Decoding the immune dance: Unraveling the interplay between beta cells and type 1 diabetes

Abstract

Background

Type 1 diabetes (T1D) is an autoimmune disease characterized by the specific destruction of insulin-producing beta cells in the pancreas by the immune system, including CD4 cells which orchestrate the attack and CD8 cells which directly destroy the beta cells, resulting in the loss of glucose homeostasis.

Scope of review

This comprehensive document delves into the complex interplay between the immune system and beta cells, aiming to shed light on the mechanisms driving their destruction in T1D. Insights into the genetic predisposition, environmental triggers, and autoimmune responses provide a foundation for understanding the autoimmune attack on beta cells. From the role of viral infections as potential triggers to the inflammatory response of beta cells, an intricate puzzle starts to unfold. This exploration highlights the importance of beta cells in breaking immune tolerance and the factors contributing to their targeted destruction. Furthermore, it examines the potential role of autophagy and the impact of cytokine signaling on beta cell function and survival.

Major conclusions

This review collectively represents current research findings on T1D which offers valuable perspectives on novel therapeutic approaches for preserving beta cell mass, restoring immune tolerance, and ultimately preventing or halting the progression of T1D. By unraveling the complex dynamics between the immune system and beta cells, we inch closer to a comprehensive understanding of T1D pathogenesis, paving the way for more effective treatments and ultimately a cure.

1. Introduction

1.1. Type 1 diabetes

The global epidemiology of type 1 diabetes (T1D) presents a significant public health challenge. Characterized by autoimmune destruction of insulin-producing pancreatic beta cells, T1D manifests in a significant proportion of new diabetes diagnoses, impacting individuals across the lifespan. As of the latest available data from the Centers for disease control and prevention (CDC), the total number of people with diabetes in the United States is 38.4 million, which is 11.6% of the US population [1]. Further, the CDC suggests that while T1D comprises a smaller percentage (5.7%) of diagnosed diabetes cases in the United States compared to type 2 diabetes, it translates to a substantial population of 1.7 million adults requiring lifelong insulin therapy [1]. Globally, estimates suggest approximately 8.4 million individuals live with T1D, with 1.5 million being children and adolescents [[2], [3], [4], [5]]. While traditionally associated with childhood onset, T1D can develop at any age, posing diagnostic challenges in adults due to the higher prevalence of type 2 diabetes [6]. Incidence of T1D varies greatly throughout populations and geographical areas. When compared to populations of African or Asian heritage, it is generally more prevalent among individuals of European descent. Scandinavian nations have the greatest incidence rates that have been documented [7]. According to JDRF Australia (jdrf.org.au), over 130,000 people in Australia have T1D making up 10% of all cases of diabetes. Approximately 3000 adults and children receive a new diagnosis of T1D each year. In Australia, T1D is diagnosed in about eight persons every day. In Australia, 88% of those with T1D are at least 21 years old. The likelihood of a T1D diagnosis is marginally (1.3 times) higher in men than in women. Compared to non-indigenous Australians, T1D diagnoses are 1.2 times more common among indigenous people. Approximately 25,000 Australians are undiagnosed and in the early stages of Type 1 Diabetes. A study of UK Biobank participants found that over 3.5% of white European individuals developed diabetes within the first 60 years of life. Individuals with a higher genetic risk of type 1 diabetes were more likely to develop the disease, even later in life (aged 31–60 years). Interestingly, these later onset cases shared similar characteristics to those diagnosed earlier in life [6]. In addition, the concept of type 1.5 diabetes previously termed Latent Autoimmune Diabetes in Adults (LADA), is gaining traction in the medical community due to its unique characteristics that blend features of both Type 1 and Type 2 diabetes. Recent genome-wide studies have shed light on the genetic intricacies of LADA, revealing that it shares significant genetic markers with Type 1 diabetes, particularly those associated with autoimmune responses [8]. This supports the hypothesis that the progression of diabetes, including LADA, may be influenced by inherited susceptibility loci. For instance, certain HLA genotypes have been linked to the progression from pre-diabetes to full-blown diabetes [9]. These findings underscore the importance of considering genetic predispositions when diagnosing and treating diabetes. Unraveling the mystery of why the immune system wages war on insulin-producing beta cells in T1D remains a captivating challenge. While islet autoimmunity, often identified by the presence of islet autoantibodies, frequently precedes the clinical presentation of T1D [10], the intricate interplay between genetic predisposition and environmental triggers, including potential viral infections, paints a complex picture. This review delves into this very question, exploring the latest advancements in our understanding of how genetic susceptibility and environmental factors culminate in the autoimmune destruction of beta cells, leading to absolute insulin deficiency.

1.2. Role of islets beta cells in T1D pathogenesis

The role of pancreatic beta cells in T1D pathogenesis, these cells are responsible for producing, storing, and releasing insulin to maintain proper blood glucose levels. The complex mechanism of insulin release involves multiple finely coordinated steps- detecting glucose levels, metabolizing glucose, generating ATP through oxidative phosphorylation, synthesizing the insulin protein itself, and finally secreting insulin from the cell in a calcium-dependent manner [11]. Dysfunction of pancreatic beta cells is crucial in the development of both type 1 and type 2 diabetes. In type 1 diabetes, the emphasis is on beta cells, which must continue releasing insulin under controlled conditions even in the presence of stressors like free fatty acids, proinflammatory cytokines, and hyperglycemia. These stresses not only dictate the potency of insulin release but also potentially contribute to the generation of autoantigens, triggering the characteristic immunological attack in T1D. Several cellular stress pathways have been implicated in beta-cell failure in T1D, including dysregulated autophagy, cellular senescence, oxidative stress response, integrated stress response (ISR), and endoplasmic reticulum (ER) stress [[12], [13], [14], [15]]. Autoimmune mediated diabetes relies on loss of tolerance to beta cell antigens by immune cells, like proinsulin identified as a crucial autoantigen based on evidence from non-obese diabetic (NOD) mice and human studies [16]. Beta-cell failure often manifests initially with increased insulin demand after meals, progressing to fasting hyperglycemia and progressive beta cell dysfunction. Scientific evidence from surgical pancreas resection suggests that at least half of beta cells must be destroyed to induce hyperglycemia [17,18]. Notably, a significant increase in blood glucose levels after meals has been observed two months prior to the presence of autoantibodies against beta cells [19]. This study showed that children who developed islet autoantibodies had early elevations in blood glucose concentrations. Specifically, a sharp and sustained rise in blood glucose was observed about 2 months before autoantibody seroconversion. This suggests that islet autoimmunity may be facilitating the progressive loss of beta cell function, in a vicious cycle of immune mediated beta cell destruction, reducing beta cell mass, causing a greater strain on the remaining beta cell mass that in turn must compensate to maintain euglycemia. The elevated glycemic levels because of the reduced beta cell mass likely leads to glucotoxicity, which perpetuates ER stress and insulin secretion defects, contributing to more loss of beta cell mass.

