An evaluation of endogenous pancreatic enzyme levels after human islet isolation

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.

Comparison of Clostripain and Neutral Protease as Supplementary Enzymes for Human Islet Isolation

Although human islet transplantation has been established as valid and safe treatment for patients with type 1 diabetes, the utilization rates of human pancreases for clinical islet transplantation are still limited and substantially determined by the quality and composition of collagenase blends. While function and integrity of collagenase has been extensively investigated, information is still lacking about the most suitable supplementary neutral proteases. The present study compared islet isolation outcome after pancreas digestion by means of collagenase used alone or supplemented with either neutral protease (NP), clostripain (CP), or both proteases. Decent amounts of islet equivalents (IEQ) were isolated using collagenase alone (3090 ± 550 IEQ/g), or in combination with NP (2340 ± 450 IEQ/g) or CP (2740 ± 280 IEQ/g). Nevertheless, the proportion of undigested tissue was higher after using collagenase alone (21.1 ± 1.1%, P < 0.05) compared with addition of NP (13.3 ± 2.2%) or CP plus NP (13.7 ± 2.6%). Likewise, the percentage of embedded islets was highest using collagenase only (13 ± 2%) and lowest adding NP plus CP (4 ± 1%, P < 0.01). The latter combination resulted in lowest post-culture overall survival (42.7 ± 3.9%), while highest survival was observed after supplementation with CP (74.5 ± 4.8%, P < 0.01). An insulin response toward glucose challenge was present in all experimental groups, but the stimulation index was significantly decreased using collagenase plus NP (2.0 ± 0.12) compared with supplementation with CP (3.16 ± 0.4, P < 0.001). This study demonstrates for the first time that it is possible to isolate significant numbers of human islets combining collagenase only with CP. The supplementation with CP is an effective means to substantially reduce NP activity, which significantly decreases survival and viability after culture. This will facilitate the manufacturing of enzyme blends with less harmful characteristics.

Islet Culture/Preservation Before Islet Transplantation

Although islet culture prior to transplantation provides flexibility for the evaluation of isolated islets and the pretreatment of patients, it is well known that isolated islets deteriorate rapidly in culture. Human serum albumin (HSA) is used for medium supplementation instead of fetal bovine serum (FBS), which is typically used for islet culture research, to avoid the introduction of xenogeneic materials. However, FBS contains several factors that are beneficial to islet viability and which also neutralize the endogenous pancreatic enzymes or exogenous enzymes left over from the isolation process. Several groups have reported the comparison of cultures at 22°C and 37°C. Recent studies have demonstrated the superiority of 4°C preservation to 22°C and 37°C cultures. We herein review the current research on islet culture/preservation for clinical islet transplantation.

Human Serum Versus Human Serum Albumin Supplementation in Human Islet Pretransplantation Culture: In Vitro and In Vivo Assessment

There is conflicting evidence favoring both the use of human serum (HS) and of human serum albumin (HSA) in human islet culture. We evaluated the effects of HS versus HSA supplementation on 1) in vitro β-cell viability and function and 2) in vivo islet graft revascularization, islet viability, β-cell death, and metabolic outcome after transplantation. Islets isolated from 14 cadaveric organ donors were cultured for 3 days in CMRL 1066 medium supplemented with HS or HSA. After 3 days in culture, β-cell apoptosis was lower in HS group (1.41 ± 0.27 vs. 2.38 ± 0.39%, p = 0.029), and the recovery of islets was 77 ± 11% and 54 ± 1% in HS- and HSA-cultured groups, respectively. Glucose-stimulated insulin secretion (GSIS) was higher in HS group (29.4, range 10.4-99.9, vs. 22.3, range 8.7-70.6, p = 0.031). In vivo viability and revascularization was determined in HS- and HSA-cultured islets transplanted into the anterior chamber of the eye of Balb/c mice (n = 14), and β-cell apoptosis in paraffin-embedded mouse eyes. Islet viability and β-cell apoptosis were similar in both groups. Revascularization was observed in one graft (HS group) on day 10 after transplantation. Islet function was determined in streptozotocin (STZ)-diabetic nude mice (n = 33) transplanted with 2,000 IEQs cultured with HS or HSA that showed similar blood glucose levels and percentage of normoglycemic animals over time. In conclusion, human islets cultured in medium supplemented with HS showed higher survival in vitro, as well as islet viability and function. The higher in vitro survival increased the number of islets available for transplantation. However, the beneficial effect on viability and function did not translate into an improved metabolic evolution when a similar number of HSA- and HS-cultured islets was transplanted.

