Serine protease mediates Ovomermis sinensis-inhibited host immune responses by inducing apoptosis: implications for successful parasitism and host mortality.

Abstract

Mermithid nematodes are entomopathogens that parasitize and kill insect hosts and are used for biological control. It is widely believed that mermithid nematodes kill their host upon nematode emergence, unlike other parasites that depend on virulence factors. In this study, we disproved this theory by demonstrating that the mermithid nematode Ovomermis sinensis mediates host mortality by serine protease-induced apoptosis. Successful parasitism of O. sinensis increased with the infection rate, and the inhibition of host immunity by O. sinensis increased with the parasitic load. A serine protease was identified from the host hemolymph. This protease belongs to the trypsin-like serine protease family, which is an apoptosis-inducing serine protease. Specifically, Os-sp was highly expressed only during the parasitic stage and could be induced by host hemocytes and the fat body. Importantly, host immune effectors (melanization, phenoloxidase activity, and encapsulation) were suppressed by the recombinant protein rOs-sp that induced apoptosis of hemocytes and fat body in a dose-dependent manner, which contributes to host death. Serine protease mediates O. sinensis-inhibited host immune responses by inducing apoptosis that is lethal to the insect host. Our findings have broader implications for understanding the mechanism of successful parasitism and killing of host by nematodes. © 2023 Society of Chemical Industry.