Endothelial cell delivery for cardiovascular therapy

Amnion-derived mesenchymal stem cells improve viability of endothelial cells exposed to shear stress in ePTFE grafts

PURPOSE:

Blood vessel reconstruction is an increasing need of patients suffering from cardiovascular diseases. For the development of microvascular prostheses, efficient endothelialization is mandatory to prevent graft occlusion. Here, we assessed the impact of amnion-derived mesenchymal stem/stromal cells (hAMSC), known for their important angiogenic potential, on the integrity and stability of endothelial cells exposed to shear stress in vascular grafts.

METHODS:

Human placental endothelial cells (hPEC) were cultured at the inner surface of an expanded polytetrafluoroethylene (ePTFE) graft positioned within a bioreactor and exposed to a minimal shear stress of 0.015 dyne/cm² or a physiological shear stress of 0.92 dyne/cm². hAMSC attached to the outer graft surface were able to interact with human placental endothelial cells by paracrine factors.

RESULTS:

Microscopical analysis and evaluation of glucose/lactate metabolism evidenced successful cell seeding of the graft: hPEC formed a stable monolayer, hAMSC showed a continuous growth during 72 h incubation. hAMSC improved the viability of hPEC exposed to 0.015 dyne/cm² as shown by a decreased lactate dehydrogenase release of 13% after 72 h compared to hPEC single culture. The viability-enhancing effect of hAMSC on hPEC was further improved by 13% under physiological shear stress. Angiogenesis array analysis revealed that hPEC exposed to physiological shear stress and hAMSC co-culture reduced the secretion of angiogenin, GRO, MCP-1, and TIMP-2.

CONCLUSION:

hAMSC exerted best survival-enhancing effects on hPEC under exposure to physiological shear stress and modulated endothelial function by paracrine factors. Our data support further studies on the development of grafts functionalized with hAMSC-derived secretomes to enable fast clinical application.

It's All in the Delivery: Designing Hydrogels for Cell and Non-viral Gene Therapies

Hydrogels provide a regenerative medicine platform with their ability to create an environment that supports transplanted or endogenous infiltrating cells and enables these cells to restore or replace the function of tissues lost to disease or trauma. Furthermore, these systems have been employed as delivery vehicles for therapeutic genes, which can direct and/or enhance the function of the transplanted or endogenous cells. Herein, we review recent advances in the development of hydrogels for cell and non-viral gene delivery through understanding the design parameters, including both physical and biological components, on promoting transgene expression, cell engraftment, and ultimately cell function. Furthermore, this review identifies emerging opportunities for combining cell and gene delivery approaches to overcome challenges to the field.

Optimizing endothelial cell functionalization for cell therapy of vascular proliferative disease using a direct contact co-culture system

Increased susceptibility to thrombosis, neoatherosclerosis, and restenosis due to incomplete regrowth of the protective endothelial layer remains a critical limitation of the interventional strategies currently used clinically to relieve atherosclerotic obstruction. Rapid recovery of endothelium holds promise for both preventing the thrombotic events and reducing post-angioplasty restenosis, providing the rationale for developing cell delivery strategies for accelerating arterial reendothelialization. The successful translation of experimental cell therapies into clinically viable treatment modalities for restoring vascular endothelium critically depends on identifying strategies for enhancing the functionality of endothelial cells (EC) derived from high cardiovascular risk patients, the target group for the majority of angioplasty procedures. Enhancing EC-associated nitric oxide (NO) synthesis by inducing overexpression of NO synthase (NOS) has shown promise as a way of increasing paracrine activity and restoring function of EC. In the present study, we developed a direct contact co-culture approach compatible with highly labile effectors, such as NO, and applied it for determining the effect of EC functionalization via NOS gene transfer on the growth of co-cultured arterial smooth muscle cells (A10 cell line) exhibiting the defining characteristics of neointimal cells. Bovine aortic endothelial cells magnetically transduced with inducible NOS-encoding adenovirus (Ad) formulated in zinc oleate-based magnetic nanoparticles (MNP[_iNOSAd]) strongly suppressed growth of proliferating A10 and attenuated the stimulatory effect of a potent mitogen, platelet-derived growth factor (PDGF-BB), whereas EC functionalization with free _iNOSAd or MNP formulated with a different isoform of the enzyme, endothelial NOS, was associated with lower levels of NO synthesis and less pronounced antiproliferative activity toward co-cultured A10 cells. These results show feasibility of applying magnetically facilitated gene transfer to potentiate therapeutically relevant effects of EC for targeted cell therapy of restenosis. The direct contact co-culture methodology provides a sensitive and reliable tool with potential utility for a variety of biomedical applications.

Cell-based strategies for vascular regeneration

Vascular regeneration is known to play an essential role in the repair of injured tissues mainly through accelerating the repair of vascular injury caused by vascular diseases, as well as the recovery of ischemic tissues. However, the clinical vascular regeneration is still challenging. Cell-based therapy is thought to be a promising strategy for vascular regeneration, since various cells have been identified to exert important influences on the process of vascular regeneration such as the enhanced endothelium formation on the surface of vascular grafts, and the induction of vessel-like network formation in the ischemic tissues. Here are a vast number of diverse cell-based strategies that have been extensively studied in vascular regeneration. These strategies can be further classified into three main categories, including cell transplantation, construction of tissue-engineered grafts, and surface modification of scaffolds. Cells used in these strategies mainly refer to terminally differentiated vascular cells, pluripotent stem cells, multipotent stem cells, and unipotent stem cells. The aim of this review is to summarize the reported research advances on the application of various cells for vascular regeneration, yielding insights into future clinical treatment for injured tissue/organ.

