Immunoevasive property of a polydnaviral product, CpBV-lectin, protects the parasitoid egg from hemocytic encapsulation of Plutella xylostella (Lepidoptera: Yponomeutidae)

Immunosuppression is the main pathological symptom of the diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae), parasitized by an endoparasitoid wasp, Cotesia plutellae (vestalis, Hymenoptera: Braconidae). C. plutellae bracovirus (CpBV), which is a symbiotic virus of C. plutellae, has been known to be the main parasitic factor in the host–parasitoid interaction. CpBV-lectin, encoded in the viral genome and expressed in P. xylostella during early parasitization stage, was suspected to play a role in immunoevasion of defense response. Here we expressed CpBV-lectin in Sf9 cells using a recombinant baculovirus for subsequent functional assays. The recombinant CpBV-lectin exhibited hemagglutination against vertebrate erythrocytes. Its hemagglutinating activity increased with calcium, but inhibited by adding EDTA, indicating its C-type lectin property. CpBV-lectin showed specific carbohydrate-binding affinity against N-acetyl glucosamine and N-acetyl neuraminic acid. The role of this CpBV-lectin in immunosuppression was analyzed by exposing hemocytes of nonparasitized P. xylostella to rat erythrocytes or FITC-labeled bacteria pretreated with recombinant CpBV-lectin, which resulted in significant reduction in adhesion or phagocytosis, respectively. The immunosuppressive activity of CpBV-lectin was further analyzed under in vitro encapsulation response of hemocytes against parasitoid eggs collected at 1- or 24-h post-parasitization. Hemocytic encapsulation was observed against 1-h eggs but not against 24-h eggs. When the 1-h eggs were pretreated with the recombinant CpBV-lectin, encapsulation response was completely inhibited, where CpBV-lectin bound to the parasitoid eggs, but not to hemocytes. These results suggest that CpBV-lectin interferes with hemocyte recognition by masking hemocyte-binding sites on the parasitoid eggs.