Apolipophorin-III inhibits BmNPV replication by reprogramming sphingolipid metabolism to accumulate ceramide

The silkworm industry faces a significant threat from Bombyx mori nucleopolyhedrovirus (BmNPV). While Apolipophorin-III (ApoLp-III) is known for its roles in lipid transport and antibacterial immunity, its function in antiviral defense and the underlying mechanisms remain poorly understood. In this study, we investigated the impact of ApoLp-III on BmNPV proliferation and explored the associated mechanism. We found that BmNPV infection significantly induced ApoLp-III expression in a tissue- and time-specific manner. Knockdown of ApoLp-III in vitro enhanced BmNPV replication, whereas its overexpression suppressed viral replication and induced G1 cell cycle arrest. Lipidomics analysis revealed that ApoLp-III overexpression triggered significant sphingolipid metabolic reprogramming, resulting in the specific accumulation of ceramide species. Furthermore, exogenous C6-ceramide treatment was found to inhibit both BmNPV proliferation and the transcription of some key genes in mTORC1 pathway. Mechanistically, this inhibition was linked to the downregulation of RPTOR, a critical component of the mTORC1 complex. Consequently, key mTORC1 effectors including c-Myc, S6K1, and PCK2 were transcriptionally downregulated, leading to G1 cell cycle arrest. Notably, pharmacological inhibition of de novo ceramide synthesis with myriocin significantly attenuated both the suppression of these key genes and the antiviral effect mediated by ApoLp-III. Our findings reveal a novel immunometabolic pathway in which ApoLp-III exerts its antiviral function by promoting ceramide accumulation, which likely inhibits mTORC1 signaling, leading to G1 cell cycle arrest and the subsequent suppression of BmNPV replication. This study identifies ApoLp-III as a key nexus linking lipid metabolism to antiviral immunity in silkworms, providing new insights for developing strategies against viral infections.