Expression and Hemocyte-Targeting of a Campoletis sonorensis Polydnavirus Cysteine-Rich Gene in Heliothis virescens Larvae

The polydnavirus associated with the parasitic wasp Campoletis sonorensis is injected into the lepidopteran insect, Heliothis virescens, during parasitization, after which viral gene products suppress the cellular immune system of the hosts. Four related cysteine-rich polydnavirus genes have been identified in parasitized H. virescens larvae and grouped into a family. In this study, we investigated the expression and hemocyte targeting of the cysteine-rich VHv1 .4 protein. Full-length and truncated VHv1 .4 proteins were produced in a bacterial expression system, and the purified proteins were used to raise polyclonal antisera. In immunoblots the VHv1 .4 protein was detected in parasitized insects as early as 6 h and throughout the entire course of parasitism. The VHv1 .4 protein appeared predominantly in the plasma fraction of hemolymph from parasitized larvae, suggesting that this protein is secreted. The VHv1 .4 protein expressed from a recombinant baculovirus was secreted in two lepidopteran cell lines and in larvae injected with the recombinant virus. Digestion with endoglycosidases suggests that the VHv1 .4 protein is glycosylated at multiple N-glycosylation sites, Immunofluorescence assays showed that the VHv1 .4 protein binds to the hemocytes, most notably the granulocytes, in H. virescens larvae. After binding, the VHv1.4 protein was internalized, probably by endocytosis. Specific binding of the VHv1 .4 to granulocytes implies an important function in the suppression of host cellular encapsulation response.