ENCAPSULATION OF FOREIGN TARGETS BY HEMOCYTES OF THE MOTH PSEUDOPLUSIA-INCLUDENS (LEPIDOPTERA, NOCTUIDAE) INVOLVES AN RGD-DEPENDENT CELL-ADHESION MECHANISM

We tested the hypothesis that an insect immune response, encapsulation , involves an RGD-dependent cell adhesion mechanism by examining the e ffects of the tetrapeptide RGDS on hemocyte spreading and encapsulatio n, Soluble RGDS at concentrations of 0.5-2 mM inhibited the spreading of the primary encapsulating hemocyte, the plasmatocyte, on the surfac e of plastic tissue culture plates. At concentrations of 5-10 mM, the spreading of granular cells was also inhibited, RGES did not inhibit p lasmatocyte or granular cell spreading, indicating that the effect was specific for RGDS. RGDS-Sepharose beads were encapsulated by hemocyte s in vitro, whereas RGES-Sepharose beads were not. Furthermore, solubl e RGDS, but not RGES, inhibited in vitro encapsulation of RGDS-Sepharo se. When injected into the hemocoel of P. includens larvae, RGDS-Sepha rose was encapsulated in 3 h, whereas RGES-Sepharose was not encapsula ted until 24 h. The only hemocyte morphotype to encapsulate RGDS-Sepha rose was the plasmatocyte. Lastly, RGDS, but not RGES, inhibited plasm atocyte spreading in response to cell-free plasma, These results indic ate that the molecular basis of cell adhesion mediating hemocyte sprea ding and encapsulation in insects involves cell adhesion molecules con taining the RGD recognition sequence.