Interleukin‐18 enhances monocyte tumor necrosis factor α and interleukin‐1β production induced by direct contact with T lymphocytes: Implications in rheumatoid arthritis

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Abstract
Objective: At sites of inflammation, T cells exert pathologic effects through direct contact with monocyte/macrophages, inducing massive up‐regulation of interleukin‐1 (IL‐1) and tumor necrosis factor α (TNFα). We examined the regulatory effects of IL‐18 on monocyte activation by direct contact with T lymphocytes in rheumatoid arthritis (RA).Methods: Activated T cells were isolated from RA synovial fluid. Resting T cells and monocytes were isolated from peripheral blood mononuclear cells. RA synovial T cells or phytohemagglutinin (PHA)–stimulated T cells were fixed by paraformaldehyde and then cocultured with monocytes at a ratio of 4:1. Levels of TNFα, IL‐1β, IL‐10, and IL‐18 were measured by enzyme‐linked immunosorbent assay. Expression of adhesion molecules, IL‐18 receptor, and TNF receptors was analyzed by flow cytometry. Expression of NF‐κB p65, phosphorylated IκBα, and phosphatidylinositol 3‐kinase (PI 3‐kinase) p110 was analyzed by Western blotting.Results: IL‐18 dose‐dependently enhanced the production of IL‐1β and TNFα, but not IL‐10, by monocytes following contact with RA synovial T cells or PHA‐prestimulated T cells. NF‐κB inhibitors N‐acetyl‐L‐cysteine and Bay 11‐7085 and PI 3‐kinase inhibitor LY294002 inhibited the enhancing effects of IL‐18, but MAPK p38 inhibitor SB203580, ERK inhibitor PD98059, and JNK inhibitor SP600125 did not. Increased levels of NF‐κB in the nucleus, phosphorylated IκB, and PI 3‐kinase were confirmed in monocytes cocultured with PHA‐prestimulated T cells, and the levels were further increased by stimulation with IL‐18. Neutralizing antibody to IL‐18 inhibited monocyte activation induced by direct contact with PHA‐prestimulated T cells. Via cell–cell contact, PHA‐prestimulated T cells increased autocrine production of IL‐18 by monocytes, which was mediated by activation of the NF‐κB and PI 3‐kinase pathways, and up‐regulated the expression of the IL‐18 receptor in monocytes. IL‐18 up‐regulated the expression of the TNF receptors vascular cell adhesion molecule 1 (VCAM‐1) and intercellular adhesion molecule 1 (ICAM‐1) on monocytes. Blocking the binding of the TNF receptors VCAM‐1 or ICAM‐1 on monocytes to their ligands on stimulated T cells suppressed the IL‐18–enhanced production of TNFα and IL‐1β in monocytes induced by contact with PHA‐prestimulated T cells.Conclusion: IL‐18 augments monocyte activation induced by contact with activated T cells in RA synovitis, which is dependent on activation of the NF‐κB and PI 3‐kinase pathways. IL‐18 up‐regulates the expression of the TNF receptors VCAM‐1 and ICAM‐1 on monocytes, which mediate the enhancing effects of IL‐18 on T cell–monocyte contact.

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