Immune Disorder in Atherosclerotic Cardiovascular Disease　― Clinical Implications of Using Circulating T-Cell Subsets as Biomarkers ―

Abstract

Atherosclerotic cardiovascular disease (ACVD) is an inflammatory phenomenon that leads to structural abnormality in the vascular lumen due to the formation of atheroma by the deposition of lipid particles and inflammatory cytokines. There is a close interaction between innate immune cells (neutrophils, monocyte, macrophages, dendritic cells) and adaptive immune cells (T and B lymphocytes) in the initiation and progression of atherosclerosis. According to novel insights into the role of adaptive immunity in atherosclerosis, the activation of CD4⁺ T cells in response to oxidized low-density lipoprotein-antigen initiates the formation and facilitates the propagation of atheroma, whereas CD8⁺ T cells cause the rupture of a developed atheroma by their cytotoxic nature. Peripheral CD4⁺ and CD8⁺ T-cell counts were altered in patients with other cardiovascular risk factors. Furthermore, on evaluation of the feasibility of immune cells as a diagnostic tool, the blood CD4⁺ (helper), CD8⁺ (cytotoxic), and CD4⁺CD25⁺Foxp3⁺ (regulatory) T cells and the ratio of CD4 to CD8 cells hold promise as biomarkers of coronary artery disease and their subtypes. T cells also could be a therapeutic target for cardiovascular diseases. The goal of this review was therefore to summarize the available information regarding immune disorders in ACVD with a special focus on the clinical implications of circulating T-cell subsets as biomarkers.

Coronary atherosclerotic disease (CAD) is an obstruction of the coronary circulation characterized by the accumulation of lipids and fibrous elements in the subendothelial space, causing conditions such as stable angina pectoris (SAP), unstable angina pectoris (UAP), myocardial infarction (MI), and sudden cardiac death.¹ Lipid accumulation is a stimulus for the influx of immune cells into the thickened intimal area, which, with time, turns into atherosclerotic plaque that can grow large enough to narrow or block the arterial lumen.² Mature plaque is characterized by the accumulation of additional innate immune cells (neutrophils, monocytes, macrophages, dendritic cells [DC]) and adaptive immune cells (T and B lymphocytes) and oxidized low-density lipoprotein (LDL) that form a core region surrounded by a cap of smooth muscle cells (SMC) and a collagen-rich matrix (Figure 1).³

Figure 1.

Infiltration of inflammatory and immune cells on immunostaining with the corresponding antibodies (antibodies against neutrophil elastase for neutrophils, CD68 for macrophages, CD3 for T cells, CD20 for B cells, and human-tryptase for mast cells) in atherosclerotic plaque.

Indirect evidence strongly supports the notion that T cells in culprit lesions are locally activated, which is a prerequisite for the involvement of antigens in plaque instability. Upon the antigen-mediated cross-linking of the T-cell receptor (TCR), T cells begin to express interleukin (IL)-2 receptors and secrete IL-2, a mechanism that facilitates preferential expansion of antigen-specific T cells.⁴ Recent research further elaborated the role of adaptive immune responses in atherosclerotic phenomena, and a more precise examination of atherosclerotic plaques showed the presence of T-lymphocyte subset cluster differentiation (CD) 4 and CD8 T cells.⁵

Status of Immune Disorder and Atherosclerotic Cardiovascular Disease Research

History of the Immune System

The immune system is the complex defense system of the body. Its main function is to distinguish between self-molecules (antigens) in the body and non-self-antigens such as bacteria, viruses, and fungi, as well as damaged tissue in the body. The immune system consists of various types of cells and soluble molecules such as complement factors and cytokines.⁶ The immune system is classified into innate immunity and adaptive immunity (Figure 2). We have also summarized the development of T cells and their subsets (Figure 2; Table 1).^7–20

Figure 2.

