MUC5B and chronic obstructive pulmonary disease: Future studies that more extensively investigate mucus hypersecretion (MUC5AC and MUC5B in the BAL), leukocyte subsets, neutrophil activation status, and soluble mediator production, focusing on some of the mediators that might be interesting in human COPD development (e.g., neutrophil elastase/other proteases) are warranted.