Autoreactive T cells in type 1 diabetes

Abstract

Type 1 diabetes (T1D) is a chronic autoimmune disease that causes severe loss of pancreatic β cells. Autoreactive T cells are key mediators of β cell destruction. Studies of organ donors with T1D that have examined T cells in pancreas, the diabetogenic insulitis lesion, and lymphoid tissues have revealed a broad repertoire of target antigens and T cell receptor (TCR) usage, with initial evidence of public TCR sequences that are shared by individuals with T1D. Neoepitopes derived from post-translational modifications of native antigens are emerging as novel targets that are more likely to evade self-tolerance. Further studies will determine whether T cell responses to neoepitopes are major disease drivers that could impact prediction, prevention, and therapy. This Review provides an overview of recent progress in our knowledge of autoreactive T cells that has emerged from experimental and clinical research as well as pathology investigations.

Figure 1. The natural history of islet autoimmunity.

To illustrate the chronic nature and heterogeneity of islet autoimmunity and highlight critical gaps in knowledge, this figure depicts two hypothetical patterns of T cell–mediated β cell destruction and loss of insulin secretion: early triggering of insulitis and semi-linear β cell destruction (blue line); or late start of the autoimmune β cell destruction, closer to clinical diagnosis (red line). This is a simplified schematic representation: loss of β cell mass and insulin secretion may not always correlate and may vary based on the relative importance of autoimmune destruction and β cell dysfunction during disease progression in a given patient. Seroconversion for T1D-associated autoantibodies to native antigens is considered the earliest step in the triggering of islet autoimmunity; it occurs in early life in children at genetic risk of T1D, but it may occur later in life in many individuals. As only a minority of islets are affected at any given time, the process proceeds asynchronously. Residual β cell mass at onset may be a function of the severity of autoimmune destruction and initial mass, which are impacted by age. It is often incomplete at diagnosis, but it continues afterward, sometimes for years. Low levels of insulin secretion are detected in many patients even decades after diagnosis: some β cell regeneration occurs after diagnosis, and it has been hypothesized that regeneration may occur during the preclinical stage. Critical questions remain unanswered: To what extent and when does the detection of humoral and cellular responses in the circulation reflect insulitis and β cell destruction, and which responses are relevant? At what time are insulitis and β cell destruction triggered during the progression of islet autoimmunity? Is there a preferred order for autoantibody and T cell responses to native antigens and neoepitopes, and what are their relative roles in insulitis, β cell destruction, and disease onset (trigger/driver responses versus secondary responses)?

Figure 2. CD8⁺ T cell responses to pancreatic β cells.

Schematic representation of major autoantigen classes that may be targeted by CD8⁺ T cells, the predominant immune cell type in the insulitis. As described in the main text, these may include native antigens and neoepitopes. The generation of these epitopes may be promoted by β cell inflammation and stress. Insulitis is often associated with an interferon response with hyperexpression of HLA class I molecules, which may be induced by viral infections. Hyperexpression of HLA class I molecules facilitates presentation of autoantigen epitopes to CD8⁺ T cells. Whether islet-infiltrating CD8⁺ T cells also target viral epitopes remains to be investigated.

Figure 3. CD4⁺ T cell responses to pancreatic β cells.

Schematic representation of major autoantigen classes that may be targeted by CD4⁺ T cells. Similar to CD8⁺ T cells, these may include native antigens and neoepitopes, which may be formed under conditions of β cell inflammation and stress. β Cells can produce several neoepitopes: for example, peptides originating from alternative splicing, insulin hybrid peptides (HP), and DRiP insulin peptides are reportedly produced in β cells (for DRiPs, the evidence is from studies of cell lines). Many autoantigens are secretory granule proteins, which may be released by β cells and acquired by APCs. Exosomes released by β cells also contain autoantigens. CD4⁺ T cells may react with antigens captured, processed, and presented by APCs in the pancreatic lymph node and in the islets. The B:9–23 epitope is produced in the secretory granules, captured, and presented by APCs, at least in the NOD mouse. The generation of neoepitopes in β cells further raises the question of whether the previously reported aberrant expression of HLA class II molecules by β cells might allow CD4⁺ T cells to recognize antigens on their surface directly.