Cloning of a cDNA encoding bovine Interleukin-18 and analysis of IL-18 expression in macrophages and its IFN-γ-inducing activity

Interleukin-18 (IL-18) is a recently described cytokine that enhances interferon-gamma (IFN-gamma) production, either independently or synergistically with IL-12, These properties identify IL-18 as an immunoregulatory cytokine that may be pivotal in host defense against intracellular pathogens. We have isolated and sequenced a cDNA encoding bovine IL-18, The open reading frame (ORF) is 582 bp in length, encoding a predicted 192 amino acid (aa) precursor protein. Multiple sequence alignment demonstrated that bovine IL-18 has 65% and 78% identity with the predicted amino acid sequences of murine and human IL-18, respectively. IL-18 mRNA was constitutively present in bovine peripheral blood monocyte-derived macrophages (MDM), with no upregulation on stimulation with lipopolysaccharide (LPS), IL-18 transcripts were weakly detected in B Lymphocytes but inducible in the B cell line BL-3, Human recombinant IL-18 (rHuIL-18) induced IFN-gamma production by PHA-stimulated peripheral blood mononuclear cells (PBMC), which was potentiated by rHuIL-12. Further, rHuIL-12 and rHuIL-18 enhanced proliferation of untreated PBMC, Antigen-specific T cell lines demonstrated IL-18-dependent enhancement of IFN-gamma production, indicating that bovine T cells are one of the leukocyte subsets that respond to IL-18, Analysis of IL-18 expression and its ability to induce IFN-gamma production by bovine lymphocytes are important considerations for understanding mechanisms of protective immunity and designing vaccines for intracellular pathogens.