Vitronectin binds to Pneumocystis carinii and mediates organism attachment to cultured lung epithelial cells

Retracing the evolution of Pneumocystis species, with a focus on the human pathogen Pneumocystis jirovecii

SUMMARYEvery human being is presumed to be infected by the fungus Pneumocystis jirovecii at least once in his or her lifetime. This fungus belongs to a large group of species that appear to exclusively infect mammals, with P. jirovecii being the only one known to cause disease in humans. The mystery of P. jirovecii origin and speciation is just beginning to unravel. Here, we provide a review of the major steps of P. jirovecii evolution. The Pneumocystis genus likely originated from soil or plant-associated organisms during the period of Cretaceous ~165 million years ago and successfully shifted to mammals. The transition coincided with a substantial loss of genes, many of which are related to the synthesis of nutrients that can be scavenged from hosts or cell wall components that could be targeted by the mammalian immune system. Following the transition, the Pneumocystis genus cospeciated with mammals. Each species specialized at infecting its own host. Host specialization is presumably built at least partially upon surface glycoproteins, whose protogene was acquired prior to the genus formation. P. jirovecii appeared at ~65 million years ago, overlapping with the emergence of the first primates. P. jirovecii and its sister species P. macacae, which infects macaques nowadays, may have had overlapping host ranges in the distant past. Clues from molecular clocks suggest that P. jirovecii did not cospeciate with humans. Molecular evidence suggests that Pneumocystis speciation involved chromosomal rearrangements and the mounting of genetic barriers that inhibit gene flow among species.

Diversity and Complexity of the Large Surface Protein Family in the Compacted Genomes of Multiple Pneumocystis Species

Pneumocystis, a major opportunistic pathogen in patients with a broad range of immunodeficiencies, contains abundant surface proteins encoded by a multicopy gene family, termed the major surface glycoprotein (Msg) gene superfamily. This superfamily has been identified in all Pneumocystis species characterized to date, highlighting its important role in Pneumocystis biology. In this report, through a comprehensive and in-depth characterization of 459 msg genes from 7 Pneumocystis species, we demonstrate, for the first time, the phylogeny and evolution of conserved domains in Msg proteins and provide a detailed description of the classification, unique characteristics, and phylogenetic relatedness of five Msg families. We further describe, for the first time, the relative expression levels of individual msg families in two rodent Pneumocystis species, the substantial variability of the msg repertoires in P. carinii from laboratory and wild rats, and the distinct features of the expression site for the classic msg genes in Pneumocystis from 8 mammalian host species. Our analysis suggests multiple functions for this superfamily rather than just conferring antigenic variation to allow immune evasion as previously believed. This study provides a rich source of information that lays the foundation for the continued experimental exploration of the functions of the Msg superfamily in Pneumocystis biology.IMPORTANCEPneumocystis continues to be a major cause of disease in humans with immunodeficiency, especially those with HIV/AIDS and organ transplants, and is being seen with increasing frequency worldwide in patients treated with immunodepleting monoclonal antibodies. Annual health care associated with Pneumocystis pneumonia costs ∼$475 million dollars in the United States alone. In addition to causing overt disease in immunodeficient individuals, Pneumocystis can cause subclinical infection or colonization in healthy individuals, which may play an important role in species preservation and disease transmission. Our work sheds new light on the diversity and complexity of the msg superfamily and strongly suggests that the versatility of this superfamily reflects multiple functions, including antigenic variation to allow immune evasion and optimal adaptation to host environmental conditions to promote efficient infection and transmission. These findings are essential to consider in developing new diagnostic and therapeutic strategies.

MUC1 mediates Pneumocystis murina binding to airway epithelial cells

Previous studies have shown that Pneumocystis binds to pneumocytes, but the proteins responsible for binding have not been well defined. Mucins are the major glycoproteins present in mucus, which serves as the first line of defence during airway infection. MUC1 is the best characterised membrane-tethered mucin and is expressed on the surface of most airway epithelial cells. Although by electron microscopy Pneumocystis primarily binds to type I pneumocytes, it can also bind to type II pneumocytes. We hypothesized that Pneumocystis organisms can bind to MUC1 expressed by type II pneumocytes. Overexpression of MUC1 in human embryonic kidney HEK293 cells increased Pneumocystis binding, while knockdown of MUC1 expression by siRNA in A549 cells, a human adenocarcinoma-derived alveolar type II epithelial cell line, decreased Pneumocystis binding. Immunofluorescence labelling indicated that MUC1 and Pneumocystis were co-localised in infected mouse lung tissue. Incubation of A549 cells with Pneumocystis led to phosphorylation of ERK1/2 that increased with knockdown of MUC1 expression by siRNA. Pneumocystis caused increased IL-6 and IL-8 secretion by A549 cells, and knockdown of MUC1 further increased their secretion in A549 cells. Taken together, these results suggest that binding of Pneumocystis to MUC1 expressed by airway epithelial cells may facilitate establishment of productive infection.

Mechanisms of Surface Antigenic Variation in the Human Pathogenic Fungus Pneumocystis jirovecii

Microbial pathogens commonly escape the human immune system by varying surface proteins. We investigated the mechanisms used for that purpose by Pneumocystis jirovecii This uncultivable fungus is an obligate pulmonary pathogen that in immunocompromised individuals causes pneumonia, a major life-threatening infection. Long-read PacBio sequencing was used to assemble a core of subtelomeres of a single P. jirovecii strain from a bronchoalveolar lavage fluid specimen from a single patient. A total of 113 genes encoding surface proteins were identified, including 28 pseudogenes. These genes formed a subtelomeric gene superfamily, which included five families encoding adhesive glycosylphosphatidylinositol (GPI)-anchored glycoproteins and one family encoding excreted glycoproteins. Numerical analyses suggested that diversification of the glycoproteins relies on mosaic genes created by ectopic recombination and occurs only within each family. DNA motifs suggested that all genes are expressed independently, except those of the family encoding the most abundant surface glycoproteins, which are subject to mutually exclusive expression. PCR analyses showed that exchange of the expressed gene of the latter family occurs frequently, possibly favored by the location of the genes proximal to the telomere because this allows concomitant telomere exchange. Our observations suggest that (i) the P. jirovecii cell surface is made of a complex mixture of different surface proteins, with a majority of a single isoform of the most abundant glycoprotein, (ii) genetic mosaicism within each family ensures variation of the glycoproteins, and (iii) the strategy of the fungus consists of the continuous production of new subpopulations composed of cells that are antigenically different.IMPORTANCEPneumocystis jirovecii is a fungus causing severe pneumonia in immunocompromised individuals. It is the second most frequent life-threatening invasive fungal infection. We have studied the mechanisms of antigenic variation used by this pathogen to escape the human immune system, a strategy commonly used by pathogenic microorganisms. Using a new DNA sequencing technology generating long reads, we could characterize the highly repetitive gene families encoding the proteins that are present on the cellular surface of this pest. These gene families are localized in the regions close to the ends of all chromosomes, the subtelomeres. Such chromosomal localization was found to favor genetic recombinations between members of each gene family and to allow diversification of these proteins continuously over time. This pathogen seems to use a strategy of antigenic variation consisting of the continuous production of new subpopulations composed of cells that are antigenically different. Such a strategy is unique among human pathogens.

