Use of a fluorescent probe to assess the activities of candidate agents against intracellular forms of Encephalitozoon microsporidia

Microsporidial Stromal Keratitis: Epidemiological Features, Slit-Lamp Biomicroscopic Characteristics, and Therapy

PURPOSE

Microsporidial stromal keratitis is a rare form of infectious keratitis, with only 7 cases reported in the United States to date. This study was performed to evaluate risk factors, clinical features, and response to therapy.

METHODS

A retrospective review of the medical records of all patients diagnosed with microsporidial stromal keratitis seen in the practices of the authors between 1999 and 2020 was performed. Diagnosis was determined by cytology or histopathology in corneal specimens. Risk factors, presence or absence of distinctive clinical features, and response to medical and surgical therapies were recorded.

RESULTS

Nine patients-7M:2F, aged 7 to 99 years-with microsporidial stromal keratitis were identified. Exposures to recreational water and hymenopteran insect bites, both epidemiologically linked risk factors for systemic microsporidial infection, were identified in our patients. Presence of stromal edema with features of disciform keratitis and a distinctive granular keratitis were observed in 6 of 9 and 5 of 9 patients, respectively. Poor response to medical therapy was noted. Penetrating keratoplasty was effective in curing the infection. Final visual acuity was 20/40 or better in 6 of 9 patients.

CONCLUSIONS

In patients with slowly progressive keratitis, history of exposure to recreational water or hymenopteran insects should be sought. In patients with corneal edema consistent with disciform keratitis, with evolution to a granular keratitis, microsporidia should be considered in the differential diagnosis. In cases of established microsporidial stromal keratitis, penetrating keratoplasty should be considered if prompt response to medical therapy is not noted.

Role of gap junctions and hemichannels in parasitic infections

In vertebrates, connexins (Cxs) and pannexins (Panxs) are proteins that form gap junction channels and/or hemichannels located at cell-cell interfaces and cell surface, respectively. Similar channel types are formed by innexins in invertebrate cells. These channels serve as pathways for cellular communication that coordinate diverse physiologic processes. However, it is known that many acquired and inherited diseases deregulate Cx and/or Panx channels, condition that frequently worsens the pathological state of vertebrates. Recent evidences suggest that Cx and/or Panx hemichannels play a relevant role in bacterial and viral infections. Nonetheless, little is known about the role of Cx- and Panx-based channels in parasitic infections of vertebrates. In this review, available data on changes in Cx and gap junction channel changes induced by parasitic infections are summarized. Additionally, we describe recent findings that suggest possible roles of hemichannels in parasitic infections. Finally, the possibility of new therapeutic designs based on hemichannel blokers is presented.

Multinucleate host cells induced by Vittaforma corneae (Microsporidia)

The microsporidium Vittaforma corneae develops within the target cell cytoplasm. In the present study, green monkey kidney (E6) cells infected at 30 degrees C, 35 degrees C or 37 degrees C with V. corneae developed enlarged multinucleate structures of up to 200 microm in any horizontal dimension made up either of a single cell or of multiple fused cells. A number of epithelial cell types (SW-480, HT-29, Caco-2 and HCT-8) were infected with V. corneae but did not induce the same highly organized structures, suggesting that for the structure to develop, the host cell must be capable of continued mitosis, and not be differentiated or be detaching from the surface matrix. Live cell imaging of infected E6 cells revealed large, multinucleate infected cells characterized by a central focus from which radiated parasite stages and host cell mitochondria. Immunocytochemistry identifying gamma and alpha tubulin suggested that a single centrally-located microtubule organizing centre governed the distribution of parasite stages and host cell organelles, with mitochondria and parasites being eventually transported towards the periphery of the structure. Whole cell patch clamp analysis of infected cells indicated an average five-fold increase in total membrane capacitance, consistent with an enlarged single cell. Scanning electron microscopy revealed cell-like protrusions around the periphery of the structure with the intervening space being made up of parasites and cell debris. Clearly in the case of V. corneae-infected E6 cells the parasite-host cell relationship involves subverting the host cell cytoskeleton and cell volume control, providing the parasite with the same protected niche as does a xenoma.

Cryptosporidium parvum: effect of multi-drug reversing agents on the expression and function of ATP-binding cassette transporters

In the present study, the gene expression of three multidrug resistance (MDR) and resistance-associated protein (MRP) transport proteins or efflux pumps was characterized and the phenotypic evidence for such pumps was demonstrated in cultured Madin-Darby canine kidney (MDCK) cells. A gradient for the fluorescent probe calcein was established between parasite and host cell suggestive of a parasite extrusion pump at the parasite-host interface. This gradient was decreased in a glucose-free medium containing 2-deoxyglucose or 3-O-methylglucose, by probenecid, and by the isoflavonoid, narigenin, suggesting that the calcein extrusion was energy-dependent and involved an MRP-like pump. While neither MDR or MRP inhibiters significantly affected transcript levels of any of the ABC transporters, transcript levels of the Cryptosporidium parvum ABC protein (CpABC1), an MRP transporter, were consistently expressed 4 logs higher than either CpABC3 or CpABC2, suggesting a prominent role in the intracellular stages of the parasite.

