A cloned chicken lymphokine homologous to both mammalian IL-2 and IL-15

IL-2, cloned in several mammalian species, plays a critical role in immune system function. Indirect evidence suggests that IL-2-like molecules also exist in lower vertebrates, although none has been cloned. In view of the commercial importance of poultry and the lack of any IL-2 sequence data in lower vertebrates, we undertook to clone a chicken IL-2-like molecule. To this end, a cDNA library was constructed from activated chicken spleen cells and screened for T cell proliferative activity. Three clones of a single gene were isolated that produced a protein stimulatory for activated T cell blasts. This protein had 24 to 25% amino acid identity and 44 to 46% similarity to both bovine IL-2 and IL-15. The homology with IL-15 was unexpected, since primate IL-15 was reported to have no homology with IL-2. However, genetic distance analysis indicated that the chicken interleukin homology is closer to the mammalian IL-2 than to the mammalian IL-15 homology. Furthermore, the chicken gene has several characteristics of mammalian IL-2s: e.g., expression only by activated T cells, mRNA with a short 5' region preceding the open reading frame, and a short leader sequence. It has one characteristic that is unique to IL-15: four conserved cysteines allowing for two intrachain disulfide bonds.