Turkey and chicken interleukin-2 cross-react in in vitro proliferation assays despite limited amino acid sequence identity.

We cloned the cDNA of turkey interleukin-2 (IL-2), initially using oligonucleotide primers based on the sequence of the chicken IL-2 gene. Compared with the only other cytokines available for comparison, the interferons (IFN), the coding regions of the turkey and chicken IL-2 genes are much less conserved (86.24% nucleotide identical and 69.93% amino acid identical). The lack of nucleotide conservation was spread across the entire length of the coding region. In comparison, the promoters of the two avian IL-2 genes shared a high degree of identity (95.71% identical over 380 nucleotides). Phylogenetic analysis shows that turkey and chicken IL-2 have diverged to a greater extent than IL-2 from closely related mammalian species. Surprisingly, considering the low level of amino acid identity, including residues known to be important in binding the IL-2 receptor in mammalian species, both turkey and chicken IL-2 cross-react in functional assays.