Participatory role of natural killer and natural killer T cells in atherosclerosis: lessons learned from in vivo mouse studies

Adenosine 5'-Monophosphate-to-Threonine Ratio Promotes Abdominal Aortic Aneurysms via Up-Regulation of HLA-DR on Natural Killer Cells: A Bidirectional Mendelian Randomized Analysis

Objective: Immune-metabolic interactions may have causal and therapeutic impacts on abdominal aortic aneurysms (AAAs). However, due to the lack of research on the relationship between immune-metabolic interactions and AAAs, further exploration of the mechanism faces challenges. Methods: A two-sample, two-step mediation analysis with Mendelian randomization (MR) based on genome-wide association studies (GWASs) was performed to determine the causal associations among blood immune cell signatures, metabolites, and AAAs. The stability, heterogeneity, and pleiotropy of the results were verified using a multivariate sensitivity analysis. Results: After multiple two-sample MRs using the AAA data from two large-scale GWAS databases, we determined that the human leukocyte antigen-DR (HLA-DR) levels on HLA-DR + natural killer (NK) cells (HLA-DR/NK) were associated with the causal effect of an AAA, with consistent results in the two databases (FinnGen: odds ratio (OR) = 1.054, 95% confidence interval (CI): 1.003-1.067, p-value = 0.036; UK Biobank: OR = 1.149, 95% CI: 1.046-1.261, p-value = 0.004). The metabolites associated with the risk of developing an AAA were enriched to find a specific metabolic model. We also found that the ratio of adenosine 5'-monophosphate (AMP) to threonine could act as a potential mediator between the HLA/NK and an AAA, with a direct effect (beta effect = 0.0496) and an indirect effect (beta effect = 0.0029). The mediation proportion was 5.56%. Conclusions: Our study found that an up-regulation of HLA-DR on HLA-DR/NK cells can increase the risk of an AAA via improvements in the AMP-to-threonine ratio, thus providing a potential new biomarker for the prediction and treatment of AAAs.

Immune and inflammatory mechanisms of abdominal aortic aneurysm

Abdominal aortic aneurysm (AAA) is a life-threatening cardiovascular disease. Immune-mediated infiltration and a destruction of the aortic wall during AAA development plays significant role in the pathogenesis of this disease. While various immune cells had been found in AAA, the mechanisms of their activation and function are still far from being understood. A better understanding of mechanisms regulating the development of aberrant immune cell activation in AAA is essential for the development of novel preventive and therapeutic approaches. In this review we summarize current knowledge about the role of immune cells in AAA and discuss how pathogenic immune cell activation is regulated in this disease.

Transcriptome and Function of Novel Immunosuppressive Autoreactive Invariant Natural Killer T Cells That Are Absent in Progressive Multiple Sclerosis

BACKGROUND AND OBJECTIVE

The aim of this study was to determine whether natural killer T (NKT) cells, including invariant (i) NKT cells, have clinical value in preventing the progression of multiple sclerosis (MS) by examining the mechanisms by which a distinct self-peptide induces a novel, protective invariant natural killer T cell (iNKT cell) subset.

METHODS

We performed a transcriptomic and functional analysis of iNKT cells that were reactive to a human collagen type II self-peptide, hCII707-721, measuring differentially induced genes, cytokines, and suppressive capacity.

RESULTS

We report the first transcriptomic profile of human conventional vs novel hCII707-721-reactive iNKT cells. We determined that hCII707-721 induces protective iNKT cells that are found in the blood of healthy individuals but not progressive patients with MS (PMS). By transcriptomic analysis, we observed that hCII707-721 promotes their development and proliferation, favoring the splicing of full-length AKT serine/threonine kinase 1 (AKT1) and effector function of this unique lineage by upregulating tumor necrosis factor (TNF)-related genes. Furthermore, hCII707-721-reactive iNKT cells did not upregulate interferon (IFN)-γ, interleukin (IL)-4, IL-10, IL-13, or IL-17 by RNA-seq or at the protein level, unlike the response to the glycolipid alpha-galactosylceramide. hCII707-721-reactive iNKT cells increased TNFα only at the protein level and suppressed autologous-activated T cells through FAS-FAS ligand (FAS-FASL) and TNFα-TNF receptor I signaling but not TNF receptor II.

