Human Antibody Responses to Two Conjugate Vaccines of Haemophilus influenzae Type B Saccharides and Diphtheria Toxin

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Abstract
Antigenicity of two Haemophilus influenzae type b (Hib) conjugate vaccines was studied by immunizing adults and 2‐year‐old children. Both vaccines induced strong anti‐Hib responses and strong antibody responses to diphtheria toxin (DT), the protein part of the conjugate. The adults' responses were stronger than the children's. A conjugate of Hib oligosaccharide and mutant diphtheria toxin (HbOC) emerged as slightly superior to a conjugate of Hib polysaccharide and diphtheria toxoid (PRP‐D). HbOC induced somewhat higher total anti‐Hib responses and significantly higher IgG1 anti‐Hib responses than PRP‐D. IgG1 and IgG2 were the main IgG subclasses of the anti‐Hib antibodies, whereas IgG1 and IgG4 were the main subclasses of the anti‐DT antibodies. Within this main rule, the ratio IgG1/IgG2 of anti‐Hib antibodies varied between individuals. The average ratio was higher than five in children but approximately one in adults. It was lower in adult recipients of the polysaccharide conjugate (0.69) than in adult recipients of the oligosaccharide conjugate (1.55). A large interindividual variation was observed in concentrations of IgG2 of Hib specificity, perhaps reflecting a small number of IgG2‐committed B‐cell clones participating in the response.