Perforin-independent beta-cell destruction by diabetogenic CD8(+) T lymphocytes in transgenic nonobese diabetic mice

Pancreatic Islet Cells Response to IFNγ Relies on Their Spatial Location within an Islet

Type 1 diabetes (T1D) is an auto-immune disease characterized by the progressive destruction of insulin-producing pancreatic beta cells. While beta cells are the target of the immune attack, the other islet endocrine cells, namely the alpha and delta cells, can also be affected by the inflammatory milieu. Here, using a flow cytometry-based strategy, we compared the impact of IFNγ, one of the main cytokines involved in T1D, on the three endocrine cell subsets isolated from C57BL/6 mouse islets. RNA-seq analyses revealed that alpha and delta cells exposed in vitro to IFNγ display a transcriptomic profile very similar to that of beta cells, with an increased expression of inflammation key genes such as MHC class I molecules, the CXCL10 chemokine and the programmed death-ligand 1 (PD-L1), three hallmarks of IFNγ signaling. Interestingly, at low IFNγ concentration, we observed two beta cell populations (responders and non-responders) based on PD-L1 protein expression. Our data indicate that this differential sensitivity relies on the location of the cells within the islet rather than on the existence of two different beta cells subsets. The same findings were corroborated by the in vivo analysis of pancreatic islets from the non-obese diabetic mouse model of T1D, showing more intense PD-L1 staining on endocrine cells close to immune infiltrate. Collectively, our work demonstrates that alpha and delta cells are as sensitive as beta cells to IFNγ, and suggests a gradual diffusion of the cytokine into an islet. These observations provide novel insights into the in situ inflammatory processes occurring in T1D progression.

Beta Cell Therapies for Preventing Type 1 Diabetes: From Bench to Bedside

Type 1 diabetes (T1D) is a chronic metabolic disease characterized by insulin deficiency, generally resulting from progressive autoimmune-mediated destruction of pancreatic beta cells. While the phenomenon of beta cell autoimmunity continues to be an active area of investigation, recent evidence suggests that beta cell stress responses are also important contributors to disease onset. Here we review the pathways driving different kinds of beta cell dysfunction and their respective therapeutic targets in the prevention of T1D. We discuss opportunities and important open questions around the effectiveness of beta cell therapies and challenges for clinical utility. We further evaluate ways in which beta cell drug therapy could be combined with immunotherapy for preventing T1D in light of our growing appreciation of disease heterogeneity and patient endotypes. Ultimately, the emergence of pharmacologic beta cell therapies for T1D have armed us with new tools and closing the knowledge gaps in T1D etiology will be essential for maximizing the potential of these approaches.

Pancreatic nerve electrostimulation inhibits recent-onset autoimmune diabetes

Vagus nerve stimulation can ameliorate autoimmune diseases such as rheumatoid arthritis by modulation of the immune system. Its efficacy for the treatment of type 1 diabetes has not been explored, in part because the nerves projecting to the pancreatic lymph nodes (pLNs) in mice are unmapped. Here, we map the nerve projecting to the pancreas and pLNs in mice and use a minimally invasive surgical procedure to implant micro-cuff electrodes onto the nerve. Pancreatic nerve electrical stimulation (PNES) resulted in β-adrenergic receptor-mediated-accumulation of B and T cells in pLNs and reduced production of pro-inflammatory cytokines following lipopolysaccharide stimulation. Autoreactive T cells showed reduced proliferation in pLNs of mice receiving PNES as compared to sham controls. In a spontaneous mouse model of autoimmune diabetes, PNES inhibited disease progression in diabetic mice.

Soluble FAS ligand is not required for pancreatic islet inflammation or beta-cell destruction in non-obese diabetic mice

CD8⁺ T cells play a central role in beta-cell destruction in type 1 diabetes. CD8⁺ T cells use two main effector pathways to kill target cells, perforin plus granzymes and FAS ligand (FASL). We and others have established that in non-obese diabetic (NOD) mice, perforin is the dominant effector molecule by which autoreactive CD8⁺ T cells kill beta cells. However, blocking FASL pharmacologically was shown to protect NOD mice from diabetes, indicating that FASL may have some role. FASL can engage with its receptor FAS on target cells either as membrane bound or soluble FASL. It has been shown that membrane-bound FASL is required to stimulate FAS-induced apoptosis in target cells, whereas excessive soluble FASL can induce NF-κB-dependent gene expression and inflammation. Because islet inflammation is a feature of autoimmune diabetes, we tested whether soluble FASL could be important in disease pathogenesis independent of its cell death function. We generated NOD mice deficient in soluble FASL, while maintaining expression of membrane-bound FASL due to a mutation in the FASL sequence required for cleavage by metalloproteinase. NOD mice lacking soluble FASL had normal numbers of lymphocytes in their spleen and thymus. Soluble FASL deficient NOD mice had similar islet inflammation as wild-type NOD mice and were not protected from diabetes. Our data indicate that soluble FASL is not required in development of autoimmune diabetes.

Mitochondrial Reactive Oxygen Species and Type 1 Diabetes

SIGNIFICANCE

The complex etiology of type 1 diabetes (T1D) is the outcome of failures in regulating immunity in combination with beta cell perturbations. Mitochondrial dysfunction in beta cells and immune cells may be involved in T1D pathogenesis. Mitochondrial energy production is essential for the major task of beta cells (the secretion of insulin in response to glucose). Mitochondria are a major site of reactive oxygen species (ROS) production. Under immune attack, mitochondrial ROS (mtROS) participate in beta cell damage. Similarly, T cell fate during immune responses is tightly regulated by mitochondrial physiology, morphology, and metabolism. Production of mtROS is essential for signaling in antigen-specific T cell activation. Mitochondrial dysfunction in T cells has been noted as a feature of some human autoimmune diseases. Recent Advances: Preclinical and clinical studies indicate that mitochondrial dysfunction in beta cells sensitizes these cells to immune-mediated destruction via direct or indirect mechanisms. Sensitivity of beta cells to mtROS is associated with genetic T1D risk loci in human and the T1D-prone nonobese diabetic (NOD) mouse. Mitochondrial dysfunction and altered metabolism have also been observed in immune cells of NOD mice and patients with T1D. This immune cell mitochondrial dysfunction has been linked to deleterious functional changes.

CRITICAL ISSUES

It remains unclear how mitochondria control T cell receptor signaling and downstream events, including calcium flux and activation of transcription factors during autoimmunity.

FUTURE DIRECTIONS

Mechanistic studies are needed to investigate the mitochondrial pathways involved in autoimmunity, including T1D. These studies should seek to identify the role of mitochondria in regulating innate and adaptive immune cell activity and beta cell failure.

