Vascular smooth muscle cells of recipient origin mediate intimal expansion after aortic allotransplantation in mice

PLX3397, a CSF1 receptor inhibitor, limits allotransplantation-induced vascular remodelling

AIMS

Graft vascular disease (GVD), a clinically important and highly complex vascular occlusive disease, arises from the interplay of multiple cellular and molecular pathways. While occlusive intimal lesions are composed predominantly of smooth-muscle-like cells (SMLCs), the origin of these cells and the stimuli leading to their accumulation in GVD are uncertain. Macrophages have recently been identified as both potential drivers of intimal hyperplasia and precursors that undergo transdifferentiation to become SMLCs in non-transplant settings. Colony-stimulating factor-1 (CSF1) is a well-known regulator of macrophage development and differentiation, and prior preclinical studies have shown that lack of CSF1 limits GVD. We sought to identify the origins of SMLCs and of cells expressing the CSF1 receptor (CSF1R) in GVD, and to test the hypothesis that pharmacologic inhibition of CSF1 signalling would curtail both macrophage and SMLC activities and decrease vascular occlusion.

METHODS AND RESULTS

We used genetically modified mice and a vascular transplant model with minor antigen mismatch to assess cell origins. We found that neointimal SMLCs derive from both donor and recipient, and that transdifferentiation of macrophages to SMLC phenotype is minimal in this model. Cells expressing CSF1R in grafts were identified as recipient-derived myeloid cells of Cx3cr1 lineage, and these cells rarely expressed smooth muscle marker proteins. Blockade of CSF1R activity using the tyrosine kinase inhibitor PLX3397 limited the expression of genes associated with innate immunity and decreased levels of circulating monocytes and intimal macrophages. Importantly, PLX3397 attenuated the development of GVD in arterial allografts.

CONCLUSION

These studies provide proof of concept for pharmacologic inhibition of the CSF1/CSF1R signalling pathway as a therapeutic strategy in GVD. Further preclinical testing of this pathway in GVD is warranted.

Impact of Local Alloimmunity and Recipient Cells in Transplant Arteriosclerosis

RATIONALE

Transplant arteriosclerosis is the major limitation to long-term survival of solid organ transplantation. Although both immune and nonimmune cells have been suggested to contribute to this process, the complex cellular heterogeneity within the grafts, and the underlying mechanisms regulating the disease progression remain largely uncharacterized.

OBJECTIVE

We aimed to delineate the cellular heterogeneity within the allografts, and to explore possible mechanisms underlying this process.

METHODS AND RESULTS

Here, we reported the transcriptional profiling of 11 868 cells in a mouse model of transplant arteriosclerosis by single-cell RNA sequencing. Unbiased clustering analyses identified 21 cell clusters at different stages of diseases, and focused analysis revealed several previously unknown subpopulations enriched in the allografts. Interestingly, we found evidence of the local formation of tertiary lymphoid tissues and suggested a possible local modulation of alloimmune responses within the grafts. Intercellular communication analyses uncovered a potential role of several ligands and receptors, including Ccl21a and Cxcr3, in regulating lymphatic endothelial cell-induced early chemotaxis and infiltration of immune cells. In vivo mouse experiments confirmed the therapeutic potential of CCL21 and CXCR3 neutralizing antibodies in transplant arteriosclerosis. Combinational use of genetic lineage tracing and single-cell techniques further indicate the infiltration of host-derived c-Kit⁺ stem cells as heterogeneous populations in the allografts. Finally, we compared the immune response between mouse allograft and atherosclerosis models in single-cell RNA-seq analysis. By analyzing susceptibility genes of disease traits, we also identified several cell clusters expressing genes associated with disease risk.

CONCLUSIONS

Our study provides a transcriptional and cellular landscape of transplant arteriosclerosis, which could be fundamental to understanding the initiation and progression of this disease. CCL21/CXCR3 was also identified as important regulators of immune response and may serve as potential therapeutic targets in disease treatment.

Discovery, synthesis and anti-atherosclerotic activities of a novel selective sphingomyelin synthase 2 inhibitor

The sphingomyelin synthase 2 (SMS2) is a potential target for pharmacological intervention in atherosclerosis. However, so far, few selective SMS2 inhibitors and their pharmacological activities were reported. In this study, a class of 2-benzyloxybenzamides were discovered as novel SMS2 inhibitors through scaffold hopping and structural optimization. Among them, Ly93 as one of the most potent inhibitors exhibited IC₅₀ values of 91 nM and 133.9 μM against purified SMS2 and SMS1 respectively. The selectivity ratio of Ly93 was more than 1400-fold for purified SMS2 over SMS1. The in vitro studies indicated that Ly93 not only dose-dependently diminished apoB secretion from Huh7 cells, but also significantly reduced the SMS activity and increased cholesterol efflux from macrophages. Meanwhile, Ly93 inhibited the secretion of LPS-mediated pro-inflammatory cytokine and chemokine in macrophages. The pharmacokinetic profiles of Ly93 performed on C57BL/6J mice demonstrated that Ly93 was orally efficacious. As a potent selective SMS2 inhibitor, Ly93 significantly decreased the plasma SM levels of C57BL/6J mice. Furthermore, Ly93 was capable of dose-dependently attenuating the atherosclerotic lesions in the root and the entire aorta as well as macrophage content in lesions, in apolipoprotein E gene knockout mice treated with Ly93. In conclusion, we discovered a novel selective SMS2 inhibitor Ly93 and demonstrated its anti-atherosclerotic activities in vivo. The preliminary molecular mechanism-of-action studies revealed its function in lipid homeostasis and inflammation process, which indicated that the selective inhibition of SMS2 would be a promising treatment for atherosclerosis.

Murine Aortic Crush Injury: An Efficient In Vivo Model of Smooth Muscle Cell Proliferation and Endothelial Function

Arterial reconstruction, whether angioplasty or bypass surgery, involves iatrogenic trauma causing endothelial disruption and vascular smooth muscle cell (VSMC) proliferation. Common murine models study small vessels such as the carotid and femoral arteries. Herein we describe an in vivo system in which both VSMC proliferation and endothelial barrier function can be simultaneously assessed in a large vessel. We studied the infrarenal aortic response to injury in C57BL/6 mice. The aorta was injured from the left renal vein to the aortic bifurcation by 30 transmural crushes of 5-seconds duration with a cotton-tipped applicator. Morphological changes were assessed with conventional histology. Aorta wall thickness was measured from the luminal surface to the adventitia. EdU integration and counter staining with DAPI and alpha-actin was used to demonstrate VSMC proliferation. Activation of ERK1/2, a known moderator of intimal hyperplasia formation, was determined by Western Blot analysis. The effect of inflammation was determined by immunohistochemistry for B-cells, T-cells, and macrophages. En face sections of endothelium were visualized with scanning electron microscopy (SEM). Endothelial barrier function was determined with Evans Blue staining. Transmural injury resulted in aortic wall thickening. This injury induced VSMC proliferation, most prominently at 3 days after injury, and early activation of ERK1/2 and decreased p27^kip1 expression. Injury did not result in increased B-cells, T-cells, or macrophages infiltration in the vessel wall. Injury caused partial endothelial cell denudation and loss of cell-cell contact. Injury resulted in a significant loss of endothelial barrier function, which returned to baseline after seven days. The murine transmural blunt aortic injury model provides an efficient system to simultaneously study both VSMC proliferation and endothelial barrier function in a large vessel.

