Loss of
secretory IgA is common in the small airways of patients with
chronic obstructive pulmonary disease and may contribute to disease pathogenesis. Using mice that lack
secretory IgA in the airways due to genetic deficiency of
polymeric Ig receptor (pIgR-/- mice), we investigated the role of neutrophils in driving the fibrotic small airway wall remodeling and
emphysema that develops spontaneously in these mice. By flow cytometry, we found an increase in the percentage of neutrophils among CD45+ cells in the lungs, as well as an increase in total neutrophils, in pIgR-/- mice compared with wild-type controls. This increase in neutrophils in pIgR-/- mice was associated with
elastin degradation in the alveolar compartment and around small airways, along with increased
collagen deposition in small airway walls. Neutrophil depletion using anti-Ly6G
antibodies or treatment with broad-spectrum
antibiotics inhibited development of both
emphysema and
small airway remodeling, suggesting that airway bacteria provide the stimulus for deleterious neutrophilic
inflammation in this model. Exogenous bacterial challenge using lysates prepared from pathogenic and nonpathogenic bacteria worsened neutrophilic
inflammation and lung remodeling in pIgR-/- mice. This phenotype was abrogated by antiinflammatory
therapy with
roflumilast. Together, these studies support the concept that disruption of the mucosal immune barrier in small airways contributes to
chronic obstructive pulmonary disease progression by allowing bacteria to stimulate chronic neutrophilic
inflammation, which, in turn, drives progressive airway wall
fibrosis and emphysematous changes in the lung parenchyma.