Type 1 diabetes-associated HLA-DQ8 transdimer accommodates a unique peptide repertoire

MHC Class II Polymorphisms, Autoreactive T-Cells, and Autoimmunity

Major histocompatibility complex (MHC) genes, also known as human leukocyte antigen genes (HLA) in humans, are the prevailing contributors of genetic susceptibility to autoimmune diseases such as Type 1 Diabetes (T1D), multiple sclerosis, and rheumatoid arthritis, among others (1–3). Although the pathways through which MHC molecules afford autoimmune risk or resistance remain to be fully mapped out, it is generally accepted that they do so by shaping the central and peripheral T-cell repertoires of the host toward autoimmune proclivity or resistance, respectively. Disease-predisposing MHC alleles would both spare autoreactive thymocytes from central tolerance and bias their development toward a pathogenic phenotype. Protective MHC alleles, on the other hand, would promote central deletion of autoreactive thymocytes and skew their development toward non-pathogenic phenotypes. This interpretation of the data is at odds with two other observations: that in MHC-heterozygous individuals, resistance is dominant over susceptibility; and that it is difficult to understand how deletion of one or a few clonal autoreactive T-cell types would suffice to curb autoimmune responses driven by hundreds if not thousands of autoreactive T-cell specificities. This review provides an update on current advances in our understanding of the mechanisms underlying MHC class II-associated autoimmune disease susceptibility and/or resistance and attempts to reconcile these seemingly opposing concepts.

Structural Characteristics of HLA-DQ that May Impact DM Editing and Susceptibility to Type-1 Diabetes

Autoreactive CD4+ T cells initiate the chronic autoimmune disease Type-1 diabetes (T1D), in which multiple environmental and genetic factors are involved. The association of HLA, especially the DR-DQ loci, with risk for T1D is well documented. However, the molecular mechanisms are poorly understood. In this review, we explore the structural characteristics of HLA-DQ and the role of HLA-DM function as they may contribute to an understanding of autoreactive T cell development in T1D.

Polyubiquitination and proteasomal turnover controls the anti-apoptotic activity of Bcl-B

Anti-apoptotic Bcl-2 family members can contribute to tumorigenesis and may convey resistance to anti-cancer regimens. Therefore, they are important targets for novel therapeutics, particularly Bcl-2 homology (BH)3 mimetics. Bcl-B (BCL-2-like protein-10) is a relatively understudied member of the Bcl-2 protein family. Its physiological function is unknown, but it has been proven to have an anti-apoptotic activity and to act as a tumor promoter in mice. In human, high Bcl-B protein expression levels correlate with poor prognosis in various carcinomas and predict treatment resistance in acute myeloid leukemia. We here report that protein expression level and anti-apoptotic activity of Bcl-B are dictated by its ubiquitination. We demonstrate that Bcl-B is polyubiquitinated at steady state, in a unique loop between the BH1 and BH2 domains. Mutagenesis identified lysine (K)128 as an acceptor site for polyubiquitin chains, and K119 and K120, but not K181, as potential ubiquitination sites. Mass spectrometry confirmed K128 as a ubiquitination site and defined the polyubiquitin chains as K48-linked, which was confirmed by linkage-specific antibodies. Accordingly, Bcl-B proved to be an instable protein that is subject to ubiquitin-dependent proteasomal degradation at steady state. At equal mRNA expression, protein expression of a lysineless, nonubiquitinated Bcl-B mutant was fivefold higher than that of wild-type Bcl-B, demonstrating that ubiquitination is a key determinant for Bcl-B protein expression levels. Ubiquitination controlled the anti-apoptotic capacity of Bcl-B, in response to a variety of conventional and novel anti-cancer drugs. Certain anti-cancer drugs, known to reduce Mcl-1 protein levels, likewise downregulated Bcl-B. Together, these data demonstrate that polyubiquitination and proteasomal turnover dictate the expression level and anti-apoptotic capacity of Bcl-B.

The multiple origins of Type 1 diabetes

It is widely accepted that Type 1 diabetes is a complex disease. Genetic predisposition and environmental factors favour the triggering of autoimmune responses against pancreatic β-cells, eventually leading to β-cell destruction. Over 40 susceptibility loci have been identified, many now mapped to known genes, largely supporting a dominant role for an immune-mediated pathogenesis. This role is also supported by the identification of several islet autoantigens and antigen-specific responses in patients with recent onset diabetes and subjects with pre-diabetes. Increasing evidence suggests certain viruses as a common environmental factor, together with diet and the gut microbiome. Inflammation and insulin resistance are emerging as additional cofactors, which might be interrelated with environmental factors. The heterogeneity of disease progression and clinical manifestations is likely a reflection of this multifactorial pathogenesis. So far, clinical trials have been mostly ineffective in delaying progression to overt diabetes in relatives at increased risk, or in reducing further loss of insulin secretion in patients with new-onset diabetes. This limited success may reflect, in part, our incomplete understanding of key pathogenic mechanisms, the lack of truly robust biomarkers of both disease activity and β-cell destruction, and the inability to assess the relative contributions of various pathogenic mechanisms at various time points during the course of the natural history of Type 1 diabetes. Emerging data and a re-evaluation of histopathological, immunological and metabolic findings suggest the hypothesis that unknown mechanisms of β-cell dysfunction may be present at diagnosis, and may contribute to the development of hyperglycaemia and clinical symptoms.

Celiac disease: an immunological jigsaw

Celiac disease (CD) is a chronic enteropathy induced by dietary gluten in genetically predisposed people. The keystone of CD pathogenesis is an adaptive immune response orchestrated by the interplay between gluten and MHC class II HLA-DQ2 and DQ8 molecules. Yet, other factors that impair immunoregulatory mechanisms and/or activate the large population of intestinal intraepithelial lymphocytes (IEL) are indispensable for driving tissue damage. Herein, we summarize our current understanding of the mechanisms and consequences of the undesirable immune response initiated by gluten peptides. We show that CD is a model disease to decipher the role of MHC class II molecules in human immunopathology, to analyze the mechanisms that link tolerance to food proteins and autoimmunity, and to investigate how chronic activation of IEL can lead to T cell lymphomagenesis.

Etiopathogenesis of insulin autoimmunity

Autoimmunity against pancreatic islet beta cells is strongly associated with proinsulin, insulin, or both. The insulin autoreactivity is particularly pronounced in children with young age at onset of type 1 diabetes. Possible mechanisms for (pro)insulin autoimmunity may involve beta-cell destruction resulting in proinsulin peptide presentation on HLA-DR-DQ Class II molecules in pancreatic draining lymphnodes. Recent data on proinsulin peptide binding to type 1 diabetes-associated HLA-DQ2 and -DQ8 is reviewed and illustrated by molecular modeling. The importance of the cellular immune reaction involving cytotoxic CD8-positive T cells to kill beta cells through Class I MHC is discussed along with speculations of the possible role of B lymphocytes in presenting the proinsulin autoantigen over and over again through insulin-carrying insulin autoantibodies. In contrast to autoantibodies against other islet autoantigens such as GAD65, IA-2, and ZnT8 transporters, it has not been possible yet to standardize the insulin autoantibody test. As islet autoantibodies predict type 1 diabetes, it is imperative to clarify the mechanisms of insulin autoimmunity.