Bordetella pertussis infection in mice: correlation of specific antibodies against two antigens, pertussis toxin, and filamentous hemagglutinin with mouse protectivity in an intracerebral or aerosol challenge system

The correlation of titers of specific serum immunoglobulin G antibodies against two antigens, pertussis toxin (PT), and filamentous hemagglutinin (FHA), which are the main components of pertussis vaccine in Japan, with mouse protectivity was examined by both intracerebral and aerosol challenge systems with virulent Bordetella pertussis cells. Titer of the antibodies was calculated from the enzyme-linked immunosorbent assay (ELISA) unit given arbitrarily to reference antibodies. PT antibody titer which protected 50% of mice was indistinguishable in both active immunization followed by intracerebral challenge and passive immunization followed by aerosol challenge. The 50% effective dose was 23 ELISA U/ml in the former mice and 24 ELISA U/mouse in the latter. In the intracerebral challenge system, FHA did not elicit a protective response but was very helpful for PT as an immunizing antigen. When anti-FHA immunoglobulin G coexisted with anti-PT immunoglobulin G in mice, the 50% effective dose of PT antibody was 4.4 or 10 ELISA U/mouse in intracerebral or aerosol challenge systems, respectively. In this active immunization system, pertussis toxoid of 1 micrograms or 0.1 microgram/mouse produced PT antibody of ca. 20 or 5 ELISA U/ml, respectively. It was concluded that pertussis toxoid or its antibody was much more potent than Formalin-treated FHA or its antibody; Formalin-treated FHA or its antibody was helpful when it was administered with pertussis toxoid toxoid or its antibody, however.