In vivo formation of complex microvessels lined by human endothelial cells in an immunodeficient mouse

Directed three-dimensional growth of microvascular cells and isolated microvessel fragments

Tissue engineering has promise as a means for repairing diseased and damaged tissues. A significant challenge in tissue construction relates to the constraints placed on tissue geometries resulting from diffusion limitations. An ability to incorporate a premade vasculature would overcome these difficulties and promote construct viability once implanted. Most in vitro microvascular fabrication strategies rely on surface-associated cell growth, manipulated cell monolayers, or random arrangement of cells within matrix materials. In contrast, we successfully suspended microvascular cells and isolated microvessel fragments within collagen and then microfluidically drove the mixtures into microfabricated network topologies. Developing within the 3D collagen matrix, patterned cells progressed into cord-like morphologies. These geometries were directed by the surrounding elastomer mold. With similar techniques, suspended fragments formed endothelial sprouts. By avoiding the addition of exogenous growth factors, we allowed constituent cells and fragments to autonomously develop within the constructs, providing a more physiologically relevant system for in vitro microvascular development. In addition, we present the first examples of directed endothelial cell sprouting from parent microvessel fragments. We believe this system may serve as a foundation for future in vivo fabrication of microvascular networks for tissue engineering applications.

Vascular development is disrupted by endothelial cell-specific expression of the anti-apoptotic protein Bcl-2

Endothelial cell (EC) apoptosis has been detected in remodelling blood vessels in vivo, and inhibition of EC apoptosis appears to alter vascular morphogenesis in vitro, suggesting that EC apoptosis may play a role in blood vessel remodelling. However, apoptotic EC are difficult to quantify in vivo, and studies of the incidence of EC apoptosis and the sites at which it occurs in vivo have produced contradictory results. Therefore, the specific biological roles played by EC apoptosis remain unclear. Here, we have used a transgenic approach to determine the biological function of EC apoptosis in vivo. Anti-apoptotic Bcl-2 transgenes were expressed in mice under control of the EC-specific tie2 promoter. These transgenic mice died during the second half of gestation. While the development and remodelling of large vessels including aortic arch arteries and great veins proceeded normally, abnormally dense and disorganised networks of small vessels were present in the skin and internal organs. In addition, vessel organisation and lumen formation were disrupted in the placental labyrinth. This study provides direct experimental evidence that endothelial cell apoptosis plays an essential role during embryogenesis. Our results suggest that EC apoptosis plays an important role in determining the structure of the microcirculation but may be dispensable for large vessel development.

Redefining endothelial progenitor cells via clonal analysis and hematopoietic stem/progenitor cell principals

The limited vessel-forming capacity of infused endothelial progenitor cells (EPCs) into patients with cardiovascular dysfunction may be related to a misunderstanding of the biologic potential of the cells. EPCs are generally identified by cell surface antigen expression or counting in a commercially available kit that identifies "endothelial cell colony-forming units" (CFU-ECs). However, the origin, proliferative potential, and differentiation capacity of CFU-ECs is controversial. In contrast, other EPCs with blood vessel-forming ability, termed endothelial colony-forming cells (ECFCs), have been isolated from human peripheral blood. We compared the function of CFU-ECs and ECFCs and determined that CFU-ECs are derived from the hematopoietic system using progenitor assays, and analysis of donor cells from polycythemia vera patients harboring a Janus kinase 2 V617F mutation in hematopoietic stem cell clones. Further, CFU-ECs possess myeloid progenitor cell activity, differentiate into phagocytic macrophages, and fail to form perfused vessels in vivo. In contrast, ECFCs are clonally distinct from CFU-ECs, display robust proliferative potential, and form perfused vessels in vivo. Thus, these studies establish that CFU-ECs are not EPCs and the role of these cells in angiogenesis must be re-examined prior to further clinical trials, whereas ECFCs may serve as a potential therapy for vascular regeneration.

Clonogenic endothelial progenitor cells are sensitive to oxidative stress

Endothelial progenitor cells (EPCs) circulate in the peripheral blood and reside in blood vessel walls. A hierarchy of EPCs exists where progenitors can be discriminated based on their clonogenic potential. EPCs are exposed to oxidative stress during vascular injury as residents of blood vessel walls or as circulating cells homing to sites of neovascularization. Given the links between oxidative injury, endothelial cell dysfunction, and vascular disease, we tested whether EPCs were sensitive to oxidative stress using newly developed clonogenic assays. Strikingly, in contrast to previous reports, we demonstrate that the most proliferative EPCs (high proliferative potential-endothelial colony-forming cells and low proliferative potential-endothelial colony-forming cells) had decreased clonogenic capacity after oxidant treatment. In addition, EPCs exhibited increased apoptosis and diminished tube-forming ability in vitro and in vivo in response to oxidative stress, which was directly linked to activation of a redox-dependent stress-induced kinase pathway. Thus, this study provides novel insights into the effect of oxidative stress on EPCs. Furthermore, this report outlines a framework for understanding how oxidative injury leads to vascular disease and potentially limits the efficacy of transplantation of EPCs into ischemic tissues enriched for reactive oxygen species and oxidized metabolites.

Gene Expression Profiling Reveals Platelet-Derived Growth Factor Receptor Alpha as a Target of Cell Contact-Dependent Gene Regulation in an Endothelial Cell–Osteoblast Co-culture Model

Angiogenesis plays an important role in bone development, repair, and remodelling. Neovascularization is furthermore a crucial step in bone tissue engineering because implantation of voluminous grafts without sufficient vascularity results in hypoxic cell death of the engineered tissue. We have previously described a co-cultivation system of human primary osteoblasts and human primary endothelial cells that was developed to improve neovascularization in bone tissue-engineering applications. In our present study, we have performed complementary deoxyribonucleic acid microarray analysis to analyze putative changes in osteoblastic gene expression upon co-cultivation of osteoblasts and endothelial cells. Transcriptional profiling revealed upregulation of 79 genes and downregulation of 62 genes in osteoblasts after co-cultivation with endothelial cells. To verify the microarray data, quantitative real-time reverse transcriptase polymerase chain reaction was carried out on selected genes. The expression of the platelet-derived growth factor receptor alpha gene in osteoblasts was analyzed in more detail, revealing that a cell contact-dependent mechanism, and not paracrine-acting diffusible factors, mediates the downregulation of this receptor in osteoblasts upon co-cultivation with endothelial cells. In summary, the data demonstrate complex gene-regulation mechanisms between endothelial cells and osteoblasts that are likely to play a role in bone morphogenesis.

Endothelial Cells Assemble into a 3-Dimensional Prevascular Network in a Bone Tissue Engineering Construct

To engineer tissues with clinically relevant dimensions, one must overcome the challenge of rapidly creating functional blood vessels to supply cells with oxygen and nutrients and to remove waste products. We tested the hypothesis that endothelial cells, cocultured with osteoprogenitor cells, can organize into a prevascular network in vitro. When cultured in a spheroid coculture model with human mesenchymal stem cells, human umbilical vein endothelial cells (HUVECs) form a 3-dimensional prevascular network within 10 days of in vitro culture. The formation of the prevascular network was promoted by seeding 2% or fewer HUVECs. Moreover, the addition of endothelial cells resulted in a 4-fold upregulation of the osteogenic marker alkaline phosphatase. The addition of mouse embryonic fibroblasts did not result in stabilization of the prevascular network. Upon implantation, the prevascular network developed further and structures including lumen could be seen regularly. However, anastomosis with the host vasculature was limited. We conclude that endothelial cells are able to form a 3-dimensional (3D) prevascular network in vitro in a bone tissue engineering setting. This finding is a strong indication that in vitro prevascularization is a promising strategy to improve implant vascularization in bone tissue engineering.

