A putative protein translation inhibitory factor encoded by Cotesia plutellae bracovirus suppresses host hernocyte-spreading behavior

An endoparasitoid, Cotesia plutellae (Hymenoptera: Braconidae), possesses a mutualistic bracovirus (CpBV), which plays significant roles in the parasitized host, Plutella xylostella (Lepidoptera: Plutellidae). CpBV15 beta, a viral gene encoded by CpBV, is expressed at early and late parasitization periods, suggesting that it functions to manipulate the physiology of the parasitized host. This paper reports a physiological function of CpBV15 beta as an immunosuppressive agent. The effect of CpBV15 beta on cellular immunity was analyzed by assessing hemocyte-spreading behavior. Parasitization by C plutellae caused altered behavior of hemocytes of P. xylostella, in which the hemocytes were not able to attach and spread on glass slides. CpBV15 beta was expressed in Sf9 cells using a baculovirus expression system and purified from the culture media. When hemocytes of nonparasitized P. xylostella were incubated with purified CpBV15 beta protein, spreading behavior was impaired in a dose-dependent manner at low micro-molar range. This inhibitory effect of CpBV15 beta could also be demonstrated on hemocytes of a non-natural host, Spodoptera exigua. CpBV15 beta protein significantly inhibited F-actin growth of hemocytes in response to an insect cytokine. Similarly, cycloheximide, a eukaryotic translation inhibitor, strongly inhibited the spreading behavior and F-actin growth of P. xylostella hemocytes. Under in vitro condition, hemocytes of nonparasitized P. xylostella released proteins into the surrounding medium. Upon incubation of hemocytes with either CpBV15 beta or cycloheximide, their ability to release protein molecules was markedly inhibited. This study suggests that CpBV15 beta suppresses hemocyte behavior by inhibiting protein translation. (c) 2007 Elsevier Ltd. All rights reserved.