2. Genetics, islet beta cell physiology, and the islet–immune interface in type 1 diabetes pathogenesis

2.1. Genetic predisposition

The pathogenesis of Type 1 diabetes (T1D) involves a complex interplay between genetic susceptibility and environmental triggers, ultimately leading to the selective destruction of pancreatic β-cells. Remarkable advancements in genetics have identified over 50 susceptibility loci that contribute to T1D risk, accounting for approximately 80% of its heritability [[20], [21], [22]]. Two key approaches are employed to understand the genetics of T1D. First, researchers follow newborn babies with a first-degree family history of diabetes [23,24]. Siblings of T1D patients have an average lifetime risk of 6–7% [25], while the risk of developing type 1 diabetes is higher in offspring of affected fathers (6%) compared to offspring of type 1 diabetic mothers (3%) [26]. Notably, the risk exceeds 70% in identical twins when observed over an extended period [27,28]. The second approach involves investigating the strong association between T1D and certain human leucocyte antigen (HLA) haplotypes, namely HLA-DR3-DQ2 and HLA-DR4-DQ8, within the major histocompatibility complex (MHC) glycoproteins encoded by the HLA region [29]. These glycoproteins allow immune cells to recognize non-self-antigens by interacting with peptide antigens. While genetic predisposition, particularly for HLA-DR3-DQ8 and DR4-DQ8 haplotypes, is well-established, genetics alone cannot account for the significant increase in global T1D incidence over the past three decades or the rising prevalence of low HLA-risk genotypes among individuals with the disease. In Scandinavia, approximately 90% of children diagnosed with T1D possess one or both high risk haplotypes [[30], [31], [32]]. Testing of newborn babies is conducted to identify those with an increased genetic predisposition, so they can be monitored for the development of specific autoantibodies that strongly indicate a heightened risk of developing type 1 diabetes [33,34]. The presence of islet autoantibodies at the time of clinical T1D diagnosis is connected to HLA-DR-DQ haplotypes, and there is a correlation between HLA haplotypes and the emergence of the first islet autoantibody and subsequent T1D development [32]. Children with HLA-DR3-DQ2 often develop glutamic acid decarboxylase autoantibodies (GADA) as the initial autoantibody, while those with HLA-DR4-DQ8 frequently exhibit insulin autoantibodies initially. Other HLA class II molecules such as HLA-DRB3, HLA-DRB4, and HLA-DRB5 may also contribute to β-cell autoimmunity and increase the risk of T1D [35]. Polymorphisms in the insulin (INS) gene, particularly in children with HLA-DR4-DQ8, also contribute to diabetes risk through increased frequencies of the INS rs689 risk genotype A/A and the A allele, which correlate with insulin autoantibodies, while PTPN22 (protein tyrosine phosphatase non-receptor type 22) gene polymorphisms also play a role; and additional loci like BACH2 (BTB Domain And CNC Homolog 2), IL2RA (Interleukin 2 Receptor Subunit Alpha), PTPN22 (protein tyrosine phosphatase non-receptor type 22), IFIH1 (Interferon-induced helicase C domain-containing protein 1), and UBASH3A (Ubiquitin Associated And SH3 Domain Containing A) have been associated with T1D, potentially influencing immune regulation and β-cell death [36,37]. Data from the Diabetes Prediction and Prevention Nutrition Study (DIPP) cohort study of 621 children, all developed one persistent antibody linked to type 1 diabetes. These antibodies fell into three categories: GADA (284 cases), IAA (268 cases), and IA-2A (40 cases). Interestingly, the type of antibody a child had depended on their genes. Children with a specific genetic profile (DR3-DQ2) almost always had GADA as their first autoantibody. In contrast, children with other genetic profiles (DR3-DQ2/DR4-DQ8, DR3-DQ/x, and DR4-DQ8/x) were more likely to have either GADA or IAA as their first antibody [38].

Several research studies have shown that specific genes, like HLA-DR4-DQ8, HLA-DR4, or HLA-DQ8 in combination with other genes (PTPN22, SH2B3 (SH2B Adaptor Protein 3), ERBB3 (Erb-B2 Receptor Tyrosine Kinase 3), RGS1 (Regulator of G Protein Signaling 1), and INS (Insulin), are linked to insulin autoantibodies. Certain gene combinations, such as HLA-DR3-DQ2 or HLA-DR4-DQ8 with PTPN22 (protein tyrosine phosphatase non-receptor type 22), ERBB3 (Erb-B2 Receptor Tyrosine Kinase 3), RGS1 (Regulator of G Protein Signaling 1), and SIRPG (Signal Regulatory Protein Gamma) may increase the risk of developing islet autoantibodies but not necessarily progressing to type 1 diabetes. GADA-only antibodies in people with specific genes (HLA-DR3-DQ2/HLA-DR3-DQ2) were associated with other genes (CCR7 (C–C Motif Chemokine Receptor 7), TNFAIP3 (TNF Alpha Induced Protein 3), SH2B3 (SH2B Adaptor Protein 3), and CD226) [39]. Various genes, including BACH2 (BTB Domain And CNC Homolog 2), IL2RA (Interleukin 2 Receptor Subunit Alpha), PTPN2 (Protein Tyrosine Phosphatase Non-Receptor Type 2), IFIH1 (Interferon Induced with Helicase C Domain 1), and UBASH3A (Ubiquitin Associated and SH3 Domain Containing A), are also linked to type 1 diabetes. Genetic factors like IL2RA polymorphisms can impact T-regulatory cell activity, influencing the risk of type 1 diabetes [40]. The INS genomic region and genes like CTLA4 (Cytotoxic T-Lymphocyte Associated Protein 4), PTPN2 (Protein Tyrosine Phosphatase Non-Receptor Type 2), interleukins, IRS-1 (Insulin Receptor Substrate 1), ICOS (Inducible T Cell Costimulator), and SUMO4 (Small Ubiquitin Like Modifier 4) also play roles in T1D susceptibility [[41], [42], [43], [44], [45], [46]].

2.2. Autoantibodies targeting beta cells

Type 1 diabetes arises from a malfunctioning immune system that mistakenly attacks the body's own insulin-producing cells in the pancreas. Understanding this relationship between autoantibodies and beta cell destruction is vital in unraveling the development of type 1 diabetes. These autoantibodies, first identified over 30 years ago, are key markers used to track the progression of the autoimmune process and evaluate treatment effectiveness during the preclinical stages and diagnosis of type 1 diabetes [47]. The presence of specific autoantibodies like islet-cell antibodies (ICA), alongside insulin autoantibodies (IAA), glutamic acid decarboxylase 65 antibodies (GAD65Ab), and IA-2 antibodies (IA-2Ab) is commonly assessed to gauge the risk of developing, and progressing to diabetes [48]. Tyrosine phosphatase-like molecules, insulinoma-associated protein 2/IA-2/ICA512/PTPRN (IA-2A), and IA-2β/PTPRN2 (IA-2βA), present on the surface of beta cells, act as specific targets for autoantibodies in T1D. Zinc Transporter 8 Antibodies (ZnT8A) detection is an additional indicator of autoimmune diabetes. Zinc transporter ZnT8/SLC30A8 (ZnT8A) and glutamic acid decarboxylase 2/GAD2/GAD65 (GADA) are generally found during clinical studies [49]. Compared to GAD65 and IA-2, which have wider tissue distribution, insulin and ZnT8 exhibit more islet-specific expression [50]. Many patients, around 80%, who are diagnosed with type 1 diabetes test positive for autoantibodies against the GAD65 protein when first evaluated clinically. Between 69 and 90% have islet-cell antibodies (ICA) and 54–75% have autoantibodies against the protein IA-2 at the time of diagnosis. More recently clinical laboratories focus on four well-characterized islet autoantibodies to predict and diagnose T1D [51]. The presence of autoantibodies against insulin itself (IAA) is more common in younger children who develop type 1 diabetes, occurring in around 70% of young children when diagnosed. IA-2 autoantibodies have been detected in 32–75% of newly diagnosed type 1 diabetes patients across different studies [52]. The Environmental Determinants of Diabetes in the Young TEDDY group's recent study highlighted differential regulation of extracellular matrix and antigen presentation proteins between individuals progressing to Type 1 Diabetes (T1D) and those individuals who maintain an autoimmune response, such as the presence of autoantibodies, without progressing to the full clinical presentation of T1D. Their proteomics analysis revealed more substantial transcriptomic changes in children with GADA-first (14 genes) compared to those with IAA-first (1 gene), particularly within the 12 months leading up to GADA seroconversion [53].

