Graft Engineering Research Articles

Physicians Poster Session: Graft Engineering and Cellular Therapy

Physicians Poster Session: Graft Engineering and Cellular Therapy

Adherent cells generated during long-term culture of human umbilical cord blood CD34+ cells have characteristics of endothelial cells and beneficial effect on cord blood ex vivo expansion.

Hematopoiesis depends on the association of hematopoietic stem cells with stromal cells that constitute the hematopoietic microenvironment. The in vitro development of the endothelial cell from umbilical cord blood (UCB) is not well established and has met very limited success. In this study, UCB CD34(+) cells were cultured for 5 weeks in a stroma-free liquid culture system using thrombopoietin, flt3 ligand, and granulocyte-colony stimulating factor. By week 4-5, we found that firmly adherent fibroblast-like cells were established. These cells showed characteristics of endothelial cells expressing von Willebrand factor, human vascular cell adhesion molecule-1, human intracellular adhesion molecule-1, human CD31, E-selectin, and human macrophage. Furthermore, when comparing an ex vivo system without an established endothelial monolayer to an ex vivo system with an established endothelial monolayer, better expansion of total nucleated cells, CD34(+) cells, and colony-forming units (CFUs)-granulocyte-macrophage and CFUs-granulocyte-erythroid-megakaryocyte-macrophage were found during culture. This phenomenon was in part due to the fact that a significant reduction of apoptotic fractions was found in the CD34(+) cells, which were cultured on the adherent monolayer for up to 5 weeks. To gather quantitative data on the number of endothelial cells derived from a given number of CD34 cells, we performed limiting dilution assay by using Poisson distribution: the number of tested cells (linear scale) producing a 37% negative culture (logarithmic scale) is the number of cells containing one endothelial cell. By this method, one endothelial cell may be found from 314 CD34(+) cells after 5 weeks of culture. These results suggest that the UCB CD34(+) cell fraction contains endothelial cell precursors, establishing the hematopoietic microenvironment and providing the beneficial effects through downregulating apoptosis on UCB expansion protocols. These observations may provide insight for future cellular therapy or graft engineering.

Oral Session: Graft Engineering and Cellular Therapy

Oral Session: Graft Engineering and Cellular Therapy

Megadose transplantation of highly purified haploidentical stem cells: current results and future prospects.

The transplantation of megadoses of haploidentical hematopoietic stem cells is for a number of children with malignant or nonmalignant diseases the only curative approach. In order to prevent severe GvHD, the removal of T lymphocytes from the stem cell graft either by positive selection of CD34+ stem cells or by negative depletion of CD3+ T lymphocytes is necessary. We present the results obtained so far by CD34+ positive selection and discuss new techniques of graft engineering which might hopefully further improve the outcome of haploidentical stem cell transplantation.

Mechanical Properties of Artificial Protein Matrices Engineered for Control of Cell and Tissue Behavior

Genetic engineering methods were used for the preparation of artificial proteins containing sequences designed to reproduce essential features of the extracellular matrix (ECM). The long-term objective of the work is to develop matrices for use in the engineering of small-diameter vascular grafts. The CS5 domain of fibronectin provides binding sites for vascular endothelial cells, while an elastin-like repeat, [(VPGIG)2(VPGKG)(VPGIG)2], controls the mechanical properties and includes sites for covalent cross-linking. Bis(sulfosuccinimidyl) suberate and disuccinimidyl suberate were used to cross-link artificial ECM protein films for uniaxial tensile testing. Variation in the amount of cross-linker and protein weight fraction allowed preparation of films with Young's moduli ranging from 0.07 to 0.97 MPa. The weight fraction of protein in the hydrated, cross-linked films was measured to be between 0.2 and 0.4; the molecular weight between cross-links (Mc) varied from 3000 to 38 000. The moduli and Mc of the ...

Unrelated and related cord blood banking and hematopoietic graft engineering.

