Is there more than one species in the genus Toxoplasma??

A complete life cycle for the ubiquitous protozoan parasite Toxoplasma was proposed over 25 years ago. Since that time, despite attempts to make the genus polyspecific, there has been only one species, Toxoplasma gondii, consistently recognised in the genus. Recent studies on taxa in genera closely related to Toxoplasma such as Neospora, Hammondia, Frenkelia, Isospora and Sarcocystis, have convincingly showed the need for a reclassification of many of the species in these genera. However, in addition to these genus level studies, over the last 10 years several laboratories have used molecular techniques including isoenzyme electrophoresis, restriction fragment length polymorphism analyses, random amplified polymorphic DNA - polymerase chain reaction, and comparisons of the small subunit ribosomal RNA gene, DNA polymerase alpha intron, and 70 kDa heat shock protein gene nucleotide sequences to investigate the genetic diversity among strains in the species T. gondii. Overall, the results of these analyses confirm that the strains in the genus Toxoplasma comprise a limited number of clonal lineages, directly correlated with their virulence in mice. The aim of this presentation is to review the molecular research in this area in order to raise the hypothesis that there may be more than one species in the genus Toxoplasma, which may contain taxa with distinct and different life cycles.

Processing of Toxoplasma ROP1 protein in nascent rhoptries

Secretion in the obligate intracellular parasite, Toxoplasma gondii, occurs through a number of regulated compartments. Among these are the apical organelles known as rhoptries which release their contents as part of the invasion process. We are interested in the processing, targeting and ultimate function of rhoptry proteins (and have focused our analyses on rhoptry protein 1 (ROP1). In this paper, we address the issue of processing: using a number of engineered forms of the ROP1 gene (introduced into a ROP1- background), we show that ROP1 is synthesized as a pre-pro-protein that is subject to proteolytic cleavages to remove the pre-sequence and the 'pro' region, at the N-terminus. Using brefeldin A (BFA) and reduced temperature we show that this processing occurs late in the secretory pathway of the parasite. Immunolocalization studies with epitope-tagged constructs indicate that processing is apparently occurring in the nascent rhoptries of dividing parasites. The results are discussed in the context of the targeting and possible function of the ROP1 protein.

Toxoplasma gondii bradyzoites form spontaneously during sporozoite-initiated development

Tachyzoites (VEG strain) that emerge from host cells infected with Toxoplasma gondii sporozoites proliferate relatively fast and double their number every 6 h. This rate of growth is intrinsic, as neither the number of host cells invaded nor host cell type appears to influence emergent tachyzoite replication. Fast tachyzoite growth was not persistent, and following approximately 20 divisions, the population uniformly shifted to slower growth. Parasites 10 days post-sporozoite infection doubled only once every 15 h and, unlike emergent tachyzoites, they grew at this slower rate over several months of continuous cell culture. The spontaneous change in tachyzoite growth rate preceded the expression of the bradyzoite-specific marker, BAG1. Within 24 h of the growth shift, 2% of the population expressed BAG1, and by 15 days post-sporozoite infection, 50% of the parasites were positive for this marker. Spontaneous BAG1 expression was not observed in sporozoites or in tachyzoites during fast growth (through day 6 post-sporozoite inoculation), although these tachyzoites could be induced to express BAG1 earlier by culturing sporozoite-infected cells at pH 8.3. However, alkaline treatment also reduced the replication of emergent tachyzoites to the rate of growth-shifted parasites, supporting a link between reduced parasite growth and bradyzoite differentiation. The shift to slower growth was closely correlated with virulence in mice, as the initially fast-growing emergent tachyzoites were avirulent (100% lethal dose, >10(4) parasites), while a mutant VEG strain (MS-J) that is unable to growth shift caused 100% mortality in mice inoculated with 10 parasites. Parasites recovered from gamma interferon knockout mice inoculated with emergent tachyzoites grew at a slow rate and expressed BAG1, confirming that the replication switch occurs in animals and in the absence of a protective immune response.

