What has been learned by cytokine targeting of asthma?

Airway inflammation in asthma involves complex, interactive, and redundant cascades mediated by an array of proinflammatory cytokines, including a type 2 (T2) pattern of injury. T2 inflammation is characterized by elevations in absolute peripheral or sputum eosinophil counts and levels of IgE (total and allergen-specific) and fractional exhaled nitric oxide, which serve as biomarkers for the presence of this type of inflammation. T2 inflammation is mediated by key "downstream" cytokines, particularly IL-4, IL-5, and IL-13, which act at the effector cell level, as well as upstream cytokines, or "alarmins," such as thymic stromal lymphopoietin, IL-25, and IL-33 generated by epithelial cells. The relevance of these pathways has led to the development of biologic therapies targeting these T2 cytokines, which have not only resulted in modifying these biomarker signatures for inflammation but have also reduced the disease burden associated with asthma exacerbations, systemic corticosteroid use, and lung function compromises. This review will summarize experiences with anticytokine biologics to highlight which specific asthma outcomes they affect and how these effects reflect inflammatory pathways modified by biologics and may relate to pathophysiologic features of asthma.