Schematic diagram of T-cell and subset development. *Adaptive immune cells; ^#innate immune cells; *^#dual properties. CD, cluster differentiation; MHC, major histocompatibility complex; NK T cells, natural killer T cells; TCR, T-cell receptor; Th, T-helper; Thf, follicular T-helper; Tregs, regulatory T cells.

Table 1. T-Cell Subsets and Their Functions

T-cell types and subset	Glyco-protein co-receptor	Chemokine expression/ production and mechanisms	Functions
CD4+	CD4	IL-2, IL-3, and TNF-α and -β: amplify the inflammatory response IFN-γ, IL-17: recruitment of T cells, macrophages to the plaque Enhances formation of foam cells	Pro-inflammatory, pro-atherogenic effect7–9
Th1	CD4	INF-γ, IL-12, IL-18, TNF-α: activate inflammatory response of monocytes/macrophages, DC Inhibit vascular SMC proliferation and reduces collagen production Upregulation of expression of MMP, Fibrous cap thinning	Pro-atherogenic effect10
Th2	CD4	Rarely detected in atherosclerotic lesions Upregulation of IL-4, IL-5, IL-13, IL-33. Downregulation of IFN-γ Stimulates production of anti-oxLDL antibodies (IgM) by B cells	Anti-atherogenic effect11
Th17	CD4	Pro-inflammatory cytokine production including TNF, IFN-γ, IL-22, IL-17	Pro-atherogenic effect12
Th9	CD4	IL-9 plays a pathogenic role Progression during experimental atherosclerosis	Pro-atherogenic effect13
Thf	CD4	Treg can switch the phenotype into pro-atherogenic Thf cells	Pro-atherogenic effect14
NKT	CD4	IL-4, IL-10, IL-13, IFN-γ, cytotoxic proteins perforin and granzyme B CD1d-NKT axis aggravates atherosclerosis	Pro-atherogenic effect15 Anti-atherogenic effect16
Tregs	CD4	Expressed as CD4+CD25+Foxp3+ IL-10, TGF-β inhibits the proliferation, activation, and differentiation of T cells toward Th1 and Th2 Inhibition of foam-cell formation	Anti-inflammatory, atheroprotective effect8,9,17
CD8+	CD8	IFN-γ promotes inflammation Dominant in advanced atherosclerotic lesions, secretes serine protease granzyme B	Pro-inflammatory, athero-destructive effect18,19
γδ T	Neither CD4 nor CD8	IL-17	No effect on early atherogenesis20

CD, cluster differentiation; DC, dendritic cell; FoxP3, forkhead box P3; IFN, interferon; IgM, immunoglobin M; IL, interleukin; MMP, matrix metalloproteinase; NKT, natural killer T cell; oxLDL, oxidized low-density lipoprotein; SMC, smooth muscle cell; TGF, tumor growth factor; Th, T-helper; Thf, follicular T-helper; TNF, tumor necrosis factor; Treg, regulatory T cell.

Innate Immunity The innate immunity cells include natural killer (NK) cells, mast cells, monocyte-derived macrophages, granulocytes, and DC. Innate immunity has no memory but the innate immunity system recognizes, responds to, and kills pathogens quickly. The innate system is triggered when pattern recognition receptors (PRR) on the innate cells recognize pathogen-associated molecular patterns (PAMP).²¹ In the arterial intima, macrophages expressing PRR mediate the degradation of lipoprotein particles and apoptotic bodies. Normally, this process in itself does not lead to inflammation, but in the long run it can lead to a massive accumulation of lipid-filled macrophages (the so-called “foam cells”) in the arterial wall. These foam cells have recently been detected to be derived mainly from SMC, because SMC are relatively incapable of releasing excess cholesterol.²²

