Göran K. Hansson

Progress and challenges in translating the biology of atherosclerosis

Atherosclerosis is a chronic disease of the arterial wall, and a leading cause of death and loss of productive life years worldwide. Research into the disease has led to many compelling hypotheses about the pathophysiology of atherosclerotic lesion formation and of complications such as myocardial infarction and stroke. Yet, despite these advances, we still lack definitive evidence to show that processes such as lipoprotein oxidation, inflammation and immunity have a crucial involvement in human atherosclerosis. Experimental atherosclerosis in animals furnishes an important research tool, but extrapolation to humans requires care. Understanding how to combine experimental and clinical science will provide further insight into atherosclerosis and could lead to new clinical applications.

The immune response in atherosclerosis: a double-edged sword

Immune responses participate in every phase of atherosclerosis. There is increasing evidence that both adaptive and innate immunity tightly regulate atherogenesis. Although improved treatment of hyperlipidaemia reduces the risk for cardiac and cerebral complications of atherosclerosis, these remain among the most prevalent of diseases and will probably become the most common cause of death globally within 15 years. This Review focuses on the role of immune mechanisms in the formation and activation of atherosclerotic plaques, and also includes a discussion of the use of inflammatory markers for predicting cardiovascular events. We also outline possible future targets for prevention, diagnosis and treatment of atherosclerosis.

The immune system in atherosclerosis

Cardiovascular disease, a leading cause of mortality worldwide, is caused mainly by atherosclerosis, a chronic inflammatory disease of blood vessels. Lesions of atherosclerosis contain macrophages, T cells and other cells of the immune response, together with cholesterol that infiltrates from the blood. Targeted deletion of genes encoding costimulatory factors and proinflammatory cytokines results in less disease in mouse models, whereas interference with regulatory immunity accelerates it. Innate as well as adaptive immune responses have been identified in atherosclerosis, with components of cholesterol-carrying low-density lipoprotein triggering inflammation, T cell activation and antibody production during the course of disease. Studies are now under way to develop new therapies based on these concepts of the involvement of the immune system in atherosclerosis.

Inflammation in Atherosclerosis From Pathophysiology to Practice

Until recently, most envisaged atherosclerosis as a bland arterial collection of cholesterol, complicated by smooth muscle cell accumulation. According to that concept, endothelial denuding injury led to platelet aggregation and release of platelet factors which would trigger the proliferation of smooth muscle cells in the arterial intima. These cells would then elaborate an extracellular matrix that would entrap lipoproteins, forming the nidus of the atherosclerotic plaque. Beyond the vascular smooth muscle cells long recognized in atherosclerotic lesions, subsequent investigations identified immune cells and mediators at work in atheromata, implicating inflammation in this disease. Multiple independent pathways of evidence now pinpoint inflammation as a key regulatory process that links multiple risk factors for atherosclerosis and its complications with altered arterial biology. Knowledge has burgeoned regarding the operation of both innate and adaptive arms of immunity in atherogenesis, their interplay, and the balance of stimulatory and inhibitory pathways that regulate their participation in atheroma formation and complication. This revolution in our thinking about the pathophysiology of atherosclerosis has now begun to provide clinical insight and practical tools that may aid patient management. This review provides an update of the role of inflammation in atherogenesis and highlights how translation of these advances in basic science promises to change clinical practice.

Innate and Adaptive Immunity in the Pathogenesis of Atherosclerosis

Abstract— 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.

Cytokine expression in advanced human atherosclerotic plaques: dominance of pro-inflammatory (Th1) and macrophage-stimulating cytokines

The atherosclerotic lesion contains large numbers of macrophages and T lymphocytes. This suggests that a cellular immune response may take place in the lesion, and oxidized lipoproteins, heat shock proteins, and micro-organisms have been implied as candidate antigens. However, the effector mechanisms elicited by this response have been largely unclear. We have therefore analyzed endarterectomy specimens by immunohistochemistry and reverse transcription-PCR to detect immune cytokines produced by immunocompetent cells of the advanced human plaque. The pro-inflammatory T cell cytokines, interleukin-2 and interferon-7, were found in a large proportion of plaques (IL-2 in 50% and interferon-gamma in 30% of plaques by immunohistochemistry and mRNA for both cytokines in 70% of plaques by PCR). In contrast, interleukin-4 and interleukin-5 were rarely observed (both cytokines in 10% of plaques by immunohistochemistry, mRNA for interleukin-4 in 10% and for interleukin-5 in 40% by PCR). This demonstrates the presence of a predominantly pro-inflammatory, Th1-type T cell response in atherosclerosis. This conclusion was further supported by the expression of the pro-inflammatory cytokine, interleukin-1 by plaque macrophages and endothelial cells. In addition, the chemokine interleukin-8 and the macrophage differentiation-stimulating cytokine, granulocyte-monocyte colony stimulating factor, were observed in plaque tissues, suggesting that the micro-environment promotes monocyte recruitment and macrophage differentiation. Occasional eosinophils and B cells were, however observed, which is compatible with a microheterogeneity within the lesion. Finally, the anti-inflammatory and fibrogenic cytokines, transforming growth factor-beta1-3 and its carrier protein, latent TGF-beta binding protein, were found in large amounts in all plaques. Together, these results show that a pro-inflammatory, Thl type cellular immune response takes place in the atherosclerotic plaque. The balance between pro-inflammatory and anti-inflammatory cytokines may be decisive for the progression of the lesion.

