Biocompatibility and immunology in the encapsulation of islets of Langerhans (bioartificial pancreas)

Successful transplantation of encapsulated islets (bioartificial pancreas) would circumvent problems of islet availability and rejection in the treatment of insulin-dependent diabetes with biological organ replacement. Alginates are widely used as a hydrogel matrix or membrane for immunoprotected transplantation. A major problem in the use of diffusion-based devices is the biocompatibility of the material used. The foreign body reaction after implantation of empty microcapsules into different compartments in rats, dogs and pigs is evaluated in this article. However, biocompatibility of the bioartificial pancreas has three different aspects: reaction of the entrapped islet to the encapsulation technique and material; reaction of the recipient against the incorporated device ( = foreign body reaction); and finally the reaction of the recipient against the encapsulated islet ( = immunology of bioartificial pancreas). It is obvious from different experiments that even if foreign body reactions (reactions against material) are almost abolished the recipient may react against material released from the encapsulated islet. In conclusion, transplantation of encapsulated islets induces various morphological reactions (i.e. inflammation and fibrosis) as a result of a variety of donor and recipient related factors. Therefore, the use of an adequate animal model that reflects the human situation is essential for progress in the development of a bioartificial pancreas.     

Contribution of glutamate decarboxylase antibodies to the reactivity of islet cell cytoplasmic antibodies

The contribution of glutamate decarboxylase (Mr 65000) antibodies to the reactivity of islet cell cytoplasmic antibodies with the 'whole' islet staining pattern from patients with newly diagnosed Type I diabetes was investigated. Diluted sera (n = 10) were preincubated with increasing concentrations of purified recombinant human islet glutamate decarboxylase (Mr 65000) and the change in islet cell cytoplasmic antibody binding was evaluated by quantitative immunocytochemistry. Binding to islet cells was partially blocked by glutamate decarboxylase in 9/10 diluted sera; the maximum blocking obtained at high concentrations of glutamate decarboxylase (5 micrograms/ml) was 36% (median, range 24-61%). In contrast, binding to islet cells in three diluted sera (two polyendocrine patients without Type I diabetes and one patient with newly diagnosed Type I diabetes) with the 'selective' islet staining pattern was totally blocked by glutamate decarboxylase. The concentration of glutamate decarboxylase required to achieve maximum blocking was less for the 'whole' islet (0.4-8.0 micrograms/microliters undiluted serum) compared to the 'selective' islet (20-645 micrograms/microliters undiluted serum) positive sera. All sera were positive for glutamate decarboxylase antibodies in an immunoprecipitation assay using 35S-methionine labelled extract of baby hamster kidney cells transfected with glutamate decarboxylase. However, the binding activity of these antibodies was less in the sera positive for the 'whole' islet compared to the 'selective' islet staining pattern. In conclusion, glutamate decarboxylase antibodies contribute partially to the reactivity of islet cell cytoplasmic antibodies of the 'whole' islet staining pattern in the sera of newly diagnosed patients with Type I diabetes, and totally to reactivity of the 'selective' islet staining pattern. The antigens recognized by the other antibodies contributing to the 'whole' islet reactivity remain to be defined.     

Surface-based hemangioma of bone: three case studies and a review of the literature

The islet organ of the Atlantic hagfish, a cyclostome, phylogenetically is the most original islet parenchyma. It is well equipped with blood vessels, but lacks nerves. The aim of the present study was to evaluate the relative proportions between regional blood flows in these animals with a microsphere technique and with laser-Doppler flowmetry. As regards the results obtained in the kidneys, gills, and islet organ, the data were essentially the same found using both methods. Approximately 20% of the injected microspheres were found in the kidneys; when corrected for weight, the gill bodies and the islet organ were also found to have the same blood perfusion. The blood flow to the brain and that of the gut and the liver were only approximately 30% and 10%, respectively, of that to the kidney. Topical application of the nitric oxide donor, sodium nitroprusside failed to affect the islet blood flow. Apart from this observation, this non-innervated "primitive" islet parenchyma did not differ from that of the amply innervated mammalian islets with regard to their high basal blood perfusion.     

