Toxoplasma gondii in primary rat CNS cells: differential contribution of neurons, astrocytes, and microglial cells for the intracerebral development and stage differentiation

Mechanisms of CNS invasion and damage by parasites

Invasion of the central nervous system (CNS) is a most devastating complication of a parasitic infection. Several physical and immunological barriers provide obstacles to such an invasion. In this broad overview focus is given to the physical barriers to neuroinvasion of parasites provided at the portal of entry of the parasites, i.e., the skin and epithelial cells of the gastrointestinal tract, and between the blood and the brain parenchyma, i.e., the blood-brain barrier (BBB). A description is given on how human pathogenic parasites can reach the CNS via the bloodstream either as free-living or extracellular parasites, by embolization of eggs, or within red or white blood cells when adapted to intracellular life. Molecular mechanisms are discussed by which parasites can interact with or pass across the BBB. The possible targeting of the circumventricular organs by parasites, as well as the parasites' direct entry to the brain from the nasal cavity through the olfactory nerve pathway, is also highlighted. Finally, examples are given which illustrate different mechanisms by which parasites can cause dysfunction or damage in the CNS related to toxic effects of parasite-derived molecules or to immune responses to the infection.

Interferon-γ restricts Toxoplasma gondii development in murine skeletal muscle cells via nitric oxide production and immunity-related GTPases

The apicomplexan parasite Toxoplasma gondii is regularly transmitted to humans via the ingestion of contaminated meat products from chronically infected livestock. This route of transmission requires intracellular development and long-term survival of the parasite within muscle tissue. In this study, we determined the cell-autonomous immunity of mature primary embryonic or C2C12 skeletal muscle cells (SkMCs) to infection with T. gondii. Non-activated SkMCs and control fibroblasts sustained parasite replication; however, interferon (IFN)-γ significantly inhibited parasite growth in SkMCs but not in fibroblasts. Intracellular parasite replication was diminished by IFN-γ whereas host cell invasion was not affected. Tumor necrosis factor (TNF) did not further increase the IFN-γ-triggered host defense of SkMCs against Toxoplasma. Remarkably, IFN-γ alone or in combination with TNF decreased the high level of T. gondii bradyzoite formation being observed in non-activated SkMCs. Stimulation of SkMCs with IFN-γ strongly triggered expression of inducible nitric oxide synthase (iNOS) transcripts, and induced significantly higher levels of nitric oxide (NO) in SkMCs than in fibroblasts. Consequently, pharmacological inhibition of iNOS partially abrogated the IFN-γ-induced toxoplasmacidal activity of SkMCs. In addition, SkMCs strongly up-regulated immunity-regulated GTPases (IRGs) following stimulation with IFN-γ. IRGs accumulated on Toxoplasma-containing vacuoles in SkMCs in a parasite strain-dependent manner. Subsequent vacuole disruption and signs of degenerating parasites were regularly recognized in IFN-γ-treated SkMCs infected with type II parasites. Together, murine SkMCs exert potent toxoplasmacidal activity after stimulation with IFN-γ and have to be considered active participants in the local immune response against Toxoplasma in skeletal muscle.

Host metabolism regulates growth and differentiation of Toxoplasma gondii

A critical step in the pathogenesis of Toxoplasma gondii is conversion from the fast-replicating tachyzoite form experienced during acute infection to the slow-replicating bradyzoite form that establishes long-lived tissue cysts during chronic infection. Bradyzoite cyst development exhibits a clear tissue tropism in vivo, yet conditions of the host cell environment that influence this tropism remain unclear. Using an in vitro assay of bradyzoite conversion, we have found that cell types differ dramatically in the ability to facilitate differentiation of tachyzoites into bradyzoites. Characterization of cell types that were either resistant or permissive for conversion revealed that resistant cell lines release low molecular weight metabolites that could support tachyzoite growth under metabolic stress conditions and thereby inhibit bradyzoite formation in permissive cells. Biochemical analysis revealed that the glycolytic metabolite lactate is an inhibitory component of supernatants from resistant cells. Furthermore, upregulation of glycolysis in permissive cells through the addition of glucose or by overexpression of the host kinase, Akt, was sufficient to convert cells from a permissive to a resistant phenotype. These results suggest that the metabolic state of the host cell may play a role in determining the predilection of the parasite to switch from the tachyzoite to bradyzoite form.

