The positive correlation between sputum MUC5AC and neutrophil numbers, neutrophil elastase concentrations, and secondary bacterial infections, and the negative correlation with SLPI and elafin concentrations observed in the current study, raise speculation that increased MUC5AC could be a further component of this mechanism, but further studies in which components of this pathway are manipulated in mouse models will be required to confirm this definitively. Here, PI3 is linked to bacterial infectious disease.