Antibodies to oxidized insulin improve prediction of type 1 diabetes in children with positive standard islet autoantibodies

Immune checkpoint modulators in early clinical development for the treatment of type 1 diabetes

INTRODUCTION

Despite the improvements of insulin therapy, people with type 1 diabetes (T1D) still suffer from a decreased quality of life and life expectancy. The search toward a cure for T1D is therefore still a scorching open field of research.

AREAS COVERED

Tackling the immune checkpoint signaling pathways has gained importance in the field of cancer immunotherapy. The same pathways can be targeted in autoimmunity with an opposite principle: to dampen the exaggerated immune response. In this review, we report a comprehensive excursus on the cellular and molecular mechanisms that lead to loss of immunological tolerance, and recent evidence on the role of immune checkpoint molecules in the development of T1D and their potential application for the mitigation of autoimmune diabetes.

EXPERT OPINION

Contrasting results about the efficacy of immune checkpoint modulators for T1D have been published, with very few molecules from preclinical studies eligible for use in humans. The heterogeneous and complex pathophysiology of T1D may explain the conflicting evidence. Designing clinical trials that acknowledge the pathophysiological and clinical complexity of T1D and that forecast the need of simultaneously tackling different disease pathways will be crucial to enhance the benefits which may be gained by such compounds.

Post-Translational Modifications and Diabetes

Diabetes and its associated complications have increasingly become major challenges for global healthcare. The current therapeutic strategies involve insulin replacement therapy for type 1 diabetes (T1D) and small-molecule drugs for type 2 diabetes (T2D). Despite these advances, the complex nature of diabetes necessitates innovative clinical interventions for effective treatment and complication prevention. Accumulative evidence suggests that protein post-translational modifications (PTMs), including glycosylation, phosphorylation, acetylation, and SUMOylation, play important roles in diabetes and its pathological consequences. Therefore, the investigation of these PTMs not only sheds important light on the mechanistic regulation of diabetes but also opens new avenues for targeted therapies. Here, we offer a comprehensive overview of the role of several PTMs in diabetes, focusing on the most recent advances in understanding their functions and regulatory mechanisms. Additionally, we summarize the pharmacological interventions targeting PTMs that have advanced into clinical trials for the treatment of diabetes. Current challenges and future perspectives are also provided.

Understanding Islet Autoantibodies in Prediction of Type 1 Diabetes

As screening studies and preventive interventions for type 1 diabetes (T1D) advance rapidly, the utility of islet autoantibodies (IAbs) in T1D prediction comes with challenges for early and accurate disease progression prediction. Refining features of IAbs can provide more accurate risk assessment. The advances in islet autoantibodies assay techniques help to screen out islet autoantibodies with high efficiency and high disease specificity. Exploring new islet autoantibodies to neoepitopes/neoantigens remains a hot research field for improving prediction and disease pathogenesis. We will review the recent research progresses of islet autoantibodies to better understand the utility of islet autoantibodies in prediction of T1D.

Autoantibody and T cell responses to oxidative post-translationally modified insulin neoantigenic peptides in type 1 diabetes

AIMS/HYPOTHESIS

Antibodies specific to oxidative post-translational modifications (oxPTM) of insulin (oxPTM-INS) are present in most individuals with type 1 diabetes, even before the clinical onset. However, the antigenic determinants of such response are still unknown. In this study, we investigated the antibody response to oxPTM-INS neoepitope peptides (oxPTM-INSPs) and evaluated their ability to stimulate humoral and T cell responses in type 1 diabetes. We also assessed the concordance between antibody and T cell responses to the oxPTM-INS neoantigenic peptides.

METHODS

oxPTM-INS was generated by exposing insulin to various reactive oxidants. The insulin fragments resulting from oxPTM were fractionated by size-exclusion chromatography further to ELISA and LC-MS/MS analysis to identify the oxidised peptide neoepitopes. Immunogenic peptide candidates were produced and then modified in house or designed to incorporate in silico-oxidised amino acids during synthesis. Autoantibodies to the oxPTM-INSPs were tested by ELISA using sera from 63 participants with new-onset type 1 diabetes and 30 control participants. An additional 18 fresh blood samples from participants with recently diagnosed type 1 diabetes, five with established disease, and from 11 control participants were used to evaluate, in parallel, CD4⁺ and CD8⁺ T cell activation by oxPTM-INSPs.

