In Vivo Recognition by the Host Adaptive Immune System of Microencapsulated Xenogeneic Cells

Designing biomaterials for the modulation of allogeneic and autoimmune responses to cellular implants in Type 1 Diabetes

The effective suppression of adaptive immune responses is essential for the success of allogeneic cell therapies. In islet transplantation for Type 1 Diabetes, pre-existing autoimmunity provides an additional hurdle, as memory autoimmune T cells mediate both an autoantigen-specific attack on the donor beta cells and an alloantigen-specific attack on the donor graft cells. Immunosuppressive agents used for islet transplantation are generally successful in suppressing alloimmune responses, but dramatically hinder the widespread adoption of this therapeutic approach and fail to control memory T cell populations, which leaves the graft vulnerable to destruction. In this review, we highlight the capacity of biomaterials to provide local and nuanced instruction to suppress or alter immune pathways activated in response to an allogeneic islet transplant. Biomaterial immunoisolation is a common approach employed to block direct antigen recognition and downstream cell-mediated graft destruction; however, immunoisolation alone still permits shed donor antigens to escape into the host environment, resulting in indirect antigen recognition, immune cell activation, and the creation of a toxic graft site. Designing materials to decrease antigen escape, improve cell viability, and increase material compatibility are all approaches that can decrease the local release of antigen and danger signals into the implant microenvironment. Implant materials can be further enhanced through the local delivery of anti-inflammatory, suppressive, chemotactic, and/or tolerogenic agents, which serve to control both the innate and adaptive immune responses to the implant with a benefit of reduced systemic effects. Lessons learned from understanding how to manipulate allogeneic and autogenic immune responses to pancreatic islets can also be applied to other cell therapies to improve their efficacy and duration. STATEMENT OF SIGNIFICANCE: This review explores key immunologic concepts and critical pathways mediating graft rejection in Type 1 Diabetes, which can instruct the future purposeful design of immunomodulatory biomaterials for cell therapy. A summary of immunological pathways initiated following cellular implantation, as well as current systemic immunomodulatory agents used, is provided. We then outline the potential of biomaterials to modulate these responses. The capacity of polymeric encapsulation to block some powerful rejection pathways is covered. We also highlight the role of cellular health and biocompatibility in mitigating immune responses. Finally, we review the use of bioactive materials to proactively modulate local immune responses, focusing on key concepts of anti-inflammatory, suppressive, and tolerogenic agents.

In vitro platform establishes antigen-specific CD8+ T cell cytotoxicity to encapsulated cells via indirect antigen recognition

The curative potential of non-autologous cellular therapy is hindered by the requirement of anti-rejection therapy. Cellular encapsulation within nondegradable biomaterials has the potential to inhibit immune rejection, but the efficacy of this approach in robust preclinical and clinical models remains poor. While the responses of innate immune cells to the encapsulating material have been characterized, little attention has been paid to the contributions of adaptive immunity in encapsulated graft destabilization. Avoiding the limitations of animal models, we established an efficient, antigen-specific in vitro platform capable of delineating direct and indirect host T cell recognition to microencapsulated cellular grafts and evaluated their consequential impacts. Using ovalbumin (OVA) as a model antigen, we determined that alginate microencapsulation abrogates direct CD8⁺ T cell activation by interrupting donor-host interaction; however, indirect T cell activation, mediated by host antigen presenting cells (APCs) primed with shed donor antigens, still occurs. These activated T cells imparted cytotoxicity on the encapsulated cells, likely via diffusion of cytotoxic solutes. Overall, this platform delivers unique mechanistic insight into the impacts of hydrogel encapsulation on host adaptive immune responses, comprehensively addressing a long-standing hypothesis of the field. Furthermore, it provides an efficient benchtop screening tool for the investigation of new encapsulation methods and/or synergistic immunomodulatory agents.

Emerging Nano- and Micro-Technologies Used in the Treatment of Type-1 Diabetes

Type-1 diabetes is characterized by high blood glucose levels due to a failure of insulin secretion from beta cells within pancreatic islets. Current treatment strategies consist of multiple, daily injections of insulin or transplantation of either the whole pancreas or isolated pancreatic islets. While there are different forms of insulin with tunable pharmacokinetics (fast, intermediate, and long-acting), improper dosing continues to be a major limitation often leading to complications resulting from hyper- or hypo-glycemia. Glucose-responsive insulin delivery systems, consisting of a glucose sensor connected to an insulin infusion pump, have improved dosing but they still suffer from inaccurate feedback, biofouling and poor patient compliance. Islet transplantation is a promising strategy but requires multiple donors per patient and post-transplantation islet survival is impaired by inflammation and suboptimal revascularization. This review discusses how nano- and micro-technologies, as well as tissue engineering approaches, can overcome many of these challenges and help contribute to an artificial pancreas-like system.