Vascularization and engraftment of a human skin substitute using circulating progenitor cell-derived endothelial cells

We seeded tissue engineered human skin substitutes with endothelial cells (EC) differentiated in vitro from progenitors from umbilical cord blood (CB-EC) or adult peripheral blood (AB-EC), comparing the results to previous work using cultured human umbilical vein EC (HUVEC) with or without Bcl-2 transduction. Vascularized skin substitutes were prepared by seeding Bcl-2-transduced or nontransduced HUVEC, CB-EC, or AB-EC on the deep surface of decellularized human dermis following keratinocyte coverage of the epidermal surface. These skin substitutes were transplanted onto C.B-17 SCID/beige mice receiving systemic rapamycin or vehicle control and were analyzed 21 d later. CB-EC and Bcl-2-HUVEC formed more human EC-lined vessels than AB-EC or control HUVEC; CB-EC, Bcl-2-HUVEC, and AB-EC but not control HUVEC promoted ingrowth of mouse EC-lined vessels. Bcl-2 transduction increased the number of human and mouse EC-lined vessels in grafts seeded with HUVEC but not with CB-EC or AB-EC. Both CB-EC and AB-EC-induced microvessels became invested by smooth muscle cell-specific alpha-actin-positive mural cells, indicative of maturation. Rapamycin inhibited ingrowth of mouse EC-lined vessels but did not inhibit formation of human EC-lined vessels. We conclude that EC differentiated from circulating progenitors can be utilized to vascularize human skin substitutes even in the setting of compromised host angiogenesis/vasculogenesis.

Targeted imaging of human endothelial-specific marker in a model of adoptive cell transfer

Imaging of endothelial-specific markers is critically important in non-invasive detection of early signs of vascular pathologies (eg inflammation, atherosclerosis and angiogenesis). A model of adoptive human endothelial cell (HUVEC) transfer was used to test-specific imaging probes for human vascular disease consisting of cross-linked iron oxide (CLIO) nanoparticles conjugated to anti-human E-selectin (CLIO-F(ab')(2)). To perform in vivo imaging of E-selectin expression in functional blood vessels, human vascular endothelium cells (HUVECs) were implanted in athymic mice in Matrigel solution, which served as a temporary neovascularization scaffold after the solidification. The formation of HUVEC-containing vessels was established by histology and microscopy. CLIO-F(ab')(2) probes were administered via an i.v. injection following the induction of E-selectin expression by IL-1beta. High-resolution MR images were obtained before and after the administration of CLIO-F(ab')(2), which showed specific hypointensity only if treated with IL-1beta. A three-times higher CLIO-induced MR signal decrease on T2(*) images was measured in HUVEC implants in response to IL-1beta treatment. Image signal intensity did not change in control animals that: (1) harbored Matrigel alone, (2) in the absence of IL-1beta treatment or (3) in animals injected with CLIO linked to the idiotype-matched control F(ab')(2). Experiments in an adoptive transfer model demonstrated that HUVEC-containing neovessels are perfused and that IL-1beta inducible E-selectin expression in these vessels is detectable with non-invasive imaging by using targeted nanoparticles.

Development and characterization of a spheroidal coculture model of endothelial cells and fibroblasts for improving angiogenesis in tissue engineering

Neovascularization is a critical step in tissue engineering applications since implantation of voluminous grafts without sufficient vascularity results in hypoxic cell death of central tissues. We have developed a three-dimensional spheroidal coculture system consisting of human umbilical vein endothelial cells (HUVECs) and human primary fibroblasts (hFBs) to improve angiogenesis in tissue engineering applications. Morphological analysis of cryosections from HUVEC/hFB cospheroids revealed a characteristic temporal and spatial organization with HUVECs located in the center of the cospheroid and a peripheral localization of fibroblasts. In coculture spheroids, the level of apoptosis of endothelial cells was strongly decreased upon cocultivation with fibroblasts. Collagen-embedded HUVEC spheroids develop numerous lumenized capillary-like sprouts. This was also apparent for HUVEC/hFB cospheroids, albeit to a lesser extent. Quantification of cumulative sprout length revealed an approximately 35% reduction in endothelial cell sprouting upon cocultivation with fibroblasts in cospheroids. The slight reduction in endothelial cell sprouting was not mediated by a paracrine mechanism but is most likely due to the formation of heterogenic cell contacts between HUVECs and hFBs within the cospheroid. The model system introduced in this study is suitable for the development of a preformed lumenized capillary-like network ex vivo and may therefore be useful for improving angiogenesis in in vivo tissue engineering applications.

Engineered skin substitutes: practices and potentials

Wound healing can be problematic in several clinical settings because of massive tissue injury (burns), wound healing deficiencies (chronic wounds), or congenital conditions and diseases. Engineered skin substitutes have been developed to address the medical need for wound coverage and tissue repair. Currently, no engineered skin substitute can replace all of the functions of intact human skin. A variety of biologic dressings and skin substitutes have however contributed to improved outcomes for patients suffering from acute and chronic wounds. These include acellular biomaterials and composite cultured skin analogs containing allogeneic or autologous cultured skin cells.

Improved microvascular network in vitro by human blood outgrowth endothelial cells relative to vessel-derived endothelial cells

Evidence suggests that bone marrow-derived cells circulating in adult blood, sometimes called endothelial progenitor cells, contribute to neovascularization in vivo and give rise to cells expressing endothelial markers in culture. To explore the utility of blood-derived cells expressing an endothelial phenotype for creating tissue-engineered microvascular networks, we employed a three-dimensional in vitro angiogenesis model to compare microvascular network formation by human blood outgrowth endothelial cells (HBOECs) with three human vessel-derived endothelial cell (EC) types: human umbilical vein ECs (HUVECs), and adult and neonatal human microvascular ECs. Under every condition investigated, HBOECs within collagen gels elongated significantly more than any other cell type. Under all conditions investigated, gel contraction and cell elongation were correlated, with HBOECs demonstrating the largest generation of force. HBOECs did not exhibit a survival advantage, nor did they enhance elongation of HUVECs when the two cell types were cocultured. Network formation of both HBOECs and HUVECs was inhibited by blocking antibodies to alpha2beta1, but not alpha(v)beta3, integrins. Taken together, these data suggest that superior network exhibited by HBOECs relative to vessel-derived endothelial cells is not due to a survival advantage, use of different integrins, or secretion of an autocrine/paracrine factor, but may be related to increased force generation.

Three-dimensional tissue culture models in cancer biology

Three-dimensional (3D) tissue culture models have an invaluable role in tumour biology today providing some very important insights into cancer biology. As well as increasing our understanding of homeostasis, cellular differentiation and tissue organization they provide a well defined environment for cancer research in contrast to the complex host environment of an in vivo model. Due to their enormous potential 3D tumour cultures are currently being exploited by many branches of biomedical science with therapeutically orientated studies becoming the major focus of research. Recent advances in 3D culture and tissue engineering techniques have enabled the development of more complex heterologous 3D tumour models.

