Optimization in osmolality and range of density of a continuous ficoll-sodium-diatrizoate gradient for isopycnic purification of isolated human islets

PRISM: A Novel Human Islet Isolation Technique

BACKGROUND

Successful pancreatic islet isolations are a key requirement for islet transplantation in selected patients with type 1 diabetes. However, islet isolation is a technically complex, time-consuming, and manual process. Optimization and simplification of the islet isolation procedure could increase islet yield and quality, require fewer operators, and thus reduce cost.

METHODS

We developed a new, closed system of tissue collection, washing, buffer change, and islet purification termed PancReatic Islet Separation Method (PRISM). In the developmental phase, pump and centrifuge speed was tested using microspheres with a similar size, shape, and density as digested pancreatic tissue. After optimization, PRISM was used to isolate islets from 10 human pancreases.

RESULTS

Islet equivalents viability (fluorescein diacetate/propidium iodide), morphology, and dynamic glucose-stimulated insulin secretion were evaluated. PRISM could be performed by 1 operator in 1 flow cabinet. A similar islet yield was obtained using PRISM compared to the traditional islet isolation method (431 234 ± 292 833 versus 285 276 ± 197 392 islet equivalents, P = 0.105). PRISM islets had similar morphology and functionality.

CONCLUSIONS

PRISM is a novel islet isolation technique that can significantly improve islet isolation efficiency using fewer operators.

Pancreatic Islet Purification from Large Mammals and Humans Using a COBE 2991 Cell Processor versus Large Plastic Bottles

The islet purification step in clinical islet isolation is important for minimizing the risks associated with intraportal infusion. Continuous density gradient with a COBE 2991 cell processor is commonly used for clinical islet purification. However, the high shear force involved in the purification method using the COBE 2991 cell processor causes mechanical damage to the islets. We and other groups have shown human/porcine islet purification using large cylindrical plastic bottles. Shear stress can be minimized or eliminated using large cylindrical plastic bottles because the bottles do not have a narrow segment and no centrifugation is required during tissue loading and the collection processes of islet purification. This review describes current advances in islet purification from large mammals and humans using a COBE 2991 cell processor versus large cylindrical plastic bottles.

"Old School" Islet Purification Based on the Unit Gravity Sedimentation as a Rescue Technique for Intraportal Islet Transplantation-A Case Report

Here, we present a case that required a supplemental "old school" islet purification for a safe intraportal infusion. Following pancreas procurement from a brain-dead 26-year-old male donor (body mass index: 21.9), 24.6 ml of islet tissue was isolated after continuous density gradient centrifugation. The islet yield was 504,000 islet equivalent (IEQ), distributed among the following three fractions: 64,161 IEQ in 0.6 ml of pellet, 182,058 IEQ in 10 ml, and 258,010 IEQ in 14 ml with 95%, 20%, and 10% purity, respectively. After a 23-h culture, we applied supplemental islet purification, based on the separation of tissue subfractions during unit gravity sedimentation, a technique developed over 60 years ago ("old school"). This method enabled the reduction of the total pellet volume to 11.6 ml, while retaining 374,940 IEQ with a viability of over 90%. The final islet product was prepared in three infusion bags, containing 130,926 IEQ in 2.6 ml of pellet, 108,079 IEQ in 4 ml of pellet, and 135,935 IEQ in 5 ml of pellet with 65%, 40%, and 30% purity, respectively, and with the addition of unfractionated heparin (70 units/kg body weight). Upon the islet infusion from all three bags, portal pressure increased from 7 to 16 mmHg. Antithrombotic prophylaxis with heparin was continued for 48 h after the infusion, with target activated partial thromboplastin time 50-60 s, followed by fractionated heparin subcutaneous injections for 2 weeks. β-Cell graft function assessed on day 75 post-transplantation was good, according to Igls criteria, with complete elimination of severe hypoglycemic episodes and 50% reduction in insulin requirements. Time spent within the target glucose range (70-180 mg/dl) improved from 42% to 98% and HbA1c declined from 8.7% to 6.7%. Supplemental "old school" islet purification allowed for the safe and successful utilization of a robust and high-quality islet preparation, which otherwise would have been discarded.

