Long-chain CoA esters activate human pancreatic beta-cell KATP channels: potential role in Type 2 diabetes

AIMS/HYPOTHESIS

The ATP-regulated potassium (KATP) channel in the pancreatic beta cell couples the metabolic state to electrical activity. The primary regulator of the KATP channel is generally accepted to be changes in ATP/ADP ratio, where ATP inhibits and ADP activates channel activity. Recently, we showed that long-chain CoA (LC-CoA) esters form a new class of potent KATP channel activators in rodents, as studied in inside-out patches.

METHODS

In this study we have investigated the effects of LC-CoA esters in human pancreatic beta cells using the inside-out and whole-cell configurations of the patch clamp technique.

RESULTS

Human KATP channels were potently activated by acyl-CoA esters with a chain length exceeding 12 carbons. Activation by LC-CoA esters did not require the presence of Mg2+ or adenine nucleotides. A detailed characterization of the concentration-dependent relationship showed an EC50 of 0.7+/-0.1 micromol/l. Furthermore, in the presence of an ATP/ADP ratio of 10 (1.1 mmol/l total adenine nucleotides), whole-cell KATP channel currents increased approximately six-fold following addition of 1 micro mol/l LC-CoA ester. The presence of 1 micro mol/l LC-CoA in the recording pipette solution increased beta-cell input conductance, from 0.5+/-0.2 nS to 2.5+/-1.3 nS.

CONCLUSION/INTERPRETATION

Taken together, these results show that LC-CoA esters are potent activators of the KATP channel in human pancreatic beta cells. The fact that LC-CoA esters also stimulate KATP channel activity recorded in the whole-cell configuration, points to the ability of these compounds to have an important modulatory role of human beta-cell electrical activity under both physiological and pathophysiological conditions.

Angiogenic capacity of endothelial cells in islets of Langerhans

Transplantation of pancreatic islets reconstitutes glucose homeostasis in diabetes mellitus. Before transplantation, islets are disrupted from the surrounding blood vessels by the isolation procedure, with the grafted tissue being subject to ischemic damage. The survival of transplanted islets is assumed to depend on effective revascularization. Perfusion studies suggest that newly formed microvessels supplying the graft with nutrients are exclusively rebuilt by the host. It is generally not known whether isolated islets contain endothelial cells (EC), which potentially participate in the revascularization process. Therefore, we tried to detect immature EC in isolated islets by transformation with polyoma middle T antigen. Endothelioma cells were generated, implicating the presence of de-differentiated EC within isolated islets. When embedded in a fibrin gel, the islets developed cellular cords consisting of EC, whereas FGF-2 and VEGF stimulated the formation of cord-like structures. Furthermore, we studied the presence of donor EC in islet grafts by using transgenic mice with an EC lineage-specific promotor-LacZ reporter construct (Tie-2LacZ). Following islet transplantation, Tie-2LacZ-positive EC of both donor and recipient were identified in the vicinity of or within the graft up to 3 wk after transplantation. In conclusion, EC and/or their progenitors with angiogenic capacity reside within isolated islets of different species, and their proliferative potential can be stimulated by various inducers. These graft-related endothelia persist after islet transplantation and are integrated within newly formed microvessels.

Metabolic effects of restoring partial beta-cell function after islet allotransplantation in type 1 diabetic patients

