Emulsion strategies in the microencapsulation of cells: Pathways to thin coherent membranes

The impact of cell surface PEGylation and short-course immunotherapy on islet graft survival in an allogeneic murine model

Islet transplantation is a promising therapy for Type 1 diabetes mellitus; however, host inflammatory and immune responses lead to islet dysfunction and destruction, despite potent systemic immunosuppression. Grafting of poly(ethylene glycol) (PEG) to the periphery of cells or tissues can mitigate inflammation and immune recognition via generation of a steric barrier. Herein, we sought to evaluate the complementary impact of islet PEGylation with a short-course immunotherapy on the survival of fully-MHC mismatched islet allografts (DBA/2 islets into diabetic C57BL/6J recipients). Anti-Lymphocyte Function-associated Antigen 1 (LFA-1) antibody was selected as a complementary, transient, systemic immune monotherapy. Islets were PEGylated via an optimized protocol, with resulting islets exhibiting robust cell viability and function. Following transplantation, a significant subset of diabetic animals receiving PEGylated islets (60%) or anti-LFA-1 antibody (50%) exhibited long-term (>100d) normoglycemia. The combinatorial approach proved synergistic, with 78% of the grafts exhibiting euglycemia long-term. Additional studies examining graft cellular infiltrates at early time points characterized the local impact of the transplant protocol on graft survival. Results illustrate the capacity of a simple polymer grafting approach to impart significant immunoprotective effects via modulation of the local transplant environment, while short-term immunotherapy serves to complement this effect.

STATEMENT OF SIGNIFICANCE

We believe this study is important and of interest to the biomaterials and transplant community for several reasons: 1) it provides an optimized protocol for the PEGylation of islets, with minimal impact on the coated islets, which can be easily translated for clinical applications; 2) this optimized protocol demonstrates the benefits of islet PEGylation in providing modest immunosuppression in a murine model; 3) this work demonstrates the combinatory impact of PEGylation with short-course immunotherapy (via LFA-1 blockage), illustrating the capacity of PEGylation to complement existing immunotherapy; and 4) it suggests macrophage phenotype shifting as the potential mechanism for this observed benefit.

Strategies to Maximize the Potential of Marine Biomaterials as a Platform for Cell Therapy

Marine biopolymers have been explored as a promising cell therapy system for efficient cell delivery and tissue engineering. However, the marine biomaterial-based systems themselves have exhibited limited performance in terms of maintenance of cell viability and functions, promotion of cell proliferation and differentiation as well as cell delivery efficiency. Thus, numerous novel strategies have been devised to improve cell therapy outcomes. The strategies include optimization of physical and biochemical properties, provision of stimuli-responsive functions, and design of platforms for efficient cell delivery and tissue engineering. These approaches have demonstrated substantial improvement of therapeutic outcomes in a variety of research settings. In this review, therefore, research progress made with marine biomaterials as a platform for cell therapy is reported along with current research directions to further advance cell therapies as a tool to cure incurable diseases.

Microfluidic conformal coating of non-spherical magnetic particles

We present the conformal coating of non-spherical magnetic particles in a co-laminar flow microfluidic system. Whereas in the previous reports spherical particles had been coated with thin films that formed spheres around the particles; in this article, we show the coating of non-spherical particles with coating layers that are approximately uniform in thickness. The novelty of our work is that while liquid-liquid interfacial tension tends to minimize the surface area of interfaces-for example, to form spherical droplets that encapsulate spherical particles-in our experiments, the thin film that coats non-spherical particles has a non-minimal interfacial area. We first make bullet-shaped magnetic microparticles using a stop-flow lithography method that was previously demonstrated. We then suspend the bullet-shaped microparticles in an aqueous solution and flow the particle suspension with a co-flow of a non-aqueous mixture. A magnetic field gradient from a permanent magnet pulls the microparticles in the transverse direction to the fluid flow, until the particles reach the interface between the immiscible fluids. We observe that upon crossing the oil-water interface, the microparticles become coated by a thin film of the aqueous fluid. When we increase the two-fluid interfacial tension by reducing surfactant concentration, we observe that the particles become trapped at the interface, and we use this observation to extract an approximate magnetic susceptibility of the manufactured non-spherical microparticles. Finally, using fluorescence imaging, we confirm the uniformity of the thin film coating along the entire curved surface of the bullet-shaped particles. To the best of our knowledge, this is the first demonstration of conformal coating of non-spherical particles using microfluidics.

