Evidence for the presence of "metabolic sterols" in Pneumocystis: identification and initial characterization of Pneumocystis carinii sterols

Efficacy of Trimethoprim–Sulfamethoxazole in Combination with an Echinocandin as a First-Line Treatment Option for Pneumocystis Pneumonia: A Systematic Review and Meta-Analysis

Although combination therapy using trimethoprim–sulfamethoxazole (TMP–SMX) plus echinocandins has been reported to reduce the mortality of patients with pneumocystis pneumonia (PCP), it remains unclear whether it is more effective than TMP–SMX monotherapy, the current first-line treatment for this disease. Hence, we performed a systematic review and meta-analysis to compare the efficacies of these treatment options for PCP. The Scopus, EMBASE, PubMed, CINAHL, and Ichushi databases were searched for studies (up to January 2022) reporting the mortality and positive response rates (fewer clinical symptoms, improved partial pressure of arterial oxygen, and resolution of pneumonitis on chest imaging) of PCP patients receiving monotherapy or combination therapy. Four studies met the inclusion criteria. All four presented mortality data and one had positive response rates. Compared with the monotherapy, the combination therapy resulted in significantly lower mortality and higher positive response rates (mortality: odds ratio (OR) 2.20, 95% confidence interval (CI) 1.46–3.31; positive response rate: OR 2.13, 95%CI 1.41–3.23), suggesting it to be an effective and promising first-line therapy for PCP. However, further safety evaluations are needed to establish this as a fact.

Human albumin enhances the pathogenic potential of Candida glabrata on vaginal epithelial cells

The opportunistic pathogen Candida glabrata is the second most frequent causative agent of vulvovaginal candidiasis (VVC), a disease that affects 70–75% of women at least once during their life. However, C. glabrata is almost avirulent in mice and normally incapable of inflicting damage to vaginal epithelial cells in vitro. We thus proposed that host factors present in vivo may influence C. glabrata pathogenicity. We, therefore, analyzed the impact of albumin, one of the most abundant proteins of the vaginal fluid. The presence of human, but not murine, albumin dramatically increased the potential of C. glabrata to damage vaginal epithelial cells. This effect depended on macropinocytosis-mediated epithelial uptake of albumin and subsequent proteolytic processing. The enhanced pathogenicity of C. glabrata can be explained by a combination of beneficial effects for the fungus, which includes an increased access to iron, accelerated growth, and increased adhesion. Screening of C. glabrata deletion mutants revealed that Hap5, a key regulator of iron homeostasis, is essential for the albumin-augmented damage potential. The albumin-augmented pathogenicity was reversed by the addition of iron chelators and a similar increase in pathogenicity was shown by increasing the iron availability, confirming a key role of iron. Accelerated growth not only led to higher cell numbers, but also to increased fungal metabolic activity and oxidative stress resistance. Finally, the albumin-driven enhanced damage potential was associated with the expression of distinct C. glabrata virulence genes. Transcriptional responses of the epithelial cells suggested an unfolded protein response (UPR) and ER-stress responses combined with glucose starvation induced by fast growing C. glabrata cells as potential mechanisms by which cytotoxicity is mediated.Collectively, we demonstrate that albumin augments the pathogenic potential of C. glabrata during interaction with vaginal epithelial cells. This suggests a role for albumin as a key player in the pathogenesis of VVC.

Candida glabrata is the overall second causative species of candidiasis in humans, but little is known about the pathogenicity mechanisms of this yeast. C. glabrata is capable of causing lethal systemic candidiasis mostly in elderly immunocompromised patients, but is also a frequent cause of vulvovaginal candidiasis. These clinical insights suggest that C. glabrata has a high virulence potential, yet little pathogenicity is observed in both in vitro and in vivo infection models. The finding that human albumin, the most abundant protein in the human body, is boosting C. glabrata pathogenicity in vitro provides novel insights into C. glabrata pathogenicity mechanisms and shows that the presence of distinct human factors can have a significant influence on the virulence potential of a pathogenic microbe.

Ibrexafungerp: A First-in-Class Oral Triterpenoid Glucan Synthase Inhibitor

Ibrexafungerp (formerly SCY-078 or MK-3118) is a first-in-class triterpenoid antifungal or “fungerp” that inhibits biosynthesis of β-(1,3)-D-glucan in the fungal cell wall, a mechanism of action similar to that of echinocandins. Distinguishing characteristics of ibrexafungerp include oral bioavailability, a favourable safety profile, few drug–drug interactions, good tissue penetration, increased activity at low pH and activity against multi-drug resistant isolates including C. auris and C. glabrata. In vitro data has demonstrated broad and potent activity against Candida and Aspergillus species. Importantly, ibrexafungerp also has potent activity against azole-resistant isolates, including biofilm-forming Candida spp., and echinocandin-resistant isolates. It also has activity against the asci form of Pneumocystis spp., and other pathogenic fungi including some non-Candida yeasts and non-Aspergillus moulds. In vivo data have shown IBX to be effective for treatment of candidiasis and aspergillosis. Ibrexafungerp is effective for the treatment of acute vulvovaginal candidiasis in completed phase 3 clinical trials.

Pneumocystis Pneumonia: Immunity, Vaccines, and Treatments

For individuals who are immunocompromised, the opportunistic fungal pathogen Pneumocystis jirovecii is capable of causing life-threatening pneumonia as the causative agent of Pneumocystis pneumonia (PCP). PCP remains an acquired immunodeficiency disease (AIDS)-defining illness in the era of antiretroviral therapy. In addition, a rise in non-human immunodeficiency virus (HIV)-associated PCP has been observed due to increased usage of immunosuppressive and immunomodulating therapies. With the persistence of HIV-related PCP cases and associated morbidity and mortality, as well as difficult to diagnose non-HIV-related PCP cases, an improvement over current treatment and prevention standards is warranted. Current therapeutic strategies have primarily focused on the administration of trimethoprim-sulfamethoxazole, which is effective at disease prevention. However, current treatments are inadequate for treatment of PCP and prevention of PCP-related death, as evidenced by consistently high mortality rates for those hospitalized with PCP. There are no vaccines in clinical trials for the prevention of PCP, and significant obstacles exist that have slowed development, including host range specificity, and the inability to culture Pneumocystis spp. in vitro. In this review, we overview the immune response to Pneumocystis spp., and discuss current progress on novel vaccines and therapies currently in the preclinical and clinical pipeline.