Overexpression of parathyroid hormone-related protein in the pancreatic islets of transgenic mice causes islet hyperplasia, hyperinsulinemia, and hypoglycemia

Parathyroid hormone-related protein (PTHrP) is produced by the pancreatic islet. It also has receptors on islet cells, suggesting that it may serve a paracrine or autocrine role within the islet. We have developed transgenic mice, which overexpress PTHrP in the islet through the use of the rat insulin II promoter (RIP). Glucose homeostasis in these mice is markedly abnormal; RIP-PTHrP mice are hypoglycemic in the postprandial and fasting states and display inappropriate hyperinsulinemia. At the end of a 24-hour fast, blood glucose values are 49 mg/dl in RIP-PTHrP mice, as compared to 77 mg/dl in normal littermates; insulin concentrations at this time are 6.3 and 3.9 ng/ml, respectively. Islet perifusion studies failed to demonstrate abnormalities in insulin secretion. In contrast, quantitative islet histomorphometry demonstrates that the total islet number and total islet mass are 2-fold higher in RIP-PTHrP mice than in their normal littermates. PTHrP very likely plays a normal physiologic role within the pancreatic islet. This role is most likely paracrine or autocrine. PTHrP appears to regulate insulin secretion either directly or indirectly, through developmental or growth effects on islet mass. PTHrP may have a role as an agent that enhances islet mass and/or enhances insulin secretion.     

Progressive pancreatic islet hyperplasia in the islet-targeted, parathyroid hormone-related protein-overexpressing mouse

PTH-related protein (PTHrP) is a paracrine/autocrine factor produced in most cell types in the body. Its functions include the regulation of cell cycle, of differentiation, of apoptosis, and of developmental events. One of the cells which produces PTHrP is the pancreatic beta cell. We have previously described a transgenic mouse model of targeted overexpression of PTHrP in the beta cell, the RIP-PTHrP mouse. These studies showed that PTHrP overexpression markedly increased islet mass and insulin secretion and resulted in hypoglycemia. Those studies were limited to RIP-PTHrP mice of 8-12 weeks of age. In the current report, we demonstrate that PTHrP overexpression induces a progressive increase in islet mass over the life of the RIP-PTHrP mouse, and that, in contrast to some other models of targeted PTHrP overexpression, the phenotype is not developmental, but occurs postnatally. The marked increase in islet mass is not associated with a measurable increase in beta cell replication rates. A further slowing in the normally low islet apoptosis rate could not be demonstrated in the RIP-PTHrP islet. Thus, the marked increase in islet mass in the RIP-PTHrP mouse is unexplained in mechanistic terms. Finally, RIP-PTHrP mice are resistant to the diabetogenic effects of streptozotocin. The mechanisms responsible for the increase in islet mass in the RIP-PTHrP mouse likely lie in either very subtle changes in islet turnover or in early steps in islet differentiation and development. The ability of PTHrP to increase islet mass and function, as well as its ability to attenuate the diabetogenic effects of streptozotocin, indicate that further study of PTHrP on islet development and function are important and may lead to therapeutic strategies in diabetes mellitus.     

Role of IAPP in the pathogenesis and development of NIDDM

Islet amyloid deposits are the characteristic lesions of the pancreas of the patients with non-insulin-dependent diabetes mellitus (NIDDM). Islet amyloid polypeptide (IAPP or amylin) is a 37 amino acid peptide, which was extracted and characterized from islet amyloid deposits in patients with NIDDM. Recent studies and characterization of IAPP cDNA suggest that IAPP is a normal islet hormone and is co-secreted with insulin from islet beta cells of the pancreas. In this paper, we review recent advance of the research for IAPP including our results, and discuss the role of IAPP in the development of NIDDM, and we propose a new type of diabetes, "amyloid diabetes".     

