Modulation of actin isoform expression in alveolar myofibroblasts (contractile interstitial cells) during pulmonary hypertension

Lungs of 37 patients with pulmonary hypertension (PHT), 5 normal human lungs, and 30 normal rat lungs, were studied using immunohistochemical stainings for actin, alpha-smooth muscle (alpha-SM) actin and desmin. The type of PHT was determined on clinicopathologic grounds (in 17 cases by catheterism); 20 patients had precapillary and 17 postcapillary PHT. In normal lungs, myofibroblasts, ie, contractile interstitial cells (CIC), distributed in alveolar septa, were not stained by alpha-SM actin antibodies. Only around the venules, were cells labeled by this antibody present. Furthermore, there were bundles of alpha-SM actin-positive cells around the openings of air sacculi into the alveolar ducts. In precapillary PHT, the distribution and immunostaining properties of interstitial cells remained unchanged; alpha-SM actin-positive cells were observed in thickened arterial intima and in plexiform lesions. In postcapillary PHT secondary to heart failure, to mitral stenosis, or in veno-occlusive disease, many interstitial cells in the alveolar septa were decorated by alpha-SM actin antibodies but not with desmin. The authors propose that, in postcapillary PHT, mechanical stretch due to capillary congestion may be responsible for the generation of cells that express an actin isoform associated with smooth muscle.