A critical role for the TLR signaling adapter Mal in alveolar macrophage-mediated protection against Bordetella pertussis

Bordetella pertussis causes whooping cough, an infectious disease of the respiratory tract that is re-emerging despite high vaccine coverage. Here we examined the role of Toll-like receptor (TLR) adapter protein Mal in the control of B. pertussis infection in the lungs. We found that B. pertussis bacterial load in the lungs of Mal-defective (Mal−/−) mice exceeded that of wild-type (WT) mice by up to 100-fold and bacteria disseminated to the liver in Mal−/− mice and 50% of these mice died from the infection. Macrophages from Mal−/− mice were defective in an early burst of pro-inflammatory cytokine production and in their ability to kill or constrain intracellular growth of B. pertussis. Importantly, the B. pertussis bacterial load in the lungs inversely correlated with the number of alveolar macrophages. Despite the maintenance and expansion of other cell populations, alveolar macrophages were completely depleted from the lungs of infected Mal−/− mice, but not from infected WT mice. Our findings define for the first time a role for a microbial pattern-recognition pathway in the survival of alveolar macrophages and uncover a mechanism of macrophage-mediated immunity to B. pertussis in which Mal controls intracellular survival and dissemination of bacteria from the lungs.