Endothelial outgrowth cells regulate coagulation, platelet accumulation, and respond to tumor necrosis factor similar to carotid endothelial cells

Surface modification of decellularized bovine carotid arteries with human vascular cells significantly reduces their thrombogenicity

BACKGROUND

Since autologous veins are unavailable when needed in more than 20% of cases in vascular surgery, the production of personalized biological vascular grafts for implantation has become crucial. Surface modification of decellularized xenogeneic grafts with vascular cells to achieve physiological luminal coverage and eventually thromboresistance is an important prerequisite for implantation. However, ex vivo thrombogenicity testing remains a neglected area in the field of tissue engineering of vascular grafts due to a multifold of reasons.

METHODS

After seeding decellularized bovine carotid arteries with human endothelial progenitor cells and umbilical cord-derived mesenchymal stem cells, luminal endothelial cell coverage (LECC) was correlated with glucose and lactate levels on the cell supernatant. Then a closed loop whole blood perfusion system was designed. Recellularized grafts with a LECC > 50% and decellularized vascular grafts were perfused with human whole blood for 2 h. Hemolysis and complete blood count evaluation was performed on an hourly basis, followed by histological and immunohistochemical analysis.

RESULTS

While whole blood perfusion of decellularized grafts significantly reduced platelet counts, platelet depletion from blood resulting from binding to re-endothelialized grafts was insignificant (p = 0.7284). Moreover, macroscopic evaluation revealed thrombus formation only in the lumen of unseeded grafts and histological characterization revealed lack of CD41 positive platelets in recellularized grafts, thus confirming their thromboresistance.

CONCLUSION

In the present study we were able to demonstrate the effect of surface modification of vascular grafts in their thromboresistance in an ex vivo whole blood perfusion system. To our knowledge, this is the first study to expose engineered vascular grafts to human whole blood, recirculating at high flow rates, immediately after seeding.

Challenges and Possibilities of Cell-Based Tissue-Engineered Vascular Grafts

There is urgent demand for biologically compatible vascular grafts for both adult and pediatric patients. The utility of conventional nonbiodegradable materials is limited because of their thrombogenicity and inability to grow, while autologous vascular grafts involve considerable disadvantages, including the invasive procedures required to obtain these healthy vessels from patients and insufficient availability in patients with systemic atherosclerosis. All of these issues could be overcome by tissue-engineered vascular grafts (TEVGs). A large body of evidence has recently emerged in support of TEVG technologies, introducing diverse cell sources (e.g., somatic cells and stem cells) and novel fabrication methods (e.g., scaffold-guided and self-assembled approaches). Before TEVG can be applied in a clinical setting, however, several aspects of the technology must be improved, such as the feasibility of obtaining cells, their biocompatibility and mechanical properties, and the time needed for fabrication, while the safety of supplemented materials, the patency and nonthrombogenicity of TEVGs, their growth potential, and the long-term influence of implanted TEVGs in the body must be assessed. Although recent advances in TEVG fabrication have yielded promising results, more research is needed to achieve the most feasible methods for generating optimal TEVGs. This article reviews multiple aspects of TEVG fabrication, including mechanical requirements, extracellular matrix components, cell sources, and tissue engineering approaches. The potential of periodic hydrostatic pressurization in the production of scaffold-free TEVGs with optimal elasticity and stiffness is also discussed. In the future, the integration of multiple technologies is expected to enable improved TEVG performance.