Magnetically enhanced cell delivery for accelerating recovery of the endothelium in injured arteries

Arterial injury and disruption of the endothelial layer are an inevitable consequence of interventional procedures used for treating obstructive vascular disease. The slow and often incomplete endothelium regrowth after injury is the primary cause of serious short- and long-term complications, including thrombosis, restenosis and neoatherosclerosis. Rapid endothelium restoration has the potential to prevent these sequelae, providing a rationale for developing strategies aimed at accelerating the reendothelialization process. The present studies focused on magnetically guided delivery of endothelial cells (EC) functionalized with biodegradable magnetic nanoparticles (MNP) as an experimental approach for achieving rapid and stable cell homing and expansion in stented arteries. EC laden with polylactide-based MNP exhibited strong magnetic responsiveness, capacity for cryopreservation and rapid expansion, and the ability to disintegrate internalized MNP in both proliferating and contact-inhibited states. Intracellular decomposition of BODIPY558/568-labeled MNP monitored non-invasively based on assembly state-dependent changes in the emission spectrum demonstrated cell proliferation rate-dependent kinetics (average disassembly rates: 6.6±0.8% and 3.6±0.4% per day in dividing and contact-inhibited EC, respectively). With magnetic guidance using a transient exposure to a uniform 1-kOe field, stable localization and subsequent propagation of MNP-functionalized EC, markedly enhanced in comparison to non-magnetic delivery conditions, were observed in stented rat carotid arteries. In conclusion, magnetically guided delivery is a promising experimental strategy for accelerating endothelial cell repopulation of stented blood vessels after angioplasty.

Tissue-engineered lymphatic graft for the treatment of lymphedema

BACKGROUND

Lymphedema is a chronic debilitating condition and curative treatment is yet to be found. Tissue engineering approach, which combines cellular components, scaffold, and molecular signals hold great potential in the treatment of secondary lymphedema with the advent of lymphatic graft to reconstruct damaged collecting lymphatic vessel. This review highlights the ideal characteristics of lymphatic graft, the limitation and challenges faced, and the approaches in developing tissue-engineered lymphatic graft.

METHODS

Literature on tissue engineering of lymphatic system and lymphatic tissue biology was reviewed.

RESULTS

The prime challenge in the design and manufacturing of this graft is producing endothelialized conduit with intraluminal valves. Suitable scaffold material is needed to ensure stability and functionality of the construct. Endothelialization of the construct can be enhanced via biofunctionalization and nanotopography, which mimics extracellular matrix. Nanocomposite polymers with improved performance over existing biomaterials are likely to benefit the development of lymphatic graft.

CONCLUSIONS

With the in-depth understanding of tissue engineering, nanotechnology, and improved knowledge on the biology of lymphatic regeneration, the aspiration to develop successful lymphatic graft is well achievable.

Endothelial cells expressing low levels of CD143 (ACE) exhibit enhanced sprouting and potency in relieving tissue ischemia

The sprouting of endothelial cells from pre-existing blood vessels represents a critical event in the angiogenesis cascade. However, only a fraction of cultured or transplanted endothelial cells form new vessels. Moreover, it is unclear whether this results from a stochastic process or instead relates to certain endothelial cells having a greater angiogenic potential. This study investigated whether there exists a sub-population of cultured endothelial cells with enhanced angiogenic potency in vitro and in vivo. First, endothelial cells that participated in sprouting, and non-sprouting cells, were separately isolated from a 3D fibrin gel sprouting assay. Interestingly, the sprouting cells, when placed back into the same assay, displayed a sevenfold increase in the number of sprouts, as compared to control cells. Angiotensin-converting enzyme (CD143) was significantly down regulated on sprouting cells, as compared to regular endothelial cells. A subset of endothelial cells with low CD143 expression was then prospectively isolated from an endothelial cell culture. Finally, these cells were found to have greater potency in alleviating local ischemia, and restoring regional blood perfusion when transplanted into ischemic hindlimbs, as compared to unsorted endothelial cells. In summary, this study indicates that low expression of CD143 can be used as a biomarker to identify an endothelial cell sub-population that is more capable to drive neovascularization.

In vitro endothelialization of cobalt chromium alloys with micro/nanostructures using adipose-derived stem cells

In this study, integrin expression, proliferation, and endothelial differentiation of adipose-derived stem cells (ADSCs) on pristine cobalt chrome (CoCr) surface, microstructured and nanostructured CoCr surfaces (obtained after treatment with piranha solution) were investigated. The results showed that proliferation of ADSCs on the substrates treated with piranha solution is not significantly different from that on the pristine substrates. However, quantitative real-time PCR analysis showed significantly enhanced up-regulation of CD31, vWF and eNOS from gene level by ADSCs on the nanostructured substrates but not on the microstructured substrates. The adsorption of vitronectin from the culture medium on the nanostructured substrates was higher than on the pristine and microstructured substrates. We speculate that this results in increased integrin αvβ3 expression in the ADSCs, which may contribute partially to the enhanced endothelial differentiation of ADSCs on the nanostructured substrates. This study shows that ADSCs can be used to endothelialize stents in vitro and the endothelial differentiation of ADSC is enhanced on the nanostructured surfaces.

The effect of substrate modulus on the growth and function of matrix-embedded endothelial cells

Endothelial cells (EC) are potent bioregulatory cells, modulating thrombosis, inflammation and control over mural smooth muscle cells and vascular health. The biochemical roles of EC are retained when cells are embedded within three-dimensional (3D) denatured collagen matrices. Though substrate mechanics have long been known to affect cellular morphology and function and 3D-EC systems are increasingly used as therapeutic modalities little is known about the effect of substrate mechanics on EC in these 3D systems. In this work, we examined the effect of isolated changes in modulus on EC growth and morphology, extracellular matrix gene expression, modulation of smooth muscle cell growth, and immunogenicity. EC growth, but not morphology was dependent on scaffold modulus. Increased scaffold modulus reduced secretion of smooth muscle cell growth inhibiting heparan sulfate proteoglycans (HSPGs), but had no effect on secreted growth factors, resulting in a loss of smooth muscle cell growth inhibition by EC on high modulus scaffolds. Expression of ICAM-1, VCAM-1 and induction of CD4(+) T cell proliferation was reduced by increased scaffold modulus, and correlated with changes in integrin α5 expression. Expression of several common ECM proteins by EC on stiffer substrates dropped, including collagen IV(α1), collagen IV(α5), fibronectin, HSPGs (perlecan and biglycan). In contrast, expression of elastin and TIMPs were increased. This work shows even modest changes in substrate modulus can have a significant impact on EC function in three-dimensional systems. The mechanism of these changes is not clear, but the data presented here within suggests a model wherein EC attempt to neutralize changes in environmental force balance by altering ECM and integrin expression, leading to changes in effects on downstream signaling and function.

