Vascular dendritic cells as gatekeepers of lipid accumulation within nascent atherosclerotic plaques

Dendritic cells in atherosclerosis: Functions in immune regulation and beyond

Chronic inflammation drives the development of atherosclerosis. Dendritic cells (DCs) are known as central mediators of adaptive immune responses and the development of immunological memory and tolerance. DCs are present in non-diseased arteries, and accumulate within atherosclerotic lesions where they can be localised in close vicinity to T cells. Recent work has revealed important functions of DCs in regulating immune mechanisms in atherogenesis, and vaccination strategies using DCs have been explored for treatment of disease. However, in line with a phenotypical and functional overlap with plaque macrophages vascular DCs were also identified to engulf lipids, thus contributing to lipid burden in the vessel wall and initiation of lesion growth. Furthermore, a function of DCs in regulating cholesterol homeostasis has been revealed. Finally, phenotypically distinct plasmacytoid dendritic cells (pDCs) have been identified within atherosclerotic lesions. This review will dissect the multifaceted contribution of DCs and pDCs to the initiation and progression of atherosclerosis and the experimental approaches utilising DCs in therapeutic vaccination strategies.

Lipid-Laden Multilocular Cells in the Aging Thymus Are Phenotypically Heterogeneous

Intrathymic lipid-laden multilocular cells (LLMC) are known to express pro-inflammatory factors that might regulate functional activity of the thymus. However, the phenotype of age-associated intrathymic LLMC is still controversial. In this study, we evaluated LLMC density in the aging thymus and better characterized their distribution, ultrastructure and phenotype. Our results show an increased density of LLMC in the thymus from 03 to 24 months of age. Morphologically, intrathymic LLMC exhibit fibroblastoid fusiform, globular or stellate shapes and can be found in the subcapsular region as well as deeper in the parenchyma, including the perivascular area. Some parenchymal LLMC were like telocytes accumulating lipids. We identified lipid droplets with different electrondensities, lipofuscin granules and autolipophagosome-like structures, indicating heterogeneous lipid content in these cells. Autophagosome formation in intrathymic LLMC was confirmed by positive staining for beclin-1 and perilipin (PLIN), marker for lipid droplet-associated proteins. We also found LLMC in close apposition to thymic stromal cells, endothelial cells, mast cells and lymphocytes. Phenotypically, we identified intrathymic LLMC as preadipocytes (PLIN⁺PPARγ2⁺), brown adipocytes (PLIN⁺UCP1⁺), macrophages (PLIN⁺Iba-1⁺) or pericytes (PLIN⁺NG2⁺) but not epithelial cells (PLIN^- panCK⁺). These data indicate that intrathymic LLMC are already present in the young thymus and their density significantly increases with age. We also suggest that LLMC, which are morphologically distinct, establish direct contact with lymphocytes and interact with stromal cells. Finally, we evidence that intrathymic LLMC correspond to not only one but to distinct cell types accumulating lipids.

Role of splenic monocytes in atherosclerosis

PURPOSE OF REVIEW

This review relates recent findings that highlight the role of the spleen as an active donor of monocytes during inflammation, with a special focus on atherosclerosis.

RECENT FINDINGS

The contribution of hypercholesterolemia and monocytes/macrophages to atherosclerotic lesion formation is undisputable. The origin of plaque macrophages is, however, still a subject of debate as to whether they derive from local amplification of (resident) macrophages or from continuous recruitment and differentiation of monocytes. Recently, the spleen has emerged as an important reservoir of monocytes that contributes to lesion growth. The regulation of monocyte mobilization from the splenic compartment has, therefore, raised a keen interest in understanding the cellular and molecular mechanisms involved in this process.

SUMMARY

Impaired regulation of cholesterol metabolism increases the proliferation of hematopoietic stem and progenitor cells in both the bone marrow and the spleen. Recent findings identified the implication of angiotensin II, red pulp macrophages and B-lymphocytes as partners of monocyte expansion in, and mobilization from the spleen. Future studies will help in understanding the mechanisms of monocyte mobilization and its precise roles in atherosclerosis, and whether modulation of the splenic components may become a promising future direction in the prevention and treatment of cardiovascular diseases.

Immune effector mechanisms implicated in atherosclerosis: from mice to humans

According to the traditional view, atherosclerosis results from a passive buildup of cholesterol in the artery wall. Yet, burgeoning evidence implicates inflammation and immune effector mechanisms in the pathogenesis of this disease. Both innate and adaptive immunity operate during atherogenesis and link many traditional risk factors to altered arterial functions. Inflammatory pathways have become targets in the quest for novel preventive and therapeutic strategies against cardiovascular disease, a growing contributor to morbidity and mortality worldwide. Here we review current experimental and clinical knowledge of the pathogenesis of atherosclerosis through an immunological lens and how host defense mechanisms essential for survival of the species actually contribute to this chronic disease but also present new opportunities for its mitigation.

Immunological aspects of atherosclerosis

Cardiovascular disease is the leading cause of death in several countries. The underlying process is atherosclerosis, a slowly progressing chronic disorder that can lead to intravascular thrombosis. There is overwhelming evidence for the underlying importance of our immune system in atherosclerosis. Monocytes, which comprise part of the innate immune system, can be recruited to inflamed endothelium and this recruitment has been shown to be proportional to the extent of atherosclerotic disease. Monocytes undergo migration into the vasculature, they differentiate into macrophage phenotypes, which are highly phagocytic and can scavenge modified lipids, leading to foam cell formation and development of the lipid-rich atheroma core. This increased influx leads to a highly inflammatory environment and along with other immune cells can increase the risk in the development of the unstable atherosclerotic plaque phenotype. The present review provides an overview and description of the immunological aspect of innate and adaptive immune cell subsets in atherosclerosis, by defining their interaction with the vascular environment, modified lipids and other cellular exchanges. There is a particular focus on monocytes and macrophages, but shorter descriptions of dendritic cells, lymphocyte populations, neutrophils, mast cells and platelets are also included.