Grouper SPL promotes STING- and IRF3-mediated antiviral immune response against iridovirus infection

Sphingosine-1-phosphate (S1P) lyase (SPL) was an intracellular enzyme that catalyzes the degradation of the bioactive lipid S1P. Singapore grouper iridovirus (SGIV), a highly pathogenic large cytoplasmic dsDNA virus, had caused significant economic losses in aquaculture and threatened biodiversity. Though SPL had been reported to be crucial for several crucial cellular functions due to its roles in S1P metabolism, the roles of SPL in fish virus infection remained poorly understood. Herein, we found the positive regulatory role of grouper SPL (EcSPL) on the host's innate immune responses against SGIV infection. EcSPL encoded a 563 amino acid protein, containing one DOPA domain (140-502 aa). Quantitative real time PCR (qPCR) analysis showed that EcSPL was constitutively expressed in all examined tissues, and the expression of EcSPL was induced by SGIV infection in a time dependent manner. Subcellular localization revealed that EcSPL was distributed in the cytoplasm, and partly colocalized with the endoplasmic reticulum, mitochondria, and lysosomes. Overexpression of EcSPL significantly reduced the transcription and protein expression of the viral genes, as well as the severity of the cytopathic effects (CPEs) caused by SGIV, thereby suppressing SGIV replication. Meanwhile, EcSPL overexpression potently increased the promoter activity of interferon1 (IFN1), interferon3 (IFN3), and nuclear factor kappa-B (NF-κB), as well as the expression of pro-inflammatory cytokines and interferon related genes. Furthermore, EcSPL overexpression further augmented the promoter activity of IFN1, IFN3, and NF-κB triggered by STING (stimulator of interferon genes) and IRF3 (interferon regulatory factor 3), but not TBK1 (TANK-binding kinase 1). Mechanistically, EcSPL interacted with EcSTING and EcIRF3. Consistently, these key adaptors were found to be co-located with EcSPL. The study provided the first evidence that EcSPL as an antiviral host factor, restricted SGIV replication by directly enhancing EcSTING- or EcIRF3-mediated antiviral immune response. Our findings revealed a novel role for EcSPL in teleost immune defense against viral infection.