The evolution of endothelial regulatory paradigms in cancer biology and vascular repair

Although the roles of endothelial cells in cancer have primarily been considered to be related to tumor perfusion, the emerging appreciation of "angiocrine" regulation adds stromal regulatory capabilities to the expanding list of endothelial functions in tumors. We posit that an understanding of the state-dependent paracrine regulatory paradigms established in vascular disease and repair will be critical for a deep understanding of tumor biology, as endothelial cells regulate diverse processes in all vascularized tissues. Here, we outline the historical developments that led to the appreciation of the paracrine regulatory functions of endothelial cells, summarize classical views of blood vessels and stroma in cancer, and attempt to merge these ideas to include the stromal regulatory endothelial cell as a critical regulator of cancer. The notion of the endothelial cell as a biochemical regulator of cancer state in constant dynamic balance with its tumor could impact diagnosis, prognosis, and treatment of cancer. Such concepts might well explain the mixed results from antiangiogenic cancer therapeutics and how certain drugs that improve vascular health correlate with improved cancer prognosis.

Review paper: Critical Issues in Tissue Engineering: Biomaterials, Cell Sources, Angiogenesis, and Drug Delivery Systems

Tissue engineering is a newly emerging biomedical technology, which aids and increases the repair and regeneration of deficient and injured tissues. It employs the principles from the fields of materials science, cell biology, transplantation, and engineering in an effort to treat or replace damaged tissues. Tissue engineering and development of complex tissues or organs, such as heart, muscle, kidney, liver, and lung, are still a distant milestone in twenty-first century. Generally, there are four main challenges in tissue engineering which need optimization. These include biomaterials, cell sources, vascularization of engineered tissues, and design of drug delivery systems. Biomaterials and cell sources should be specific for the engineering of each tissue or organ. On the other hand, angiogenesis is required not only for the treatment of a variety of ischemic conditions, but it is also a critical component of virtually all tissue-engineering strategies. Therefore, controlling the dose, location, and duration of releasing angiogenic factors via polymeric delivery systems, in order to ultimately better mimic the stem cell niche through scaffolds, will dictate the utility of a variety of biomaterials in tissue regeneration. This review focuses on the use of polymeric vehicles that are made of synthetic and/or natural biomaterials as scaffolds for three-dimensional cell cultures and for locally delivering the inductive growth factors in various formats to provide a method of controlled, localized delivery for the desired time frame and for vascularized tissue-engineering therapies.

Magnetic nanoparticles for targeted vascular delivery

Magnetic targeting has shown promise to improve the efficacy and safety of different classes of therapeutic agents by enabling their active guidance to the site of disease and minimizing dissemination to nontarget tissues. However, its translation into clinic has proven difficult because of inherent limitations of traditional approaches inapplicable for deep tissue targeting in human subjects and a need for developing well-characterized and fully biocompatible magnetic carrier formulations. A novel magnetic targeting scheme based on the magnetizing effect of deep-penetrating uniform fields is presented as an example of a strategy providing a potentially clinically viable solution for preventing injury-triggered reobstruction of stented blood vessels (in-stent restenosis). The design of optimized magnetic carrier formulations and experimental results showing the feasibility of uniform field-controlled targeting for site-specific vascular delivery of small-molecule pharmaceuticals, biotherapeutics, and cells are discussed in the context of antirestenotic therapy. The versatility of this approach applicable to different classes of therapeutic agents exerting their antirestenotic effects through distinct mechanisms prompts exploring the utility of uniform field-mediated magnetic stent targeting for combination therapies with enhanced efficiencies and improved safety profiles. Additional improvements in terms of site specificity and protracted carrier retention at the site of injury may be expected from the development and use of magnetic carriers exhibiting affinity for arterial wall-specific antigens.

Cell-seeding techniques in vascular tissue engineering

Previous studies have demonstrated the benefits of cell seeding in the construction of tissue-engineered vascular grafts (TEVG). However, seeding methods are diverse and no method is clearly superior in either promoting seeding efficiency or improving long-term graft function. As we head into an era during which a variety of different TEVG are under investigation in clinical trials around the world, it is important to consider the regulatory issues surrounding the translation of these technologies. In this review, we summarize important advances in the field of vascular tissue engineering, with particular attention on cell-seeding techniques for TEVG development and special emphasis placed on regulatory issues concerning the clinical translation of these various methods.

Silver nanoparticles inhibit VEGF-and IL-1beta-induced vascular permeability via Src dependent pathway in porcine retinal endothelial cells

The aim of this study is to determine the effects of silver nanoparticles (Ag-NP) on vascular endothelial growth factor (VEGF)-and interleukin-1 beta (IL-1β)-induced vascular permeability, and to detect the underlying signaling mechanisms involved in endothelial cells. Porcine retinal endothelial cells (PRECs) were exposed to VEGF, IL-1β and Ag-NP at different combinations and endothelial cell permeability was analyzed by measuring the flux of RITC-dextran across the PRECs monolayer. We found that VEGF and IL-1β increase flux of dextran across a PRECs monolayer, and Ag-NP block solute flux induced by both VEGF and IL-1β. To explore the signalling pathway involved VEGF- and IL-1β-induced endothelial alteration, PRECs were treated with Src inhibitor PP2 prior to VEGF and IL-1β treatment, and the effects were recorded. Further, to clarify the possible involvement of the Src pathways in endothelial cell permeability, plasmid encoding dominant negative(DN) and constitutively active(CA) form of Src kinases were transfected into PRECs, 24 h prior to VEGF and IL-1β exposure and the effects were recorded. Overexpression of DN Src blocked both VEGF-and IL-1β-induced permeability, while overexpression of CA Src rescues the inhibitory action of Ag-NP in the presence or absence of VEGF and IL-1β. Further, an in vitro kinase assay was performed to identify the presence of the Src phosphorylation at Y419. We report that VEGF and IL-1β-stimulate endothelial permeability via Src dependent pathway by increasing the Src phosphorylation and Ag-NP block the VEGF-and IL-1β-induced Src phosphorylation at Y419. These results demonstrate that Ag-NP may inhibit the VEGF-and IL-1β-induced permeability through inactivation of Src kinase pathway and this pathway may represent a potential therapeutic target to inhibit the ocular diseases such as diabetic retinopathy.

