Superiority of small islets in human islet transplantation

Microassay for glucose-induced preproinsulin mRNA expression to assess islet functional potency for islet transplantation

BACKGROUND

The capacity for insulin synthesis in islets is important for islet transplantation to succeed. We developed a microassay that evaluates the potency of human islets by measuring changes in glucose-induced human insulin gene (INS) expression using a single islet in octuplicate samples.

METHODS

Poly (A) messenger RNA (mRNA) was purified from a set of single handpicked human islets. Glucose-induced mature (postspliced) and premature (prespliced) insulin mRNA were quantified by reverse-transcriptase polymerase chain reaction using several insulin mRNA primers designed at different locations including, intron, exon, and an exon-intron junction.

RESULTS

The synthesis of premature INS mRNA was significantly increased in islets exposed to high glucose for 16 vs. 4 hr (P<0.01), whereas mature INS mRNA showed no difference. Glucose-induced premature INS mRNA synthesis was attenuated in heat-damaged islets. Stimulation index (SI) calculated by normalizing premature by mature INS mRNA (SI_INS mRNA) positively correlated with SI of insulin release (SI_16h insulin) from the same set of islets during 16-hr incubation in high or low glucose media, and SI of glucose-mediated insulin release obtained from the same islet lot in a perifusion system (n=12). Furthermore, linear multiple regression analysis using SI_INS mRNA and SI_16h insulin predicted islet transplantation outcome in nonobese diabetic (NOD) scid mice (n=8).

CONCLUSION

The measurement of glucose-induced premature INS mRNA normalized by mature INS mRNA can be used to assess the functional quality of human islets and may predict islet function after transplantation in type 1 diabetic patients.

Enhanced prediction of porcine islet yield and posttransplant outcome using a combination of quantitative histomorphometric parameters and flow cytometry

Prediction of islet yield and posttransplant outcome is essential for clinical porcine islet xenotransplantation. Although several histomorphometric parameters of biopsied porcine pancreases are predictive of islet yield, their role in the prediction of in vivo islet potency is unknown. We investigated which histomorphometrical parameter best predicts islet yield and function, and determined whether it enhanced the predictive value of in vitro islet function tests for the prediction of posttransplant outcome. We analyzed the histomorphometry of pancreases from which 60 adult pig islet isolations were obtained. Islet function was assessed using the beta-cell viability index based on flow cytometry analysis, oxygen consumption rate, ADP/ATP ratio, and/or concurrent transplantation into NOD/SCID mice. Receiver operating characteristic (ROC) analysis revealed that only islet equivalent (IEQ)/cm(2) and the number of islets >200 microm in diameter significantly predicted an islet yield of >2000 IEQ/g (p < 0.001 for both) and in vivo islet potency (p = 0.024 and p = 0.019, respectively). Although not predictive of islet yield, a high proportion of large islets (>100 microm in diameter) best predicted diabetes reversal (p = 0.001). Multiple regression analysis revealed that the beta-cell viability index (p = 0.003) and the proportion of islets >100 microm in diameter (p = 0.048) independently predicted mean posttransplant blood glucose level (BGL). When BGL was estimated using both these parameters [area under the ROC curve (AUC), 0.868; 95% confidence interval (CI), 0.730-1.006], it predicted posttransplant outcome more accurately than the beta-cell viability index alone (AUC, 0.742; 95% CI, 0.544-0.939). In conclusion, we identified the best histomorphometric predictors of islet yield and posttransplant outcome. This further enhanced the predictive value of the flow cytometry analysis. These parameters should be useful for predicting islet yield and in vivo potency before clinical adult porcine islet xenotransplantation.

