Immunoglobulin carbohydrate requirement for formation of an IgG-IgG complex

In addition to their Fc oligosaccharides, some immunoglobulin molecules have oligosaccharides linked to variable segments of H or L chains. These Fab oligosaccharides are potential determinants of antibody specificity. This possibility was considered in a study of the IgG antiglobulin from a patient with IgG-IgG complexes. F(ab')2 fragments of the antiglobulin retained the ability to form complexes with normal IgG as detected by analytical ultracentrifugation. Removal of F(ab')2 sialic acids by neuraminidase abolished complex formation. Recombination experiments further localized antiglobulin activity to the L chains. Antiglobulin activity of the recombinant molecules was shown by analytical ultracentrifugation and by column chromatography with molecules containing 125I-labeled L chains. L chains from the subject's IgG were enriched in sialic acids. Thus, a sialic acid-containing oligosaccharide on the L chain of this antiglobulin is required for its binding action.