Adaptive Immunity The adaptive immune system is composed of highly specialized cells and processes that eliminate or prevent pathogenic growth. The activation of the adaptive immune system requires antigen presentation by specific antigen-presenting cells (APC).⁷ Adaptive immunity is further divided into (1) humoral immunity, which is mediated by B cells and antibodies; and (2) cell-mediated immunity, which is mediated by T cells that activate macrophages for killing or that lyse infected cells.^15,16 T lymphocytes are characterized by a surface CD3 and consist of two major functional subsets: T-helper (Th) lymphocytes expressing the cell-surface molecule CD4, and cytotoxic T lymphocytes (CTL) expressing the cell-surface molecule CD8. The γδT cells, however, are another T-cell subset: they have γδTCR and are negative for CD4 and CD8, and they have CD277 as the antigen-presenting molecule.²⁰ The CD4 T cells are subdivided into the Th1, Th2, Th9, Th17, follicular T-helper (Thf), and T regulatory (Treg) groups, each associated with atherosclerotic phenomena and a characteristic profile of cytokines (Table 1).^11–13

CD4⁺ T cells bind antigen in association with major histocompatibility complex (MHC)-II molecules. The main function of Th cells is to assist other cells of the immune system such as B cells, phagocytic cells, and CD8⁺ T cells to perform their immune functions. The CTL that recognize antigens loaded on MHC class I molecules are called CD8⁺ T cells. These cells contribute to resistance against intracellular infections by killing target cells via a release of toxins.²³ Engelbertsen et al reported that high numbers of Th2 cells were independently associated with decreased mean common carotid intima-media thickness and increased serum Th2-related cytokine IL-4, and inversely correlated with reduced risk of MI and stroke.²⁴ One comprehensive review article highlighted the roles of IL-17 and Th17 cells in the pathophysiology of atherosclerosis.²⁵ Another study clarified the importance of the blood Treg/Th17 cell ratio in human atherosclerotic plaque progression.²⁶

Immune Disorder Begets Atherosclerotic Cardiovascular Disease (ACVD)

The deep involvement of adaptive immunity in inflammatory and metabolic disorders has been further shown by the increased risk of ACVD in patients with disorders of the immune system, including HIV and autoimmune diseases. Brown et al investigated Kawasaki disease patients with coronary artery aneurysms, and found significantly increased levels of CD8⁺ T cells in the coronary artery atherosclerotic plaques despite an increased circulating CD4:CD8 ratio, suggesting the possibility of selective recruitment of activated CD8⁺ T cells into the lesions of the coronary aneurysm.²⁷ This suggests the ability of coronary arteries to recruit activated CD8⁺ T cells selectively during some inflammatory pathological conditions.²⁷ In an investigation of the variations in CD4 and CD8 levels according to age, compared with young adults, the numbers of CD4 and CD8 T cells in older individuals were lower, suggesting either a decrease in the thymic release of these cells²⁸ or a reduction in the diversity of TCR. Another study suggested that these lower cell numbers might be associated with a previous cytomegalovirus infection.²⁹ Patients, however, with atherosclerotic diseases, which are usually elderly patients, have relatively higher proportions of both CD4 and CD8 T cells compared with younger individuals. This suggests a role of adaptive immunity as a major contributor in the course of atheroma formation and development, despite the relatively lower numbers of total CD4 and/or CD8 T cells in elderly individuals. Complications of atherosclerosis are clearly age related, and explorations of the hypothesis that age-related dysfunction of the immune system has pathogenic relevance may be useful.

Harmful Effects of Immune Overreaction: Mechanisms of Immune Disorder-Related ACVD

Pathogenesis of Atherosclerosis

Inflammation is an important contributing factor for plaque development, and today atherosclerosis is regarded as a chronic inflammatory disease in which both innate and adaptive immune responses play important roles.³⁰ In brief, LDL in the blood enter the artery wall and adhere to matrix proteins, where they may be modified by oxidation and aggregation.³¹ These modified lipids activate the endothelial cells to express pro-inflammatory molecules including adhesion molecules such as P-selectin, E-selectin, intercellular adhesion molecule (ICAM)-1, and vascular cell adhesion molecule (VCAM)-1, thereby enabling immune cells such as monocytes, T cells, B cells, NK cells, and DC to attach and migrate into the vessel wall.³²

