[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.

Re-examination of resistance of Toxoplasma gondii tachyzoites and bradyzoites to pepsin and trypsin digestion

The effect of digestion in trypsin and acid pepsin on Toxoplasma gondii tachyzoites and bradyzoites was re-evaluated because of recent use of this method to distinguish tachyzoites from bradyzoites. Toxoplasma gondii tachyzoites survived better in 0.5% trypsin solution for 1 h than in 1.0% solution, and occasionally survived for 2 h in acid pepsin solution. Extracellular tachyzoites (> or = 1000) were also infectious orally to mice and cats. Bradyzoites survived equally in trypsin and acid pepsin solutions but the digestion of brain tissue in 0.5% trypsin solution was better than in acid pepsin solution. The resistance to digestion in acid pepsin solution is not a reliable method to distinguish tachyzoites from bradyzoites.

The surface of Toxoplasma: more and less

As for any intracellular parasite, the surface of the Apicomplexan parasite Toxoplasma gondii must fulfil many functions including a role in attachment, signalling, invasion, transport and interaction with the immune response of the host. In this review, we describe the current state of knowledge on the molecules that are found on the surface of the different developmental stages of this parasite and speculate as to how at least some of these multiple functions are fulfilled. Special emphasis is given to the growing family of surface antigens that are related to the tachyzoite-specific surface antigen 1. We conclude that the surface (of tachyzoites, at least) is both more and less complex than previously thought: there are more proteins present but their sequences suggest that the majority may share a similar overall structure typified by surface antigen 1.

Ultrastructure of early stages of infections in mice fed Toxoplasma gondii oocysts

Transmission electron microscopy was used to study Toxoplasma gondii infections in the small intestines of Swiss-Webster mice at 2-48 h post-feeding of oocysts (p.f.). Sporozoites passed through intestinal epithelial cells (enterocytes and goblet cells) and infected all cells except red blood cells in the lamina propria. Parasites in intestinal epithelial cells or in cells in the lamina propria were located within a single type of parasitophorous vacuole, which contained exocytosed electron-dense material and well-developed tubulovesicular membranous networks. Sporozoites did not infect intraepithelial lymphocytes (IELs), but at 48 h p.f. IELs had become infected with tachyzoites arising from those that had developed in the lamina propria. At 48 h p.f., the lamina propria contained numerous tachyzoites, much cellular debris, and few intact cells. The intestinal epithelium exhibited limited cytopathological changes except for villar fusion, slight vacuolation, and cell separation at the bases of enterocytes.

Molecular analysis and characterization of a protein involved in the replication of intracellular Toxoplasma gondii

Previous studies in our laboratory have identified a cytoplasmic protein (p97) of T. gondii that is involved in the process of intracellular parasite replication. Monoclonal antibody inhibits parasite replication in vitro and recognizes a protein of approximate 97 kDa by Western blot analysis. Using biotinylation, we demonstrate that p97 is not expressed on the surface of the tachyzoite. Polyclonal sera raised against the purified native protein was used to isolate a cDNA of 3.3 kb from a library. The product of this gene expresses a protein of approximate Mr 97 kDa that is reactive to the antibody (1B8) raised against the native antigen. The protein sequence of this product suggests that it is within the cytoplasm as suggested by the lack of a signal sequence or hydrophobic trans-membrane domain. This protein fails to dissociate into a monomer in the presence of non-ionic detergents as shown by gel filtration and density gradient. Southern blot analysis demonstrates a homologous gene sequence in two closely related Apicomplexa, Neospora caninum and Besnoitia jellisoni suggesting this protein is conserved among certain species of the Sarcocystidae.

