Phenotypic characterization of alveolar capillary endothelial cells, alveolar epithelial cells and alveolar macrophages in patients with pulmonary fibrosis, with special reference to MHC class II antigens.

Major histocompatibility complex (MHC) class II antigens are essential in the presentation of antigens to T lymphocytes, and cells expressing MHC class II antigens are known to play a role as antigen presenting cells (APC). We investigated the distribution of MHC class II antigens and the reactivity of monoclonal antibodies OKM1 and OKM5 in normal and fibrotic lungs immunohistochemically. The results showed that alveolar capillary endothelial cells (ACEnd) expressed MHC class II antigens and were reactive with OKM5 in both normal lungs and the non-thickened parts of alveolar septa of pulmonary fibrosis. However, ACEnd did not express MHC class II antigens and were not reactive with OKM5 in thickened alveolar septa of pulmonary fibrosis. Type II alveolar epithelial cells (AEp) proliferating and replacing type I AEp in pulmonary fibrosis expressed MHC class II antigens strongly. Alveolar macrophages expressed MHC class II antigens strongly and reacted with OKM1 in pulmonary fibrosis, especially in alveolar spaces. These findings suggest that the phenotypic changes of ACEnd may be involved in the process of pulmonary fibrosis, and type II AEp and alveolar macrophages in the parts of thickened alveolar septa may play a role as APC.