Role of interleukin-17 and the neutrophil in asthma

Recent clinical evidence shows that acute, severe exacerbations of asthma are associated with recruitment and activation of neutrophils in the airways. There is also experimental evidence from rodents that T-lymphocytes are involved in the recruitment of neutrophils following allergen challenge in sensitised airways. This review addresses the potential role of neutrophils and the cytokine interleukin-17 (IL-17) in severe asthma. IL-17 is produced and released as a free protein from T-lymphocytes of the memory (CD45RO+) subset. Evidence from rats in vivo suggests that IL-17 can recruit and activate neutrophils in the airways; the recruitment is mediated by the rat correlate to the neutrophil chemoattractant interleukin-8 (IL-8) macrophage inflammatory protein-2 (MIP-2). Endogenous peptidases modulate neutrophil recruitment by acting on NK-1 receptors in rat airways in vivo. Human bronchial epithelial cells in vitro respond to stimulation with IL-17 by increasing the production and release of the human neutrophil chemoattractant IL-8. This release of IL-8 is functionally significant; it causes neutrophil chemotaxis in vitro. Furthermore, this IL-8 release is sensitive to a glucocorticoid and is potentiated by the pro-inflammatory cytokine, tumour necrosis factor-alpha (TNF-alpha) in vitro. In addition, IL-17 stimulates human bronchial epithelial cells in vitro to release the neutrophil-activating factor IL-6. This effect of IL-17 on IL-6, and IL-8 is in part mediated via mitogen-activated protein kinases. In conclusion, as indicated in rat airways in vivo and in human bronchial epithelial cells in vitro, IL-17 may constitute a link between the activation of certain T-lymphocytes and mobilisation of neutrophils in the airways, via induced release of C-X-C chemokines and tachykinins. Further studies are required to answer the question whether free, soluble IL-17 protein plays this role in the airways of patients with severe asthma.