Rapid Restoration of Vascularity and Oxygenation in Mouse and Human Islets Transplanted to Omentum May Contribute to Their Superior Function Compared to Intraportally Transplanted Islets

Transplantation of islets into the liver confers several site‐specific challenges, including a delayed vascularization and prevailing hypoxia. The greater omentum has in several experimental studies been suggested as an alternative implantation site for clinical use, but there has been no direct functional comparison to the liver. In this experimental study in mice, we characterized the engraftment of mouse and human islets in the omentum and compared engraftment and functional outcome with those in the intraportal site. The vascularization and innervation of the islets transplanted into the omentum were restored within the first month by paralleled ingrowth of capillaries and nerves. The hypoxic conditions in the islets early posttransplantation were transient and restricted to the first days. Newly formed blood vessels were fully functional, and the blood perfusion and oxygenation of the islets became similar to that of endogenous islets. Furthermore, islet grafts in the omentum showed at 1 month posttransplantation functional superiority to intraportally transplanted grafts. We conclude that in contrast to the liver the omentum provides excellent engraftment conditions for transplanted islets. Future studies in humans will be of great interest to investigate the capability of this site to also harbor larger grafts without interfering with islet functionality.     

Identification and Characterization of microRNAs Associated With Human β‐Cell Loss in a Mouse Model

Currently there is no effective approach for monitoring early β‐cell loss during islet graft rejection following human islet transplantation (HIT). Due to ethical and technical constraints, it is difficult to directly study biomarkers of islet destruction in humans. Here, we established a humanized mouse model with induced human β‐cell death using adoptive lymphocyte transfer (ALT). Human islet grafts of ALT‐treated mice had perigraft lymphocyte infiltration, fewer insulin⁺ β cells, and increased β‐cell apoptosis. Islet‐specific miR‐375 was used to validate our model, and expression of miR‐375 was significantly decreased in the grafts and increased in the circulation of ALT‐treated mice before hyperglycemia. A NanoString expression assay was further used to profile 800 human miRNAs in the human islet grafts, and the results were validated using quantitative real‐time polymerase chain reaction. We found that miR‐4454 and miR‐199a‐5p were decreased in the human islet grafts following ALT and increased in the circulation prior to hyperglycemia. These data demonstrate that our in vivo model of induced human β‐cell destruction is a robust method for identifying and characterizing circulating biomarkers, and suggest that miR‐4454 and miR‐199a‐5p can serve as novel biomarkers associated with early human β‐cell loss following HIT.     

Unmethylated Insulin DNA Is Elevated After Total Pancreatectomy With Islet Autotransplantation: Assessment of a Novel Beta Cell Marker

Beta cell death may occur both after islet isolation and during infusion back into recipients undergoing total pancreatectomy with islet autotransplantation (TPIAT) for chronic pancreatitis. We measured the novel beta cell death marker unmethylated insulin (INS) DNA in TPIAT recipients before and immediately after islet infusion (n = 21) and again 90 days after TPIAT, concurrent with metabolic functional assessments (n = 25). As expected, INS DNA decreased after pancreatectomy (p = 0.0002). All TPIAT recipients had an elevated unmethylated INS DNA ratio in the first hours following islet infusion. In four samples (three patients), INS DNA was also assessed immediately after islet isolation and again before islet infusion to assess the impact of the isolation process: Unmethylated and methylated INS DNA fractions both increased over this interval, suggesting death of beta cells and exocrine tissue before islet infusion. Higher glucose excursion with mixed‐meal tolerance testing was associated with persistently elevated INS DNA at day 90. In conclusion, we observed universal early elevations in the beta cell death marker INS DNA after TPIAT, with pronounced elevations in the islet supernatant before infusion, likely reflecting beta cell death induced by islet isolation. Persistent posttransplant elevation of INS DNA predicted greater hyperglycemia at 90 days.     