Apoptosis in the pancreatic islet cells of the neonatal rat is associated with a reduced expression of insulin-like growth factor II that may act as a survival factor

Islet cell ontogeny will define adult beta-cell mass and will consist of a balance of islet cell birth and death. We have investigated the ontogeny of factors that may be related to developmental apoptosis in the islets, insulin-like growth factor II (IGF-II) and inducible nitric oxide synthase (iNOS), in pancreata of young Wistar rats. Pancreata were collected from rats of 21 days gestation to 29 days postnatal age. In situ hybridization and immunohistochemistry showed that IGF-II was expressed and present in fetal and neonatal islet cells, but declined rapidly 2 weeks after birth. Little IGF-I was associated with fetal or postnatal islets. Apoptosis in islet cells was visualized by molecular histochemistry for DNA breakage in tissue sections. Apoptosis was low in the fetus, but increased in incidence postnatally so that 13% of islet cells were undergoing apoptosis on postnatal day 14, with the incidence declining thereafter. Immunohistochemistry for iNOS showed that it was expressed within beta-cells and was most abundant 12 days after birth. When islets were isolated from rat pancreata 20-22 days after birth, islet cell viability, DNA synthetic rate, and insulin release were reduced after incubation with interleukin-1beta, tumor necrosis factor, or interferon-gamma. An increased rate of islet cell survival was found after simultaneous incubation with IGF-I or -II. Cytokine-mediated islet cell death involved the induction of apoptosis. Islets isolated from neonatal rats were not killed after exposure to these cytokines at the same concentrations, but cytokine-induced cell death was seen when neonatal islets were incubated with a neutralizing antibody against IGF-II. These experiments show that a peak of islet cell apoptosis that is maximal in the rat pancreas 14 days after birth is temporally associated with a fall in the islet cell expression of IGF-II. IGF-II was shown to function as an islet survival factor in vitro. The induction of islet cell apoptosis in vivo may involve an increased expression of iNOS within beta-cells.     

Treatment with growth hormone and dexamethasone in mice transgenic for human islet amyloid polypeptide causes islet amyloidosis and beta-cell dysfunction

Islet amyloid derived from islet amyloid polypeptide (IAPP) is a well-recognized feature of type II diabetes. However, the mechanism of islet amyloidogenesis is unknown. In vitro studies suggest that amino acid residues 20-29 in human, but not mouse, IAPP confer amyloidogenicity consistent with the absence of spontaneous islet amyloidosis in mice. Several clinical and in vitro studies suggest that increased synthetic rates of IAPP predispose to IAPP-amyloidosis. In the present study, we sought to test the hypothesis that pharmacological induction of insulin resistance in a mouse transgenic (TG) for human IAPP would induce islet amyloid and beta-cell dysfunction. TG and non-transgenic (N-TG) control mice were treated with both rat growth hormone (12 micrograms/day) and dexamethasone (0.24 mg/day) (dex/GH) or received no treatment for 4 weeks, after which animals were killed to examine islet morphology. Treatment with dex/GH caused hyperglycemia (7.3 +/- 0.4 vs. 5.2 +/- 0.1 mmol/l, TG vs. N-TG, P < 0.001) associated with a decreased plasma insulin concentration (595 +/- 51 vs. 996 +/- 100 pmol/l, TG vs. N-TG, P < 0.05) in TG versus control mice. Islet amyloid was induced in treated TG mice but not in control mice. Islet amyloid was identified in both intra- and extracellular deposits, the former being associated with evidence of beta-cell degeneration. We conclude that dex/GH treatment in mice TG for human IAPP induces IAPP-derived islet amyloid, hyperglycemia, and islet dysfunction. The present model recapitulates the islet morphology and phenotype of type II diabetes.     