Device design and materials optimization of conformal coating for islets of Langerhans

Encapsulation of islets of Langerhans may represent a way to transplant islets in the absence of immunosuppression. Traditional methods for encapsulation lead to diffusional limitations imposed by the size of the capsules (600-1,000 μm in diameter), which results in core hypoxia and delayed insulin secretion in response to glucose. Moreover, the large volume of encapsulated cells does not allow implantation in sites that might be more favorable to islet cell engraftment. To address these issues, we have developed an encapsulation method that allows conformal coating of islets through microfluidics and minimizes capsule size and graft volume. In this method, capsule thickness, rather than capsule diameter, is constant and tightly defined by the microdevice geometry and the rheological properties of the immiscible fluids used for encapsulation within the microfluidic system. We have optimized the method both computationally and experimentally, and found that conformal coating allows for complete encapsulation of islets with a thin (a few tens of micrometers) continuous layer of hydrogel. Both in vitro and in vivo in syngeneic murine models of islet transplantation, the function of conformally coated islets was not compromised by encapsulation and was comparable to that of unencapsulated islets. We have further demonstrated that the structural support conferred by the coating materials protected islets from the loss of function experienced by uncoated islets during ex vivo culture.

Oxygen supply to encapsulated therapeutic cells

Therapeutic cells encapsulated in immunobarrier devices have promise for treatment of a variety of human diseases without immunosuppression. The absence of sufficient oxygen supply to maintain viability and function of encapsulated tissue has been the most critical impediment to progress. Within the framework of oxygen supply limitations, we review the major issues related to development of these devices, primarily in the context of encapsulated islets of Langerhans for treating diabetes, including device designs and materials, supply of tissue, protection from immune rejection, and maintenance of cell viability and function. We describe various defensive measures investigated to enhance survival of transplanted tissue, and we review the diverse approaches to enhancement of oxygen transport to encapsulated tissue, including manipulation of diffusion distances and oxygen permeability of materials, induction of neovascularization with angiogenic factors and vascularizing membranes, and methods for increasing the oxygen concentration adjacent to encapsulated tissue so as to exceed that in the microvasculature. Recent developments, particularly in this latter area, suggest that the field is ready for clinical trials of encapsulated therapeutic cells to treat diabetes.

Polymers in cell encapsulation from an enveloped cell perspective

In the past two decades, many polymers have been proposed for producing immunoprotective capsules. Examples include the natural polymers alginate, agarose, chitosan, cellulose, collagen, and xanthan and synthetic polymers poly(ethylene glycol), polyvinyl alcohol, polyurethane, poly(ether-sulfone), polypropylene, sodium polystyrene sulfate, and polyacrylate poly(acrylonitrile-sodium methallylsulfonate). The biocompatibility of these polymers is discussed in terms of tissue responses in both the host and matrix to accommodate the functional survival of the cells. Cells should grow and function in the polymer network as adequately as in their natural environment. This is critical when therapeutic cells from scarce cadaveric donors are considered, such as pancreatic islets. Additionally, the cell mass in capsules is discussed from the perspective of emerging new insights into the release of so-called danger-associated molecular pattern molecules by clumps of necrotic therapeutic cells. We conclude that despite two decades of intensive research, drawing conclusions about which polymer is most adequate for clinical application is still difficult. This is because of the lack of documentation on critical information, such as the composition of the polymer, the presence or absence of confounding factors that induce immune responses, toxicity to enveloped cells, and the permeability of the polymer network. Only alginate has been studied extensively and currently qualifies for application. This review also discusses critical issues that are not directly related to polymers and are not discussed in the other reviews in this issue, such as the functional performance of encapsulated cells in vivo. Physiological endocrine responses may indeed not be expected because of the many barriers that the metabolites encounter when traveling from the blood stream to the enveloped cells and back to circulation. However, despite these diffusion barriers, many studies have shown optimal regulation, allowing us to conclude that encapsulated grafts do not always follow nature's course but are still a possible solution for many endocrine disorders for which the minute-to-minute regulation of metabolites is mandatory.

