Pro-angiogenic CD14(++) CD16(+) CD163(+) monocytes accelerate the in vitro endothelialization of soft hydrophobic poly (n-butyl acrylate) networks

Chitosan-based biomaterials promote bone regeneration by regulating macrophage fate

The development of various osteogenic biomaterials has not only promoted the development of bone tissue engineering but also provided more possibilities for bone defect repair. However, most previous studies have focused on the interaction of biomaterials on endogenous or exogenous stem cells involved in the bone regeneration process while neglecting the effect of changes in the immune microenvironment of bone defect sites on bone regeneration after biomaterial implantation into the host. With the development of bone immunology, the role of various immune cells, especially macrophages, in bone regeneration has gradually attracted the attention of researchers. An increasing number of studies have begun to target macrophages to better promote bone regeneration by modulating the fate of macrophages in a spatiotemporally ordered manner to mimic the changes in the immune microenvironment of bone defect sites during the natural repair process of bone tissue. Chitosan is one of the most abundant natural polysaccharides in the world. In recent years, various chitosan-based biomaterials have been widely used in macrophage fate modulation and bone regeneration. In this review, we review the interaction between macrophages and scaffold materials, general information about chitosan, the modulation of macrophage fate by chitosan-based biomaterials, and their application in bone regeneration.

Self-healing amino acid-bearing acrylamides/n-butyl acrylate copolymers via multiple noncovalent bonds

Four amino acid-bearing acrylamides, N-acryloyl-l-threonine (AThrOH), N-acryloyl-l-glutamic acid (AGluOH), N-acryloyl-l-phenylalanine (APheOH), and N-acryloyl-l, l-diphenylalanine (APhePheOH), were selected for copolymerization with n-butyl acrylate (nBA) to develop amino acid-based self-healable copolymers. A series of copolymers comprising amino acid-bearing acrylamides and nBA with tunable comonomer compositions and molecular weights were synthesized by free radical and reversible addition-fragmentation chain-transfer copolymerization. Self-healing and mechanical properties originated from the noncovalent bonds between the carboxyl, hydroxyl, and amide groups, and π-π stacking interactions among the amino acid residues in the side chains were evaluated. Among these copolymers, P(nBA-co-AGluOH) with suitable comonomer compositions and molecular weights (nBA : AGluOH = 82 : 18, Mn = 18 300, Mw/Mn = 2.58) exhibited good mechanical properties (modulus of toughness = 17.3 MJ m-3) and self-healing under ambient conditions. The multiple noncovalent bonds of P(nBA-co-AGluOH)s were also efficient in improving the optical properties with an enhanced refractive index and good transparency.

Macrophages and fibroblasts in foreign body reactions: How mechanical cues drive cell functions?

Biomaterials, when implanted in the human body, can induce a series of cell- and cytokine-related reactions termed foreign body reactions (FBRs). In the progression of FBRs, macrophages regulate inflammation and healing by polarizing to either a pro-inflammatory or pro-healing phenotype and recruit fibroblasts by secreting cytokines. Stimulated by the biomaterials, fibrotic capsule is formed eventually. The implant, along with its newly formed capsule, introduces various mechanical cues that influence cellular functions. Mechanosensing proteins, such as integrins or ion channels, transduce extracellular mechanical signals into cytoplasm biochemical signals in response to mechanical stimuli. Consequently, the morphology, migration mode, function, and polarization state of the cells are affected. Modulated by different intracellular signaling pathways and their crosstalk, the expression of fibrotic genes increases with fibroblast activation and fibroblast to myofibroblast transition under stiff or force stimuli. However, summarized in most current studies, the outcomes of macrophage polarization in the effect of different mechanical cues are inconsistent. The underlying mechanisms should be investigated with more advanced technology and considering more interfering aspects. Further research is needed to determine how to modulate the progression of fibrotic capsule formation in FBR artificially.

Fucoidan and topography modification improved in situ endothelialization on acellular synthetic vascular grafts

Thrombogenesis remains the primary failure of synthetic vascular grafts. Endothelial coverage is crucial to provide an antithrombogenic surface. However, most synthetic materials do not support cell adhesion, and transanastomotic endothelial migration is limited. Here, a surface modification strategy using fucoidan and topography was developed to enable fast in situ endothelialization of polyvinyl alcohol, which is not endothelial cell-adhesive. Among three different immobilization approaches compared, conjugation of aminated-fucoidan promoted endothelial monolayer formation while minimizing thrombogenicity in both in vitro platelet rich plasma testing and ex vivo non-human primate shunt assay. Screening of six topographical patterns showed that 2 μm gratings increased endothelial cell migration without inducing inflammation responses of endothelial cells. Mechanistic studies demonstrated that fucoidan could attract fibronectin, enabling integrin binding and focal adhesion formation and activating focal adhesion kinase (FAK) signaling, and 2 μm gratings further enhanced FAK-mediated cell migration. In a clinically relevant rabbit carotid artery end-to-side anastomosis model, 60% in situ endothelialization was observed throughout the entire lumen of 1.7 mm inner diameter modified grafts, compared to 0% of unmodified graft, and the four-week graft patency also increased. This work presents a promising strategy to stimulate in situ endothelialization on synthetic materials for improving long-term performance.