Positron emission tomography in clinical islet transplantation

The fate of islets in clinical transplantation is unclear. To elude on this positron emission tomography combined with computed tomography (PET/CT) was performed for 60 min during islet transplantation in five patients receiving six transplants. A fraction of the islets (23%) were labeled with 18F-fluorodeoxyglucose ([(18)F]FDG) and carefully mixed with unlabeled islets just prior to intraportal transplantation. The peak radioactivity concentration in the liver was found at 19 min after start of islet infusion and corresponded to only 75% of what was expected, indicating that islets are lost during the transplantation procedure. No accumulation of radioactivity was found in the lungs. A nonphysiological peak of C-peptide was found in plasma during and immediately after transplantation in all subjects. Distribution in the liver was heterogeneous with wide variations in location and concentration. Islets found in areas with concentrations of >400 IEQ/cc liver tissue varied between 1% and 32% of the graft in different subjects. No side effects attributed to the PET/CT procedure were found. Clinical outcome in all patients was comparable to that previously observed indicating that the [(18)F]FDG labeling procedure did not harm the islets. The technique has potential to be used to assess approaches to enhance islet survival and engraftment in clinical transplantation.

Quantitative assessment of islet cell products: estimating the accuracy of the existing protocol and accounting for islet size distribution

The ability to consistently and reliably assess the total number and the size distribution of isolated pancreatic islet cells from a small sample is of crucial relevance for the adequate characterization of islet cell preparations used for research or transplantation purposes. Here, data from a large number of isolations were used to establish a continuous probability density function describing the size distribution of human pancreatic islets. This function was then used to generate a polymeric microsphere mixture with a composition resembling those of isolated islets, which, in turn, was used to quantitatively assess the accuracy, reliability, and operator-dependent variability of the currently utilized manual standard procedure of quantification of islet cell preparation. Furthermore, on the basis of the best fit probability density function, which corresponds to a Weibull distribution, a slightly modified scale of islet equivalent number (IEQ) conversion factors is proposed that incorporates the size distribution of islets and accounts for the decreasing probability of finding larger islets within each size group. Compared to the current calculation method, these factors introduce a 4-8% downward correction of the total IEQ estimate, but they reflect a statistically more accurate contribution of differently sized islets.

Bioengineering of a functional sheet of islet cells for the treatment of diabetes mellitus

The present study was designed to establish a novel tissue engineering approach for diabetes mellitus (DM) by fabricating a tissue sheet composed of pancreatic islet cells for in vivo transplantation. Pancreatic islet cell suspensions were obtained from Lewis rats, and plated onto temperature-responsive culture dishes coated with extracellular matrix (ECM) proteins. After the cells reached confluency, islet cells cultured on laminin-5 coated dishes were successfully harvested as a uniformly spread tissue sheet by lowering the culture temperature to 20 degrees C for 20 min. The functional activity of the islet cell sheets was confirmed by histological examination and Insulin secretion assay prior to in vivo transplantation. Histological examination revealed that the harvested islet cell sheet was comprised of insulin- (76%) and glucagon- (19%) positive cells, respectively. In vivo functionality of the islet cell sheet was maintained even 7 days after transplantation into the subcutaneous space of Lewis rats. The present study describes an approach to generate a functional sheet of pancreatic islet cells on laminin-5 coated temperature-responsive dishes, which can be subsequently transplanted in vivo. This study serves as the foundation for the creation of a novel cell-based therapy for DM to provide patients an alternative method.

Transfection of rat pancreatic islet tissue by polymeric gene vectors

BACKGROUND

In vitro genetic modification has been regarded as one option to improve the viability and functionality of pancreatic islets when used for transplantation in patients with diabetes, either as naked islets or in a type of bioartificial pancreas. In this approach, vector safety and poor transfection efficiency are major concerns.

METHODS

In this study, the influence of in vitro transfection conditions on polyplexes constructed of polyethyleneimine (PEI) and plasmid DNA (pDNA) on the transfection efficiency was investigated by varying the transfection medium, the pDNA dose, and the amines of polycation/phosphates of pDNA (N/P) ratio.

RESULTS

Ca2+-containing Krebs-Ringer-HEPES medium was more effective than RPMI 1640 medium by increasing transfection efficiency (2.5-fold). An increase in pDNA dose slightly reduced the transfection efficiency but had minimal influence on islet loss. However, the N/P ratio had a large effect on islet viability and transfection efficiency. For example, the PEI/pDNA ratio at N/P = 10 caused greater islet loss (56% vs. 28%) and 30-fold less transfection efficiency than at N/P = 5. Even under a set of best conditions selected from this study, mostly a fraction of cells located in the peripheral regions of an islet were transfected, and the viability and insulin secretion from the treated islets were not altered. However, it was found that the extent of apoptosis was noticeably higher (approximately 16%) than in untreated islets (approximately 2%).