Clinical application of microencapsulated islets: actual prospectives on progress and challenges

After 25 years of intense pre-clinical work on microencapsulated intraperitoneal islet grafts into non-immunosuppressed diabetic recipients, the application of this procedure to patients with type 1 diabetes mellitus has been a significant step forward. This result, achieved in a few centers worldwide, underlies the safety of biopolymers used for microencapsulation. Without this advance, no permission for human application of microcapsules would have ever been obtained after years of purification technologies applied to the raw alginates. To improve safety of the encapsulated islet graft system, renewed efforts on the capsules' bioengineering, as well as on insulin-producing cells within the capsular membranes, are in progress. It is hoped that advances in these two critical aspects of the cell encapsulation technology will result in wider human application of this system.

Advances in biocompatibility and physico-chemical characterization of microspheres for cell encapsulation

Cell encapsulation has already shown its high potential and holds the promise for future cell therapies to enter the clinics as a large scale treatment option for various types of diseases. The advancement in cell biology towards this goal has to be complemented with functional biomaterials suitable for cell encapsulation. This cannot be achieved without understanding the close correlation between cell performance and properties of microspheres. The ongoing challenges in the field of cell encapsulation require a critical view on techniques and approaches currently utilized to characterize microspheres. This review deals with both principal subjects of microspheres characterization in the cell encapsulation field: physico-chemical characterization and biocompatibility. The up-to-day knowledge is summarized and discussed with the focus to identify missing knowledge and uncertainties, and to propose the mandatory next steps in characterization of microspheres for cell encapsulation. The primary conclusion of this review is that further success in development of microspheres for cell therapies cannot be accomplished without careful selection of characterization techniques, which are employed in conjunction with biological tests.

A Technology Platform to Test the Efficacy of Purification of Alginate

Alginates are widely used in tissue engineering technologies, e.g., in cell encapsulation, in drug delivery and various immobilization procedures. The success rates of these studies are highly variable due to different degrees of tissue response. A cause for this variation in success is, among other factors, its content of inflammatory components. There is an urgent need for a technology to test the inflammatory capacity of alginates. Recently, it has been shown that pathogen-associated molecular patterns (PAMPs) in alginate are potent immunostimulatories. In this article, we present the design and evaluation of a technology platform to assess (i) the immunostimulatory capacity of alginate or its contaminants, (ii) where in the purification process PAMPs are removed, and (iii) which Toll-like receptors (TLRs) and ligands are involved. A THP1 cell-line expressing pattern recognition receptors (PRRs) and the co-signaling molecules CD14 and MD2 was used to assess immune activation of alginates during the different steps of purification of alginate. To determine if this activation was mediated by TLRs, a THP1-defMyD88 cell-line was applied. This cell-line possesses a non-functional MyD88 coupling protein, necessary for activating NF-κB via TLRs. To identify the specific TLRs being activated by the PAMPs, we use different human embryonic kidney (HEK) cell-line that expresses only one specific TLR. Finally, specific enzyme-linked immunosorbent assays (ELISAs) were applied to identify the specific PAMP. By applying this three-step procedure, we can screen alginate in a manner, which is both labor and cost efficient. The efficacy of the platform was evaluated with an alginate that did not pass our quality control. We demonstrate that this alginate was immunostimulatory, even after purification due to reintroduction of the TLR5 activating flagellin. In addition, we tested two commercially available purified alginates. Our experiments show that these commercial alginates contained peptidoglycan, lipoteichoic acid, flagellin, and even lipopolysaccharides (LPS). The platform presented here can be used to evaluate the efficacy of purification procedures in removing PAMPs from alginates in a cost-efficient manner.

Conformal polymer coatings for pancreatic islets transplantation

The aim of this work was to improve an aqueous two-phase system methodology for fabrication of coherent microcapsules. Simulated microgravity was investigated as tool to improve the cell cluster morphology in order to increase the overall quality of conformal polymer coatings, while the application of two concentric alginate layers and the use of barium instead of calcium as gelling ion was evaluated. Simulated microgravity enabled improvement of neonatal porcine cell cluster sphericity however the freely floating cells, originated during incubation and often found on the capsule surface, raised immunological concerns. Overall, these technical changes translated into improving quality of microcapsules, in terms of either morphologic aspects or the membrane's functional performance. Preparation procedure did not seem to adversely affect viability of the embodied cells. Moreover, the employed alginates high biocompatibility, per se, would promote a good encapsulated cell engraftment. Minimization of last generation microcapsule's size, made of highly purified alginates, represents a further advance on the new horizons of cell therapy for the treatment of a wide variety of chronic disorders, including insulin-dependent diabetes mellitus.