Tumor-Released Products Promote Bone Marrow-Derived Macrophage Survival and Proliferation

Macrophages play a central role within the tumor microenvironment, with relevant implications for tumor progression. The modulation of their phenotype is one of the mechanisms used by tumors to escape from effective immune responses. This study was designed to analyze the influence of soluble products released by tumors, here represented by the tumor-conditioned media of two tumor cell lines (3LL from Lewis lung carcinoma and MN/MCA from fibrosarcoma), on murine macrophage differentiation and polarization in vitro. Data revealed that tumor-conditioned media stimulated macrophage differentiation but influenced the expression levels of macrophage polarization markers, cytokine production, and microRNAs of relevance for macrophage biology. Interestingly, tumor-derived soluble products supported the survival and proliferation rate of bone marrow precursor cells, an effect observed even with mature macrophages in the presence of M2 but not M1 inducers. Despite presenting low concentrations of macrophage colony-stimulating factor (M-CSF), tumor-conditioned media alone also supported the proliferation of cells to a similar extent as exogenous M-CSF. This effect was only evident in cells positive for the expression of the M-CSF receptor (CD115) and occurred preferentially within the CD16⁺ subset. Blocking CD115 partially reversed the effect on proliferation. These results suggest that tumors release soluble products that not only promote macrophage development from bone marrow precursors but also stimulate the proliferation of cells with specific phenotypes that could support protumoral functions.

Response of Endothelial Cells to Gelatin-Based Hydrogels

The establishment of confluent endothelial cell (EC) monolayers on implanted materials has been identified as a concept to avoid thrombus formation but is a continuous challenge in cardiovascular device engineering. Here, material properties of gelatin-based hydrogels obtained by reacting gelatin with varying amounts of lysine diisocyanate ethyl ester were correlated with the functional state of hydrogel contacting venous EC (HUVEC) and HUVEC's ability to form a monolayer on these hydrogels. The density of adherent HUVEC on the softest hydrogel at 37 °C (G' = 1.02 kPa, E = 1.1 ± 0.3 kPa) was significantly lower (125 mm^-1) than on the stiffer hydrogels (920 mm^-1; G' = 2.515 and 5.02 kPa, E = 4.8 ± 0.8 and 10.3 ± 1.2 kPa). This was accompanied by increased matrix metalloprotease activity (9 pmol·min^-2 compared to 0.6 pmol·min^-2) and stress fiber formation, while cell-to-cell contacts were comparable. Likewise, release of eicosanoids (e.g., prostacyclin release of 1.7 vs 0.2 pg·mL^-1·cell^-1) and the pro-inflammatory cytokine MCP-1 (8 vs <1.5 pg·mL^-1·cell^-1) was higher on the softer than on the stiffer hydrogels. The expressions of pro-inflammatory markers COX-2, COX-1, and RAGE were slightly increased on all hydrogels on day 2 (up to 200% of the control), indicating a weak inflammation; however, the levels dropped to below the control from day 6. The study revealed that hydrogels with higher moduli approached the status of a functionally confluent HUVEC monolayer. The results indicate the promising potential especially of the discussed gelatin-based hydrogels with higher G' as biomaterials for implants foreseen for the venous system.

Substrate-enzyme affinity-based surface modification strategy for endothelial cell-specific binding under shear stress