Fungal Strategies to Evade the Host Immune Recognition

The recognition of fungal cells by the host immune system is key during the establishment of a protective anti-fungal response. Even though the immune system has evolved a vast number of processes to control these organisms, they have developed strategies to fight back, avoiding the proper recognition by immune components and thus interfering with the host protective mechanisms. Therefore, the strategies to evade the immune system are as important as the virulence factors and attributes that damage the host tissues and cells. Here, we performed a thorough revision of the main fungal tactics to escape from the host immunosurveillance processes. These include the composition and organization of the cell wall, the fungal capsule, the formation of titan cells, biofilms, and asteroid bodies; the ability to undergo dimorphism; and the escape from nutritional immunity, extracellular traps, phagocytosis, and the action of humoral immune effectors.

Pneumocystis Carinii Pneumonia in Adult Non-HIV Disorders

Pneumocystis carinii pneumonia (PCP) remains a serious infection in the immunocompromised host (in the absence of HIV infection) and presents significant management and diagnostic challenges to ICU physicians. Non-HIV PCP is generally abrupt in onset, and follows a fulminate course with high rates of hospitalization, ICT admission, respiratory failure, and requirement for intubation. Mortality is generally high, especially if mechanical ventilation is required. Non-invasive ventilatory support may be considered, although the rapid progression to respiratory failure often necessitates intubation at the time of presentation. Bronchoscopy is often required to establish the diagnosis, and empirical antimicrobial treatment specifically targeted to P. carinii should be initiated while awaiting confirmation. Adjunctive corticosteroids may accelerate recovery, although their use has not yet been established in non-HIV PCP. For the ICU physicians to diagnose PCP, the non-specific presentation of an acute febrile illness and respiratory distress with diffuse pulmonary infiltrates requires a high clinical index of suspician, familiarity with clinical conditions associated with increased risk for PCP, and a low threshold for bronchoscopy to establish the diagnosis.

Conserved Patterns of Microbial Immune Escape: Pathogenic Microbes of Diverse Origin Target the Human Terminal Complement Inhibitor Vitronectin via a Single Common Motif

Pathogenicity of many microbes relies on their capacity to resist innate immunity, and to survive and persist in an immunocompetent human host microbes have developed highly efficient and sophisticated complement evasion strategies. Here we show that different human pathogens including Gram-negative and Gram-positive bacteria, as well as the fungal pathogen Candida albicans, acquire the human terminal complement regulator vitronectin to their surface. By using truncated vitronectin fragments we found that all analyzed microbial pathogens (n = 13) bound human vitronectin via the same C-terminal heparin-binding domain (amino acids 352-374). This specific interaction leaves the terminal complement complex (TCC) regulatory region of vitronectin accessible, allowing inhibition of C5b-7 membrane insertion and C9 polymerization. Vitronectin complexed with the various microbes and corresponding proteins was thus functionally active and inhibited complement-mediated C5b-9 deposition. Taken together, diverse microbial pathogens expressing different structurally unrelated vitronectin-binding molecules interact with host vitronectin via the same conserved region to allow versatile control of the host innate immune response.

Pathobiology of Pneumocystis pneumonia: life cycle, cell wall and cell signal transduction

Pneumocystis is a genus of ascomycetous fungi that are highly morbid pathogens in immunosuppressed humans and other mammals. Pneumocystis cannot easily be propagated in culture, which has greatly hindered understanding of its pathobiology. The Pneumocystis life cycle is intimately associated with its mammalian host lung environment, and life cycle progression is dependent on complex interactions with host alveolar epithelial cells and the extracellular matrix. The Pneumocystis cell wall is a varied and dynamic structure containing a dominant major surface glycoprotein, β-glucans and chitins that are important for evasion of host defenses and stimulation of the host immune system. Understanding of Pneumocystis cell signaling pathways is incomplete, but much has been deduced by comparison of the Pneumocystis genome with homologous genes and proteins in related fungi. In this mini-review, the pathobiology of Pneumocystis is reviewed, with particular focus on the life cycle, cell wall components and cell signal transduction.

Vitronectin expression in the airways of subjects with asthma and chronic obstructive pulmonary disease

Vitronectin, a multifunctional glycoprotein, is involved in coagulation, inhibition of the formation of the membrane attack complex (MAC), cell adhesion and migration, wound healing, and tissue remodeling. The primary cellular source of vitronectin is hepatocytes; it is not known whether resident cells of airways produce vitronectin, even though the glycoprotein has been found in exhaled breath condensate and bronchoalveolar lavage from healthy subjects and patients with interstitial lung disease. It is also not known whether vitronectin expression is altered in subjects with asthma and COPD. In this study, bronchial tissue from 7 asthmatic, 10 COPD and 14 control subjects was obtained at autopsy and analyzed by immunohistochemistry to determine the percent area of submucosal glands occupied by vitronectin. In a separate set of experiments, quantitative colocalization analysis was performed on tracheobronchial tissue sections obtained from donor lungs (6 asthmatics, 4 COPD and 7 controls). Vitronectin RNA and protein expressions in bronchial surface epithelium were examined in 12 subjects who undertook diagnostic bronchoscopy. Vitronectin was found in the tracheobronchial epithelium from asthmatic, COPD, and control subjects, although its expression was significantly lower in the asthmatic group. Colocalization analysis of 3D confocal images indicates that vitronectin is expressed in the glandular serous epithelial cells and in respiratory surface epithelial cells other than goblet cells. Expression of the 65-kDa vitronectin isoform was lower in bronchial surface epithelium from the diseased subjects. The cause for the decreased vitronectin expression in asthma is not clear, however, the reduced concentration of vitronectin in the epithelial/submucosal layer of airways may be linked to airway remodeling.

Pneumocystis

Since its initial misidentification as a trypanosome some 100 years ago, Pneumocystis has remained recalcitrant to study. Although we have learned much, we still do not have definitive answers to such basic questions as, where is the reservoir of infection, how does Pneumocystis reproduce, what is the mechanism of infection, and are there true species of Pneumocystis? The goal of this review is to provide the reader the most up to date information available about the biology of Pneumocystis and the disease it produces.

Host cell invasion by medically important fungi

To infect the host and cause disease, many medically important fungi invade normally nonphagocytic host cells, such as endothelial cells and epithelial cells. Host cell invasion is a two-step process consisting of adherence followed by invasion. There are two general mechanisms of host cell invasion, induced endocytosis and active penetration. Furthermore, fungi can traverse epithelial or endothelial cell barriers either by proteolytic degradation of intercellular tight junctions or via a Trojan horse mechanism in which they are transported by leukocytes. Although these mechanisms of host cell invasion have been best studied using Candida albicans and Cryptococcus neoformans, it is probable that other invasive fungi also use one or more of these mechanisms to invade host cells. Identification of these invasion mechanisms holds promise to facilitate the development of new approaches to inhibit fungal invasion and thereby prevent disease.

