Regulatory T cell responses: potential role in the control of atherosclerosis

The double-edged sword of statin immunomodulation

Statin drugs are widely prescribed to achieve aggressive low-density lipoprotein lowering in order to decrease cardiovascular disease. Although some of the immunomodulatory effects of statins may stabilize atherosclerotic plaque, they may be harmful in certain segments of the population. Recently, statins have been shown to increase the concentration of regulatory T cells (Tregs), in vivo. There is evidence that this increases the risk of many cancers, particularly in the elderly. Furthermore, a statin induced increase in Tregs may be detrimental in neurodegenerative disorders, such as amyotrophic lateral sclerosis; and a myriad of infectious diseases. These include, but are not limited to, human immunodeficiency virus, hepatitis B virus, hepatitis C virus, and varicella zoster virus. These issues need our attention, and call for a heightened state of vigilance among those prescribing statins.

A vaccine against atherosclerosis: myth or reality?

Atherosclerosis is a chronic inflammatory disease that develops in the context of enhanced serum lipid levels. Nowadays, many studies focus on the modulation of inflammatory responses to reduce atherosclerosis. The most powerful strategy to achieve this is vaccination. In several immune diseases vaccination is shown to be very effective, resulting in a drastic decline in the incidence of the disease. But is vaccination also realistic in atherosclerosis? In this article, several approaches to vaccinate against atherosclerosis are described. Vaccination (based on protein or DNA) against bioactive molecules and disease-related proteins successfully reduces experimental atherosclerosis. In addition, passive immunization with antibodies against atherosclerosis-specific antigens and tolerance induction, in which antigen-specific regulatory T cells are elicited, are described. In the near future, we expect an increased interest in vaccination against atherosclerosis and, maybe, the myth may become reality when the first clinical trials are performed.

Immunomodulation of the inflammatory response in atherosclerosis

PURPOSE OF REVIEW

Cardiovascular disease, as manifested in the formation of atherosclerotic lesions, can be described as a chronic inflammatory autoimmune-like disease that proceeds in the context of enhanced plasma lipid levels. Modulation of the immune response may therefore form a valuable therapy in addition to standardized cholesterol and blood pressure-lowering therapies. The purpose of this review is to describe a number of recent approaches to immunomodulate atherosclerosis: immunization against mediators involved in atherosclerosis, such as cytokines and modified low-density lipoprotein; intervention in cytokine pathways; intervention in co-stimulatory pathways; activation of regulatory T cells; and modulation of natural killer T cells.

RECENT FINDINGS

The most recent findings point to an important role for regulatory T cells in atherosclerotic lesion formation. The function of the regulatory T cells is modulated by chemokines and by co-stimulatory pathways, whereas the function of these cells can be strongly upregulated by anti-CD3 treatment and tolerance induction.

SUMMARY

In the near future the first exponents of this approach, such as immunization and enhancement of the function of regulatory T cells, may enter the first phase of clinical trials, and may ultimately add to the current therapies in atherosclerosis.

Induction of oral tolerance to HSP60 or an HSP60-peptide activates T cell regulation and reduces atherosclerosis

OBJECTIVE

HSP60-specific T cells contribute to the development of the immune responses in atherosclerosis. This can be dampened by regulatory T cells activated via oral tolerance induction, and we explored the effect of oral tolerance induction to HSP60 and the peptide HSP60 (253 to 268) on atherosclerosis.

METHODS AND RESULTS

HSP60 and HSP60 (253 to 268) were administered orally to LDLr(-/-) mice before induction of atherosclerosis and resulted in a significant 80% reduction in plaque size in the carotid arteries and in a 27% reduction in plaque size at the aortic root. Reduction in plaque size correlated with an increase in CD4(+)CD25(+)Foxp3(+) regulatory T cells in several organs and in an increased expression of Foxp3, CD25, and CTLA-4 in atherosclerotic lesions of HSP60-treated mice. The production of interleukin (IL)-10 and transforming growth factor (TGF)-beta by lymph node cells in response to HSP60 was observed after tolerance induction.