Combinatorial coating of adhesive polypeptide and anti-CD34 antibody for improved endothelial cell adhesion and proliferation

Improved attachment, adhesion and proliferation of the surrounding mature endothelial cells (ECs) and circulating endothelial progenitor cells (EPCs) is of primary importance to realize the in situ rapid re-endothelialization of cardiovascular stents. To achieve this, a combinatorial coating of synthesized mussel adhesive polypeptide mimics as well as anti-CD34 antibody was constructed onto the devices through a novel adsorption method in this study. To immobilize the polypeptide and target antibody effectively, polycaprolactone (PCL) was first spin-coated onto the substrate as intermediate. The immobilization of polypeptide and antibody was confirmed by the changes of water contact angles and the attachment, growth of ECs and EPCs on the substrates, respectively. The results showed that after adhesive polypeptide or/and antibody immobilization, the hydrophilicity of coated PCL substrate (PCLS) was obviously improved. The amount of the immobilized antibody, determined by enzymelinked immunoassay (ELISA) method, was enhanced with the increase of antibody concentrations in the range from 5 to 25 mug/ml. The coatings after BSA blocking prevented the unspecific protein adsorption as monitored by fluorescent microscopy. The results of in vitro cell culture showed that compared with the PCLS, polypeptide/anti-CD34 antibody coating could effectively enhance the attachment, growth and adhesion of ECs and EPCs, in particular EPCs. A platelet adhesion experiment revealed that the blood compatibility of the PCLS after polypeptide/anti-CD34 antibody coating was also obviously improved. The results showed that the surface modification with adhesive polypeptide and anti-CD34 antibody will be a promising coating technique for the surface modification of the intravascular prostheses for rapid re-endothelialization.

Effects of exogenous peripheral-blood-derived endothelial progenitor cells or unfractionated bone-marrow-derived cells on neointimal formation and inflammation in cholesterol-fed, balloon-denuded, and radiated iliac arteries of inbred rabbits

BACKGROUND

Injection of bone marrow cells (BMC) and endothelial progenitor cells (EPC) or application of stem-cell-mobilizing factors has been associated both with reduction or exacerbation of atherosclerosis and with unstable plaque phenotype. The discrepancies may reflect the cell type, dosing, duration, and route of administration of cells in these studies. The aim of this study was to determine the effects of peripheral-blood-derived endothelial progenitor cells (PBEPC) or unfractionated BMC obtained from inbred siblings on neointimal formation and inflammation in cholesterol-fed, balloon-denuded, and radiated rabbit iliac arteries.

METHODS

Rabbits were fed a 1.0% cholesterol diet for 14 days, followed by endothelial denudation in both iliac arteries, and continued on a 0.15% cholesterol diet. On day 42, denuded areas were radiated, and animals were randomized. The first group received PBEPC (n=5), the second group received BMC (n=4), and the third group received heparinized (20 IU) saline (Control; n=3). PBEPC were characterized by flow cytometry. Cells (5x10(6)) or saline was administered twice through the ear vein: the first time at 1 h after radiation and the second time at 2 weeks after radiation. Four weeks after radiation, the animals were sacrificed, and arterial segments were processed for morphometry.

RESULTS

Administration of BMC or PBEPC from inbred siblings had no adverse effect. Lumen area (0.93+/-0.53 mm(2)), neointimal area (0.65+/-0.29 mm(2)), percent stenosis (44+/-21), and macrophage score (0.6+/-0.3) in controls were similar to those in cell-treated groups.

CONCLUSION

This study demonstrates that, in the current animal model, either PBEPC or BMC failed to affect neointimal formation or inflammation.

Vascular tissue engineering with magnetic nanoparticles: seeing deeper

The endothelium bares a paramount therapeutic and diagnostic significance in vascular disease. The current work presents a novel strategy based on the use of superparamagnetic nanoparticles to obtain an endothelial cell lining on the luminal surface of vascular conduits, which can be detected non-invasively in a clinical Magnetic Resonance Imaging (MRI) scanner. Human umbilical vein endothelial cells (HUVECs) were prelabeled with clinically approved superparamagnetic nanoparticles. Cell viability and eNOS expression were not affected by the labelling procedure. Magnetically labelled cells were delivered onto the lumen of a PTFE tubular graft by a customised electromagnet. The endothelium was detected in a 1,5T MRI scanner. Magnetic cell delivery provides an efficient technique to seed tubular scaffolds enabling the non-invasive depiction of the cells from the substrate, thus providing a reliable tool to assess the quality of cell delivery procedures.

Challenges and emerging technologies in the immunoisolation of cells and tissues

Protection of transplanted cells from the host immune system using immunoisolation technology will be important in realizing the full potential of cell-based therapeutics. Microencapsulation of cells and cell aggregates has been the most widely explored immunoisolation strategy, but widespread clinical application of this technology has been limited, in part, by inadequate transport of nutrients, deleterious innate inflammatory responses, and immune recognition of encapsulated cells via indirect antigen presentation pathways. To reduce mass transport limitations and decrease void volume, recent efforts have focused on developing conformal coatings of micron and submicron scale on individual cells or cell aggregates. Additionally, anti-inflammatory and immunomodulatory capabilities are being integrated into immunoisolation devices to generate bioactive barriers that locally modulate host responses to encapsulated cells. Continued exploration of emerging paradigms governed by the inherent challenges associated with immunoisolation will be critical to actualizing the clinical potential of cell-based therapeutics.