The first successful transplantation of umbilical-cord blood (CB) was performed in 1988 to treat a boy with Fanconi's anemia, using CB from his HLA full-matched sister. A few years later, CB transplantation (CBT) was also performed in an adult recipient, however major obstacles still prevent a wider application of CBT in this age group. The principle limiting-factor is the low numbers of nucleated (NC) and CD34+ cells available for transplantation compared to a typical bone marrow (BM)/peripheral blood (PB) allograft, resulting in a lower engraftment success as well as delayed hematopoietic recovery with its characteristic complications, including infections and transplant related mortality (TRM). Other problems include uncertainty regarding potency and efficacy of graft versus leukemia (GvL)/tumor effects in this kind of transplant, considering the reduced graft versus host disease (GvHD) manifestations and immunologic prematurity. These subjects are reviewed with orientation to technical methods directed to improve CB grafts and graft engineering.

Mediastinal fat: a source of cells for tissue engineering of coronary artery bypass grafts

Mediastinal fat: a source of cells for tissue engineering of coronary artery bypass grafts

Mediastinal fat: a source of cells for tissue engineering of coronary artery bypass grafts

Mediastinal fat: a source of cells for tissue engineering of coronary artery bypass grafts

A large-scale method for T cell depletion: towards graft engineering of mobilized peripheral blood stem cells.

We have investigated the feasibility and efficacy of large-scale T cell depletion from granulocyte colony-stimulating factor (G-CSF) mobilized peripheral blood stem cells (PBSC). The method is based on the use of a CD3 antibody conjugated to magnetic microbeads and magnetic activated cell sorting (Clinimacs). A total of eight large-scale experiments were performed. In four experiments, CD3(+) T cells were depleted from PBSC obtained from volunteers mobilized with G-CSF whereas, in four experiments, T cells were depleted from PBSC from stem cell donors, in which the CD34(+) stem cells had been removed for allogeneic transplantation by positive selection prior to T cell depletion. The mean number of processed mononuclear cells (MNCs) was 3.3 x 10(10) (range 1.5 x 10(10)-5.1 x 10(10)) with a mean T cell proportion of 35.8% (range 16.7-64.0%). After T cell depletion, the percentage of contaminating T cells was 0.15% (range 0.01-1.01%) with a mean log(10) depletion of 3.4 (range 2.8-4.1). The mean recovery of CD3-negative MNCs after depletion was 76% (range 52-100%). The mean recovery of CD34(+) stem cells in the four evaluable experiments was 82% (range 75-92%). In vitro colony assays and in vivo NOD/SCID repopulation assays showed that this large-scale T cell depletion method has no negative impact on the function of the hematopoietic precursor cells. Therefore, we conclude that this T cell depletion method is a valuable tool for further graft engineering strategies involving mobilized PBSCs.

In Vivo Vascular Engineering of Vein Grafts: Directed Migration of Smooth Muscle Cells by Perivascular Release of Elastase Limits Neointimal Proliferation

Saphenous vein bypass grafting for coronary revascularization procedures remains limited by accelerated neointima formation. It was hypothesized that creation of a modified chemotactic gradient in vivo could guide migration of smooth muscle cells (SMCs) peripherally instead of in a luminal direction and reduce intimal hyperplasia during vein graft arterialization. Surgical bypass vein grafting to femoral arteries was performed in adult male New Zealand White rabbits (n = 8 per treatment group; five for 7 d and three for 28 d). Controlled-release microspheres delivering elastase or buffered polymer only were administered perivascularly at the vein graft site. At 7 days, five vein grafts per group were harvested and cross-sections were immunostained with anti-proliferating cell nuclear antigen (PCNA) to determine the number and distribution of proliferating SMCs. At 28 days, three vein grafts per group were harvested and intima-to-media (I/M) ratios were calculated after staining with Verhoeff von Gieson-Masson trichrome stain. Significant early outward-directed elastin degradation resulted from elastase treatment. Concurrently, proliferating SMCs migrated peripherally. PCNA(+) cells in the outer half of the wall increased 2.37 fold compared to procedural controls (P <.0001). Directional shifts in SMC migration underlie these results because overall SMC proliferation was not significantly different. At 28 days after vein graft surgery, a 38% reduction (P =.0008) in neointima was observed relative to procedural controls. Directional guidance of SMC responses through perivascular elastase release achieves favorable vein graft remodeling characteristics, including limited neointima development. This represents practical evidence that SMC migration can be directionally guided in vivo in a vein graft model and that plaque progression can be prevented by redistributing elastin without decreasing functional vein graft wall stability.

