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Understanding Islet Cell Transplantation for Type 1 Diabetes
Type 1 diabetes is a chronic autoimmune disorder characterized by the destruction of insulin-producing beta cells in the pancreas, leading to insulin deficiency and chronic hyperglycemia. The main current therapeutic strategies for clinically overt type 1 diabetes—primarily exogenous insulin administration combined with blood glucose monitoring—fail to fully mimic physiological insulin regulation, often resulting in suboptimal or insufficient glycemic control. For millions of patients worldwide, managing this condition requires constant vigilance, multiple daily insulin injections, and careful monitoring of blood sugar levels to prevent dangerous complications.
Islet cell transplantation has emerged as a promising avenue for functionally replacing endogenous insulin production and achieving long-term glycemic stability. In islet transplantation, islets (which contain β cells and other cell types) are isolated from donor cadaveric pancreases and transplanted into people with type 1 diabetes. The transplanted islets then start to produce insulin in response to blood glucose levels. This approach represents a significant advancement over traditional insulin therapy, offering the potential for patients to achieve near-normal blood sugar control without the need for constant insulin injections.
Islet transplantation was recently approved by the U.S. Food and Drug Administration for adults with type 1 diabetes complicated by recurrent severe hypoglycemia events. Deceased donor islet transplantation was recently approved by the U.S. Food and Drug Administration as the first cellular therapy (Lantidra; CellTrans, Inc.) for adults with type 1 diabetes who are unable to approach target HbA1c because of current repeated severe hypoglycemia events despite intensive diabetes management and education. This landmark approval represents decades of research and clinical development in the field of cell-based therapies for diabetes.
Long-term follow-up of the Clinical Islet Transplantation Consortium multicenter phase 3 trial of islet-alone transplantation involving 48 individuals from this population demonstrated islet graft survival in 84% of recipients, with HbA1c maintained at less than 7.0% in 77% and at or below 6.5% in 74%, absence of severe hypoglycemia events in more than 90%, and approximately 50% remaining insulin independent at a median follow-up of 6 years. These impressive results demonstrate the transformative potential of islet transplantation for carefully selected patients with difficult-to-manage diabetes.
The Critical Challenge: Immune Rejection
Despite the remarkable success of islet transplantation, one of the most significant barriers to widespread adoption remains the body’s immune response to transplanted cells. Because such transplantations occur in the allogeneic setting, recipients require immunosuppressive therapy. This chronic and systemic adjuvant treatment can lead to toxicity, increased risks of infection and tumor development, and ultimately a decreased quality of life for patients.
The medications needed to suppress immune rejection of the islets must be continued for the life of the transplant, and they come with significant risks. Their use increases susceptibility to bacterial and viral infections; can cause fatigue, decreased kidney function, mouth sores, and gastrointestinal problems; and may increase the long-term risk of developing certain cancers. These immunosuppressants are also thought to affect the long-term viability of the transplanted islets, as studies suggest that they are toxic to the islets over time.
Kidney function declined at a greater rate in the islet transplant cohort when compared with standard care, an effect likely explained by the ongoing requirement for calcineurin inhibitor–based immunosuppression to protect the islet graft from alloimmune rejection and autoimmune recurrence. This finding underscores the urgent need for alternative approaches that can protect transplanted islets without requiring lifelong systemic immunosuppression.
The need for systemic immunosuppression remains the primary barrier to making islet transplantation a more widespread therapy for patients with type 1 diabetes. Thus, an important future research goal is the achievement of “immunological tolerance” for the transplanted cells, meaning that immunosuppression drugs would only be needed for a short time or even not at all. This is where encapsulation technologies enter the picture as a potentially game-changing solution.
What Are Encapsulation Technologies?
Encapsulation is a technology of enclosing living cells with a semi-permeable membrane. Cell microencapsulation technology involves immobilization of cells within a polymeric semi-permeable membrane. It permits the bidirectional diffusion of molecules such as the influx of oxygen, nutrients, growth factors essential for cell metabolism and the outward diffusion of waste products and therapeutic proteins. At the same time, the semi-permeable nature of the membrane prevents immune cells and antibodies from destroying the encapsulated cells, regarding them as foreign invaders.
In one strategy, called encapsulation, islets (including those from donors as well as progenitor cell-derived islet-like clusters and organoids grown in the laboratory) are coated with a material that protects them from being attacked by the recipient’s immune system and promotes their healthy functioning. The fundamental principle behind encapsulation is elegant in its simplicity: create a protective barrier that allows essential nutrients and oxygen to reach the transplanted cells while simultaneously preventing immune cells from attacking them.
A bioartificial pancreas is defined as a pancreatic islet construct based on encapsulation of islet cells within a semipermeable membrane so that the cells can be protected from the host’s immune system while they secrete insulin to regulate blood sugar. This concept represents a sophisticated bioengineering approach that seeks to replicate the natural function of the pancreas while protecting the transplanted cells from immune destruction.