Construction and characterization of a thrombin-resistant designer FGF-based collagen binding domain angiogen

Humans demonstrate limited spontaneous endothelialization of prosthetic bypass grafts. However the local application of growth factors to prosthetic grafts or to injured blood vessels can provide an immediate effect on endothelialization. Novel chimeric proteins combining potent angiogens with extracellular matrix binding domains may localize to exposed matrices and provide sustained activity to promote endothelial regeneration after vascular interventions. We have ligated a thrombin-resistant mutant of fibroblast growth factor (FGF)-1 (R136K) with a collagen binding domain (CBD) in order to direct this growth factor to sites of exposed vascular collagen or selected bioengineered scaffolds. While FGF-1 and R136K are readily attracted to a variety of matrix proteins, R136K-CBD demonstrated selective and avid binding to collagen approximately 4x that of FGF-1 or R136K alone (P<0.05). The molecular stability of R136K-CBD was superior to FGF-1 and R136K. Its chemotactic activity was superior to R136K and FGF-1 (11+/-1% vs. 6+/-2% and 4+/-1%; P<0.01). Its angiogenic activity was similar to R136K and significantly greater than control by day 2 (P<0.01). After day 3, FGF-1-treated endothelial cell's (EC) sprouts had regressed back to levels insignificant compared to the control group (P=0.17), while both R136K and R136K-CBD continued to demonstrate greater sprout lengthening as compared to control (P<0.0002). The mitogenic activity of all growth factors was greater than control groups (20% PBS); in all comparisons (P<0.0001). This dual functioning angiogen provides proof of concept for the application of designer angiogens to matrix binding proteins to intelligently promote endothelial regeneration of selected matrices.

The application of microbubbles for targeted drug delivery

Interest in microbubbles as vehicles for drug delivery has grown in recent years, due in part to characteristics that make them well suited for this role and in part to the need the for localized delivery of drugs in a number of applications. Microbubbles are inherently small, allowing transvascular passage, they can be functionalized for targeted adhesion, and can be acoustically driven, which facilitates ultrasound detection, production of bioeffects and controlled release of the cargo. This article provides an overview of related microbubble biofluid mechanics and reviews recent developments in the application of microbubbles for targeted drug delivery. Additionally, related advances in non-bubble microparticles for drug delivery are briefly described in the context of targeted adhesion.

Cardiovascular gene delivery: The good road is awaiting

Atherosclerotic cardiovascular disease is a leading cause of death worldwide. Despite recent improvements in medical, operative, and endovascular treatments, the number of interventions performed annually continues to increase. Unfortunately, the durability of these interventions is limited acutely by thrombotic complications and later by myointimal hyperplasia followed by progression of atherosclerotic disease over time. Despite improving medical management of patients with atherosclerotic disease, these complications appear to be persisting. Cardiovascular gene therapy has the potential to make significant clinical inroads to limit these complications. This article will review the technical aspects of cardiovascular gene therapy; its application for promoting a functional endothelium, smooth muscle cell growth inhibition, therapeutic angiogenesis, tissue engineered vascular conduits, and discuss the current status of various applicable clinical trials.

Matrix embedding alters the immune response against endothelial cells in vitro and in vivo

BACKGROUND

Endothelial cell (EC) dysfunction represents the first manifestation of atherosclerotic disease. Restoration of endothelium via seeding or transfection is hampered by local alterations in flow, inflammation, and metabolic activation. Perivascular EC matrix implants are shielded from these forces and still control vascular repair. The host immune response to such implants, however, remains largely unknown. We investigated the effect of embedding of ECs within 3-dimensional matrices on host immune responses in vitro and in vivo.

METHODS AND RESULTS

We compared expression of major histocompatibility complex (MHC), costimulatory, and adhesion molecules by free aortic ECs or ECs embedded in Gelfoam matrices by flow-cytometry. T-cell proliferation was assessed by [3H] thymidine incorporation. Humoral immune response (ELISA and FACS analysis) and cellular (histopathology) infiltration were investigated after subcutaneous injection of free porcine aortic ECs (PAEs) or of a Gelfoam/EC block, or after concomitant injection of PAEs adjacent to Gelfoam in rats. Aortic ECs embedded in Gelfoam expressed lower levels of MHC class II, costimulatory, and adhesion molecules compared with free ECs (P<0.001), and induced 3-fold less proliferation of human CD4+ T-cells (P<0.0005). Implantation of a Gelfoam/EC block in rats nearly abrogated the immune response with 1.75- to 9.0-fold downregulation in tumor necrosis factor-alpha, interleukin-6, monocyte chemotactic protein-1, and PAE-specific immunoglobulin G (P<0.005) and 3.3- to 4.5-fold reduction in leukocytic tissue infiltration. Injecting PAEs adjacent to Gelfoam induced a significant response comparable to that of free implanted PAEs.

CONCLUSIONS

Embedding ECs within 3-dimensional matrices alters the host immune response by inhibiting expression of MHC class II, costimulatory, and adhesion molecules, offering the rationale to develop novel therapies for vascular diseases.

Design and evaluation of a novel flow chamber for measuring cell adhesion to absorbable polymer films

There is great interest in improving cellular attachment to synthetic materials, particularly for developing small diameter tissue-engineered vascular grafts. However, limited research has been conducted to evaluate the adhesion characteristics of different cell types to absorbable substrates. Tissue engineered vessels typically fail as a result of delamination of the endothelial cell layer when exposed to fluid or blood flow. The focus of this research was to design and evaluate a flow chamber, using fibroblasts, smooth muscle cells, and endothelial cells, to probe the bounds of the system. A flow chamber was designed and fabricated to compare the relative adhesion characteristics of cells to absorbable polymer films. A preliminary investigation of mouse fibroblast (3T3M) adhesion to semicrystalline poly-L-lactide (PLL) films was conducted to determine general operating specifications. Cell coverage on films was evaluated using a live-dead assay and image analysis; following exposure to flow, tests were similarly conducted. Based on these results, additional studies were conducted to compare the adhesion of rat aortic smooth muscle cells (SMC) and endothelial cells (EC) on PLL films.