Successful intraportal islet transplantation normalizes glucose metabolism in diabetic humans. To date, full function is not routinely achieved after islet transplantation in humans, with most grafts being characterized by only partial function. Moreover, the duration of full function is variable and cannot be sufficiently predicted with available methods. In contrast, most grafts retain partial function for a long time. We hypothesized that partial function can restore normal protein and lipid metabolism in diabetic individuals. We studied 45 diabetic patients after islet transplantation. Labeled glucose and leucine were infused to assess whole-body glucose and protein turnover in 1) 6 type 1 diabetic patients with full function after intraportal islet transplantation (FF group; C-peptide > 0.6 nmol/l; daily insulin dosage 0.03 +/- 0.02 U x kg(-1) body wt x day(-1); fasting plasma glucose < 7.7 mmol/l; HbA1c < or = 6.5%), 2) 17 patients with partial function (PF group; C-peptide > 0.16 nmol/l; insulin dosage < 0.4 U x kg(-1) body wt x day(-1)), 3) 9 patients with no function (NF group; C-peptide < 0.16 nmol/l; insulin dosage > 0.4 U x kg(-1) body wt x day(-1)), and 4) 6 patients with chronic uveitis as control subjects (CU group). Hepatic albumin synthesis was assessed in an additional five PF and five healthy volunteers by means of a primed-continuous infusion of [3,3,3-2H3]leucine. The insulin requirement was 97% lower than pretransplant levels for the FF group and 57% lower than pretransplant levels for the PF group. In the basal state, the PF group had a plasma glucose concentration slightly higher than that of the FF (P = 0.249) and CU groups (P = 0.08), but was improved with respect to the NF group (P < 0.01). Plasma leucine (101.1 +/- 5.9 micromol/l) and branched-chain amino acids (337.6 +/- 16.6 micromol/l) were similar in the PF, FF, and CU groups, and significantly lower than in the NF group (P < 0.01). During insulin infusion, the metabolic clearance rate of glucose was defective in the NF group versus in the other groups (P < 0.01). Both the basal and insulin-stimulated proteolytic and proteosynthetic rates were comparable in the PF, FF, and CU groups, but significantly higher in the NF group (P = 0.05). In addition, the PF group had a normal hepatic albumin synthesis. Plasma free fatty acid concentrations in the PF and FF groups were similar to those of the CU group, but the NF group showed a reduced insulin-dependent suppression during the clamp. We concluded that the restoration of approximately 60% of endogenous insulin secretion is capable of normalizing the alterations of protein and lipid metabolism in type 1 diabetic kidney recipients, notwithstanding chronic immunosuppressive therapy. The results of the present study indicate that "success" of islet transplantation may be best defined by a number of metabolic criteria, not just glucose concentration/metabolism alone.

Prevention of early islet graft failure by selective inducible nitric oxide synthase inhibitors after pig to nude rat intraportal islet transplantation

BACKGROUND

Clinical and experimental data indicate that early failure of intraportally grafted islets is caused by inflammation including secretion of cytokines and nitric oxide. Direct inducible nitric oxide synthase suppression may avoid detrimental effects associated with steroid administration. We compared the efficiency of selective and unselective inducible nitric oxide synthase inhibitors with dexamethasone to suppress nitric oxide generation after intraportal islet xenotransplantation into nude rats.

METHODS

Nonfasting serum glucose levels were daily evaluated after intraportal transplantation of 4000 freshly isolated pig islets into diabetic nude rats (85 mg/kg streptozotocin) either sham-treated with saline (n=21) or continuously infused for 7 days with L-NG-monomethyl-arginine (n=7), S-methyl-isothiourea (n=15), or S-(2-aminoethyl)-isothiourea (n=19) in a dosage of 240, 100, or 50 mg/kg/day, respectively. Dexamethasone was injected i.p. twice as a daily bolus of 20 mg/kg (n=10) starting 1 day pretransplant. The nitrate/nitrite serum level was quantified colorimetrically 0, 24, and 48 hr posttransplant.

RESULTS

Saline treatment partially resulted in graft function (4/21) throughout the observation period (21 days). L-NG-monomethyl-arginine-treated rats showed sustained hyperglycemia (0/7) not different from diabetic controls. Normoglycemia was observed after treatment with dexamethasone (6/10, P<0.05 versus saline and L-NG-monomethyl-arginine), S-methyl-isothiourea (10/15, P<0.01), or S-(2-aminoethyl)-isothiourea (15/19, P<0.001). Graft function was associated with complete suppression of nitric oxide generation after S-methyl-isothiourea and S-(2-aminoethyl)-isothiourea treatment (P<0.001 versus saline) and partial suppression after dexamethasone treatment (P<0.05).

CONCLUSIONS

Our observation of long-term function of xenogeneic islets in an inflammatory environment without interference of reactive T cells revealed the potency of highly selective isothioureas to completely suppress inducible nitric oxide synthase making reduction of islet-toxic immunosuppression feasible.

