The antigen location of Cryptosporidium parvum, which stimulates antibody formation in humans and animals, was investigated using infected human sera. Immuno-electron microscopy revealed that antigenicity-inducing humoral immunity was located at various developmental stages of parasites, including asexual, sexual stages, and oocysts. The amount of antigen-stimulating IgG antibodies was particularly high on the oocyst wall. The sporozoite surface was shown to give stimulation on IgG and IgM antibody formation. Trophozoites implicated the lowest antigenicity to humoral immunity, both IgG and IgM, by showing the least amount of gold labeling. Immunogold labeling also provided clues that antigens were presented to the host-cell cytoplasm via feeder organelles and host-parasite junctions.

Pneumocystis carinii causes serious pulmonary infection in immunosuppressed patients. This study was undertaken to observe the cytoskeletal proteins of P. carinii by immuno-electron microscopy. P. carinii infection was experimentally induced by immunosuppression of Sprague-Dawley rats for seven weeks, and their lungs were used for the observations of this study. The gold particles localized actin, tropomyosin, and tubulin. The actin was irregularly scattered in the cytoplasm of the trophic forms but was much more concentrated in the inner space of the cell wall of the cystic forms called the inner electron-lucent layer. No significant amount of tropomyosin was observed in either trophic forms or cystic forms. The tubulin was distributed along the peripheral cytoplasm and filopodia of both the trophic and cystic forms rather than in the inner side of the cytoplasm. Particularly, in the cystic forms, the amount of tubulin was increased and located mainly in the inner electron-lucent layer of the cell wall where the actin was concentrated as well. The results of this study showed that the cell wall of P. carinii cystic forms is a structure whose inner side is rich in actin and tubulin. The location of the actin and tubulin in P. carinii suggests that the main role of these proteins is an involvement in the protection of cystic forms from the outside environment by maintaining rigidity of the cystic forms.