Genomics and evolution of Pneumocystis species

The genus Pneumocystis comprises highly diversified fungal species that cause severe pneumonia in individuals with a deficient immune system. These fungi infect exclusively mammals and present a strict host species specificity. These species have co-diverged with their hosts for long periods of time (> 100 MYA). Details of their biology and evolution are fragmentary mainly because of a lack of an established long-term culture system. Recent genomic advances have unlocked new areas of research and allow new hypotheses to be tested. We review here new findings of the genomic studies in relation with the evolutionary trajectory of these fungi and discuss the impact of genomic data analysis in the context of the population genetics. The combination of slow genome decay and limited expansion of specific gene families and introns reflect intimate interactions of these species with their hosts. The evolutionary adaptation of these organisms is profoundly influenced by their population structure, which in turn is determined by intrinsic features such as their self-fertilizing mating system, high host specificity, long generation times, and transmission mode. Essential key questions concerning their adaptation and speciation remain to be answered. The next cornerstone will consist in the establishment of a long-term culture system and genetic manipulation that should allow unravelling the driving forces of Pneumocystis species evolution.

A Molecular Window into the Biology and Epidemiology of Pneumocystis spp

Pneumocystis, a unique atypical fungus with an elusive lifestyle, has had an important medical history. It came to prominence as an opportunistic pathogen that not only can cause life-threatening pneumonia in patients with HIV infection and other immunodeficiencies but also can colonize the lungs of healthy individuals from a very early age. The genus Pneumocystis includes a group of closely related but heterogeneous organisms that have a worldwide distribution, have been detected in multiple mammalian species, are highly host species specific, inhabit the lungs almost exclusively, and have never convincingly been cultured in vitro, making Pneumocystis a fascinating but difficult-to-study organism. Improved molecular biologic methodologies have opened a new window into the biology and epidemiology of Pneumocystis. Advances include an improved taxonomic classification, identification of an extremely reduced genome and concomitant inability to metabolize and grow independent of the host lungs, insights into its transmission mode, recognition of its widespread colonization in both immunocompetent and immunodeficient hosts, and utilization of strain variation to study drug resistance, epidemiology, and outbreaks of infection among transplant patients. This review summarizes these advances and also identifies some major questions and challenges that need to be addressed to better understand Pneumocystis biology and its relevance to clinical care.

Identification and Functional Ascertainment of the Pneumocystis jirovecii Potential Drug Targets Gsc1 and Kre6 Involved in Glucan Synthesis

The most efficient drug against the human pathogenic fungus Pneumocystis jirovecii is cotrimoxazole targeting the folate biosynthesis. However, resistance toward it is emerging and adverse effects occur in some patients. Studies in rodent models suggested that echinocandins could be useful to treat Pneumocystis pneumonia. Echinocandins inhibit the catalytic subunit Gsc1 of the enzymatic complex ensuring the synthesis of 1,3-β glucan, an essential constituent of cell walls of most fungi. Besides, inhibitors of the enzyme Kre6 involved in the synthesis of 1,6-β glucan, another essential component of fungal walls, were recently described. We identified and functionally characterized these two potential drug targets in the human pathogen P. jirovecii by rescue of the null allele of the orthologous gene in Saccharomyces cerevisiae. The P. jirovecii proteins Gsc1 and Kre6 identified using those of the relative Pneumocystis carinii as the query sequence showed high sequence identity to the putative fungal orthologs (53-97% in conserved functional domains). The expression of their encoding genes on plasmid rescued the increased sensitivity to, respectively, caspofungin or calcofluor white of the corresponding S. cerevisiae null allele. The uniqueness and likely essentiality of these proteins suggest that they are potential good drug targets.

Pneumocystis jirovecii--from a commensal to pathogen: clinical and diagnostic review

Pneumocystis pneumonia is an opportunistic disease caused by invasion of unicellular fungus Pneumocystis jirovecii. Initially, it was responsible for majority of morbidity and mortality cases among HIV-infected patients, which later have been reduced due to the introduction of anti-retroviral therapy, as well as anti-Pneumocystis prophylaxis among these patients. Pneumocystis pneumonia, however, is still a significant cause of mortality among HIV-negative patients being under immunosuppression caused by different factors, such as transplant recipients as well as oncologically treated ones. The issue of pneumocystosis among these people is particularly emphasized in the article, since rapid onset and fast progression of severe symptoms result in high mortality rate among these patients, who thereby represent the group of highest risk of developing Pneumocystis pneumonia. In contrast, fungal invasion in immunocompetent people usually leads to asymptomatic colonization, which frequent incidence among healthy infants has even suggested the possibility of its association with sudden unexpected infant death syndrome. In the face of emerging strains with different epidemiological profiles resulting from genetic diversity, including drug-resistant genotypes, the colonization phenomenon desires particular attention, discussed in this article. We also summarize specific and sensitive methods, required for detection of Pneumocystis invasion and for distinguish colonization from the disease.

Sterols of Saccharomyces cerevisiae erg6 Knockout Mutant Expressing the Pneumocystis carinii S-Adenosylmethionine:Sterol C-24 Methyltransferase

The AIDS-associated lung pathogen Pneumocystis is classified as a fungus although Pneumocystis has several distinct features such as the absence of ergosterol, the major sterol of most fungi. The Pneumocystis carinii S-adenosylmethionine:sterol C24-methyltransferase (SAM:SMT) enzyme, coded by the erg6 gene, transfers either one or two methyl groups to the C-24 position of the sterol side chain producing both C28 and C29 24-alkylsterols in approximately the same proportions, whereas most fungal SAM:SMT transfer only one methyl group to the side chain. The sterol compositions of wild-type Sacchromyces cerevisiae, the erg6 knockout mutant (Δerg6), and Δerg6 expressing the P. carinii or the S. cerevisiae erg6 gene were analyzed by a variety of chromatographic and spectroscopic procedures to examine functional complementation in the yeast expression system. Detailed sterol analyses were obtained using high performance liquid chromatography and proton nuclear magnetic resonance spectroscopy ((1)H-NMR). The P. carinii SAM:SMT in the Δerg6 restored its ability to produce the C28 sterol ergosterol as the major sterol, and also resulted in low levels of C29 sterols. This indicates that while the P. carinii SAM:SMT in the yeast Δerg6 cells was able to transfer a second methyl group to the side chain, the action of Δ(24(28)) -sterol reductase (coded by the erg4 gene) in the yeast cells prevented the formation and accumulation of as many C29 sterols as that found in P. carinii.