Pretransplant induction of HSP-70 in isolated adult pig islets decreases early islet xenograft survival

The heat-induced HSP-70 expression protects rat islet single cells against lysis mediated by nitric oxide (NO), reactive oxygen, and streptozotocin. The present study was performed to investigate the potential antiinflammatory effect of pretransplant heat shock in adult pig islets for subsequent early islet xenograft survival. Maximum HSP-70 expression in freshly isolated pig islets was induced by hyperthermia at 43 degrees C for 90 min prior to islet regeneration at 37 degrees C for 4-6 h. Heat-stressed and sham-treated islets were incubated in 0.6 mM H2O2 or 1.5 mM Na-nitroprusside at 37 degrees C for 20 h. Early graft survival was evaluated in normoglycemic Lewis rats after simultaneous, contralateral transplantation of heat-shocked islets and sham-treated islets into the renal subcapsular space of the same recipient. Prior hyperthermia significantly reduced specific lysis of islets exposed to NO or H2O2, although protection was only marginal. No differences were observed between viability of heat-shocked and sham-treated islets after NO exposure. In contrast, prior heat shock increased islet viability after H2O2 treatment. The finding that hyperthermia reduced recovery of initially grafted pig insulin 48 h after transplantation by 30% compared to controls contrasted significantly with an increased insulin recovery in heat-exposed islets at the end of simultaneous 37 degrees C culture. The observation, that the heat-induced HSP-70 expression decreases early islet xenograft survival as reflected by recovery of grafted insulin, implies an enhancement of islet immunogenicity and the induction of apoptosis. Future experiments aiming at augmentation of intrinsic defense mechanisms should consider detrimental effects associated with induction of heat shock proteins.

Incompatibility between human blood and isolated islets of Langerhans: a finding with implications for clinical intraportal islet transplantation?

The remarkable difference in success rates between clinical pancreas transplantation and islet transplantation is poorly understood. Despite the same histocompatibility barrier and similar immunosuppressive treatments in both transplantation procedures, human intraportal islet transplantation has a much inferior success rate than does vascularized pancreas transplantation. Thus far, little attention has been directed to the possibility that islets transplanted into the blood stream may elicit an injurious incompatibility reaction. We have tested this hypothesis in vitro with human islets and in vivo with porcine islets. Human islets were exposed to nonanticoagulated human ABO-compatible blood in surface-heparinized polyvinyl chloride tubing loops. Heparin and/or the soluble complement receptor 1 (sCR1) TP10 were tested as additives. Adult porcine islets were transplanted intraportally into pigs, and the liver was recovered after 60 min for immunohistochemical staining. Human islets induced a rapid consumption and activation of platelets. Neutrophils and monocytes were also consumed, and the coagulation and complement systems were activated. Upon histological examination, islets were found to be embedded in clots and infiltrated with CD11+ leukocytes. Furthermore, the cellular morphology was disrupted. When heparin and sCR1 were added to the blood, these events were avoided. Porcine islets retrieved in liver biopsies after intraportal islet allotransplantation showed a morphology similar to that of human islets perifused in vitro. Thus, exposure of isolated islets of Langerhans to allogenic blood resulted in significant damage to the islets, a finding that could explain the unsatisfactory clinical results obtained with intraportal islet transplantation. Because administration of heparin in combination with a soluble complement receptor abrogated these events, such treatment would presumably improve the outcome of clinical islet transplantation by reducing both initial islet loss and subsequent specific immune responses.

Significant progress in porcine islet mass isolation utilizing liberase HI for enzymatic low-temperature pancreas digestion

BACKGROUND

Frequent success in human islet isolation is prevented by the large variability of scarce organ donors; this favors the future utilization of pigs as donors for clinical islet xenotransplantation. Porcine-specific difficulties of islet isolation are attributed to the intrinsic fragility of islets during pancreas digestion.

METHODS

To preserve islet integrity during efficient pancreas dissociation, porcine pancreata (n=48) were distended after cold storage with cold University of Wisconsin solution containing Liberase HI and digested at 24-28 degrees C using digestion-filtration. Pancreata distended with University of Wisconsin solution containing well-proven crude collagenase and digested at 32-34 degrees C served as controls (n=46). Monolayer Ficolldiatrizoate gradient purification was performed in a Cobe 2991.