Toxoplasma gondii invasion and replication in astrocyte primary cultures and astrocytoma cell lines: systematic review of the literature

Toxoplasma gondii is a cosmopolitan protozoan which infects all homoeothermic species, including humans. This parasite may cause severe neurological problems in congenitally infected newborns or immunocompromised individuals, but it also provokes psychiatric and neurological disorders as well as behavioural and sensory deficit. There is controversy regarding the effect of T. gondii upon astrocytes, which may serve as parasite proliferation recipients or protective immune response activators within the central nervous system. This apparent contradiction could partially be due to the infection degree obtained in the different experiments reported. Thus, we decided to systematically review the in vitro models used to study these phenomena. Fifteen articles from which direct invasion and replication data could be gathered were found. Very heterogeneous results emerged, mainly due to diversity of models in relation to parasite strain (virulence), host species, parasite dose and evaluation times after infection. Also, the results were measured in diverse ways, i.e. some reported percent infected cells, while others informed parasites pervacuole or cell, or parasitic vacuoles per cell. Very few conclusions could be drawn, among them that human astrocytoma cell lines and mouse astrocytes seem more susceptible to infection and less resistant to tachyzoite proliferation than human primary culture astrocytes. The present study supports the need to reanalyse T. gondii astrocyte invasion and replication processes, especially with the use of actual technology, which allows detailed mechanistic studies.

Myenteric neuronal plasticity induced by Toxoplasma gondii (genotype III) on the duodenum of rats

The effects of acute and chronic infection caused by Toxoplasma gondii on duodenal myenteric neurons were analyzed. Eighteen rats were assigned into four groups: Acute Control Group (ACG, n=4); Acute Experimental Group (AEG, n=4); Chronic Control Group (CCG, n=5); and Chronic Experimental Group (CEG, n=5). Rats from the AEG and CEG were inoculated orally with 105 genotype III (BTU-II strain) tachyzoites of T. gondii isolated from a dog with neurological signs. Acute groups were killed after 24 hours after the inoculation and the chronic groups after 30 days. Whole-mount from the duodenum were stained with Giemsa. The population density of myenteric neurons, as well the body cell, nuclear and cytoplasmic area were analyzed. Both acute and chronic toxoplasmic infection did not provoke neuronal loss. On the other hand, plastic alterations were observed: decreasing of the nuclear and cytoplasmic area during the acute phase and neuronal hypertrophy during the chronic phase.

Mechanisms of Toxoplasma gondii persistence and latency

Toxoplasma gondii is an obligate intracellular protozoan parasite that causes opportunistic disease, particularly in immunocompromised individuals. Central to its transmission and pathogenesis is the ability of the proliferative stage (tachyzoite) to convert into latent tissue cysts (bradyzoites). Encystment allows Toxoplasma to persist in the host and affords the parasite a unique opportunity to spread to new hosts without proceeding through its sexual stage, which is restricted to felids. Bradyzoite tissue cysts can cause reactivated toxoplasmosis if host immunity becomes impaired. A greater understanding of the molecular mechanisms orchestrating bradyzoite development is needed to better manage the disease. Here, we will review key studies that have contributed to our knowledge about this persistent form of the parasite and how to study it, with a focus on how cellular stress can signal for the reprogramming of gene expression needed during bradyzoite development.

Toxoplasma on the brain: understanding host-pathogen interactions in chronic CNS infection

Toxoplasma gondii is a prevalent obligate intracellular parasite which chronically infects more than a third of the world's population. Key to parasite prevalence is its ability to form chronic and nonimmunogenic bradyzoite cysts, which typically form in the brain and muscle cells of infected mammals, including humans. While acute clinical infection typically involves neurological and/or ocular damage, chronic infection has been more recently linked to behavioral changes. Establishment and maintenance of chronic infection involves a balance between the host immunity and parasite evasion of the immune response. Here, we outline the known cellular interplay between Toxoplasma gondii and cells of the central nervous system and review the reported effects of Toxoplasma gondii on behavior and neurological disease. Finally, we review new technologies which will allow us to more fully understand host-pathogen interactions.