Analysis of current antifungal agents and their targets within the Pneumocystis carinii genome

Pneumocystis pneumonia (PCP) remains a leading opportunistic infection in patients with weakened immune systems. The fungus causing the infection belongs to the genus, Pneumocystis, and its members are found in a large variety of mammals. Adaptation to the lung environment of a host with an intact immune system has been a key to its successful survival. Unfortunately, the metabolic strategies used by these fungi to grow and survive in this context are largely unknown. There were considerable impediments to standard approaches for investigation of this unique pathogen, the most problematic being the lack of a long term in vitro culture system. The absence of an ex vivo cultivation method remains today, and many fundamental scientific questions about the basic biology, metabolism, and life cycle of Pneumocystis are unanswered. Recent progress in sequencing of the Pneumocystis carinii genome, a species infecting rats, permitted a more informative search for genes and biological pathways within this pathogen that are known to be targets for existing antifungal agents. In this work, we review the classes of antifungal drugs with respect to their potential applicability to the treatment of PCP. Classes covered in the review are the azoles, polyenes, allylamines, and echinocandins. Factors limiting the use of standard antifungal treatments and the currently available alternatives (trimethoprim-sulfamethoxazole, atovaquone, and pentamidine) are discussed. A summary of genomic sequences within Pneumocystis carinii associated with the corresponding targeted biological pathways is provided. All sequences are available via the Pneumocystis Genome Project at http://pgp.cchmc.org/.

Clearance in vivo of instilled [h]cholesterol from the rat lung

Phospholipids and lung surfactant proteins are known to be recycled within the lung alveolus mainly by uptake into type II epithelial cells that secrete lipid-enriched lung surfactant. Dipalmitoyl phosphatidylcholine (DPPC) is the major component of lung surfactant lipids and cholesterol is the second most abundant. However, cholesterol turnover in vivo has not been measured and it is not known how long steroidal compounds persist in the lung in intact animals. Here we report on experiments in which radiolabeled cholesterol was instilled into the lungs of rats, then at various postinstillation periods, radioactive sterols in lavage fluid, and in postlavage whole lungs were measured in individual animals. Radioactive sterols in the lungs remained high for a week and were still detectable 46 days later. The clearance rate during the initial postinstillation week was approximately 10% per day. Both radioactive free and esterified sterols were recovered from bronchoalveolar lavage fluid and postlavage lungs.

Functional characterization and localization of Pneumocystis carinii lanosterol synthase

Organisms in the genus Pneumocystis are ubiquitous, opportunistic pathogenic fungi capable of causing a lethal pneumonia in immunocompromised mammalian hosts. Pneumocystis spp. are unique members of the fungal kingdom due to the absence of ergosterol in their cellular membranes. Although these organisms were thought to obtain cholesterol by scavenging, transcriptional analyses indicate that Pneumocystis carinii encodes gene homologs involved in sterol biosynthesis. To better understand the sterol pathway in these uncultivable fungi, yeast deletion strains were used to interrogate the function and localization of P. carinii lanosterol synthase (ERG7). The expression of PcErg7p in an ERG7-null mutant of the yeast Saccharomyces cerevisiae did not alter its growth rate and produced a functional lanosterol synthase, as evidenced by the presence of lanosterol detected by gas chromatographic analysis in levels comparable to that produced by the yeast enzyme. Western blotting and fluorescence microscopy revealed that, like the S. cerevisiae Erg7p, the PcErg7p localized to lipid particles in yeast. Using fluorescence microscopy, we show for the first time the presence of apparent lipid particles in P. carinii and the localization of PcErg7p to lipid particles in P. carinii. The detection of lipid particles in P. carinii and their association with PcErg7p therein provide strong evidence that the enzyme serves a similar function in P. carinii. Moreover, the yeast heterologous system should be a useful tool for further analysis of the P. carinii sterol pathway.

Pneumocystis carinii: a fungus resistant to antifungal therapies - mechanisms of action of antipneumocystis drugs

Pneumocystis carinii is a pathogen that causes a potentially lethal pneumonia in patients with AIDS and other immunodeficiency states. This review discusses the mechanisms of action of four classes of antipneumocystis agents: inhibitors of ergosterol synthesis and function, 1,3-beta-glucan synthase inhibitors, antifolates and DNA binding agents. Investigations of P. carinii's biologic pathways affected by the antipneumocystis actions of each of these classes of agents has generated important insights into the organism's basic biology and supports the organism's classification as a fungus. In addition, this review discusses some recent P. carinii research and its potential impact on drug development.

Transcriptome of Pneumocystis carinii during fulminate infection: carbohydrate metabolism and the concept of a compatible parasite

Members of the genus Pneumocystis are fungal pathogens that cause pneumonia in a wide variety of mammals with debilitated immune systems. Little is known about their basic biological functions, including life cycle, since no species can be cultured continuously outside the mammalian lung. To better understand the pathological process, about 4500 ESTS derived from sequencing of the poly(A) tail ends of P. carinii mRNAs during fulminate infection were annotated and functionally characterized as unassembled reads, and then clustered and reduced to a unigene set with 1042 members. Because of the presence of sequences from other microbial genomes and the rat host, the analysis and compression to a unigene set was necessarily an iterative process. BLASTx analysis of the unassembled reads (UR) vs. the Uni-Prot and TREMBL databases revealed 56% had similarities to existing polypeptides at E values of<or=10(-6), with the remainder lacking any significant homology. The most abundant transcripts in the UR were associated with stress responses, energy production, transcription and translation. Most (70%) of the UR had similarities to proteins from filamentous fungi (e.g., Aspergillus, Neurospora) and existing P. carinii gene products. In contrast, similarities to proteins of the yeast-like fungi, Schizosaccharomyces pombe and Saccharomyces cerevisiae, predominated in the unigene set. Gene Ontology analysis using BLAST2GO revealed P. carinii dedicated most of its transcripts to cellular and physiological processes ( approximately 80%), molecular binding and catalytic activities (approximately 70%), and were primarily derived from cell and organellar compartments (approximately 80%). KEGG Pathway mapping showed the putative P. carinii genes represented most standard metabolic pathways and cellular processes, including the tricarboxylic acid cycle, glycolysis, amino acid biosynthesis, cell cycle and mitochondrial function. Several gene homologs associated with mating, meiosis, and sterol biosynthesis in fungi were identified. Genes encoding the major surface glycoprotein family (MSG), heat shock (HSP70), and proteases (PROT/KEX) were the most abundantly expressed of known P. carinii genes. The apparent presence of many metabolic pathways in P. carinii, sexual reproduction within the host, and lack of an invasive infection process in the immunologically intact host suggest members of the genus Pneumocystis may be adapted parasites and have a compatible relationship with their mammalian hosts. This study represents the first characterization of the expressed genes of a non-culturable fungal pathogen of mammals during the infective process.

