RESUMO
The related tyrosine phosphatase-like proteins (PTP) IA-2 and IA-2beta are autoantigens of type 1 diabetes. Autoantibodies are predominantly against IA-2. We utilized the close homology between IA-2 and IA-2beta PTP domains to design chimeras and mutants in order to identify humoral IA-2-specific epitopes. Fifteen sera with antibodies to IA-2 specific PTP domain epitopes were tested against IA-2beta(741-848)/IA-2(795-889)/IA-2beta(943-1033), IA-2beta(741-848)/IA-2(795-845)/IA-2beta(900-1033), and IA-2beta(741-898)/IA-2(845-875)/IA-2beta(930-1033)chimeras. Two sera bound IA-2beta(741-848)/IA-2(795-889)/IA-2beta(943-1033)and IA-2beta(741-848)/IA-2(795-845)/IA-2beta(900-1033)only indicating that the IA-2 specific residues 859, 862, and/or 867 were critical for antibody binding. Mutation of glutamine 862 abolished binding in one of these sera. Seven sera bound only the IA-2beta(741-848)/IA-2(795-889)/IA-2beta(943-1033)chimera, indicating that binding required IA-2 specific amino acids within both 795-845 and 846-875, or that IA-2 residues 876-888 were important for binding. Mutation of glutamine 862 abolished binding in two of these sera, and mutation of residues 876, 877, 878, and 880 markedly reduced binding in two others. Six sera bound all three chimeras indicating that they contained multiple IA-2 specific PTP domain antibodies. In three of these sera, mutation of residues at positions 876, 877, 878, 880, and/or residues 862 and 822 reduced antibody binding by more than 50%. These findings indicate that glutamine at position 862, and residues 876-880 of the WPD loop of IA-2 are important for several of the IA-2 specific PTP domain epitopes.
Assuntos
Especificidade de Anticorpos/imunologia , Autoanticorpos/imunologia , Diabetes Mellitus/imunologia , Mapeamento de Epitopos , Proteínas de Membrana/química , Proteínas de Membrana/imunologia , Proteínas Tirosina Fosfatases/química , Proteínas Tirosina Fosfatases/imunologia , Adolescente , Adulto , Autoantígenos , Criança , Pré-Escolar , Diabetes Mellitus/genética , Epitopos/química , Epitopos/genética , Epitopos/imunologia , Feminino , Humanos , Lactente , Masculino , Proteínas de Membrana/genética , Proteínas de Membrana/metabolismo , Mutação/genética , Estrutura Terciária de Proteína , Proteína Tirosina Fosfatase não Receptora Tipo 1 , Proteínas Tirosina Fosfatases/genética , Proteínas Tirosina Fosfatases/metabolismo , Proteínas Tirosina Fosfatases Classe 8 Semelhantes a ReceptoresRESUMO
The related tyrosine phosphatase-like proteins islet Ag (IA)-2 and IA-2beta are autoantigens of type 1 diabetes in humans. Autoantibodies are predominantly against IA-2, and IA-2-specific epitopes are major autoantibody targets. We used the close homology of IA-2 and IA-2beta to design chimeras and mutants to identify humoral IA-2-specific epitopes. Two major IA-2 epitopes that are absent from the related autoantigens IA-2beta and IA-2Delta 13 splice variant ICA512.bdc were found contiguous to each other within IA-2 juxtamembrane amino acids 611-620 (epitope JM1) and 621-630 (epitope JM2). JM1 and JM2 are recognized by sera from 67% of patients with IA-2 Abs, and relatives of patients with type 1 diabetes having Abs to either JM epitope had a >50% risk for developing type 1 diabetes within 6 years, even in the absence of diabetes-associated HLA genotypes. Remarkably, the presence of Abs to one of these two epitopes was mutually exclusive of the other; JM2 Abs and not JM1 Abs were found in relatives with HLA DR3/4, DR4/13, or DR1/4 genotypes; and the binding of autoantibodies to the JM2 epitope, but not the JM1 epitope, markedly affected proteolysis of IA-2. This is a unique demonstration of HLA-associated B cell responses to epitopes within a single autoantigen in humans and is consistent with modification of Ag processing by specific Ab-influencing peptide presentation by HLA molecules.
