Regulation of immune responses by a tumor necrosis factor in pearl oysters: insights from <italic>PmTNF</italic> gene expression and function

Tumor necrosis factor (TNF) is a multifunctional cytokine that regulates cellular processes such as inflammation, apoptosis, differentiation, and proliferation and activates various functions of the immune system. This article reports the discovery and characterization of a novel tumor necrosis factor gene in the pearl oyster <i>Pinctada fucata martensii</i>, which is named <i>PmTNF</i>. The deduced PmTNF protein sequence displays the typical structural characteristics of a TNF domain, and phylogenetic analysis of the sequences of PmTNF and its putative orthologs shows that they conform to the current taxonomy. Analysis of <i>PmTNF</i> mRNA expression via real-time PCR reveals its constitutive expression in all the examined tissues, with the highest expression in the gills. Furthermore, <i>PmTNF</i> expression in the gills varies upon exposure to pathogen-derived stimuli, with modest upregulation in response to lipopolysaccharides, but with significant downregulation in response to polyinosinic:polycytidylic acid. Nucleus insertion surgery induces an increase in <i>PmTNF</i> mRNA level in the gills at 12 h postoperation. Knocking down <i>PmTNF</i> through RNA interference significantly inhibits the expressions of immune-related genes in the NF-κB signaling pathway in the gills by 24 h ( <i>P</i> < 0.05). The function of <i>PmTNF</i> is further characterized by studying the activity of an engineered recombinant PmTNF protein (rPmTNF) <i>in vivo</i>. Upon nuclear insertion, treatment with rPmTNF for 6 h upregulates several genes in the NF-κB pathway. Similarly, rPmTNF increases the activities of the antioxidant enzymes, including superoxide dismutase, glutathione and peroxidase, which reflect the total antioxidant capacity. Collectively, these results indicate that <i>PmTNF</i> participates in pearl oyster immunity by modulating the NF-κB pathway and activating the antioxidant defense system.