DISCUSSION

Based on their immunomodulatory properties, NKT cells have a potential value in the treatment of autoimmune diseases, such as MS. These significant findings suggest that endogenous peptide ligands can be used to expand iNKT cells, without causing a cytokine storm, constituting a potential immunotherapy for autoimmune conditions, including PMS.

Natural Killer Cells: Friend or Foe in Metabolic Diseases?

The worldwide epidemic of metabolic diseases, especially obesity and other diseases caused by it, has shown a dramatic increase in incidence. A great deal of attention has been focused on the underlying mechanisms of these pathological processes and potential strategies to solve these problems. Chronic inflammation initiated by abdominal adipose tissues and immune cell activation in obesity is the major cause of the consequent development of complications. In addition to adipocytes, macrophages and monocytes, natural killer (NK) cells have been verified to be vital components involved in shaping the inflammatory microenvironment, thereby leading to various obesity-related metabolic diseases. Here, we provide an overview of the roles of NK cells and the interactions of these cells with other immune and nonimmune cells in the pathological processes of metabolic diseases. Finally, we also discuss potential therapeutic strategies targeting NK cells to treat metabolic diseases.

Cholesterol-independent effects of atorvastatin prevent cardiovascular morbidity and mortality in a mouse model of atherosclerotic plaque rupture

Because cholesterol-independent effects of statins are difficult to determine in patients, we studied these pleiotropic effects in apolipoprotein E-deficient (ApoE(-/-)) mice with a mutation in the fibrillin-1 gene (Fbn1(C1039G+/-)). These mice develop exacerbated atherosclerosis and spontaneous plaque ruptures, accompanied by myocardial infarctions (MI) and sudden death. ApoE(-/-)Fbn1(C1039G+/-) mice were fed a Western diet (WD). At week 10 of WD, mice were divided in a control (WD), atorvastatin (10mg/kg/day + WD) and cholesterol withdrawal group (cholW, normal chow). The latter was included to compare the effects of atorvastatin with dietary lipid lowering. Fifteen weeks later, the mice were sacrificed. CholW, but not atorvastatin, reduced plasma cholesterol. Survival increased from 50% to 90% both in cholW and atorvastatin treated mice. CholW as well as atorvastatin treatment increased plaque collagen and fibrous cap thickness, but they did not affect the amount of plaque macrophages and T cells. MMP-2 and MMP-9 activity was significantly lower and the expression of MMP-12, TNF-α and IL-1β was strongly reduced in both treatment groups. Blood monocytes and neutrophils returned to baseline levels (ApoE(-/-) mice before the onset of atherosclerosis). Importantly, atorvastatin but not cholW significantly reduced coronary stenosis (from 50 to 28%) and the occurrence of MI (from 43 to 10%). In conclusion, independent of cholesterol lowering, atorvastatin significantly reduced mortality, plaque vulnerability and inflammation to the same extent as cholW. In addition, atorvastatin but not cholW reduced coronary stenosis and the occurrence of MI. These data unequivocally illustrate the significance of the pleiotropic effects of atorvastatin in the prevention of cardiovascular morbidity and mortality.