Kinetics in parasite abundance in susceptible and resistant mice infected with an avirulent strain of Toxoplasma gondii by using quantitative competitive PCR

The kinetics of changes in Toxoplasma gondii abundance were evaluated with a quantitative competitive (QC)-polymerase chain reaction (PCR) assay at various sites in both C57BL/6 and BALB/c mice. Higher mortality was apparent in C57BL/6 mice than in BALB/c mice when infected with a high dose of cysts. There were significant differences in cyst number when infected with a low dose of cysts, although there was no significant difference in mortality between the 2 mouse strains. One day after infection with a low dose of an avirulent Fukaya strain, T. gondii was detected in peripheral blood, mesenteric lymph nodes, spleen, lungs, and brain. Two weeks after infection, the number of T. gondii in the brain greatly increased in C57BL/6 mice but not in BALB/c mice. Thus, it would appear that the first to second week after infection is a critical period in determining T. gondii abundance. QC PCR allows the detection of low numbers of T. gondii at an early stage of infection in the murine model. This is useful for the early diagnosis of toxoplasmosis and to understand reactivation of toxoplasmosis.

Interferon consensus sequence binding protein-deficient mice display impaired resistance to intracellular infection due to a primary defect in interleukin 12 p40 induction

Mice lacking the transcription factor interferon consensus sequence binding protein (ICSBP), a member of the interferon regulatory factor family of transcription proteins, were infected with the intracellular protozoan, Toxoplasma gondii. ICSBP-deficient mice exhibited unchecked parasite replication in vivo and rapidly succumbed within 14 d after inoculation with an avirulent Toxoplasma strain. In contrast, few intracellular parasites were observed in wild-type littermates and these animals survived for at least 60 d after infection. Analysis of cytokine synthesis in vitro and in vivo revealed a major deficiency in the expression of both interferon (IFN)-gamma and interleukin (IL)-12 p40 in the T. gondii exposed ICSBP-/- animals. In related experiments, macrophages from uninfected ICSBP-/- mice were shown to display a selective impairment in the mRNA expression of IL-12 p40 but not IL-1alpha, IL-1beta, IL-1Ra, IL-6, IL-10, or TNF-alpha in response to live parasites, parasite antigen, lipopolysaccharide, or Staphylococcus aureus. This selective defect in IL-12 p40 production was observed regardless of whether the macrophages had been primed with IFN-gamma. We hypothesize that the impaired synthesis of IL-12 p40 in ICSBP-/- animals is the primary lesion responsible for the loss in resistance to T. gondii because IFN-gamma-induced parasite killing was unimpaired in vitro and, more importantly, administration of exogenous IL-12 in vivo significantly prolonged survival of the infected mice. Together these findings implicate ICSBP as a major transcription factor which directly or indirectly regulates IL-12 p40 gene activation and, as a consequence, IFN-gamma-dependent host resistance.

Bradyzoite-induced murine toxoplasmosis: stage conversion, pathogenesis, and tissue cyst formation in mice fed bradyzoites of different strains of Toxoplasma gondii

The development of Toxoplasma gondii was studied in mice fed bradyzoites. At one hour after oral inoculation (HAI), bradyzoites were found in cells of the surface epithelium and the lamina propria of the small intestine, primarily the ileum. Division into two tachyzoites was first observed at 18 HAI in the intestine. At 24 HAI, organisms were also seen in mesenteric lymph nodes. Organisms were first detected in the brain at six days after oral inoculation with bradyzoites (DAI) but not consistently until 10 DAI. Immunohistochemical staining with bradyzoite specific (BAG-5 antigen) anti-serum showed that bradyzoites retained their BAG-5 reactivity even after the first division into two tachyzoites in the intestine at 18 HAI. BAG-5 positive organisms were not seen 2-5 DAI. BAG-5 antigens reappeared in T. gondii at 6 DAI. Whole mice and individual tissues of mice fed bradyzoites were bioassayed in cats and mice for the presence of bradyzoites. Feces of cats fed murine tissues were examined for oocyst shedding for short prepatent periods. Bradyzoites were present in the intestines of mice up to 12 HAI but not at 18 HAI, and tachyzoites and not bradyzoites disseminated to other tissues from the intestine. Bradyzoites were again detected 6 DAI. Using the mouse bioassay, T. gondii was first detected in peripheral blood at 24 HAI and more consistently at 48 HAI. Using a pepsin-digestion procedure and mouse bioassay, organisms were demonstrated in many tissues of mice 15 and 49 DAI.

