Pertussis Toxin Plays an Early Role in Respiratory Tract Colonization by Bordetella pertussis

Bordetella pertussis is a gram-negative bacterial pathogen that colonizes the human respiratory tract, leading to a severe paroxysmal coughing disease known as whooping cough. In the absence of a human challenge model of B. pertussis infection, studies of respiratory tract colonization and disease caused by B. pertussis have been limited to animal models, of which the most well established and frequently used is the mouse intranasal inoculation model. In this model, although overt symptomatic disease is not elicited, several characteristics of the human infection are reproduced, such as multiplication and clearance of the bacteria, limitation of infection to the respiratory tract, increased severity of infection in young animals, and various systemic physiological changes (8, 25, 29, 37). Recent studies have shown that this may also be a useful model for the preclinical assessment of acellular pertussis vaccine efficacy (6, 19). However, several aspects of the pathogenic mechanisms employed by B. pertussis and the immune response to this infection remain poorly understood. Pertussis toxin (PT) is a multisubunit exotoxin that is uniquely produced by B. pertussis and is considered one of the important virulence factors of B. pertussis. The holotoxin has an AB5 structure (33, 35) consisting of an enzymatically active A subunit (S1) that ADP-ribosylates the alpha subunits of several heterotrimeric G proteins in mammalian cells (12, 22) and a B oligomer that binds unidentified glycoconjugate receptors on cells (2, 39). PT disrupts signaling pathways in mammalian cells with a wide range of downstream effects (28, 40). When administered to experimental animals, purified PT can elicit almost all of the systemic symptoms associated with pertussis disease in humans, such as lymphocytosis, insulinemia or hypoglycemia, and histamine sensitivity (21, 23, 24), but its role in the severe coughing disease is uncertain. It is also unclear whether PT has a role in initial respiratory tract infection and colonization by B. pertussis. Early studies with Tn5 mutants of B. pertussis deficient in PT production demonstrated a reduced lethality for intranasally inoculated infant mice and a more rapid clearance from the lungs after a sublethal dose compared to the wild-type strain (5, 37, 38). Similar effects on virulence in 3- to 4-week-old mice were reported for a different pair of PT-deficient mutants of B. pertussis (13). Data from a more recent study demonstrated that a PT-deficient mutant of B. pertussis showed a similar time course profile of colonization in 4-week-old intranasally inoculated mice as the wild-type strain (1). Collectively, these data indicate a role for PT in the overall virulence of B. pertussis in respiratory tract infection but do not provide evidence of a role for PT in initial colonization. In this study, we assessed the role of PT as a colonization factor for B. pertussis in the respiratory tracts of young adult mice. From our results, we conclude that PT is a significant colonization factor that plays an early role in this host-pathogen interaction.