Autoimmune CD8+ T cells in type 1 diabetes: from single-cell RNA sequencing to T-cell receptor redirection

Type 1 diabetes (T1D) is an organ-specific autoimmune disease caused by pancreatic β cell destruction and mediated primarily by autoreactive CD8+ T cells. It has been shown that only a small number of stem cell-like β cell-specific CD8+ T cells are needed to convert normal mice into T1D mice; thus, it is likely that T1D can be cured or significantly improved by modulating or altering self-reactive CD8+ T cells. However, stem cell-type, effector and exhausted CD8+ T cells play intricate and important roles in T1D. The highly diverse T-cell receptors (TCRs) also make precise and stable targeted therapy more difficult. Therefore, this review will investigate the mechanisms of autoimmune CD8+ T cells and TCRs in T1D, as well as the related single-cell RNA sequencing (ScRNA-Seq), CRISPR/Cas9, chimeric antigen receptor T-cell (CAR-T) and T-cell receptor-gene engineered T cells (TCR-T), for a detailed and clear overview. This review highlights that targeting CD8+ T cells and their TCRs may be a potential strategy for predicting or treating T1D.

The investigation of the effects of monosodium glutamate on healthy rats and rats with STZ-induced diabetes

Monosodium glutamate (MSG, E621) is a flavor-enhancing food additive used widely in the food preparation industry and consumed regularly. It is considered that long-term consumption of MSG causes metabolic syndrome and obesity. Diabetes mellitus (DM) is a chronic metabolic disease characterized by high blood sugar, polyuria, polydipsia, and polyphagia, in which insulin secreted from pancreatic β cells is inadequate for maintaining blood glucose homeostasis. Rats were application 65 mg/kg streptozotocin (STZ) solution intraperitoneally and a diabetes model was created. For this purpose, freshly prepared STZ was injected into the peritoneum. Tumor necrosis factor-α, interleukin (IL)-10, IL-6, and IL-1β levels in STZ, MSG, and STZ + MSG groups were found to be significantly increased in inflammation parameters measured on the 28th day of administration when compared to the Control Group (p < 0.001). Also, although malondialdehyde (MDA) levels increased significantly in the STZ + MSG group when compared to the control group (p < 0.001), glutathione (GSH), and superoxide dismutase (SOD) levels were significantly decreased in the STZ, MSG, and STZ + MSG groups when compared to the control group (p < 0.001). Also, although glucose levels increased significantly in STZ and STZ + MSG at the end of the 28th day (p < 0.01), insulin levels decreased in STZ, MSG, and STZ + MSG groups when compared to the control groups (p < 0.01). As a result, it was found that STZ and MSG application significantly increased cytokine production, increased MDA, which is an oxidant parameter in pancreatic tissue, and decreased antioxidants (GSH and SOD) when compared to the control groups. It was also found that MSG disrupted the normal histological structure in pancreatic cells, and the damage was much more in both exocrine and endocrine pancreatic areas in the STZ + MSG group when compared to the STZ and MSG groups. It was considered that with the increased use of MSG, the susceptibility to DM might increase along with tissue damage significantly in diabetic groups, therefore, MSG must be used in a limited and controlled manner.

A Supportive Role of Mesenchymal Stem Cells on Insulin-Producing Langerhans Islets with a Specific Emphasis on The Secretome

Type 1 Diabetes (T1D) is a chronic autoimmune disease characterized by a gradual destruction of insulin-producing β-cells in the endocrine pancreas due to innate and specific immune responses, leading to impaired glucose homeostasis. T1D patients usually require regular insulin injections after meals to maintain normal serum glucose levels. In severe cases, pancreas or Langerhans islet transplantation can assist in reaching a sufficient β-mass to normalize glucose homeostasis. The latter procedure is limited because of low donor availability, high islet loss, and immune rejection. There is still a need to develop new technologies to improve islet survival and implantation and to keep the islets functional. Mesenchymal stem cells (MSCs) are multipotent non-hematopoietic progenitor cells with high plasticity that can support human pancreatic islet function both in vitro and in vivo and islet co-transplantation with MSCs is more effective than islet transplantation alone in attenuating diabetes progression. The beneficial effect of MSCs on islet function is due to a combined effect on angiogenesis, suppression of immune responses, and secretion of growth factors essential for islet survival and function. In this review, various aspects of MSCs related to islet function and diabetes are described.

