Novel Site-Specific Mast Cell Subpopulations in the Human Lung.

BACKGROUND: Lung mast cells are stereotypically divided into connective tissue (MCTC) and mucosal (MCT) mast cells. This study tests the hypothesis that each of these subtypes can be divided further into site-specific populations created by the microenvironment within each anatomic lung compartment. METHODS: To study mast cells under non-inflamed conditions surgical resections and bronchial and transbronchial biopsies from non-smoking individuals were obtained to investigate morphometric and molecular characteristics of mast cell populations in multiple lung structures by immunohistochemistry and electron microscopy. RESULTS: MCT and MCTC coexisted at all compartments with MCT being the prevailing type in bronchi, bronchioles and the alveolar parenchyma. MCTC were more abundant in pulmonary vessels and the pleura. Each of the MCTC and MCT phenotypes could be further differentiated into site-specific populations. MCTC was of significantly larger size in pulmonary vessels than in small airway walls (p<0.001) while a reversed pattern was observed for MCT (p<0.001). Within each MCTC and MCT population there was also distinct site-specific expression pattern of the IgE-receptor, IL-9 receptor, renin, histidine decarboxylase, VEGF, FGF, 5-Lipoxygense, and LTC4-synthase; e.g. bronchial MCT consistently expressed more histidine decarboxylase than alveolar MCT (p<0.004). Renin content was high among vascular MCTC but markedly reduced among MCTC in other compartments (p<0.0002). Notably, for both MCTC and MCT IgE-receptor was highly expressed in conducting airways but virtually absent in alveolar parenchyma. CONCLUSION: Our findings demonstrate novel site-specific sub-populations of lung MCTC and MCT. This observation is suggested to have important implications in unravelling the recently proposed role of mast cells in a variety of pulmonary diseases.