Neutralization Versus Reinforcement of Proinflammatory Cytokines to Arrest Autoimmunity in Type 1 Diabetes

As physiological pathways of intercellular communication produced by all cells, cytokines are involved in the pathogenesis of inflammatory insulitis as well as pivotal mediators of immune homeostasis. Proinflammatory cytokines including interleukins, interferons, transforming growth factor-β, tumor necrosis factor-α, and nitric oxide promote destructive insulitis in type 1 diabetes through amplification of the autoimmune reaction, direct toxicity to β-cells, and sensitization of islets to apoptosis. The concept that neutralization of cytokines may be of therapeutic benefit has been tested in few clinical studies, which fell short of inducing sustained remission or achieving disease arrest. Therapeutic failure is explained by the redundant activities of individual cytokines and their combinations, which are rather dispensable in the process of destructive insulitis because other cytolytic pathways efficiently compensate their deficiency. Proinflammatory cytokines are less redundant in regulation of the inflammatory reaction, displaying protective effects through restriction of effector cell activity, reinforcement of suppressor cell function, and participation in islet recovery from injury. Our analysis suggests that the role of cytokines in immune homeostasis overrides their contribution to β-cell death and may be used as potent immunomodulatory agents for therapeutic purposes rather than neutralized.

Perforin facilitates beta cell killing and regulates autoreactive CD8+ T-cell responses to antigen in mouse models of type 1 diabetes

In type 1 diabetes, cytotoxic CD8(+) T lymphocytes (CTLs) directly interact with pancreatic beta cells through major histocompatibility complex class I. An immune synapse facilitates delivery of cytotoxic granules, comprised mainly of granzymes and perforin. Perforin deficiency protects the majority of non-obese diabetic (NOD) mice from autoimmune diabetes. Intriguingly perforin deficiency does not prevent diabetes in CD8(+) T-cell receptor transgenic NOD8.3 mice. We therefore investigated the importance of perforin-dependent killing via CTL-beta cell contact in autoimmune diabetes. Perforin-deficient CTL from NOD mice or from NOD8.3 mice were significantly less efficient at adoptive transfer of autoimmune diabetes into NODRag1(-/-) mice, confirming that perforin is essential to facilitate beta cell destruction. However, increasing the number of transferred in vitro-activated perforin-deficient 8.3 T cells reversed the phenotype and resulted in diabetes. Perforin-deficient NOD8.3 T cells were present in increased proportion in islets, and proliferated more in response to antigen in vivo indicating that perforin may regulate the activation of CTLs, possibly by controlling cytokine production. This was confirmed when we examined the requirement for direct interaction between beta cells and CD8(+) T cells in NOD8.3 mice, in which beta cells specifically lack major histocompatibility complex (MHC) class I through conditional deletion of β2-microglobulin. Although diabetes was significantly reduced, 40% of these mice developed diabetes, indicating that NOD8.3 T cells can kill beta cells in the absence of direct interaction. Our data indicate that although perforin delivery is the main mechanism that CTL use to destroy beta cells, they can employ alternative mechanisms to induce diabetes in a perforin-independent manner.

MDA5 Is Critical to Host Defense during Infection with Murine Coronavirus

Infection with the murine coronavirus mouse hepatitis virus (MHV) activates the pattern recognition receptors melanoma differentiation-associated gene 5 (MDA5) and Toll-like receptor 7 (TLR7) to induce transcription of type I interferon. Type I interferon is crucial for control of viral replication and spread in the natural host, but the specific contributions of MDA5 signaling to this pathway as well as to pathogenesis and subsequent immune responses are largely unknown. In this study, we use MHV infection of the liver as a model to demonstrate that MDA5 signaling is critically important for controlling MHV-induced pathology and regulation of the immune response. Mice deficient in MDA5 expression (MDA5(-/-) mice) experienced more severe disease following MHV infection, with reduced survival, increased spread of virus to additional sites of infection, and more extensive liver damage than did wild-type mice. Although type I interferon transcription decreased in MDA5(-/-) mice, the interferon-stimulated gene response remained intact. Cytokine production by innate and adaptive immune cells was largely intact in MDA5(-/-) mice, but perforin induction by natural killer cells and levels of interferon gamma, interleukin-6 (IL-6), and tumor necrosis factor alpha (TNF-α) in serum were elevated. These data suggest that MDA5 signaling reduces the severity of MHV-induced disease, at least in part by reducing the intensity of the proinflammatory cytokine response.

IMPORTANCE

Multicellular organisms employ a wide range of sensors to detect viruses and other pathogens. One such sensor, MDA5, detects viral RNA and triggers induction of type I interferons, chemical messengers that induce inflammation and help regulate the immune responses. In this study, we sought to determine the role of MDA5 during infection with mouse hepatitis virus, a murine coronavirus used to model viral hepatitis as well as other human diseases. We found that mice lacking the MDA5 sensor were more susceptible to infection than were mice with MDA5 and experienced decreased survival. Viral replication in the liver was similar in mice with and without MDA5, but liver damage was increased in MDA5(-/-) mice, suggesting that the immune response is causing the damage. Production of several proinflammatory cytokines was elevated in MDA5(-/-) mice, suggesting that MDA5 may be responsible for keeping pathological inflammatory responses in check.

Perforin is a novel immune regulator of obesity-related insulin resistance

Obesity-related insulin resistance is associated with an influx of pathogenic T cells into visceral adipose tissue (VAT), but the mechanisms regulating lymphocyte balance in such tissues are unknown. Here we describe an important role for the immune cytotoxic effector molecule perforin in regulating this process. Perforin-deficient mice (Prf1(null)) show early increased body weight and adiposity, glucose intolerance, and insulin resistance when placed on high-fat diet (HFD). Regulatory effects of perforin on glucose tolerance are mechanistically linked to the control of T-cell proliferation and cytokine production in inflamed VAT. HFD-fed Prf1(null) mice have increased accumulation of proinflammatory IFN-γ-producing CD4(+) and CD8(+) T cells and M1-polarized macrophages in VAT. CD8(+) T cells from the VAT of Prf1(null) mice have increased proliferation and impaired early apoptosis, suggesting a role for perforin in the regulation of T-cell turnover during HFD feeding. Transfer of CD8(+) T cells from Prf1(null) mice into CD8-deficient mice (CD8(null)) resulted in worsening of metabolic parameters compared with wild-type donors. Improved metabolic parameters in HFD natural killer (NK) cell-deficient mice (NK(null)) ruled out a role for NK cells as a single source of perforin in regulating glucose homeostasis. The findings support the importance of T-cell function in insulin resistance and suggest that modulation of lymphocyte homeostasis in inflamed VAT is one possible avenue for therapeutic intervention.

Pancreatic beta cells are highly susceptible to oxidative and ER stresses during the development of diabetes

The complex interplay of many cell types and the temporal heterogeneity of pancreatic islet composition obscure the direct role of resident alpha and beta cells in the development of Type 1 diabetes. Therefore, in addition to studying islets isolated from non-obese diabetic mice, we analyzed homogeneous cell populations of murine alpha (αTC-1) and beta (NIT-1) cell lines to understand the role and differential survival of these two predominant islet cell populations. A total of 56 proteins in NIT-1 cells and 50 in αTC-1 cells were differentially expressed when exposed to proinflammatory cytokines. The major difference in the protein expression between cytokine-treated NIT-1 and αTC-1 cells was free radical scavenging enzymes. A similar observation was made in cytokine-treated whole islets, where a comprehensive analysis of subcellular fractions revealed that 438 unique proteins were differentially expressed under inflammatory conditions. Our data indicate that beta cells are relatively susceptible to ER and oxidative stress and reveal key pathways that are dysregulated in beta cells during cytokine exposure. Additionally, in the islets, inflammation also leads to enhanced antigen presentation, which completes a three-way insult on beta cells, rendering them targets of infiltrating T lymphocytes.