Though the precise mechanisms are not fully elucidated, mounting evidence indicates that autoantibodies may have a pivotal role in initiating and perpetuating type 1 diabetes. They affect T-cell responses by changing how antigens are processed and presented, potentially revealing new self-components [48,[54], [55], [56]]. The production of islet autoantibodies by B cells hinges on their activation by T-helper cells recognizing the same antigen, a phenomenon termed linked recognition. In the case of type 1 diabetes, modulation of GAD65 presentation to T-cells by disease-associated GAD65 autoantibodies initiates the breakdown of immunological tolerance to the pancreatic β-cells and thus alters the focus of the T-cell response. Along with CD4+ T cells, newly diagnosed type 1 diabetes patients and high-risk participants were shown to have MHC class I HLA-A∗0201-restricted CD8+ cytotoxic T cells reactive against the GAD autoantigen, but not healthy control subjects expressing HLA-A [[57], [58], [59], [60], [61]]. It has also been reported that the early onset of diabetes is predicted by the presence of insulin autoantibody in NOD mice [[62], [63], [64], [65]]. It has also been reported that diabetes can be transferred in an accelerated fashion with CD4+ T cell clones specific to the insulin B chain in NOD scid mice [66]. Furthermore, transgenic production of mouse proinsulin II in NOD mice prevented progression of diabetes [67]. These findings suggest that inducing tolerance can suppress T-cell responses to specific autoantigens.

3. Insulitis and immune cell infiltration

Insulitis, characterized by immune cell infiltration into the islets of Langerhans, is strongly associated with Type 1 diabetes. The term “insulitis” refers to inflammation of the pancreatic islets, primarily affecting the insulin-producing beta cells. Histopathological examination of pancreatic tissue reveals specific changes in islets, such as lobular loss, which indicates the presence of over 10 insulin-deficient islets. In contrast, non-Type 1 diabetes does not show insulin-deficient islets in pancreatic sections [68]. Insulitis involves various immune cells, including T cells, B cells, infiltrating macrophages, T helper cells, and NK cells. CD8 T-cells are prominent in post cadaveric histological evaluation, possibly due to the time frame of examination, as CD4+ T cells are likely to have extravasated out of the pancreas and migrated back to the secondary lymphoid organs [69]. CD8 T cells specifically targeting beta cells expressing elevated levels of HLA class I (and class II) molecules arrive after CD4 infiltrate. This is supported by the presence of viral infections, which are linked to increased HLA expression and implicated in type 1 diabetes [[70], [71], [72]]. Insulitis diagnosis requires the presence of lesions in at least three islets, either at the islet–exocrine interface or in the islet parenchyma, along with a minimum CD45+ cell threshold [68]. CD20+ B-cells play a significant role by producing autoantibodies that target beta cell antigens, and also serve as key professional antigen presenting cells necessary for overcoming a major check point in disease progression in the NOD mouse [73,74] triggering immune responses and immune complex formation, leading to beta cell destruction. CD8+ T cells drive immune cell infiltration and beta cell destruction in insulitis by targeting beta cell antigens and releasing cytotoxic molecules. Macrophages maintain a steady presence and become predominant in insulin-deficient islets [75]. During the early stages of islet infiltration, T cells interact with CD11c-expressing antigen-presenting cells (APCs), leading to their activation and increased production of IFN-γ. This results in inflammation and the upregulation of vascular adhesion molecules and chemokine production. Various immune cells release pro-inflammatory cytokines such as IL-1, IL-6, and TNF-alpha, which contribute to the inflammatory response and amplify the immune attack on beta cells. These inflammatory changes within the islets promote the recruitment and infiltration of various immune cells, including T cells, mononuclear phagocytic cells (MNPs), B cells, and others, into the islet environment [[76], [77], [78], [79]]. In some instances, insulitis is accompanied by fibrosis and hyaluronan accumulation within the islets, which may alter tissue stiffness and contribute to organ inflammation [[80], [81], [82]]. Research on NOD mice has demonstrated that, as the disease progresses, there is an increased infiltration of CD4+ and CD8+ T lymphocytes into the islets, along with a decrease in islet mass [83]. Furthermore, neutrophils infiltrating the pancreas significantly influence the onset and progression of type 1 diabetes by modulating the CXCL8-CXCR1/2 pathway [84].

3.1. Beta cell stress and apoptosis

In diabetic conditions, beta cells, which are essential for maintaining glucose homeostasis through insulin secretion, are vulnerable to damage and dedifferentiation due to factors such as hyperglycemia, hyperlipidemia, and inflammation. This damage is primarily caused by the promotion of endoplasmic reticulum (ER) stress, mitochondrial dysfunction, and oxidative stress. In Type 1 diabetes, infiltrating macrophages release pro-inflammatory cytokines like tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β), as well as chemokines such as CXCL9 and CXCL10. These molecules attract immune cells, including dendritic cells, macrophages, and T lymphocytes, to the islets. Activated T cells that recognize beta cell-specific antigens infiltrate the inflamed islets and target the beta cells for attack [65,85]. Cytokines play a critical role in inducing stress and apoptosis (programmed cell death) in beta cells. Studies have shown that blocking the action of pro-inflammatory cytokines like IL-1β, IFN-γ, IL-6, and TNF-α using antibodies or soluble decoy receptors can prevent or inhibit the development of Type 1 diabetes mellitus (T1DM) in animal models such as NOD mice and BB rats [86], underscoring the importance of cytokines in the progression of Type 1 diabetes and suggests that targeting these molecules at the level of the immune cells as well as the ensuing effect they have on immune cells could be a potential therapeutic approach.

3.2. Endoplasmic reticulum stress

The generation of endoplasmic reticulum (ER) stress and/or death in beta cells by cytokines is crucial, and important general characteristics of cytokine-induced apoptosis are conserved in mice, rats, and humans, even though the exact mechanisms of response to cytokines vary between species [87]. Specifically, proinflammatory molecules such as IL-1β, TNF-α, and IFN-γ are known to induce ER stress. Factors like chemical exposure, viral infections, fluctuating blood sugar levels, and the demands of insulin production can exacerbate ER stress in beta cells, contributing to the development of type 1 diabetes [[88], [89], [90]]. When cells experience high demand on their endoplasmic reticulum (ER), they trigger a response called the unfolded protein response (UPR). The UPR helps alleviate ER stress and restore protein folding balance. This process includes slowing down protein synthesis, boosting the production of chaperones to aid in folding, and activating ER-associated degradation to eliminate misfolded proteins [91]. Three UPR stress sensors, PKR-like endoplasmic reticulum kinase (PERK), activating transcription factor 6 (ATF6), and inositol-requiring enzyme 1α (IRE1α), activate this response. The UPR's translational control aspect is governed by the phosphorylation of eIF2 by PERK. This phosphorylation event involves eIF2, a translation initiation factor that binds with guanosine triphosphate (GTP) and transports the initiator Met-tRNAiMet to the translation apparatus. When the α subunit of eIF2 is phosphorylated at Ser-51, it inhibits eIF2B's function, which is necessary for swapping eIF2-GDP (guanosine diphosphate) with the active eIF2-GTP form. This leads to a decrease in protein production, which halts the entry of new polypeptides into the ER, giving the cell sufficient time to carry out the transcriptome modifications required by the UPR [92]. In stress conditions, the presence of binding-immunoglobulin protein (BiP), which holds the three UPR sensors in an inactivated state is essentially titrated away from binding the three UPR sensors, allowing them to control protein cargo handling in the ER. However, in many instances this regulation is overcome and protein misfolding increases often leading to the release of post translationally modified proteins, which serve as neoantigens activating the self-reactive T cells in type 1 diabetes [15]. Mice that are deficient in the PERK protein exhibit reduced phosphorylation (activation) of the eIF2α molecule, leading to increased global translation of mRNA transcripts. This results in heightened endoplasmic reticulum (ER) stress, which is accompanied by the loss or destruction of beta cells in these mice [93,94]. Chronic ER stress induces PERK to stimulate the translation of the gene encoding activating transcription factor 4 (ATF4), which in turn triggers the apoptotic-promoting gene encoding C/EBP-homologous protein (CHOP)/growth arrest- and DNA damage-inducible gene 153 CHOP/GADD153 [95].