Engineering immunomodulatory biomaterials for type 1 diabetes

A cure for type 1 diabetes (T1D) would help millions of people worldwide, but remains elusive thus far. Tolerogenic vaccines and beta cell replacement therapy are complementary therapies that seek to address aberrant T1D autoimmune attack and subsequent beta cell loss. However, both approaches require some form of systematic immunosuppression, imparting risks to the patient. Biomaterials-based tools enable localized and targeted immunomodulation, and biomaterial properties can be designed and combined with immunomodulatory agents to locally instruct specific immune responses. In this Review, we discuss immunomodulatory biomaterial platforms for the development of T1D tolerogenic vaccines and beta cell replacement devices. We investigate nano- and microparticles for the delivery of tolerogenic agents and autoantigens, and as artificial antigen presenting cells, and highlight how bulk biomaterials can be used to provide immune tolerance. We examine biomaterials for drug delivery and as immunoisolation devices for cell therapy and islet transplantation, and explore synergies with other fields for the development of new T1D treatment strategies.

Long-Term Function of Alginate-Encapsulated Islets

Human trials have demonstrated the feasibility of alginate-encapsulated islet cells for the treatment of type 1 diabetes. Encapsulated islets can be protected from the host's immune system and remain viable and functional following transplantation. However, the long-term success of these therapies requires that alginate microcapsules maintain their immunoprotective capacity and stability in vivo for sustained periods. In part, as a consequence of different encapsulation strategies, islet encapsulation studies have produced inconsistent results in regard to graft functioning time, stability, and overall metabolic benefits. Alginate composition (proportion of M- and G-blocks), alginate purity, the cross-linking ions (calcium or barium), and the presence or absence of additional polymer coating layers influence the success of cell encapsulation. This review summarizes the outcomes of long-term studies of alginate-encapsulated islet transplants in animals and humans and provides a critical discussion of the graft failure mechanisms, including issues with graft biocompatibility, transplantation site, and integrity of the encapsulated islet grafts. Strategies to improve the mechanical stability of alginate capsules and methods for monitoring graft survival and function in vivo are presented.

Immunological and technical considerations in application of alginate-based microencapsulation systems

Islets encapsulated in immunoprotective microcapsules are being proposed as an alternative for insulin therapy for treatment of type 1 diabetes. Many materials for producing microcapsules have been proposed but only alginate does currently qualify as ready for clinical application. However, many different alginate-based capsule systems do exist. A pitfall in the field is that these systems are applied without a targeted strategy with varying degrees of success as a consequence. In the current review, the different properties of alginate-based systems are reviewed in view of future application in humans. The use of allogeneic and xenogeneic islet sources are discussed with acknowledging the different degrees of immune protection the encapsulation system should supply. Also issues such as oxygen supply and the role of danger associated molecular patterns (DAMPS) in immune activation are being reviewed. A common property of the encapsulation systems is that alginates for medical application should have an extreme high degree of purity and lack pathogen-associated molecular patterns (PAMPs) to avoid activation of the recipient’s immune system. Up to now, non-inflammatory alginates are only produced on a lab-scale and are not yet commercially available. This is a major pitfall on the route to human application. Also the lack of predictive pre-clinical models is a burden. The principle differences between relevant innate and adaptive immune responses in humans and other species are reviewed. Especially, the extreme differences between the immune system of non-human primates and humans are cumbersome as non-human primates may not be predictive of the immune responses in humans, as opposed to the popular belief of regulatory agencies. Current insight is that although the technology is versatile major research efforts are required for identifying the mechanical, immunological, and physico-chemical requirements that alginate-based capsules should meet for successful human application.

