Wall shear modulation of cytokines in early vein grafts

Integration of substrate- and flow-derived stresses in endothelial cell mechanobiology

Endothelial cells (ECs) lining all blood vessels are subjected to large mechanical stresses that regulate their structure and function in health and disease. Here, we review EC responses to substrate-derived biophysical cues, namely topography, curvature, and stiffness, as well as to flow-derived stresses, notably shear stress, pressure, and tensile stresses. Because these mechanical cues in vivo are coupled and are exerted simultaneously on ECs, we also review the effects of multiple cues and describe burgeoning in vitro approaches for elucidating how ECs integrate and interpret various mechanical stimuli. We conclude by highlighting key open questions and upcoming challenges in the field of EC mechanobiology.

Distinct subsets of T cells and macrophages impact venous remodeling during arteriovenous fistula maturation

Patients with end-stage renal failure depend on hemodialysis indefinitely without renal transplantation, requiring a long-term patent vascular access. While the arteriovenous fistula (AVF) remains the preferred vascular access for hemodialysis because of its longer patency and fewer complications compared with other vascular accesses, the primary patency of AVF is only 50-60%, presenting a clinical need for improvement. AVF mature by developing a thickened vascular wall and increased diameter to adapt to arterial blood pressure and flow volume. Inflammation plays a critical role during vascular remodeling and fistula maturation; increased shear stress triggers infiltration of T-cells and macrophages that initiate inflammation, with involvement of several different subsets of T-cells and macrophages. We review the literature describing distinct roles of the various subsets of T-cells and macrophages during vascular remodeling. Immunosuppression with sirolimus or prednisolone reduces neointimal hyperplasia during AVF maturation, suggesting novel approaches to enhance vascular remodeling. However, M2 macrophages and CD4+ T-cells play essential roles during AVF maturation, suggesting that total immunosuppression may suppress adaptive vascular remodeling. Therefore it is likely that regulation of inflammation during fistula maturation will require a balanced approach to coordinate the various inflammatory cell subsets. Advances in immunosuppressive drug development and delivery systems may allow for more targeted regulation of inflammation to improve vascular remodeling and enhance AVF maturation.

Activation of Interleukin-1 Beta in Arterialized Vein Grafts and the Influence of the -511C/T IL-1β Gene Polymorphism

The interleukin-1 family is associated with innate immunity and inflammation. The latter has been linked to the genesis of cardiovascular diseases. We, therefore, investigated whether interleukin-1 beta (IL-1β) is activated during arterialization of vein grafts. First, we examined the activation of IL-1β using the rat arterialized jugular vein serially sampled for up to 90 days. IL-1β expression increased 18 times on day 1 in the arterialized rat jugular vein and remained five times above nonarterialized vein levels for up to 90 days. Similarly, IL-1β expression increased early (1-5 days) in human vein graft autopsy samples compared with late phases (1-4 years). Activation was also detected in ex vivo arterialized human saphenous veins. Upon stratification of the results, we uncovered a T allele promoter attenuating effect in IL-1β activation in response to hemodynamic stress. Altogether, the results show that IL-1β is activated during arterialization of vein grafts in rats and humans, and this response is modulated by -511C/T IL-1β gene polymorphism. It is tempting to speculate that the activation of IL-1β, and consequently local inflammation, modulates early vascular remodeling and that the gene polymorphism may be useful in predicting outcomes or assisting in interventions.

Inflammation in Vein Graft Disease

Bypass surgery is one of the most frequently used strategies to revascularize tissues downstream occlusive atherosclerotic lesions. For venous bypass surgery the great saphenous vein is the most commonly used vessel. Unfortunately, graft efficacy is low due to the development of vascular inflammation, intimal hyperplasia and accelerated atherosclerosis. Moreover, failure of grafts leads to significant adverse outcomes and even mortality. The last couple of decades not much has changed in the treatment of vein graft disease (VGD). However, insight is the cellular and molecular mechanisms of VGD has increased. In this review, we discuss the latest insights on VGD and the role of inflammation in this. We discuss vein graft pathophysiology including hemodynamic changes, the role of vessel wall constitutions and vascular remodeling. We show that profound systemic and local inflammatory responses, including inflammation of the perivascular fat, involve both the innate and adaptive immune system.

Vein graft failure: from pathophysiology to clinical outcomes

Occlusive arterial disease is a leading cause of morbidity and mortality worldwide. Aside from balloon angioplasty, bypass graft surgery is the most commonly performed revascularization technique for occlusive arterial disease. Coronary artery bypass graft surgery is performed in patients with left main coronary artery disease and three-vessel coronary disease, whereas peripheral artery bypass graft surgery is used to treat patients with late-stage peripheral artery occlusive disease. The great saphenous veins are commonly used conduits for surgical revascularization; however, they are associated with a high failure rate. Therefore, preservation of vein graft patency is essential for long-term surgical success. With the exception of 'no-touch' techniques and lipid-lowering and antiplatelet (aspirin) therapy, no intervention has hitherto unequivocally proven to be clinically effective in preventing vein graft failure. In this Review, we describe both preclinical and clinical studies evaluating the pathophysiology underlying vein graft failure, and the latest therapeutic options to improve patency for both coronary and peripheral grafts.

