Monocyte-Macrophages and T Cells in Atherosclerosis Ira Tabas, Andrew H. Lichtman  Immunity  Volume 47, Issue 4, Pages 621-634 (October 2017) DOI: 10.1016/j.immuni.2017.09.008 Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 1 Regulation of Innate Immune Processes Related to Monocyte-Macrophages in Atherosclerosis Lesional macrophages originate from bone-marrow-derived hematopoietic stem and progenitor cells (HSPCs), which give rise to circulating monocytes. In certain instances, these stem cells first populate the spleen and then undergo extramedullary hematopoiesis. Proliferation and release of HSPCs can be exacerbated by elevated cellular cholesterol and by somatic mutations leading to clonal hematopoiesis, such as those that occur in aging and myeloproliferative disease (MPD). This process can also be stimulated by stress-induced activation of the sympathetic nervous system (SNS). The major subpopulation of monocytes that contribute to atherosclerosis progression are Ly6chi monocytes, which enter lesions in response to subendothelially retained apolipoprotein-B-containing lipoproteins (LPs) and subsequent chemokine release by activated endothelial cells. After differentiation into macrophages, these myeloid cells undergo a variety of phenotypic changes under the influence of the factors listed in the figure. Those macrophages on the inflammatory end of the spectrum secrete proteins and carry out processes that promote atherosclerosis progression, whereas those on the resolution end of the spectrum promote lesion regression. Immunity 2017 47, 621-634DOI: (10.1016/j.immuni.2017.09.008) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 2 Intracellular Atherosclerosis-Influencing Effects of Excess Cholesterol on Myeloid Cells and T Cells In the setting of hypercholesterolemia or defects in cholesterol efflux, HSPCs accumulate excess cholesterol. The consequence is enhanced IL-3 and GM-CSF growth factor signaling, leading to HSPC proliferation and monocytosis. With aging, clonal hematopoiesis can occur as a result of loss-of-function mutations in a number of genes, including TET2. This process might also contribute to monocytosis. Monocytosis is associated with increased accumulation of inflammatory monocyte-derived macrophages in atherosclerotic lesions and a higher risk of atherosclerotic vascular disease in humans. These lesional macrophages are also subject to intracellular cholesterol accumulation because of their internalization of subendothelial apoB LPs. Excess cholesterol in macrophages has multiple effects that enhance lesion inflammation, including activation of toll-like receptors (TLRs) and inflammasomes. The result is increased production of inflammatory chemokines and cytokines, including inflammasome-derived IL-1β and IL-1β-induced IL-6. Moreover, changes associated with clonal hematopoiesis, e.g., loss of TET2 function, can also activate the inflammasome in macrophages, further fueling lesional inflammation. T cells do not have the capacity to accumulate large amounts of excess cholesterol, but several studies have shown that perturbations of T cell cholesterol metabolism can affect T cell differentiation and activation. Impaired ABCG1 cholesterol efflux from T cells results in enhanced Treg differentiation, which reduces atherosclerotic lesion development and inflammation. In contrast, impaired esterification of cholesterol by deficiency or inhibition of acyl-coenzyme A:cholesterol acyltransferase increases the formation of lipid rafts in CD8+ effector T cells and thereby enhances immune synapse formation and killing functions of these cells. The net effect of increases in T cell cholesterol on lesion development and inflammation are likely to reflect changes in the Teff-Treg balance and the influence of Teff cells on lesional macrophages. Immunity 2017 47, 621-634DOI: (10.1016/j.immuni.2017.09.008) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 3 Regulation and Impact of Adaptive Immune Processes Related to T Cells in Atherosclerosis T lymphocyte responses that affect atherosclerosis include a balance between inflammatory effector T (Teff) cells, mainly interferon-γ-producing T helper (Th) cells, and anti-inflammatory regulatory T cells (Treg). Pro-atherogenic Th effector cells differentiate from thymic-derived naive T cell precursors in secondary lymphoid organs (SLOs), such as lymph nodes, in response to antigen presentation of LDL-derived peptides by dendritic cells (DCs), some of which could have migrated from the arterial wall. Treg cells develop in the thymus, and peripheral Treg (pTreg) cells can also be differentiated from peripheral naive T cells in SLOs. The direction of differentiation of naive T cells into different Th subsets and Treg cells in SLOs can be influenced by systemic and local metabolic conditions. Treg and Th cells migrate into developing atherosclerotic lesions and modulate the local inflammatory microenvironment, in large part by influencing macrophage phenotypes. Conversely, resolving or inflammatory macrophage phenotypes can shift the plaque T cell balance toward Treg and Th phenotypes, respectively. Change in the balance between Treg and Th cells could reflect phenotypic plasticity by permitting re-differentiation between regulatory and inflammatory phenotypes. The Treg-Th-cell balance can also be influenced by systemic and plaque metabolic conditions. See main text for details. Immunity 2017 47, 621-634DOI: (10.1016/j.immuni.2017.09.008) Copyright © 2017 Elsevier Inc. Terms and Conditions