Glycosylation is influential in murine IgG3 self-association

In mice and humans, antibodies of the IgG3 isotype are unique in that they spontaneously self-associate. A consequence is the formation of cryoglobulins from some, but not all IgG3 molecules. Little is known about the structural basis of murine IgG3 self-association. A region of the CH3 domain that is unique to IgG3 antibodies is the presence of an extra glycosylation site at residues 471-473. It is known that glycosylation greatly influences solubility. It has also been shown by X-ray crystallography that glycosylated residues (specifically sialic acid) are influential in the contacts of the CH1 to CH2 as well as the CH2 to CH2 domains in a human IgG1 antibody. These findings provided evidence that a direct interaction exists between the glycosylated residues and other residues within the constant and/or variable domains. It was, therefore, important to determine whether the glycosylated residue in the CH3 domain of the IgG3 constant region is influential in self-association. We have found that removing the glycosylation site by site-directed mutagenesis of an IgG3 RF significantly reduced the self-associating ability of this antibody.