Oxidized LDL are then engulfed by the macrophages, which are then transformed into foam cells. This foam cell accumulation is called the “fatty streak”. Macrophages and T cells are localized particularly in the region where the plaque grows.³³ After entering the arterial wall, T cells will become activated by interacting with APC, in particular macrophages and DC. Macrophages are also the source of the cytokine tumor necrosis factor (TNF) that promotes the recruitment of T cells in the process of atherosclerotic changes.³³ The majority of the T cells, macrophages, and mast cells together contribute to the inflammatory response and plaque progression in the arterial wall by secreting cytokines such as interferon-γ (IFN-γ), IL-1, and TNF and growth factors that promote the migration and proliferation of SMC.³⁴

CD4⁺ Activation as a Proatherogenic Agent

Most of the CD4⁺ effector T cells detected in atherosclerotic lesions express the αβTCR and have a Th1 phenotype. Th1 cells are derived from a naïve CD4⁺ T-cell precursor after antigen stimulation and co-stimulating activation in the presence of certain cytokines such as IL-12 and IFN-γ. The defining characteristic of Th1 cells is their production of IFN-γ, a pro-inflammatory cytokine that activates mainly macrophages.¹⁰ In addition to IFN-γ, Th1 cells release IL-2, IL-3, and TNF-α and -β, which amplify the inflammatory response.¹⁰ As noted in several studies, chronic repeated antigen exposure resulted in the dominance of the Th1 subtype and IFN-γ expression by CD4⁺ T cells, which is analogous to the predominance of Th1 cells in atherosclerosis, indicating that many CD4⁺ T cells and large amounts of IFN-γ are present together in human and mouse atherosclerotic plaques.¹⁴ IFN-γ leads to several processes, including the recruitment of T cells and macrophages to the plaque, the formation of foam cells by an enhancement of macrophage uptake of lipids, and an increase in the number of APC. IFN-γ also facilitates the production of immunoglobulin G(IgG)2a antibodies by B cells and enhances the secretion of Th1-promoting cytokines, which continue to drive the pathogenic process that, once started, appears to trigger a cascade of proatherogenic processes.^14,35 These suggest a role for various T-cell subsets in the development and progression of atherosclerosis. CD4⁺ T cells in general are believed to exacerbate atherosclerotic disease.

CD8⁺ Activation as an Athero-Destructive Agent

In studies of the roles of CTL (also known as CD8⁺ T cells) in atherosclerosis, the arterial intima and adventitia of patients with symptomatic atherosclerosis had higher numbers of CD8⁺ T cells than the arterial media.³⁶ In addition, the increased number of intimal CD8⁺ T cells was directly correlated with the severity of atherosclerotic lesions. The arterial adventitia and the intima in arterial sectors of advanced atherosclerotic lesions were also freely accessible to effector CTL.³⁶ Experimental studies using apolipoprotein E-deficient (ApoE^−/−) mice showed that CD8⁺ T cells are able to promote the formation of an atherosclerotic lesion when their stimulation was induced by the expression of a foreign antigen in the vascular wall.³⁷

Although relatively lower percentages of CD8 cells compared with CD4 cells have been observed in early atherosclerotic plaques, in advanced lesions the CD8 percentage can be as high as 50%. Due to its lytic activity, CD8 causes the expansion of the necrotic core of an atheroma and the inflammation and thinning of the atherosclerotic cap.³⁸ After activation, CD8⁺ T cells secrete large amounts of the serine protease granzyme B, by which cytotoxic cells may accelerate the apoptosis of vascular SMC and plaque destabilization in atherosclerotic lesions.³⁵

Tregs as an Athero-Protective Agent

The development and maturation of Tregs can be achieved both in the thymus and in the periphery. Tregs that are generated in the thymus are called natural Tregs (nTregs), and they exist in the absence of antigenic stimulation. The important role of nTregs is to induce and maintain immune tolerance.¹⁷ The Tregs that develop and mature in the periphery are called inducible Tregs. nTregs express CD4, CD25 (IL-2R), and the transcription factor forkhead box P3 (FoxP3), and nTregs negatively regulate pro-inflammatory immune effects via the cytokines IL-10 and tumor growth factor-β (TGF-β; Table 1). In the periphery, Tregs are derived from naïve CD4⁺ T cells during an active immune response, and they also express CD4⁺ CD25⁺ but do not require FoxP3 expression to be functional.³⁹

