Objective:To study whether interleukin (IL)-18 is involved in the activation of monocytes through direct contact with T cells and the related intracellular mechanism. Methods: T cells and monocytes were isolated and purified from the peripheral blood of healthy donors by magnetic beads. Phytohemagglutinin (PHA) pre-stimulated T cells were fixed by 1% paraformaldehyde and were then co-cultured with monocytes at a T cellmonocyte ratio of 41. TNF-α and IL-18 levels in the supernatants were assayed by ELISA. Expression of IL-18 receptor α chain (IL-18Rα) on the surface of monocytes was analyzed by flow cytometry.Results: Monocytes activated by PHA-stimulated T cells produced significantly more TNF-α than by unstimulated T cells; non-cultured T cells or monocytes hardly produced any TNF-α. Upon direct cellular contact, PHA pre-stimulated T cells also up-regulated IL-18Rα expression on the surface of monocytes and induced IL-18 production in monocytes, which could be suppressed by nuclear factor (NF)-κB inhibitor (N-acetyl-L-cysteine, NAC) or phosphatidyl-inositol (PI) 3 kinase inhibitor (LY294002), but not by mitogen activated protein kinase (MAPK) inhibitor (SB203580). Neutralizing anti-IL-18 monoclonal antibody dose-dependently inhibited the production of TNF-α by monocyte-stimulated T cells. IL-18 failed to induce TNF-α production by cultured monocytes alone, while dose-dependently enhanced TNF-α production in monocyte-stimulated T cells, which could be inhibited by NAC or LY294002, but not by SB203580.Conclusion: By direct cellular contact T cells can stimulate monocytes to produce TNF-α and IL-18, up-regulate IL-18 receptor expression in monocytes, and activate intracellular NF-κB and PI3 kinase pathways. IL-18 can enhance T cell ability to stimulate TNF-α production by monocytes, which is dependent on the activation of NF-κB and PI3 kinase pathways.