The Biology of Hemodialysis Vascular Access Failure

Arteriovenous fistulas (AVFs) are essential for patients and clinicians faced with end-stage renal disease (ESRD). While this method of vascular access for hemodialysis is preferred to others due to its reduced rate of infection and complications, they are plagued by intimal hyperplasia. The pathogenesis of intimal hyperplasia and subsequent thrombosis is brought on by uremia, hypoxia, and shear stress. These forces upregulate inflammatory and proliferative cytokines acting on leukocytes, fibroblasts, smooth muscle cells, and platelets. This activation begins initially with the progression of uremia, which induces platelet dysfunction and primes the body for an inflammatory response. The vasculature subsequently undergoes changes in oxygenation and shear stress during AVF creation. This propagates a strong inflammatory response in the vessel leading to cellular proliferation. This combined response is then further subjected to the stressors of cannulation and dialysis, eventually leading to stenosis and thrombosis. This review aims to help interventional radiologists understand the biological changes and pathogenesis of access failure.

CARD9 mediates necrotic smooth muscle cell-induced inflammation in macrophages contributing to neointima formation of vein grafts

AIMS

Inflammation plays an important role in the neointima formation of grafted veins. However, the initiation of inflammation in grafted veins is still unclear. Here, we investigated the role and underlying mechanism of an innate immunity signalling protein, caspase-associated recruitment domain 9 (CARD9) in vein grafts in mice.

METHODS AND RESULTS

In early murine vein grafts, we observed robust death of smooth muscle cells (SMCs), which was accompanied by infiltration of macrophages and expression of pro-inflammatory cytokines. Meanwhile, SMC necrosis was associated with the expression of pro-inflammatory cytokines in macrophages in vitro. To explore the mediators of necrotic SMC-induced inflammation in grafted veins from mice, we examined the expression of CARD family proteins and found CARD9 highly expressed in infiltrated macrophages of grafted veins. CARD9-knockout (KO) inhibited necrotic SMC-induced pro-inflammatory cytokine expression and NF-κB activation. Furthermore, CARD9-KO suppressed necrotic SMC-induced expression of VEGF in macrophages. Finally, CARD9-KO decreased neointima formation of grafted veins in mice.

CONCLUSION

The innate immune protein CARD9 in macrophages may mediate necrotic SMC-induced inflammation by activating NF-κB and contributed to neointima formation in the vein grafts.

A Multiscale Computational Framework to Understand Vascular Adaptation

The failure rate for vascular interventions (vein bypass grafting, arterial angioplasty/stenting) remains unacceptably high. Over the past two decades, researchers have applied a wide variety of approaches to investigate the primary failure mechanisms, neointimal hyperplasia and aberrant remodeling of the wall, in an effort to identify novel therapeutic strategies. Despite incremental progress, specific cause/effect linkages among the primary drivers of the pathology, (hemodynamic factors, inflammatory biochemical mediators, cellular effectors) and vascular occlusive phenotype remain lacking. We propose a multiscale computational framework of vascular adaptation to develop a bridge between theory and experimental observation and to provide a method for the systematic testing of relevant clinical hypotheses. Cornerstone to our model is a feedback mechanism between environmental conditions and dynamic tissue plasticity described at the cellular level with an agent based model. Our implementation (i) is modular, (ii) starts from basic mechano-biology principle at the cell level and (iii) facilitates the agile development of the model.

Time and flow-dependent changes in the p27(kip1) gene network drive maladaptive vascular remodeling

OBJECTIVE

Although clinical studies have identified that a single nucleotide polymorphism in the p27(kip1) gene is associated with success or failure after vein bypass grafting, the underlying mechanisms for this difference are not well defined. Using a high-throughput approach in a flow-dependent vein graft model, we explored the differences in p27(kip1)-related genes that drive the enhanced hyperplastic response under low-flow conditions.

METHODS

Bilateral rabbit carotid artery interposition grafts with jugular vein were placed with a unilateral distal outflow branch ligation to create differential flow states. Grafts were harvested at 2 hours and at 1, 3, 7, 14, and 28 days after implantation, measured for neointimal area, and assayed for cell proliferation. Whole-vessel messenger RNA was isolated and analyzed using an Affymetrix (Santa Clara, Calif) gene array platform. Ingenuity Pathway Analysis (Ingenuity, Redwood City, Calif) was used to identify the gene networks surrounding p27(kip1). This gene set was then analyzed for temporal expression changes after graft placement and for differential expression in the alternate flow conditions.

