The macrophage scavenger receptor type A directs modified proteins to antigen presentation

CD4+ T cells in atherosclerosis

Atherosclerosis is an inflammatory disease. T lymphocytes, occurring concomitantly with macrophages, are found in atherosclerotic lesions with substantial numbers in all stages. Most of the T cells in the lesions are CD4(+) T cells. The finding of activated T cells and macrophages in lesions and cloning of T cells specific for modified low-density lipoproteins from lesions suggest that a cell-mediated immune reaction is taking place in atherosclerosis. This review provides an overview of our current understanding of the roles of CD4(+) T cell subpopulations in atherosclerosis.

Innate and adaptive immunity in the pathogenesis of atherosclerosis

This review considers critically the evidence for the involvement of mediators of innate and acquired immunity in various stages of atherosclerosis. Rapidly mobilized arms of innate immunity, including phagocytic leukocytes, complement, and proinflammatory cytokines, contribute to atherogenesis. In addition, adaptive immunity, with its T cells, antibodies, and immunoregulatory cytokines, powerfully modulates disease activity and progression. Atherogenesis involves cross talk between and shared pathways involved in adaptive and innate immunity. Immune processes can influence the balance between cell proliferation and death, between synthetic and degradative processes, and between pro- and antithrombotic processes. Various established and emerging risk factors for atherosclerosis modulate aspects of immune responses, including lipoproteins and their modified products, vasoactive peptides, and infectious agents. As we fill in the molecular details, new potential targets for therapies will doubtless emerge.

The many roles of the class A macrophage scavenger receptor

The class A macrophage scavenger receptor (SR-A) is the prototypic example of a group of plasma membrane receptors collectively known as scavenger receptors. SR-A displays the ability to bind and endocytose large quantities of modified lipoprotein. Hence, it is thought to be one of the main receptors involved in mediating lipid influx into macrophages (Mphi), which promotes their conversion into foam cells that are abundant in the atherosclerotic lesion. However, as a result of increased interest and research effort and through the development of specific reagents and animal models, it is now appreciated to be multifunctional. These roles include Mphi growth and maintenance, adhesion to the substratum, cell-cell interactions, phagocytosis, and host defense. In this review, we attempt to summarize the evidence and argue that these kinds of activities underlie the biological versatility of Mphi.

Pharmacokinetic analysis of in vivo disposition of succinylated proteins targeted to liver nonparenchymal cells via scavenger receptors: importance of molecular size and negative charge density for in vivo recognition by receptors

In vivo disposition characteristics of succinylated (Suc-) proteins were studied after intravenous injection in mice in relation to their molecular characteristics as negatively charged macromolecules. Recombinant superoxide dismutase (SOD; molecular mass, 32 kDa), bovine serum albumin (BSA; molecular mass, 67 kDa), and bovine IgG (molecular mass, 150 kDa) were used to produce succinylated derivatives with different degrees of modification. (111)In-labeled Suc-SODs were rapidly excreted into the urine with no significant hepatic uptake. In contrast, (111)In-Suc-BSA and Suc-IgG were significantly taken up by liver nonparenchymal cells via scavenger receptors (SRs) according to the degree of succinylation and the dose injected. Interestingly, highly succinylated BSAs exhibited significant accumulation in the kidney at higher doses when the hepatic uptake was saturated. Pharmacokinetic analysis demonstrated that the hepatic uptake of succinylated proteins depended on the molecular size and the estimated surface density of succinylated amino residues. Further analysis based on a physiological pharmacokinetic model, involving a saturable process with Michaelis-Menten kinetics, revealed that the surface density of negative charges was correlated with the affinity of larger succinylated proteins for the hepatic SRs. Thus, the present study has provided useful basic information for a therapeutic strategy and the molecular design of succinylated proteins for use as drug carriers and therapeutic agents per se for SR-mediated targeting in vivo.

