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

In most Lepidoptera, plasmatocytes and granulocytes are the two hemocyte cl asses capable of adhering to foreign targets. Previously, we identified pla smatocyte spreading peptide (PSP1) from the moth Pseudoplusia includens and reported that it induced plasmatocytes to rapidly spread on foreign surfac es. Here we examine whether the response of plasmatocytes to PSP1 was influ enced by cell density or culture conditions, and whether PSP1 affected the adhesive state of granulocytes. Plasmatocyte spreading rates were clearly a ffected by cell density in the absence of PSP1 but spreading was density in dependent in the presence of PSP1. PSP1 also induced plasmatocytes in agaro se-coated culture wells to form homotypic aggregations rather than spread o n the surface of culture wells. In contrast, granulocytes rapidly spread in a density independent manner in the absence of PSP1, but were dose-depende ntly inhibited from spreading by the addition of peptide. An anti-PSP1 poly clonal antibody neutralized the spreading activity of synthetic PSP1. This antibody also neutralized the plasmatocyte spreading activity of granulocyt e-conditioned medium, and significantly delayed plasmatocyte spreading when cells were cultured at a high density in unconditioned medium. These resul ts suggested that the spreading activity derived from granulocytes is due i n part to PSP1. Pretreatment of plasmatocytes with trypsin had no effect on PSP1-induced aggregation but PSP1-induced aggregations were readily dissoc iated by trypsin. This suggested that PSP1 is not an adhesion factor but in duces adhesion by stimulating a change in the cell surface of plasmatocytes . Synthetic PSP1 also induced aggregation of plasmatocytes from other Lepid optera indicating that regulation of hemocyte activity by PSP1-related pept ides may be widespread. Arch. Insect Biochem, Physiol. 42:213-223, 1999. (C ) 1999 Wiley-Liss, Inc.