A Perspective on Bioactive Cell Microencapsulation

ECM-based bioactive microencapsulation significantly improves islet function and graft performance

Microencapsulation is a promising strategy to prolong the survival and function of transplanted pancreatic islets for diabetes therapy, albeit its translation has been impeded by incoherent graft performance. The use of decellularized ECM has lately gained substantial research momentum due to its innate capacity to augment the function of cells originating from the same tissue type. In the present study, the advantages of both these approaches are leveraged in a porcine pancreatic ECM (pECM)-based microencapsulation platform, thus significantly enhancing murine pancreatic islet performance. pECM-encapsulated islets sustain high insulin secretion levels in vitro, surpassing those of islets encapsulated in conventional alginate microcapsules. Moreover, pECM-encapsulated islet cells proliferate and produce an enriched intra-islet ECM framework, displaying a distinctive structural rearrangement. The beneficial effect of pECM encapsulation is further reinforced by the temporary protection against cytokine-induced cytotoxicity. In-vivo, this platform significantly improves glucose tolerance and achieves glycemic correction in 100% of immunocompetent diabetic mice without any immunosuppression, compared to only 50% mice achieved glycemic correction by alginate encapsulation. Altogether, the results presented herein reveal that pECM-based microencapsulation offers a natural pancreatic niche that can restore the function of isolated pancreatic islets and deliver them safely, avoiding the need for immunosuppression. STATEMENT OF SIGNIFICANCE: Aiming to improve pancreatic islet transplantation outcomes in diabetic patients, we developed a microencapsulation platform based on pancreatic extracellular matrix (pECM). In these microcapsules the islets are entrapped within a pECM hydrogel that mimics the natural pancreatic microenvironment. We show that pECM encapsulation supports the islets' viability and function in culture, and provides temporal protection against cytokine-induced stress. In a diabetic mouse model, pECM encapsulation significantly improved glucose tolerance and achieved glycemic correction without any immunosuppression. These results reveal the potential of pECM encapsulation as a viable treatment for diabetes, providing a solid scientific basis for more advanced preclinical studies.

Artificial cells for the treatment of liver diseases

Liver diseases have become an increasing health burden and account for over 2 million deaths every year globally. Standard therapies including liver transplant and cell therapy offer a promising treatment for liver diseases, but they also suffer limitations such as adverse immune reactions and lack of long-term efficacy. Artificial cells that mimic certain functions of a living cell have emerged as a new strategy to overcome some of the challenges that liver cell therapy faces at present. Artificial cells have demonstrated advantages in long-term storage, targeting capability, and tuneable features. This article provides an overview of the recent progress in developing artificial cells and their potential applications in liver disease treatment. First, the design of artificial cells and their biomimicking functions are summarized. Then, systems that mimic cell surface properties are introduced with two concepts highlighted: cell membrane-coated artificial cells and synthetic lipid-based artificial cells. Next, cell microencapsulation strategy is summarized and discussed. Finally, challenges and future perspectives of artificial cells are outlined. STATEMENT OF SIGNIFICANCE: Liver diseases have become an increasing health burden. Standard therapies including liver transplant and cell therapy offer a promising treatment for liver diseases, but they have limitations such as adverse immune reactions and lack of long-term efficacy. Artificial cells that mimic certain functions of a living cell have emerged as a new strategy to overcome some of the challenges that liver cell therapy faces at present. This article provides an overview of the recent progress in developing artificial cells and their potential applications in liver disease treatment, including the design of artificial cells and their biomimicking functions, two systems that mimic cell surface properties (cell membrane-coated artificial cells and synthetic lipid-based artificial cells), and cell microencapsulation strategy. We also outline the challenges and future perspectives of artificial cells.

Polymer microcapsules and microbeads as cell carriers for in vivo biomedical applications

Polymer microcarriers are being extensively explored as cell delivery vehicles in cell-based therapies and hybrid tissue and organ engineering. Spherical microcarriers are of particular interest due to easy fabrication and injectability. They include microbeads, composed of a porous matrix, and microcapsules, where matrix core is additionally covered with a semipermeable membrane. Microcarriers provide cell containment at implantation site and protect the cells from host immunoresponse, degradation and shear stress. Immobilized cells may be genetically altered to release a specific therapeutic product directly at the target site, eliminating side effects of systemic therapies. Cell microcarriers need to fulfil a number of extremely high standards regarding their biocompatibility, cytocompatibility, immunoisolating capacity, transport, mechanical and chemical properties. To obtain cell microcarriers of specified parameters, a wide variety of polymers, both natural and synthetic, and immobilization methods can be applied. Yet so far, only a few approaches based on cell-laden microcarriers have reached clinical trials. The main issue that still impedes progress of these systems towards clinical application is limited cell survival in vivo. Herein, we review polymer biomaterials and methods used for fabrication of cell microcarriers for in vivo biomedical applications. We describe their key limitations and modifications aiming at improvement of microcarrier in vivo performance. We also present the main applications of polymer cell microcarriers in regenerative medicine, pancreatic islet and hepatocyte transplantation and in the treatment of cancer. Lastly, we outline the main challenges in cell microimmobilization for biomedical purposes, the strategies to overcome these issues and potential future improvements in this area.