In Vivo Cell Seeding With Anti-CD34 Antibodies Successfully Accelerates Endothelialization but Stimulates Intimal Hyperplasia in Porcine Arteriovenous Expanded Polytetrafluoroethylene Grafts

Background— The patency of AV expanded polytetrafluoroethylene (ePTFE) grafts for hemodialysis is impaired by intimal hyperplasia (IH) at the venous outflow tract. The absence of a functional endothelial monolayer on the prosthetic grafts is an important stimulus for IH. In the present study, we evaluated the feasibility of capturing endothelial progenitor cells in vivo using anti-CD34 antibodies on ePTFE grafts to inhibit IH in porcine AV ePTFE grafts.

Methods and Results— In 11 pigs, anti-CD34–coated ePTFE grafts were implanted between the carotid artery and internal jugular vein. Bare ePTFE grafts were implanted at the contralateral side. After 3 (n=2) or 28 (n=9) days, the pigs were terminated, and the AV grafts were excised for histological analysis and SEM. At 3 and 28 days after implantation, 95% and 85% of the coated graft surface was covered by endothelial cells. In contrast, no cell coverage was observed in the bare graft at 3 days, whereas at 28 days, bare grafts were partly covered with endothelial cells (32%; P =0.04). Twenty-eight days after implantation, IH at the venous anastomosis was strongly increased in anti-CD34–coated grafts (5.96±1.9 mm 2 ) compared with bare grafts (1.70±0.4 mm 2 ; P =0.03). This increase in IH coincided with enhanced cellular proliferation at the venous anastomosis.

Conclusions— Autoseeding with anti-CD34 antibodies results in rapid endothelialization within 72 hours. Despite persistent endothelial graft coverage, IH at the outflow tract is increased profoundly at 4 weeks after implantation. Further modifications are required to stimulate the protective effects of trapped endothelial cells.

Paradoxical Effects of Iron Chelation on Growth of Vascular Endothelial Cells

Endothelial cell (EC) and vascular smooth muscle cell (VSMC) interactions play critical roles in restenosis following vascular injury. We examined the effects of intracellular iron chelation on endothelial cell cycle progression and VSMC modulation of endothelial cell growth. A diffusible, lipid-soluble iron chelator that rapidly enters cells, desferri-exochelin 772SM (D-Exo), was studied in human endothelial cells and VSMCs. In both cell types D-Exo reversibly halted cell cycle progression from G0/G1 phase to S phase and from S phase to G2/M phase and increased expression of hypoxia-inducible factor 1alpha (HIF-1alpha). D-Exo increased secretion of vascular endothelial growth factor (VEGF), a downstream target of HIF-1alpha, in VSMCs, but there was no VEGF production in endothelial cells. D-Exo was 25-fold more potent than the lipid-insoluble iron chelator deferoxamine, which does not readily enter cells. Intracellular iron chelation with D-Exo directly inhibits endothelial cell growth but indirectly stimulates endothelial cell growth by increasing VEGF release by VSMCs.

Endothelial and Smooth Muscle Cell Seeding onto Processed Ex Vivo Arterial Scaffolds Using 3D Vascular Bioreactors

Biomaterials derived from ex vivo tissues offer a viable alternative to synthetic materials for organ replacement therapies. In this study, we describe the use of a tissue engineering scaffold derived from ex vivo arterial tissue to assess vascular cell adhesion within a three-dimensional perfusion bioreactor. With the aim of maximizing seeding efficiency, five methods for endothelial cell (EC) and three independent methods for vascular smooth muscle cell (VSMC) adhesion were explored. Seeded constructs were maintained in vascular bioreactors under pulsatile flow conditions, culminating at 165 ml/min at 1.33 Hz to validate cell attachment and retention over time. Progressive modification of the seeding and flow regime protocols resulted in an increased of EC retention from 5.1 to 634 cells/mm2. Seeding VSMCs as sheets rather than cell suspensions bound and stabilized surface EC matrix fibers, resulting in multiple cell layers adhered to the scaffold with cells migrating to the medial/adventitial boundary. In conjunction with the bioscaffold, the vascular perfusion system serves as a useful tool to analyze cell adhesion and retention by allowing controlled manipulation of seeding and perfusion conditions.

Endothelium-Targeted Gene and Cell-Based Therapies for Cardiovascular Disease

Most common cardiovascular diseases are accompanied by endothelial dysfunction. Because of its predominant role in the pathogenesis of cardiovascular disease, the vascular endothelium is an attractive therapeutic target. The identification of promoter sequences capable of rendering endothelial-specific transgene expression together with the recent development of vectors with enhanced tropism for endothelium may offer opportunities for the design of new strategies for modulation of endothelial function. Such strategies may be useful in the treatment of chronic diseases such as hypertension, atherosclerosis, and ischemic artery disease, as well as in acute myocardial infarction and during open heart surgery for prevention of ischemia and reperfusion (I/R)-induced injury. The recent identification of putative endothelial progenitor cells in peripheral blood may allow the design of autologous cell-based strategies for neovascularization of ischemic tissues and for the repair of injured blood vessels and bioengineering of vascular prosthesis. "Proof-of-concept" for some of these strategies has been established in animal models of cardiovascular disease. However the successful translation of these novel strategies into clinical application will require further developments in vector and delivery technologies. Further characterization of the processes involved in mobilization, migration, homing, and incorporation of endothelial progenitor cells into the target tissues is necessary, and the optimal conditions for therapeutic application of these cells need to be defined and standardized.

Local drug delivery in restenosis injury: thermoresponsive co-polymers as potential drug delivery systems

The success of percutaneous transluminal coronary angioplasty in treatment of acute coronary syndromes has been compromised by the incidence of restenosis. The physical insult of balloon insertion can damage or remove the endothelial monolayer, thereby generating a prothrombotic surface. The resulting inappropriate response to injury can also lead to penetration of inflammatory cells, conversion of the underlying media to a synthetic phenotype, deposition of extracellular matrix, constrictive remodeling, and neointimal hyperplasia. While stent implantation at the time of balloon insertion has offset some of these events, inflammatory responses to the implanted biomaterial (stent) and intimal hyperplasia are still prominent features of the procedure, leading in 20-30% of cases to in-stent restenosis within a year. Systemic delivery of drugs designed to offset in-stent restenosis injury has been largely unsuccessful, which has led to the development of strategies for coating stents with drugs for local delivery. Drug-eluting stents constitute an innovative means of further reducing the incidence of restenosis injury and clinical trials have shown encouraging results. This review focuses on properties of a class of environment-sensitive hydrogels, the N-isopropylacrylamide-based thermoresponsive co-polymers, on their potential roles as stent coatings, on their demonstrated ability to incorporate and release drugs that modify vascular endothelial and smooth muscle cell functions, and on issues that still await clarification, prior to their adoption in a clinical setting.

