Variability of outer membrane protein P1 and its evaluation as a vaccine candidate against experimental otitis media due to nontypeable Haemophilus influenzae: An unambiguous, multifaceted approach

Candidate vaccine antigens for preventing otitis media caused by nontypeabl e Haemophilus influenzae (NTHI) should possess one or more conserved epitop es. We sought to evaluate the candidacy of P1, a surface-expressed outer me mbrane protein knowing that this antigen is subject to diversifying selecti on. Therefore, we selected NTHI strains from among >500 phylogenically vari ant isolates representative of the diversity found in natural populations o f H. influenzae. Twenty-three variants of P1 (less than or equal to 95% sim ilarity) were identified among 42 strains. When chinchillas were immunized with recombinant P1 (rP1) obtained from one of these isolates (BCH-3), all animals developed antibodies specific for rP1. Immunized animals were prote cted against disease when challenged with BCH-3, but not with an ompP1 muta nt of BCH-3 or a strain (BCH-2) possessing a heterologous P1 (91% identity) . We conclude that (i) while P1 induces protection against NTHI-mediated ot itis media, development of a polyvalent vaccine reflecting the variability of P1 would be necessary to construct an efficacious vaccine and (ii) use o f a phylogenically characterized collection of representative isolates in c oncert with gene sequencing, cloning, gene inactivation, and animal testing offers an efficient, rational, and rigorous strategy for evaluating the po tential problems associated with variability of vaccine targets and specifi city of related immune responses.