DNA Methylation Signature of Post-injury Neointimal Cells During Vascular Remodeling in the Rat Balloon Injury Model

Vascular smooth muscle cell (VSMC) accumulation in the neointimal is a common feature in vascular diseases such as atherosclerosis, transplant arteriosclerosis and restenosis. In this study, we isolated the neointimal cells and uninjured residential vascular smooth muscle cells by laser micro dissection and carried out single-cell whole-genome methylation sequencing. We also sequenced the bisulfite converted genome of circulating bone-marrow-derived cells such as peripheral blood mononuclear cells (PBMC) and bone marrow mononuclear cells (BMMC). We found totally 2,360 differential methylation sites (DMS) annotated to 1,127 gene regions. The majority of differentially methylated regions (DMRs) were located in intergenic regions, outside those CpG islands and island shores. Interestingly, exons have less DMRs than promotors and introns, and CpG islands contain more DMRs than islands shores. Pearson correlation analysis showed a clear clustering of neointimal cells with PBMC/BMMC. Gene set enrichment analysis of differentially methylated CpG sites revealed that many genes were important for regulation of VSMC differentiation and stem cell maintenance. In conclusion, our results showed that neointimal cells are more similar to the progenitor cells in methylation profile than the residential VSMCs at the 30^th day after the vascular injury.

Histone deacetylases and cardiovascular cell lineage commitment

Cardiovascular diseases (CVDs), which include all diseases of the heart and circulation system, are the leading cause of deaths on the globally. During the development of CVDs, choric inflammatory, lipid metabolism disorder and endothelial dysfunction are widely recognized risk factors. Recently, the new treatment for CVDs that designed to regenerate the damaged myocardium and injured vascular endothelium and improve recovery by the use of stem cells, attracts more and more public attention. Histone deacetylases (HDACs) are a family of enzymes that remove acetyl groups from lysine residues of histone proteins allowing the histones to wrap the DNA more tightly and commonly known as epigenetic regulators of gene transcription. HDACs play indispensable roles in nearly all biological processes, such as transcriptional regulation, cell cycle progression and developmental events, and have originally shown to be involved in cancer and neurological diseases. HDACs are also found to play crucial roles in cardiovascular diseases by modulating vascular cell homeostasis (e.g., proliferation, migration, and apoptosis of both ECs and SMCs). This review focuses on the roles of different members of HDACs and HDAC inhibitor on stem cell/ progenitor cell differentiation toward vascular cell lineages (endothelial cells, smooth muscle cells and Cardiomyocytes) and its potential therapeutics.

Regulation of smooth muscle cells in development and vascular disease: current therapeutic strategies

Vascular smooth muscle cells (SMCs) exhibit extensive phenotypic diversity and rapid growth during embryonic development, but maintain a quiescent, differentiated state in adult. The pathogenesis of vascular proliferative diseases involves the proliferation and migration of medial vascular SMCs into the vessel intima, possibly reinstating their embryonic gene expression programs. Multiple mitogenic stimuli induce vascular SMC proliferation through cell cycle progression. Therapeutic strategies targeting cell cycle progression and mitogenic stimuli have been developed and evaluated in animal models of atherosclerosis and vascular injury, and several clinical studies. Recent discoveries on the recruitment of vascular progenitor cells to the sites of vascular injury suggest new therapeutic potentials of progenitor cell-based therapies to accelerate re-endothelialization and prevent engraftment of SMC-lineage progenitor cells. Owing to the complex and multifactorial nature of SMC regulation, combinatorial antiproliferative approaches are likely to be used in the future in order to achieve maximal efficacy and reduce toxicity.

Neutrophils are responsible for impaired medial smooth muscle cell recovery and exaggerated allograft vasculopathy in aortic allografts exposed to prolonged cold ischemia

BACKGROUND

Ischemia and reperfusion injury is critical in allograft vasculopathy (AV) development. We have shown that neutrophil-mediated medial smooth muscle cell (SMC) loss precedes AV and that prolonged cold ischemia (CI) impairs medial SMC recovery and accelerates AV development. We hypothesize that neutrophils (NØs) are responsible for failed medial SMC recovery that precedes AV.

METHODS

Aortic transplants were performed between fully disparate C3H/HeJ murine donors and wild-type C57BL/6 (WT B6), B6.129S7-Rag1 (Rag1(-/-); intact innate but no adaptive immunity), and B6.129S-Cybb (NOX2(-/-); NØ loss-of-function) recipients under cyclosporine A immunosuppression. Grafts were exposed to 20 or 60 minutes CI before transplant and harvested at 1 day, 2 weeks, and 8 weeks after transplant. Some WT B6 recipients were treated with remote ischemic pre-conditioning (rIPC). Grafts were assessed for medial SMCs, NØs, and lesion area.

RESULTS

The 60-minute vs 20-minute CI grafts exhibited reduced SMC recovery at 2 weeks in WT B6 and Rag1(-/-) recipients (WT B6: p = 0.0009; Rag1(-/-): p = 0.0006). NØ influx was greater in Rag1(-/-) recipients of 60-minute vs 20-minute CI grafts at 1 day (p = 0.0002). The difference in 2-week medial SMC recovery between ischemia groups was abrogated in NOX2(-/-) recipients. At 8 weeks, NOX2(-/-) and rIPC recipients of 60-minute CI grafts exhibited smaller neointimal lesions than B6 recipients (NOX2(-/-): p = 0.0009; rIPC: p = 0.0005).

CONCLUSIONS

Impaired medial SMC recovery in murine aortic allografts at 2 weeks occurs in the absence of adaptive immunity. Enhanced medial SMC recovery and reduced neointimal lesion formation in NOX2(-/-) and rIPC recipients of 60-minute CI grafts suggest a causal role for NØs in impaired medial SMC repopulation and the development of AV.

Endothelial dysfunction and cardiac allograft vasculopathy

Cardiac allograft vasculopathy remains a major challenge to long-term survival after heart transplantation. Endothelial injury and dysfunction, as a result of multifactorial immunologic and nonimmunologic insults in the donor and the recipient, are prevalent early after transplant and may be precursors to overt cardiac allograft vasculopathy. Current strategies for managing cardiac allograft vasculopathy, however, rely on the identification and treatment of established disease. Improved understanding of mechanisms leading to endothelial dysfunction in heart transplant recipients may provide the foundation for the development of sensitive screening techniques and preventive therapies.

