Hypercholesterolemia exacerbates transplant arteriosclerosis via increased neointimal smooth muscle cell accumulation: studies in apolipoprotein E knockout mice

Revisiting Biomarkers of Cardiac Allograft Vasculopathy: Addressing the Achilles Heel of Heart Transplantation

Nearly half of heart transplant recipients will be diagnosed with cardiac allograft vasculopathy (CAV) within five years after transplantation. Advanced CAV can lead to worsening heart failure as well as arrhythmias and sudden cardiac death. The only curative therapy for end-stage CAV is re-transplantation. Current diagnostic methods are invasive and limited by poor sensitivity in early disease. Despite its high prevalence in the post-transplantpopulation, the underlying pathophysiology of this condition has yet to be fully described. It is thought to be primarily related to endothelial dysfunction, immune activation, and cardiometabolic disease. Biomarkers reflecting these underlying processes, particularly endothelial injury and immune activation, have shown early promise in discriminating prevalent CAV. Next-generation sequencing technologies such as proteomic and transcriptomic profiling have also provided further insight into the pathophysiology of CAV through the identification of novel biomarkers. Ultimately, these biomarkers may have a role in not only diagnosing CAV but also highlighting potential targets for disease-specific therapies. In this article, we review the current data for biomarkers in CAV and discuss future directions for biomarker identification..

Hyperlipidemia Promotes Anti-Donor Th17 Responses That Accelerate Allograft Rejection

Hyperlipidemia occurs in 95% of organ transplant recipients, however its effect on organ allograft rejection has not been investigated. We found that induction of hyperlipidemia in mice caused a significant acceleration of rejection of cardiac allografts. Accelerated rejection was associated with an aggressive T cell infiltrate that mediated significant tissue damage as well as increased serum levels of the proinflammatory cytokines IL-2, IL-6, and IL-17. Hyperlipidemic mice had an increased number of Th17 cells in their periphery and rejecting allografts from hyperlipidemic mice contained significant numbers of IL-17 producing T cells that were not detectable in transplants harvested from controls. Neutralization or genetic ablation of IL-17 prolonged survival of cardiac allografts transplanted into hyperlipidemic recipients, suggesting that IL-17 production promotes accelerated rejection. Analysis of alloreactive T cell frequencies directly ex vivo in naïve mice revealed that the frequency of donor reactive IL-17 producing cells in hyperlipidemic was increased prior to antigen exposure, suggesting that hyperlipidemia was sufficient to alter T cell alloreactivity and promote anti-donor Th17 responses on first exposure to antigen. Together, our data suggest that hyperlipidemia alters rejection by altering the types of T cell subsets that respond to donor antigen by promoting Th17 biased anti-donor reactivity.

Hyperlipidemia Alters Regulatory T Cell Function and Promotes Resistance to Tolerance Induction Through Costimulatory Molecule Blockade

Recent work from our laboratory has shown that hyperlipidemia promotes accelerated rejection of vascularized cardiac allografts in mice by inducing anti-donor Th17 reactivity and production of IL-17. Here, we show that hyperlipidemia also affects FoxP3(+) regulatory T cells (Tregs). Hyperlipidemia promotes the development of Tregs that express low levels of CD25. Hyperlipidemia also promotes a decrease in central Tregs and an increase in effector Tregs that appears to account for the increase in the frequency of CD25(low) Tregs. Alterations in Treg subsets also appear to lead to alterations in Treg function. The ability of FoxP3(+) , CD25(high) , CD4(+) Tregs from hyperlipidemic mice to inhibit proliferation of effector T cells stimulated with anti-CD3 and CD28 was reduced when compared with Tregs from control mice. Regulatory T cells isolated from hyperlipidemic recipients exhibit increased activation of Akt, and a reduction in Bim levels that permits the expansion of FoxP3(+) CD25(low) CD4(+) T cells. Hyperlipidemic mice were also resistant to tolerance induction using costimulatory molecule blockade consisting of anti-CD154 and CTLA4Ig, a strategy that requires Tregs. Together, our data suggest that hyperlipidemia profoundly affects Treg subsets and function as well as the ability to induce tolerance.

