Experimental recurrent pneumococcal otitis media. Protection and serum antibodies

To elucidate immune mechanisms in otitis media, middle ear infection was induced in 12 rats by intratympanic inoculation of Streptococcus pneumoniae type 3, either the ipsilateral or the contralateral middle ear being re-challenged 4 weeks later. Otomicroscopy inspection confirmed the presence of acute otitis media (AOM) in all rats after the first challenge. After re-challenge, protection against AOM was noted both in the ipsilateral and contralateral ears. Serum antibody concentrations increased after the initial challenge, reaching a maximum at 4-7 days, but had decreased to pre-immune levels at re-challenge, after which no new increase was noted. Serum IgG-antibodies to pneumococcal type 3-polysaccharide were triggered following the initial induction of unilateral pneumococcal AOM, but mucosal immune mechanisms are argued to be a more probable explanation of the ipsilateral protection seen after re-challenge.