| Summary: | Polydnavirus (PDV) is a parasitic factor of endoparasitic wasps and contributes greatly to overcoming the immune response of parasitized hosts. Protein tyrosine phosphatases (PTPs) regulate a wide variety of biological processes at the post-transcriptional level in mammals, but knowledge of PDV PTP action during a parasitoid–host interaction is limited. In this study, we characterized a PTP gene, <i>CvBV_12-6</i>, derived from <i>Cotesia vestalis</i> bracovirus (CvBV), and explored its possible regulatory role in the immune response of the host <i>Plutella xylostella</i>. Our results from qPCR show that <i>CvBV_12-6</i> was highly expressed in hemocytes at an early stage of parasitization. To explore <i>CvBV_12-6</i> function, we specifically expressed <i>CvBV_12-6</i> in <i>Drosophila melanogaster</i> hemocytes. The results show that Hml-Gal4 > <i>CvBV_12-6</i> suppressed the phenoloxidase activity of hemolymph in <i>D. melanogaster</i>, but exerted no effect on the total count or the viability of the hemocytes. In addition, the Hml-Gal4 > <i>CvBV_12-6</i> flies exhibited decreased antibacterial abilities against <i>Staphylococcus aureus.</i> Similarly, we found that <i>CvBV_12-6</i> significantly suppressed the melanization of the host <i>P. xylostella</i> 24 h post parasitization and reduced the viability, but not the number, of hemocytes. In conclusion, <i>CvBV_12-6</i> negatively regulated both cellular and humoral immunity in <i>P. xylostella</i>, and the related molecular mechanism may be universal to insects.
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