Posttransplant oxygen inhalation improves the outcome of subcutaneous islet transplantation: A promising clinical alternative to the conventional intrahepatic site

Subcutaneous tissue is a promising site for islet transplantation, due to its large area and accessibility, which allows minimally invasive procedures for transplantation, graft monitoring, and removal of malignancies as needed. However, relative to the conventional intrahepatic transplantation site, the subcutaneous site requires a large number of islets to achieve engraftment success and diabetes reversal, due to hypoxia and low vascularity. We report that the efficiency of subcutaneous islet transplantation in a Lewis rat model is significantly improved by treating recipients with inhaled 50% oxygen, in conjunction with prevascularization of the graft bed by agarose–basic fibroblast growth factor. Administration of 50% oxygen increased oxygen tension in the subcutaneous site to 140 mm Hg, compared to 45 mm Hg under ambient air. In vitro, islets cultured under 140 mm Hg oxygen showed reduced central necrosis and increased insulin release, compared to those maintained in 45 mm Hg oxygen. Six hundred syngeneic islets subcutaneously transplanted into the prevascularized graft bed reversed diabetes when combined with postoperative 50% oxygen inhalation for 3 days, a number comparable to that required for intrahepatic transplantation; in the absence of oxygen treatment, diabetes was not reversed. Thus, we show oxygen inhalation to be a simple and promising approach to successfully establishing subcutaneous islet transplantation.     

Validation of the BETA‐2 Score: An Improved Tool to Estimate Beta Cell Function After Clinical Islet Transplantation Using a Single Fasting Blood Sample

The beta score, a composite measure of beta cell function after islet transplantation, has limited sensitivity because of its categorical nature and requires a mixed‐meal tolerance test (MMTT). We developed a novel score based on a single fasting blood sample. The BETA‐2 score used stepwise forward linear regression incorporating glucose (in millimoles per liter), C‐peptide (in nanomoles per liter), hemoglobin A1c (as a percentage) and insulin dose (U/kg per day) as continuous variables from the original beta score data set (n = 183 MMTTs). Primary and secondary analyses assessed the score's ability to detect glucose intolerance (90‐min MMTT glucose ≥8 mmol/L) and insulin independence, respectively. A validation cohort of islet transplant recipients (n = 114 MMTTs) examined 12 mo after transplantation was used to compare the score's ability to detect these outcomes. The BETA‐2 score was expressed as follows (range 0–42): A score <20 and ≥15 detected glucose intolerance and insulin independence, respectively, with >82% sensitivity and specificity. The BETA‐2 score demonstrated greater discrimination than the beta score for these outcomes (p < 0.05). Using a fasting blood sample, the BETA‐2 score estimates graft function as a continuous variable and shows greater discrimination of glucose intolerance and insulin independence after transplantation versus the beta score, allowing frequent assessments of graft function. Studies examining its utility to track long‐term graft function are required.     

Transplantation of macroencapsulated human islets within the bioartificial pancreas βAir to patients with type 1 diabetes mellitus

Macroencapsulation devices provide the dual possibility of immunoprotecting transplanted cells while also being retrievable, the latter bearing importance for safety in future trials with stem cell–derived cells. However, macroencapsulation entails a problem with oxygen supply to the encapsulated cells. The βAir device solves this with an incorporated refillable oxygen tank. This phase 1 study evaluated the safety and efficacy of implanting the βAir device containing allogeneic human pancreatic islets into patients with type 1 diabetes. Four patients were transplanted with 1‐2 βAir devices, each containing 155 000‐180 000 islet equivalents (ie, 1800‐4600 islet equivalents per kg body weight), and monitored for 3‐6 months, followed by the recovery of devices. Implantation of the βAir device was safe and successfully prevented immunization and rejection of the transplanted tissue. However, although beta cells survived in the device, only minute levels of circulating C‐peptide were observed with no impact on metabolic control. Fibrotic tissue with immune cells was formed in capsule surroundings. Recovered devices displayed a blunted glucose‐stimulated insulin response, and amyloid formation in the endocrine tissue. We conclude that the βAir device is safe and can support survival of allogeneic islets for several months, although the function of the transplanted cells was limited (Clinicaltrials.gov: NCT02064309).     