Peculiarities of host cholesterol transport to the unique intracellular vacuole containing Toxoplasma

The intracellular protozoan Toxoplasma gondii is auxotrophic for low-density lipoprotein (LDL)-derived cholesterol (C). We previously showed that T. gondii scavenges this essential lipid from host endolysosomal compartments and that C delivery to the parasitophorous vacuole (PV) does not require transit through host Golgi or endoplasmic reticulum. In this study, we explore the itinerary of C from the host endolysosomes to the PV. Labeled C incorporated into LDL is rapidly detected in intravacuolar parasites and partially esterified by the parasites. In contrast to diverse mammalian organelles, the post-endolysosomal transfer of C to the PV does not involve the host plasma membrane as an intermediate. Nevertheless, the PV membrane is accessible to extracellular sterol acceptors, suggesting C trafficking from intracellular parasites to host plasma membrane. C movement to the PV requires temperatures permissive for vesicular transport, metabolic energy and functional microtubules. Host caveolae vesicles and the sterol carrier protein-2 do not participate in this process. Proteolytic treatment of purified PV or free parasites abolishes C acquisition by the parasites. Altogether, these results support a vesicular transport system from host endolysosomes to the PV, and a requirement for PV membrane and parasite plasma membrane proteins in C delivery to T. gondii.

Sterol metabolism in the opportunistic pathogen Pneumocystis: advances and new insights

Pneumocystis can transiently colonize healthy individuals without causing adverse symptoms, and most people test positive for exposure to this organism early in life. However, it can cause Pneumocystis pneumonia (PcP) in people with impaired immune systems and is a major cause of death in HIV/AIDS. Although it has close affinities to the Ascomycetes, Pneumocystis has features unlike those of any single group of fungi. For example, Pneumocystis does not synthesize ergosterol, which is consistent with the inefficacy of amphotericin B and some triazoles in clearing PcP. Pneumocystis sterols include distinct delta7 24-alkylsterols. Metabolic radiolabeling experiments demonstrated that P. carinii synthesizes sterols de novo. Cholesterol is the most abundant sterol in Pneumocystis; most, if not all, is scavenged from the mammalian host lung by the pathogen. The P. carinii erg7, erg6, and erg11 genes have been cloned, sequenced, and expressed in heterologous systems. The recombinant P. carinii S-adenosyl-L-methionine:C-24 sterol methyl transferase (SAM:SMT) has a preference for lanosterol over zymosterol as substrate, and the enzyme can catalyze the transfer of either one or two methyl groups to the C-24 position of the sterol side chain. Two different sterol compositions were detected among human-derived P. jirovecii; one was dominated by C28 and C29 sterols, and the other had high proportions of higher molecular mass components, notably the C32 sterol pneumocysterol. The latter phenotype apparently represents organisms blocked at 14alpha-demethylation of the sterol nucleus. These studies suggest that SAM:SMT is an attractive drug target for developing new chemotherapy for PcP.

Sterol composition of Pneumocystis jirovecii with blocked 14alpha-demethylase activity

Several drugs that interact with membrane sterols or inhibit their syntheses are effective in clearing a number of fungal infections. The AIDS-associated lung infection caused by Pneumocystis jirovecii is not cleared by many of these therapies. Pneumocystis normally synthesizes distinct C28 and C29 24-alkylsterols, but ergosterol, the major fungal sterol, is not among them. Two distinct sterol compositional phenotypes were previously observed in P. jirovecii. One was characterized by delta7 C28 and C29 24-alkylsterols with only low proportions of higher molecular mass components. In contrast, the other type was dominated by high C31 and C32 24-alkylsterols, especially pneumocysterol. In the present study, 28 molecular species were elucidated by nuclear magnetic resonance analysis of a human lung specimen containing P. jirovecii representing the latter sterol profile phenotype. Fifteen of the 28 had the methyl group at C-14 of the sterol nucleus and these represented 96% of the total sterol mass in the specimen (excluding cholesterol). These results strongly suggest that sterol 14alpha-demethylase was blocked in these organisms. Twenty-four of the 28 were 24-alkylsterols, indicating that methylation of the C-24 position of the sterol side chain by S-adenosyl-L-methionine:sterol C-24 methyl transferase was fully functional.

Sterol biosynthesis in Pneumocystis: unique steps that define unique targets

Pneumocystis lacks ergosterol, and several antimycotics that bind ergosterol in fungal membranes or inhibit its synthesis are ineffective against Pneumocystis pneumonia. The organism synthesizes C(28) and C(29) Delta(7) 24-alkylsterols, 24-alkyllanosterol derivatives, and Delta(5) 24-alkylsterols, which may be produced by modifying scavenged Delta(5) sterols. Mammals cannot desaturate C-22 and alkylate C-24 of sterols, thus, these processes are particularly attractive targets for antifungal drug development. Recent data indicate that C-22 desaturation is not, but C-24 alkylation is an attractive target in P. carinii. The P. carinii S-adenosyl-L-methionine:sterol C-24 methyl transferase (SAM:SMT) has unique properties; it prefers lanosterol as its sterol substrate.

Evidence for multiple sterol methyl transferase pathways in Pneumocystis carinii

The sterol composition of Pneumocystis carinii, an opportunistic pathogen responsible for life-threatening pneumonia in immunocompromised patients, was determined. Our purpose was to identify pathway-specific enzymes to impair using sterol biosynthesis inhibitors. Prior to this study, cholesterol 15 (ca. 80% of total sterols), lanosterol 1, and several phytosterols common to plants (sitosterol 31, 24alpha-ethyl and campesterol, 24alpha-methyl 30) were demonstrated in the fungus. In this investigation, we isolated all the previous sterols and many new compounds from P. carinii by culturing the microorganism in steroid-immunosuppressed rats. Thirty-one sterols were identified from the fungus (total sterol = 100 fg/cell), and seven sterols were identified from rat chow. Unusual sterols in the fungus not present in the diet included, 24(28)-methylenelanosterol 2; 24(28)E-ethylidene lanosterol 3; 24(28)Z-ethylidene lanosterol 4; 24beta-ethyllanosta-25(27)-dienol 5; 24beta-ethylcholest-7-enol 6; 24beta-ethylcholesterol 7; 24beta,-ethylcholesta-5,25(27)-dienol 8; 24-methyllanosta-7-enol 9; 24-methyldesmosterol 10; 24(28)-methylenecholest-7-enol 11; 24beta-methylcholest-7-enol 12; and 24beta-methylcholesterol 13. The structural relationships of the 24-alkyl groups in the sterol side chain were demonstrated chromatographically relative to authentic specimens, by MS and high-resolution 1H NMR. The hypothetical order of these compounds poses multiple phytosterol pathways that diverge from a common intermediate to generate 24beta-methyl sterols: route 1, 1 --> 2 --> 11 --> 12 --> 13; route 2, 1 --> 2 --> 9 --> 10 --> 13; or 24beta-ethyl sterols: route 3, 1 --> 2 --> 4 --> 6 --> 7; route 4, 1 --> 2 --> 5 --> 8 --> 7. Formation of 3 is considered to form an interrupted sterol pathway. Taken together, operation of distinct sterol methyl transferase (SMT) pathways that generate 24beta-alkyl sterols in P. carinii with no counterpart in human biochemistry suggests a close taxonomic affinity with fungi and provides a basis for mechanism-based inactivation of SMT enzyme to treat Pneumocystis pneumonia.