The history of encapsulation technology dates back several decades. In 1964, the idea of encapsulating cells within ultra thin polymer membrane microcapsules so as to provide immunoprotection to the cells was proposed by Thomas Chang who introduced the term “artificial cells” to define this concept of bioencapsulation. The system was further advanced by Lim and Sun, who pioneered the microencapsulation of islets, creating the first bioartificial endocrine pancreas. Since these pioneering efforts, the field has evolved dramatically with advances in materials science, nanotechnology, and bioengineering.
Types of Encapsulation Methods
Two main encapsulation approaches have been widely studied: microencapsulation and nanoencapsulation. Each approach offers distinct advantages and faces unique challenges in protecting transplanted islet cells from immune rejection while maintaining their viability and function. Understanding these different methods is crucial for appreciating the complexity and potential of encapsulation technology in diabetes treatment.
Microencapsulation
Microencapsulation refers to a spherical system ranging in size from approximately tens of microns to 1.5 mm. This approach involves coating individual islet cells or small clusters of islets with a thin layer of biocompatible material, typically creating spherical capsules that can be implanted into the patient’s body. The most commonly used material for microencapsulation is alginate, a naturally derived polysaccharide extracted from brown seaweed.
Alginate-polylysine-alginate (APA) microcapsules immobilizing xenograft islet cells were developed. The study demonstrated that when these microencapsulated islets were implanted into diabetic rats, the cells remained viable and controlled glucose levels for several weeks. This early success in animal models demonstrated the feasibility of the microencapsulation approach and sparked decades of subsequent research.
Alginate-based microencapsulation has several advantages. The material is biocompatible, relatively inexpensive, and can be processed under mild conditions that do not harm the encapsulated cells. The gelation process occurs rapidly when alginate solution comes into contact with calcium ions, allowing for efficient encapsulation of large numbers of islets. However, alginate microcapsules have also faced significant challenges, particularly regarding the foreign body response and fibrotic overgrowth that can occur after implantation.
Microspheres for islet encapsulation have enabled long-term glycemic control in rodent models of diabetes; however, humans transplanted with equivalent microsphere formulations have experienced only transient islet graft function owing to a vigorous foreign-body response, to pericapsular fibrotic overgrowth and, in upright bipedal species, to the sedimentation of the microspheres within the peritoneal cavity. This disconnect between success in rodent models and challenges in human applications has been one of the major obstacles in translating microencapsulation technology to clinical practice.
To address these challenges, researchers have developed chemically modified alginate formulations. In conjunction with a minimally invasive transplantation technique into the bursa omentalis of non-human primates, the most promising chemically modified alginate derivative (Z1-Y15) protected viable and glucose-responsive allogeneic islets for 4 months without the need for immunosuppression. A recent study using triazole-modified alginate hydrogel appears to prevent excessive fibrosis common with larger animal models (non-human primates) and may be useful in prolonging encapsulated islet graft.
Microencapsulation requires more complex and individualized fabrication processes, as opposed to macroencapsulation devices that may be easier to manufacture, are more easily retrievable after implantation, and are more favourable for commercialization. Despite these manufacturing challenges, microencapsulation remains an active area of research due to its potential to provide immunoprotection without the need for large implantable devices.
Macroencapsulation
Macroencapsulation takes a different approach by encasing many islet cells within a larger device or capsule. These devices typically consist of a chamber or pouch that contains multiple islets, surrounded by a semi-permeable membrane. Macroencapsulation devices offer several potential advantages, including easier retrieval if complications arise, more straightforward manufacturing processes, and the ability to incorporate additional features such as oxygen generators or vascularization-promoting structures.
The Theracyte device is immunoisolating, and is composed of a two-membrane pouch. The outer membrane has a 5 μm pore size to support cell infiltration and to promote angiogenesis throughout the device. The inner membrane has a pore size diameter of 0.4 μm for immunoisolating the islets adjacent to the vasculature. This dual-membrane design represents an innovative approach to balancing the competing needs of immune protection and adequate vascularization.
ViaCyte has since developed a system known as Encaptra, which has a single membrane that is immunoisolating to protect the transplanted cells from direct interaction with immune cells, while allowing oxygen and nutrients to pass. Encapsulated stem cell-derived beta cells exert glucose control in patients with type 1 diabetes. These clinical developments demonstrate that macroencapsulation devices are progressing from laboratory research to real-world applications.
Several devices that have been developed include Theracyte™ from TheraCyte Inc., βAir from BetaO2 Technologies, the Cell Pouch System from Sernova, and PEC-Encap (VC-01) and PEC-Direct (VC-02) from ViaCyte (now acquired by Vertex Pharmaceuticals). Each of these devices represents a unique approach to solving the challenges of islet encapsulation, with different designs, materials, and implantation sites.
Another macroencapsulation device that uses microfabrication technology is called the Nanogland. It consists of an outer membrane with parallel nanochannels (3.6–40 nm) and perpendicular microchannels (20–60 microns) surrounding islets. The nanochannels are designed to provide immunoprotection and the microchannels are thought to help with engraftment. Subcutaneous implantation of the Nanogland with human islets in mice showed the survival of implants for more than 120 days.