RESULTS

Purified yield of islet equivalents per pancreas (mean+/-SEM) was almost doubled by Liberase HI compared with crude collagenase (526,480+/-46,560 vs. 270,270+/-19,420; P < 0.0001) and also significantly increased comparing islet equivalents per gram of pancreas (4,210+/-320 vs. 2,640+/-245; P=0.0004). Islet integrity was better preserved during Liberase HI digestion compared with crude collagenase digestion as indicated by isolation index (2.1+/-0.1 vs. 1.4+/-0.1; P<0.0001). Purity, viability, and in vitro function of islets did not differ between experimental groups. Preserved in vivo function of islets isolated by Liberase HI was demonstrated after subcapsular transplantation into 16 diabetic nude rats.

CONCLUSIONS

If the problems related to xenograft rejection and xenosis could be solved, low-temperature digestion of porcine pancreata using Liberase HI could serve as an essential prerequisite for successful 1:1 xenotransplantation of pig islets into type 1 diabetic human recipients.

Long-term survival, morphology and in vitro function of isolated pig islets under different culture conditions

BACKGROUND

Islet culture aims to optimize islet survival and to reduce islet immunogenicity. To achieve these objectives, culture periods at 37 degrees C and 22-24 degrees C are mainly used.

METHODS

This study compares the influence of donor age (juvenile vs. adult), temperature (22 degrees C vs. 37 degrees C), and serum supplementation (10% newborn calf serum [NCS] with 10% pig serum) on morphological integrity and in vitro function of porcine islets during long-term culture (LTC).

RESULTS

After 21 days at 22 degrees C, the survival rate of cultured islets isolated from juvenile donors was lower than of adult islets (23+/-0.9% vs. 88+/-2.8%, P<0.001). Compared with 37 degrees C, LTC at 22 degrees C increased survival of adult islets and DNA recovery (92+/-2.5% vs. 45+/-4.8%, P<0.001; 72+/-4.1% vs. 30+/-5.1%, P<0.001) and reduced viability (62+/-8% vs. 89+/-5%, P<0.05). LTC at 22 degrees C was associated with a reduction of insulin content (85+/-9 vs. 152+/-10 microU/islet equivalents [IEQ], P<0.01), 24 hr-insulin secretion (82+/-7 vs. 552+/-91 microU/ day/IEQ, P<0.001), and integrated dynamic insulin response to glucose (1093+/-124 vs. 3074+/-708 microU/60 min/100 IEQ, P<0.05), compared with 37 degrees C LTC. Histologic analysis revealed disintegration of islet periphery after 22 degrees C, whereas smoothly shaped islets were present after 37 degrees C LTC. Integrity after 14 days at 37 degrees C was significantly better preserved when medium CMRL 1066 was supplemented with 10% porcine serum, compared with 10% NCS (40+/-2.3% vs. 21+/-6.7%, P<0.05), contrasting with 22 degrees C (52+/-4.0% vs. 59+/-3.7%, not significant).

CONCLUSIONS

This study demonstrates that survival of cultured porcine islets is increased at 22 degrees C, whereas in vitro function and viability are better preserved at 37 degrees C. Survival at 37 degrees C can be improved by adding homologous serum to the medium.

Large variability of the intracellular ATP content of human islets isolated from different donors

Observations in experimental heart, liver, kidney and pancreas transplantation indicated that graft function and survival correlates significantly with ATP content of transplanted tissue. The ATP content of cells can be reduced by several factors i.e. the nutritional donor status, storage technique, warm ischemia and cold ischemia time. This study investigates the intracellular ATP content of isolated human islets for the first time. Quantified samples of freshly isolated (digestion-filtration, continuous ficoll gradient purification) and cultured (22 degrees C, CMRL+10% FCS) islet equivalents (IEQ) of consecutively processed human pancreata from multiorgan donors (UW vascular flush) were shock frozen in liquid nitrogen and stored at -196 degrees C until rapid thawing, sonification and subsequent luminometric determination of ATP (Luciferin-Luciferase-reaction) and assessment of islet protein (IP). The ATP content was analysed for freshly isolated and subsequently 5+/-1 days cultured islets (n=10). The ATP content of freshly isolated human islets was 130.4+/-53.4 pg/microg IP (mean+/-SEM) corresponding to 20.7+/-6.3 pg/IEQ. After culture ATP content increased to 265.5+/-113.3 pg/microg IP (204.2+/-41.5%) corresponding to 43.7+/-15.3 pg/IEQ (216.1+/-34.9%; p<0.05). The coefficient of variation was 129.5%, 96.5% (fresh) and 135.0%, 111.0% (cultured) for ATP/microg IP and ATP/IEQ, respectively. The present data show that: (1) the ATP content of freshly isolated human islets varies enormously; (2) intraislet ATP levels increase significantly during 22 degrees C culture suggesting that the capacity to produce ATP is maintained despite hypothermic environment. More data are necessary to clarify the relevance of intraislet ATP content for graft function and survival after islet transplantation.