Endothelial progenitor cell sprouting in spheroid cultures is resistant to inhibition by osteoblasts: A model for bone replacement grafts

Survival of tissue transplants generated in vitro is strongly limited by the slow process of graft vascularization in vivo. A method to enhance graft vascularization is to establish a primitive vascular plexus within the graft prior to transplantation. Endothelial cells (EC) cultured as multicellular spheroids within a collagen matrix form sprouts resembling angiogenesis in vitro. However, osteoblasts integrated into the graft suppress EC sprouting. This inhibition depends on direct cell-cell-interactions and is characteristic of mature ECs isolated from preexisting vessels. In contrast, sprouting of human blood endothelial progenitor cells is not inhibited by osteoblasts, making these cells suitable for tissue engineering of pre-vascularized bone grafts.

Therapeutic neovascularization: contributions from bioengineering

A number of pathological entities and surgical interventions could benefit from therapeutic stimulation of new blood vessel formation. Although strategies designed for promoting neovascularization have shown promise in preclinical models, translation to human application has met with limited success when angiogenesis is used as the single therapeutic mechanism. While clinical protocols continue to be optimized, a number of exciting new approaches are being developed. Bioengineering has played an important role in the progress of many of these innovative new strategies. In this review, we present a general outline of therapeutic neovascularization, with an emphasis on investigations using engineering principles to address this vexing clinical problem. In addition, we identify some limitations and suggest areas for future research.

Self-assembling short oligopeptides and the promotion of angiogenesis

Because an adequate blood supply to and within tissues is an essential factor for successful tissue regeneration, promoting a functional microvasculature is a crucial factor for biomaterials. In this study, we demonstrate that short self-assembling peptides form scaffolds that provide an angiogenic environment promoting long-term cell survival and capillary-like network formation in three-dimensional cultures of human microvascular endothelial cells. Our data show that, in contrast to collagen type I, the peptide scaffold inhibits endothelial cell apoptosis in the absence of added angiogenic factors, accompanied by enhanced gene expression of the angiogenic factor VEGF. In addition, our results suggest that the process of capillary-like network formation and the size and spatial organization of cell networks may be controlled through manipulation of the scaffold properties, with a more rigid scaffold promoting extended structures with a larger inter-structure distance, as compared with more dense structures of smaller size observed in a more compliant scaffold. These findings indicate that self-assembling peptide scaffolds have potential for engineering vascularized tissues with control over angiogenic processes. Since these peptides can be modified in many ways, they may be uniquely valuable in regeneration of vascularized tissues.

Inosculation of tissue-engineered capillaries with the host's vasculature in a reconstructed skin transplanted on mice

The major limitation for the application of an autologous in vitro tissue-engineered reconstructed skin (RS) for the treatment of burnt patients is the delayed vascularization of its relatively thick dermal avascular component, which may lead to graft necrosis. We have developed a human endothelialized reconstructed skin (ERS), combining keratinocytes, fibroblasts and endothelial cells (EC) in a collagen sponge. This skin substitute then spontaneously forms a network of capillary-like structures (CLS) in vitro. After transplantation to nude mice, we demonstrated that CLS containing mouse blood were observed underneath the epidermis in the ERS in less than 4 days, a delay comparable to our human skin control. In comparison, a 14-day period was necessary to achieve a similar result with the non-endothelialized RS. Furthermore, no mouse blood vessels were ever observed close to the epidermis before 14 days in the ERS and the RS. We thus concluded that the early vascularization observed in the ERS was most probably the result of inosculation of the CLS network with the host's capillaries, rather than neovascularization, which is a slower process. These results open exciting possibilities for the clinical application of many other tissue-engineered organs requiring a rapid vascularization.

Attenuation of retinal vascular development and neovascularization during oxygen-induced ischemic retinopathy in Bcl-2−/− mice

Bcl-2 is a death repressor that protects cells from apoptosis mediated by a variety of stimuli. Bcl-2 expression is regulated by both pro- and anti-angiogenic factors; thus, it may play a central role during angiogenesis. However, the role of bcl-2 in vascular development and growth of new vessels requires further delineation. In this study, we investigated the physiological role of bcl-2 in development of retinal vasculature and retinal neovascularization during oxygen-induced ischemic retinopathy (OIR). Mice deficient in bcl-2 exhibited a significant decrease in retinal vascular density compared to wild-type mice. This was attributed to a decreased number of endothelial cells and pericytes in retinas from bcl-2-/- mice. We observed, in bcl-2-/- mice, delayed development of retinal vasculature and remodeling, and a significant decrease in the number of major arteries, which branch off from near the optic nerve. Interestingly, hyaloid vessel regression, an apoptosis-dependent process, was not affected in the absence of bcl-2. The retinal vasculature of bcl-2-/- mice exhibited a similar sensitivity to hyperoxia-mediated vessel obliteration compared to wild-type mice during OIR. However, the degree of ischemia-induced retinal neovascularization was significantly reduced in bcl-2-/- mice. These results suggest that expression of bcl-2 is required for appropriate development of retinal vasculature as well as its neovascularization during OIR.

Endothelial cell modulation of bone marrow stromal cell osteogenic potential

In the context of bone development and regeneration, the intimate association of the vascular endothelium with osteogenic cells suggests that endothelial cells (ECs) may directly regulate the differentiation of osteoprogenitor cells. To investigate this question, bone marrow stromal cells (BMSCs) were cultured: in the presence of EC-conditioned medium, on EC extracellular matrix, and in EC cocultures with and without cell contact. RNA and protein were isolated from ECs and analyzed by reverse transcriptase-polymerase chain reaction and Western blotting, respectively, for expression of bone morphogenetic protein 2 (BMP-2). In animal studies, BMSCs and ECs were cotransplanted into severe combined immunodeficient mice on biodegradable polymer matrices, and histomorphometric analysis was performed to determine the extent of new bone and blood vessel formation. ECs significantly increased BMSC osteogenic differentiation in vitro only when cultured in direct contact. ECs expressed BMP-2, and experiments employing interfering RNA inhibition confirmed its production as contributing to the increased BMSC osteogenic differentiation. In vivo, cotransplantation of ECs with BMSCs resulted in greater bone formation than did transplantation of BMSCs alone. These data suggest that ECs function not only to form the microvasculature that delivers nutrients to developing bone but also to modulate the differentiation of osteoprogenitor cells in vitro and in vivo.

Diffusion Limits of an in Vitro Thick Prevascularized Tissue

Although tissue engineering promises to replace or restore lost function to nearly every tissue in the body, successful applications are currently limited to tissue less than 2 mm in thickness. in vivo capillary networks deliver oxygen and nutrients to thicker (> 2 mm) tissues, suggesting that introduction of a preformed in vitro vascular network may be a useful strategy for engineered tissues. This article describes a system for generating capillary-like networks within a thick fibrin matrix. Human umbilical vein endothelial cells, growing on the surface of microcarrier beads, were embedded in fibrin gels a known distance (Delta = 1.8-4.5 mm) from a monolayer of human dermal fibroblasts. The distance of the growth medium, which contained vascular endothelial growth factor and basic fibroblast growth factor, from the beads, C, was varied from 2.7 to 7.2 mm. Capillaries with visible lumens sprouted in 2-3 days, reaching lengths that exceeded 500 microm within 6-8 days. On day 7, capillary network formation was largely independent of C; however, a strong inverse correlation with Delta was observed, with the maximum network formation at Delta = 1.8 mm. Surprisingly, the thickness of the gel was not a limiting factor for oxygen diffusion as these tissue constructs retained a relatively high oxygen tension of > 125 mmHg. We conclude that diffusion of oxygen in vitro is not limiting, allowing the development of tissue constructs on the order of centimeters in thickness. In addition, diffusion of fibroblast-derived soluble mediators is necessary for stable capillary formation, but is significantly impeded relative to that of nutrients present in the medium.

