IFN-γ in turtle: Conservation in sequence and signalling and role in inhibiting iridovirus replication in Chinese soft-shelled turtle Pelodiscus sinensis

The IFN-γ gene was identified in a turtle, the Chinese soft-shelled turtle, Pelodiscus sinensis, with its genome consisting of 4 exons and 3 introns. The deduced amino acid sequence of this gene contains a signal peptide, an IFN-γ family signature motif 130IQRKAVNELFPT, an NLS motif 155KRKR and three potential N-glycosylation sites. As revealed by real-time quantitative PCR, the gene was constitutively expressed in all tested organs/tissues, with higher level observed in blood, intestine and thymus. An induced expression of IFN-γ at mRNA level was observed in peripheral blood leucocytes (PBLs) in response to in vitro stimulation of LPS and PolyI:C. The overexpression of IFN-γ in the Chinese soft-shelled turtle artery (STA) cell line resulted in the increase in the expression of transcriptional regulators, such as IRF1, IRF7 and STAT1, and antiviral genes, such as Mx, PKR, implying possibly the existence of a conserved signalling network and role for IFN-γ in the turtle. Furthermore, the infection of soft-shelled turtle iridovirus (STIV) in the cell line transfected with IFN-γ may cause the cell death as demonstrated with the elevated lactate dehydrogenase (LDH) level and cell mortality. However, the mechanism involved in the antiviral activity may require further investigation.