Trimetazidine Preconditioning Potentiates the Effect of Mesenchymal Stem Cells Secretome on the Preservation of Rat Pancreatic Islet Survival and Function In Vitro

Islet transplantation offers improved glycemic control in individuals with type 1 diabetes mellitus. However, in vitro islet culture is associated with islet apoptosis and eventually will lose their functionality prior to transplantation. In this study, we examined the effects of mesenchymal stem cells (MSCs) secretome preconditioned with diazoxide (DZ) and trimetazidine (TMZ) on rat islet cells during pre-transplant culture. With and without preconditioned hAD-MSCs' concentrated conditioned media (CCM) were added to the culture medium containing rat islets every 12 h for 24 and 48 h, after testing for selected cytokine concentrations (interleukin (IL)-4, IL-6, IL-13). Insulin content, glucose-stimulated insulin secretion, islet cell apoptosis, and mRNA expression of pro-apoptotic (BAX, BAK-1, and PUMA) and anti-apoptotic factors (BCL-2, BCL-xL, and XIAP) in rat islets were assessed after 24 and 48 h of culture. The protein level of IL-6 and IL-4 was significantly higher in TMZ-MSC-CM compared to MSC-non-CM. In rat isolated islets, normalized secreted insulin in the presence of 16.7 mM glucose was significantly higher in treated islet groups compared to control islets at both 24 and 48 h cultivation. Also, the percentage of apoptotic islet cells TMZ-MSC-CCM-treated islets was significantly lower compared to MSC-CM and MSC-CCM-treated islets in both 24 and 48 h cultivation. Consistent with the number of apoptotic cells, after 24 h culture, the expression of BCL-2 and BCL-xL genes in the control islets was lower than all treatment islet groups and in 48 h was lower than only TMZ-MSC-CM-treated islets. Also, the expression of the XIAP gene in control islets was significantly lower compared to the TMZ-MSC-CCM-treated islets at both at 24 and 48 h. In addition, mRNA level of the BAX gene in TMZ-MSC-CCM-treated islets was significantly lower compared to other groups at 48 h. Our findings revealed that TMZ proved to be more effective than DZ and could enhance the potential of hAD-MSCs-CM to improve the function and viability of islets prior to transplantation.

Interleukin-4 reduces insulin secretion in human islets from healthy but not type-2 diabetic donors

Type 2 diabetes (T2D) is associated with low-grade inflammation. Here we investigate if the anti-inflammatory cytokine interleukin-4 (IL-4) affects glucose-stimulated insulin secretion (GSIS) in human islets from non-diabetic (ND) and type-2 diabetic (T2D) donors. We first confirmed that GSIS is reduced in islets from T2D donors. Treatment with IL-4 for 48 h had no further effect on GSIS in these islets but significantly reduced secretion in ND islets. Acute treatment with IL-4 for 1 h had no effect on GSIS in ND islets which led us to suspect that IL-4 affects a slow cellular mechanism such as gene transcription. IL-4 has been reported to regulate miR-378a-3p and, indeed, we found that this microRNA was increased with IL-4 treatment. However, overexpression of miR-378a-3p in the human beta cell line EndoC-βH1 did not affect GSIS. MiR-378a-3p is transcribed from the same gene as peroxisome proliferator-activated receptor gamma co-activator 1 beta (PCG-1β) and we found that IL-4 treatment showed a clear tendency to increased gene expression of PCG-1β. PCG-1β is a co-activator of peroxisome proliferator-activated receptor gamma (PPARγ) and, the gene expression of PPARγ was also increased with IL-4 treatment. Our data suggests that the protective role of IL-4 on beta cell survival comes at the cost of lowered insulin secretion, presumably involving the PPARγ-pathway.

