Immunization with outer membrane protein P6 from nontypeable Haemophilus influenzae induces bactericidal antibody and affords protection in the chinchilla model of otitis media

Abstract

The role of nontypeable Haemophilus influenzae (NTHi) outer membrane protein (OMP) P6 in the pathogenesis of otitis media (OM) has not been defined. OMPs, fimbriae, pili, and lipooligosaccharide are several types of surface antigens of NTHi that are currently being evaluated as potential vaccine candidates. P6 is antigenically conserved among both nontypeable and type b H. influenzae strains and elicits bactericidal as well as protective antibodies; however, initial evaluation of a vaccine mixture of P6 combined with other NTHi OMPs failed to induce bactericidal antibody or protection in the chinchilla model of OM. We undertook an assessment of the ability of immunization with isolated P6 lipoprotein alone to confer protection. Chinchillas were immunized with P6 and challenged 10 days after the final immunization with either 3 x 10(3) CFU of NTHi delivered directly into the middle ear to induce OM or 5 x 10(8) CFU of NTHi delivered intranasally to establish nasopharyngeal colonization. All immunized animals responded with elevated serum titers of anti-P6 antibody, which also demonstrated bactericidal activity against homologous as well as a heterologous NTHi isolate. By 14 days post-transbullar challenge, the number of chinchillas with middle ear fluid and the incidence of NTHi culture-positive middle ear fluids were reduced 48 and 51%, respectively, in the P6-immunized chinchillas relative to the sham-immunized cohort. Nasopharyngeal colonization levels were comparable in the two cohorts. These data demonstrate that active immunization with P6 results in the production of NTHi-specific bactericidal antibody in the chinchilla and also affords a reduction in the incidence of NTHi-induced OM; however, parenteral immunization does not appear to affect the extent or duration of nasopharyngeal colonization by NTHi.