Transplantation-induced endothelial dysfunction as studied in rat aorta allografts

TET2 Protects Against Vascular Smooth Muscle Cell Apoptosis and Intimal Thickening in Transplant Vasculopathy

BACKGROUND

Coronary allograft vasculopathy (CAV) is a devastating sequela of heart transplant in which arterial intimal thickening limits coronary blood flow. There are currently no targeted therapies to prevent or reduce this pathology that leads to transplant failure. Vascular smooth muscle cell (VSMC) phenotypic plasticity is critical in CAV neointima formation. TET2 (TET methylcytosine dioxygenase 2) is an important epigenetic regulator of VSMC phenotype, but the role of TET2 in the progression of CAV is unknown.

METHODS

We assessed TET2 expression and activity in human CAV and renal transplant samples. We also used the sex-mismatched murine aortic graft model of graft arteriopathy (GA) in wild-type and inducible smooth muscle-specific Tet2 knockout mice; and in vitro studies in murine and human VSMCs using knockdown, overexpression, and transcriptomic approaches to assess the role of TET2 in VSMC responses to IFNγ (interferon γ), a cytokine elaborated by T cells that drives CAV progression.

RESULTS

In the present study, we found that TET2 expression and activity are negatively regulated in human CAV and renal transplant samples and in the murine aortic graft model of GA. IFNγ was sufficient to repress TET2 and induce an activated VSMC phenotype in vitro. TET2 depletion mimicked the effects of IFNγ, and TET2 overexpression rescued IFNγ-induced dedifferentiation. VSMC-specific TET2 depletion in aortic grafts, and in the femoral wire restenosis model, resulted in increased VSMC apoptosis and medial thinning. In GA, this apoptosis was tightly correlated with proliferation. In vitro, TET2-deficient VSMCs undergo apoptosis more readily in response to IFNγ and expressed a signature of increased susceptibility to extrinsic apoptotic signaling. Enhancing TET2 enzymatic activity with high-dose ascorbic acid rescued the effect of GA-induced VSMC apoptosis and intimal thickening in a TET2-dependent manner.

CONCLUSIONS

TET2 is repressed in CAV and GA, likely mediated by IFNγ. TET2 serves to protect VSMCs from apoptosis in the context of transplant vasculopathy or IFNγ stimulation. Promoting TET2 activity in vivo with systemic ascorbic acid reduces VSMC apoptosis and intimal thickening. These data suggest that promoting TET2 activity in CAV may be an effective strategy for limiting CAV progression.

A novel method for long-lasting preservation of arterial grafts

BACKGROUND

Autologous venous grafts generally give best results for arterial bypass grafting in cases of arterial stenosis. When no suitable venous graft can be found, synthetic prosthetic graft may be an alternative. Prostheses are easily accessible but susceptible to infection. In these cases, the replacement of infected prosthesis by the human arterial allograft is the best treatment option. The question arises whether we could prepare a graft meeting mechanical conditions of an artery immunologically inert and resistant to bacterial infection.

MATERIALS AND METHODS

LEW and BN rat aortic segments were placed in dehydrated sodium chloride and stored for 1 to 12 mo. Then, they were transplanted orthotopically as aortic grafts for 3 to 15 mo in syngenic and allogenic combination. No immunosuppression was used. Patency, pulsation, and frequency of development of aneurysms were studied. The tensile strength and maximum intraluminal pressures were measured. Morphology of grafts was evaluated on histology and electron microscopy. The endothelial and infiltrating cells were identified.

RESULTS

Transplanted allogeneic aortic grafts preserved in anhydrous sodium chloride up to 12 mo remained patent for 15 mo. Hypertrophy of intima with endothelial cells lining the inner surface and single muscle cells between elastic fibers were seen. Normal structure of collagen and elastic fibers was maintained. Only minor-host mononuclear infiltrates were seen around the preserved allografts.

CONCLUSIONS

Rat aortas preserved in anhydrous sodium chloride retain patency and function even 15 mo after transplantation. Such grafts retain their wall structure and evoke only minor recipient reaction. Our results confirm that anhydrous sodium chloride may be used for arterial grafts preservation. Low immunogenicity is additional advantage.