Adduction of soluble proteins with malondialdehyde-acetaldehyde (MAA) induces antibody production and enhances T-cell proliferation

BACKGROUND

The alcohol metabolites malondialdehyde and acetaldehyde can combine to form stable adducts (MAA) which are found in the livers of humans and rats after significant alcohol ingestion. While adducted proteins induce antibody responses in the absence of adjuvants, the mechanisms by which these responses occur are unknown. Thus, it was the purpose of these studies to investigate how MAA modification stimulates antibody and T-cell responses in the absence of adjuvants.

METHODS

Hen egg lysozyme (HEL) was modified with increasing levels of MAA and was used as an immunogen, and antibody and T-cell responses were determined. The role of scavenger receptors in the immunogenicity of MAA-adducted proteins was also investigated.

RESULTS

Maximum antibody response was induced after immunization with 1.8 nM MAA/nM HEL, and was primarily an IgG1 response to HEL as determined by inhibition ELISAs. T-cell proliferative responses after immunization with HEL-MAA were solely to HEL. Immunization with a scavenger receptor ligand in conjunction with HEL-MAA increased the predominant IgG1 response and sharply decreased the IgG2a response by approximately 50%. Binding of HEL-MAA by splenocytes was determined by flow cytometry to be approximately 15% greater than HEL alone, showing a doubling of the geometric mean fluorescence. Also, most of the cells that bound HEL-MAA were class II positive, indicating that antigen-presenting cells can bind the MAA-adducted HEL, and potentially initiate immune responses.

CONCLUSIONS

MAA modification of proteins induces antibody and T-cell proliferative responses in vivo. Initial studies suggest that these responses may be mediated by scavenger receptors that recognize MAA-adducted proteins. This suggests a mechanism by which proteins modified with oxidative products associated with chronic ethanol consumption may alter immune responses that may play an active role in the development and/or progression of alcoholic liver disease.

Murine sclerodermatous graft-versus-host disease, a model for human scleroderma: cutaneous cytokines, chemokines, and immune cell activation

Murine sclerodermatous graft-vs-host disease (Scl GVHD) models human scleroderma, with prominent skin thickening, lung fibrosis, and up-regulation of cutaneous collagen mRNA. Fibrosis in Scl GVHD may be driven by infiltrating TGF-beta1-producing mononuclear cells. Here we characterize the origin and types of those cutaneous effector cells, the cytokine and chemokine environments, and the effects of anti-TGF-beta Ab on skin fibrosis, immune cell activation markers, and collagen and cytokine synthesis. Donor cells infiltrating skin in Scl GVHD increase significantly at early time points post-transplantation and are detectable by PCR analysis of Y-chromosome sequences when female mice are transplanted with male cells. Cutaneous monocyte/macrophages and T cells are the most numerous cells in Scl GVHD compared with syngeneic controls. These immune cells up-regulate activation markers (MHC class II I-A(d) molecules and class A scavenger receptors), suggesting Ag presentation by cutaneous macrophages in early fibrosing disease. Early elevated cutaneous mRNA expression of TGF-beta1, but not TGF-beta2 or TGF-beta3, and elevated C-C chemokines macrophage chemoattractant protein-1, macrophage inflammatory protein-1alpha, and RANTES precede subsequent skin and lung fibrosis. Therefore, TGF-beta1-producing donor mononuclear cells may be critical effector cells, and C-C chemokines may play important roles in the initiation of Scl GVHD. Abs to TGF-beta prevent Scl GVHD by effectively blocking the influx of monocyte/macrophages and T cells into skin and by abrogating up-regulation of TGF-beta1, thereby preventing new collagen synthesis. The Scl GVHD model is valuable for testing new interventions in early fibrosing diseases, and chemokines may be new potential targets in scleroderma.

Scavenger receptors in innate immunity

Scavenger receptors (SR) are expressed by myeloid cells (macrophages and dendritic cells) and certain endothelial cells. They play an important role in uptake and clearance of effete components, such as modified host molecules and apoptotic cells. They bind and internalise micro-organisms and their products including Gram-positive bacteria (lipoteichoic acid), Gram-negative bacteria (lipopolysaccharide), intracellular bacteria and CpG DNA. SR can alter cell morphology and their expression is affected by various cytokines. SR are involved in lipid metabolism and bind modified low-density lipoproteins.