Inflammatory Ly-6C(hi) monocytes play an important role in the development of severe transplant arteriosclerosis in hyperlipidemic recipients

Objective

Transplant arteriosclerosis (TA) restricts long-term survival of heart transplant recipients. Although the role of monocyte/macrophages is well established in native atherosclerosis, it has been studied to a much lesser extent in TA. Plasma cholesterol is the most important non-immunologic risk factor for development of TA but the underlying mechanisms are largely unknown. We hypothesized that monocyte/macrophages might play an important role in the pathogenesis of TA under hyperlipidemic conditions.

Methods

We studied TA in fully mismatched arterial allografts transplanted into hyperlipidemic ApoE^−/− recipients compared to wild-type controls. The recruitment of distinct monocyte populations into the grafts was tracked by in vivo labelling with fluorescent microspheres. We used antibody-mediated depletion protocols to dissect the relative contribution of T lymphocytes and monocytes to disease development.

Results

In the hyperlipidemic environment the progression of TA was highly exacerbated and the inflammatory CD11b⁺CD115⁺Ly-6C^hi monocytes were preferentially recruited into the neointima. The number of macrophage-derived foam cells present in the grafts strongly correlated with plasma cholesterol and disease severity. Depletion of Ly-6C^hi monocytes and neutrophils significantly inhibited macrophage accumulation and disease progression. The accelerated monocyte recruitment occurs through a T cell-independent mechanism, as T cell depletion did not influence macrophage accumulation into the grafts.

Conclusions

Our study identifies for the first time the involvement of inflammatory Ly-6C^hi monocytes into the pathogenesis of TA, particularly in conditions of hyperlipidemia. Targeted therapies modulating the recruitment and activation of these cells could potentially delay coronary allograft vasculopathy and improve long-term survival of heart transplant recipients.

Pharmacotherapy of hyperlipidemia in pediatric heart transplant recipients: current practice and future directions

Lipoprotein abnormalities are fairly common after pediatric heart transplantation. Graft coronary artery disease (GCAD) limits long-term survival and has been linked to elevated serum triglyceride levels and decreased high-density lipoprotein levels. Histologically, GCAD represents intimal hyperplasia of the coronary vessel and is best imaged by intravascular ultrasound.A number of pharmacologic agents are available for the management of lipid disorders but experience with these drugs has mainly been in adults. HMG-CoA reductase inhibitors (statins) are currently used by many adult transplantation centers to alter lipid profiles in the hope of reducing GCAD. The use of statins among pediatric heart transplant centers is more limited. Although rhabdomyolysis is a concern with these agents, the incidence among individuals receiving immunosuppressant therapy is low. Aside from their lipid-lowering properties, statins may also protect against graft failure and rejection.

Absence of heme oxygenase-1 exacerbates myocardial ischemia/reperfusion injury in diabetic mice

Increased production of reactive oxygen species contributes to the etiology of diabetes complications. Pathophysiological stimuli that increase oxidative stress upregulate heme oxygenase (HO)-1, a cytoprotective heme-degrading enzyme. We hypothesized that HO-1 may be important in myocardial injury that is exacerbated by diabetes. To test this hypothesis, the left anterior descending coronary arteries of nondiabetic and diabetic wild-type (HO-1(+/+)) and HO-1 null (HO-1(-/-)) mice were ligated for 1 h followed by 24 h reperfusion. The absence of HO-1 significantly increased myocardial infarct size (36.4 +/- 2.0 vs. 21.4 +/- 1.8% in HO-1(+/+) mice), while cardiac-specific overexpression of HO-1 protected against myocardial ischemic injury in diabetic mice. Despite similar high blood glucose levels, diabetic HO-1(-/-) mice had fourfold higher oxidative stress and larger infarcts (56.0 +/- 2.8%) than diabetic HO-1(+/+) mice (30.8 +/- 6.1%). Moreover, hyperglycemia increased the mortality of HO-1(-/-) mice (31.3%) after ischemia/reperfusion injury, and 55% of diabetic HO-1(-/-) mice had mural thrombi in the left ventricles. The increased mortality of diabetic HO-1(-/-) mice may be in part due to formation of left ventricular mural thrombi. Our data demonstrate that the absence of HO-1 renders animals more susceptible to myocardial ischemia/reperfusion damage and diabetes worsens the injury.

