Interaction between immunocompetent cells in atherosclerosis

Specific arterial sites, such as branches, bifurcations, and curvatures, cause characteristic alterations in the flow of blood, including decreased shear stress and increased turbulence. At these sites, specific molecules form on the endothelium that are responsible for the adherence, migration, and accumulation of monocytes and T-cells. Such adhesion molecules, which act as receptors for glycoconjugates and integrins present on monocytes and T-cells, include several selectins, intercellular adhesion molecules, and vascular-cell adhesion molecules. Molecules associated with the migration of leukocytes across the endothelium, such as platelet-endothelial-cell adhesion molecules, act in conjunction with chemoattractant molecules generated by the endothelium, smooth muscle, and monocytes - such as monocyte chemotactic protein-1, osteopontin, and modified LDL - to attract monocytes and T-cells into the artery. The nature of the flow - that is, whether shear stress or turbulence is high or low - appears to be important in determining whether lesions occur at these vascular sites. Changes in flow alter the expression of genes that have elements in their promoter regions that respond to shear stress. For example, the genes for intercellular adhesion molecule-1, platelet-derived growth factor B chain, and tissue factor in endothelial cells have these elements, and their expression is increased by reduced shear stress. Thus, alterations in blood flow appear to be critical in determining which arterial sites are prone to have lesions. Rolling and adherence of monocytes and T-cells occur at these sites as a result of the up-regulation of adhesion molecules on both the endothelium and the leukocytes.

Chemokines may be responsible for the chemotaxis and accumulation of macrophages in fatty streaks. Activation of monocytes and T-cells leads to up-regulation of receptors on their surfaces, such as the mucin-like molecules that bind selectins, integrins that bind adhesion molecules of the immunoglobulin superfamily, and receptors that bind chemoattractant molecules. These ligand-receptor interactions further activate mononuclear cells, induce cell proliferation, and help define and localize the inflammatory response at the sites of lesions. Thus, adherence of monocytes and T-cells may occur after an increase in one or more of the adhesion molecules, which may act in concert with chemotactic molecules such as monocyte chemotactic protein-1, interleukin-8, or modified LDL. Comparison of the relative roles of these molecules in inflammation in the arteries and the microvasculature may provide clues to the relative feasibility of modifying the inflammatory process at these sites, and thus of modifying atherosclerosis.

A recently discovered class of molecules, the disintegrins, sometimes called metalloproteinase-like, disintegrin-like, cysteine-rich proteins (MDCs), has been identified in endothelium, smooth muscle, and macrophages. These transmembrane proteins, which appear to be involved in cell-cell interactions, contain a metalloproteinase sequence in their extracellular segment that permits them to activate molecules such as tumor necrosis factor-α. They are not found in normal arteries, but one of them, MDC15, is present in lesions of atherosclerosis. Adhesion molecules such as L-selectin can be cleaved from the surface of leukocytes by a metalloproteinase (L-selectin sheddase), which suggests that in situations of chronic inflammation it may be possible to measure the “shed” molecules, such as the different adhesion molecules, in plasma, as markers of a sustained inflammatory response. Disintegrins may participate in these shedding processes. If shedding occurs, it may be detectable in different types of inflammatory responses. Increased plasma concentrations of shed molecules might then be used to identify patients at risk for atherosclerosis or other inflammatory diseases.

The ubiquitous monocyte, the precursor of macrophages in all tissues, is present in every phase of atherogenesis. Monocyte-derived macrophages are scavenging and antigen-presenting cells, and they secrete cytokines, chemokines, growth-regulating molecules, and metalloproteinases and other hydrolytic enzymes. The continuing entry, survival, and replication of mononuclear cells in lesions depend in part on factors such as macrophage colony-stimulating factor and granulocyte-macrophage colony-stimulating factor for monocytes and interleukin-2 for lymphocytes. Continued exposure to macrophage colony-stimulating factor permits macrophages to survive in vitro and possibly to multiply within the lesions. In contrast, inflammatory cytokines such as interferon-γ activate macrophages and under certain circumstances induce them to undergo programmed cell death (apoptosis). If this occurs in vivo, macrophages may become involved in the necrotic cores characteristic of advanced, complicated lesions. Initially, the only cells thought to proliferate during expansion of atherosclerotic lesions were smooth-muscle cells. However, replication of monocyte-derived macrophages and T-cells is probably of equal importance. The ability of macrophages to produce cytokines (such as tumor necrosis factor-α, interleukin-1, and transforming growth factor-β), proteolytic enzymes (particularly metalloproteinases), and growth factors (such as platelet-derived growth factor and insulin-like growth factor-I) may be critical in the role of these cells in the damage and repair that ensue as the lesions progress.