Isokinetic gradient centrifugation prolongs survival of pig islets xenografted into mice

Highly purified porcine islets were prepared by isokinetic gradients performed subsequently to isopycnic gradients. This additional purification step separates ductal, vascular, and lymphoid tissue effectively from endocrine tissue. Although ductal, vascular, and lymphoid tissue comprises only a minor contamination of the islet suspensions, a significant prolongation of the survival of porcine islets xenografted into streptozotocin diabetic C57BL/6 mice can be achieved by the elimination of the non-endocrine tissue. Rejection after islet transplantation is delayed from 2.2+/-0.4 days (n=27) to 13.1+/-2.1 days (n=36), respectively, when conventionally purified and highly purified islets are compared. Irrespective of the purification state, pretreatment of islets by low temperature culture had no effect on xenograft survival.

Analysis of the cellular reaction towards microencapsulated xenogeneic islets after intraperitoneal transplantation

Xenotransplantation of encapsulated islets of Langerhans is a possibility to overcome problems of human organ donor shortage in islet transplantation. Preexisting natural xenoantibodies are known to play a major role in the rejection of vascularized xenografts. Only little is known about the mechanism of rejection of non-vascularized cellular xenotransplants. In this study we introduce a method for the characterization of xenograft rejection of encapsulated islets by FACS analysis of peritoneal cells. Pig islets were transplanted intraperitoneally into non-diabetic Lewis rats either encapsulated or non-encapsulated. Animals receiving empty capsules and sham-operated animals served as controls. After 7 days a peritoneal lavage was performed. The total cell number and the viability of the cells were determined. Cells were analysed after staining with a panel of antibodies for the detection of T-lymphocytes, B-lymphocytes, macrophages, MHC class II molecules. Total cell number was highest after microencapsulated transplantation (149.4+/-30.1x10(6)) compared with empty capsules (41.4+/-19.7x10(6)) and non-encapsulated porcine islets (18.1+/-3.3x10(6)). The percentage of CD 3 positive T-lymphocytes rose to 44.5+/-11.5% in case of microencapsulated xenografts compared with 19.2+/-8.2% for non-encapsulated xenografts and 4.9+/-2.4% for empty controls. B-lymphocytes were detected in only small amounts. MHC class II expression on macrophages as activation marker was significantly increased after encapsulated transplantation (60.2+/-8.9% vs 15.2+/-7.0% for free islets and 4.9+/-1.2% for empty controls). The discrepancy between the macrophage activation due to encapsulated xenogeneic islets in comparison to empty capsules made from the same material clearly indicates that the reaction is not only material related but that a recognition of the encapsulated islet takes place despite the effective inhibition of a direct cell-to-cell contact. This recognition occurs on a T-cell level as well as on the macrophage level. 7 days after transplantation the reaction towards encapsulated xenografts is even more intense than to non-encapsulated xenografts. This might be due either to the time course of the rejection process or to a prolongation of the activation because antigen elimination is hindered by the capsule.

