Regulation of immune responses by a tumor necrosis factor in pearl oysters: insights from PmTNF 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 Pinctada fucata martensii, which is named PmTNF. 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 PmTNF mRNA expression via real-time PCR reveals its constitutive expression in all the examined tissues, with the highest expression in the gills. Furthermore, PmTNF 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 PmTNF mRNA level in the gills at 12 h postoperation. Knocking down PmTNF through RNA interference significantly inhibits the expressions of immune-related genes in the NF-κB signaling pathway in the gills by 24 h ( P < 0.05). The function of PmTNF is further characterized by studying the activity of an engineered recombinant PmTNF protein (rPmTNF) in vivo. 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 PmTNF participates in pearl oyster immunity by modulating the NF-κB pathway and activating the antioxidant defense system.