Background Interleukin-18 (IL-18) is a member of the IL-1 family of proteins that exerts proinflammatory effects.¹ It was formally known as IGIF and is a pivotal cytokine for the development of Th1 responses.² IL-18 is structurally related to IL-1b both cytokines need ICE or caspase-1 for cleavage of the precursor to release the bioactive molecules. Apart from immune-stimulatory activity, IL-18 induces production of TNF and IL-1 in vitro. IL-18 synthesis is found in both articular chondrocytes and osteoblasts and, with respect to cartilage, IL-18 promotes gene expression of nitric oxide synthase, inducible cyclooxygenase, IL-6 and stromelysin.

Objectives The goal of the present study was to investigate whether IL-18 mediates cartilage degradation directly or via induction of other cytokines, such as IL-1 and IFN-g.

Methods Patellae of C57Bl/6, IL-1b deficient, Balb/C and IFN-g deficient mice were used for in vitro cartilage degradation experiments with either murine IL-18 or IL-1b. Patellar cartilage was pre-labelled with 35S-sulphate. The explants were cultured for 24 h, 48 h or 72 h in either IGF-1 (0.25 ug/ml), IGF-1/IL-18 (10 and 100 ng/ml) or IGF-1/IL-1b (10 ng/ml). Patellae were washed in saline, fixed in 4% formaldehyde and subsequently decalcified in 5% formic acid. Patellae were punched out of the adjacent tissue, dissolved and the 35S content was measured by liquid scintillation counting. For IL-1 inhibition we added IL-1Ra (10 ug/ml) or an ICE inhibitor (2.5 uM) to the culture system.

Results IL-18 exposure for 24 or 48 h did not induce cartilage degradation, determined as release of prelabelled proteoglycans. Cartilage degradation by IL-18 was only found after a 72 h culture period In contrast, IL-1b did already induce marked cartilage degradation after 24 h. To investigate whether generation of another destructive cytokine was responsible for the delay of cartilage degradation we added IL-1Ra or ICE inhibitors to the culture system. Blocking of IL-1 resulted in almost complete protection against IL-18 mediated cartilage degradation. Additionally, studies in IL-1b -/- mice showed that IL-1b was pivotal in IL-18-induced cartilage degradation. Since IL-18 is a potent inducer of IFN-g, we investigated whether IFN-g induction by IL-18 was involved. It was revealed that IL-18 mediated cartilage destruction was partly dependent on IFN-g induction.

Conclusion The present study demonstrated that IL-18 induces cartilage destruction in vitro. Here we showed that IL-1b generation, due to IL-18 exposure was essential for marked cartilage degradation. In addition, it was shown that induction of IFN-g was also involved in IL-18-induced cartilage degradation. These findings implicated that IL-18 contributed to cartilage destruction by induction of IL-1b and IFN-g. Inhibition of both IL-18 and IL-1b generation by ICE inhibitors might be a protective therapy in RA.

References

1. Okamura H, et al. Nature 1995;378:88

2. Akira S. Curr Opin Immunol. 2000;12:59