Purification of Pneumocystis carinii trophozoites and identification of their circulating antigens

Cell wall antigens of Pneumocystis carinii trophozoites and cysts: purification and carbohydrate analysis of these glycoproteins

We have purified and biochemically analyzed individual cell wall glycoproteins of Pneumocystis carinii. Our results show that corresponding core glycoproteins constitute the cell wall antigens in both trophozoites and cysts, and glycosylation of these glycoproteins does not appear to be significantly altered during development. Cysts and trophozoites in rat-derived organism preparations were separated from each other by counterflow centrifugal elutriation, then treated with Zymolyase to obtain the cell wall fractions. Gel electrophoresis patterns of these fractions from both life-cycle stages were qualitatively similar. Ten major antigenic glycoproteins in these fractions were purified by preparative continuous elution gel electrophoresis. All ten glycoproteins from cysts and trophozoites contained mannose, glucose, galactose, and N-acetylglucosamine, and some contained traces of fucose. The glycoproteins of cysts had more mannose than their trophozoite counterparts. The trophozoite glycoproteins differed from those of the cyst by the presence of xylose. To examine the species-specificity of glycoprotein glycosylation, preparations of human-derived P. carinii (comprised of mixed life-cycle stages) were also examined and found to contain the same sugars as those found in rat-derived organisms. Most of the purified rat-derived glycoproteins bound Concanvalin A, which was abolished by treatment with N-glycanase. This suggested that the majority of the oligosaccharides were N-linked to the proteins, but attempts to identify carbohydrate linkage sites by amino acid sequencing were hampered by apparent modifications of residues. The peptides derived by cyanogen bromide cleavage revealed distinct size patterns for each glycoprotein, suggesting that they were distinct proteins. Most of the glycoproteins reacted with monoclonal antibodies which recognize a highly conserved epitope on rat P. carinii. Four of the individually purified glycoprotein preparations elicited in vitro cellular immune responses, implicating their involvement in the recognition of P. carinii by host T cells. The identification and characterization of P. carinii cell wall proteins will be helpful in analyzing the relationship of the organism to its mammalian host.

Pneumocystis carinii pneumonia: the status of Pneumocystis biochemistry

Pneumocystis carinii pneumonia remains a prevalent opportunistic disease among immunocompromised individuals. Although aggressive prophylaxis has decreased the number of acute P. carinii pneumonia cases, many patients cannot tolerate the available drugs, and experience recurrence of the infection, which can be fatal. It is now generally agreed that the organism should be placed with the fungi, but the identification of extant fungal species representing its closest kins, remains debated. Most recent data indicate that P. carinii represents a diverse group of organisms. Since the lack of methods for the continuous subcultivation of this organism hampered P. carinii research, molecular cloning and nucleotide sequencing approaches led the way for understanding the biochemical nature of this pathogen. However, within the last 5 years, the development of improved protocols for isolating and purifying viable organisms from infected mammalian host lungs has enabled direct biochemical and metabolism studies on the organism. The protein moiety of the major high mol. wt surface antigen, represented by numerous isoforms, is encoded by different genes. These proteins are post-transcriptionally modified by carbohydrates and lipids. The organism has the shikimic acid pathway that leads to the formation of compounds which mammals cannot synthesise (e.g., folic acid), hence drugs that inhibit these pathways are effective against the pathogen. Ornithine decarboxylase has now been detected; rapid and complete depletion of polyamines occurs in response to difluoromethylornithine (DFMO). Instead of ergosterol (the major sterol of higher fungi), P. carinii synthesises distinct delta7, C-24-alkylated sterols. An unusual C32 sterol, pneumocysterol, has been identified in human-derived P. carinii. Another signature lipid discovered is cis-9,10-epoxy stearic acid. CoQ10, identified as the major ubiquinone homologue, is synthesised de novo by P. carinii. Atovaquone and other hydroxynaphthoquinone drugs with anti-P. carinii activity probably inhibit pathogen respiration as CoQ analogues. Unlike its effects on Plasmodium, atovaquone does not inhibit the P. carinii dihydroorotate dehydrogenase and pyrimidine metabolism.

Fractionation of Pneumocystis carinii developmental stages by counterflow centrifugal elutriation and sequential filtrations

Immunomagnetic sorting, sequential filtrations, and counterflow centrifugal elutriation were compared for their ability to obtain enriched populations of Pneumocystis carinii developmental stages from infected rat-lung homogenates. Elutriation combined with sequential filtrations resulted in highly (> 95%) enriched populations of P. carinii cysts and trophozoites with excellent viability. This approach offers advantages over previously described methods of obtaining enriched P. carinii cell populations and should have important applications to research on this organism.