Increased airway smooth muscle is observed in large and small airways in asthma. Semi-quantitative estimates suggest that cells containing alpha smooth muscle actin (alpha-SMA) are also increased in the lung parenchyma. This study quantified and characterized alpha-SMA positive cells (alpha-SMA+) in the lung parenchyma of non-asthmatic and asthmatic individuals.
Post-mortem sections of peripheral lung from cases of fatal asthma (FA), persons with asthma dying of non-respiratory causes (NFA) and non-asthma control subjects (NAC) were stained for alpha-SMA, quantified using point-counting and normalised to alveolar basement membrane length and interstitial area.
alpha-SMA+ fractional area was increased in alveolar parenchyma in both FA (14.7 +/- 2.8% of tissue area) and NFA (13.0 +/- 1.2%), compared with NAC (7.4 +/- 2.4%), p <0.05 The difference was greater in upper lobes compared with lower lobes (p <0.01) in both asthma groups. Similar changes were observed in alveolar ducts and alveolar walls. The electron microscopic features of the alpha-SMA+ cells were characteristic of myofibroblasts.
We conclude that in asthma there is a marked increase in alpha-SMA+ myofibroblasts in the lung parenchyma. The physiologic consequences of this increase are unknown.