Genome-Wide RNAi Screen in \(IFN-\gamma-Treated\) Human Macrophages Identifies Genes Mediating Resistance to the Intracellular Pathogen Francisella tularensis

Abstract

Interferon-gamma ((IFN-\gamma)) inhibits intracellular replication of Francisella tularensis in human monocyte-derived macrophages (HMDM) and in mice, but the mechanisms of this protective effect are poorly characterized. We used genome-wide RNA interference (RNAi) screening in the human macrophage cell line THP-1 to identify genes that mediate the beneficial effects of (IFN-\gamma) on F. tularensis infection. A primary screen identified ~200 replicated candidate genes. These were prioritized according to mRNA expression in (IFN-\gamma-primed) and F. tularensis-challenged macrophages. A panel of 20 top hits was further assessed by re-testing using individual shRNAs or siRNAs in THP-1 cells, HMDMs and primary human lung macrophages. Six of eight validated genes tested were also found to confer resistance to Listeria monocytogenes infection, suggesting a broadly shared host gene program for intracellular pathogens. The F. tularensis-validated hits included ‘druggable’ targets such as TNFRSF9, which encodes CD137. Treating HMDM with a blocking antibody to CD137 confirmed a beneficial role of CD137 in macrophage clearance of F. tularensis. These studies reveal a number of important mediators of (IFN-\gamma) activated host defense against intracellular pathogens, and implicate CD137 as a potential therapeutic target and regulator of macrophage interactions with Francisella tularensis.