ER calcium homeostasis is crucial in type 1 diabetes, with proteins such as SERCA2 and Wfs1 regulating ER calcium levels to ensure proper protein folding. Alterations in these proteins can cause ER stress, leading to glucose intolerance and beta cell death [96]. In response to various stressors like viral infections, inflammation, changes in the gut microbiome, and nutrient availability, cells activate the integrated stress response (ISR) as a protective mechanism. However, chronic ISR activation can contribute to diabetes pathogenesis. Evidence suggests that in individuals who are pre-diabetic and positive for type 1 diabetes autoantibodies, the ISR is dysregulated. This dysregulation appears to result from repressed or reduced expression of key ISR kinase genes, such as PERK, GCN2, and PKR [96].

3.3. Oxidative stress

Oxidative stress, characterized by an imbalance between free radicals and antioxidant defenses, plays a significant role in beta cell dysfunction and diabetes pathogenesis. When beta cells experience oxidative and ER stress, it induces deleterious events leading to inflammation and cell death. This process activates nearby immune cells in the islets, such as macrophages and T cells [97]. Beta cells are particularly vulnerable to oxidative stress due to hydrogen peroxide (H₂O₂) generated during insulin processing. If not fully neutralized, H₂O₂ can form more reactive oxygen species (ROS) like hydroxyl radicals (•OH), damaging cellular constituents [98]. Beta cells have comparatively low levels of antioxidant defenses, making them susceptible to uncontrolled oxidative damage to proteins, lipids, and DNA. Oxidative stress regulation of insulin secretion involves ROS at basal levels, influenced by inflammation, hyperlipidemia, and hyperglycemia. These conditions increase ROS production, affecting β-cell function by targeting potassium-ATP channels and cell metabolism. Genetic removal of these channels lessens oxidative stress impacts on β cells [99]. Beta cells have antioxidant defenses, including catalase (CAT), glutathione peroxidase (GPx), thioredoxin (TXN), and peroxiredoxins. However, their lower expression of CAT and GPx1 leads to higher ROS accumulation, and subsequent progression to inflammation [100,101]. Hyperglycemia and hyperlipidemia exacerbate oxidative stress by diminishing antioxidant defenses, increasing pro-inflammatory cytokines. Evidence includes a decreased ratio of reduced to oxidized glutathione under hyperglycemia and reduced expression of antioxidant enzymes SOD1 and SOD2 in hyperlipidemia [102,103]. High blood sugar, lipids, and inflammation further increase ROS production through mitochondrial dysfunction and ER stress. Inflammation impairs mitochondria, increasing ROS and disrupting redox balance, which boosts nitric oxide (NO) production [104,105]. Recent research shows that elevated glucose levels and saturated free fatty acids (FFAs) disrupt ER function in beta cells, impairing proinsulin synthesis, folding, and processing. Prolonged ER stress promotes apoptosis, increases oxidative stress, and activates mitochondrial apoptosis pathways, leading to beta cell decline [106]. Toxic environmental factors trigger pathways increasing ROS in beta cells, making oxidative stress a crucial determinant of their fate in diabetic conditions. Oxidative stress disrupts vital pathways necessary for beta cell survival and function, activating AMP-activated protein kinase (AMPK) and c-Jun N-terminal kinase (JNK) while inhibiting the mTOR pathway [107]. ROS-induced AMPK activation in INS-1 cells protects β cells by reducing oxidative stress and increasing autophagy [108]. AMPK activation also enhances insulin secretion and beta cell proliferation via miR-184 upregulation [109,110]. In rat beta cells, ROS-mediated AMPK activation increases phosphorylated extracellular signal-regulated kinase (pERK), impairing beta cell proliferation and reducing overall beta cell mass [111]. Prolonged pAMPK activation, while initially protective, can be detrimental to β cell viability and function. Persistent γ2 AMPK activity in mice led to suppressed insulin release, decreased baseline β cell activity, and increased disallowed gene expression [112]. mTOR pathway downregulation also triggers mitochondria-mediated apoptosis via Thioredoxin-interacting protein (TXNIP) upregulation, causing mitochondrial dysfunction and reducing beta cell proliferation [107]. Additionally, ROS produced by NOX2 through p53, p38MAPK, and JNK pathways triggers β cell apoptosis [113]. Overexpression of JNK alone does not induce β cell death but impairs insulin signaling, suggesting that oxidative stress triggers additional factors for JNK-mediated apoptosis [114].

Furthermore, elevated NO levels deplete ER calcium and impair ER protein folding, ultimately causing pancreatic β cell apoptosis.

3.4. Impact of chronic stress on beta cell apoptosis

Prolonged stress triggers the body's “fight or flight” response, leading to increased cortisol and adrenaline release, which can strain beta cells and disrupt glucose-insulin balance [115]. Chronic stress can also activate the sympathetic nervous system, promoting beta cell death. Additionally, insulin resistance, associated with long-term stress, further complicates glucose homeostasis. Persistent hyperglycemia can result from disrupted glucose balance [116]. Chronic stress triggers the sympathetic nervous system, releasing norepinephrine and other neurotransmitters that promote beta cell death [117]. Insulin resistance, linked to long-term stress, increases insulin demands, placing beta cells under greater stress and accelerating apoptosis risk [118]. Unregulated stress causes irreversible cell damage, including DNA damage and senescence. Senescent beta cells in NOD mice and individuals with T1D show signs of DNA damage-associated senescence early in disease development, with increased expression of p21, p16INKa, Bcl-2, and SA-βgal activity [[119], [120], [121]].

4. Contribution of immune cells in beta cell apoptosis

In Type 1 Diabetes (T1D), the innate and adaptive immune system plays a critical role in the destruction of insulin-producing beta cells in the pancreas. Inadequate education of T cells in the thymus during selection leads to seeding the periphery with self-reactive T cells that recognize beta cell associated antigens [75]. More recently it has been reported that post-translational modifications (PTMs) of beta cell proteins, owing to disruption in ER homeostasis brings about the production of neoantigens with enhanced immunogenicity in pancreatic beta cells [15]. ER homeostasis disruption initiates autoimmune inflammation by recruiting dendritic cells to transport antigen to draining lymph nodes, where self-reactive T cells are presented with antigen. These T cells then return to the islets, inducing site-specific production of inflammatory cytokines such as INF-α, INF-β, IFN-γ, TNF, and IL-1β. This process activates beta cell gene networks controlled by transcription factors NF-κB and STAT-1, leading to beta cell apoptosis [122,123]. NF-κB activation leads to NO and chemokine production, ER calcium depletion, and beta cell death via MAP kinase activation, ER stress, and mitochondrial death signal release. Cytokines create a positive feedback loop, converting macrophages to M1 phenotype, boosting cytokine production, and increasing beta cell death [77]. Furthermore, studies derived from both human and animal samples show that beta cells can express the IFN-γ-inducible chemokine CXCL10, which encourages T cell infiltration and might hasten the demise of beta cells [124,125]. Previous research in NOD mice indicates that autoimmune diabetes is strongly dependent on MHC class II allele I-Ag7 and requires CD4 T cells, CD8 T cells, and B cells [[126], [127], [128]]. Current research findings imply that presenting antigen to CD4 T lymphocytes via pancreatic islet dendritic cells and macrophages is a major factor in stimulating beta cell death. CD8+ cytotoxic T cells are particularly implicated in beta cell destruction. These T cells recognize beta cell antigens and release cytotoxic molecules, such as perforin and granzymes, leading to beta cell apoptosis [75,129]. Increased CD8 T cell infiltration and direct beta cell death via perforin and granzyme B are caused by the ensuing inflammation, while natural (nTregs) and peripheral (pTregs) seek to temper and resolve this reaction by releasing TGFβ and IL-10 [75]. Recent research has shown that dendritic cells, acting as the primary presenter are critical in the initiation of CD4⁺ T cell activation, and the expression of CD40L, critical for T-B cell interaction and linked recognition for autoantibody production. B lymphocytes play a role in the pathogenesis of Type 1 diabetes through the production of autoantibodies and regulating the activation, proliferation, and cytokine production of self-reactive CD4⁺ T cells [75]. In summary, IL-1β signaling mediated by cytokine production from immune cells, acts as a key regulator of a multifaceted gene network. This network influences β-cell differentiation, endoplasmic reticulum calcium homeostasis, immune cell recruitment and activation, and ultimately contributes to β-cell death through apoptosis.

