Discriminative T cell recognition of cross-reactive islet-antigens is associated with HLA-DQ8 transdimer-mediated autoimmune diabetes

Human leukocyte antigen (HLA)-DQ8 transdimer (HLA-DQA1*0501/DQB1*0302) confers exceptionally high risk in autoimmune diabetes. However, little is known about HLA-DQ8 transdimer-restricted CD4 T cell recognition, an event crucial for triggering HLA-DQ8 transdimer-specific anti-islet immunity. Here, we report a high degree of epitope overlap and T cell promiscuity between susceptible HLA-DQ8 and HLA-DQ8 transdimer. Despite preservation of putative residues for T cell receptor (TCR) contact, stronger disease-associated responses to cross-reactive, immunodominant islet epitopes are elicited by HLA-DQ8 transdimer. Mutagenesis at the α chain of HLA-DQ8 transdimer in complex with the disease-relevant GAD65_250-266 peptide and in silico analysis reveal the DQ α52 residue located within the N-terminal edge of the peptide-binding cleft for the enhanced T cell reactivity, altering avidity and biophysical affinity between TCR and HLA-peptide complexes. Accordingly, a structurally promiscuous but nondegenerate TCR-HLA-peptide interface is pivotal for HLA-DQ8 transdimer-mediated autoimmune diabetes.

Antigen-Specific T Cell Analysis Reveals That Active Immune Responses to β Cell Antigens Are Focused on a Unique Set of Epitopes

CD38 is an activation marker that is present on recently activated T cells, but absent on resting memory T cells. In this study, we show that CD45RO⁺CD38⁺ β cell Ag-specific CD4⁺ T cells were present at higher frequencies in type 1 diabetes subjects compared with those in healthy subjects. These results imply an ongoing β cell immunity years after onset of diabetes and suggest these activated T cells have an active role in the disease process. The Ag specificities of these activated T cells were determined by a novel CD154 T cell epitope mapping assay. Although each patient usually had a unique set of epitopes recognized by these T cells, two epitopes, DR0401-restricted modified preproinsulin peptide 78-90^K88S and zinc transport 8 266-285, were repeatedly identified in multiple subjects. Identifying these T cells and their specific antigenic epitopes might provide immunotherapeutic targets for personalized therapies.

T cell promiscuity between susceptible MHC alleles potentiates autoimmune diabetes (HUM3P.259)

Major histocompatibility complex (MHC) genes, also known as human leukocyte antigen (HLA) genes in humans, are the prevailing risk factors in multiple autoimmune diseases, including multiple sclerosis (MS), autoimmune diabetes (type 1 diabetes or T1D), and rheumatoid arthritis (RA), yet MHC-linked susceptibility is not fully understood. In T1D, HLA-DQ8trans (DQ8trans), a trans-complementary molecule encoded by DQA1*0501 (HLA-DQ2α) and DQB1*0302 (HLA-DQ8β), confers exceptionally high risk. In contrast to the notion that DQ8trans expands a unique pathogenic T cell repertoire due to its trans-complementary nature, here we report a high degree of epitope overlap between DQ8trans and another high-risk allele, HLA-DQ8 (DQ8; DQA1*0301/DQB1*0302).T cells specific for shared islet-epitopes are cross-reactive to both molecules. Cross-reactivity is driven mainly by the shared β-chain and is observed ex vivo in polyclonal T cell populations in T1D patients. Stronger disease-associated responses to cross-reactive epitopes are elicited by DQ8trans, as the HLA-DQ2 α-chain enhances peptide-MHC-TCR interaction.These results suggest that a functional epistatic interaction between DQ8 and DQ8trans promotes the pathogenesis of T1D and provide an important general mechanism whereby disease risk may be altered primarily byT cell promiscuity among susceptible HLA haplotypes.

Specific immunotherapy modifies allergen-specific CD4(+) T-cell responses in an epitope-dependent manner

BACKGROUND

Understanding the mechanisms by which the immune system induces and controls allergic inflammation at the T-cell epitope level is critical for the design of new allergy vaccine strategies.

OBJECTIVE

We sought to characterize allergen-specific T-cell responses linked with allergy or peripheral tolerance and to determine how CD4(+) T-cell responses to individual allergen-derived epitopes change over allergen-specific immunotherapy.

METHODS

Timothy grass pollen (TGP) allergy was used as a model for studying grass pollen allergies. The breadth, magnitude, epitope hierarchy, and phenotype of the DR04:01-restricted TGP-specific T-cell responses in 10 subjects with grass pollen allergy, 5 nonatopic subjects, and 6 allergy vaccine-treated subjects was determined by using an ex vivo peptide-MHC class II tetramer approach.

RESULTS

CD4(+) T cells in allergic subjects are directed to a broad range of TGP epitopes characterized by defined immunodominance hierarchy patterns and with distinct functional profiles that depend on the epitope recognized. Epitopes that are restricted specifically to either TH2 or TH1/TR1 responses were identified. Allergen-specific immunotherapy was associated with preferential deletion of allergen-specific TH2 cells and without a significant change in the frequency of TH1/TR1 cells.

CONCLUSIONS

Preferential allergen-specific TH2 cell deletion after repeated high-dose antigen stimulation can be another independent mechanism to restore tolerance to allergen during immunotherapy.

Dissecting the molecular mechanisms for the increased risk of T1D in HLA-DQ8/DQ2 heterozygous individuals (P5182)

HLA-DQ8trans is the HLA molecule that confers the highest risk for human type 1 diabetes (T1D), yet the reason for this risk remains elusive. Using tetramer-guided epitope mapping, we discovered a high degree of peptide overlap between DQ8 and DQ8trans for antigens from influenza A viruses, with only one peptide exclusively presented by DQ8trans. Among influenza epitopes shared by DQ8, DQ2 and DQ8trans, cross-reactivity was observed between DQ8 and DQ8trans, but not between DQ8 and DQ2 or between DQ8trans and DQ2. In agreement with foreign antigen observations, glutamic acid decarboxylase 65 (GAD65)-, and islet tyrosine phosphatase (IA-2)-specific T cell clones isolated from DQ8 or DQ8/DQ2 patients responded to peptides loaded on DQ8 or DQ8trans molecules. The autoantigenic peptides bound to DQ8 and DQ8trans in the same binding registry, but peptides presented by DQ8trans bound with higher affinity to both T cell clones and in vitro expanded T cells. Proliferation assays and cytokine ELISA revealed that DQ8trans elicited stronger functional responses from cross-reactive autoantigenic T cells and created an environment that favored the expansion of self-reactive T cells by preferentially promoting the secretion of IFN-gamma from cross-reactive T cells. Overall, our data suggest that the presence of higher affinity HLA-DQ8trans molecules enhances self-reactive DQ8 and DQ8trans cross-reactive T cell responses, leading to increased susceptibility to T1D in DQ8/DQ2 individuals.