Alginate‐microencapsulation of human stem cell–derived β cells with CXCL12 prolongs their survival and function in immunocompetent mice without systemic immunosuppression

Pancreatic β‐cell replacement by islet transplantation for the treatment of type 1 diabetes (T1D) is currently limited by donor tissue scarcity and the requirement for lifelong immunosuppression. The advent of in vitro differentiation protocols for generating functional β‐like cells from human pluripotent stem cells, also referred to as SC‐β cells, could eliminate these obstacles. To avoid the need for immunosuppression, alginate‐microencapsulation is widely investigated as a safe path to β‐cell replacement. Nonetheless, inflammatory foreign body responses leading to pericapsular fibrotic overgrowth often causes microencapsulated islet‐cell death and graft failure. Here we used a novel approach to evade the pericapsular fibrotic response to alginate‐microencapsulated SC‐β cells; an immunomodulatory chemokine, CXCL12, was incorporated into clinical grade sodium alginate to microencapsulate SC‐β cells. CXCL12 enhanced glucose‐stimulated insulin secretion activity of SC‐β cells and induced expression of genes associated with β‐cell function in vitro. SC‐β cells co‐encapsulated with CXCL12 showed enhanced insulin secretion in diabetic mice and accelerated the normalization of hyperglycemia. Additionally, SC‐β cells co‐encapsulated with CXCL12 evaded the pericapsular fibrotic response, resulting in long‐term functional competence and glycemic correction (>150 days) without systemic immunosuppression in immunocompetent C57BL/6 mice. These findings lay the groundwork for further preclinical translation of this approach into large animal models of T1D.     

Shielding islets with human amniotic epithelial cells enhances islet engraftment and revascularization in a murine diabetes model

Hypoxia is a major cause of considerable islet loss during the early posttransplant period. Here, we investigate whether shielding islets with human amniotic epithelial cells (hAECs), which possess anti‐inflammatory and regenerative properties, improves islet engraftment and survival. Shielded islets were generated on agarose microwells by mixing rat islets (RIs) or human islets (HI) and hAECs (100 hAECs/IEQ). Islet secretory function and viability were assessed after culture in hypoxia (1% O₂) or normoxia (21% O₂) in vitro. In vivo function was evaluated after transplant under the kidney capsule of diabetic immunodeficient mice. Graft morphology and vascularization were evaluated by immunohistochemistry. Both shielded RIs and HIs show higher viability and increased glucose‐stimulated insulin secretion after exposure to hypoxia in vitro compared with control islets. Transplant of shielded islets results in considerably earlier normoglycemia and vascularization, an enhanced glucose tolerance, and a higher β cell mass. Our results show that hAECs have a clear cytoprotective effect against hypoxic damages in vitro. This strategy improves β cell mass engraftment and islet revascularization, leading to an improved capacity of islets to reverse hyperglycemia, and could be rapidly applicable in the clinical situation seeing that the modification to HIs are minor.     

Long-term control of diabetes in a nonhuman primate by two separate transplantations of porcine adult islets under immunosuppression

Porcine islet transplantation is an alternative to allo-islet transplantation. Retransplantation of islets is a routine clinical practice in islet allotransplantation in immunosuppressed recipients and will most likely be required in islet xenotransplantation in immunosuppressed recipients. We examined whether a second infusion of porcine islets could restore normoglycemia and further evaluated the efficacy of a clinically available immunosuppression regimen including anti-thymocyte globulin for induction; belimumab, sirolimus, and tofacitinib for maintenance and adalimumab, anakinra, IVIg, and tocilizumab for inflammation control in a pig to nonhuman primate transplantation setting. Of note, all nonhuman primates were normoglycemic after the retransplantation of porcine islets without induction therapy. Graft survival was >100 days for all 3 recipients, and 1 of the 3 monkeys showed insulin independence for >237 days. Serious lymphodepletion was not observed, and rhesus cytomegalovirus reactivation was controlled without any serious adverse effects throughout the observation period in all recipients. These results support the clinical applicability of additional infusions of porcine islets. The maintenance immunosuppression regimen we used could protect the reinfused islets from acute rejection.     

BMX‐001, a novel redox‐active metalloporphyrin,improves islet function and engraftment in a murine transplant model