Management of hyperlipidaemia associated with heart transplantation

The past 20 years have seen considerable advances in the field of organ transplantation that have together led to a notable increase in survival rates and a reduction in postoperative morbidity of transplant recipients. However, these advances have been accompanied by the appearance of other complications of transplantation, such as post-transplant hyperlipidaemia, hypertension and graft coronary vasculopathy (GCV). GCV is an accelerated form of atherosclerosis in transplanted hearts that has proven to be one of the most important late complications of heart transplantation and is the single most limiting factor for long-term survival. The most important factors favouring the development of hyperlipidaemia after heart transplantation are inappropriate diet in combination with reduced physical activity, adverse effects of immunosuppressive therapy (ciclosporin [cyclosporin], corticosteroids) and polygenic hypercholesterolaemia in combination with ischaemic cardiomyopathy. The treatment of hyperlipidaemia in heart transplant recipients results in a variety of complications and side effects. In particular, interactions between lipid-lowering drugs and immunosuppressive therapy have been observed. Early attempts at treatment with bile acid binding agents and nicotinic acid derivatives often proved insufficiently effective, and led to unacceptable adverse effects and significant disturbances of ciclosporin metabolism. Fibric acid derivatives provided moderate reductions in triglyceride and total cholesterol levels that were mostly--with the exception of gemfibrozil--accompanied by significant impairment of renal function. Probucol achieved only an unsatisfactory reduction in low-density lipoprotein (LDL) cholesterol. Omega-3 fatty acids lower cholesterol levels and improve endothelial function in heart transplant recipients; however, the significance of these effects is still under discussion. As in the general patient population, use of HMG-CoA reductase inhibitors (statins) achieved significant reductions in cholesterol levels. Use of these substances has resulted in significantly extended long-term survival times, significantly less GCV and fewer severe graft rejections. Selective cholesterol absorption inhibitors, administered with or without statins, could provide another treatment option for heart transplant patients with hypercholesterolaemia. In severe familial hypercholesterolaemia, which is rarely observed in heart transplant recipients, treatment with statins can be combined with extracorporeal cholesterol elimination procedures such as heparin induced extracorporeal LDL cholesterol precipitation (HELP). HELP enables total cholesterol levels to be kept within any desired target range, and has been used successfully and without adverse effects in heart transplant recipients.

Safety and efficacy of atorvastatin in heart transplant recipients

BACKGROUND

Pravastatin and simvastatin prolong survival and reduce transplant-related coronary vasculopathy, although low-density lipoprotein (LDL) lowering with these agents is only modest. The objective of this study was to assess the safety of moderate dose atorvastatin and its efficacy when prior treatment with another statin had failed to lower LDL to < 100 mg/dl.

METHODS

Data from 185 patients were retrospectively evaluated for adverse events, duration of exposure (person-days), and the mean atorvastatin dose exposure. Changes in lipid parameters, and prednisone and cyclosporine doses were determined.

RESULTS

SAFETY

48 patients received atorvastatin for 24,240 person-days at a mean dose exposure of 21 +/- 10 mg. Rhabdomyolysis, myositis, myalgias, and hepatotoxicity occurred in 0, 2, 2, and 0 patients, respectively. All events occurred at the 10-mg dose, within the first 3 months, and were rapidly reversible with atorvastatin discontinuation.

