Pneumocystis carinii pneumonia: a light microscopical and ultrastructural study

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

Electron microscopy as a tool for identifying new pathogens

Electron microscopy (EM) is still an important tool for the investigation of infectious diseases, despite the introduction of powerful new methods, mainly involving the polymerase chain reaction. Particularly in the field of parasitic protozoology associated with AIDS, where many new species of human pathogens have been recognized in tissue biopsies, EM remains an essential 'catch-all' diagnostic method. The resolved ultrastructural details of these newly recognized parasites allows a unique insight into the biology of these organisms. The information produced by EM is different, but complementary, to that provided by alternative methods.

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.

Molecular genetic distinction of Pneumocystis carinii from rats and humans

Pneumocystis carinii from rats and from humans were compared with respect to electrophoretic karyotype, presence of DNA sequences known to be repeated in rat-derived P. carinii, overall DNA sequence homology, and the sequences at two genetic loci. The organisms from each host species were different in each respect. Neither of two repeated DNAs from rat-derived P. carinii was found in the genome of human-derived organisms, and total DNA from rat-derived P. carinii failed to hybridize to human-derived P. carinii DNA. The sequences of the alpha-tubulin genes from the two P. carinii were strikingly different and the base composition of the alpha-tubulin gene from rat-derived P. carinii was rich in adenine and thymine, while the base composition of this gene from human-derived P. carinii was rich in guanine and cytosine. The sequence from the 18S rRNA gene of human-derived P. carinii was twice as divergent from that of rat-derived P. carinii as the sequence from the corresponding region of Candida albicans was from that of Candida tropicalis. These data show that rats and humans can harbor distinct types of P. carinii that are sufficiently different to suggest that P. carinii from the two hosts could be different species.

Nude rats as a model of natural Pneumocystis carinii pneumonia: sequential morphological study of lung lesions

A spontaneous infection with Pneumocystis carinii (P.c.) caused enzootic fatal pneumonia in a long-term experiment with athymic Rowett Nude (Han:RNU rnu/rnu) and New Zealand Nude (Han:NZNU rnuN/rnuN) rats. In order to reproduce the infection and to characterize the early pathogenesis of lung lesions, 13 young athymic Han:RNU and Han:NZNU rats from P.c.-free breeding colonies were housed together in one room with chronically P.c.-infected rats. They were killed after 8, 12, 16, 20 or 24 weeks, and their lungs were examined for P.c. infection by means of light microscopy, immunohistochemistry and transmission electron microscopy. Lung lesions progressed from a mild interstitial pneumonia with scattered alveolar macrophages (8 weeks) to a severe diffuse interstitial pneumonia with widespread areas of distended alveoli, filled with foamy material. In all lungs, P.c. antigen was detected immunohistochemically, developing from small intra-alveolar aggregations of organisms into large multifocal clusters. Ultrastructurally, P.c. trophozoites and cysts were seen attached to type I pneumocytes and lying free in the alveolar lumen. Chronic severe P.c. pneumonia (20 and 24 weeks) was characterized by masses of P.c. trophozoites in the alveoli and alveolar walls, extensive proliferation of type II pneumocytes and interstitial fibrosis. The easy and consistent horizontal transmission of spontaneous P.c. pneumonia to previously non-infected athymic Han:RNU and Han:NZNU rats and the similarity of the disease to human infection demonstrate both rat strains to be excellent models for studying pulmonary pneumocystosis of immunodeficient human patients.

Evidence for two genetic variants of Pneumocystis carinii coinfecting laboratory rats

Pneumocystis carinii pneumonia is an oftentimes fatal infection for hosts in an immunocompromised state. The disease occurs in a wide variety of mammals, but the etiologic agent of this disease has been referred to as P. carinii regardless of the host species. However, even within a single host species, such as laboratory rats, distinct varieties of P. carinii have been identified from differences in the electrophoretic migration of chromosomes in agarose gels. Here we present evidence indicating that some laboratory rats can contain two different genetic variants of P. carinii that differ not only in electrophoretic karyotype but also in the presence of a particular repeated DNA sequence, in the presence of an intron in the 18S ribosomal RNA gene, and in the sequence of part of the 18S rRNA gene. Most of the rat colonies studied were infected with P. carinii that contained the repeated DNA and the 18S rRNA gene intron. The other type of rat-derived P. carinii, which lacked the repeated DNA and the intron in the 18S rRNA gene, was found as a coinfection with the first. Parasite populations from different coinfected rats contained the two variants in different proportions.