Excellence of the two-layer method (University of Wisconsin solution/perfluorochemical) in pancreas preservation before islet isolation

BACKGROUND: In islet transplantation pancreatic preservation before islet isolation is an obstacle compromising islet yield and viability. We tested the feasibility of a two-layer method (University of Wisconsin solution [UW]/perfluorochemical) for pancreatic preservation before islet isolation. METHODS: Dog pancreases were processed into pure islets by the method of Ricordi preceded by five different preservations (groups 1-a and 1-b, the two-layer method for 3 and 24 hours; groups 2-a and 2-b, simple cold storage in UW for 3 and 24 hours; group 3, without preservation). Islet yields and functional success after autotransplantation into the liver were compared among the groups. RESULTS: Postpurification islet equivalents (IE)/gm pancreas and functional success rate were 5600 (mean), 83% in group 1-a; 4000, 56% in group 1-b; 4700, 33% in group 2-a; 1300, 0% in group 2-b; and 5000, 89% in group 3 (p < 0.05; 2b versus 1-a, 1-b, and 3), respectively. There was no statistical difference among groups 1-a, 1-b, and 3 in terms of islet yield and function (p > 0.2). CONCLUSIONS: The two-layer method is more effective than conventional simple cold storage in UW for pancreatic preservation before islet isolation. Clinical trials with the two-layer method are warranted.     

Cytotoxic effects of islet cell surface antibodies

Antibodies against rat islet cells were produced by immunisation of rabbits with neonatal rat islet cells. In the presence of complement the rabbit anti-rat islet cell surface sera were strongly cytotoxic for both, neonatal rat islet cells and spleen lymphocytes as revealed by the high percentage of 51Cr release from these cells. However, after absorption with rat lymphocytes and rat liver powder the cytotoxicity of the islet cell antisera for rat lymphocytes was drastically reduced while the release of 51Cr from islet cells was only slightly influenced. These data indicate that islet cell specific antibodies were still present in the antisera after absorption. Native normal rabbit serum was also cytotoxic for neonatal rat islet cells and spleen lymphocytes. The release of 51Cr from islet cells and lymphocytes was vigorously reduced after absorption of the normal rabbit serum with lymphocytes and was paralleled by a similar decrease of insulin release from intact islets under conditions where the active insulin secretion was blocked pharmacologically. Intact islets prelabeled with 51Cr were also used as targets but this approach was less suitable for the detection of cytotoxic islet cell surface antibodies.     

TNF-alpha down-regulates type 1 cytokines and prolongs survival of syngeneic islet grafts in nonobese diabetic mice

Administration of TNF-alpha to autoimmune diabetes-prone nonobese diabetic mice and biobreeding rats inhibits diabetes development; however, the mechanism(s) of diabetes prevention by TNF-alpha has not been established. We used the model of syngeneic islet transplantation into diabetic nonobese diabetic mice to study the effects of TNF-alpha administration on the types of mononuclear cells and cytokines expressed in the islet grafts and on autoimmune diabetes recurrence. Twice daily i.p. injections of TNF-alpha (20 microg/day) from day 1 to day 30 after islet transplantation significantly prolonged islet graft survival; thus, 70% (16 of 23) of mice treated with TNF-alpha were normoglycemic at 30 days after islet transplantation compared with none (0 of 14) of vehicle-treated control mice. Islet grafts and spleens from TNF-alpha-treated mice at 10 days after islet transplantation contained significantly fewer CD4+ and CD8+ T cells, and significantly decreased mRNA levels of type 1 cytokines (IFN-gamma, IL-2, and TNF-beta) than islet grafts and spleens from control mice. Regarding type 2 cytokines, IL-4 mRNA levels were not changed significantly in islet grafts or spleens of TNF-alpha-treated mice, whereas IL-10 mRNA levels were decreased significantly in islet grafts of TNF-alpha-treated mice and not significantly changed in spleens. TGF-beta mRNA levels in islet grafts and spleens were similar in TNF-alpha-treated and control mice. These results suggest that TNF-alpha partially protects beta cells in syngeneic islet grafts from recurrent autoimmune destruction by reducing CD4+ and CD8+ T cells and down-regulating type 1 cytokines, both systemically and locally in the islet graft.     