EFFICACY

Thirty-four patients evaluable for efficacy analyses had a pre-atorvastatin LDL of 145 +/- 38 mg/dl on the following statins: pravastatin (n = 30, 40 +/- 0mg), fluvastatin (n = 3, 33 +/- 12 mg), simvastatin (n = 1, 40 mg). After atorvastatin (21 +/- 9 mg/day) for 133 +/- 67 days, LDL was reduced to 97 +/- 24 mg/dl (relative reduction 31 +/- 20%, p < 0.0001). At the end of the observation period (418 +/- 229 days, atorvastatin final dose 24 +/- 14 mg/day), LDL was further decreased to 88 +/- 23 mg (relative reduction 37 +/- 17%, p < 0.0001).

CONCLUSION

Atorvastatin, when used at moderate doses and with close biochemical and clinical monitoring, appears to be safe and is effective in aggressively lowering LDL in heart transplant recipients when treatment with other statins has failed to achieve LDL goals.

Acute rejection and cardiac graft vasculopathy in the absence of donor-derived ICAM-1 or P-selectin

BACKGROUND

ICAM-1 and P-selectin are molecules that facilitate adhesion of circulating leukocytes to vessel walls. We have investigated the role of donor-derived ICAM-1 and P-selectin in acute and chronic cardiac allograft rejection.

METHODS

C57BL/6J (H-2(b)) mice were used as donors for heterotopic heart transplantation into CBA/Ca (H-2(k)) recipients. The donors were wild-type or homozygous for gene mutations of ICAM-1 or P-selectin. We measured acute rejection in non-immunosuppressed recipients by daily palpation and sacrificed mice at Days 2, 4, and 6 for immunohistochemical analysis. For chronic rejection, recipients received monoclonal antibody against CD4+ T cells. We removed hearts at Days 60 to 62 for histologic assessment of vasculopathy using quantitative morphometry to measure intimal thickening.

RESULTS

Time (days) to rejection was 7.1 +/- 0.57 for wild-type (n = 10), 7.0 +/- 0.71 for ICAM-1 -/- (not significantly different, n = 7) and 6.1 +/- 0.33 (p = 0.001) for P-selectin -/- donors. ICAM-1 deficiency was associated with delayed infiltrate at Day 4 compared with wild-type. In the model of chronic rejection, elastin-positive vessels showed a mean occlusion of 34% +/- 3% in transplanted wild-type hearts; vessels were divided into those showing 0% to 20%, 20% to 50%, and 50% to 100% occlusion. We observed no difference in the number of affected vessels or the amount of vascular thickening in donors lacking ICAM-1 or P-selectin compared with wild-type controls.

CONCLUSIONS

The absence of ICAM-1 or P-selectin in donor tissues neither lengthens the time of allograft survival nor inhibits the vascular lesions associated with chronic rejection. Indeed, the absence of P-selectin may exacerbate alloimmune injury.

Apolipoprotein E inhibits neointimal hyperplasia after arterial injury in mice

The potential cytostatic function of apolipoprotein (apo) E in vivo was explored by measuring neointimal hyperplasia in response to vascular injury in apoE-deficient and apoE-overexpressing transgenic mice. Results showed a significant increase in medial thickness, medial area, and neointimal formation after vascular injury in both apoE knockout and wild-type C57BL/6 mice. Immunochemical analysis with smooth muscle alpha-actin-specific antibodies revealed that the neointima contained proliferating smooth muscle cells. Neointimal area was 3.4-fold greater, and the intima/medial ratio as well as stenotic luminal area was more pronounced in apoE(-/-) mice than those observed in control mice (P < 0.05). The human apoE3 transgenic mice in FVB/N genetic background were then used to verify a direct effect of apoE in protection against neointimal hyperplasia in response to mechanically induced vascular injury. Results showed that neointimal area was reduced threefold to fourfold in mice overexpressing the human apoE3 transgene (P < 0.05). Importantly, suppression of neointimal formation in the apoE transgenic mice also abolished the luminal stenosis observed in their nontransgenic FVB/N counterparts. These results documented a direct role of apoE in modulating vascular response to injury, suggesting that increasing apoE level may be beneficial in protection against restenosis after vascular surgery.