Relationship between the host cell mitochondria and the parasitophorous vacuole in cells infected with Encephalitozoon microsporidia

Encephalitozoon microsporidia proliferate and differentiate within a parasitophorous vacuole. Using the fluorescent probe, calcein, and the mitochondrial probe, MitoTracker-CMXRos, a vital method was developed that confirmed ultrastructural reports that the host cell mitochondria frequently lie in immediate proximity to the parasitophorous vacuole. Morphometry failed to demonstrate any infection-induced increase in host cell mitochondria as there was no correlation between the mitochondrial volume and the extent of infection as judged by the parasitophorous vacuole volume. The total ATP concentration of infected cells did not differ from that of uninfected cells in spite of the increased metabolic demands of the infection. Treatment with 10(-6) M albendazole, more than ten times the antiparasitic IC50 dose, and demecolcine had no subjective effect on the proximity of mitochondria to the parasitophorous vacuole membrane when studied by either transmission electron microscopy or by confocal microscopy even though these drug concentrations affected microtubule structure. Thus, once the association between mitochondria and the parasitophorous vacuole has been established, host cell microtubule integrity is probably not required for its maintenance. It is unlikely that the antimicrosporidial action of albendazole involves physically uncoupling developing parasite stages from host cell organelle metabolic support.

Role of P glycoprotein in the course and treatment of Encephalitozoon microsporidiosis

Encephalitozoon microsporidia are obligate intracellular protozoan parasites that proliferate and differentiate within a parasitophorous vacuole inside host cells that are usually epithelial in nature. Isolates of the three species of the Encephalitozoon microsporidia, E. cuniculi, E. hellem, and E. intestinalis, were obtained from AIDS patients and cultured in green monkey (E6) kidney cells. Anti-P-glycoprotein (anti-Pgp) and anti-multidrug resistance-associated protein (anti-MRP) monoclonal antibodies were used to probe for multidrug resistance (MDR) pump epitopes and verapamil- or cyclosporin A- and probenecid-modulated intracellular calcein fluorescence were used to assess the expression of Pgp and MRP respectively in uninfected and infected cells. Pgp, but not MRP, was detected immunocytochemically and by verapamil- and cyclosporin A-potentiated intracellular fluorescence in both host cells and parasite developing stages. When an in vitro infection assay was employed, verapamil and cyclosporin A acted as chemosensitizing agents for the antiparasitic drug albendazole. These observations suggest that inhibiting host cell and perhaps parasite MDR pumps may increase the efficacy of antiparasitic agents in these and other microsporidia species.

Drug treatment of microsporidiosis

Microsporidia are ubiquitous organisms that are emerging pathogens in humans. These are most likely zoonotic and/or waterborne infections. In the immunosuppressed host, such as those treated with immunosuppressive drugs or infected with human immunodeficiency virus particularly at advanced stages of the disease, microsporidia can produce a wide range of clinical diseases. The most common manifestation is gastrointestinal tract infection; however, encephalitis, ocular infection, sinusitis, myositis and disseminated infection have also been described. In addition, these organisms have been reported in immune competent individuals. Multiple genera are involved in these infections and different organisms can result in distinct clinical pictures. Differences in clinical and parasitologic response to various therapeutic agents have emerged from clinical, as well as in vitro and in vivo studies. Currently there are no precisely defined guidelines for the optimal treatment of microsporidial infections. This article reviews the available data on compounds with in vitro activity and/or in vivo efficacy for microsporidial infections. Copyright 2000 Harcourt Publishers Ltd.

Infection by microsporidia disrupts the host cell cycle

Microsporidia of the genus Encephalitozoon infect mammalian cells and have become a source of morbidity and mortality in immunocompromised humans. Encephalitozoon microsporidia develop and mature within parasitophorous vacuoles, enlarging the vacuole over time until it eventually occupies most of the cytoplasm of the host cell. The ability of the host cell to accommodate such a large burden for several days suggests that the parasite subverts normal host cell processes to ensure optimal environmental conditions for its growth and development. Since this environment would be threatened if cell division of the host cell occurred, we have formulated the hypothesis that infection with Encephalitozoon microsporidia induces an arrest in the cell cycle of the host cell. In support of this hypothesis, we have found that mitotic index and DNA duplication are reduced in infected cells as compared to uninfected cells. The number of host cell nuclei in S phase is increased. The levels of cyclin D1 and the percentage of cells in G1 are reduced; however, the levels of cyclin B1 are elevated even though the percentage of cells in G2/M is decreased. These results suggest that host cells infected with Encephalitozoon microsporidia are blocked at multiple points in the cell cycle.