Protein tyrosine phosphatases (PTPs) in diabetes: causes and therapeutic opportunities

Protein tyrosine phosphatases (PTPs) have an emerging paradigm for the development of antidiabetic drugs. Herein, we provide a comprehensive overview of the relevance of PTPs to type 2 diabetes (T2D) and the therapeutic opportunities thereof, while critically evaluating the potential challenges for PTP inhibitors to be next generation antidiabetics. This review briefly discusses the structure and function of PTPs. An account of importance and relevance of PTPs in various human diseases is presented with special attention to diabetes. The PTPs relevant to T2D have been targeted by small molecule inhibitors such as natural products and synthetic compounds as well as antisense nucleic acids. This review will give better understanding of the important concepts helpful in outlining the strategies for the development of new therapeutic agents with promising antidiabetic activities.

Cytokines and the immune response in obesity-related disorders

The increasing prevalence of obesity and the associated morbidity and mortality are important public health problems globally. There is an important relationship between an unhealthy lifestyle and increased serum inflammatory cytokines. Adipocytes secrete several pro-inflammatory cytokines involved in the recruitment and activation of macrophages resulting in chronic low-grade inflammation. Increased cytokines in obese individual are related to the progression of several disorders including cardiovascular disease, hypertension, and insulin resistance. In present review we have summarized the crucial roles of cytokines and their inflammatory functions in obesity-related immune disorders.

Sphingolipids in Type 1 Diabetes: Focus on Beta-Cells

Type 1 diabetes (T1DM) is a chronic autoimmune disease, with a strong genetic background, leading to a gradual loss of pancreatic beta-cells, which secrete insulin and control glucose homeostasis. Patients with T1DM require life-long substitution with insulin and are at high risk for development of severe secondary complications. The incidence of T1DM has been continuously growing in the last decades, indicating an important contribution of environmental factors. Accumulating data indicates that sphingolipids may be crucially involved in T1DM development. The serum lipidome of T1DM patients is characterized by significantly altered sphingolipid composition compared to nondiabetic, healthy probands. Recently, several polymorphisms in the genes encoding the enzymatic machinery for sphingolipid production have been identified in T1DM individuals. Evidence gained from studies in rodent islets and beta-cells exposed to cytokines indicates dysregulation of the sphingolipid biosynthetic pathway and impaired function of several sphingolipids. Moreover, a number of glycosphingolipids have been suggested to act as beta-cell autoantigens. Studies in animal models of autoimmune diabetes, such as the Non Obese Diabetic (NOD) mouse and the LEW.1AR1-iddm (IDDM) rat, indicate a crucial role of sphingolipids in immune cell trafficking, islet infiltration and diabetes development. In this review, the up-to-date status on the findings about sphingolipids in T1DM will be provided, the under-investigated research areas will be identified and perspectives for future studies will be given.

Cytokines in type 1 diabetes: mechanisms of action and immunotherapeutic targets

Cytokines play crucial roles in orchestrating complex multicellular interactions between pancreatic β cells and immune cells in the development of type 1 diabetes (T1D) and are thus potential immunotherapeutic targets for this disorder. Cytokines that can induce regulatory functions-for example, IL-10, TGF-β and IL-33-are thought to restore immune tolerance and prevent β-cell damage. By contrast, cytokines such as IL-6, IL-17, IL-21 and TNF, which promote the differentiation and function of diabetogenic immune cells, are thought to lead to T1D onset and progression. However, targeting these dysregulated cytokine networks does not always result in consistent effects because anti-inflammatory or proinflammatory functions of cytokines, responsible for β-cell destruction, are context dependent. In this review, we summarise the current knowledge on the involvement of well-known cytokines in both the initiation and destruction phases of T1D and discuss advances in recently discovered roles of cytokines. Additionally, we emphasise the complexity and implications of cytokine modulation therapy and discuss the ways in which this strategy has been translated into clinical trials.

New Insights Into the Role of Autoreactive CD8 T Cells and Cytokines in Human Type 1 Diabetes

Since the establishment of the network for pancreatic organ donors with diabetes (nPOD), we have gained unprecedented insight into the pathology of human type 1 diabetes. Many of the pre-existing "dogmas", mostly derived from studies of animal models and sometimes limited human samples, have to be revised now. For example, we have learned that autoreactive CD8 T cells are present even in healthy individuals within the exocrine pancreas. Furthermore, their "attraction" to islets probably relies on beta-cell intrinsic events, such as the over-expression of MHC class I and resulting presentation of autoantigens such as (prepro)insulin. In addition, we are discovering other signs of beta-cell dysfunction, possibly at least in part due to stress, such as the over-expression of certain cytokines. This review summarizes the latest developments focusing on cytokines and autoreactive CD8 T cells in human type 1 diabetes pathogenesis.