Progression of cardiac allograft vascular disease as assessed by serial intravascular ultrasound: correlation to immunological and non-immunological risk factors

OBJECTIVE

To characterise the severity and progression of cardiac allograft vascular disease (CAVD) in a large patient cohort, and to evaluate possible immunological and non-immunological risk factors for progression.

DESIGN

A prospective observational study using intravascular ultrasound.

SETTING

Two university hospitals.

PATIENTS AND MAIN OUTCOME MEASURES

Changes in focal plaque, lumen, and total vessel area (worst site method) were assessed at baseline and after 12.1 (2.8) months (mean (SD)) of follow up in a cohort of 96 patients (79 male, 17 female; mean age 48.7 (9.6) years; time post-transplant 26.0 (32.4) months).

RESULTS

Overall, the mean (SD) intimal index of worst sites increased by 6.7 (8.8)%. The increase in the first 12 months was 7.5 (9.4)%, v 5.9 (8.0)% after the first year (NS). Analysing immunological and non-immunological risk factors (age, underlying disease, sex, donor age, immunosuppression, cytomegalovirus, rejection episodes, cholesterol), low density lipoprotein (LDL) cholesterol was found to be the most important predictor of severe progression (as defined by an increase in intimal index of >/= 15% (p = 0.01).

CONCLUSIONS

Progression of CAVD is characterised by a continuing increase in intimal hyperplasia, especially within the first year after heart transplantation. LDL cholesterol is an important predictor of major progression.

Orthotopic aortic transplantation in mice: a new model of allograft arteriosclerosis

BACKGROUND

Graft arteriosclerosis is a major cause of death after allotransplantation of organs such as the heart or the kidney. Aortic allotransplantation in mice is a useful experimental model to study the mechanisms of this pathology. However, the conventional heterotopic aortic model is limited by a high morbidity and is technically difficult to perform. We developed a new simple method for aortic transplantation in mice.

METHODS

The infrarenal aorta from the donor mouse was anastomosed to the recipient's aorta at the same position using a sleeve technique. Orthotopic aortic transplantation was performed in 45 mice, 5 isografts and 40 allografts. No immunosuppression was given, and the mice were killed at day 15 or 30. The graft was examined macroscopically, and several histologic sections were made.

RESULTS

The overall survival rate was 78%. The incidence of thrombosis was low (4 cases) compared with previously published series. Histology of aortas revealed typical aspects of rejection in the allografts with a chronic picture at day 30. No significant lesion was observed in isografts.

CONCLUSIONS

We have developed a model of orthotopic aortic transplantation in mice. This new model is easy to carry out and has a low incidence of thrombosis, probably because there is no size discrepancy between donor and recipient aortic segment.

Chronic allograft rejection. Do the Th2 cells preferentially induced by indirect alloantigen recognition play a dominant role?

Chronic rejection has been the major obstacle to the long-term allograft survival in the clinic. Although the etiology of this rejection reaction is multifactorial, alloantigen-specific immune activation plays the most critical role. We herein hypothesize that CD4+ Th2 cells that are preferentially induced by the indirect recognition of allogeneic histocompatibility antigens late in transplantation may play the most critical role in the initiation and/or maintenance of chronic allograft rejection. Immunosuppression used to prevent acute rejection and the nature of antigen-presenting cells and alloligands in the graft may all contribute to immune deviation to the Th2 response. This response may be further perpetuated by type 2 cytokines conceivably produced by activated macrophages, NK cells, and CD8+ T cells in the graft. Cytokines and growth factors induced by this type 2 response, in turn, allow for activation of B, endothelial, and smooth muscle cells that collectively contribute to the pathogenesis of chronic allograft rejection by producing alloantibodies and growth hormones required for interstitial fibrosis, extracellular matrix deposition, and vascular neointimal hyperplasia.