Vascular smooth muscle cell apoptosis promotes transplant arteriosclerosis through inducing the production of SDF-1α

Transplant arteriosclerosis is a leading cause of late allograft loss. Medial smooth muscle cell (SMC) apoptosis is considered to be an important event in transplant arteriosclerosis. However, the precise contribution of medial SMC apoptosis to transplant arteriosclerosis and the underlying mechanisms remain unclear. We transferred wild-type p53 to induce apoptosis of cultured SMCs. We found that apoptosis induces the production of SDF-1α from apoptotic and neighboring viable cells, resulting in increased SDF-1α in the culture media. Conditioned media from Ltv-p53-transferred SMCs activated PI3K/Akt/mTOR and MAPK/Erk signaling in a SDF-1α-dependent manner and thereby promoted mesenchymal stem cell (MSC) migration and proliferation. In a rat aorta transplantation model, lentivirus-mediated BclxL transfer selectively inhibits medial SMC apoptosis in aortic allografts, resulting in a remarkable decrease of SDF-1α both in allograft media and in blood plasma, associated with diminished recruitment of CD90(+)CD105(+) double-positive cells and impaired neointimal formation. Systemic administration of rapamycin or PD98059 also attenuated MSC recruitment and neointimal formation in the aortic allografts. These results suggest that medial SMC apoptosis is critical for the development of transplant arteriosclerosis through inducing SDF-1α production and that MSC recruitment represents a major component of vascular remodeling, constituting a relevant target and mechanism for therapeutic interventions.

The indirect alloimmune response causes microvascular endothelial dysfunction-a possible role for alloantibody

BACKGROUND

The causes of endothelial dysfunction after cardiac transplantation are unknown. Here, we have investigated whether the indirect alloimmune response mediates endothelial dysfunction in a major histocompatibility complex class I mismatch model.

METHODS

PVG.RT1 rat hearts were transplanted into thymectomized CD8 T-cell-depleted allogeneic (PVG.R8) or syngeneic (PVG.RT1) recipients. Alloantibody was assessed at 2, 4, and 8 weeks. Cardiac allograft vasculopathy, the nature of the inflammatory infiltrate, and origin of endothelial cells were examined at 1, 2, 4, and 8 weeks. Endothelial function was assessed by Langendorff preparations at 1, 2, and 4 weeks.

RESULTS

Recipients produced alloantibody and showed luminal occlusion at 1 (17.7%±8.0%), 2 (23.2%±4.9%), 4 (34.3%±5.0%), and 8 weeks (58.1%±1.8%) posttransplantation. The major inflammatory features of the allografts consisted of CD11b monocytes, CD4 T cells, and C4d deposition. At 1 week, the basal coronary flow and the vasodilator response to 5-hydroxytrytamine of syngeneic and allografted hearts were inhibited compared with normal hearts. At 4 weeks, the basal coronary flow of allografts was 54% lower than syngrafts (P<0.01), and 5- hydroxytrytamine and sodium nitroprusside did not evoke an increase in coronary flow in the allograft heart compared with syngeneic controls (P<0.01). Culture of aortic rings with antibody to major histocompatibility complex class I inhibited endothelium-dependent vasodilation to acetylcholine.

CONCLUSION

Transient microvascular endothelial dysfunction occurred in syngeneic and allogeneic cardiac grafts after transplantation. Syngeneic but not allogeneic grafts recovered, suggesting the indirect immune response, consisting of CD4 T cells, monocytes, and antibody, mediates endothelial dysfunction. A possible role for alloantibody in endothelial dysfunction is discussed.

Vascular endothelial function in health and diseases

The vascular endothelium constitutes approximately 1% of body mass (1kg) and has a surface area of approximately 5000m(2). The endothelium is a multifunctional endocrine organ strategically placed between the vessel wall and the circulating blood, and has a key role in vascular homeostasis. The endothelium is both a target for and mediator of cardiovascular disease. The endothelium releases several relaxing and constricting factors, which can affect vascular homeostasis. Endothelial dysfunction, whether caused by physical injury or cellular damage, leads to compensatory responses that alter the normal homeostatic properties of the endothelium. In this review, we summarized some physiological aspects of endothelial function and then we discussed endothelial dysfunction during some pathological conditions.

An interleukin-6-neutralizing antibody prevents cyclosporine-induced nephrotoxicity in mice

INTRODUCTION

Chronic use of cyclosporine A (CyA) induces nephrotoxicity primarily due to endothelial dysfunction. In our previous studies, potential mechanisms were identified in vitro and implicated nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and interleukin-6 (IL-6) as key components in causing endothelial dysfunction. In this study, we tested the hypothesis that NADPH oxidase activity and IL-6 are key components in renal damage in an in vivo model.