The presence of Tregs in human atherosclerotic lesions was found to be reduced, but Tregs were present in all developmental stages of atherosclerotic plaque. CD4⁺ CD25⁺ Tregs may exert their suppressive role on macrophage foam cell formation, and they can redirect the macrophage differentiation toward an anti-inflammatory cytokine-producing phenotype.^8,9 The major cellular source that produces IL-10 and TGF-β is represented by Tregs, which indicates an active role of Tregs in the control of the atherosclerotic process. This suggests the important protective role of Tregs against atherogenesis. Tregs have been shown to be protective in murine atherogenesis and to reduce plaque progression in atherosclerotic animals.^40,41 Tregs can prevent autoimmunity by competing with other T-cell subsets for complexes of antigen and MHC-II on APC.

A clinical study showed that, in a prospective cohort, the number of circulating Treg cells in peripheral blood was inversely associated with the development of MI and coronary events in general.⁴² All of these new findings highlighted the importance of Tregs in slowing down the inflammatory process building up in the arterial wall. The control of the differentiation of Tregs and their function may provide the potential for Tregs to be used as therapeutic targets for the prevention of atherosclerosis.

Clinical Assessment of Peripheral Blood CD4⁺ Cells, CD8⁺ Cells, and CD4⁺:CD8⁺ Ratio

Accumulating evidence indicates that circulating CD4⁺, CD8⁺, and CD4⁺ CD25⁺ Foxp3⁺ T-cell counts were altered in patients with other cardiovascular risk factors. Moreover, on evaluation of the feasibility of immune cells as a diagnostic tool, these circulating T cells, their subtypes and their ratio were found to hold promise as biomarkers of coronary artery disease (Table 2).^{8,9,18,19,24,26,28,29,35–38,40–57} Peripheral measurements of T-cell subsets and their levels in ACVD patients are summarized in Table 2.

Table 2. Levels of Peripheral T-Cell Subsets in Cardiac Disease

Study	Subjects	ACVD or CVD risk factors	CD4	CD8	Treg	CD4/CD8 ratio	Implications/mechanisms
Wigren et al (2012)42	Humans Blood	CAD			↓		Development of MI and coronary events
Gao et al (2017)43	Humans Blood	CAD				↑
Jonasson et al (2003)19	Humans Blood	UAP			↑
Liuzzo et al (1999)44	Humans Blood	UAP SAP	↑	↑
Backteman et al (2012)45	Humans Blood	CAD-SAP ACS	↑ ↑	↓ ↑	↑ ↓	↑	Progressive atherosclerotic lesion Plaque more vulnerable to rupture
Cheng et al (2011)46	Humans Blood	UAP STEMI			↓ ↑
Syrjälä et al (1991)47	Humans Blood	AMI				↓	Poor prognosis
Boag et al (2015)48	Humans Blood	AMI/PCI		↑			Microvascular obstruction
Yang et al (2006)49	Animals Blood	AMI	↑				Myocardial ischemia-reperfusion injury
Zhu et al (2014)50	Humans Blood	Population- based cohort			↓		Increased MI risk
Nikolich-Žugich (2014)28 Komarowska et al (2015)29	Humans Blood	Elderly vs. young	↓	↓			Adaptive immune impairment Cytomegalovirus infection
Ramos et al (2017)51	Humans Blood	CHF (ischemia/ idiopathic)	↑	→		↑
Bansal et al (2017)52	Mice Blood	CHF	↑	→			Enhancing cardiac remodeling
Krikke et al (2014)53 Castilho et al (2016)54 Riddler (2003)55	Humans Blood	CAD/HIV(+)	↓ ↓	↓		↓	Immunodeficiency syndrome Increased coronary events
Koch et al (2007)56	Humans Blood	Smoking	↑			↑	Correlated with smoking
Kolbus et al (2013)18	Humans Blood	Metabolic risk factors		↑			Correlated with a high waist-hip ratio, high fasting plasma glucose, insulin, and triglyceride levels Low fraction of CD8+ T cells reduces risk of the development of AMI
Tanigawa et al (2004)57	Humans Blood	Metabolic risk factors	↑				Correlated with insulin resistance and MetS Total lymphocytes and CD3+ cells also increased
Aukrust et al (2008)35 Gewaltig et al (2008)36 Hedrick (2015)37 Lichtman et al (2013)38 de Boer et al (2007)8 Lin et al (2010)9 Ait-Oufella et al (2006)40 Mor et al (2007)41	ApoE−/− mice Adventitia/intima		↑	↑	↓		CD8+ T cells: produces IFN-γ, IL-2, IL-3, and TNF-α in early AS plaques CD8+ T cells: produces cytotoxic serine protease granzyme B/causes advanced AS plaque more vulnerability and rupture Treg: produces IL-10 and TGF-β Suppresses macrophage foam cell formation and differentiation and plaque progression
Engelbertsen et al (2013)24	Human blood	CAD					↑Th2 associated with reduced risk of MI
Potekhina et al (2015)26	Human blood	CAD					↓Treg/Th17 ratio associated with more severe atherosclerosis

