Identification and Functional Characterization of Toxoneuron nigriceps Ovarian Proteins Involved in the Early Suppression of Host Immune Response

SIMPLE SUMMARY: The endoparasitoid of the tobacco budworm Heliothis virescens, Toxoneuron nigriceps, has several strategies to survive, including venom and calyx fluid. This latter contains a Polydnavirus and Ovarian Proteins (OPs). They are injected into the host body together with the egg. Although much research has focused on venom protein components, little is known about OPs. OPs can disrupt the cellular immune response of the host, acting on host hemocytes, the immune cells. In this study we investigated the action of HPLC fractions derived from OPs. Two fractions caused multiple and significant changes in hemocytes, including cellular oxidative stress and actin cytoskeleton disruption, which might explain the high incidence of hemocyte death and loss of function. Furthermore, using a transcriptome and proteomic approach, we identify the proteins of the two fractions that may be involved in the observed host hemocyte alterations. Our results will help to better understand the OP components and their involvement in parasitization strategies. ABSTRACT: The endophagous parasitoid Toxoneuron nigriceps (Viereck) (Hymenoptera, Braconidae) of the larval stages of the tobacco budworm Heliothis virescens (Fabricius) (Lepidoptera, Noctuidae) injects the egg, the venom, the calyx fluid, which includes a Polydnavirus (T. nigriceps BracoVirus: TnBV) and the Ovarian Proteins (OPs) into the host body during oviposition. The host metabolism and immune system are disrupted prematurely shortly after parasitization by the combined action of the TnBV, venom, and OPs. OPs are involved in the early suppression of host immune response, before TnBV infects and expresses its genes in the host tissues. In this work, we evaluated the effect of HPLC fractions deriving from in toto OPs. Two fractions caused a reduction in hemocyte viability and were subsequently tested to detect changes in hemocyte morphology and functionality. The two fractions provoked severe oxidative stress and actin cytoskeleton disruption, which might explain the high rate of hemocyte mortality, loss of hemocyte functioning, and hence the host’s reduced hemocyte encapsulation ability. Moreover, through a transcriptome and proteomic approach we identify the proteins of the two fractions: eight proteins were identified that might be involved in the observed host hemocyte changes. Our findings will contribute to a better understanding of the secreted ovarian components and their role in parasitoid wasp strategy for evading host immune responses.