The chinchilla experimental model of
otitis media was used to examine the importance of serum
antibodies in protection against disease caused by nontypable Haemophilus influenzae. An immune serum pool was prepared by immunizing chinchillas with killed bacterial cells of nontypable H. influenzae 3245. Pooled preimmune or immune serum from these immunized animals was administered intravenously to a group of nonimmune chinchillas 1 day before intrabullar challenge with strain 3245. Of 5 animals receiving preimmune serum, 5 developed
otitis media compared with 0 of 10 animals receiving immune serum (P = 0.008). The immune serum pool contained
antibodies directed against both surface-exposed outer
membrane proteins and
lipopolysaccharide (LPS). The 39-kilodalton major outer
membrane protein was the immunodominant
surface protein. Anti-LPS
antibodies were removed from the immune serum pool by affinity chromatography, and affinity-purified anti-LPS
antibodies were recovered. Immune serum, immune serum absorbed of LPS
antibodies, or affinity-purified LPS
antibodies were then administered to another group of experimental animals 1 day before bacterial challenge. Of four animals that received the affinity-purified LPS
antibodies, four developed
otitis compared with zero of four animals that received the immune serum or zero of four animals that received the LPS-absorbed immune serum (P = 0.028). These studies indicate that passive immunization with immune serum is protective in experimental nontypable H. influenzae
otitis media and that
bacterial outer membrane proteins may be the principal targets of protective antibody.