Acute regeneration and chronic acellular transformation of rabbit cryopreserved aortic allografts

An analysis of rabbit cryopreserved aortic allografts excised on postoperative days (POD) 2, 5, 11, 60, 210, 360, and 720, as well as controls that were untransplanted native aortas and cryopreserved aortas, was performed. On POD2, the number of medial smooth muscle cells in the allografts was reduced to approximately 50%. Ki-67 analysis revealed that medial smooth muscle cells in the allografts proliferated from the 2nd day. By the 11th day, their proliferation ceased and the number of medial smooth muscle cells was restored to almost at the same level as in the controls. Polymorphic microsatellite DNA marker analysis disclosed that the restored medial smooth muscle cells were of donor origin. From 7 months through 2 years, the media of cryopreserved aortic allografts were transformed into acellular structures, in which the elastic fibers were preserved. On the other hand, newly accumulated smooth muscle cells were observed in the adventitia just outside of acellular media after 7 months. In some cases, scattered lamellar calcium deposition was observed in the same regions. This study presents a comprehensive documentation of regeneration and acellular transformation in cryopreserved aortic allografts based on short and long-term analysis.

Progenitor cells in arteriosclerosis: good or bad guys?

Accumulating evidence indicates that the mobilization and recruitment of circulating or tissue-resident progenitor cells that give rise to endothelial cells (ECs) and smooth muscle cells (SMCs) can participate in atherosclerosis, neointima hyperplasia after arterial injury, and transplant arteriosclerosis. It is believed that endothelial progenitor cells do exist and can repair and rejuvenate the arteries under physiologic conditions; however, they may also contribute to lesion formation by influencing plaque stability in advanced atherosclerotic plaque under specific pathologic conditions. At the same time, smooth muscle progenitors, despite their capacity to expedite lesion formation during restenosis, may serve to promote atherosclerotic plaque stabilization by producing extracellular matrix proteins. This profound evidence provides support to the hypothesis that both endothelial and smooth muscle progenitors may act as a double-edged sword in the pathogenesis of arteriosclerosis. Therefore, the understanding of the regulatory networks that control endothelial and smooth muscle progenitor differentiation is undoubtedly fundamental both for basic research and for improving current therapeutic avenues for atherosclerosis. We update the progress in progenitor cell study related to the development of arteriosclerosis, focusing specifically on the role of progenitor cells in lesion formation and discuss the controversial issues that regard the origins, frequency, and impact of the progenitors in the disease.

Essential role for calcium waves in migration of human vascular smooth muscle cells

Vascular smooth muscle cell (SMC) migration is characterized by extension of the lamellipodia at the leading edge, lamellipodial attachment to substrate, and release of the rear (uropod) of the cell, all of which enable forward movement. However, little is known regarding the role of intracellular cytosolic Ca(2+) concentration ([Ca(2+)](i)) in coordinating these distinct activities of migrating SMCs. The objective of our study was to determine whether regional changes of Ca(2+) orchestrate the migratory cycle in human vascular SMCs. We carried out Ca(2+) imaging using digital fluorescence microscopy of fura-2 loaded human smooth muscle cells. We found that motile SMCs exhibited Ca(2+) waves that characteristically swept from the rear of polarized cells toward the leading edge. Ca(2+) waves were less evident in nonpolarized, stationary cells, although acute stimulation of these SMCs with the agonists platelet-derived growth factor-BB or histamine could elicit transient rise of [Ca(2+)](i). To investigate a role for Ca(2+) waves in the migratory cycle, we loaded cells with the Ca(2+) chelator BAPTA, which abolished Ca(2+) waves and significantly reduced retraction, supporting a causal role for Ca(2+) in initiation of retraction. However, lamellipod motility was still evident in BAPTA-loaded cells. The incidence of Ca(2+) oscillations was reduced when Ca(2+) release from intracellular stores was disrupted with the sarcoplasmic reticulum Ca(2+)-ATPase inhibitor thapsigargin or by treatment with the inositol 1,4,5-trisphosphate receptor blocker 2-aminoethoxy-diphenyl borate or xestospongin C, implicating Ca(2+) stores in generation of waves. We conclude that Ca(2+) waves are essential for migration of human vascular SMCs and can encode cell polarity.

Flanking recipient vasculature, not circulating progenitor cells, contributes to endothelium and smooth muscle in murine allograft vasculopathy

OBJECTIVE

The prevailing view assumes that circulating endothelial and smooth muscle progenitor cells participate in allograft vasculopathy (AV), although the seminal studies in the field were not designed to distinguish between circulating and migrating cells of recipient origin. We developed a double-transplantation technique to overcome this problem and reinvestigated the origin of endothelial cells (ECs) and smooth muscle cells (SMCs) in murine AV.

METHODS AND RESULTS

Carotid artery segments from BALB/c mice were allografted to apolipoprotein E(-/-) B6 mice with or without a "flanking" isograft interpositioned between the allograft and the recipient artery. Either recipient mice or interpositioned isografts expressed enhanced green fluorescent protein, and consequently, cells migrating into the allograft from the flanking vasculature could easily be tracked and distinguished from recruited circulating cells. Without immunosuppression, allograft donor cells vanished as expected, and AV developed by replacement and accumulation of ECs and SMCs of recipient origin. The double transplantation models revealed that all ECs and SMCs in AV had migrated into the allograft from the flanking vasculature without any contribution from putative progenitor cells in the blood.

CONCLUSIONS

Migrating cells from the flanking vasculature, not circulating progenitor cells, are the source of recipient-derived ECs and SMCs in murine AV.

Bioengineered vascular graft grown in the mouse peritoneal cavity

BACKGROUND

We tested the hypothesis that the mouse peritoneum can function like a bioreactor to generate directed bio-engineered tissues such as those used for bypass grafting. Additionally, we reasoned that the mouse animal model would allow us to elucidate the underlying cellular and molecular mechanisms that are responsible for the generation of tissue in peritoneal cavity.

METHODS

Plastic tubes (two tubes/mouse) were implanted into the peritoneal cavity of three strains of mice (C57BL/6, BALB/c, and MRL). The tubes were harvested, tissue capsule surrounding the tubes was removed, and analyzed by immunostaining (five capsules/five mice/strain) and microarray (three capsules/three mice/strain). In addition, the tissue capsules that were harvested from MRL mice (n = 21) were grafted into abdominal aorta of the same mice as autografts. The patency of all grafts was monitored by micro-ultrasound, and their functionality was assessed by laser Doppler imaging of blood flow in femoral arteries. Venous (n = 13) and arterial isografts (n = 11) were used as positive controls. In a negative control group (five mice/strain), the abdominal aorta was occluded by double ligation with 9-0 silk.