The Pneumocystis Ace2 transcription factor regulates cell wall-remodeling genes and organism virulence

Pneumocystis carinii (Pc) β-glucans are major components of the organism cell wall; yet, the regulation of Pc cell wall genesis and remodeling is not well understood. Ace2 transcription factors, which are present in many fungi, regulate glucanases and other enzymes needed for cell wall remodeling. The cloning and heterologous expression of PcAce2 in ace2Δ Saccharomyces cerevisiae demonstrated that PcAce2 can restore the defective glucanase and endochitinase gene expression of the mutant as well as regulate cell wall β-glucan biosynthetic genes. Furthermore, when a reconstructed yeast system was used, PcAce2 activated the transcription of the Pneumocystis gsc1 β-glucan synthetase, confirming the activity of a Pc transcription factor on a native Pneumocystis promoter and gene for the first time. We further observed that Pneumocystis binding to host extracellular matrix proteins and lung epithelial cells induced the phosphorylation (activation) of the PcAce2 transcription factor. Finally, we present a novel method that confirms the role of PcAce2 in modulating organism virulence using ace2Δ Candida glabrata infection in neutropenic mice. Together, these results indicate that the adherence of Pc to lung matrix proteins and epithelial cells leads to the activation of the Ace2 transcription factor, which regulates cell wall degradation and biosynthesis genes that are required for cell wall remodeling.

Colonization by Pneumocystis jirovecii and its role in disease

Although the incidence of Pneumocystis pneumonia (PCP) has decreased since the introduction of combination antiretroviral therapy, it remains an important cause of disease in both HIV-infected and non-HIV-infected immunosuppressed populations. The epidemiology of PCP has shifted over the course of the HIV epidemic both from changes in HIV and PCP treatment and prevention and from changes in critical care medicine. Although less common in non-HIV-infected immunosuppressed patients, PCP is now more frequently seen due to the increasing numbers of organ transplants and development of novel immunotherapies. New diagnostic and treatment modalities are under investigation. The immune response is critical in preventing this disease but also results in lung damage, and future work may offer potential areas for vaccine development or immunomodulatory therapy. Colonization with Pneumocystis is an area of increasing clinical and research interest and may be important in development of lung diseases such as chronic obstructive pulmonary disease. In this review, we discuss current clinical and research topics in the study of Pneumocystis and highlight areas for future research.

Pneumocystis carinii interactions with lung epithelial cells and matrix proteins induce expression and activity of the PcSte20 kinase with subsequent phosphorylation of the downstream cell wall biosynthesis kinase PcCbk1

Eukaryotic cell proliferation and phenotype are highly regulated by contact-dependent mechanisms. We have previously shown that the binding and interaction of the opportunistic fungal pathogen Pneumocystis carinii to lung epithelial cells and extracellular matrix proteins induces mRNA expression of both the mitogen-activated protein (MAP) kinase P. carinii Ste20 (PcSte20) and the cell wall-remodeling enzyme PcCbk1 (16). Herein, we report that in addition to PcSte20 mRNA expression being upregulated, Pneumocystis PcSte20 kinase activity is increased upon interacting with these same lung targets. This activity is also significantly suppressed by Clostridium difficile toxin B, a pan-specific inhibitor of small GTPases, demonstrating the potential role of a Cdc42-like molecule in this signaling cascade. We further observed that the PcSte20 kinase physically interacts with a specific region of the P. carinii cell wall biosynthesis kinase, PcCbk1, a downstream kinase important for mating projection formation and cell wall remodeling. This direct binding was mapped to a specific region of the PcCbk1 protein. We also demonstrated that PcSte20 obtained from whole P. carinii lysates has the ability to phosphorylate PcCbk1 after the organism interacts with lung epithelial cells and extracellular matrix components. These observations provide new insights into P. carinii signaling induced by interactions of this important opportunistic fungal pathogen with lung epithelial cells and matrix.

Some Opportunistic Parasitic Infections in AIDS: Candidiasis, Pneumocystosis, Cryptosporidiosis, Toxoplasmosis

Almost 80% of patients with AIDS die from infections other than human immunodeficiency virus (HIV). These infections usually occur late in the course of disease when CD4(+) T-cell count has fallen below 200 permm(3) cells per milliliter. Most of these infections are caused by organisms that do not normally afflict healthy individuals and are thus considered to be opportunistic. In this article, Lloyd Kasper and Dominique Buzoni-Gatel review the host-parasite interaction for four important pathogens: Candida albicans and Pneumocystis carinii (usually non-invasive pathogens), Cryptosporidium parvum (invades the cells but remains localized in the gut) and Toxoplasma gondii (penetrates through the gut to cause systemic infection). These organisms, which generally cause limited or even insignificant clinical evidence of infection in the normal host, were chosen because of their high prevalence in AIDS patients and because they exhibit different invasive abilities. The reason why individuals with AIDS are susceptible to this particular group of pathogens is uncertain.

Update on the diagnosis and treatment of Pneumocystis pneumonia

Pneumocystis is an opportunistic fungal pathogen that causes an often-lethal pneumonia in immunocompromised hosts. Although the organism was discovered in the early 1900s, the first cases of Pneumocystis pneumonia in humans were initially recognized in Central Europe after the Second World War in premature and malnourished infants. This unusual lung infection was known as plasma cellular interstitial pneumonitis of the newborn, and was characterized by severe respiratory distress and cyanosis with little or no fever and no pathognomic physical signs. At that time, only anecdotal cases were reported in adults and usually these patients had a baseline malignancy that led to a malnourished state. In the 1960-1970s additional cases were described in adults and children with hematological malignancies, but Pneumocystis pneumonia was still considered a rare disease. However, in the 1980s, with the onset of the HIV epidemic, Pneumocystis prevalence increased dramatically and became widely recognized as an opportunistic infection that caused potentially life-treating pneumonia in patients with impaired immunity. During this time period, prophylaxis against this organism was more generally instituted in high-risk patients. In the 1990s, with widespread use of prophylaxis and the initiation of highly active antiretroviral therapy (HAART) in the treatment of HIV-infected patients, the number of cases in this specific population decreased. However, Pneumocystis pneumonia still remains an important cause of severe pneumonia in patients with HIV infection and is still considered a principal AIDS-defining illness. Despite the decreased number of cases among HIV-infected patients over the past decade, Pneumocystis pneumonia continues to be a serious problem in immunodeficient patients with other immunosuppressive conditions. This is mostly due to increased use of immunosuppressive medications to treat patients with autoimmune diseases, following bone marrow and solid organ transplantation, and in patients with hematological and solid malignancies. Patients with hematologic disorders and solid organ and hematopoietic stem cell transplantation are currently the most vulnerable groups at risk for developing this infection. However, any patient with an impaired immunity, such as those receiving moderate doses of oral steroids for greater than 4 weeks or those receiving other immunosuppressive medications are at also at significant risk.

EXPRESSION OF ANGIOSTATIN, INTEGRINαvβ3, AND VITRONECTIN IN HUMAN LUNGS IN SEPSIS

Angiostatin, integrin alphavbeta3, and vitronectin play important roles in inflammation. However, there is very little information on expression of these molecules in the lungs of humans with sepsis. Therefore, as a first step to eventually study the function of these molecules, the authors conducted an immunohistochemical study to evaluate their expression in lungs of normal (N = 8) and sepsis patients (N = 8). In normal lungs, angiostatin expression was minimal in the alveolar septa and alveolar macrophages, and absent in large blood vessels, bronchioles, and interstitium. In sepsis patients, the staining was intense in the septa, neutrophils, alveolar macrophages, and large blood vessels. Integrin alphavbeta3 staining was observed in occasional bronchiolar epithelial cells and a few alveolar macrophages in the normal lungs. The integrin was expressed extensively and intensely in bronchiolar epithelium and alveolar macrophages, and with lesser intensity in large blood vessels in inflamed lungs. Compared to the normal lung, vitronectin expression was increased in alveolar macrophages and in vascular smooth muscles in inflamed lungs. These data show cell-specific increase in the expression of integrin alphavbeta3, angiostatin, and vitronectin in inflamed lungs of sepsis patients. Because all these molecules can have significant influence on inflammation, the data reported in this manuscript create a need for further investigation.

