During persistence of nonencapsulated Haemophilus influenzae in the respiratory tracts of patients with chronic bronchitis, the major outer membrane proteins (MOMPs) P2 and P5 show antigenic drift. The hypothesis that appearance of antigenic variants is the consequence of antibody-dependent selection was tested in a rabbit model. Persistence of H. influenzae d1 was achieved in subcutaneous tissue cages for up to 948 days. During persistence in the rabbits, similar changes in MOMP P2 of H. influenzae occurred, as observed in isolates from chronic bronchitis patients. In rabbits vaccinated with strain d3 and in nonvaccinated rabbits, antigenic drift occurred later than in rabbits vaccinated with strain d1. High titers of antibodies against H. influenzae were measured in tissue cage fluid and serum. Vaccination of the rabbits with H. influenzae d1 or d3, an antigenic variant of strain d1, resulted neither in eradication of H. influenzae d1 nor in increased antibody titers in serum and tissue cage fluid. The sera of nonvaccinated rabbits during persistence had no strain d1-specific bactericidal activity in the presence of complement. Vaccination with H. influenzae d1 induced serum bactericidal activity against strain d1 in the presence of complement. However, a variant of strain d1 appearing in the tissue cages was not killed by this serum bactericidal activity. We conclude that immunological pressure leads to the selection of MOMP variants of H. influenzae and that these variants escape the antibody-mediated strain-specific bactericidal activity against H. influenzae.

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