Safe haven: the cell biology of nonfusogenic pathogen vacuoles

Our understanding of both membrane traffic in mammalian cells and the cell biology of infection with intracellular pathogens has increased dramatically in recent years. In this review, we discuss the cell biology of the host-microbe interaction for four intracellular pathogens: Chlamydia spp., Legionella pneumophila, Mycobacterium spp., and the protozoan parasite Toxoplasma gondii. All of these organisms reside in vacuoles inside cells that have restricted fusion with host organelles of the endocytic cascade. Despite this restricted fusion, the vacuoles surrounding each pathogen display novel interactions with other host cell organelles. In addition to the effect of infection on host membrane traffic, we focus on these novel interactions and relate them where possible to nutrient acquisition by the intracellular organisms.

Oocyst-induced murine toxoplasmosis: life cycle, pathogenicity, and stage conversion in mice fed Toxoplasma gondii oocysts

The development of sporozoites to tachyzoites and bradyzoites was studied in mice after feeding 1-7.5 x 10(7) Toxoplasma gondii oocysts. Within 2 hr after inoculation (HAI), sporozoites had excysted and penetrated the small intestinal epithelium. At 2 HAI, most sporozoites were in surface epithelial cells and in the lamina propria of the ileum, and by 8 HAI, T. gondii was also seen in mesenteric lymph nodes. At 12 HAI, sporozoites had divided into 2 tachyzoites in the lamina propria of the small intestine. By 48 HAI, there was a profuse growth of tachyzoites in the intestine and mesenteric lymph nodes of mice fed 7.5 x 10(7) oocysts. Parasites had disseminated via the blood and lymph to other organs by 4 days after inoculation (DAI). Toxoplasma gondii was first isolated from peripheral blood at 4 HAI. Tissue cysts were visible histologically in the brain at 8 DAI. By using immunohistochemical staining with anti-bradyzoite-specific (BAG-5 antigen) serum, BAG-5-positive organisms were first seen at 5 DAI in the intestine and at 8 DAI in the brain. Using the bioassay in cats, bradyzoites were first detected in mouse tissues between 6 and 7 DAI, and they were found in intestines before they were found in the brain. Cats fed murine tissues containing bradyzoites shed oocysts in their feces with a short (< 10 days) prepatent period, whereas cats fed tissues containing tachyzoites did not shed oocysts within 3 wk. Using a pepsin-digestion procedure and mouse bioassay, bradyzoites were first detected in brain tissue at 7 DAI and in many organs of mice at 51 and 151 DAI. Individual bradyzoites, small and large tissue cysts, and tachyzoites were seen in the brains of mice at 87 and 236 DAI.

Mechanisms of innate resistance to Toxoplasma gondii infection

The interaction of protozoan parasites with innate host defences is critical in determining the character of the subsequent infection. The initial steps in the encounter of Toxoplasma gondii with the vertebrate immune system provide a striking example of this important aspect of the host-parasite relationship. In immuno-competent individuals this intracellular protozoan produces an asymptomatic chronic infection as part of its strategy for transmission. Nevertheless, T. gondii is inherently a highly virulent pathogen. The rapid induction by the parasite of a potent cell-mediated immune response that both limits its growth and drives conversion to a dormant cyst stage explains this apparent paradox. Studies with gene-deficient mice have demonstrated the interleukin-12 (IL-12)-dependent production of interferon gamma (IFN-gamma) to be of paramount importance in controlling early parasite growth. However, this seems to be independent of nitric oxide production as mice deficient in inducible nitric oxide synthase (iNOS) and tumour necrosis factor receptor were able to control early growth of T. gondii, although, they later succumbed to infection. Nitric oxide does, however, seem to be important in controlling persistent infection; treating chronic infection with iNOS metabolic inhibitors results in disease reactivation. Preliminary evidence implicates neutrophils in effector pathways against this parasite distinct from that described for macrophages. Once initiated, IL-12-dependent IFN-gamma production in synergy with other proinflammatory cytokines can positively feed back on itself to induce 'cytokine shock'. Regulatory cytokines, particularly IL-10, are essential to down-regulate inflammation and limit host pathology.