Islet and stem cell encapsulation for clinical transplantation

Over the last decade, improvements in islet isolation techniques have made islet transplantation an option for a certain subset of patients with long-standing diabetes. Although islet transplants have shown improved graft function, adequate function beyond the second year has not yet been demonstrated, and patients still require immunosuppression to prevent rejection. Since allogeneic islet transplants have experienced some success, the next step is to improve graft function while eliminating the need for systemic immunosuppressive therapy. Biomaterial encapsulation offers a strategy to avoid the need for toxic immunosuppression while increasing the chances of graft function and survival. Encapsulation entails coating cells or tissue in a semipermeable biocompatible material that allows for the passage of nutrients, oxygen, and hormones while blocking immune cells and regulatory substances from recognizing and destroying the cell, thus avoiding the need for systemic immunosuppressive therapy. Despite advances in encapsulation technology, these developments have not yet been meaningfully translated into clinical islet transplantation, for which several factors are to blame, including graft hypoxia, host inflammatory response, fibrosis, improper choice of biomaterial type, lack of standard guidelines, and post-transplantation device failure. Several new approaches, such as the use of porcine islets, stem cells, development of prevascularized implants, islet nanocoating, and multilayer encapsulation, continue to generate intense scientific interest in this rapidly expanding field. This review provides a comprehensive update on islet and stem cell encapsulation as a treatment modality in type 1 diabetes, including a historical outlook as well as current and future research avenues.

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.

Noninvasive evaluation of the vascular response to transplantation of alginate encapsulated islets using the dorsal skin-fold model

Alginate encapsulation reduces the risk of transplant rejection by evading immune-mediated cell injury and rejection; however, poor vascular perfusion results in graft failure. Since existing imaging models are incapable of quantifying the vascular response to biomaterial implants after transplantation, in this study, we demonstrate the use of in vivo laser speckle imaging (LSI) and wide-field functional imaging (WiFI) to monitor the microvascular environment surrounding biomaterial implants. The vascular response to two islet-containing biomaterial encapsulation devices, alginate microcapsules and a high-guluronate alginate sheet, was studied and compared after implantation into the mouse dorsal window chamber (N = 4 per implant group). Images obtained over a 14-day period using LSI and WiFI were analyzed using algorithms to quantify blood flow, hemoglobin oxygen saturation and vascular density. Using our method, we were able to monitor the changes in the peri-implant microvasculature noninvasively without the use of fluorescent dyes. Significant changes in blood flow, hemoglobin oxygen saturation and vascular density were noted as early as the first week post-transplant. The dorsal window chamber model enables comparison of host responses to transplanted biomaterials. Future experiments will study the effect of changes in alginate composition on the vascular and immune responses.

The TheraCyte™ device protects against islet allograft rejection in immunized hosts

Clinically, many candidates for islet transplantation are already immunized, which increases their risk of graft rejection. Encapsulation of pancreatic islets using the TheraCyte™ device has been shown to protect against allograft rejection in nonimmunized recipients. However, the capacity of the TheraCyte™ device to prevent rejection in immunized recipients has not yet been studied. In this study, the protective capacity of the TheraCyte™ device was evaluated in an allogeneic rat model. Lewis rats were used as islet donors, and nonimmunized (control) and alloimmunized, diabetic Wistar-Furth (WF) rats were used as recipients. Graft survival was shorter in immunized recipients than in nonimmunized recipients (mean survival, 5.3 ± 2.7 and 9.3 ± 1.6 days, respectively, p < 0.01) when nonencapsulated islets were transplanted under the kidney capsule. When islets were transplanted into the TheraCyte™ device, graft function was maintained during the 6-month study period in both immunized and nonimmunized rats. In oral glucose tolerance tests performed at 1 month after transplantation, both groups had similar insulin and blood glucose levels indicating similar metabolic functions. Volume densities and absolute volumes of tissue inside the devices 6 months after transplantation were also comparable between the two groups, indicating that both groups maintained similar amounts of endocrine tissue. A higher number of IFN-γ-producing CD8+ T-cells were detected in immunized WF rats compared to control WF rats transplanted with encapsulated islets. This suggests that donor-specific alloreactivity in recipient rats was sustained throughout the study period. This study suggests that the TheraCyte™ device protects islet allografts also in immunized recipients. Our results further highlight the potential for using macroencapsulation to avoid immunosuppressive therapy in clinical islet transplantation.