METHODS

Male mice C57B/6 mice from Jackson Laboratory (Bar Harbor, ME) at 6-8 wks were subjected to a low-salt diet throughout the trial. After 1 week on a low-salt diet, the mice were injected daily with treatments in 50 muL vehicle composed of 75% cremaphor (Sigma, St. Louis, MO) and ethanol for 5 wks. A vehicle-alone group was also set aside. Mice were weighed and 25 mg/kg/day cyclosporine (Novartis Pharma, St. Louis, MO) was injected daily. Apocynin (Calbiochem, Gibbstown, NJ) 20 mg/kg were injected either alone or concomitantly with CyA. Another group of mice were administered IL-6 antibody (Cat no. MAB406; R&D Systems, Minneapolis, MN) at 2 mug/day along with CyA. The kidneys were removed en bloc immediately and submitted in formalin for paraffin sections. Trichrome stains were performed. Slides were blinded and 10 photographs of cortical areas per treatment group were taken, which covered an estimate of 10% surface area in random fashion. Areas of renal damage, which were determined by tubular necrosis, were identified and quantified by amount of necrosis per photograph. Each photograph was divided into 10 blocks, and the number of blocks that contained necrotic tubules per photo was recorded.

RESULTS

The two control mice (low salt only) had no damage. The four vehicle mice had trace amounts of tubular necrosis. CyA treatment group demonstrated the highest amount of damage (29/70; 41%). CyA with apocynin, a specific NADPH oxidase inhibitor, was found to have 36% (22/60) damage, whereas the CyA with IL-6 antibody only was observed to have 15% (6/40) damage. Comparing imaging analysis, there was no difference between mice treated with CyA alone and with CyA with apocynin. However, the amount of damage in mice treated with CyA and IL-6 antibody was found to be significantly lower than both CyA and CyA with apocynin.

CONCLUSIONS

CyA action as a calcineurin inhibitor has allowed prolongation of kidney transplants, but its chronic use has led to devastating consequences such as allograft nephropathy. Previously, we have identified potential mechanisms of CyA-induced endothelial dysfunction in vitro. The current study identifies increased IL-6 expression as a mechanism by which CyA induces renal damage and that the use of an IL-6-neutralizing antibody may be useful in reducing CyA-induced renal damage.

Cardiac allograft vasculopathy: The achilles’ heel of long-term survival after cardiac transplantation

Over the past 40 years, cardiac transplantation has evolved as the single best long-term option for eligible candidates with end-stage heart failure. Approximately 2000 transplants are performed annually in the United States, and with the institution of calcineurin-based immunotherapy, surveillance biopsies, and programmatic-based patient care, life expectancy at 1 and 12 years is 85% and 50%, respectively. Cardiac allograft vasculopathy (CAV) is the number one cause of death after the first year of transplantation. The incidence of CAV remains as high as 50% at 5 years, with life expectancy significantly abbreviated once it is recognized. Although current immunotherapy has reduced the likelihood of cellular rejection, it has not impacted CAV substantially. Better treatment of established risk factors and the advent of newer antiproliferative immunotherapy may hold promise in treating CAV. However, future therapies must address the multitude of mechanisms underlying CAV. This manuscript reviews the pathophysiology, clinical manifestations, screening, and diagnostic strategies for cardiac allograft vasculopathy while emphasizing current treatment paradigms designed to stave off or retard the progression of CAV.

T cell-mediated vascular dysfunction of human allografts results from IFN-gamma dysregulation of NO synthase

Allograft vascular dysfunction predisposes to arteriosclerosis and graft loss. We examined how dysfunction develops in transplanted human arteries in response to circulating allogeneic T cells in vivo using immunodeficient murine hosts. Within 7-9 days, transplanted arteries developed endothelial cell (EC) dysfunction but remained sensitive to exogenous NO. By 2 weeks, the grafts developed impaired contractility and desensitization to NO, both signs of VSMC dysfunction. These T cell-dependent changes correlated with loss of eNOS and expression of iNOS--the latter predominantly within infiltrating T cells. Neutralizing IFN-gamma completely prevented both vascular dysfunction and changes in NOS expression; neutralizing TNF reduced IFN-gamma production and partially prevented dysfunction. Inhibiting iNOS partially preserved responses to NO at 2 weeks and reduced graft intimal expansion after 4 weeks in vivo. In vitro, memory CD4+ T cells acted on allogeneic cultured ECs to reduce eNOS activity and expression of protein and mRNA. These effects required T cell activation by class II MHC antigens and costimulators (principally lymphocyte function-associated antigen-3, or LFA-3) on the ECs and were mediated by production of soluble mediators including IFN-gamma and TNF. We conclude that IFN-gamma is a central mediator of vascular dysfunction and, through dysregulation of NOS expression, links early dysfunction with late arteriosclerosis.

