A novel molecular interaction in bronchiolitis obliterans syndrome in lung transplantation patients: the role of SERPINA3 and osteoprotegerin

Background and objectiveChronic lung allograft dysfunction, significantly limits survival after lung transplantation. The obstructive phenotype bronchiolitis obliterans syndrome (BOS) is characterized by the abnormal activation of epithelium in airways accompanied by fibrotic changes with excessive deposition of extracellular matrix, resulting in narrowing and ultimately obliteration of airways. Mast cells, characterized by mediators such as tryptase and chymase, play a role in lung fibrosis. The proteins, osteoprotegerin (OPG) and SERPIN family member A 3 (SERPINA3) have previously been associated with progression of lung fibrosis. Tryptase-mast cells can produce OPG, while chymase released from mast cells interacts with SERPINA3. This study aimed to investigate if and how SERPINA3, OPG and tryptase/chymase-positive mast cells are related to fibrotic airway obliteration, and potentially show an association with BOS severity. MethodsSERPINA3 levels in serum from patients with or without BOS were examined using ELISA. In lung tissue from patients with BOS, we identified non-cartilaginous airways and classified them into normal, partially and completely obstructed airways. Immunohistochemistry was used for detection of SERPINA3, OPG, chymase and tryptase. Colocalization of, and interactions between, SERPINA3, OPG and tryptase were assessed by immunofluorescence, proximity ligation assay, and AlphaFold modeling. ResultsSERPINA3 levels in serum from BOS patients were higher compared to non-BOS patients. A low percentage of SERPINA3 and OPG was detected in partially and completely obstructed airways. Cells positive for OPG and SERPINA3 colocalized with tryptase-mast cells in airways from BOS lung tissue. OPG colocalized with SERPINA3 and their staining positively correlated in partially obstructed airways. In completely obstructed airways, OPG, SERPINA3 and tryptase staining areas all positively correlated with each other. ConclusionSERPINA3 and OPG are expressed by mast cells and these proteins may form a complex in lung tissue with potential for contributing to airway remodeling in BOS.