The diagnosis of Pneumocystis carinii pneumonia by cytologic evaluation of Papanicolaou and Leishman-stained bronchoalveolar specimens in patients with the acquired immunodeficiency syndrome

The presence of foamy alveolar casts or flocculent material in Papanicolaou and Leishman-stained smears of bronchoalveolar lavage (BAL) fluid is said to be indicative of infection with Pneumocystis carinii. We have investigated the sensitivity and specificity of this method of diagnosing pneumocystis pneumonia in patients with the acquired immunodeficiency syndrome (AIDS). Patients (n = 114) with diffuse lung infiltrates were submitted to fibreoptic bronchoscopy and BAL. Seventy of them were patients with AIDS. The other 44 individuals were not infected by the human immunodeficiency virus (HIV). Pneumocystis carinii organisms were identified on Grocott's methenamine silver (GMS)-stained BAL smears in 30 patients with AIDS. Flocculent material was present in the Papanicolaou and Leishman-stained smears from all of these cases. Conversely, P. carinii were not seen on GMS-stained smears in the remaining 84 individuals with or without AIDS. No flocculent material was observed in Papanicolaou or Leishman-stained smears in these 84 patients. We concluded that the presence of flocculent material in Papanicolaou or Leishman-stained smears of BAL fluid is indicative of P. carinii pneumonia in patients with AIDS.

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.

Development and resolution of Pneumocystis carinii pneumonia in severe combined immunodeficient mice: a morphological study of host inflammatory responses

The development and resolution of naturally-acquired Pneumocystis carinii pneumonia was studied in a severe combined immunodeficient (SCID) mouse model by light and electron microscopies. Initial infection was evident in 3-week-old SCID mice and started as focal alveolar colonization in the areas near terminal airways. Pronounced pulmonary inflammation occurred in animals of 10 weeks or older and the infection intensity reached a plateau in animals 12 weeks of age. At this stage of disease, the histopathological features of P. carinii infection in SCID mice were similar to those of immunodeficient man. Reconstitution of SCID mice with immunocompetent spleen cells at day 0 induced substantial pulmonary inflammation that was evident already by day 7 and most severe and extensive by day 12. The clearance of P. carinii did not begin until after day 12 and was almost completed by day 17. Alveolar macrophages in mice killed between days 12 and 15, at the time when P. carinii are being rapidly cleared, appeared active but phagocytosis of P. carinii was not commonly observed by either light or electron microscopy. These results suggest that (1) the presence of non-lymphoid inflammatory cells in SCID mice is not sufficient to control P. carinii infection; (2) the clearance of P. carinii from the lungs of reconstituted SCID mice requires local recruitment of large numbers of inflammatory cells with an active appearance; and (3) intracellular killing of P. carinii by phagocytosis does not appear to be a major mechanism in host defences against P. carinii infection in this model.

[Study on the therapeutic effects of interferon and gamma-globulin in experimental Pneumocystis carinii pneumonia]

This study was performed to observe the therapeutic effects of interferon-gamma(IFN-gamma) and gamma-globulin(gamma-globulin) in experimental Pneumocystis carinii pneumonia of immune suppressed mice. After 9 weeks, trimethoprim-sulfamethoxazole(TMP-SMZ; 10-50 mg/mouse/day), mouse IFN-gamma(5 x 10(4) units/mouse/day) and mouse gamma-globulin(20 mg/mouse/day) were administered to the mice for 3 weeks by the experimental group. The therapeutic efficacy was evaluated by body weights, histopathologic and electron microscopic findings of the lungs, and number of P. carinii cysts by Gomori's methenamine silver stain. Body weights of the mice were significantly increased in the group of combination therapy of TMP-SMZ with IFN-gamma or gamma-globulin, and in the group of TMP-SMZ treatment (p < 0.05), however, little effect was found in the group of gamma-globulin alone. Histopathologic findings of P. carinii pneumonia were much improved in the group of combination therapy of TMP-SMZ with IFN-gamma. Treatment with either TMP-SMZ or IFN-gamma significantly reduced the number of cysts in the P. carinii pneumonia, but gamma-globulin alone was ineffective. In electron microscopic findings of P. carinii pneumonia, the number of trophozoites and cysts were reduced by treatment with either TMP-SMZ or IFN-gamma, and most of the cysts were empty or containing one or two intracystic bodies. The present results suggested, that combination therapy of TMP-SMZ with IFN-gamma had synergistic effects in treatment of P. carinii pneumonia in experimental mice.

Improved intracellular morphology of Pneumocystis carinii from rat lung by postfixation with a mixture of potassium ferrocyanide and osmium tetroxide

Pneumocystis carinii infected rat lungs were postfixed with a mixture of OsO4 and K4Fe(CN)6. A marked improvement in staining of cell membranes, endoplasmic reticulum, nuclear membranes and glycogen was observed. These improvements were seen in both the trophic and cystic forms of the organisms. The addition of K4Fe(CN)6 did not improve the staining of cell walls, microtubules or ribosomes. Trophozoites were seen attached to both type 1 and type 2 pneumocytes by filopodia and/or intercalation of the cell body of P. carinii with the host lung cells. It is expected that the improvement in ultrastructural detail will allow better understanding of the ultrastructure of P. carinii and provide insights into the modes of action of various antimicrobial compounds on this organism.

