Induction of hemocyte apoptosis by Ovomermis sinensis: Implications for host immune suppression

The entomopathogenic nematode, Ovomermis sinensis, is a parasite of some common lepidopteran pests. O. sinensis is able to overcome the immune system of its hosts and eventually kill the hosts when it emerges. We provide insight into how the mermithid nematode overcomes the immune response of the host Helicoverpa armigera. Our results indicate that O. sinensis actively inhibits host immune responses as evidenced by hemocyte nodulation, spreading behavior, lysozyme activity and melanization. However, O. sinensis did not inhibit host immune responses through immune gene activation. Moreover, the research on the immune depressive strategies of O. sinensis revealed that the parasite did not inhibit host effector molecules, but did reduce the number of hemocytes. Flow cytometry analysis revealed that the host hemocytes were apoptotic within 24 h, and no hemocytes were present after 72 h. In addition, our in vivo and in vitro studies showed that the survival rate of O. sinensis was increased when hemocyte proliferation was inhibited. Our findings suggest that O. sinensis inhibited host immune responses by inducing apoptosis of host hemocytes.