Induction, differentiation, and remodeling of blood vessels after transplantation of Bcl-2-transduced endothelial cells

Implants of collagen-fibronectin gels containing Bcl-2-transduced human umbilical vein endothelial cells (Bcl-2-HUVECs) induce the formation of human endothelial cell (EC)/murine vascular smooth muscle cell (VSMC) chimeric vessels in immunodeficient mice. Microfil casting of the vasculature 60 d after implantation reveals highly branched microvascular networks within the implants that connect with and induce remodeling of conduit vessels arising from the abdominal wall circulation. Approximately 85% of vessels within the implants are lined by Bcl-2-positive human ECs expressing VEGFR1, VEGFR2, and Tie-2, but not integrin alpha(v)beta(3). The human ECs are seated on a well formed human laminin/collagen IV-positive basement membrane, and are surrounded by mouse VSMCs expressing SM-alpha actin, SM myosin, SM22alpha, and calponin, all markers of contractile function. Transmission electron microscopy identified well formed EC-EC junctions, chimeric arterioles with concentric layers of contractile VSMC, chimeric capillaries surrounded by pericytes, and chimeric venules. Bcl-2-HUVEC-lined vessels retain 70-kDa FITC-dextran, but not 3-kDa dextran; local histamine rapidly induces leak of 70-kDa FITC-dextran or India ink. As in skin, TNF induces E-selectin and vascular cell adhesion molecule 1 only on venular ECs, whereas intercellular adhesion molecule-1 is up-regulated on all human ECs. Bcl-2-HUVEC implants are able to engraft within and increase perfusion of ischemic mouse gastrocnemius muscle after femoral artery ligation. These studies show that cultured Bcl-2-HUVECs can differentiate into arterial, venular, and capillary-like ECs when implanted in vivo, and induce arteriogenic remodeling of the local mouse vessels. Our results support the utility of differentiated EC transplantation to treat tissue ischemia.

Murine endothelial cell lines as models of tumor endothelial cells

Identification of appropriate models for in vivo and in vitro preclinical testing of inhibitors of tumor angiogenesis and progression is vital to the successful development of anticancer therapeutics. Although the focus is on human molecular targets, most preclinical in vivo efficacy testing occurs in mice. The goal of the current studies was to identify a murine endothelial cell line to model tumor endothelium for studying the antiangiogenic activity of therapeutic compounds in vitro. In situ hybridization was performed on three s.c. grown syngeneic murine tumors (B16 melanoma, Lewis lung carcinoma, and CT26 colon carcinoma) to assess expression of murine homologs of human tumor endothelial cell markers in the vasculature of these tumor models. Seven murine endothelial cell lines were characterized for expression of the murine homologs of recognized endothelial cell surface markers as well as for tumor endothelial cell surface markers. The seven murine endothelial cell lines had similar generation times and five of the seven lines were able to form tubes on Matrigel. Real-time-PCR and flow cytometry analysis were used to evaluate relative mRNA and protein expression of murine homologs of several recognized endothelial cell surface markers in the seven cell lines. The expression of the mRNA for the murine homologs of five tumor endothelial cell surface markers was also evaluated. The 2H11 cell line expressed all five of the tumor endothelial cell surface markers as well as several well-recognized endothelial cells markers. The 2H11 cell line responds to known and novel antiangiogenic agents by inhibition of proliferation and tube formation. These cells can be used in in vitro angiogenesis assays for evaluating the potential antiangiogenic properties and interspecies cross-reactivity of novel compounds.

Tissue-engineered microvessels on three-dimensional biodegradable scaffolds using human endothelial progenitor cells

Tissue engineering may offer patients new options when replacement or repair of an organ is needed. However, most tissues will require a microvascular network to supply oxygen and nutrients. One strategy for creating a microvascular network would be promotion of vasculogenesis in situ by seeding vascular progenitor cells within the biopolymeric construct. To pursue this strategy, we isolated CD34(+)/CD133(+) endothelial progenitor cells (EPC) from human umbilical cord blood and expanded the cells ex vivo as EPC-derived endothelial cells (EC). The EPC lost expression of the stem cell marker CD133 but continued to express the endothelial markers KDR/VEGF-R2, VE-cadherin, CD31, von Willebrand factor, and E-selectin. The cells were also shown to mediate calcium-dependent adhesion of HL-60 cells, a human promyelocytic leukemia cell line, providing evidence for a proinflammatory endothelial phenotype. The EPC-derived EC maintained this endothelial phenotype when expanded in roller bottles and subsequently seeded on polyglycolic acid-poly-l-lactic acid (PGA-PLLA) scaffolds, but microvessel formation was not observed. In contrast, EPC-derived EC seeded with human smooth muscle cells formed capillary-like structures throughout the scaffold (76.5 +/- 35 microvessels/mm(2)). These results indicate that 1) EPC-derived EC can be expanded in vitro and seeded on biodegradable scaffolds with preservation of endothelial phenotype and 2) EPC-derived EC seeded with human smooth muscle cells form microvessels on porous PGA-PLLA scaffolds. These properties indicate that EPC may be well suited for creating microvascular networks within tissue-engineered constructs.

Modulation ofIn VitroAngiogenesis in a Three-Dimensional Spheroidal Coculture Model for Bone Tissue Engineering

One of the major challenges in tissue engineering of bone substitutes remains vascularization of the transplant. We have developed a three-dimensional collagen-based coculture system to assess interactions between human endothelial cells (hECs) and human osteoblasts (hOBs) in vitro. Human umbilical vein endothelial cells (HUVECs) were grown as three-dimensional multicellular spheroids and seeded in a collagen matrix to assess sprouting of the spheroids, that is, formation of tubelike structures resembling early capillaries. Direct cell contact between hOBs and HUVECs was established by incorporating hOBs into the EC spheroids, thus forming heterogeneous cospheroids. Spatial organization of cospheroids and sprout configuration were assessed by immunohistochemical wholemount staining techniques and confocal laser microscopy. Cumulative sprout length of spheroids was quantitatively analyzed by digital imaging planimetry. In this model HUVECs and hOBs formed heterogeneous cospheroids with distinct spatial organization. The ability of HUVEC spheroids to form tubelike structures on angiogenic stimulation with vascular endothelial growth factor and basic fibroblast growth factor was suppressed in heterogeneous HUVEC/hOB cospheroids. The model system introduced in this study may be useful to assess the mechanisms involved in regulating angiogenesis during bone formation and to further investigate the mechanisms by which heterotypic cell-cell interactions inhibit endothelial tube formation for applications in bone tissue engineering.

