Possible Role of IL-25 in Eosinophilic Lung Inflammation in Patients with Chronic Eosinophilic Pneumonia

Interleukin (IL)-25 and IL-33 induce IL-5 production by various types of cells, such as type 2 helper T (Th2) cells and type 2 innate lymphoid cells. The number of Th2 cells and concentration of IL-5 in the bronchoalveolar lavage fluid (BALF) are increased in patients with eosinophilic pneumonia (EP). To examine the contribution of IL-25 and IL-33 to eosinophilic inflammation of the lung in humans, we evaluated IL-5, IL-25 and IL-33 levels in the BALF of patients with EP. IL-5, IL-25, and IL-33 concentrations in the BALF were measured by enzyme-linked immunosorbent assay in patients with acute eosinophilic pneumonia (AEP), chronic eosinophilic pneumonia (CEP), idiopathic pulmonary fibrosis (IPF), and sarcoidosis. The absolute number of eosinophils, and IL-5 levels, but not IL-33 levels, in the BALF were significantly higher in patients with EP than in patients with IPF and sarcoidosis. IL-25 levels in the BALF were significantly higher in patients with CEP, but not in patients with AEP, than in patients with IPF and sarcoidosis. The absolute number of eosinophils was significantly correlated with the IL-5 concentration in the BALF of patients with EP. IL-5 concentrations were significantly correlated with IL-25 concentrations in the BALF of patients with CEP, but not in patients with AEP. IL-5 levels were not correlated with IL-33 levels in the BALF of patients with EP. Our findings suggest that IL-25 plays an important role via IL-5 in eosinophilic lung inflammation in patients with CEP.