One of the critical challenges for macroencapsulation devices is ensuring adequate oxygen supply to the encapsulated islets. Anderson and his colleagues reported an islet-encapsulation device that also carries an on-board oxygen generator. This generator consists of a proton-exchange membrane that can split water vapor (found abundantly in the body) into hydrogen and oxygen. The hydrogen diffuses harmlessly away, while oxygen goes into a storage chamber that feeds the islet cells through a thin, oxygen-permeable membrane. They showed that these encapsulated pancreatic islet cells could survive in the body for at least 90 days. In mice that received the implants, the cells remained functional and produced enough insulin to control the animals’ blood sugar levels.
However, not all macroencapsulation approaches have been successful. VX-264, an investigational islet cell therapy encapsulated in a proprietary macroencapsulation device developed by Vertex, completed Phase 1/2 dosing. However, the analysis did not meet its efficacy endpoint, resulting in the termination of the clinical trial. This setback highlights the ongoing challenges in developing effective macroencapsulation systems and the need for continued research and refinement.
Nanoencapsulation
Nanoencapsulation, by contrast, refers to nanometer-scale coatings or layers directly deposited on the islet surface. Unlike other encapsulate methods that immobilize the cells or substances to be encapsulated in a micron-sized gel matrix, nanoencapsulation methods are usually based on the formation of nanomembranes around cells or organs. Nanoencapsulation is a technology for encapsulating islets through conformal coating, mostly relying on the use of a nozzle method. As compared with conventional microcapsules, conformational coating allows for the formation of thin-films covering each individual islet.
Both the size of the resulting materials and the thickness of the film are adjusted to the size and morphology of individual islets. This technology gives rise to nanocapsules, for which the thickness of the protecting membrane favors the bi-directional diffusion of oxygen, nutrients and metabolites. The ultra-thin nature of nanoencapsulation coatings offers significant advantages in terms of nutrient and oxygen diffusion compared to thicker microencapsulation layers.
Nanoencapsulation represents the cutting edge of encapsulation technology, leveraging advances in nanotechnology and materials science to create protective barriers that are just nanometers thick. This approach minimizes the diffusion distance for oxygen and nutrients while still providing effective immune protection. The conformal coating technique ensures that each islet is individually protected with a coating that precisely matches its shape and size.
Various materials and methods have been explored for nanoencapsulation, including layer-by-layer assembly of polyelectrolytes, chemical vapor deposition, and plasma polymerization. Each method offers different advantages in terms of coating uniformity, thickness control, and biocompatibility. The goal is to create a coating that is thin enough to allow rapid diffusion of oxygen and nutrients, yet robust enough to provide effective immune protection over extended periods.
Biomaterials Used in Encapsulation
The choice of biomaterial is critical to the success of any encapsulation strategy. The ideal encapsulation material must meet several demanding requirements: it must be biocompatible, mechanically stable, permeable to oxygen and nutrients, impermeable to immune cells and antibodies, and resistant to degradation in the body’s environment. Researchers have explored a wide range of natural and synthetic materials in the quest for the optimal encapsulation biomaterial.
Alginate and Modified Alginates
Alginate remains the most widely studied material for islet encapsulation due to its biocompatibility, ease of processing, and ability to form gels under mild conditions. However, standard alginate formulations have shown limitations in clinical applications, particularly regarding foreign body responses and fibrotic overgrowth. This has led to extensive research into chemically modified alginate formulations designed to reduce these adverse reactions.
Three chemically modified, immune-modulating alginate formulations elicited a reduced foreign body response. The Z1-Y15 chemical modification specifically modulates macrophage activation upstream, which in turn significantly reduces the recruitment of myofibroblasts: the major contributor to downstream fibrosis. These modified alginate formulations represent a significant advancement in addressing one of the major challenges of encapsulation technology.
The development of triazole-modified alginate and other chemically modified formulations demonstrates the importance of understanding the molecular interactions between biomaterials and the immune system. By carefully engineering the chemical properties of alginate, researchers can modulate the host response and reduce the fibrotic reactions that have plagued earlier encapsulation attempts.
Silk-Based Materials
Treated silk proteins have low antigenicity and rarely cause immune reactions when implanted in vivo. The performance of islets encapsulated in silk materials was significantly enhanced by co-encapsulation with fibroin, a protein presenting strong mechanical properties and low immunogenicity. Co-encapsulation with mesenchymal stromal cells resulted in a 2.3 fold increase of the stimulation index and additional co-encapsulation of fibroin led to 4.4 fold enhancement, as compared with pure silk encapsulated islets.
Silk-based materials offer unique advantages including excellent mechanical properties, controllable degradation rates, and the ability to be processed into various forms including films, hydrogels, and porous scaffolds. The natural origin of silk proteins and their long history of use in medical applications provide additional confidence in their biocompatibility and safety profile.
Synthetic Polymers
By using a highly porous and durable nanofibrous skin made by electrospinning a biocompatible medical‐grade thermoplastic silicone‐polycarbonate‐urethane (TSPU) and an alginate hydrogel core, researchers developed an implantable nanofiber‐integrated cell encapsulation (NICE) device that offers enhanced biocompatibility, safety, and scalability for large‐scale production, ensuring the safe delivery and protection of xenogeneic stem cell‐derived islets. To further improve the biocompatibility of the encapsulation device in large animals, researchers reported a zwitterionic polyurethanes (ZPU) nanoporous encapsulation device using electrospinning technique.
