Insects rely on their immune defense system for protection against foreign substances invading the host hemocoel. The humoral and cellular immune systems have a well-evolved relationship. The cellular immune response, encompassing processes such as encapsulation and nodule formation, is associated with local melanin formation on the surface of the foreign substance. Hemocytes identified as the hyper-spread cells (HSCs) of Mythimna separata larvae deposit melanin around their peripheral edge. Some granulocytes carrying pro-phenoloxydase settle near to the HSCs and may collaborate with them in melanin formation. The HSC population serves as a signal marker on the foreign substance to other hemocytes involved in encapsulation. In the present study, we examined the proteins associated with inhibiting the adhesion and extension of HSCs on foreign substances, focusing on the elimination of HSCs from the parasitized host. The Meteorus pulchricornis (Wesmael) (Hymenoptera, Braconidae) wasp produces virus-like particles (VLPs) consisting of proteinaceous substances, and injects them with its egg into the host hemocoel. The VLPs regulate the host cellular immune system to facilitate the development of the parasitoid eggs and larvae. We found that Mp18 and Mp19 proteins (mainly Mp19) contained in the M. pulchricornis virus-like particle (MpVLP) led to the inhibition of actin formation and filopodia extension in adhesive hemocytes such as granulocytes and plasmatocytes. Double knockdown by injection of dsRNA from Mp18 and Mp19 genes into the abdomen of female wasps rescued hemocyte adhesion. Furthermore, the same protein identified with anti-Mp19 antibody was released from teratocytes at a later developmental stage. In addition, a parasitoid larva in the late stages of development expressed the same Mp19 gene, whereas smaller amount of Mp18 mRNA was observed. This result indicates that during the larval stage, the teratocytes and larvae express the Mp19 gene without temporal overlap; the protein makes the HSC and the related hemocytes inactive. This is supported by the existence of small particles around teratocytes and host tissues stained with anti-Mp19 antibody from 7 to 8 d after parasitization.
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