Characterization of a novel ADAM protease expressed by Pneumocystis carinii

Pneumocystis species are opportunistic fungal pathogens that cause severe pneumonia in immunocompromised hosts. Recent evidence has suggested that unidentified proteases are involved in Pneumocystis life cycle regulation. Proteolytically active ADAM (named for "a disintegrin and metalloprotease") family molecules have been identified in some fungal organisms, such as Aspergillus fumigatus and Schizosaccharomyces pombe, and some have been shown to participate in life cycle regulation. Accordingly, we sought to characterize ADAM-like molecules in the fungal opportunistic pathogen, Pneumocystis carinii (PcADAM). After an in silico search of the P. carinii genomic sequencing project identified a 329-bp partial sequence with homology to known ADAM proteins, the full-length PcADAM sequence was obtained by PCR extension cloning, yielding a final coding sequence of 1,650 bp. Sequence analysis detected the presence of a typical ADAM catalytic active site (HEXXHXXGXXHD). Expression of PcADAM over the Pneumocystis life cycle was analyzed by Northern blot. Southern and contour-clamped homogenous electronic field blot analysis demonstrated its presence in the P. carinii genome. Expression of PcADAM was observed to be increased in Pneumocystis cysts compared to trophic forms. The full-length gene was subsequently cloned and heterologously expressed in Saccharomyces cerevisiae. Purified PcADAMp protein was proteolytically active in casein zymography, requiring divalent zinc. Furthermore, native PcADAMp extracted directly from freshly isolated Pneumocystis organisms also exhibited protease activity. This is the first report of protease activity attributable to a specific, characterized protein in the clinically important opportunistic fungal pathogen Pneumocystis.

Pneumocystis jiroveci Pneumonia

Pneumocystis pneumonia (PCP) is one of the most common pulmonary infections in persons with impaired cell-mediated immunity, and particularly those infected with human immunodeficiency virus (HIV).1–7 Pneumocystis was first described in the lungs of guinea pigs, during experiments on American trypanosomiasis by Carlos Chagas8 in 1909 and by Antonio Carinii9 in 1910. Both considered the cysts of Pneumocystis as part of the trypanosome’s life cycle. Shortly afterward the Delanoes10 found identical forms in the lungs of rats that had not been infected with trypanosomes and recognized the organism as a separate species. The name Pneumocystis carinii, was given to this organism as a generic name (Greek:pneumon, “lung”; kystis, “cyst”), honoring Carinii.11

Pneumocystis PCINT1, a molecule with integrin-like features that mediates organism adhesion to fibronectin

Pneumocystis species cause severe pneumonia during chronic immunosuppression, especially in patients with AIDS or malignancy. Adhesion of Pneumocystis to extracellular matrix proteins, particularly fibronectin, associated with alveolar epithelial cell surfaces, triggers organism proliferative pathways. Herein, we report the characterization of a novel Pneumocystis molecule with considerable structural features of an integrin-like extracellular matrix adhesion receptor. A PCINT1115 bp probe was initially identified from partial sequence present within the Pneumocystis genome project database. A full-length 3018 bp cDNA was subsequently obtained with extensive homology to the C-terminal region of Candida albicans INT1 (31% blastx), a gene originally described as encoding an integrin-like molecule implicated in adhesion, growth, and virulence. Sequence analysis of PCINT1 indicated that the Pneumocystis molecule contained both a putative internal RGD motif and four Metal Ion-Dependent Attachment Sites (MIDAS) motifs required for coordination of divalent cations, as well as a specific tyrosine residue found in the cytoplasmic tails of some integrin receptors and C. albicans INT1. Northern, Western and immunofluorescence studies demonstrated that the trophic forms of Pneumocystis, known to be the life cycle forms that tightly adhere to lung epithelium, expressed the molecule to a substantially greater degree than cystic forms. Heterologous expression of PCINT1 in yeast followed by application to human fibronectin-coated surfaces demonstrated these yeast display PCINT1 on their surfaces and subsequently gain the ability to bind fibronectin in a cation dependent fashion. Taken together, these results indicate that Pneumocystis expresses a novel integrin-like PCINT1 molecule sufficient to mediate interactions with extracellular matrix fibronectin, an integral component of host-cell organism interactions during this infection.

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.

Current insights into the biology and pathogenesis of Pneumocystis pneumonia

The fungal infection Pneumocystis pneumonia is the most prevalent opportunistic infection in patients with AIDS. Although the analysis of this opportunistic fungal pathogen has been hindered by the inability to isolate it in pure culture, the use of molecular techniques and genomic analysis have brought insights into its complex cell biology. Analysis of the intricate relationship between Pneumocystis and the host lung during infection has revealed that the attachment of Pneumocystis to the alveolar epithelium promotes the transition of the organism from the trophic to the cyst form. It also revealed that Pneumocystis infection elicits the production of inflammatory mediators, culminating in lung injury and impaired gas exchange. Here we discuss these and other recent findings relating to the biology and pathogenesis of this intractable fungus.

The immunology of parasite infections in immunocompromised hosts

Immune compromise can modify the severity and manifestation of some parasitic infections. More widespread use of newer immnosuppressive therapies, the growing population of individuals with immunocompromised states as well as the prolonged survival of these patients have altered the pattern of parasitic infection. This review article discusses the burden and immunology of parasitic infections in patients who are immunocompromised secondary to congenital immunodeficiency, malnutrition, malignancy, and immunosuppressive medications. This review does not address the literature on parasitic infections in the setting of HIV-1 infection.

Haemophilus influenzae surface fibrils contribute to serum resistance by interacting with vitronectin

Vitronectin inhibits the membrane attack complex of the complement system and is found both in plasma and the extracellular matrix. In this study, we have identified the outer membrane protein Haemophilus surface fibrils (Hsf) as the major vitronectin-binding protein in encapsulated H. influenzae type b. A H. influenzae mutant devoid of Hsf showed a significantly decreased binding to both soluble and immobilized vitronectin as compared with the wild-type counterpart. Moreover, Escherichia coli-expressing Hsf at the surface strongly adhered to immobilized vitronectin. Importantly, the H. influenzae Hsf mutant had a markedly reduced survival as compared with the wild-type bacterium when incubated with normal human serum. A series of truncated Hsf fragments were recombinantly manufactured in E. coli. The vitronectin binding regions were located within two separate binding domains. In conclusion, Hsf interacts with vitronectin and thereby inhibits the complement-mediated bactericidal activity, and thus is a major H. influenzae virulence factor.