Establishing an endothelial cell (EC) monolayer on top of the blood contacting surface of grafts is considered to be a promising approach for creating a hemocompatible surface. Here we utilized the high affinity interactions between the EC plasma membrane expressed enzyme called endothelin converting enzyme-1 (ECE-1) and its corresponding substrate big Endothelin-1 (bigET-1) to engineer an EC-specific binding surface. Since enzymatic cleavage of substrates require physical interaction between the enzyme and its corresponding substrate, it was hypothesized that a surface with chemically immobilized synthetic bigET-1 will preferentially attract ECs over other types of cells found in vascular system such as vascular smooth muscle cells (VSMCs). First, the expression of ECE-1 was significantly higher in ECs, and ECs processed synthetic bigET-1 to produce ET-1 in a cell number-dependent manner. Such interaction between ECs and synthetic bigET-1 was also detectible in blood. Next, vinyl-terminated self-assembled monolayers (SAMs) were established, oxidized and activated on a glass substrate as a model to immobilize synthetic bigET-1 via amide bonds. The ECs cultured on the synthetic bigET-1-immobilized surface processed larger amount of synthetic bigET-1 to produce ET-1 compared to VSMCs (102.9±5.13 vs. 9.75±0.74 pg/ml). The number of ECs bound to the synthetic bigET-1-immobilized surface during 1 h of shearing (5dyne/cm2) was approximately 3-fold higher than that of VSMCs (46.25±12.61 vs. 15.25±3.69 cells/100×HPF). EC-specific binding of synthetic bigET-1-immobilized surface over a surface modified with collagen, a common substance for cell adhesion, was also observed. The present study demonstrated that using the substrate-enzyme affinity (SEA) of cell type-specific enzyme and its corresponding substrate can be an effective method to engineer a surface preferentially binds specific type of cells. This novel strategy might open a new route toward rapid endothelialization under dynamic conditions supporting the long-term patency of cardiovascular implants.

Protein-Stabilizing Effect of Amphiphilic Block Copolymers with a Tertiary Sulfonium-Containing Zwitterionic Segment

Tertiary sulfonium-containing zwitterionic block copolymers consisting of N-acryloyl-l-methionine methyl sulfonium salt (A-Met(S+)-OH) and n-butyl acrylate (BA) were newly synthesized to develop a novel protein stabilizer. The zwitterionic block copolymers were prepared by reversible addition-fragmentation chain-transfer (RAFT) polymerization of BA using a hydrophilic macro-chain-transfer agent (CTA) obtained from N-acryloyl-l-methionine (A-Met-OH) and subsequent postmodification. RAFT polymerization of A-Met-OH using poly(BA) macro-CTA, followed by postmodification, also afforded the target poly(A-Met(S+)-OH)-b-poly(BA). The block copolymers stabilized horseradish peroxidase (HRP) during storage at 37 °C for 5 days, and the protein-stabilizing effect was enhanced with increase in the A-Met(S+)-OH content. In particular, the block copolymer with ∼85% A-Met(S+)-OH content showed a significant protein-stabilizing effect at a temperature (37 °C) higher than the room temperature, which is highly desirable for practical and industrial applications. The addition of sucrose into the block copolymer-protein solution led to a considerable increase in the HRP activity under the same conditions. Excellent alkaline phosphatase stabilization at 37 °C for 12 days was also achieved using the block copolymers. The zwitterionic block copolymers with the optimal hydrophilic/hydrophobic balance were found to serve as efficient protein-stabilizing agents, in comparison with the corresponding homopolymer and random copolymers. Dynamic light scattering, zeta potential, transmission electron microscopy, and circular dichroism measurements revealed that the zwitterionic block copolymer stabilizes an enzyme by wrapping with a slight change in the size, whereas the secondary and ordered structures of the enzyme are maintained.

Implantation of VEGF-functionalized cell-free vascular grafts: regenerative and immunological response

Recently, our group demonstrated that immobilized VEGF can capture flowing endothelial cells (ECs) from the blood in vitro and promote endothelialization and patency of acellular tissue-engineered vessels (A-TEVs) into the arterial system of an ovine animal model. Here, we demonstrate implantability of submillimeter diameter heparin and VEGF-decorated A-TEVs in a mouse model and discuss the cellular and immunologic response. At 1 mo postimplantation, the graft lumen was fully endothelialized, as shown by expression of EC markers such as CD144, eNOS, CD31, and VEGFR2. Interestingly, the same cells coexpressed leukocyte/macrophage (Mϕ) markers CD14, CD16, VEGFR1, CD38, and EGR2. Notably, there was a stark difference in the cellular makeup between grafts containing VEGF and those containing heparin alone. In VEGF-containing grafts, infiltrating monocytes (MCs) converted into anti-inflammatory M2-Mϕs, and the grafts developed well-demarcated luminal and medial layers resembling those of native arteries. In contrast, in grafts containing only heparin, MCs converted primarily into M1-Mϕs, and the endothelial and smooth muscle layers were not well defined. Our results indicate that VEGF may play an important role in regulating A-TEV patency and regeneration, possibly by regulating the inflammatory response to the implants.-Smith, R. J., Jr., Yi, T., Nasiri, B., Breuer, C. K., Andreadis, S. T. Implantation of VEGF-functionalized cell-free vascular grafts: regenerative and immunological response.