Detection of human Toxoplasma-specific immunoglobulins A, M, and G with a recombinant Toxoplasma gondii rop2 protein

The Toxoplasma gondii rhoptry protein Rop2 was expressed in Escherichia coli as a fusion protein containing 44 kDa of the 55-kDa mature Rop2, supplied with six histidyl residues at the N-terminal end (Rop2196-561). Humoral response during Toxoplasma infection of humans was analyzed by immunoglobulin G (IgG), IgA, and IgM enzyme-linked immunosorbent assay with Rop2196-561 as the antigen substrate. The analyzed sera were divided according to T. gondii-specific serological tests (IgG, IgA, or IgM indirect immunofluorescence and IgA or IgM immunosorbent agglutination assay) as group A (IgG+ IgA- IgM-; n = 35), group B (IgG+ IgA+ IgM+; n = 21), group C (IgG+ IgA+ IgM-; n = 5), and group D (IgG+ IgA- IgM+; n = 16). Twenty-six T. gondii-seronegative sera from individuals with other infections were also included (group E). Anti-Rop2 IgG antibodies were detected in 82.8% of group A sera and in 97.6% of the sera with acute-phase marker immunoglobulins (groups B, C, and D). The percentage of IgA antibody reactivity against Rop2196-561 was 17.1% in group A, 50% in group D, and 80.8% in groups B and C. The percentage of IgM antibody reactivity was 0% in groups A and C and 62% in groups B and D. Sera from group E failed to show IgA, IgM, or IgG antibody reactivity. Since T. gondii Rop2 elicits a strong humoral response from an early stage of infection, it is suggested that recombinant Rop2196-561 would be suitable for use in diagnostic systems, in combination with other T. gondii antigens, to detect specific IgG, IgA, and IgM antibodies.

Determination of the inhibitory effect on Toxoplasma growth in the serum of AIDS patients during acute therapy for toxoplasmic encephalitis

In 26 AIDS patients treated for toxoplasmic encephalitis, the inhibitory effect on Toxoxplasma growth of sequential sera taken before and after initiation of therapy was determined using a culture-based immunoassay and compared with pyrimethamine blood levels. A marked inhibition of Toxoplasma growth was observed 1 day after initiation of therapy with pyrimethamine plus sulfadiazine and was maintained between 67% and 93% throughout the 15-day follow-up period. The degree of inhibition was not correlated to pyrimethamine blood levels and seemed highly potentiated when sulfadiazine rather than macrolides was given in combination with pyrimethamine.

Assessment of the activity of atovaquone-loaded nanocapsules in the treatment of acute and chronic murine toxoplasmosis

The aim of this work was to develop a new pharmaceutical form of atovaquone and to study its activity against Toxoplasma gondii in vitro and in vivo. Nanocapsules were chosen as the oral dosage form of administration. An analytical method was developed to determine the drug content in nanocapsules. The stability of these nanocapsules were assessed by following drug content, size, pH and osmolarity for a period of six months. The in vitro activity of atovaquone-loaded nanocapsules against tachyzoites of T. gondii (RH stain) was comparable to its suspension form. In vivo studies were carried out in murine models of acute and chronic toxoplasmosis. Mice acutely infected with the virulent RH strain were orally treated with a dose regimen of 15 mg/kg/day for 10 days, starting from day 1 post-infection. 75% of the mice receiving atovaquone-loaded nanocapsules survived 30 days post-infection, compared to none of untreated controls and none of mice treated with the suspension with the same dose regimen. In mice chronically infected by the COUL or the ME49 strain (Type II strains), then treated for six weeks, treatment with atovaquone (15 mg/kg/d, nanoparticles or suspension) resulted in a decrease of brain parasitic burden, which was significantly more pronounced in ME49-infected mice and in those treated with drug-loaded nanocapsules. These results show that the sensibility of T. gondii to atovaquone is different according to the strains and that the activity of atovaquone in the treatment of toxoplasmosis is enhanced when administered in nanoparticular form.

Targeted disruption of the GRA2 locus in Toxoplasma gondii decreases acute virulence in mice

Following invasion into the host cell, the protozoan Toxoplasma gondii secretes a variety of proteins that modify the parasitophorous vacuole. Within the vacuole, the 28-kDa dense granule protein known as GRA2 is specifically targeted to the tubulovesicular network which forms connections with the vacuolar membrane. To investigate the importance of GRA2, we derived from strain RH a mutant T. gondii line in which GRA2 was disrupted by replacement with the marker Ble (selecting for phleomycin resistance). The Deltagra2 mutant invaded and grew normally in both fibroblasts and macrophages in vitro; however, it was less virulent during acute infection in mice. The survival rate of mice inoculated with Deltagra2 was significantly higher; some infected mice survived the acute infection, whereas all mice infected with the wild-type strain RH succumbed to early death. Chronic infection by Deltagra2 was detected by positive serology, immunohistochemical detection of parasites and cysts in the brain, and reisolation of parasites by bioassay at 6 weeks postinfection. Thus, absence of GRA2 partially attenuates the virulence of T. gondii during the acute phase of infection and allows for establishment of chronic infection by the otherwise highly virulent RH strain. These results establish that GRA2 plays an important role during in vivo infection and provide a potential model for examining acute pathogenesis by T. gondii.

