An immunohistochemical study of alveolar macrophages, with special reference to class II MHC expression.

Abstract

To elucidate the role of alveolar macrophages in inflammatory and fibrosing processes of lung, the paraquat-induced alveolitis in rats was investigated by the immunohistochemical method with especial references to the class II major histocompatibility antigen complex (MHC) expression. Two days after the peritoncal injection of paraquat, the class II MHC expression by type II alveolar epithelial cells was observed prior to obvious histological and immunohistochemical changes except for pulmonary edema. From four days after the injection, alveolar macrophages increased in number, and expressed class II MHC, which was not demonstrated in the normal control. The intensity of the expression was variable, and alveolar macrophages containing abundant dust particles were still absent for class II MHC. On the contrary, the class II MHC expression by alveolar epithelial cells turned obscure. Two weeks after the injection, alveolar fibrosis developed and inflammatory cell infiltration was almost disappeared. A small number of class II MHC negative foamy cells were observed in the alveolar spaces. Alveolar capillary endothelial cells and alveolar septal cells, which expressed class II MHC in the normal control, did not show significant changes in paraquat-induced alveolitis. These results suggest that resident alveolar macrophages with the active phagocytic activity are not the main component in inflammatory reactions in the lung. In contrast, type II alveolar epithelial cells participate invariably in inflammatory process of alveolitis.