Cell encapsulation: Overcoming barriers in cell transplantation in diabetes and beyond

Cell-based therapy is emerging as a promising strategy for treating a wide range of human diseases, such as diabetes, blood disorders, acute liver failure, spinal cord injury, and several types of cancer. Pancreatic islets, blood cells, hepatocytes, and stem cells are among the many cell types currently used for this strategy. The encapsulation of these "therapeutic" cells is under intense investigation to not only prevent immune rejection but also provide a controlled and supportive environment so they can function effectively. Some of the advanced encapsulation systems provide active agents to the cells and enable a complete retrieval of the graft in the case of an adverse body reaction. Here, we review various encapsulation strategies developed in academic and industrial settings, including the state-of-the-art technologies in advanced preclinical phases as well as those undergoing clinical trials, and assess their advantages and challenges. We also emphasize the importance of stimulus-responsive encapsulated cell systems that provide a "smart and live" therapeutic delivery to overcome barriers in cell transplantation as well as their use in patients.

Comparison of different cationic polymers efficacy in fabrication of alginate multilayer microcapsules

In past decades, alginate-based multilayer microcapsules have been given important attention in various pharmaceutical investigations. Alginate-poly l lysine-alginate (APA) is studied the most. Due to the similarity between the structure of polyethyleneimine (PEI) and poly-L-lysine (PLL) and also lower price of PEI than PLL, this study was conducted to compare the efficacy of linear (LPEI) and branch (BPEI) forms of PEI with PLL as covering layers in fabrication of microcapsules. The microcapsules were fabricated using electrostatic bead generator and their shape/size, surface roughness, mechanical strength, and interlayer interactions were also investigated using optical microscopy, AFM, explosion test and FTIR, respectively. Furthermore, cytotoxicity was evaluated by comparing the two anionic final covering layers alginate (Alg) and sodium cellulose sulphate (NCS) using MTT test. BPEI was excluded from the rest of the study due to its less capacity to strengthen the microcapsules and also the aggregation of the resultant alginate-BPEI-alginate microcapsules, while LPEI showed properties similar to PLL. MTT test also showed that NCS has no superiority over Alg as final covering layer. Therefore, it is concluded that, LPEI could be considered as a more cost effective alternative to PLL and a promising subject for future studies.

Induced Cell Turnover: A Novel Therapeutic Modality for In Situ Tissue Regeneration

Induced cell turnover (ICT) is a theoretical intervention in which the targeted ablation of damaged, diseased, and/or nonfunctional cells is coupled with replacement by partially differentiated induced pluripotent stem cells in a gradual and multiphasic manner. Tissue-specific ablation can be achieved using pro-apoptotic small molecule cocktails, peptide mimetics, and/or tissue-tropic adeno-associated virus-delivered suicide genes driven by cell type-specific promoters. Replenishment with new cells can be mediated by systemic administration of cells engineered for homing, robustness, and even enhanced function and disease resistance. Otherwise, the controlled release of cells can be achieved using implanted biodegradable scaffolds, hydrogels, and polymer matrixes. In theory, ICT would enable in situ tissue regeneration without the need for surgical transplantation of organs produced ex vivo, and addresses non-transplantable tissues (such as the vasculature, lymph nodes, and the nervous system). This article outlines several complimentary strategies for overcoming barriers to ICT in an effort to stimulate further research at this promising interface of cell therapy, tissue engineering, and regenerative medicine.

Compartmentalized Spherical Collagen Microparticles for Anisotropic Cell Culture Microenvironments

This paper describes a new fabrication method for obtaining anisotropic spherical hydrogel microparticles with different types of extracellular matrix (ECM) hemispheres for use in 3D cell culture. To fabricate the microparticles, a mixture of an ECM precursor solution and sodium alginate is ejected into a calcium chloride solution under large centrifugal acceleration by a centrifuge-based microfluidic device; the calcium alginate hydrogel plays a significant role as a "sacrificial gelation template" to maintain the ECM molecules in each hemisphere. This fabrication method enables gaining control of the hemispherical volume, density, and type of ECM. Using the microparticles, cells could be successfully encapsulated in each hemisphere selectively with high viability, which are then suitable for culture in the microparticles to form microtissues. It is believed that the proposed anisotropic ECM microparticles will facilitate the coculture of multiple cell types in different ECMs, which is similar to in vivo microenvironments, facilitating control of cell behavior in an anisotropic microenvironment for the benefit of large-scale and quantitative analyses in vitro.