Natural regulatory T cells control the development of atherosclerosis in mice

Atherosclerosis is an immunoinflammatory disease elicited by accumulation of lipids in the artery wall and leads to myocardial infarction and stroke. Here, we show that naturally arising CD4+CD25+ regulatory T cells, which actively maintain immunological tolerance to self and nonself antigens, are powerful inhibitors of atherosclerosis in several mouse models. These results provide new insights into the immunopathogenesis of atherosclerosis and could lead to new therapeutic approaches that involve immune modulation using regulatory T cells.

Protective immunity against atherosclerosis carried by B cells of hypercholesterolemic mice

Atherosclerosis is characterized by vascular inflammation and associated with systemic and local immune responses to oxidized LDL (oxLDL) and other antigens. Since immunization with oxLDL reduces atherosclerosis, we hypothesized that the disease might be associated with development of protective immunity. Here we show that spleen-associated immune activity protects against atherosclerosis. Splenectomy dramatically aggravated atherosclerosis in hypercholesterolemic apoE knockout (apoE degrees ) mice. Transfer of spleen cells from atherosclerotic apoE degrees mice significantly reduced disease development in young apoE degrees mice. To identify the protective subset, donor spleen cells were divided into B and T cells by immunomagnetic separation before transfer. Protection was conferred by B cells, which reduced disease in splenectomized apoE degrees mice to one-fourth of that in splenectomized apoE degrees controls. Protection could also be demonstrated in intact, nonsplenectomized mice and was associated with an increase in antibody titers to oxLDL. Fewer CD4(+) T cells were found in lesions of protected mice, suggesting a role for T-B cell cooperation. These results demonstrate that B cell-associated protective immunity develops during atherosclerosis and reduces disease progression.

HYPERCHOLESTEROLEMIA IS ASSOCIATED WITH A T HELPER (TH) 1/TH2 SWITCH OF THE AUTOIMMUNE RESPONSE IN ATHEROSCLEROTIC APO E-KNOCKOUT MICE

Atherosclerosis is an inflammatory-fibrotic response to accumulation of cholesterol in the artery wall. In hypercholesterolemia, low density lipoproteins (LDL) accumulate and are oxidized to proinflammatory compounds in the arterial intima, leading to activation of endothelial cells, macrophages, and T lymphocytes. We have studied immune cell activation and the autoimmune response to oxidized LDL in atherosclerotic apo E-knockout mice. Autoantibodies to oxidized LDL exhibited subclass specificities indicative of T cell help, and the increase in antibody titers in peripheral blood was associated with increased numbers of cytokine-expressing T cells in the spleen. In addition to T cell-dependent antibodies, IgM antibodies to oxidized LDL were also increased in apo E-knockout mice. This suggests that both T cell-dependent and T cell-independent epitopes may be present on oxidized LDL. In moderate hypercholesterolemia, IgG antibodies were largely of the IgG2a isotype, suggesting that T cell help was provided by proinflammatory T helper (Th) 1 cells, which are prominent components of atherosclerotic lesions. In severe hypercholesterolemia induced by cholesterol feeding of apo E-knockout mice, a switch to Th2-dependent help was evident. It was associated with a loss of IFN-gamma-producing Th1 cells in the spleen, whereas IL-4-producing Th2 cells were more resistant to hypercholesterolemia. IFN-gamma but not IL-4 mRNA was detected in atherosclerotic lesions of moderately hypercholesterolemic apo E-knockout mice, but IL-4 mRNA appeared in the lesions when mice were made severely hypercholesterolemic by cholesterol feeding. These data show that IFN-gamma-producing Th1 cells infiltrate atherosclerotic lesions and provide T cell help for autoimmune responses to oxidized LDL in apo E-knockout mice. However, severe hypercholesterolemia is associated with a switch from Th1 to Th2, which results not only in the formation of IgG1 autoantibodies to oxidized LDL, but also in the appearance of Th2-type cytokines in the atherosclerotic lesions. Since the two subsets of T cells counteract each other, this switch may have important consequences for the inflammatory/immune process in atherosclerosis.

Nuclear factor kappa-B and the heart

Nuclear factor kappa-B (NFkappaB), a redox-sensitive transcription factor regulating a battery of inflammatory genes, has been indicated to play a role in the development of numerous pathological states. Activation of NFkappaB induces gene programs leading to transcription of factors that promote inflammation, such as leukocyte adhesion molecules, cytokines, and chemokines, although some few substances with possible anti-inflammatory effects are also NFkappaB regulated. The present article reviews basic regulation of NFkappaB and its activation, cell biological effects of NFkappaB activation and the role of NFkappaB in apoptosis. Evidence involving NFkappaB as a key factor in the pathophysiology of ischemia-reperfusion injury and heart failure is discussed. Although activation of NFkappaB induces pro-inflammatory genes, it has lately been indicated that the transcription factor is involved in the signaling of endogenous myocardial protection evoked by ischemic preconditioning. A possible role of NFkappaB in the development of atherosclerosis and unstable coronary syndromes is discussed. Nuclear factor kappa-B may be a new therapeutic target for myocardial protection.