Airway smooth muscle mass reduction after Bronchial Thermoplasty in asthmatics correlates with FEV1

Background: The mechanism of action of Bronchial Thermoplasty (BT) and its related responder profile is unclear. Objective: Investigate if BT-induced airway smooth muscle mass (ASM) change correlates with FEV1. Methods: 16 severe asthma patients were analyzed. Before and after BT, pre- and post-bronchodilator FEV1 was measured and bronchial biopsies were taken and stained with ASM-specific desmin and ASM/myofibroblast/myoepithelial α-smooth muscle actin (α-SMA). Results: BT treatment reduced ASM mass significantly as assessed by both desmin (55%) and α-SMA analysis (40%)(Fig1A). Baseline pre-and post-bronchodilator FEV1 correlated negatively with desmin-determined baseline ASM mass (resp. R=-0.61;p=0.01 and R=-0.62;p=0.01). Furthermore desmin-determined ASM mass decrease correlated negatively with baseline pre- and post-bronchodilator FEV1 (resp. R=-0.61;p=0.01 and R=-0.54;p=0.03 Fig1B). Patients with FEV1 80%(n=10): 2.4%(0.4;6.3 IQR (p=0.01)Fig1C). This correlation was not found with α-SMA staining. Conclusions: The correlation between ASM and FEV1 seems to depend on the fully differentiated contractile ASM only. Patients with a FEV1< 80% show the most reduction of ASM mass after BT treatment. Whether this “obstructive” asthma phenotype qualifies as a responder criteria needs to be determined.