Migratory activation of primary cortical microglia upon infection with Toxoplasma gondii

Disseminated toxoplasmosis in the central nervous system (CNS) is often accompanied by a lethal outcome. Studies with murine models of infection have focused on the role of systemic immunity in control of toxoplasmic encephalitis, while knowledge remains limited on the contributions of resident cells with immune functions in the CNS. In this study, the role of glial cells was addressed in the setting of recrudescent Toxoplasma infection in mice. Activated astrocytes and microglia were observed in the close vicinity of foci with replicating parasites in situ in the brain parenchyma. Toxoplasma gondii tachyzoites were allowed to infect primary microglia and astrocytes in vitro. Microglia were permissive to parasite replication, and infected microglia readily transmigrated across transwell membranes and cell monolayers. Thus, infected microglia, but not astrocytes, exhibited a hypermotility phenotype reminiscent of that recently described for infected dendritic cells. In contrast to gamma interferon-activated microglia, Toxoplasma-infected microglia did not upregulate major histocompatibility complex (MHC) class II molecules and the costimulatory molecule CD86. Yet Toxoplasma-infected microglia and astrocytes exhibited increased sensitivity to T cell-mediated killing, leading to rapid parasite transfer to effector T cells in vitro. We hypothesize that glial cells and T cells, besides their role in triggering antiparasite immunity, may also act as "Trojan horses," paradoxically facilitating dissemination of Toxoplasma within the CNS. To our knowledge, this constitutes the first report of migratory activation of a resident CNS cell by an intracellular parasite.

The diagnosis of a personality disorder increases the likelihood for seropositivity to Toxoplasma gondii in psychiatric patients

Individuals serologically positive for the chronic infection with the parasite Toxoplasma gondii (TG) display certain personality traits differently from uninfected individuals. Experimental data in mice demonstrate that TG infection modulates behaviour. However, psychiatric patients with a personality disorder have not yet been investigated systematically. In our sample containing 896 psychiatric inpatients with the primary diagnoses of schizophrenia, major depression, schizoaffective or bipolar disorder and 214 psychiatrically unaffected controls (same geographic region, sampled during same time period) we analysed for effects of the additional diagnosis of a personality disorder in the patients. Psychiatrically, a patient can meet the criteria of a personality disorder additionally to any of the mentioned primary diagnoses. We applied logistic regression and cross-table statistics, separated groups by the presence/absence of a personality disorder (ICD-10) and adjusted for age between groups. We found that among all patients the additional diagnosis of a personality disorder was significantly associated with TG infection. Furthermore, only in the patients with an additional personality disorder medium titre responses (1:16-1:64) were associated with chronic course and high C-reactive protein (CRP) levels whereas high titre response (>1:64) was associated with a more acute recurrent clinical course. In the older individuals only there was a preponderance of medium titre responses (1:16-1:64) among the patients with personality disorder compared to those without and controls. We conclude that TG infection and the host's response to it make a difference for the diagnosis of a personality disorder. Our data support that TG infection can modulate human behaviour and personality traits.

Toxoplasma gondii: determinants of tachyzoite to bradyzoite conversion

Apicomplexa are primarily obligate intracellular protozoa that have evolved complex developmental stages important for pathogenesis and transmission. Toxoplasma gondii, responsible for the disease toxoplasmosis, has the broadest host range of the Apicomplexa as it infects virtually any warm-blooded vertebrate host. Key to T. gondii's pathogenesis is its ability to differentiate from a rapidly replicating tachyzoite stage during acute infection to a relatively non-immunogenic, dormant bradyzoite stage contained in tissue cysts. These bradyzoite cysts can reconvert back to tachyzoites years later causing serious pathology and death if a person becomes immune-compromised. Like the sexual stage sporozoites, bradyzoites are also orally infectious and a major contributor to transmission. Because of the critical role of stage conversion to pathogenesis and transmission, a major research focus is aimed at identifying molecular mediators and pathways that regulate differentiation. Tachyzoite to bradyzoite development can occur spontaneously in vitro and be induced in response to exogenous stress including but not limited to host immunity. The purpose of this review is to explore the potential contributors to stage differentiation in infection and how a determination is made by the parasite to differentiate from tachyzoites to bradyzoites.