Pneumocystis

Pneumocystis organisms can cause pneumonia in mammals that lack a strong immune defense. The genus Pneumocystis contains many different organisms that can be distinguished by DNA sequence analysis. These different organisms are different species of yeast-like fungi that are most closely related to the ascomycete, Schizosaccharomyces pombe. Each species of Pneumocystis appears to be specific for the mammal in which it is found. The species that infects humans is Pneumocystis jiroveci. P. jiroveci has not been found in any other mammal and the species of Pneumocystis found in other mammals have not been seen in humans. Genetic variation among P. jiroveci samples is common, suggesting that there are many strains. Strain analysis shows that adults can be infected by more than one strain, and suggests that pneumonia can be the result of infection occurring proximal to the time of disease, rather than to reactivation of dormant organisms acquired in early childhood. Nevertheless, long-term colonisation may be occurring. A large fraction of normal children and animals show evidence of infection. A Pneumocystis species that grows in rats has been shown to possess a complex genetic system for surface antigen variation, a strategy employed by other microbes that dwell in immunocompetent hosts. These findings, together with strong host specificity, suggest that Pneumocystis species may be obligate parasites. The source of infection is not clear. Pneumocystis DNA is detectable in the air, but is scarce except in environments occupied by individuals with Pneumocystis pneumonia. In a few cases, there is direct evidence of person to person transmission. In general, however, patients and their contacts have been found to have different strains of P. jiroveci.

Is Pneumocystis a plant?

Pneumocystis, an AIDS-associated opportunistic pathogen of the lung has some unusual features. This article focuses on work done by my group to understand the organism's distinct sterols. Although Pneumocystis is closely related to fungi, it lacks the major fungal sterol, ergosterol. Several delta(7) 24-alkysterols synthesized by P. carinii are the same as those reported in some basidiomycete rust fungi. The 24-alkylsterols are synthesized by the action of S-adenosyl-L-methionine:C-24 sterol methyl transferase (SAM:SMT). Fungal SAM:SMT enzymes normally transfer only one methyl group to the C-24 position of the sterol side chain and the cells accumulate C28 24-alkylsterols. In contrast, the P. carinii SAM:SMT and those of some plants catalyze one or two methyl transfer reactions producing both C28 and C29 24-alkylsterols. However, unlike most fungi, plants, and the kinetoplastid flagellates Leishmania and Trypanosoma cruzi, P. carinii does not appear to form double bonds at C-5 of the sterol nucleus and C-22 of the sterol side chain. Furthermore, the P. carinii SAM:SMT substrate preference for C30 lanosterol differs from that of homologous enzymes in any other organisms studied. C31 24-Methylenelanosterol and C32 pneumocysterol, products of SAM:SMT activity on lanosterol, can accumulate in high amounts in some, but not all, human-derived Pneumocystis jiroveci populations.

Comprehensive and definitive structural identities of Pneumocystis carinii sterols

Pneumocystis causes a type of pneumonia in immunodeficient mammals, such as AIDS patients. Mammals cannot alkylate the C-24 position of the sterol side chain, nor can they desaturate C-22. Thus, the reactions leading to these sterol modifications are particularly attractive targets for the development of drugs against fungal and protozoan pathogens that make them. In the present study, the definitive structures of 43 sterol molecular species in rat-derived Pneumocystis carinii were elucidated by nuclear magnetic resonance spectroscopy. Ergosterol, Delta(5,7) sterols, trienes, and tetraenes were not among them. Most (32 of the 43) were 24-alkylsterols, products of S-adenosyl-L-methionine:C-24 sterol methyl transferase (SAM:SMT) enzyme activity. Their abundance is consistent with the suggestion that SAM:SMT is highly active in this organism and that the enzyme is an excellent anti-Pneumocystis drug target. In contrast, the comprehensive analysis strongly suggest that P. carinii does not form Delta(22) sterols, thus C-22 desaturation does not appear to be a drug target in this pathogen. The lanosterol derivatives, 24-methylenelanost-8-en-3 beta-ol and (Z)-24-ethylidenelanost-8-en-3 beta-ol (pneumocysterol), previously identified in human-derived Pneumocystis jiroveci, were also detected among the sterols of the rat-derived P. carinii organisms.

The Pneumocystis carinii drug target S-adenosyl-L-methionine:sterol C-24 methyl transferase has a unique substrate preference

Pneumocystis is an opportunistic pathogen that can cause pneumonitis in immunodeficient people such as AIDS patients. Pneumocystis remains difficult to study in the absence of culture methods for luxuriant growth. Recombinant protein technology now makes it possible to avoid some major obstacles. The P. carinii expressed sequence tag (EST) database contains 11 entries of a sequence encoding a protein homologous to S-adenosyl-L-methionine (SAM):C-24 sterol methyl transferase (SMT), suggesting high activity of this enzyme in the organism. We sequenced the erg6 cDNA, identified the putative peptide motifs for the sterol and SAM binding sites in the deduced amino acid sequence and expressed the protein in Escherichia coli. Unlike SAM:SMT from other organisms, the P. carinii enzyme had higher affinities for lanosterol and 24-methylenelanosterol than for zymosterol, the preferred substrate in other fungi. Cycloartenol was not a productive substrate. With lanosterol and 24-methylenelanosterol as substrates, the major reaction products were 24-methylenelanosterol and pneumocysterol respectively. Thus, the P. carinii SAM:SMT catalysed the transfer of both the first and the second methyl groups to the sterol C-24 position, and the substrate preference was found to be a unique property of the P. carinii SAM:SMT. These observations, together with the absence of SAM:SMT among mammals, further support the identification of sterol C-24 alkylation reactions as excellent targets for the development of drugs specifically directed against this pathogen.

Heterogeneity of Pneumocystis sterol profiles of samples from different sites in the same pair of lungs suggests coinfection by distinct organism populations

Sterol profiles of samples taken from different sites of a Pneumocystis-infected human lung showed large variations in pneumocysterol similar to those that occur among samples from different patients. Thus, the influence of diet or drugs on pneumocysterol accumulation was ruled out, suggesting distinct phenotypic populations as the basis for the heterogeneity.