Synthetic polymers offer the advantage of precise control over material properties, including mechanical strength, permeability, and degradation rate. Advanced manufacturing techniques such as electrospinning enable the creation of nanofibrous structures with high surface area and controlled pore sizes, optimizing the balance between immune protection and nutrient transport.
Advantages of Encapsulation Technologies
Encapsulation technologies offer several compelling advantages that make them an attractive approach for improving islet transplantation outcomes. These benefits address many of the key limitations that have prevented islet transplantation from becoming a widely available treatment option for type 1 diabetes.
Elimination of Chronic Immunosuppression
Encapsulated islets equipped with adequate barrier to host immune cells and antibodies would advance islet transplantation without use of toxic immunosuppressive drugs to prevent transplant rejection while addressing donor islet shortage. Both encapsulation methods aim to reduce immune rejection and eliminate the need for systemic immunosuppression, offering a promising path to improved islet viability and functionality in type 1 diabetes treatment.
The ability to protect transplanted islets without requiring lifelong immunosuppressive drugs represents perhaps the most significant advantage of encapsulation technology. Cell encapsulation could reduce the need for long-term use of immunosuppressive drugs after an organ transplant to control side effects. This would dramatically expand the pool of patients who could benefit from islet transplantation, as many patients currently cannot tolerate or are unwilling to accept the risks associated with chronic immunosuppression.
By eliminating the need for immunosuppressive drugs, encapsulation technology could make islet transplantation appropriate for a much broader population of type 1 diabetes patients, not just those with the most severe and difficult-to-manage disease. This could transform islet transplantation from a last-resort treatment for a small subset of patients into a viable option for many more individuals struggling with diabetes management.
Extended Islet Survival and Function
Combining design principles promoted islet viability for the duration of the study (4 months) post transplantation into non-human primates without the use of any immunosuppression. Islet xenograft survival, rapid lowering of blood glucose and long-term glycemic control for more than 200 days was achieved without any immunosuppressants. These results demonstrate that properly designed encapsulation systems can support long-term islet survival and function without the need for immunosuppressive drugs.
The protective environment created by encapsulation can potentially extend the functional lifespan of transplanted islets beyond what is achievable with immunosuppression alone. By shielding the islets from immune attack and providing a stable microenvironment, encapsulation may help preserve islet function over extended periods, reducing or eliminating the need for repeat transplantations.
Enabling Use of Alternative Cell Sources
The use of microencapsulation would protect the islet cells from immune rejection as well as allow the use of animal cells or genetically modified insulin-producing cells. Encapsulation has been tested on all of primary human islets, porcine islets and stem cell-derived islets, and it is feasible for such platform technologies to be developed to suit different cell types and disease applications.
One of the most exciting advantages of encapsulation technology is its potential to enable the use of alternative cell sources beyond human cadaveric islets. The scarcity of organ donors poses a significant limitation to these procedures. Because of its current limitations, and because the needed cadaver-derived islets are in short supply, islet transplantation is only appropriate for a small subset of people with type 1 diabetes.
Encapsulation could enable the use of porcine islets, which are available in virtually unlimited quantities and have been shown to function effectively in preclinical studies. In further attempts to reduce immune rejection after xenogeneic islet transplantation, porcine islets may be encapsulated in a protective layer to avoid immune cell recognition. In one study, neonatal porcine islets were encapsulated in a stable and permeable alginate gel and enclosed in a biocompatible, immunoprotective membrane, and transplanted in the abdominal cavities of immunocompetent diabetic mice.
Additionally, encapsulation technology could facilitate the use of stem cell-derived islets, which represent another potentially unlimited source of insulin-producing cells. Research in beta cell replacement has focused on developing scalable solutions, such as stem cell-derived islets, combined with localized immunosuppression. Preliminary results of ongoing clinical trials suggest that the transplantation of stem cell–derived β-cells can consistently restore insulin independence in immunosuppressed recipients with type 1 diabetes, thus signaling the profound progress made in generating an unlimited and a uniform supply of cells for transplant.
Retrievability and Safety
Macroencapsulation devices offer the additional advantage of being retrievable if complications arise. Unlike dispersed microencapsulated islets or directly transplanted islets, macroencapsulation devices can be surgically removed if necessary. This retrievability provides an important safety feature, allowing for intervention if the device fails or causes adverse effects. The devices were shown to retain their integrity after they were retrieved and re-transplanted in new immunocompetent diabetic mice.
Clinical Progress and Recent Developments
The field of encapsulated islet transplantation has seen remarkable progress in recent years, with several approaches advancing to clinical trials and showing promising results. These developments demonstrate that encapsulation technology is moving from laboratory research to real-world clinical applications.
Stem Cell-Derived Islets in Clinical Trials
Using more mature stem cell-derived β-cells, Vertex Pharmaceuticals initiated a phase 1/2 clinical trial (VX-880) in 2021, with cells transplanted intraportally into the liver under full-dose immunosuppression. By June 2024, 12 patients had been dosed; 11 of 12 had marked reduction or complete insulin independence, and all had HbA1c less than 7.0% and percentage of time spent with glucose in target range greater than 70% on continuous glucose monitoring. The three patients with over a year of follow-up met the primary endpoint of severe hypoglycemic episode elimination with HbA1c less than 7.0% and the secondary endpoint of insulin independence.