RESULTS

Outflow branch ligation resulted in an eightfold difference in mean flow rates throughout the 28-day perfusion period (P < .001). Flow reduction led to a robust hyperplastic response, resulting in a significant increase in intimal area by 7 days (0.13 ± 0.04 mm(2) vs 0.014 ± 0.006 mm(2); P < .005) and progressive growth to 28 days (1.37 ± 0.05 mm(2) vs 0.39 ± 0.06 mm(2); P < .001). At 7 days, low-flow grafts demonstrated a burst of actively dividing intimal cells (36.4 ± 9.4 cells/mm(2) vs 11.5 ± 1.9 cells/mm(2); P = .04). Sixty-five unique genes within the microarray were identified as components of the p27(kip1) network. At a false discovery rate of 0.05, 26 genes demonstrated significant temporal changes, and two dominant patterns of expression were identified. Class comparison analysis identified differential expression of 11 genes at 2 hours and seven genes and 14 days between the high-flow and low-flow grafts (P < .05). At 2 hours, oncostatin M and cadherin 1 were the most differentially expressed. Cadherin 1 and protein kinase B exhibited the greatest differential expression at 14 days.

CONCLUSIONS

Alterations in flow and shear stress result in divergent patterns of vein graft remodeling. Associated with the dramatic increase in neointimal expansion observed in low-flow vs high-flow grafts is a subset of differentially expressed p27(kip1)-associated genes that correlate with critical stages in the adaptive response. These represent potential biologic targets whose activity may be altered to augment maladaptive vascular remodeling.

Vein graft adaptation and fistula maturation in the arterial environment

Veins are exposed to the arterial environment during two common surgical procedures, creation of vein grafts and arteriovenous fistulae (AVF). In both cases, veins adapt to the arterial environment that is characterized by different hemodynamic conditions and increased oxygen tension compared with the venous environment. Successful venous adaptation to the arterial environment is critical for long-term success of the vein graft or AVF and, in both cases, is generally characterized by venous dilation and wall thickening. However, AVF are exposed to a high flow, high shear stress, low-pressure arterial environment and adapt mainly via outward dilation with less intimal thickening. Vein grafts are exposed to a moderate flow, moderate shear stress, high-pressure arterial environment and adapt mainly via increased wall thickening with less outward dilation. We review the data that describe these differences, as well as the underlying molecular mechanisms that mediate these processes. Despite extensive research, there are few differences in the molecular pathways that regulate cell proliferation and migration or matrix synthesis, secretion, or degradation currently identified between vein graft adaptation and AVF maturation that account for the different types of venous adaptation to arterial environments.

Fluid shear stress-induced osteoarthritis: roles of cyclooxygenase-2 and its metabolic products in inducing the expression of proinflammatory cytokines and matrix metalloproteinases

The mechanical overloading of cartilage is involved in the pathophysiology of osteoarthritis (OA) by both biochemical and mechanical pathways. The application of fluid shear stress to chondrocytes recapitulates the earmarks of OA, as evidenced by the release of proinflammatory cytokines (PICs), matrix metalloproteinases (MMPs), and apoptotic factors. Dysregulations or mutations in these genes might directly cause OA in addition to determining the stage at which OA becomes apparent, the joint sites involved, and the severity of the disease and how rapidly it progresses. However, the underlying mechanisms remain unknown. In this review, we propose that the dysregulation of cyclooxygenase-2 (COX-2) is associated with fluid shear stress-induced OA via its metabolic products at different stages of the disease. Indeed, high fluid shear stress rapidly induces the production of PICs and MMPs via COX-2-derived prostaglandin (PG)E2 at the early stage of OA. In contrast, prolonged shear exposure (>12 h) aggravates the condition by concurrently up-regulating the expression of proapoptotic genes and down-regulating the expression of antiapoptotic genes in a 15-deoxy-Δ (12,14)-prostaglandin J2 (15d-PGJ2)-dependent manner at the late stage of disease. These observations may help to resolve long-standing questions in OA progression and provide insight for development of strategies to treat and combat OA.

Perivascular innate immune events modulate early murine vein graft adaptations

OBJECTIVE

Innate immunity drives numerous cardiovascular pathologies. Vein bypass grafting procedures are frequently accompanied by low-grade wound contamination. We hypothesized that a peri-graft innate immune challenge, via an outside-in route, augments inflammatory responses, which subsequently drive a component of negative vein graft wall adaptations; moreover, adipose tissue mediates this immune response.