Genetic and biochemical analysis of development in Toxoplasma gondii

Toxoplasma gondii has recently come under intense study as a model for intracellular parasitism because it has a number of properties that facilitate experimental manipulation. Attention is now being turned towards understanding the developmental biology of this complex parasite. The differentiation between the two asexual stages, the rapidly growing tachyzoites and the more slowly dividing, encysted bradyzoites, is of particular interest. Progression from the former to the latter is influenced by the host's immune response. This paper describes current progress on a number of research fronts, all aimed at understanding the triggers that push the tachyzoite-bradyzoite equilibrium in one or other direction and the changes that occur in gene expression (and ultimately metabolism and function). Chief among the techniques used for these studies are genetics and molecular genetics. Recent progress in these areas is described.

Conformationally restricted analogues of trimethoprim: 2,6-diamino-8-substituted purines as potential dihydrofolate reductase inhibitors from Pneumocystis carinii and Toxoplasma gondii

Twenty-two 2,6-diamino-8-substituted purines (2-23) were synthesized, in which rotation around the two flexible bonds of trimethoprim (TMP), linking the pyrimidine ring to the side chain phenyl ring, was restricted by incorporation into a purine ring, in an attempt to increase the potency and selectivity of TMP against dihydrofolate reductase (DHFR) from the organisms that often cause fatal opportunistic infections in patients with AIDS, i.e., Pneumocystis carinii (pc) and Toxoplasma gondii (tg). The syntheses of analogues 2-20 were achieved via a one-pot reaction of 2,4,5,6-tetraaminopyrimidine and the appropriately substituted benzaldehyde or phenyl acetaldehyde, in acidic methoxyethanol. Analogues 21-23 were synthesized via nucleophilic displacement of 2,6-diamino-8-(chloromethyl)purine with the appropriate anilines or 2-naphthalenethiol. The compounds were evaluated as inhibitors of pcDHFR and tgDHFR with rat liver (rl) DHFR as the mammalian reference enzyme. Compound 11, the 3',4'-dichlorophenyl analogue, was as potent as TMP and had a selectivity ratio of 13 for pcDHFR, which ranked it as one of the three most selective inhibitors of pcDHFR (compared to rlDHFR) known to date. It also displayed a selectivity ratio of 38 for tgDHFR. None of the other analogues showed any improvement compared to TMP in potency or selectivity. In the preclinical in vitro screening program of the National Cancer Institute, compound 11 showed a GI50 of 10(-6) M for the inhibition of the growth of 17 tumor cell lines.

Characterization of anti-Toxoplasma activity of SDZ 215-918, a cyclosporin derivative lacking immunosuppressive and peptidyl-prolyl-isomerase-inhibiting activity: possible role of a P glycoprotein in Toxoplasma physiology

The immunosuppressive agent cyclosporin A (CsA) also possesses broad-spectrum antimicrobial activity. Previous investigators have reported that the obligate intracellular protozoan Toxoplasma gondii is sensitive to CsA. We have measured the sensitivity of Toxoplasma to 26 CsA derivatives that maintain only a subset of the parent compound's activity. We identified one compound, SDZ 215-918, that is a particularly potent inhibitor of parasite invasion and replication, with a 50% inhibitory concentration of 0.45 microg/ml, which is 10-fold lower than that of CsA. Kinetic studies demonstrate that activity has a rapid onset (half-life, < or = 20 min) and is initially reversible, although long-term exposure (> 24 h) to 5 microg/ml is lethal; in contrast, this concentration had no effect on host cell protein synthesis or cell division. SDZ 215-918 acts directly on the parasite, as demonstrated by inhibition of macromolecular synthesis in host-free extracellular parasites. Inhibition of invasion is due to a reduction in parasite motility. SDZ 215-918 does not bind to cyclophilins, the ubiquitous cyclosporin-binding proteins, but is a potent inhibitor of the mammalian P glycoprotein, a member of the ATP binding cassette transporter superfamily and the pump responsible for multidrug resistance in cancer and parasite cell lines. SDZ 215-918 blocks the efflux of rhodamine 123 from extracellular parasites, consistent with inhibition of a P glycoprotein-like pump. We suggest that a P glycoprotein or a related transporter plays a crucial role in the biology of Toxoplasma and may be a novel target for antiparasitic compounds. Preliminary studies with animals indicate that SDZ 215-918 inhibits parasite growth in vivo; its relationship to CsA may make it suitable for clinical development.

