Epitope specificities of the group Y and W-135 polysaccharides of Neisseria meningitidis

Abstract

Previous studies have identified the length dependency of several polysaccharide (PS) protective epitopes. We have investigated whether meningococcal polysaccharides Y and W-135 possess such epitopes. Oligosaccharides (OSs) consisting of one or more disaccharide repeating units (RU) were derived from the capsular PSs of group Y and W-135 meningococci (GYMP and GWMP, respectively) by mild acid hydrolysis. The relative affinities of anticapsular antibodies binding to derivative OSs of different chain lengths were measured in inhibition enzyme-linked immunosorbent assays. As OS size increased from two to three RU, there was a notable increase in binding inhibition of rabbit anti-group Y antiserum. This pattern of antibody binding inhibition was also observed for rabbit antiserum to group W-135, though the inhibition increase was much more pronounced. In the cases of both OS species, the concentration of inhibiting antigen required to achieve 50% inhibition of rabbit immunoglobulin binding increased progressively as the inhibiting disaccharide chain length increased from 1 RU through greater than 50 RU. These data suggest that antibodies directed against both of these meningococcal PSs recognize conformational epitopes only fully expressed in higher-molecular-weight forms of these antigens.

FIG. 1.

(a) Structures of meningococcal polysaccharides. (b) Space-filling model of Neisseria meningitidis group Y polysaccharide structure. Green arrows indicate hydrophobic surfaces; blue arrows indicate hydrophilic surfaces. (c) Space-filling model of Neisseria meningitidis group W-135 polysaccharide structure. Green arrows indicate hydrophobic surfaces; red arrows indicate hydrophilic surfaces.

FIG. 2.

(a) ELISA titration of rabbit antisera on GYMP-HSA-coated plate. (b) ELISA titration of rabbit antisera on GWMP-HSA-coated plate. •, anti-N. meningitidis group B; ○, anti-N. meningitidis group C; ▾, anti-N. meningitidis group W-135; ▵, anti-N. meningitidis group Y. OD 450, optical density at 450 nm.

FIG. 3.

Inhibition ELISA of rabbit anti-N. meningitidis Y antiserum on GYMP-HSA-coated plate. The mean molecular weight of larger fragments (≥18 RU) was determined by SEC-MALLS.

FIG. 4.

Inhibition ELISA of rabbit anti-N. meningitidis W-135 antiserum on GWMP-HSA-coated plate. The mean molecular weight of larger fragments (≥12 RU) was determined by SEC-MALLS.