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‐18Rα) in AR blood monocytes remain obscure. We, therefore, investigated IL‐18, IL‐18BPa and IL‐18Rα 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‐18Rα in the monocytes of AR patients. House dust mite extract, Artemisia sieversiana wild extract and Platanus pollen extract enhanced IL‐18Rα 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‐18Rα+ monocytes increased in the blood, and the number of IL‐18Rα+ macrophages increased in the nasal lavage fluid of WT mice. In conclusion, IL‐18 may serve as a causative factor for AR.