Advances in our understanding of the pathogenesis of fibrotic lung disease

Production of interleukin 13 by alveolar macrophages from normal and fibrotic lung

Human interleukin 13 (IL-13) is a cytokine that has a profound effect on primary immune cells by inducing immunoglobulin production, proliferation of B cells, and the differentiation of cells of the monocytic lineage. IL-13 can inhibit the production of inflammatory cytokines by both macrophages and monocytes. Previously, IL-13 expression has been reported only in cells of the T-cell lineage and the mast cell line HMC-1. We now report the presence of IL-13 mRNA and protein in human alveolar macrophages (AMs) analyzed by the reverse transcription-polymerase chain reaction (RT-PCR) and enzyme-linked immunoabsorbent assay (ELISA), respectively, and IL-13 protein in bronchoalveolar lavage fluid (BALF) of subjects with pulmonary fibrosis. We have investigated 13 patients from 49 to 75 yr of age with forms of pulmonary fibrosis, and eight healthy volunteers from 24 to 61 yr of age. Their AMs were obtained by bronchoalveolar lavage (BAL) and purified by adherence. The proportion of BAL purified AMs expressing IL-13 mRNA was increased in those subjects with fibrotic lung disease, in comparison with those from control subjects (11 of 13 versus 2 of 8, P < 0.01). IL-13 protein was detectable in the BALF of 8 of 13 patients with pulmonary fibrosis, but in none of the control subjects. AMs of four subjects with systemic sclerosis were cultured and IL-13 protein was increased in the culture supernatants when compared to the control subjects, although this did not reach significance. These findings show that IL-13 mRNA is not only a product of T cells, but is also expressed in both normal AMs and those from subjects with pulmonary fibrosis, and that at least some of the IL-13 mRNA is translated into protein and secreted in subjects with pulmonary fibrosis. We hypothesize that IL-13 may be expressed by normal human AMs as part of the homeostatic control process but its production may be increased in the presence of inflammatory lung disease.

Apposition of fibroblasts to mast cells and lymphocytes in normal human lung and in cryptogenic fibrosing alveolitis. Ultrastructure and cell perimeter measurements

The perimeters of mast cells and lymphocytes in human lungs were measured in electron micrographs by digitizer to determine the percentage of perimeter apposed to fibroblast (PPAF). Fibroblasts were apposed to the majority of mast cells. The median PPAF for mast cells in normal lung was 50.3 per cent, and in cryptogenic fibrosing alveolitis (CFA), 35.7 per cent. Although the value in CFA was lower, the difference was not statistically significant (median difference 11.8; 95 per cent confidence interval (-19.6, 25.1); P = 0.65). The PPAF range overall was 3.8-94.1 per cent. There was similar apposition of fibroblasts to lymphocytes, and no statistical differences were found when median PPAF results for mast cells and lymphocytes were compared for normal and CFA lung. The high degree of percentage apposition, accurately quantified in this study, shows that fibroblasts, mast cells, and lymphocytes are ideally arranged structurally in normal alveolar walls, to facilitate the many physiological interactions which are currently being uncovered. The present study also shows that apposition persists in pathological states, e.g., CFA lung, but because all these cells are increased in number in CFA lung, apposition is easier to identify here than in normal lung.