Interleukin-33 improves local immunity during Gram-negative pneumonia by a combined effect on neutrophils and inflammatory monocytes

Pneumonia represents a major health care burden and Gram-negative bacteria provide an increasing therapeutic challenge at least in part through the emergence of multidrug-resistant strains. IL-33 is a multifunctional cytokine belonging to the IL-1 family that can affect many different cell types. We sought here to determine the effect of recombinant IL-33 on the host response during murine pneumonia caused by the common Gram-negative pathogen Klebsiella pneumoniae. IL-33 pretreatment prolonged survival for more than 1 day during lethal airway infection and decreased bacterial loads at the primary site of infection and distant organs. Postponed treatment with IL-33 (3 h) also reduced bacterial growth and dissemination. IL-33-mediated protection was not observed in mice deficient for the IL-33 receptor component IL-1 receptor-like 1. IL-33 induced a brisk type 2 response, characterized by recruitment of type 2 innate lymphoid cells to the lungs and enhanced release of IL-5 and IL-13. However, neither absence of innate lymphoid cells or IL-13, nor blocking of IL-5 impacted on IL-33 effects in mice infected with Klebsiella. Likewise, IL-33 remained effective in reducing bacterial loads in mice lacking B, T, and natural killer T cells. Experiments using antibody-mediated cell depletion indicated that neutrophils and inflammatory monocytes were of importance for antibacterial defense. The capacity of IL-33 to restrict bacterial growth in the lungs was strongly reduced in mice depleted of both neutrophils and inflammatory monocytes, but not in mice selectively depleted of either one of these cell types. These results suggest that IL-33 boosts host defense during bacterial pneumonia by a combined effect on neutrophils and inflammatory monocytes.