The reversal of diabetes by pancreatic islet transplantation

PD-1/PD-L1, PD-1/PD-L2, and other co-inhibitory signaling pathways in transplantation

Transplantation of cells, tissues and vascularized solid organs is a successful therapeutic intervention for many end-stage chronic diseases. The combination of co-stimulatory blockade with the delivery of negative signals to T cells through co-inhibitory receptors would provide a robust approach to modulating T-cell receptor signaling and improving alloantigen-specific control of transplant rejection. This approach based on fundamental knowledge of APC/T-cell interactions may complement conventional therapies in the near future to reinforce long-term allograft survival, and permit minimal immunosuppression. The focus of this review was primarily on two major co-inhibitory signaling pathways, namely PD-1/PD-L1/PD-L2 and BTLA/CD160/HVEM/LIGHT that have been thoroughly characterized in murine models of transplantation using genetically modified mice, specific monoclonal antibodies and fusion proteins.

Prolongation of islet allograft survival following ex vivo transduction with adenovirus encoding a soluble type 1 TNF receptor–Ig fusion decoy

Islet transplantation is a viable long-term therapeutic alternative to daily insulin replacement for type I diabetes. The allogeneic nature of the transplants poses immunological challenges for routine clinical utility. Gene transfer of immunoregulatory molecules and those that improve insulin release kinetics provides rational approaches to facilitate allogeneic islet transplantation as a potential therapy. We have examined the efficacy of a soluble type 1 tumor necrosis factor receptor (TNFR) immunoglobulin-Fc fusion transgene (TNFR-Ig) to protect human islets from cytokine-induced apoptosis in culture, as well as in facilitating allogeneic islet transplants in diabetic mice. Cultured human islets were transduced with an adenoviral vector encoding human TNFR-Ig (Ad-TNFR-Ig). TNFR-Ig protein was secreted by cultured islets, as well as by transduced mouse islet transplants recovered from mouse recipients. Glucose-induced insulin release kinetics were comparable among untransduced, Ad-TNFR-Ig-infected human islets and vector-transduced islets exposed to cytokines. In parallel, Ad-TNFR-Ig-infected islets were protected from cytokine-induced apoptosis activation. Finally, diabetic mice transplanted with allogeneic islets expressing TNFR-Ig returned to and maintained normoglycemia significantly longer than untransduced islet recipients. These data support the potential utility of TNFR-Ig gene transfer to islets as a means of facilitating allogeneic islet transplantation.

Antigen Concentration and Precursor Frequency Determine the Rate of CD8+ T Cell Tolerance to Peripherally Expressed Antigens

Expression of transgene-encoded proteins in the pancreatic islets can cause peripheral deletion of T cells. However, tolerance has not been observed in all transgenic models. It has been proposed that the determining factor for successful peripheral tolerance is the amount of Ag cross-presented by quiescent APCs. Using InsHA mice, which demonstrate peripheral tolerance to the influenza virus hemagglutinin (HA) expressed in the pancreatic islet β cells, we have investigated the consequences when different amounts of HA are expressed. As compared with InsHA mice that are heterozygous for the InsHA transgene, homozygous InsHA mice demonstrated enhanced activation and proliferation of Kd-restricted HA-specific CD8+ T cells in the pancreatic lymph nodes. However, despite such activation, insulitis was not observed, and the T cells were gradually functionally deleted. Deletion of these activated cells occurred much more rapidly in homozygous than in heterozygous InsHA mice. These data demonstrate that there is a direct correlation between the amount of HA expressed in the periphery, and both the degree of T cell proliferation in the pancreatic lymph nodes and the rate of tolerance of HA-specific CD8+ T cells. This strongly supports the hypothesis that activation of T cells through cross-presentation of peripheral Ags in a noninflammatory environment is an important part of the normal mechanism of tolerance to Ags expressed in the pancreatic islets.

Development and optimization of the human allogeneic mixed lymphocyte islet (MLIC) and acinar (MLAC) coculture system

Although human islet allotransplantation has been shown to be effective in restoring normoglycaemia to diabetic recipients, the long-term success rate is relatively low. The poor results of human islet transplantation are surprising given the relative ease of establishing functioning islet allografts in rodent models with minimal immunosuppression and have been attributed to a number of factors of which rejection may play a major part. The impracticality of studying the immunogenicity of human islets in vivo has led us to develop a model specifically to study the allogeneic immune response to the major components of isolated human islet preparations. To try and overcome the variability associated with this methodology, this study was designed to develop and optimize the mixed lymphocyte coculture as an in vitro model for the initial alloresponse to both freshly isolated intact human islets (MLIC) and acinar tissue pieces (MLAC). For this, titration of the islets and acinar tissue, as well as kinetic studies of the response, were used to select optimum conditions for the MLIC which were shown to be ten islets and ten acinar tissue pieces with a coculture duration of 7-9 days. The optimized studies included the use of non-tissue culture microwells to deter fibroblast growth, RPMI + 10% human AB serum to reduce the background lymphocyte response and a low concentration of dithizone to stain the islets prior to handpicking. Freshly isolated human islets were found to stimulate allogeneic lymphocytic proliferation; however, differences in the relative response to islets and acinar tissue led us to test the effect of soluble products from the acinar cells on lymphocyte proliferation in the mixed lymphocyte reaction. An inhibitory effect of the soluble products of acinar cells on the allogeneic lymphocyte proliferative response was found to contribute to the reduced response of the MLAC compared to the MLIC. Human islets were shown to stimulate an allogeneic immune response in vitro, and this MLIC model provides a method for assessing the specific immunogenicity of human isolated islet preparations and associated acinar tissue in transplantation.

