Pneumocystis carinii: inhibition of lung cell growth mediated by parasite attachment

Pneumocystis carinii glycoprotein A binds macrophage mannose receptors

Pneumocystis carinii causes life-threatening pneumonia in patients with impaired immunity. Recent studies suggest that alveolar macrophages interact with P. carinii through macrophage mannose receptors. However, the ligand(s) on P. carinii that is recognized by these receptors has not been fully defined. P. carinii contains a major mannose-rich surface antigen complex termed glycoprotein A (gpA). It was therefore hypothesized that gpA binds directly to macrophage mannose receptors and mediates organism attachment to these phagocytes. To assess this, gpA was purified from P. carinii by continuous-elution gel electrophoresis. 125I-labeled gpA bound to alveolar macrophages in a saturable fashion. In addition, gpA binding was substantially inhibited by both alpha-mannan and EDTA, further suggesting that gpA interacts with macrophage mannose receptors. Macrophage membrane proteins capable of binding to gpA were isolated with a gpA-Sepharose column. A 165-kDa membrane-associated protein was specifically eluted from the gpA-Sepharose column with EDTA (20 mM). This protein was identified as the macrophage mannose receptor by immunoprecipitation with a polyclonal anti-mannose receptor antiserum. To further investigate the role of gpA in P. carinii-macrophage interactions, 51Cr-labeled P. carinii cells were incubated with macrophages in the presence of increasing concentrations of soluble gpA, and organism attachment was quantified. Soluble gpA (2.5 mg/dl) competitively inhibited P. carinii attachment to alveolar macrophages by 51.3% +/- 3.7% (P = 0.01). Our findings demonstrate that gpA present on P. carinii interacts directly with mannose receptors, thereby mediating organism attachment to alveolar macrophages.

Pneumocystitis carinii organisms from in vitro culture are highly infectious to the nude rat

Many in vitro systems have been used to cultivate Pneumocystis, but only limited parasite growth has been obtained by different authors. A reliable in vitro system enabling a sustained propagation of Pneumocystis appears to be an important condition for a better definition of the transmission of P. carinii pneumonia. In this work, Pneumocystis in vitro culture was performed on monolayers of L2 rat lung epithelial-like cells. Ultrastructural assessment revealed that culture parasites were structurally intact. Pneumocystis culture samples were intratracheally inoculated into corticosteroid-treated nude rats (nonlatently infected by P. carinii), which developed P. carinii pneumonia at 40 days postinoculation. The infectious power of parasites obtained in vitro was 7-10 times higher than that of parasites freshly extracted from parasitized rat lung. In summary, the present results show that it is possible to obtain in vitro highly infectious Pneumocystis forms, and this study provides a promising infectivity test for use by investigators working on Pneumocystis in vitro systems.

The yeast killer phenomenon: a hypothetical way to control Pneumocystis carinii pneumonia

Pneumocystis carinii is an important agent of pneumonia in immunocompromised individuals, especially in acquired immunodeficiency syndrome AIDS patients P. carinii attaches specifically to type 1 pneumocytes. Although this phenomenon must play a marked role in pneumocystosis pathophysiology, no therapeutic molecules able to inhibit specifically the parasite attachment were found. A killer toxin, secreted by the yeast Pichia anomala, induced a significant decrease in P. carinii in vitro attachment and inhibited the parasite infectivity in SCID mice. Killer toxins cannot be used as systemic antibiotics. However, it was possible to produce antiidiotypic antibodies against a monoclonal antibody specific of the toxin active site. These antilds were shown to mimic the in vitro killer effect for the toxin and were called 'antibiobodies'. The susceptibility of P. carinii to the antimicrobial activity of the killer toxin made it possible to hypothesize that the killer phenomenon could constitute a new way for the treatment and prophylaxis of P. carinii infections.