CONCLUSIONS

These results suggest that the gene delivery efficacy to isolated islets can be improved by manipulating the transfection conditions. Polymeric vectors will broaden the options for islet transfection, which is currently limited to viral vectors.

Immune cell populations in nonhuman primate islets

Islet transplantation is a promising cellular therapy for the treatment of type 1 diabetes (T1D). The immunogenicity of isolated islets has been of interest to the transplant community for many years, as upon transplantation, islets are damaged or destroyed through specific and nonspecific inflammatory and immune events. Antigen presenting cells (APC) are crucial intermediates in the generation of both innate and specific immune responses, and it has long been understood that some APC are resident in islets in situ, as well as after isolation. Our aim was to identify and characterize intraislet resident populations of APC and other immune cells in islets from nonhuman primates (Macaca fascicularis) in situ (pancreas biopsies obtained prerecovery) and after isolation using immunohistochemistry, confocal microscopy, and flow cytometry. The numbers of cells obtained in situ are similar to those in islets postisolation. Each isolated islet equivalent contains an average of 21.8 immune cells, 14.7 (67%) of which are APC. Many of these APC are dentritic cells and, surprisingly, 50% are B lymphocytes. The number of islet-resident immune cells increases with islet size, with greater numbers in large versus small islets (p < 0.001). The APC were localized around the exterior or spread evenly throughout the islets, with no definitive orientation identified. This knowledge will be useful to develop tailored modulation strategies to decrease immunogenicity, enhance engraftment, and ultimately prevent islet rejection.

ATP measurement predicts porcine islet transplantation outcome in nude mice

Current nude mice islet transplantation studies cannot be used prospectively. Therefore, to predict transplantation outcomes, reliable and rapid assays for islet quality assessment are warranted. This study evaluated the predictive power of the porcine islet ATP content on the outcomes of islet transplantation in nude mice. Here, we report that the ATP measurement using a small number of handpicked islets with a diameter of 100 to 150 mum is a good predictor of islet graft efficacy in nude mice. Using receiver-operator characteristic analysis, the area under the curve of the ATP content using a small number of handpicked islets was 0.867 (95% confidence interval 0.744-0.989, P<0.001). The sensitivity and the specificity measured were 83.3% and 73.3%, respectively. In conclusion, a simple and a rapid measurement of intraislet ATP content could be a promising substitute for current nude mice islet transplantation studies.

2007 update on allogeneic islet transplantation from the Collaborative Islet Transplant Registry (CITR)

As of October 1, 2007, 25 North American medical institutions and one European islet transplant center reported detailed information to the Registry on 315 allograft recipients, of which 285 were islet alone (IA) and 30 were islet after kidney (IAK). Of the 114 IA recipients expected at 4 years after their last infusion, 12% were insulin independent, 16% were insulin dependent with detectable C-peptide, 40% had no detectable C-peptide, and 32% had missing C-peptide data or were lost to follow-up. Of the IA recipients, 72% achieved insulin independence at least once over 3 years and multiple infusions. Factors associated with achievement of insulin independence included islet size >1.0 expressed as IEQs per islet number [hazard ratio (HR) = 1.5, p = 0.06], additional infusions given (HR = 1.5, p = 0.01), lower pretransplant HbA(1c) (HR = 1.2 each %-age unit, p = 0.02), donor given insulin (HR = 2, p = 0.003), daclizumab given at any infusion (HR = 1.9, p = 0.06), and shorter cold storage time (HR = 1.04, p = 0.03), mutually adjusted in a multivariate model. Severe hypoglycemia prevalence was reduced from 78-83% preinfusion to less than 5% throughout the first year post-last infusion, and to 18% adjusted for missing data at 3 years post-last infusion. In Year 1 post-first infusion for IA recipients, 53% experienced a Grade 3-5 or serious adverse event (AE) and 35% experienced a severe AE related to either an infusion procedure or immunosuppression. In Year 1 post-first infusion, 33% of IA subjects and 35% of IAK subjects had an AE related to the infusion procedure, while 35% of IA subjects and only 27% of IAK subjects had an AE related to the immunosuppression therapy. Five deaths were reported, of which two were classified as probably related to the infusion procedure or immunosuppression, and 10 cases of neoplasm, of which two were classified as probably related to the procedure or immunosuppression. Islet transplantation continues to show short-term benefits of insulin independence, normal or near normal HbA(1C) levels, and sustained marked decrease in hypoglycemic episodes.