The transcription factor STAT6 plays a critical role in promoting beta cell viability and is depleted in islets of individuals with type 1 diabetes

AIMS/HYPOTHESIS

In type 1 diabetes, selective beta cell loss occurs within the inflamed milieu of insulitic islets. This milieu is generated via the enhanced secretion of proinflammatory cytokines and by the loss of anti-inflammatory molecules such as IL-4 and IL-13. While the actions of proinflammatory cytokines have been well-studied in beta cells, the effects of their anti-inflammatory counterparts have received relatively little attention and we have addressed this.

METHODS

Clonal beta cells, isolated human islets and pancreas sections from control individuals and those with type 1 diabetes were employed. Gene expression was measured using targeted gene arrays and by quantitative RT-PCR. Protein expression was monitored in cell extracts by western blotting and in tissue sections by immunocytochemistry. Target proteins were knocked down selectively with interference RNA.

RESULTS

Cytoprotection achieved with IL-4 and IL-13 is mediated by the early activation of signal transducer and activator of transcription 6 (STAT6) in beta cells, leading to the upregulation of anti-apoptotic proteins, including myeloid leukaemia-1 (MCL-1) and B cell lymphoma-extra large (BCLXL). We also report the induction of signal regulatory protein-α (SIRPα), and find that knockdown of SIRPα is associated with reduced beta cell viability. These anti-apoptotic proteins and their attendant cytoprotective effects are lost following siRNA-mediated knockdown of STAT6 in beta cells. Importantly, analysis of human pancreas sections revealed that STAT6 is markedly depleted in the beta cells of individuals with type 1 diabetes, implying the loss of cytoprotective responses.

CONCLUSIONS/INTERPRETATION

Selective loss of STAT6 may contribute to beta cell demise during the progression of type 1 diabetes.

The competitive nature of signal transducer and activator of transcription complex formation drives phenotype switching of T cells

Signal transducers and activators of transcription (STATs) are key molecular determinants of T-cell fate and effector function. Several inflammatory diseases are characterized by an altered balance of T-cell phenotypes and cytokine secretion. STATs, therefore, represent viable therapeutic targets in numerous pathologies. However, the underlying mechanisms by which the same STAT proteins regulate both the development of different T-cell phenotypes and their plasticity during changes in extracellular conditions remain unclear. In this study, we investigated the STAT-mediated regulation of T-cell phenotype formation and plasticity using mathematical modelling and experimental data for intracellular STAT signalling proteins. The close fit of our model predictions to the experimental data allows us to propose a potential mechanism for T-cell switching. According to this mechanism, T-cell phenotype switching is the result of the relative redistribution of STAT dimer complexes caused by the extracellular cytokine-dependent STAT competition effects. The developed model predicts that the balance between the intracellular STAT species defines the amount of the produced cytokines and thereby T-cell phenotypes. The model predictions are consistent with the experimentally observed interferon-γ to interleukin-10 switching that regulates human T helper type 1/type 1 regulatory T-cell responses. The proposed model is applicable to a number of STAT signalling circuits.

Protein kinase- and lipase inhibitors of inositide metabolism deplete IP7 indirectly in pancreatic β-cells: Off-target effects on cellular bioenergetics and direct effects on IP6K activity