Islet transplantation has become a well‐established therapy for select patients with type 1 diabetes. Viability and engraftment can be compromised by the generation of oxidative stress encountered during isolation and culture. We evaluated whether the administration of BMX‐001 (MnTnBuOE‐2‐PyP⁵⁺ [Mn(III) meso‐tetrakis‐(N‐ b ‐butoxyethylpyridinium‐2‐yl)porphyrin]) and its earlier derivative, BMX‐010 (MnTE‐2‐PyP [Mn(III) meso‐tetrakis‐(N‐methylpyridinium‐2‐yl)porphyrin]) could improve islet function and engraftment outcomes. Long‐term culture of human islets with BMX‐001, but not BMX‐010, exhibited preserved in vitro viability. Murine islets isolated and cultured for 24 hours with 34 μmol/L BMX‐001 exhibited improved insulin secretion (n = 3 isolations, P < .05) in response to glucose relative to control islets. In addition, 34 μmol/L BMX‐001–supplemented murine islets exhibited significantly reduced apoptosis as indicated by terminal deoxynucleotidyl transferase dUTP nick end labeling, compared with nontreated control islets (P < .05). Murine syngeneic islets transplanted under the kidney capsule at a marginal dose of 150 islets revealed 58% of 34 μmol/L BMX‐001–treated islet recipients became euglycemic (n = 11 of 19) compared with 19% of nontreated control islet recipients (n = 3 of 19, P < .05). Of murine recipients receiving a marginal dose of human islets cultured with 34 μmol/L BMX‐001, 92% (n = 12 of 13) achieved euglycemia compared with 57% of control recipients (n = 8 of 14, P = .11). These results demonstrate that the administration of BMX‐001 enhances in vitro viability and augments murine marginal islet mass engraftment.     

Formation of amyloid in encapsulated human pancreatic and human stem cell-generated beta cell implants

Detection of amyloid in intraportal islet implants of type 1 diabetes patients has been proposed as cause in their functional decline. The present study uses cultured adult human islets devoid of amyloid to examine conditions of its formation. After intraportal injection in patients, amyloid deposits <15 µm diameter were identified in 5%–12% of beta cell containing aggregates, 3–76 months posttransplant. Such deposits also formed in glucose-controlling islet implants in the kidney of diabetic mice but not in failing implants. Alginate-encapsulated islets formed amyloid during culture when functional, and in all intraperitoneal implants that corrected diabetes in mice, exhibiting larger sizes than in functioning nonencapsulated implants. After intraperitoneal injection in a patient, retrieved single capsules presented amyloid near living beta cells, whereas no amyloid occurred in clustered capsules with dead cells. Amyloid was also demonstrated in functional human stem cell-generated beta cell implants in subcutaneous devices of mice. Deposits up to 35 µm diameter were localized in beta cell-enriched regions and related to an elevated IAPP over insulin ratio in the newly generated beta cells. Amyloid in device-encapsulated human stem cell-generated beta cell implants marks the formation of a functional beta cell mass but also an imbalance between its activated state and its microenvironment.     

Islet damage during isolation as assessed by miRNAs and the correlation of miRNA levels with posttransplantation outcome in islet autotransplantation

High‐quality pancreatic islets are essential for better posttransplantation endocrine function in total pancreatectomy with islet autotransplantation (TPIAT), yet stress during the isolation process affects quality and yield. We analyzed islet‐enriched microRNAs (miRNAs) ‐375 and ‐200c released during isolation to assess damage and correlated the data with posttransplantation endocrine function. The absolute concentration of miR‐375, miR‐200c, and C‐peptide was measured in various islet isolation steps, including digestion, dilution, recombination, purification, and bagging, in 12 cases of TPIAT. Posttransplantation glycemic control was monitored through C‐peptide, hemoglobin A_1c, insulin requirement, and SUITO index. The amount of miR‐375 released was significantly higher during enzymatic digestion followed by the islet bagging (P < .001). Mir‐200c mirrored these changes, albeit at lower concentrations. In contrast, the C‐peptide amount was significantly higher in the purification and bagging steps (P < .001). Lower amounts of miR‐375 were associated with a lower 6‐month insulin requirement (P = .01) and lower hemoglobin A_1c (P = .04). Measurement of the absolute quantity of miRNA‐375 and ‐200c released during islet isolation is a useful tool to assess islet damage. The quantity of released miRNA is indicative of posttransplantation endocrine function in TPIAT patients.     