Activated macrophages express class II histocompatibility antigens such as HLA-DR that allow them to present antigens to T-lymphocytes. Thus, it is not surprising that cell-mediated immune responses may be involved in atherogenesis, since both CD4 and CD8 T-cells are present in the lesions at all stages of the process. T-cells are activated when they bind antigen processed and presented by macrophages. T-cell activation results in the secretion of cytokines, including interferon-γ and tumor necrosis factor-α and -β, that amplify the inflammatory response. Smooth-muscle cells from the lesions also have class II HLA molecules on their surfaces, presumably induced by interferon-γ, and can also present antigens to T-cells. One possible antigen may be oxidized LDL, which can be produced by macrophages. Heat-shock protein 60 may also contribute to autoimmunity. This and other heat-shock proteins perform several functions, including the assembly, intracellular transport, and breakdown of proteins and the prevention of protein denaturation. These proteins may be elevated on endothelial cells and participate in immune responses.

An immunoregulatory molecule, CD40 ligand, can be expressed by macrophages, T-cells, endothelium, and smooth muscle in atherosclerotic lesions in vivo, and its receptor, CD40, is expressed on the same cells. Both are up-regulated in lesions of atherosclerosis, providing further evidence of immune activation in the lesions. Furthermore, CD40 ligand induces the release of interleukin-1β by vascular cells, potentially enhancing the inflammatory response. Inhibition of CD40 with blocking antibodies reduces lesion formation in apolipoprotein E-deficient mice.

Platelet adhesion and mural thrombosis are ubiquitous in the initiation and generation of the lesions of atherosclerosis in animals and humans. Platelets can adhere to dysfunctional endothelium, exposed collagen, and macrophages. When activated, platelets release their granules, which contain cytokines and growth factors that, together with thrombin, may contribute to the migration and proliferation of smooth-muscle cells and monocytes. Activation of platelets leads to the formation of free arachidonic acid, which can be transformed into prostaglandins such as thromboxane A₂, one of the most potent vasoconstricting and platelet-aggregating substances known, or into leukotrienes, which can amplify the inflammatory response. Plaque rupture and thrombosis are notable complications of advanced lesions that lead to unstable coronary syndromes or myocardial infarction. Platelets are important in maintaining vascular integrity in the absence of injury and protecting against spontaneous hemorrhage. Activated platelets can accumulate on the walls of arteries and recruit additional platelets into an expanding thrombus. An important component of the platelets is the glycoprotein IIb/IIIa receptor, which belongs to the integrin superfamily of adhesion-molecule receptors and appears on the surface of platelets during platelet activation and thrombus formation. These receptors serve an important hemostatic function, and antagonists to them prevent thrombus formation in patients who have had a myocardial infarction.

There particular aspects of the chronic inflammatory response in atherosclerosis that can be used to advantage. At least three different types of macrophages, each regulated by different T-cell cytokines (interferon-γ, interleukin-2, interleukin-4, and interleukin-10) have been identified. There differences in arterial endothelium and microvascular endothelium such that different types of monocytes are attracted to each, and could one take advantage of such differences. One might try to use such differences to modify the inflammatory response so as to emphasize its protective rather than its destructive characteristics. Atherosclerosis is clearly an inflammatory disease and does not result simply from the accumulation of lipids. If we can selectively modify the harmful components of inflammation in the arteries and leave the protective aspects intact, we may create new avenues for the diagnosis and management of disease of patients with cardiovascular disease who do not have hypercholesterolemia.