Featured research: Inflammatory and immune cells in atherosclerosis T-cells
This study demonstrated that activation of invariant natural killer T (iNKT) cells decreased fibrous cap thickness by 33% and increased the number of buried fibrous caps as indicator of plaque rupture by a factor of almost 3 in brachiocephalic arteries of Apo E -/- mice fed a high fat diet. Genetic inactivation of iNKT cells led to the opposite changes in comparison with control mice. These changes corresponded to up- and downregulation of IFN-gamma and MMP-2 gene expression. Hence, this study supports a role for iNKT cells not only in atherogenesis but also plaque stability.
The study identified 56 subsets of CD4 T cells and T effector memory (TEM) cells as the subset that correlated independently the best with carotid intima-media thickeness. TEM levels were also higher in patients with CAD compared with controls but did not differ by acute or stable CAD presentation. In Apo E -/- and LDL-R -/- mice fed a Western diet for 16 weeks, TEM levels significantly increased in correlation with the extent of aortic root atherosclerosis. Thus, this study points out TEM as a particular CD4 T cell subset of potential further interest in the atherosclerotic disease process.
This study showed that genetic absence of CD4 T cells reduced the incidence of development of advance atherosclerotic plaques in Apo E -/- mice with renovascular hypertension by left renal artery clipping by 50%. Plaque size was reduced by 44%, and the absence of CD4 led to a 75% reduction in lipid core fraction and a three time higher SMC count. Re-infusion of CD4 in the genetically CD4 T cell deficient greatly increased atherosclerotic lesion size and decreased SMC content. Hence, this study supports the contributory role of CD4 cells to atherosclerosis in a renovascular hypertension model.
Macrophages
This study identified a new subtype of macrophage induced by exposure and uptake of hemoglobin. These henceforth coined “hemoglobin-stimulated macrophages” have increased ferroportin expression and reduced iron and ROS load. These characteristics are linked to activation of liver X receptor alpha and thereby ABC transporter expression, which confers resistance to cholesterol loading and foam cell formation. In atherosclerotic lesions this new type of macrophages can be detected by the expression of mannose and CD163 receptors at sites of intraplaque hemorrhage. Hence, this study provides new information on how intraplaque hemorrhage can influence the plaque microenvironment.
Folate receptor beta (FRbeta) is an exclusive marker of macrophage activation. Remarkably, an anti-FRbeta immunotoxin or antibody approach significantly decreased the number of FRbeta and TNF-alpha expressing macrophages and atherosclerotic burden in Apo E -/- mice. Thus, this study point out new potential for selective targeting of pro-atherogenic plaque macrophages.
Mast cells
This study examined carotid endarterectomy specimens and found a noteworthy correlation of the number of mast cells per carotid plaque along with other typical features of an unstable plaque phenotype. In particular, a highly significant association between mast cell count and microvessel density was noted. Intriguingly, over a follow-up time of 3 years, the rate of cardiovascular events was approx. 50% higher in patients with high compared with those with a low plaque mast cell count. Hence, this study brings a new spotlight on the pathological significance of mast cells in atherosclerosis.
This study utilizing a carotid collar model in Apo E -/- mice on a high cholesterol diet showed that recruitment and activation of mast cells to the vulnerable plaque region upstream the cast reduced the content of vascular smooth muscle cells in the plaque cap by 80%. This phenomenon was prevented by toll-like receptor 4 antagonism. In in-vitro experiments, both TLR4 antagonist and antibody approach inhibited mast cell-associated VSMC apoptosis as did inhibition of chymase. Thus, these findings link apoptosis of vascular smooth muscle cells to release of chymase from mast cells with activation of the TLR4 pathway and provide further mechanistic evidence for the role of mast cells in atherosclerosis.
Immunization
This study showed that SQ immunization against heat shock protein 65 reducedatherosclerosis extent in Apo E -/- mice. This effect was independent from regulatory T cell induction. Hence, the current study suggests that hsp65 immunization may lead a protective humoral response.
Spotlight: plaque neovascularization and plaque vulnerability
In this autopsy-based study on 121 apparently healthy heart transplant donors, neovessels on coronary artery cross-sections were found in 15% of 363 lesions, namely in those with greater intima thickness. In a multivariate analysis, only macrophage and T-cell infiltration and plaque fibrosis remained significantly associated with neovascularization. Thus, the current study adds to the current literature on this topic by showing the early onset of the neovascularization process, preceding plaque complication.
This study showed that in thoracic aortic plaques of rabbits on a cholesterol-rich diet for five months total cholesterol content of erythrocyte membranes significantly increased in association with lipid core size. Even the extent of the presence of erythrocytes within the lipid core was strongly associated with lipid core size. Hence, this study supports the concept of RBC-related cholesterol loading of the atherosclerotic plaque.
In this carotid endarterectomy sample-based study, it was found that 25% of plaquesshowed platelets in intraplaque hemorrhages. In 15% of the plaques, platelets were noted around microvessels even in the absence of erythrocyte leakage. In only 5% of the plaques, platelets were present in a mural thrombus attached to the luminal endothelium. Platelet staining was associated with microvessel density and IL-8 levels. Thus, this study points out very clearly that platelet enter the atherosclerotic plaque not via the main lumen but rather its neovasculature.
While not geared specifically to look at plaque neovascularization, this study nevertheless indicates that red blood cells are a carrier not only for cholesterol and iron oxidative stress via hemoglobin but also for MPO. Thus, this study adds to the notion of the significance of intraplaque RBCs for plaque dynamics.
