Distinct Responses of Thitarodes xiaojinensis β-1,3-Glucan Recognition Protein-1 and Immulectin-8 to Ophiocordyceps sinensis and Cordyceps militaris Infection

Key Points Recognition of fungi by T. xiaojinensis βGRP1 is vital for proPO activation. A surface layer prevents O. sinensis from βGRP1 detection. IML8 binds to βGRP1 in the presence of C. militaris and promotes encapsulation. Melanization and encapsulation are prominent defense responses against microbes detected by pattern recognition receptors of their host insects. In the ghost moth Thitarodes xiaojinensis, an activated immune system can melanize and encapsulate the fungus Cordyceps militaris. However, these responses were hardly detected in the host hemolymph postinfection of another fungus Ophiocordyceps sinensis. The immune interaction between O. sinensis and the host remains largely unknown, which hinders the artificial cultivation of Chinese cordyceps. We found that T. xiaojinensis β-1,3-glucan recognition protein-1 (βGRP1) was needed for prophenoloxidase activation induced by C. militaris. Failure of βGRP1 to recognize O. sinensis is a primary reason for the lack of melanization in the infected host. Lyticase or snailase treatment combined with binding and immunofluorescence detection showed the existence of a protective layer preventing the fungus from βGRP1 recognition. Coimmunoprecipitation and mass spectrometry analysis indicated that βGRP1 interacted with immulectin-8 (IML8) via binding to C. militaris. IML8 promotes encapsulation. This study suggests the roles of T. xiaojinensis βGRP1 and IML8 in modulating immune responses against C. militaris. Most importantly, the data indicate that O. sinensis may evade melanization by preventing βGRP1 recognition.