Isolation of Pneumocystis carinii cysts by flow cytometry

Pneumocystis carinii cysts were separated as enriched populations from suspensions of lung homogenate obtained from infected rats containing all developmental stages of this organism. Isolation of cyst populations was achieved by incubating filtered lung homogenate with the fluorescent phospholipid analog 1-palmitoyl-2-C6-NBD-phosphatidylcholine. Whereas cysts did not fluorescence as a result of outer-wall restraint of lipid integration, trophozoites and young intermediate stages readily incorporated the fluorescently labeled lipid analog into their outer membrane. The two distinct labeling patterns displayed by cysts and other developmental phases of P. carinii constitute a novel, easy, and reproducible means of isolating cysts from infected lung homogenate by flow cytometry.

Pneumocystis carinii pneumonia in simian immunodeficiency virus infection: immunohistological and scanning and transmission electron microscopical studies

Pneumocystis carinii pneumonia occurred in 6 of 17 rhesus monkeys infected with simian immunodeficiency virus and was studied by immunohistochemistry and by scanning and transmission electron microscopy. A monoclonal antibody/streptavidin-biotin-peroxidase staining method was highly sensitive for detecting the organisms in small, early lesions and was much more sensitive and specific than traditional silver impregnation methods. Reprocessing of paraffin wax-embedded lung tissue for scanning electron microscopy and use of a video printer to produce a photographic montage of light microscopic lesions allowed the same areas of tissue to be examined and compared by both methods. The ultrastructural morphology of P. carinii in the rhesus monkey was identical to that in man, as were the histological and electron microscopic lesions, including pulmonary fibrosis. Trophozoites were seen attached to alveolar type I epithelium mainly by intimate apposition to the plasma membrane, but scanning electron microscopy also showed attachment by elongated filopodia. Few macrophages were present in infected alveoli, and though phagocytosis followed by digestion of P. carinii trophozoites was observed, it appeared to occur at a very low level.

Opportunistic protozoan infections in human immunodeficiency virus disease: review highlighting diagnostic and therapeutic aspects

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Why have Pneumocystis carinii trophozoites been ignored?

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Measurement of lung tissue mass in Pneumocystis carinii pneumonia

Transmission-emission scanning of the thorax with a flood source of technetium-99m (99mTc), autologous 99mTc labelled red blood cells, and diethylenetriaminepenta-acetic acid (99mTc DTPA) allows measurement of thoracic tissue thickness and blood and interstitial volume per pixel of the gamma camera image. Volume of blood or interstitium per pixel divided by pixel area gives the thoracic tissue thickness for these two compartments. A measure of lung tissue thickness may be obtained by subtracting chest wall thickness as measured on a lateral chest radiograph. As part of the evaluation of this technique 13 patients were scanned before treatment for proved or presumed pneumocystis pneumonia and their results were compared with those of 12 normal young men, approximately matched for age. In the patients with pneumocystis pneumonia lung tissue thickness at the base of the right lung averaged 3.4 cm (71%) more than that in the normal subjects, and interstitial thickness was 1.2 cm (150%) more than in the normal subjects. After treatment 10 of the patients with pneumocystis pneumonia were scanned again. Lung tissue thickness remained greater than the control value by 1.2 cm and interstitial tissue thickness by 0.5 cm. Blood thickness remained unchanged. Lung tissue and interstitial tissue thickness was correlated with a numerical score of the changes in the chest radiograph. The changes in the properties of the lung tissue compartments in the patients presumably reflect the mural and intra-alveolar inflammation found in pneumocystis pneumonia. Transmission scanning alone measures the increase of lung tissue thickness as well as transmission-emission scanning. It may be of value in monitoring the progress of this condition during treatment.

A clinical, histopathologic, and electron microscopic study of Pneumocystis carinii choroiditis

We studied the clinical and histopathologic features of Pneumocystis carinii choroiditis in three patients with acquired immunodeficiency syndrome. In two cases, a provisional diagnosis of disseminated P. carinii infection was made by ophthalmologic examination. The characteristic fundus changes in this infection consisted of numerous slightly elevated, plaque-like, yellow-white lesions located in the choroid and unassociated with signs of intraocular inflammation. The diagnosis was confirmed by postmortem examination of the eyes and other organs. Histopathologically, the globes showed many choroidal infiltrates that were eosinophilic, acellular, vacuolated, and frothy. Several such infiltrates were noted within the choroidal vessels and choriocapillaries. Gomori's methenamine silver stain demonstrated many cystic and crescentic organisms. Electron microscopy disclosed thick-walled cystic organisms and large numbers of trophozoites.

