Follicular regulatory T cell in atherosclerosis

Atherosclerosis is a chronic inflammatory disease involving the infiltration of immune cells, such as monocytes/macrophages, neutrophils, T cells, and B cells, into the inner layer of vessel walls. T and B cell functions in the process of atherogenesis, as well as their mutual regulation, have been investigated but several aspects remain to be clarified. In the present review, we give a brief overview of the functions of follicular regulatory T cell (Tfr) on follicular T (Tfh) and B cell regulation related to atherosclerosis pathogenesis, including their influence on lymphangiogenesis and lipoprotein metabolism. We will also discuss their potential therapeutics properties in the resolution of established atherosclerotic lesions.


INTRODUCTION
Cardiovascular diseases (CVD) are the leading cause of mortality worldwide. 1 Atherosclerosis is the most studied CVD. Atherosclerosis is a chronic inflammatory disease characterized by the accumulation of lipids in the artery walls and involving both the innate and adaptive immune systems. 2 The infiltration of proinflammatory monocytes in the artery walls in presence of elevated concentrations of circulating lipids triggers atherogenesis. [3][4][5] During this complex process, circulating monocytes infiltrate the inner layer of vessel walls and differentiate into macrophages. This phenotypical switch allows the integration of lipids in excess in the artery walls leading to inflammation, foam cell formation, and establishment of plaques composed by the infiltrated elements, connective-tissue components, but also other particles. 6,7 Over time, plaques enlarge and can break, causing thrombus formation, leading to ischemia, and myocardial infarction. 8 Although the specific roles of T and B cell subtypes are not yet totally understood, their importance in the establishment and development of atherosclerosis has been clearly established. [9][10][11][12][13] In this review, we will try to give an overview of the functions of immune cells in the regulation of atherosclerosis with particular emphasis on regulatory B cells (Breg), follicular T cell (Tfh), and follicular regulatory T cell (Tfr).
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T CELLS IN ATHEROSCLEROSIS
During the development of atherosclerotic lesions, T cells are recruited at a very early stage. 14 In the presence of low-density lipoprotein (LDL), adhesion molecules and chemoattractants located and expressed in the intima lead to the infiltration of T cells into the plaques. 15,16 Once in the artery wall, antigens such as LDL and/or oxidized LDL activate the lymphocytes. 17,18 T cells' activation increases inflammation through proinflammatory cytokines release and creates an amplification loop between T cells and innate-immunity cells, such as macrophages. 19 To study atherosclerosis, transgenic mice depleted of Apoe or ldlr genes are the most currently used models. 20 In this context, Kyaw et al. determined that the majority of T cells present in atherosclerotic plaques are CD4 + T cells, although CD8 + T cells are present mainly in human and mouse advanced plaques 21 with functions of atherosclerotic plaque destabilization. 22 CD8 + T cells indeed trigger the apoptosis of macrophages, smooth muscle cells, and endothelial cells by its cytotoxic and inflammatory ability, leading to necrotic cores creation. 22 CD8 + are also able to regulate Tfh 23,24 known to be proatherogenic. [25][26][27] CD4 + T cells have also been shown to have a proatherogenic function. 28,29 Immunosuppressed Apoe −/− mice are atheroprotected, but adoptive transfer of CD4 + cells abolishes this TA B L E 1 Breg, Tfh, and Tfr cell subsets proprieties in murine atherosclerosis: discriminative biological markers; principal cytokines secreted; localization during atherosclerosis; and influence on the plaques development. GCs: germinal centers. TGF-: Transforming growth factor beta. IFN-: Interferon gamma. For more information on B subsets cells 70 27 To determine the exact function of T cells in atherogenesis, each subset needs to be thoroughly investigated.
Th1 cell population is widely present in plaques. Their ability to secrete proatherogenic cytokines certainly inhibits the atheroprotective action of other subsets. 37 Th2 and Th17 cell populations are also found but their functions are still unclear. 38,39 The functions of Treg cells in atherosclerosis have been largely investigated and show an overall atheroprotective role. 40,41 It is also important to consider the possible atherogenic effect of T cell populations led by their potential ability to regulate cholesterol levels through the lymphatic system. [42][43][44] It has been thus shown that areas with higher density of lymphatic vessels show less atherosclerotic plaques 44,45 probably by their ability to stimulate a process called reverse cholesterol transport (RCT). 43 In the context of cholesterol homeostasis, Treg cells seem to have opposite functions to other T cell subsets. 46 In Apoe −/− mice and in humans with coronary artery disease, Treg cells are impaired, although they are known to initially increase in numbers in early lesions. 47 Over the longer term, atherosclerosis is associated with a reduction of Treg cells. 48

F I G U R E 1 Mutual regulation between Tfr, Tfh, and Breg cells and their influence on lymphangiogenesis.
Tfr cell population is a key player in the repression of atherosclerosis. Its atheroprotective function is derived by its capacity to inhibit Tfh cells via TGF-secretion, 93 its direct up-regulation of lymphangiogenesis, 68 and also by the creation of a positive loop between Tfr and Breg cells, itself a Tfr cell activator. 27 In presence of Breg cells, the differentiation of CD4 + cells in Tfh cells during atherosclerosis is diminished 83

T AND B CELLS AND THEIR MUTUAL REGULATION IN ATHEROSCLEROSIS
The recent studies from our group indicate that Tfr cells exert part of their atheroprotective functions through the regulation of Breg cells differentiation. 27 Our findings indicate also that Tfr cells control Breg cell population through cell-cell interaction. It thereby seems that Breg and Tfr cell populations are mutually self-regulated, leading to an amplification loop of regulation dampening Tfh cells frequency and, in turn, atherogenesis (Fig. 1). Consistently, whereas Tfh cell are the main producers of IL-4 in GCs, IL-4 is able to indirectly inhibit Breg cells expansion. 81 The lack of Tfh cells regulation leads to nonantigenspecific B cells expansion, leading to autoimmune pathologies. 35,82 The association of atherosclerosis and autoimmune diseases is corre-

CONCLUSION
The role of immune cells in atherosclerosis has already been well studies, particularly T and B cells. Recent discoveries of new lymphocyte subsets, however, require constant updating of our knowledge to have a better understanding of the disease. The ability of Tfh and Tfr cells to modulate atherosclerosis permitted to highlight the mutual regulation between B and T cells. Although further investigations are required in the field of immunomodulation of atherosclerosis, the atheroprotective properties of Tfr population through the control of Tfh cells, Breg cells, lymphangiogenesis, and lipid metabolism make Tfr cell population a key player of atherogenesis (Fig. 1). Moreover, the appearance of new T cell subsets spotlights the importance to further consider T and B cells subsets interactions, their localization and their functions, not only in the artery wall but also on the lymphatic system, all promising lines of research. More largely, the study of immune system and its potential outcomes will certainly participate to unravel the complicated multifactorial physiopathology of atherosclerosis.