Encapsulated islets transplantation: Past, present and future

Islet transplantation could become an ideal treatment for severe diabetes to prevent hypoglycemia shock and irreversible diabetic complications, once some of the major and unresolved obstacles are overcome, including limited donor supplies and side effects caused by permanent immunosuppressant use. Approximately 30 years ago, some groups succeeded in improving the blood glucose of diabetic animals by transplanting encapsulated islets with semi-permeable membranes consisting of polymer. A semi-permeable membrane protects both the inner islets from mechanical stress and the recipient's immune system (both cellular and humoral immunities), while allowing bidirectional diffusion of nutrients, oxygen, glucose, hormones and wastes, i.e., immune-isolation. This device, which enables immune-isolation, is called encapsulated islets or bio-artificial pancreas. Encapsulation with a semi-permeable membrane can provide some advantages: (1) this device protects transplanted cells from the recipient's immunity even if the xenogeneic islets (from large animals such as pig) or insulin-producing cells are derived from cells that have the potential for differentiation (some kinds of stem cells). In other words, the encapsulation technique can resolve the problem of limited donor supplies; and (2) encapsulation can reduce or prevent chronic administration of immunosuppressants and, therefore, important side effects otherwise induced by immunosuppressants. And now, many novel encapsulated islet systems have been developed and are being prepared for testing in a clinical setting.

Responsive micromolds for sequential patterning of hydrogel microstructures

Microscale hydrogels have been shown to be beneficial for various applications such as tissue engineering and drug delivery. A key aspect in these applications is the spatial organization of biological entities or chemical compounds within hydrogel microstructures. For this purpose, sequentially patterned microgels can be used to spatially organize either living materials to mimic biological complexity or multiple chemicals to design functional microparticles for drug delivery. Photolithographic methods are the most common way to pattern microscale hydrogels but are limited to photocrosslinkable polymers. So far, conventional micromolding approaches use static molds to fabricate structures, limiting the resulting shapes that can be generated. Herein, we describe a dynamic micromolding technique to fabricate sequentially patterned hydrogel microstructures by exploiting the thermoresponsiveness of poly(N-isopropylacrylamide)-based micromolds. These responsive micromolds exhibited shape changes under temperature variations, facilitating the sequential molding of microgels at two different temperatures. We fabricated multicompartmental striped, cylindrical, and cubic microgels that encapsulated fluorescent polymer microspheres or different cell types. These responsive micromolds can be used to immobilize living materials or chemicals into sequentially patterned hydrogel microstructures which may potentially be useful for a range of applications at the interface of chemistry, materials science and engineering, and biology.

Transplantation of microencapsulated cells expressing VEGF improves angiogenesis in implanted xenogeneic acellular dermis on wound

BACKGROUND

Cell-based gene therapy using cells that express angiogenic factors is an alternative technique for therapeutic angiogenesis in transplantation of xenogeneic acellular dermis matrix (ADM). However, immune rejection is a substantial obstacle to implantation of genetically engineered allogeneic or xenogeneic cells.

OBJECTIVE

To evaluate application of microencapsulated cells that express vascular endothelial growth factor (VEGF) in xenogeneic ADM transplants to improve wound angiogenesis and healing.

MATERIALS AND METHODS

NIH3T3 cells were genetically modified to secrete VEGF and enveloped in semipermeable microcapsules. Microencapsulated VEGF-NIH3T3 cells were implanted in defects on the dorsa of guinea pigs with xenogeneic ADM and autologous split-thickness skin grafts. Cell structure and microencapsulation were observed at microscopy, and expression of VEGF was detected using an enzyme-linked immunosorbent assay (ELISA) and immunochemistry. Extent of angiogenesis in the ADM and the survival rate of the composite skin were evaluated after 2 weeks. In addition, expression of human VEGF and CD31 in the implanted acellular dermis was assessed, and microvessel density was calculated.

RESULTS

Microencapsulated VEGF-expressing NIH3T3 cells were prepared successfully, and demonstrated proliferation and viability, and expressed VEGF both in vitro and in vivo. Extent of angiogenesis and survival rate of the composite skin containing the microencapsulated VEGF-expressing cells were significantly greater than in controls. Microencapsulated VEGF-expressing NIH3T3 cells augmented early angiogenesis in ADM implanted on wound and improved healing.

CONCLUSION

Microencapsulated xenogeneic cell-based gene therapy may be a novel approach to therapeutic angiogenesis in transplantation of xenogeneic ADM skin.