Mapping a quantitative trait locus involved in melanotic encapsulation of foreign bodies in the malaria vector, Anopheles gambiae

A Plasmodium-refractory strain of Anopheles gambiae melanotically encapsulates many species of Plasmodium, whereas wild-type mosquitoes are usually susceptible. This encapsulation trait can also be observed by studying the response of refractory and susceptible strains to intrathoracically injected CM-Sephadex beads. We report the results of broad-scale quantitative trait locus (QTL) mapping of the encapsulation trait using the bead model system. Interval mapping using the method of maximum likelihood identified one major QTL, Pen1. The 13.7-cM interval containing Pen1 was defined by marker AGH157 at 8E and AGH46 at 7A on 2R. Pen1 was associated with a maximum LOD score of 9.0 and accounted for 44% of the phenotypic variance in the distribution of phenotypes in the backcross. To test if this QTL is important for encapsulation of Plasmodium berghei, F2 progeny were infected with P. berghei and evaluated for degree of parasite encapsulation. For each of the two markers that define the interval containing Pen1, a significant difference of encapsulation was seen in progeny with at least one refractory allele in contrast with homozygous susceptible progeny. These results suggest that Pen1 is important for melanotic encapsulation of Plasmodium as well as beads.     

Love, courage, and honor

OBJECTIVES: Despite the increasing success of whole-organ pancreas transplantation, the success of clinical islet allografts has remained limited. One of the factors limiting the success is the difficulty in monitoring islet allografts after transplantation. The aim of these studies is to develop a method of "biopsying" human islet allografts using a forearm islet implantation site. METHODS: A subtherapeutic number of isolated human islets were placed in the forearm under the muscle fascia in three human recipients with Type I insulin dependent diabetes. All of the recipients had undergone successful cadaveric renal transplantation at least one year prior and were maintained on their baseline immunosuppression. Aliquots of the islet grafts were removed 7 and 14 days to assess engraftment and graft infiltrate. To verify that the islets were viable, 400 were handpicked and transplanted into B6-scid mice made diabetic with streptozotocin. RESULTS: The biopsy site was found in all three cases. In one patient, no islets were recovered. In two other patients, viable islet tissue was recovered 7 days after transplantation. Immunohistology at 7 days showed the presence of both insulin and glucagon-staining cells in the islets. At 14 days in these two patients, a mononuclear cell infiltrate was observed in the explanted islet biopsies. Immunohistology showed the relative absence of insulin-staining cells with intact glucagon-staining cells. This finding is consistent with recurrent autoimmunity in the islet grafts. DISCUSSION: This preliminary study shows that the forearm biopsy site is a useful method to retrieve human islet grafts after transplantation. The islets engraft and are easily found in the first weeks after transplantation. These data suggest that recurrent autoimmunity may affect islet allografts. Further studies will be needed to determine if the histology in the forearm will correlate with the fate of intraportal or intraperitoneal islet allografts. Although they were shown to reduce the incidence of early allograft failure, their influence on the long-term graft survival remains to be proven.     