MICA/B expression in macrophage foam cells infiltrating atherosclerotic plaques

Infiltrating macrophages accumulate in fatty streak lesions and transform into foam cells, leading to the formation of atherosclerotic plaques. Inflammatory mechanisms underlying the plaque formation mediated by NKG2D-positive lymphocytes such as CD8+ T cells, natural killer cells and natural killer T cells have been extensively investigated. Yet, the involvement of the NKG2D system itself remains poorly understood. Recent work in mouse models has shown that blockade of an NKG2D receptor-ligand interaction reduces plaque formation and suppresses inflammation in aortae. In this study, we conducted immunohistochemical analysis of NKG2D ligand expression in autopsy-derived aortic specimens. Foam cells expressing NKG2D ligands MICA/B were found in advanced atherosclerotic lesions accompanied by a large necrotic core or hemorrhage. Human monocyte-derived macrophages treated in vitro with acetylated low-density lipoproteins enhanced expression of MICA/B and scavenger receptor A, thus accounting for NKG2D ligand expression in foam cells infiltrating atherosclerotic plaques. Our results suggest that, as in mice, the NKG2D system might be involved in the development of atherosclerosis in humans.

Altered expression of Raet1e, a major histocompatibility complex class 1-like molecule, underlies the atherosclerosis modifier locus Ath11 10b

RATIONALE

Quantitative trait locus mapping of an intercross between C57.Apoe⁻/⁻ and FVB.Apoe⁻/⁻ mice revealed an atherosclerosis locus controlling aortic root lesion area on proximal chromosome 10, Ath11. In a previous work, subcongenic analysis showed Ath11 to be complex with proximal (10a) and distal (10b) regions.

OBJECTIVE

To identify the causative genetic variation underlying the atherosclerosis modifier locus Ath11 10b.

METHODS AND RESULTS

We now report subcongenic J, which narrows the 10b region to 5 genes, Myb, Hbs1L, Aldh8a1, Sgk1, and Raet1e. Sequence analysis of these genes revealed no amino acid coding differences between the parental strains. However, comparing aortic expression of these genes between F1.Apoe⁻/⁻ Chr10SubJ((B/F)) and F1.Apoe⁻/⁻ Chr10SubJ((F/F)) uncovered a consistent difference only for Raet1e, with decreased, virtually background, expression associated with increased atherosclerosis in the latter. The key role of Raet1e was confirmed by showing that transgene-induced aortic overexpression of Raet1e in F1.Apoe⁻/⁻ Chr10SubJ((F/F)) mice decreased atherosclerosis. Promoter reporter constructs comparing C57 and FVB sequences identified an FVB mutation in the core of the major aortic transcription start site abrogating activity.

CONCLUSIONS

This nonbiased approach has revealed Raet1e, a major histocompatibility complex class 1-like molecule expressed in lesional aortic endothelial cells and macrophage-rich regions, as a novel atherosclerosis gene and represents one of the few successes of the quantitative trait locus strategy in complex diseases.

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.

The CD1d-natural killer T cell axis in atherosclerosis

A key role for 'lipid-sensing' CD1-restricted natural killer T (NKT) cells in the pathogenesis of atherosclerosis has been suggested. However, the biology of NKT cells remains poorly characterized, as in different experimental settings their activation was reported to both stimulate and suppress innate and adaptive immune responses. Most of the data from experimental models suggest that NKT cells are proatherogenic; however, it is debated whether the increase in atherosclerosis observed following NKT cell stimulation is a consequence of the inability to induce functional NKT cells rather than the proatherogenic nature of NKT cells. CD1d-expressing antigen-presenting cells and NKT cells were detected in mouse and human atherosclerotic lesions. Furthermore, several lysophospholipids and glycosphingolipids, known to accumulate in atherosclerotic plaques, are antigenic for human NKT cell clones. Lipid transfer proteins, such as apolipoprotein E and microsomal triglyceride transfer protein, are central to NKT cell responses. All these data suggest a profound relation between lipid metabolism, CD1d-NKT cell axis activation and atherosclerosis. In this review, we summarize the advances and gaps in our knowledge of NKT cell biology in the context of atherosclerosis as well as the possibility of influencing NKT cell polarization toward an atheroprotective phenotype.