↑, increased; ↓, decreased; →, no change; --, unknown. ACS, acute coronary syndrome; ACVD, atherosclerotic cardiovascular disease; AMI, acute myocardial infarction; ApoE^−/−, apolipoprotein E deficient; AS, atherosclerosis; CAD, coronary atherosclerotic disease; CHF, chronic heart failure; DCM, dilated cardiomyopathy; IFN, interferon; IL, interleukin; MI, myocardial infarction; MetS, metabolic syndrome; MVO, microvascular obstruction; PCI, percutaneous coronary intervention; SAP, stable angina pectoris; STEMI, ST-elevation myocardial infarction; TGF, transforming growth factor; TNF, tumor necrosis factor; Treg, regulatory T cell; UAP, unstable stable angina pectoris.

Alteration of Peripheral Blood Circulating T Cells and CVD Risk Factors

Tobacco intake is one of the major contributors to atherosclerotic disease. Its association with T cells has been examined in relation to many cardiorespiratory conditions, and cigarette smoking has been shown to significantly raise the CD4 level: for example, a significantly higher number of CD4⁺ T cells and a significantly higher CD4⁺:CD8⁺ ratio in the peripheral blood of smokers was observed compared with non-smokers.⁵⁶ A high fraction of CD8⁺ T cells was shown to be associated with several characteristics of insulin resistance, including low plasma high-density lipoprotein, cholesterol and high waist-hip ratio, high insulin, triglyceride, and fasting plasma glucose level.¹⁸ That study also showed that individuals with a low fraction of CD8⁺ T cells have a decreased risk of the development of acute MI.¹⁸

These findings suggest that CD8⁺ T cell number could be used as a surrogate marker in the disease process. The total CD8⁺ T-cell population was associated with an increased risk of MI, a decreased release of pro-inflammatory cytokines from activated leucocytes, and metabolic signs of insulin resistance; in addition, the subpopulation of IFN-γ-producing CD8⁺ T cells was associated with enhanced release of pro-inflammatory cytokines from activated leukocytes.¹⁸ In an investigation of the relationship between altered cellular immune status and clustered features of the metabolic syndrome, total leukocyte, total lymphocyte, CD3⁺ T cell, and CD4⁺ T cell counts were significantly correlated with the number of components of metabolic syndrome.⁵⁷

