Haemophilus influenzae vaccine candidate outer membrane protein P6 is not conserved in all strains.

An outer membrane protein of nontypeable Haemophilus influenzae (NTHi), P6, is a vaccine candidate because it has been characterized as conserved among all H. influenzae strains. Among 151 isolates from children, age 6 to 30 months, evaluating NTHi nasopharyngeal (NP) and oropharyngeal (OP) colonization and tympanocentesis confirmed acute otitis media we identified 14 strains (9.3%) that had variant protein sequences of P6. One atypical omp P6 isolate had sequence mutations in the binding site of a proposed major antigenic epitope of omp P6 identified by monoclonal antibody 7F3. Eight strains (5.3%) had non-homologous variations in amino acids that could result in significant changes to the protein structure of P6, and 5 other strains had amino acid substitutions at four previously described key residue sites. These results show that NTHi omp P6 is not invariant in its structure among respiratory isolates from children.