An apolipophorin secreted by endoparasitoid larvae of Microplitis mediator regulates immune responses of the host Mythimna separata

Endoparasitoid wasps lay eggs into their host insect hemocoel and the eggs and larvae face immune challenges from the hosts. While maternal factors such as venom and polydnaviruses are known to modulate host immunity, the larval-derived factors are largely unknown. Two proteins secreted by Microplitis mediator larvae were previously identified: an arylphorin (MmAry) capable to suppress the host immunity and an apolipophorin-II/I (MmApo) whose functions remain to be elucidated. MmApo is a high molecular weight protein consisting of MmApo II (80.4 kDa) and MmApo I (309.5 kDa) which contains distinct functional domains in each. After injection of recombinant MmApoLPD, which represents a 59.5 kDa fraction of MmApo II, in the hemolymph of Mythimna separata larvae, the activity of phenoloxidase (PO) and the activation of prophenoloxidase (PPO) were suppressed. Additionally, injection of recombinant MmApoLPD downregulated the expression of antimicrobial peptide (AMP) genes (cecropin, gloverin, and lysozyme). The injection of recombinant MmApoVWD, which represents the von Willebrand factor type D domain (16.5 kDa) of MmApo I, diminished hemocyte-mediated phagocytosis and encapsulation. Further, pull-down experiments revealed a direct interaction between MmApo and the host plasma protein Inter-α-trypsin inhibitor heavy chain H4-like (MsITIH4). Neutralization of MmApo by the injection of its antibody reduced pupation rate of the parasitoids, while supplementation by the co-injection of recombinant MmApoLPD and MmApoVWD enhanced the rate of parasitoids pupation. Collectively, this study found that MmApo plays a role as a parasitic effector along with MmAry to modulate the host immune responses to protect the parasitoid larvae for successful development.