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 cell s. Like pilus expression of other bacteria, expression of Hib pili und ergoes phase variation. We observed that Hib in the piliated phase (Hi b p+) bound monoclonal antibodies directed against six conserved, surf ace-exposed, nonpilus Hib outer membrane epitopes to a greater extent than Hib in the nonpiliated phase (Hib p-). However, after extended in cubation, p+ and p- cells bound these antibodies in a similar fashion. The differential in nonpilus antibody binding to p+ and p- Hib was no t related to the presence of the type b capsule. In addition, Hib p+ o rganisms whose pilin gene was insertionally inactivated and did not pr oduce pili and Hib in the nonpiliated phase bound the nonpilus Hib ant ibodies similarly. Hib p+ and p- organisms did not differ in their bin ding of anti-type b capsule antibody, and the binding was specific for the epitopes recognized by the antibodies. In complement-dependent ba ctericidal assays, the nonpilus antibodies killed Hib p+ more effectiv ely than Hib p-. The increased binding to, and killing of, Hib p+ by a variety of nonpilus antibodies may be important for host defense agai nst invasive Hib.