Risk factors for islet loss during culture prior to transplantation

Culturing islets can add great flexibility to a clinical islet transplant program. However, a reduction in the islet mass has been frequently observed during culture and its degree varies. The aim of this study was to identify the risk factors associated with a significant islet loss during culture. One-hundred and four islet preparations cultured in an attempt to use for transplantation constituted this study. After culture for 20 h (median), islet yield significantly decreased from 363 309 +/- 12 647 to 313 035 +/- 10 862 islet equivalent yield (IE) (mean +/- SE), accompanied by a reduction in packed tissue volume from 3.9 +/- 0.1 to 3.0 +/- 0.1 ml and islet index (IE/islet particle count) from 1.20 +/- 0.04 to 1.05 +/- 0.04. Culture did not markedly alter islet purity or percent of trapped islet. Morphology score and viability were significantly improved after culture. Of 104 islet preparations, 37 suffered a substantial islet loss (> 20%) over culture. Factors significantly associated with risk of islet loss identified by univariate analysis were longer cold ischemia time, two-layer method (TLM) preservation, lower islet purity, and higher islet index. Multivariate analysis revealed that independent predictors of islet loss were higher islet index and the use of TLM. This study provides novel information on the link between donor- isolation factors and islet loss during culture.

Topographical associations between islet endocrine cells and duct epithelial cells in the adult human pancreas

OBJECTIVES

The pancreatic ducts, endocrine islets and exocrine acini are three functionally related components. From birth to adulthood, the islets and ducts are regarded as independent entities. The objective of this study is to investigate the topographical associations between the islet endocrine cells and duct epithelial cells in the adult human pancreas.

MATERIALS AND METHODS

Panels of immunomarkers for the exocrine acinar cells (amylase), duct cells [cytokeratin 19 (CK19)], endocrine cells (chromogranin A, neurone specific enolase, synaptophysin) and islet hormones (glucagon, insulin, somatostatin, pancreatic polypeptide) were applied to sequential pancreatic tissue sections obtained from autopsy specimens of 10-nondiabetic human adults. Double immunofluorescent staining with CK19 and islet hormones was performed to confirm the islet to duct interrelationship.

RESULTS

Sequential sectioning and immunostaining showed that 45% of the 172 islets examined appeared as single endocrine cell units or small clusters of < 10 endocrine cells on at least one plane of section. A topographical association was found between the islet endocrine cells and duct epithelial cells. Topographical associations with CK 19-stained duct cells occurred in 10.9% of the islet insulin-containing beta-cells and in 8.9% of the islet glucagon-producing alpha-cells. The frequency of topographical associations increased toward the more distally located duct systems. The CK19-stained duct cells and amylase-labelled acinar cells were less frequently in association with other islet hormone-producing cells.

CONCLUSIONS

Topographical associations between islet endocrine cells and pancreatic duct cells are frequent in adult human pancreas. The islet-duct association suggests possible functional interactions between the two interrelated pancreatic compartments.

Has time come for new goals in human islet transplantation?

The enthusiasm regarding clinical islet transplantation has been dampened by the long-term results. Concerns about the associated risks of life-long immunosuppression and the striking imbalance between potential recipients and available donor pancreata warrant changes in some of the current goals. Islet transplantation will never be a cure of type 1 diabetes in the majority of patients with no secondary complications, but is a valid option for a limited number of patients with brittle diabetes waiting for an organ or after organ transplantation. Furthermore, insulin independence should not be the main goal of islet transplantation, but avoidance of severe hypoglycemia and good glycemic control, which can be achieved with a relatively small functional beta-cell mass. Therefore, initially one islet infusion is sufficient. Retransplantation at a later time point remains an option, if glucose control deteriorates. Efforts to improve islet transplantation should no longer focus on islet isolation and immunosuppression, but rather on the low posttransplant survival rate of islets caused by activation of the coagulation pathway and the limited oxygen delivery to the islets. Transplantation of smaller islets be it naturally small or size tailored reaggregated islets has the potential to facilitate these processes.