A novel in situ model to study Pneumocystis carinii adhesion to lung alveolar epithelial cells

Pneumocystis carinii, an extracellular parasite thriving in the lungs of immunosuppressed mammals, is a major cause of death in AIDS patients in the USA. As a prelude to growth, the parasite adheres mostly to type I pneumocytes lining the alveolar spaces. The mechanism of adherence remains unknown, largely because of difficulties in isolating type I pneumocytes and maintaining them in vitro. As a first step to understand P. carinii adherence to its natural substrate, we developed an in situ method to directly study parasite binding to lung alveolar cells. We used formaldehyde-fixed paraffin-embedded sections of normal rat lung as substrate for adhesion. As in its binding to the lungs in vivo, P. carinii adhered preferentially to type I pneumocytes. Adherence was saturable, time and dose dependent, and selectively blocked by glycoconjugates, in particular bovine submaxillary mucin, fetuin, and asialofetuin, suggesting that it may be mediated by a lectin type of interaction. Further, IgG of rats with P. carinii pneumonia inhibited adherence, suggesting that it may react with parasite ligands involved in the recognition of type I cell receptors. Our results demonstrate the usefulness of the in situ model for studying the mechanisms of P. carinii adherence to alveolar cells. In addition, this method may be valuable for identifying neutralizing antibodies and drugs potentially useful for controlling the infection in vivo.

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

Pneumocystis carinii attaches to alveolar epithelial cells during the development of pneumonia. Adhesive proteins found within the alveolar space have been proposed to mediate P. carinii adherence to lung cells. Vitronectin (Vn), a 75-kDa glycoprotein present in the lower respiratory tract, has substantial cell-adhesive properties and might participate in the host-parasite interaction during P. carinii pneumonia. To address whether Vn binds to P. carinii, we studied the interaction of radiolabeled Vn with purified P. carinii organisms. Vn bound to P. carinii, occupying an estimated 5.47 x 10(5) binding sites per organism, with an affinity constant, Kd, of 4.24 x 10(-7) M. Interestingly, the interaction of Vn with P. carinii was not mediated through the Arg-Gly-Asp cell-adhesive domain of Vn. Addition of Arg-Gly-Asp-Ser (RGDS) peptides did not inhibit binding. In contrast, Vn binding to P. carinii was substantially inhibited by the addition of heparin or by digesting the organisms with heparitinase, suggesting that P. carinii may interact with the glycosaminoglycan-binding domain of Vn. To determine whether Vn might enhance P. carinii attachment to lung epithelial cells, we studied the binding of 51Cr-labeled P. carinii to cultured A549 lung cells. Addition of Vn resulted in significantly increased binding of P. carinii to A549 cells, whereas a neutralizing anti-Vn serum substantially reduced the binding of P. carinii to A549 cells. These data suggest that Vn binds to P. carinii and that Vn might provide an additional means by which P. carinii attaches to respiratory epithelial cells.

Pneumocystis carinii pneumonia in cancer patients

Concern has been arisen about the recently reported increasing incidence of PCP in patients with cancer and the potential transmissibility of this infection. Whether or not there is an increase in the incidence of P. carinii infections, PCP should be considered in the differential diagnosis of pulmonary infiltrates in bone marrow transplant recipients, in patients with hematologic neoplasms and in patients with primary or metastatic brain neoplasms. Intensity of immunosuppression plays a crucial role, especially long-term (> 2 months) corticosteroid treatment. PCP is usually manifested clinically during augmentation or during tapering of corticosteroid dose. Thus, if the chest radiograph of a high-risk patient shows diffuse infiltrates, bronchoscopy and bronchoalveolar lavage should be done immediately. Treatment options are the same as for the AIDS population, except that TMP-SMX is tolerated better in non-AIDS patients. The role of supportive care, including mechanical ventilation in such patients should not be underestimated. Oral therapy with dapsone-trimethoprim or with atovaquone, can be as effective as conventional therapy in mild disease, permitting treatment on an outpatient basis. PCP is often preventable and our understanding has improved about when prophylaxis should be initiated. In the future, the emergence of new technologies for diagnosis and of new agents for treatment and prophylaxis, will bring us closer to the goal of controlling this serious infection.

