IRF10, which has been reported to play important roles in immune responses to both viral and bacterial infections in teleosts, can inhibit the activation of IFN promoters and negatively regulate fish antiviral gene expression to prevent an excessive immune response [10, 11, 32]. In the present study, a non-mammalian IRF10 gene was cloned from common carp, and the antiviral and antibacterial immune functions of CcIRF10 were investigated. The predicted CcIRF10 protein contains two conserved functional domains, the N-terminal DBD and the C-terminal IAD, suggesting that the functions of IRF10 may be conserved throughout vertebrates. The DBD contains a highly conserved five-tryptophan repeat that can bind to IFN-stimulating response elements (ISREs) and IRF-regulatory elements (IRF-Es) in target promoters [4, 33]. Notably, the five well-conserved tryptophan residues in D. rerio, C. idella, G. gallus, and C. carpio are all at positions 10, 25, 37, 57 and 76 in the N-terminus. The IAD, which is responsible for homo-/heterodimer interactions of the IRFs and association with other transcription factors, is less conserved than the DBD [5]. Phylogenetic analysis of predicted IRF10 protein sequences of C. carpio and other vertebrate species supported the division of IRF10 into two branches: teleost and bird. These results match the established evolutionary relationships among teleost and other vertebrate species and support the authenticity of the nomenclature for IRF10.
IRF10 was first identified in G. gallus and is highly expressed in white blood cells and splenic lymphocytes, whereas low expression levels are found in other tissues [8]. In the present study, constitutive expression of the CcIRF10 gene was detectable in all eleven tissues of C. carpio when analysed by real-time PCR, although there were differences in the levels of expression. This ubiquitous tissue expression pattern supports the findings of previous studies on IRFs in teleosts, including M. albus [17], G. morhua [13], D. rerio [34], O. mykiss [35], turbot (Scophthalmus maximus) [36], P. olivaceus [12, 37], rock bream (Oplegnathus fasciatus) [38], blunt snout bream (Megalobrama amblycephala) [39] and half-smooth tongue sole (Cynoglossus semilaevis) [40]. CcIRF10 was found to be most highly expressed in gonads (Fig. 2), which is different from the expression patterns in P. olivaceus, C. idella and M. albus. In P. olivaceus, the expression of IRF10 has been found to be very high in the gills, intestine, trunk kidney, heart, stomach, head kidney and PBLs; in C. idella, IRF10 expression has been found to be high in all tested tissues, with the highest expression in the thymus and gills; and in M. albus, the highest expression level has been observed in intestine, whereas the lowest level has been found in the liver [12, 15, 17]. However, D. rerio IRF10 is highly expressed in the testis, which is also a reproductive organ [9]. These results suggest that IRF10 may not only play an important role in the immune system but also likely participate in the regulation of the reproductive system in teleosts.
Previous studies on C. carpio have shown that the expression of IRF1, IRF3, IRF5 and IRF7 is upregulated upon stimulation with poly I:C or viruses [25-27]. In the present study, following poly I:C injection, the induction of CcIRF10 in the foregut (27.5-fold) was much stronger than that in the other tissues (4.5- to 7.5-fold), revealing the important role of CcIRF10 in the mucosal immune system response to poly I:C (Fig. 3). Moreover, similar results were observed in PBLs and HKLs with poly I:C stimulation (Fig. 5). The expression of IRF10 in P. olivaceus is also upregulated by poly I:C stimulation in PBLs [12]. In G. morhua, two isoforms of IRF10 have been identified, and the expression of the long isoform reaches its peak at 6 hpi at 16 °C and 24 hpi at 10 °C, whereas the highest expression of the short isoform is observed at 6 hpi at both 16 °C and 10 °C after poly I:C stimulation [13, 41]. Moreover, IRF10 in P. olivaceus, C. idella, M. albus and zebrafish embryo fibroblast-like ZF4 cells can be upregulated by viruses (viral haemorrhagic septicaemia virus [VHSV] or grass carp haemorrhagic virus [GCHV]) or poly I:C [9, 12]. The observed induction of IRF10 expression by various viruses and poly I:C suggests that the fish IRF10 may play a crucial role in protecting the host from viral infection.