Inositol pyrophosphates have emerged as important regulators of many critical cellular processes from vesicle trafficking and cytoskeletal rearrangement to telomere length regulation and apoptosis. We have previously demonstrated that 5-di-phosphoinositol pentakisphosphate, IP₇, is at a high level in pancreatic β-cells and is important for insulin exocytosis. To better understand IP₇ regulation in β-cells, we used an insulin secreting cell line, HIT-T15, to screen a number of different pharmacological inhibitors of inositide metabolism for their impact on cellular IP₇. Although the inhibitors have diverse targets, they all perturbed IP₇ levels. This made us suspicious that indirect, off-target effects of the inhibitors could be involved. It is known that IP₇ levels are decreased by metabolic poisons. The fact that the inositol hexakisphosphate kinases (IP6Ks) have a high K_m for ATP makes IP₇ synthesis potentially vulnerable to ATP depletion. Furthermore, many kinase inhibitors are targeted to the ATP binding site of kinases, but given the similarity of such sites, high specificity is difficult to achieve. Here, we show that IP₇ concentrations in HIT-T15 cells were reduced by inhibitors of PI3K (wortmannin, LY294002), PI4K (Phenylarsine Oxide, PAO), PLC (U73122) and the insulin receptor (HNMPA). Each of these inhibitors also decreased the ATP/ADP ratio. Thus reagents that compromise energy metabolism reduce IP₇ indirectly. Additionally, PAO, U73122 and LY294002 also directly inhibited the activity of purified IP6K. These data are of particular concern for those studying signal transduction in pancreatic β-cells, but also highlight the fact that employment of these inhibitors could have erroneously suggested the involvement of key signal transduction pathways in various cellular processes. Conversely, IP₇’s role in cellular signal transduction is likely to have been underestimated.

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In pancreatic β-cells several inhibitors of signal transduction reduce IP₇ levels.

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There is a positive correlation between IP₇ reduction and decrease in ATP/ADP.

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Inhibitors deplete IP₇ levels indirectly by decreasing ATP/ADP levels.

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Some purportedly specific cell-signaling inhibitors directly target IP6K activity.

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Caution is required in interpreting data obtained using inhibitors of inositide metabolism.

Differentially expressed JAK-STAT signaling pathway genes and target microRNAs in the spleen of necrotic enteritis-afflicted chicken lines

The JAK signal transducer and STAT signaling pathway is an important regulator of cell proliferation, differentiation, survival, motility, apoptosis, immune response, and development. In this study, we used RNA-Sequencing, qRT-PCR, and bioinformatics tools to investigate the differential expression of JAK-STAT pathway genes, their interactions, and regulators in the spleen of two genetically disparate chicken lines (Marek's disease-resistant line 6.3 and MD-susceptible line 7.2) induced necrotic enteritis (NE) disease by co-infection with Eimeria maxima and Clostridium perfringens. Using RNA-Seq analysis, we identified a total of 116 JAK-STAT pathway genes that were differentially expressed in the spleen of these chickens. All of the identified genes were analyzed through clustering, mapping to the KEGG chicken JAK-STAT pathway, and the Pathway Studio program. Of the 116 JAK-STAT pathway genes, 20 were further verified by qRT-PCR. According to the RNA-Seq results, several key genes, including STAT1-6, JAK1-3, TYK2, AKT1, AKT3, SOCS1-5, PIAS1, PIAS2, PIAS4, SHP1, SHP2, and PIK3, showed marked differential expression in the two lines, relative to their respective controls. Moreover, the RNA-Seq results of many key genes were highly correlated with the qRT-PCR results. Finally, we predicted 63 mature miRNAs that variably target JAK-STAT pathway genes and are differentially expressed in the spleen of chickens of both lines. To the best of our knowledge, this study is the first to analyze most of the genes, interactions, and regulators of the JAK-STAT pathway in the innate immune response to NE disease in chickens.

DPP-4 (CD26) inhibitor sitagliptin exerts anti-inflammatory effects on rat insulinoma (RINm) cells via suppressing NF-κB activation

Dipeptidyl peptidase-4 (CD26), a cell surface glycoprotein, is expressed by a variety of cells. It has been shown that dipeptidyl peptidase-4 (CD26) is involved in T cell activation. Nonetheless, its role in inflammatory effects in islet β cells has not been well investigated. In this study, we used sitagliptin, a classic inhibitor of dipeptidyl peptidase-4 (CD26), to research the effect of dipeptidyl peptidase-4 (CD26) on the activation of NF-κB, the expression of inflammatory cytokines, and cell apoptosis in rat insulinoma cells. Results showed that dipeptidyl peptidase-4 (CD26) was expressed on the surface of rat insulinoma cells. Lipopolysaccharide-induced NF-κB activation and expression of inflammatory cytokines were suppressed by sitagliptin treatment in rat insulinoma cells. Furthermore, sitagliptin treatment reduced cell apoptosis stimulated by lipopolysaccharide. Taken together, this study showed for the first time that sitagliptin suppressed NF-κB activation and inflammatory cytokines expression in rat insulinoma cells, suggesting that the dipeptidyl peptidase-4 inhibitor may exert direct anti-inflammatory effects in islet β cells.