RESULTS

The implanted plastic tubes required at least 8 weeks of incubation in the peritoneum of the three strains of mice in order to generate useful grafts. No vascular cells were found in the tissue capsules. Microarray analysis of tissue capsules revealed that the capsular cells express a gene expression program that is vastly shared among the three strains of mice, and the cells exhibit a high degree of plasticity. The micro-ultrasound analysis of the grafts showed that 62% of autografts remained patent compared with 77% of venous isografts and 91% of arterial isografts. The laser Doppler imaging analysis showed that blood flow dropped by 40% and 35% in the autografts and vein isografts, respectively, 1 day after surgery. The flow, however, rebounded to the level of arterial isografts 1 month post-surgery and remained unchanged among all grafts for the next 4 months. Immunostaining of the autografts showed a thick vessel wall with endothelial cells that lined the lumen and smooth muscle cells that constituted the graft wall.

CONCLUSION

The mouse peritoneal cavity of mice has the ability to function like a bioreactor to generate bio-engineered tissues. The tissue capsules harvested from peritoneal cavity of a mouse are composed of nonvascular cells that display phenotype of progenitor cells. After grafting, however, the capsule autografts become arterialized and remained patent for at least 4 months after surgery, similar to venous or arterial isografts.

Neutrophil mediated smooth muscle cell loss precedes allograft vasculopathy

BACKGROUND

Cardiac allograft vasculopathy (AV) is a pathological process of vascular remodeling leading to late graft loss following cardiac transplantation. While there is consensus that AV is alloimmune mediated, and evidence that the most important alloimmune target is medial smooth muscle cells (SMC), the role of the innate immune response in the initiation of this disease is still being elucidated. As ischemia reperfusion (IR) injury plays a pivotal role in the initiation of AV, we hypothesize that IR enhances the early innate response to cardiac allografts.

METHODS

Aortic transplants were performed between fully disparate mouse strains (C3H/HeJ and C57BL/6), in the presence of therapeutic levels of Cyclosporine A, as a model for cardiac AV. Neutrophils were depleted from some recipients using anti-PMN serum. Grafts were harvested at 1,2,3,5d and 1,2wk post-transplant. Ultrastructural integrity was examined by transmission electron microscopy. SMC and neutrophils were quantified from histological sections in a blinded manner.

RESULTS

Grafts exposed to cold ischemia, but not transplanted, showed no medial SMC loss and normal ultrastructural integrity. In comparison, allografts harvested 1d post-transplant exhibited > 90% loss of SMC (p < 0.0001). SMC partially recovered by 5d but a second loss of SMC was observed at 1wk. SMC loss at 1d and 1wk post-transplant correlated with neutrophil influx. SMC loss was significantly reduced in neutrophil depleted recipients (p < 0.01).

CONCLUSIONS

These novel data show that there is extensive damage to medial SMC at 1d post-transplant. By depleting neutrophils from recipients it was demonstrated that a portion of the SMC loss was mediated by neutrophils. These results provide evidence that IR activation of early innate events contributes to the etiology of AV.

Decreased transplant arteriosclerosis in endothelial nitric oxide synthase-deficient mice

BACKGROUND

Occlusive vascular changes, characterized by the formation of a neointima with lumen obstruction, are key histologic findings of allograft arteriosclerosis. Vascular integrity of the graft is critically dependent on nitric oxide (NO), synthesized by NO synthases (NOS), of which three isoforms have been located in the arterial wall: endothelial NOS (eNOS), inducible NOS, and neuronal NOS (nNOS). We have studied the role of NOS in a murine model of aortic allograft rejection.

METHODS

The descending thoracic aorta of donor mice (BALB/c mice) was transplanted into two groups of recipients: (a) C57BL/6J and (b) C57BL/6J mice homozygous (-/-) for a knockout of the eNOS gene (eNOS(-/-)).

RESULTS

After 4 weeks, pronounced neointima formation, upregulated expression of adhesion molecules, and increased infiltration by inflammatory cells were demonstrated in wild-type recipient mice, whereas eNOS(-/-) recipient mice were protected from neointima development by a significantly increased synthesis of NO, as shown by increased formation of cGMP; this was mainly explained by upregulation of inducible NOS and nNOS.

CONCLUSIONS

Upregulation of inducible NOS and nNOS isoforms may be beneficial in preventing allograft arteriosclerosis in the early posttransplant period.

Naringenin decreases progression of atherosclerosis by improving dyslipidemia in high-fat-fed low-density lipoprotein receptor-null mice

OBJECTIVE

Naringenin is a citrus flavonoid that potently inhibits the assembly and secretion of apolipoprotein B100-containing lipoproteins in cultured hepatocytes and improves the dyslipidemia and insulin resistance in a mouse model of the metabolic syndrome. In the present study, we used low-density lipoprotein receptor-null mice fed a high-fat diet (Western, TD96125) to test the hypothesis that naringenin prevents atherosclerosis.

METHODS AND RESULTS

Three groups (chow, Western, and Western plus naringenin) were fed ad libitum for 6 months. The Western diet increased fasting plasma triglyceride (TG) (5-fold) and cholesterol (8-fold) levels compared with chow, whereas the addition of naringenin significantly decreased both lipids by 50%. The Western-fed mice developed extensive atherosclerosis in the aortic sinus because plaque area was increased by 10-fold compared with chow-fed animals. Quantitation of fat-soluble dye (Sudan IV)-stained aortas, prepared en face, revealed that Western-fed mice also had a 10-fold increase in plaque deposits throughout the arch and in the abdominal sections of the aorta, compared with chow. Atherosclerosis in both areas was significantly decreased by more than 70% in naringenin-treated mice. Consistent with quantitation of aortic lesions, the Western-fed mice had a significant 6-fold increase in cholesterol and a 4-fold increase in TG deposition in the aorta compared with chow-fed mice. Both were reduced more than 50% by naringenin. The Western diet induced extensive hepatic steatosis, with a 10-fold increase in both TG and cholesteryl ester mass compared with chow. The addition of naringenin decreased both liver TG and cholesteryl ester mass by 80%. The hyperinsulinemia and obesity that developed in Western-fed mice was normalized by naringenin to levels observed in chow-fed mice.

CONCLUSIONS

These in vivo studies demonstrate that the citrus flavonoid naringenin ameliorates the dyslipidemia in Western-fed low-density lipoprotein receptor-null mice, leading to decreased atherosclerosis; and suggests a potential therapeutic strategy for the hyperlipidemia and increased risk of atherosclerosis associated with insulin resistance.

Contribution of B cells and antibody to cardiac allograft vasculopathy

BACKGROUND

The aim of this study was to determine the role of alloantibody in the development of cardiac allograft vasculopathy (AV). AV is the main pathologic indicator of chronic cardiac graft rejection resulting in graft loss at 10 years posttransplant. In AV, a neointimal lesion forms resulting in luminal occlusion and damage to the transplanted organ. AV is T-cell mediated, but the role played by B cells and antibody in AV development has been controversial. No studies have been conducted in the presence of a clinically relevant immunosuppressant. In our study, we use cyclosporin A, a calcineurin inhibitor.

