Identification of pregnancy-associated CA125-reactive protein as a carbohydrate-binding immunoglobulin G

Cancer antigen 125 (CA125), also referred to as mucin 16, is expressed under both normal and pathological conditions and the complexity of its structure indicates multifunctionality, i.e. both the protein and carbohydrate parts may be involved in diverse interactions at different levels of cell and tissue organization. Its biological role is not understood, but involvement in immune response modulation and influence on cell adhesion have been speculated. This study aimed at isolation and characterization of endogenous ligands for CA125 as an initial step in gaining insight into its activity. A CA125-reactive fraction was separated from human placental extract by affinity chromatography. The isolated preparation was characterized by SDS–PAGE, immunoblotting, peptide mass fingerprinting and binding assay. The CA125-reactive fraction from placental extract was identified as carbohydrate-binding IgG. The glycan composition of inhibitors of carbohydrate-binding pointed to sialic acid as one determinant for recognition but indicated that sialylation was not alone and that glycotopes containing galactose, N-acetylgalactosamine and N-acetylglucosamine were also important. CA125-reactive IgG could be selectively enriched using fetuin as the ligand and represents a distinct IgG subfraction differing from abundant natural carbohydrate-binding antibodies. Taking advantage of the particular properties of ligands for CA125 may have biomedical potential for use as biological modifiers or delivery agents and have an impact beyond pregnancy, since many immunoregulatory molecular pathways are common to embryonic development and malignant transformation.