These impressive results with VX-880 demonstrate the potential of stem cell-derived islets to restore insulin independence and achieve excellent glycemic control. However, it’s important to note that these trials still require immunosuppression. The next frontier is combining stem cell-derived islets with encapsulation technology to eliminate the need for immunosuppressive drugs.
Autologous Stem Cell-Derived Islet Transplantation
A first-in-human phase I clinical trial assessed the feasibility of autologous transplantation of chemically induced pluripotent stem-cell-derived islets (CiPSC islets) beneath the abdominal anterior rectus sheath for type 1 diabetes treatment. The patient achieved sustained insulin independence starting 75 days post-transplantation. The patient’s time-in-target glycemic range increased from a baseline value of 43.18% to 96.21% by month 4 post-transplantation, accompanied by a decrease in glycated hemoglobin, an indicator of long-term systemic glucose levels at a non-diabetic level.
Thereafter, the patient presented a state of stable glycemic control, with time-in-target glycemic range at greater than 98% and glycated hemoglobin at around 5%. This remarkable result demonstrates the potential of autologous stem cell-derived islets to restore normal glucose control. While this trial still used immunosuppression, the use of autologous cells (derived from the patient’s own tissues) represents an important step toward reducing immune rejection.
Encapsulated Cell Therapy Trials
In 2017, ViaCyte conducted phase 1/2 clinical trial (VC‐02) utilizing the PEC‐Encap system, which encapsulated pluripotent stem cell‐derived pancreatic endoderm cells. While early results from this trial showed that the encapsulated cells could survive and produce C-peptide (a marker of insulin production), the trial also revealed challenges related to vascularization and fibrotic responses that limited the effectiveness of the approach.
CRISPR Therapeutics (previously in conjunction with ViaCyte) is conducting first-in-human Phase I clinical trials with an investigational, allogeneic, gene-edited, hypoimmune stem cell-derived pancreatic endoderm cells for type 1 diabetes. The cells are also encapsulated in a device to be implanted in patients without immunosuppressive therapy. This approach combines multiple cutting-edge technologies—gene editing, stem cell differentiation, and encapsulation—to create a comprehensive solution to the challenges of islet transplantation.
Expansion of FDA-Approved Islet Transplantation
On November 25, 2024, the University of Illinois Health in Chicago initiated LANTIDRA therapy in partnership with CellTrans. Throughout 2024, CellTrans engaged in extensive discussions with regional and national islet transplant programs, aiming to launch a multicenter implementation by 2025. LANTIDRA has been covered by most private insurers in the U.S. for patients with brittle type 1 diabetes. Additionally, the FDA has recently approved LANTIDRA’s shipping protocols for the shelf life of LANTIDRA up to 48 hours, facilitating broader distribution.
While LANTIDRA represents unencapsulated islet transplantation requiring immunosuppression, its approval and expanding availability create important infrastructure and clinical experience that will support the eventual translation of encapsulated islet therapies to widespread clinical use.
Challenges Facing Encapsulation Technologies
Despite the significant promise of encapsulation technologies, several substantial challenges must be overcome before these approaches can achieve widespread clinical success. Understanding these challenges is essential for appreciating the complexity of developing effective encapsulation systems and the work that remains to be done.
Foreign Body Response and Fibrosis
The major limitations for large clinical application include the great variability of biomaterials, with insufficient biocompatibility leading to some degree of foreign body reaction and progressive fibrotic reactions. Transplantation of the capsules leads to a host response that will depend on multiple factors (for example, cells, materials, transplant site and so on). Shortly after transplantation into tissues, the host response to transplantation and the material can consist of an inflammatory response with nearby blood vessels. Over time, the inflammatory response would ideally resolve without fibrosis and would allow for vascular growth adjacent to the capsule for nutrient and hormone exchange. However, shed antigens released from the islet may contribute to immune cell recruitment and activation.
The foreign body response represents one of the most significant obstacles to successful encapsulation. When the body recognizes an implanted material as foreign, it initiates an inflammatory cascade that can lead to the formation of a dense fibrotic capsule around the implanted device or microcapsules. This fibrotic tissue acts as a barrier that restricts the diffusion of oxygen and nutrients to the encapsulated islets, potentially leading to islet dysfunction and death.
Activated macrophages are known to recruit myofibroblasts, which deposit extracellular matrix proteins (collagen I/III, laminin, fibrinogen) in conjunction with macrophages to form the nutrient restrictive matrix. Understanding the cellular and molecular mechanisms underlying the foreign body response has been crucial for developing strategies to mitigate this reaction.
Encapsulated islet viability in larger animal models (non-human primates, pigs, dogs) is more challenging compared to rodents due to robust immune response causing more fibrosis of encapsulating device impairing nutrient exchange. This further highlights the disconnect between non-human primates and the most predictive mouse model for testing islet cell encapsulation technologies. This species-specific difference in foreign body responses has been a major challenge in translating promising results from rodent studies to human applications.
