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

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.

The lipids of Pneumocystis carinii

Information about a number of Pneumocystis carinii lipids obtained by the analyses of organisms isolated and purified from infected lungs of corticosteroid-immunosuppressed rats has been reported in recent years. Of the common opportunistic protists associated with AIDS (Cryptosporidium, Toxoplasma, and the microsporidia), more is currently known about the lipids of P. carinii than the others. Lipids that are synthesized by the organism but not by humans are attractive targets for drug development. Thus, the elucidation of delta 7C-24-alykylated sterol and cis-9,10-epoxystearic acid biosyntheses in P. carinii is currently being examined in detail, since these have been identified as P. carinii-specific lipids. The development of low-toxicity drugs that prevent sterol C-24 alkylation and the specific inhibition of the lipoxygenase that forms cis-9,10-epoxystearic acid might prove fruitful. Although humans can synthesize coenzyme Q10, the anti-P. carinii activity and low toxicity of ubiquinone analogs such as atovaquone suggest that the electron transport chain in the pathogen may differ importantly from that in the host. Although resistance to atovaquone has been observed, development of other naphthoquinone drugs would provide a broader armamentarium of drugs to treat patients with P. carinii pneumonia. Studies of bronchoalveolar lavage fluid and of infected lungs have demonstrated that the infection causes a number of chemical abnormalities. Bronchoalveolar lavage fluid obtained after the removal of lung cellular material and the organisms has been shown to contain larger amounts of surfactant proteins and smaller amounts of phospholipids than do comparable samples from P. carinii-free lungs. Increased phospholipase activity, inhibition of surfactant secretion by type II cells, and uptake and catabolism of lipids by the pathogen may explain this phenomenon related to P. carinii pneumonia. Although not yet thoroughly examined, initial studies on the uptake and metabolism of lipids by P. carinii suggest that the organism relies heavily on exogenous lipid nutrients.

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.

Uptake and metabolism of L-serine by Pneumocystis carinii carinii

Serine is an important amino acid that is utilized in the biosyntheses of proteins and lipids. It is directly incorporated into the head group of phosphatidylserine, which in turn can be converted to other phospholipids. Also, it is required for the formation of long chain bases, precursors of sphingolipids. Uptake and incorporation of radiolabeled serine into both lipids and acid-precipitable material were demonstrated in Pneumocystis carinii carinii organism preparations freshly isolated from infected rat lungs. Radioactivity in proteins was about double that observed in lipids. Liquid scintillation spectrometry of metabolically radiolabeled lipids separated by thin-layer chromatography showed 53% of the total radioactivity were in phosphatidylserine, 12% in phosphatidylethanolamine, 24% in ceramides, and 11% in long chain bases and other compounds. Four long chain bases were detected by thin-layer chromatography in hydrolyzed P. carinii ceramides metabolically labeled with radioactive serine. Phytosphingosine and dihydrosphingosine were tentatively identified by their migrations on thin-layer plates. Radiolabeled ethanolamine was incorporated into P. carinii phosphatidylethanolamine, but relatively low incorporation of radiolabeled choline into phosphatidylcholine occurred. The observations made in this study indicated that P. carinii has the biosynthetic capacity to metabolize phospholipid head groups and to de novo synthesize sphingolipids. L-Cycloserine and beta-Cl-D-alanine, inhibitors of long chain base synthesis, reduced the incorporation of serine into P. carinii long chain bases and ceramides, which supported the conclusion that the pathogen synthesizes sphingolipids.