Improved yield of canine islet isolation from deceased donors

BACKGROUND

Canine diabetes is a strikingly prevalent and growing disease, and yet the standard treatment of a twice-daily insulin injection is both cumbersome to pet owners and only moderately effective. Islet transplantation has been performed with repeated success in canine research models, but has unfortunately not been made available to companion animals. Standard protocols for islet isolation, developed primarily for human islet transplantation, include beating-heart organ donation, vascular perfusion of preservation solutions, specialized equipment. Unfortunately, these processes are prohibitively complex and expensive for veterinary use. The aim of the study was to develop a simplified approach for isolating canine islets that is compatible with the financial and logistical restrictions inherent to veterinary medicine for the purpose of translating islet transplantation to a clinical treatment for canine diabetes.

RESULTS

Here, we describe simplified strategies for isolating quality islets from deceased canine donors without vascular preservation and with up to 90 min of cold ischemia time. An average of more than 1500 islet equivalents per kg of donor bodyweight was obtained with a purity of 70% (N = 6 animals). Islets were 95% viable and responsive to glucose stimulation for a week. We found that processing only the body and tail of the pancreas increased isolation efficiency without sacrificing islet total yield. Islet yield per gram of tissue increased from 773 to 1868 islet equivalents when the head of the pancreas was discarded (N = 3/group).

CONCLUSIONS

In summary, this study resulted in the development of an efficient and readily accessible method for obtaining viable and functional canine islets from deceased donors. These strategies provide an ethical means for obtaining donor islets.

Comparison of Purification Solutions With Different Osmolality for Porcine Islet Purification

The osmolality of the purification solution is one of the most critical variables in human islet purification during islet isolation. We previously reported the effectiveness of a combined continuous density/osmolality gradient for the supplemental purification of human islets. We herein applied a combined continuous density/osmolality gradient for regular purification. The islets were purified with a continuous density gradient without osmolality preparation [continuous density/normal osmolality (CD/NO)] or continuous density/osmolality solution with osmolality preparation by 10× Hank's balanced salt solution (HBSS) [continuous density/continuous osmolality (CD/CO)]. The osmolality of the low-density solution was 400 mOsm/kg in both groups and that of the high-density solution was 410 mOsm/kg in the CD/NO group and 500 mOsm/kg in the CD/CO group. Unexpectedly, we noted no significant differences between the two solutions in terms of the islet yield, rate of viability and purity, score, stimulation index, or the attainability and suitability of posttransplantation normoglycemia. Despite reports that the endocrine and exocrine tissues of pancreata have distinct osmotic sensitivities and that high-osmolality solutions result in greater purification efficiency, the isolation and transplant outcomes did not markedly differ between the two purification solutions with different osmolalities in this study.

Pancreatic Islets: Methods for Isolation and Purification of Juvenile and Adult Pig Islets

The current situation of organ transplantation is mainly determined by the disbalance between the number of available organs and the number of patients on the waiting list. This obvious dilemma might be solved by the transplantation of porcine organs into human patients. The metabolic similarities which exist between both species made pancreatic islets of Langerhans to that donor tissue which will be most likely transplanted in human recipients. Nevertheless, the successful isolation of significant yields of viable porcine islets is extremely difficult and requires extensive experiences in the field. This review is focussing on the technical challenges, pitfalls and particularities that are associated with the isolation of islets from juvenile and adult pigs considering donor variables that can affect porcine islet isolation outcome.

Clinical Islet Isolation

The overarching success of islet transplantation relies on the success in the laboratory to isolate the islets. This chapter focuses on the processes of human islet cell isolation and the ways to optimally provide islet cells for transplantation. The major improvements in regards to the choice of enzyme type, way the digested pancreas tissue is handled to best separate islets from the acinar and surrounding tissues, the various methods of purification of the islets, their subsequent culture and quality assurance to improve outcomes to culminate in safe and effective islet transplantation will be discussed. After decades of improvements, islet cell isolation and transplantation now clearly offer a safe, effective and feasible therapeutic treatment option for an increasing number of patients suffering from type 1 diabetes specifically for those with severe hypoglycaemic unawareness.

An effective method to release human islets from surrounding acinar cells with agitation in high osmolality solution

INTRODUCTION

Islet purification is mainly performed by the density gradient method. However, purification of the embedded islets that are surrounded by exocrine tissue should be difficult, because their density is similar to exocrine tissue. In this study, we performed chart review to assess the relationship between the ratio of embedded islets and efficacy of purification. Then, we tested several conditions of a new method to free the islets from surrounded exocrine tissues using high osmolality solution with gentle agitation.

