technology

 

In the past few years, considerable evidence has accumulated in favor of the paradigm that atherosclerosis is a chronic disease in which inflammatory and immune responses contribute to the initiation, progression, and destabilization of atherosclerotic lesions.^1,2  The innate and adaptive immune responses have both been shown to modulate atherosclerosis.^3,4  Like many other biologic systems, it has become apparent that activation of the immune system has two juxtaposed roles in atherosclerosis, with evidence of both athero-promoting and athero-protecting effects.^3,4,5  Selective activation of the athero-protective pathway of the immune response might offer a potentially novel therapeutic or preventive approach against this prevalent disease.

 

Adaptive immune response occurs when a specific antigen, processed and presented by antigen-presenting cells, is recognized by the immune system, leading to proliferation of T and B cells. (see Figure 1).

 

 

Figure 1.  A schematic representation of the potential juxtaposed roles of the adaptive immune response to specific antigens.  The athero-promoting component results from T helper-1 cell and natural killer T-cell activation triggered by the presentation of antigens by the major histocompatibility complex class II or CD1 molecules.  The atheroprotective component is mediated by the secretion of anti-inflammatory cytokines (interleukin-10 and transforming growth factor-β), mediated by T-helper-2, T-helper-3, and regulatory T cells, and antibody response mediated by B cells.

 

MHC class II, major histocompatibility complex class II; NK T cell, natural killer T cell; Ox-LDL, oxidized LDL cholesterol; PAMP, pathogen-associated molecular patterns; SR, scavenger receptor; TLR, Toll-like receptor; Treg, regulatory T cells. (Shah, Nature Clinical Practice Cardiovascular Medicine 2005)

 

Following uptake by the scavenger receptors on macrophages, and possibly dendritic cells, oxidized LDL is processed and epitopes derived from the LDL particle are presented by major histocompatibility complex class II proteins for recognition by specific CD4+ T cells.  When T cells encounter their specific antigens on a major histocompatibility complex class II molecule, an adaptive immune response is activated, including clonal proliferation of the T cell and production of cytokines, and subsequent activation of B cells to produce immunoglobulins

 

Findings from several experimental and clinical studies have demonstrated that autoantibodies to autoantigens, such as oxidized LDL, heat-shock protein 60, and β-2 glycoprotein, are present in atherosclerosis.^6-9  Furthermore, T cells reactive to oxidized LDL have been identified within the atherosclerotic lesions¹⁰ and the cytokines of both Th1 and Th2 systems have been shown to modulate atherosclerosis.^3,4  Evidence reported so far suggests that activation of an adaptive immune response to heat-shock protein 60 and β-2 glycoprotein is athero-promoting, whereas the adaptive immune response to oxidized LDL might be either athero-promoting or athero-protecting.^{3,4,5,11,12,13} Palinski et al.¹⁴ first demonstrated athero-protecting effects of immunization with malondialdehyde-modified LDL in Watanabe rabbits.  The following year, further studies demonstrated athero-protecting effects of immunization with native and ex vivo oxidized homologous LDL in cholesterol-fed rabbits.^15,16 

 

These observations of athero-protecting effects of immunization with oxidized LDL have also been demonstrated in hypercholesterolemic mice.^17,18,19  The available experimental evidence thus suggests that, while adaptive immunity in hypercholesterolemic animals is predominantly athero-promoting, active immunization with oxidized LDL inhibits atherosclerosis.  This athero-protective effect is possibly achieved by shifting the endogenous immune response against oxidized LDL from a pro-inflammatory Th1 response towards an anti-inflammatory Th2 or Th3 response.

 

 

Figure 2.  Adaptive immune responses to oxidized LDL.  Oxidized LDL antigens are presented by macrophage MHC class II proteins for recognition by antigen-specific CD4+ T cells.  Activated CD4+ T cells may differentiate into pro-inflammatory Th1 cells, Th2 cells promoting antibody production, Treg cells that suppress antigen-induced activation of other CD4+ T cells, or TGF-β–producing Th3 cells.  Presentation of lipid antigens by the macrophage class I–like molecule CD1 results in activation of NK1.1+ CD4+ cells promoting Th1 and Th2 responses.  The balance between pro-inflammatory and anti-inflammatory T-cell subsets has a major influence on disease activity and progression. (Nilsson ATVB 2005)

 

Oxidized LDL is believed to have a key role in atherosclerosis because it causes intimal inflammation and foam-cell formation.  Oxidation of polyunsaturated fatty acids in phospholipids and cholesteryl esters generates breakdown products such as malondialdehyde and 4-hydroxynonenal, which form covalent adducts with amino acids containing free amino groups in apolipoprotein B-100, the main protein component of LDL.  Oxidation of LDL also leads to degradation of apolipoprotein B-100 into numerous peptide fragments.²⁰  It is thought that these modifications make oxidized LDL a target for the immune system (see Figure 2). 

 

Several clinical studies have shown the presence of circulating autoantibodies to oxidized LDL, in healthy individuals and patients with cardiovascular disease; however, the relationship between the antibody titer and cardiovascular disease has been inconsistent.^7,21,22,23  Some antigenic sequences that are normally absent or concealed become available after the oxidation of LDL.²⁴  The structure of the apolipoprotein B-100 component of LDL was studied to identify potential antigenic epitopes that could be responsible for the athero-protective effects of immunization with oxidized LDL.²⁵  This effort included the synthesis of 302 peptide sequences, spanning the entire human apolipoprotein B-100 molecule, each 20 amino acids long with an overlap of five amino acids.²⁶  Of these potential antigenic epitopes, 102 have been found to be associated with an antibody response in pooled human serum.²⁷  It is clear that select peptide sequences, but not others, provoke an athero-protective response in hypercholesterolemic mice when incorporated into a vaccine formulation with alum as an adjuvant; a 40–70% reduction in aortic atherosclerosis and a reduction in plaque inflammation was observed.^26,27 

 

Furthermore, studies have shown that such athero-protection can be passively transferred to nonimmunized mice through adoptive transfer of splenocytes from immunized mice.²⁷  Passive immunization, using a selected monoclonal antibody to one of the athero-protective peptide epitopes, has also been shown to reduce atherosclerosis in hyperlipidemic mice.²⁸ 

 

In summary, observations from experimental studies have shown that immunization favorably changes the composition of established plaques, indicated by decreased plaque inflammation and increased collagen content.^19,26,27  Modulation of immune responses involved in atherosclerosis using active immunization with antibodies represents a singularly promising approach to the management of atherosclerotic cardiovascular disease.

 

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