Leukotriene B4 and interleukin-8 in human immunodeficiency virus-related pulmonary disease

STUDY OBJECTIVE

To investigate the pathogenesis of lung injury in Pneumocystic carinii pneumonia and nonspecific interstitial pneumonitis (NIP), common pulmonary complications of human immunodeficiency virus (HIV) infection. The efficacy of corticosteroid therapy in P carinii pneumonia and the observation that bronchoalveolar lavage (BAL) neutrophilia predicts a poor prognosis support the premise that the lung injury of P carinii pneumonia is due to the host's inflammatory response to the infection.

DESIGN

In vitro measurements on previously collected BAL fluid samples.

SETTING

The Clinical Center of the National Institutes of Health, a research hospital and tertiary care referral center.

PATIENTS

Five normal volunteers, 5 asymptomatic HIV-positive patients, 10 HIV-positive patients with NIP (5 asymptomatic and 5 with respiratory symptoms), and 19 HIV-positive patients with P carinii pneumonia.

MEASUREMENTS AND RESULTS

BAL leukotriene B4 (LTB4), interleukin 8 (IL-8), and phospholipase A2 (PLA2) were measured. IL-8 and PLA2 were elevated in patients with P carinii pneumonia, and IL-8 correlated with BAL fluid absolute neutrophil count. LTB4, IL-8, and PLA2 levels were elevated in patients with NIP; LTB4 and PLA2 levels correlated with absolute neutrophil count, and IL-8 correlated with alveolar-arterial oxygen pressure difference. IL-8 was elevated in the asymptomatic HIV-positive patients, and there was a trend toward elevation of PLA2 in this group.

CONCLUSION

IL-8 appears to play a role in the pathogenesis of lung injury in P carinii pneumonia and may be the principal neutrophil chemotaxin in this disease; PLA2 may also be involved in the pathogenesis of P carinii pneumonia. Both LTB4 and IL-8 may be involved in the recruitment of neutrophils and subsequent lung injury of NIP. These data suggest that there are varying mechanisms by which inflammatory cells are recruited to the lung in different HIV-related lung diseases.

Activity of bilobalide, a sesquiterpene from Ginkgo biloba, on Pneumocystis carinii

The sesquiterpene bilobalide, extracted from Ginkgo biloba leaves, was tested in vitro and in vivo for the ability to inhibit Pneumocystis carinii growth. Bilobalide was inhibitory to trophozoites cultured on human embryonic lung fibroblasts (HEL 299) at approximately the same concentration as trimethoprim plus sulfamethoxazole (lowest effective concentration, 50 micrograms of bilobalide per ml versus 9/45 microgram of trimethoprim-sulfamethoxazole per ml), inducing microscopically detectable morphological changes in the cytoplasm of the parasite. In pharmacologically immunosuppressed Sprague-Dawley rats transtracheally infected with a suspension of about 5 x 10(6) P. carinii trophozoites per ml, the daily intraperitoneal administration of bilobalide (10 mg/kg of body weight for 8 days) lowered the number of organisms by approximately 2 logs (that is, about 99%). There was no apparent toxicity either in uninfected HEL 299 feeder cells or in infected and uninfected animals. These studies suggest that the sesquiterpene bilobalide might be useful for therapy of and prophylaxis against P. carinii infections in humans.

In situ hybridization analysis of developmental stages of Pneumocystis carinii that are transcriptionally active for a major surface glycoprotein gene