Microencapsulated Cells for Cancer Therapy

The microencapsulation of different types of cells that are able to produce therapeutic factors is being investigated for the treatment of several human diseases. Most efforts are focused on chronic and degenerative diseases as this strategy could become an alternative to some commonly used parenteral treatments that need to be repeatedly administered. But, this approach has also been investigated in the field of oncology with the aim of providing immunomodulatory antibodies that are able to enhance the patient's inherent immune response against the tumor. These kind of treatments would provide the patient with the therapeutic drug produced in situ, de novo, and in a sustained way, making the therapy more comfortable.Although different devices are nowadays available to produce cell-enclosing alginate-microcapsules, here, we describe the most important steps and advices in order to fabricate alginate-poly-L-lysine-alginate microcapsules containing hybridoma cells for cancer management using an electrostatic bead generator, and how to evaluate the viability of those cells over the time.

Sizes and Sufficient Quantities of MSC Microspheres for Intrathecal Injection to Modulate Inflammation in Spinal Cord Injury

Microencapsulation of mesenchymal stem cells (MSC) in alginate facilitates cell delivery, localization and survival, and modulates inflammation in vivo. However, we found that delivery of the widely used ~0.5 mm diameter encapsulated MSC (eMSC) by intrathecal injection into spinal cord injury (SCI) rats was highly variable. Injections of smaller (~0.2 mm) diameter eMSC into the lumbar spine were much more reproducible and they increased the anti-inflammatory macrophage response around the SCI site. We now report that injection of small eMSC >2 cm caudal from the rat SCI improved locomotion and myelin preservation 8 weeks after rat SCI versus control injections. Because preparation of sufficient quantities of small eMSC for larger studies was not feasible and injection of the large eMSC is problematic, we have developed a procedure to prepare medium-sized eMSC (~0.35 mm diameter) that can be delivered more reproducibly into the lumbar rat spine. The number of MSC incorporated/capsule in the medium sized capsules was ~5-fold greater than that in small capsules and the total yield of eMSC was ~20-fold higher than that for the small capsules. Assays with all three sizes of eMSC capsules showed that they inhibited TNF-α secretion from activated macrophages in co-cultures, suggesting no major difference in their anti-inflammatory activity in vitro. The in vivo activity of the medium-sized eMSC was tested after injecting them into the lumbar spine 1 day after SCI. Histological analyses 1 week later showed that eMSC reduced levels of activated macrophages measured by IB4 staining and increased white matter sparing in similar regions adjacent to the SCI site. The combined results indicate that ~0.35 mm diameter eMSC reduced macrophage inflammation in regions where white matter was preserved during critical early phases after SCI. These techniques enable preparation of eMSC in sufficient quantities to perform pre-clinical SCI studies with much larger numbers of subjects that will provide functional analyses of several critical parameters in rodent models for CNS inflammatory injury.

Encapsulated cells for long-term secretion of soluble VEGF receptor 1: Material optimization and simulation of ocular drug response

Anti-angiogenic therapies with vascular endothelial growth factor (VEGF) inhibiting factors are effective treatment options for neovascular diseases of the retina, but these proteins can only be delivered as intravitreal (IVT) injections. To sustain a therapeutic drug level in the retina, VEGF inhibitors have to be delivered frequently, every 4-8weeks, causing inconvenience for the patients and expenses for the healthcare system. The aim of this study was to investigate cell encapsulation as a delivery system for prolonged anti-angiogenic treatment of retinal neovascularization. Genetically engineered ARPE-19 cells secreting soluble vascular endothelial growth factor receptor 1 (sVEGFR1) were encapsulated in a hydrogel of cross-linked collagen and interpenetrating hyaluronic acid (HA). The system was optimized in terms of matrix composition and cell density, and long-term cell viability and protein secretion measurements were performed. sVEGFR1 ARPE-19 cells in the optimized hydrogel remained viable and secreted sVEGFR1 at a constant rate for at least 50days. Based on pharmacokinetic/pharmacodynamic (PK/PD) modeling, delivery of sVEGFR1 from this cell encapsulation system is expected to lead only to modest VEGF inhibition, but improvements of the protein structure and/or secretion rate should result in strong and prolonged therapeutic effect. In conclusion, the hydrogel matrix herein supported the survival and protein secretion from the encapsulated cells. The PK/PD simulation is a convenient approach to predict the efficiency of the cell encapsulation system before in vivo experiments.