Effects of metalloporphyrins on reducing inflammation and autoimmunity

SIGNIFICANCE

High levels of reactive oxygen species can facilitate DNA and protein damage beyond the control of endogenous antioxidants, resulting in oxidative stress. Oxidative stress then triggers inflammation, which can lead to pathological conditions. In genetically susceptible individuals, the conglomeration of oxidative stress and inflammation can enhance autoreactive immune cell activation, causing beta-cell destruction in autoimmune type 1 diabetes. As a means of shielding pancreatic islets, manganese porphyrin (MnP) oxidoreductant treatment has been tested in a number of reported studies.

RECENT ADVANCES

MnP affects both innate and adaptive immune cell responses, blocking nuclear factor kappa-B activation, proinflammatory cytokine secretion, and T helper 1 T-cell responses. As a result, MnP treatment protects against type 1 diabetes onset in nonobese diabetic mice and stabilizes islets for cellular transplantation.

CRITICAL ISSUES

MnP displays global immunosuppressive properties, exemplified by decreased cytokine production from all T-helper cell subsets. This quality may impact infection control in the setting of autoimmunity. Nonetheless, because of their cytoprotective and immunomodulatory function, MnPs should be considered as a safer alternative to other clinical immunosuppressive agents (i.e., rapamycin) for transplantation.

FUTURE DIRECTIONS

Although MnP likely affects only redox-sensitive targets, the mechanism behind global T-cell immunosuppression and the outcome on infection clearance will have to be elucidated. Based on the increased primary engraftment seen with MnP use, protection against primary nonfunction in porcine to human xenotransplants would likely be enhanced. Further, a better understanding of MnP oxidoreductase function may allow for its use in other chronic inflammatory conditions.

BAFF regulates activation of self-reactive T cells through B-cell dependent mechanisms and mediates protection in NOD mice

Targeting the BAFF/APRIL system has shown to be effective in preventing T-cell dependent autoimmune disease in the NOD mouse, a spontaneous model of type 1 diabetes. In this study we generated BAFF-deficient NOD mice to examine how BAFF availability would influence T-cell responses in vivo and the development of spontaneous diabetes. BAFF-deficient NOD mice which lack mature B cells, were protected from diabetes and showed delayed rejection of an allogeneic islet graft. Diabetes protection correlated with a failure to expand pathogenic IGRP-reactive CD8(+) T cells, which were maintained in the periphery at correspondingly low levels. Adoptive transfer of IGRP-reactive CD8(+) T cells with B cells into BAFF-deficient NOD mice enhanced IGRP-reactive CD8(+) T-cell expansion. Furthermore, when provoked with cyclophosphamide, or transferred to a secondary lymphopenic host, the latent pool of self-reactive T cells resident in BAFF-deficient NOD mice could elicit beta cell destruction. We conclude that lack of BAFF prevents the procurement of B-cell-dependent help necessary for the emergence of destructive diabetes. Indeed, treatment of NOD mice with the BAFF-blocking compound, BR3-Fc, resulted in a delayed onset and reduced incidence of diabetes.

Perforin and human diseases

Natural killer (NK) cells and cytotoxic T lymphocytes (CTL) use a highly toxic pore-forming protein perforin (PFN) to destroy cells infected with intracellular pathogens and cells with pre-cancerous transformations. However, mutations of PFN and defects in its expression can cause an abnormal function of the immune system and difficulties in elimination of altered cells. As discussed in this chapter, deficiency of PFN due to the mutations of its gene, PFN1, can be associated with malignancies and severe immune disorders such as familial hemophagocytic lymphohistiocytosis (FHL) and macrophage activation syndrome. On the other hand, overactivity of PFN can turn the immune system against autologous cells resulting in other diseases such as systemic lupus erythematosus, polymyositis, rheumatoid arthritis and cutaneous inflammation. PFN also has a crucial role in the cellular rejection of solid organ allografts and destruction of pancreatic β-cells resulting in type 1 diabetes. These facts highlight the importance of understanding the biochemical characteristics of PFN.

Genetic deletion of granzyme B does not confer resistance to the development of spontaneous diabetes in non-obese diabetic mice

Granzyme B (GzmB) and perforin are proteins, secreted mainly by natural killer cells and cytotoxic T lymphocytes that are largely responsible for the induction of apoptosis in target cells. Because type 1 diabetes results from the selective destruction of β cells and perforin deficiency effectively reduces diabetes in non-obese diabetic (NOD) mice, it can be deduced that β cell apoptosis involves the GzmB/perforin pathway. However, the relevance of GzmB remains totally unknown in non-obese diabetic (NOD) mice. In this study we have focused on GzmB and examined the consequence of GzmB deficiency in NOD mice. We found that NOD.GzmB(-/-) mice developed diabetes spontaneously with kinetics similar to those of wild-type NOD (wt-NOD) mice. Adoptive transfer study with regulatory T cell (Treg )-depleted splenocytes (SPCs) into NOD-SCID mice or in-vivo Treg depletion by anti-CD25 antibody at 4 weeks of age comparably induced the rapid progression of diabetes in the NOD.GzmB(-/-) mice and wt-NOD mice. Expression of GzmA and Fas was enhanced in the islets from pre-diabetic NOD.GzmB(-/-) mice. In contrast to spontaneous diabetes, GzmB deficiency suppressed the development of cyclophosphamide-promoted diabetes in male NOD mice. Cyclophosphamide treatment led to a significantly lower percentage of apoptotic CD4(+) , CD8(+) and CD4(+) CD25(+) T cells in SPCs from NOD.GzmB(-/-) mice than those from wt-NOD mice. In conclusion, GzmB, in contrast to perforin, is not essentially involved in the effector mechanisms for β cell destruction in NOD mice.

Accelerated type 1 diabetes induction in mice by adoptive transfer of diabetogenic CD4+ T cells

The nonobese diabetic (NOD) mouse spontaneously develops autoimmune diabetes after 12 weeks of age and is the most extensively studied animal model of human Type 1 diabetes (T1D). Cell transfer studies in irradiated recipient mice have established that T cells are pivotal in T1D pathogenesis in this model. We describe herein a simple method to rapidly induce T1D by adoptive transfer of purified, primary CD4+ T cells from pre-diabetic NOD mice transgenic for the islet-specific T cell receptor (TCR) BDC2.5 into NOD.SCID recipient mice. The major advantages of this technique are that isolation and adoptive transfer of diabetogenic T cells can be completed within the same day, irradiation of the recipients is not required, and a high incidence of T1D is elicited within 2 weeks after T cell transfer. Thus, studies of pathogenesis and therapeutic interventions in T1D can proceed at a faster rate than with methods that rely on heterogenous T cell populations or clones derived from diabetic NOD mice.