Reduced transplant arteriosclerosis in plasminogen-deficient mice

Recent gene targeting studies indicate that the plasminogen system is implicated in cell migration and matrix degradation during arterial neointima formation and atherosclerotic aneurysm formation. This study examined whether plasmin proteolysis is involved in accelerated posttransplant arteriosclerosis (graft arterial disease). Donor carotid arteries from wild-type B10.A2R mice were transplanted into either plasminogen wild-type (Plg+/+) or homozygous plasminogen-deficient (Plg-/-) recipient mice with a genetic background of 75% C57BL/6 and 25% 129. Within 15 d after allograft transplantation, leukocytes and macrophages infiltrated the graft intima in Plg+/+ and Plg-/- recipient mice to a similar extent. In Plg+/+ recipients, the elastic laminae in the transplant media exhibited breaks through which macrophages infiltrated before smooth muscle cell proliferation, whereas in Plg-/- recipients, macrophages failed to infiltrate the transplant media which remained structurally more intact. After 45 d of transplantation, a multilayered smooth muscle cell-rich transplant neointima developed in Plg+/+ hosts, in contrast to Plg-/- recipients, in which the transplants contained a smaller intima, predominantly consisting of leukocytes, macrophages, and thrombus. Media necrosis, fragmentation of the elastic laminae, and adventitial remodeling were more pronounced in Plg+/+ than in Plg-/- recipient mice. Expression of the plasminogen activators (PA), urokinase-type PA (u-PA) and tissue-type PA (t-PA), and expression of the matrix metalloproteinases (MMPs), MMP-3, MMP-9, MMP-12, and MMP-13, were significantly increased within 15 d of transplantation when cells actively migrate. These data indicate that plasmin proteolysis plays a major role in allograft arteriosclerosis by mediating elastin degradation, macrophage infiltration, media remodeling, medial smooth muscle cell migration, and formation of a neointima.

Nitric oxide and endothelin in the development of cardiac allograft vasculopathy. Potential targets for therapeutic interventions

Extensive research has been carried out in recent years to discover the potential risk factors contributing to cardiac allograft atherogenesis. Injury to endothelial cells has been regarded as an important early mechanism in the development of transplant atherosclerosis; it leads to the manifestation of epicardial and microvascular endothelial dysfunction and development of intimal hyperplasia. Moreover, continuous minor endothelial cell damage contributes to endothelial dysfunction which reflects one of the first measurable steps in the cascade of atherogenesis without macroscopic evidence of vascular lesions. The discovery of two important vasoactive substances nitric oxide (NO) and endothelin (ET) has brought new insights but also new unsolved questions regarding the mechanisms leading to atherosclerosis. To date it is known that both substances play a major role in both prevention and development of atherosclerosis. NO appears to be protective in low concentrations by inhibiting leukocyte and platelet activation/adherence and smooth muscle cell proliferation. Impaired endothelial NO production, as one cause of endothelial dysfunction may occur in early stages of atherosclerosis before macroscopic lesions are evident. In addition, increased endothelin release also results in endothelial dysfunction by inducing vasoconstriction; it promotes vascular lesion formation due to endothelial- and vascular smooth muscle cell proliferation. Direct and indirect manipulation of both the NO and ET signal transduction systems may provide novel preventive and therapeutic approaches for limiting transplant atherogenesis and to treat native atherosclerosis. This review summarizes important experimental and clinical evidence which points to nitric oxide and endothelin as potential therapeutic targets in the process of cardiac allograft vasculopathy.

Chronic rejection: increasing evidence for the importance of allogen-independent factors

Chronic rejection is a process caused by an interplay of different risk factors. Early injury regardless of type, seems to be an important prognostic event. Later insults appear to contribute. As the individual components of the process are increasingly dissected and understood, means to modulate or normalize them will be forthcoming.