Bio-microfluidics: biomaterials and biomimetic designs

Bio-microfluidics applies biomaterials and biologically inspired structural designs (biomimetics) to microfluidic devices. Microfluidics, the techniques for constraining fluids on the micrometer and sub-micrometer scale, offer applications ranging from lab-on-a-chip to optofluidics. Despite this wealth of applications, the design of typical microfluidic devices imparts relatively simple, laminar behavior on fluids and is realized using materials and techniques from silicon planar fabrication. On the other hand, highly complex microfluidic behavior is commonplace in nature, where fluids with nonlinear rheology flow through chaotic vasculature composed from a range of biopolymers. In this Review, the current state of bio-microfluidic materials, designs and applications are examined. Biopolymers enable bio-microfluidic devices with versatile functionalization chemistries, flexibility in fabrication, and biocompatibility in vitro and in vivo. Polymeric materials such as alginate, collagen, chitosan, and silk are being explored as bulk and film materials for bio-microfluidics. Hydrogels offer options for mechanically functional devices for microfluidic systems such as self-regulating valves, microlens arrays and drug release systems, vital for integrated bio-microfluidic devices. These devices including growth factor gradients to study cell responses, blood analysis, biomimetic capillary designs, and blood vessel tissue culture systems, as some recent examples of inroads in the field that should lead the way in a new generation of microfluidic devices for bio-related needs and applications. Perhaps one of the most intriguing directions for the future will be fully implantable microfluidic devices that will also integrate with existing vasculature and slowly degrade to fully recapitulate native tissue structure and function, yet serve critical interim functions, such as tissue maintenance, drug release, mechanical support, and cell delivery.

Encapsulated arrays of self-assembled microtissues: an alternative to spherical microcapsules

Micro-encapsulation and immuno-isolation of allogenic and xenogenic tissues and cells is a promising method for the treatment of a variety of metabolic disorders. Many years have been spent optimizing spherical microcapsules, yet micro-encapsulation has not achieved its full clinical potential. As an alternative to spherical microcapsules, this study presents an alginate-encapsulated array of self-assembled three-dimensional (3D) microtissues. Monodispersed HepG2 cells were seeded onto a micro-molded agarose gel. Cells settled to the bottom of the mold recesses and self-assembled 3D microtissues (n = 822) within 24 h. This array of densely packed microtissues was encapsulated in situ using alginate. When separated from the agarose micro-mold, the encapsulated array had HepG2 microtissues in close proximity to its surface. This surface could be further modified by a simple dipping process. Microtissue size, viability, and albumin secretion were all controllable by the number of cells seeded onto the original agarose micro-mold, and microtissue shape and spacing were controllable by the design of the micro-mold. This approach to encapsulation and the use of self-assembled/self-packing 3D microtissues offers new design possibilities that may help to address certain limitations of conventional microcapsules.

Engineered 3D tissue models for cell-laden microfluidic channels

Delivery of nutrients and oxygen within three-dimensional (3D) tissue constructs is important to maintain cell viability. We built 3D cell-laden hydrogels to validate a new tissue perfusion model that takes into account nutrition consumption. The model system was analyzed by simulating theoretical nutrient diffusion into cell-laden hydrogels. We carried out a parametric study considering different microchannel sizes and inter-channel separation in the hydrogel. We hypothesized that nutrient consumption needs to be taken into account when optimizing the perfusion channel size and separation. We validated the hypothesis by experiments. We fabricated circular microchannels (r = 400 microm) in 3D cell-laden hydrogel constructs (R = 7.5 mm, volume = 5 ml). These channels were positioned either individually or in parallel within hydrogels to increase nutrient and oxygen transport as a way to improve cell viability. We quantified the spatial distribution of viable cells within 3D hydrogel scaffolds without channels and with single- and dual-perfusion microfluidic channels. We investigated quantitatively the cell viability as a function of radial distance from the channels using experimental data and mathematical modeling of diffusion profiles. Our simulations show that a large-channel radius as well as a large channel to channel distance diffuse nutrients farther through a 3D hydrogel. This is important since our results reveal that there is a close correlation between nutrient profiles and cell viability across the hydrogel.

IL-10 secretion increases signal persistence of HEMA-MMA-microencapsulated luciferase-modified CHO fibroblasts in mice

Microencapsulation of cells in a polymer membrane [e.g., poly(hydroxyethyl methacrylate-co-methyl methacrylate) (HEMA-MMA)] has been proposed as a vehicle for the delivery of therapeutic biomolecules, but cells (especially xenogeneic cells) survive only for short times, limiting the utility of this approach. Murine interleukin-10 (mIL-10) has been shown to downregulate the xenogeneic immune response, and we tested the hypothesis that mIL-10 produced by microencapsulated Chinese hamster ovary (CHO) cells would modulate the transplant-site environment leading to prolonged cell function in a xenogeneic model without other immunomodulatory agents. Prior to encapsulation, CHO cells were genetically engineered to express mIL-10 and a firefly bioluminescence protein, luciferase, which allowed for noninvasive tracking of transplanted cells in vivo with the Xenogen IVIS Imaging System. This nondestructive imaging system was sufficiently sensitive to detect photon signal emitted by a single capsule containing around 800 luciferase-transduced CHO (CHO(LUC)) cells in vitro, and to track changes in luciferase expression in vivo over time. Effective modulation of the transplantation-site environment with mIL-10 secreted from capsules was evident by greater luciferase expression at 10 and 21 days after transplantation relative to encapsulated luciferase-transfected cells that did not produce mIL-10. Longer duration effects require further investigation to extend this proof-of-concept study.

