The related cytokine genes IL-3, IL-5 and GM-CSF map to the (extended) TH2 cytokine locus of the mammalian genome. For chicken an additional related cytokine gene, KK34, was reported downstream of the IL-3 plus GM-CSF cluster, but hitherto it was believed that mammalian genomes lack this gene. However, the present study identifies an intact orthologue of chicken KK34 gene in many mammals like cattle and pig, while remnants of KK34 can be found in human and mouse. Bovine KK34 was found to be transcribed, and its recombinant protein could induce STAT5 phosphorylation and proliferation of lymphocytes upon incubation with bovine PBMCs. This concludes that KK34 is a fourth functional cytokine of the IL-3/IL-5/GM-CSF/KK34-family (alias IL-5 family) in mammals. While analyzing KK34, the present study also made new identifications of cytokine genes in the extended TH2 cytokine loci for reptiles, birds and marsupials. This includes a hitherto unknown cytokine gene in birds and reptiles which we designated “IL-5famE”. Other newly identified genes are KK34, GM-CSF(-like), IL-5, and IL-13 in reptiles, and IL-3 in marsupials.
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