Xinran Zhao: Effects of mast cell proteases on lung structural cells: Implications for asthma

Abstract

Asthma is a prevalent chronic inflammatory condition that impacts the airways of the lungs. Its hallmark symptoms include wheezing, difficulty of breathing, chest tightness and coughing. Allergic asthma represents the most common form of this condition. While previous investigations have established a pivotal role of mast cells in asthma, the precise underlying mechanisms remain unclear. Mast cells are immune cells with a high content of secretory granules, containing a diverse array of bioactive substances such as histamine, cytokines, proteoglycans, and mast cell-restricted proteases such as tryptase, chymase, and carboxypeptidase A3. This thesis aims to investigate the regulatory role of mast cell-restricted proteases in modulating airway responses across the three layers of the airway wall, including human lung fibroblasts (HLF), human small airway epithelial cells (HSAEC), and human bronchial/tracheal smooth muscle cells (SMC).

In Paper I, primary HLFs were subjected to in vitro challenges with two distinct mast cell proteases: human β-tryptase and recombinant human chymase. The chymase-treated group exhibited significant morphological alterations compared to minimal effects observed in the tryptase-treated group. Additionally, chymase demonstrated the ability to modulate the extracellular matrix (ECM), cytokine, and chemokine output from primary HLFs. Transcriptome analysis revealed that chymase induced a proinflammatory gene transcription profile in HLFs, whereas tryptase had minimal effects.

In Paper II, the effect of mast cell proteases on primary HSAEC monolayers was evaluated. The HSAECs were relatively refractory to tryptase. In contrast, chymase was found to suppress the expression of ECM-related genes, degrade fibronectin, and reduce the migratory capacity of HSAECs. Chymase also degraded the cell-cell contact protein E-cadherin on the epithelial cell surface.

In Paper III, the impact of mast cell proteases on primary human SMCs was investigated. Chymase induced a profound reduction in SMC metabolic activity and cell proliferation, while tryptase had minimal effects. Our findings also revealed that chymase partially suppressed SMC contractility and enhanced the migratory capacity of airway SMCs. Chymase also caused reorganization of major cytoskeletal components and degraded tight junction proteins expressed by SMCs.

Our investigations increased the current knowledge of the effects of mast cell proteases on lung structural cells. These findings have the potential to unravel novel mechanisms of relevance in the pathology of asthma.