CONCLUSIONS

Oral tolerance induction to HSP60 and a small HSP60-peptide leads to an increase in the number of CD4(+)CD25(+)Foxp3(+) regulatory T cells, resulting in a decrease in plaque size as a consequence of increased production of IL-10 and TGF-beta. We conclude that these beneficial results of oral tolerance induction to HSP60 and HSP60 (253 to 268) may provide new therapeutic approaches for the treatment of atherosclerosis.

The effect of HMG-CoA reductase inhibitors on naturally occurring CD4+CD25+ T cells

Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) are in widespread use due to their LDL reducing properties and concomitant improvement of clinical outcome in patients with and without preexisting atherosclerosis. Considerable evidence suggests that immune mediated mechanisms play a dominant role in the beneficial effects of statins. Naturally occurring CD4(+)CD25(+) regulatory T cells (Tregs) have a key role in the prevention of various inflammatory and autoimmune disorders by suppressing immune responses. We tested the hypothesis that statins influence the circulating number and the functional properties of Tregs. We studied the effects of in vivo and in vitro statin treatment of human and murine mononuclear cells on the number of Tregs and the expression level of their master transcription regulator, Foxp3. Atorvastatin, but not mevastatin nor pravastatin, treatment of human peripheral blood mononuclear cells (PBMCs) increased the number of CD4(+)CD25(high) cells, and CD4(+)CD25(+)Foxp3(+) cells. These Tregs, induced by atorvastatin, expressed high levels of Foxp3, which correlated with an increased regulatory potential. Furthermore, co-culture studies revealed that atorvastatin induced CD4(+)CD25(+)Foxp3(+) Tregs were derived from peripheral CD4(+)CD25(-)Foxp3(-) cells. Simvastatin and pravastatin treatment in hyperlipidemic subjects increased the number of Tregs. In C57BL/6 mice however, no effect of statins on Tregs was evident. In conclusion, statins appear to significantly influence the peripheral pool of Tregs in humans. This finding may shed light on the mechanisms governing the plaque stabilizing properties of statins.

Indoleamine 2,3-dioxygenase enzyme activity correlates with risk factors for atherosclerosis: the Cardiovascular Risk in Young Finns Study

Indoleamine 2,3 dioxygenase (IDO), an enzyme involved in the catabolism of tryptophan, suppresses T cell activity and is up-regulated by various inflammatory stimuli. The ratio of kynurenine, the main metabolite of tryptophan, to tryptophan (kyn/trp) reflects IDO activity. We calculated IDO activity and measured carotid intima-media thickness (IMT), a presymptomatic predictor of atherosclerosis, in 986 young adults (544 female, 442 male) for whom data on levels of high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), triglyceride, high sensitive C-reactive protein (CRP), body mass index (BMI), waist circumference, waist-to-hip ratio, systolic and diastolic blood pressure and smoking habits were available. IDO activity correlated significantly with IMT in female subjects, but not in males. In a multivariate linear regression model, IDO did not correlate independently with IMT in female subjects. However, IDO activity correlated significantly with several risk factors for atherosclerosis in females, i.e. with age, LDL-C, BMI, weakly with CRP and inversely with HDL-C and triglyceride. In males IDO activity correlated significantly with CRP and inversely with HDL-C. In conclusion, our results suggest that the IDO enzyme is involved in the immune regulation of early atherosclerosis, particularly in young female adults, and could constitute a novel marker of immune activation in early atherosclerosis in females.

Induction of oral tolerance to oxidized low-density lipoprotein ameliorates atherosclerosis

BACKGROUND

Oxidation of low-density lipoprotein (LDL) and the subsequent processing of oxidized LDL (oxLDL) by macrophages results in activation of specific T cells, which contributes to the development of atherosclerosis. Oral tolerance induction and the subsequent activation of regulatory T cells may be an adequate therapy for the treatment of atherosclerosis.

METHODS AND RESULTS

Tolerance to oxLDL and malondialdehyde-treated LDL (MDA-LDL) was induced in LDL receptor-/- mice fed a Western-type diet by oral administration of oxLDL or MDA-LDL before the induction of atherogenesis. Oral tolerance to oxLDL resulted in a significant attenuation of the initiation (30% to 71%; P<0.05) and progression (45%; P<0.05) of atherogenesis. Tolerance to oxLDL induced a significant increase in CD4+ CD25+ Foxp3+ cells in spleen and mesenteric lymph nodes, and these cells specifically responded to oxLDL with increased transforming growth factor-beta production. Tolerance to oxLDL also increased the mRNA expression of Foxp3, CTLA-4, and CD25 in the plaque. In contrast, tolerance to MDA-LDL did not affect atherogenesis.

