Alteration in FAD-glucose dehydrogenase activity and hemocyte behavior contribute to initial disruption of Manduca sexta immune response to Cotesia congregata parasitoids

Cotesia congregata and Manduca sexta were used as a model system to study the mechanism and effect of a polydnavirus (PDV). We hypothesized that (1) FAD-glucose dehydrogenase (GLD) (EC 1.1.9910) hemolymph titer would increase in response to parasitism, (2)hemocyte targeting behavior would be altered by parasitism, and (3) changes observed in GLD activity and hemocyte behavior immediately post-parasitization would be due to the presence of PDV. GLD specific activity was measured at four time points early during parasitism using a spectrophotometric enzyme assay, Hemocyte behavior was measured using direct observations of hemocyte response to a foreign target in vitro. Results demonstrate that GLD increases immediately post-oviposition and post-injection of purified PDV, indicating that virions elicit nonself recognition. This increase relative to unparasitized controls also occurs in response to trioxsalen-UV inactivated virus, indicating that the initial disruption of the host immune response is not dependent upon viral transcription. Further, we demonstrate that plasmatocytes are actively spreading and aggregating but are not targeting nonself material in both parasitized and polydnavirus treatments. These results indicate that purified PDV is recognized as nonself and is triggering an immediate cellular immune response prior to viral transcription.