Bcl-2 Transduction Protects Human Endothelial Cell Synthetic Microvessel Grafts from Allogeneic T Cells In Vivo

T cell interactions with vascular endothelial cells (EC) are of central importance for immune surveillance of microbes and for pathological processes such as atherosclerosis, allograft rejection, and vasculitis. Animal (especially rodent) models incompletely predict human immune responses, in particular with regard to the immunological functions of EC, and in vitro models may not accurately reflect in vivo findings. In this study, we describe the development of an immunodeficient SCID/bg murine model combining a transplanted human synthetic microvascular bed with adoptive transfer of human T lymphocytes allogeneic to the cells of the graft that more fully recapitulates T cell responses in natural tissues. Using this model, we demonstrate that transduced Bcl-2 protein in the engrafted EC effectively prevents injury even as it enhances T cell graft infiltration and replication.

Stem cell-derived angiogenic/vasculogenic cells: possible therapies for tissue repair and tissue engineering

1. The recent ability to isolate stem cells and study their specific capacity of self-renewal with the formation of different cell types has opened up exciting vistas to help the repair of damaged tissue and even the formation of new tissue. In the present review, we deal with the characteristics and sources that stem cells can be derived and cultured from. 2. We focus on the role that stem cell-derived vascular cells or endothelial progenitor cells (EPC) may play in (re)vascularization of ischaemic and engineered tissues. This so-called vasculogenesis resembles the embryological process in which 'haemangioblasts' differentiate in blood cells, as well as in primitive vessels. Although also derived from the blood-forming bone marrow, in adult life vasculogenic stem cells contribute only little to the regular vascular repair mechanisms: namely (i) angiogenesis (outgrowth of vessels from existing vessels); and (ii) arteriogenesis (monocyte-aided increase in the calibre of existing arteriolar collaterals). 3. Most attempts to increase vascular repair by stem cells involve the use of growth factors, which mobilize stem cells from bone marrow into the blood, sometimes combined with isolation and reinfusion of these cells after ex vivo expansion and differentiation into EPC. 4. Clear improved perfusion of ischaemic sites and new vasculature has been observed in vivo mostly in animal models. Specific homing or administration of these cells and regulated and quantitative expansion and (final) differentiation at these vascular (repair) sites are less studied, but are paramount for efficacy and safety. 5. In conclusion, the use of embryonic stem cells will still encounter ethical objections. Moreover, special attention and measures are needed to cope with the allogeneic barriers that these cells usually encounter. In general, the long and complicated ex vivo cultures to obtain sufficient offspring from the very small numbers of stem cells that can be obtained as starting material will be costly and cumbersome. Both basic research on conceptual matters and cost-effective development of the product itself will have to go a long way before the clinical use of some volume can be expected.

The role of angiopoietins in the development of endothelial cells from cord blood CD34+ progenitors

Circulating endothelial progenitors contribute to neovascularization at sites of injury and tumorigenesis in postnatal life. Yet, the molecular mechanisms initiating the endothelial developmental program of these precursors remain elusive. Here we provide evidence that endothelial development from progenitors circulating in human cord blood requires angiopoietins, a set of growth factors also involved in vascular branching during embryogenesis. We show that cord blood cells with the potential for endothelial development reside in a CD34(+)CD11b+ subset capable of autonomously producing and binding angiopoietins. Functionally, endogenous angiopoietin-1 regulates initial endothelial cell commitment, whereas angiopoietin-2 enhances expansion of the endothelial cell progeny. These findings suggest a role for angiopoietins as regulators of endothelial development from circulating progenitors and imply a function of angiopoietins at distinct developmental steps in postnatal angiogenesis.

Rapid Perfusion and Network Remodeling in a Microvascular Construct After Implantation

OBJECTIVE

We have previously demonstrated the ability to construct 3-dimensional microvascular beds in vitro via angiogenesis from isolated, intact, microvessel fragments that retain endothelial cells and perivascular cells. Our objective was to develop and characterize an experimental model of tissue vascularization, based on the implantation of this microvascular construct, which recapitulated angiogenesis, vessel differentiation, and network maturation.

METHODS AND RESULTS

On implantation in a severe combined-immunodeficient mouse model, vessels in the microvascular constructs rapidly inosculated with the recipient host circulation. Ink perfusion of implants via the left ventricle of the host demonstrated that vessel inosculation begins within the first day after implantation. Evaluation of explanted constructs over the course of 28 days revealed the presence of a mature functional microvascular bed. Using a probe specific for the original microvessel source, 91.7%+/-11% and 88.6%+/-19% of the vessels by day 5 and day 28 after implantation, respectively, were derived from the original microvessel isolate. Similar results were obtained when human-derived microvessels were used to build the microvascular construct.

CONCLUSIONS

With this model, we reproduce the important aspects of vascularization, angiogenesis, inosculation, and network remodeling. Furthermore, we demonstrate that the model accommodates human-derived vessel fragments, enabling the construction of human-mouse vascular chimeras.

Creation of long-lasting blood vessels

The construction of stable blood vessels is a fundamental challenge for tissue engineering in regenerative medicine. Although certain genes can be introduced into vascular cells to enhance their survival and proliferation, these manipulations may be oncogenic. We show here that a network of long-lasting blood vessels can be formed in mice by co-implantation of vascular endothelial cells and mesenchymal precursor cells, by-passing the need for risky genetic manipulations. These networks are stable and functional for one year in vivo.

Protease nexin-1 inhibits plasminogen activation-induced apoptosis of adherent cells

Degradation of adhesive glycoproteins by plasmin is implicated in cell migration. In this study, we further explored the role of plasminogen activation in cell adhesion and survival and show that uncontrolled plasminogen activation at the cell surface may induce cell detachment and apoptosis. We hypothesized that this process could be prevented in adherent cells by expression of protease nexin-1, a potent serpin able to inhibit thrombin, plasmin, and plasminogen activators. Using two- and three-dimensional culture systems, we demonstrate that Chinese hamster ovary fibroblasts constitutively express tissue-type plasminogen activator and efficiently activate exogenously added plasminogen in a specific and saturable manner (K(m) = 46 nm). The formation of plasmin results in proteolysis of fibronectin and laminin, which is followed by cell detachment and apoptosis. Protease nexin-1 expressed by transfected cells significantly inhibited the activity of plasmin and tissue-type plasminogen activator via the formation of inhibitory complexes and prevented cell detachment and apoptosis. In conclusion, protease nexin-1 may be an important anti-apoptotic factor for adherent cells. This cell model could be a useful tool to evaluate therapeutic agents such as serpins in vascular pathologies involving pericellular protease-protease inhibitor imbalance.

Treatment of melanoma cells with a bcl-2/bcl-xL antisense oligonucleotide induces antiangiogenic activity

We have recently reported that bcl-2 overexpression and hypoxia synergistically interact to modulate vascular endothelial growth factor (VEGF) and in vivo angiogenesis in tumour cells through VEGF mRNA stabilization and hypoxia-inducible factor 1-mediated transcriptional activity. Bcl-2 antisense treatment has shown promising clinical results in patients with malignant melanoma. In the present study, we demonstrated that the bcl-2/bcl-xL bispecific antisense oligonucleotide 4625 inhibits bcl-2 expression and angiogenesis in two bcl-2 overexpressing clones derived from the M14 human melanoma cell line. The antiangiogenic effect was determined in in vitro and in vivo angiogenesis assays. In particular, a reduction of hypoxia-induced VEGF secretion was observed after 4625 treatment, and the conditioned medium (CM) of bcl-2 overexpressing clones treated with 4625 and exposed to hypoxic conditions resulted in decreased endothelial cell proliferation when compared to CM of untreated control cells. In addition, we found that CM of 4625 antisense-treated bcl-2 transfectants inhibited in vivo vessel formation in matrigel plugs implanted subcutaneously in C57/B16 mice. Our findings confirm that bcl-2 plays a crucial role in melanoma angiogenesis and demonstrate for the first time that downregulation of bcl-2 by antisense treatment has potential to inhibit angiogenesis independent of its effect on cell survival. The use of 4625 in cancer therapy is suggested as an approach to facilitate simultaneously tumour cell apoptosis and inhibit tumour angiogenesis.

