Ex vivo observation of insect hemocyte behavior against beads and nematodes in the presence of insect plasma

Abstract

Recognition of foreign targets by insect hemocytes is a crucial first step for insect immunity against invading multicellular organisms in the hemocoel. To understand the mechanism of recognition, we observed the hemocyte behavior of Galleria mellonella Linnaeus (Lepidoptera: Pyralidae) larvae against beads and the nonparasitic nematode Caenorhabditis elegans (Maupas) (Rhabditida: Rhabditidae) in the presence of plasma ex vivo using time-lapse microscopy. Both granular cells and plasmatocytes adhered to and spread on the surface of beads and nematodes. In addition, the spread plasmatocytes actively moved over the beads and nematodes. These results suggest that not only granular cells but also plasmatocytes can recognize foreign targets in the presence of insect plasma and that spread plasmatocytes can actively search for foreign targets. Hemocyte adhesion to beads and nematodes ex vivo was similar to that of the in vivo 1 h after injection. A divalent cation chelator inhibited the spreading and adhesion of plasmatocytes ex vivo, but it did not affect the adhesion of granular cells. The present method enables the analysis of acute hemocyte response against foreign targets in the presence of plasma.