Cyclosporine treatment preserves coronary resistance artery function in rat cardiac allografts

BACKGROUND

A marked decline in vascular myogenic response occurs during the course of rat cardiac allograft rejection. Two important contributory features are an inducible nitrous oxide synthase (iNOS)-catalyzed, NO-mediated vasodilation and a loss of smooth muscle function. In this study, we examine the effect of cyclosporine immunosuppressive therapy on the alleviation of arterial dysfunction of coronary resistance arteries in allografts using pressure myography.

METHODS

Rats receiving heterotopic abdominal cardiac transplantation were treated with cyclosporine (5 mg/kg), Cremophore or distilled water. Coronary septal arteries (internal diameter 200 microm) were dissected from isograft (Lewis to Lewis) and allograft (Fisher to Lewis) rat hearts at Day 21 post-transplantation and mounted on a pressure myograph. Pressure-induced vasoconstriction was measured before and after iNOS inhibition with aminoguanidine (AG; 100 micromol/liter). Both endothelium-based (ACh-induced) and endothelium-independent (sodium nitroprusside-induced) vasorelaxation were also recorded in each group.

RESULTS

Pressure-induced myogenic contraction was reduced in allograft coronary arteries at Day 21 post-transplantation compared with matched isografts (p < 0.05). AG potentiated myogenic tone in allograft arteries, but had no effect on untreated Day 21 isograft vessels, indicating the presence of iNOS-based relaxation only in allograft vessels. Depolarization-induced vasoconstriction was lower in allograft compared with isograft arteries (p < 0.05). Cyclosporine therapy also improved depolarization-induced constriction in allograft vessels compared with untreated groups (p < 0.05). Furthermore, cyclosporine therapy preserved endothelium-based and endothelium-independent vasorelaxation in allograft arteries at Day 21 post-transplantation.

CONCLUSIONS

Cyclosporine immunosuppressive therapy has a significant effect on the alleviation of early endothelial and smooth muscle dysfunction in coronary allograft arteries.

Transplantation-induced functional/morphological changes in rat aorta allografts differ from those in arteries of rat kidney allografts

The functional/morphological changes observed in rat aorta allografts were compared with those seen in the arteries of rat kidney allografts. Untreated allografts (F344-to-LEW) were collected at various times post-transplantation (Tx). Vascular smooth muscle cell (SMC) constriction to phenylephrine (Phe) and endothelial cell (EC)-dependent relaxation to acetylcholine (Ach) were assessed. Neointima formation in graft vessels was assessed by histology. In aorta allografts, the effects of Phe and Ach were irreversibly abolished within 3-2 weeks post-Tx. Neointima formation was consistently detected between 4 and 8 weeks post-Tx. In kidney allografts, sign of vasculopathy was seen in 10, 30 and 40% of resistance arteries at 8, 16 and 33 weeks post-Tx, respectively. In the main renal artery, substantial neointima formation was not apparent before 33 weeks post-Tx, the vasoconstrictor effect of Phe was fully maintained until then, and Ach-induced vasorelaxation was irreversibly reduced by approximately 70% from week 2 post-Tx onwards. These results indicate that the post-Tx functional/morphological changes seen in aorta allografts do not reflect those seen in arteries of kidney allografts. Hence, renal arteries from rat kidney allografts can be considered as a more relevant model to study the cascade of events leading to Tx-induced CGA in solid organ allografts.

Interferon-gamma plays a nonredundant role in mediating T cell-dependent outward vascular remodeling of allogeneic human coronary arteries

Vascular remodeling (change in vessel diameter) rather than intimal hyperplasia is the most important predictor of luminal loss in immune-mediated arterial injury, yet little is known about its mechanisms. Here, we show that outward vascular remodeling and intimal thickening, two manifestations of arteriosclerosis with opposing effects on luminal size, result from immune effector mechanisms that are T-cell dependent and interferon (IFN)-gamma mediated. In our in vivo model of human coronary artery injury by allogeneic peripheral blood mononuclear cells, both processes occur concurrently and are characterized by T-cell infiltrates with a predominantly IFN-gamma-producing cytokine profile. Neutralization of IFN-gamma inhibits the arterial and intimal expansion, whereas administration of IFN-gamma enhances these effects. The nonredundant role of IFN-gamma in T-cell-dependent remodeling of human coronary arteries demonstrated here presents a new therapeutic target for preservation of vessel lumen in arteriosclerosis.

