Ex vivo sputum analysis reveals impairment of protease-dependent mucus degradation by plasma proteins in acute asthma.
BACKGROUND
Airway mucus plugs, composed of mucin glycoproteins mixed with plasma proteins, are an important cause of airway obstruction in acute severe asthma, and they are poorly treated with current therapies.


OBJECTIVE
To investigate mechanisms of airway mucus clearance in health and in acute severe asthma.


METHODS
We collected airway mucus from patients with asthma and nonasthmatic control subjects, using sputum induction or tracheal aspiration. We used rheological methods complemented by centrifugation-based mucin size profiling and immunoblotting to characterize the physical properties of the mucus gel, the size profiles of mucins, and the degradation products of albumin in airway mucus.


RESULTS
Repeated ex vivo measures of size and entanglement of mucin polymers in airway mucus from nonasthmatic control subjects showed that the mucus gel is normally degraded by proteases and that albumin inhibits this degradation. In airway mucus collected from patients with asthma at various time points during acute asthma exacerbation, protease-driven mucus degradation was inhibited at the height of exacerbation but was restored during recovery. In immunoblots of human serum albumin digested by neutrophil elastase and in immunoblots of airway mucus, we found that albumin was a substrate of neutrophil elastase and that products of albumin degradation were abundant in airway mucus during acute asthma exacerbation.


CONCLUSIONS
Rheological methods complemented by centrifugation-based mucin size profiling of airway mucins in health and acute asthma reveal that mucin degradation is inhibited in acute asthma, and that an excess of plasma proteins present in acute asthma inhibits the degradation of mucins in a protease-dependent manner. These findings identify a novel mechanism whereby plasma exudation may impair airway mucus clearance.
Cardiovascular Research Institute, University of California, San Francisco, San Francisco, California, USA.
Anh L
AL
Innes
Stephen D
SD
Carrington
David J
DJ
Thornton
Sara
S
Kirkham
Karine
K
Rousseau
Ryan H
RH
Dougherty
Wilfred W
WW
Raymond
George H
GH
Caughey
Susan J
SJ
Muller
John V
JV
Fahy
D000208:Acute Disease; D000328:Adult; D000368:Aged; D001249:Asthma / Q000188:drug therapy / Q000378:metabolism*; D004548:Elasticity; D015180:Electrophoresis, Gel, Two-Dimensional; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D015151:Immunoblotting; D008297:Male; D008875:Middle Aged; D008970:Molecular Weight; D009077:Mucins / Q000032:analysis*; D009079:Mucociliary Clearance / Q000187:drug effects*; D053481:Secretory Leukocyte Peptidase Inhibitor / Q000494:pharmacology*; D015842:Serine Proteinase Inhibitors / Q000494:pharmacology*; D013183:Sputum / Q000737:chemistry* / Q000187:drug effects; D014783:Viscosity; D055815:Young Adult
D009077:Mucins; D053481:Secretory Leukocyte Peptidase Inhibitor; D015842:Serine Proteinase Inhibitors