Childhood adversity and immune and inflammatory biomarkers associated with cardiovascular risk in youth: a systematic review

BACKGROUND

Research suggests that adverse experiences in childhood affect the development of cardiovascular disease (CVD), and immune and inflammation dysregulation has been postulated to play role. However, it is unclear whether the effects of social adversity on immune-related biomarkers are evident in early life, and if these biomarkers may provide an early risk marker for targeting prevention and intervention. The purpose of this review is to evaluate research on the relationship between adversity and CVD-relevant immune biomarkers in youth, assess the consistency of the findings, and consider what additional research is needed.

METHODS

PubMed and PsycINFO searches were conducted through September 2011. Studies were selected using criteria related to the childhood exposure, biomarker outcome, age range, and sample selection. Twenty articles were identified, examining associations between childhood adversity and immune biomarkers (assessed during childhood) that are potential risk markers for CVD later in life.

RESULTS

Although childhood adversity was not consistently related to youth levels of inflammatory and other immune markers relevant to CVD, a trend toward positive findings was observed. No detectable patterns were evident based on measure of adversity, biomarker, study design, or sample size.

CONCLUSIONS

Overall, our findings suggest this avenue of research is worth continued investigation. We offer recommendations for future research related to (1) study design and sample, (2) definition and measurement of adversity, (3) statistical analysis, and (4) outcomes that will help distinguish whether there are immunologic alterations related to adversity and subsequent CVD risk that can be reliably detected in childhood.

Immune activation resulting from NKG2D/ligand interaction promotes atherosclerosis

BACKGROUND

The interplay between the immune system and abnormal metabolic conditions sustains and propagates a vicious feedback cycle of chronic inflammation and metabolic dysfunction that is critical for atherosclerotic progression. It is well established that abnormal metabolic conditions, such as dyslipidemia and hyperglycemia, cause various cellular stress responses that induce tissue inflammation and immune cell activation, which in turn exacerbate the metabolic dysfunction. However, molecular events linking these processes are not well understood.

METHODS AND RESULTS

Tissues and organs of humans and mice with hyperglycemia and hyperlipidemia were examined for expression of ligands for NKG2D, a potent immune-activating receptor expressed by several types of immune cells, and the role of NKG2D in atherosclerosis and metabolic diseases was probed with the use of mice lacking NKG2D or by blocking NKG2D with monoclonal antibodies. NKG2D ligands were upregulated in multiple organs, particularly atherosclerotic aortas and inflamed livers. Ligand upregulation was induced in vitro by abnormal metabolites associated with metabolic dysfunctions. Using apolipoprotein E-deficient mouse models, we demonstrated that preventing NKG2D functions resulted in a dramatic reduction in plaque formation, suppressed systemic and organ inflammation mediated by multiple immune cell types, and alleviated abnormal metabolic conditions.

CONCLUSIONS

The NKG2D/ligand interaction is a critical molecular link in the vicious cycle of chronic inflammation and metabolic dysfunction that promotes atherosclerosis and might be a useful target for therapeutic intervention in the disease.

Strain-dependent susceptibility for hypertension in mice resides in the natural killer gene complex

Hypertension is associated with chronic vascular inflammation. We tested the hypothesis that the sensitivity to develop hypertension and vascular remodeling depends on the immunological background. Blood pressure, vascular remodeling, endothelial function, vascular architecture (number of collateral arteries), and expression of inflammatory cytokines were determined in mice that received N(G)-nitro-l-arginine methyl ester (l-NAME) to inhibit nitric oxide synthesis. We studied C57BL/6, BALB/c, and BALB.B6-Cmv1r mice, a congenic strain where the natural killer (NK) gene complex of C57BL/6 mice is introduced in the BALB/c background. During a 4-wk treatment with l-NAME, blood pressure initially increased in both C57BL/6 and BALB/C mice, but after 4 wk, only C57BL/6 mice showed a significant increase in mean arterial blood pressure (+53 mmHg; P < 0.001) and small artery inward remodeling. Endothelial function and vascular design were significantly different between C57BL/6 mice and BALB/C mice. The inflammatory response was similar in C57BL/6 and BALB/C mice, except for the leukocyte marker CD11b. Cellular colocalization of CD11b with NK1.1 indicated the recruitment of NK cells in C57BL/6 mice. Congenic BALB.B6-Cmv1r mice showed the same endothelial response and vascular architecture as BALB/c mice. However, BALB.B6-Cmv1r mice displayed a similar sensitivity to hypertension and vascular remodeling as C57BL/6 mice. In conclusion, we have identified the NK gene complex as an important determinant in the genetically determined sensitivity to develop l-NAME-induced hypertension in mice.

