Abstract
Acute viral bronchiolitis causes significant mortality in the developing world, is the number one cause of infant hospitalisation in the developed world, and is associated with the later development of chronic lung diseases such as asthma. A vaccine against respiratory syncytial virus (RSV), the leading cause of viral bronchiolitis in infancy, remains elusive, and hence new therapeutic modalities are needed to limit disease severity. However, much remains unknown about the underlying pathogenic mechanisms. Neutrophilic inflammation is the predominant phenotype observed in infants with both mild and severe disease, however, a clear understanding of the beneficial and deleterious effects of neutrophils is lacking. In this review, we describe the multifaceted roles of neutrophils in host defence and antiviral immunity, consider their contribution to bronchiolitis pathogenesis, and discuss whether new approaches that target neutrophil effector functions will be suitable for treating severe RSV bronchiolitis.
Highlights
IntroductionRespiratory syncytial virus (RSV) is the leading viral cause of lower respiratory infections (LRI)
Respiratory syncytial virus (RSV) is the leading viral cause of lower respiratory infections (LRI)among young infants [1,2]
The expression of TLR4 is lower on blood and airway neutrophils of infants with severe RSV infection compared with uninfected controls [122]
Summary
Respiratory syncytial virus (RSV) is the leading viral cause of lower respiratory infections (LRI). Neutrophils contain cytoplasmic granules, which store antimicrobial molecules and facilitate cross-talk with other immune cells [16] These granules (summarised in Table 1) are subdivided into primary or azurophilic granules (containing, e.g., myeloperoxidase, CD63), secondary or specific granules (containing, e.g., lipocalin-2 and lactoferrin), and tertiary or gelatinase granules (containing, e.g., cathelicidins and neutrophil collagenase) [16]. ROS production can activate azurophilic granules to release myeloperoxidase (MPO) and neutrophil elastase (NE) If these enzymes translocate to the nucleus, they can disrupt chromatin packaging [15,17], and promote the release of chromatin, which interacts with other granular and cytosolic proteins to form ‘NETs’ [30]. NETosis are being assessed for the treatment of infectious diseases (including bacterial sepsis), autoimmune diseases (including systemic lupus erythematosus [40], rheumatoid arthritis [41]) and chronic lung disorders (such as cystic fibrosis [42])
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