MATERIALS AND METHODS

First, we performed chart review of our human islet isolation. Second, embedded islet-enriched human islet fractions (embedded islets >50%) were suspended in University of Wisconsin (UW) solution (UW group, 320 mOsm/kg/H(2)0) or osmolality-adjusted UW solution (400, 500, and 600 mOsm/kg/H(2)0; 400 group, 500 group, and 600 group, respectively). Each tube was gently shaken at 4°C. The tissue samples were taken before shaking and after 15, 30, and 60 minutes. Islet yield, percentage of embedded islets, and viabilities were assessed.

RESULTS

The chart review revealed that high ratio of embedded islets deteriorated the efficacy of islet purification. The islet yield in all groups except for the 600 group did not change at 15 minutes, but it decreased in all groups at 60 minutes. The average percentage of embedded islets before shaking was 62.6%. Although percentage of embedded islets were decreasing in all groups, it was < 20% at 15 minutes in the 500 and 600 groups whereas it was >44% in the UW group, which indicated that higher osmolality would have a greater effect. Viability was >95% in all groups at 30 minutes.

CONCLUSIONS

The embedded islets deteriorated the efficacy of islet purification. Gentle agitation of embedded islets in high osmolality (500 mOsm/kg/H(2)O, 15 minutes) could release islets from surrounded exocrine tissue.

Improved Islet Purity by the Hypertonic-Hypotonic Method

Introduction

Islet purification is usually performed using the density gradient separation method, but the purity of islets is low because exocrine cells and the embedded islets are hard to remove by using only the density gradient method. The aim of this study was to establish a new islet purification process comprising a hypertonic-hypotonic treatment step followed by a density gradient centrifugation step to improve the purity of islets.

Methods

The Plackett-Burman method was used to determine which factors had a significant influence on the purity of islets obtained after the hypertonic-hypotonic treatment step.

Results

The hypertonic solution concentration and the incubation time were both found to have a significant effect on islet purity. The purity of islets obtained using the modified purification process was significantly higher than that of islets obtained by density gradient alone (97% vs. 87.23%). Importantly, good cell viability and normal insulin secretion ability of islets were maintained following the modified purification.

Conclusions

The new purification process allows isolation of islets with improved purity and does not compromise the viability or function of the islets.

Comparison of Incubation Solutions Prior to the Purification of Porcine Islet Cells

For pancreatic islet transplantation, one of the most important steps of islet isolation is islet purification. The most common method of islet purification is density gradient centrifugation because there are differences in density between islets and acinar tissue. However, the density of islets/acinar tissue depends on several conditions, such as the incubation time before purification and the osmolality of the preincubation solution. In this study, we evaluated the impact of using two different preincubation solutions before purification. We used the University of Wisconsin (UW) solution and a new preservation solution (HN-1), which we recently developed. There were no significant differences between the two solutions in terms of the islet yield, rate of viability, and purity or stimulation index after purification. There were also no differences in the attainability and suitability of posttransplantation normoglycemia. Our study shows that the HN-1 solution is equivalent to the UW solution for preincubation before islet purification.

Evaluation of osmolality of density gradient for human islet purification

For pancreatic islet transplantation, the most common method of islet purification is density gradient centrifugation because of the differences in density between islets and acinar tissue. The density of islets/acinar tissue depends on several conditions, such as osmolality of purification solution. In this study, we evaluated the osmolality of iodixanol-controlled density gradients (400, 450, and 500 mOsm/kg) on the islet purification step. The density of the purification solutions was controlled by changing the volumetric ratio of iodixanol and the purification solutions (iodixanol-Kyoto solutions; IK solutions). The osmolality of density gradients was controlled by addition of 10× Hanks balanced salt solution (HBSS) solution. Density of both islets and acinar tissue increased relative to increase of the osmolality of purification solutions. There were no significant differences among the three groups on islet yield after density-adjusted purification and the rate of postpurification recovery. In vitro and in vivo assays suggest that the quality of islets was similar among the three groups. Our data suggest that efficacy of purification and quality of isolated islets is similar when the osmolality of purification solutions is between 400 and 500 mOsm/kg and density adjustment is applied. Since the density of islet and acinar tissue is changed according to osmolality, the density adjustment is important when using several osmolality solutions.