METHODS

The inferior vena cava from a donor mouse was implanted into the common carotid artery of a recipient mouse utilizing a validated cuff technique (9-week-old male C57BL/6J mice). Slow-release low-dose (5 μg) lipopolysaccharide (LPS) (n = 9) or vehicle (n = 9) was applied peri-graft; morphologic analysis was completed (day 28). In parallel, vein-grafted mice received peri-graft LPS (n = 12), distant subcutaneous LPS (n = 6), or vehicle (n = 12), then day-1 and -3 harvest of grafts and adipose tissue for cytokines and toll-like receptor (TLR) signaling mRNA expression (qRT-PCR).

RESULTS

All recipient mice survived, and all vein grafts were patent. Acute low-dose local LPS challenge enhanced vein graft lumen loss (P = .04) and tended to augment intimal hyperplasia (P = .06). The surgical trauma of vein grafting universally upregulated key pro- and anti-inflammatory mediators within the day-1 graft wall, but varied on TLR signaling gene expression. Local and distant LPS accentuated these patterns until at least postoperative day 3. LPS challenge enhanced the inflammatory response in adipose tissue (locally > distantly); local LPS upregulated adipose TLR-4 dramatically.

CONCLUSIONS

Perivascular and distant inflammatory challenges potentiate the magnitude and duration of inflammatory responses in the early vein graft wall, negatively modulating wall adaptations, and thus, potentially contribute to vein graft failure. Furthermore, surgery activates innate immunity in adipose tissue, which is augmented (regionally > systemically) by LPS. Modulation of these local and distant inflammatory signaling networks stands as a potential strategy to enhance the durability of vascular interventions such as vein grafts.

Lack of interleukin-1 signaling results in perturbed early vein graft wall adaptations

BACKGROUND

Vein grafts fail as the result of wall maladaptations to surgical injury and hemodynamic perturbations. Interleukin-1 signaling has emerged as an important mediator of the vascular response to trauma and hemodynamically induced vascular lesions. We therefore hypothesized that interleukin-1 signaling drives early vein graft wall adaptations.

METHODS

Using interleukin-1 type I receptor knockout (IL-1RI(-/-)) and wild-type (B6129SF2/J) mice, we investigated morphologic changes 28 days after interposition isograft from donor inferior vena cava to recipient carotid artery, without (n = 19) or with (n = 13) outflow restriction. The impact of mouse strain on the response to vein arterialization also was evaluated between B6129SF2/J (n = 18) and C57BL/6J (n = 19) mice.

RESULTS

No differences were observed in the traditional end points of intimal thickness and calculated luminal area, yet media+adventitia thickness of the vein graft wall of IL-1RI(-/-) mice was 44% to 52% less than wild-type mice, at the both proximal (P < .01, P < .01) and distal (P = .054, P < .01) portions of vein grafts, for both normal flow and low flow, respectively. Compared with the C57BL/6J strain, B6129SF2/J mice exhibited no difference in vein graft intimal thickness but 2-fold greater media+adventitia thickness (P < .01).

CONCLUSION

When lacking IL-1 signaling, the vein graft wall adapts differently compared with the injured artery, showing typical intima hyperplasia although attenuated media+adventitia thickening. B6129SF2/J mice exhibit more media+adventitia response than C57BL/6J mice. The inflammatory networks that underlie the vein response to arterialization hold many roles in the adaptation of the total wall; thus, the utility of anti-inflammatory approaches to extend the durability of vein grafts comes into question.

Is endoscopic harvesting bad for saphenous vein graft patency in coronary surgery?

PURPOSE OF REVIEW

Endoscopic vein harvest (EVH) has quickly been adopted as the standard-of-care for coronary artery bypass grafting (CABG). Despite clear advantages in terms of wound morbidity, healing, pain, and patient satisfaction, data from recent large clinical trials have called the safety of this technique into question.

RECENT FINDINGS

Post-hoc analyses of a variety of prospective trials have suggested EVH is associated with decreased graft patency, higher rates of cardiovascular complications (e.g. myocardial infarction, need for repeat revascularization) and mortality. Imaging studies of veins procured by EVH have revealed retained clot and vascular injury, particularly during the 'learning curve' of the technician. These findings may alter the quality of the conduit and, therefore, the outcome of the bypass graft. Elucidating the mechanisms that underlie any differences in results produced by the open and endoscopic procedures would help better inform clinical practice and the development of targeted strategies to improve EVH.

SUMMARY

Clear clinical advantages over traditional open vein harvest have allowed EVH to rapidly become the standard-of-care for harvesting of one or more vein grafts during CABG. The quality of these conduits, suggested to be equivalent by early studies, has come into question as groups with varying levels of experience have adopted the endoscopic technique. Elucidating the principles of 'best practice' for vein harvest will likely help shorten the learning curve and improve the safety of EVH.

