Interleukin-32 (IL-32) induces various inflammatory molecules in human monocytes and differentiation of monocytes into macrophage-like cells. This study was undertaken to evaluate the effects of IL-32gamma, the most biologically active isoform, on the differentiation and activation of osteoclasts.
CD14+ monocytes were obtained from healthy volunteers, and samples of synovial tissue and synovial fluid were obtained from patients with rheumatoid arthritis (RA) and patients with osteoarthritis (OA). The concentration and expression levels of IL-32gamma in RA and OA samples were evaluated by enzyme-linked immunosorbent assay and immunoblotting, respectively. To examine the osteoclastogenic effects and functional activities, isolated monocytes were treated with either IL-32gamma or IL-17 in the presence or absence of soluble RANKL (sRANKL) on a culture system and on Osteologic disks. The expression of RANKL and osteoprotegerin (OPG) messenger RNA (mRNA) in RA fibroblast-like synoviocytes (FLS) was measured using reverse transcription-polymerase chain reaction (PCR) and real-time PCR.
The concentration and expression levels of IL-32gamma were higher in the RA samples than in the OA samples. Upon costimulation with sRANKL, the osteoclast count and resorbed area increased more significantly in the IL-32gamma-stimulated cultures than in those stimulated with IL-17. In the IL-32gamma-treated group without sRANKL stimulation, osteoclasts were differentiated, but the cells displayed low resorption activity. In RA FLS, RANKL mRNA expression increased in the presence of both IL-32gamma and IL-17. However, transcription of OPG decreased following IL-32gamma stimulation, resulting in a significant increase in the RANKL:OPG ratio.
Our results suggest that IL-32gamma is a potent mediator of active osteoclast generation in the presence of sRANKL. Moreover, this novel cytokine creates more favorable conditions for osteoclastogenesis in the RA joint by increasing the RANKL:OPG ratio in FLS.