Glucose sensitivity of porcine and human islets in vitro

Preliminary experiments about the suitability of different commonly used culture media in our laboratory indicated, that prolonged exposure to high glucose concentrations during low temperature culture (LTC) impairs the viability of long term cultured human islets. As a consequence of the heterogeneity of tested media the present study was aimed to evaluate the influence of different glucose concentrations on survival, viability and in-vitro function of cultured human islets in order to optimize islet survival until transplantation and to compare species dependent differences in glucose sensitivity. Quantified aliquots of freshly isolated (digestion-filtration, ficoll gradient purification) islets from consecutively processed human (n=6) and porcine (n=11) pancreata were subjected to different glucose concentrations (human islets: 500, 750, 1000 and 2000 mg/l; porcine islets: 1000 and 2000 mg/l) in CMRL (22 degrees C) for 8-10 days. After LTC survival, viability and glucose-stimulated insulin release of incubated tissue was assessed. A reduction of glucose concentration promotes survival and viability of human islets but impairs in vitro function at the same time, presumably due to a reduced glucose oxidation as expressed by the significantly reduced stimulation index. In contrast to these findings in the human, elevated glucose concentration in porcine islet culture increases survival but reduces the glucose-stimulated insulin release and the viability of cultured islets. The contradiction of the results in regard to islet survival related to islet viability are still unclear in the pig and needs further evaluation.

Cytokine mRNA expression in peripheral blood cells of immunosuppressed human islet transplant recipients

The macrophage derived cytokines interleukin-beta (IL-1beta), and tumor necrosis factor alpha (TNFalpha), and the T-cell derived cytokine interferon gamma (IFNgamma) have been implicated to play an important role in early attack on islet cells during human islet transplantation (ITx). Therefore, the aim of this study was to investigate the influence of the current immunosuppressive induction therapy in clinical islet transplantation on mRNA expression of these cytokines in blood cells, compared to lipopolysaccharide (LPS) induced cytokine release in vitro and to plasma levels. The cytokine release correlated to lymphocyte counts and significantly decreased after ATG, and partially recovered 2 weeks after ITx. Unexpectedly, there was no correlation between mRNA expression for IL-1beta in total blood and the number of lymphocytes and monocytes remaining after anti thymocyte globulin (ATG)-therapy. Even when the blood was nearly totally depleted from mononuclear cells, high amounts of IL-1beta mRNA could be detected. However, IL-1beta secretion could not be stimulated in vitro. Our results show that application of ATG during ITx might contribute to graft survival during the early posttransplant period by suppression of the synthesis of monocyte derived cytokines IL-1beta and TNFalpha.

Critical islet mass for successful porcine islet autotransplantation

A major reason for the failure of clinical islet transplantations may be a limited islet mass. The aim of this study was to determine the critical islet mass necessary for normalization of glucose metabolism in a porcine model. Diabetes was induced by total pancreatectomy. The splenic lobe of the pancreas was intraductally distended with UW-solution containing 2.67-3.33 mg/ml collagenase, and the distended pancreas was digested in a continuous digestion filtration device. The islets were purified on a isoosmotic Ficoll-sodium-diatrizoate gradient. The survival period of the diabetic recipients in group 2 and 3 receiving, respectively, a low (2.14+/-0.39 microL/kg body weight) and a high (4.99+/-0.83 microL/kg body weight) islet mass was significantly prolonged compared to that of diabetic recipients in group 1 receiving no islet transplantation. However, the survival period of the recipients in group 2 was not significantly different to that in group 3. Three recipients of an islet mass of >5 microl/kg body weight became normoglycemic (fasting blood glucose <100 mg/dl) for more than two months. Furthermore, the glucose and insulin release reactions to the glucose challenge were comparable to that before pancreatectomy. Contrarily, another five diabetic recipients of an islet mass of <4 microL/kg body weight became a fasting blood glucose level of <200 mg/dl. The glucose and insulin release reactions to the glucose challenge were improved only, but not normalized compared to that before pancreatectomy. The data presented in this study demonstrate that metabolic normalization in pancreatectomized diabetic minipigs can be established by autotransplantation of an islet mass of >5 microl/kg body weight.

