Characterization of a human monoclonal immunoglobulin M (IgM) antibody (IgM(BEN)) specific for Vi capsular polysaccharide of Salmonella typhi

Abstract

A search for human monoclonal antibodies to protective antigens of bacteria revealed an immunoglobulin M λ chain [IgM(λ); designated IgM(BEN)] reactive with the Vi capsular polysaccharide of Salmonella typhi. Vi, a linear homopolymer of α(1→4)GalApNAc that is O acetylated at C-3, is a licensed vaccine for typhoid fever. Immunologic properties of IgM(BEN) were compared to those of burro globulin prepared by intravenous injections of S. typhi (B339-340). IgM(BEN) and B339-340 yielded identical precipitin lines with Vi by double immunodiffusion. IgM(BEN) and B339-340 produced similar precipitation results with Vi and its derivatives prepared by de-O- acetylation, carboxyl reduction, and removal or replacement of the N-acetyl at C-2 with O-acetyl. B339-340 yielded maximal precipitation with VI (0.41 mg of antibody per ml with 1.4 μg of Vi); next was carboxyl-reduced, O- acetylated Vi, which precipitated 0.325 mg of antibody per ml with 2.5 μg of Vi. IgM(BEN) yielded maximal precipitation with de-O-acetylated, carboxyl- reduced Vi (~11.0 mg of antibody per ml with ~ 1.3 μg of antigen); next were de-O-acetylated Vi (9.89 mg/ml) and Vi (9.19 mg/ml). The precipitin curves and equivalence points of these three antigens were similar. Pneumococcus type 1, which contains GalApNAc, did not precipitate with Vi or its derivatives. These slight differences in specificity between IgM(BEN) and B339-340 were related to our proposed structure of Vi. We plan to use IgM(BEN) as a reference for measurement of vaccine-induced Vi antibodies.