Immune mechanisms in atherosclerosis; Focus on the role of the complement system

Atherosclerosis is an inflammatory disease characterized by accumulation of lipids in the inner layer of the arterial wall. During atherogenesis, various structures that are recognized as non-self by the immune system, such as modified lipoproteins, are deposited in the arterial wall. Accordingly, atherosclerotic lesions and blood of humans and animals with atherosclerotic lesions show signs of activation of both innate and adaptive immune responses. Although immune attack is initially a self-protective reaction, which is meant to destroy or remove harmful agents, a chronic inflammatory state in the arterial wall accelerates atherosclerosis. Indeed, various modulations of the immune system of atherosclerosis-prone animals have provided us with convincing evidence that immunological mechanisms play an important role in the pathogenesis of atherosclerosis. This thesis focuses on the role of complement system, a player of the innate immunity, in atherosclerosis. Complement activation via any of the three different pathways (classical, alternative, lectin) proceeds as a self-amplifying cascade, which leads to the generation of opsonins, anaphylatoxins C3a and C5a, and terminal membrane-attack complex (MAC, C5b-9), all of which regulate the inflammatory response and act in concert to destroy their target structures. To prevent uncontrolled complement activation or its attack against normal host cells, complement needs to be under strict control by regulatory proteins. The complement system has been shown to be activated in atherosclerotic lesions, modified lipoproteins and immune complexes containing oxLDL, for instance, being its activators. First, we investigated the presence and role of complement regulators in human atherosclerotic lesions. We found that inhibitors of the classical and alternative pathways, C4b-binding protein and factor H, respectively, were present in atherosclerotic lesions, where they localized in the superficial proteoglycan-rich layer. In addition, both inhibitors were found to bind to arterial proteoglycans in vitro. Immunohistochemical stainings revealed that, in the superficial layer of the intima, complement activation had been limited to the C3 level, whereas in the deeper intimal layers, complement activation had proceeded to the terminal C5b-9 level. We were also able to show that arterial proteoglycans inhibit complement activation in vitro. These findings suggested to us that the proteoglycan-rich layer of the arterial intima contains matrix-bound complement inhibitors and forms a protective zone, in which complement activation is restricted to the C3 level. Thus, complement activation is regulated in atherosclerotic lesions, and the extracellular matrix is involved in this process. Next, we studied whether the receptors for the two complement derived effectors, anaphylatoxins C3a and C5a, are expressed in human coronary atherosclerotic lesions. Our results of immunohistochemistry and RT-PCR analysis showed that, in contrast to normal intima, C3aR and C5aR were highly expressed in atherosclerotic lesions. In atherosclerotic plaques, the principal cells expressing both C3aR and C5aR were macrophages. Moreover, T cells expressed C5aR, and a small fraction of them also expressed C3aR, mast cells expressed C5aR, whereas endothelial cells and subendothelial smooth muscle cells expressed both C3aR and C5aR. These results suggested that intimal cells can respond to and become activated by complement-derived anaphylatoxins. Finally, we wanted to learn, whether oxLDL-IgG immune complexes, activators of the classical complement pathway, could have direct cellular effects in atherogenesis. Thus, we tested whether oxLDL-IgG immune complexes affect the survival of human monocytes, the precursors of macrophages, which are the most abundant inflammatory cell type in atherosclerotic lesions. We found that OxLDL-IgG immune complexes, in addition to transforming monocytes into foam cells, promoted their survival by decreasing their spontaneous apoptosis. This effect was mediated by cross-linking Fc receptors with ensuing activation of Akt-dependent survival signaling. Our finding revealed a novel mechanism by which oxLDL-IgG immune complexes can directly affect the accumulation of monocyte-macrophages in human atherosclerotic lesions and thus play a role in atherogenesis.

Constructions and situations : a constructivist reading of Hegel's System

The object of this work is Hegel's Logic, which comprises the first third of his philosophical System that also includes the Philosophy of Nature and the Philosophy of Spirit. The work is divided into two parts, where the first part investigates Hegel s Logic in itself or without an explicit reference to rest of Hegel's System. It is argued in the first part that Hegel's Logic contains a methodology for constructing examples of basic ontological categories. The starting point on which this construction is based is a structure Hegel calls Nothing, which I argue to be identical with an empty situation, that is, a situation with no objects in it. Examples of further categories are constructed, firstly, by making previous structures objects of new situations. This rule makes it possible for Hegel to introduce examples of ontological structures that contain objects as constituents. Secondly, Hegel takes also the very constructions he uses as constituents of further structures: thus, he is able to exemplify ontological categories involving causal relations. The final result of Hegel's Logic should then be a model of Hegel s Logic itself, or at least of its basic methods. The second part of the work focuses on the relation of Hegel's Logic to the other parts of Hegel's System. My interpretation tries to avoid, firstly, the extreme of taking Hegel's System as a grand metaphysical attempt to deduce what exists through abstract thinking, and secondly, the extreme of seeing Hegel's System as mere diluted Kantianism or a second-order investigation of theories concerning objects instead of actual objects. I suggest a third manner of reading Hegel's System, based on extending the constructivism of Hegel's Logic to the whole of his philosophical System. According to this interpretation, transitions between parts of Hegel's System should not be understood as proofs of any sort, but as constructions of one structure or its model from another structure. Hence, these transitions involve at least, and especially within the Philosophy of Nature, modelling of one type of object or phenomenon through characteristics of an object or phenomenon of another type, and in the best case, and especially within the Philosophy of Spirit, transformations of an object or phenomenon of one type into an object or phenomenon of another type. Thus, the transitions and descriptions within Hegel's System concern actual objects and not mere theories, but they still involve no fallacious deductions.