Mechanisms of tissue injury in desquamative interstitial pneumonitis

To evaluate a contribution of immunologic factors to the pathogenesis of desquamative interstitial pneumonitis (DIP), lung biopsy specimens from four patients were studied for immunoglobulin deposits in tissue and cellular characteristics by immunologic, ultrastructural and histochemical methods. Accumulations of large cells with vacuolated cytoplasm within the distal air spaces and marked increase in the numbers of type II pneumocytes lining pulmonary alveoli were observed in all cases. The cells in air spaces were identified as macrophages containing intracellular lysozyme and alpha-naphthyl acetate esterase. Deposits of immunoglobulin G (IgG) and the third component of complement were found in distal air spaces and on the surfaces of the accumulated macrophages. The interstitial fibrosis was not a significant feature in our patients. Circulating immune complexes and a decreased IgG level were detected in serum during the acute phase of the disease. IgG levels returned to normal and were no longer detectable during convalescence in one patient followed sequentially. The formation and deposition of complement-fixing antibody and/or immune complexes may be responsible for the local accumulation and activation of macrophages and for tissue damages. Release of lysosomal enzymes by alveolar macrophages phagocytosing the complexes could also contribute to the alveolar injury, whereas the proliferation of type II pneumocytes may be a reparative tissue reaction to immunologically-mediated injury.