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Immunological cross-reaction between a naturally occurring galactan, agarose, and an LPS locus for immune lysis of Neisseria meningitidis by human sera.

An immunological cross-reaction between agarose, a naturally occurring galactan, and an antigenic determinant which is a locus for human bactericidal antibody within the LPS of group Y strains of N. meningitidis was investigated. Bactericidal antibody in the convalescent serum of a child from whom a group Y, type IX strain was isolated could be absorbed by highly purified agarose in bead form (Sepharose), but not by a dextran gel (Sephadex). It was inhibited by agarose as a linear polymer and by the strain's LPS, but not by a heterologous LPS from a group B, type II strain of N. meningitidis, nor by the homologous capsular polysaccharide. Both LPS contained galactose; neither was anti-complementary. Agarose antiserum, raised in a rabbit, was bactericidal for the group Y strain, but not for the group B strain. Bactericidal activity in the agarose antiserum could be inhibited by agarose. Immunization with agarose induced haemagglutinating antibody against the group Y strain's LPS which could be absorbed by the group Y, but not the group B strain, and could be reduced by absorption with Sepharose. Absorption with Sepharose also removed the lytic activity against group Y strains of serotypes II and IV from a normal human serum without group Y capsular polysaccharide antibody. We conclude that the cross-reaction resides in the fine structure of the galactose constituent of the group Y strains' LPS.