Influence of different surface treatments of poly(n-butyl acrylate) networks on fibroblasts adhesion, morphology and viability

BACKGROUND

Physical and chemical characteristics of implant materials determine the fate of long-term cardiovascular devices. However, there is still a lack of fundamental understanding of the molecular mechanisms occurring in the material-tissue interphase. In a previous study, soft covalently crosslinked poly(n-butyl acrylate) networks (cPnBA) were introduced as sterilizable, non-toxic and immuno-compatible biomaterials with mechanical properties adjustable to blood vessels. Here we study the influence of different surface treatments in particular oxygen plasma modification and fibrinogen deposition as well as a combinatorial approach on the adhesion and viability of fibroblasts.

MATERIAL AND METHODS

Two types of cPnBA networks with Young's moduli of 0.19±0.01 MPa (cPnBA04) and 1.02±0.01 MPa (cPnBA73) were synthesized and post-modified using oxygen plasma treatment (OPT) or fibrinogen coating (FIB) or a combination of both (OPT+FIB). The water contact angles of the differently post-treated cPnBAs were studied to monitor changes in the wettability of the polymer surfaces. Because of the key role of vascular fibroblasts in regeneration processes around implant materials, here we selected L929 fibroblasts as model cell type to explore morphology, viability, metabolic activity, cell membrane integrity as well as characteristics of the focal adhesions and cell cytoskeleton on the cPnBA surfaces.

RESULTS

Compared to non-treated cPnBAs the advancing water-contact angles were found to be reduced after all surface modifications (p < 0.05, each), while lowest values were observed after the combined surface treatment (OPT+FIB). The latter differed significantly from the single OPT and FIB. The number of adherent fibroblasts and their adherence behavior differed on both pristine cPnBA networks. The fibroblast density on cPnBA04 was 743±434 cells·mm-2, was about 6.5 times higher than on cPnBA73 with 115±73 cells·mm-2. On cPnBA04 about 20% of the cells were visible as very small, round and buckled cells while all other cells were in a migrating status. On cPnBA73, nearly 50% of fibroblasts were visible as very small, round and buckled cells. The surface functionalization either using oxygen plasma treatment or fibrinogen coating led to a significant increase of adherent fibroblasts, particularly the combination of both techniques, for both cPnBA networks. It is noteworthy to mention that the fibrinogen coating overruled the characteristics of the pristine surfaces; here, the fibroblast densities after seeding were identical for both cPnBA networks. Thus, the binding rather depended on the fibrinogen coating than on the substrate characteristics anymore. While the integrity of the fibroblasts membrane was comparable for both polymers, the MTS tests showed a decreased metabolic activity of the fibroblasts on cPnBA.

CONCLUSION

The applied surface treatments of cPnBA successfully improved the adhesion of viable fibroblasts. Under resting conditions as well as after shearing the highest fibroblast densities were found on surfaces with combined post-treatment.

Transparent Substrates Prepared From Different Amorphous Polymers Can Directly Modulate Primary Human B cell functions

Manipulation of B cell functions such as antibody and cytokine secretion, is of clinical and biotechnological interest and can be achieved by soluble ligands activating cell surface receptors. Alternatively, the exposure to suitable solid substrates would offer the possibility to transiently induced cell signaling, since the signaling is interrupted when the cells are removed from the substrate. Cell/substrate interactions are mediated by physical valences such as, hydrogen bonds or hydrophobic forces on the substrate surface. Therefore, in this study B cells were cultivated on polymeric substrates, differing in their chemical composition and thus their capacity to undergo physical interactions. Activated B cells cultivated on polystyrene (PS) showed an altered cytokine response indicated by increased IL-10 and decreased IL-6 secretion. Interestingly, B cells cultivated on polyetherurethane (PEU), which has among all tested polymers the highest potential to form strong hydrogen bonds showed an impaired activation, which could be restored by re-cultivation on tissue culture polystyrene. The results indicate that B cell behavior can transiently be manipulated solely by interacting with polymeric surface, which could be explained by receptor activation mediated by physical interaction with the substrate or by altering the availability of the soluble stimulatory reagents by adsorption processes.