Mesenchymal stem cells--a new pathway for tissue engineering in reconstructive surgery

Mesenchymal stem cells (MSC) have the capacity to differentiate into chondrocytes with the synthesis of cartilage. This report presents the use of human adult bone marrow derived mesenchymal stem cells for tissue engineering of autologous cartilage grafts. Human bone marrow aspirates were obtained from the iliac crest and fractionated on a Percoll gradient. The isolated hMSC were plated at 20 x 10 (6) cells per 100 mm (2) culture dish. After 21 days in culture at 37 degrees C with 5 % CO 2, the adherent multiplied MSC were trypsinized, counted, and tested for viability by trypan blue assay. The hMSCs were loaded into a sterile 15 ml polypropylene tube (0.5 Mio cells/ml) and centrifuged on the bottom of the tube at 500 g for 5 minutes. The MSC were cultivated for 3 weeks in vitro in a specific chondrogenetic medium composed of Dulbecco's Modified Eagles Medium-High Glucose supplemented with 10 ng/ml transforming growth factor-beta 1, 1 % ITS-Premix medium, 80 micro M ascorbic acid, and 100 nM dexamethasone. Histological and immunohistochemical studies performed after 3 weeks in three dimensional culture demonstrated the expression of cartilage specific collagen type II and X as well as proteoglycans. Human adult mesenchymal stem cells derived from bone marrow aspirates have the ability to differentiate into chondrocytes under specific culture conditions by growth factors. The use of adult mesenchymal stem cells may be a promising tool for tissue engineering of autologous cartilage grafts in reconstructive surgery in the future.

Allogeneic hematopoietic cell transplantation for multiple myeloma

At the present time, allogeneic hematopoetic stem cell transplantation (AHSCT) is the only proven treatment for multiple myeloma (MM) that is potentially curative. This conclusion is based on observations of patients who have undergone AHSCT and are living disease-free for 5-15 years. While patients who receive either allogeneic or autologous stem cell transplants for MM have similar 3-5 year survival, only allograft recipients appear to enjoy long-term disease free survival. This is most likely due to an allogeneic graft-versus-myeloma effect, demonstrated most dramatically by complete responses observed after the simple infusion of donor lymphocytes for patients who have relapsed after a prior allograft. The very high transplant related mortality associated with standard allogeneic stem cell transplantation is currently the major limitation to wider use of this potentially curative treatment modality. Complications are age-related and thus standard AHSCT is offered only to patients under age 55; further limiting the utility of this treatment. The challenge for clinical investigators will be to reduce the incidence of post-transplant complications for patients receiving AHSCT for MM. These strategies include the use of non-ablative conditioning regimens, the use of peripheral blood stem cells rather than bone marrow, graft engineering and targeted conditioning therapies such as bone-seeking radioisotopes. In one such approach, tandem autologous/non-ablative allogeneic transplants have been shown to result in relatively low mortality, high complete response rates and 1-year survivals of 81%. Further follow-up and randomized trials will help to define the utility of this strategy.

The storage and re-infusion of autologous blood and BM as back-up following failed primary hematopoietic stem-cell transplantation: a survey of European practice.