Mouse vein graft hemodynamic manipulations to enhance experimental utility

Mouse models serve as a tool to study vein graft failure. However, in wild-type mice, there is limited intimal hyperplasia, hampering efforts to identify anti-intimal hyperplasia therapies. Furthermore, vein graft wall remodeling has not been well quantified in mice. We hypothesized that simple hemodynamic manipulations can reproducibly augment intimal hyperplasia and remodeling end points in mouse vein grafts, thereby enhancing their experimental utility. Mouse inferior vena cava-to-carotid interposition isografts were completed using an anastomotic cuff technique. Three flow restriction manipulations were executed by ligating outflow carotid branches, creating an outflow common carotid stenosis, and constructing a midgraft stenosis. Flowmetry and ultrasonography were used perioperatively and at day 28. All ligation strategies decreased the graft flow rate and wall shear stress. Morphometry showed that intimal thickness increased by 26% via carotid branch ligation and by 80% via common carotid stenosis. Despite similar mean flow rates and shear stresses among the three manipulations, the flow waveform amplitudes were lowest with common carotid stenosis. The disordered flow of the midgraft stenosis yielded poststenotic dilatation. The creation of an outflow common carotid stenosis generates clinically relevant (poor runoff) vein graft low wall shear stress and offers a technically flexible method for enhancing the intimal hyperplasia response. Midgraft stenosis exhibits poststenotic positive wall remodeling. These reproducible approaches offer novel strategies for increasing the utility of mouse vein graft models.

Differential endothelial gap junction expression in venous vessels exposed to different hemodynamics

After being anastomosed with the artery, vein graft is exposed to abruptly increased hemodynamic stresses. These hemodynamic stresses may change the profile of endothelial gap junction expression as demonstrated in the artery, which may subsequently play active roles in physiological adaptation or pathophysiological changes of the vein grafts. We investigated the endothelial expression of gap junction in the venous vessels exposed to different hemodynamic stresses. Immunocytochemical analysis of the endothelial Cx expression was performed by observing the whole mounts of inferior vena cava (IVC) of aortocaval fistula (ACF) rats or IVC-banded ACF rats using confocal microscope. Immunocytochemical analysis demonstrated that in the endothelium of the native vein, the gap-junctional spot numbers (GJSNs) and the total gap-junctional areas (TGJAs) of Cx40 and Cx43 were lower than those of the thoracic aorta and that Cx37 was hardly detectable. In the IVCs of ACF rats, which were demonstrated to be exposed to a hemodynamic condition of high flow velocity and low pressure, the GJSNs and the TGJAs of all three Cxs were increased. In the IVCs of IVC-banded ACF rats, which were exposed to a hemodynamic condition of high pressure and low flow velocity, the GJSNs and the TGJAs of Cx37 increased markedly and those of Cx40 and Cx43 remained without significant changes. In conclusion, the endothelial expressions of gap junctions in the native veins were lower than those of the arteries. When exposed to different hemodynamic stresses, the gap junctions were expressed in specific patterns.

Mechanisms of vein graft adaptation to the arterial circulation: insights into the neointimal algorithm and management strategies

For patients with coronary artery disease or limb ischemia, placement of a vein graft as a conduit for a bypass is an important and generally durable strategy among the options for arterial reconstructive surgery. Vein grafts adapt to the arterial environment, and the limited formation of intimal hyperplasia in the vein graft wall is thought to be an important component of successful vein graft adaptation. However, it is also known that abnormal, or uncontrolled, adaptation may lead to abnormal vessel wall remodeling with excessive neointimal hyperplasia, and ultimately vein graft failure and clinical complications. Therefore, understanding the venous-specific pathophysiological and molecular mechanisms of vein graft adaptation are important for clinical vein graft management. Of particular importance, it is currently unknown whether there exist several specific distinct molecular differences in the venous mechanisms of adaptation that are distinct from arterial post-injury responses; in particular, the participation of the venous determinant Eph-B4 and the vascular protective molecule Nogo-B may be involved in mechanisms of vessel remodeling specific to the vein. This review describes (1) venous biology from embryonic development to the mature quiescent state, (2) sequential pathologies of vein graft neointima formation, and (3) novel candidates for strategies of vein graft management. Scientific inquiry into venous-specific adaptation mechanisms will ultimately provide improvements in vein graft clinical outcomes.

Rationale and practical techniques for mouse models of early vein graft adaptations

Mouse models serve as relatively new yet powerful research tools to study intimal hyperplasia and wall remodeling of vein bypass graft failure. Several model variations have been reported in the past decade. However, the approach demands thoughtful preparation, selected sophisticated equipment, microsurgical technical expertise, advanced tissue processing, and data acquisition. This review compares several described models and aims (building on our personal experiences) to practically aid the investigators who want to utilize mouse models of vein graft failure.