NK cells, essential for early antiviral defense, play a key role in eliminating infected cells and tumors by producing cytokines like IFNγ. This contributes directly and indirectly to beta cell destruction in Type 1 Diabetes (T1D). In both T1D patients and mouse models, pancreatic NK cells are associated with destructive infiltrates and show increased expression of CD25, CD69, PD1, and KLRG1. They also exhibit heightened proliferation, spontaneous IFNγ production, and expression of CD107a, indicating their cytotoxic function. Notably, NK cells are present in the pancreas of NOD mice even before T cell infiltration, suggesting a potential sentinel role in pancreatic immunity. This observation holds true even in NOD–Rag mice lacking mature B and T cells. NK cells may play a pathogenic role in various T1D mouse models, including NOD mice, SOCS1-transgenic NOD mice infected with coxsackie virus B4, IFNβ-transgenic mice, and CTLA4-specific antibody-treated BDC2.5 TCR-transgenic NOD mice [[130], [131], [132]]. This study suggests that NKG2D and NKp46 receptor inhibition might prevent type 1 diabetes (T1D) in non-obese diabetic (NOD) mice. This finding implies a potential contribution of natural killer (NK) cell recognition of pancreatic β-cells to T1D pathogenesis [133]. Depleting TReg cells in BDC2.5 TCR-transgenic NOD mice increased activated pancreatic NK cells, boosting IFNγ production. This enhances the effector function of diabetogenic CD4+ T cells [134].

5. Effects of cytokine signaling in beta cell functions and survival

Cytokines are pivotal in type 1 diabetes (T1D) development by affecting insulin-production/secretion from beta cells in the pancreas directly, as well as optimally activating immune cells to full effector function and targeted destruction of the islet beta cells. Dysregulated cytokines, essential for immune signaling, contribute significantly to T1D pathophysiology. Some cytokines like IL-10, TGF-β, and IL-33 can help prevent beta cell destruction and restore immune tolerance. Conversely, cytokines such as TNF, IL-6, IL-17, and IL-21 promote T1D by activating harmful immune cells. However, targeting these cytokines for therapy may have inconsistent results due to their dual pro- and anti-inflammatory roles, as well as the timing with which they are liberated may affect beta cell destruction or survival [86]. Additionally, Tregs appear to be more reliant on the IL-2/STAT5 signaling in response to active stimulation, whereas effector T cells are triggered via the MAPK, PI3K/AKT, and STAT5 pathways. In autoimmune diabetes studies, a combination of IL-2 and sirolimus effectively prevented β cell destruction in NOD mice. Sirolimus inhibits T-cell proliferation, while IL-2 induces T-cell death, synergistically working together. Low-dose IL-2 increases and activates Treg populations precisely, aiding T1D patients in regaining immunological tolerance [135]. Type-2 cytokines like IL-4, IL-5, IL-9, and IL-13 show promise in suppressing type-1-derived autoimmunity [136]. IL-5 promotes the growth of CD19 + IgM + B cells in NOD mice, stimulating B cells to secrete IL-10, which suppresses diabetes pathophysiology. Elevated IL-9 expression is observed in type 1 diabetes patients, while systemic delivery or genetic overexpression of IL-4 delays insulitis onset and reduces diabetes risk [137,138]. Adoptive transfer of dendritic cells expressing soluble IL-4 or local IL-4 expression in beta cells postpones diabetes onset in NOD mice [139,140]. Although IL-10 has immunosuppressive effects, its role in T1D remains controversial. IL-10 treatment enhances β-cell activity, inhibits insulitis development, and prevents diabetes onset in animal models by increasing Treg frequencies, Th2-type cytokine levels, and suppressing Th1-type cytokines. IL-10 promotes apoptosis of double-negative (CD4−CD8−) T cells, which have strong immune tolerance characteristics, and their depletion increases diabetes susceptibility in NOD mice [141,142]. Additionally, type 1 IFN signals can suppress IL-10 signals in effector T cells and Tregs through the JAK/STAT pathway, influencing T1D progression. TGF-β signaling is critical for natural Treg development and induces Foxp3 expression. It primarily targets CD4+ T cells to suppress T1D, as evidenced by diabetes development prevention upon TGF-β signaling removal in Tregs and rapid T1D onset upon removal in effector CD4+ T cells and thymic T cells in BDC2.5 NOD animals [141,142].

Classical cytokines like IL-1, IL-6, TNF-α, and IFNs drive inflammation in type-1 diabetes. IL-1, especially, is known to cause β-cell dysfunction and death [143]. β cells are more susceptible to IL-1 because they have higher IL-1 receptor density and unique NF-κB and MAPK signaling characteristics [144]. Antibody disorders are mediated in their onset and progression by IL-6. It has been demonstrated that a significant case of insulitis, along with the infiltration of B cells, macrophages, and T cells, is linked to the overexpression of IL-6 in local pancreatic beta cells. Additional research highlights the significance of the IL-6 pathway by demonstrating that elevated expression of IL-6R in CD4+ and CD8+ T cells from T1D patients is coordinated by STAT3 activation [145]. Members of the IFN family, IFN-α and IFN-γ, are linked to both the initiation and destruction phases of T1D. IFN-α directly contributes to the death of β-cells by inducing endoplasmic reticulum stress and mediating apoptosis in human β cells by promoting the overexpression of HLA-I molecules [146,147]. Previous work has demonstrated that, IFN-γ like TNF-α [148] can potentiate destruction early in the immune response, however later it plays a role in facilitating the resolution of the immune response by driving the expression of PDL-1 on target tissue to quell the ensuing immune response [149]. Resolution of the adaptive immune response, post pathogen clearance occurs when IFN-γ signaling up-regulates PD-L1 expression on target tissue to ensure limited immunopathology post infection clearance [150]. This ensures that as the pathogen is cleared the ensuing immune cells begin the contraction phase of the response thus inhibiting anti-viral CD8+ T-cell cytotoxicity of uninfected cells. IFN-γ triggers the local pancreatic cells to express MHC-I and MHC-II abnormally, which might lead to the death of β cells through an inflammatory process [151,152]. Furthermore, by incubating with IFN-γ, IL-1, and TNF-α, innate-like mucosal-associated invariant T (MAIT) cells become full effectors capable of destroying a human beta cell line [153]. IL-15 can expedite Type 1 Diabetes in vivo as well as play a role in controlling diabetogenic effects through Treg activation. NK cells and diabetogenic T cells can both be activated simultaneously to counteract the harmful effects of diabetes. Furthermore, current research investigation revealed that the IL-12 family (IL-12, IL-23, IL-27 and IL-35), Th17 cytokines (IL-17, IL-22 and IL-25) and other cytokines, such as IL-15, IL-21, IL-33 and IL-7 also have a vital role in the initiation of type 1 diabetes disease and enhanced beta cell destruction [86]. Taken together, cytokine networks play crucial roles in orchestrating inflammation during both the initiation and destruction phases of T1D, as well as in the resolution of the ensuing immune response, limiting immunopathology. Immune cell plasticity might influence cytokine activity, which could lead to the pathogenic or regulatory effects of cytokines in the various T1D microenvironments (Figure 1).

