Faculty Bibliography

Light chains of the serologically and chemically defined V region sub-subgroup kappa IIIb are preferentially associated with several types of human IgM kappa (monoclonal) autoantibodies and are remarkably homologous in primary structure, as evidenced by partial amino acid sequence data. To establish the extent of homology among such proteins, we have determined the complete variable region (V) sequence of the light chains of four monoclonal IgM kappa autoantibodies, of which two (GAR and GOT) are rheumatoid factors (RFs), the third (SON) has anti-apo beta lipoprotein specificity, and the fourth (PIE) binds specifically to intermediate filaments. The region encoded by the V kappa segment gene (positions 1-95) in all four light (L) 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 96-108) is identical after position 96 for the two RFs GAR and GOT (J kappa 2), but different in proteins SON (J kappa 4) and PIE (J kappa 1). The amino acid residue at position 96, located in CDR3 at the site of combinatoriaL joining of the V kappa and J kappa 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 particular V kappa germ line gene, kappa IIIb, is expressed as a phylogenetic response to certain self antigens or as part of a selection process by which these autoimmune responses are regulated.

The predominance of the relatively uncommon V region subgroup isotype kappa III among the light chains of human monoclonal (IgM kappa) anti-IgG antibodies, (i.e., rheumatoid factors), was further documented through sequence analyses of ten such autoantibodies isolated from IgM-anti-IgG cold-insoluble immune complexes (mixed cryoglobulins). The amino-terminal sequence of all ten kappa-chains was characteristic for kappa III proteins and virtually identical to that of a prototype kappa III light chain. Similar sequence identity was found for kappa-chains isolated from three IgM kappa autoantibodies that formed cold-insoluble immune complexes with low-density lipoprotein (LDL). The thirteen light chains were found to be virtually identical in sequence for the first framework region (FR); ten of these proteins sequenced through the first complementarity-determining region (CDR) and into the second FR were markedly similar. The second CDR of five proteins was almost identical in sequence to that of the prototype kappa III-chain. Concordance was also demonstrated between the structural classification of the light chains as kappa III and their immunochemical classification as members of this V region subgroup. Serologic analyses of light chains isolated from seven IgM kappa autoantibodies (six anti-IgG, one anti-LDL) and of one intact IgM kappa anti-LDL antibody showed that each had antigenic determinants common to kappa II proteins. These light chains also expressed the antigenic determinant(s) of a V-region sub-subgroup of kappa III proteins designated kappa IIIb. Our studies confirm the preferential association of kappa III (and kappa IIIb) light chains with IgM kappa anti-IgG antibodies and demonstrate a similar association for IgM kappa anti-LDL antibodies. The finding that these and other types of IgM kappa autoantibodies, e.g., cold agglutinins, have remarkably similar light chains suggests an inherent restriction in the immune response to self-antigens.

We have determined the amino acid sequence of the Fv [variable heavy (VH) and variable light (VL)] region of a human monoclonal IgM-kappa with antibody activity against 3,4-pyruvylated galactose, isolated from the plasma of patient WEA with Waldenstrom macroglobulinemia. The VH region has 114 residues, belongs to subgroup III, and has a very short third complementarity-determining region (CDR3), probably due to a small D segment/or an unusual D-J rearrangement (D, diversity; J, joining). The VL region has 108 residues and belongs to subgroup V kappa I. Compared to other members of the human VHIII and V kappa I families, WEA Fv does not appear to have significant differences within the framework residues but has unique CDRs that might be responsible for the particular antibody activity. Another IgM-kappa (GAL), which has an as-yet-undetermined antibody activity, shares a striking homology in V kappa with WEA, including an identical CDR1.