Cell intrinsic immunity by human Guanylate Binding Proteins against Toxoplasma gondii and Salmonella Typhimurium

Abstract

Cell intrinsic defence mechanisms are employed by most cells of a multicellular organism and aid the immune systems in combating infections with intracellular pathogens like the parasite Toxoplasma gondii or the bacterium Salmonella Typhimurium. Interferon-induced GTPases are key components of cell intrinsic defence of rodents and humans. One class of GTPases, the Guanylate Binding Proteins (GBPs), protect against pathogens by targeting and disrupting their intracellular compartments, modulating host cell death, inflammasomes, autophagy and oxidative responses. The function of human GBPs was studied here by developing and employing HRMAn (Host Response to Microbe Analysis), an image analysis platform combining machine learning algorithms and deep learning. HRMAn performs unbiased quantification of host-pathogen interactions and has the capability to learn phenotypes from data. HRMAn can automatically recognise, classify and quantify pathogen killing, replication and cellular responses. Using three in vitro macrophage models, it could be established that human GBP1, GBP2 and GBP5 contribute to Toxoplasma growth control and that GBP1 additionally has the ability to kill parasites and control infection with Salmonella. In contrast to other human cell types, GBP1 expressed in macrophages is able to target parasites and their vacuoles and cytosolic bacteria. Recruitment of GBP1 is controlled by post-translational modifications including isoprenylation or phosphorylation by PIM family kinases. New findings presented here, show that Toxoplasma-induced death of human macrophages required functional GBP1. Its ability to target Toxoplasma vacuoles and parasites through its GTPase activity are critical. Mechanistically, GBP1 releases Toxoplasma-DNA into the cytosol for detection by AIM2, which in turn induces GSDMD-independent, ASC- and CASP8-dependent apoptosis. Similarly, GBP1 targets cytosolic Salmonella, where it facilitates caspase-4 recruitment and activation leading to increased pyroptosis. A regulatory mechanism can inactivate GBP1 through caspase-1-mediated cleavage. The human GBP protein family therefore are essential regulators of cell intrinsic macrophage defence and gatekeepers of host cell death.