Islet Transplantation: Microencapsulation, Nanoencapsulation, and Hypoimmune Engineering

Abstract

Islet transplantation represents a promising curative approach for type 1 diabetes by restoring glucose-responsive insulin secretion. However, the requirement for lifelong immunosuppression to prevent immune rejection can lead to significant side effects. Emerging strategies such as microencapsulation, nanoencapsulation, and hypoimmune engineering are being developed to protect transplanted islets from immune attack, thereby enhancing their viability and function. This review critically examines these innovative technologies, highlighting the methodologies, materials, experimental and clinical outcomes, as well as the challenges they face and potential solutions to overcome those challenges.

Keywords: cell encapsulation; hypoimmune engineering; islet transplantation; type 1 diabetes.

FIGURE 1

Methods for islet microencapsulation. (a) Syringe pump‐based droplet generation. An alginate solution with the cell suspension is extruded from the syringe and crosslinked by bivalent cations. (b) Microfluidics‐based droplet generation. The aqueous solution with the cell suspension forms droplets in an organic phase. (c) Optimized microfluidic device. Adapted from Tomei et al. (2014). (d) Interfacial polymerization. The adsorbed CaCO₃ on the cell surface is released by acetic acid, and the released Ca ions induce alginate crosslinking. Adapted from Mao et al. (2017). (e) Comparison of droplet formation and interfacial polymerization in cell microencapsulation: The former method is associated with the formation of empty microcapsules and a large polymer volume, whereas the latter achieves nearly 100% encapsulation efficiency and uses a much smaller polymer volume.

FIGURE 2

Representative materials for islet microencapsulation. (a) Chemical structure of alginic acid and alginate crosslinking by bivalent cations such as Ca²⁺ or Ba²⁺. Adapted from Liu et al. (2019). (b) Chemical structures of zwitterionically modified alginate. Sulfobetaine and carboxybetaine monomers were used for the conjugation. Adapted from Liu et al. (2019). (c) Chemical structures of PEG and PEG derivatives with functional groups. Multi‐arm PEG derivatives can be crosslinked to form hydrogels via different addition reactions (Tomei et al. ; Stock et al. 2020). (d) Chemical structure of gelatin methacryloyl (GelMA) showing only representative amino acid residues in the backbone. (e) Chemical structure of chitosan, primarily composed of d‐glucosamine units that are the deacetylated form of N‐acetyl‐d‐glucosamine.

FIGURE 3

In vivo evaluation of microencapsulated islets. (a) Blood glucose monitoring of diabetic nonhuman primates receiving porcine islets. Left: Recipient ID# NHP 8C4‐40, Right: Recipient ID# NHP 8C4‐16. Adapted from Safley et al. (2018). (b) Time progression profiles of HbA1c and TEF. Different groups are shown in (●) or (○). Encapsulated neonatal porcine islets were transplanted twice into type 1 diabetic patients. The second transplantation was conducted 3 months after the initial procedure. Adapted from Matsumoto et al. (2016). (c) Blood glucose monitoring of diabetic mice after syngeneic (left) and allogeneic (right) islet transplantation. Adapted from Weaver et al. (2019).

FIGURE 4

Methods for cell nanoencapsulation. (a) The iterative layer‐by‐layer coating process involves the sequential deposition of oppositely charged biomaterials onto the islet surface, forming stable multilayers. This technique enhances biocompatibility and protection by allowing precise control over layer composition and thickness. (b) The schematic illustrates nano‐scale encapsulation of pancreatic islets through layer‐by‐layer deposition of charged polysaccharide multilayers. Chitosan‐PC (+) is shown in blue, alginate (−) in red, and chondroitin‐4‐sulfate‐PC (−) in green. Adapted from Zhi et al. (2012). (c) DNA‐templated nanoencapsulation without iterative cell treatment. Cholesterol‐conjugated DNA initiators are displayed on the cell surface, where they trigger the automatic assembly of DNA‐polymer complexes, followed by polyelectrolyte complexation. Adapted from Shi et al. (2019). (d) The schematic illustrates a stepwise method for coating islets with collagen, PD‐TAMP, and alginate. Islets were first incubated in collagen solution at 37°C to facilitate collagen fiber formation and PD‐TAMP binding, followed by sequential layering of PD‐TAMP and collagen. Lastly, a thin alginate hydrogel layer was applied via surface‐triggered in situ gelation (STIG) to stabilize the coating. Adapted from Le Tran et al. (2022).

FIGURE 5

Therapeutic outcomes of nanoencapsulated islets. (a) Allogeneic transplantation of both nonencapsulated and nanoencapsulated Balb/c islets successfully reversed STZ‐induced hyperglycemia in diabetic C57BL/6 mice. In the right graph, upright arrows indicate the day of nephrectomy. Adapted from Zhi et al. (2012). (b) Non‐fasting blood glucose levels in β cell spheroid transplanted mice. Histological examination of implants collected 30 days post‐implantation from streptozotocin‐treated BALB/C mice, shown through hematoxylin and eosin (H&E) staining and insulin immunostaining in each group. Scale bars: 200 μm (top images) and 100 μm (bottom images). Adapted from Kim et al. (2021). (c) Nonfasting blood glucose levels in control mice (C), untreated diabetic mice (DM), diabetic mice transplanted with uncoated human islets (TU), and diabetic mice receiving nanoencapsulated human islets (TN). Adapted from Syed et al. (2018). (d) Nonfasting blood glucose levels in diabetic C57BL/6 mice following transplantation of control islets, DOPA‐NPs‐coated islets, or FK506/DOPA‐NPs‐coated islets. Graft survival rates for each group are shown. Adapted from Pham et al. (2018).

FIGURE 6

Surface engineering via the Removal and/or display of specific molecules. (a) Selective removal and retention of cell surface proteins. The knockout of HLA class I and class II molecules enables immune evasion from CD8⁺ cytotoxic T cells and CD4⁺ helper T cells. Selective retention of specific HLA antigens prevents the activation of natural killer (NK) cells. (b) Expression of specific cell‐protecting ligands or Secretory cytokines. Self‐protective molecules, including PD‐L1, FasL, CTLA4‐Ig, HLA‐E, HLA‐C, HLA‐G, and CD47, can be overexpressed on the cell surface, or secreted cytokines such as CD47, TGF‐β, IL‐10, and IL‐12 can be released from cells to reduce immunogenicity.

FIGURE 7

Process of genetic engineering for the Removal and/or display of specific molecules. The CRISPR–Cas system is used to selectively disrupt B2M, CITTA, or both, enabling targeted depletion of HLA class molecules. Meanwhile, lentiviral vectors facilitate the stable integration of one or more self‐protective molecules, promoting the development of immune‐evasive cell populations.

FIGURE 8

Microenvironmental engineering strategies for immune evasion. (a) Genetically engineered MSCs overexpressing immunomodulatory molecules, including CTLA4‐Ig and PD‐L1, to establish an immune‐privileged microenvironment. (b) Development of immunomodulatory microgels with FasL constructs to induce localized immune suppression and attenuate allogeneic immune rejection.