CONCLUSIONS

OxLDL-specific T cells, present in LDL receptor-/- mice and important contributors in the immune response leading to atherosclerotic plaque, can be counteracted by oxLDL-specific CD4+ CD25+ Foxp3+ regulatory T cells activated via oral tolerance induction to oxLDL. We conclude that the induction of oral tolerance to oxLDL may be a promising strategy to modulate the immune response during atherogenesis and a new way to treat atherosclerosis.

Intra-tumoural regulatory T cells: a potential new target in cancer immunotherapy

We hypothesised that T(reg) cells preferentially expand/infiltrate inside murine mesotheliomas. Immunotherapy based on the manipulation of T(reg) cell populations should therefore be targeted to the tumour site. The AE17 murine mesothelioma model was used for this study. Both intra-tumoural T(reg) cells and those in the periphery of tumour-bearing mice were identified by flow cytometry. The effect on tumour growth of intra-tumoural depletion of T(reg) cells using the PC61 anti-CD25 mAb was then examined. We identified CD4+ T(reg) cells co-expressing both the CD25 cell surface marker and the transcription factor Foxp3 within murine mesotheliomas. These intra-tumoural T(reg) cells increase significantly as a percentage of total CD4+ T cells within the tumour as it grows. We showed that the depletion of intra-tumoural T(reg) cells with anti-CD25 mAb injected directly into the tumours can cause significantly reduced tumour growth. Localised, intra-tumoural depletion of T(reg) cells is a new, clinically relevant treatment option for established tumours.

Common angiotensin receptor blockers may directly modulate the immune system via VDR, PPAR and CCR2b

Background

There have been indications that common Angiotensin Receptor Blockers (ARBs) may be exerting anti-inflammatory actions by directly modulating the immune system. We decided to use molecular modelling to rapidly assess which of the potential targets might justify the expense of detailed laboratory validation. We first studied the VDR nuclear receptor, which is activated by the secosteroid hormone 1,25-dihydroxyvitamin-D. This receptor mediates the expression of regulators as ubiquitous as GnRH (Gonadatrophin hormone releasing hormone) and the Parathyroid Hormone (PTH). Additionally we examined Peroxisome Proliferator-Activated Receptor Gamma (PPARgamma), which affects the function of phagocytic cells, and the C-CChemokine Receptor, type 2b, (CCR2b), which recruits monocytes to the site of inflammatory immune challenge.

Results

Telmisartan was predicted to strongly antagonize (Ki≈0.04nmol) the VDR. The ARBs Olmesartan, Irbesartan and Valsartan (Ki≈10 nmol) are likely to be useful VDR antagonists at typical in-vivo concentrations. Candesartan (Ki≈30 nmol) and Losartan (Ki≈70 nmol) may also usefully inhibit the VDR. Telmisartan is a strong modulator of PPARgamma (Ki≈0.3 nmol), while Losartan (Ki≈3 nmol), Irbesartan (Ki≈6 nmol), Olmesartan and Valsartan (Ki≈12 nmol) also seem likely to have significant PPAR modulatory activity. Olmesartan andIrbesartan (Ki≈9 nmol) additionally act as antagonists of a theoretical modelof CCR2b. Initial validation of this CCR2b model was performed, and a proposed model for the AngiotensinII Type1 receptor (AT2R1) has been presented.

Conclusion

Molecular modeling has proven valuable to generate testable hypotheses concerning receptor/ligand binding and is an important tool in drug design. ARBs were designed to act as antagonists for AT2R1, and it was not surprising to discover their affinity for the structurally similar CCR2b. However, this study also found evidence that ARBs modulate the activation of two key nuclear receptors-VDR and PPARgamma. If our simulations are confirmed by experiment, it is possible that ARBs may become useful as potent anti-inflammatory agents, in addition to their current indication as cardiovascular drugs.