Legionella pneumophila is one example among many species of pathogenic bacteria that replicate within mammalian macrophages during infection. The immune signaling factor interferon gamma (IFN-γ) blocks L. pneumophila replication in macrophages and is an essential component of the immune response to L. pneumophila and other intracellular pathogens. However, to date, no study has identified the exact molecular factors induced by IFN-γ that are required for its activity. We generated macrophages lacking different combinations of IFN-γ-induced genes in an attempt to find a genetic background in which there is a complete loss of IFN-γ-mediated restriction of L. pneumophila. We identified six genes that comprise the totality of the IFN-γ-dependent restriction of L. pneumophila replication in macrophages. Our results clarify the molecular basis underlying the potent effects of IFN-γ and highlight how redundancy downstream of IFN-γ is key to prevent exploitation of macrophages by pathogens.

DNA double-strand breaks (DSBs) are a particularly deleterious class of DNA damage that threatens genome integrity. DSBs are repaired by three pathways: nonhomologous-end joining (NHEJ), homologous recombination (HR), and single-strand annealing (SSA). () is the ortholog of and human, and has been shown to suppress crossovers in mitotic cells and repair mitotic DNA gaps via HR. To further elucidate the role of DmBlm in repair of a simple DSB, and in particular recombination mechanisms, we utilized the Direct Repeat of (DR-) and Direct Repeat of (DR-) repair assays in multiple mutant allele backgrounds. null and helicase-dead mutants both demonstrated a decrease in repair by noncrossover HR, and a concurrent increase in non-HR events, possibly including SSA, crossovers, deletions, and NHEJ, although detectable processing of the ends was not significantly impacted. Interestingly, gene conversion tract lengths of HR repair events were substantially shorter in null but not helicase-dead mutants, compared to heterozygote controls. Using DR-, we found that, in contrast to Sgs1, DmBlm is not required for suppression of recombination between diverged sequences. Taken together, our data suggest that DmBlm helicase function plays a role in HR, and the steps that contribute to determining gene conversion tract length are helicase-independent.

18Interferon gamma (IFNγ) restricts the intracellular replication of many pathogens, but 19how IFNγ confers cell-intrinsic pathogen resistance remains unclear. For example, 20 intracellular replication of the bacterial pathogen Legionella pneumophila in 21 macrophages is potently curtailed by IFNγ, but consistent with prior results, no 22Importance 41 Legionella pneumophila is one example among many species of pathogenic bacteria 42 that replicate within mammalian macrophages during infection. The immune signaling 43 factor interferon gamma (IFNγ) blocks L. pneumophila replication in macrophages and 44 is an essential component of the immune response to L. pneumophila and other 45 intracellular pathogens. However, to date, no study has determined the exact molecular 46 factors induced by IFNγ that are required for its activity. We generated macrophages 47 lacking different combinations of IFNγ-induced genes in an attempt to find a genetic 48 background in which there is a complete loss of IFNγ-mediated restriction of L. 49 pneumophila. We successfully identified six genes that comprise the totality of the IFNγ-50 dependent restriction of L. pneumophila replication in macrophages. Our results clarify 51 the molecular basis underlying the potent effects of IFNγ and highlight how redundancy 52 downstream of IFNγ is key to prevent exploitation of the macrophage niche by 53 pathogens. 54 IFNγ that can restrict L. pneumophila in the absence of iNOS. 93 Previous work has attempted to address the possibility of redundancy in the 94 IFNγ-dependent immune response to L. pneumophila. Pilla et al generated quadruple 95 knockout (QKO) mice deficient in Nos2, Cybb (cytochrome b(558) subunit beta, 96 encoding NADPH oxidase 2 aka NOX2), Irgm1 (immunity-related GTPase family M 97 member 1), and Irgm3 (immunity-related GTPase family M member 3), all induced by 98 IFNγ (20). NOX2 partners with phagosomal oxidase components to generate reactive 99 oxygen species, which, like NO, can cause direct toxicity to phagocytized pathogens in 100 6 neutrophils and macrophages (22, 23). IRGM1 and IRGM3 are antimicrobial GTPases 101 that may participate in the disruption of membrane-bound, pathogen-containing 102 compartments within phagocytes (20, 24). Remarkably, Pilla et al observed that 103 macrophages derived from QKO mice retained potent restriction of L. pneumophila 104 replication when stimulated with IFNγ, and further implicated the bacterial 105 lipopolysaccharide detector caspase 11 (CASP11), which when activated can trigger 106 host macrophage pyroptosis, in some of the residual IFNγ-dependent restriction of L. 107 pneumophila replication in macrophages (20). 108 Recently, Naujoks et al implicated immune responsive gene 1 (IRG1, encoded by 109 the gene Acod1) in the IFNγ-dependent immune response to L. pneumophila, 110demonstrating that driving Acod1 expression in macrophages was sufficient to suppress 111 L. pneumophila replication (21). However, the study did not address whether 112 macrophages deficient in IRG1 were impaired in t...