: Toxoneuron nigriceps (Viereck) (Hymenoptera, Braconidae) is an endophagous parasitoid of the larval stages of Heliothis virescens (Fabricius) (Lepidoptera, Noctuidae). During oviposition, T. nigriceps injects into the host body the egg, the venom, the calyx fluid, which contains a Polydnavirus (T. nigriceps BracoVirus: TnBV), and the Ovarian Proteins (OPs). OPs, originating from the ovarian calyx cells, are involved in the induction of precocious symptoms in the host immune system alteration. To functionally characterize the OPs, we carried out two approaches. Firstly, incubating hemocytes with OPs in toto, detecting several alterations on host cells: the OP injection induced an extensive oxidative stress and a disorganization of actin cytoskeleton. These alterations can explain the loss of haemocyte functionality (the reduction in encapsulation ability by the host) and the high-level of haemocyte mortality. Then, we evaluated the effect of HPLC fractions deriving from in toto OPs. Among the 28 analyzed fractions, two fractions caused a reduction in hemocyte viability and were tested to detect changes in hemocyte morphology and functionality. In accordance with previous results, obtained with in toto OPs, the two fractions caused severe oxidative stress, actin cytoskeleton disruption, loss of hemocyte encapsulation ability and high mortality rate. Moreover, a transcriptome and proteomic approach was applied to identify the proteins of the two fractions: eight proteins that might be involved in the observed host hemocyte changes were detected. Our findings will contribute to a better understanding of the ovarian components and their role in parasitic wasp strategy to escape the host immune responses
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