Single-Nucleus Sequencing of Fat Body Reveals Distinct Metabolic and Immune Response Landscapes in Silkworm Larvae after Bombyx mori Nucleopolyhedrovirus Infection

The fat body plays a central role in the regulation of the life cycle of insects and acts as the major site for detoxification, nutrient storage, energy metabolism, and innate immunity. However, the diversity of cell types in the fat body, as well as how these cell subsets respond to virus infection, remains largely unknown. We used single-nucleus RNA sequencing to identify 23 distinct clusters representing adipocyte, hemocyte, epithelial cell, muscle cell, and glial cell types in the fat body of silkworm larvae. Further, by analysis of viral transcriptomes in each cell subset, we reveal that all fat body cells could be infected by Bombyx mori nucleopolyhedrovirus (BmNPV) at 72 h postinfection, and that the majority of infected cells carried at least a medium viral load, whereas most cells infected by BmNPV at 24 h postinfection had only low levels of infection. Finally, we characterize the responses occurring in the fat body cell clusters on BmNPV infection, which, on one hand, mainly reduce their metabolic functions, involving energy, carbohydrates, lipids, and amino acids, but, on the other hand, initiate a strong antiviral response. Our single-nucleus RNA sequencing analysis reveals the diversity of insect fat body cells and provides a resource of gene expression profiles for a systems-level understanding of their response to virus infection.