Significance of noncapsular antigens in protection against experimental Haemophilus influenzae type b disease: cross-reactivity.

A bacterial strain, tentatively identified as an Actinobacillus species, was found to asymptomatically colonize the pharynx of some rats and to bear cell wall antigens which cross-react with noncapsular antigens of Haemophilus influenzae type b (HIb). The cross-reacting antigens appeared to be a heterogeneous mixture with varying molecular size and charge. The antigenic moieties are probably carbohydrate in nature. Antisera raised with this strain had both immunochemical and biological (bactericidal, opsonizing, and protective against experimental infection) activity against HIb. These findings lend further evidence to the idea that noncapsular antigens are important in the induction of resistance to HIb disease. The findings also raise the possibility of using bacteria which cross-react with noncapsular antigens for immunization against HIb disease in humans through nasopharyngeal or enteric colonization.