Pathogenic mechanisms in type 1 diabetes: the islet is both target and driver of disease

Recent advances in our understanding of the pathogenesis of type 1 diabetes have occurred in all steps of the disease. This review outlines the pathogenic mechanisms utilized by the immune system to mediate destruction of the pancreatic beta-cells. The autoimmune response against beta-cells appears to begin in the pancreatic lymph node where T cells, which have escaped negative selection in the thymus, first meet beta-cell antigens presented by dendritic cells. Proinsulin is an important antigen in early diabetes. T cells migrate to the islets via the circulation and establish insulitis initially around the islets. T cells within insulitis are specific for islet antigens rather than bystanders. Pathogenic CD4⁺ T cells may recognize peptides from proinsulin which are produced locally within the islet. CD8⁺ T cells differentiate into effector T cells in islets and then kill beta-cells, primarily via the perforin-granzyme pathway. Cytokines do not appear to be important cytotoxic molecules in vivo. Maturation of the immune response within the islet is now understood to contribute to diabetes, and highlights the islet as both driver and target of the disease. The majority of our knowledge of these pathogenic processes is derived from the NOD mouse model, although some processes are mirrored in the human disease. However, more work is required to translate the data from the NOD mouse to our understanding of human diabetes pathogenesis. New technology, especially MHC tetramers and modern imaging, will enhance our understanding of the pathogenic mechanisms.

B-cell cross-presentation of autologous antigen precipitates diabetes

For autoimmune conditions like type 1 diabetes to progress, self-reactive CD8⁺ T cells would need to interact with peptide-antigen cross-presented on the surface of antigen-presenting cells in a major histocompatibility complex (MHC) class I-restricted fashion. However, the mechanisms by which autoantigen is cross-presented remain to be identified. In this study, we show cross-presentation of islet-derived autoantigens by B cells. B cells engage self-reactive CD8⁺ T cells in the pancreatic lymph node, driving their proliferative expansion and differentiation into granzyme B⁺interferon-γ⁺lysosomal-associated membrane protein 1⁺ effector cells. B-cell cross-presentation of insulin required proteolytic cleavage and endosomal localization and was sensitive to inhibitors of protein trafficking. Absent B-cell MHC class I, or B-cell receptor restriction to an irrelevant specificity, blunted the expansion of self-reactive CD8⁺ T cells, suggesting B-cell antigen capture and presentation are critical in vivo events for CD8 activation. Indeed, the singular loss of B-cell MHC class I subverted the conversion to clinical diabetes in NOD mice, despite the presence of a pool of activated, and B cell-dependent, interleukin-21-expressing Vβ4⁺CD4⁺ T cells. Thus, B cells govern the transition from clinically silent insulitis to frank diabetes by cross-presenting autoantigen to self-reactive CD8⁺ T cells.

Cytotoxic mechanisms employed by mouse T cells to destroy pancreatic β-cells

Several cytotoxic mechanisms have been attributed to T cells participating in β-cell death in type 1 diabetes. However, sensitivity of β-cells to these mechanisms in vitro and in vivo is likely to be different. Moreover, CD4⁺ and CD8⁺ T cells may use distinct mechanisms to cause β-cell demise that possibly involve activation of third-party cytotoxic cells. We used the transfer of genetically modified diabetogenic T cells into normal, mutant, and bone marrow chimeric recipients to test the contribution of major cytotoxic mechanisms in β-cell death. We found that 1) the killing of β-cells by CD4⁺ T cells required activation of the recipient's own cytotoxic cells via tumor necrosis factor-α (TNF-α); 2) CD8⁺ T-cell cytotoxic mechanisms destroying β-cells were limited to perforin and Fas ligand, as double knockouts of these molecules abrogated the ability of T cells to cause diabetes; and 3) individual CD8⁺ T-cell clones chose their cytotoxic weaponry by a yet unknown mechanism and destroyed their targets via either Fas-independent or Fas-dependent (~40% of clones) pathways. Fas-dependent destruction was assisted by TNF-α.

Granzyme B is dispensable in the development of diabetes in non-obese diabetic mice

Pancreatic beta cell destruction in type 1 diabetes is mediated by cytotoxic CD8(+) T lymphoctyes (CTL). Granzyme B is an effector molecule used by CTL to kill target cells. We previously showed that granzyme B-deficient allogeneic CTL inefficiently killed pancreatic islets in vitro. We generated granzyme B-deficient non-obese diabetic (NOD) mice to test whether granzyme B is an important effector molecule in spontaneous type 1 diabetes. Granzyme B-deficient islet antigen-specific CD8(+) T cells had impaired homing into islets of young mice. Insulitis was reduced in granzyme B-deficient mice at 70 days of age (insulitis score 0.043±0.019 in granzyme B-deficient versus 0.139±0.034 in wild-type NOD mice p<0.05), but was similar to wild-type at 100 and 150 days of age. We observed a reduced frequency of CD3(+)CD8(+) T cells in the islets and peripheral lymphoid tissues of granzyme B-deficient mice (p<0.005 and p<0.0001 respectively), but there was no difference in cell proportions in the thymus. Antigen-specific CTL developed normally in granzyme B-deficient mice, and were able to kill NOD islet target cells as efficiently as wild-type CTL in vitro. The incidence of spontaneous diabetes in granzyme B-deficient mice was the same as wild-type NOD mice. We observed a delayed onset of diabetes in granzyme B-deficient CD8-dependent NOD8.3 mice (median onset 102.5 days in granzyme B-deficient versus 57.50 days in wild-type NOD8.3 mice), which may be due to the delayed onset of insulitis or inefficient priming at an earlier age in this accelerated model of diabetes. Our data indicate that granzyme B is dispensable for beta cell destruction in type 1 diabetes, but is required for efficient early activation of CTL.

Apoptosis of human islet cells by cytokines

FasL, perforin, TNFα, IL-1 and NO have been considered as effector molecule(s) leading to β-cell death in autoimmune diabetes. However, the real culprit(s) of β-cell destruction have long been elusive despite intense investigation. Previously we have suggested IFNγ/TNFα synergism as the final effector molecules in autoimmune diabetes of NOD mice. A combination of IFNγ and TNFα but neither cytokine alone, induced classical caspase-dependent apoptosis in murine insulinoma and pancreatic islet cells. IFNγ treatment conferred susceptibility to TNFα-induced apoptosis on otherwise resistant murine insulinoma cells by STAT1 activation followed by IRF-1 induction. Here we report that IFNγ/TNFα synergism induces apoptosis of human pancreatic islet cells. We also observed STAT1 activation followed by IRF-1 induction by IFNγ treatment in human islet cells. Taken together, we suggest that IFNγ/TNFα synergism could be involved in human islet cell death in type 1 diabetes, similar to murine type 1 diabetes.