Encapsulated cell therapy for neurodegenerative diseases: from promise to product

Delivering therapeutic molecules, including trophic factor proteins, across the blood brain barrier to the brain parenchyma to treat chronic neurodegenerative diseases remains one of the great challenges in biology. To be effective, delivery needs to occur in a long-term and stable manner at sufficient quantities directly to the target region in a manner that is selective but yet covers enough of the target site to be efficacious. One promising approach uses cellular implants that produce and deliver therapeutic molecules directly to the brain region of interest. Implanted cells can be precisely positioned into the desired region and can be protected from host immunological attack by encapsulating them and by surrounding them within an immunoisolatory, semipermeable capsule. In this approach, cells are enclosed within a semiporous capsule with a perm selective membrane barrier that admits oxygen and required nutrients and releases bioactive cell secretions while restricting passage of larger cytotoxic agents from the host immune defense system. Recent advances in human cell line development have increased the levels of secreted therapeutic molecules from encapsulated cells, and membrane extrusion techniques have led to the first ever clinical demonstrations of long-term survival and function of encapsulated cells in the brain parenchyma. As such, cell encapsulation is capable of providing a targeted, continuous, de novo synthesized source of very high levels of therapeutic molecules that can be distributed over significant portions of the brain.

Characterization of a bioactive fiber scaffold with entrapped HUVECs in coaxial electrospun core-shell fiber

Human umbilical vein endothelial cells (HUVECs) were successfully entrapped in polyethylene oxide (PEO) core /polycaprolactone (PCL) shell electrospun fibers thus creating a "bioactive fiber." The viability and release of biomolecules from the entrapped cells in the bioactive fibers were characterized. A key modification to the core solution was the inclusion of 50% fetal bovine serum (FBS), which improved cell viability substantially. The fluorescein diacetate (FDA) staining revealed that the entrapped cells were intact and viable immediately after the electrospinning process. A long-term cell viability assay using AlamarBlue® showed that cells were viable for over two weeks. Secreted Interleukin-8 (IL-8) was monitored as a candidate released protein, which can also act as an indicator of HUVEC stress. These results demonstrated that HUVECs could be entrapped within the electrospun scaffold with the potential of controllable cell deposition and the creation of a bioactive fibrous scaffold with extended functionality.

Three-dimensional scaffolds for tissue engineering applications: role of porosity and pore size

Tissue engineering applications commonly encompass the use of three-dimensional (3D) scaffolds to provide a suitable microenvironment for the incorporation of cells or growth factors to regenerate damaged tissues or organs. These scaffolds serve to mimic the actual in vivo microenvironment where cells interact and behave according to the mechanical cues obtained from the surrounding 3D environment. Hence, the material properties of the scaffolds are vital in determining cellular response and fate. These 3D scaffolds are generally highly porous with interconnected pore networks to facilitate nutrient and oxygen diffusion and waste removal. This review focuses on the various fabrication techniques (e.g., conventional and rapid prototyping methods) that have been employed to fabricate 3D scaffolds of different pore sizes and porosity. The different pore size and porosity measurement methods will also be discussed. Scaffolds with graded porosity have also been studied for their ability to better represent the actual in vivo situation where cells are exposed to layers of different tissues with varying properties. In addition, the ability of pore size and porosity of scaffolds to direct cellular responses and alter the mechanical properties of scaffolds will be reviewed, followed by a look at nature's own scaffold, the extracellular matrix. Overall, the limitations of current scaffold fabrication approaches for tissue engineering applications and some novel and promising alternatives will be highlighted.

Immunotherapeutic organoids: a new approach to cancer treatment

Therapeutic monoclonal antibodies have revolutionized the treatment of cancer and other diseases. However, several limitations of antibody-based treatments, such as the cost of therapy and the achievement of sustained plasma levels, should be still addressed for their widespread use as therapeutics. The use of cell and gene transfer methods offers additional benefits by producing a continuous release of the antibody with syngenic glycosylation patterns, which makes the antibody potentially less immunogenic. In vivo secretion of therapeutic antibodies by viral vector delivery or ex vivo gene modified long-lived autologous or allogeneic human mesenchymal stem cells may advantageously replace repeated injection of clinical-grade antibodies. Gene-modified autologous mesenchymal stem cells can be delivered subcutaneously embedded in a non-immunogenic synthetic extracellular matrix-based scaffold that guarantees the survival of the cell inoculum. The scaffold would keep cells at the implantation site, with the therapeutic protein acting at distance (immunotherapeutic organoid), and could be retrieved once the therapeutic effect is fulfilled. In the present review we highlight the practical importance of living cell factories for in vivo secretion of recombinant antibodies.