Background: There are several clinical variants of severe difficult-to-treat asthma: asthma with persistent airflow limitation, brittle asthma, and fatal asthma; but the differences between the pathogenetic mechanisms underlying the disease heterogeneity are unknown.
Objectives: The aim was to evaluate the morphological and molecular characteristics of brittle asthma type I and asthma with persistent airflow limitation compared to mild-to-moderate asthma, by the analysis of the cellular structure and gene expression in the bronchial mucosa.
Methods: Bronchoscopic evaluation was performed in 42 asthmatic patients: 10 with brittle asthma, 10 with severe asthma with persistent airflow limitation, and 22 with mild-to-moderate asthma. Morphometric and cytological analyses of the bronchial mucosa were performed. The mRNA levels for the ADRB2, HRH1, and CHRM3 genes in the bronchial mucosa were measured by quantitative real-time polymerase chain reaction (PCR).
Results: A predominance of eosinophils (29.48/mm(2), 95% confidence interval [CI] 25.24-33.72) and neutrophils (40.13/mm(2), 95% CI 32.77-47.49) was observed in patients with mild-to-moderate asthma; however, histiocytes-macrophages (65.80/mm(2), 95% CI 56.95-74.65) and lymphocytes (52.94/mm(2), 95% CI 42.83-63.06) were more common in patients with brittle asthma, and neutrophil counts (81.11/mm(2), 95% CI 58.33-103.89) were significantly increased in subjects with persistent airflow limitation. An increase in the expression of the M(3)-cholinoreceptor and the beta(2)-adrenoreceptor genes was demonstrated in severe asthmatics compared to mild-to-moderate asthma patients. Significantly higher levels of CHRM3 (57.17%, 95% CI 55.04-59.29) and HRH1 (38.82%, 95% CI 35.84-41.81) mRNAs were observed in patients with brittle asthma. The level of ADRB2 gene expression (71.41%, 95% CI 63.54-85.09) was maximal in patients with asthma with persistent airflow limitation.
Conclusions: There is evidence of significantly different morphological characteristics and molecular mechanisms of inflammation and bronchoconstriction underlying the clinical heterogeneity of severe asthma.