Peripheral Blood Circulating T-Cell Level in CAD

Many studies have demonstrated that the number of Tregs is reduced in patients with unstable coronary artery disease, whereas other studies reported an increase in Tregs in acute ST-elevation MI patients.⁴⁶ A large prospective population-based cohort study also noted an association between an increased risk of MI and reduced Tregs in the circulation.⁵⁰ In a recent study, CD4 cells and CD8 cells in peripheral blood were measured in individuals aged >60 years who had been diagnosed with atherosclerotic CAD with >50% obstructive plaques in the coronary arteries.⁴³ The CD4/CD8 ratio was found to be independently higher in the CAD group as compared with the non-CAD group after adjusting for classical risk factors.⁴³ In another cross-sectional study, a peripheral blood expansion of CD8⁺ T cells was observed in CAD patients, and more extensively in patients <60 years with unstable angina.¹⁹ Thus, a role of T cells in >50% obstructive plaques has been observed in a wide age range of individuals with significant atherosclerosis without MI.

An increased secretion of INF-γ from CD8⁺ T cells was observed in UAP patients compared with SAP patients and controls, whereas the absolute lymphocyte number was not significantly different between the SAP, UAP, and control groups; in addition, the level of CD4⁺ IL2⁺ T cells was significantly higher in the SAP group compared with the UAP and control groups.⁴⁴ Thoracic lymph nodes in CAD patients had a lymphocyte subpopulation profile differing substantially from that in the patients’ blood, including a higher proportion of B cells, a lower proportion of CD8⁺ T cells, a 2-fold higher CD4/CD8 ratio, and an enrichment of Tregs as well as CD4⁺ CD69⁺.⁴⁵ An enhanced peripheral expression of either CD4 or CD8 has been encountered in patients with acute coronary syndrome (ACS), with a concomitant deficiency of Tregs. The CD4 activity was increased in patients with progressive atherosclerotic lesions or SAP, whereas in UAP the CD8 activity would make plaque more vulnerable to rupture. Further studies regarding the role of CTL in the progression and vulnerability of atherosclerotic plaques are necessary for a better understanding of CAD complications.

Peripheral Blood Circulating T Cells in CAD With Chronic Heart Failure (CHF)

Lymphopenia was observed in patients with CHF.⁵⁸ In a later comparative study of ischemic CAD patients with CHF and control subjects, the level of CD4 cells was increased, leading to an elevated CD4:CD8 ratio in which the number of CD8 cells tended to be decreased, although not significantly.⁵¹ In a study of CHF due to different causes (i.e., ischemic or idiopathic CHF), the level of IFN-γ positive CD4⁺ T cells was higher in ischemic cardiomyopathy than in idiopathic dilated cardiomyopathy.⁵⁹ In a mouse model with ischemia-driven heart failure, a very significant increase was seen in the CD4 population (and in that of other inflammatory cells) in failing hearts, whereas the CD8 population generally remained static in peripheral tissues in the same model, indicating a higher level of CD4 in heart failure as the agent enhancing the cardiac remodeling in CHF.⁵² Collectively, the aforementioned findings suggest that in patients with severe ischemic CHF, a significant activation of the immune system occurs, possibly contributing to disease decompensation and progression. Future studies evaluating immune responses in acute heart failure secondary to acute MI would be of great interest.

Acquired Immunodeficiency Promotes CVD

Chronic HIV infection – which is well known to depress the immune function of the host – is correlated with a high risk of coronary artery disease that progresses rapidly from endothelial impairment to subclinical atherosclerosis and, in some cases, to advanced atherosclerotic plaque and ACS.⁶⁰ In particular, generalized CD8⁺ T-cell activation as well as CD4⁺ T-cell activation senescence were found to correlate positively with atherosclerosis in HIV-infected individuals treated with an effective antiretroviral therapy.⁵³ AIDS not only reduces the CD4 and CD8 cell counts but also poses an independent risk for CAD. In a prospective clinical study of patients with AIDS after virological stabilization by antiretroviral therapy, the patients were analyzed for CAD events and non-cardiac mortality during 14 years of follow-up. A significant inverse CD4:CD8 ratio was seen in all of the patients who had CAD events during the follow-up, but not in the patients who had other causes of mortality.⁵⁴ Evaluation of HIV patients indicated increased LDL with a decreased level of CD4 cells, and after those patients started antiretroviral therapy there was a significant improvement in the CD4 count, along with a reduction of the LDL level, which is a major contributor to future CAD events.⁵⁵ Thus, in patients with chronic HIV infection, lack of circulating CD4⁺ helper T cells appear to promote initiation and progression of atherosclerotic plaques, although increase in the CD4 population is associated with ACVD in other conditions. Alterations in monocyte subpopulation and Treg dysfunction in chronic HIV infection patients may contribute to increase ACVD.