Vascular remodeling
This study showed that positive remodeling compared with negative remodeling of the culprit lesion in acute coronary syndromes (ACS) portrays a poorer outcome over a median follow-up time of nearly 6 years. Major adverse cardiac events (death, ACS, and revascularization) rates were two times higher in patients with positive compared with negative remodeling. This association remained significant in the multivariate analysis (HR 2.4). Hence, this study points out positive remodeling of the actual ACS culprit lesion as a marker for a more aggressive disease style.
Endothelial dysfunction
This study outlined that alpha 1 AMP-activated protein kinase deletion aborts the otherwise observed increase in eNOS expression and serine1177 phosphorylation in mice undergoing voluntary exercise training. Furthermore, this deletion led to a loss of cell senescence protection. As such, this study provided important new information on the molecular mechanisms underlying the benefit of exercise for vascular health.
This study showed that reconstitution of eNOS activity only in the endothelium of eNOS -/- mice restored endothelium-dependent vasoreactivity responses but did not affect the elevated systolic blood pressure readings observed in the eNOS -/- mice. Hence, this study added new important data on the lack of contribution of eNOS activity to the resting systolic blood pressure.
This study showed that endothelium-dependent vasoreactivity can be impaired in the segment distal to an everolimus-eluting stent as a function of time from stent implantation. Compared with paclitaxel-eluting stents endothelium-dependent vasomotion is much improved proximally and distally to everolimus-eluting stents.
This study demonstrated that paclitaxel at ten times lower concentration caused reactive oxygen species production in human coronary artery endothelial cells, which was related to the much greater potential of paclitaxel to induce NFkB-mediated upregulation of p47phox. Hence, this study provided valuable mechanistic insight explaining the greater potential for abnormal vasoreactivity with the use of Paclitaxel-eluting stents.
Biomarkers
This study use urine proteomics to identify novel biomarkers of atherosclerosis in Apo E -/- mice. Sixteen polypeptides were identified, and three of these, EGF, collagen, and alpha1-antitrypsin were then noted to be expressed in the plaque as well. For collagen and alpha1-antitrypsin, urinary excretion levels correlated significantly with intraplaque content. Thus, this study stimulates novel approaches in the atherosclerosis biomarker arena.
Therapy
This study, performed in male LDLr -/- mice on an atherogenic diet, showed that adenoviral gene transfer of LDLr decreased non-HDL levels by 95% and induced atherosclerotic lesion regression by 25-30% along wit increased collagen content, reduction of necrotic core size, decreased expression of inflammatory genes, decreased macrophage content with a shift to the M2 phenotype. Adenoviral apolipoprotein A-I transfer by itself increased collagen content but did not reduce lesion size. In combination with LDLr transfer, Apo A-I transfer exerted additive effects, esp. on inflammatory gene expression. Hence, this study is an important contribution to highlight the relative benefit of LDL- and HDL-directed therapy and is in agreement with the current recommendations of making LDL the first treatment goal and parameter.
This study on peripheral blood mononuclear cells from healthy volunteers showed that ox-LDL and AngII induce maturation of dendritic cells and their capacity to stimulate T-cell proliferation. This was associated with upregulation of SR-A and LOX-1, and neutralizing antibodies to these partly inhibited the Ox-LDL effects. Ox-LDL also induced secretion of AngII from dendritic cells, and lorsartan antagonized the activating effects of ox-LDL on dendritic cells. Thus, this study points out important new insight into the anti-atherosclerotic mechanism of ARB therapy.
This study observed a significant reduction of tissue oxidative stress, endothelial dysfunction, and atherosclerotic plaques in Apo E -/- knock out mice on a HC diet by eplerenone. These effects were independent of the AT1 receptor as tested in double Apo and AT1R knockout mice. Hence, this study highlights that aldosterone antagonism exerts beneficial vascular effects independent from AT1 receptor signaling.
Miscellaneous
This study demonstrated significantly increased aortic leukocyte infiltration, oxidative stress, and endothelial dysfunction in hypercholesterolemic Apo E -/- mice with additional genetic deletion of the cannabinoid receptor-2 (CB2). This could not be overcome by treatment with a CB2 agonist, which improved aforementioned changes in hypercholesterolemic Apo E -/- mice without genetic CB2 deficiency. CB2 deficiency in bone marrow-derived cells alone was sufficient to cause increased vascular leukocyte infiltration. Further studies indicated that signaling via CB2 increases cytokine release and migration capacity of monocytes. Moreover, CB2 agonism decreased ROS in endothelial and vascular smooth muscle cells (VSMC) and VSMC proliferation.
This study outlined that neither activation nor deletion of the CB2 receptor influence atherosclerotic lesion size or composition in LDLr -/- mice on a high cholesterol diet. Hence, taken together with study P3163, there is currently conflicting data on the role of CB2 receptor signaling in the atherosclerotic disease process, which may relate to differences in experimental models.
This study showed that Apo E -/- mice rendered diabetic by streptozotocin develop vascular oxidative stress, endothelial dysfunction, and atherosclerosis. Atherosclerotic plaque area in particular is 90% reduced by additional AT1R deletion. The AT1R antagonist telmisartan yielded similarly beneficial results. In both circumstances, PPARgamma functionality was required for the benefit. Hence, this study points out the pivotal role of AT1 receptor signaling for atherosclerosis in this experimental diabetes model.
This study demonstrated that circulating endothelial progenitor cell (EPC) are reduced despite high circulating levels of the SDF-1alpha, a strong mobilizing factor for bone marrow (BM)-derived progenitor cells, in Apo E -/- mice on a high cholesterol diet. Within the BM, EPC apoptosis rates were not affected and EPC proliferation rates were actually higher in Apo E -/- mice. However, BM senescence was significantly increased in these mice, possibly accounting for the reduced mobilization capacity. Thus, this study provided important information on the mechanisms of reduced EPC levels in atherosclerosis.