Oxygen and Nutrient Diffusion Limitations
Hypoxia activates the apoptosis signal in beta cells leading to decrease islet viability. In addition, the effective diffusional distance of the islet graft to the nearest blood vessel is 150–200 µm, but the macrocapsule diameter is greater than 1000 µm; this also causes a time lag in insulin response time to changes in host’s blood glucose.
Ensuring adequate oxygen supply to encapsulated islets represents a critical challenge. Islets are highly metabolically active tissues that require substantial oxygen to function properly. In the native pancreas, islets are richly vascularized, with blood vessels in close proximity to every islet cell. However, encapsulation creates a physical barrier between the islets and the host’s blood supply, increasing the diffusion distance for oxygen and potentially creating hypoxic conditions within the capsule.
The oxygen diffusion limitation is particularly problematic for macroencapsulation devices, which contain large numbers of islets within a single chamber. Islets in the center of the device may be far from the nearest blood vessels, leading to oxygen gradients within the device. This can result in central necrosis, where islets in the middle of the device die due to insufficient oxygen while those near the periphery survive.
Enhancing microvasculature has the potential to significantly enhance the survival of encapsulated islets. Various strategies have been explored to address the oxygen limitation, including incorporating oxygen-generating systems, promoting vascularization around the device, and optimizing device geometry to minimize diffusion distances.
Biocompatibility and Material Optimization
The long-term durability of the biomaterials in vivo will need to be tested and optimized in an application specific manner. For translational purposes, production of the encapsulation materials/devices need to conform with good manufacturing practices and ISO standards normally under the regulation of medical devices.
Developing biomaterials that are truly biocompatible over the long term remains a significant challenge. Materials that perform well in short-term studies may elicit adverse reactions when implanted for months or years. The body’s response to implanted materials can change over time, with initially mild reactions potentially progressing to more severe fibrosis or material degradation.
Additionally, the manufacturing and quality control requirements for clinical-grade encapsulation materials are stringent. There are many gold standard biomaterials used for encapsulation of islets that are straightforward to mass produce. However, ensuring consistent quality, sterility, and performance across large-scale production batches presents significant technical and regulatory challenges.
Transplantation Site Selection
The choice of transplantation site significantly impacts the success of encapsulated islet transplantation. Different anatomical locations offer different advantages and disadvantages in terms of oxygen availability, ease of implantation, retrievability, and host immune responses. The peritoneal cavity has been widely studied due to its large volume and relative ease of access, but issues with capsule sedimentation and clumping have been problematic.
Pericapsular fibrotic overgrowth scores were further reduced when Z1-Y15 spheres were transplanted into the bursa omentalis site compared to the general intraperitoneal space, which may be indicative of a reduction in material fibrosis by limiting sphere clumping. In vitro assessments performed on the retrieved Z1-Y15 encapsulated islets indicate functional engrafted endocrine tissue, which further suggests that the bursa omentalis transplantation site (pO2 levels of 35.0 ± 3.2 mmHg) can support encapsulated islets if devoid of fibrotic overgrowth to ensure free nutritional exchange.
Other potential transplantation sites being explored include subcutaneous spaces, the omentum, and even intramuscular locations. Each site presents unique challenges and opportunities, and identifying the optimal location for encapsulated islet transplantation remains an active area of research.
Scale-Up and Manufacturing Challenges
Producing sufficient quantities of encapsulated islets for clinical use presents significant manufacturing challenges. A typical islet transplant requires hundreds of thousands to millions of islets, all of which must be encapsulated with consistent quality. For microencapsulation approaches, this means producing millions of individual microcapsules, each meeting strict specifications for size, permeability, and mechanical properties.
Quality control is particularly challenging for encapsulated islet products. Each batch must be tested for islet viability, function, capsule integrity, sterility, and freedom from endotoxins. The encapsulation process itself can stress the islets, potentially reducing their viability and function. Optimizing encapsulation protocols to minimize islet damage while maintaining high throughput is an ongoing challenge.
Emerging Strategies to Overcome Challenges
Researchers are actively developing innovative strategies to address the challenges facing encapsulation technologies. These emerging approaches leverage advances in materials science, bioengineering, immunology, and cell biology to create more effective encapsulation systems.
Advanced Biomaterial Design
Based on previous studies that generally used one or two combined strategies to protect islet graft function, a multifunctional encapsulated hydrogel model with multiple functions is the way forward for development. With the continuous progress of technology, additional modifications of polymers should achieve higher degree of biological compatibility.
Next-generation biomaterials are being designed with multiple functional properties to address several challenges simultaneously. These multifunctional materials may incorporate anti-inflammatory agents, pro-angiogenic factors, or immunomodulatory molecules to actively shape the host response rather than simply providing a passive barrier. Chemical modifications to traditional materials like alginate are being refined to minimize foreign body responses while maintaining mechanical stability and permeability.
Researchers are also exploring biomimetic materials that more closely resemble the natural extracellular matrix of the pancreas. By incorporating specific proteins, growth factors, or structural features found in the native islet microenvironment, these materials aim to better support islet survival and function.
