Plasmatocyte spreading peptide induces spreading of plasmatocytes but represses spreading of granulocytes

In most Lepidoptera, plasmatocytes and granulocytes are the two hemocyte classes capable of adhering to foreign targets. Previously, we identified plasmatocyte spreading peptide (PSP1) from the moth Pseudoplusia includens and reported that it induced plasmatocytes to rapidly spread on foreign surfaces. Here we examine whether the response of plasmatocytes to PSP1 was influenced by cell density or culture conditions, and whether PSP1 affected the adhesive state of granulocytes. Plasmatocyte spreading rates were clearly affected by cell density in the absence of PSP1 but spreading was density independent in the presence of PSP1. PSP1 also induced plasmatocytes in agarose‐coated culture wells to form homotypic aggregations rather than spread on the surface of culture wells. In contrast, granulocytes rapidly spread in a density independent manner in the absence of PSP1, but were dose‐dependently inhibited from spreading by the addition of peptide. An anti‐PSP1 polyclonal antibody neutralized the spreading activity of synthetic PSP1. This antibody also neutralized the plasmatocyte spreading activity of granulocyte‐conditioned medium, and significantly delayed plasmatocyte spreading when cells were cultured at a high density in unconditioned medium. These results suggested that the spreading activity derived from granulocytes is due in part to PSP1. Pretreatment of plasmatocytes with trypsin had no effect on PSP1‐induced aggregation but PSP1‐induced aggregations were readily dissociated by trypsin. This suggested that PSP1 is not an adhesion factor but induces adhesion by stimulating a change in the cell surface of plasmatocytes. Synthetic PSP1 also induced aggregation of plasmatocytes from other Lepidoptera indicating that regulation of hemocyte activity by PSP1‐related peptides may be widespread. Arch. Insect Biochem. Physiol. 42:213–223, 1999. © 1999 Wiley‐Liss, Inc.