Experimental recurrent otitis media induced by Haemophilus influenzae: protection and serum antibodies

Prediction of upper respiratory tract bacteria in acute otitis media

CONCLUSIONS

Thorough otomicroscopical examination of the tympanic membrane in acute otitis media (AOM) might distinguish AOM episodes caused by different bacteria. It thus might be a way to select appropriate treatment for each patient without raising the number of dangerous complications.

OBJECTIVES

The aim of this study was to see if it might be possible to predict the causative bacterium by judging the otomicroscopical appearance of the tympanic membrane in episodes of AOM.

PATIENTS AND METHODS

The study was prospective. Patients suffering from non-perforated AOM were included. The tympanic membrane was photographed. A prediction of the causative bacterium was made and tympanocentesis was performed. Effusion from the middle ear and a nasopharyngeal swab were obtained for bacterial culturing. The causative bacteria were categorized into gram-positive (Streptococcus pneumoniae and S. pyogenes) or gram-negative (non-typable Haemophilus influenzae and Moraxella catarrhalis).

RESULTS

A total of 82 patients were included in the study. A correct prediction was made in 47/63, a false prediction in 16/63 (kappa 0.48, p<0.001).

Immunization and protection in pneumococcal otitis media studied in a rat model

The recent and growing problem of bacterial resistance to common antibiotics has generated great interest in different methods for prevention of infections. The treatment of the pathogens causing upper airway infections and especially acute otitis media (AOM) is especially interesting in this context because these infections are a common cause of prescription of antibiotics all over the world. Both in AOM and recurrent AOM, Streptococcus pneumoniae, the most frequently occurring bacterium is isolated in 30-50% of all AOM attacks. In the last decade, multiresistant S. pneumoniae have emerged as a major problem. Thus, it is important to explore possibilities that immunization may protect against pneumococcal OM. In a well-defined animal model using Sprague-Dawley rats, we have investigated the effects of different routes of immunization with different antigens and whole cells. Together with otomicroscopical evaluation of middle ear (ME) status, samples for bacterial cultivation as well as for studies of histopathological changes have been collected. Antibody titers have been followed during and after pneumococcal AOM by an enzyme-linked immunosorbent assay (ELISA) method.

The acylated form of protein D of Haemophilus influenzae is more immunogenic than the nonacylated form and elicits an adjuvant effect when it is used as a carrier conjugated to polyribosyl ribitol phosphate

The nonacylated form of protein D (PDm) of Haemophilus influenzae has been shown to induce the production of antibodies that are bactericidal to homologous and heterologous nontypeable H. influenzae (NTHi) strains. In this study, immunization of rats with lipoprotein D (LPD) induced higher levels of anti-protein D immunoglobulin G and A serum antibodies than immunization with PDm, and the bactericidal activities of sera from LPD-immunized rats were greater than those of sera from PDm-immunized rats. Immunization with LPD or PDm did not prevent the development of acute otitis media (AOM) when rats were challenged with 10(4) CFU of an NTHi strain. However, on the eighth day of bacterial challenge, 50% (5 of 10) of LPD-immunized rats had recovered from otitis media and 30% (3 of 10) had negative middle ear cultures, whereas only 30% (3 of 10) of PDm-immunized rats had recovered, though none was culture positive. Immunization with an inactivated homologous bacterial strain elicited 70% protection (i.e., 7 of 10 rats) in the rat otitis media model. LPD and PDm were also conjugated to the H. influenzae type b (Hib) capsular polysaccharide, polyribosyl ribitol phosphate (PRP), to test protein D-conjugated PRP vaccine's potential for protection against Hib infection. When two LPD-conjugated and two PDm-conjugated PRP vaccines, each containing a different protein concentration, and a tetanus toxoid-conjugated vaccine (ACT-HIB) were tested in the experimental model of rat otitis induced with a Hib strain (Minn A), both of the LPD-conjugated and one of the PDm-conjugated vaccines induced significant protection from AOM, the level of protection being highest in animals given the vaccine with the highest LPD content. Sera from these rats also manifested the highest anti-PRP and anti-LPD antibody levels and the highest bactericidal activities against a Hib strain and an NTHi strain.

A resolved pneumococcal infection protects against nontypeable Haemophilus influenzae: an evaluation of different routes of whole cell immunization in protection against experimental acute otitis media

A conferred cross-protection between Haemophilus influenzae type b (Hib) and nontypeable H. influenzae (NTHi) was demonstrated in a previous study of experimental recurrent otitis media. To explore cross-protection further, and to compare oral administration of whole cells with two more conventional routes for vaccination against acute otitis media (AOM), a total number of 79 rats were immunized perorally, subcutaneously and intrabullarly with H. influenzae or pneumococci and thereafter challenged in the middle ear with NTHi or Hib 4 or 9 weeks later. Otomicroscopic changes, bacterial cultures, and serum IgG antibody levels were monitored. The study demonstrated that while peroral administration did not elicit any protection, a resolved pneumococcal AOM could reduce the susceptibility to reinfection with NTHi. In the latter case no cross-reacting antibodies were detected, but the protective rate was 50% or more, and it was comparable with that found after subcutaneous or intrabullar immunization with homologous NTHi or Hib strains. The results suggest that the protection of the rat middle ear mucosa may involve unspecific responses.

Biological activity of serum antibodies to a nonacylated form of lipoprotein D of Haemophilus influenzae

Protein D, a surface-exposed 42-kDa membrane lipoprotein, is well conserved among both type b and nontypeable Haemophilus influenzae strains, and it is considered a vaccine against H. influenzae infections. Here, we report the large-scale purification of a nonacylated form of protein D (PDm) from the periplasmic space of Escherichia coli overexpressing PDm. Screening of human sera for levels of antibodies to PDm demonstrated that the immunoglobulin G (IgG) antibody level is above background levels in infants less than 6 months of age. Following a drop to background values in the age group 6 months to 1 year, IgG antibody levels start to increase, together with IgA antibody levels, after 1 year of age. The first appearance of serum IgM antibodies is in 6-month- to 1-year-old infants whose IgG antibody levels have dropped to the postnatal background level. Affinity-purified antibodies from humans and from PDm-immunized rats detected epitopes of protein D which are normally exposed on the bacterial surface. Affinity-isolated human anti-PDm antibodies eluted in acidic buffer were not bactericidal against H. influenzae. Loss of bactericidal activity may occur in this buffer, as was demonstrated in pooled human sera with high bactericidal activity after incubation in the same buffer. Hyperimmunization of rats with PDm induced high levels of serum IgG and IgA antibodies against PDm and significant bactericidal activity against homologous and heterologous H. influenzae strains.

Effect of Haemophilus influenzae type b conjugate vaccine in combination with peroral immunization with Escherichia coli on experimental otitis media

The protective ability of a conjugated Haemophilus influenzae type b vaccine, ACT-HIB, used singly or in combination with orally administered Escherichia coli, was investigated in a rat model for acute otitis media. The humoral response to ACT-HIB was also analyzed. The study demonstrated that ACT-HIB vaccination resulted in a prompt antibody response, and that ACT-HIB was efficient in preventing middle ear infections caused by Haemophilus influenzae type b. The efficiency increased if the vaccine was combined with Escherichia coli. The results suggest that Escherichia coli could possibly be useful in the future as a vaccine vehicle, and since Haemophilus influenzae acute mastoiditis seems to be almost exclusively due to serotype b, the incidence of this infection may be reduced with the conjugated Haemophilus influenzae type b vaccines.