Certain Diet and Lifestyle May Contribute to Islet β-cells Protection in Type-2 Diabetes via the Modulation of Cellular PI3K/AKT Pathway

PI3K/AKT pathway has been shown to play a pivotal role on islet β-cell protection, enhancing β-cell survival by stimulating cell proliferation and inhibiting cell apoptosis. Accordingly, this pathway appears to be crucial in type-2 diabetes. Understanding the regulations of this pathway may provide a better efficacy of new therapeutic approaches. In this review, we summarize advances on the involvement of the PI3K/AKT pathway in hypothetical intra-cellular signaling of islet β-cells. As recent findings may show the nutritional regulation of the survival pathway in the islet β-cells through activation of the PI3K/AKT pathway, we also review studies on the features of several diets, correlated lifestyle, and its signaling pathway involved in type-2 diabetes. The molecular mechanisms contributing to the disease are the subject of considerable investigation, as a better understanding of the pathogenesis will lead to novel therapies against a condition of the disease.

The impact of anti-inflammatory cytokines on the pancreatic β-cell

Considerable efforts have been invested to understand the mechanisms by which pro-inflammatory cytokines mediate the demise of β-cells in type 1 diabetes but much less attention has been paid to the role of anti-inflammatory cytokines as potential cytoprotective agents in these cells. Despite this, there is increasing evidence that anti-inflammatory molecules such as interleukin (IL)-4, IL-10 and IL-13 can exert a direct influence of β-cell function and viability and that the circulating levels of these cytokines may be reduced in type 1 diabetes. Thus, it seems possible that targeting of anti-inflammatory pathways might offer therapeutic potential in this disease. In the present review, we consider the evidence implicating IL-4, IL-10 and IL-13 as cytoprotective agents in the β-cell and discuss the receptor components and downstream signaling pathways that mediate these effects.

Modulation of the peroxiredoxin system by cytokines in insulin-producing RINm5F cells: down-regulation of PRDX6 increases susceptibility of beta cells to oxidative stress

Peroxiredoxins are a family of six antioxidant enzymes (PRDX1-6), and may be an alternative system for the pancreatic beta cells to cope with oxidative stress. This study investigated whether the main diabetogenic pro-inflammatory cytokines or the anti-inflammatory cytokine IL-4 modulate PRDXs levels and putative intracellular pathways important for this process in the insulin-producing RINm5F cells. RINm5F cells expressed significant amounts of PRDX1, PRDX3 and PRDX6 enzymes. Only PRDX6 was modulated by cytokines, showing both mRNA and protein down-regulation following incubation of RINm5F cells with TNF-alpha and IFN-gamma but not with IL-1beta. Separately IFN-gamma or TNF-alpha decreased PRDX6 protein but not mRNA levels. The blockage of the JNK signalling and of the calpains and proteasome proteolysis systems restored PRDX6 protein levels. IL-4 alone did not modulate PRDXs levels. However, pre/co-incubation with IL-4 substantially prevented the decrease in PRDX6 induced by pro-inflammatory cytokines. Knockdown of PRDX6 increased susceptibility of RINm5F cells to the deleterious effects of pro-inflammatory cytokines and to oxidative stress. These results show that, from the PRDXs significantly expressed in RINm5F cells, only PRDX6 is modulated by the diabetogenic cytokines IFN-gamma and TNF-alpha. This PRDX6 down-regulation depends on the calpain and proteasome systems and JNK signalling. PRDX6 is an important enzyme for protection against oxidative stress and the interaction between pro- and anti-inflammatory cytokines might be important to determine the antioxidant capacity of the cells.