Pneumocystis: immune recognition and evasion

Pulmonary infection caused by the opportunistic fungal organism Pneumocystis continues to be a leading AIDS defining illness. The initiation of highly active antiretroviral therapy (HAART) in the HIV-infected population has led to a significant reduction in the incidence of Pneumocystis pneumonia (PCP), although recent trends suggest the incidence has plateaued rather than decreased. Host defense against Pneumocystis involves a delicate, concerted balance between the inflammatory response and immune-mediated clearance. Innate cellular immunity is a cornerstone in this response as it provides the initial recognition event that precipitates an immune response, ultimately leading to clearance of the organism from the host. This review will focus on carbohydrate moieties found in the Pneumocystis cell wall and the immune events that occur following their recognition.

Pneumocystis carinii activates the NF-kappaB signaling pathway in alveolar epithelial cells

Pneumocystis carinii pneumonia (PcP) is a clinically important infection of immunocompromised patients. Although the interaction of Pneumocystis with the alveolar epithelium has been well documented, very little information regarding the epithelial response to Pneumocystis is currently available. In order to study Pneumocystis-epithelium interactions, a murine cell line derived specifically from an alveolar epithelial cell (AEC) was utilized. The coculture of murine AECs with mouse Pneumocystis induced a dose- and time-dependent release of the CXC chemokine MIP-2. Importantly, the specific removal of Pneumocystis from the preparation, or the pretreatment of AECs with sulfasalazine, a potent and specific inhibitor of NF-kappaB, nearly completely abrogated the chemokine response to Pneumocystis. Since the murine MIP-2 promoter contains consensus kappaB binding sequences, the ability of Pneumocystis to stimulate NF-kappaB signaling in AECs was examined. Pneumocystis stimulation of an AEC line stably transfected with a kappaB-dependent reporter construct triggered the NF-kappaB signaling pathway and reporter production. These data were confirmed in gel shift assays, providing direct evidence that Pneumocystis induced the nuclear translocation of the p50/p65 heterodimeric form of NF-kappaB. Maximal NF-kappaB activation was dependent upon direct contact with viable Pneumocystis organisms. These data demonstrate that Pneumocystis activates NF-kappaB signaling in AECs and establish a reporter cell line for studying NF-kappaB activation in AECs. Given the global regulatory functions of the NF-kappaB family, these findings suggest that Pneumocystis directly alters AEC gene expression in a manner that promotes pulmonary immune and inflammatory responses.

Immunity against the opportunistic fungal pathogen Pneumocystis

Species of the genus Pneumocystis exist as opportunistic fungal pathogens and are associated with severe pneumonia and pulmonary complications in immunocompromised individuals. Although prophylactic therapy for Pneumocystis has significantly decreased the overall incidence of infection, more than 80% of cases in current patient populations are considered breakthrough cases. In the HIV-infected population, in the years following the initiation of highly active antiretroviral therapy (HAART), significant reductions in the incidence of Pneumocystis infection were observed, although trends over the last several years suggest that the incidence of Pneumocystis has plateaued rather than decreased. Furthermore, with the more prominent usage of immunosuppressive therapies, the frequency of Pneumocystis infection in the HIV-negative population, such as those with hematologic malignancies and those who have undergone transplantation, has risen significantly. Investigating host defense mechanisms against P. carinii has historically been problematic due to the difficulty in achieving continuous in vitro propagation of proliferating Pneumocytis organisms. Nevertheless, clinical and experimental studies have documented that host defense against Pneumocystis involves a concerted effort between innate, cell-mediated (T lymphocyte) and humoral (B lymphocyte) responses. However, the pulmonary environment is a tissue site where heightened inflammatory responses can often lead to inflammation-mediated injury, thereby contributing to the pathogenesis of Pneumocystis infection. Accordingly, clearance of Pneumocystis from the pulmonary environment is dependent on a delicate equilibrium between the inflammatory response and immune-mediated clearance of the organism. Furthermore, innate and adaptive responses against Pneumocystis are strikingly similar to those against other medically-important fungi, thus providing additional evidence that Pneumocystis exists as a fungal organism.

Pneumocystis jiroveci Pneumonia in Adult Patients with AIDS

Pneumocystis jiroveci (P. carinii) is an opportunistic pathogen that has gained particular prominence since the onset of the AIDS epidemic. Among several important advances in diagnosis and management, appropriately targeting chemoprophylaxis to HIV-infected patients at high clinical risk for P. jiroveci pneumonia and the introduction of effective combination anti-retroviral therapy (including highly active antiretroviral therapy [HAART]) have contributed to the reduced incidence of P. jiroveci pneumonia. Despite the success of these clinical interventions, P. jiroveci pneumonia remains the most common opportunistic pneumonia and the most common life-threatening infectious complication in HIV-infected patients. Trimethoprim/sulfamethoxazole (cotrimoxazole) remains the first-line agent for effective therapy and chemoprophylaxis, and corticosteroids represent an important adjunctive agent in the treatment of moderate-to-severe P. jiroveci pneumonia. However, problems of chemoprophylaxis and treatment failures, high rates of adverse drug reactions and drug intolerance to first-line antimicrobials, high rates of relapse or recurrence with second-line agents, and newer concerns about the development of P. jiroveci drug resistance represent formidable challenges to the management and treatment of AIDS-related P. jiroveci pneumonia. With the expanding global problem of HIV infection, the intolerance or unavailability of HAART to many individuals and limited access to healthcare for HIV-infected patients, P. jiroveci pneumonia will remain a major worldwide problem in the HIV-infected population. New drugs under development as anti-Pneumocystis agents such as echinocandins and pneumocandins, which inhibit beta-glucan synthesis, or sordarins, which inhibit fungal protein synthesis, show promise as effective agents. Continued basic research into the biology and genetics of P. jiroveci and host defense response to P. jiroveci will allow the development of newer antimicrobials and immunomodulatory therapeutic agents to more effectively treat life-threatening pneumonia caused by this organism.

Lung epithelial cells and extracellular matrix components induce expression of Pneumocystis carinii STE20, a gene complementing the mating and pseudohyphal growth defects of STE20 mutant yeast

Pneumocystis carinii causes severe pneumonia in immunocompromised hosts. The binding of P. carinii to alveolar epithelial cells and extracellular matrix constituents such as fibronectin and vitronectin is a central feature of infection, which initiates proliferation of the organism. Herein, we demonstrate that P. carinii binding to lung cells specifically alters the gene expression of the organism, regulating fungal growth. Subtractive hybridization was performed to isolate P. carinii genes expressed following binding to mammalian extracellular matrix constituents. P. carinii STE20 (PCSTE20), a gene participating in mating and pseudohyphal growth of other fungi, was identified following adherence to the extracellular matrix constituents fibronectin, vitronectin, collagen, and lung epithelial cells. The expression of PCSTE20 and a related P. carinii mitogen-activated protein kinase (MAPK) kinase gene, also implicated in signaling of mating, were both specifically upregulated by binding to matrix protein. The expression of general cyclin-dependent kinases and other MAPKs not involved in mating pathways were not altered by organism binding. PCSTE20 expression was also strongly enhanced following organism attachment to A549 lung epithelial cells. When expressed in a Saccharomyces cerevisiae ste20Delta mutant, PCSTE20 suppressed defects in both mating and pseudohyphal growth. These findings are consistent with the observed proliferation and filopodial extension of Pneumocystis organisms adherent to the epithelium in the lungs of immunocompromised hosts. PCSTE20 expression appears to represent a significant component in the regulation of the life cycle of this intractable opportunistic pathogen.

