The two groups of zebrafish virus-induced interferons signal via distinct receptors with specific and shared chains.

Because the availability of fish genomic data, the number of reported sequences for fish type II helical cytokines is rapidly growing, featuring different IFNs including virus-induced IFNs (IFNphi) and IFN-gamma, and IL-10 with its related cytokines (IL-20, IL-22, and IL-26). Many candidate receptors exist for these cytokines and various authors have postulated which receptor chain would be involved in which functional receptor in fish. To date, only the receptor for zebrafish IFNphi1 has been identified functionally. Three genes encoding virus-induced IFNphis have been reported in zebrafish. In addition to these genes clustered on chromosome 3, we have identified a fourth IFNphi gene on chromosome 12. All these genes possess the intron-exon organization of mammalian lambda IFNs. In the zebrafish larva, all induce the expression of reporter antiviral genes; protection in a viral challenge assay was observed for IFNphi1 and IFNphi2. Using a combination of gain- and loss-of-function experiments, we also show that all zebrafish IFNphis do not bind to the same receptor. Two subgroups of fish virus-induced IFNs have been defined based on conserved cysteines, and we find that this subdivision correlates with receptor usage. Both receptor complexes include a common short chain receptor (CRFB5) and a specific long chain receptor (CRFB1 or CRFB2).

Comparative genomic analysis reveals independent expansion of a lineage-specific gene family in vertebrates: the class II cytokine receptors and their ligands in mammals and fish.

BACKGROUND: The high degree of sequence conservation between coding regions in fish and mammals can be exploited to identify genes in mammalian genomes by comparison with the sequence of similar genes in fish. Conversely, experimentally characterized mammalian genes may be used to annotate fish genomes. However, gene families that escape this principle include the rapidly diverging cytokines that regulate the immune system, and their receptors. A classic example is the class II helical cytokines (HCII) including type I, type II and lambda interferons, IL10 related cytokines (IL10, IL19, IL20, IL22, IL24 and IL26) and their receptors (HCRII). Despite the report of a near complete pufferfish (Takifugu rubripes) genome sequence, these genes remain undescribed in fish. RESULTS: We have used an original strategy based both on conserved amino acid sequence and gene structure to identify HCII and HCRII in the genome of another pufferfish, Tetraodon nigroviridis that is amenable to laboratory experiments. The 15 genes that were identified are highly divergent and include a single interferon molecule, three IL10 related cytokines and their potential receptors together with two Tissue Factor (TF). Some of these genes form tandem clusters on the Tetraodon genome. Their expression pattern was determined in different tissues. Most importantly, Tetraodon interferon was identified and we show that the recombinant protein can induce antiviral MX gene expression in Tetraodon primary kidney cells. Similar results were obtained in Zebrafish which has 7 MX genes. CONCLUSION: We propose a scheme for the evolution of HCII and their receptors during the radiation of bony vertebrates and suggest that the diversification that played an important role in the fine-tuning of the ancestral mechanism for host defense against infections probably followed different pathways in amniotes and fish.