TLR-independent neutrophil-derived IFN-γ is important for host resistance to intracellular pathogens

IFN-gamma is a major cytokine that is critical for host resistance to a broad range of intracellular pathogens. Production of IFN-gamma by natural killer and T cells is initiated by the recognition of pathogens by Toll-like receptors (TLRs). In an experimental model of toxoplasmosis, we have identified the presence of a nonlymphoid source of IFN-gamma that was particularly evident in the absence of TLR-mediated recognition of Toxoplasma gondii. Genetically altered mice lacking all lymphoid cells due to deficiencies in Recombination Activating Gene 2 and IL-2R gamma(c) genes also produced IFN-gamma in response to the protozoan parasite. Flow-cytometry and morphological examinations of non-NK/non-T IFN-gamma(+) cells identified neutrophils as the cell type capable of producing IFN-gamma. Selective elimination of neutrophils in TLR11(-/-) mice infected with the parasite resulted in acute susceptibility similar to that observed in IFN-gamma-deficient mice. Similarly, Salmonella typhimurium infection of TLR-deficient mice induces the appearance of IFN-gamma(+) neutrophils. Thus, neutrophils are a crucial source for IFN-gamma that is required for TLR-independent host protection against intracellular pathogens.