A Combined Continuous Density/Osmolality Gradient for Supplemental Purification of Human Islets

For islet transplantation, islet purification minimizes the risks associated with islet infusion through the portal vein. However, islet purification may result in decreased numbers of islets recovered from digested tissue. In this study, we evaluated the effectiveness of performing supplemental purification (SP) after regular purification (RP). We designed the densities of low- and high-density solutions based on the outcome of RP. Moreover, a combined continuous osmolality/continuous density gradient for the SP was used in this study. Low-density/osmolality (1.075-1.110 g/cm³/400-410 mOsm/kg) and high-density/osmolality (1.090-1.125 g/cm³/495-505 mOsm/kg) solutions were produced by changing the volumetric ratio of iodixanol, 10 × HBSS, and RP solutions. The percentage of islet recovery (postpurification IE/prepurification IE × 100) after RP was 77.3 ± 5.6%, and the percentage of islet recovery after addition of SP was 85.3 ± 5.4%. In vitro and in vivo assessments showed that islet viability and function were not altered by the additional purification step. These data suggest that the addition of SP could contribute approximately 8% to islet recovery with viability and potency comparable to that obtained by RP and, therefore, that usage of the combined continuous density and continuous osmolality gradient for SP could efficiently improve islet equivalents in the final preparation.

Simplified method to isolate highly pure canine pancreatic islets

OBJECTIVES

The canine model has been used extensively to improve the human pancreatic islet isolation technique. At the functional level, dog islets show high similarity to human islets and thus can be a helpful tool for islet research. We describe and compare 2 manual isolation methods, M1 (initial) and M2 (modified), and analyze the variables associated with the outcomes, including islet yield, purity, and glucose-stimulated insulin secretion (GSIS).

METHODS

Male mongrel dogs were used in the study. M2 (n = 7) included higher collagenase concentration, shorter digestion time, faster shaking speed, colder purification temperature, and higher differential density gradient than M1 (n = 7).

RESULTS

Islet yield was similar between methods (3111.0 ± 309.1 and 3155.8 ± 644.5 islets/g, M1 and M2, respectively; P = 0.951). Pancreas weight and purity together were directly associated with the yield (adjusted R(2) = 0.61; P = 0.002). Purity was considerably improved with M2 (96.7% ± 1.2% vs 75.0% ± 6.3%; P = 0.006). M2 improved GSIS (P = 0.021). Independently, digestion time was inversely associated with GSIS.

CONCLUSIONS

We describe an isolation method (M2) to obtain a highly pure yield of dog islets with adequate β-cell glucose responsiveness. The isolation variables associated with the outcomes in our canine model confirm previous reports in other species, including humans.

A method for murine islet isolation and subcapsular kidney transplantation

Since the early pioneering work of Ballinger and Reckard demonstrating that transplantation of islets of Langerhans into diabetic rodents could normalize their blood glucose levels, islet transplantation has been proposed to be a potential treatment for type 1 diabetes ^1,2. More recently, advances in human islet transplantation have further strengthened this view ^1,3. However, two major limitations prevent islet transplantation from being a widespread clinical reality: (a) the requirement for large numbers of islets per patient, which severely reduces the number of potential recipients, and (b) the need for heavy immunosuppression, which significantly affects the pediatric population of patients due to their vulnerability to long-term immunosuppression. Strategies that can overcome these limitations have the potential to enhance the therapeutic utility of islet transplantation.

Islet transplantation under the mouse kidney capsule is a widely accepted model to investigate various strategies to improve islet transplantation. This experiment requires the isolation of high quality islets and implantation of islets to the diabetic recipients. Both procedures require surgical steps that can be better demonstrated by video than by text. Here, we document the detailed steps for these procedures by both video and written protocol. We also briefly discuss different transplantation models: syngeneic, allogeneic, syngeneic autoimmune, and allogeneic autoimmune.

Influence of the purification of human adult pancreatic islets on insulin secretion

BACKGROUND/AIM

The most effective method for human adult pancreatic islets purification is density-gradient centrifugation. The aim of this study was to analyze the effects of non-automated purification on preservation of functional capacity of human adult pancreatic islet cells.

METHODS

Human pancreata were obtained after pancreatectomy in the patients with chronic pancreatitis or benign tumors. Pancreatic islets were purified by non-automated method in discontinuous Ficoll density gradient. The samples were divided in 2 fractions: purified (P) and non-purified (NP) cultures. Islets were stained with diphenyl-thiocarbazone. The efficiency of separation was determined by comparing percentage of stained cells in P and NP cultures on day 1, 3 and 7 of shortterm cultivation. Glucose-stimulated insulin secretion was expressed as stimulation index (SI).

