Bordetella pertussis isolates in Finland: serotype and fimbrial expression

Abstract

Background: Bordetella pertussis causes whooping cough or pertussis in humans. It produces several virulence factors, of which the fimbriae are considered adhesins and elicit immune responses in the host. B. pertussis has three distinct serotypes Fim2, Fim3 or Fim2,3. Generally, B. pertussis Fim2 strains predominate in unvaccinated populations, whereas Fim3 strains are often isolated in vaccinated populations. In Finland, pertussis vaccination was introduced in 1952. The whole-cell vaccine contained two strains, 18530 (Fim3) since 1962 and strain 1772 (Fim2,3) added in 1976. After that the vaccine has remained the same until 2005 when the whole-cell vaccine was replaced by the acellular vaccine containing pertussis toxin and filamentous hemagglutinin. Our aims were to study serotypes of Finnish B. pertussis isolates from 1974 to 2006 in a population with > 90% vaccination coverage and fimbrial expression of the isolates during infection. Serotyping was done by agglutination and serotype-specific antibody responses were determined by blocking ELISA.

Results: Altogether, 1,109 isolates were serotyped. Before 1976, serotype distributions of Fim2, Fim3 and Fim2,3 were 67%, 19% and 10%, respectively. From 1976 to 1998, 94% of the isolates were Fim2 serotype. Since 1999, the frequency of Fim3 strains started to increase and reached 83% during a nationwide epidemic in 2003. A significant increase in level of serum IgG antibodies against purified fimbriae was observed between paired sera of 37 patients. The patients infected by Fim3 strains had antibodies which blocked the binding of monoclonal antibodies to Fim3 but not to Fim2. Moreover, about one third of the Fim2 strain infected patients developed antibodies capable of blocking of binding of both anti-Fim2 and Fim3 monoclonal antibodies.

Conclusion: Despite extensive vaccinations in Finland, B. pertussis Fim2 strains were the most common serotype. Emergence of Fim3 strains started in 1999 and coincided with nationwide epidemics. Results of serotype-specific antibody responses suggest that Fim2 strains could express Fim3 during infection, showing a difference in fimbrial expression between in vivo and in vitro.

Figure 1

Serotype distributions of B. pertussis isolates in Finland from 1974 to 2006. A total of 1,109 isolates were collected and serotyped.

Figure 2

Blocking of binding of mAbFim2 and mAbFim3 to Fim2-expressing strain S1 (left) and of mAbFim3 and mAbFim2 to Fim3-expressing strain S3 (right) by human IgG antibodies to fimbriae. Patients 48I and 10II were infected by Fim2 strains and patient 2026 by Fim3 strain. Blocking of binding of mAbFim2 and mAbFim3 to S1 and S3 by normal sheep serum in PBS (NSS-PBS) was not observed. Data were from a single experiment. Three sera with different blocking levels were tested twice and the results obtained were same. In each experiment, both positive and negative controls were included, and all the experiments were performed by an experienced technician.

Figure 3

Detection of epitope-specific antibody responses to fimbriae in paired sera from Fim2 infected patients. Group 1 included 12 patients who had significant increases in level of IgG antibodies to fimbriae and in blocking level of mAbFim2 to S1(Fim2) between paired sera (p < 0.01) but had no change in blocking level of mAbFim3 to S3. Group 2 consisted of 11 patients who had significant increases in level of IgG antibodies to fimbriae and in blocking levels of both mAbFim2 to S1(Fim2) and mAbFim3 to S3(Fim3) between paired sera (p < 0.01 for both). Group 3 included 11 patients who had no change either in level of IgG antibodies to fimbriae or in blocking levels of both mAbFim2 to S1(Fim2) and mAbFim3 to S3(Fim3) between paired sera.