Animal models are used for the evaluation of the safety and immunogenicity of acellular pertussis vaccines before administration to humans. The aerosol challenge model provides a reproducible system for the study of virulence factors and immunity involved in respiratory infection and subsequent disease. Clinical studies have shown that acellular pertussis antigens are protective against disease; however, to date, there is no laboratory correlate of vaccine mediated protection. In some recent publications, authors have suggested that when mice were immunized with DTP vaccines [with either whole-cell or acellular pertussis component] and then challenged with an aerosol or intranasal B. pertussis, the clearance of infection from the lungs paralleled the ability of these vaccines to protect children against pertussis. The aerosol model has been evaluated in our laboratory; despite very good intra- and inter-assay reproducibility, the rate of bacterial clearance does not distinguish between mice immunized with vaccines of high and low clinical efficacy. FY2000 activities: We participated in an international, multi-center collaborative study that evaluated three basic protection mouse models [intranasal, intracerebral, and aerosal] for the assay of vaccines containing an acellular pertussis component. Our laboratory tested five vaccines, twice in an intranasal, and once in an aerosol infection model. In all assay systems, there was a clear difference between immunized mice and unimmunized controls. Additionally, a three-component vaccine and a two-component vaccine from the same manufacturer were distinguished from each other in the intranasal models. However, the remaining samples were less clearly distinguished from one another, with the ordering of some samples appearing different in different assays. We participated in review of the data and writing of the technical report of the study which will be presented to WHO's Expert Committee on Biological Standardization in October 2000. The conclusion of the study was that further collaboration will be required to define and standardize a model for use in the characterization of acellular pertussis vaccines. In the past year, we published the results of a study designed to further dissect the immune mechanisms responsible for protection using a variety of techniques and reagents. To investigate the fundamental nature of protective immunity to Bordetella pertussis, we studied intranasal immunization of adult mice with formalin-fixed B. pertussis (FFBP), followed by aerosol challenge. Mice given two doses of FFBP intranasally completely cleared a subsequent pertussis aerosol challenge from tracheae and lungs, but there was no correlation between levels of specific antibody and clearance of bacteria. Transfer of immune serum before aerosol challenge had minimal effects on bacterial burdens, however, pertussis-specific T cells producing interferon gamma were detected in draining lymph nodes of FFBP-immunized mice. Complete protection in B cell knockout mice resulted from transfer of pertussis-immune B cells at a stage when little if any specific antibody was detectable. Immunization of mice lacking CD4(+) T cells did not lead to protection while mice lacking CD8(+) T cells were protected. Taken together, these data indicate that protective immunity to pertussis is dependent on both CD4(+) T cells and B cells, and both cell types provide significant functions other than specific antibody production.
