Identification and characterisation of molecularly-distinct Drosophilamacrophage subpopulations with enhanced inflammatory responses to injury

Macrophage heterogeneity is a firmly established principle in vertebrates, encompassing tissue-resident subpopulations and polarised activation states. However, there is much discussion over the exact types and functions of these different macrophage populations, presenting the need for a simpler model system. Drosophila melanogaster (Drosophila) possess cells that are functionally equivalent to macrophages (plasmatocytes of the hemocyte lineage), but to date these cells have been largely considered to inhabit homogeneous populations, with only limited evidence suggesting macrophage heterogeneity might exist in flies. However, our work and the work of others indicates that Drosophila embryonic macrophages, much like their vertebrate counterparts, do not respond uniformly to a variety of immune stimuli, including phagocytic challenge and tissue damage, leading us to hypothesise that macrophage heterogeneity is an evolutionarily conserved facet of cellular innate immunity that is present in Drosophila. By screening the Vienna Tilling array library, we have identified enhancer-GAL4 lines that label molecularly-distinct subpopulations of Drosophilamacrophages in vivo. We have extensively evaluated the functions of these subpopulations, identifying enhancer lines that label subpopulations of macrophages. Importantly, some of these subpopulations of macrophages exhibit enhanced inflammatory responsesto injury within the developing embryo. Furthermore, several subpopulations migrate at faster rates and/or display reduced rates of apoptotic cell clearance in vivo. Moreover, we have recently begun to investigate the underlying mechanisms behind these behaviours, uncovering roles for metabolism and ROS. Specifically, we have demonstrated that increased glycolysis within hemocytes induces more effective response to injury. We have definitively demonstrated macrophage heterogeneity in Drosophilafor the first time and developed novel tools in which to examine macrophage subpopulations. This work extends the capacity of Drosophilaas a model organism for studying macrophage behaviour in vivo and shows that macrophage heterogeneity is a key feature of cellular innate immune systems that is conserved across evolution.