METHODS

Two models of B-cell deficiency were used as recipients of a C3H/HeJ abdominal aortic graft; grafts were harvested at 8 weeks. T- and B-cell immunodeficient mice (RAG1-/-) received passively transferred anti-C3H antibody, raised in B6 mice. Cyclosporin A was administered daily to both control and experimental groups. Alpha-actin staining was used to identify myofibroblasts in the neointima.

RESULTS

Lesions in B-cell-deficient B6 mice were not significantly different in size from those of control mice. Lesions in both B-cell-deficient and wild-type mice showed similar levels of alpha-actin positivity. Passive transfer of antibody to RAG1-/- mice resulted in small, alpha-actin-positive lesions.

CONCLUSIONS

B cells are not required for the development of AV, but the presence of an alloantibody can contribute to AV. We hypothesize that the alloantibody mediates AV by initiating complement-mediated killing of smooth muscle cells, based on an in vitro work. Of interest, we found that the neointimal lesions of B-cell-deficient mice and mice that received antibody showed the presence of alpha-actin in myofibroblasts.

Contribution of pre-existing vascular disease to allograft vasculopathy in a murine model

Allograft vasculopathy (AV) has emerged as a major obstacle for long-term graft survival after cardiac transplantation. The shortage of donor hearts has meant fewer restrictions have been placed on acceptable hearts over the past few years resulting in an increase in the number of older hearts in the donor pool. This increase has subsequently led to the increase of donor hearts containing pre-existing disease. The importance of this pre-existing donor vascular disease in AV outcomes remains controversial. In this study we address this by taking advantage of the fact that B6 Apolipoprotein-E knockout mice develop atherosclerotic lesions in their aortic tracts that closely model human naturally occurring vascular disease. By using these mice as donors, we transplant known levels of pre-existing disease into fully disparate (C3H) recipients. Cyclosporin A is used to prevent acute rejection and allow for allograft vasculopathy. We found that pre-existing lesions are retained in this model after transplantation and that they contribute to increase in lesion size and to increased lumenal narrowing. The de novo AV lesions overlay the pre-existing lesions and this leads to areas of eccentric lesion formation in the vessels with likely accompanying exacerbation of flow perturbation.

Smooth muscle progenitor cells: friend or foe in vascular disease?

The origin of vascular smooth muscle cells that accumulate in the neointima in vascular diseases such as transplant arteriosclerosis, atherosclerosis and restenosis remains subject to much debate. Smooth muscle cells are a highly heterogeneous cell population with different characteristics and markers, and distinct phenotypes in physiological and pathological conditions. Several studies have reported a role for bone marrow-derived progenitor cells in vascular maintenance and repair. Moreover, bone marrow-derived smooth muscle progenitor cells have been detected in human atherosclerotic tissue as well as in in vivo mouse models of vascular disease. However, it is not clear whether smooth muscle progenitor cells can be regarded as a 'friend' or 'foe' in neointima formation. In this review we will discuss the heterogeneity of smooth muscle cells, the role of smooth muscle progenitor cells in vascular disease, potential mechanisms that could regulate smooth muscle progenitor cell contribution and the implications this may have on designing novel therapeutic tools to prevent development and progression of vascular disease.

Mesenchymal stem cells-derived vascular smooth muscle cells release abundant levels of osteoprotegerin

Although several studies have shown that the serum levels of osteoprotegerin (OPG) are significantly elevated in patients affected with atherosclerotic lesions in coronary and peripheral arteries, the cellular source and the role of OPG in the physiopathology of atherosclerosis are not completely defined. Therefore, we aimed to investigate the potential contribution of mesenchymal stem cells in the production/release of OPG. OPG was detectable by immunohistochemistry in aortic and coronary atherosclerotic plaques, within or in proximity of intimal vascular smooth muscle cells (SMC). In addition, bone marrow mesenchymal stem cell (MSC)-derived vascular SMC as well as primary aortic SMC released in the culture supernatant significantly higher levels of OPG with respect to MSC-derived endothelial cells (EC) or primary aortic EC. On the other hand, in vitro exposure to full-length human recombinant OPG significantly increased the proliferation rate of aortic SMC cultures, as monitored by bromodeoxyuridine incorporation. Taken together, these data suggest that OPG acts as an autocrine/paracrine growth factor for vascular SMC, which might contribute to the progression of atherosclerotic lesions.

Vascular progenitor cells and translational research: the role of endothelial and smooth muscle progenitor cells in endogenous arterial remodelling in the adult

There has been much recent research into the therapeutic use of stem and progenitor cells for various diseases. Alongside this, there has also been considerable interest in the normal roles that endogenous precursor cells may play in both physiological and pathological settings. In the present review, we focus on two types of progenitor cell which are of potential relevance to vascular homoeostasis, namely the EPC (endothelial progenitor cell) and the smooth muscle progenitor cell. We discuss evidence for their existence and sources in adults, and the various techniques currently used to identify these cells. We examine data obtained from studies using different methods of progenitor identification and relate these to each other, in order to provide a framework in which to interpret the literature in this area. We review evidence for the influence of these vascular progenitor cells upon vascular function and the development and progression of atherosclerosis.

Stem Cells and Transplant Arteriosclerosis

Stem cells can differentiate into a variety of cells to replace dead cells or to repair damaged tissues. Recent evidence indicates that stem cells are involved in the pathogenesis of transplant arteriosclerosis, an alloimmune initiated vascular stenosis that often results in transplant organ failure. Although the pathogenesis of transplant arteriosclerosis is not yet fully understood, recent developments in stem cell research have suggested novel mechanisms of vascular remodeling in allografts. For example, stem cells derived from the recipient may repair damaged endothelial cells of arteries in transplant organs. Further evidence suggests that stem cells or endothelial progenitor cells may be released from both bone marrow and non–bone marrow tissues. Vascular stem cells appear to replenish cells that died in donor vessels. Concomitantly, stem/progenitor cells may also accumulate in the intima, where they differentiate into smooth muscle cells. However, several issues concerning the contribution of stem cells to the pathogenesis of transplant arteriosclerosis are controversial, eg, whether bone marrow–derived stem cells can differentiate into smooth muscle cells that form neointimal lesions of the vessel wall. This review summarizes recent research on the role of stem cells in transplant arteriosclerosis, discusses the mechanisms of stem cell homing and differentiation into mature endothelial and smooth muscle cells, and highlights the controversial issues in the field.

