Allergic rhinitis
AR also known as hay fever, represents the most common allergic disease and is characterized by increased circulating IgE levels and/or positive skin prick test. This is triggered by various environmental allergens including pollen, molds, house dust mite and animal dander, and results in a cascade of type 2 immune response in which type 2 cytokine production and eosinophil numbers are increased . AR manifests in the upper airways and often coexists with asthma44. Indeed, an estimated 10-40% of AR patients suffer from asthma45. Mucosal inflammation in AR and asthma shares many features, which has led to the “united airway concept”46 and the idea that inflammation in AR can progressively extend to the lower airways47. Even though AR and asthma often co-exist, many studies examining AR subjects aimed to uncover unique AR-specific miRNA signatures (Table 2 ). Indeed, a subset of circulating miRNAs in plasma, miR-206, -338-3p, -329 and -26a, were found to be differentially expressed in patients with AR, but not in healthy individuals or those with non-allergic asthma. Random forest model prediction suggested that a subset of six miRNAs allowed for high accuracy in distinguishing between these three groups48.
In nasal biopsies, out-off-season AR patients displayed higher miR-7 and miRPlus-E1194 expression, whereas let-7, miR-498,-187, -874, -143, -886, -224, and -767 were decreased compared to non-allergic patients undergoing inferior turbinate surgery49,50. The reduced levels of let-7e were confirmed by an additional study, which also showed increased levels of miR-155, a miRNA involved in type 2 immune responses (see above51,52), miR-205 and miR-498 in nasal biopsies of patients with current AR symptoms50. miR-498 was also increased in the nasal mucosa of subjects suffering from perennial allergy, while miR-18a expression was significantly lower in subjects with perennial allergy compared to subjects with sensitization to seasonal allergens48.
The correlation of miRNAs with AR symptom severity (Total Nasal Symptoms Score) revealed 3 down-regulated (miR-572, -1228-, -483) and 9 up-regulated miRNAs in nasal mucosa (miR-1908, -126, -92a, -125a, 19a, 26a, 106a, -181c, -3177). Of the identified miRNAs, miR-126, -19a and -26a specifically and sensitively predicted AR disease activity in a receiver operating characteristic (ROC) analysis53, thus suggesting that miRNAs may be potential biomarkers in the prognosis of AR.