Pneumocystis carinii f. sp. carinii synthesizes de novo four homologs of ubiquinone

Ubiquinone, coenzyme Q, plays a pivotal role in electron transport and is a target for chemotherapy against a number of eukaryotic infectious agents, including Pneumocystis carinii. Coenzyme Q10 was previously identified as the major ubiquinone homolog in P. carinii isolated and purified from rat lungs; CoQ9 was also present. In contrast, CoQ9 and CoQ8 (but not CoQ10) were detected in the lungs of uninfected rat controls. These observations suggested that the pathogen synthesizes CoQ10, and perhaps CoQ9 as well. In the present study, CoQ biosynthesis in P. carinii was examined in greater detail. Radiolabeled mevalonate, a precursor of the CoQ polyprenyl chain, was incorporated in vitro into P. carinii ubiquinones. Incorporation of radiolabeled mevalonate into P. carinii CoQ was not enhanced by treating cells with lovastatin, suggesting that the cells did not transport the drug, or that a lovastatin-insensitive pathway for de novo synthesis of isoprenoids may also function in this organism. Radiolabeled precursors of the ring moiety, including shikimic acid, p-hydroxybenzoic acid, and tyrosine were also incorporated into P. carinii CoQ. Unexpectedly, it was found that not only CoQ9 and CoQ10, but also CoQ7, and CoQ8, were metabolically radiolabeled by all the precursors tested, indicating that the organism synthesizes CoQ7, CoQ8, CoQ9, and CoQ10. Metabolic radiolabeling of ubiquinones in rat lung controls was not detected in experiments using either radioactive mevalonate or p-hydroxybenzoate. Thus the incorporations measured using purified P. carinii preparations were due to the enzymes of the organism.

Inhibitors of sterol biosynthesis and amphotericin B reduce the viability of pneumocystis carinii f. sp. carinii

Pneumocystis carinii synthesizes sterols with a double bond at C-7 of the sterol nucleus and an alkyl group with one or two carbons at C-24 of the side chain. Also, some human-derived Pneumocystis carinii f. sp. hominis strains contain lanosterol derivatives with an alkyl group at C-24. These unique sterols have not been found in other pathogens of mammalian lungs. Thus, P. carinii may have important differences in its susceptibility to drugs known to block reactions in ergosterol biosynthesis in other fungi. In the present study, inhibitors of 3-hydroxy-3-methyglutaryl coenzyme A reductase, squalene synthase, squalene epoxidase, squalene epoxide-lanosterol cyclase, lanosterol demethylase, Delta(8) to Delta(7) isomerase, and S-adenosylmethionine:sterol methyltransferase were tested for their effects on P. carinii viability as determined by quantitation of cellular ATP levels in a population of organisms. Compounds within each category varied in inhibitory effect; the most effective included drugs targeted at squalene synthase, squalene epoxide-lanosterol cyclase, and Delta(8) to Delta(7) isomerase. Some drugs that are potent against ergosterol-synthesizing fungi had little effect against P. carinii, suggesting that substrates and/or enzymes in P. carinii sterol biosynthetic reactions are distinct. Amphotericin B is ineffective in clearing P. carinii infections at clinical doses; however, this drug apparently binds to sterols and causes permeability changes in P. carinii membranes, since it reduced cellular ATP levels in a dose-dependent fashion.

C27 to C32 sterols found in Pneumocystis, an opportunistic pathogen of immunocompromised mammals

Pneumocystis carinii is the paradigm of opportunistic infections in immunocompromised mammals. Prior to the acquired immunodeficiency syndrome (AIDS) pandemic and the use of immunosuppressive therapy in organ transplant and cancer patients, P. carinii was regarded as a curiosity, rarely observed clinically. Interest in this organism exploded when it was identified as the agent of P. carinii pneumonia (PcP), the direct cause of death among many AIDS patients. Aggressive prophylaxis has decreased the number of acute PcP cases, but it remains among the most prevalent opportunistic infections found within this patient population. The taxonomic assignment of P. carinii has long been argued; molecular genetics data now demonstrate that it is a fungus. Several antimycotic drugs are targeted against ergosterol or its biosynthesis, but these are not as effective against PcP as they are against other fungal infections. This can now be explained in part by the identification of the sterols of P. carinii. The organism lacks ergosterol but contains distinct C28 and C29 delta7 24-alkylsterols. Also, 24-methylenelanost-8-en-3beta-ol (C31) and pneumocysterol, (24Z)-ethylidenelanost-8-en-3beta-ol (C32) were recently identified in organisms infecting humans. Together, the delta7 24-alkylsterols and pneumocysterol are regarded as signature lipids of the pathogen that can be useful for the diagnosis of PcP, since no other lung pathogen is known to contain them. Cholesterol (C27), the dominant sterol component in P. carinii, is probably totally scavenged from the host. De novo synthesis of sterols has been demonstrated by the presence of lovastatin-sensitive 3-hydroxy-3-methylglutaryl-CoA reductase activity, the incorporation of radiolabeled mevalonate and squalene into P. carinii sterols, and the reduction in cellular ATP in cells treated with inhibitors of enzymes in sterol biosynthesis.

Sterols of Pneumocystis carinii hominis organisms isolated from human lungs

The opportunistic pathogen Pneumocystis carinii causes pneumonia (P. carinii pneumonia, or PCP) in immunocompromised individuals such as AIDS patients. Rat-derived P. carinii carinii organisms have distinct sterols which are not synthesized by mammals and not found in other microbes infecting mammalian lungs. The dominant sterol present in the organism is cholesterol (which is believed to be scavenged from the host), but other sterols in P. carinii carinii have an alkyl group at C-24 of the sterol side chain (C(28) and C(29) 24-alkylsterols) and a double bond at C-7 of the nucleus. Recently, pneumocysterol (C(32)), which is essentially lanosterol with a C-24 ethylidene group, was detected in lipids extracted from a formalin-fixed human P. carinii-infected lung, and its structures were elucidated by gas-liquid chromatography, mass spectrometry, and nuclear magnetic resonance spectrometry in conjunction with analyses of chemically synthesized authentic standards. The sterol composition of isolated P. carinii hominis organisms has yet to be reported. If P. carinii from animal models is to be used for identifying potential drug targets and for developing chemotherapeutic approaches to clear human infections, it is important to determine whether the 24-alkylsterols of organisms found in rats are also present in organisms in humans. In the present study, sterol analyses of P. carinii hominis organisms isolated from cryopreserved human P. carinii-infected lungs and from bronchoalveolar lavage fluid were performed. Several of the same distinct sterols (e.g., fungisterol and methylcholest-7-ene-3beta-ol) previously identified in P. carinii carinii were also present in organisms isolated from human specimens. Pneumocysterol was detected in only some of the samples.