HIV-1 viral protein R compromises cellular immune function in vivo

HIV-1 viral protein R (Vpr) is a virion-associated gene product that profoundly affects T cell proliferation, induces apoptosis and can affect cytokine production in part through interfering with NF-kappa B-mediated transcription from host cells. Collectively, these effects support that Vpr could influence immune activation in vivo. However, this effect of Vpr has not been explored previously. Here we examined the effect of Vpr expression in an in vivo model system on the induction of antigen-specific immune responses using a DNA vaccine model. Vpr co-vaccination significantly altered the immune response to co-delivered antigen. Specifically, in the presence of Vpr, inflammation was markedly reduced compared to antigen alone. Vpr reduced antigen-specific CD8-mediated cytotoxic T lymphocyte activity and suppressed T(h)1 immune responses in vivo as evidenced by lower levels of IFN-gamma. In the presence of Vpr, there is a profound shift in isotype towards a T(h)2 response as determined by the IgG2a:IgG1 ratio. The data support that Vpr compromises antigen-specific immune responses and ultimately effector cell function, thus confirming a strong selective advantage to the virus at the expense of the host.

Immune mechanisms in atherosclerosis

Atherosclerosis is an inflammatory disease. Its lesions are filled with immune cells that can orchestrate and effect inflammatory responses. In fact, the first lesions of atherosclerosis consist of macrophages and T cells. Unstable plaques are particularly rich in activated immune cells, suggesting that they may initiate plaque activation. We have seen a rapid increase in the understanding of the mechanisms that govern the recruitment, differentiation, and activation of immune cells in atherosclerosis. Experimental research has identified several candidate antigens, and there are encouraging data suggesting that immune modulation as well as immunization can reduce the progression of the disease. This review provides an overview of our current understanding of the role of immune mechanisms in atherosclerosis.

The chemistry and biological effects of malondialdehyde-acetaldehyde adducts

This article represents the proceedings of a workshop at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Geoffrey M. Thiele and Simon Worrall. The presentations were (1) The chemistry of malondialdehyde-acetaldehyde (MAA) adducts, by Dean J. Tuma; (2) The formation and clearance of MAA adducts in ethanol-fed rats, by Simon Worrall; (3) Immune responses to MAA adducts may play a role in the development of alcoholic liver disease, by Lynell W. Klassen; (4) Unique biological responses to MAA-modified proteins that may play a role in the development and/or progression of alcoholic liver disease, by Geoffrey M. Thiele; (5) MAA-adducted bovine serum albumin activates protein kinase C and stimulates interleukin-8 release in bovine bronchial epithelial cells, by Todd A. Wyatt; and (6) An enzyme immune assay for serum antiacetaldehyde adduct antibody using low-density lipoprotein-adduct and its significance in alcoholic liver injury and ALDH2 heterozygotes, by Naruhiko Nagata.

The role of vitamin E in atherogenesis: linking the chemical, biological and clinical aspects of the disease

Atherosclerosis is a disease involving both oxidative modifications and disbalance of the immune system. Vitamin E, an endogenous redox-active component of circulating lipoproteins and (sub)cellular membranes whose levels can be manipulated by supplementation, has been shown to play a role in the initiation and progression of the disease. Recent data reveal that the activities of vitamin E go beyond its redox function. Moreover, it has been shown that vitamin E can exacerbate certain processes associated with atherogenesis. In this essay we review the role of biology of atherosclerosis, and suggest that these two facets decide the clinical manifestation and outcome of the disease.