A. hydrophila, a well-known fish-pathogenic bacterium, is primarily found in temperate and freshwater environments and causes infections in various organisms. Fish are becoming increasingly susceptible to A. hydrophila because of the increasingly intensive rearing methods used in aquaculture [42]. Moreover,A. hydrophila breakouts have caused great economic losses around the world [43]. To gain insights into the immune mechanism of CcIRF10 in the antibacterial response, its expression pattern in response to A. hydrophila was investigated using real-time PCR. When fish were challenged with A. hydrophila, the levels of CcIRF10 were upregulated in all four tissues, with the highest induction in the foregut (Fig. 4). IRF10 of P. olivaceus can also be induced by Edwardsiella tarda and Streptococcus iniae [37]. Upon Aeromonas salmonicida infection, the greatest expression of the long isoform of G. morhua IRF10 occurs at 24 hpi, whereas the highest expression of the short isoform is observed at 6 hpi at 10 °C and 16 °C, suggesting the distinct roles of the two isoforms in the immune system of G. morhua [13, 41]. It should be noted that E. coioides IRF10 is responsive to both poly I:C stimulation and Vibrio parahaemolyticus infection but increases to a greater extent after poly I:C stimulation [10]. In accordance with these results, our study showed that the fold change in CcIRF10 induced by poly I:C stimulation (4.5- to 27.5-fold) was greater than that induced by A. hydrophila infection (3.0- to 13.7-fold). LPS is the major component of the outer membranes of gram-negative bacteria, but it has been reported that lower vertebrates (such as fish) may be resistant to the toxic effects of LPS [44]. Therefore, it was unsurprising that CcIRF10 was not upregulated by LPS in the PBLs (Fig. 5A). PGN, a major component of the bacterial cell wall, consists of sugars and amino acids. Similar to the findings of a previous study regarding head kidney macrophages of O. mykiss, CcIRF10 was upregulated by PGN stimulation in vitro [15]. Hence, our in vivo and in vitro findings, together with the previous analogous results, suggest that CcIRF10 is more susceptible to poly I:C than to A. hydrophila infection and plays a substantial role in the foregut, which is a mucosal immune organ. Moreover, fish IRF10 may play essential roles in both antiviral and antibacterial defence, as reported for G. gallus IRF10.
In mammals, IFNs are natural glycoproteins produced by cells of the innate and adaptive immune systems in most vertebrates in response to challenge by viruses, bacteria, fungi, parasites, and tumour cells [45]. In addition, IFNs can also be produced by non-immune cells such as fibroblasts and epithelial cells [45]. Similarly, in fish, IFNs play a crucial role in innate immunity [46, 47]. To investigate the regulatory role of CcIRF10 in the IFN response, we detected the mRNA expression of type I IFN, ISGs (PKR and ISG15) and TNFα in CcIRF10-transfected EPC cells. The transcription of PKR, ISG15 and TNFα was downregulated in EPC cells transfected with the pcIRF10 plasmid (Fig. 6). This result is in line with the findings of a previous study that overexpression of D. rerio IRF10 downregulated the expression of IFN stimulated genes induced by poly I:C and promoted the replication of spring viremia of carp virus (SVCV) in EPC cells [9]. Moreover, this study found that the ISRE site in the promoter was responsible for DrIRF10-mediated inhibition of gene expression of IFNs and ISGs [9, 48]. The mechanism involved in the inhibition of IFN signalling pathway in common carp may be similar, which needs our further study. However, G. gallus IRF10 can upregulate the expression of MHC class I and GBP [8], suggesting that the function of IRF10 might be different between fish and birds.