Mannose receptor of Epinephelus coioides exerts antiviral activity against red-spotted grouper nervous necrosis virus and regulates apoptosis and inflammation

Mannose receptor (MR), a pattern recognition receptor, plays a critical role in innate immune responses by binding to pathogen-associated molecular patterns. The antiviral role of MR has been described extensively in mammals but not well characterized in teleosts. In this study, we investigated the role of Epinephelus coioides MR in red spotted grouper nervous necrosis virus (RGNNV) infection by cloning and characterizing EcMR. The sequence analysis showed that EcMR contained a signal peptide, ricin-like beta-type clover domain (RICIN), fibronectin type II domain (FN II), eight tandem C-type lectin-like domains (CLECTs), and a transmembrane domain. The function of EcMR was analyzed using constructs expressing three segments of the full-length protein including EcMR1 (RICIN-FN II), EcMR2 (CLECT 1-3), and EcMR3 (CLECT 4-8). Overexpression of EcMR1, EcMR2, and EcMR3 significantly reduced RGNNV coat protein expression at both mRNA and protein levels. Conversely, RGNNV replication was increased by EcMR knockdown. EcMR was found to inhibit apoptosis signaling and inflammation in GF-1 grouper fin cells, as determined by immunofluorescence analysis and flow cytometry. The increases in apoptosis-related gene expression (FADD, Fas, Bax, and p53) and apoptosis-related enzyme activities (caspase-3, -8, and - 9) in cells overexpressing EcMR1, EcMR2, and EcMR3 and the contrary effect observed upon EcMR knockdown indicated that EcMR blocks apoptosis induced by RGNNV. EcMR also exhibited an anti-inflammatory function through suppression of proinflammatory cytokine production. These results demonstrate that EcMR contributes to the response of E. coioides to virus infection by modulating virus-induced apoptosis and inflammation, which may have important implications for commercial aquaculture practices.