Quantitative and morphometric changes of subpopulations of myenteric neurons in swines with toxoplasmosis

The consequences of the infection caused by Toxoplasma gondii in myenteric neurons of the jejunum of swines reactive to NADH-diaphorase and NADPH-diaphorase were evaluated in this study. Ten 88-day-old mixed-breed swines (Pietrain and Wessex) were assigned into two groups: Control (n=5) and Experimental (n=5), which orally received 5000 sporulated oocysts from a genotype III T. gondii strain. After 30days, the animals were anesthetized, having part of their jejunum removed and stained with NADPH-diaphorase and NADH-diaphorase. NADPHd-p neurons (nitrergic) presented increase of the number of cells per ganglion and hypertrophy. The number of NADHd-p neurons (metabolic more active) and their nuclear area decreased.

Treating ocular toxoplasmosis: current evidence

The current treatment of ocular toxoplasmosis is controversial. The mainstay of treatment has been pyrimethamine and sulphonamides with or without systemic corticosteroids, but the actual evidence that antibiotics have a beneficial effect in recurrent toxoplasmic retinochoroiditis is unsupported by randomised placebo controlled trials. Thus far there have only been three studies looking at the efficacy of antibiotic treatment, all of which were methodologically weak and two of which were perfomed more than 30 years ago. All studies reported adverse effects from treatment. There is an urgent need for further randomised, double blind, placebo controlled studies for lesions in all parts of the retina and to test the efficacy of adjunctive corticosteroid treatment.

The role of DNA microarrays in Toxoplasma gondii research, the causative agent of ocular toxoplasmosis

Ocular toxoplasmosis, which is caused by the protozoan parasite Toxoplasma gondii, is the leading cause of retinochoroiditis. Toxoplasma is an obligate intracellular pathogen that replicates within a parasitophorous vacuole. Infections are initiated by digestion of parasites deposited in cat feces or in undercooked meat. Parasites then disseminate to target tissues that include the retina where they then develop into long-lived asymptomatic tissue cysts. Occasionally, cysts reactivate and growth of newly emerged parasites must be controlled by the host’s immune system or disease will occur. The mechanisms by which Toxoplasma grows within its host cell, encysts, and interacts with the host’s immune system are important questions. Here, we will discuss how the use of DNA microarrays in transcriptional profiling, genotyping, and epigenetic experiments has impacted our understanding of these processes. Finally, we will discuss how these advances relate to ocular toxoplasmosis and how future research on ocular toxoplasmosis can benefit from DNA microarrays.

Alterations of the myenteric plexus of the ileum and the descending colon caused by Toxoplasma gondii (genotype III)

Alterations caused by a genotype III strain of Toxoplasma gondii were assessed with respect to the number and the morphometry of the myenteric neurons in the terminal ileum and the descending colon. Eighteen rats were divided into four groups: Acute Control Group (ACG, n=4); Acute Experimental Group (AEG, n=4); Chronic Control Group (CCG, n=5) and Chronic Experimental Group (CEG, n=5). NaCl solution was administered through gavage to the animals in the ACG and CCG. Toxoplasma gondii tachyzoites (10(4)) from a genotype III strain were orally administered to the AEG and CEG. Acute Groups were died after 24 hours, and the Chronic Groups after 30 days. Neuronal loss was not observed in both organs. The neurons atrophied in the terminal ileum as the opposite occurred with the neurons at the descending colon during the chronic phase of infection. In the terminal ileum, the neurons atrophied during the chronic phase of the infection as no alteration was found during the acute phase. For the descending colon, the neurons became hypertrophic during the chronic infection in opposition to the atrophy found during the acute phase.