Rapid assessment of islet viability with acridine orange and propidium iodide

A simple, rapid method for estimating the viability of isolated islets of Langerhans with fluorescent dyes is described. Low concentrations of acridine orange and propidium iodide (AO/PI) were used to visualize living and dead islet cells simultaneously. AO/PI-stained islets can be divided into three distinct groups. Group A islets fluoresce green, contain insulin, and have normal ultrastructure; group C islets fluoresce primarily red, contain little or no insulin, and have cells with disrupted cellular membranes. Group B islets fluoresce red, green, and yellow. The yellow color is due to the addition of two primary colors from the superimposed red and green fluorescing cells. In this assay, the interpretation that red islet cells are dead and green islet cells are alive was confirmed by sequentially staining single islet cells with AO/PI and trypan blue. The observation that red islets are dead was confirmed by heat-killing, enzymatically damaging, treating with ethanol, or depriving islets of nutrients and observing the red fluorescence. This assay should be useful in studies where the assessment of islet viability is essential.

Pancreatic islet allograft function in nonimmunosuppressed conscious mice

CBA/H (H-2k) male mice were made diabetic by giving Streptozotocin 300 mg/kg intravenously. They were transplanted beneath the renal capsule with seven clusters of BALB/c (H-2d) adult islets, cultured for seven days in 95% O2 and 5% CO2. Basal blood glucose and weight gain returned to normal within 7 to 10 days. Blood glucose and serum insulin responses to intragastric glucose, intraperitoneal arginine and intraperitoneal theophylline were assessed after restoration of normoglycemia and compared with the responses in a control group of CBA mice. The animals were pretreated with intraperitoneal phentolamine and propranolol to reduce the stress of these procedures. Essentially normal carbohydrate tolerance was maintained in the transplanted animals but the insulin responses were markedly reduced, suggesting an increased sensitivity to insulin. The insulin content of the grafted tissue was also found to be considerably lower than that in the normal pancreas. This study demonstrates that in the mouse, adult pancreatic islet allografts can respond to stimuli that require functional adaptation by the islets. Further studies are required in larger animals before islet transplantation can be applied to the management of insulin dependent diabetic patients.

Nonenzymic in vitro isolation of perinatal islets of Langerhans

We have developed a method to circumvent the use of exogenous proteolytic enzymes in the isolation of islets of Langerhans from the perinatal rodent pancreas. Advantage is taken of the propensity of fibroblastlike cells to attach and migrate on polystyrene at low-serum concentrations (5%). In contrast, at this serum level, rat islet epithelial cells tend not to adhere to the substrate. At 3 d of culture, islets are visible at the edges of the explants. With further fibroblast outgrowth the majority of islets are freefloating by 7 d. Simple agitation of the medium and centrifugation yields approximately 50 micrograms of islet tissue per perinatal pancreas. Further purification of the islets can be obtained by subculture. Rat islets can be maintained in this manner for several months in Medium F12 supplemented with 25% horse serum in an atmosphere of 5% CO2 and air at 37 degrees C. Hormone content of the islet tissue remains constant during prolonged subculture and such islets continue to exhibit appropriate insulin and glucagon responses to glucose and theophylline. The morphological integrity of the endocrine cells within the cultured islets was confirmed by immunocytochemistry and ultrastructural study. Nonendocrine cells are not identifiable within the long-term cultured islets.

Differences in susceptibility to rejection of mouse pancreatic islet allografts disparate for class I or class II major histocompatibility antigens

Pancreatic islet B cells express class I but not class II antigens, and removal of Ia positive passenger cells from H-2 allogeneic islets by anti-Ia serum and complement leads to permanent allograft survival. A test was made of whether the same result can be achieved by genetically removing the Ia stimulus by performing mouse islet allografts in congenic donor-recipient combinations differing at the H-2 K only, D only, or K + D regions. Mice disparate for class I antigens (H-2 K, D, and K + D) alone reject islet allografts, suggesting that Ia positive passenger cells may be involved in presentation of class I disparities. Established islet allografts appear to be sensitive to rejection induced by injection of donor strain splenocytes when donor and recipient differ for class I (H-2 D alone and D + I) but not class II (H-2 I alone) antigens. These results are consistent with the hypothesis that pancreatic islet allografts do not express class II target antigens, but do express class I antigens that in long-established pancreatic islet allografts are capable of acting as targets but not in initiating an immune response.