The pro-apoptotic BH3-only protein Bid is dispensable for development of insulitis and diabetes in the non-obese diabetic mouse

Type 1 diabetes is caused by death of insulin-producing pancreatic beta cells. Beta-cell apoptosis induced by FasL may be important in type 1 diabetes in humans and in the non-obese diabetic (NOD) mouse model. Deficiency of the pro-apoptotic BH3-only molecule Bid protects beta cells from FasL-induced apoptosis in vitro. We aimed to test the requirement for Bid, and the significance of Bid-dependent FasL-induced beta-cell apoptosis in type 1 diabetes. We backcrossed Bid-deficient mice, produced by homologous recombination and thus without transgene overexpression, onto a NOD genetic background. Genome-wide single nucleotide polymorphism analysis demonstrated that diabetes-related genetic regions were NOD genotype. Transferred beta cell antigen-specific CD8+ T cells proliferated normally in the pancreatic lymph nodes of Bid-deficient mice. Moreover, Bid-deficient NOD mice developed type 1 diabetes and insulitis similarly to wild-type NOD mice. Our data indicate that beta-cell apoptosis in type 1 diabetes can proceed without Fas-induced killing mediated by the BH3-only protein Bid.

Intracellular pathways of pancreatic β-cell apoptosis in type 1 diabetes

BACKGROUND

Apoptosis of β cells is a feature of type 1 diabetes. It is also increasingly recognized in type 2 diabetes and islet graft rejection.

METHODS

We have studied the intracellular pathways that regulate β-cell apoptosis in type 1 and 2 diabetes. We have examined the role of Bid, a pro-apoptotic member of the Bcl-2 family, using islets from mice deficient in Bid. We also studied the Bcl-2 family molecules involved in killing by using high concentrations of reducing sugars such as glucose or ribose.

RESULTS

We found that Bid-deficient islets are protected from recombinant human perforin and granzyme B, as well as from Fas-mediated killing. This makes Bid a target for protection of β cells from multiple insults relevant to type 1 diabetes. In contrast to granzyme B and death receptor signalling, we found that islets lacking Bim or Puma were protected from glucose toxicity.

CONCLUSIONS

Our data indicate that different stimuli activate different initiator molecules in the Bcl-2-regulated pathway in β cells.

Oxidative stress and redox modulation potential in type 1 diabetes

Redox reactions are imperative to preserving cellular metabolism yet must be strictly regulated. Imbalances between reactive oxygen species (ROS) and antioxidants can initiate oxidative stress, which without proper resolve, can manifest into disease. In type 1 diabetes (T1D), T-cell-mediated autoimmune destruction of pancreatic β-cells is secondary to the primary invasion of macrophages and dendritic cells (DCs) into the islets. Macrophages/DCs, however, are activated by intercellular ROS from resident pancreatic phagocytes and intracellular ROS formed after receptor-ligand interactions via redox-dependent transcription factors such as NF-κB. Activated macrophages/DCs ferry β-cell antigens specifically to pancreatic lymph nodes, where they trigger reactive T cells through synapse formation and secretion of proinflammatory cytokines and more ROS. ROS generation, therefore, is pivotal in formulating both innate and adaptive immune responses accountable for islet cell autoimmunity. The importance of ROS/oxidative stress as well as potential for redox modulation in the context of T1D will be discussed.

Induction of pathogenic cytotoxic T lymphocyte tolerance by dendritic cells: a novel therapeutic target

IMPORTANCE OF THE FIELD

Dendritic cells (DCs) have an important role, both direct and indirect, in controlling the expansion and function of T cells. Of the different subsets of T cells, cytotoxic T lymphocytes (CTLs/CD8(+) T cells) have been implicated in the pathogenesis and development of many diseases, including various forms of autoimmunity and transplant rejection. It may therefore be of therapeutic benefit to control the function of CTL in order to modulate disease processes and to ameliorate disease symptoms. Currently, pharmacological approaches have been employed to either directly or indirectly modulate the function of T cells. However, these treatment strategies have many limitations. Many experimental data have suggested that it is possible to alter CTL activity through manipulation of DC.

AREAS COVERED IN THIS REVIEW

Novel strategies that condition DCs to influence disease outcome through manipulation of CTL activity, both directly and indirectly. This includes the modulation of co-stimulation, negative co-stimulation, as well as manipulation of the cytokine milieu during CTL generation. Furthermore, DCs may also impact CTL activity through effects on effector and regulatory cells, along with manipulation of bioenergetic regulation, apoptotic-cell mediated tolerance and through the generation of exosomes. The implications of related interventions in the clinical arena are in turn considered.

WHAT THE READER WILL GAIN

Insight into such indirect methods of controlling CTL activity allows for an understanding of how disease-specific T cells may be regulated, while also sparing other aspects of adaptive immunity for normal physiological function. Such an approach towards the treatment of disease represents an innovative therapeutic target in the clinical arena.

TAKE HOME MESSAGE

There are numerous innovative methods for using DCs to control CTL responses. Manipulation of this interaction is thus an attractive avenue for the treatment of disease, particularly those of immune dysregulation, such as seen in autoimmunity and transplantation. With the number of studies moving into clinical stages constantly increasing, further advances and successes in this area are inevitable.

Dendritic cells recruit T cell exosomes via exosomal LFA-1 leading to inhibition of CD8+ CTL responses through downregulation of peptide/MHC class I and Fas ligand-mediated cytotoxicity

Active T cells release bioactive exosomes (EXOs). However, its potential modulation in immune responses is elusive. In this study, we in vitro generated active OVA-specific CD8(+) T cells by cultivation of OVA-pulsed dendritic cells (DC(OVA)) with naive CD8(+) T cells derived from OVA-specific TCR transgenic OTI mice and purified EXOs from CD8(+) T cell culture supernatant by differential ultracentrifugation. We then investigated the suppressive effect of T cell EXOs on DC(OVA)-mediated CD8(+) CTL responses and antitumor immunity. We found that DC(OVA) uptake OTI T cell EXOs expressing OVA-specific TCRs and Fas ligand via peptide/MHC Ag I-TCR and CD54-LFA-1 interactions leading to downregulation of peptide/MHC Ag I expression and induction of apoptosis of DC(OVA) via Fas/Fas ligand pathway. We demonstrated that OVA-specific OTI T cell EXOs, but not lymphocytic choriomeningitis virus-specific TCR transgenic mouse CD8(+) T cell EXOs, can inhibit DC(OVA)-stimulated CD8(+) CTL responses and antitumor immunity against OVA-expressing B16 melanoma. In addition, these T cell EXOs can also inhibit DC(OVA)-mediated CD8(+) CTL-induced diabetes in transgenic rat insulin promoter-mOVA mice. Interestingly, the anti-LFA-1 Ab treatment significantly reduces T cell EXO-induced inhibition of CD8(+) CTL responses in both antitumor immunity and autoimmunity. EXOs released from T cell hybridoma RF3370 cells expressing OTI CD8(+) TCRs have a similar inhibitory effect as T cell EXOs in DC(OVA)-stimulated CTL responses and antitumor immunity. Therefore, our data indicate that Ag-specific CD8(+) T cells can modulate immune responses via T cell-released EXOs, and T cell EXOs may be useful for treatment of autoimmune diseases.

