Expression of Interleukin (IL)-18 and Functional IL-18 Receptor on Human Vascular Endothelial Cells, Smooth Muscle Cells, and Macrophages: Implications for Atherogenesis

Although considerable evidence implicates the cytokine interferon (IFN)-γ in atherogenesis, the proximal inducers and the range of sources of its expression remain unknown. This study tested the hypothesis that interleukin (IL)-18 regulates IFN-γ expression during atherogenesis. Indeed, human atheroma in situ expressed IL-18 and elevated levels of its receptor subunits, IL-18Rα/β, compared with nondiseased arterial tissue. IL-18 occurred predominantly as the mature, 18-kD form and colocalized with mononuclear phagocytes (MØ), while endothelial cells (ECs), smooth muscle cells (SMCs), and MØ all expressed IL-18Rα/β. Correspondingly in vitro, only MØ expressed IL-18, while all three cell types displayed the IL-18Rα/β complex constitutively, exhibiting enhanced expression upon stimulation with LPS, IL-1β, or tumor necrosis factor (TNF)-α. IL-18 signaling evoked effectors involved in atherogenesis, e.g., cytokines (IL-6), chemokines (IL-8), intracellular adhesion molecules (ICAM)-1, and matrix metalloproteinases (MMP-1/-9/-13), demonstrating functionality of the receptor on ECs, SMCs, and MØ. Finally, IL-18, particularly in combination with IL-12, induced the expression of IFN-γ in cultured MØ and, surprisingly, in SMCs (but not in ECs). The expression of functional IL-18 and IL-18 receptor on human atheroma-associated ECs, SMCs, and MØ, and its unexpected ability to induce IFN-γ expression in SMCs, suggests a novel paracrine proinflammatory pathway operating during atherogenesis.