RESULTS

We observed antibody and T cell responses to three out of six LC-MS/MS-identified insulin peptide candidates: A:12-21 (SLYQLENYCN, native insulin peptide 3 [Nt-INSP-3]), B:11-30 (LVEALYLVCGERGFFYTPKT, Nt-INSP-4) and B:21-30 (ERGFFYTPKT, Nt-INSP-6). For Nt-INSP-4 and Nt-INSP-6, serum antibody binding was stronger in type 1 diabetes compared with healthy control participants (p≤0.02), with oxidised forms of ERGFFYTPKT, oxPTM-INSP-6 conferring the highest antibody binding (83% binders to peptide modified in house by hydroxyl radical [^●OH] and >88% to in silico-oxidised peptide; p≤0.001 vs control participants). Nt-INSP-4 induced the strongest T cell stimulation in type 1 diabetes compared with control participants for both CD4⁺ (p<0.001) and CD8⁺ (p=0.049). CD4⁺ response to oxPTM-INSP-6 was also commoner in type 1 diabetes than in control participants (66.7% vs 27.3%; p=0.039). Among individuals with type 1 diabetes, the CD4⁺ response to oxPTM-INSP-6 was more frequent than to Nt-INSP-6 (66.7% vs 27.8%; p=0.045). Overall, 44.4% of patients showed a concordant autoimmune response to oxPTM-INSP involving simultaneously CD4⁺ and CD8⁺ T cells and autoantibodies.

CONCLUSIONS/INTERPRETATION

Our findings support the concept that oxidative stress, and neoantigenic epitopes of insulin, may be involved in the immunopathogenesis of type 1 diabetes.

Oxidized Proteins Differentially Affect Maturation and Activation of Human Monocyte-Derived Cells

In cancer, antigen-presenting cells (APC), including dendritic cells (DCs), take up and process proteins to mount adaptive antitumor immune responses. This often happens in the context of inflamed cancer, where reactive oxygen species (ROS) are ubiquitous to modify proteins. However, the inflammatory consequences of oxidized protein uptake in DCs are understudied. To this end, we investigated human monocyte-derived cell surface marker expression and cytokine release profiles when exposed to oxidized and native proteins. Seventeen proteins were analyzed, including viral proteins (e.g., CMV and HBV), inflammation-related proteins (e.g., HO1 and HMGB1), matrix proteins (e.g., Vim and Coll), and vastly in the laboratory used proteins (e.g., BSA and Ova). The multifaceted nature of inflammation-associated ROS was mimicked using gas plasma technology, generating reactive species cocktails for protein oxidation. Fourteen oxidized proteins led to elevated surface marker expression levels of CD25, CD40, CD80, CD86, and MHC-II as well as strongly modified release of IL6, IL8, IL10, IL12, IL23, MCP-1, and TNFα compared to their native counterparts. Especially IL8, heme oxygenase 2, and vimentin oxidation gave pronounced effects. Furthermore, protein kinase phospho-array studies in monocyte-derived cells pulsed with native vs. oxidized IL8 and insulin showed enhanced AKT and RSK2 phosphorylation. In summary, our data provide for the first time an overview of the functional consequences of oxidized protein uptake by human monocyte-derived cells and could therefore be a starting point for exploiting such principle in anticancer therapy in the future.

Biomarkers of autoimmunity and beta cell metabolism in type 1 diabetes

Posttranslational protein modifications (PTMs) are an inherent response to physiological changes causing altered protein structure and potentially modulating important biological functions of the modified protein. Besides cellular metabolic pathways that may be dictated by PTMs, the subtle change of proteins also may provoke immune attack in numerous autoimmune diseases. Type 1 diabetes (T1D) is a chronic autoimmune disease destroying insulin-producing beta cells within the pancreatic islets, a result of tissue inflammation to specific autoantigens. This review summarizes how PTMs arise and the potential pathological consequence of PTMs, with particular focus on specific autoimmunity to pancreatic beta cells and cellular metabolic dysfunction in T1D. Moreover, we review PTM-associated biomarkers in the prediction, diagnosis and in monitoring disease activity in T1D. Finally, we will discuss potential preventive and therapeutic approaches of targeting PTMs in repairing or restoring normal metabolic pathways in pancreatic islets.