Expression of Toxoplasma gondii-specific heat shock protein 70 during In vivo conversion of bradyzoites to tachyzoites

Stage conversion between bradyzoites and tachyzoites was investigated in C57BL/6 mice chronically infected with the ME-49 strain of Toxoplasma gondii. In order to promote bradyzoite-tachyzoite conversion, mice were treated in vivo with neutralizing doses of anti-gamma interferon (IFN-gamma) or anti-tumor necrosis factor alpha (TNF-alpha) antibodies. Expression of parasite-specific antigens SAG-1, SAG-2, and heat shock protein 70 (Hsp-70) was visualized in the central nervous system by immunocytochemistry and measured by photometric assay. The immunosuppressive effect of anti-IFN-gamma or anti-TNF-alpha treatment was immediate, leading to parasite stage conversion as indicated by the increased expression of tachyzoite-specific antigens (SAG-1 and SAG-2) and by rapid parasite replication. We also observed expression of high levels of Hsp-70 during a short period of conversion of bradyzoites to tachyzoites. Our data suggest that Hsp-70 may have an important role in the process of bradyzoite-tachyzoite conversion during the reactivation of chronic toxoplasmosis.

Toxoplasma gondii: an ultrastructural study of host-cell invasion by the bradyzoite stage

The invasion of Toxoplasma gondii tachyzoites and bradyzoites was followed in bovine kidney cells via electron microscopy. The process of invasion differed between bradyzoites and tachyzoites. In the early stages of entry there was evidence of localised formation of membrane projections in the host cell adjacent to the parasite. Parasite reorientation and rhoptry release appeared to be necessary for invasion; however, the tight junction could not be clearly discerned and there was no evidence of constriction or of any membrane shedding from the parasite. The resulting parasitophorous vacuole was smaller than the tachyzoite vacuole and parasites were frequently found to lie immediately under the host cell membrane. The vacuole was rapidly adapted by the release and formation of an intra-phagosomal membrane network, while the parasitophorous vacuole formed a relationship with host-cell endoplasmic reticulum.

A cell cycle model for the tachyzoite of Toxoplasma gondii using the Herpes simplex virus thymidine kinase

Toxoplasma gondii (RH strain) tachyzoites were transfected with a plasmid containing a fusion of the chloramphenicol acetyl transferase and the Herpes simplex virus-2 thymidine kinase coding regions and transgenic parasites obtained by chloramphenicol selection. CTK11, a single high expressing clone was isolated based on immunofluorescence and contained approximately five integrated copies of the fusion sequence. Lysates prepared from this clone displayed thymidine kinase activity of 2.9 pmol min(-1) microg(-1) protein, whereas thymidine kinase activity was not detected in lysates from the parental RH strain. Growth of CTK11 tachyzoites was fully inhibited in 5 microM ganciclovir and thymidine and in 2.5 microM 5-bromo-2'-deoxyuridine. While the inhibitory effects of ganciclovir were lethal, low concentrations of thymidine (10 microM) were largely reversible. Asynchronously growing CTK11 tachyzoites were found to contain major G1 (1 N) and S phase (1 N+) distributions as determined by relative propidium iodide fluorescence and with reference to the haploid (1 N) DNA content of a T. gondii sporozoite population. CTK11 tachyzoites blocked 4 h in 10 microM thymidine exhibited mean fluorescence consistent with a 1 N complement of DNA indicating growth was arrested in G1. Following the removal of excess thymidine, parasites immediately entered S phase, thus confirming the late G1 block. Parasites with a 2 N complement of DNA (G2 + M) first appear at 2 h post-release, while 1 N (G1) parasites re-appear at 3 h suggesting the length of S phase is < or = 2 h and that of G2 + M is < or = 1 h. Within 7 h, parasites had transited G2 + M and much of G1 and re-entered S of the subsequent cell cycle--a time consistent with the doubling of these parasites in culture. Thus, the CTK11 tachyzoite cell cycle is similar to those of higher eukaryotic cells and is characterized by major G1 and S phases and a relatively short G2 + M.