Surfactant protein D-mediated aggregation of Pneumocystis carinii impairs phagocytosis by alveolar macrophages

Pneumocystis carinii remains an important and potentially fatal cause of opportunistic pneumonia. Animal studies reveal that substantial quantities of surfactant protein D (SP-D) accumulate in the airspaces during P. carinii pneumonia and are particularly abundant in aggregates of organisms. Due to the multimeric structure of SP-D, we hypothesized that SP-D mediates aggregation of the organism. From previous clinical studies it is known that aggregated organisms are conspicuous in sections of lung tissue and bronchoalveolar lavage (BAL) fluids of humans with active P. carinii pneumonia. Herein, we observe that SP-D levels increased at least fourfold in BAL fluids of patients with P. carinii pneumonia. Next, a spectrophotometric sedimentation assay was developed to assess the aggregation of P. carinii in vitro by SP-D. P. carinii organisms were first stripped with glutathione to remove bound SP-D and subsequently incubated in the presence of SP-D and 2 mM calcium. P. carinii incubated with natural SP-D (10 micro g/ml) containing dodecamers and higher-order forms exhibited aggregation and enhanced sedimentation compared to that of glutathione-stripped P. carinii. Aggregation was also enhanced by the concentrated supernatant of rat BAL fluid, and this effect was abolished by the selective removal of SP-D from the lavage fluid. P. carinii aggregation was reduced by maltose, mannose, and EDTA, consistent with the role of the SP-D C-type lectin domain (CRD) in the aggregation event. Comparisons of different molecular forms of SP-D showed that dodecamers-but not trimeric subunits-mediate optimal aggregation of P. carinii. Aggregation of P. carinii by SP-D was shown to be responsible for the impaired phagocytosis of the organisms by alveolar macrophages. Thus, SP-D-mediated aggregation of P. carinii may represent one means by which the organism avoids elimination by the host.

The Haemophilus ducreyi serum resistance antigen DsrA confers attachment to human keratinocytes

Haemophilus ducreyi is the etiologic agent of the sexually transmitted genital ulcer disease chancroid. H. ducreyi serum resistance protein A (DsrA) is a member of a family of multifunctional outer membrane proteins that are involved in resistance to killing by human serum complement. The members of this family include YadA of Yersinia species, the UspA proteins of Moraxella catarrhalis, and the Eib proteins of Escherichia coli. The role of YadA, UspA1, and UspA2H as eukaryotic cell adhesins and the function of UspA2 as a vitronectin binder led to our investigation of the cell adhesion and vitronectin binding properties of DsrA. We found that DsrA was a keratinocyte-specific adhesin as it was necessary and sufficient for attachment to HaCaT cells, a keratinocyte cell line, but was not required for attachment to HS27 cells, a fibroblast cell line. We also found that DsrA was specifically responsible for the ability of H. ducreyi to bind vitronectin. We then theorized that DsrA might use vitronectin as a bridge to bind to human cells, but this hypothesis proved to be untrue as eliminating HaCaT cell binding of vitronectin with a monoclonal antibody specific to integrin alpha(v)beta(5) did not affect the attachment of H. ducreyi to HaCaT cells. Finally, we wanted to examine the importance of keratinocyte adhesion in chancroid pathogenesis so we tested the wild-type and dsrA mutant strains of H. ducreyi in our swine models of chancroid pathogenesis. The dsrA mutant was less virulent than the wild type in both the normal and immune cell-depleted swine models of chancroid infection.

Characterization of the expression site of the major surface glycoprotein of human-derived Pneumocystis carinii

The major surface glycoprotein (MSG) of Pneumocystis carinii, a pathogen responsible for pulmonary infection in AIDS and other immunocompromised patients, is an abundant surface protein that potentially allows the organism to evade host defences by antigenic variation. MSG is encoded by a multicopy gene family; in two specific forms of rat-derived P. carinii, regulation of MSG expression uses a single expression site, termed the upstream conserved sequence (UCS), through two related but distinct mechanisms. In the current study, the UCS of the MSG from human-derived P. carinii was obtained using an RNA ligase-mediated rapid amplification of cDNA ends technique. Southern blot analysis demonstrated that the UCS was present in a single copy per genome, whereas multiple copies of the downstream MSG gene were present. Sequencing and restriction fragment length polymorphism analysis of polymerase chain reaction products amplified from pulmonary samples of patients with P. carinii pneumonia demonstrated that multiple MSG genes were expressed in a given host, and that different patterns of MSG expression were seen among different patients. Tandem repeats present in the single intron occurred with varying frequency in different patient isolates, potentially providing a new method for typing human isolates. Thus, human-derived P. carinii regulates MSG expression in a manner similar to P. carinii f. sp. carinii and, in immunosuppressed patients, in whom immune pressures that probably drive antigenic variation are functioning inadequately, P. carinii can express a broad repertoire of MSG variants.

Involvement of alpha(v)beta3 integrin-like receptor and glycosaminoglycans in Candida albicans germ tube adhesion to vitronectin and to a human endothelial cell line

The present study was undertaken to investigate the expression of alpha(v)beta3 and alpha(v)beta5 integrin-like vitronectin receptors (VNRs) on Candida albicans germ tube and their involvement in its adhesion to vitronectin (VN) and human endothelial cells. By immunofluorescence and FACS analysis, several monoclonal antibodies directed against human alpha(v) or beta3 integrin subunit or alpha(v)beta3 and alpha(v)beta5 heterodimers, positively stained C. albicans germ tubes. C. albicans germ tubes specifically adhered (45-50%) to VN and this adhesion was markedly inhibited by RGD-, but not RGE-containing peptides. Adhesion of C. albicans germ tubes to VN was strongly inhibited by anti-alphav, anti-beta3 or anti-alpha(v)beta3, but not by alpha(v)beta5 monoclonal antibody. C. albicans germ tube adhesion to VN was also inhibited by glycosaminoglycans (GAGs) such as heparin or chondroitin sulphate. Finally, we show that C. albicans germ tubes adhere to the human EA.hy 926 endothelial cell line. This adhesion is markedly blocked by anti-beta3 monoclonal antibody, GRGDSP peptide or heparin, and is completely abolished by their combination. Overall these results indicate that C. albicans germ tube adherence to VN and to a human endothelial cell line is mediated by alpha(v)beta3, but not by alpha(v)beta5-like integrin, and depends on GAGs which may act by regulating alpha(v)beta3 integrin-like/VN adhesive interaction.