Glucose diffusion in pancreatic islets of Langerhans

We investigate the time required for glucose to diffuse through an isolated pancreatic islet of Langerhans and reach an equilibrium. This question is relevant in the context of in vitro electrophysiological studies of the response of an islet to step changes in the bath glucose concentration. Islet cells are electrically coupled by gap junctions, so nonuniformities in islet glucose concentration may be reflected in the activity of cells on the islet periphery, where electrical recordings are made. Using a mathematical model of hindered glucose diffusion, we investigate the effects of the islet porosity and the permeability of a surrounding layer of acinar cells. A major factor in the determination of the equilibrium time is the transport of glucose into islet beta-cells, which removes glucose from the interstitial spaces where diffusion occurs. This transport is incorporated by using a model of the GLUT-2 glucose transporter. We find that several minutes are required for the islet to equilibrate to a 10 mM change in bath glucose, a typical protocol in islet experiments. It is therefore likely that in electrophysiological islet experiments the glucose distribution is nonuniform for several minutes after a step change in bath glucose. The delay in glucose penetration to the inner portions of the islet may be a major contributing factor to the 1-2-min delay in islet electrical activity typically observed after bath application of a stimulatory concentration of glucose.     

Elderly hospitalized patients'' perceptions of the interaction with the social worker during discharge planning

During parasitization of Heliothis virescens, Campoletis sonorensis deposits an egg along with venom, polydnavirus particles (CsPDV), and ovarian proteins (OPs). Oviposited eggs are not encapsulated, while washed eggs are encapsulated rapidly by H. virescens. Early protection from encapsulation is afforded by a group of 29-36 kD OP glycoforms. These glycoforms are endocytosed by host hemocytes within 30 min post-parasitization (pp) and disrupt hemocyte spreading behavior and egg encapsulation through at least 24 h p.i. Purified CsPDV does not protect eggs from encapsulation early, but disrupts hemocyte spreading and egg encapsulation from 24 h through at least 5 days p.i. Functional activity of CsPDV appears to be correlated with time-dependent accumulation of virus-specific proteins in parasitized insects. We propose that the fast-acting 29-36 kD OP protects Campoletis eggs from encapsulation until accumulation of CsPDV proteins which sustain immunosuppression.     

Interaction of cholecystokinin (CCK-33) and its C-terminal fragments: CCK-8 and CCK-4 with alpha- and beta-adrenoceptor agonists and antagonists in the cardiovascular system of diabetic rats. Part B

Although neurotransmitters are present in pancreatic islets of Langerhans and can be shown to alter hormone secretion, their precise physiological roles in islet function and their cellular mechanisms of action are unclear. Recent research has identified specific neurotransmitter receptor isoforms in islets that may be important physiologically, because selective receptor agonists activate islet ion channels, modify intracellular [Ca2+], and affect secretion. This article focuses on the putative roles of acetylcholine, glutamate, and GABA in islet function. It has been hypothesized that acetylcholine potentiates insulin secretion by either promoting Ca release from cellular stores, activating a store depletion-activated channel, or activating a novel Na channel. GABA and glutamate, in contrast, have been proposed to mediate a novel paracrine signaling pathway whereby alpha- and beta-cells communicate within the islet. The evidence supporting these hypotheses will be critically evaluated.     

Respiratory chain activity and mitochondrial DNA content of nonpurified and purified pancreatic islet cells

Considerable interest has recently focused on the possible role of alterations in mitochondrial activity and mutations in the mitochondrial genome for the development of non-insulin-dependent diabetes. Our study aimed at investigating the normal mitochondrial respiratory chain activity of nonpurified and purified islet cells to further explore whether some diabetic states are associated with alterations of mitochondrial oxidative processes. For this purpose, pancreatic islets were isolated from Wistar rats. Unpurified islet cells were obtained in the presence of trypsin and DNAse, and purified beta and non-beta cells were prepared by autofluorescence-activated sorting using a flowcytometer. Intact cell respiration and substrate oxidation in digitonin-permeabilized cells were measured polarographically with a Clark oxygen electrode in a micro-water-jacketed cell. Specific activity of the individual complexes of the respiratory chain was determined spectrophotometrically in unpurified islet cells. The relative amount of mitochondrial (mtDNA) and nuclear (nDNA) DNA in all three cell populations and in rat brain and skeletal muscle was estimated by dot blotting. The intact cell respiration of unpurified islet cells corresponds to the mean of values obtained for beta and non-beta islet cells. Oxidation rates of different substrates by permeabilized beta cells were lower than those for unpurified and non-beta cells. The amount of mtDNA relative to nDNA was similar in all three groups of cells, and was also similar to that obtained from brain and skeletal muscle. In summary, we have described mitochondrial respiratory chain activity in unpurified, beta, and non-beta islet cells. Our results represent an initial step in investigating the potential pathogenic role that alterations in oxidative phosphorylation could play in some diabetic states.     