Biodegradable polyester elastomers in tissue engineering

Tissue engineering often makes use of biodegradable scaffolds to guide and promote controlled cellular growth and differentiation in order to generate new tissue. There has been significant research regarding the effects of scaffold surface chemistry and degradation rate on tissue formation and the importance of these parameters is widely recognised. Nevertheless, studies describing the role of mechanical stimuli during tissue development and function suggest that the mechanical properties of the scaffold will also be important. In particular, scaffold mechanics should be taken into account if mechanical stimulation, such as cyclic strain, will be incorporated into strategies to grow improved tissues or the target tissue to be replaced has elastomeric properties. Biodegradable polyesters, such as polyglycolide, polylactide and poly(lactide-co-glycolide), although commonly used in tissue engineering, undergo plastic deformation and failure when exposed to long-term cyclic strain, limiting their use in engineering elastomeric tissues. This review will cover the latest advances in the development of biodegradable polyester elastomers for use as scaffolds to engineer tissues, such as heart valves and blood vessels.

Biomatériaux vasculaires : du génie biologique et médical au génie tissulaire

Biomaterials are already widely used in medical sciences. The field of biomaterials began to shift to produce materials able to stimulate specific cellular responses at the molecular level. The combined efforts of cell biologists, engineers, materials scientists, mathematicians, geneticists, and clinicians are now used in tissue engineering to restore, maintain, or improve tissue functions or organs. This rapidly expanding approach combines the fields of material sciences and cell biology for the molecular design of polymeric scaffolds with appropriate 3D configuration and biological responses. Future developments for new blood vessels will require improvements in technology of materials and biotechnology together with the increased knowledge of the interactions between materials, blood, and living tissues. Biomaterials represent a crucial mainstay for all these studies.

Cellular engineering of conduits for coronary and lower limb bypass surgery: role of cell attachment peptides and pre-conditioning in optimising smooth muscle cells (SMC) adherence to compliant poly(carbonate-urea)urethane (MyoLink) scaffolds

OBJECTIVE

We are developing a hybrid arterial bypass graft of compliant poly(carbonate-urea)urethane (MyoLink), endothelial and smooth muscle cells (SMCs). To enhance adhesion of SMCs we assessed various attachment factors and the effect of pre-conditioning on cell retention.

METHODS

MyoLink segments were coated with either RGD, superfibronectin, fibronectin, fibronectin-like engineered polymer protein (FEPP), FEPP plus or type 1 collagen overnight. (111)Indium-radiolabelled SMCs were placed onto MyoLink segments for 48 h before being aspirated, then lavaged off. All grafts, aspirates and lavages were counted in a gamma counter. SMC viability on the MyoLink segments was also assessed for viability using the Alamar blue redox assay. Separately, MyoLink grafts lined with radiolabelled SMCs were divided into a pre-conditioned group, exposed to subarterial pulsatile flow whilst another group were held in static culture. After 1-week, grafts were exposed to arterial pulsatile flow whilst radioactivity was assessed using a gamma camera.

RESULTS

Only FEPP plus significantly enhanced SMC attachment: mean of 32+/-6% cell attachment compared to 21+/-5% for uncoated control. Cell viability was enhanced by all attachment factors except fibronectin. Pre-conditioning was shown to significantly enhance the retention of SMCs onto the MyoLink once exposed to pulsatile arterial flow: the final attachment was 57+/-7% for the static and 76+/-7% for the pre-conditioned group.

CONCLUSIONS

FEPP plus enhances SMC attachment to MyoLink. We believe this is because of its repeating sequences of RGD and its positive charge. Pre-conditioning enhances the retention of SMCs to MyoLink once exposed to pulsatile arterial flow.

New strategies to prevent restenosis

The Holy Grail of cardiovascular pharmacology has been the search for an effective therapy targeting restenosis after angioplasty and/or intra-arterial stenting. The failure of promising therapeutics in clinical trials underscores the complexity and redundancy of the signaling cascades regulating mitogenesis and fibrogenesis. Novel therapeutic modalities have potential to target dysfunctional signaling elements directly in vascular smooth muscle cells. Significant progress in the treatment against restenosis will require the exploitation and cross-fertilization of developments in the fields of pharmacology, bioengineering, genetics, and molecular biology. Collaboration among researchers in these fields will be essential.

Endothelium properties of a tissue-engineered blood vessel for small-diameter vascular reconstruction

PURPOSE

A tissue-engineered blood vessel (TEBV) produced in vitro by the self-assembly method was developed in our laboratory for the replacement of small-diameter blood vessels. The interior of this vessel is covered by an endothelium. The aim of the present study was to evaluate whether the endothelial layer would make a favorable contribution at the time of implantation of the TEBV by investigating in vitro the hemocompatible properties of the endothelial cells covering its interior.

METHODS

The secretion of the von Willebrand factor (vWF) and expression of thrombomodulin by the endothelium were assessed, and the adhesive molecules E-selectin and intercellular adhesion molecule-1 (ICAM-1) were quantified as a function of maturation time. To evaluate the functional response of the endothelium on injury, the cellular response to physiological stimulatory factors (thrombin and lipopolysaccharide [LPS]) was analyzed.

RESULTS

The endothelial cells formed a confluent monolayer displaying favorable hemocompatible properties (78% +/- 10% of cells expressing thrombomodulin with only 12 +/- 3 mU/10(6) cells of vWF secreted over a 2-hour period), which acquired their full expression after a culture period of 4 days. Moreover, pro-adhesive properties toward inflammatory cells were not observed. The cells were also able to respond to physiological-stimulating agents (thrombin and LPS) and demonstrated a statistically significant overexpression of the corresponding molecules under the conditions tested.