Engraftment of a vascularized human skin equivalent

Clinical performance of currently available human skin equivalents is limited by failure to develop perfusion. To address this problem we have developed a method of endothelial cell transplantation that promotes vascularization of human skin equivalents in vivo. Enhancement of vascularization by Bcl-2 overexpression was demonstrated by seeding human acellular dermis grafts with human umbilical vein endothelial cells (HUVEC) transduced with the survival gene Bcl-2 or an EGFP control transgene, and subcutaneous implantation in immunodeficient mice (n=18). After 1 month the grafts with Bcl-2-transduced cells contained a significantly greater density of perfused HUVEC-lined microvessels (55.0/mm3) than controls (25.4/mm3,P=0.026). Vascularized skin equivalents were then constructed by sequentially seeding the apical and basal surfaces of acellular dermis with cultured human keratinocytes and Bcl-2-transduced HUVEC, respectively. Two weeks after orthotopic implantation onto mice, 75% of grafts (n=16) displayed both a differentiated human epidermis and perfusion through HUVEC-lined microvessels. These vessels, which showed evidence of progressive maturation, accelerated the rate of graft vascularization. Successful transplantation of such vascularized human skin equivalents should enhance clinical utility, especially in recipients with impaired angiogenesis.

A tissue-engineered endothelialized dermis to study the modulation of angiogenic and angiostatic molecules on capillary-like tube formation in vitro

BACKGROUND

Because angiogenesis is a major feature of different physiological and pathological situations, the identification of factors that stimulate or inhibit this process and the elucidation of their mechanisms of action are most certainly of clinical relevance. We have produced a new model of endothelialized reconstructed dermis that promotes the spontaneous formation of a human capillary-like network and its stabilization in vitro for a period longer than 1 month.

OBJECTIVES

The aim of this work was to describe the three-dimensional structure of the capillary-like network. Thereafter we strove to study, quantitatively and qualitatively, the influence of angiogenic and angiostatic drugs on capillary-like tube (CLT) formation in vitro in the model.

METHODS

The endothelialized dermis was prepared by coculturing two human cell types, dermal fibroblasts and umbilical vein endothelial cells, in a collagen sponge biomaterial.

RESULTS

The visualization by confocal microscopy of the tubes present in the model showed that the endothelial structures were not cord-like but rather CLTs with well-defined lumina. Moreover, these tubes were organized in a complex network of branching structures. When angiogenic factors (vascular endothelial growth factor 10 ng mL-1 or basic fibroblast growth factor 10 ng mL-1) were added to the model, 1.8 and 1.4 times more capillaries, respectively, were observed, whereas the addition of progesterone (10 microg x mL(-1)) reduced by 2.4 times the number of tubes compared with the control.

CONCLUSIONS

These results suggest that this model is a highly efficient assay for the screening of potentially angiogenic and angiostatic compounds.

Molecular regulation of vessel maturation

The maturation of nascent vasculature, formed by vasculogenesis or angiogenesis, requires recruitment of mural cells, generation of an extracellular matrix and specialization of the vessel wall for structural support and regulation of vessel function. In addition, the vascular network must be organized so that all the parenchymal cells receive adequate nutrients. All of these processes are orchestrated by physical forces as well as by a constellation of ligands and receptors whose spatio-temporal patterns of expression and concentration are tightly regulated. Inappropriate levels of these physical forces or molecules produce an abnormal vasculature--a hallmark of various pathologies. Normalization of the abnormal vasculature can facilitate drug delivery to tumors and formation of a mature vasculature can help realize the promise of therapeutic angiogenesis and tissue engineering.

Altered angiogenesis and survival in human tumor-derived endothelial cells

Knowledge on the functional properties of tumor-derived endothelial cells (TEC) can be relevant for the development of antiangiogenic therapeutic strategies. In the present study, we obtained and characterized endothelial cell lines from human renal carcinomas. TEC did not undergo senescence and showed constant expression of markers of endothelial activation and angiogenesis. In vitro, TEC, in contrast to normal endothelial cells, were resistant to apoptosis, proadhesive for renal carcinoma cells, and able to grow and organize in the absence of serum in persistent capillary-like structures. In vivo, TEC were able to grow in immunodeficient mice and to form vascular structures connected with the circulation. At a molecular level, gene array analysis showed an increased expression of genes involved in survival and cell adhesion compared with expression in normal microvascular endothelial cells. Moreover, expression of angiopoietin-1 and vascular endothelial growth factor (VEGF)-D and the Akt survival pathway were up-regulated. Inhibition of interaction of VEGFR-2 or VEGFR-3 with VEGF-D but not of Tie-2-angiopoietin-1 interaction with soluble receptors abrogated Akt activation and survival of TEC. These results indicate that at least some of the TEC within a tumor display abnormal characteristics in terms of survival and angiogenic properties and also indicate the presence of a functional autocrine pathway related to VEGF-D.

Systemic delivery of human growth hormone using genetically modified tissue-engineered microvascular networks: prolonged delivery and endothelial survival with inclusion of nonendothelial cells

Endothelial cells have the potential to provide efficient long-term delivery of therapeutic proteins to the circulation if a sufficient number of genetically modified endothelial cells can be incorporated into the host vasculature and if these cells persist for an adequate period of time. Here we describe the ability of nonendothelial cells to modulate the survival of implanted endothelial cells and their incorporation into host vasculature. Bovine aortic endothelial cells (BAECs) suspended in Matrigel and cultured in vitro remained spherical and decreased in number over time. Subcutaneous implantation of gels containing BAECs secreting human growth hormone (hGH) in mice initially resulted in detectable plasma hGH levels, which were undetectable after 2 weeks. When mixed with fibroblasts and suspended in Matrigel, hGH-secreting BAECs formed microvascular networks in vitro. Implantation of these gels resulted in plasma hGH levels that decreased slightly over 2 weeks and then remained stable for at least 6 weeks. BAECs incorporated into blood vessels within both the implant and fibrous capsule that surrounded and invaded implants. Within implants containing BAECs and fibroblasts, viable BAECs were present for at least 6 weeks at a higher density than in implants containing BAECs alone at 3 weeks. These results indicate that implanted BAECs can incorporate into host blood vessels and that inclusion of fibroblasts in this system prolongs BAEC survival and hGH delivery.