Viability and recovery of frozen-thawed human islets and in vivo quality control by xenotransplantation

Cryopreservation of islets of Langerhans offers advantages for the transplantation into diabetic patients. In this study two different methods of cryopreservation were compared with respect to islet viability and recovery after cryostorage. It was also investigated whether human islet survival in mice was affected by cryopreservation. Aliquots of human islets were cryopreserved conventionally or vitrified, respectively. After rapid thawing, islet viability and islet equivalent (IEQ) recovery rate were determined. Aliquots of freshly isolated or conventionally cryopreserved islets were transplanted beneath the kidney capsule of non-diabetic C57BL/6 mice. After three days renal insulin content was determined. Islet cell viability was 17.3+/-8.0% for vitrified and 51.8+/-3.0% for conventionally cryopreserved islets; the recovery rate was 84.8+/-12.2% and 92.8+/-12.4%, respectively. Insulin recovery after transplantation was 25.6+/-7.3% for fresh and 24.1+/-7.4% for cryopreserved islets. This study suggests that the conventional method of cryopreservation is superior to vitrification with respect to islet viability after thawing. We found no significant difference between fresh and cryopreserved islets with respect to insulin recovery after transplantation into mice.

Evidence for a significant correlation of donor pancreas morphology and the yield of isolated purified human islets

In clinical islet transplantation to patients with type 1 diabetes mellitus, the number of isolated and purified islet has been identified as a key determinant for functional success of the islet graft. With improved isolation methods based on the original procedure published by Ricordi et al. yield and function of isolated islets were considerably enhanced. However, there is still a large variance in the number, purity, viability and secretory capacity of islets isolated from brain-dead human donor pancreata, significantly hampering utilization of human islet preparations derived from a single donor for one diabetic recipient. The reasons for the limited success in islet isolation and purification have not been clarified in detail yet. Recent studies have indicated, that donor preconditions, and a number of technical factors during organ procurement and the islet isolation process itself are critical to successful islet isolation. This study aimed at identifying distinct morphological and histopathological characteristics of the donor pancreas as determinants for the outcome of human islet isolation and purification.

[Graft rejection in experimental xenogenic transplantation of isolated islands of Langerhans of the pig]

Prevention of the occurrence of diabetes-specific vascular complications is the final aim of clinical islet transplantation. Pancreatic islets isolated from adult pigs may be a suitable tissue source to transplant a large number of type 1 diabetic patients. Acute cellular rejection may be finally overcome by clinically applicable protocols for tolerance induction. However, primary nonfunction of the graft, as regularly observed in the porcine islet-to-rat xenotransplantation model, may be an additional problem. In this paper, species-specific inflammatory and immunological mechanisms are discussed which prevent early porcine islet graft function in rats but not in mice.

[Isolation of islands of Langerhans from human and porcine pancreas for transplantation to humans]

The enormous variability of donor factors and organ procurement related variables prevent a constant isolation success, thus reducing the potential number of clinical islet transplants. Since the availability of intact and viable pancreatic donor tissue intended for islet transplantation is limited, the porcine pancreas was selected as a potential source of xenogeneic islets for human recipients. The differences of islet histomorphology between porcine and human pancreas result in a higher intrinsic fragility of porcine islets during collagenase digestion. Nevertheless, if the isolation method is modified to inhibit factors potentially toxic to pig islets, reproducibility of isolation success is higher in the pig as in the human due to a lower variability in donor characteristics and the opportunity of preselection in regard to age and race. If xenograft rejection can be overcome and the risk of xenosis can be minimized, the logistic prerequisites for xenotransplantation of large amounts of viable pig islets into human recipients with insulin dependent diabetes are fulfilled.

Assessment of intracellular insulin content during all steps of human islet isolation procedure

This study investigated the recovery of pancreatic insulin content during human islet isolation prior to and after digestion-filtration, continuous Hanks-Ficoll gradient purification (n = 20), and 3-4 day culture at 22 degrees C (n = 6). The native insulin content varied in a wide range from 28.4 U to 360.8 U/pancreas. After digestion the initially measured average insulin content of 115.8 +/- 20.8 U/pancreas (mean +/- SEM) increased to 264.6 +/- 22.8% (p < 0.001). This increase of insulin during pancreas digestion was attributed to the asymetrical distribution of insulin within the pancreas. Sampling of insulin within the pancreatic caput seemed not to be representative for the insulin content of the complete native organ, because the ratio of insulin per gram tissue within the pancreatic cauda compared to the caput (n = 5) was 2.4 +/- 0.4 (p < 0.05). After purification total insulin recovery was 55.3 +/- 4.8% (p < 0.001). Because recovery of islet equivalent number (IEQ) (83.7 +/- 4.4%) exceeded insulin recovery, insulin/IEQ ratio decreased from 656.8 +/- 70.6 microU/IEQ before purification to 436.4 +/- 58.1 microU/IEQ (p < 0.001) after purification. After 22 degrees C culture (n = 6) recovery of insulin and IEQ was 80.1 +/- 8.1% (p < 0.05) and 92.8 +/- 3.5% (p = NS), respectively. Insulin content per IEQ decreased to 85.8 +/- 6.5% (p < 0.05). This study clearly shows that most of islet insulin is lost during purification. This seems to be caused rather by an amplified insulin release than by the loss of islets itself. This release may facilitate the separation of endocrine and exocrine tissue by gradient centrifugation, but may also accelerate islet exhaustion detrimental for long-term insulin independence.

