Circulating blood cells function as a surveillance system for damaged tissue in Drosophila larvae

Insects have an open circulatory system in which the heart pumps blood (hemolymph) into the body cavity, where it directly bathes the internal organs and epidermis. The blood contains free and tissue-bound immune cells that function in the inflammatory response. Here, we use live imaging of transgenic Drosophila larvae with fluorescently labeled blood cells (hemocytes) to investigate the circulatory dynamics of larval blood cells and their response to tissue injury. We find that, under normal conditions, the free cells rapidly circulate, whereas the tissue-bound cells are sessile. After epidermal wounding, tissue-bound cells around the wound site remain sessile and unresponsive, whereas circulating cells are rapidly recruited to the site of damage by adhesive capture. After capture, these cells distribute across the wound, appear phagocytically active, and are subsequently released back into circulation by the healing epidermis. The results demonstrate that circulating cells function as a surveillance system that monitors larval tissues for damage, and that adhesive capture, an important mechanism of recruitment of circulating cells to inflammatory sites in vertebrates, is shared by insects and vertebrates despite the vastly different architectures of their circulatory systems.