Morphological and quantitative assessment of mast cells in rheumatoid arthritis associated non-specific interstitial pneumonia and usual interstitial pneumonia

Background: The role of mast cells in extra-articular manifestations of rheumatoid arthritis (RA) has not been studied so far. Objective: To characterise and quantify mast cells in RA associated interstitial pneumonia (IP) by an immunohistological study. Methods: Lung biopsy specimens from 15 patients with RA associated IP, 12 patients with idiopathic IP, and 5 control patients were stained with antibodies directed against tryptase (mast cell marker). Morphological characterisation of stained specimens was carried out and staining was quantified by computer assisted image analysis. Results: Tryptase staining showed the marked presence of mast cells in idiopathic IP and in RA associated IP. A significant difference in stained tissue area was found between RA associated IP (2.0%, IQR 2.0-3.2%, p = 0.015) and idiopathic IP (3.1%, IQR 1.8-3.7%, p = 0.003) compared with control tissue specimens (1.0%, IQR 0.7-1.5%). The extent of mast cell infiltration correlated well and inversely with pulmonary function variables. Conclusions: Mast cell infiltrates are present in RA associated IP and idiopathic IP. The observed correlation of pulmonary function and mast cell numbers would be consistent with the proposed role of mast cell mediators in the promotion of fibrogenesis. The findings provide a rationale for studying functional aspects of mast cell involvement in the pathogenesis of RA associated lung disease.