Figure 1.

Beta cell destruction in T1D. In Type 1 diabetes, a combination of genetic predisposition and environmental factors likely triggers the development of the disease. One potential scenario involves environmental factors causing oxidative stress in the form of reactive oxygen species (ROS). This stress may lead pancreatic beta cells to release inflammatory mediators like tumor necrosis factor-alpha (TNF-α), interleukin-1 beta (IL-1β), and interferon-gamma (IFN-γ) as an alarm signal. The process begins with the influence of two cell signaling molecules, TNF-alpha (TNF-α) and interferon-gamma (IFN-γ). These molecules bind to their respective receptors on beta cells, triggering a series of events inside the cells. As a result of this binding, enzymes called JAK1 and JAK2 are activated. These enzymes then phosphorylate STAT1, a protein that regulates gene expression. Phosphorylated STAT1 moves into the nucleus of the cell and turns on the production of an enzyme called inducible nitric oxide synthase (iNOS). The iNOS enzyme produces nitric oxide (NO), which can damage beta cells in several ways. It can increase the production of reactive oxygen species (ROS), disrupt the function of mitochondria, and inhibit DNA repair mechanisms. The diagram also shows another pathway that can lead to beta cell destruction. This pathway involves interleukin-1 beta (IL-1β) and interleukin-1 receptor type I (IL-1R1). When IL-1β binds to IL-1R1, it activates other signaling molecules that can ultimately lead to the activation of two important factors: NF-κB and JNK. NF-κB is a protein that regulates the expression of genes involved in inflammation and cell survival. JNK is a protein involved in cell stress and programmed cell death (apoptosis). The activation of these factors can further contribute to the destruction of beta cells. The figure was created with BioRender.com (accessed on 06 May 2024).

6. Beta cells antigen presentation trigger immune response in type 1 diabetes

The cytokine networks discussed earlier orchestrate the inflammatory response in T1D. Understanding how this response is initiated is crucial. Environmental triggers, such as pathogen infections, initiate a “danger” defense response in infected tissues. This response involves the release of cytokines and chemokines, which mobilize the immune system to the target organ. While the immune response usually clears the pathogen and allows tissue repair, in individuals with a predisposition for failed central tolerance, self-reactive immune cells are activated. This leads to a prolonged, chronic inflammation and autoimmune attack. In type 1 diabetes (T1D), autoimmune attack on beta cells releases beta cell antigens. These antigens are taken up by antigen-presenting cells (APCs) and presented to autoreactive T cells. The activation of T cells results in the release of inflammatory cytokines like IFN-γ, TNF-α, and IL-1β from macrophages and dendritic cells, causing beta cell dysfunction, insulin secretion defects, and eventually beta cell destruction. MHC polymorphisms in NOD mice facilitate self-peptide binding and APC-T cell interactions [154]. Islet antigens such as insulin, proinsulin, and IA-2 are presented via HLA binding/tetramers. MHC class I activates CD8+ T cells, while CD4+ T cells recognizing MHC class II, provide the necessary IFN- γ that matures the CD8+ T cells to become cytotoxic imparting effector function. CD4+ T cells also aid in autoantibody generation. Neoepitope generation signals the loss of immune tolerance, with post-translational modifications (PTMs) influencing protein properties crucial for immune tolerance [155]. ER stress triggers the unfolded protein response (UPR) to maintain ER balance. However, overactive UPR under stress can lead to abnormal protein modifications or neoepitope generation [156,157]. In T1D, hybrid insulin peptides (HIPs) create chimeric antigens, breaking immune tolerance [158,159].

While interferon-alpha (IFN-α) within the pancreas of T1D patients promotes the upregulation of MHC class I and costimulatory molecules on beta cells, facilitating presentation of autoantigens and activation of cytotoxic CD8+ T cells, research suggests IFN-α, either independently or in combination with IFN-γ and TNF-α, can also induce MHC class II gene expression in human islets [160], this coupled with CIITA isoform expression, enables IFN family members to enhance MHC and antigen presentation molecules on islet beta cells [161]. Inflammatory cytokine stimulation or intrinsic beta cell dysfunction leads to neoantigen generation, MHC expression, and co-stimulatory molecule expression, conferring antigen-presenting properties to beta cells. Interestingly, viral infection for example, with Coxsackievirus which has a tissue tropism for β cells, is linked to Type 1 Diabetes (T1D) by triggering type I interferon (IFN) production in both beta cells and plasmacytoid dendritic cells (pDCs) (Figure 2). Plasmacytoid dendritic cells, unlike beta cells, can produce substantial amounts of IFN-a and IFN-b. These interferons induce elevated class I expression, licensing beta cells to present antigen efficiently to T cells mediating beta cell pathogenesis [162]. Plasmacytoid dendritic cells (pDCs) in pancreatic islets, generate interferon-alpha (IFN-α) secretion against viral infections, promoting T cells infiltration in the islets through the pancreatic vasculature, leading to damage of beta cells [163,164]. It is becoming increasingly evident that the interplay between immune signatures, viral infection, and the dysregulation of protein cargo handling machinery within beta cells can contribute to the development of type 1 diabetes (T1D) [165].

Figure 2.

Environmental triggers and immune response. The figure illustrates the complex interplay between environmental factors, viral infection, and the immune system in the development of type 1 diabetes (T1D). The process starts with environmental factors and/or viral infection that can damage beta cells. This damage leads to the release of cellular components, including autoantigens. Beta cells can exhibit some features of antigen-presenting cells (APCs) by expressing MHC class I molecules under stress conditions. This allows them to directly present antigens to CD8+ T cells, potentially contributing to their activation and proliferation. However, beta cells typically lack MHC class II molecules and crucial co-stimulatory molecules like B7-1 (CD80) and B7-2 (CD86) required for full activation of CD4+ T cells. This significantly limits their ability to effectively induce a strong CD4+ T cell response on their own. The combined effects of these immune system attacks ultimately lead to the destruction of beta cells and T1D. The figure was created with BioRender.com (accessed on 06 May 2024).

7. Viral infection

While ER stress affects all pancreatic beta cells, not every individual experiencing this stress develops T1D [166]. This suggests a complex interplay between ER stress, genetic predisposition, and environmental triggers in disease manifestation. Progression to T1D is often a slow chronic developing process manifested over time, particularly in individuals with a genetic susceptibility to autoimmunity, indicating that environmental factors might accelerate disease onset. Several environmental variables, including viral infections, are linked to T1D initiation [[167], [168], [169]]. Viral infections impact biological systems by inducing the unfolded protein response (UPR) (Figure 3). The UPR, a complex cytoprotective signaling network, restores ER homeostasis and promotes cell survival under ER stress [170]. High viral protein synthesis, changes to ER membranes, and calcium concentration abnormalities within the ER activate the UPR [171]. Enteroviruses, particularly those within the human enterovirus B (HEV-B) species, such as coxsackievirus B (CVB) and echovirus (EV), are potential contributors to T1D development [[172], [173], [174]].

Figure 3.

