LAUSR

&NA; Activated bronchial epithelial cells (BEC) release various alarmins, including thymic stromal lymphopoietin (TSLP), that drive type 2 inflammation. We hypothesize that BEC‐derived factors promote in situ eosinophil differentiation and maturation, a process that is driven by an IL‐5‐rich microenvironment in asthmatic airways. To assess the eosinophilopoietic potential of epithelial‐derived factors, eosinophil/basophil colony forming units (Eo/B‐CFU) were enumerated in 14‐day methylcellulose cultures of blood‐derived nonadherent mononuclear cells incubated with BEC supernatants (BECSN) from healthy nonatopic controls (n = 8), mild atopic asthmatics (n = 9), and severe asthmatics (n = 5). Receptor‐blocking antibodies were used to evaluate the contribution of alarmins. Modulation of the mRNA expression of transcription factors that are crucial for eosinophil differentiation was evaluated. BECSN stimulated the clonogenic expansion of eosinophil progenitors in vitro. In the presence of IL‐5, Eo/B‐CFU numbers were significantly greater in cocultures of BESCN from severe asthmatics compared with other groups. This was attenuated in the presence of a TSLP (but not an IL‐33) receptor‐blocking antibody. Recombinant human TSLP (optimal at 100 pg/ml) stimulated Eo/B‐CFU growth, which was significantly enhanced in the presence of IL‐5 (1 ng/ml). Overnight culture of CD34+ cells with IL‐5 and TSLP synergistically increased GATA‐binding factor 2 and CCAAT/enhancer‐binding protein &agr; mRNA expression. The eosinophilopoietic potential of factors derived from BEC is increased in severe asthma. Our data suggest that TSLP is a key alarmin that is produced by BECs and promotes in situ eosinophilopoiesis in a type 2‐rich microenvironment.