An aegerolysin-like protein from Heliothis virescens ascovirus 3h (HvAV-3h) shows immune suppression and antibacterial activity

Aegerolysins are lipid-binding proteins associated with multiple functions, including membrane pore-formation, insecticidal toxicity and defence against predators. Whilst distributed over the kingdoms of the Tree of Life, ascoviruses are the only representative viruses that encode an aegerolysin-like protein. Ascoviruses are entomopathogenic and possess a large dsDNA genome. The present study aimed to functionally characterize the aegerolysin-like protein of Heliothis virescens ascovirus 3h (HvAV-3h), encoded by ORF85, and to explore its potential roles in the interaction between the ascovirus and its host. Our results demonstrate the importance of this species-specific protein to HvAV-3h replication in host cells. <i>In vivo</i>, silencing of this gene for 12-72 h significantly increased the expression of some innate immunity-associated genes, including Toll (114-fold), IMD (44.7-fold) and Hopscotch (22.9-fold). In parallel, we detected significant gradual increases in MyD88 and Relish and decreases in PIAS. Moreover, histopathological analyses of infected larval tissues indicated reduced tissue damage after 72 h of ORF85 gene silencing. The prokaryotic expression of the HvAV-3h aegerolysin, followed by feeding to third-instar <i>Spodoptera exigua</i> larvae for 24 or 48 h led to significant reductions in larval weight. Moreover, the <i>in vitro</i> treatment demonstrated a bactericidal action against <i>Lysinibacillus xylanilyticus</i>, a bacterial resident of some insect guts. Overall, our findings suggest that the protein encoded by ORF85 is associated with the pathogenicity of HvAV-3h and its ability to replicate in host cells. Additionally, aegerolysin may inhibit or kill specific bacterial species in the host microbiome during infection, potentially modulating the host immune response.