Steinernema glaseriSurface Coat Protein Suppresses the Immune Response ofPopillia japonica(Coleoptera: Scarabaeidae) Larvae

The host immune response is a key obstacle to entomopathogenic nematodes in making the transition from the free-living state to parasitism. The entomopathogenic nematode Steinernema glaseri has evolved mechanisms to evade immune encapsulation in larvae of the Japanese beetle, Popillia japonica. Host intrahemocoelic injection tests show that live axenic nematodes of S. glaseri not only avoid host melanotic encapsulation but also protect dead nematodes injected after the live ones. This result indicates that the nematodes release anti-immune factor(s). We extracted the nematode surface coat proteins and found that at least one protein (SCP3a) from the S. glaseri surface coat can suppress the host immune system. This suppression protects unrelated nematode species from encapsulation and latex beads from phagocytosis. We conclude that S. glaseri uses an anti-immune protein to defeat the host immune system, thereby protecting itself from encapsulation. Presumably its symbiotic bacteria are similarly protected from phagocytosis.