Novel cell‐permeable p38‐MAPK inhibitor efficiently prevents porcine islet apoptosis and improves islet graft function

During islet transplantation, mitogen‐activated protein kinase (MAPK) p38 is preferentially activated in response to the isolation of islets and the associated inflammation. Although therapeutic effects of p38 inhibitors are expected, the clinical application of small‐molecule inhibitors of p38 is not recommended because of their serious adverse effects on the liver and central nervous system. Here we designed peptides to inhibit p38, which were derived from the sites on p38 that mediate binding to proteins such as MAPK kinases. Peptide 11R‐p38I₁₁₀ significantly inhibited the activation of p38. To evaluate the effects of 11R‐p38I₁₁₀, porcine islets were incubated with 10 µmol/L 11R‐p38I₁₁₀ or a mutant form designated 11R‐mp38I₁₁₀. After islet transplantation, blood glucose levels reached the normoglycemic range in 58.3% and 0% of diabetic mice treated with 11R‐p38I₁₁₀ or 11R‐mp38I₁₁₀, respectively. These data suggest that 11R‐p38I₁₁₀ inhibited islet apoptosis and improved islet function. Peptide p38I₁₁₀ is a noncompetitive inhibitor of ATP and targets a unique docking site. Therefore, 11R‐p38I₁₁₀ specifically inhibits p38 activation, which may avoid the adverse effects that have discouraged the clinical use of small‐molecule inhibitors of p38. Moreover, our methodology to design “peptide inhibitors” could be used to design other inhibitors derived from the binding sites of proteins.     

Key Matrix Proteins Within the Pancreatic Islet Basement Membrane Are Differentially Digested During Human Islet Isolation

Clinical islet transplantation achieves insulin independence in selected patients, yet current methods for extracting islets from their surrounding pancreatic matrix are suboptimal. The islet basement membrane (BM) influences islet function and survival and is a critical marker of islet integrity following rodent islet isolation. No studies have investigated the impact of islet isolation on BM integrity in human islets, which have a unique duplex structure. To address this, samples were taken from 27 clinical human islet isolations (donor age 41–59, BMI 26–38, cold ischemic time < 10 h). Collagen IV, pan‐laminin, perlecan and laminin‐α5 in the islet BM were significantly digested by enzyme treatment. In isolated islets, laminin‐α5 (found in both layers of the duplex BM) and perlecan were lost entirely, with no restoration evident during culture. Collagen IV and pan‐laminin were present in the disorganized BM of isolated islets, yet a significant reduction in pan‐laminin was seen during the initial 24 h culture period. Islet cytotoxicity increased during culture. Therefore, the human islet BM is substantially disrupted during the islet isolation procedure. Islet function and survival may be compromised as a consequence of an incomplete islet BM, which has implications for islet survival and transplanted graft longevity.     

Pancreatic islets engineered with a FasL protein induce systemic tolerance at the induction phase that evolves into long‐term graft‐localized immune privilege

We have previously shown that pancreatic islets engineered to transiently display a modified form of FasL protein (SA‐FasL) on their surface survive indefinitely in allogeneic recipients without a need for chronic immunosuppression. Mechanisms that confer long‐term protection to allograft are yet to be elucidated. We herein demonstrated that immune protection evolves in two distinct phases; induction and maintenance. SA‐FasL‐engineered allogeneic islets survived indefinitely and conferred protection to a second set of donor‐matched, but not third‐party, unmanipulated islet grafts simultaneously transplanted under the contralateral kidney capsule. Protection at the induction phase involved a reduction in the frequency of proliferating alloreactive T cells in the graft‐draining lymph nodes, and required phagocytes and TGF‐β. At the maintenance phase, immune protection evolved into graft site‐restricted immune privilege as the destruction of long‐surviving SA‐FasL‐islet grafts by streptozotocin followed by the transplantation of a second set of unmanipulated islet grafts into the same site from the donor, but not third party, resulted in indefinite survival. The induced immune privilege required both CD4⁺CD25⁺Foxp3⁺ Treg cells and persistent presence of donor antigens. Engineering cell and tissue surfaces with SA‐FasL protein provides a practical, efficient, and safe means of localized immunomodulation with important implications for autoimmunity and transplantation.     

