In vivoandin vitrophagocytosis by hemocytes of the stick insectBacillus rossius

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Abstract
Two main hemocyte subpopulations, plasmatocytes and granulocytes, constitute the cell content of the hemolymph in the stick insect Bacillus rossius. These cells can be distinguished by their phagocytic response to the in vitro and in vivo addition of polystyrene latex particles and bacteria. In monolayers of cultured hemocytes, latex particles accumulated in plasmatocytes. When latex beads were injected into the hemocoel, after 18 hours the particles were observed to accumulate only in plasmatocytes. Using FITC‐labelled E. coli the same observations were repeated, and the bacteria were found to be phagocytosed by plasmatocytes and not by granulocytes both in vitro and in vivo. In vitro phagocytosis of FITC‐labelled E. coli monitored by flow cytometric analysis showed that the process is rapid and complete in one hour. These results suggest that stick insects possess an immune system in which the hemocyte subpopulations are differentially specialized with respect to phagocytosis. Plasmatocytes appear to be fully responsible for phagocytic processes in vitro and in vivo.