Paclitaxel arrests growth of intracellular Toxoplasma gondii

Addition of paclitaxel (Taxol) at a concentration of 1 microM to Toxoplasma gondii-infected human foreskin fibroblasts arrested parasite multiplication. Division of the T. gondii tachyzoite nucleus was inhibited, leading to syncytium-like parasite structures within the fibroblasts by 24 h after infection and treatment of the cultures. By 4 days after infection and treatment of the cultures with paclitaxel, this inhibition was irreversible, since the arrested intracellular form was incapable of leaving the host cell, infecting new cells, and initiating the growth of tachyzoites with normal morphology. Specifically, when paclitaxel was added to infected cells for 4 days and then removed by washing and the infected, paclitaxel-treated cells were cultured for 4 more days, there were no remaining T. gondii organisms with normal morphology. Syncytium-like structures in the cultures that were infected and treated with paclitaxel for 8 days were similar in appearance to those in preparations of infected paclitaxel-treated fibroblasts that had been cultured for 24 to 48 h. Pretreatment of the tachyzoites for 1 h with paclitaxel followed by the removal of the paclitaxel by repeatedly centrifuging and resuspending the parasites in fresh medium without paclitaxel and then adding fresh medium prior to culture of the parasites with fibroblasts did not prevent their invasion of fibroblasts but did affect their subsequent ability to replicate within fibroblasts. Pretreatment of the fibroblasts with paclitaxel also diminished subsequent replication of T. gondii in such host cells after 8 days. Thus, paclitaxel alters the ability of T. gondii to replicate in host cells. Inhibition of parasite microtubules by such compounds at concentrations which do not interfere with the function of host cell microtubules may be useful for development of novel medicines to treat T. gondii infections in the future.

Host cell surface sialic acid residues are involved on the process of penetration of Toxoplasma gondii into mammalian cells

Tachyzoites of Toxoplasma gondii are able to infect several cell types tested (wild-type chinese hamster ovary (CHO) cells and glycosylation mutants, Vero and LLCMK2 cells). However, the extent of infection varied. Mutant cells which present few or no surface-exposed sialic acid residues were infected to a lower extent. Similar results were obtained if sialic acid residues were removed by previous neuraminidase treatment. Addition of sialic acid residues to surface-exposed glycoconjugates using fetuin as a sialic acid donor and the trans-sialidase of Trypanosoma cruzi rendered the cells more easily infected by Toxoplasma gondii. These observations indicate that surface-exposed carbohydrate residues of the host cell are involved on the process of Toxoplasma gondii-host cell recognition.

Bradyzoite development in Toxoplasma gondii and the hsp70 stress response

Toxoplasma gondii is a well-described ubiquitous Apicomplexan protozoan parasite that is an important opportunistic pathogen. The factors affecting the transition of tachyzoites to the latent bradyzoite stage remain to be defined. The induction of bradyzoite development in vitro has been linked to temperature, pH, mitochondrial inhibitors, sodium arsenite, and many of the other stressors associated with heat shock protein (hsp) induction. There is evidence for other organisms that hsps are developmentally regulated. Therefore, we examined whether hsp induction is an early event in bradyzoite differentiation. Extracellular and intracellular T. gondii cells, after exposure to pH 8.1 or 7.1, were analyzed for the expression of inducible hsp70 by using monoclonal antibody C92F3A-5 (specific to hsp70). Western blotting demonstrated that a 72-kDa protein reactive with C92F3A-5 (hsp70), which we believe is part of the hsp70 family, is induced during bradyzoite development. By immunofluorescence and immunoelectron microscopy, we were able to demonstrate that hsp70 staining colocalized to T. gondii expressing bradyzoite-specific antigens and the presence of hsp70 in bradyzoites isolated from mouse brain. Quercetin, a bioflavonoid which inhibits the synthesis of hsp90, hsp70, and hsp27, suppresses the induction of bradyzoite development in vitro. Reverse transcription-PCR with conserved hsp70 primers demonstrated an increase in hsp70 in T. gondii on exposure to conditions which induce bradyzoite formation. A T. gondii hsp70 was subsequently cloned and sequenced by using this amplified fragment. We believe our evidence suggests that hsps are important in the process of bradyzoite differentiation.

