Hemocytes from the tobacco hornworm Manduca sexta have distinct functions in phagocytosis of foreign particles and self dead cells

Phagocytosis is an important innate immune response against microbial infections and an effective mechanism to eliminate apoptotic cells. In vertebrates, phagocytes such as macrophages and dendritic cells are involved in phagocytosis. We demonstrate here that insect hemocytes have distinct functions in phagocytosis of foreign particles and self dead cells. Plasmatocytes from the tobacco hornworm Manduca sexta were major hemocytes involved in phagocytosis of non-self microsphere beads, whereas granulocytes were apparently the only hemocytes that phagocytose self dead cells. We also showed that M. sexta immulectin-2, a pattern recognition receptor that protects larvae from bacterial infection, has an opsonic activity in phagocytosis. Immulectin-2 bound to the surface of granulocytes from the naïve larvae, but more immulectin-2 bound to plasmatocytes when larvae were injected with microsphere beads. Coupling of immulectin-2 onto microsphere beads enhanced in vitro phagocytosis of the beads. Our results suggest that insect hemocytes can have specialized functions similar to vertebrate phagocytes in phagocytosis, and pattern recognition receptors may function as opsonins to enhance phagocytosis.