Asthma, chronic obstructive pulmonary disease, and idiopathic pulmonary fibrosis are representative chronic respiratory diseases (CRDs). Although they differ in terms of disease presentation, they are all thought to arise from unresolved inflammation. Neutrophils are not only the first responders to acute inflammation, but they also help resolve the inflammation. Notably, emerging clinical studies show that CRDs are associated with systemic and local elevation of neutrophils. Moreover, murine studies suggest that airway-infiltrating neutrophils not only help initiate airway inflammation but also prolong the inflammation. Given this background, this review describes neutrophil-mediated immune responses in CRDs and summarizes the completed, ongoing, and potential clinical trials that test the therapeutic value of targeting neutrophils in CRDs. The review also clarifies the importance of understanding how neutrophils interact with other immune cells and how these interactions contribute to chronic inflammation in specific CRDs. This information may help identify future therapeutic strategies for CRDs.

Although cigarette smoking is known to exacerbate asthma, only a few clinical asthma studies have been conducted involving smokers. Here we show, by comparing paired sputum and blood samples from smoking and non-smoking patients with asthma, that smoking associates with significantly higher frequencies of pro-inflammatory, natural-cytotoxicity-receptor-non-expressing type 3 innate lymphoid cells (ILC3) in the sputum and memory-like, CD45RO-expressing ILC3s in the blood. These ILC3 frequencies positively correlate with circulating neutrophil counts and M1 alveolar macrophage frequencies, which are known to increase in uncontrolled severe asthma, yet do not correlate with circulating eosinophil frequencies that characterize allergic asthma. In vitro exposure of ILCs to cigarette smoke extract induces expression of the memory marker CD45RO in ILC3s. Cigarette smoke extract also impairs the barrier function of airway epithelial cells and increases their production of IL-1β, which is a known activating factor for ILC3s. Thus, our study suggests that cigarette smoking increases local and circulating frequencies of activated ILC3 cells, plays a role in their activation, thereby aggravating non-allergic inflammation and the severity of asthma.

The standard treatment for allergic-airway inflammation, which is the dominant asthma endotype, is a steroid. However, steroid-refractory asthma is a significant problem. Innate-lymphoid cells (ILCs) produce type-2 cytokines as Th2 cells and play critical roles in asthma pathogenesis. Limited evidence from the asthma-mouse models and human studies suggests that ILC2s may participate in steroid-resistant asthma. Here, we showed that lung ILC2s, but not Th2 cells, can develop steroid resistance that maintains their survival, cytokine production, and pathogenic activities during steroid treatment. Such steroid-resistant ILC2s are associated with the presence of multiple ILC2-stimulating cytokines and the emergence of multipotent IL-5+IL-13+IL-17A+ ILC2s, and the Janus-kinase (JAK) 3/signal-transducer-and-activator-of-transcription (STAT) 3,5, and 6 pathway participates in the acquisition of steroid-resistant ILC2s. JAK3-inhibitor treatment significantly reduced the survival, proliferation, and cytokine production of multipotent ILC2s in vitro ameliorated ILC2-dependent Alternaria-induced asthma. Moreover, JAK3-inhibitor combined with a steroid strongly inhibited steroid-resistant asthma. Therefore, sustained asthmatic conditions may induce multipotent ILC2s that promote steroid-resistant asthma, and combining JAK3-inhibitor with steroid may be a treatment option for steroid-refractory asthma.