Macrophage activation occurs by pathogen recognition through the Toll-like receptors (TLRs) to stimulate innate immune responses. Interferon-gamma (IFNg) augments the inflammatory production of tumour necrosis factor, interleukin-6 and anti-microbial nitric oxide and has been shown to induce cell death in human macrophages co-stimulated with TLR agonists. This underappreciated mode of activation-induced cell death in innate immune cells has been reported, but the essential genetic and biochemical requirements have yet to be elucidated. This project aims to genetically delineate the molecular mechanism underpinning IFNg and TLR activation-induced cell death and define how this novel macrophage killing pathway impacts innate immune responses.
Using a panel of genetic knockout mice lacking essential cell death regulators, our research has identified critical death effector components, including a single apoptotic caspase, necessary for IFNg and TLR-induced macrophage killing. Our data also demonstrates that the highly inflammatory cell death pathways, pyroptosis and necroptosis, are dispensable for IFNg and TLR killing. Unexpectedly, we document an important role for inducible nitric oxide synthase (iNOS) in IFNg and TLR-induced cell death that is likely to promote apoptotic caspase activity. Importantly, this physiologically relevant mode of cell death may act to impact innate immune responses upon infection.