Progenitor cells and vascular disease

Vascular progenitor cells have been the focus of much attention in recent years; both from the point of view of their pathophysiological roles and their potential as therapeutic agents. However, there is as yet no definitive description of either endothelial or vascular smooth muscle progenitor cells. Cells with the ability to differentiate into mature endothelial and vascular smooth muscle reportedly reside within a number of different tissues, including bone marrow, spleen, cardiac muscle, skeletal muscle and adipose tissue. Within these niches, vascular progenitor cells remain quiescent, until mobilized in response to injury or disease. Once mobilized, these progenitor cells enter the circulation and migrate to sites of damage, where they contribute to the remodelling process. It is generally perceived that endothelial progenitors are reparative, acting to restore vascular homeostasis, while smooth muscle progenitors contribute to pathological changes. Indeed, the number of circulating endothelial progenitor cells inversely correlates with exposure to cardiovascular risk factors and numbers of animal models and human studies have demonstrated therapeutic roles for endothelial progenitor cells, which can be enhanced by manipulating them to overexpress vasculo-protective genes. It remains to be determined whether smooth muscle progenitor cells, which are less well studied than their endothelial counterparts, can likewise be manipulated to achieve therapeutic benefit. This review outlines our current understanding of endothelial and smooth muscle progenitor cell biology, their roles in vascular disease and their potential as therapeutic agents.

Vascular progenitor cells and atherosclerosis

Vascular regeneration occurs throughout life as a dynamic process. Millions of new endothelial cells are created with essentially the same number of cells undergoing programmed cell death or necrosis every day. As a result, the human vascular tree could be considered to essentially replace its entire endothelial population over a specified number of years. Within this network there is a compartment of vascular progenitor cells that appear to govern this homeostasis throughout life, continuously repopulating cells that die by apoptosis or necrosis. This delicate equilibrium appears to be disrupted in atherosclerotic disease processes as patients with known ischemic heart disease risk factors have been found to have lower numbers of circulating endothelial progenitor cells, which may tip the balance in favor of lesion formation, rather than repair. The aim of this article is to discuss the types of vascular progenitor cells and the mechanisms behind their mobilization, homing and differentiation into mature endothelial cells capable of vascular repair.

The role of vascular stem cells in atherogenesis and post-angioplasty restenosis

It is well known that atherosclerosis prevails in elderly populations as ageing acts as a recognized risk factor for this disease. Although the pathogenic factors leading to atherosclerosis are highly heterogeneous, traditionally speaking, the causative risk factors include hyperlipidemia, hypertension, diabetes mellitus and smoking, which can damage to endothelial function, and subsequently promote lipid penetration and inflammatory cell infiltration. Damaged endothelial cells (ECs) may be replaced by neighboring cell division, while damaged smooth muscle cells (SMCs) may be replaced by medial SMCs emigrating into the intima during atherogenesis. However, this standpoint is challenged by recent findings that vascular progenitor/stem cells (VPCs) may contribute to atherogenesis and post-angioplasty restenosis. VPCs are a group of primitive cells that have the potential to produce mature, functional cells in the vascular wall. VPCs residing in bone marrow, vascular wall or circulating in the peripheral blood may be stimulated by a variety of pathogenic factors. These stem cells then participate in regeneration, repair and remodeling of the injured arterial wall. This new concept may bring about a great breakthrough in understanding the pathogenesis of atherosclerosis and develop novel therapeutic strategies for coronary heart disease. This article will mainly review the role of VPCs in atherogenesis, thus providing a novel understanding about the pathophysiology of atherosclerosis.

Low Molecular Weight Fucoidan Prevents Neointimal Hyperplasia After Aortic Allografting

BACKGROUND

Fucoidan, a new low molecular weight sulfated polysaccharide (LMWF), has previously been shown to mobilize bone marrow-derived progenitors cells via stimulation of stromal derived factor (SDF)-1 release. Mobilized progenitor cells have been suggested to repair intimal lesions after immune-mediated endothelial injury and thus prevent intimal proliferation. The aim of this study was to evaluate the effect of LMWF treatment in a rat aortic allograft model of transplant arteriosclerosis (TA).

METHODS

Aortic grafts were performed in Brown Norway (BN, donor) and Lewis (Lew, recipient) rats. The recipient rats were treated with LMWF (5 mg/kg/day) and sacrificed at 30 days. To determine the role of SDF-1 in mediating the effects of LMWF, a specific inhibitor of the SDF-1 receptor CXCR4, AMD 3100 (20 microg/kg/day), was used. The grafted segments were evaluated by morphometric (histochemical) analyses.

RESULTS

Untreated aortic allografts exhibited severe intimal proliferation, indicative of TA. In contrast, LMWF treatment significantly prevented allograft intimal proliferation as compared with controls (5.7+/-3 vs. 66.2+/-6 microm, P<0.01) and permitted a normalization of the intima/media ratio (0.1+/-0.1 vs. 1.7+/-0.3, P<0.01). Further, LMWF treatment stimulated allograft reendothelialization, as evidenced by strong intimal endothelial nitric oxide synthase antibody and CD31 signals. Unexpectedly, AMD treatment failed to prevent the protective effect of LMWF on intimal thickening and AMD treatment alone was found to reduced intimal proliferation in allografts.

CONCLUSIONS

We found that LMWF treatment reduced intimal thickness and induced the presence of an endothelial cell lining in the vascular graft at 30 days. Our findings may suggest a novel therapeutic strategy in the prevention of TA.

Vascular Remodeling in Transplant Vasculopathy

As therapeutic strategies to prevent acute rejection progressively improve, transplant vasculopathy (TV) constitutes the single most important limitation for long-term functioning of solid organ allografts. In TV, allograft arteries characteristically develop severe, diffuse intimal hyperplastic lesions that eventually compromise luminal flow and cause ischemic graft failure. Traditional immunosuppressive strategies that check acute allograft rejection do not prevent TV; indeed 50% of transplant recipients will have significant disease within five years of organ transplantation, and 90% will have significant TV a decade after their surgery. TV can involve the entire length of the transplanted arterial bed, including penetrating intraorgan arterioles. Indeed, the luminal narrowing of such penetrating vessels may be the most functionally significant because arterioles represent the major contributors to tissue vascular resistance. Because of the diffuseness of TV involvement in the allograft vascular bed, the only currently definitive therapy requires re-transplantation. Nevertheless, as we better understand the pathogenesis and critical mediators of these lesions, pharmacological advances can be anticipated. Other articles in this thematic review series focus on the specifics of the inciting injury, the cytokines and chemokines that drive TV development, and the nature of the recruited cells in TV lesions, as well as the pathogenic similarities between TV and other vascular lesions such as atherosclerosis. This review focuses on the mechanisms of vascular wall remodeling in TV, including the intimal accumulation of smooth muscle-like cells and associated extracellular matrix, medial smooth muscle cell degeneration, and adventitial fibrosis. A brief overview highlights the aneurysmal changes that can accrue when vessel wall inflammation has a cytokine profile distinct from the typical proinflammatory interferon-gamma-dominated milieu.

