Development of pertussis serological potency test. Serological assessment of antibody response induced by whole cell vaccine as an alternative to mouse protection in an intracerebral challenge model.

The current potency test for pertussis vaccines, the mouse protection test (MPT), has many disadvantages. However, no alternative is yet available. The purpose of this study is to develop a serological alternative for the MPT based on in vitro assessment of the humoral immune response against pertussis in mice. After immunization with pertussis whole cell vaccine, the MPT shows a normal primary and secondary antibody response. Moreover, the i.c. challenge has a distinct booster effect on the pertussis IgG response. The relationship between the concentration of IgG antibodies against the surface-antigens of pertussis bacteria and the survival of mice after the i.c. challenge was demonstrated in a modified MPT (R = 0.91). To this end a protecting antibody level of > or = 45 EU/ml was selected as a level at which concentration most of the mice survived. Survival of mice in the MPT could be predicted, based on the antibody concentration at the day of challenge. Potencies estimated with the predicted and actual survival corresponded well (P = 0.990). This confirmed the essential role of vaccine induced pertussis antibodies in the protection against a lethal i.c. challenge and offered a possibility to develop a pertussis potency test based on serology. We developed a model in which mice (20-24 g) are immunized (i.p.) with graded doses of vaccine and bled after four weeks. Sera are titrated in Bordetella pertussis whole cell ELISA and potency based on vaccine dose dependent antibody response is estimated by means of a parallel line analysis. The potency of vaccines tested in the Pertussis Serological Potency Test (PSPT) and MPT are significantly similar, a P-value of 0.92 was found by means of the chi 2 test. Compared to the MPT, the PSPT is more reproducible as is indicated by its smaller 95% confidence intervals. Moreover, by using the PSPT the animal distress can be reduced to an acceptable level and the PSPT also results in a reduction of more than 25% in use of mice.