ER stress. The figure depicts the unfolded protein response (UPR) in the endoplasmic reticulum (ER), which is an organelle found in eukaryotic cells. The ER is responsible for protein folding, maturation, and transport. When proteins in the ER misfold, it triggers ER stress. The UPR is a signaling pathway that helps cells cope with ER stress. The three main ER stress sensors are PERK, IRE1, and ATF6. These sensors are located on the ER membrane. Under normal conditions, BiP (binding immunoglobulin protein) binds to the luminal domains of the three sensors, keeping them inactive. However, when ER stress occurs, unfolded proteins accumulate in the ER lumen, causing BiP to dissociate from the sensors. PERK, upon activation, phosphorylates eukaryotic translation initiation factor 2 alpha (eIF2α), which reduces overall protein translation. However, it selectively increases the translation of ATF4, a transcription factor that promotes the expression of genes involved in amino acid synthesis and ER protein folding. Activated IRE1 splices XBP1 mRNA, which encodes a transcription factor that upregulates the expression of genes involved in ER protein folding, degradation, and ER-associated protein degradation (ERAD). IRE1 can also activate the c-Jun N-terminal kinase (JNK) pathway, which can lead to cell death if ER stress is prolonged. ATF6 is transported to the Golgi apparatus where it is cleaved and activated. Activated ATF6 translocate to the nucleus and induces the expression of genes involved in protein folding, chaperone expression, and ER expansion. Overall, the UPR helps cells to restore ER homeostasis by reducing protein translation, increasing protein folding capacity, and degrading misfolded proteins. However, if ER stress is not resolved, the UPR can trigger cell death. The figure was created with BioRender.com (accessed on 06 May 2024).

Viruses can stimulate the immune system, potentially leading to autoimmune conditions like T1D. During viral infections, viruses often spread to secondary target organs, including the pancreas [175]. CVB-infected islet cells, once phagocytosed by resident APCs, can optimally activate these presenters, stimulating beta-cell antigen-specific T cells and generating proinflammatory cytokines and chemokines [176,177].

The escape of autoreactive T cells is crucial in targeting beta cells in T1D. This aberrant escape is linked to genetic susceptibility leading to defective tolerance [178]. Children with T1D have higher viral infection rates compared to their siblings or peers, indicating a potential trigger [179,180]. Prolonged beta-cell stress triggers the UPR to a terminal point, inducing proapoptotic pathways like CHOP to eliminate infected islets [181]. Studies have shown that IRE1α, a primary UPR regulator, is activated before the onset of insulitis in virus-induced diabetic models [182]. Additionally, islet cells of T1D patients have elevated CHOP protein levels, mediating ER stress-induced apoptosis [183].

ER stress caused by CVB infections activates the PERK and IRE1α pathways in both rat and human beta cells. While PERK pathway stimulation does not result in the overexpression of ATF4, ATF3, and CHOP or beta-cell death, IRE1α-mediated JNK1 activation is necessary for CVB replication [184]. As with many viruses it appears that CVB modifies the beta cells' UPR to promote its replication while averting premature beta-cell death. Additionally, viruses like porcine circovirus type 2 (PCV2) and Coxsackievirus B3 induce ER-stress-mediated apoptosis by activating CHOP production [185,186]. In conclusion, the interaction between ER stress, genetic predisposition, and environmental triggers, particularly viral infections, likely plays a critical role in the pathogenesis of T1D. Understanding these mechanisms is essential for developing strategies to prevent or treat T1D (Figure 3).

It has been proposed that viruses with a tissue tropism for islets induce T1D through several non-mutually exclusive mechanisms that involve disrupting the delicate balance of ER-protein handling, processing, and regulation [187]. Infection with virus leads to dysregulation of all these factors, as well as liberating proinflammatory cytokines that activate both innate and adaptive immune system. One such mechanism is molecular mimicry, where viral peptides exhibit high homology to islet peptides, leading to cross-reactivity and the activation of autoreactive T-lymphocytes against the body's own tissues [[188], [189], [190]]. Although reported for viral myocarditis, molecular mimicry is still controversial in the progression of islet beta cell destruction [191]. Another mechanism involves persistent viral infections, which can lead to prolonged activation of the immune system, resulting in continuous presentation of viral peptides and production of pro-inflammatory cytokines. This state of chronic inflammation can progressively promote a break in self-tolerance in those individuals with a predisposition to self-reactivity and the development of islet autoimmunity [188]. Viral evolution within a single host (intra-host evolution) can generate genetic alterations in the viral genome. These modifications may then contribute to the establishment of persistent infections, modulation of viral replication rates, and evasion of the host's immune response. Ultimately, the manifestations mentioned, such as islet autoimmunity and the development of type 1 diabetes, can be attributed to the viruses' ability to infect, and induce ER stress upon capturing the host protein machinery, and activating the islet's anti-viral response mobilizing the immune system and triggering the immune system to the islets.

Eukaryotic cells utilize a highly conserved catabolic process termed autophagy to maintain cellular homeostasis. This process is inducible by various stress conditions, including viral infection, endoplasmic reticulum (ER) stress, oxidative stress, and nutrient deprivation [192]. There are susceptibility loci for T1D found in many genes related to selective autophagy [193]. There has been a great deal of interest on how autophagy functions in type 1 diabetes. Dysfunctional autophagy is deleterious to β-cell health and normal insulin release [194,195]. However, blocking autophagy resulted in prolonged ER stress and β-cell death [196]. Beta cell function is also regulated by organelle-specific autophagy, including lipophagy (which breaks down lipid droplets), crinophagy (which breaks down insulin granules), and mitophagy (which breaks down mitochondria) [195]. It has been demonstrated that LC3 accumulates in insulin-positive cells in autoantibody-positive donors. LC3 and p62 are autophagosome indicators that are used to identify defective autophagy when they accumulate [197,198]. Nevertheless, there is no obvious abnormality in β cells when the p62 gene is deleted, indicating that p62 is not necessary for regular β-cell activity [199]. Prior research has implicated a cellular catabolic process as essential for maintaining pancreatic beta cell homeostasis. Dysfunction in this process has been linked to impaired pancreatic beta cell function and programmed cell death [200]. Additionally, autophagy is evidently involved in the cellular response to viral infection as well as the possible production of proinflammatory cytokines during viral infection [[201], [202], [203]]. We can hypothesize that, in these circumstances, autophagy plays a critical role in restoring β-cell homeostasis. This suggests that islet autophagy dysfunction may contribute to β-cell dysfunction and autoimmunity in T1D pathogenesis (Figure 4).

Figure 4.

Autophagy dysfunction in beta cells. The figure illustrates the process of how autophagy, a cellular cleaning process, can become compromised in beta cells, which are the cells in the pancreas that produce insulin. The figure shows a healthy beta cell on the left and a stressed beta cell on the right. In the healthy beta cell, there is an autophagosome, which is a sac-like structure that engulfs unnecessary or damaged cellular components. The autophagosome then fuses with a lysosome, which is a cellular waste disposal unit, and the engulfed components are degraded. In the stressed beta cell, the autophagosome formation is compromised, leading to a buildup of nonfunctional lysosomes and defective degradation of cellular components. This can lead to endoplasmic reticulum (ER) stress, which is a condition in which the ER, a cellular organelle responsible for protein folding, becomes overloaded. ER stress can lead to the release of neo antigens or peptides, which are molecules that can trigger an immune response. In the context of the figure, this immune response can be harmful to the beta cells, leading to their destruction. The figure was created with BioRender.com (accessed on 06 May 2024).