RESULTS

The results obtained showed a statistically significant difference (p < 0.01) between P and NP cultures. P cultures had higher percentages of stained cells (70.43 +/- 3.97%, 73.77 +/- 4.22% and 71.34 +/- 4.69% on the first, third and seventh day of cultivation, respectively) than NP cultures (53.68 +/- 1.71%, 57.14 +/- 3.94% and 43.97 +/- 4.56%, respectively). P cultures had higher values of SI for the first, third and seventh day of cultivation than NP cultures (0.45 +/- 0.08, 0.80 +/- 0.21, 1.28 +/- 0.15 and 0.46 +/- 0.10, 0.752 +/- .0.16, 0.76 +/- 0.11 for P and NP cultures respectively). The difference was statistically significant on day seven (p = 0.01).

CONCLUSION

Although during purification process islets were exposed to a number of insults that might result in cellular damage and functional impairment, our assessments showed that islets in P cultures preserved their functional capacity better than islets in NP cultures, since they had greater insulin secretion.

Human islet separation utilizing a closed automated purification system

A central step within the human islet isolation process is the separation of islets from contaminating exocrine tissue utilizing linear, continuous density gradients manufactured by means of manually controlled standard gradient makers (SGM). The present study was performed to develop a closed, automated purification system (APS) that customizes density gradient profiles aiming to standardize and optimize human islet purification. Digested human pancreata were pooled, split evenly, and incubated in UW solution according to our standard protocol (n = 11). Continuous density gradient centrifugation was performed in parallel in two refrigerated COBE 2991 cell separators loaded with light (1.076 g/ml) and heavy (1.097 g/ml) Ficoll utilizing either an SGM or two computer-controlled pumps connected to Ficoll-containing bags. Quality control included islet equivalent (IE) yield, purity, in vitro function, and islet cytokine expression. Gradient profiles demonstrated that the APS readily customizes linear and nonlinear gradients. In comparison to the SGM, the APS recovered a higher percentage of the expected volume of continuous gradients (90.0 +/- 1.1% vs. 98.2 +/- 2.0%, p < 0.05). Islet yield (120,468 +/- 15,970 vs. 114,570 +/- 15,313 IE, NS) and purity (51.7 +/- 4.8% vs. 54.4 +/- 4.9%, NS) were nearly identical utilizing the SGM or APS. Decreased MCP-1, IL-6, and IL-8 expression indicated that APS-purified islets were possibly exposed to less proinflammatory stress. Compared to standard procedures, similar success and gentle continuous density gradient separation of human islets is feasible utilizing the APS. The APS facilitates the standardization of this complex procedure according to cGMP standards.

Iodixanol-controlled density gradient during islet purification improves recovery rate in human islet isolation

BACKGROUND

For pancreatic islet transplantation, islet purification minimizes the risks associated with islet infusion through the portal vein by reducing the amount of transplanted tissue. However, the purification step may result in decreased numbers of islets recovered from digested tissue and be traumatic to the islets. In this study, we evaluated the effectiveness of iodixanol-controlled density gradients on the islet purification step.

METHODS

For 14.3% of the isolations, the density was 1.085 g/cm3, 32.1% were 1.090 g/cm3, 46.4% were 1.095 g/cm3, 3.6% were 1.100 g/cm3, and 3.6% were 1.105 g/cm3, indicating that the density varies with each isolation. This has profound implications for the difficulty of islet purification. According to the density of digested tissue before purification, the density of the purification solutions was controlled by changing the volumetric ratio of iodixanol and the purification solutions (iodixanol-Kyoto [IK] solutions).

RESULTS

Islet yield after purification and rate of postpurification recovery were significantly higher in the IK group than with standard continuous gradient purification by Ficoll solutions (islet yield=Ficoll group: 377,230+/-50,207 islet equivalents, IK group: 594,136+/-50,570 islet equivalents, P less than 0.01; percentage of recovery=Ficoll group: 55.6%+/-5.8%, IK group: 84.9%+/-4.2%, P less than 0.01). In vitro and in vivo assays suggest that the quality of islets was similar between the two groups.

CONCLUSION

Our data suggest that using an iodixanol-controlled density gradient improves the islet recovery rate in human islet isolation. On the basis of these data, we now use this purification method for clinical islet transplantation.