LIGHT AND ELECTRON-MICROSCOPY STUDY OF CARBOHYDRATE ANTIGENS FOUND INTHE ELECTRON-LUCENT LAYER OF PNEUMOCYSTIS-CARINII CYSTS

The localization and biochemical nature of antigens found in the elect ron-lucent layer (ELL) of Pneumocystis carinii cysts using polyclonal rabbit antibodies are described. These antigens, specific for the cyst ic stages of the parasite, were shared by organisms from different hos ts, suggesting that they represent functionally important components o f the cyst cell wall. The binding sites were situated on an interwoven net of fibrils in the ELL produced by mild to strong proteolysis, Deg radation of this residue by glucanase and chitinase confirms that this layer contains branched glucan and chitin. In contrast, the prompt su sceptibility of the polysaccharide-rich ELL to proteolysis reveals tha t proteins are also relevant in building up the cyst-wall glucan skele ton. It is therefore concluded that the formation of the Pneumocystis cyst wall shows differences to the typical fungal cell-wall architectu re. The taxonomical debate regarding this unique protist is on-going, and consideration of these immunological and morphological findings ma y be useful for the study of the biology and phylogeny of Pneumocystis .