RESUMEN
Potency assay in mice to evaluate the immunogenic properties of a number of candidate Lymphocytosis Promoting Factor (LPF) antigen in whole cell and acellular pertussis vaccines is described. Potency was estimated both by the conventional WHO intracerebral challenge test and an LPF-toxin challenge method. The latter method involved intraperitoneal injection of dilutions of test and reference preparations followed by challenging the animals 28 days later by LPF combined with AlPO4 adjuvant and estimating the total white blood cells three days after challenge. The method has been in routine use for the past four years and has given consistent results with reproducible specific activity and ED50 values. LPF antigen prepared by different chemical treatment or by genetic alteration of the molecule showed differences in specific activity, although different batches made by the same procedure appeared to give consistent values. Only two preparations, did not give specific activity close to what was given by LPF in whole cell vaccine by the LPF challenge method. Some preparations did not give any activity. In the intracerebral challenge method, many preparations failed to protect, while some gave a low degree of protection. The content of residual ADP ribosylating activity also varied. These three tests can be used for characterizing the LPF antigen of acellular pertussis vaccines.