Pneumocystis carinii: electron microscopic investigation on the interaction of trophozoite and alveolar lining cell.

The habitat, behaviour of Pneumocystis carinii, and the interaction between the organisms and host alveolar cells were studied electron microscopically in corticosteroid-treated rats. The trophozoites of P. carinii used to line up along the type I alveolar epithelial cells at the early stage of infection. No organism was found being adhere to type II cells which have numerous microvilli on their surface. By adding tannic acid to the fixative we could preserve the alveolar lining layer, in which the organisms were found all the time. Therefore P. carinii is not exposed to alveolar air but is living in the liquid layer. Those findings are important in considering nutrition of the organism as well as pathogenicity to the host. The trophozoites and type I cells were in close contact mostly with their smooth surface rather than with tubular expansions of the organism. The cytoplasm of type I cell was often found protruded along the pellicle of the organism as if it surrounded the organism. Phagocytosis of P. carinii by alveolar macrophage and neutrophil was often seen in the alveolar lumen when the infection advanced. Sometimes the trophozoites were found sub- and intra- epithelially . Those trophozoites seemed alive from their ultrastructures. However, it remained unsolved, whether those figures were the resultants of active invasion of the trophozoite or of being surrounded by the epithelial cells.