Perforin: structure, function, and role in human immunopathology

The secretory granule-mediated cell death pathway is the key mechanism for elimination of virus-infected and transformed target cells by cytotoxic lymphocytes. The formation of the immunological synapse between an effector and a target cell leads to exocytic trafficking of the secretory granules and the release of their contents, which include pro-apoptotic serine proteases, granzymes, and pore-forming perforin into the synapse. There, perforin polymerizes and forms a transmembrane pore that allows the delivery of granzymes into the cytosol, where they initiate various apoptotic death pathways. Unlike relatively redundant individual granzymes, functional perforin is absolutely essential for cytotoxic lymphocyte function and immune regulation in the host. Nevertheless, perforin is still the least studied and understood cytotoxic molecule in the immune system. In this review, we discuss the current state of affairs in the perforin field: the protein's structure and function as well as its role in immune-mediated diseases.

The Type 1 Diabetes PhysioLab Platform: a validated physiologically based mathematical model of pathogenesis in the non-obese diabetic mouse

Type 1 diabetes is an autoimmune disease whose clinical onset signifies a lifelong requirement for insulin therapy and increased risk of medical complications. To increase the efficiency and confidence with which drug candidates advance to human type 1 diabetes clinical trials, we have generated and validated a mathematical model of type 1 diabetes pathophysiology in a well-characterized animal model of spontaneous type 1 diabetes, the non-obese diabetic (NOD) mouse. The model is based on an extensive survey of the public literature and input from an independent scientific advisory board. It reproduces key disease features including activation and expansion of autoreactive lymphocytes in the pancreatic lymph nodes (PLNs), islet infiltration and beta cell loss leading to hyperglycaemia. The model uses ordinary differential and algebraic equations to represent the pancreas and PLN as well as dynamic interactions of multiple cell types (e.g. dendritic cells, macrophages, CD4+ T lymphocytes, CD8+ T lymphocytes, regulatory T cells, beta cells). The simulated features of untreated pathogenesis and disease outcomes for multiple interventions compare favourably with published experimental data. Thus, a mathematical model reproducing type 1 diabetes pathophysiology in the NOD mouse, validated based on accurate reproduction of results from multiple published interventions, is available for in silico hypothesis testing. Predictive biosimulation research evaluating therapeutic strategies and underlying biological mechanisms is intended to deprioritize hypotheses that impact disease outcome weakly and focus experimental research on hypotheses likely to provide insight into the disease and its treatment.

Beta cell apoptosis in diabetes

Apoptosis of beta cells is a feature of both type 1 and type 2 diabetes as well as loss of islets after transplantation. In type 1 diabetes, beta cells are destroyed by immunological mechanisms. In type 2 diabetes abnormal levels of metabolic factors contribute to beta cell failure and subsequent apoptosis. Loss of beta cells after islet transplantation is due to many factors including the stress associated with islet isolation, primary graft non-function and allogeneic graft rejection. Irrespective of the exact mediators, highly conserved intracellular pathways of apoptosis are triggered. This review will outline the molecular mediators of beta cell apoptosis and the intracellular pathways activated.

Active CD4+ helper T cells directly stimulate CD8+ cytotoxic T lymphocyte responses in wild-type and MHC II gene knockout C57BL/6 mice and transgenic RIP-mOVA mice expressing islet beta-cell ovalbumin antigen leading to diabetes

CD4+ helper T (Th) cells play crucial role in priming, expansion and survival of CD8+ cytotoxic T lymphocytes (CTLs). However, how CD4+ Th cell's help is delivered to CD8+ T cells in vivo is still unclear. We previously demonstrated that CD4+ Th cells can acquire ovalbumin (OVA) peptide/major histocompatibility complex (pMHC I) and costimulatory CD80 by OVA-pulsed DC (DC(OVA)) stimulation, and then stimulate OVA-specific CD8+ CTL responses in C57BL/6 mice. In this study, we further investigated CD4+ Th cell's effect on stimulation of CD8 CTL responses in major histocompatibility complex (MHC II) gene knockout (KO) mice and transgenic rat insulin promoter (RIP)-mOVA mice with moderate expression of self OVA by using CD4+ Th cells or Th cells with various gene deficiency. We demonstrated that the in vitro DC(OVA)-activated CD4+ Th cells (3 x 10(6) cells/mouse) can directly stimulate OVA-specific CD8+ T-cell responses in wild-type C57BL/6 mice and MHC II gene KO mice lacking CD4+ T cells. A large amount of CD4+ Th cells (12 x 10(6) cells/mouse) can even overcome OVA-specific immune tolerance in transgenic RIP-mOVA mice, leading to CD8+ CTL-mediated mouse pancreatic islet destruction and diabetes. The stimulatory effect of CD4+ Th cells is mediated by its IL-2 secretion and CD40L and CD80 costimulations, and is specifically delivered to OVA-specific CD8+ T cells in vivo via its acquired pMHC I complexes. Therefore, the above elucidated principles for CD4+ Th cells will have substantial implications in autoimmunity and antitumor immunity, and regulatory T-cell-dependent immune suppression.

Intracellular versus extracellular granzyme B in immunity and disease: challenging the dogma

The cytotoxic granzyme B (GrB)/perforin pathway has been traditionally viewed as a primary mechanism that is used by cytotoxic lymphocytes to eliminate allogeneic, virally infected and/or transformed cells. Although originally proposed to have intracellular and extracellular functions, upon the discovery that perforin, in combination with GrB, could induce apoptosis, other potential functions for this protease were, for the most part, disregarded. As there are 5 granzymes in humans and 11 granzymes in mice, many studies used perforin knockout mice as an initial screen to evaluate the role of granzymes in disease. However, in recent years, emerging clinical and biochemical evidence has shown that the latter approach may have overlooked a critical perforin-independent, pathogenic role for these proteases in disease. This review focuses on GrB, the most characterized of the granzyme family, in disease. Long known to be a pro-apoptotic protease expressed by cytotoxic lymphocytes and natural killer cells, it is now accepted that GrB can be expressed in other cell types of immune and nonimmune origin. To the latter, an emerging immune-independent role for GrB has been forwarded due to recent discoveries that GrB may be expressed in nonimmune cells such as smooth muscle cells, keratinocytes, and chondrocytes in certain disease states. Given that GrB retains its activity in the blood, can cleave extracellular matrix, and its levels are often elevated in chronic inflammatory diseases, this protease may be an important contributor to certain pathologies. The implications of sustained elevations of intracellular and extracellular GrB in chronic vascular, dermatological, and neurological diseases, among others, are developing. This review examines, for the first time, the multiple roles of GrB in disease pathogenesis.

In vivo effects of cytokines on pancreatic beta-cells in models of type I diabetes dependent on CD4(+) T lymphocytes

CD4(+) T cells can actively kill beta-cells in type I diabetes as well as help CD8(+) T cells become cytolytic. Cytokines have the potential to kill beta-cells, or upregulate Fas on beta-cells, and increase their susceptibility to FasL. We investigated the direct effects of cytokines on beta-cells in perforin-deficient non-obese diabetic (NOD) mice and NOD4.1 TCR transgenic mice, two models in which CD8(+) T cells play a less dominant role. Inhibiting the effects of cytokines by the overexpression of suppressor of cytokine signalling-1 (SOCS1) in beta-cells did not reduce diabetes or insulitis in perforin-deficient NOD, NOD4.1 or interleukin (IL)-1 receptor-deficient NOD4.1 mice. SOCS1 overexpression prevented Fas upregulation on NOD4.1 beta-cells, but did not prevent islet destruction because SOCS1 transgenic islets were killed when grafted into NOD4.1.scid mice. Likewise, Fas-deficient NOD.lpr islets were destroyed in NOD4.1 mice. Although blocking the effects of interferon (IFN)gamma on beta-cells did not affect diabetes in NOD4.1 mice, global deficiency of IFNgammaR2 reduced diabetes and insulitis, suggesting that IFNgamma is involved in CD4(+) T-cell activation or migration. Our data show that beta-cells under attack by CD4(+) T cells are not destroyed by the effects of cytokines including IFNgamma and IL-1 or Fas-dependent cytotoxicity.

