Plasmatocyte spreading peptide does not induce Microplitis demolitor polydnavirus-infected plasmatocytes to spread on foreign surfaces.

Abstract

Capsule formation by the moth Pseudopulsia includens requires that plasmatocytes change from being nonadhesive cells in circulation to strongly adhesive cells capable of attaching to the foreign target and one another. This change in adhesive state is induced by Plasmatocyte Spreading Peptide (PSP1); a 23 amino acid peptide isolated from P. includens plasma. Plasmatocytes from hosts parasitized by Microplitis demolitor remain in a nonadhesive state after infection by Microplitis demolitor polydnavirus (MdPDV). This alteration in plasmatocyte function prevents P. includens from encapsulating the developing parasitoid. In the current study, we examined whether MdPDV infection eliminates PSP1-responsive plasmatocytes from circulation or disrupts the ability of PSP1 to induce adhesion and spreading of plasmatocytes to foreign surfaces. In vivo experiments revealed that infection of P. includens by MdPDV induced an increase in the total number of hemocytes in circulation but reduced the proportion of hemocytes in circulation that were plasmatocytes. However, plasmatocytes normally capable of responding to PSP1 were not eliminated from circulation. Both in vivo and in vitro experiments indicated that plasmatocytes inoculated with MdPDV lost the capacity to respond to PSP1 4-6 h post-infection. Infection of P. includens with MdPDV reduced expression levels of prepro-PSP1 mRNA in hemocytes but did not appear to alter expression levels in fat body.