Posttranslational modifications in diabetes: Mechanisms and functions

As one of the most widespread chronic diseases, diabetes and its accompanying complications affect approximately one tenth of individuals worldwide and represent a growing cause of morbidity and mortality. Accumulating evidence has proven that the process of diabetes is complex and interactive, involving various cellular responses and signaling cascades by posttranslational modifications (PTMs). Therefore, understanding the mechanisms and functions of PTMs in regulatory networks has fundamental importance for understanding the prediction, onset, diagnosis, progression, and treatment of diabetes. In this review, we offer a holistic summary and illustration of the crosstalk between PTMs and diabetes, including both types 1 and 2. Meanwhile, we discuss the potential use of PTMs in diabetes treatment and provide a prospective direction for deeply understanding the metabolic diseases.

Diabetes and COVID-19: Short- and Long-Term Consequences

When the corona pandemic commenced more than two years ago, it was quickly recognized that people with metabolic diseases show an augmented risk of severe COVID-19 and an increased mortality compared to people without these comorbidities. Furthermore, an infection with SARS-CoV-2 has been shown to lead to an aggravation of metabolic diseases and in single cases to new-onset metabolic disorders. In addition to the increased risk for people with diabetes in the acute phase of COVID-19, this patient group also seems to be more often affected by long-COVID and to experience more long-term consequences than people without diabetes. The mechanisms behind these discrepancies between people with and without diabetes in relation to COVID-19 are not completely understood yet and will require further research and follow-up studies during the following years. In the current review, we discuss why patients with diabetes have this higher risk of developing severe COVID-19 symptoms not only in the acute phase of the disease but also in relation to long-COVID, vaccine breakthrough infections and re-infections. Furthermore, we discuss the effects of lockdown on glycemic control.

SARS-CoV-2 induced post-translational protein modifications: A trigger for developing autoimmune diabetes?

Emerging evidence indicates a bi-directional relationship between SARS-CoV-2 and diabetes. The possibility exists that SARS-CoV-2 could induce diabetes, but it is not yet clear whether this might be a fulminant-type diabetes, autoimmune diabetes, or a new-onset transient hyperglycaemia. This viewpoint discusses mechanisms by which SARS-CoV-2 might trigger type 1 diabetes mellitus (T1DM). Specifically, we looked at the role of post-translational protein modifications (PTMs) and the generation of neoepitopes as a potential mechanism in the induction of islet autoimmunity, and the pathways via which coronavirus infections might exacerbate the formation of PTMs and, in so doing, provoke the onset of T1DM.

Hydroxyl radical induced structural perturbations make insulin highly immunogenic and generate an auto-immune response in type 2 diabetes mellitus

Reactive oxygen species (ROS) cause oxidative damage to proteins and generate deleterious by-products which induce a breakdown of immune tolerance and produce antibodies against host macromolecules with implication in human diseases. This study characterizes the hydroxyl radical (OH) modifications of insulin, evaluates its cytotoxicity and immunogenicity, and probes its role in type 2 diabetes (T2DM) autoimmunity. The results demonstrate susceptibility of insulin to modifications induced by OH, causing exposure of its chromophoric aromatic amino acid residues, quenching of tyrosine fluorescence intensity, loss of α-helix and gain in β content. Modification causes re-arrangement of native interactions of the aromatic residues in insulin. It enhanced the carbonyl content in insulin, exposed its hydrophobic patches and generated non-fibrillar, amorphous type of aggregates that are cytotoxic in nature. Native insulin induced low titre antibodies in immunized rabbits, whereas OH modified insulin generated a strong immune response. Competitive ELISA studies showed high specificity of antibodies generated against OH modified insulin towards the modified protein. Cross reaction studies showed the presence of common antigenic determinants on various oxidised proteins. Since T2DM patients show increased ROS production, oxidation of insulin is expected to occur, which might amplify autoimmune reactions against insulin. True to the assumption, direct binding ELISA showed the presence of anti-OH insulin circulating antibodies in T2DM patients which are specific for the oxidized insulin. In conclusion, insulin loses structural integrity to OH, forms cytotoxic amorphous aggregates, turns highly immunogenic and elicits humoral response in T2DM patients.