Innate immunity and intestinal microbiota in the development of Type 1 diabetes

Type 1 diabetes (T1D) is a debilitating autoimmune disease that results from T-cell-mediated destruction of insulin-producing beta-cells. Its incidence has increased during the past several decades in developed countries, suggesting that changes in the environment (including the human microbial environment) may influence disease pathogenesis. The incidence of spontaneous T1D in non-obese diabetic (NOD) mice can be affected by the microbial environment in the animal housing facility or by exposure to microbial stimuli, such as injection with mycobacteria or various microbial products. Here we show that specific pathogen-free NOD mice lacking MyD88 protein (an adaptor for multiple innate immune receptors that recognize microbial stimuli) do not develop T1D. The effect is dependent on commensal microbes because germ-free MyD88-negative NOD mice develop robust diabetes, whereas colonization of these germ-free MyD88-negative NOD mice with a defined microbial consortium (representing bacterial phyla normally present in human gut) attenuates T1D. We also find that MyD88 deficiency changes the composition of the distal gut microbiota, and that exposure to the microbiota of specific pathogen-free MyD88-negative NOD donors attenuates T1D in germ-free NOD recipients. Together, these findings indicate that interaction of the intestinal microbes with the innate immune system is a critical epigenetic factor modifying T1D predisposition.

Cytotoxic T-lymphocyte-mediated killing of human pancreatic islet cells in vitro

Cytotoxic T lymphocytes (CTL) are believed to play an essential role in beta-cell destruction leading to development of type 1 diabetes and allogeneic islet graft failure. We aimed to identify the mechanisms used by CTL to kill human beta cells. CTL clones that recognize epitopes from influenza virus and Epstein-Barr virus restricted by human leukocyte antigen (HLA)-A0201 and -B0801, respectively, were used to investigate the susceptibility of human beta cells to CTL. In a short-term (5-hour) assay, CTL killed human islet cells of the appropriate major histocompatibility complex (MHC) class I type that had been pulsed with viral peptides. Killing was increased by pretreating islets with interferon gamma that increases MHC class I on target cells. Killing was abolished by incubation of CTL with the perforin inhibitor concanamycin A. The Fas pathway did not contribute to killing because blocking with neutralizing anti-Fas ligand antibody did not significantly reduce beta-cell killing. In conclusion, we report a novel way of investigating the interaction between CTL and human islets. Human islets were rapidly killed in vitro by MHC class I-restricted CTL predominantly by the granule exocytosis pathway.

Distinct local immunogenic stimuli dictate differential requirements for CD4+ and CD8+ T cell subsets in the pathogenesis of spontaneous autoimmune diabetes

The strong MHC class II association in human as well as murine Type 1 diabetes (T1D) suggests a central role for CD4+T cells in the disease pathogenesis. Nonetheless, CD8+T cells also play a role in the pathogenic process. We describe how CD4+ or CD8+T cells can contribute differentially to the pathogenesis of T1D using the HLA-DQ8 transgenic mouse models. HLA-DQ8 transgenic mice expressing the costimulatory molecule, B7.1 (RIP.B7.1), or the proinflammatory cytokine, TNF-alpha (RIP.TNF) or both (RIP.B7.RIP.TNF) under the control of rat insulin promoter (RIP) were used. Our observations indicate that in the RIP-B7 model, CD4+T cells were absolutely required for diabetes to occur. However, when CD8+ T cells were also present, the incidence of diabetes increased. On the other hand, in the RIP-TNF model, CD8+T cells were absolutely required for diabetes to occur. Interestingly, when CD4+T cells were also present, the incidence of diabetes decreased. In the RIP-B7.RIP-TNF double transgenic mouse model, either CD4+ or CD8+T cells were sufficient to precipitate diabetes in 100% of the animals. Thus, the relative roles of CD4+ or CD8+T cells in the pathogenesis of T1D are possibly determined by the local inflammatory stimuli.

The Countervailing Actions of Myeloid and Plasmacytoid Dendritic Cells Control Autoimmune Diabetes in the Nonobese Diabetic Mouse

Islet Ag-specific CD4(+) T cells receive antigenic stimulation from MHC class II-expressing APCs. Herein, we delineate the direct in vivo necessity for distinct subsets of macrophages and dendritic cells (DC) in type 1 diabetes mellitus of the NOD mouse by using diphtheria toxin-mediated cell ablation. The ablation of macrophages had no impact on islet Ag presentation or on the induction of insulitis or diabetes in either transfer or spontaneous models. However, the ablation of CD11b(+)CD11c(+) DC led to the loss of T cell activation, insulitis, and diabetes mediated by CD4(+) T cells. When the specific myeloid DC subset was "added-back" to mice lacking total DC, insulitis and diabetes were restored. Interestingly, when NOD mice were allowed to progress to the insulitis phase, the ablation of DC led to accelerated insulitis. This accelerated insulitis was mediated by the loss of plasmacytoid DC (pDC). When pDC were returned to depleted mice, the localized regulation of insulitis was restored. The loss of pDC in the pancreas itself was accompanied by the localized loss of IDO and the acceleration of insulitis. Thus, CD11c(+)CD11b(+) DC and pDC have countervailing actions in NOD diabetes, with myeloid DC providing critical antigenic stimulation to naive CD4(+) T cells and pDC providing regulatory control of CD4(+) T cell function in the target tissue.

The vicious cycle of apoptotic beta-cell death in type 1 diabetes

Autoimmune insulitis, the cause of type 1 diabetes, evolves through several discrete stages that culminate in beta-cell death. In the first stage, antigenic epitopes of B-cell-specific peptides are processed by antigen presenting cells in local lymph nodes, and auto-reactive lymphocyte clones are propagated. Subsequently, cell-mediated and direct cytokine-mediated reactions are generated against the beta-cells, and the beta-cells are sensitized to apoptosis. Ironically, the beta-cells themselves contribute some of the cytokines and chemokines that provoke the immune reaction within the islets. Once this vicious cycle of autoimmunity is fully developed, the fate of the beta-cells in the islets is sealed, and clinical diabetes inevitably ensues. Differences in various aspects of these concurrent events appear to underlie the significant discrepancies in experimental data observed in experimental models that simulate autoimmune insulitis.

