Cloning, chromosomal location, and tissue expression of the gene for pig interleukin-18

Interleukin 18 (IL-18), a recently described cytokine, plays an important r ole in the cell-mediated immune response, in particular through its ability to induce the production of interferon (IFN)-gamma. We cloned pig IL-18 cD NA from the intestinal epithelial cell line IPI-2I using a reverse transcri ptase-polymerase chain reaction method with primers derived from the human IL-18 sequence. The amino acid sequence deduced from pig IL-18 cDNA encodes a 192 amino-acid polypeptide that exhibits 92, 90, 81, and 71% similarity to IL-18 from horse, dog, human, and rodents (mouse and rat), respectively. Structural comparison of the IL-18 protein with IL-1 alpha and IL-1 beta s howed that IL-18 shares several characteristics with the IL-1 cytokine fami ly: the IL-1 signature-like sequence, a potential caspase-1 (ICE) cleavage site, and the presence of 12 predicted beta strands. Fluorescence in situ h ybridization was used to localize the IL-18 gene on the short arm (p13) of pig chromosome 9. Analysis of IL-18 expression in different organs of pigle ts demonstrated that IL-18 mRNA is weakly expressed in the kidney and the l ung. By contrast, we observed highly constitutive expression of IL-18 mRNA in the spleen, mesenteric lymph nodes, and the intestine, particularly in t he small intestine, indicating a potential role for IL-18 as a first line o f host defense in the intestinal mucosa.