Assessment of plasma microvesicles to monitor pancreatic islet graft dysfunction: Beta cell‐ and leukocyte‐derived microvesicles as specific features in a pilot longitudinal study

Markers of early pancreatic islet graft dysfunction and its causes are lacking. We monitored 19 type 1 diabetes islet‐transplanted patients for up to 36 months following last islet injection. Patients were categorized as Partial (PS) or complete (S) Success, or Graft Failure (F), using the β‐score as an indicator of graft function. F was the subset reference of maximum worsened graft outcome. To identify the immune, pancreatic, and liver contribution to the graft dysfunction, the cell origin and concentration of circulating microvesicles (MVs) were assessed, including MVs from insulin‐secreting β‐cells typified by polysialic acid of neural cell adhesion molecule (PSA‐NCAM), and data were compared with values of the β‐score. Similar ranges of PSA‐NCAM⁺‐MVs were found in healthy volunteers and S patients, indicating minimal cell damage. In PS, a 2‐fold elevation in PSA‐NCAM⁺‐MVs preceded each β‐score drop along with a concomitant rise in insulin needs, suggesting β‐cell damage or altered function. Significant elevation of liver asialoglycoprotein receptor (ASGPR)⁺‐MVs, endothelial CD105⁺‐MVs, neutrophil CD66b⁺‐MVs, monocyte CD 14⁺‐MVs, and T4 lymphocyte CD4⁺‐MVs occurred before each β‐score drop, CD8⁺‐MVs increased only in F, and B lymphocyte CD19⁺‐MVs remained undetectable. In conclusion, PSA‐NCAM⁺‐MVs are noninvasive early markers of transplant dysfunction, while ASGPR⁺‐MVs signal host tissue remodeling. Leukocyte MVs could identify the cause of graft dysfunction.     

Ten-year outcomes of islet transplantation in patients with type 1 diabetes: Data from the Swiss-French GRAGIL network

To describe the 10-year outcomes of islet transplantation within the Swiss-French GRAGIL Network, in patients with type 1 diabetes experiencing high glucose variability associated with severe hypoglycemia and/or with functional kidney graft. We conducted a retrospective analysis of all subjects transplanted in the GRAGIL-1c and GARGIL-2 islet transplantation trials and analyzed components of metabolic control, graft function and safety outcomes over the 10-year period of follow-up. Forty-four patients were included between September 2003 and April 2010. Thirty-one patients completed a 10-year follow-up. Ten years after islet transplantation, median HbA1c was 7.2% (6.2–8.0) (55 mmol/mol [44–64]) versus 8.0% (7.1–9.1) (64 mmol/mol [54–76]) before transplantation (p < .001). Seventeen of 23 (73.9%) recipients were free of severe hypoglycemia, 1/21 patients (4.8%) was insulin-independent and median C-peptide was 0.6 ng/ml (0.2–1.2). Insulin requirements (UI/kg/day) were 0.3 (0.1–0.5) versus 0.5 (0.4–0.6) before transplantation (p < .001). Median (IQR) β-score was 1 (0–4) (p < .05 when comparing with pre-transplantation values) and 51.9% recipients had a functional islet graft at 10 years. With a 10-year follow-up in a multicentric network, islet transplantation provided sustained improvement of glycemic control and was efficient to prevent severe hypoglycemia in almost 75% of the recipients.     

Co‐localized immune protection using dexamethasone‐eluting micelles in a murine islet allograft model

The broad application of ß cell transplantation for type 1 diabetes is hindered by the requisite of lifelong systemic immunosuppression. This study examines the utility of localized islet graft drug delivery to subvert the inflammatory and adaptive immune responses. Herein, we have developed and characterized dexamethasone (Dex) eluting Food and Drug Administration‐approved micro‐Poly(lactic‐co‐glycolic acid) micelles and examined their efficacy in a fully major histocompatibility complex‐mismatch murine islet allograft model. A clinically relevant dose of 46.6 ± 2.8 μg Dex per graft was confirmed when 2 mg of micelles was implemented. Dex‐micelles + CTLA‐4‐Ig (n = 10) resulted in prolonged allograft function with 80% of the recipients demonstrating insulin independence for 60 days posttransplant compared to 40% in empty micelles + CTLA‐4‐Ig recipients (n = 10, P = .06). Recipients of this combination therapy (n = 8) demonstrated superior glucose tolerance profiles, compared to empty micelles + CTLA‐4‐Ig recipients (n = 4, P < .05), and significantly reduced localized intragraft proinflammatory cytokine expression. Histologically, increased insulin positive and FOXP3⁺ T cells were observed in Dex‐micelles + CTLA‐4‐Ig grafts compared to empty micelles + CTLA‐4‐Ig grafts (P < .01 and P < .05, respectively). Localized drug delivery via micelles elution has the potential to alter the inflammatory environment, enhances allograft survival, and may be an important adjuvant approach to improve clinical islet transplantation outcomes.