Plasmatocyte sensitivity to plasmatocyte spreading peptide (PSP) fluctuates with the larval molting cycle

Plasmatocyte spreading peptide (PSP) is a cytokine from the moth Pseudoplusia includens that activates a class of hemocytes called plasmatocytes to bind and spread on foreign surfaces. Previous structure-function studies on PSP used plasmatocytes collected from P. includens larvae that were in the late stages of the last (fifth) instar. Here, we report that plasmatocyte sensitivity to PSP varied significantly during the fourth and fifth instar. PSP weakly activated plasmatocytes early in the instar when hemolymph juvenile hormone (JH) titers were relatively high and ecdysteroid titers were low, but strongly activated plasmatocytes late in the instar after JH titers declined and ecdysteroid titers rose. In contrast, plasmatocytes did not vary in their response to plasma, which contains other factors besides PSP that affect plasmatocyte function. In vitro assays indicated that 20-hydroxyecdysone (20E) dose-dependently synergized PSP activity, whereas the JH analog methoprene antagonized PSP activity. Methoprene had no effect on adhesion and spreading of granular cells, but plasmatocytes from larvae topically treated with methoprene exhibited a reduction in sensitivity to PSP. Collectively, these results indicate that plasmatocyte sensitivity to PSP fluctuates in relation to the molting cycle, and that PSP activity is affected by juvenoids and ecdysone.