Stage-specific expression of a selectable marker in Toxoplasma gondii permits selective inhibition of either tachyzoites or bradyzoites

The establishment of culture conditions suitable for inducing differentiation of Toxoplasma gondii tachyzoites into parasites resembling the latent bradyzoite form has opened this important developmental transition to experimental analysis. In order to develop a genetic marker suitable for positive and negative selection during parasite differentiation. the T. gondii HXGPRT gene was placed under control of 5' flanking sequences derived from two bradyzoite-specific genes: BAG1 and LDH2. Random transgene integration at undefined genomic loci resulted in modest regulation (approximately 5-6-fold induction) above relatively high background levels (approximately 4% of wild-type controls). Integration of transgenes at a defined genomic position was achieved by targeting the uracil phosphoribosyl transferase (UPRT) locus using flanking homologous sequences and fluorouracil selection. This strategy was found to provide the added advantage of enhancing bradyzoite induction frequencies under conditions of pyrimidine starvation (low CO2). Constructs integrated in the direction of normal UPRT transcription exhibited moderate levels of inducibility, but transgenes integrated in the opposite direction were dramatically induced under differentiation conditions: 50-100-fold above the very low levels observed in tachyzoites (< 1% control). Positive selection (using mycophenolic acid) was shown to inhibit tachyzoites but not bradyzoites, while negative selection (using 8-azahypoxanthine) inhibited bradyzoites only. Stage-specific regulation of the HXGPRT selectable marker should permit genetic selections for the identification of mutants in the bradyzoite differentiation process.

Toxoplasmosis, behaviour and personality

The clinical sequelae of acute and congenital toxoplasmosis are well established, but that of chronic toxoplasma infection remains uncertain. In rodents, chronic toxoplasma infection is associated with altered behaviour leading to an enhanced risk of feline predation and a putative selective advantage to the parasite. It is proposed that neurotropic cysts of toxoplasma exert an effect on animal behaviour, either directly or via the release of metabolic products. Long-standing toxoplasma infection in humans has been linked to cerebral tumour formation and personality shift. In view of the vast population with chronic toxoplasma infection, further studies of the clinical sequelae of this condition are required.

Time lapse video microscopy and ultrastructure of penetrating sporozoites, types 1 and 2 parasitophorous vacuoles, and the transformation of sporozoites to tachyzoites of the VEG strain of Toxoplasma gondii

Videomicroscopy and transmission electron microscopy were used to study the interaction of Toxoplasma gondii sporozoites with cultured cardiopulmonary artery endothelial, embryonic bovine tracheal and Madin-Darby bovine kidney cells. No moving junction or exocytosis of rhoptries, micronemes, and dense granules was detected during the initial penetration of sporozoites into cultured cells, whereas constriction of the sporozoite and partial exocytosis of rhoptries occurred during movement of the sporozoite from the first parasitophorous vacuole (PV1) into the second vacuole (PV2). The PV1 was unusually large, lacked a tubulovesicular membrane network (TMN), and had an indistinct parasitophorous vacuolar membrane (PVM). Comparatively, the PV2 was small, had a distinct PVM, contained a well-developed TMN, and was surrounded by numerous host cell mitochondria. Sporozoites that passed completely through cells carried with them an envelope of host cell membranes and cytoplasm. Cultured cells occasionally endocytosed sporozoites that were enveloped by host cell material. After formation of the PV2, sporozoites replicated by endodyogeny to form tachyzoites.