An abundant glycoprotein on the surface of Pneumocystis carinii, termed gpA or gp120, is thought to play a role in the interaction of this opportunistic pathogen with its host. Using RNA:RNA hybridization techniques, the in situ expression of gpA mRNA in developmental forms of the organism was investigated in a ferret model of P. carinii pneumonia. The results suggested that the relative abundance of gpA-specific mRNA was variable in different developmental stages of ferret P. carinii. P. carinii localized along the epithelial lining of alveoli were transcriptionally active. Immunocytochemical detection of gpA and Giemsa staining suggested that many of these organisms were trophic forms of P. carinii. While no detectable gpA mRNA signal was found in the majority of P. carinii cysts, a portion of identifiable cysts co-localized with significant levels of gpA mRNA signal. Differential staining of the cyst wall with Gomori's methenamine silver suggested that the transcriptionally active P. carinii cysts were the intermediate or precyst forms of the organism, while the cysts with no detectable mRNA signal were either mature or empty (excysted). Alveolar macrophages were observed surrounded by transcriptionally active organisms; however, no gpA-transcriptional activity was detected within macrophages. Taken together, the results suggest that transcription of gpA occurs in forms of P. carinii that are actively replicating, and in close proximity or contact with, alveolar epithelial cells.

Fibronectin. A versatile matrix protein with roles in thoracic development, repair and infection

Fibronectin, a dimeric cell-adhesive extracellular matrix glycoprotein, is secreted by mesenchymal cells and assembled into insoluble matrices which have important biological functions in embryologic development as well as in tissue response to injury. Fibronectin interacts with numerous cell types including mesenchymal cells and inflammatory cells which bear appropriate fibronectin receptors. In vitro, fibronectin serves as an adhesive substrate and promotes cell proliferation and cytodifferentiation. During development, fibronectin-rich matrices are deposited in specific location and regulate the directional migration of embryonic cells. In particular, fibronectin matrices appear to be of critical importance to normal cardiopulmonary development. Following embryologic development, the tissue expression of fibronectin is greatly reduced, but increases markedly following tissue injury, where newly expressed fibronectin matrices appear critical to tissue repair. Recent evidence has documented increased expression of fibronectin in numerous pulmonary conditions including the adult respiratory distress syndrome (ARDS), bronchiolitis obliterans organizing pneumonia (BOOP) and idiopathic pulmonary fibrosis (IPF). Additionally, fibronectin also interacts with a large number of microorganisms and therefore also is potentially important in microbial adherence to airway epithelium and subsequent infections of the respiratory system.

120-kD surface glycoprotein of Pneumocystis carinii is a ligand for surfactant protein A

Pneumocystis carinii is the most common cause of life-threatening pneumonia in immunocompromised patients. In the current study, surfactant protein A (SP-A), the major nonserum protein constituent of pulmonary surfactant, is demonstrated to bind P. carinii in a specific and saturable manner. SP-A is surface bound and does not appear to be internalized or degraded by the P. carinii organism. Furthermore, SP-A binding to P. carinii is time- and calcium-dependent and is competitively inhibited by mannosyl albumin. In the absence of calcium or the presence of excess mannosyl albumin, SP-A binding to P. carinii is reduced by 95 and 71%, respectively. SP-A avidly binds P. carinii with a Kd of 8 x 10(-9) M and an estimated 8.4 x 10(6) SP-A binding sites per P. carinii organism, as determined from Scatchard plots. SP-A is shown to bind P. carinii in vivo, and a putative binding site for SP-A on P. carinii is demonstrated to be the mannoserich surface membrane glycoprotein gp120. These findings suggest that P. carinii can interact with the phospholipid-rich material in the alveolar spaces by specifically binding a major protein constituent of pulmonary surfactant.

Pneumocystis carinii attachment to cultured lung cells by pneumocystis gp 120, a fibronectin binding protein