Differential effects of interleukin-13 and interleukin-6 on Jak/STAT signaling and cell viability in pancreatic β-cells

Pro-inflammatory cytokines are important mediators of β-cell demise in type 1 diabetes, and similar mechanisms are increasingly implicated in type 2 diabetes, where a state of chronic inflammation may persist. It is likely that the actions of anti-inflammatory cytokines are also altered in diabetes. Cytokines are released from immune cells, which may be recruited to the islets in diabetes, but they can also be produced by islet endocrine cells in response to environmental factors, including enteroviral infection. Since enteroviral infection of islet cells may influence the development of diabetes in humans, we examined the actions of two cytokines, IL-13 and IL-6, whose expression are reported to be altered in β-cells during enteroviral infection. Human and rodent islet cells were shown to express receptors for both IL-13 and IL-6, and treatment with either cytokine resulted in the rapid phosphorylation of STAT3 and STAT6. However, while β-cells were protected against a range of cytotoxic insults during exposure to IL-13, treatment with IL-6 enhanced cytotoxicity and western blotting revealed that IL-13 induced one specific isoform of phospho-STAT6 preferentially. Upon incubation with both cytokines together, the isoform of STAT6 that was upregulated by IL-13 alone was again induced, and the effects of IL-6 on β-cell viability were attenuated. Overall, the results suggest that induction of specific isoforms of STAT family transcription factors may underlie the cytoprotective actions of IL-13, and they imply that selective targeting of specific STAT-mediated signaling components could provide a means to ameliorate the loss of β-cell viability in diabetes.

Changes in microRNA expression contribute to pancreatic β-cell dysfunction in prediabetic NOD mice

During the initial phases of type 1 diabetes, pancreatic islets are invaded by immune cells, exposing β-cells to proinflammatory cytokines. This unfavorable environment results in gene expression modifications leading to loss of β-cell functions. To study the contribution of microRNAs (miRNAs) in this process, we used microarray analysis to search for changes in miRNA expression in prediabetic NOD mice islets. We found that the levels of miR-29a/b/c increased in islets of NOD mice during the phases preceding diabetes manifestation and in isolated mouse and human islets exposed to proinflammatory cytokines. Overexpression of miR-29a/b/c in MIN6 and dissociated islet cells led to impairment in glucose-induced insulin secretion. Defective insulin release was associated with diminished expression of the transcription factor Onecut2, and a consequent rise of granuphilin, an inhibitor of β-cell exocytosis. Overexpression of miR-29a/b/c also promoted apoptosis by decreasing the level of the antiapoptotic protein Mcl1. Indeed, a decoy molecule selectively masking the miR-29 binding site on Mcl1 mRNA protected insulin-secreting cells from apoptosis triggered by miR-29 or cytokines. Taken together, our findings suggest that changes in the level of miR-29 family members contribute to cytokine-mediated β-cell dysfunction occurring during the initial phases of type 1 diabetes.

The humanized NOD/SCID mouse as a preclinical model to study the fate of encapsulated human islets

Despite encouraging results in animal models, the transplantation of microencapsulated islets into humans has not yet reached the therapeutic level. Recent clinical trials using microencapsulated human islets in barium alginate showed the presence of dense fibrotic overgrowth around the microcapsules with no viable islets. The major reason for this is limited understanding of what occurs when encapsulated human islets are allografted. This warrants the need for a suitable small animal model. In this study, we investigated the usefulness of NOD/SCID mice reconstituted with human PBMCs (called humanized NOD/SCID mice) as a preclinical model. In this model, human T cell engraftment could be achieved, and CD45+ cells were observed in the spleen and peripheral blood. Though the engrafted T cells caused a small fibrotic overgrowth around the microencapsulated human islets, this failed to stop the encapsulated islets from functioning in the diabetic recipient mice. The ability of encapsulated islets to survive in this mouse model might partly be attributed to the presence of Th2 cytokines IL-4 and IL-10, which are known to induce graft tolerance. In conclusion, this study showed that the hu-NOD/SCID mouse is not a suitable preclinical model to study the allograft rejection mechanisms of encapsulated human islets. As another result, the maintained viability of transplanted islets on the NOD/SCID background emphasized a critical role of protective mechanisms in autoimmune diabetes transplanted subjects due to specific immunoregulatory effects provided by IL-4 and IL-10.