The Effects of Topographic Micropatterning on Endothelial Colony-Forming Cells

Artificial small-diameter vascular grafts remain an unmet need in modern medicine, due to the thrombosis and neointimal hyperplasia that plague currently available synthetic devices. Tissue engineering techniques, including in vitro endothelialization, could offer a solution to this problem. A potential minimally invasive source of patient autologous endothelium is endothelial colony-forming cells (ECFCs), endothelial-like outgrowth products of circulating progenitors. While ECFCs respond to shear stress similar to mature endothelial cells (ECs), their response to luminal topographic micropatterning (TMP), a biomaterial modification with the potential to flow-independently, enhance the attachment, migration, gene expression, and function of mature ECs, remains unstudied. In this study, case-matched carotid endothelial cells (CaECs) and blood-derived ECFCs are statically cultured on polyurethane substrates with micropatterned pitches (pitch = peak to peak distance) ranging from 3-to 14 μm. On all pattern pitches tested, both CaECs and ECFCs showed significant and robust alignment to the angle of the micropatterns. Using a novel cell-by-cell image analysis technique, it was found that actin fibers similarly and significantly aligned to the angle of micropatterned features on all pitches tested. Microtubules analyzed through the same novel approach showed significant alignment on most pitches examined, with a greater variation in fiber angle overall. Interestingly, only CaECs showed significant cellular elongation, and notably to a lower degree than previously seen either in vivo due to flow or in vitro due to spatial growth restriction micropatterning, but consistent with earlier studies of TMP. Neither cell type displayed any significant micropattern-driven changes in the expression of KLF-2 or the downstream adhesion molecules it regulates. These results demonstrate that TMP flow-independently affects ECFC morphology, but that alignment alone is insufficient to drive protective changes in EC and ECFC function.

Static spatial growth restriction micropatterning of endothelial colony forming cells influences their morphology and gene expression

BACKGROUND

Endothelialization of small diameter synthetic vascular grafts is a potential solution to the thrombosis and intimal hyperplasia that plague current devices. Endothelial colony forming cells, which are blood-derived and similar to mature endothelial cells, are a potential cell source. Anisotropic spatial growth restriction micropatterning has been previously shown to affect the morphology and function of mature endothelial cells in a manner similar to unidirectional fluid shear stress. To date, endothelial colony forming cells have not been successfully micropatterned. This study addresses the hypothesis that micropatterning of endothelial colony forming cells will induce morphological elongation, cytoskeletal alignment, and changes in immunogenic and thrombogenic-related gene expression.

METHODS

Spatially growth restrictive test surfaces with 25 μm-wide lanes alternating between collagen-I and a blocking polymer were created using microfluidics. Case-matched endothelial colony forming cells and control mature carotid endothelial cells were statically cultured on either micropatterned or non-patterned surfaces. Cell elongation was quantified using shape index. Using confocal microscopy, cytoskeletal alignment was visualized and density and apoptotic rate were determined. Gene expression was measured using quantitative PCR to measure KLF-2, eNOS, VCAM-1, and vWF.

RESULTS

Endothelial colony forming cells were successfully micropatterned for up to 50 hours. Micropatterned cells displayed elongation and actin alignment. Micropatterning increased the packing densities of both cell types, but did not affect apoptotic rate, which was lower in endothelial colony forming cells. KLF-2 gene expression was increased in micropatterned relative to non-patterned endothelial colony forming cells after 50 hours. No significant differences were seen in the other genes tested.

CONCLUSIONS

Endothelial colony forming cells can be durably micropatterned using spatial growth restriction. Micropatterning has a significant effect on the gross and subcellular morphologies of both cell types. Further study is required to fully understand the effect of micropatterning on endothelial colony forming cell gene expression.

Bioactive Anti-Thrombotic Modification of Decellularized Matrix for Vascular Applications

The decellularized matrix derived from porcine small intestinal submucosa (SIS) is a widely used biomaterial being investigated for numerous applications. Currently, thrombus deposition and neointimal hyperplasia have limited the use of SIS in some vascular applications. To limit these detrimental processes, this work applies bioactive, endothelial-inspired properties to the material. SIS is modified with the endothelial cell membrane protein thrombomodulin and the glycosaminoglycan heparin to facilitate protein C activation and anticoagulant activity, respectively. Modifying SIS with thrombomodulin alone enables robust activated protein C (APC) generation, and thrombomodulin activity is maintained after prolonged exposure to fluid shear and blood plasma. Heparin-modified SIS has a potent anticoagulant activity. When both modifications are applied sequentially, SIS modified first with thrombomodulin then with heparin retains the full activity of each individual modification. Tubular SIS devices are connected to a baboon arteriovenous shunt to quantify thrombus deposition on these materials. After being exposed to flowing whole blood for 60 min, SIS devices modified first with thrombomodulin then with heparin have significantly less platelet accumulation compared to unmodified SIS devices. These studies demonstrate that modifying SIS with thrombomodulin and heparin confers APC generation and anticoagulant activity that results in reduced thrombogenesis.