Pneumocystis carinii is an extracellular organism which is thought to require attachment to alveolar epithelial cells for its growth and replication in humans. Fibronectin (Fn) binding to P. carinii is essential for optimal P. carinii attachment. This study demonstrates that gp120, a 110-120-kD membrane glycoprotein on P. carinii, mediates attachment of the organism to cultured lung cells and is the site of Fn binding to P. carinii. A 51Cr-labeled P. carinii binding assay was used to quantify attachment of the organism to the alveolar epithelial cell line A549. Addition of free gp120, purified from whole P. carinii organisms, caused a significant decrease in attachment of P. carinii to A549 cells from 44.2 +/- 5.5% to 22.4 +/- 4.2% (P less than 0.01). Preincubation of the P. carinii organisms with a polyclonal antibody to gp120 also resulted in a marked decrease in P. carinii attachment to A549 cells from 46.8% +/- 5.2% to 21.3 +/- 4.8% (P less than 0.01). Furthermore, addition of free gp120 to P. carinii organisms caused a significant reduction in specific binding of 125I-Fn to P. carinii (from 83.3 +/- 8.5 ng to 47.1 +/- 5.9 ng, P less than 0.01). Similarly, anti-gp 120 antibody decreased specific Fn binding to P. carinii from 74.3 +/- 8.4 ng to 25.5 +/- 5.3 ng (P less than 0.001). Solubilized P. carinii organisms separated by gel electrophoresis and blotted with 125I-Fn demonstrated specific binding of the 125I-Fn to gp120. In addition, a specific anti-beta 1-integrin antiserum reacted with gp120 by Western blot, suggesting structural homology between gp120 and the beta-subunit of integrins. Thus, the data suggest that the P. carinii membrane glycoprotein gp120 functions as a Fn binding protein and is required for optimal P. carinii attachment to alveolar epithelial cells.

Pneumocystis carinii, an opportunist in immunocompromised patients

Pneumocystis carinii has been recognized as a cause of pneumonia in immunocompromised patients for over 40 years. Until the 1980s, Pneumocystis pneumonia (pneumocystosis) was most often seen in patients undergoing chemotherapy for malignancy or transplantation. Infection could be prevented by trimethoprim-sulfamethoxazole prophylaxis; thus, it was an uncommon clinical problem. With the onset of the AIDS epidemic, Pneumocystis pneumonia has become a major problem in the United States because it develops in approximately 80% of patients with AIDS and because almost two-thirds of patients have adverse reactions to anti-Pneumocystis drugs. Thus, physicians and laboratories in any community may be called upon to diagnose and provide care for patients with Pneumocystis pneumonia. The classification of the organism is currently controversial, but it is either a protozoan or a fungus. P. carinii appears to be acquired during childhood by inhalation and does not cause clinical disease in healthy persons but remains latent. If the person becomes immunosuppressed, the latent infection may become activated and lead to clinical disease. Damage of type I pneumocytes by Pneumocystis organisms leads to the foamy alveolar exudate which is characteristic of the disease. Diagnosis is established by morphologic demonstration of Pneumocystis organisms in material from the lungs. Current efforts to find better anti-Pneumocystis drugs should provide more effective therapy and prophylaxis.

Mechanism of Pneumocystis carinii attachment to cultured rat alveolar macrophages

Pneumocystis carinii (PC) pneumonia begins as an intra-alveolar process resulting in injury to the alveolar epithelium with subsequent invasion of the lung interstitium. The clearance of PC organisms from the alveolar space is a critical function of alveolar macrophages (AM), the resident alveolar phagocytic cells. In this study the mechanism of PC attachment to AM was determined using 51Cr-labeled organisms, with PC attachment reaching a maximum of 18.9 +/- 2.5% after 4 h. Attachment was significantly decreased by preincubation of the AM with a monoclonal anti-fibronectin antibody directed against the cell attachment site of fibronectin (from 17.8 +/- 2.2% to 8.3 +/- 1.0%, P less than 0.01), or by addition of the fibronectin cell binding site analogue Arg-Gly-Asp-Ser (RGDS) (from 18.1 +/- 2.3% to 2.9 +/- 0.8%, P less than 0.01). An anti-fibronectin monoclonal antibody directed against the heparin binding domain of fibronectin had no effect on PC attachment. Addition of the specific calcium ion chelating agent EGTA to the culture media similarly decreased attachment from 16.9 +/- 2.0% to 5.1 +/- 1.1% (P less than 0.01). Fibronectin-mediated attachment of PC to AM did not result in phagocytosis of the organisms by the AM as determined by chemiluminescence measurements. Therefore, the data indicate that PC attachment to AM is a calcium-dependent process mediated by the cell binding domain of fibronectin which does not trigger a phagocytic response by the AM.