Paradoxical effect of pilus expression on binding of antibodies by Haemophilus influenzae

Haemophilus influenzae type b (Hib) pili are surface proteins that are associated with the ability of Hib to attach to human epithelial cells. Like pilus expression of other bacteria, expression of Hib pili undergoes phase variation. We observed that Hib in the piliated phase (Hib p⁺) bound monoclonal antibodies directed against six conserved, surface-exposed, nonpilus Hib outer membrane epitopes to a greater extent than Hib in the nonpiliated phase (Hib p^-). However, after extended incubation p⁺ and p^- cells bound these antibodies in a similar fashion. The differential in nonpilus antibody binding to p⁺ and p^- Hib was not related to the presence of the type b capsule. In addition, Hib p⁺ organisms whose pilin gene was insertionally inactivated and did not produce pili and Hib in the nonpiliated phase bound the nonpilus Hib antibodies similarly. Hib p⁺ and p^- organisms did not differ in their binding of anti-type b capsule antibody, and the binding was specific for the epitopes recognized by the antibodies. In complement-dependent bactericidal assays, the nonpilus antibodies killed Hib p⁺ more effectively than Hib p^-. The increased binding to, and killing of, Hib p⁺ by a variety of nonpilus antibodies may be important for host defense against invasive Hib.