Principals of neovascularization for tissue engineering

The goals in tissue engineering include the replacement of damaged, injured or missing body tissues with biological compatible substitutes such as bioengineered tissues. However, due to an initial mass loss after implantation, improved vascularization of the regenerated tissue is essential. Recent advances in understanding the process of blood vessel growth has offered significant tools for therapeutic neovascularization. Several angiogenic growth factors including vascular endothelial cell growth factor (VEGF) and basic fibroblast growth factor (bFGF) were used for vascularization of ischemic tissues. Three approaches have been used for vascularization of bioengineered tissue: incorporation of angiogenic factors in the bioengineered tissue, seeding endothelial cells with other cell types and prevascularization of matrices prior to cell seeding. This paper reviews the process of blood vessel growth and tissue vascularization, and discuss strategies for efficient vascularization of engineered tissues.

Tissue engineering strategies for in vivo neovascularisation

Neovascularisation is a promising alternative therapeutic approach to re-establish blood flow in ischaemic tissues of patients suffering from coronary artery or peripheral artery disease. Often, these patients are not suitable candidates for current revascularisation procedures such as coronary angioplasty or bypass surgery. Several strategies are presently under investigation to induce vascularisation by stimulating the body's natural processes of vasculogenesis, angiogenesis and arteriogenesis. These strategies involve transplantation of various cell types into the ischaemic site and the delivery of recombinant angiogenic agents through direct protein administration or gene transfer. We will examine the basic approaches for these neovacularisation strategies and their therapeutic potential as demonstrated in animal models and human trials to date.

Fibroblast-dependent differentiation of human microvascular endothelial cells into capillary-like 3-dimensional networks

An in vitro model has been developed to study migration, survival, proliferation, and capillary-like differentiation of human microvascular endothelial cells (HMVECs) in an environment that avoids tumor promoters and complex matrices. HMVEC monolayers were plated, then induced to form three-dimensional, capillary-like networks by overlaying with human type I collagen followed by a second overlay of collagen with embedded fibroblasts. Detachment and migration of endothelial cells into the matrix was triggered within hours by the overlaying collagen, and the fibroblasts stimulated survival and formation of cords, vacuoles, tubes, and, after 4 to 5 days, capillary networks. The differentiation into branching capillary-like structures was dependent on direct fibroblast-endothelial cell contact and was not achieved when fibroblasts were replaced by seven types of melanoma cells, which included radial and vertical growth phase primary and metastatic stages. Vascular endothelial growth factor (VEGF), when overexpressed in fibroblasts, stimulated endothelial cell proliferation and migration, whereas angiopoietin-1 (Ang-1) had only motogenic effects. Neutralizing antibodies against VEGF and blocking antibodies for VEGF-receptor 2 (VEGFR2) significantly inhibited but not completely obliterated capillary network formation, suggesting that the VEGF signaling pathway is important but not exclusive and that other fibroblast-derived soluble factors and fibroblast-endothelial cell contact are essential for endothelial cell survival and differentiation.

Human dermal microvascular endothelial cells form vascular analogs in cultured skin substitutes after grafting to athymic mice

Cultured skin substitutes (CSS) consisting of autologous fibroblasts and keratinocytes combined with biopolymers are an adjunctive treatment for large excised burns. CSS containing two cell types are limited by anatomical deficiencies, including lack of a vascular plexus, leading to slower vascularization after grafting than split-thickness autograft. To address this limitation, CSS were prepared containing human keratinocytes, fibroblasts, and dermal microvascular endothelial cells (HDMEC) isolated from a single skin sample. After 16 days in culture, control CSS and CSS containing HDMEC (CSS+EC) were grafted to full-thickness wounds in athymic mice. In CSS+EC in vitro, HDMEC persisted in the dermal substitutes and formed multicellular aggregates. One wk after grafting, HDMEC in CSS+EC organized into multicellular structures, some containing lumens. By 4 wk after grafting, HDMEC were found in linear and circular organizations resembling vascular analogs associated with basement membrane deposition. In some cases, colocalization of HDMEC with mouse perivascular cells was observed. The results demonstrate HDMEC transplantation in a clinically relevant cultured skin model, persistence of HDMEC after grafting, and HDMEC organization into vascular analogs in vitro and in vivo. All cells were derived from the same donor tissue, indicating the feasibility of preparing CSS containing autologous HDMEC for grafting to patients.

Inhibition of programmed cell death impairs in vitro vascular-like structure formation and reduces in vivo angiogenesis

Tissue remodeling during embryonic development and in the adult organism relies on a subtle balance between cell growth and apoptosis. As angiogenesis involves restructuring of preexisting endothelium, we examined the role of apoptosis in new vessel formation. We show that apoptosis occurs before capillary formation but not after vessels have assembled. Using the human umbilical vein endothelial cell (HUVEC) in vitro Matrigel angiogenesis model, we show that vascular-like structure formation requires apoptotic cell death through activation of a caspase-dependent mechanism and mitochondrial cytochrome c release. Vascular-like structure formation was further blocked by caspase inhibitors such as z-VAD or Ac-DEVD-CHO, using HUVEC and human lung microvascular endothelial cells. Overexpression of anti-apoptotic human Bcl-2 or baculovirus p35 genes in HUVEC altered endothelial cell rearrangement during in vitro angiogenesis, causing impaired vessel-like structure formation. Caspase inhibitors blocked VEGF- or bFGF-induced HUVEC angiogenesis on 2- or 3-D collagen gels, respectively, confirming that apoptosis was not the result of nonspecific cell death after seeding on the matrix. In an in vivo angiogenesis assay, caspase inhibitors blocked VEGF-dependent vascular formation at the alignment step, as demonstrated histologically. This evidence indicates that endothelial cell apoptosis may be relevant for precise vascular tissue rearrangement in in vitro and in vivo angiogenesis.

Vascular morphogenesis and differentiation after adoptive transfer of human endothelial cells to immunodeficient mice

To establish a model for adoptive transfer of endothelial cells, we transferred human umbilical vein endothelial cells (HUVECs) to immunodeficient mice (Rag 2(-/-)). HUVECs were suspended as single cells in Matrigel and injected subcutaneously in the abdominal midline. Within 10 days after injection, HUVECs expressed pseudopod-like extensions and began to accumulate in arrays. By day 20, we observed human vessels that contained erythrocytes, and on day 30 we observed perivascular cells that expressed smooth muscle actin, thus resembling mature vessels. Throughout the experimental period, HUVECs bound Ulex europaeus lectin and expressed CD31, VE-cadherin, von Willebrand factor, as well as ICAM-2. A fraction of the cells also expressed the proliferation marker Ki67. Moreover, the sialomucin CD34, which is rapidly down-regulated in cultured HUVECs, was reinduced in vivo. However, we found no reinduction of CD34 in cells cultured inside or on top of Matrigel in vitro. We also injected cells suspended in Matrigel around the catheter tip of implanted osmotic pumps. Delivery of recombinant human interferon-gamma by this route led to strong induction of MHC class II and ICAM-1 on the human vessels. In conclusion, isolated human endothelial cells can integrate with the murine vascular system to form functional capillaries and regain in vivo properties.

Building drug delivery into tissue engineering

The creation of efficient methods for manufacturing biotechnology drugs--many of which influence fundamental but complex cell behaviours, such as proliferation, migration and differentiation--is creating new opportunities for tissue repair. Many agents are potent and multifunctional; that is, they produce different effects within different tissues. Therefore, control of tissue concentration and spatial localization of delivery is essential for safety and effectiveness. Synthetic systems that can control agent delivery are particularly promising as materials for enhancing tissue regeneration. This review discusses the state of the art in controlled-release and microfluidic drug delivery technologies, and outlines their potential applications for tissue engineering.