Principles and therapeutic implications of angiogenesis, vasculogenesis and arteriogenesis

The vasculature is the first organ to arise during development. Blood vessels run through virtually every organ in the body (except the avascular cornea and the cartilage), assuring metabolic homeostasis by supplying oxygen and nutrients and removing waste products. Not surprisingly therefore, vessels are critical for organ growth in the embryo and for repair of wounded tissue in the adult. Notably, however, an imbalance in angiogenesis (the growth of blood vessels) contributes to the pathogenesis of numerous malignant, inflammatory, ischaemic, infectious and immune disorders. During the last two decades, an explosive interest in angiogenesis research has generated the necessary insights to develop the first clinically approved anti-angiogenic agents for cancer and blindness. This novel treatment is likely to change the face of medicine in the next decade, as over 500 million people worldwide are estimated to benefit from pro- or anti-angiogenesis treatment. In this following chapter, we discuss general key angiogenic mechanisms in health and disease, and highlight recent developments and perspectives of anti-angiogenic therapeutic strategies.

Characterization of neointima lesions associated with arteriovenous fistulas in a mouse model

Arteriovenous fistulas (AVFs) are usually used for vascular access in the provision of hemodialysis, but AVFs have a 1-year patency rate of only about 60% owing to stenosis. As the molecular mechanisms behind AVF neointimal hyperplasia remain largely unknown, representative models in transgenic mice could be useful to study this process at the genetic level. Hence, we characterized neointimal lesion formation in a model of AVF recently developed in the mouse, where the common carotid artery was end-to-side sutured to jugular vein in C57BL/6J mice. At the site of anastomosis, arterial wall thickening was observed as early as 1 week after surgery (fourfold) and progressed to six- and 10-fold original thickness in carotid arteries after 2 and 3 weeks, respectively. The lumen of the carotid artery was significantly narrowed owing to neointima hyperplasia, and thrombosis was observed in the vein wall opposite to the anastomosed artery. Histological and immunohistochemical analyses revealed that 3-week neointimal lesions consisted of abundant smooth muscle cells (alpha-actin(+)) and a small number of membrane attack complex-1+ macrophages. Furthermore, using chimeric mice receiving bone marrow from transgenic mice expressing the LacZ gene in smooth muscle (SM-LacZ), it was found that bone marrow stem cells did not contribute to smooth muscle cell accumulation in neointimal lesions of AVF arteries. Thus, this model, which reproduces many of the features of human AVF, should prove useful for our understanding of the mechanism of neointimal formation and to evaluate the effects of drugs and gene therapy on this disease.

PAF-receptor is preferentially expressed in a distinct synthetic phenotype of smooth muscle cells cloned from human internal thoracic artery: functional implications in cell migration

Platelet-activating-Factor (PAF) and its structural analogues formed upon low density lipoprotein oxidation are involved in atherosclerotic plaque formation and may signal through PAF-receptor (PAF-R) expressed in human macrophages and in certain smooth muscle cells (SMCs) in the media, but rarely in the intima of human plaques. Our aim was to determine which SMC phenotype expresses PAF-R and whether this receptor is functional in cell migration. Circulating SMC progenitors and two phenotypically distinct clones of proliferative, epithelioid phenotype vs contractile, spindle-shaped SMCs from the media of adult internal thoracic artery were studied for the presence of PAF-receptor (PAF-R). The levels of specific mRNA were obtained by reverse transcription/real-time PCR, the protein expression was deduced from immunohistochemistry staining, and the functional transmigration assay was performed by Boyden chamber-type chemotaxis assay. Only SMCs of spindle-shape and synthetic phenotype expressed both mRNA and PAF-R protein and in the functional test migrated at low concentrations of PAF. Two unrelated, specific PAF-R antagonists inhibited PAF-induced migration, but did not modify the migration initiated by PDGF. The presence of functional PAF-R in arterial spindle-shaped SMCs of synthetic phenotype may be important for their migration from the media into the intima and atherosclerotic plaques formation.

The impact of progenitor cells in atherosclerosis

During the pathogenesis of arteriosclerosis, endothelial cells on the arterial wall damaged by various means were initially thought to be replaced by replication of neighboring cells. Smooth-muscle cells (SMCs) were also thought to migrate from the media into the intima, where they constituted arteriosclerotic lesions. This concept has been challenged, however, by the discovery that progenitor cells in the circulation and adventitia contribute to endothelial repair and SMC accumulation. Studies have demonstrated that atherosclerosis is a pathophysiologic process initiated by endothelial death in specific areas, such as bifurcation regions, and with subsequent replacement by endothelial progenitor cells. Differentiation of the neoendothelial cells into mature endothelium takes several days or weeks, during which LDL deposits in the intima. Blood mononuclear cells also adhere to neoendothelial cells and migrate into the subendothelial space. Meanwhile, progenitor cells from blood and the adventitia migrate into the intima, where they proliferate and differentiate into neo-SMCs. All risk factors for atherosclerosis can exert their effects on the vessel wall partly via increase in endothelial turnover, inhibition of progenitor-cell differentiation, and promotion of smooth-muscle and macrophage accumulation in lesions. Thus, progenitor cells comprise the main cell source responsible for the formation of atherosclerotic lesions, which appear in the context of inflammatory disease. Here I provide an update on research and discuss the role of progenitor cells in the pathogenesis of atherosclerosis.

Adventitial progenitor cells contribute to arteriosclerosis

Accumulating evidence indicates the involvement of vascular progenitor cells in the development of arteriosclerosis, including transplant arteriosclerosis, angioplasty-induced restenosis, vein graft atherosclerosis, and spontaneous atherosclerosis. Recently, it was found that the adventitia of the arterial wall contains a large number of progenitor cells, which can differentiate into smooth muscle cells in vitro and in vivo. These progenitor cells were able to migrate from the adventitia into the intima, where they accumulate to contribute to atherosclerotic lesions of vein grafts in apoE-deficient mice. Thus, these cells may be a source of smooth muscle cells and might have implications for cellular, genetic, and tissue engineering approaches to vascular disease.

An in vivo analysis of hematopoietic stem cell potential: hematopoietic origin of cardiac valve interstitial cells

Recent studies evaluating hematopoietic stem cell (HSC) potential raise the possibility that, in addition to embryonic sources, adult valve fibroblasts may be derived from HSCs. To test this hypothesis, we used methods that allow the potential of a single HSC to be evaluated in vivo. This was achieved by isolation and clonal expansion of single lineage-negative (Lin-), c-kit(+), Sca-1(+), CD34- cells from the bone marrow of mice that ubiquitously express enhanced green fluorescent protein (EGFP) combined with transplantation of individual clonal populations derived from these candidate HSCs into a lethally irradiated congenic non-EGFP mouse. Histological analyses of valve tissue from clonally engrafted recipient mice revealed the presence of numerous EGFP+ cells within host valves. A subpopulation of these cells exhibited synthetic properties characteristic of fibroblasts, as evidenced by their expression of mRNA for procollagen 1alpha1. Further, we show by Y-chromosome-specific fluorescence in situ hybridization analysis of female-to-male transplanted mice that the EGFP+ valve cells are the result of HSC-derived cell differentiation and not the fusion of EGFP+ donor cells with host somatic cells. Together, these findings demonstrate HSC contribution to the adult valve fibroblast population.

