The overall objectives of this research are to acquire further knowledge of antibody structure and diversity through correlative immunochemical, biochemical, physicochemical, genetic, and functional properties of the homogeneous immunoglobulins found in patients with multiple myeloma and related B-cell malignancies. Studies of the monoclonal immunoglobulin components in these and other patients with neoplastic and inflammatory diseases are continuing. We have found that the light chains of the serologically and chemically defined, V-region subgroup kappa-IIIb (that are preferentially associated with several types of human IgM-kappa monoclonal autoantibodies) are remarkably homologous in primary structure. The complete V-region sequence of the light chains of four monoclonal IgM-kappa autoantibodies, two of which (GAR and GOT) are rheumatoid factors and the third (PIE), binds specifically to intermediate filaments. The region encoded by the Vkappa segment gene (positions 1 to 95) in all four light chains is virtually identical in sequence, differing by only one residue in the FR3 of protein SON and in the first CDR of protein GOT. Further, the CDR3 of kappa chain SON contains an additional residue (prolyl) located at the carboxyl-terminus of the V segment. The region encoded by the J gene (positions 96 to 108) is identical after position 96 for the two rheumatoid factors GAR and and GOT (Jkappa2) but different in proteins SON (Jkappa4) and PIE (Jkappa1). The amino-acid residue at position 96, located in CDR3 at the site of combinatorial joining of the Vkappa and Jkappa gene segments and involved as a contacting residue in the hapten binding site, is different in all four light chains. These results demonstrate the extensive homology in sequence among light chains of IgM-kappa autoantibodies and indicate that a particulate Vkappa germline gene, kappa-IIIb, is expressed as a phylogenetic response to certain selfantigens or as part of a selection process by which these autoimmune responses are regulated. To determine the contribution of germline Vkappa-II genes to kappa-chain diversity, we have identified a human Vkappa-II-synthesizing lymphoblastoid cell line, GM 607, and characterized it by cloning and sequencing its rearranged Vkappa-II gene. Blot and cross-hybridization with this gene probe revealed approximately 7 to 10 germline Vkappa-II subgroup genes. These findings provide further evidence for a limited number of Vkappa germline genes in the human genome, estimated to be 20 to 30, and implicate somatic mutation as responsible for the V-region sequence diversity of human kappa light chains. (AB)