Interleukin-18 mRNA, but not interleukin-18 receptor mRNA, is constitutively expressed in islet beta-cells and upregulated by interferon-gamma

Interleukin-18 (IL-18) mRNA is expressed in islets of NOD mice during the e arly stages of insulitis and IL-18 has therefore been implicated as a contr ibuting factor in immune-mediated beta-cell destruction. However, a recent study failed to show any effect of human IL-18 on the function of isolated rat islets, Since species differences have been shown between human and mur ine IL-18, the aims of this study were to investigate 1) if species homolog ous IL-18 alone or following IL-12 pre-exposure affected rat islet function , 2) if IL-18 dose-dependently modulated IL-1 beta or interferon-gamma (IFN -gamma) + tumor necrosis factor-alpha (TNF-alpha) actions on islet function , and 3) if IL-18 and IL-18 receptor (IL-18R) were expressed in rat islet b eta-cells. Insulin release and nitric oxide (NO) production from isolated r at islets were measured after incubation with or without cytokines, RT-PCR was used to quantitate mRNA expression of IL-18 and the IL-18R signaling ch ain (IL-18R beta), There were no significant effects of 0.625-10 nM recombi nant murine (rm) IL-18 alone on accumulated or glucose-challenged insulin r elease or NO production after 24 hours. Fifteen pg/ml of recombinant human (rh) IL-1 beta as well as 200 U/ml recombinant rat (rr) IFN-gamma + 250 U/m l rhTNF-alpha significantly increased islet NO production and inhibited bot h accumulated and glucose-challenged islet insulin release. However, rmIL-1 8 failed to modulate these effects of IL-1 beta or IFN-gamma + TNF-alpha. A lthough IL-12 induces IL-18R expression in Th1 and B lymphocytes, 24-hours rmIL-12 preincubation neither sensitized islets to effects of 10 nM of rm o r rrIL-18 alone nor primed the islets to IL-1 beta actions on insulin relea se and NO production. IL-18R beta mRNA, which was expressed in human periph eral blood mononuclear cells (PBMC), was not expressed in rat insulinoma (R IN) cells or in isolated rat islets, even after exposure to IL-1 beta and/o r IFN-gamma + TNF-alpha or IL-12. IL-18 mRNA was constitutively expressed i n RIN cells, in FACS-purified rat beta-cells and in intact rat and mouse is lets, and was up-regulated by IFN-gamma in an interferon regulatory factor- 1- IRF-1) and NO - independent manner. However, IL-18 protein was undetecta ble in lysates and supernates of RIN cells by ECL, Western blotting and imm unoprecipitation, In conclusion, we show for the first time that IL-18 but not IL-18R is expressed in rodent islet beta-cells. The physiological impor tance and pathological role of IL-18 originating from islet beta-cells dese rve further investigation.