Intensive (pasture) beef cattle operations: the perspective of New Zealand

Beef production in New Zealand has characteristics typical of a temperate climate and pasture-based animal husbandry. The specific pathogens which may contaminate fresh beef and which are empirically considered to be of public health importance are similar to those in other countries with temperate climates, i.e. Salmonella, Campylobacter, Escherichia coli O157:H7, Listeria monocytogenes and Toxoplasma gondii. With the exception of T. gondii, it is likely that almost all transmission of these hazards through consumption of beef results from unseen microbial cross-contamination from gastrointestinal sources during slaughter, dressing and further processing. Gaining comprehensive information on carcass contamination levels is an essential first step in establishing food safety objectives for a particular beef production system, and in designing risk-based hazard analysis and critical control point (HACCP) plans. It is likely that the lower mean and maximum numbers of indicator micro-organisms on New Zealand carcasses (when compared with other countries) are in part due to the pre-slaughter cleanliness status of cattle reared under temperate, pasture conditions. Similarly, the failure to detect specific pathogens of gastrointestinal origin in a comprehensive baseline survey most probably reflects the limited pathway for faecal contamination during slaughter and dressing under processing conditions in New Zealand. The New Zealand example provides strong evidence for the need to design HACCP plans according to the specific national (or regional) situation. Reducing all pathways for faecal contamination of products to the maximum extent practicable will be the most important factor in achieving desired food safety objectives for fresh beef. Variable densities of microbial pathogens in gastrointestinal contents are also likely to have a significant effect on subsequent contamination levels of beef carcasses: however, effective controls for limiting the presence of most pathogens of concern in the live animal have yet to be identified.

Tissue cyst tropism in Toxoplasma gondii: a comparison of tissue cyst formation in organs of cats, and rodents fed oocysts

The persistence of Toxoplasma gondii tissue cysts in organs of cats (definitive host) and rodents (intermediate hosts) was studied. Nine cats, 12 rats, and 12 mice were fed T. gondii oocysts and their organs were digested in pepsin and then bioassayed for bradyzoites in mice. Of 9 cats killed 37 or 51 days after feeding 10(2) (2 cats), 10(3) (3 cats) or 10(4) (4 cats) oocysts of the VEG strain, tissue cysts were found in each cat; in the tongue of 9, in the heart of 5, in the brain of 4, and in the eyes of 1 cat. The dose had no effect on the distribution of tissue cysts in cats. Twelve rats were each fed 10(5) oocysts of the VEG strain of T. gondii and killed 21, 29, 64 or 237 days later. At each time-period, 11 tissues of 3 rats were pooled and bioassayed in mice. Tissue cysts were found in the brain, skeletal muscle, heart and kidneys of rats at each killing time; in the lungs, intestines, and mesenteric lymph nodes in 3 of 4 instances; in the tongue, liver, and eyes in 2 instances and in the spleen in 1 instance. Also, using the same procedures and sampling the same 11 tissues as used for rats, tissue cysts were seen in all organs except in the tongue and liver of 3 mice killed on day 82 after feeding the VEG strain. In 9 mice (3 with each strain) fed oocysts of the ME-49, GT-1, or P89 T. gondii strain and killed 62-130 days later, tissue cysts were found consistently only in the brain. Thus, in rats and mice, most tissue cysts were found in the brain and rarely in the tongue. This was in marked contrast to the distribution of tissue cysts in cats.

