An fMLP receptor is involved in activation of phagocytosis by hemocytes from specific insect species

In mammalian phagocytes, the bacterial formylated peptide fMLP functions both as a potent enhancer of phagocytosis and chemoattractant. fMLP has been reported to be chemotactic for hemocytes of two marine invertebrates, and of the insect Manduca sexta (Lepidoptera). Whether fMLP is also able to activate phagocytosis has not been explored in hemocytes of any invertebrate. To determine the effect of fMLP on insect hemocyte phagocytosis, in vitro phagocytosis assays were performed with hemocytes from the insects: Gromphadorhina portentosa (Blattodea), Acheta domesticus (Orthoptera), Zophobas morio (Coleoptera), and Galleria mellonella (Lepidoptera). Phagocytosis of latex, zymosan (yeast), Gram-positive and Gram-negative bacteria was measured by flow cytometry, in the presence of increasing fMLP concentrations. G. portentosa hemocytes showed no enhancement of phagocytosis by fMLP. A. domesticus hemocytes had increased phagocytosis of latex and Gram-negative bacteria in the presence of fMLP. Z. morio hemocytes increased phagocytosis of latex, yeast, and Gram-negative bacteria after fMLP stimulation. Galleria mellonella hemocytes increased phagocytosis of latex after fMLP stimulation. Treating hemocytes with Pertussis toxin, a known inhibitor of the signaling pathway initiated by the mammalian fMLP receptor, returned phagocytosis to basal levels. Also, hemocytes from all insect species tested presented a similar chemotactic response to fMLP. These data suggest that, whereas the ability of hemocytes to chemotactically-respond to fMLP is conserved in insects ranging from Blattodea to Lepidoptera, the ability to respond to fMLP by activating phagocytosis is restricted to specific insect species.