Meningitis caused by Haemophilus influenzae in infant rats: protective immunity and antibody priming by gastrointestinal colonization with Escherichia coli.

Infant rats aged five to seven days were fed Escherichia coli O75:K100:H5, E. coli O13:K92:H4, or saline and five weeks later were inoculated with Haemophilus influenzae type b. The incidence of bacteremia and meningitis was significantly less (P less than 0.05) for rats fed E. coli that possessed K100 capsular antigen (cross-reactive with type b capsular antigen) than for rats fed E. coli K92 or saline. Antibody to capsular antigen was not detectable in sera obtained from rats prior to challenge with H. influenzae type b, but five days after challenge, antibody levels were significantly higher (P less than 0.001) in rats colonized with E. coli K100 than in controls. These results, together with data from passive-immunization studies, suggested that the protection against infection with H. influenzae type b was due to priming or serum anticapsular antibody, although a protective role for cell-mediated immunity and/or secretory antibody could not be ruled out. E. coli K100 primed rats vaccinated with purified H. influenzae type b antigen for a significantly increased, although transient, anticapsular antibody response.