Recognition of tissue cyst-specific antigens in reactivating toxoplasmosis

Current serological tests do not discriminate between asymptomatic latent Toxoplasma gondii infection and reactivating toxoplasmosis, but timely therapeutic intervention before the development of symptoms would lead to major reductions in morbidity and permanent disability. This study developed a new enzyme-linked immunosorbent assay (ELISA) for antibody to T. gondii tissue cyst antigens and screened tissue cyst antigens by Western blot analysis to test the hypothesis that antibody recognition of T. gondii tissue cyst-derived antigen is a good indicator of reactivation disease. A total of 187 sera was tested by Sabin-Feldman dye test and tissue cyst ELISA, AIDS patients and patients with ocular disease were considered separately, as the exposure to parasite antigens may be different in these two groups. The dye test did not discriminate between immunocompetent and immunocompromised T. gondii seropositive patients or between active and quiescent toxoplasmosis. Tissue cyst ELISA demonstrated a raised specific antibody response in immunocompetent T. gondii seropositive patients and in quiescent HIV positive sera. These data support th view that the tissue cyst population is in a state of dynamic equilibrium. It is proposed that, in the immunocompetent host, tissue cyst development and rupture are under some degree of immune control, but that in the immunocompromised host this equilibrium is disturbed and reactivation disease results. Data from patients with reactivating ocular toxoplasmosis demonstrate that tissue cyst-specific antibody levels are not different in active and quiescent disease and indeed they are not significantly different from immunocompetent T. gondii seronegative sera. In the Western blot analysis of 57 HIV positive patient sera, eight antigens (65, 57, 49, 47, 36, 28, 26 and 18 kDa) were consistently recognised by one third or more of the sera tested, but no single antigen was diagnostic of quiescent or active toxoplasmosis. It is concluded that tissue cyst-derived antigens are not a reliable serological marker of reactivating toxoplasmosis.

Experimental infection of murine splenic lymphocytes and granulocytes with Toxoplasma gondii RH tachyzoites

Toxoplasma gondii, an intracellular protozoan infecting many kinds of eukaryotic cells, has been used to experimentally infect macrophages, epithelial cells, fibroblasts, and various cancer cells, but rarely T and B lymphocytes or granulocytes. The present study was performed to determine the susceptibility of murine (BALB/c or CBA) splenic T and B lymphocytes, and granulocytes to infection with T. gondii RH tachyzoites. The ultrastructure of the infected host cells was observed by TEM, and the degree of intracellular parasite proliferation was quantified using 3H-uracil uptake assay. At 24 hrs post-culture, the host cell cytoplasm was found to contain 1 or 2, or a maximum of 7-8 tachyzoites. Infected T lymphocytes demonstrated a peripherally displaced nucleus, a parasitophorous vacuole enveloping the parasite, and an increased number of mitochondria. In B lymphocytes infected with tachyzoites, RER was not well developed compared to uninfected B lymphocytes. Uninfected granulocytes contained many electron-dense granules, but T. gondii-infected granulocytes demonstrated a decreased number of granules. Based on the 3H-uracil uptake assay, the susceptibility of T and B lymphocytes, and granulocytes, to infection with T. gondii tachyzoites was fairly high irrespective of cell type and strain of mouse. This strongly suggests deterioration in the functioning of infected host immune cells.

Cytokine responses induced by Toxoplasma gondii in astrocytes and microglial cells

To investigate the role of astroglia in intracerebral immune response to Toxoplasma gondii, astrocytes cultured from mouse brain were inoculated with mouse-virulent or -avirulent toxoplasma strains. In comparison to microglia/ brain macrophages, astrocytes as host cells allowed stronger proliferation of avirulent parasites. Toxoplasma infection of astroglia was accompanied by release of interleukin- (IL)1 alpha, IL-6, and granulocyte/macrophage colony-stimulating factor (GM-CSF) activity, whereas alternative challenge by lipopolysaccharide (LPS) evoked no IL-1 response and significantly higher titers of IL-6 and GM-CSF. At the mRNA level, both stimuli induced transcription of all three cytokines in astrocytes. Secretion of IL-1 and IL-6 upon infection was triggered by T. gondii brady- and tachyzoites in a time- and dose-dependent manner. Heat killing of parasites, but not an exposure to polymyxin B, abrogated their cytokine-inducing activity, thus indicating that an LPS-independent stimulus is provided by T. gondii. When administered in combination, LPS synergistically augmented the IL-1-inducing effect of toxoplasma infection. In comparison, T. gondii-induced, but not an LPS-triggered, IL-6 response of astrocytes resisted to antagonization with IL-10. The IL-6 response of parasitized astroglia was up-regulated by external tumor necrosis factor (TNF)-alpha and transforming growth factor (TGF)-beta 1, with only TNF-alpha enhancing simultaneous release of IL-1. Substantial secretion of IL-10 and TNF-alpha was detected in T. gondii-infected microglia, but not in astrocyte cultures. A possibly autocrine stimulation of infected astroglia via IL-1 was found to be unlikely, since addition of IL-1 receptor antagonist did not affect the release of IL-6 and GM-CSF while inhibiting these responses in IL-1-treated cells. The findings substantiate a separate, T. gondii-induced pathway of astroglia activation characterized by the release of IL-1 which may drive local inflammatory reaction both at initial infection of the brain and during reactivating toxoplasmosis.

