Airway smooth muscle as a target in asthma.

Traditionally, the contractile properties of airway smooth muscle have been regarded as its sole contribution to the pathogenesis of asthma. However, our understanding of the role that this structural cell plays in asthma is changing. Airway smooth muscle can undergo hyperplasia and/or hypertrophy leading to structural changes in the airway wall which contribute to the development of persistent airway obstruction and increased non-specific airway hyperresponsiveness in chronic severe asthma. Many studies in vitro have characterized airway smooth muscle proliferation induced by various pro-inflammatory mediators, growth factors and components of the extracellular matrix, but the mediator(s) responsible for the observed increase in airway wall smooth muscle content in vivo remain to be determined. In addition to geometric obstruction by increased airway wall thickening, proliferating airway smooth muscle cells undergo phenotypic modulation from a contractile to synthetic-proliferative state where additional functions of airway smooth muscle such as cytokine/chemokine and extracellular matrix secretion may become more apparent. This may be especially relevant in the diseased airway where the content of airway smooth muscle as a fraction of the total cells present in the airway wall is already increased. Airway smooth muscle cells may also interact by direct contact with immunocytes such as T lymphocytes through expression of cell adhesion molecules with the result that myocyte DNA synthesis is induced. As additional functions of airway smooth muscle are described, a more contemporary view is emerging that airway smooth cells may adopt an immuno-effector role in chronic asthma by proliferating, secreting cytokines, expressing adhesion molecules and by interacting with various inflammatory cells. This may involve changes in the phenotypic status of airway smooth muscle, and as a result, these cells may play an active role in perpetuating and orchestrating airway inflammation in the remodelled airway. An important phase for future airway smooth muscle research will be to determine the extent that these putative mechanisms exist in vivo in the pathogenesis of chronic severe asthma.