Immune cell populations in nonhuman primate islets

TCF7 is not essential for glucose homeostasis in mice

Objective

Glucose-dependent insulinotropic polypeptide (GIP) and Glucagon-like peptide-1 (GLP-1) are incretin hormones that exert overlapping yet distinct actions on islet β-cells. We recently observed that GIP, but not GLP-1, upregulated islet expression of Transcription Factor 7 (TCF7), a gene expressed in immune cells and associated with the risk of developing type 1 diabetes. TCF7 has also been associated with glucose homeostasis control in the liver. Herein we studied the relative metabolic importance of TCF7 expression in hepatocytes vs. islet β-cells in mice.

Methods

Tcf7 expression was selectively inactivated in adult mouse hepatocytes using adenoviral Cre expression and targeted in β-cells using two different lines of insulin promoter-Cre mice. Glucose homeostasis, plasma insulin and triglyceride responses, islet histology, hepatic and islet gene expression, and body weight gain were evaluated in mice fed regular chow or high fat diets. Tcf7 expression within pancreatic islets and immune cells was evaluated using published single cell RNA-seq (scRNA-seq) data, and in islet RNA from immunodeficient Rag2^−/−Il2rg^−/− mice.

Results

Reduction of hepatocyte Tcf7 expression did not impair glucose homeostasis, lipid tolerance or hepatic gene expression profiles linked to control of metabolic or immune pathways. Similarly, oral and intraperitoneal glucose tolerance, plasma insulin responses, islet histology, body weight gain, and insulin tolerance were not different in mice with targeted recombination of Tcf7 in insulin-positive β-cells. Surprisingly, islet Tcf7 mRNA transcripts were not reduced in total islet RNA containing endocrine and associated non-endocrine cell types from Tcf7^{βcell−/−} mice, despite Cre-mediated recombination of islet genomic DNA. Furthermore, glucose tolerance was normal in whole body Tcf7^−/− mice. Analysis of scRNA-seq datasets localized pancreatic Tcf7 expression to islet progenitors during development, and immune cells, but not within differentiated islet β-cells or endocrine lineages within mature islets. Moreover, the expression of Tcf7 was extremely low in islet RNA from Rag2^−/−Il2rg^−/− mice and, consistent with expression within immune cells, Tcf7 was highly correlated with levels of Cd3g mRNA transcripts in RNA from wild type mouse islets.

Conclusions

These findings demonstrate that Tcf7 expression is not a critical determinant of glucose homeostasis in mice. Moreover, the detection of Tcf7 expression within islet mRNA is attributable to the expression of Tcf7 RNA in islet-associated murine immune cells, and not in islet β-cells.

•Reduction of hepatocyte Tcf7 does not impair glucose homeostasis.

•Targeting beta cell Tcf7 using insulin-promoter-Cre does not reduce islet Tcf7 expression.

•RNA-seq localizes pancreatic Tcf7 to islet progenitors and lymphocytes.

•Tcf7 expression is markedly reduced in islet RNA from Rag2^−/−Il2rg^−/− mice.

Redox-Dependent Inflammation in Islet Transplantation Rejection

Type 1 diabetes is an autoimmune disease that results in the progressive destruction of insulin-producing pancreatic β-cells inside the islets of Langerhans. The loss of this vital population leaves patients with a lifelong dependency on exogenous insulin and puts them at risk for life-threatening complications. One method being investigated to help restore insulin independence in these patients is islet cell transplantation. However, challenges associated with transplant rejection and islet viability have prevented long-term β-cell function. Redox signaling and the production of reactive oxygen species (ROS) by recipient immune cells and transplanted islets themselves are key players in graft rejection. Therefore, dissipation of ROS generation is a viable intervention that can protect transplanted islets from immune-mediated destruction. Here, we will discuss the newly appreciated role of redox signaling and ROS synthesis during graft rejection as well as new strategies being tested for their efficacy in redox modulation during islet cell transplantation.