Microglia in infectious diseases of the central nervous system

Microglia are the resident macrophage population in the central nervous system (CNS) parenchyma and, as such, are poised to provide a first line of defense against invading pathogens. Microglia are endowed with a vast repertoire of pattern recognition receptors that include such family members as Toll-like receptors and phagocytic receptors, which collectively function to sense and eliminate microbes invading the CNS parenchyma. In addition, microglial activation elicits a broad range of pro-inflammatory cytokines and chemokines that are involved in the recruitment and subsequent activation of peripheral immune cells infiltrating the infected CNS. Studies from several laboratories have demonstrated the ability of microglia to sense and respond to a wide variety of pathogens capable of colonizing the CNS including bacterial, viral, and fungal species. This review will highlight the role of microglia in microbial recognition and the resultant antipathogen response that ensues in an attempt to clear these infections. Implications as to whether microglial activation is uniformly beneficial to the CNS or in some circumstances may exacerbate pathology will also be discussed.

Communication between Toxoplasma gondii and its host: impact on parasite growth, development, immune evasion, and virulence

Toxoplasma gondii is an obligate intracellular protozoan parasite that can infect most warm-blooded animals and cause severe and life-threatening disease in developing fetuses and in immune-compromised patients. Although Toxoplasma was discovered over 100 years ago, we are only now beginning to appreciate the importance of the role that parasite modulation of its host has on parasite growth, bradyzoite development, immune evasion, and virulence. The goal of this review is to highlight these findings, to develop an integrated model for communication between Toxoplasma and its host, and to discuss new questions that arise out of these studies.

Visualization of Toxoplasma gondii stage conversion by expression of stage-specific dual fluorescent proteins

To recognize the stage conversion of Toxoplasma gondii between tachyzoite and bradyzoite in live host cells, a transgenic T. gondii line, which expressed stage-specific red and green fluorescence, was constructed. T. gondii PLK strain tachyzoites were stably transformed with genes encoding red fluorescent protein (DsRed Express) and green fluorescent protein (GFP) under the control of tachyzoite-specific SAG1 and bradyzoite-specific BAG1 promoters, respectively. The resulting transgenic parasite was designated PLK/DUAL. When PLK/DUAL was cultured in pH 7.0 medium, the PLK/DUAL zoites expressed red fluorescence, but no detectable levels of green fluorescence were observed. The PLK/DUAL zoites reacted with anti-SAG1 antibody, but not anti-BAG1 antiserum. When PLK/DUAL was cultured under high pH conditions, or in the presence of the p38 MAPK inhibitor SB202190, a small number of zoites expressed green fluorescence and were BAG1 positive. C57BL/6J mice were infected with PLK/DUAL tachyzoites. During the acute and reactivating phase, zoites expressed red fluorescence. However, green fluorescence was not detectable. By contrast, latent cysts expressed green fluorescence. The stage-specific dual fluorescence of PLK/DUAL facilitates identification of the parasitic stage in live cells, with the advantage that fixation or immunostaining is not required.

A possible relationship between Toxoplasma gondii and schizophrenia: A seroprevalence study

Objective. Schizophrenia is a pervasive neuropsychiatric disease of uncertain etiology. We aimed to investigate a possible association between Toxoplasma gondii infection and schizophrenia in this study. Method. We selected individuals with schizophrenia (n=88) and tested them with commercially available enzyme-linked immunosorbent assay (ELISA) for anti-Toxoplasma IgG and IgM antibodies and compared these seropositivity rate to those of controls without psychiatric disease (n=88). Results. The rate of IgG antibody in the schizophrenia patients (47.7%) was higher than the control groups (20.4%) (P<0.001). We did not find any anti-Toxoplasma IgM seropositivity in both schizophrenia patients and control group. In schizophrenic patients with and without anti-Toxoplasma IgG groups statistical analysis did not reveal any correlation between demographic variables and Toxoplasma infection. Conclusion. Our findings supported previous studies indicate that T.gondii may play a role in etiopathogenesis in some cases of schizophrenia.