8. Beyond immune suppression: restoring tolerance and preserving beta cells in type 1 diabetes

Murine models have provided vital knowledge regarding the role the viral infection plays in providing the micro-milieu necessary for activation of the immune response toward the infected islets. The issue lies in the aberrant autoimmune T cells that have escaped tolerance and are now poised to respond to the beta cell antigens and targeted destruction of virally infected and non-infected islet beta cells. Our understanding of the mechanisms driving islet infiltration and the interplay between target beta cells and immune cells in human disease remains incomplete. The transfer of effective treatments from these models to human T1D has proven challenging. The role of pancreatic islet beta cells in breaking immune tolerance in diabetes is intricate and multifaceted, particularly in T1D, which is primarily characterized by their autoimmune destruction (Figure 5). Despite multiple attempts, there are currently no authorized treatments targeting the fundamental causes to prevent or reverse the disease [204]. While current immune intervention strategies demonstrate some success in delaying disease progression, their inability to re-establish immunological tolerance likely contributes to the impermanence of their clinical benefit [204,205]. Understanding immunological homeostasis in health and autoimmune diseases is crucial. Insights from tolerance restoration in autoimmune diseases, transplantation, and tolerance breakdown in cancer are valuable for determining druggable targets for therapeutic intervention. Beta cells play a pivotal role in dialoguing with the immune system, affecting immunological tolerance. Factors contributing to tolerance breakdown include autoimmune responses, genetic predisposition, environmental triggers, immune dysfunctions, and the gut microbiome. β-cell tolerance breakdown accelerates the progression of and presentation of autoantigens to T cells, triggering an autoimmune reaction. This activates immune cells like cytotoxic T cells and macrophages, causing local inflammation and beta cell loss. Cytokines released by immune cells, such as IL-1β and TNF-α, further contribute to beta cell apoptosis. Additionally, beta cells have been found to express molecules involved in the recruitment and activation of dendritic cells, further amplifying the immune response. Ultimately, the multifaceted role of islet beta cells in breaking immune tolerance in diabetes involves insulin production, antigen presentation, chemokine release, and immune cell recruitment, collectively contributing to the development and progression of the disease [206].

Figure 5.

The shifting landscape of immune response in type 1 diabetes. The figure depicts the cycle of breaking and restoring immune tolerance to pancreatic beta-cells in type 1 diabetes. Beta-cells, along with their macroenvironment and microenvironment, play a critical role in maintaining immunological tolerance. The green zone in the figure represents factors that contribute to immunological tolerance, while the red zone signifies factors that lead to a loss of immune tolerance and islet pathology. A healthy state is represented by the green zone, where factors promote a tolerogenic environment for beta cells. Beta cells that are well-rested and experience minimal stress or production of reactive oxygen species (ROS) are less likely to be seen as targets by the immune system. This likely refers to potential therapeutic interventions aimed at retraining the immune system to recognize beta cells as “self” again. Cytokines are signaling molecules that influence immune response. A shift in the cytokine profile towards a more tolerogenic one could prevent beta cell destruction. The red zone depicts factors that contribute to the breakdown of immune tolerance and ultimately lead to islet pathology, the cellular damage characteristic of type 1 diabetes. When stressed, beta cells may produce molecules (neoantigens) that appear foreign to the immune system. Additionally, endoplasmic reticulum (ER) stress, where protein folding is disrupted, can also contribute to neoantigen production. Infiltration of immune cells, particularly T cells, into the pancreatic islets where beta cells reside is a hallmark of type 1 diabetes. These infiltrating cells can directly attack and destroy beta cells. Factors that make beta cells more visible or “recognizable” by the immune system can increase the risk of attack. The figure was created with BioRender.com (accessed on 06 May 2024).

Immune checkpoint inhibitors (ICIs) like CTLA-4, PD-1, LAG-3, and PD-L1 antibodies boost cancer immunity but can trigger autoimmune diabetes, despite intact Treg regulation [207,208]. Considering that tumor-specific neoantigens are crucial targets in immune checkpoint inhibitor (ICI) treatments for cancer; They may also play a key role in the development of autoimmune diabetes, including both ICI-associated and classic type 1 diabetes (T1D). It is worth noting that neoantigens can arise from various modifications, including post-transcriptional, post-translational, and proteasomal changes. One example is the formation of hybrid insulin peptides (HIPs), which result from the fusion of insulin with other protein fragments. HIPs have been shown to activate T-cells in individuals with T1D [158,[208], [209], [210]]. Furthermore, the presence of islet autoreactive T-cells in healthy individuals, even without disease development, suggests underlying disturbances in normal physiological processes that trigger the expression of certain factors in the islet. These factors may serve as alerts to the immune system, raising questions about the mechanisms behind end organ autoimmunity and why the islet, specifically the beta cell, becomes the target of this immune response. A possible explanation lies in the intrinsic nature of the beta cell's function. These cells dynamically sense glucose levels and continuously produce insulin through ATP-dependent insulin granule exocytosis, operating under constant demand throughout an individual's lifetime. This persistent activity could lead to perturbations in normal physiology, triggering the expression of factors that attract the attention of the immune system and potentially initiate an autoimmune response against the beta cells. Despite extensive research, the exact mechanisms and interplay between genetic, environmental, and physiological factors that determine why some individuals develop type 1 diabetes while others do not are still not fully understood.

Therapeutic interventions for Type 1 Diabetes (T1D) frequently focus on modulating the immune response. Monoclonal antibodies, such as teplizumab and rituximab, are being investigated for their potential to transiently halt beta-cell destruction and decelerate disease progression in recently diagnosed T1D patients and individuals with high susceptibility [205,211]. Immune regulation offers an alternative approach to impede the ongoing loss of beta-cell function. This can be achieved by hindering the migration or co-stimulation of T cells, potentially through the use of therapeutic agents such as alefacept or abatacept [212,213]. Therefore, a therapy plan for T1D before, at the time of diagnosis, or even long after might involve maintaining the surviving beta-cell population and restoring their function. Similarly, verapamil, a calcium channel blocker, increased beta-cell survival in mice by reducing thioredoxin-interacting protein expression [214,215]. Clinical trials demonstrated improved beta-cell function and decreased insulin needs with vildagliptin, liraglutide, and exenatide, DPP-4 inhibitors, and GLP-1 receptor agonists. However, the degree of beta-cell function restoration varied among trials [216]. Therefore, in patients with newly diagnosed T1D, allogeneic or autologous fecal microbiome transplantation was carried out to stop the disease's progression and fully preserve beta-cell function for a minimum of one year [217].

9. Conclusion

While our knowledge of the mechanisms underlying beta cell destruction in type 1 diabetes (T1D) has advanced significantly, critical questions remain unanswered. We understand how, where, and what contributes to beta cell destruction, but the fundamental question of why the immune system specifically targets these cells is still unresolved. The immune system's primary role is to recognize and eliminate pathogens or abnormal cells, yet in T1D, it erroneously identifies the body's own beta cells as foreign or harmful, triggering an autoimmune response that leads to their destruction. Although other cells may express similar antigens, the factors that make beta cells the primary target of this misdirected immune attack remain unclear. Several potential explanations exist. The unique characteristics of beta cells, such as their role in insulin production and glucose homeostasis, may render them particularly susceptible to immune assault. The specialized metabolic demands and protein processing requirements of beta cells could create an environment conducive to autoimmune targeting. Genetic predisposition leading to immune tolerance defects and ER stress induced β cell protein handling dysregulation can lead to aberrant recognition and responding to post translationally modified proteins (neoantigens), for which tolerance is lacking. Furthermore, the inflammatory signature released by stressed beta cells may resemble that of virus-infected cells, inadvertently activating the immune system and prompting an autoimmune response. Unraveling the complexities behind why the immune system selectively targets beta cells in T1D is crucial. Elucidating these mechanisms could reveal potential therapeutic targets to prevent or halt beta cell destruction, ultimately leading to improved management and treatment strategies for this debilitating condition.

Funding

This work was funded by National Institute of Diabetes & Digestives & Kidney (NIDDK) grant – NIH R01 DK132583.

CRediT authorship contribution statement

Saptarshi Roy: Writing – review & editing, Writing – original draft, Conceptualization. Pravil Pokharel: Writing – review & editing, Writing – original draft. Jon D. Piganelli: Writing – review & editing, Writing – original draft, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.

Data availability

No data was used for the research described in the article.