Combining submerged electrospray and UV photopolymerization for production of synthetic hydrogel microspheres for cell encapsulation

Microencapsulation within hydrogel microspheres holds much promise for drug and cell delivery applications. Synthetic hydrogels have many advantages over more commonly used natural materials such as alginate, however their use has been limited due to a lack of appropriate methods for manufacturing these microspheres under conditions compatible with sensitive proteins or cells. This study investigated the effect of flow rate and voltage on size and uniformity of the hydrogel microspheres produced via submerged electrospray combined with UV photopolymerization. In addition, the mechanical properties and cell survival within microspheres was studied. A poly(vinyl alcohol) (PVA) macromer solution was sprayed in sunflower oil under flow rates between 1-100 µL/min and voltages 0-10 kV. The modes of spraying observed were similar to those previously reported for electrospraying in air. Spheres produced were smaller for lower flow rates and higher voltages and mean size could be tailored from 50 to 1,500 µm. The microspheres exhibited a smooth, spherical morphology, did not aggregate and the compressive modulus of the spheres (350 kPa) was equivalent to bulk PVA (312 kPa). Finally, L929 fibroblasts were encapsulated within PVA microspheres and showed viability >90% after 24 h. This process shows great promise for the production of synthetic hydrogel microspheres, and specifically supports encapsulation of cells.

Immunoisolation of pancreatic islet grafts with no recipient's immunosuppression: actual and future perspectives

In spite of steady and remarkable progress, islet transplantation in patients with type 1 diabetes mellitus (T1DM) continues to face two major bottlenecks: inadequate availability of human pancreatic donors and necessity to totally immunosuppress the graft recipients lifelong. Microencapsulation of the islet grafts within highly biocompatible and selective permeable biomembranes could obviate use of the immunosuppressants, while potentially offering the opportunity to use a wide array of insulin-producing cells, in active development, including xenogeneic pig islets. Although macrodevices and microcapsules, which essentially differ by size/configuration, and both serve for immunoisolation devices, have been used for many years with initial human applications, new products on development in both areas might open new perspectives for more focused use in patients with T1DM. Physical-chemical properties and material engineering of these devices are critically reviewed to assess where we actually stand and where the future expansion of these technologies may go.

Enhancing clinical islet transplantation through tissue engineering strategies

Clinical islet transplantation (CIT), the infusion of allogeneic islets within the liver, has the potential to provide precise and sustainable control of blood glucose levels for the treatment of type 1 diabetes. The success and long-term outcomes of CIT, however, are limited by obstacles such as a nonoptimal transplantation site and severe inflammatory and immunological responses to the transplant. Tissue engineering strategies are poised to combat these challenges. In this review, emerging methods for engineering an optimal islet transplantation site, as well as novel approaches for improving islet cell encapsulation, are discussed.

Controlling surface porosity and release from hydrogels using a colloidal particle coating

Recent studies suggest that coating microcapsules by a shell composed of impenetrable colloidal particles (thereby forming 'colloidosomes') can be used to control surface porosity, and therefore, permeability. The voids between the particles in the coating define the size of the surface pores available for transport. However, to date, data demonstrating this selectivity has been largely qualitative. In this paper we examine, quantitatively, the effect of a surface coating (shell), composed of colloidal particles, on release from hydrogels. We find that the presence of a colloidal shell does indeed reduce the rate of transport of three model molecules: Aspirin, caffeine, and FITC-dextran with MW of approximately 3000-5000. Contrary to expectation, however, we find that for all three molecules the reduction in transport rate is largely independent of the dimensions of the particles composing the shell, despite differences that range over three orders of magnitude. In the case of the small molecules, caffeine and aspirin, the colloidal shell reduces the effective diffusion coefficient by a factor of 3. In the case of dextran, the suppression in the release rate due to the colloidal shell was much larger. These results are explained using a simple diffusion model that accounts for the volume fraction and diameter of the colloidal particles in the shell, and the size of the diffusing molecules.