Stress-related and spontaneous stage differentiation of Toxoplasma gondii

Toxoplasma gondii is an obligatory intracellular parasitic protozoan that infects a variety of avian and mammalian hosts including up to one third of the human population worldwide. Developmental differentiation between distinct stages, i.e. sporozoites, tachyzoites and bradyzoites is fundamental for the parasite life cycle and for transmission between hosts. It is also interconnected with the pathogenesis of overt toxoplasmosis and makes T. gondii an important opportunistic pathogen of humans. In order to delineate the underlying mechanisms, several cell culture differentiation systems have been developed which mimic the transition from fast-replicating tachyzoites to slowly proliferating bradyzoites in vitro. Since exogenous stress factors, i.e. alkaline pH, IFN-gamma and other proinflammatory cytokines, chemicals or drugs, heat shock, and deprivation of nutrients have been shown to increase the efficacy of bradyzoite development in vitro, Toxoplasma stage differentiation is largely viewed as a stress-related response to hostile environmental conditions. However, tachyzoite to bradyzoite differentiation also occurs spontaneously in vitro and this raises questions about the importance of stress conditions for triggering stage conversion. High frequencies of spontaneous bradyzoite development in primary and permanent skeletal muscle cells, i.e. cells that preferentially harbour bradyzoite-containing tissue cysts in vivo suggest that the host cell type may be critical. Furthermore, the host cell transcriptome, including the expression of distinct host cell genes, has recently been shown to trigger bradyzoite development and cyst formation. Together, these results strongly indicate that the complex cellular environment, besides exogenous stress factors, may govern the developmental differentiation of T. gondii.

Primary culture of skeletal muscle cells as a model for studies of Toxoplasma gondii cystogenesis

Toxoplasma gondii is a protozoan pathogen of birds and mammals, including humans. The infective stage, the bradyzoite, lives within cysts, which occur predominantly in cells of the central nervous system and skeletal and cardiac muscles, characterizing the chronic phase of toxoplasmosis. In the present study, we employed for the first time primary mouse culture of skeletal muscle cells (SkMC) infected with bradyzoites, as a cellular model for cystogenesis. The interconversion of bradyzoite and tachyzoite was analyzed by immunofluorescence using 2 stage-specific antibodies, i.e., anti-bradyzoite (anti-BAG1) and anti-tachyzoite (anti-SAG1). After 24 hr of interaction only bradyzoites were multiplying, as revealed by anti-BAG1 incubation; interconversion to tachyzoites was not observed. After 48 hr of infection, 2 types of vacuoles were seen, i.e., BAG1+ and SAG1+, indicating the presence of bradyzoites as well as their interconversion to tachyzoites. After 96 hr of infection, BAG1+ vacuoles presented a higher number of parasites when compared to 48 hr, indicating multiplication of bradyzoites without interconversion. Using ultrastructural analysis, bradyzoites were found to adhere to the cell membranes via both the apical and posterior regions or were associated with SkMC membrane expansions. During bradyzoite invasion of SkMC, migration of the rough endoplasmic reticulum (RER) profiles to the parasite invasion site was observed. Later, RER profiles were localized between the mitochondria and parasitophorous vacuole membrane (PVM) that contained the parasite. After 31 days of parasite-host cell infection, RER profiles and mitochondria were not observed in association with the cyst wall. Alterations of the PVM, including increased thickness and electrondensity gain on its inner membrane face, were observed 48 hr after infection. Cystogenesis was complete 96 hr after infection, resulting in the formation of the cyst wall, which displayed numerous membrane invaginations. In addition, an electron-dense granular region enriched with vesicles and tubules was present, as well as numerous intracystic bradyzoites. These results show that the in vitro T. gondii model and SkMC are potential tools for both the study of cystogenesis using molecular approaches and the drug screening action on tissue cysts and bradyzoites.

A controlled prospective study of toxoplasma gondii infection in individuals with schizophrenia: beyond seroprevalence