Advances in the life cycle of Toxoplasma gondii

This paper reviews recent studies on the life cycle of Toxoplasma gondii. Tachyzoites, bradyzoites, and sporozoites are the three infectious stages of T. gondii. Humans and animals become infected mainly by ingesting bradyzoites or oocytes. After ingestion, both bradyzoites and sporozoites convert to tachyzoites inside tissues. The conversion of tachyzoites to bradyzoites and bradyzoites to tachyzoites is of biological and clinical significance because bradyzoites are less susceptible to chemotherapy and reactivation of bradyzoites to tachyzoites is considered the cause of fatal toxoplasmosis in AIDS patients. Of all the methods currently available to assess stage conversion of T. gondii, feeding infective stages to cats is the most reliable method. Felidae, the definitive hosts of T. gondii excrete oocysts 3-10 days after ingesting tissue cysts/bradyzoites, > or = 18 days after ingesting oocysts, and > or = 13 days after ingesting tachyzoites.

Ultrastructure of the oocysts, sporocysts, and sporozoites of Toxoplasma gondii

Transmission and scanning electron microscopy were used to study the ultrastructure of the oocysts, sporocysts, and sporozoites of the VEG strain of Toxoplasma gondii and to compare the ultrastructure of sporozoites with tachyzoites (from the peritoneum of mice) and bradyzoites (from brain tissue cysts in mice). Oocysts were surrounded by a thin veil of finely reticulate material. The oocyst wall consisted of 3 layers and contained a previously unknown disc-shaped micropyle that appeared as a depression in the oocyst wall. The sporocyst contained 4 sporozoites and a residuum of lipid and amylopectin granules. The sporocyst wall was 3-layered with the innermost layer consisting of 4 curved plates held together at sutures by an interposed strip. Exposure to excysting fluid caused the interposed strip to separate from the curved plates, which curled inward releasing the sporozoites. Sporozoites had a posteriorly located nucleus and all the organelles typical for coccidian zoites. Sporozoites, tachyzoites, and bradyzoites had similar numbers of rhoptries but differed in the numbers and sizes of micronemes, dense granules, amylopectin granules, and lipid bodies.

Structures of Toxoplasma gondii tachyzoites, bradyzoites, and sporozoites and biology and development of tissue cysts

Infections by the protozoan parasite Toxoplasma gondii are widely prevalent world-wide in animals and humans. This paper reviews the life cycle; the structure of tachyzoites, bradyzoites, oocysts, sporocysts, sporozoites and enteroepithelial stages of T. gondii; and the mode of penetration of T. gondii. The review provides a detailed account of the biology of tissue cysts and bradyzoites including in vivo and in vitro development, methods of separation from host tissue, tissue cyst rupture, and relapse. The mechanism of in vivo and in vitro stage conversion from sporozoites to tachyzoites to bradyzoites and from bradyzoites to tachyzoites to bradyzoites is also discussed.

Selection and characterization of Toxoplasma gondii mutants resistant to artemisinin

Toxoplasma gondii infection, like malaria, is sensitive to inhibition by artemisinin (ART). Mechanisms of action for ART in malaria treatment have been proposed, but little is known about its effects in T. gondii infection. To better understand its inhibitory effects on T. gondii, mutants resistant to ART were selected by progressive culture in permissive levels of the drug. Five clonal isolates were established and characterized. The isolates were approximately 65-fold less sensitive to ART than is the parental RH and showed cross-resistance to the ART derivatives dihydroartemisinin and artemether. In addition to ART resistance, 1 clone (C9) formed morphologically unusual parasitophorous vacuoles and another (A2) was avirulent for mice and protected mice from challenge with the wild type. These clonal T. gondii mutant isolates will be useful for the study of not only the mechanism of action of ART but also parasite vacuole biology and virulence factors.