Non-Genetically Encoded Epitopes Are Relevant Targets in Autoimmune Diabetes

Islet antigen reactive T cells play a key role in promoting beta cell destruction in type 1 diabetes (T1D). Self-reactive T cells are typically deleted through negative selection in the thymus or deviated to a regulatory phenotype. Nevertheless, those processes are imperfect such that even healthy individuals have a reservoir of potentially autoreactive T cells. What remains less clear is how tolerance is lost to insulin and other beta cell specific antigens. Islet autoantibodies, the best predictor of disease risk, are known to recognize classical antigens such as proinsulin, GAD65, IA-2, and ZnT8. These antibodies are thought to be supported by the expansion of autoreactive CD4⁺ T cells that recognize these same antigenic targets. However, recent studies have identified new classes of non-genetically encoded epitopes that may reflect crucial gaps in central and peripheral tolerance. Notably, some of these specificities, including epitopes from enzymatically post-translationally modified antigens and hybrid insulin peptides, are present at relatively high frequencies in the peripheral blood of patients with T1D. We conclude that CD4⁺ T cells that recognize non-genetically encoded epitopes are likely to make an important contribution to the progression of islet autoimmunity in T1D. We further propose that these classes of neo-epitopes should be considered as possible targets for strategies to induce antigen specific tolerance.

Autoantigen Treatment in Type 1 Diabetes: Unsolved Questions on How to Select Autoantigen and Administration Route

Autoantigen treatment has been tried for the prevention of type 1 diabetes (T1D) and to preserve residual beta-cell function in patients with a recent onset of the disease. In experimental animal models, efficacy was good, but was insufficient in human subjects. Besides the possible minor efficacy of peroral insulin in high-risk individuals to prevent T1D, autoantigen prevention trials have failed. Other studies on autoantigen prevention and intervention at diagnosis are ongoing. One problem is to select autoantigen/s; others are dose and route. Oral administration may be improved by using different vehicles. Proinsulin peptide therapy in patients with T1D has shown possible minor efficacy. In patients with newly diagnosed T1D, subcutaneous injection of glutamic acid decarboxylase (GAD) bound to alum hydroxide (GAD-alum) can likely preserve beta-cell function, but the therapeutic effect needs to be improved. Intra-lymphatic administration may be a better alternative than subcutaneous administration, and combination therapy might improve efficacy. This review elucidates some actual problems of autoantigen therapy in the prevention and/or early intervention of type 1 diabetes.

Prediction and Prevention of Type 1 Diabetes

Type 1 Diabetes (T1D) is one of the most common chronic autoimmune diseases in children. The disease is characterized by the destruction of beta cells, leading to hyperglycemia, and to a lifelong insulin-dependent state. Although several studies in the last decades have added relevant insights, the complex pathogenesis of the disease is not yet completely understood. Recent studies have been focused on several factors, including family history and genetic predisposition (HLA and non-HLA genes) as well as environmental and metabolic biomarkers, with the aim of predicting the development and progression of T1D. Once a child becomes symptomatic, beta cell mass has already reached a critical threshold (usually a residual of 20-30% of normal amounts), thus representing only the very late phase of the disease. In particular, this final stage follows two preceding asymptomatic stages, which have been precisely identified. In view of the long natural history and complex pathogenesis of the disease, many strategies may be proposed for primary, secondary, and tertiary prevention. Strategies of primary prevention aim to prevent the onset of autoimmunity against beta cells in asymptomatic individuals at high risk for T1D. In addition, the availability of novel humoral and metabolic biomarkers that are able to characterize subjects at high risk of progression, have stimulated several studies on secondary and tertiary prevention, aimed to preserve residual beta cell destruction and/or to prolong the remission phase after the onset of T1D. This review focuses on the major current knowledge on prediction and prevention of T1D in children.