title: VH and V kappa segment structure of anti-insulin IgG autoantibodies in patients with insulin-dependent diabetes mellitus. Evidence for somatic selection.
abstract: In some patients with insulin-dependent (type I) diabetes mellitus (IDDM), autoantibodies to insulin are present at diagnosis. After initiation of the treatment with not only animal but also human insulin, anti-insulin, mainly IgG, autoantibodies become a major component of the autoimmune response in virtually all IDDM patients. Their structure, however, is still relatively unknown. We analyzed the structure of the VH and V kappa segments of three human IgG mAb derived from three IDDM patients. The sequences of VH genes of two IgG, mAb13 and mAb48, were 98.3 and 96.6% identical with those of the H11 and 1.9III genes (VHIII family), respectively. The sequence of the VH gene of the third IgG, mAb49, was 98.6% identical with that of the 51p1 gene (VHI family). All three IgG mAb used V kappa III segments. The V kappa III gene sequences of mAb13 and mAb49 were 97.9 and 98.9% identical, respectively, to that of the kv3g gene; the mAb48 V kappa gene sequence was 96.5% identical to that of the kv328 gene. The VH and/or V kappa segments of these anti-insulin IgG mAb are similar to Ig V genes expressed in the fetal, and adult normal and autoimmune B cell repertoires. The nucleotide differences displayed by the three anti-insulin IgG mAb VH gene sequences, when compared with those of the closest reported germ-line genes, were concentrated in the CDR (6.2 x 10(-2) and 0.8 x 10(-2) difference/base in CDR and FR, respectively; p < 0.01, chi 2 test), and yielded a significantly higher putative replacement (R) to silent (S) mutation ratio in the CDR (12.0) than in the framework (0.2). The concentration of nucleotide differences in the CDR and their high R:S putative mutation ratios were consistent with the hypothesis that these expressed VH genes underwent a process of somatic mutation and Ag-driven clonal selection. That such differences constituted somatic point-mutations was formally proved in IgG mAb13, by differentially targeted PCR amplification and Southern blot hybridization of the mAb13-producing cell line DNA. The putative germ-line gene that gave rise to the expressed VH segment was cloned using genomic DNA from PMN of the same patient whose B cells were used for the generation of this mAb. Overall, in the anti-insulin IgG mAb VH and V kappa III genes, the (putative and verified) somatic point-mutations yielded 27 amino acid replacements, of which 14 nonconserved. Four of these resulted in positively charged residues, three Arg and one His.(ABSTRACT TRUNCATED AT 400 WORDS)
authors.affiliation: Department of Pathology, New York University School of Medicine, New York 10016.
authors.forename: H
authors.initials: H
authors.lastname: Ikematsu
authors.forename: Y
authors.initials: Y
authors.lastname: Ichiyoshi
authors.forename: E W
authors.initials: EW
authors.lastname: Schettino
authors.forename: M
authors.initials: M
authors.lastname: Nakamura
authors.forename: P
authors.initials: P
authors.lastname: Casali
mesh_terms: D000595:Amino Acid Sequence; D000911:Antibodies, Monoclonal; D000915:Antibody Affinity; D001323:Autoantibodies / Q000235:genetics*; D001483:Base Sequence; D003001:Cloning, Molecular; D017931:DNA Primers / Q000737:chemistry; D003922:Diabetes Mellitus, Type 1 / Q000276:immunology*; D005803:Genes, Immunoglobulin; D006801:Humans; D007143:Immunoglobulin Heavy Chains / Q000235:genetics*; D007135:Immunoglobulin Variable Region / Q000235:genetics*; D007145:Immunoglobulin kappa-Chains / Q000235:genetics*; D007328:Insulin / Q000276:immunology*; D008969:Molecular Sequence Data; D017354:Point Mutation; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D012689:Sequence Homology, Nucleic Acid
chemical_list: D000911:Antibodies, Monoclonal; D001323:Autoantibodies; D017931:DNA Primers; D007143:Immunoglobulin Heavy Chains; D007135:Immunoglobulin Variable Region; D007145:Immunoglobulin kappa-Chains; D007328:Insulin