Steroid-Resistant Asthma and Neutrophils.

Asthma patients are classified by phenotype and endotype. Although symptoms in most asthma patients are well controlled by glucocorticoid treatment, certain populations of severe eosinophilic asthma patients in T-helper 2 (Th2)/type 2 asthma and neutrophilic asthma patients in non-Th2/type 2 asthma show insensitivity to inhaled or oral glucocorticoid therapy. In some cases of severe eosinophilic asthma, eosinophils remain in the lungs despite glucocorticoid therapy. It was reported that interleukin (IL)-33-induced activation of type 2 innate lymphoid cells (ILC2) was resistant to glucocorticoid treatment in certain allergic conditions. Regarding neutrophilic airway inflammation in steroid-resistant asthma, IL-17 derived from Th17 cells and IL-8 and tumor necrosis factor-α derived mainly from macrophages were reported to be involved in the pathogenesis. Recently, "NETosis," a specific cell death of neutrophils, has been reported to be involved in asthmatic airway inflammation. When NETosis is induced in asthma, aggravation of inflammation and delay of tissue repair could occur, suggesting that NETosis may be associated with the development of steroid-resistant asthma. This article reviews the pathogenesis of steroid-resistant asthma by focusing mainly on neutrophils.