Chronic overproduction of islet amyloid polypeptide/amylin in transgenic mice: lysosomal localization of human islet amyloid polypeptide and lack of marked hyperglycaemia or hyperinsulinaemia

Type 2 (non-insulin-dependent) diabetes mellitus is characterised by hyperglycaemia, peripheral insulin resistance, impaired insulin secretion and pancreatic islet amyloid formation. The major constituent of islet amyloid is islet amyloid polypeptide (amylin). Islet amyloid polypeptide is synthesized by islet beta cells and co-secreted with insulin. The ability of islet amyloid polypeptide to form amyloid fibrils is related to its species-specific amino acid sequence. Islet amyloid associated with diabetes is only found in man, monkeys, cats and racoons. Pharmacological doses of islet amyloid polypeptide have been shown to inhibit insulin secretion as well as insulin action on peripheral tissues (insulin resistance). To examine the role of islet amyloid polypeptide in the pathogenesis of Type 2 diabetes, we have generated transgenic mice with the gene encoding either human islet amyloid polypeptide (which can form amyloid) or rat islet amyloid polypeptide, under control of an insulin promoter. Transgenic islet amyloid polypeptide mRNA was detected in the pancreas in all transgenic mice. Plasma islet amyloid polypeptide levels were significantly elevated (up to 15-fold) in three out of five transgenic lines, but elevated glucose levels, hyperinsulinaemia and obesity were not observed. This suggests that insulin resistance is not induced by chronic hypersecretion of islet amyloid polypeptide. Islet amyloid polypeptide immunoreactivity was localized to beta-cell secretory granules in all mice. Islet amyloid polypeptide immunoreactivity in beta-cell lysosomes was seen only in mice with the human islet amyloid polypeptide gene, as in human beta cells, and might represent an initial step in intracellular formation of amyloid fibrils.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hypotensive action of bromocriptine in the DOCA-salt hypertensive rat: contribution of spinal dopamine receptors

Clinical islet transplantation is potentially the treatment of choice for people with type I diabetes. Rates of insulin independence in islet transplant recipients are disappointingly low, and the relative contribution of the rejection response compared with the loss of islet function is still unclear. We have compared the mixed lymphocyte islet coculture (MLIC) with the mixed lymphocyte acinar cell coculture (MLAC) and the mixed lymphocyte response (MLR) as in vitro models of allograft rejection to MHC and tissue-specific antigens expressed by human islets and acinar cells. The reduced number of MHC class II antigen-positive cells in islets and acinar tissue compared to those in the stimulator lymphocyte population of the MLR, correlated with a reduced proliferative response in the MLIC and MLAC. Enhancement of MHC class II antigen expression by islets using TNFalpha and IFNgamma did not increase their stimulatory capacity in the islet cocultures, which may have been due to a corresponding absence of B7 expression. The lack of T cell proliferation to acinar cells despite cytokine-induced enhancement of MHC class II expression and detectable B7 expression appeared to be due to the inhibitory effect of exocrine enzymes on lymphocyte proliferation. In conclusion, we suggest that a rejection response to islets and acinar tissue is possible due to the accompanying MHC class II-positive cells and that, in this model, islet and acinar-specific antigens do not significantly contribute to that response. Acinar cells may have the potential to stimulate lymphocytes directly, but this was not evident by proliferation in the MLAC. Rejection appears to contribute to the low survival rate of human islet allografts, but it is unlikely that this is the sole explanation, and other factors should be considered.