The function of scavenger receptors expressed by macrophages and their role in the regulation of inflammation

Macrophages are able to recognise and internalise apoptotic and foreign cells from the tissue microenvironment and, where pertinent, initiate inflammation and macrophage activation. Regulation of this protective response is important so as to prevent inappropriate activation of host cells and excessive tissue damage. Scavenger receptors (SRs) are widely expressed by macrophages and are able to bind a diverse array of endogenous and foreign molecules. Studies have concentrated on the role of the receptors in atherosclerosis as they can endocytose modified lipoproteins, however, this review will detail the accumulating evidence that SRs function in innate immunity and macrophage control during inflammation.

The role of adaptive immunity in atherosclerosis

Atherosclerosis is an inflammatory disease induced by a lipid metabolic disturbance at sites of hemodynamic strain in the vasculature. Studies in both man and experimental animal models show an involvement of innate and adaptive immune mechanisms in the disease process. Our recent studies in apoE-knockout mice show that the level of hypercholesterolemia affects the functional properties of the immune response. Modulating immune activity by injections of polyclonal immunoglobulins inhibits disease progression, suggesting that immunomodulation may be useful to treat atherosclerosis. Analysis of T cell receptor (TCR) mRNA in atherosclerotic lesions shows expansions of T cells expressing TCR-V beta 6, a receptor type that is also expressed by T cells recognizing oxidized low density lipoprotein (oxLDL). This suggests that oxLDL is an autoantigen that induces strong, local T cell responses in the plaque. Further characterization of this and other candidate antigens, such as heat shock proteins and macromolecular components of Chlamydia pneumoniae, may provide important information on which specific interference with the disease process could be based.

The role of scavenger receptors in the innate immune system

Akey aspect of the innate immune system is the ability to discriminate between self and infectious nonself. This is achieved through pattern recognition receptors which directly recognise molecular epitopes expressed by microbes. Scavenger receptors (SRs) have been studied primarily due to their ability to bind and internalise modified lipoproteins, suggesting an important role in foam cell formation and the pathogenesis of atherosclerosis. However, the ability of some SRs to function as pattern recognition receptors through their binding of a wide variety of pathogens indicates a potential role in host defence. This review will detail our current understanding of the function of SRs in innate immunity, and in the initiation of aquired immune responses.

Class A type II scavenger receptor mediates silica-induced apoptosis in Chinese hamster ovary cell line

Macrophage scavenger receptors are known to bind endotoxins, oxidized low-density lipoproteins (Ox-LDL), and other proteins with clustered negative charges. Recent evidence indicates some particulates may also bind to the scavenger receptor and initiate apoptosis. In this study, chinese hamster ovary (CHO) cells stabily transfected with the murine class A type II scavenger receptor (SR-A II) were exposed to crystalline silica to examine the role of this receptor in apoptosis. In a 24-h culture, silica (250 microg/ml) induced significant cell injury (necrosis and apoptosis) in transfected cells (MSR II) but not in the control cells (KA-7). This effect was specific to silica, as a control particle titanium dioxide had no cytotoxic effects on the MSR II cells at equal particle mass concentrations. Furthermore, silica-induced apoptosis in the MSR II cells could be eliminated by preincubating the cells with SR-A II antagonists: polyinosinic acid or maleylated bovine serum albumin. This study further supports the hypothesis that the SR-A II is directly involved with silica toxicity and that certain scavenger receptor ligands may have an important role in regulating macrophage apoptosis.

Inflammation and immune response in atherosclerosis

Atherosclerosis is an inflammatory disease with a significant autoimmune component. Studies using transgenic murine models have clarified that recruitment of mononuclear leukocytes through vascular leukocyte-adhesion molecules and chemokines, differentiation of monocytes to macrophages, and endocytosis through scavenger receptors all are of decisive importance for atherosclerosis in hypercholesterolemic mice. T and B cells modulate disease progression and lesion development is reduced in mice lacking adaptive immunity. In particular, local immune responses eliciting Th1 effector mechanisms appear to be proatherogenic, whereas protective immune responses can be induced by immunization with oxidized low-density lipoprotein. Thus, innate immunity is necessary for atherosclerosis, whereas adaptive immunity is an important modulator of disease development.