Vitronectin and fibronectin function as glucan binding proteins augmenting macrophage responses to Pneumocystis carinii

beta-glucans represent major structural components of fungal cell walls. We recently reported that Pneumocystis carinii beta-glucans stimulate alveolar macrophages to release proinflammatory cytokines. Macrophage activation by beta-glucan is augmented by serum, implying the presence of circulating factors that interact with beta-glucans and enhance their ability to stimulate macrophages. Using beta-glucan-enriched cell wall fractions from P. carinii and Saccharomyces cerevisiae, two prominent proteins were precipitated from serum and demonstrated to be vitronectin (VN) and fibronectin (FN) by immune analysis. Preincubation of beta-glucan with VN or FN enhanced macrophage activation in response to this cell wall component. Because VN and FN accumulate in the lungs during P. carinii pneumonia, we further investigated hepatic and pulmonary expression of VN and FN messenger RNA during infection. P. carinii pneumonia in rodents is associated with increased hepatic expression of VN and FN as well as increased local expression of FN in the lung. Because interleukin (IL)-6 represents the major regulator of VN and FN expression during inflammatory conditions, we measured macrophage IL-6 release in response to stimulation with P. carinii beta-glucan. Stimulation of macrophages with P. carinii beta-glucan induced significant release of IL-6. Elevated concentrations of IL-6 were noted in the blood of infected animals compared with uninfected control animals. These studies indicate that VN and FN bind to beta-glucan components of P. carinii and augment macrophage inflammatory responses. P. carinii cell wall beta-glucan stimulates secretion of IL-6 by macrophages, thereby enhancing hepatic synthesis of both VN and FN, and lung synthesis of FN during pneumonia.

Genetics of surface antigen expression in Pneumocystis carinii

This article reviews the molecular genetic data pertaining to the major surface glycoprotein (MSG) gene family of Pneumocystis carinii and its role in surface variation and compares this fungal system to antigenic variation systems in the protozoan Trypanosoma brucei and the bacteria Borrelia spp.

Pneumocystis carinii uses a functional cdc13 B-type cyclin complex during its life cycle

Pneumocystis carinii causes severe pneumonia in immunocompromised patients. Recent studies indicate that P. carinii uses a Cdc2 cyclin-dependent kinase to control its proliferation. To further study the regulation of the life cycle of P. carinii, we characterized the P. carinii B-type cyclin termed Cdc13, whose binding to Cdc2 is necessary for kinase activity. Antibodies to B-type cyclins (Cdc13) specifically immunoprecipitated Cdc2/ Cdc13 complexes with associated kinase activity from P. carinii extracts. To clone P. carinii cdc13, degenerate polymerase chain reaction was undertaken using primers generated from amino-acid motifs conserved in fungal Cdc13 proteins. This amplicon was used to obtain full-length genomic and complementary DNA (cDNA) clones. A specific synthetic peptide antibody generated to P. carinii Cdc13 further demonstrated differential Cdc2/Cdc13 activity over the life cycle of P. carinii, with greater activity in cysts compared with trophic forms of the organism. Finally, P. carinii cdc13 cDNA was used to rescue mutant Schizosaccharomyces pombe strains containing temperature-sensitive deficiencies of endogenous Cdc13 activity, thus verifying function of the P. carinii Cdc13 protein. Therefore, P. carinii contains a Cdc13 cyclin, which is variably active over its life cycle and which promotes fungal proliferation.

Major surface glycoprotein genes from Pneumocystis carinii f. sp. ratti

Pneumocystis carinii occurs in a variety of mammals, each of which harbors one or more genetically distinct "special forms" of the microbe. Laboratory rats can be infected by two special forms, P. carinii f. sp. ratti and P. carinii f. sp. carinii. P. carinii f. sp. carinii has a variable antigen, the major surface glycoprotein (MSG), the expression of which is controlled by genetic recombination. Recombination may involve the CRJE, a 23-bp DNA sequence element invariant among P. carinii f. sp. carinii MSG genes. To better understand the role of the CRJE in MSG gene expression and to explore the possible role of MSG in P. carinii infection in rats, P. carinii f. sp. ratti MSG genes were studied. These genes were found to be related to MSG genes of P. carinii f. sp. carinii, but less so than MSG genes from P. carinii f. sp. carinii are to each other. P. carinii f. sp. ratti MSG genes were present throughout the genome and were expressed as an abundant mRNA species slightly smaller than that found in P. carinii f. sp. carinii. P. carinii f. sp. ratti MSG transcripts included a CRJE-like sequence only 78% identical to the CRJE of P. carinii f. sp. carinii. Comparison of MSG proteins from the two rat special forms of P. carinii to those from human, ferret, and mouse P. carinii did not support the hypothesis that growth in the rat lung requires certain primary MSG peptide sequences.

Sulfated polysaccharide-directed recruitment of mammalian host proteins: a novel strategy in microbial pathogenesis

Fundamental to the virulence of microbial pathogens is their capacity for adaptation and survival within variable, and often hostile, environments encountered in the host. We describe a novel, extragenomic mechanism of surface modulation which may amplify the adaptive and pathogenic potential of numerous bacterial species, including Staphylococcus, Yersinia, and pathogenic Neisseria species, as well as Helicobacter pylori and Streptococcus pyogenes. The mechanism involves specific bacterial recruitment of heparin, glycosaminoglycans, or related sulfated polysaccharides, which in turn serve as universal binding sites for a diverse array of mammalian heparin binding proteins, including adhesive glycoproteins (vitronectin and fibronectin), inflammatory (MCP-3, PF-4, and MIP-1alpha) and immunomodulatory (gamma interferon) intermediates, and fibroblast growth factor. This strategy impacts key aspects of microbial pathogenicity as exemplified by increased bacterial invasion of epithelial cells and inhibition of chemokine-induced chemotaxis. Our findings illustrate a previously unrecognized form of parasitism that complements classical virulence strategies encoded within the microbial genome.

The carboxyl terminus of Pneumocystis carinii glycoprotein A encodes a functional glycosylphosphatidylinositol signal sequence

Pneumocystis carinii pneumonia is a hallmark disease associated with AIDS. An abundant glycoprotein, termed gpA, on the surface of P. carinii is considered an important factor in host-parasite interactions. The primary structure of ferret P. carinii gpA contains a carboxyl-terminal sequence characteristic of a signal for glycosylphosphatidylinositol (GPI) anchors. Here we report the capacity for this gpA carboxyl sequence to direct attachment of a secreted protein, human growth hormone (hGH), to the membranes of COS cells. A control fusion protein (hGHDAF37) was obtained which, under the direction of the GPI signal from decay accelerating factor, directs hGH cell surface expression. A construct (phGH2-1A30) was created similar to hGHDAF37 by fusing hGH to the putative GPI signal sequence encoded in the terminal 30 residues from a ferret P. carinii gpA cDNA clone. By indirect immunofluorescent staining, hGH was detected on the surface of COS cells transfected with phGH2-1A30; this surface location was confirmed by confocal laser cytometry. Metabolic labeling with [3H]ethanolamine and subsequent immunopurification of hGH from cells transfected with phGH2-1A30 confirmed that a lipid moiety characteristic of a conventional GPI anchor was linked covalently to hGH, and cell surface hGH2-1A30 fusion protein was sensitive to enzymatic cleavage by phosphatidylinositol-phospholipase C. Furthermore, hGH2-1A30 recombinant protein cofractionated with 5'-nucleotidase, a classical GPI-anchored membrane marker. Together, these results indicate that the carboxyl-terminal residues of ferret P. carinii gpA constitute a biologically functional GPI consensus domain, thus providing a potential mechanism for antigenic variation of P. carinii gpA during P. carinii pneumonia.

