Identification and expression analysis of the sting gene, a sensor of viral DNA, in common carp Cyprinus carpio

Stimulator of interferon gene (sting) was identified and characterized from common carp Cyprinus carpio. The sting messenger (m)RNA encoded a polypeptide of 402 amino acids with a calculated molecular mass of 46·184 kDa and an isoelectronic point of 6·08. The deduced protein of sting contained a signal peptide, three transmembrane motifs in the N-terminal region and four putative motifs (RXR) found in resident endoplasmic reticulum proteins. mRNA expression of sting was present in twelve investigated tissues, and was up-regulated by koi herpesvirus (KHV) in vivo and in vitro. The transcription of sting was altered by poly(I:C) and poly(dT:dA) stimulation in vitro. The findings suggested that sting is an inducible gene involved in innate immunity against DNA- and RNA-derived pathogens. To investigate defence mechanisms in C. carpio development, sting level in embryos, larvae and juvenile fish was monitored following KHV challenge. The sting message was negligible in embryos prior to hatching, but observed at higher transcriptional levels throughout larval and juvenile stages. Investigation showed the mRNA expression profiles of genes encoding for proteins promoting various functions in the interferon pathway, from pattern recognition receptors to antiviral genes, to be significantly induced in all examined organs by in vivo infection with KHV. Following KHV infection, the ifn message was significantly downregulated in spleen, head kidney, brain and hepatopancreas but notably up-regulated in gill, intestine and skin, suggesting that ifn induction might be related to the mucosal immune system and virus anti-ifn mechanisms. These results provided the basis for further research into the role and mechanisms of sting in fishes.