Progenitor cells in vascular disease

Stem cell research has the potential to provide solutions to many chronic diseases via the field of regeneration therapy. In vascular biology, endothelial progenitor cells (EPCs) have been identified as contributing to angiogenesis and hence have therapeutic potential to revascularise ischaemic tissues. EPCs have also been shown to endothelialise vascular grafts and therefore may contribute to endothelial maintenance. EPC number has been shown to be reduced in patients with cardiovascular disease, leading to speculation that atherosclerosis may be caused by a consumptive loss of endothelial repair capacity. Animal experiments have shown that EPCs reendothelialise injured vessels and that this reduces neointimal formation, confirming that EPCs have an atheroprotective effect. Smooth muscle cell accumulation in the neointimal space is characteristic of many forms of atherosclerosis, however the source of these cells is now thought to be from smooth muscle progenitor cells (SMPCs) rather than the adjacent media. There is evidence for the presence of SMPCs in the adventitia of animals and that SMPCs circulate in human blood. There is also data to support SMPCs contributing to neointimal formation but their origin remains unknown. This article will review the roles of EPCs and SMPCs in the development of vascular disease by examining experimental data from in vitro studies, animal models of atherosclerosis and clinical studies.

The role of stem cells in the response to myocardial and vascular wall injury

Myocardium has long been considered a terminally differentiated tissue, with injury invariably leading to replacement with fibrosis. However, new reports suggest potential roles for circulating or endogenous stem cells in repopulating myocardium after irreversible injury. Unfortunately, these benefits may represent a double-edged sword. While offering exciting possibilities for therapy following myocardial infarction (MI), stem cells are also increasingly implicated in contributing to a number of vascular pathologies, including the formation of graft arterial disease (GAD) after cardiac transplantation. In this review, the function of stem cells in repopulating infarcted myocardium and their role in the pathogenesis of intimal hyperplastic lesions such as GAD will be discussed.

Bone marrow-derived recipient cells in murine transplanted hearts: potential roles and the effect of immunosuppression

Currently, there is intense debate regarding the origin of reparative cells in injured hearts and vasculature. To determine the contribution of recipient bone marrow (BM)-derived cells to the regeneration of cells in the vasculature of transplanted hearts and to examine the effect of immunosuppression on this phenomenon, we evaluated the fate of green fluorescent protein (GFP)-positive recipient BM cells in non-GFP-expressing cardiac allografts. C57BL/6 BM-GFP chimeric recipients underwent cardiac transplantation. Allografts were immunosuppressed with tacrolimus for 14 or 30 days post-transplantation or were saline treated. Hearts were excised and stained with markers for endothelial cells (EC) or smooth muscle cells (SMC). Colocalization with BM-derived recipient cells was evaluated using confocal microscopy with three-dimensional image analysis. Immunosuppression with tacrolimus did not affect the frequency of recipient BM-derived cell chimerism as EC or SMC phenotypes. A higher frequency of EC chimerism was found at 14 days as compared to 30 days post-transplantation in allograft hearts. BM-derived recipient cells are recruited to areas of donor vascular injury with intercalation of recipient EC and SMC in the setting of ongoing alloimmune recognition of the allograft. Our findings confirm that immunosuppression with tacrolimus does not affect the frequency of recipient BM-derived cell repopulation at an early time point 14 days post-transplantation. EC repopulation by BM-derived recipient cells was found to be an early event in transplanted allograft hearts, which decreased in frequency over time.

Role of progenitor cells in transplant arteriosclerosis

To date, chronic transplant dysfunction (CTD) is recognized as the major cause of transplant loss long term after transplantation. CTD has the remarkable histologic feature that the luminal areas of the intragraft arteries become obliterated as a result of occlusive neointima formation. Neointimal lesions contain predominantly vascular smooth muscle cells (VSMCs) and extracellular matrix admixed with inflammatory cells. At the luminal side, neointimal lesions are covered with a monolayer of endothelial cells (ECs). The etiology of transplant arteriosclerosis (TA) is largely unknown, and adequate prevention and treatment protocols are not available. In contrast to the largely accepted "response-to-injury" hypothesis for the development of TA that attributes an important role to graft-derived ECs and VSMCs, recent data indicate that host-derived vascular progenitor cells play a major role in the development of TA. The process leading to TA appears to be heterogeneous, and neointimal ECs and VSMCs can be recruited from different sources, possibly depending on the severity and duration of vascular damage. These data suggest a significant role of host-derived circulating EC/VSMC progenitor cells, which may be partly bone marrow derived. Circulating vascular progenitor cells are potential targets for therapeutic intervention to ameliorate TA development. Therefore, identification of mediators and cellular mechanisms that promote recruitment of vascular progenitors to sites of injury is warranted to dissect their detrimental and possible beneficial effects in the development of TA.

Impairment of recipient cytolytic activity attenuates allograft vasculopathy

We investigated the role of CD4+ and CD8+ T subsets as well as T cell cytolytic effector mechanisms in the aortic allograft model of allograft vasculopathy using CD4 and CD8 gene knockout mice (CD4(-/-), CD8(-/-)) and mice deficient in cytolytic effector pathways. Medial apoptosis at 2 weeks was reduced in CD8(-/-) mice and in mice where cytotoxic T cell activity was compromised. At 8 weeks, substantial medial damage was observed in wild-type (WT) and CD4(-/-) recipients but medial preservation was evident in CD8(-/-) mice and in mice with impaired cytotoxic T cell activity. The intima/media ratio, a comprehensive measure of allograft vasculopathy, was similar in WT and CD4(-/-) recipients but was significantly reduced in CD8(-/-) mice and mice with impaired cytotoxic T cell activity. These data indicate that CD8+ T cells contribute to the vascular remodeling that is characteristic of allograft vasculopathy. They also show that CD8+ T cells participate in allograft vasculopathy in the absence of CD4+ T cell help. We further demonstrated that WT mice exhibited robust allograft vasculopathy in the presence of cyclosporin A immunosuppression but that allograft vasculopathy was ablated in cyclosporin-treated CD8(-/-) mice. This supports the hypothesis that non-CD8+ T cell effector mechanisms are sensitive to calcineurin inhibitor therapy but that CD8+ T cell-mediated allograft vasculopathy is refractory to such treatment. Taken together, our data suggest that CD8+ T cells contribute to the induction of vascular remodeling in allograft vasculopathy and provide evidence that novel therapies which target CD8+ T cell effector function might be effective in mitigating AV in the clinical setting.