A small phagocytosis stimulating factor is released by and acts on phagocytosing Galleria mellonella haemocytes in vitro

We established an in vitro transfer system with monolayers of isolated plasmatocytes from Galleria mellonella. The plasmatocytes represent the main phagocytically active haemocyte type in this lepidopteran insect. Plasmatocytes to which hydrophilic silica beads were added as a phagocytosing agent served as ‘donor’ cells. Supernatants from these donor cultures were transferred to freshly prepared ‘recipient’ plasmatocyte monolayers. Subsequently, FITC (fluorescein-isothiocyanate) labelled yeast cells were added to the recipient monolayers and the phagocytic activity was determined using an FITC quenching assay with trypan blue. The phagocytic activity in plasmatocyte monolayers which received supernatants from phagocytically active donor cells was significantly higher than the activity of cells receiving supernatants from non-activated donor cells. Time course studies revealed that the inducing capacity of the donor cell supernatants was highest 2–4h after starting phagocytosis of the silica beads. Isolation of the responsible phagocytosis stimulating factor is still underway. From our investigations we can conclude that it must be a very small (