Background Thymic stromal lymphopoietin (TSLP), a pleiotropic cytokine mainly expressed by epithelial cells, plays a key role in asthma pathobiology. In humans, TSLP exists in two variants: the long form TSLP (lfTSLP) and a shorter TSLP isoform (sfTSLP), overlapping the lfTSLP C-terminus. Macrophages (HLMs) and mast cells (HLMCs) are in close proximity in the human lung and play central roles in different asthma phenotypes. Methods Immunofluorescence and Western blot were employed to localize intracellular TSLP. Limited proteolysis and mass spectrometry allowed the identification of cleavage sites of TSLP caused by tryptase and chymase. ELISA assays were employed to measure TSLP and VEGF-A. Results TSLP was detected in highly purified (≥ 99%) macrophages isolated from human lung and subcellularly localized in the cytoplasm by confocal microscopy and Western blot. IL-4 and lipopolysaccharide induced the release of TSLP from HLMs. HLMCs contain and release tryptase and chymase that specifically cleaved TSLP. Mass spectrometric analyses of TSLP treated with tryptase showed the production of 1-97 and 98-132 fragments. Chymase treatment of TSLP generated two peptides 1-36 and 37-132. HLM activation by lfTSLP induced VEGF-A, the most potent angiogenic factor, release. The four TSLP fragments generated by tryptase and chymase failed to activate HLMs. sfTSLP neither activated HLMs nor interfered with activating property of lfTSLP on HLMs. Conclusions Given the close proximity between mast cells and macrophages in the human lung, our results illuminate a new circuit between HLMs and mast cells. These findings have potential relevance in understanding novel aspects of asthma pathobiology.

Asthma is a chronic inflammatory airway disease resulting in airflow obstruction, which in part can become irreversible to conventional therapies, defining the concept of airway remodeling. The introduction of biologics in severe asthma has led in some patients to the complete normalization of previously considered irreversible airflow obstruction. This highlights the need to distinguish a “fixed” bronchial obstruction due to structural changes unresponsive to current therapies, from a “reversible” one as demonstrated by lung function normalization during biological therapies not previously obtained even with high dose systemic glucocorticoids. The mechanisms by which exposure to environmental factors initiates the inflammatory responses that trigger airway remodeling are still incompletely understood. Alarmins represent tissue-derived cytokines that initiate immunologic events leading to inflammatory airway remodeling. Biological therapies can improve airflow obstruction by addressing these airway inflammatory changes. In addition, biologics might prevent and possibly even revert “fixed” remodeling due to structural changes. Hence, it appears clinically important to separate the therapeutic effects (early and late) of biologics as a new paradigm to evaluate the effects of these drugs and future treatments on airway remodeling in severe asthma.