Sphingosine-1-phosphate improves endothelialization with reduction of thrombosis in recellularized human umbilical vein graft by inhibiting syndecan-1 shedding in vitro

Sphingosine-1-phosphate (S1P) has been known to promote endothelial cell (EC) proliferation and protect Syndecan-1 (SDC1) from shedding, thereby maintaining this antithrombotic signal. In the present study, we investigated the effect of S1P in the construction of a functional tissue-engineered blood vessel by using human endothelial cells and decellularized human umbilical vein (DHUV) scaffolds. Both human umbilical vein endothelial cells (HUVEC) and human cord blood derived endothelial progenitor cells (EPC) were seeded onto the scaffold with or without the S1P treatment. The efficacy of re-cellularization was determined by using the fluorescent marker CellTracker CMFDA and anti-CD31 immunostaining. The antithrombotic effect of S1P was examined by the anti-aggregation tests measuring platelet adherence and clotting time. Finally, we altered the expression of SDC1, a major glycocalyx protein on the endothelial cell surface, using MMP-7 digestion to explore its role using platelet adhesion tests in vitro. The result showed that S1P enhanced the attachment of HUVEC and EPC. Based on the anti-aggregation tests, S1P-treated HUVEC recellularized vessels when grafted showed reduced thrombus formation compared to controls. Our results also identified reduced SDC1 shedding from HUVEC responsible for inhibition of platelet adherence. However, no significant antithrombogenic effect of S1P was observed on EPC. In conclusion, S1P is an effective agent capable of decreasing thrombotic risk in engineered blood vessel grafts.

STATEMENT OF SIGNIFICANCE

Sphingosine-1phosphate (S1P) is a low molecular-weight phospholipid mediator that regulates diverse biological activities of endothelial cell, including survival, proliferation, cell barrier integrity, and also influences the development of the vascular system. Based on these characters, we the first time to use it as an additive during the process of a small caliber blood vessel construction by decellularized human umbilical vein and endothelial cell/endothelial progenitor. We further explored the function and mechanism of S1P in promoting revascularization and protection against thrombosis in this tissue engineered vascular grafts. The results showed that S1P could not only accelerate the generation but also reduce thrombus formation of small caliber blood vessel.

The pathology of the foreign body reaction against biomaterials

The healing process after implantation of biomaterials involves the interaction of many contributing factors. Besides their in vivo functionality, biomaterials also require characteristics that allow their integration into the designated tissue without eliciting an overshooting foreign body reaction (FBR). The targeted design of biomaterials with these features, thus, needs understanding of the molecular mechanisms of the FBR. Much effort has been put into research on the interaction of engineered materials and the host tissue. This elucidated many aspects of the five FBR phases, that is protein adsorption, acute inflammation, chronic inflammation, foreign body giant cell formation, and fibrous capsule formation. However, in practice, it is still difficult to predict the response against a newly designed biomaterial purely based on the knowledge of its physical-chemical surface features. This insufficient knowledge leads to a high number of factors potentially influencing the FBR, which have to be analyzed in complex animal experiments including appropriate data-based sample sizes. This review is focused on the current knowledge on the general mechanisms of the FBR against biomaterials and the influence of biomaterial surface topography and chemical and physical features on the quality and quantity of the reaction. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 927-940, 2017.

Human Endothelial Cell Models in Biomaterial Research

Endothelial cell (EC) models have evolved as important tools in biomaterial research due to ubiquitously occurring interactions between implanted materials and the endothelium. However, screening the available literature has revealed a gap between material scientists and physiologists in terms of their understanding of these biomaterial-endothelium interactions and their relative importance. Consequently, EC models are often applied in nonphysiological experimental setups, or too extensive conclusions are drawn from their results. The question arises whether this might be one reason why, among the many potential biomaterials, only a few have found their way into the clinic. In this review, we provide an overview of established EC models and possible selection criteria to enable researchers to determine the most reliable and relevant EC model to use.

Influence of the coating with extracellular matrix and the number of cell passages on the endothelialization of a polystyrene surface

The proper morphology and function of the vascular endothelium are prerequisites for a sufficient supply of the tissues. Endothelial cell (EC) dysfunction can lead to circulatory disorders and the development of cardiovascular diseases. The endothelialization of cardiovascular implants is a sophisticated task since EC miss their natural environment and physiological stimuli in vitro. In addition, different studies revealed that the EC behavior and morphology depended on the substrate and the passage number of the EC. Therefore, the comparison of endothelialization studies is very difficult, when passage and substrate are unknown.The aim of this study was to investigate the growth potential and cell morphology of human venous endothelial cells (HUVEC) as a function of different cell passages and different substrates (pristine polystyrene, tissue-typical ECM-coated polystyrene). The study revealed that HUVEC morphology and growth potential were significantly different on pristine polystyrene compared to the basal lamina-like ECM-coated polystyrene surface. Furthermore, it became obvious that the passage of the cells affected the endothelialization of the polystyrene surface significantly. In conclusion, this study emphasized the need for a critical consideration of EC data whereas a simple comparison of results is not possible if EC age and passage is unknown.