Submerged electrosprays: A versatile approach for microencapsulation

The unearthing of fundamental science and technology associated with microencapsulation is an ongoing exciting scientific endeavour focused on by several scientists. Encapsulated structures (containing either a gas, molecules or materials) previously have been shown as having widespread applications across the physical and life sciences. In particular, such methodologies used for forming encapsulations in medical-related studies have shown great promise from diagnostics and imaging, to gene therapy and drug delivery which are only a few amongst several other applications. At present there are numerous 'jet-based' manifestations available for microencapsulation, these primarily root from either capillary or channel-based techniques. The driving mechanisms employed in these approaches exploit aerodynamic/pressure gradients to piezoelectricity. In this paper submerged electrosprays a multipurpose electric field driven jet-based technique is explored for forming near mono-dispersed encapsulations sized in the micrometer range. Our studies elucidate the ability to form microencapsulations containing either a gas or a micro/nanoparticulate-based material suspension as capsules sized in the ranges approximately 65-80 microm, approximately 8-25 microm to approximately 3-14 microm, respectively. We believe this technique can significantly contribute to the microencapsulation field of research based on both the size of the generated encapsulations to the containment of immiscible high viscosity particulate-based suspensions. Furthermore our investigations show the ability to control the production of these encapsulations in terms of both their size and rate of generation with ease; hence demonstrating this electrospray-assisted microencapsulation route as having a wide range of applications. It should be noted that in its present form this technique could be explored for generating emulsions with a variety of materials especially with living organisms for applications in the clinical and biomedical areas of research.

Thrombosis and inflammation in intraportal islet transplantation: a review of pathophysiology and emerging therapeutics

With the inception of the Edmonton Protocol, intraportal islet transplantation (IPIT) has re-emerged as a promising cell-based therapy for type 1 diabetes. However, current clinical islet transplantation remains limited, in part, by the need to transplant islets from 2-4 donor organs, often through several separate infusions, to reverse diabetes in a single patient. Results from clinical islet transplantation and experimental animal models now indicate that the majority of transplanted islets are destroyed in the immediate post-transplant period, a process largely facilitated by deleterious inflammatory responses triggered by islet-derived procoagulant and proinflammatory mediators. Herein, mechanisms that underlie the pathophysiology of thrombosis and inflammation in IPIT are reviewed, and emerging approaches to improve islet engraftment through attenuation of inflammatory responses are discussed.

Challenges and emerging technologies in the immunoisolation of cells and tissues

Protection of transplanted cells from the host immune system using immunoisolation technology will be important in realizing the full potential of cell-based therapeutics. Microencapsulation of cells and cell aggregates has been the most widely explored immunoisolation strategy, but widespread clinical application of this technology has been limited, in part, by inadequate transport of nutrients, deleterious innate inflammatory responses, and immune recognition of encapsulated cells via indirect antigen presentation pathways. To reduce mass transport limitations and decrease void volume, recent efforts have focused on developing conformal coatings of micron and submicron scale on individual cells or cell aggregates. Additionally, anti-inflammatory and immunomodulatory capabilities are being integrated into immunoisolation devices to generate bioactive barriers that locally modulate host responses to encapsulated cells. Continued exploration of emerging paradigms governed by the inherent challenges associated with immunoisolation will be critical to actualizing the clinical potential of cell-based therapeutics.

Polymer Chemistry in Diabetes Treatment by Encapsulated Islets of Langerhans: Review to 2006

Polymeric materials have been successfully used in numerous medical applications because of their diverse properties. For example, development of a bioartificial pancreas remains a challenge for polymer chemistry. Polymers, as a form of various encapsulation device, have been proposed for designing the semipermeable membrane capable of long-term immunoprotection of transplanted islets of Langerhans, which regulate the blood glucose level in a diabetic patient. This review describes the current situation in the field, discussing aspects of material selection, encapsulation devices, and encapsulation protocols. Problems and unanswered questions are emphasized to illustrate why clinical therapies with encapsulated islets have not been realized, despite intense activity over the past 15 years. The review was prepared with the goal to address professionals in the field as well as the broad polymer community to help in overcoming final barriers to the clinical phase for transplantation of islets of Langerhans encapsulated in a polymeric membrane.