Traditionally, autologous BM or PBSC have been stored as a secondary source ('back-up') of hematopoietic stem cells (HSC) prior to allogeneic and autologous HSC transplantation. We conducted an audit of a single transplant center practice for providing back-up HSC and compared this practice with other European centers. Laboratory records relating to the collection and re-infusion of consecutive HSC harvests were reviewed for 515 transplants (300 autologous and 215 allogeneic HSC transplants). In our experience, 2.3% (five of 215) of allogeneic HSC transplants required secondary HSC rescue for failure to engraft or graft failure (MUD, n = 2; un-manipulated sibling BMT, n = 1; T-cell depleted sibling BMT, n = 2). For autologous transplants, 4.7% (14 of 300) required rescue due to failure to engraft or late graft failure (ABMT for AML, n = 8; CD34+ cell selection/ex vivo expanded, n = 4; ABMT/PBSCT, n = 2). Among the European centers, 69.7% replied to a postal questionnaire, demonstrating that 81.4% and 45.6% of centers stored a secondary HSC source for manipulated and unmanipulated MUD BMT, respectively; 50% and 11.6% of centers stored a secondary source of HSC for manipulated and unmanipulated matched sibling BMT, respectively; 36.4% and 12.7% of centers stored HSC for manipulated and unmanipulated matched sibling PBSCT, respectively. In the autologous setting, 15.2% and 62.1% of centers stored back-up for unmanipulated and manipulated BMT, respectively and 19.5% and 68.5% stored back-up for unmanipulated and manipulated PBSCT, respectively, when myeloablative conditioning regimens were used. These data suggest that a small minority of patients require a secondary source of HSC rescue, most commonly in transplants with higher risk of graft failure. This is reflected in the practice across Europe of storing 'back-up' HSC. Guidelines should accommodate the need for storage of a secondary source of HSC only in those transplants associated with a higher risk of graft failure, especially in relation to graft engineering.

The use of graft engineering in allogeneic transplantation for myeloid malignancies

Although methods for engineering of allogeneic stem cell grafts are available for decades, the use of manipulated vs. unmanipulated grafts is still mainly dependent on the policy of individual centers. In transplantation regimens using engineered grafts not only the manipulation of the graft itself, but also the additional use of other transplant components and the impact of modifications of the conditioning regimen must be taken into consideration. The most frequently used technique for graft engineering remains T-cell depletion for GvHD prophylaxis, now mostly performed by positive selection of CD34+ cells. With the currently available techniques, e.g. with the Clinimacs device (Miltenyi) a highly effective reduction of T-cells with high recovery rates of CD34+ cells can reproducibly be achieved. For compensation of the reduced graft versus leukemia effect different strategies are used. Donor lymphocyte infusions are given after T-cell depleted transplants either prophylactically, based on the monitoring of hematopoietic chimerism or of minimal residual disease. With incremental doses of T-cells infused, the risk of inducing severe GvHD can be reduced and the strategy has been shown to be effective especially in CML. Several groups have started to use HSV-tk gene transduced lymphocytes for better control of severe GvHD by the potential to induce suicide of the T-cells with gancyclovir. This approach is highly attractive in concepts using manipulated stem cell grafts although recent findings have caused a debate on its efficacy and safety. The use of manipulated grafts can further be influenced by the choice of the conditioning regimen. The application of radiolabeled antibodies in addition to the conventional conditioning increases the antileukemic activity and may allow to utilize the advantage of better GvHD control with T-cell depleted transplants in diseases where the GvL effect is less active than in CML, e.g. in high risk AML.

A hybrid compliant vascular graft seeded with microvascular endothelial cells extracted from human omentum.