Cytotoxic T-cells from T-cell receptor transgenic NOD8.3 mice destroy beta-cells via the perforin and Fas pathways

Cytotoxic T-cells are the major mediators of beta-cell destruction in type 1 diabetes, but the molecular mechanisms are not definitively established. We have examined the contribution of perforin and Fas ligand to beta-cell destruction using islet-specific CD8(+) T-cells from T-cell receptor transgenic NOD8.3 mice. NOD8.3 T-cells killed Fas-deficient islets in vitro and in vivo. Perforin-deficient NOD8.3 T-cells were able to destroy wild-type but not Fas-deficient islets in vitro. These results imply that NOD8.3 T-cells use both pathways and that Fas is required for beta-cell killing only when perforin is missing. Consistent with this theory, transgenic NOD8.3 mice with beta-cells that do not respond to Fas ligation were not protected from diabetes. We next investigated the mechanism of protection provided by overexpression of suppressor of cytokine signaling-1 (SOCS-1) in beta-cells of NOD8.3 mice. SOCS-1 islets remained intact when grafted into NOD8.3 mice and were less efficiently killed in vitro. However, addition of exogenous peptide rendered SOCS-1 islets susceptible to 8.3 T-cell-mediated lysis. Therefore, NOD8.3 T-cells use both perforin and Fas pathways to kill beta-cells and the surprising blockade of NOD8.3 T-cell-mediated beta-cell death by SOCS-1 overexpression may be due in part to reduced target cell recognition.

Effect of antibody to CD134 on perforin-mediated cytolysis in human peripheral blood mononuclear cells

CD134 (OX40) is a new member of the tumor necrosis factor receptor (TNFR) superfamily. The influence of CD134 on perforin-mediated cytolysis remains obscure. In this study, human peripheral blood mononuclear cells (PBMC) were induced with phytohemagglutinin (50 microg/mL), by using anti-CD134 blocking monoclonal antibody (MAb; 1, 5, and 10 microg/mL) for 6, 12, 24, and 48 h. Perforin-mediated cytolysis against human erythrocytes of PBMC was detected. The expression of perforin mRNA and protein was also measured by reverse transcription-polymerase chain reaction and flow cytometry. Our data showed that the perforin-mediated cytolysis of PBMC was downregulated by various concentrations of anti-CD134 blocking MAb for different times and reached a minimum at 24 h at any concentration. Anti-CD134 blocking MAb induced downregulation of perforin-mediated cytolysis of PBMC in a dose-dependent manner in the range of 1-5 microg/mL. Perforin- mediated cytolysis of PBMC reached a plateau when the concentration of anti-CD134 blocking MAb exceeded 5 microg/mL. Anti-CD134 blocking MAb also induced the inhibition of the expression of perforin protein and mRNA in PBMC. In conclusion, anti-CD134 blocking MAb inhibits perforin-mediated cytolysis of PBMC by, at least in some part, decreasing the production of preforin, which may be useful in treating several autoimmune diseases.

CD8+ T lymphocyte-mediated loss of marginal metallophilic macrophages following infection with Plasmodium chabaudi chabaudi AS

The splenic architecture is essential for the quick resolution of a primary infection with Plasmodium. A critical component of this architecture is the marginal zone (MZ), an area of the spleen that separates the reticuloendothelial red pulp of the spleen from the lymphoid white pulp compartment. There are two unique macrophage populations found in the MZ: MZ macrophages (MZM) found on the outer border of the MZ, and marginal metallophilic macrophages (MMM) found on the inner border, adjacent to the white pulp. We investigated the homeostasis of MMM and MZM following infection with Plasmodium chabaudi and demonstrated that a complete loss of both MMM and MZM occurred by the time of peak parasitemia, 8 days after infection. The loss was not induced by up-regulation of the inflammatory cytokines TNF or IFN-gamma. In contrast, following only CD8+ T cell depletion (not dendritic cell), MMM but not MZM were retained, implicating CD8+ T cells in the P. chabaudi-induced loss of MMM. Retention of MMM occurred in mice deficient in CD95, CD95-ligand, and perforin, indicating that these signals are involved in the death pathway of MMM. These data have significant implications for the understanding of the immune-mediated pathology of the spleen as a result of infection with Plasmodium.

Granzyme B-mediated death of pancreatic beta-cells requires the proapoptotic BH3-only molecule bid

Perforin-deficient NOD mice are protected from diabetes, suggesting that cytotoxic granule contents of CD8(+) T-cells have a significant role in killing beta-cells. Despite this, cytotoxic granule effects on human or mouse pancreatic islets have not been reported. We tested the susceptibility of human and mouse islet cells to purified recombinant perforin and granzyme B and measured apoptotic death using a number of assays. Perforin and granzyme B impaired insulin secretion from islet cells, and this was accompanied by cytochrome c release, caspase activation, and DNA fragmentation. Granzyme B-mediated apoptotic changes only occurred in the presence of perforin. When compared with hemopoietic cells, traditionally used as targets to measure cytotoxic T-cell function in vitro, islet cells were relatively resistant to perforin and granzyme B. Inhibition of caspases prevented DNA fragmentation but not cytochrome c release, indicating that mitochondrial disruption due to granzyme B is independent of caspase activation. Consistent with this, islet cells from mice deficient in the BH3-only protein Bid were resistant to cytochrome c release and were protected from apoptosis after exposure to perforin/granzyme B. Our data suggest that Bid cleavage by granzyme B precedes mitochondrial disruption and apoptosis in pancreatic islets.

Perforin and Fas induced by IFNgamma and TNFalpha mediate beta cell death by OT-I CTL

Direct interaction between auto-reactive CTL and specific peptide-MHC class I complexes on pancreatic beta cells is critical in mediating beta cell destruction in type I diabetes. We used mice with genetic modifications in three major pathways used by CTL, perforin, Fas and pro-inflammatory cytokines to assess the relative contribution of these mechanisms to beta cell death. In vitro-activated ovalbumin (OVA)-specific CTL, from OT-I TCR-transgenic mice, specifically killed transgenic beta cells expressing OVA (from RIP-mOVA mice) in a 16-h cytotoxicity assay. Perforin-deficient CTL had a reduced ability to kill OVA-expressing islets in vitro (22.1 +/- 3.8%) compared with wild-type CTL (71.4 +/- 4.6%). Fas-deficient islets were only slightly protected from wild-type CTL but were completely protected from the residual killing observed with perforin-deficient CTL. Residual cytotoxicity in perforin-deficient CTL was also prevented by overexpression of SOCS-1, which blocks multiple cytokine signaling pathways. It was also prevented by pre-incubation with anti-tumor necrosis factor-alpha (anti-TNFalpha) antibody or by blocking IFNgamma responsiveness through expressing a dominant negative IFNgamma receptor. Perforin-deficient CTL produced IFNgamma and TNFalpha that was shown to directly induce islet Fas expression during the assays. This suggests that Fas-deficiency, SOCS-1 overexpression and blockade of IFNgamma and TNFalpha all protect beta cells from residual cytotoxicity of perforin-deficient CTL by blocking Fas upregulation. These findings indicate that wild-type CTL destroy antigen-expressing islets via a perforin-dependent mechanism. However, in the absence of perforin, the Fas/FasL pathway provides an alternative mechanism dependent on islet cell Fas upregulation by cytokines IFNgamma and TNFalpha.