Inflammatory response in islet transplantation

Islet cell transplantation is a promising beta cell replacement therapy for patients with brittle type 1 diabetes as well as refractory chronic pancreatitis. Despite the vast advancements made in this field, challenges still remain in achieving high frequency and long-term successful transplant outcomes. Here we review recent advances in understanding the role of inflammation in islet transplantation and development of strategies to prevent damage to islets from inflammation. The inflammatory response associated with islets has been recognized as the primary cause of early damage to islets and graft loss after transplantation. Details on cell signaling pathways in islets triggered by cytokines and harmful inflammatory events during pancreas procurement, pancreas preservation, islet isolation, and islet infusion are presented. Robust control of pre- and peritransplant islet inflammation could improve posttransplant islet survival and in turn enhance the benefits of islet cell transplantation for patients who are insulin dependent. We discuss several potent anti-inflammatory strategies that show promise for improving islet engraftment. Further understanding of molecular mechanisms involved in the inflammatory response will provide the basis for developing potent therapeutic strategies for enhancing the quality and success of islet transplantation.

Do post-translational beta cell protein modifications trigger type 1 diabetes?

Type 1 diabetes is considered an autoimmune disease characterised by specific T cell-mediated destruction of the insulin-producing beta cells. Yet, except for insulin, no beta cell-specific antigens have been discovered. This may imply that the autoantigens in type 1 diabetes exist in modified forms capable of specifically triggering beta cell destruction. In other immune-mediated diseases, autoantigens targeted by the immune system have undergone post-translational modification (PTM), thereby creating tissue-specific neo-epitopes. In a similar manner, PTM of beta cell proteins might create beta cell-specific neo-epitopes. We suggest that the current paradigm of type 1 diabetes as a classical autoimmune disease should be reconsidered since the immune response may not be directed against native beta cell proteins. A modified model for the pathogenetic events taking place in islets leading to the T cell attack against beta cells is presented. In this model, PTM plays a prominent role in triggering beta cell destruction. We discuss literature of relevance and perform genetic and human islet gene expression analyses. Both direct and circumstantial support for the involvement of PTM in type 1 diabetes exists in the published literature. Furthermore, we report that cytokines change the expression levels of several genes encoding proteins involved in PTM processes in human islets, and that there are type 1 diabetes-associated polymorphisms in a number of these. In conclusion, data from the literature and presented experimental data support the notion that PTM of beta cell proteins may be involved in triggering beta cell destruction in type 1 diabetes. If the beta cell antigens recognised by the immune system foremost come from modified proteins rather than native ones, the concept of type 1 diabetes as a classical autoimmune disease is open for debate.

Candidate genes expressed in human islets and their role in the pathogenesis of type 1 diabetes

In type 1 diabetes (T1D), the insulin-producing β cells are destroyed by an immune-mediated process leading to complete insulin deficiency. There is a strong genetic component in T1D. Genes located in the human leukocyte antigen (HLA) region are the most important genetic determinants of disease, but more than 40 additional loci are known to significantly affect T1D risk. Since most of the currently known genetic candidates have annotated immune cell functions, it is generally considered that most of the genetic susceptibility in T1D is caused by variation in genes affecting immune cell function. Recent studies, however, indicate that most T1D candidate genes are expressed in human islets suggesting that the functions of the genes are not restricted to immune cells, but also play roles in the islets and possibly the β cells. Several candidates change expression levels within the islets following exposure to proinflammatory cytokines highlighting that these genes may be involved in the response of β cells to immune attack. In this review, the compiling evidence that many of the candidate genes are expressed in islets and β cells will be presented. Further, we perform the first systematic human islet expression analysis of all genes located in 50 T1D-associated GWAS loci using a published RNA sequencing dataset. We find that 336 out of 857 genes are expressed in human islets and that many of these interact in protein networks. Finally, the potential pathogenetic roles of some candidate genes will be discussed.