We report the development of a hybrid vascular graft using an innovative compliant poly(carbonate-urea)urethane unlike any previous polyurethane MyoLink as a permanent scaffold. The engineered graft has a hierarchical arterial structure: a monolayer of oriented microvascular endothelial cells (MVECs) and 3-D matrix (human collagen Type 4/dermatan sulfate) bonded onto MyoLink. The grafts' clinical feasibility was evaluated by determining optimal MVEC seeding density, incubation time, viability, and adhesion of these cells when exposed to arterial shear stress. MVECs from human omentum were isolated by a new centrifugation protocol, radiolabeled and seeded onto hybrid graft 2 to 18 x 105 cells/cm(2) for 24 h at 37 degrees C and for 1 to 5 h at 6 x 105 cells/cm(2), washed 3 times, and gamma counted. Qualitative assessment of seeding density/incubation was also undertaken with scanning electron microscopy (SEM) and viability tested with a modified Alamar Blue assay. A pulsatile flow phantom was used to subject the hybrid graft (200 mm length, 5 mm internal diameter) seeded with radiolabeled MVECs (6 x 105 cells/cm(2) at 2 h) to arterial shear and dynamic scintigraphy images acquired in real time using a nuclear medicine gamma camera system during 14 h of perfusion (n = 6). The optimal seeding density was 6 x 105 cells/cm(2), and qualitative SEM confirmed this. Incubating cells for 2 h produced significantly greater cell attachment than was seen for 1 h incubation (p < 0.05), and there was no significant difference in adhesion between cells incubated over the 2 h. Exposure of grafts to acute shear stress resulted in significantly higher proportions of initial cells attached to hybrid grafts compared to native MyoLink grafts (67 +/- 3% versus 55 +/- 2%, p < 0.001). As shown here, tissue engineering of native Myolink grafts significantly reduces the seeding density and incubation time to produce a monolayer onto which cells adhere to better.

Late-Breaking Clinical Trials at the American Heart Association’s Scientific Sessions 2001

The PRoject of Ex-vivo Vein graft ENgineering via Transfection (PREVENT) II trial found that a double-stranded piece of DNA called the E2F duplex decoy significantly reduced failure of vein grafts after coronary artery bypass surgery (CABG), according to Eberhard Grube, MD, of the Heart Center in Sieburg, Germany, during the American Heart Association’s 2001 Scientific Sessions in Anaheim, Calif. During the Late-Breaking Clinical Trials Plenary Session on November 12, 2001, Dr Grube said that his decoy blocks vascular smooth muscle proliferation, which is the core problem in vein graft failure. Eberhard Grube, MD (photo by Paul C. SoRelle) The graft is treated during CABG after the vein has been removed from the leg. A solution containing the E2F decoy is placed in a tube with a stopcock on one side and a syringe at the other, which is used to apply pressure. The vein is placed in the tube and pressure is applied via the syringe to ≈6 lbs/in2 for 10 minutes, Dr Grube said. That is enough to insure that 80% of the cells in the graft will take up the decoy. The transfected graft then is used to bypass blocked coronary arteries. In the study, 101 patients who had undergone CABG were randomly assigned to the treatment group, and 99 were assigned to placebo. In angiographic studies performed on 61 placebo patients and 75 treated patients who received a total of 309 grafts, the E2F decoy was associated with a 30% relative reduction in a composite index of vein graft failure and death, said Dr Grube. In addition, there was a 30% reduction in the size of vessel wall volume. According to Dr Grube, the results are similar to those in the PREVENT I trial, which used E2F to treat bypass veins in peripheral artery disease. He …

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.

Tissue engineering of small caliber vascular grafts.

Previous tissue engineering approaches to create small caliber vascular grafts have been limited by the structural and mechanical immaturity of the constructs. This study uses a novel in vitro pulse duplicator system providing a 'biomimetic' environment during tissue formation to yield more mature, implantable vascular grafts. Vascular grafts (I.D. 0.5 cm) were fabricated from novel bioabsorbable polymers (polyglycolic-acid/poly-4-hydroxybutyrate) and sequentially seeded with ovine vascular myofibroblasts and endothelial cells. After 4 days static culture, the grafts (n=24) were grown in vitro in a pulse duplicator system (bioreactor) for 4, 7, 14, 21, and 28 days. Controls (n=24) were grown in static culture conditions. Analysis of the neo-tissue included histology, scanning electron microscopy (SEM), and biochemical assays (DNA for cell content, 5-hydroxyproline for collagen). Mechanical testing was performed measuring the burst pressure and the suture retention strength. Histology showed viable, dense tissue in all samples. SEM demonstrated confluent smooth inner surfaces of the grafts exposed to pulsatile flow after 14 days. Biochemical analysis revealed a continuous increase of cell mass and collagen to 21 days compared to significantly lower values in the static controls. The mechanical properties of the pulsed vascular grafts comprised supra-physiological burst strength and suture retention strength appropriate for surgical implantation. This study demonstrates the feasibility of tissue engineering of viable, surgically implantable small caliber vascular grafts and the important effect of a 'biomimetic' in vitro environment on tissue maturation and extracellular matrix formation.