Immune and inflammatory mechanisms of atherosclerosis (*)

Atherosclerosis is an inflammatory disease of the wall of large- and medium-sized arteries that is precipitated by elevated levels of low-density lipoprotein (LDL) cholesterol in the blood. Although dendritic cells (DCs) and lymphocytes are found in the adventitia of normal arteries, their number is greatly expanded and their distribution changed in human and mouse atherosclerotic arteries. Macrophages, DCs, foam cells, lymphocytes, and other inflammatory cells are found in the intimal atherosclerotic lesions. Beneath these lesions, adventitial leukocytes organize in clusters that resemble tertiary lymphoid tissues. Experimental interventions can reduce the number of available blood monocytes, from which macrophages and most DCs and foam cells are derived, and reduce atherosclerotic lesion burden without altering blood lipids. Under proatherogenic conditions, nitric oxide production from endothelial cells is reduced and the burden of reactive oxygen species (ROS) and advanced glycation end products (AGE) is increased. Incapacitating ROS-generating NADPH oxidase or the receptor for AGE (RAGE) has beneficial effects. Targeting inflammatory adhesion molecules also reduces atherosclerosis. Conversely, removing or blocking IL-10 or TGF-beta accelerates atherosclerosis. Regulatory T cells and B1 cells secreting natural antibodies are atheroprotective. This review summarizes our current understanding of inflammatory and immune mechanisms in atherosclerosis.

CXCR6 promotes atherosclerosis by supporting T-cell homing, interferon-gamma production, and macrophage accumulation in the aortic wall

BACKGROUND

T lymphocytes are thought to be important in atherosclerosis, but very little is known about the mechanisms of lymphocyte recruitment into atherosclerosis-prone aortas. In this study we tested the hypothesis that CXCR6, a chemokine receptor that is expressed on a subset of CD4+ T helper 1 cells and natural killer T cells, is involved in lymphocyte homing into the aortic wall and modulates the development and progression of atherosclerosis.

METHODS AND RESULTS

To investigate the role of CXCR6 in the development and progression of atherosclerosis, we bred CXCR6-deficient (CXCR6(GFP/GFP)) mice with apolipoprotein E-deficient (ApoE(-/-)) mice. We found that CXCR6(GFP/GFP)/ApoE(-/-) mice fed a Western diet for 17 weeks or a chow diet for 56 weeks had decreased atherosclerosis compared with ApoE(-/-) controls. Flow cytometry analysis of the aortas from CXCR6(GFP/GFP)/ApoE(-/-) mice showed that the reduction of atherosclerosis was accompanied by a decreased percentage of CXCR6+ T cells within the aortas. Short-term homing experiments demonstrated that CXCR6 is involved in the recruitment of CXCR6+ leukocytes into the atherosclerosis-prone aortic wall. The reduced percentage of CXCR6+ T cells within the aortas resulted in significantly diminished production of interferon-gamma and reduction of CD11b+/CD68+ macrophages in the aorta.

CONCLUSIONS

These data provide evidence for a proatherosclerotic role of CXCR6. Absence of CXCR6 alters the recruitment of CXCR6+ leukocytes and modulates the local immune response within the aortic wall.