Innate players in Th2 and non-Th2 asthma: emerging roles for the epithelial cell, mast cell, and monocyte/macrophage network

Asthma is one of the most common chronic respiratory diseases and is characterized by airway inflammation, increased mucus production, and structural changes in the airways. Recently, there is increasing evidence that the disease is much more heterogeneous than expected, with several distinct asthma endotypes. Based on the specificity of T cells as the best-known driving force in airway inflammation, bronchial asthma is categorized into T helper cell 2 (Th2) and non-Th2 asthma. The most studied effector cells in Th2 asthma include T cells and eosinophils. In contrast to Th2 asthma, much less is known about the pathophysiology of non-Th2 asthma, which is often associated with treatment resistance. Besides T cells, the interaction of myeloid cells such as monocytes/macrophages and mast cells with the airway epithelium significantly contributes to the pathogenesis of asthma. However, the underlying molecular regulation and particularly the specific relevance of this cellular network in certain asthma endotypes remain to be understood. In this review, we summarize recent findings on the regulation of and complex interplay between epithelial cells and the "nonclassical" innate effector cells mast cells and monocytes/macrophages in Th2 and non-Th2 asthma with the ultimate goal of providing the rationale for future research into targeted therapy regimens.