A glycoconjugate vaccine for Neisseria meningitidis induces antibodies in human infants that afford protection against meningococcal bacteremia in a neonate rat challenge model

Abstract

The functional activities of serum samples from human infants immunized with a glycoconjugate vaccine for Neisseria meningitidis serogroup C were assessed in a complement-mediated antibody-dependent serum bactericidal assay (SBA) and in a neonate rat model of protection from bacteremia. Selective serum samples from individual human infants were combined to make a panel of 11 serum pools to obtain a sufficient volume for testing. Each pool was assayed (i) for the anti-N. meningitidis serogroup C capsular polysaccharide (PS) immunoglobulin G (IgG) concentration as determined by reactivity in a direct-binding enzyme-linked immunosorbent assay, (ii) for bactericidal activity against N. meningitidis serogroup C strain C11, and (iii) for the ability to reduce bacteremia after passive transfer into a neonate rat model. Representative serum samples from infants who were not previously immunized with any N. meningitidis serogroup C vaccine served as a negative control. The prepared serum pools ranged in antibody concentration from 0.18 to 17.31 micro g of IgG specific for N. meningitidis serogroup C PS per ml. For this serum panel, a direct relationship between concentrations of anti-N. meningitidis serogroup C PS-specific IgG and serum SBA titers (r = 0.9960) was observed. Passive transfer to neonate rats demonstrated the ability of postimmunization serum samples to significantly reduce (> or =2-log(10) reduction compared to control animals) the level of bacteremia following a challenge. Of 79 neonate rats that received > or =0.031 micro g of human infant anti-N. meningitidis serogroup C PS IgG, 75 (94.9%) had a > or =2-log(10) reduction in bacteremia, whereas of the animals that received <0.031 micro g of antigen-specific IgG, 10.3% (4 of 39 rats) showed a > or =2-log(10) reduction in bacteremia. It was concluded that the anti-N. meningitidis serogroup C PS IgG antibody induced by this glycoconjugate vaccine had in vitro functional activity (as determined by a SBA) and also afforded protection against meningococcal bacteremia in an animal model.

FIG. 1.

Linear relationship between the anti-MnCPS IgG concentration and the bactericidal titer of 11 pools of human infant serum samples (r = 0.9960). Any bactericidal titer of <10 was assigned a value of 5 (one half of the lowest serum dilution tested). BC₅₀ titer, reciprocal of the highest dilution of antiserum that kills ≥50% of the target cells introduced into the SBA.

FIG. 2.

Correlation between the anti-MnCPS IgG concentration and the bactericidal titer of 16 different adult serum samples (r = 0.708). Any bactericidal titer of <4 was assigned a value of 2 (one half of the lowest serum dilution tested). BC₅₀ titer, reciprocal of the highest dilution of antiserum that kills ≥50% of the target cells introduced into the SBA.