DESCRIPTION (provided by applicant): IFN-γ is a key cytokine that mediates host resistance to a variety of intracellular pathogen. Until recently, it was largely acknowledged that lymphoid cells are the sole sources for IFN-γ. We have recently established that neutrophils are an emerging cellular source of IFN-γ, a key cytokine that mediates host defense to Toxoplasma gondii and other intracellular pathogens. Production of IFN-γ by neutrophils, in contrast to lymphoid cells, is Toll-like receptor (TLR) and IL-12-independent and the events associated with IFN-γ production by neutrophils are not understood. We have also observed that neutrophils express IFN-γ during their lineage development in the bone marrow niche independently of microbes. IFN-γ accumulates in neutrophilic granules and is released upon induction of neutrophil degranulation. We propose to build upon these findings to gain a deeper understanding of how neutrophil-derived IFN-γ expression is regulated, and to investigate the physiological significance of neutrophil-derived IFN-γ in mouse and human models of toxoplasmosis. In Aim 1, we will identify neutrophil precursors involved in IFN-γ production in naïve and Toxoplasma gondii-infected mice and determine the transcription factors that regulate IFN-γ production in human and mouse neutrophil. In Aim 2, we will use mice harboring neutrophil-specific IFN-γ deficiency to test the hypothesis that neutrophil-derived IFN-γ coordinates innate and adaptive immune responses to Toxoplasma gondii. These studies will advance our understanding of how human and mouse innate immune systems mediate the protective TLR-independent effects of IFN-γ.