Protozoal infections in the acquired immunodeficiency syndrome

Several protozoa have emerged as the major opportunistic infections and cause of death in patients with acquired immunodeficiency syndrome (AIDS). Pneumocystis carinii pneumonia is the leading cause of death in AIDS patients. Electron microscopy (EM) usually shows numerous trophozoites and cysts of Pneumocystis filling up the entire alveolar space, while only cysts are seen under the light microscope. The focal thickening of cyst wall of Pneumocystis, as demonstrated by EM and manifested as a "parentheses" shaped structure with silver stain, serves as a diagnostic marker for Pneumocystis. Freeze-fracture EM has demonstrated the intimate contact between Pneumocystis trophozoites and the type I pneumocytes, which may contribute to the alveolar-capillary block, leading to severe respiratory distress. However, EM is seldom needed for the diagnosis of this infection. Toxoplasma encephalitis, which is an unusual clinical manifestation in cases of toxoplasmosis reported previously, has become a common complication and one of the major causes of death in patients with AIDS. Because subclinical infection by Toxoplasma is common, serologic tests usually offer no definite answers as to whether the infection is acute or chronic, active or past. The small size and its non-specificity in both morphology and tissue affinity make light microscopic diagnosis of toxoplasmosis difficult. Only immunologic staining, such as immunoperoxidase and immunofluorescence, can help to achieve a definite positive identification of the organism. When special antibodies or facility for such staining is not available, EM is the final resort for identifying Toxoplasma by showing the apical complex with the characteristic sausage-shaped rhoptries. Cryptosporidiosis, practically unknown before the AIDS outbreak, has become one of the most common intestinal protozoa in both immunocompromised and immunocompetent patients. The protracted and sometimes fatal course of cryptosporidiosis in immunocompromised patients can be explained by the presence of autoinfective oocysts (thin-walled oocysts), as detected by EM, and by recycling of first-generation schizonts observed experimentally. While diagnosis of cryptosporidiosis can be made by detection of oocysts in stools in most cases, EM is still the last resort for a definitive identification of Cryptosporidium species. While the incidence of isosporiasis is still low, it has been found more frequently in patients with AIDS than in the general population. The parasite, Isospora belli, being a coccidian as is the Cryptosporidium species, is similar to the latter in its life cycle and clinical manifestations.(ABSTRACT TRUNCATED AT 400 WORDS)

Histoenzymological study of selected dehydrogenase enzymes in Pneumocystis carinii

The metabolic activity of Pneumocystis carinii cysts was studied histochemically by a tetrazolium dye technique to assess substrate-specific dehydrogenase activity. Lactate dehydrogenase, succinate dehydrogenase, and glutamate dehydrogenase produced moderate-to-strong reactions in the cysts, whereas glucose-6-phosphate dehydrogenase had little if any reactivity. These results suggest that pneumocystis cysts have some of the enzymes necessary for glycolysis, Krebs cycle activity, and intermediary protein metabolism. These studies provide a method of directly assessing metabolic pathways in P. carinii which circumvents the uncertainties of specificity inherent in previous investigations with partially purified suspensions.

Adult respiratory distress syndrome--a review

Adult respiratory distress syndrome (ARDS) represents a common pathway of damage to the lungs by a wide variety of different agents. The important aetiological factors and mechanisms of lung injury are considered. Electron microscopic features as well as light microscopy are described. Factors that may modify the pathological picture are discussed. Probably the most important of these is oxygen. This gas is said to produce interstitial pulmonary fibrosis but this concept is once again questioned.

[Pulmonary infections caused by Pneumocystis carinii]

The following topics are discussed: 1. Pneumocystis carnii: appearance, antigenic structure, pathogenicity...; 2. Human pneumocytosis: clinical, diagnostic, prognostic and therapeutic aspects; 3. Pneumocystis carnii and the pneumocystoses: coexistence, theoretical and practical consequences, difficulties in interpretation of bronchiolo-alveolar lavage.

Pathologic features of the lung in the immunocompromised host

Acute pulmonary disease is a major complication of immunodeficiency, and it has become increasingly important with the expanded use of immunosuppressive drugs. When routine clinical evaluation fails to identify a specific etiologic agent, a morphologic diagnosis is pursued by means of one or more invasive procedures. Interpretation of the material obtained by these procedures poses a challenge to pathologists. In this paper, the important histopathologic patterns of pulmonary disease likely to be encountered in this setting are reviewed, with emphasis on differential diagnosis. In addition, various diagnostic techniques are discussed and compared, with regard to interpretation of findings and diagnostic yields.

Pneumocystis carinii: the continuing enigma

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