Background
Recent observations indicate that epithelial alarmins thymic stromal lymphopoetin (TSLP), interleukin (IL)-33, and IL-25 are key signalling molecules in asthmatic airway inflammation.

Aim
The aim was to test if combined pre-treatment with TSLP, IL-33 and IL-25 (alarmin mix) affected bronchoconstriction in isolated human small airways.

Methods
Human lung tissue was obtained with consent from 17 patients undergoing lobectomies. Intact segments of isolated small bronchi (inner ? 0.5-2 mm) were exposed to the alarmin mix (100 ng/mL each) for 48 hours. The segments were placed in organ baths where contractile responses to first histamine and then anti-human IgE antibody (anti-IgE, 0.5 and 5 ?g/mL) were studied in myographs, and mediator release was analysed.

Results
Pre-treatment with the alarmin mix enhanced the maximal contractile response (36.9±6.8 vs. 14.5±4.9%) to anti-IgE (0.5 ?g/mL) and the time to reach peak contraction in response to anti-IgE (5 ?g/mL) was shortened (24.4±1.0 vs. 28.2±1.2 min) compared to untreated. One hour after anti-IgE exposure, levels of prostaglandin (PG)D₂ (146.8±24.3 vs. 87.4±9.0 ng/mg tissue) and histamine (2.9±0.7 vs. 0.8±0.2-fold change from baseline) were increased in the bath fluid from the alarmin mix pre-treated group compared to untreated. In contrast, the pre-treatment with the alarmin mix did not affect the histamine-mediated contractile response.

Conclusions
Epithelial alarmins enhanced the IgE-dependent bronchoconstriction by increasing the release of mast cell mediators PGD₂ and histamine without changing smooth muscle responsiveness to histamine. These findings highlight the potential of alarmins as regulators of airway mast cell responsiveness.