Anusha N. Seneviratne

Toll-like receptors and macrophage activation in atherosclerosis.

Atherosclerosis is a multi-factorial inflammatory disease and is the primary initiator of coronary artery and cerebrovascular disease. Initially believed to be exclusively lipid-driven, recent evidence demonstrates that inflammation is a significant driving force of the disease. Cellular components of innate immunity, for example monocytes and macrophages, play a predominant role in atherosclerosis. Toll-like receptors (TLRs) are the most characterised innate immune receptors and recent evidence demonstrates an important role in atherogenesis. Engagement of TLRs results in the transcription of pro-inflammatory cytokines, foam cell formation and activation of adaptive immunity. Recently they have also been implicated in protection from vascular disease. In this review, we detail the role of the innate immune system, specifically macrophages and TLR signalling, in atherosclerosis and acute cardiovascular complications, and thereby identify the potential of TLRs to act as therapeutic targets.

Biomechanical factors and macrophages in plaque stability

Thin-cap fibroatheromas (TCFAs) or vulnerable atherosclerotic plaques are considered a high-risk phenotype for acute cardiovascular events. TCFAs are identified by a thin rupture-prone fibrous cap, a large necrotic core, and a high content of leucocytes. Atherogenesis is dependent upon complex patterns of blood flow. Slow-flowing blood imposing low shear stress on the arterial wall up-regulates inflammatory signalling in endothelial cells and leucocytes, and modulates microRNAs to promote inflammation and monocyte recruitment. Hence, low shear stress is believed to promote conditions conducive to vulnerable plaque development. In this review, we explore how biomechanical factors modulate macrophage phenotype and plaque stability.

Intimal hyperplasia following implantation of helical-centreline and straight-centreline stents in common carotid arteries in healthy pigs: influence of intraluminal flow†

Intimal hyperplasia (IH) is a leading cause of obstruction of vascular interventions, including arterial stents, bypass grafts and arteriovenous grafts and fistulae. Proposals to account for arterial stent-associated IH include wall damage, low wall shear stress (WSS), disturbed flow and, although not widely recognized, wall hypoxia. The common non-planarity of arterial geometry and flow, led us to develop a bare-metal, nitinol, self-expanding stent with three-dimensional helical-centreline geometry. This was deployed in one common carotid artery of healthy pigs, with a straight-centreline, but otherwise identical (conventional) stent deployed contralaterally. Both stent types deformed the arteries, but the helical-centreline device additionally deformed them helically and caused swirling of intraluminal flow. At sacrifice, one month post stent deployment, histology revealed significantly less IH in the helical-centreline than straight-centreline stented vessels. Medial cross-sectional area was not significantly different in helical-centreline than straight-centreline stented vessels. By contrast, luminal cross-sectional area was significantly larger in helical-centreline than straight-centreline stented vessels. Mechanisms considered to account for those results include enhanced intraluminal WSS and enhanced intraluminal blood–vessel wall mass transport, including of oxygen, in the helical-centreline stented vessels. Consistent with the latter proposal, adventitial microvessel density was lower in the helical-centreline stented than straight-centreline stented vessels.

Low shear stress induces M1 macrophage polarization in murine thin-cap atherosclerotic plaques.

Macrophages, a significant component of atherosclerotic plaques vulnerable to acute complications, can be pro-inflammatory (designated M1), regulatory (M2), lipid- (Mox) or Heme-induced (Mhem). We showed previously that low (LSS) and oscillatory (OSS) shear stress cause thin-cap fibroatheroma and stable smooth muscle cell-rich plaque formation respectively in ApoE-knockout (ApoE(-/-)) mice. Here we investigated whether different shear stress conditions relate to specific changes in macrophage polarization and plaque morphology by applying a shear stress-altering cast to the carotid arteries of high fat-fed ApoE(-/-) mice. The M1 markers iNOS and IRF5 were highly expressed in macrophage-rich areas of LSS lesions compared to OSS lesions 6weeks after cast placement, while the M2 marker Arginase-1, and Mox/Mhem markers HO-1 and CD163 were elevated in OSS lesions. Our data indicates shear stress could be an important determinant of macrophage polarization in atherosclerosis, with low shear promoting M1 programming.

Interferon Regulatory Factor 5 Controls Necrotic Core Formation in Atherosclerotic Lesions by Impairing Efferocytosis.

Background: Myeloid cells are central to atherosclerotic lesion development and vulnerable plaque formation. Impaired ability of arterial phagocytes to uptake apoptotic cells (efferocytosis) promotes lesion growth and establishment of a necrotic core. The transcription factor interferon regulatory factor (IRF)-5 is an important modulator of myeloid function and programming. We sought to investigate whether IRF5 affects the formation and phenotype of atherosclerotic lesions. Methods: We investigated the role of IRF5 in atherosclerosis in 2 complementary models. First, atherosclerotic lesion development in hyperlipidemic apolipoprotein E-deficient (ApoE-/-) mice and ApoE-/- mice with a genetic deletion of IRF5 (ApoE-/-Irf5-/-) was compared and then lesion development was assessed in a model of shear stress-modulated vulnerable plaque formation. Results: Both lesion and necrotic core size were significantly reduced in ApoE-/-Irf5-/- mice compared with IRF5-competent ApoE-/- mice. Necrotic core size was also reduced in the model of shear stress-modulated vulnerable plaque formation. A significant loss of CD11c+ macrophages was evident in ApoE-/-Irf5-/- mice in the aorta, draining lymph nodes, and bone marrow cell cultures, indicating that IRF5 maintains CD11c+ macrophages in atherosclerosis. Moreover, we revealed that the CD11c gene is a direct target of IRF5 in macrophages. In the absence of IRF5, CD11c- macrophages displayed a significant increase in expression of the efferocytosis-regulating integrin-&bgr;3 and its ligand milk fat globule-epidermal growth factor 8 protein and enhanced efferocytosis in vitro and in situ. Conclusions: IRF5 is detrimental in atherosclerosis by promoting the maintenance of proinflammatory CD11c+ macrophages within lesions and controlling the expansion of the necrotic core by impairing efferocytosis.

Innate Immune Receptors in Atherosclerosis

The inflammatory response is an important process, aiming to restore tissue homeostasis following tissue injury or infection. Acute inflammation is a tightly controlled process. If an inflammatory stimulus persists or if normal immune function is perturbed, inflammation may become chronic. Atherosclerosis is a chronic inflammatory disorder involving components of both the innate and adaptive immune systems (Ross, 1999). The innate immune system provides the first line of defence against invading pathogens. Innate immune detection of pathogens relies on a set of pattern recognition receptors (PRRs) that recognise and respond to conserved pathogen-associated molecular patterns (PAMPs). Growing evidence supports roles for PRRs in the initiation and progression of atherosclerosis. In this chapter, the agonists, signalling pathways, expression and functions of PRRs, in particular in reference to atherosclerosis, will be discussed. The potential therapeutic benefit of targeting PRRs for treatment of atherosclerosis will also be explored.