Interplay of CCR2 signaling and local shear force determines vein graft neointimal hyperplasia in vivo

Leukocytes play a central role in vein graft neointimal hyperplasia, which is significantly augmented under low shear conditions. The current concept is that shear force regulates leukocyte adhesion predominately through up-regulation of chemokines and growth factors within the graft wall. Using rabbit and murine vein graft models, we demonstrate that CC chemokine receptor 2/monocyte chemoattractant protein-1 mediated monocyte recruitment and a low shear environment act synergistically to augment neointimal hyperplasia development and removal of either of the conditions leads to a significant reduction in neointimal thickening. We propose a novel concept that the shear stress response element phenotypically stems from the complex interplay of the biological and physical microenvironments.

Degradation of the internal elastic laminae in vein grafts of rats with aortocaval fistulae: potential impact on graft vasculopathy

The internal elastic lamina (IEL) of vein grafts may be modified when exposed to arterialized hemodynamics. We investigated changes of the IEL in the inferior vena cava (IVC) of rats with aortocaval fistulae (ACF). In the IVC of ACF rats, both a markedly increased flow velocity and a mildly increased pressure were demonstrated. In the lower segment where hemodynamic changes were prominent, neointimal hyperplasia was prominently found. The IEL of the IVC in sham-operated rats, observed by confocal microscopy, was composed of parallel elastic fibers. In ACF rats, the IEL degenerated progressively after surgery. The elastic fibers were stretched and gradually became sparse, a change that was more prominent in the lower segment. Eight weeks after surgery, the IEL hardly existed in some areas of the lower segment. Electron microscopy revealed decreased densities and diameters of elastic fibers. Reverse transcriptase-polymerase chain reaction analysis revealed an up-regulation of potent elastases, cathepsins K and S, and matrix metalloproteinase-2 in the IVC of ACF rats. Results of immunohistochemical studies localized cathepsin expression predominantly to the luminal endothelium lining the IEL, suggesting involvement of elastinolysis in the degradation of the IEL. We demonstrated the degradation of the IEL in the vein graft of ACF rats, especially in the segment exposed to prominent hemodynamic changes. IEL degradation may contribute to the development of neointimal hyperplasia in vein grafts.

Adventitial delivery of platelet-derived endothelial cell growth factor gene prevented intimal hyperplasia of vein graft

BACKGROUND

Platelet-derived endothelial cell growth factor (PD-ECGF), also known as thymidine phosphorylase (TP) reportedly inhibits vascular smooth muscle cells (VSMCs) migration and proliferation. We hypothesized that adventitial administration of the PD-ECGF/TP gene will suppress intimal hyperplasia and prevent vein graft failure.

METHODS

The study used 68 female rabbits. Rabbit jugular vein was autogenously transplanted into carotid artery with a cuff anastomotic technique. To define vascular wall gene transfer efficiency, poloxamer hydrogel (20%) containing plasmid vector encoding the LacZ gene and different concentrations of trypsin (0%, 0.1%, 0.25%, and 0.5%, n = 5 for each group) was applied to the adventitia of the vein graft. Gene transfer efficiency was evaluated 7 days later by X-gal staining. An additional 48 rabbits received poloxamer hydrogel (20%) containing 0.25% trypsin and the human PD-ECGF/TP gene, LacZ gene, or saline. Intima thickness was evaluated at 2 and 8 weeks after grafting (n = 8 for each group at each time point). Transgene expression was examined by reverse transcriptase-polymerase chain reaction, immunoblotting assay, and immunohistochemical staining. Immunohistochemical staining was also used to determine VSMC proliferation, heme oxygenase-1 expression, and macrophage infiltration.

RESULTS

Incorporation of trypsin into the poloxamer hydrogel significantly increased vessel wall gene transfer. Trypsin at 0.25% and 0.5% resulted in higher gene transfer at the same level without effecting intimal hyperplasia and inflammation; thus, trypsin at 0.25% concentration was used for subsequent experiments. Compared with the LacZ and saline groups, grafts receiving the PD-ECGF/TP gene significantly reduced intimal thickness at 2 and 8 weeks after treatment. The ratio of proliferative VSMC was lower in PD-ECGF/TP treated grafts. Histologic examination of the PD-ECGF/TP transgene grafts demonstrated high expression of heme oxygenase-1, which has been reported to inhibit VSMC proliferation, suggesting that heme oxygenase-1 may be important in the inhibition effect of PD-ECGF/TP on VSMC. No neoplastic or morphologic changes were found in the remote organs.

CONCLUSIONS

A safe and highly efficient gene transfer method was developed by using poloxamer hydrogel and a low concentration of trypsin. Neointimal hyperplasia was significantly reduced by adventitial application of the PD-ECGF/TP gene to the vein graft. Our data suggest that adventitial delivery of the PD-ECGF/TP gene after grafting may be promising method for preventing vein graft failure.

