Identification of a subgroup of black South Africans with type 1 diabetes who are older at diagnosis but have lower levels of glutamic acid decarboxylase and islet antigen 2 autoantibodies.

AIMS: To compare the age at diagnosis and prevalence of islet autoantibody [glutamic acid decarboxylase (65 kDa) 65 and islet antigen 2] positivity in black and white participants with type 1 diabetes in South Africa, and to analyse the relationship between age at diagnosis and the presence of autoantibodies. METHODS: Participants were recruited from diabetes outpatient departments and autoantibodies to glutamic acid decarboxylase (65 kDa) and islet antigen 2 were measured by enzyme-linked immunosorbent assay. RESULTS: We recruited 472 (353 black and 119 white) participants with type 1 diabetes. Age at diagnosis of diabetes was later in black (19.7 ± 10.5) than in white participants (12.7 ± 10.8 years; P < 0.001) with a median (interquartile range) disease duration of 5.0 (2.0-10.0) and 8.5 (4.0-20.0) years (P < 0.001), respectively. An older age at diagnosis (≥ 21 years) was more frequent in black (152 of 340, 45%) than in white participants (24 of 116, 21%; P < 0.001). The prevalence of islet antigen 2 autoantibodies was 19% (66/352) in black and 41% in white participants (48/118; P < 0.001). There was no significant difference in glutamic acid decarboxylase (65 kDa) autoantibody positivity between black (212/353, 60%) and white participants (77/117, 66%; P = 0.269). In black, but not white, participants the prevalence of both glutamic acid decarboxylase (65 kDa) and islet antigen 2 autoantibody positivity was significantly lower in participants diagnosed at age ≥ 21 years (P < 0.001 for both comparisons). CONCLUSIONS: The older age at diagnosis, lower prevalence of islet antigen 2 autoantibodies and a distinct subgroup of participants with type 1 diabetes with age at diagnosis of > 20 years in the black compared to white population suggest a difference in the immunological aetiology of type 1 diabetes in these two population groups.

Longitudinal epitope analysis of insulin-binding antibodies in type 1 diabetes.

Autoantibodies to insulin (IAA) are one of the first markers of the autoimmune process leading to type 1 diabetes (T1D). While other autoantibodies in T1D have been studied extensively, relatively little is known about IAA and their binding specificities, especially after insulin treatment is initiated. We hypothesize that insulin antibodies (IA) that develop upon initiation of insulin treatment differ in their epitope specificities from IAA. We analysed insulin antibody binding specificities in longitudinal samples of T1D patients (n = 49). Samples were taken at clinical diagnosis of disease and after insulin treatment was initiated. The epitope specificities were analysed using recombinant Fab (rFab) derived from insulin-specific monoclonal antibodies AE9D6 and CG7C7. Binding of radiolabelled insulin by samples taken at onset of the disease was significantly reduced in the presence of rFab CG7C7 and AE9D6. rFab AE9D6 competed sera binding to insulin significantly better than rFab CG7C7 (P = 0.02). Binding to the AE9D6-defined epitope in the initial sample was correlated inversely with age at onset (P = 0.005). The binding to the AE9D6-defined epitope increased significantly (P < 0.0001) after 3 months of insulin treatment. Binding to the CG7C7-defined epitope did not change during the analysed period of 12 months. We conclude that epitopes recognized by insulin binding antibodies can be identified using monoclonal insulin-specific rFab as competitors. Using this approach we observed that insulin treatment is accompanied by a change in epitope specificities in the emerging IA.

Epitope analysis of insulin autoantibodies using recombinant Fab.

Autoantibodies to insulin are often the first autoantibodies detected in young children with type 1 diabetes and can be present before the onset of clinical diabetes. These autoantibodies and their epitopes are, however, not well characterized. We explored the use of monoclonal antibodies and their recombinant Fab as reagents for epitope analysis. In this study we cloned and characterized the recombinant Fab of the insulin-specific monoclonal antibody CG7C7. We found the epitope of this antibody to be located predominantly at the A-chain loop of the insulin molecule. The recombinant Fab was then used to compete for insulin binding against insulin autoantibodies present in sera from patients with type 1 or type 1.5 diabetes. In competition experiments with sera positive for autoantibodies to insulin the recombinant Fab significantly reduced the binding to [125I]-insulin by sera of type 1 (n = 35) and type 1.5 diabetes [latent autoimmune diabetes in adults (LADA)] (n = 14) patients (P < 0.0001). We conclude that competition between insulin-specific monoclonal antibodies or their recombinant Fab and insulin autoantibodies should prove useful in the epitope analysis of autoantibodies to insulin.