Probing the function of Bordetella bronchiseptica adenylate cyclase toxin by manipulating host immunity

We have examined the role of adenylate cyclase-hemolysin (CyaA) by constructing an in-frame deletion in the Borderella bronchiseptica cyaA structural gene and comparing wild-type and cyaA deletion strains in natural host infection models. Both the wild-type strain RB50 and its adenylate cyclase toxin deletion (ΔcyaA) derivative efficiently establish persistent infections in rabbits, rats, and mice following low-dose inoculation. In contrast, an inoculation protocol that seeds the lower respiratory tract revealed significant differences in bacterial numbers and in polymorphonuclear neutrophil recruitment in the lungs from days 5 to 12 postinoculation. We next explored the effects of disarming specific aspects of the immune system on the relative phenotypes of wild-type and ΔcyaA bacteria. SCID, SCID-beige, or RAG-1(-/-) mice succumbed to lethal systemic infection following high- or low-dose intranasal inoculation with the wild- type strain but not the ΔcyaA mutant. Mice rendered neutropenic by treatment with cyclophosphamide or by knockout mutation in the granulocyte colony- stimulating factor locus were highly susceptible to lethal infection by either wild-type or ΔcyaA strains. These results reveal the significant role played by neutrophils early in B. bronchiseptica infection and by acquired immunity at later time points and suggest that phagocytic cells are a primary in vivo target of the Borderella adenylate cyclase toxin.