Regulation of neutrophils by interferon-γ limits lung inflammation during tuberculosis infection

Resistance to Mycobacterium tuberculosis requires the host to restrict bacterial replication while preventing an over-exuberant inflammatory response. Interferon (IFN)gamma is crucial for activating macrophages and also regulates tissue inflammation. We dissociate these two functions and show that IFN-gamma(-/-) memory CD4(+) T cells retain their antimicrobial activity but are unable to suppress inflammation. IFN-gamma inhibits CD4(+) T cell production of IL-17, which regulates neutrophil recruitment. In addition, IFN-gamma directly inhibits pathogenic neutrophil accumulation in the infected lung and impairs neutrophil survival. Regulation of neutrophils is important because their accumulation is detrimental to the host. We suggest that neutrophilia during tuberculosis indicates failed Th1 immunity or loss of IFN-gamma responsiveness. These results establish an important antiinflammatory role for IFN-gamma in host protection against tuberculosis.