The Fc region of IgG bears two oligosaccharides of variable composition. The serum level of one variant which lacks terminal galactose and sialic acid (agalactosyl IgG) is raised in a number of autoimmune diseases and animal models thereof. Here it is shown that such changes in IgG glycosylation occur during non-pathological humoral immune responses. It was found that if specific pathogen free (SPF) CBA/Ca mice are transferred from a sterile to a conventional environment, their levels of total serum IgG rise whereas the degree of IgG galactosylation falls. Next, mice were immunised with bovine serum albumin (BSA) in incomplete Freund's adjuvant. As anti-BSA titres rose the antibodies became less galactosylated and later, as the titres fell, the antibodies became more galactosylated. By contrast, there was little or no variation in the relative galactosylation of total IgG. It is considered that the galactosylation of IgG antibodies varies during an immune response.
It has been postulated that agalactosyl immunoglobulin G (IgG) self-associates to form pathological aggregates in the rheumatoid joint. To examine this hypothesis, IgG aggregates from synovial fluid (SF) of 22 patients with RA were prepared by precipitation with polyethylene glycol (PEG) 6000. The PEG precipitates and SFs were reduced with 2-mercaptoethanol (2ME) and bound to protein G. This procedure isolated the IgG in the PEG precipitates from other contaminating glycosylated proteins. The levels of galactose and N-acetylglucosamine (GlcNAc) residues present on the reduced IgG were quantified by their ability to bind the lectins Ricinus communis (RCA)120 and Bandeiraea simplicifolia (BS) II. Proportionally less galactose (expressed as a ratio of bound RCA120 to BS II) was present on the IgG from the PEG precipitates than on the IgG in the paired SF (P = 0.001). However, in many cases more RCA120 as well as BS II bound to IgG from PEG precipitates than from the corresponding SF. It is considered that agalactosyl IgG occurs preferentially in RA SF PEG precipitates and that this IgG may also exhibit increased Fab glycosylation.
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