IgG subclass responses to Bordetella pertussis filamentous haemagglutinin and pertussis toxin in whooping cough

The IgG subclass response pattern to two major protective antigens of Bordetella pertussis—filamentous haemagglutinin (FHA) and pertussis toxin (PT)—was measured by enzyme-linked immunosorbent assay (ELISA) in paired sera from 46 patients with culture-confirmed whooping cough. Variations in specific subclasses related to antigen and to previous immunization status were noted. The dominant response to both antigens was of the IgG1 subclass and closely followed the total IgG response for both unimmunized children (n = 18) and previously DTP-immunized children and adults (n = 28). Neither IgG2 nor IgG4 to FHA were seen in the unimmunized children but were found in immunized children and adults. An IgG2 response to PT was noted in 70–80% of all patients irrespective of previous immunization status. An observed correlation between IgG2 and neutralizing antibodies to PT was further investigated. It was found that 10 patients without neutralizing antibodies also lacked IgG2 to PT. The study thus revealed differences in IgG subclass responses to two bacterial protein antigens, with an indication that the IgG2 response to PT reflects or correlates to the development of neutralizing antibodies to this toxin. The finding could explain discrepancies between ELISAs and neutralization tests for antibodies to bacterial toxins and have implications for vaccine research.