Emerging roles of T helper subsets in the pathogenesis of asthma

The cardinal features of asthma include pulmonary inflammation and airway hyperresponsiveness (AHR). Classically, asthma, specifically allergic asthma, has been attributed to a hyperactive Th2 cell immune response. However, the Th2 cell-mediated inflammation model has failed to adequately explain many of the clinical and molecular aspects of asthma. In addition, the outcomes of Th2-targeted therapeutic trials have been disappointing. Thus, asthma is now believed to be a complex and heterogeneous disorder, with several molecular mechanisms underlying the airway inflammation and AHR that is associated with asthma. The original classification of Th1 and Th2 pathways has recently been expanded to include additional effector Th cell subsets. These include Th17, Th9 and Treg cells. Emerging data highlight the involvement of these new Th cell subsets in the initiation and augmentation of airway inflammation and asthmatic responses. We now review the roles of these recently classified effector Th cell subsets in asthmatic inflammation and the insights they may provide in addition to the traditional Th2 paradigm. The hope is that a clearer understanding of the inflammatory pathways involved and the mediators of inflammation will yield better targeted therapeutics.