Evaluation of mixtures of purified Haemophilus influenzae outer membrane proteins in protection against challenge with nontypeable H. influenzae in the chinchilla otitis media model

Abstract

Nontypeable Haemophilus influenzae (NTHi) is one of the leading causative agents of bacterial otitis media, and no vaccine has been shown to be effective against it. Three outer membrane lipoproteins of NTHi have been investigated extensively and are leading candidates for inclusion in a vaccine against this organism. Hi-PAL (P6), recombinant PCP (rPCP), and e (P4) proteins are antigenically conserved among NTHi strains and elicit bactericidal and protective antibodies. A genetic fusion of the rPCP and Hi-PAL proteins has also been reported. Mixtures of these proteins were used for active immunization experiments in the chinchilla model of otitis media. Chinchillas were immunized either with a mixture of all three lipoproteins or with the mixture of rPCP-PAL hybrid plus e protein. When these animals were challenged with a NTHi strain injected directly into the middle ears, no protection from infection or disease, as measured by otoscopy, was observed in either group. However, effusion and inflammation measured by tympanometry were significantly reduced in animals immunized with the three lipoproteins. Animals that had been immunized with either whole NTHi cells or total outer membranes and then challenged with the homologous strain were significantly protected from both infection and disease, as determined by tympanometry and otoscopy. Unlike other animals antisera, chinchilla antisera against the purified proteins had no bactericidal activity against NTHi but did fix complement on the cell surface. Thus, the chinchilla immune responses to mixtures of these lipoproteins differ from the immune responses observed in other animal species. Further evaluation of these proteins for their vaccine potential remains to be done.