Anti-capsular polysaccharide antibodies reduce nasopharyngeal colonization by Haemophilus influenzae type b in infant rats.

Haemophilus influenzae type b (Hib) conjugate vaccines reduced oropharyngeal carriage of Hib among children in Finland and the United States. To study the mechanisms of this reduction, a colonization model in infant rats with passively administered antibodies was developed. Of the pups, 94% were colonized 48 h and 64% 7 days after intranasal inoculation with approximately 2500 cfu of Hib. Intranasally administered anti-Hib antibodies, including human IgG and both serum and secretory IgA (sIgA) as well as murine monoclonal anti-Hib capsular polysaccharide (PS) of IgG1 isotype, given simultaneously with, before, or after bacteria, significantly reduced nasopharyngeal colonization by Hib. Hib colonization was also significantly reduced when the antibodies were given intraperitoneally, with a resultant anti-Hib PS serum concentration of > or = 7 micrograms/mL. Thus, anti-Hib PS antibodies, both sIgA and IgG, can function on the mucosal surface and prevent colonization.