Cytokines and the early vein graft: strategies to enhance durability

This brief review focuses on experimental studies linking the proinflammatory cytokine tumor necrosis factor-alpha to accelerated vein graft failure in the broader historical context of vein graft research. From some perspectives, the field appears ripe for transfer of cytokine knowledge and therapeutic approaches that have evolved in other systems to vascular surgery problems. However, the complexity of vein graft disease suggests that more robust research approaches, such as broadening of the scope beyond focus on single mediators and neointimal hyperplasia, will be necessary to reach translatable strategies to prolong human vein graft durability.

TNF-alpha and shear stress-induced large artery adaptations

BACKGROUND

Tumor necrosis factor-alpha (TNF-alpha) up-regulation has been associated with both low and high shear-induced arterial remodeling. To address this apparent paradox and to define the biology of TNF-alpha signaling in large arteries, we tested the hypotheses that differential temporal expression of TNF-alpha drives shear-regulated arterial remodeling.

MATERIALS AND METHODS

Both low- and high-shear environments in the same rabbit were surgically created for common carotid arteries. Common carotid arteries (n = 60 total) were harvested after d0, d1, d3, d7, and d14 and analyses included morphology, TNF-alpha, and IL-10 mRNA quantitation. In separate experiments, animals received pegylated soluble TNF-alpha Type 1 receptor (PEG sTNF-RI) or vehicle via either short- or long-term dosing to define the effect of TNF-alpha blockade.

RESULTS

The model yielded a 14-fold shear differential (P < 0.001) with medial thickening under low shear (P = 0.025), and evidence of outward remodeling with high shear (P = 0.007). Low shear immediately up-regulated TNF-alpha expression approximately 50 fold (P < 0.001) at d1. Conversely, high shear-induced delayed and sustained TNF-alpha expression (22-fold at d7, P = 0.012; 23-fold at d14, P = 0.007). Both low and high shear gradually induced IL-10 expression (P = 0.002 and P = 0.004, respectively). Neither short-term (5-day) nor long-term (14-day) blockage of TNF-alpha signaling resulted in treatment-induced changes in the remodeling of low- or high-shear arteries.

CONCLUSIONS

Shear stress differentially and temporally regulates TNF-alpha expression in remodeling large arteries. However, TNF-alpha blockage did not substantially impact the final shear-induced morphology, suggesting that large arteries can remodel in response to flow perturbations independent of TNF-alpha signaling.

Multi-cell agent-based simulation of the microvasculature to study the dynamics of circulating inflammatory cell trafficking

Leukocyte trafficking through the microcirculation and into tissues is central in angiogenesis, inflammation, and the immune response. Although the literature is rich with mechanistic detail describing molecular mediators of these processes, integration of signaling events and cell behaviors within a unified spatial and temporal framework at the multi-cell tissue-level is needed to achieve a fuller understanding. We have developed a novel computational framework that combines agent-based modeling (ABM) with a network flow analysis to study monocyte homing. A microvascular network architecture derived from mouse muscle was incorporated into the ABM. Each individual cell was represented by an individual agent in the simulation. The network flow model calculates hemodynamic parameters (blood flow rates, fluid shear stress, and hydrostatic pressures) throughout the simulated microvascular network. These are incorporated into the ABM to affect monocyte transit through the network and chemokine/cytokine concentrations. In turn, simulated monocytes respond to their local mechanical and biochemical environments and make behavioral decisions based on a rule set derived from independent literature. Simulated cell behaviors give rise to emergent leukocyte rolling, adhesion, and extravasation. Molecular knockout simulations were performed to validate the model, and predictions of monocyte adhesion, rolling, and extravasation show good agreement with the independently published corresponding mouse studies.

TGF-beta- and CTGF-mediated fibroblast recruitment influences early outward vein graft remodeling

Luminal shearing forces have been shown to impact both geometric remodeling and the development of intimal hyperplasia. Less well studied is the influence of intramural wall stresses on vessel growth and adaptation. Using a vein graft-fistula configuration to isolate the impact of circumferential wall stress, we identify the reorganization of adventitial myofibroblasts as the dominant histological event that limits early outward remodeling of vein grafts in response to elevated wall stress. We hypothesize that increased production of transforming growth factor-beta (TGF-beta) and connective tissue growth factor (CTGF) induces recruitment of myofibroblasts, promotes adventitial reorganization, and limits early outward remodeling in response to increased intramural wall stress. Vein grafts with a distal arteriovenous fistula in the neck of rabbits were constructed, resulting in a fourfold differential in circumferential wall stress. Using this model, we demonstrate 1) elevated wall stress augments the production of TGF-beta and CTGF, 2) increased TGF-beta expression and CTGF expression are correlated with the enhanced differentiation from fibroblasts to myofibroblasts, as evidenced by the significant increase in the alpha-actin-positive cells in adventitia, and 3) the levels of TGF-beta, CTGF, and alpha-actin are inversely correlated with the magnitude of outward remodeling of the graft wall. Increased wall stress after vein graft implantation appears to induce a TGF-beta- and CTGF-mediated recruitment of adventitial fibroblasts and a conversion to a myofibroblast phenotype. Although important in the maintenance of wall stability in the face of an increased mechanical load, this adventitial adaptation limits early outward remodeling of the vein conduit and may prove deleterious in maintaining long-term vein graft patency.

