Bacterial challenge initiates endosome-lysosome response in Drosophila immune tissues

An effective innate immune response is critical for the protection of an organism against pathogen and environmental challenge. There is emerging evidence that an effective immune response depends heavily on the traffic and function of endosomes and lysosomes. However, there is very little understanding of the dynamics of an innate immune response, especially in vivo. Toward this aim, we have used two-photon microscopy to visualize the response to bacterial infection of the endosome-lysosome system in immune response tissues using intact Drosophila larvae. First, we set up the conditions to image intact larva in vivo and more specifically GFP-labeled endosomes-lysosomes in the fat body, and compared their distribution and size with those in tissue explanted ex vivo. Notably, we observed significant expansion of both Rab5 and Rab7 endosomal compartments upon both tissue isolation and minor aseptic wounding, indicating significant differences between live and explanted tissue. We also observed changes in endosome-lysosome vesicles within internal immune response tissues following in vivo bacterial infection by the oral route (to avoid a wounding response). We conclude that there are significant changes to the architecture of endosomes and lysosomes during an innate immune response, setting the scene for mechanistic studies to identify the signaling pathways that orchestrate this process.