Tissue-engineered skin. Current status in wound healing

Tissue-engineered skin is a significant advance in the field of wound healing and was developed due to limitations associated with the use of autografts. These limitations include the creation of a donor site which is at risk of developing pain, scarring, infection and/or slow healing. A number of products are commercially available and many others are in development. Cultured epidermal autografts can provide permanent coverage of large area from a skin biopsy. However, 3 weeks are needed for graft cultivation. Cultured epidermal allografts are available immediately and no biopsy is necessary. They can be cryopreserved and banked, but are not currently commercially available. A nonliving allogeneic acellular dermal matrix with intact basement membrane complex (Alloderm) is immunologically inert. It prepares the wound bed for grafting allowing improved cultured allograft 'take' and provides an intact basement membrane. A nonliving extracellular matrix of collagen and chondroitin-6-sulfate with silicone backing (Integra) serves to generate neodermis. A collagen and glycosaminoglycan dermal matrix inoculated with autologous fibroblasts and keratinocytes has been investigated but is not commercially available. It requires 3 to 4 weeks for cultivation. Dermagraft consists of living allogeneic dermal fibroblasts grown on degradable scaffold. It has good resistance to tearing. An extracellular matrix generated by allogeneic human dermal fibroblasts (TransCyte) serves as a matrix for neodermis generation. Apligraf is a living allogeneic bilayered construct containing keratinocytes, fibroblasts and bovine type I collagen. It can be used on an outpatient basis and avoids the need for a donor site wound. Another living skin equivalent, composite cultured skin (OrCel), consists of allogeneic fibroblasts and keratinocytes seeded on opposite sides of bilayered matrix of bovine collagen. There are limited clinical data available for this product, but large clinical trials are ongoing. Limited data are also available for 2 types of dressing material derived from pigs: porcine small intestinal submucosa acellular collagen matrix (Oasis) and an acellular xenogeneic collagen matrix (E-Z-Derm). Both products have a long shelf life. Other novel skin substitutes are being investigated. The potential risks and benefits of using tissue-engineered skin need to be further evaluated in clinical trials but it is obvious that they offer a new option for the treatment of wounds.

Upregulation of the cochaperone Mdg1 in endothelial cells is induced by stress and during in vitro angiogenesis

Angiogenesis research has focused on receptors and ligands mediating endothelial cell proliferation and migration. Little is known about the molecular mechanisms that are involved in converting endothelial cells from a proliferative to a differentiated state. Microvascular differentiation gene 1 (Mdg1) has been isolated from differentiating microvascular endothelial cells that had been cultured in collagen type I gels (3D culture). In adult human tissue Mdg1 is expressed in endothelial and epithelial cells. Sequence analysis of the full-length cDNA revealed that the N-terminal region of the putative Mdg1-protein exhibits a high sequence similarity to the J-domain of Hsp40 chaperones. We show that this region functions as a bona fide J-domain as it can replace the J-domain of Escherichia coli DnaJ-protein. Mdg1 is also upregulated in primary endothelial and mesangial cells when subjected to various stress stimuli. GFP-Mdg1 fusion constructs showed the Mdg1-protein to be localized within the cytoplasm under control conditions. Stress induces the translocation of Mdg1 into the nucleus, where it accumulates in nucleoli. Costaining with Hdj1, Hdj2, Hsp70, and Hsc70 revealed that Mdg1 colocalizes with Hsp70 and Hdj1 in control and stressed HeLa cells. These data suggest that Mdg1 is involved in the control of cell cycle arrest taking place during terminal cell differentiation and under stress conditions.

Human endothelial cell presentation of antigen and the homing of memory/effector T cells to skin

Dermal microvascular endothelial cells (ECs) form a continuous lining that normally bars blood-borne T lymphocytes from entering the skin, but as part of the response to foreign antigen, dermal ECs undergo alterations in their surface proteins so as to provide signals to circulating T cells that lead to their activation and recruitment. Several observations suggest that human dermal microvascular ECs may help initiate cutaneous immune reactions by presentation of cognate antigens to circulating T memory cells: (1) antigen-specific inflammatory responses in the skin, as in other organs, involve accumulation of memory and effector T cell populations that are enriched in cells specific for the eliciting antigen; (2) recall responses to intradermal protein antigens in the skin start very rapidly within two hours of challenge; (3) dermal microvascular ECs in humans and other large mammals basally display high levels of class I and class II MHC molecules, the only known purpose of which is to present antigenic peptides to lymphocytes; (4) the lumen of dermal capillaries are narrower than the diameter of circulating T cells, ensuring surface contact; and (5) cultured human ECs effectively present antigens to resting memory T cells isolated from the circulation. Upon contact with activated T cells or their secreted products (cytokines), dermal ECs themselves become activated, increasing their capacity to recruit memory and effector T cell populations in an antigen-independent manner. Specifically, activated ECs express inducible leukocyte adhesion molecules such as E-selectin, ICAM-1, and VCAM-1; and several lines of evidence, including neutralizing antibody experiments and gene knockouts, have supported a role of these molecules in T cell recruitment. Dermal ECs have unique expression patterns of adhesion molecules that can determine the subsets of memory T cells that are recruited into the skin. For example, slow internalization of E-selectin allows more persistent expression of this protein on the surface of dermal ECs, favoring interactions with CLA-1+ T cells. VCAM-1 expression, normally confined to venular EC may extend to capillaries within the dermal papillae and contribute to epidermal inflammation, recruiting alpha4beta7 integrin-expressing T cells that also express the cadherin-binding integrin alphaEbeta7. New models involving transplantation of normal and genetically modified human dermal ECs into immunodeficient mice may be used to further explore these properties.

Engineering and characterization of functional human microvessels in immunodeficient mice

SUMMARY

Current model systems used to investigate angiogenesis in vivo rely on the interpretation of results obtained with nonhuman endothelial cells. Recent advances in tissue engineering and molecular biology suggest the possibility of engineering human microvessels in vivo. Here we show that human dermal microvascular endothelial cells (HDMEC) transplanted into severe combined immunodeficient (SCID) mice on biodegradable polymer matrices differentiate into functional human microvessels that anastomose with the mouse vasculature. HDMEC were stably transduced with Flag epitope or alkaline phosphatase to confirm the human origin of the microvessels. Endothelial cells appeared dispersed throughout the sponge 1 day after transplantation, became organized into empty tubular structures by Day 5, and differentiated into functional microvessels within 7 to 10 days. Human microvessels in SCID mice expressed the physiological markers of angiogenesis: CD31, CD34, vascular cellular adhesion molecule 1 (VCAM-1), and intercellular adhesion molecule 1 (ICAM-1). Human endothelial cells became invested by perivascular smooth muscle alpha-actin-expressing mouse cells 21 days after implantation. This model was used previously to demonstrate that overexpression of the antiapoptotic protein Bcl-2 in HDMEC enhances neovascularization, and that apoptotic disruption of tumor microvessels is associated with apoptosis of surrounding tumor cells. The proposed SCID mouse model of human angiogenesis is ideally suited for the study of the physiology of microvessel development, pathologic neovascular responses such as tumor angiogenesis, and for the development and investigation of strategies designed to enhance the neovascularization of engineered human tissues and organs.