Differential expression of Galα1,3Gal epitope in polymeric and monomeric IgM secreted by mouse myeloma cells deficient in α2,6-sialyltransferase

IgM are glycoproteins secreted by plasma cells as (mu 2L2)(5)+J or (mu 2L2)(6) polymers. In most species, mu- and J-chains bear five and one N-glycans, respectively. Here we compare the terminal glycosylation patterns of 4-hydroxy-3-nitrophenylacetyl (NP)-specific IgM secreted by transfectants of the J558L mouse myeloma deficient in the alpha 2,6 sialyltransferase [alpha 2,6ST(N)] or by a hybridoma expressing this enzyme(B1.8 cells).The absence of alpha 2,6-sialylation results in an increased addition of alpha 1,3-galactosyl residues to mu- and J-chain N-glycans. Since alpha 1,3-galactosyltransferase (alpha 1,3Gal-T) is similarly expressed in the two cell lines, these results indicate that a competition reaction occurs in vivo between alpha 2,6ST(N) and alpha 1,3Gal-T, In the alpha 2,6ST(N) deficient transfectants, mu-chains lacking the C-subterminal Cys575 residue, which are secreted mainly in the form of mu 2L2 monomers, are more efficiently capped by alpha 1,3-galactosyl residues, confirming that polymerization significantly reduces the accessibility of Ct-chain glycans to the Golgi processing enzymes involved in the biogenesis of antennary sugars. Functional assays indicate that IgM sialylation affects antigen-binding and complement-dependent hemolysis of haptenated red blood cells.