MiR-145 facilitates proliferation and migration of endothelial progenitor cells and recanalization of arterial thrombosis in cerebral infarction mice via JNK signal pathway

Arterial thrombosis in cerebral infarction severely affects patients' lives. Classical treatment including surgery and medication both had significantly adverse effects, making it necessary to find novel strategy. Endothelial progenitor cells (EPCs) have been shown to enhance the recanalization of thrombosis, while leaving its molecular mechanism unclear. EPCs were separated from peripheral blood, and were transfected by microRNA (miR)-145. The growth, proliferation and migration abilities were quantified by MTT, clone formation and Transwell assays, respectively. Cell apoptosis was evaluated by flow cytometry. The activation of JNK signaling pathway was measured by Western blotting, followed by JNK inhibitor SP600125. In a mouse cerebral infarction model, miR-145 transfected EPCs were injected to observe the condition of arterial thrombosis. MiR-145 transfection enhanced growth, migration and proliferation of EPCs without induction of apoptosis. MiR-145 exerts its effects via JNK signaling pathway, as the blocking inhibited cell migration/proliferation. In vivo injection of miR-145 transfected EPCs also potentiated cell proliferation and migration, in addition to the recanalization of arterial thrombosis. MiR-145 facilitates proliferation and migration of EPCs and recanalization of arterial thrombosis in cerebral infarction mice via JNK signal pathway. This study provided new insights regarding infarction treatment.

In vivo assessment of two endothelialization approaches on bioprosthetic valves for the treatment of chronic deep venous insufficiency

Chronic deep venous insufficiency is a debilitating disease with limited therapeutic interventions. A bioprosthetic venous valve could not only replace a diseased valve, but has the potential to fully integrate into the patient with a minimally invasive procedure. Previous work with valves constructed from small intestinal submucosa (SIS) showed improvements in patients' symptoms in clinical studies; however, substantial thickening of the implanted valve leaflets also occurred. As endothelial cells are key regulators of vascular homeostasis, their presence on the SIS valves may reduce the observed thickening. This work tested an off-the-shelf approach to capture circulating endothelial cells in vivo using biotinylated antikinase insert domain receptor antibodies in a suspended leaflet ovine model. The antibodies on SIS were oriented to promote cell capture and showed positive binding to endothelial cells in vitro; however, no differences were observed in leaflet thickness in vivo between antibody-modified and unmodified SIS. In an alternative approach, valves were pre-seeded with autologous endothelial cells and tested in vivo. Nearly all the implanted pre-seeded valves were patent and functioning; however, no statistical difference was observed in valve thickness with cell pre-seeding. Additional cell capture schemes or surface modifications should be examined to find an optimal method for encouraging SIS valve endothelialization to improve long-term valve function in vivo. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1610-1621, 2016.

Engineering an endothelialized vascular graft: a rational approach to study design in a non-human primate model

After many years of research, small diameter, synthetic vascular grafts still lack the necessary biologic integration to perform ideally in clinical settings. Endothelialization of vascular grafts has the potential to improve synthetic graft function, and endothelial outgrowth cells (EOCs) are a promising autologous cell source. Yet no work has established the link between endothelial cell functions and outcomes of implanted endothelialized grafts. This work utilized steady flow, oscillatory flow, and tumor necrosis factor stimulation to alter EOC phenotype and enable the formulation of a model to predict endothelialized graft performance. To accomplish this, EOC in vitro expression of coagulation and inflammatory markers was quantified. In parallel, in non-human primate (baboon) models, the platelet and fibrinogen accumulation on endothelialized grafts were quantified in an ex vivo shunt, or the tissue ingrowth on implanted grafts were characterized after 1mth. Oscillatory flow stimulation of EOCs increased in vitro coagulation markers and ex vivo platelet accumulation. Steady flow preconditioning did not affect platelet accumulation or intimal hyperplasia relative to static samples. To determine whether in vitro markers predict implant performance, a linear regression model of the in vitro data was fit to platelet accumulation data-correlating the markers with the thromboprotective performance of the EOCs. The model was tested against implant intimal hyperplasia data and found to correlate strongly with the parallel in vitro analyses. This research defines the effects of flow preconditioning on EOC regulation of coagulation in clinical vascular grafts through parallel in vitro, ex vivo, and in vivo analyses, and contributes to the translatability of in vitro tests to in vivo clinical graft performance.