Induction of fibrinogen expression in the lung epithelium during Pneumocystis carinii pneumonia

Pneumocystis carinii is an important pulmonary pathogen responsible for morbidity and mortality in patients with AIDS. The acute-phase response (APR), the primary mechanism used by the body to restore homeostasis following infection, is characterized by increased levels of circulating fibrinogen (FBG). Although the liver is the primary site of increased FBG synthesis during the APR, we unexpectedly discovered that FBG is synthesized and secreted by lung alveolar epithelial cells in vitro during an inflammatory stimulus. Therefore, we sought to determine whether lung epithelial cells produce FBG in vivo using animal models of P. carinii pneumonia (PCP). Inflammation was noted by an influx of macrophages to P. carinii-infected alveoli. Northern hybridization revealed that gamma-FBG mRNA increased two- to fivefold in P. carinii-infected lung tissue, while RNA in situ hybridization demonstrated increased levels of gamma-FBG mRNA in the lung epithelium. Immunoelectron microscopy detected lung epithelial cell-specific production of FBG, suggesting induction of a localized inflammatory response resembling the APR. A systemic APR was confirmed by a two- to fivefold upregulation of the levels of hepatic gamma-FBG mRNA in animals with PCP, resulting in a corresponding increase in levels of FBG in plasma. Furthermore, immunoelectron microscopy revealed the presence of FBG at the junction of cell membranes of trophic forms of P. carinii organisms aggregated along the alveolar epithelium. These results implicate FBG in the pathogenesis of PCP in a manner similar to that of the adhesive glycoproteins fibronectin and vitronectin, which are known to participate in intra-alveolar aggregation of organisms and adherence of P. carinii to the lung epithelium.

Isolation and characterization of two proteins from Moraxella catarrhalis that bear a common epitope

The UspA1 and UspA2 proteins of Moraxella catarrhalis are potential vaccine candidates for preventing disease caused by this organism. We have characterized both proteins and evaluated their vaccine potential using both in vitro and in vivo assays. Both proteins were purified from the O35E isolate by Triton X-100 extraction, followed by ion-exchange and hydroxyapatite chromatography. Analysis of the sequences of internal peptides, prepared by enzymatic and chemical cleavage of the proteins, revealed that UspA1 and UspA2 exhibited distinct structural differences but shared a common sequence including an epitope recognized by the monoclonal antibody 17C7. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), purified UspA1 exhibited a molecular weight of approximately 350, 000 when unheated and a molecular weight of 100,000 after being heated for 10 min at 100 degreesC. In contrast, purified UspA2 exhibited an apparent molecular weight of 240,000 by SDS-PAGE that did not change with the length of time of heating. Their sizes as determined by gel filtration were 1,150,000 and 830,000 for UspA1 and UspA2, respectively. Preliminary results indicate the proteins have separate functions in bacterial pathogenesis. Purified UspA1 was found to bind HEp-2 cells, and sera against UspA1, but not against UspA2, blocked binding of the O35E isolate to the HEp-2 cells. UspA1 also bound fibronectin and appears to have a role in bacterial attachment. Purified UspA2, however, did not bind fibronectin but had an affinity for vitronectin. Both proteins elicited bactericidal antibodies in mice to homologous and heterologous disease isolates. Finally, mice immunized with each of the proteins, followed by pulmonary challenge with either the homologous or a heterologous isolate, cleared the bacteria more rapidly than mock-immunized mice. These results suggest that UspA1 and UspA2 serve different virulence functions and that both are promising vaccine candidates.

Vitronectin binds to the gonococcal adhesin OpaA through a glycosaminoglycan molecular bridge

Several bacterial pathogens including Neisseria gonorrhoeae bind the human serum glycoprotein vitronectin. We aimed at defining the gonococcal receptor for vitronectin. Ligand blots demonstrated that vitronectin bound specifically to the heparin-binding outer-membrane protein OpaA, but that coating OpaA with the sulphated polysaccharide heparin was required for the interaction to occur. Bound vitronectin could be dissociated from OpaA-heparin-vitronectin complexes by the addition of excess heparin, indicating that sulphated polysaccharides provided the main linkage between the two proteins. Binding assays with intact micro-organisms substantiated the requirement of sulphated polysaccharides such as heparin and dextran sulphate for the efficient binding of vitronectin to OpaA+ gonococci. This was underscored by the increased binding of vitronectin to gonococci that had been preincubated with saturating concentrations of dextran sulphate, as opposed to the inhibition of vitronectin binding observed when bacteria were incubated simultaneously with vitronectin and saturating concentrations of dextran sulphate. Binding assays with dextran sulphates of various sizes indicated that vitronectin binding correlated with the size of the polysaccharide rather than with the amount of OpaA produced by the bacteria. The inability of zero-length cross-linking agents to couple vitronectin to OpaA provided further evidence that sulphated polysaccharides formed the linkage between vitronectin and OpaA. Infection experiments demonstrated that proteoglycan-deficient Chinese hamster ovary cells efficiently internalized dextran sulphate/vitronectin-coated gonococci, suggesting that soluble sulphated polysaccharides could substitute for cell surface glycosaminoglycans in the internalization process. On the basis of our results, we propose a novel mechanism of vitronectin binding in which sulphated polysaccharides act as molecular bridges, linking the glycosaminoglycan-binding sites of vitronectin and gonococcal OpaA.

Characterization of a mitogen-activated protein kinase from Pneumocystis carinii

The pathogenic fungus Pneumocystis carinii causes severe pneumonia in patients with impaired immunity, particularly patients with acquired immunodeficiency syndrome. The life cycle of P. carinii is poorly understood, and the inability to continuously culture P. carinii is a major limitation in understanding its cell biology. In fungi homologous to P. carinii, pheromone mating factors signal through a mitogen-activated protein kinase (MAPK) signal transduction cascade, resulting in mitotic cell cycle arrest and entry into a pathway of conjugation, cellular differentiation, and proliferation. Using degenerate PCR and library screening, we have identified a MAPK cDNA in P. carinii that is highly homologous to fungal MAPKs involved in the pheromone mating signal transduction cascade, and we demonstrate MAPK activity in P. carinii lysates with a specific antiserum derived from the translated P. carinii MAPK cDNA sequence.

Interaction of rat Pneumocystis carinii and rat alveolar epithelial cells in vitro

During Pneumocystis carinii pneumonia, P. carinii trophic forms adhere tightly to type I alveolar epithelial cells (AECs). However, the manner in which the interaction between P. carinii organisms and AECs results in clinical pneumonia has not been explored. To investigate this interaction in vitro, we established a culture system using rat P. carinii and primary cultures of rat AECs. We hypothesized that binding of P. carinii to AECs would alter the metabolic, structural, and barrier functions of confluent AECs. Using fluorescently labeled P. carinii, we demonstrated that P. carinii bound to AECs in a dose-dependent manner. During P. carinii-AEC interaction, both the AECs and the P. carinii organisms remained metabolically active. Immunofluorescent staining demonstrated that AEC expression of the junctional proteins E-cadherin and occludin and the structural protein cytokeratin 8 were unaffected by P. carinii binding. To evaluate the effect of P. carinii on AEC barrier function, transepithelial resistance across AEC monolayers was measured during interaction with organisms. Culture with P. carinii did not result in loss of AEC barrier function but in fact increased AEC transepithelial resistance in a dose- and time-dependent manner. We conclude that the direct interaction of P. carinii with AECs does not disrupt AEC metabolic, structural, or barrier function. Therefore, we speculate that additional inflammatory cells and/or their signals are required to induce the epithelial derangements characteristic of P. carinii pneumonia.