GRA7, an excretory 29 kDa Toxoplasma gondii dense granule antigen released by infected host cells

Monoclonal antibody (mAb) TxE2, reactive with Toxoplasma gondii excretory products, detects an acidic 29 kDa protein (p29) which, in 2D gel electrophoresis, exhibits a migration pattern distinct from those of the toxoplasmic excretory proteins described so far. The sequence of seven peptides from tryptic digestion of isolated p29 allowed the design of primers to obtain the coding DNA sequence. The full-length gene was amplified from genomic DNA of T. gondii strain BK and the sequence was identical with that of the corresponding cDNA, providing evidence for an intron-free gene structure. A single mRNA transcript of 1.3 kb was detected by Northern blot analysis. The deduced 236 amino acid protein contains a putative N-terminal signal peptide, one site of potential N-linked glycosylation, and, close to the C-terminus, a further hydrophobic, putative transmembrane domain. With synthetic peptides spanning the sequence of p29, the epitope for mAb TxE2 was mapped adjacent to the putative signal sequence. The antigen, which represents almost 0.5% of T. gondii protein, is expressed in strains of all three intraspecies subgroups, and is associated with the parasite dense granules as demonstrated by immunoelectron microscopy. In tachyzoite-infected cells, p29 accumulates within the parasitophorous vacuole and co-localizes with its delimiting membrane. In bradyzoite-infected cells, p29 is present within the host cell cytoplasm as detected by immunofluorescence staining, and, furthermore, in the supernatant of cyst-bearing cell culture lacking extracellular parasites as shown by enzyme-linked immunosorbent assay (ELISA). Thus, p29 which is named dense granule protein (GRA)7 may indicate the presence of intracellular toxoplasma.

Gene knock-outs and allelic replacements in Toxoplasma gondii: HXGPRT as a selectable marker for hit-and-run mutagenesis

The hypoxanthine-xanthine-guanine phosphoribosyl transferase (HXGPRT) gene of the protozoan parasite Toxoplasma gondii encodes a safe, practical genetic marker suitable for both positive and negative selection. Taking advantage of the ability to control homologous versus nonhomologous recombination in haploid T. gondii tachyzoites by manipulating the length of homologous DNA sequence, we have explored the possibility of 'hit-and-run' mutagenesis to introduce gene knock-outs (or allelic replacements) at loci for which no known selection or screen is available. Using the uracil phosphoribosyl transferase (UPRT) locus as a target, a genomic clone containing approximately 8 kb encompassing the UPRT gene (but lacking essential coding sequence) was fused to a cDNA-derived HXGPRT 'minigene', which lacks sufficient contiguous genomic sequence for homologous recombination. After transfection of circular plasmid DNA, positive selection for HXGPRT activity identified stable transformants, > 30% of which were found to have integrated at the UPRT locus as 'pseudodiploids' (produced by single-site homologous recombination between the circular plasmid and genomic DNA). Upon removal of mycophenolic acid, resolution of pseudodiploids by spontaneous intrachromosomal homologous recombination was selected using 6-thioxanthine, yielding a 1:1 ratio of UPRT knock-out parasites to wild-type revertants, at frequencies of approximately 10(-6) per parasite doubling. Applications of 'hit-and-run' technology relative to other gene targeting strategies are discussed.

Growth inhibitory effect of bovine lactoferrin to Toxoplasma gondii tachyzoites in murine macrophages: tyrosine phosphorylation in murine macrophages induced by bovine lactoferrin

Previous studies have shown that lactoferrin induces growth inhibitory effects in mouse macrophages against intracellular Toxoplasma gondii, and these effects were not mediated by the oxygen-dependent and inorganic nitrogen-dependent pathway. To clarify the mechanism of anti-Toxoplasma gondii activity induced by lactoferrin, we examined whether lactoferrin promoted the phosphorylation of tyrosine residues in macrophage proteins. In immunoblotting assays using anti-[phosphorylated tyrosine] monoclonal antibody, phosphorylation of tyrosine residues was detected in protein(s) of approximately 30 kDa in macrophages incubated with lactoferrin. Inhibition of the lactoferrin-induced tyrosine-phosphorylation by genistein led to loss of the lactoferrin-induced growth inhibitory effect against the parasites. These findings suggest that lactoferrin induces tyrosine-phosphorylation in macrophages, and the tyrosine-phosphorylation seems to be associated with the induction of the growth inhibitory activity exerted against intracellular Toxoplasma gondii.