Sterol composition and biosynthesis in Trypanosoma cruzi amastigotes

A detailed analysis of the endogenous sterols present in the clinically relevant intracellular (amastigote) stages of Trypanosoma cruzi, is presented. The parasites were grown in cultured Vero cells in the absence or presence of different sterol biosynthesis inhibitors, including the C14alpha demethylase inhibitor ketoconazole and two inhibitors of delta24(25)-sterol methyl transferase, 20 piperidin-2-yl-5alpha-pregnan-3beta-20-R-diol (22,26-azasterol) and 24-(R,S),25-epiminolanosterol. Amastigotes were isolated and purified from their host cells and neutral lipids were extracted, separated and analyzed by chromatographic and mass spectrometric methods. Control (untreated) amastigotes contained as main endogenous sterols 24-methyl-cholesta-7-en-3beta-ol (ergosta-7-en-3beta-ol) and its 24-ethyl analog, plus smaller amounts of their precursor, ergosta-7,24(28)dien-3beta-ol; these cells also contained cholesterol (up to 80% by weight of total sterols), probably derived from host cells. Amastigotes that proliferated in the presence of 10 nM ketoconazole (minimal inhibitory concentration, MIC) for 24 h had a sharply reduced content of endogenous 4-desmethyl sterols with a concomitant accumulation of 24-methyl-dihydrolanosterol and 24-methylene-dihydrolanosterol. On the other hand, amastigotes incubated during the same period of time with the two inhibitors of 24(25)-SMT at their respective MICs (100-300 nM) accumulated large amounts of C27 sterols whose structure suggested, in the case of 22,26-azasterol, that delta14 sterol reductase was also inhibited. Ketoconazole produced a dose-dependent reduction in the incorporation of [2-(14)C]-acetate into the parasite's endogenous C4-desmethyl sterols with an IC50 of 50 nM, indistinguishable from the value reported previously for the extracellular epimastigote form. Taken together, the results showed that amastigotes have a simpler sterol biosynthetic pathway than that previously described for epimastigotes, lacking both delta5 and delta22 reductases. They also suggest that the 100-fold higher potency of antifungal azoles as antiproliferative agents against amastigotes, when compared with epimastigotes, is most probably due to a smaller pool of endogenous sterols in the intracellular parasites.

Pneumocysterol [(24Z)-ethylidenelanost-8-en-3beta-ol], a rare sterol detected in the opportunistic pathogen Pneumocystis carinii hominis: structural identity and chemical synthesis

Pneumocystis carinii pneumonia (PcP) remains among the most prevalent opportunistic infections among AIDS patients. Currently, drugs used clinically for deep mycosis act by binding ergosterol or disrupting its biosynthesis. Although classified as a fungus, P. carinii lacks ergosterol. Instead, the pathogen synthesizes a number of distinct Delta7, 24-alkylsterols, despite the abundance of cholesterol, which it can scavenge from the lung alveolus. Thus, the pathogen-specific sterols appear vital for organism survival and proliferation. In the present study, high concentrations of a C32 sterol were found in human-derived P. carinii hominis. The definitive structural identities of two C-24 alkylated lanosterol compounds, previously not reported for rat-derived P. carinii carinii, were determined by using GLC, MS, and NMR spectroscopy together with the chemical syntheses of authentic standards. The C31 and C32 sterols were identified as euphorbol (24-methylenelanost-8-en-3beta-ol) and pneumocysterol [(24Z)-ethylidenelanost-8-en-3beta-ol], respectively. The identification of these and other 24-alkylsterols in P. carinii hominis suggests that (i) sterol C-24 methyltransferase activities are extraordinarily high in this organism, (ii) 24-alkylsterols are important components of the pathogen's membranes, because the addition of these side groups onto the sterol side chain requires substantial ATP equivalents, and (iii) the inefficacy of azole drugs against P. carinii can be explained by the ability of this organism to form 24-alkysterols before demethylation of the lanosterol nucleus. Because mammals cannot form 24-alkylsterols, their biosyntheses in P. carinii are attractive targets for the development of chemotherapeutic strategies against this opportunistic infection.

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.

Clinically used antimicrobial drugs against experimental pneumocystosis, singly and in combination: analysis of drug interactions and efficacies

We analyzed single drugs and combinations of drugs used clinically in the treatment of opportunistic infections and other conditions for their activities against Pneumocystis carinii pneumonia in immunosuppressed rats. When they were used alone, atovaquone, rifabutin, and dapsone were more active than clarithromycin or trimethoprim. Drug combinations were evaluated for synergistic activity by an analysis of variance model for two-way factorial experiments and a response surface model. Atovaquone combined with trimethoprim trimethoprim and some combinations of dapsone and clarithromycin was synergistic; however, the activities of combinations of atovaquone and rifabutin, atovaquone and clarithromycin, and atovaquone and dapsone were simply additive. Lovastatin, which inhibits 3-hydroxy-methylglutaryl coenzyme A reductase, was inactive whether it was used alone or in combination with other agents. None of the synergistic drug combinations was as effective as trimethoprim-sulfamethoxazole. We conclude that the rat model can be used to test combinations of anti-P. carinii agents for synergistic activity by well-established statistical techniques. While some combinations of clinically used antimicrobial drugs have enhanced anti-P. carinii activity, further studies are needed before clinical trials can be contemplated.

Identification of C31 and C32 sterols in Pneumocystis carinii hominis-infected human lungs

Two sterols in autopsied whole lung specimens obtained from Pneumocystis carinii pneumonia patients were detected by gas-liquid chromatography and their structures were elucidated by mass spectrometry and nuclear magnetic resonance spectrometry. Both were in the lanosterol series; the C31 sterol, with a methyl group at C-24, was identified as euphorbol, and the more abundant C32 sterol, with an ethyl group at C-24, is given the trivial name pneumocysterol.

Pneumocystis carinii: an atypical fungal micro-organism

The purpose of this review is to assist mycologists in having a better understanding of Pneumocystis carinii and the disease that it causes. Now considered to be a fungus, P. carinii is unusual in its life cycle and relationship with the host. P. carinii pneumonia (PCP) pathogenesis, immunology and host defence mechanisms are examined, as well as epidemiological and control strategies. Most pneumocystosis pathophysiological changes result from the parasite's attachment and proliferation in the lungs, resulting in a filling of the alveoli with masses of the micro-organism. Pathological changes include an increase in alveolar capillary membrane permeability and injury to the alveolar epithelium, which may be mediated by the release of degradative enzymes from the pathogen. A host response takes place by hypertrophy, and hyperplasia involving type II epithelial alveolar cells. P carinii interacts with pulmonary surfactants by binding to the hydrophilic proteins A and D, and by modifying their phospholipid composition. Alveolar macrophages and CD4+ T cells play a key role in the host's defence against Pneumocystis. The epidemiology of PCP remains poorly understood. Airborne transmission has been established, but the actual infective form and its source remains unknown. Studies concerning P. carinii genetic diversity have shown that the parasite polymorphism is related, at least partially, to the host species. A strong host-species specificity in P. carinii has been found. From an epidemiological perspective, there appears to be no animal reservoir for the agent of human PCP. Thus, this disease should not be considered to be zoonotic. Although a significant decrease in the incidence of pneumocystosis has been obtained when employing chemoprophylaxis, anti-P. carinii drugs are not completely successful, often inducing deleterious side-effects. For these reasons, new prophylactic and therapeutic strategies need to be developed. One approach could be based on the anti-P. carinii effect of yeast killer toxins and antibiotic anti-idiotypic antibodies.