A novel macroencapsulating immunoisolatory device: the preparation and properties of nanomat-reinforced amphiphilic co-networks deposited on perforated metal scaffold

This paper describes the design and preparation of the non-biological components (the "hardware") of a conceptually novel bioartificial pancreas (BAP) to correct diabetes. The key components of the hardware are (1) a thin (5-10 microm) semipermeable amphiphilic co-network (APCN) membrane [i.e., a membrane of cocontinuous poly(dimethyl acryl amide) (PDMAAm)/polydimethylsiloxane (PDMS) domains cross-linked by polymethylhydrosiloxane (PMHS)] expressly created for macroencapsulation and immunoisolation of a tissue graft; (2) an electrospun nanomat of PDMS-containing polyurethane to reinforce the water-swollen APCN membrane; and (3) a perforated hollow-ribbon nitinol scaffold to stiffen and provide geometric stability to the construct. The reinforcement of water-swollen hydrogels with an electrospun nanomat is a generally applicable new method for hydrogel reinforcement. Details of device design and preparation are discussed. The advantages and disadvantages of micro- and macro-immunoisolation are analyzed, and the requirements for the ideal immunoisolatory membrane are presented. Burst pressure, and glucose and insulin permeabilities of representative devices have been determined and the effect of device composition and wall thickness on these properties is discussed.

A survey of bioengineering research in Canada-2007

Research activity in bioengineering at Canadian universities has been surveyed. Details were provided by chemical engineering departments in response to a common request for information on activities by individual researchers and for key publications. The information provided has been grouped by topics within the broad theme of "Bioengineering," and contributions from individual departments have been summarized within these topics. Although many aspects of bioengineering research are being pursued in Canada, it would appear as though environmental biotechnology, biomaterials, and tissue/cell culture are the most active areas under investigation.

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.

Effects of biomaterial-induced inflammation on fibrosis and rejection

Evidence is emerging that biomaterials cause inflammation by ligating innate immune receptors on antigen presenting cells. Although inflammation is usually viewed as detrimental, it has unexpected and potentially beneficial effects on fibrosis and transplant rejection. For example, the magnitude of inflammation due to a biomaterial is not predictive of the extent of fibrosis. Similarly, biomaterials do not always show adjuvancy. Some biomaterials suppressed T cell rejection responses in vivo and in vitro, while others non-specifically stimulated T cell proliferation. Understanding these complex inter-relationships is the key to designing a biomaterial that stimulates regeneration and induces tolerance in tissue engineering applications.

Transplantation of microencapsulated genetically modified xenogeneic cells augments angiogenesis and improves heart function

Cell-based gene therapy offers an alternative strategy for therapeutic angiogenesis for the management of myocardial infarction (MI). However, immune rejection poses a significant obstacle to the implantation of genetically engineered allogeneic or xenogeneic cells. In the present study, an ex vivo gene therapy approach utilizing cell microencapsulation was employed to deliver vascular endothelial growth factor (VEGF) to ischemic myocardium. Chinese hamster ovary (CHO) cells were genetically modified to secrete VEGF and enveloped into semipermeable microcapsules. In vitro assay indicated that the microencapsulated engineered CHO cells could secrete VEGF as high as 3852 pg ml(-1) per 48 h at day 8 after encapsulation. Then the microencapsulated CHO cells were implanted into the injured myocardium in a rat MI model, while engineered CHO cells, blank microcapsules and serum-free culture media were implanted as controls. The humoral immunity to xenogeneic CHO cells were evaluated and we found that the titer of anti-CHO antibodies was significantly lower in the microencapsulated CHO transplantation group than the group receiving unencapsulated CHO cells at two weeks after implantation. However, 1 week later, there was almost no difference between these groups. Histology and western blotting confirmed that the microencapsulated CHO cells maintained their original structure and VEGF secretion three weeks after implantation. The capillary density in the treatment region was also significantly higher in the microencapsulated CHO cell group than control groups, which was consistent with gross heart functional improvement. These data suggest that microencapsulated xenogeneic cell-based gene therapy might be a novel approach for therapeutic angiogenesis in ischemic heart disease.