Optimising islet engraftment is critical for successful clinical islet transplantation

Clinical islet transplantation is currently being explored as a treatment for persons with type 1 diabetes and hypoglycaemia unawareness. Although 'proof-of-principle' has been established in recent clinical studies, the procedure suffers from low efficacy. At the time of transplantation, the isolated islets are allowed to embolise the liver after injection in the portal vein, a procedure that is unique in the area of transplantation. A novel view on the engraftment of intraportally transplanted islets is presented that could explain the low efficacy of the procedure.

Rat pancreatic islet size standardization by the "hanging drop" technique

INTRODUCTION

Rejection and hypoxia are the main factors that limit islet engraftment in the recipient liver in the immediate posttransplant period. Recently authors have reported a negative relationship of graft function and islet size, concluding that small islets are superior to large islets. Islets can be dissociated into single cells and reaggregated into so called "pseudoislets," which are functionally equivalent to intact islets but exhibit reduced immunogenicity. The aim of our study was develop a technique that enabled one to obtain pseudoislets of defined, preferably small, dimensions.

MATERIALS AND METHODS

Islets were harvested from Lewis rats by the collagenase digestion procedure. After purification, the isolated islets were dissociated into single cells by trypsin digestion. Fractions with different cell numbers were seeded into single drops onto cell culture dishes, which were inverted and incubated for 5 to 8 days under cell culture conditions. Newly formed pseudoislets were analyzed for dimension, morphology, and cellular composition.

RESULTS

The volume of reaggregated pseudoislets strongly correlated with the cell number (r(2) = .995). The average diameter of a 250-cell aggregate was 95 +/- 8 microm (mean +/- SD) compared with 122 +/- 46 microm of freshly isolated islets. Islet cell loss may be minimized by performing reaggregation in the presence of medium glucose (11 mmol/L) and the GLP-1 analogue Exendin-4. Morphology, cellular composition, and architecture of reaggregated islets were comparable to intact islets.

CONCLUSION

The "hanging drop" culture method allowed us to obtain pseudoislets of standardized size and regular shape, which did not differ from intact islets in terms of cellular composition or architecture. Further investigations are required to minimize cell loss and test in vivo function of transplanted pseudoislets.

Investigating the role of islet cytoarchitecture in its oscillation using a new beta-cell cluster model

The oscillatory insulin release is fundamental to normal glycemic control. The basis of the oscillation is the intercellular coupling and bursting synchronization of β cells in each islet. The functional role of islet β cell mass organization with respect to its oscillatory bursting is not well understood. This is of special interest in view of the recent finding of islet cytoarchitectural differences between human and animal models. In this study we developed a new hexagonal closest packing (HCP) cell cluster model. The model captures more accurately the real islet cell organization than the simple cubic packing (SCP) cluster that is conventionally used. Using our new model we investigated the functional characteristics of β-cell clusters, including the fraction of cells able to burst f_b, the synchronization index λ of the bursting β cells, the bursting period T_b, the plateau fraction p_f, and the amplitude of intracellular calcium oscillation [Ca]. We determined their dependence on cluster architectural parameters including number of cells n_β, number of inter-β cell couplings of each β cell n_c, and the coupling strength g_c. We found that at low values of n_β, n_c and g_c, the oscillation regularity improves with their increasing values. This functional gain plateaus around their physiological values in real islets, at n_β∼100, n_c∼6 and g_c∼200 pS. In addition, normal β-cell clusters are robust against significant perturbation to their architecture, including the presence of non-β cells or dead β cells. In clusters with n_β>∼100, coordinated β-cell bursting can be maintained at up to 70% of β-cell loss, which is consistent with laboratory and clinical findings of islets. Our results suggest that the bursting characteristics of a β-cell cluster depend quantitatively on its architecture in a non-linear fashion. These findings are important to understand the islet bursting phenomenon and the regulation of insulin secretion, under both physiological and pathological conditions.