Attachment of Pneumocystis carinii to rat pneumocytes

Male Sprague-Dawley rats were immunosuppressed by subcutaneous injections of cortisone acetate for 8 weeks to induce Pneumocystis carinii pneumonia. Rats were killed with ether, their lungs were filled in situ with cold glutaraldehyde, and sections were examined by transmission electron microscopy. P. carinii trophozoites were observed in lungs for as long as 6 weeks after cessation of corticosteroid injections. Trophozoites were attached most frequently to type I pneumocytes but were seen on several occasions to be attached to type II pneumocytes. In attachment, trophozoites in contact with alveolar epithelial cells produced cytoplasmic modifications of three basic forms: one was an invagination of parasite surface to pinch cytoplasmic extensions on the surface of the epithelial cells. A second was an extension that was inserted into a crevice of the host surface and enlarged at the tip to form an anchor. The third was an extension that remained pointed at the tip and was recurved to form a hook. None of these modifications was seen on the surface of the trophozoites. Filopodia did not appear to be involved in attachment but were seen on both attached and lumenal surfaces of the trophozoites. Trophozoites and cysts were also observed to be partially or completely enveloped by extensions of the host cell's cytoplasm. In these instances, parasites had rounded surfaces but appeared viable. No fusion of cell membranes between parasite and host cells was ever seen. These observations may help explain why P. carinii can attach tenaciously to lung epithelium without cell-membrane fusion, production of a glycocalyx, or invasion of host cells.