Ultrastructural evidence of early endothelial damage in coronary arteries of rat cardiac allografts

BACKGROUND

Events that occur early after transplantation, particularly immune recognition of allo-endothelium, initiate transplant vascular disease (TVD). Previous work suggests an important compromise of endothelial integrity as the allo-immune milieu evolves, although mechanisms by which integrity is altered remain unclear. Increased vascular permeability caused by endothelial damage may allow inflammatory cells, lipoproteins, other proteins, and plasma fluid to enter the sub-endothelial space, thereby contributing to the initiation of atherosclerosis. In this study, we examined endothelial integrity in coronary arteries and the proximal aorta after cardiac transplantation in rats.

METHODS

We used Lewis-to-Lewis and Lewis-to-F344 rat heterotopic cardiac transplant models. We studied the effects of cyclosporine (5mg/kg/day) therapy compared with saline-treated controls. En face silver nitrate staining was performed to demonstrate endothelial cell borders and gaps. We used scanning electron microscopy to extend silver nitrate findings and to further define the presence and nature of endothelial disruptions. We used transmission electron microscopy to further characterize immune cell identity and interaction with endothelium.

RESULTS

Syngrafts and cyclosporine-treated allografts showed normal-looking endothelium similar to that observed in arteries from native hearts. However, saline-treated allografts displayed progressive endothelial destruction, including large intercellular gaps, missing cells, and areas of bare extracellular matrix. Exfoliated surfaces were covered by platelets at various stages of adhesion, activation, and spreading. Similarly, we observed numerous leukocytes as either adherent to the endothelial lining or transmigrating into the sub-endothelial space. Cessation of cyclosporine therapy was associated with the development of similar abnormalities.

CONCLUSIONS

Our findings indicate that, especially when immunosuppression is insufficient, early endothelial damage may promote vascular permeability and thereby initiate TVD.

Endothelial cell chimerism after renal transplantation in a rat model

BACKGROUND

Endothelial chimerism, that is, the replacement of damaged donor endothelium by recipient precursors, has been proposed to reduce the allogeneic stimulus and graft rejection. However, both its mechanism and consequence are poorly understood. In this study, we set up a rat model of renal transplantation to investigate the phenomenon of endothelial chimerism and the relationship among the factors involved in its induction, such as preischemic injury, severity of graft rejections, and the role of cyclosporine A toxicity.

METHODS

Female Lewis rats received renal transplants from male Brown Norway kidneys. Groups were divided according to ischemia or nonischemia of the donor kidney and cyclosporine treatment or nonimmunosuppression. To investigate the endothelial chimerism, an in situ hybridization by an X-chromosome-specific DNA probe was performed. The severity of allograft rejection was scored according to the Banff '97 classification.

RESULTS

In grafts without preischemic injury or without cyclosporine A treatment, a low degree of endothelial chimerism was detected, although severe vascular rejection and tissue necrosis developed. More chimerism was found in recipients receiving an ischemic kidney followed by immunosuppressive treatment, although less severe rejection developed. In recipients receiving an ischemic kidney without cyclosporine A treatment, the highest degree of endothelial chimerism occurred.

CONCLUSIONS

Endothelial chimerism demonstrated in rats after renal transplantation may be caused by endothelial damage induced by vascular rejection or ischemia. Ischemia had the strongest association with the induction of chimerism, which may function as a synergistic promoter. A low-dose cyclosporine A treatment was shown to inhibit endothelial replacement.

Endothelial response to hypoxia: physiologic adaptation and pathologic dysfunction

When subjected to a period of oxygen deprivation, endothelial cells exhibit a characteristic pattern of responses that can be considered either adaptive or pathologic, depending on the circumstances. In this review, the molecular basis for these responses is detailed. Hypoxia shifts the endothelial phenotype towards one in which anticoagulant properties are diminished, permeability and leukoadhesivity are increased, and proinflammatory features dominate the endovascular milieu. Of all the different points of intersection between the coagulation and inflammatory axes in the vasculature, perhaps most fundamentally, hypoxia alters several key transcriptional factors, including early growth response gene 1 (Egr1) and hypoxia-inducible factor (HIF) 1, which coordinate separate programs of gene activation. The preponderance of forces in the hypoxic endovascular environment, perhaps designed as an evolutionary adaptation to oxygen deprivation, can trigger severe, pathologic, clinical consequences in the setting of tissue ischemia.