A spore counting method and cell culture model for chlorine disinfection studies of Encephalitozoon syn. Septata intestinalis

The microsporidia have recently been recognized as a group of pathogens that have potential for waterborne transmission; however, little is known about the effects of routine disinfection on microsporidian spore viability. In this study, in vitro growth of Encephalitozoon syn. Septata intestinalis, a microsporidium found in the human gut, was used as a model to assess the effect of chlorine on the infectivity and viability of microsporidian spores. Spore inoculum concentrations were determined by using spectrophotometric measurements (percent transmittance at 625 nm) and by traditional hemacytometer counting. To determine quantitative dose-response data for spore infectivity, we optimized a rabbit kidney cell culture system in 24-well plates, which facilitated calculation of a 50% tissue culture infective dose (TCID(50)) and a minimal infective dose (MID) for E. intestinalis. The TCID(50) is a quantitative measure of infectivity and growth and is the number of organisms that must be present to infect 50% of the cell culture wells tested. The MID is as a measure of a system's permissiveness to infection and a measure of spore infectivity. A standardized MID and a standardized TCID(50) have not been reported previously for any microsporidian species. Both types of doses are reported in this paper, and the values were used to evaluate the effects of chlorine disinfection on the in vitro growth of microsporidia. Spores were treated with chlorine at concentrations of 0, 1, 2, 5, and 10 mg/liter. The exposure times ranged from 0 to 80 min at 25 degrees C and pH 7. MID data for E. intestinalis were compared before and after chlorine disinfection. A 3-log reduction (99.9% inhibition) in the E. intestinalis MID was observed at a chlorine concentration of 2 mg/liter after a minimum exposure time of 16 min. The log(10) reduction results based on percent transmittance-derived spore counts were equivalent to the results based on hemacytometer-derived spore counts. Our data suggest that chlorine treatment may be an effective water treatment for E. intestinalis and that spectrophotometric methods may be substituted for labor-intensive hemacytometer methods when spores are counted in laboratory-based chlorine disinfection studies.

Molecular techniques for detection, species differentiation, and phylogenetic analysis of microsporidia

Microsporidia are obligate intracellular protozoan parasites that infect a broad range of vertebrates and invertebrates. These parasites are now recognized as one of the most common pathogens in human immunodeficiency virus-infected patients. For most patients with infectious diseases, microbiological isolation and identification techniques offer the most rapid and specific determination of the etiologic agent. This is not a suitable procedure for microsporidia, which are obligate intracellular parasites requiring cell culture systems for growth. Therefore, the diagnosis of microsporidiosis currently depends on morphological demonstration of the organisms themselves. Although the diagnosis of microsporidiosis and identification of microsporidia by light microscopy have greatly improved during the last few years, species differentiation by these techniques is usually impossible and transmission electron microscopy may be necessary. Immunfluorescent-staining techniques have been developed for species differentiation of microsporidia, but the antibodies used in these procedures are available only at research laboratories at present. During the last 10 years, the detection of infectious disease agents has begun to include the use of nucleic acid-based technologies. Diagnosis of infection caused by parasitic organisms is the last field of clinical microbiology to incorporate these techniques and molecular techniques (e.g., PCR and hybridization assays) have recently been developed for the detection, species differentiation, and phylogenetic analysis of microsporidia. In this paper we review human microsporidial infections and describe and discuss these newly developed molecular techniques.

In vitro susceptibilities of the microsporidia Encephalitozoon cuniculi, Encephalitozoon hellem, and Encephalitozoon intestinalis to albendazole and its sulfoxide and sulfone metabolites

In vitro comparisons demonstrated that the efficacy of albendazole, albendazole-sulfoxide, and albendazole-sulfone against pathogenic Encephalitozoon species was proportional to the degree of oxidation at a concentration of >10(-3) microgram/ml. Furthermore, at a concentration of <10(-2) microgram/ml, benzimidazoles were more effective against Encephalitozoon cuniculi and Encephalitozoon hellem than against Encephalitozoon intestinalis.

Purification of Encephalitozoon cultures contaminated by mycoplasmas by murine intraperitoneal inoculation

Encephalitozoon species are strict intracellular microsporidia. Cocultures with eukaryotic cell lines can become accidently contaminated by mycoplasmas. We propose a decontamination protocol based on differential cell targeting after intraperitoneal inoculation in mice. Mycoplasma-free microsporidia were isolated from the brains and spleens of inoculated mice 24 h postinoculation by using the centrifugation shell vial system. Identification was confirmed by direct sequencing of PCR-amplified 16S rRNA.