CONCLUSIONS

These results indicate that the endothelium of the tissue-engineered blood vessel produced by the self-assembly approach displays advantageous qualities with regard to the vessel's future implantation as a small-diameter vascular prosthesis.

Expression of adhesion molecules and cytokines in vitro by endothelial cells seeded on various polymer surfaces coated with titaniumcarboxonitride

Although endothelial cell (EC) seeding improves the patency of vascular prostheses, the detachment of adherent ECs after the restoration of circulation remains one of the major obstacles. Polymer surfaces for endothelialization can be optimized. In this study, polyethylene terephthalate (PET), polypropylene (PP), polytetrafluoroethylene (PTFE), polyurethane (PUR), and silicone were coated with a titaniumcarboxonitride (Ti(C,N,O)) layer by a plasma-assisted chemical vapor deposition process to verify the effect of titanium onto human saphenous vein ECs. Almost confluent EC monolayers were evaluated for 1) proliferation activity and 2) expression of adhesion molecules using cellular enzyme-linked immunosorbent assay and release of cytokines. The results showed that all titanium-coated polymers and uncoated PET have no toxic effect on human saphenous vein ECs excepting uncoated PTFE, PP, and silicone. Moreover, growing ECs showed an insignificant decrease in cytokine production and an unessential change in basal expression of adhesion molecules. Tumor necrosis factor-alpha-induced response depends on polymer surface: for example, intercellular adhesion molecule-1 expression decreased. E-selectin expression was unchanged for culturing ECs on coated PET, PP, and PTFE and reduced for polyurethane and silicone. Vascular cell adhesion molecule-1 expression was unchanged for coated PUR and silicone and reduced for PET, PP, and PTFE. In summary, titanium-coating layers promote adhesion of human ECs on polymer vascular grafts with no proinflammatory reaction of ECs.

Covalent immobilization of chitosan/heparin complex with a photosensitive hetero-bifunctional crosslinking reagent on PLA surface

Chitosan (CS) was covalently immobilized onto polylactic acid (PLA) film surface using the photosensitive hetero-bifunctional crosslinking reagent, 4-azidobenzoic acid, which was previously bonded to chitosan by reaction between an acid group of the crosslinking reagent and a free amino group of chitosan. The immobilization was accomplished by irradiating with ultraviolet light, the modified chitosan being coated on the film surface to photolyze azide groups, thus crosslinking chitosan and PLA together. The hydroxyl and amino groups of chitosan may provide the opportunity for them being further derived by chemically immobilizing a variety of functional groups. These modifications could be carried out to tailor PLA biomaterial to meet the specific needs of different biomedical applications. For example, chitosan molecules immobilized on the PLA could be modified by heparin (Hp) solution to form a polyelectrolyte complex on the PLA surface. Platelet adhesion assay showed that PLA surface modified by chitosan/heparin complex could inhibit platelet adhesion and activation. Cell culture assay indicated that PLA surface with CS/Hp complex showed enhanced cell adhesion.

Angiogenic bFGF expression from gas-plasma treated scaffolds

PURPOSE

In vivo experiments indicate that gas-plasma-treated D,L-polylactide polymers expressing basic fibroblast growth factor (bFGF) exhibit enhanced angiogenesis. bFGF is not a single entity, but it is instead a family of isoforms. Consequently, we sought to determine which bFGF isoforms and levels initiate angiogenesis in nude mice peritoneums.

METHODS

Cytoplasmic and nuclear bFGF were characterized for nude mice peritoneums incubated with nontreated scaffolds containing HAEC (CW), its respective polymer-only scaffolds (Cp) and gas-plasma treated scaffolds with HAEC (TW) and without cells (Tp). NuPAGE electrophoresis and WesternBreeze Chemiluminescent kits were used to analyze relative bFGF densities and molecular weights. VEGF was quantified using ImageJ.

RESULTS

bFGF bands were located at molecular weights of 24, 48, 58, 72 and 80 kDa, depending on whether they were from cytoplasms or nuclei. At 12, 24 and 72 days, 58-kDa bFGF bands were observed from nuclei of TW and Tp, 80-kDa bFGF bands were only observed in cytoplasmic fractions < or = 24 days. Total cytoplasmic and nuclear bFGF intensities increased from 12 to 24 days, then declined by 72 days.

CONCLUSIONS

(1) Gas-plasma treated scaffolds up-regulate bFGF isoforms. (2) bFGF was expressed in the nuclei; however, 80-kDa bFGF was seen only in cytoplasms.

Characterization of nanoparticle uptake by endothelial cells

Endothelium is an important target for drug or gene therapy because of its important role in the biological system. In this paper, we have characterized nanoparticle uptake by endothelial cells in cell culture. Nanoparticles were formulated using poly DL-lactide-co-glycolide polymer containing bovine serum albumin as a model protein and 6-coumarin as a fluorescent marker. It was observed that the cellular uptake of nanoparticles depends on the time of incubation and the concentration of nanoparticles in the medium. The uptake of nanoparticles was rapid with confocal microscopy demonstrating their localization mostly in the cytoplasm. The mitogenic study demonstrated biocompatability of nanoparticles with the cells. The study thus demonstrates that nanoparticles could be used for localizing therapeutic agents or gene into endothelial cells. Nanoparticles localized in the endothelium could provide prolonged drug effects because of their sustained release characterics, and also could protect the encapsulated agent from enzymatic degradation.

Cellular engineering of vascular bypass grafts: role of chemical coatings for enhancing endothelial cell attachment

Surgical treatment of vascular disease has become common. The use of synthetic materials is limited to grafts larger than 5-6mm, because of the frequency of occlusion observed with small-diameter prosthetics. An alternative would be a hybrid or tissue-engineered graft with the surface coated with a monolayer of the patient's own cells. Currently, to be effective, high-density seeding regimens have to be undertaken. This is because endothelial cells (ECs) are washed off the graft lumen once exposed to physiological blood flow. EC attachment has been shown to be significantly improved by pre-coating with substances known to attach ECs selectively. The review examines the various types of coating and bonding technology used to date to enhance endothelial cell attachment onto the surface of prosthetic vascular bypass grafts.