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

Abstract

An endoparasitoid, Cotesia plutellae (Hymenoptera: Braconidae), possesses a mutualistic bracovirus (CpBV), which plays significant roles in the parasitized host, Plutella xylostella (Lepidoptera: Plutellidae). CpBV15 β, 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 β as an immunosuppressive agent. The effect of CpBV15 β 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 β 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 β protein, spreading behavior was impaired in a dose-dependent manner at low micro-molar range. This inhibitory effect of CpBV15 β could also be demonstrated on hemocytes of a non-natural host, Spodoptera exigua. CpBV15 β 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 β or cycloheximide, their ability to release protein molecules was markedly inhibited. This study suggests that CpBV15 β suppresses hemocyte behavior by inhibiting protein translation.