Pertussis serological potency test as an alternatively to the intracerebral mouse protection test.

The current potency test for pertussis vaccines, the intracerebral protection test (MPT), is still the only mandatory laboratory model available. This test, however, is a valid, but inhumane and imprecise test and therefore a good candidate for replacement. Recently we have developed the Pertussis Serological Potency Test (PSPT) as an alternative for the MPT. The PSPT is based on in vitro assessment of the humoral immune response against the whole range of surface -antigens of B. pertussis in mice after immunisation with Whole Cell Vaccine (WCV). We have demonstrated a relationship between the mean pertussis antibody concentration at the day of challenge and the proportion of surviving mice at each vaccine dose in the MPT (R = 0.91). The PSPT is a model in which mice (20-24 g) are immunised i.p. with graded doses of vaccine and bled after four weeks. Sera are titrated in a whole cell ELISA and potency based on the vaccine dose-dependent antibody response is estimated by means of a parallel line analysis. In an in-house validation study 13 WCVs were tested in the PSPT and MPT. Homogeneity of both tests was proven by means of the chi-square test; potencies were significantly similar (p = 0.95). 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. Additional experiments showed that estimation of WCV-potency in the PSPT based on specific antibody responses against protective antigens (PT, FHA, 69- and 92-kDa OMPS) was not possible or did not correlate with protection in MPT. Sera obtained from the PSPT showed a correlation between pertussis antibody levels and complement-mediated killing by pertussis antibodies in in vitro assays. In conclusion, the PSPT is a promising substitute for the MPT though further validation and additional studies on functional validity should finally warrant replacement of the MPT by this serological model.