Genes and environment: a moving target?
Studies from multiple populations have shown shifts of the typical HLA class II gene associations during the last few decades. Indeed, fewer patients carry the high risk heterozygous HLA-DR/DQ genotype, except among younger children [59–63], in whom the disease is becoming more common . In contrast, more patients now carry moderate risk HLA types and genotypes. Such shifts in HLA associations may be explained by stronger environmental pressures that enhance HLA-mediated genetic predisposition and/or broaden the spectrum of diabetogenic gene–environment interactions. Viruses, and especially enteroviruses, rank at the top of the list of environmental factors that have been linked to T1DM .
Several lines of evidence support a role for viruses :
- diabetes incidence correlates with the seasonality of enterovirus infections;
- reports of T1DM epidemic bouts;
- associations of T1DM with immune responses to enterovirus and detection of enteroviral RNA, which correlate with islet autoimmunity and β-cell function ;
- reports demonstrate higher T1DM risk in the offspring of mothers with enteroviral infections  and, related to this observation, enteroviruses may infect the thymus and alter thymic selection processes in turn favoring autoimmunity ;
- he identification of a susceptibility gene controlling innate responses to enterovirus : the IFIH1 gene encodes for the interferon induced with helicase C domain protein 1, which recognizes enteroviral double-stranded RNA (dsRNA) in infected cells and promotes interferon and NFκB responses, followed by the production of inflammatory cytokines. Experimental data suggest that viral infection may lead to sustained interferon responses and upregulation of HLA class I expression in the presence of predisposing IFIH1 alleles, which would in turn increase the potential to expose self-antigens and trigger autoreactive responses. In contrast, T1DM-protective IFIH1 variants may be associated with reduced responses and decreased likelihood to trigger autoimmunity (Figure 29.1);
- growing evidence that enterovirus can infect β cells in T1DM patients [69,70]: studies in large cohorts of deceased donors show that the presence of viral antigen is detected much more frequently in the pancreas of T1DM patients compared to patients with T2DM and nondiabetic subjects . Further, enteroviruses can infect and damage β cells, induce expression of HLA class I antigens and alpha-interferon , which could amplify inflammation and provide a link to the triggering of islet autoimmunity. Enterovirus infections may also induce functional changes and influence β-cell replication [65,71].
Researchers are testing the hypothesis that multiple, acute viral infections and/or chronic viral infections may repeatedly trigger and sustain autoimmunity over time. Future studies could possibly link one or more serotypes to T1DM, with obvious implications for improved prediction, prevention, and treatment against the triggering and progression of islet autoimmunity.
Multiple T-cell types and autoreactive T-cell specificities mediate T1DM
The inflammatory lesion known as insulitis is considered the pathologic hallmark of T1DM, typically seen during the prediabetic stage or around the time of onset: it consists of the infiltration of the pancreatic islets by lymphocytes and other inflammatory cells. Of great significance is the observation that β cells of NOD mice contain granules enriched in a diabetogenic insulin peptide (B9-23), which is up-taken by intra-islet DCs and presented to CD4 T cells in the PLN [72,73]. This finding calls attention to the β cell itself as the source of a peptide antigen which, at least in the NOD mouse, is the main driver of islet autoimmunity . Once activated in the PLN, autoreactive CD8 T cells recognize MHC class I restricted islet autoantigens directly on the β-cell surface and mediate their cytotoxic effects through multiple effector molecules, including perforin, Fas ligand, and Th1 cytokines, such as IFN-γ and TNF-α. CD4 T cells are linked with the triggering of the autoimmune process and support activation and recruitment of cytotoxic CD8 T cells into the islets. However, CD4 T cells also stimulate β-cell killing via the secretion of cytokines and via signaling through death receptors (Fas, TNF-α) . While T1DM is traditionally considered to be mediated by Th1 type T cells, growing evidence implicates Th17T cells, which have been shown to have deleterious effects on β cells .
In both patients and animal models, studies have identified multiple CD4 and CD8 T cells reacting against a variety of islet autoantigens , including multiple epitopes from the autoantigens insulin/proinsulin, GAD65, IA-2, IGRP, and ZnT8. In patients, most studies are limited to peripheral blood samples; however, progress has been made in validating that these cells are linked to disease pathology in the target organ: (1) insulin autoreactive T cells were found in the PLN of T1DM patients ; (2) in pancreas transplant recipients, proinsulin/insulin and GAD-specific CD4 T cells were detected in both the circulation and the pancreas transplant\ lymph node in biopsies in association with recurrent diabetes, as demonstrated by insulitis in the transplanted pancreas . Recent studies also provide functional evidence for the pathogenic role of human autoreactive T cells: (1) GAD-reactive CD4 T cells from the above pancreas transplant patients with recurrent diabetes killed human islets, in vivo, when co-transplanted under the kidney capsule of immunodeficient mice ; (2) proinsulin-specific CD8 T cells from the peripheral blood of T1DM patients kill β cells in vitro ; (3) autoreactive T cells were directly demonstrated in the insulitis lesion, in the pancreas of an organ donor with T1DM, using MHC tetramers .