Radiobinding assay for detecting autoantibodies to single epitopes

Influence of HLA-DR and -DQ alleles on autoantibody recognition of distinct epitopes within the juxtamembrane domain of the IA-2 autoantigen in type 1 diabetes

AIMS/HYPOTHESIS

Insulinoma-associated protein 2 (IA-2) is a major target of autoimmunity in type 1 diabetes. When first detected, IA-2-autoantibodies commonly bind epitopes in the juxtamembrane (JM) domain of IA-2 and antibody responses subsequently spread to the tyrosine phosphatase domain. Definition of structures of epitopes in the JM domain, and genetic requirements for autoimmunity to these epitopes, is important for our understanding of initiation and progression of autoimmunity. The aims of this study were to investigate the contribution of individual amino acids in the IA-2 JM domain to antibody binding to these epitopes and the role of HLA genotypes in determining epitope specificity.

METHODS

Regions of the JM domain recognised by autoantibodies were identified by peptide competition and inhibitory effects of alanine substitutions of residues within the JM region. Antibody binding was determined by radioligand binding assays using sera from patients genotyped for HLA-DRB1 and -DQB1 alleles.

RESULTS

Patients were categorised into two distinct groups of JM antibody reactivity according to peptide inhibition. Inhibition by substitutions of individual amino acids within the JM domain differed between patients, indicating heterogeneity in epitope recognition. Cluster analysis defined six groups of residues having similar inhibitory effects on antibody binding, with three clusters showing differences in patients affected or unaffected by peptide. One cluster demonstrated significant differences in antibody binding between HLA-DRB1*04 and HLA-DRB1*07 patients and within DRB1*04 individuals; antibody recognition of a second cluster depended on expression of HLA-DQB1*0302.

CONCLUSIONS/INTERPRETATION

The results identify amino acids contributing to distinct epitopes on IA-2, with both HLA-DR and HLA-DQ alleles influencing epitope specificity.

Relationships between major epitopes of the IA-2 autoantigen in Type 1 diabetes: Implications for determinant spreading

Diversification of autoimmunity to islet autoantigens is critical for progression to Type 1 diabetes. B-cells participate in diversification by modifying antigen processing, thereby influencing which peptides are presented to T-cells. In Type 1 diabetes, JM antibodies are associated with T-cell responses to PTP domain peptides. We investigated whether this is the consequence of close structural alignment of JM and PTP domain determinants on IA-2. Fab fragments of IA-2 antibodies with epitopes mapped to the JM domain blocked IA-2 binding of antibodies that recognise epitopes in the IA-2 PTP domain. Peptides from both the JM and PTP domains were protected from degradation during proteolysis of JM antibody:IA-2 complexes and included those representing major T-cell determinants in Type 1 diabetes. The results demonstrate close structural relationships between JM and PTP domain epitopes on IA-2. Stabilisation of PTP domain peptides during proteolysis in JM-specific B-cells may explain determinant spreading in IA-2 autoimmunity.

IA-2 autoantibody affinity in children at risk for type 1 diabetes

Autoantibodies to insulinoma-associated protein 2 (IA-2A) are associated with increased risk for type 1 diabetes. Here we examined IA-2A affinity and epitope specificity to assess heterogeneity in response intensity in relation to pathogenesis and diabetes risk in 50 children who were prospectively followed from birth. At first IA-2A appearance, affinity ranged from 10(7) to 10(11)L/mol and was high (>1.0×10(9)L/mol) in 41 (82%) children. IA-2A affinity was not associated with epitope specificity or HLA class II haplotype. On follow-up, affinity increased or remained high, and IA-2A were commonly against epitopes within the protein tyrosine phosphatase-like IA-2 domain and the homologue protein IA-2β. IA-2A were preceded or accompanied by other islet autoantibodies in 49 (98%) children, of which 34 progressed to diabetes. IA-2A affinity did not stratify diabetes risk. In conclusion, the IA-2A response in children is intense with rapid maturation against immunogenic epitopes and a strong association with diabetes development.

Genetic and diabetic auto-antibody markers in Saudi children with type 1 diabetes

Human leukocyte antigen DRB1 and DQB1 contribute to the genetic susceptibility of type 1 diabetes (T1DM), and they are involved in the induction of the autoimmune destruction of pancreatic beta cells precipitating the disease. The objective of this study was to examine diabetic auto-antibodies (ICA-512, GAD65) and the HLA-DR/DQ genotype among T1DM Saudi children in a cross-sectional study conducted at King Khalid University and National Guard Hospitals, Riyadh. Subjects included in this study were 103 Saudi patients and 180 healthy controls. In all, 41% of patients were positive for ICA512 73.3% positive for GAD65, and 27.3% had both antibodies. The risk alleles were DRB1*0301 (odds ratio [OR] = 11.1); DRB1*0405 (OR = 6.02); DRB1*0401 (OR = 5.8); DQB1*0201 (OR, 17.69) and DQB1*0302 (OR = 3.77). In addition, the DRB1*03/04-DQB1*02/0302 (OR = 123.4) is positively associated with T1DM. However, DRB1*0403 (OR = 0.27), DRB1*1101 (OR = 0.049), DRB1*1307 (OR = 0.28), DRB1*1501 (OR = 0.12), DQB1*0301 (OR = 0.03), DQB1*0401 (OR = 0.04), and DQB1*0602 (OR = 0.16) were protective. Among GAD-positive patients, 81% were DRB1*0301, 68.75% were DQB1*0201, 62.5% were DRB1*0405, 43.75% were DQB1*0302, and 43.7% were DRB1*03/04. Among ICA512-positive patients, all were DRB1*0301, 66.6% were DQB1*0201, 55% were DRB1*0405, 33.3% were DQB1*0302, and 55% were DRB1*03/04. In conclusion, these results show a strong association of HLA-DQB1*0201/0302 and DRB1*03/04 with T1DM. Thus, combining genetic markers with autoantibody is useful in a screening program for early detection of T1DM among Saudi children.