Role of IL-18 in atopic asthma is determined by balance of IL-18/IL-18BP/IL-18R

It is recognized that IL-18 is related to development of asthma, but role of IL-18 in asthma remains controversial and confusing. This is largely due to lack of information on expression of IL-18 binding protein (BP) and IL-18 receptor (R) in asthma. In this study, we found that plasma levels of IL-18 and IL-18BP were elevated in asthma. The ratio between plasma concentrations of IL-18 and IL-18BP was 1:12.8 in asthma patients. We demonstrated that 13-fold more monocytes, 17.5-fold more neutrophils and 4.1-fold more B cells express IL-18BP than IL-18 in asthmatic blood, suggesting that there is excessive amount of IL-18BP to abolish actions of IL-18 in asthma. We also discovered that more IL-18R+ monocytes, neutrophils and B cells are located in asthmatic blood. Once injected, IL-18 eliminated IL-18R+ monocytes in blood, but up-regulated expression of IL-18R in lung macrophages of OVA-sensitized mice. Our data clearly indicate that the role of IL-18 in asthma is very likely to be determined by balance of IL-18/IL-18BP/IL-18R expression in inflammatory cells. Therefore, IL-18R blocking or IL-18BP activity enhancing therapies may be useful for treatment of asthma.