Co-Encapsulation Strategies
Mesenchymal Stromal Cells reduce the immune response by releasing cytokines and growth factors and also have the potential to induce angiogenesis and repair of damaged tissues. Co-encapsulating islets with supportive cell types represents a promising strategy to enhance islet survival and function. Mesenchymal stromal cells, endothelial cells, or other supportive cell types can be included within the encapsulation device to provide trophic support, promote vascularization, or modulate immune responses.
The incorporation of extracellular matrix components, endothelial cells and vascular endothelial growth factor into the bio-ink can make the printed model more similar to the living environment of islet cells, thus enhancing their biological function. This approach of creating a more complete microenvironment within the encapsulation device may better support long-term islet survival and function.
3D Printing and Advanced Manufacturing
3D printing technology can achieve fast manufacturing throughput and maintain high cell vitality. Overall, 3D printing is seen as one of the most promising encapsulation approaches because it can produce clinically relevant multi-component devices in a short period of time.
Three-dimensional bioprinting offers unprecedented control over the architecture and composition of encapsulation devices. This technology enables the creation of complex, multi-layered structures with precisely controlled pore sizes, material compositions, and spatial arrangements of different cell types. Bioprinting can produce devices with optimized geometries that minimize diffusion distances while maximizing mechanical stability.
The ability to rapidly prototype and test different device designs using 3D printing accelerates the development process. Researchers can quickly iterate through multiple design variations to identify optimal configurations for specific applications. Additionally, 3D printing may enable personalized device designs tailored to individual patients’ needs.
Combination with Gene Editing
This approach is facilitated by advances in gene editing technologies, such as CRISPR-Cas9, which enable the precise alteration of immune-related pathways to diminish graft immunogenicity. Hypoimmune engineering has the potential to redefine the therapeutic landscape of cell therapy, such as islet transplantation.
Combining encapsulation with gene editing to create hypoimmune islets represents a powerful synergistic approach. Gene-edited islets with reduced immunogenicity may require less robust immune protection, allowing for thinner encapsulation barriers that better support oxygen and nutrient diffusion. Alternatively, encapsulation could provide an additional layer of protection for gene-edited cells, further reducing the risk of immune rejection.
Islet cells overexpressing PD-L1 provided sustained blood glucose homeostasis, with human C-peptide levels correlating with glycemic control for more than 50 days. Engineering islets to express immunomodulatory molecules like PD-L1 can help create a local immunosuppressive environment that complements the physical barrier provided by encapsulation.
Oxygen Delivery Systems
Innovative approaches to ensuring adequate oxygen supply are being developed to address one of the most critical limitations of encapsulation. Beyond the oxygen-generating devices mentioned earlier, researchers are exploring oxygen-carrying materials, perfluorocarbon-based oxygen delivery systems, and device designs that promote rapid vascularization around the implant.
Some approaches involve pre-vascularization strategies, where the implantation site is prepared in advance to promote blood vessel formation before the encapsulated islets are implanted. This can help ensure that an adequate vascular network is in place to support the encapsulated islets from the moment of implantation.
Immunomodulatory Approaches
More recent advances in islet transplantation derive from islet encapsulation devices, biomaterial platforms releasing immunomodulatory compounds or surface-modified with immune regulating ligands, islet engineering and co-transplantation with accessory cells.
Rather than relying solely on physical barriers, next-generation encapsulation systems are incorporating active immunomodulatory strategies. These may include controlled release of anti-inflammatory drugs, incorporation of immunomodulatory molecules on the capsule surface, or engineering the capsule material itself to have immunomodulatory properties. By actively modulating the local immune environment, these approaches aim to prevent the foreign body response and promote long-term biocompatibility.
Future Directions and Clinical Translation
Avoiding the risks of chronic immunosuppression represents the next frontier. Several strategies have entered or are approaching clinical investigation, including immune-isolating islets, engineering immune-privileged islet implantation sites, rendering islets immune evasive, and inducing immune tolerance in transplanted islets. The field of encapsulated islet transplantation stands at an exciting juncture, with multiple promising approaches advancing toward clinical application.
Regulatory Pathways and Approval
Navigating the regulatory landscape for encapsulated islet products presents unique challenges. These products combine biological components (the islets) with medical devices (the encapsulation system), requiring careful consideration of regulatory requirements for both aspects. Regulatory agencies must evaluate not only the safety and efficacy of the encapsulated islets but also the biocompatibility and performance of the encapsulation materials and devices.
The authors discuss the significance of this approval and the critical steps necessary to broaden patient access, such as scaling up production, clinical integration, reimbursement frameworks, post-marketing surveillance, and patient education initiatives. The approval of LANTIDRA has established important precedents and pathways that will facilitate the regulatory approval of future encapsulated islet products.
Addressing the Donor Shortage
NIDDK is currently supporting research to characterize and generate new sources of insulin-producing cells and to eliminate the need for immunosuppressive medicines. To help overcome the shortage of cadaveric islets, research is building on an NIDDK-supported landmark discovery that progenitor cells could be used to produce large quantities of β-like cells in the laboratory.