Molecularly Engineered Islet Cell Clusters for Diabetes Mellitus Treatment

Pancreatic islet transplantation is a promising method for curing diabetes mellitus. We proposed in this study a molecularly engineered islet cell clusters (ICCs) that could overcome problems posed by islet transplantation circumstances and host's immune reactions. A gene containing highly releasable exendin-4, an insulinotropic protein, was delivered into single islet cells to enhance glucose sensitivity; thereafter, the cells were reaggregated into small size ICCs. Then the surface of ICCs was modified with biocompatible poly(ethylene glycol)-lipid (PEG) (C18) for preventing immune reactions. The regimen of ICCs with low doses of anti-CD154 mAb and tacrolimus could effectively maintain the normal glucose level in diabetic mice. This molecularly engineered PEG-Sp-Ex-4 ICC regimen prevented cell death in transplantation site, partly through improving the regulation of glucose metabolism and by preventing hypoxia- and immune response-induced apoptosis. Application of this remedy is also potentially far-reaching; one would be to help overcome islet supply shortage due to the limited availability of pancreas donors and reduce the immunosuppressant regimens to eliminate their adverse effects.

Outcome of adrenal tissue fragments allotransplantation: the impact of cryopreservation

Cryopreservation is thought to have the potential to preserve tissue for transplantation. In addition, it can also be used for decreasing tissue immunogenicity, which might be important for prolonging allograft survival. In the present study we examined the impact of cryopreservation at various cooling rates on the outcome of allotransplantation of murine adrenal tissue fragments (ATFr). ATFr were cryopreserved with a cooling rate at 1; 10; 40 and more than 100 °C/min. After thawing it was found that the number of the cells expressing markers of dendritic cells (CD11c) and macrophages (CD11b) in the suspension obtained from ATFr decreased with increasing cooling rate. After allotransplantation the survival rates of adrenalectomized mice and the blood serum levels of corticosterone were higher in recipients of cryopreserved ATFr. By immunohistochemistry, cryopreserved allografts displayed a decreased infiltration by CD4+ and CD8+ T-lymphocytes as compared to fresh grafts. These findings suggest that cryopreserved allografts cause a less severe rejection by decreasing graft immunogenicity.

Donor islet endothelial cells in pancreatic islet revascularization

OBJECTIVE

Freshly isolated pancreatic islets contain, in contrast to cultured islets, intraislet endothelial cells (ECs), which can contribute to the formation of functional blood vessels after transplantation. We have characterized how donor islet endothelial cells (DIECs) may contribute to the revascularization rate, vascular density, and endocrine graft function after transplantation of freshly isolated and cultured islets.

RESEARCH DESIGN AND METHODS

Freshly isolated and cultured islets were transplanted under the kidney capsule and into the anterior chamber of the eye. Intravital laser scanning microscopy was used to monitor the revascularization process and DIECs in intact grafts. The grafts' metabolic function was examined by reversal of diabetes, and the ultrastructural morphology by transmission electron microscopy.

RESULTS

DIECs significantly contributed to the vasculature of fresh islet grafts, assessed up to 5 months after transplantation, but were hardly detected in cultured islet grafts. Early participation of DIECs in the revascularization process correlated with a higher revascularization rate of freshly isolated islets compared with cultured islets. However, after complete revascularization, the vascular density was similar in the two groups, and host ECs gained morphological features resembling the endogenous islet vasculature. Surprisingly, grafts originating from cultured islets reversed diabetes more rapidly than those originating from fresh islets.

CONCLUSIONS

In summary, DIECs contributed to the revascularization of fresh, but not cultured, islets by participating in early processes of vessel formation and persisting in the vasculature over long periods of time. However, the DIECs did not increase the vascular density or improve the endocrine function of the grafts.

Mesenchymal Stromal Cells as a Therapeutic Strategy to Support Islet Transplantation in Type 1 Diabetes Mellitus

Type 1 diabetes is an autoimmune disorder that leads to destruction of pancreatic β islet cells and is a growing global health issue. While insulin replacement remains the standard therapy for type 1 diabetes, exogenous insulin does not mimic the physiology of insulin secretion. Transplantation of pancreatic islets has the potential to cure this disease; however, there are several major limitations to widespread implementation of islet transplants. The use of mesenchymal stromal cells (MSCs) in the treatment of type 1 diabetes has been investigated as an adjunct therapy during islet graft administration to prevent initial islet loss and promote engraftment and revascularization of islets. In this review we will discuss the results of recent MSC studies in animal models of diabetes with a focus on islet transplantation and explore the potential for these findings to be extended to clinical use for the treatment of type 1 diabetes.