The role of glycosylation in the structure and function of human immunoglobulin A1

Similar to most proteins in nature, immunoglobulins are modified by the posttranslational addition of carbohydrates. One of the antibodies involved in defense at the mucosal surfaces of the lungs and gut, immunoglobulin A1 (IgA1), possesses two types of carbohydrate modification: N-linked glycosylation sites in the second and secreted tailpiece domains and 5 closely grouped sites of O-linked glycosylation in the IgA1 hinge.;To understand the role of the N-linked carbohydrates in the structure and function of human IgA1, site-directed IgA1 mutants that lack either one or both of the N-linked carbohydrate sites have been produced. When the mutant heavy chains are expressed in myeloma lines producing the relevant kappa light chain, efficient secretion of the monomer and dimer forms of IgA1 is seen. In addition, higher polymer forms of the IgA molecules lacking the third domain carbohydrate, either singly or in the double mutant, are present.;Analysis of the IgA1 proteins has shown that the proteins retain biologic function and differences between the various mutants and wild-type IgA are observed. All the IgA1 molecules retained the ability to bind to the polymeric immunoglobulin receptor. C3 binding was observed by all the IgA molecules, with the IgA mutants showing a reduced ability to bind C3; however, IgA did not activate complement as determined by Factor B and terminal complex binding. The ability of bacterial IgA receptors Arp4, Arp60, Sir and Bac to bind IgA is altered by the lack of N-linked carbohydrate. The lack of carbohydrate increased IgA resistance to cleavage by IgA1 proteases secreted by pathogenic bacteria.;We have also examined the role of the O-linked carbohydrate sites in the IgA1 hinge by expressing wild-type IgA1 in the O-linked glycosylation-deficient Chinese hamster ovary (CHO) cell line ldlD. While production of IgA1 from wild-type CHO cell lines was possible, the absence of O-linked glycosylation from the hinge appeared to prevent secretion of the antibody from the cell; instead, the antibody was retained inside the cell and rapidly degraded.;These studies indicate the importance of glycosylation in IgA1 and support an important role for glycosylation in determining immunoglobulin structure and function.