The development of unlimited sources of insulin-producing cells through stem cell technology, combined with encapsulation to eliminate the need for immunosuppression, could finally make islet transplantation a widely available treatment option. With advancements in stem cell technology, unlimited stem cell‐derived islets can be differentiated in vitro and proved functional in vivo in different preclinical animal models. Thus, stem cell-derived islets emerged as a promising alternative to human primary islets.
The combination of stem cell-derived islets with advanced encapsulation technologies represents perhaps the most promising path forward for making islet transplantation accessible to the millions of people living with type 1 diabetes worldwide. This approach addresses both major limitations of current islet transplantation: the shortage of donor islets and the need for chronic immunosuppression.
Personalized Medicine Approaches
Future encapsulated islet therapies may incorporate personalized medicine approaches, tailoring the treatment to individual patient characteristics. This could include using autologous stem cell-derived islets to eliminate allogeneic immune responses, customizing device designs based on patient anatomy, or selecting specific encapsulation materials based on individual immune profiles.
The use of patient-specific induced pluripotent stem cells to generate autologous islets represents an exciting possibility. While this approach is more complex and expensive than using allogeneic cells, it could potentially eliminate both alloimmune and autoimmune rejection, especially when combined with appropriate encapsulation and immunomodulation strategies.
Expanding Applications Beyond Type 1 Diabetes
Macroencapsulation devices have been shown to be applied to cardiovascular diseases and CAR-T cell therapy and shown promising results. These clinical trials highlight the broad applications of this therapy beyond diabetes. The encapsulation technologies being developed for islet transplantation have potential applications far beyond type 1 diabetes.
Encapsulation could enable cell-based therapies for a wide range of conditions, including other endocrine disorders, neurological diseases, liver failure, and cancer. The principles and technologies being refined for islet encapsulation can be adapted to protect and deliver many different types of therapeutic cells. Success in islet encapsulation could therefore catalyze a broader revolution in cell-based medicine.
Long-Term Vision
More advances are needed to achieve a better islet immunoisolation without impeding nutritional transport and therapeutic delivery of insulin within appropriately designed encapsulation matrix that resembles the native pancreatic microenvironment. Also, more studies of efficacy in preclinical trials with larger animal models are needed as in vitro and preclinical rodent studies often do not always translate to human response. In conclusion, careful optimization of the encapsulation technology will accelerate its clinical translation and conventional use as a therapeutic option in diabetes mellitus.
By combining expertise across disciplines ranging from electrical engineering to immunology, researchers can begin to address the multiple challenges that are involved in translating encapsulated cell therapy from the laboratory to the clinic. Future success requires a willingness to collaborate, to combine new ‘device’ technologies with ‘cell’ technologies, and to understand the limitations of the biological environment in which human cell therapy must exist.
The ultimate vision for encapsulated islet transplantation is a one-time procedure that provides long-term or even permanent restoration of normal glucose control without the need for insulin injections or immunosuppressive drugs. While significant challenges remain, the remarkable progress made in recent years suggests that this vision is increasingly achievable. Continued research, clinical trials, and refinement of encapsulation technologies are bringing us closer to making this transformative treatment a reality for people with type 1 diabetes.
Conclusion
Encapsulation technologies represent one of the most promising frontiers in the treatment of type 1 diabetes. By providing a protective barrier that shields transplanted islet cells from immune attack while allowing the passage of nutrients, oxygen, and insulin, encapsulation offers the potential to eliminate the need for chronic immunosuppression—one of the major barriers preventing islet transplantation from becoming a widely available treatment option.
The field has made remarkable progress from the early conceptual work of Thomas Chang in the 1960s to today’s sophisticated encapsulation systems incorporating advanced biomaterials, gene-edited cells, oxygen delivery systems, and immunomodulatory strategies. Clinical trials are demonstrating that encapsulated islets can survive, function, and provide glycemic control in patients, validating the fundamental concept while also revealing the challenges that must be overcome.
Significant obstacles remain, including foreign body responses, fibrosis, oxygen diffusion limitations, and the need for improved biocompatible materials. However, researchers are actively developing innovative solutions to these challenges through advanced biomaterial design, 3D printing, co-encapsulation strategies, and combination approaches that integrate encapsulation with gene editing and immunomodulation.
The convergence of multiple technological advances—including stem cell-derived islets, sophisticated encapsulation systems, gene editing, and advanced manufacturing—is creating unprecedented opportunities to finally realize the full potential of islet transplantation. When combined with unlimited sources of insulin-producing cells from stem cell technologies, encapsulation could transform islet transplantation from a treatment available only to a small subset of patients into a widely accessible therapy that could benefit millions of people living with type 1 diabetes.
As research continues and clinical trials advance, the dream of a functional cure for type 1 diabetes through encapsulated islet transplantation is becoming increasingly tangible. While challenges remain, the progress made to date provides strong reason for optimism that encapsulation technologies will play a central role in the future treatment of diabetes and potentially many other diseases amenable to cell-based therapies.
For more information about islet transplantation and diabetes research, visit the National Institute of Diabetes and Digestive and Kidney Diseases, the American Diabetes Association, the JDRF, Nature’s islet transplantation research, and Frontiers in Immunology for the latest developments in this rapidly evolving field.