Objective: To determine whether IL18 has any indirect effects on osteoclastogenesis mediated by T cells in RA synovium, and compare its effects with those of IL1β and TNFα.

Methods: Resting T cells were isolated from peripheral blood of healthy donors, and stimulated with 2 μg/ml phytohaemagglutinin (PHA) and 0.5 ng/ml IL2 for 24 hours. Synovial T cells were isolated from RA synovial tissue. The levels of soluble receptor activator of the NF-κB ligand (RANKL), osteoprotegerin (OPG), IFNγ, M-CSF, and GM-CSF were determined by ELISA. Membrane bound RANKL expression was analysed by flow cytometry. Commercially available human osteoclast precursors were cocultured with T cells to induce osteoclast formation, which was determined with tartrate resistant acid phosphatase staining and pit formation assay.

Results: In PHA prestimulated T cells or RA synovial T cells, IL18, IL1β, or TNFα increased soluble RANKL production and membrane bound RANKL expression in a dose dependent manner. IL18, IL1β, and TNFα did not induce M-CSF, GM-CSF, IFNγ, or OPG production in PHA prestimulated T cells or RA synovial T cells. IL18 increased the number of osteoclasts and bone resorption area on dentine slices in the coculture of human osteoclast precursors with PHA prestimulated T cells or RA synovial T cells; its ability was equivalent to that of IL1β, but less potent than that of TNFα. In the coculture system, OPG completely blocked osteoclast induction by IL18 or IL1β, and greatly inhibited induction by TNFα.

Conclusion: IL18, IL1β, or TNFα can indirectly stimulate osteoclast formation through up regulation of RANKL production from T cells in RA synovitis; IL18 is as effective as IL1β, but less potent than TNFα.