MICROPLITIS-DEMOLITOR POLYDNAVIRUS INDUCES APOPTOSIS OF A SPECIFIC HEMOCYTE MORPHOTYPE IN PSEUDOPLUSIA-INCLUDENS

Microplitis demolitor polydnavirus (MdPDV) is associated with Micropli tis demolitor, a parasitic wasp that attacks the larval stage of the l epidopteran Pseudoplusia includens. Previously, we observed that MdPDV induced several alterations in the granular cells and plasmato-cytes of P. includens, the primary haemocytes involved in regulating the cel lular immune response toward M. demolitor and other parasites. In exam ining the mechanisms underlying immunosuppression of this host, we fou nd that MdPDV induced apoptosis of granular cells. Granular cells unde rwent apoptosis both when virus was injected into the haemocoel of P. includens larvae and after infection with MdPDV in vitro. Characterist ics of MdPDV-induced apoptosis included condensation of chromatin, cel l surface blebbing and fragmentation of DNA into a 200 bp ladder. Alth ough MdPDV induced changes in the ability of plasmatocytes to adhere t o foreign surfaces, apoptosis of this morphotype was not observed. Exa mples from the literature suggest that some viruses promote their own survival by suppressing apoptosis of host cells. However, since polydn aviruses are likely to be transmitted vertically, we suggest that MdPD V promotes its own survival by inducing apoptosis of host immune cells which would otherwise kill the developing M. demolitor egg.