Allergens elicit upregulated IL‐18 and IL‐18Rα expression in blood monocytes of patients with allergic rhinitis

Interleukin (IL)-18 is a potentially important molecule in allergic rhinitis (AR). However, expressions of IL-18, IL-18 binding protein isoform a (IL-18BPa) and IL-18 receptor alpha (IL-18R alpha) in AR blood monocytes remain obscure. We, therefore, investigated IL-18, IL-18BPa and IL-18R alpha expressions in monocytes using flow cytometry, murine AR model and quantitative real-time polymerase chain reaction (PCR). The results showed that the numbers of IL-18(+) monocytes increased, whereas IL-18BPa(+) monocytes decreased in the peripheral blood of AR patients. It was also observed that Platanus pollen extract provoked elevated expressions of IL-18 and IL-18R alpha in the monocytes of AR patients. House dust mite extract, Artemisia sieversiana wild extract and Platanus pollen extract enhanced IL-18R alpha protein and mRNA expression in the isolated primary monocytes from AR patients. Using ELISA kits, we observed that the levels of total IL-18 and free IL-18 in the plasma of perennial AR (pAR) and seasonal AR (sAR) patients were elevated, and the molar concentration ratio of free IL-18BPa/free IL-18 was 16.5 for healthy control subjects and 9.7 for patients with sAR, indicating that IL-18 likely plays a role in sAR. In the murine AR model, the number of IL-18R alpha(+) monocytes increased in the blood, and the number of IL-18R alpha(+) macrophages increased in the nasal lavage fluid of WT mice. In conclusion, IL-18 may serve as a causative factor for AR.