Toxoplasma gondii: the growth characteristics of three virulent strains

We have studied the phenotype of three mouse virulent strains of Toxoplasma gondii (RH, Martin and ENT), monitoring cellular factors which may relate to virulence. There was variation between these three strains in three separate criteria: invasion, growth and tachyzoite-bradyzoite interconversion. The ENT strain exhibited consistently higher invasion rates, a shorter doubling time and a lower frequency of bradyzoite production than Martin or RH strains. In addition to variation in growth rate, there were also differences in the morphology of the parasites, with the ENT strain exhibiting highly synchronous division giving rise to characteristic rosettes. The Martin strain produced bradyzoites at a higher frequency and, in culture, parasites were often seen in tight clusters, which were reminiscent of early tissue cysts. These phenotypic variations amongst mouse-virulent strains of the parasite may imply underlying genetic differences within the group.

Inducible nitric oxide is essential for host control of persistent but not acute infection with the intracellular pathogen Toxoplasma gondii

The induction by IFN-gamma of reactive nitrogen intermediates has been postulated as a major mechanism of host resistance to intracellular pathogens. To formally test this hypothesis in vivo, the course of Toxoplasma gondii infection was assessed in nitric oxide synthase (iNOS)-/- mice. As expected, macrophages from these animals displayed defective microbicidal activity against the parasite in vitro. Nevertheless, in contrast to IFN-gamma-/- or IL-12 p40-/- animals, iNOS-deficient mice survived acute infection and controlled parasite growth at the site of inoculation. This early resistance was ablated by neutralization of IFN-gamma or IL-12 in vivo and markedly diminished by depletion of neutrophils, demonstrating the existence of previously unappreciated NO independent mechanisms operating against the parasite during early infection. By 3-4 wk post infection, however, iNOS knockout mice did succumb to T. gondii. At that stage parasite expansion and pathology were evident in the central nervous system but not the periphery suggesting that the protective role of nitric oxide against this intracellular infection is tissue specific rather than systemic.

Bradyzoite-specific gene expression in Toxoplasma gondii requires minimal genomic elements

BAG1 is a small heat-shock protein of Toxoplasma gondii that is specifically expressed in the cyst-forming bradyzoite stage of the parasite. Upregulation of BAG1 mRNA occurs early during the differentiation pathway from tachyzoites to bradyzoites. In order define genomic elements involved in bradyzoite-specific gene regulation, chloramphenicol acetyltransferase (CAT)-reporter gene studies were performed with 5' flanking sequences of the BAG1 gene. Tachyzoites, transiently transfected with the BAG1/cat construct, exhibited very low CAT activity (200 fold less than in parasites transfected with a tubulin promoter/cat construct). After induction of bradyzoite differentiation by alkaline pH shift, however, CAT activity increased 50 fold, demonstrating bradyzoite-specific expression of the CAT reporter gene under control of 5' flanking sequences of BAG1. Stage-specific regulation of BAG1/CAT was independent of the 3'-flanking region, since constructs containing 3'-flanking sequences of the tachyzoite-specific SAG1 gene showed identical regulation to those containing the BAG1 3'-flanking region. The kinetics of BAG1/CAT induction in stably transfected parasites is similar to the kinetics of endogenous BAG1 expression: increased CAT activity was first detected on day 3 after alkaline pH shift (20 fold) and was dramatically upregulated 250 fold on day 4. A series of deletions in the BAG1 5'-flanking sequences demonstrated that a 324 nucleotide (nt) fragment, starting 60 nt upstream of the BAG1 transcription start, is sufficient to confer stage-specific regulation on the CAT reporter. These deletion analyses demonstrate that bradyzoite-specific expression of a heterologeous reporter gene requires only minimal genomic sequences.