Human antibody response to Helicobacter pylori lipopolysaccharide: presence of an immunodominant epitope in the polysaccharide chain of lipopolysaccharide

Abstract

We have examined the antibody response to Helicobacter pylori lipopolysaccharides (LPS) in humans. We used sera from patients with gastroduodenal diseases and healthy adults infected or not infected with H. pylori. Data from the experiments for antibody binding to LPS suggested that the polysaccharide chains from many H. pylori strains showed high immunogenicity in humans. Sera from most (above 70%) H. pylori-infected individuals contained immunoglobulin G (IgG) antibodies against the polysaccharide region highly immunogenic H. pylori LPS. The IgG titers of individual serum samples that reacted strongly with highly immunogenic LPS were quite similar (r2 = 0.84 to 0.98). The results suggest wide distribution among H. pylori strains of a highly antigenic epitope in the polysaccharide moieties of their LPS. Also, the similarity in the titers of individual serum samples against highly immunogenic LPS points to the existence of epitopes sharing a common structural motif. However, some strains showed low antigenicity, even those with polysaccharide-carrying LPS. The dominant subclass of IgG that reacted with the highly immunogenic LPS was IgG2, which was preferentially raised against polysaccharide antigens. Recently, a structure that mimics that of the Lewis antigens was identified in the O-polysaccharide fraction of H. pylori LPS; however, no correlation between antigenicity of the polysaccharide chain in humans and the presence of Lewis antigens was found. The IgA and IgM titers against H. pylori LPS seemed to be mostly nonspecific and directed against lipid A. In a few cases, however, sera from individuals infected with H. pylori gave strong IgA and IgM titers against the highly immunogenic polysaccharide. In conclusion, the LPS of many H. pylori strains possess an antigenic epitope in their polysaccharide regions that is immunogenic in humans. However, our results show that the antigenic epitope is unlikely to be immunologically related to structures mimicking Lewis antigens.

FIG. 1

Antigenicity of H. pylori LPS from clinical isolates. Antigenicity is expressed as the mean values of the ELISA reading (A₄₅₀) for a random selection of 10 serum samples from patients and healthy adults with H. pylori infection. Smooth and rough chemotypes were discriminated by SDS-PAGE and silver staining as described previously (2).

FIG. 2

Determination of the IgG subclass of highly immunogenic H. pylori LPS-specific antibodies in human sera. LPS derived from strain GU2 was coated onto a microplate. Human serum (1,000-fold dilution) was applied to the plate. Subsequently, human IgG subclass-specific murine monoclonal antibodies, peroxidase-conjugated goat anti-mouse IgG antibodies, and TMB substrate solution were applied. Binding activity was expressed as absorbance at 450 nm.

FIG. 3

Relationship of the IgM titer of human sera to H. pylori LPS derived from a strain with high antigenicity (GU2), a strain with low antigenicity (CA6), and a rough strain (CG10) and S. minnesota rough (Re-type) LPS as determined by ELISA. Human sera were diluted 1:300. The human sera were donated by patients with gastroduodenal disease (•), healthy adults with H. pylori infection (▪), and healthy adults without H. pylori infection (▴).

FIG. 4

Relationship of the titer of IgA in human sera to H. pylori LPS derived from a strain with high antigenicity (GU2), a strain with low antigenicity (CA6), and a rough strain (CG10) and S. minnesota rough (Re-type) LPS as determined by ELISA. Human serum was diluted 1:300. The human sera were donated by patients with gastroduodenal disease (•), healthy adults with H. pylori infection (▪), and healthy adults without H. pylori infection (▴).