Vacuolar-type H+-ATPase regulates cytoplasmic pH in Toxoplasma gondii tachyzoites

Cytoplasmic pH (pHi) regulation was studied in Toxoplasma gondii tachyzoites by using the fluorescent dye 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein. Their mean baseline pHi (7.07+/-0.06; n=5) was not significantly affected in the absence of extracellular Na+, K+ or HCO3(-) but was significantly decreased in a dose-dependent manner by low concentrations of N, N'-dicyclohexylcarbodi-imide (DCCD), N-ethylmaleimide (NEM) or bafilomycin A1. Bafilomycin A1 also inhibited the recovery of tachyzoite pHi after an acid load with sodium propionate. Similar concentrations of DCCD, NEM and bafilomycin A1 produced depolarization of the plasma membrane potential as measured with bis-(1,3-diethylthiobarbituric)trimethineoxonol (bisoxonol), and DCCD prevented the hyperpolarization that accompanies acid extrusion after the addition of propionate, in agreement with the electrogenic nature of this pump. Confocal laser scanning microscopy indicated that, in addition to being located in cytoplasmic vacuoles, the vacuolar (V)-H+-ATPase of T. gondii tachyzoites is also located in the plasma membrane. Surface localization of the V-H+-ATPase was confirmed by experiments using biotinylation of cell surface proteins and immunoprecipitation with antibodies against V-H+-ATPases. Taken together, the results are consistent with the presence of a functional V-H+-ATPase in the plasma membrane of these intracellular parasites and with an important role of this enzyme in the regulation of pHi homoeostasis in these cells.

Toxoplasma gondii-infected cells are resistant to multiple inducers of apoptosis

Infection with certain intracellular pathogens, including viruses and bacteria, may induce host cell apoptosis. On the other hand, infection with some viruses inhibits apoptosis. Complex protozoan parasites, including Toxoplasma gondii and members of Plasmodium, Leishmania, and Microsporidia, are also obligate intracellular pathogens, yet relatively little is known regarding their subversion of host cell functions. We now report that cells infected with T. gondii are resistant to multiple inducers of apoptosis, including Fas-dependent and Fas-independent CTL-mediated cytotoxicity, IL-2 deprivation, gamma irradiation, UV irradiation, and the calcium ionophore beauvericin. Inhibition of such a broad array of apoptosis inducers suggests that a mechanism common to many, or perhaps all, apoptotic pathways is involved. The inhibitory activity requires live intracellular parasite and ongoing protein synthesis. Despite T. gondii-mediated inhibition of DNA fragmentation, infected cells can still be lysed by CTL.

Human platelet inhibition of Toxoplasma gondii growth

The human platelet contribution against the intracellular growth of the parasite in vitro in human pulmonary fibroblasts was explored. It was observed that tachyzoites of Toxoplasma gondii induced activation of human platelets and additionally that platelets mediated inhibition of intracellular growth in a virulent T. gondii strain. A prominent role for platelet-derived growth factor (PDGF) was demonstrated in this phenomenon, by testing human recombinant PDGF-AA, -AB and -BB and antibodies to human PDGF-AB that partially reversed its effects. Moreover, the effect of PDGF was significantly higher if the host cells were treated 2 h before parasite infection. PDGF was not directly 'toxic' to free tachyzoites, but only affected parasites within host cells. PDGF-mediated inhibition may involve the cyclooxygenase cycle of the fibroblasts being partially reversed by the cyclooxygenase inhibitors, acetylsalicylic acid and indomethacin. However, a thromboxane synthetase pathway was not implicated. PDGF action against intracellular tachyzoites may also include increased IL-6 production in fibroblasts. Finally, transforming growth factor-beta 1 (TGF-beta1), another component of alpha-granules released at the same time as PDGF, may not be antagonistic to the PDGF parasite inhibitory effect in confluent host cells.

[An immunochemical study of the antigens from Toxoplasma gondii tachyzoites obtained in different cultivation systems]

The protein composition and immunochemical properties of Toxoplasma gondii tachyzoites cultured on Vero cells and on mice were studied. Despite the fact that the main components of both preparations were shown to be proteins with molecular weights of 47, 34, 24, and 22 kDa, Toxoplasma-infected human sera antibodies interact mainly with the antigens of 66, 62, 57, 42, 38, 37, 36, 31, and 24 kDa. Comparing efficiency of enzyme immunoassay using the antigens of the tachyzoites obtained in different culture systems showed that the preparation of cultured Vero cells is similar to those of peritoneal exudates from infected mice and may be successfully used for the detection of antitoxoplasma antibodies in the sera of infected subjects.