Meeting summary of the Tenth International Symposium on autologous blood and marrow transplantation.

The First International Symposium on Autologous Blood and Marrow Transplantation was held in 1984 in Houston just prior to the Annual Meeting of the International Society of Experimental Hematology (ISEH). At that time, only feasibility studies were reported, and the role of high-dose with autologous stem cell support was totally unclear. In subsequent symposia, held every other year, clinical indications for this modality of treatment were established including not only hematological malignancies but also solid tumors. These symposia have been instrumental for progress in the field from initial studies to design of multi-institutional phase II and III studies in acute leukemia [1,2], Hodgkin's disease [3,4], non-Hodgkin's lymphoma [5,6], and multiple myeloma [7,8]. The role of high-dose chemotherapy in breast cancer was explored initially by a few U.S. teams [9–11] and later on by many groups worldwide [12,13]. In the late 1980s, the use of peripheral blood as source of stem cells emerged [14], and this symposium became instrumental in promoting the use of colony-stimulating factor (CSF)–activated progenitor cells from bone marrow as well as from peripheral blood [15–17]. The availability of larger numbers of stem cells collected from peripheral blood after mobilization not only enhanced hematopoietic recovery [18] but also opened the possibility to lower the intensity of the conditioning regimen without compromising engraftment in the allogeneic setting [19]. The so-called “mini-allo transplants” were included in the last three symposia focusing on graft vs tumor effect in various diseases. Graft engineering to maximize quality and quantity of stem cells and to minimize contamination of tumor cells was from the outset an important part of each meeting. Over the last two decades, novel approaches leading to stem-cell expansion [20,21], stem-cell selection [22], and detection of minimal residual disease [23–26] have been presented.

The predictive value of helper T lymphocyte precursor frequencies for graft-versus-host disease and graft-versus-leukaemia effects in allogeneic bone marrow transplantation.

Helper T lymphocyte precursor (HTLp) frequencies determined by limiting dilution analysis were studied in the graft-versus-host direction to assess the predictive value for outcome in allogeneic BMT. The HTLp frequencies correlated with the degree of HLA disparity. HTLp frequencies from 28 HLA-identical sibling BMT pairs had a median of 1:557 362 (range 1:9511 to <1:2 500 000). The HTLp frequencies from 20 HLA-matched unrelated and partially HLA-matched related BMT pairs had a median of 1:88 110 (range 1:4139-1:736 123). The HLA-identical sibling BMT pairs were split evenly into high and low HTLp frequency groups above and below 1:500 000. There was a trend towards a higher risk for acute GVHD > or =grade II (P = 0.075) in the high frequency group. There was no difference in TRM. The high HTLp frequency group had a significantly higher risk for chronic GVHD (P = 0.04), a significantly lower risk for relapse (P = 0.01), as well as a significantly better overall survival (P = 0.045) and leukaemia-free survival (P = 0.008). The HLA-matched unrelated and partially HLA-matched related BMT pairs were split evenly into high and low HTLp frequency groups above and below 1:90 000. There was a significantly higher risk for acute GVHD > or = grade II (P = 0.007) in the high HTLp frequency group. There was a trend towards a higher TRM in the high HTLp frequency group (P = 0.05). There were no differences in chronic GVHD, risk of relapse, overall survival and leukaemia-free survival. Analyzing all 48 patients the risk of acute GVHD > or = grade II and TRM was significantly higher (P = 0.012 and 0.021, respectively) with HTLp frequencies >1:100 000 and there was a trend towards a higher risk of relapse (P = 0.058) with low HTLp frequencies <1:400 000. Patients in the intermediate HTLp frequency group 1:100 000-1:400 000 had a trend towards improved survival (P = 0.059). The HTLp frequency seems to detect clinically significant differences in alloreactivity, that can be useful in donor selection and graft engineering.