Composition of Pneumocystis carinii neutral lipids and identification of coenzyme Q10 as the major ubiquinone homolog

The lipids of purified preparations of Pneumocystis carinii carinii freshly isolated from infected rats were analyzed and compared with those of whole lungs from normal and methylprednisolone-immunosuppressed uninfected rats. In this study, the neutral lipid fraction was examined in detail; the relative concentrations of individual classes making up this fraction were quantified. Of particular interest was the nature of the organism's ubiquinone (coenzyme Q, CoQ) fraction because atovaquone, a hydroxynaphtho-quinone (566C80) analog of ubiquinone, is efficacious in the treatment of P. carinii pneumonia. The ubiquinone concentration in both P. carinii and lung tissues was relatively low compared to that present in rat heart and liver tissues. Two homologs were identified in the organism: CoQ10 was the predominant homolog with lesser amounts of CoQ9 present. In contrast, the lungs of normal and immunosuppressed uninfected rats had CoQ9 and lesser amounts of CoQ8, but no detectable CoQ10. Furthermore, radiolabeled mevalonic acid was incorporated in vitro into the ubiquinone fraction of P. carinii indicating that the organism has the de novo branch of the isoprenoid biosynthetic pathway leading to polyprenyl formation. Hence, it was concluded that CoQ10 (if not both CoQ10 and CoQ9) in P. carinii was not scavenged from the host but was synthesized by the organism. Although lung tissues contained substantial free fatty acids, the organism was enriched in these lipids. The high concentration of free fatty acids and relatively low level of triglycerides in P. carinii suggest that fatty acids may represent major carbon sources for ATP production by the organism.

Fatty acid composition of the major phospholipids of Pneumocystic carinii: comparison with those in the lungs of normal and methylprednisolone-immunosuppressed rats

Large numbers of viable organisms can be isolated from the corticosteroid-immunosuppressed rat model of Pneumocystis carinii pneumonia. With the development of purification protocols that provide organism preparations of high purity, meaningful lipid biochemical analyses of this important opportunistic pathogen can now be conducted. The phospholipid class composition of the pathogen was reported earlier, together with observations of changes that occur in the rat lungs in response to methylprednisolone immunosuppression treatment. In this report, analyses of the effects of corticosteroids on the fatty acid compositions of the major lung phospholipids, individually isolated and purified by thin-layer chromatography, were elucidated and quantified by gas-liquid chromatography. In response to methylprednisolone, there was a relative increase in palmitate and there were decreases in several unsaturated fatty acids of the rat whole-lung total polar lipids leading to a doubling of the saturation index. Reciprocal changes in the relative concentrations of palmitate and stearate in phosphatidylethanolamine, phosphatidylinositol, lysophosphatidylcholine, and cardiolipin were observed, suggesting that there is tight control of acylation of these phospholipids in the lung. Detailed phospholipid fatty acid analyses were also performed with mixed life cycle stages of P. carinii organisms. The most abundant phospholipids, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol, had much higher concentrations of oleic acid and lower concentrations of palmitate in P. carinii than in lung tissue. Sphingomyelin in lung tissue and P. carinii differed from the glycerophospholipids by the presence of high levels of saturated C(22) and C(24) fatty acids. This study represents the most comprehensive fatty acid analysis of rat lung phospholipids and the changes that occur in response to corticosteroid treatment. It is the first report about the fatty acids of individual phospholipids of the opportunistic protist P. carinii carinii.

Phospholipid composition of Pneumocystis carinii carinii and effects of methylprednisolone immunosuppression on rat lung lipids

The phospholipid class composition of Pneumocystis carinii carinii freshly isolated from infected lungs generally resembled that of the host lung, suggesting that the parasite scavenges lung alveolar lipids. However, subtle quantitative differences were demonstrated, indicating that the pathogen has the metabolic capacity to de novo synthesize, or at least tailor, its lipids. The concentration of phosphatidylcholine, the major lung surfactant lipid, in the organism was lower than that in lungs of normal and immunosuppressed uninfected rats, and the concentration of phosphatidylinositol was higher. Phosphonolipids were not detected in the organism by chemical analysis and nuclear magnetic resonance spectrometry. The immunosuppressive regimen alone caused increases in both surfactant protein A and the lipid content of the whole lung. The lungs of rats that were subjected to corticosteroid immunosuppression and had heavy parasite loads had dramatically elevated surfactant protein A levels, whereas the lipid contents of these lungs were not different from lipid contents in whole lungs of immunosuppressed uninfected rats. P. carinii was found to concentrate lipids, indicating that a large amount of the lipids in the whole infected rat lung was within the parasites residing in the organ. These observations have important implications relevant to the use of corticosteroid therapy for P. carinii pneumonitis.

Phytosterols are present in Pneumocystis carinii

Although originally classified as a protozoan, Pneumocystis carinii is now considered to have fungal characteristics. Drugs typically used for the treatment of fungal infections target ergosterol. Because P. carinii is an important pathogen in AIDS and other immunocompromised patients, knowledge of the sterol content of this organism may be useful as a basis for developing new treatment strategies or for improving diagnosis. P. carinii organisms were harvested from infected rat lungs and were purified by filtration. Control preparations from uninfected animals were identically prepared. Lipids were extracted from the organisms and control preparations and were separated into neutral lipid, glycolipid, and phospholipid fractions by silicic acid chromatography. The neutral lipid fraction was further treated by alkaline hydrolysis and was analyzed by reversed-phase high-pressure liquid chromatography (HPLC), gas chromatography (GC), and GC-mass spectrometry (GC-MS). As shown by HPLC, the neutral lipid fraction from infected rats contained a minimum of six peaks, while in control preparations a single peak with a retention time identical to that of cholesterol was observed. The predominant sterol in these preparations was positively identified by GC-MS as cholesterol and constituted 80 to 90% of the total. The remaining peaks had relative retention times similar to those of phytosterols by both HPLC and GC, and the similarity of these sterols to those derived from plants and fungi was confirmed by MS. Ergosterol, however, was not present. These results provide further evidence for a close phylogenetic relationship between P. carinii and fungi and suggest that these sterols could be used as targets for drug development and for improving diagnosis.