Macrophages maintain tissue homeostasis by removing old, damaged and apoptotic cells. During metamorphosis, holometabolous insects, including fruit flies, go through dramatic transformation — the larval fat body undergoes cell death and is eventually replaced by the adult fat body. Although it has been shown that macrophages infiltrate the dying larval adipose tissue, their role in metamorphosis is not well understood. Here, Adam Bajgar and colleagues find that macrophages convert engulfed larval adipocytes into suitable nutrients to be utilised by other tissues during post-metamorphic development. The authors first characterise the ultrastructure of the fat body over the course of metamorphosis. They observe that macrophages participate in adipose tissue remodelling, by engulfing lipid droplets, RNA-protein granules and other cellular debris from the adipocytes. Then, using lipidomics analysis and transcriptomic profiling, the authors find that the macrophages from newly emerged flies transiently adopt adipocyte-like metabolic features to mobilise nutritionally rich substances from larval adipocytes. Finally, the authors genetically remove the ability of macrophages to produce lipoproteins, such that the macrophages cannot redistribute nutrients from larval adipocytes. They observe delayed ovarian maturation in those flies. Overall, the findings uncover a metabolic role of macrophages during post-metamorphic development.