Improved glucose counterregulation and autonomic symptoms after intraportal islet transplants alone in patients with long-standing type I diabetes mellitus

BACKGROUND

Defective glucose counterregulation and hypoglycemia unawareness are both well-recognized risk factors for recurrent episodes of severe hypoglycemia in patients with type I diabetes. At present, no conventional therapy is available to routinely overcome these acquired impairments in long-standing diabetes.

METHODS

To test the hypothesis that successful intraportal islet transplantation could improve this syndrome, hormonal counterregulatory responses and symptoms were studied during stepped hypoglycemic clamp tests before and after intraportal islet transplantation in three patients with type I diabetes who were prone to severe hypoglycemia.

RESULTS

As compared with matched nondiabetic control subjects, before islet transplantation, glucagon responses were absent while epinephrine and cortisol responses were either markedly decreased or absent in all diabetic subjects. One patient also had decreased norepinephrine and growth hormone responses. Autonomic warning symptoms were absent in all patients during hypoglycemia. One month after successful islet transplantation, there was no improvement in the glucagon response. However, glycemic thresholds and/or peak incremental responses of epinephrine, norepinephrine, and cortisol improved in all patients. Moreover, all patients had developed autonomic warning symptoms so that glycemic thresholds were detectable within the examined range.

CONCLUSION

We conclude that intraportal islet transplantation does not restore hypoglycemia-induced glucagon secretion, but it improves the responses of most counterregulatory hormones and hypoglycemic warning symptoms even in long-standing type I diabetes.

Successful islet auto- and allotransplantation in diabetic pigs

BACKGROUND

Because of its anatomical and physiological similarities to humans, the pig appears to be a suitable large animal model for preclinical studies of islet transplantation. The aim of this study was to investigate islet auto- and allotransplantation in a pig model with diabetes induced by total pancreatectomy.

METHODS

Porcine islets were isolated by a continuous digestion-filtration device at 32 degrees C and purified by a discontinuous iso-osmolar Ficoll-sodium-diatrizoate gradient on a Cobe 2991. The purified islets were autografted into the liver or the renal subcapsular space. The liver appears to be a more suitable site for the islet grafts than the renal subcapsular space, and the minimal amount of islets for reversal of diabetes is >5 microl/kg of body weight.

RESULTS

Persistent normoglycemia (fasting blood glucose level: 72.4+/-44.38 mg/dl) with a normal insulin secretion response to glucose stimulation was successfully achieved in five of six diabetic pigs by implanting a sufficient islet mass into the liver. Triple-drug immunosuppressive therapy with cyclosporine, azathioprine, and prednisolone did not prevent porcine islet allografts from experiencing early failure. However, the addition of 15-deoxyspergualin to the triple-drug immunosuppressive regimen significantly prolonged the function of the islet allografts. When antithymocyte globulin was added to the above-mentioned immunosuppressive drug regimen, the normoglycemic period was prolonged to more than 1 month (fasting blood glucose level: 75.4+/-17 mg/dl).

CONCLUSION

We conclude that autotransplantation with a sufficient islet mass can induce normoglycemia with a normal insulin secretion response to glucose stimulation in pancreatectomized diabetic pigs and that allotransplantation can be successfully achieved when 15-deoxyspergualin and antithymocyte globulin are combined with the triple-drug immunosuppression described above. However, this immunosuppressive protocol results in a high rate of infectious complications.