Transcriptomic Analysis Insight into the Immune Modulation during the Interaction of Ophiocordyceps sinensis and Hepialus xiaojinensis

Simple Summary Ophiocordyceps sinensis parasitizes the ghost moth larva (Thitarodes spp.) and produces a fruit-body, which is a well-known Traditional Chinese Medicine (TCM). The infection process is time-consuming and inefficient, restricting the large-scale cultivation of this fungus. This study aimed to reveal the biology of immune modulation during the interaction between O. sinensis and its host (Hepialus xiaojinensis) using high-throughput RNA sequencing. In H. xiaojinensis, 345 immune-related gene candidates were identified and were further classified according to the functions of pathogen recognition, signal transduction and immune response. According to the results of the analysis, the putative pathways of immune modulation in H. xiaojinensis in response to O. sinensis infection were sketched. Meanwhile, gene families probably involved in O. sinensis pathogenicity were identified, mainly including serine carboxypeptidase, peroxidase, metalloprotease peptidase, aminopeptidases, cytochrome P450, and oxidoreductase. Our findings provided an insight into the biology of fungi–insect host interaction, paving the way for large-scale artificial cultivation of the valued fungus as well as biological pest control. Abstract Ophiocordyceps sinensis (Berk.) is an entomopathogenic fungus that can infect the larva of the ghost moth, Hepialus xiaojinensis, causing mummification after more than one year. This prolonged infection provides a valuable model for studying the immunological interplay between an insect host and a pathogenic fungus. A comparative transcriptome analysis of pre-infection (L) and one-year post-infection (IL) larvae was performed to investigate the immune response in the host. Here, a total of 59,668 unigenes were obtained using Illumina Sequencing in IL and L. Among the 345 identified immune-related genes, 83 out of 86 immune-related differentially expressed genes (DEGs) had a much higher expression in IL than in L. Furthermore, the immune-related DEGs were classified as pathogen recognition receptors (PRRs), signal modulators or transductors, and immune effector molecules. Serpins and protease inhibitors were found to be upregulated in the late phase of infection, suppressing the host’s immune response. Based on the above analysis, the expression levels of most immune-related genes would return to the baseline with the immune response being repressed in the late phase of infection, leading to the fungal immunological tolerance after prolonged infection. Meanwhile, the transcriptomes of IL and the mummified larva (ML) were compared to explore O. sinensis invasion. A total of 1408 novel genes were identified, with 162 of them annotated with putative functions. The gene families likely implicated in O. sinensis pathogenicity have been identified, primarily including serine carboxypeptidase, peroxidase, metalloprotease peptidase, aminopeptidases, cytochrome P450, and oxidoreductase. Furthermore, quantitative real-time PCR (qPCR) was used to assess the expression levels of some critical genes that were involved in immune response and fungal pathogenicity. The results showed that their expression levels were consistent with the transcriptomes. Taken together, our findings offered a comprehensive and precise transcriptome study to understand the immune defense in H. xiaojinensis and O. sinensis invasion, which would accelerate the large-scale artificial cultivation of this medicinal fungus.