Vascularization and Improved In Vivo Survival of VEGF-Secreting Cells Microencapsulated in HEMA-MMA

Vascularization caused by encapsulated cells engineered to secrete vascular endothelial growth factor (VEGF) improved the in vivo survival of the encapsulated cells in a syngeneic mouse Matrigel plug model. Murine fibroblast cells (L929) were engineered to secrete recombinant human vascular endothelial growth factor (rhVEGF(165)). Transfected and nontransfected L929 cells were microencapsulated in a 75:25 hydroxyethyl methacrylate-methyl methacrylate (HEMA-MMA) copolymer. Capsules containing transfected cells induced vascularization in vivo at 1 and 3 weeks postimplantation. In histological sections, a significant positive correlation was seen between the number of capsules and blood vessel density for VEGF-secreting cell capsule implants. New vessels, many positively stained for smooth muscle cells and pericytes, were seen surrounding these VEGF-secreting cell capsule explants. Few vessels were seen in nontransfected L929 capsule implants. The viability of transfected and nontransfected encapsulated cells was assessed on explantation. Although the viability of all encapsulated cells decreased at both 1 and 3 weeks, encapsulated VEGF-secreting cells retained more of the viability than did encapsulated nontransfected control cells. Genetically modified cells promoted vascularization in this context and appeared to enhance the viability of the encapsulated cells, although the extent of the functional benefit was less than expected. Additional effort is required to enhance the benefit, to quantify it, and to understand further the host response to HEMA-MMA microencapsulated cells and tissue constructs, more generally.

A cell-laden microfluidic hydrogel

The encapsulation of mammalian cells within the bulk material of microfluidic channels may be beneficial for applications ranging from tissue engineering to cell-based diagnostic assays. In this work, we present a technique for fabricating microfluidic channels from cell-laden agarose hydrogels. Using standard soft lithographic techniques, molten agarose was molded against a SU-8 patterned silicon wafer. To generate sealed and water-tight microfluidic channels, the surface of the molded agarose was heated at 71 degrees C for 3 s and sealed to another surface-heated slab of agarose. Channels of different dimensions were generated and it was shown that agarose, though highly porous, is a suitable material for performing microfluidics. Cells embedded within the microfluidic molds were well distributed and media pumped through the channels allowed the exchange of nutrients and waste products. While most cells were found to be viable upon initial device fabrication, only those cells near the microfluidic channels remained viable after 3 days, demonstrating the importance of a perfused network of microchannels for delivering nutrients and oxygen to maintain cell viability in large hydrogels. Further development of this technique may lead to the generation of biomimetic synthetic vasculature for tissue engineering, diagnostics, and drug screening applications.

Immune mechanisms associated with the rejection of encapsulated neonatal porcine islet xenografts

BACKGROUND

The immune mechanisms associated with the rejection of microencapsulated neonatal porcine islets (NPI) are not clearly understood. Therefore, in this study we characterized the immune cells and molecules that are involved in this process by examining the microencapsulated NPI xenografts at various time points post-transplantation in B6 mice.

METHODS

Microencapsulated NPI were transplanted into streptozotocin-induced diabetic immune-competent B6 and immune-deficient B6 rag-/- mice and blood glucose levels were monitored twice a week. Encapsulated NPI were then recovered from B6 mice at various time points post-transplantation to characterize the islets and immune response using immunohistochemical and RT-PCR analyses. To determine which T-cell subpopulation is important for the rejection of encapsulated NPI, B6 rag-/- mice with established microencapsulated NPI xenografts were reconstituted with either CD4(+) or CD8(+) T cells and a return to the diabetic state was noted. For controls, adoptive transfer experiments involved reconstitution of B6 rag-/- mice with established microencapsulated NPI with non-fractionated lymph node cells or non-reconstituted mice.