Peripheral Blood Circulating T-Cells in Acute MI Patients With Underwent Angioplasty

In a clinical study from the 1990s, a reversed CD4/CD8 ratio was observed as a common phenomenon in patients who had acute MI, and the reversed ratio usually rapidly returned to the normal range; in addition, a permanently low peripheral CD4/CD8 ratio was shown to be a sign of poor prognosis.⁴⁷ In a more recent trial evaluating ischemia/reperfusion injury in patients following primary percutaneous coronary intervention, the patients with MI were analyzed in a prospective as well as retrospective fashion, and it was observed that peripheral lymphopenia was associated with poor prognosis after MI.⁴⁸ The lymphocyte count fell ≤90 min after reperfusion, primarily because of the interstitial infiltration of plasma T cells mediated by the chemokine fractalkine, and the reduction of CD8⁺ T cells was greater than that of CD4⁺ T cells. In addition, a lower CD8⁺ T-cell count was associated with significant microvascular obstruction.⁴⁸ Experimental studies, however, strongly indicated that CD4⁺ T cells, but not CD8⁺ T cells, contribute to myocardial ischemia-reperfusion injury involving IFN-γ expression.⁴⁹ Taking the these findings into consideration, it could be concluded that INF-γ-mediated CD8 cytotoxic activity is higher in UAP or acute MI, leading to a higher number of CD8 T cells in the circulation, whereas an IL-2-mediated progression of the inflammatory process is facilitated by CD4 cells in chronic atherosclerosis. An increased level of CD4 cells may thus be observed.

Clinical Implications

Compelling evidence obtained in research regarding human and mouse immune cell heterogeneity points to a scenario in which immune cell subsets actively change and modulate the development of ACVD. Patients with CAD are, prototypically, obese smokers with hypertension, diabetes mellitus, and/or hypercholesterolemia. All of these comorbidities and some of the drugs they necessitate (e.g., angiotensin-converting enzyme inhibitors⁶¹ and statins⁶²) have been shown to influence lymphocyte function.

CD4⁺ T cells become activated upon the recognition of self-antigens during MI, and they infiltrate both the infarcted and remote myocardium. CD4⁺ T cells add to the immediate ischemia/reperfusion injury but are also required for efficient healing and might also attenuate chronic remodeling after acute MI. T-cell activation is related to reperfusion injury, and thus T-cell suppression therapy during reperfusion might be beneficial. On clinical testing cyclosporine showed some positive findings. After experimental MI, Tregs attenuated fibrosis in response to experimental aortic constriction and improved remodeling. As shown in Table 2, evaluation of blood CD4⁺, CD8⁺, and CD4⁺ CD25⁺ Foxp3⁺ Treg cells and the CD4:CD8 ratio has been significant in all ACVD. Further investigation of how peripheral T-cell subsets in relation to coronary intervention therapy determine the severity of atherosclerotic vascular lesions should be conducted. Nevertheless, it is clear that in order to advance our understanding of how each subset of T cells contributes to the initiation and progression of atherosclerosis, it is necessary to study each cell type in isolation to determine the types of chemokine signals initiated by each subset of T cells.

Sources of Funding

This work was supported in part by grants from the National Natural Science Foundation of China (nos. 81260068, 81560240, and 81770485).

Disclosures

The authors declare no conflicts of interest.

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