Tumor necrosis factor-alpha and the early vein graft

BACKGROUND

Tumor necrosis factor-alpha (TNF-alpha) has been implicated in the blood vessel wall response to hemodynamic forces. We hypothesized that TNF-alpha activity drives neointimal hyperplasia (NIH) during vein graft arterialization and that anti-TNF-alpha therapy would inhibit NIH.

METHODS

Rabbits underwent bilateral vein grafting using jugular vein. All distal branches except the occipital artery were unilaterally ligated to create distinct flow environments between the bilateral grafts. Vein grafts were harvested sequentially up to 28 days for TNF-alpha messenger RNA (mRNA) quantitation. In separate experiments, animals received short-term or long-term dosing with pegylated soluble TNF-alpha type I receptor (PEG sTNF-RI) or vehicle. After 14 to 28 days, grafts were analyzed for morphometry, proliferation, apoptosis, and PEG sTNF-RI distribution.

RESULTS

Quantitative mRNA assay (TaqMan) revealed shear-dependent (P < .001) and time-dependent (P < .001) TNF-alpha expression. TNF-alpha induction was maximal at day 1 and gradually decreased over time, but was persistently elevated even 4 weeks later (P < .001). Low shear (associated with increased NIH) resulted in significantly higher TNF-alpha mRNA expression (P = .03). PEG sTNF-RI was found in high concentrations in the serum and localized to NIH. The high-flow and low-flow vein grafts from treated animals demonstrated similar volumes of NIH compared with controls. PEG-sTNF-RI had only modest impact on vascular wall cell turnover, as reflected by terminal deoxynucleotide transferase-mediated deoxy uridine triphosphate nick-end labeling (P = .064) and anti-Ki-67 (P = .12) assays.

CONCLUSIONS

Placement of a vein into the arterial circulation acutely upregulates TNF-alpha; this expression level correlates with the degree of subsequent NIH. Pharmacologic interruption of this signaling pathway has no significant impact on NIH or wall cellular proliferation/apoptosis, suggesting that early vein graft adaptations can proceed via TNF-alpha-independent mechanisms.

Early Differential MMP-2 and -9 Dynamics During Flow-Induced Arterial and Vein Graft Adaptations

BACKGROUND

Arteries and vein grafts respond differently to reductions in flow, with arteries demonstrating inward remodeling through only limited structural reorganization of the media and vein grafts developing a thickened intima, with little change in the external diameter. In an effort to mechanistically explore the biology of this contrasting behavior, we hypothesized that this differential response in flow-mediated remodeling is driven by unique temporal expression patterns and functional activities of the matrix metalloproteinase (MMP)-2 and -9, key effectors of blood vessel architecture remodeling.

MATERIAL AND METHODS

In rabbits, bilateral carotid vein grafting was coupled with unilateral partial distal ligation to create a sevenfold flow differential between sides. Vein grafts and proximal carotid arteries were then harvested for morphological and biochemical studies at time points ranging from 1 to 14 days.

RESULTS

Vein grafts demonstrated an early, transient increase in pro-MMP-9 within adherent monocytes and endothelial cells. This was followed by a delayed increase in smooth muscle cell active MMP-2, which was coincident with the onset of intimal thickening at days 7 and 14 and significantly elevated by low flow. In contrast, arteries showed no elevation in pro MMP-9 above baseline, but demonstrated a transient increase in latent MMP-2 preceding the flow-mediated change in vessel diameter.

CONCLUSIONS

These data provide evidence for unique and independent roles of MMP-2 and -9 in the process of hemodynamically driven vascular remodeling.

Pharmacologic inhibition of vein graft neointimal hyperplasia

Although arterial conduits are widely used and have improved the long-term results of coronary artery bypass grafting, vein grafts remain important additional conduits in coronary surgery. Newer studies show a saphenous vein graft patency of 60% or more at 10 years postoperatively. The pathology of vein graft disease consists of thrombosis, neointimal hyperplasia, and vein graft atherosclerosis, which limit graft longevity. Therapeutic strategies to prevent vein graft disease include external stenting, pharmacotherapy, and gene therapy. The potential benefits of a pharmacologic approach are as follows: (1) Drugs with a broad clinical experience can be used; (2) side effects of systemic application can be minimized by local therapy; and (3) no vascular injury, such as pressurizing the vein for a viral transfection approach, is necessary. The different sites for pharmacotherapy in vein graft disease are reviewed in this article.