Supplements in human islet culture: human serum albumin is inferior to fetal bovine serum

Culture of human islets before clinical transplantation or distribution for research purposes is standard practice. At the time the Edmonton protocol was introduced, clinical islet manufacturing did not include culture, and human serum albumin (HSA), instead of fetal bovine serum (FBS), was used during other steps of the process to avoid the introduction of xenogeneic material. When culture was subsequently introduced, HSA was also used for medium supplementation instead of FBS, which was typically used for research islet culture. The use of HSA as culture supplement was not evaluated before this implementation. We performed a retrospective analysis of 103 high-purity islet preparations (76 research preparations, all with FBS culture supplementation, and 27 clinical preparations, all with HSA supplementation) for oxygen consumption rate per DNA content (OCR/DNA; a measure of viability) and diabetes reversal rate in diabetic nude mice (a measure of potency). After 2-day culture, research preparations exhibited an average OCR/DNA 51% higher (p < 0.001) and an average diabetes reversal rate 54% higher (p < 0.05) than clinical preparations, despite 87% of the research islet preparations having been derived from research-grade pancreata that are considered of lower quality. In a prospective paired study on islets from eight research preparations, OCR/DNA was, on average, 27% higher with FBS supplementation than that with HSA supplementation (p < 0.05). We conclude that the quality of clinical islet preparations can be improved when culture is performed in media supplemented with serum instead of albumin.

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.

Insulin degradation by acinar cell proteases creates a dysfunctional environment for human islets before/after transplantation: benefits of α-1 antitrypsin treatment

BACKGROUND

Pancreatic acinar cells are commonly cotransplanted along with islets during auto- and allotransplantations. The aims of this study were to identify how acinar cell proteases cause human islet cell loss before and after transplantation of impure islet preparations and to prevent islet loss and improve function with supplementation of α-1 antitrypsin (A1AT).

METHODS

Acinar cell protease activity, insulin levels, and percent islet loss were measured after culture of pure and impure clinical islet preparations. The effect of proteases on ultrastructure of islets and β-cell insulin granules were examined by transmission electron microscopy. The number of insulin granules and insulin-labeled immunogold particles were counted. The in vivo effect of proteases on islet function was studied by transplanting acinar cells adjacent to islet grafts in diabetic mice. The effects of A1AT culture supplementation on protease activity, insulin levels, and islet function were assessed in pure and impure islets.

RESULTS

Islet loss after culture was significantly higher in impure relative to pure preparations (30% vs. 14%, P<0.04). Lower islet purity was associated with increased protease activity and decreased insulin levels in culture supernatants. Reduced β-cell insulin granules and insulin degradation by proteases were confirmed by transmission electron microscopy. Transplantations in mice showed delayed islet graft function when acinar cells were transplanted adjacent to the islets under the kidney capsule. Supplementation of A1AT to impure islet cultures maintained islet cell mass, restored insulin levels, and preserved islet functional integrity.

CONCLUSION

Culture of impure human islet fractions in the presence of A1AT prevents insulin degradation and improves islet recovery.

Gene expression profiling of cultured human islet preparations

The expression of functional and regulatory genes by islet cells is a key determinant for the success of islet transplantation. The aim of this study is twofold: first, to characterize the cluster of genes expressed in human islet isolations; and second, to validate the capability of gene array technology to assess with accuracy the expression of various transcripts. RNA from isolated islet preparations obtained from three independent donors was converted to cDNA and then transcribed to cRNA. Individual cRNA preparations were then hybridized to U133A microarrays carrying approximately 23,000 genes, and analyzed using GeneSpring (SiliconGenetics, Redwood City, CA) software. Real-time reverse transcription-polymerase chain reaction was performed to validate results obtained by microarray analysis. Microarray analysis identified the expression of about 7,000 genes transcribed in cultured human islet preparations. Enzymes represented the most abundant class of genes identified, followed by nuclear binding proteins, signal transduction molecules, transport proteins, and growth factor receptors and their ligands. Real-time polymerase chain reaction confirmed the identification of various islet-specific genes detected by microarray analysis, but also showed that such genes as pancreatic duodenal homeobox 1 protein and glucagon-like peptide 1 receptor, which were not detected by gene array, can be readily identified and quantified. In addition, gene array produced a suboptimal quantification of genes expressed in large amounts by islet cells. Indeed, the abundance of mRNA for insulin when compared with the level of somatostatin mRNA was not as different as one would have predicated based on the classic knowledge of islet physiology. Gene array analysis appears to be a valuable tool to obtain preliminary information of genes expressed by a given tissue. The expression levels of transcripts expressed in very low or very high quantities need to be confirmed by an independent technique.