Regulation of phagocyte triglyceride by a STAT-ATG2 pathway controls mycobacterial infection.

Mycobacterium tuberculosis remains a global threat to human health, yet the molecular mechanisms regulating immunity remain poorly understood. Cytokines can promote or inhibit mycobacterial survival inside macrophages and the underlying mechanisms represent potential targets for host-directed therapies. Here we show that cytokine-STAT signalling promotes mycobacterial survival within macrophages by deregulating lipid droplets via ATG2 repression. In Drosophila infected with Mycobacterium marinum, mycobacterium-induced STAT activity triggered by unpaired-family cytokines reduces Atg2 expression, permitting deregulation of lipid droplets. Increased Atg2 expression or reduced macrophage triglyceride biosynthesis, normalizes lipid deposition in infected phagocytes and reduces numbers of viable intracellular mycobacteria. In human macrophages, addition of IL-6 promotes mycobacterial survival and BCG-induced lipid accumulation by a similar, but probably not identical, mechanism. Our results reveal Atg2 regulation as a mechanism by which cytokines can control lipid droplet homeostasis and consequently resistance to mycobacterial infection in Drosophila.

An improved flow cytometry assay to monitor phagosome acidification.

Phago-lysosome formation is important for cell-autonomous immunity to intracellular pathogens, antigen presentation and metabolism. A hallmark feature of phago-lysosomal compartments is that they undergo progressive luminal acidification controlled by the activation of vacuolar V-ATPase. Acidification is required for many enzymatic processes taking place in phago-lysosomes, like proteolysis, and supports the microbicidal activity of macrophages. Here we present a new quantitative methodology to assess phagosome acidification by flow cytometry based on the use of bi-fluorescent particles. This method relies on the use of UV polystyrene beads labelled with the acid sensor pHrodo-succinimidyl ester (pHrodo(TM) SE red) and enables us to dissociate particle association with phagocytes from their engulfment in acidified compartments. This methodology is well suited to monitor the acidification of phagosomes formed in vivo after fluorescent bead administration.

Intracellular infections in Drosophila melanogaster: host defense and mechanisms of pathogenesis.

The fruit-fly Drosophila melanogaster has emerged as a powerful model to study innate immunity against intracellular pathogens. To combat infection, the fly relies on multiple lines of defense, many of which are shared with mammals and arthropod vectors of human diseases. In addition to conserved immune pathways, the ease of performing sophisticated genetic screens has allowed the identification of novel host immune factors and novel pathogen virulence factors. Recently, some groups have exploited this to simultaneously analyze the host and pathogen genetics of intracellular infection. This review aims to unravel the Drosophila immune response against intracellular pathogens, highlighting recent discoveries.

MEF2 is an in vivo immune-metabolic switch.

Infections disturb metabolic homeostasis in many contexts, but the underlying connections are not completely understood. To address this, we use paired genetic and computational screens in Drosophila to identify transcriptional regulators of immunity and pathology and their associated target genes and physiologies. We show that Mef2 is required in the fat body for anabolic function and the immune response. Using genetic and biochemical approaches, we find that MEF2 is phosphorylated at a conserved site in healthy flies and promotes expression of lipogenic and glycogenic enzymes. Upon infection, this phosphorylation is lost, and the activity of MEF2 changes--MEF2 now associates with the TATA binding protein to bind a distinct TATA box sequence and promote antimicrobial peptide expression. The loss of phosphorylated MEF2 contributes to loss of anabolic enzyme expression in Gram-negative bacterial infection. MEF2 is thus a critical transcriptional switch in the adult fat body between metabolism and immunity.