RESULTS

All B6 recipients of microencapsulated NPI remained diabetic throughout the study while B6 rag-/- recipients achieved normoglycemia and maintained normoglycemia for up to 100 days post-transplantation. Encapsulated NPI recovered from B6 mice at early time points (day 7 and day 14) post-transplantation were surrounded with very few layers of immune cells that increased with time post-transplantation. The extent of cellular overgrowth on the surface of encapsulated NPI has a significant correlation with islet cell death and the presence of CD4(+) T cells, B cells and macrophages. Mouse IgG antibody and complement as well as cytokines [gamma-interferon (IFN-gamma), interleukin10 (IL10)] and chemokines (monocyte chemotactic protein-1 and macrophage inflammatory protein-1alpha and beta) were detected within the microcapsules at several time points post-transplantation suggesting that these molecules can traverse the microcapsule. Mouse anti-porcine IgG antibodies in recipient sera were found to peak at 30 days post-transplantation indicating leakage of porcine xenoantigens. In contrast, microencapsulated NPI recovered from B6 rag-/- mice had no cellular overgrowth on the surface. Complement and cytokines (IL 10 but not IFN-gamma) including chemokines were detected within the microcapsules at several days post-transplantation. We also found that B6 rag-/- mice reconstituted with non-fractionated lymph node cells or CD4(+) T cells but not CD8(+) T cells became diabetic demonstrating that CD4(+) T cells are the necessary T-cell subtype for microencapsulated NPI rejection. In contrast, non-reconstituted B6 rag-/- mice remained normoglycemic for the entire duration of the study.

CONCLUSIONS

Our results demonstrate that CD4(+) T cells, B cells and macrophages are the immune cells recruited to and involved in the rejection of encapsulated NPI. Immune molecules secreted by these cells as well as complement can traverse the microcapsule membrane and are responsible for destroying the NPI cells. Treatment regimens which target these molecules may modify the rejection of encapsulated NPI and lead to prolonged islet xenograft survival.

Suppressed splenocyte proliferation following a xenogeneic skin graft due to implanted biomaterials

BACKGROUND

Immune system responses to antigens in the context of biomaterials are poorly understood. Biomaterial implantation results in an inflammatory reaction, which is anticipated to alter the adaptive immune response, in the case presented here, to a skin xenograft. Our earlier work showed unexpectedly low splenocyte proliferation following a xenogeneic cell implant in tandem with a biomaterial in the form of a microcapsule. Here we explore whether that effect was due to the cells or the biomaterial, and attempt to dissect the mechanism of immune deviation.

METHODS

We assessed the immune response of Balb/c mice to hamster skin grafts accompanied by one of three implants: encapsulated xenogeneic cells; free cells accompanied by the same encapsulation biomaterials; and the encapsulation biomaterials without cells. Cells were encapsulated in a hydroxyethyl methacrylate-methyl methacrylate copolymer then embedded in an agarose gel. Splenocyte proliferation upon re-challenge in vitro, antibody titer in serum, and Th1/2 polarization (by cytokines in splenocyte challenge supernatants and antibody isotypes in serum) were measured.

RESULTS

All skin grafts with encapsulation materials (even without cells) suppressed subsequent splenocyte proliferation at 10 days postimplant, although this effect disappeared by two months. In contrast, the antibody response was equal to or greater than that for a skin graft alone. Th1/2 polarization could not explain these observations because it did not correlate with the suppression of splenocyte proliferation.

CONCLUSIONS

Implanted biomaterials caused nonspecific, transient suppression of splenocyte responses to hamster cells following hamster skin grafts, which is potentially important in the context of tissue engineering.

Microencapsulating hepatocytes

BACKGROUND

Because standardization of the cell microencapsulation procedure has not yet been achieved, we performed hepatocyte microencapsulation using alginate (ALG)-poly-l-lysine (PLL)-ALG (APA) polymer.

METHODS

Hepatocytes were microencapsulated using a binozzle air-jet droplet generator. Our study aims were to: (1) clarify how ALG concentration affects the quality of ALG beads; (2) determine how the PLL concentration affects the quality of microcapsules (MCs); (3) ascertain the influence of liquefaction time by sodium citrate (SC) on the quality of the MCs; (4) and clarify how temperature and solution pH, respectively, affect the viability of the hepatocytes inside the MCs.

RESULTS

The concentration of ALG must be > or = 3% (w/v) to generate droplets with satisfactory homogeneity in size and roundness (P < .01). The total quantity of PLL molecules is the essential component for MCs (P < .01). As our results show, the numeric ratio of PLL (milligrams) to MCs (milliliters) is roughly 25:1. SC incubation for 8 minutes resulted in the proper thickness of the MC wall; however, the time varied according to the size of the MCs (P < .05). Temperature and pH, although both difficult to control, exerted great influences on cell viability: 4 degrees C and pH 7.2 were found to be optimal by this study (P < .05).

CONCLUSIONS

Concentrations of ALG and PLL exerted decisive effects on the quality and strength of MCs. Higher concentrations were suggested. Because temperature and pH greatly affected cell viability, they must be properly monitored.