG Fc receptors (Fc gamma Rs) plays a role in atherogenesis. Ox-LDL may also be cleared directly by Fc gamma Rs, as shown for murine Fc gamma RII-B2. In humans, the homologous Fc gamma R is Fc gamma RIIA (CD32), which is abundantly expressed on monocytes and macrophages and shares 60% sequence identity with murine Fc gamma RII-B2. As murine Fc gamma RII-B2 and human Fc gamma RIIA also share similar IgG ligand-binding properties, the purpose of this study was to test the hypothesis that human CD32 is a receptor for oxLDL. For these studies we used transfected Chinese hamster ovary (CHO) cells, monocytes, and cell lines that functionally express either of two Fc gamma RIIA subtypes (R131 or H131) and assayed binding or degradation of several preparations of oxLDL. The integrity of all oxLDL preparations was checked by studying their ability to react with CHO cells expressing human type I scavenger receptors and by other characteristics of lipoprotein oxidation. Although we showed that each preparation of oxLDL could recognize class A or class B scavenger receptors, we did not detect any differences in the binding or degradation of any type of oxLDL preparation among control versus CHO cell transfectants. Using monocytes that express Fc gamma RIIA and CD36, we showed that the binding of oxLDL was inhibited by antibodies to CD36, but not by Fc gamma RIIA antibodies. Thus, the data do not support the hypothesis that human Fc gamma RIIA is by itself a receptor for oxLDL. We conclude that human CD32 can mediate uptake of lipoprotein immune complexes, but does not mediate uptake of oxLDL in the absence of anti-oxLDL antibodies. OxLDL may interact with human mononuclear phagocytes directly via other types of receptors, such as class A and class B scavenger receptors or CD68.

Targeting lipoproteins to Fc gamma receptors with bispecific antibodies

The data presented here support use of bispecific antibodies (BsAb) for studying the role of each of the different types of IgG Fc receptors (Fc gamma Rs) in uptake and metabolism of low-density lipoprotein (LDL) immune complexes. The bispecific anti-Fc gamma R x anti-LDL antibodies used in these studies were effective in specifically triggering metabolic uptake and degradation of LDL immune complexes (LDL-IC) through each type of Fc gamma R. Using LDL-IC prepared with LDL aggregates, foam cell formation was induced with relatively acute stimulation. Thus, these conditions will be appropriate for studying lipoprotein metabolism in the context of specific Fc gamma R under a variety of conditions to determine if, in fact, there are differences in sterol metabolism associated with the different types of Fc gamma R and for comparing metabolism to that mediated by the other important pathways of lipoprotein uptake (native LDL receptors and scavenger receptors). The results of these studies will reveal which of these pathways are potentially most important in foam cell formation and might suggest the possibility that macrophage foam cell formation could be altered by redirecting LDL to particular pathways.