Toxoplasma gondii (TG) infection has been reported to be more frequent in schizophrenia. The interaction of the lifelong persisting parasite with the host's immune system involves T-cell/interferon-gamma-induced degradation of tryptophan and provides a challenge to the host well beyond a possible role in the etiology of schizophrenia. The hypothesis we tested in this study was that TG infection may be more frequent (serofrequency) and/or more intense (serointensity) in patients with schizophrenia or major depression compared with psychiatrically healthy controls. In addition, these measures are associated with the clinical course. We did a cross-sectional, prospective investigation of individuals with schizophrenia (n = 277) and major depression (n = 465) admitted to our department (2002-2005) and of healthy controls (n = 214), with all groups adjusted for age and geographic home region. Serofrequency was comparable between the groups, but serointensity was significantly higher in the patients. In individuals with schizophrenia, serointensity was significantly positively associated with C-reactive protein levels and leukocyte counts, and first-episode patients yielded significantly higher serotiters. Immunomodulatory medication was associated with decreased serotiters. In addition, the route of infection appears to differ between patients and controls. Thus, our results support increased host responses to TG infection in the patients, as well as increased titers in first-episode patients with schizophrenia; this may relate to the shifted T-helper 1/2 status described in these patients. Therefore, we suggest that TG infection, particularly in individuals with schizophrenia, is an important environmental factor in the interaction between psychiatric vulnerability, genetic background, immunomodulation, and the neurotransmitter systems.

Changes in the expression of human cell division autoantigen-1 influence Toxoplasma gondii growth and development

Toxoplasma is a significant opportunistic pathogen in AIDS, and bradyzoite differentiation is the critical step in the pathogenesis of chronic infection. Bradyzoite development has an apparent tropism for cells and tissues of the central nervous system, suggesting the need for a specific molecular environment in the host cell, but it is unknown whether this environment is parasite directed or the result of molecular features specific to the host cell itself. We have determined that a trisubstituted pyrrole acts directly on human and murine host cells to slow tachyzoite replication and induce bradyzoite-specific gene expression in type II and III strain parasites but not type I strains. New mRNA synthesis in the host cell was required and indicates that novel host transcripts encode signals that were able to induce parasite development. We have applied multivariate microarray analyses to identify and correlate host gene expression with specific parasite phenotypes. Human cell division autoantigen-1 (CDA1) was identified in this analysis, and small interfering RNA knockdown of this gene demonstrated that CDA1 expression causes the inhibition of parasite replication that leads subsequently to the induction of bradyzoite differentiation. Overexpression of CDA1 alone was able to slow parasite growth and induce the expression of bradyzoite-specific proteins, and thus these results demonstrate that changes in host cell transcription can directly influence the molecular environment to enable bradyzoite development. Investigation of host biochemical pathways with respect to variation in strain type response will help provide an understanding of the link(s) between the molecular environment in the host cell and parasite development.

Macrophageal/microglial cell activation and cerebral injury induced by excretory–secretory products secreted by Paragonimus westermani

Cerebral paragonimiasis causes various neurological disorders including seizures, visual impairment and hemiplegia. The excretory-secretory product (ESP) released by Paragonimus westermani has a cysteine protease activity and plays important roles in its migration in the host tissue and modulation of host immune responses. To gain more insight into the pathogenesis of ESP in the brain, we investigated the inflammatory reaction and cerebral injury following microinjection of ESP into rat striatum. The size of injury was maximally observed 3 days after microinjection of ESP and then declined to control levels as astrocytes have repopulated the injury. ED1-positive monocytes and microglia were confluently found inside the injury. The mRNA expression of inducible nitric oxide synthase (iNOS) occurred as early as 9h after ESP injection and then declined to control levels within 1 day. The iNOS inhibitor aminoguanidine largely decreased the expression of iNOS but did not reduce the size of lesion caused by ESP. Interestingly, however, heat inactivation of ESP caused a decrease of injury formation with no altered expression of iNOS. The data indicate that ESP produces brain tissue injury by recruiting activated monocytes/microglia via heat-labile protease activity.

Association between prenatal treatment and clinical manifestations of congenital toxoplasmosis in infancy: a cohort study in 13 European centres

AIM

To determine the effectiveness of prenatal treatment for clinical manifestations of congenital toxoplasmosis.

METHODS

We prospectively identified 255 live-born infants with congenital toxoplasmosis using prenatal or neonatal screening. We determined the effect of prenatal treatment on the risks of intracranial or ocular lesions in infancy, accounting for gestational age at maternal seroconversion.

RESULTS

Prenatal treatment within 4 wk of seroconversion reduced the risk of intracranial lesions compared with no treatment (odds ratio, OR 0.28; 95% CI: 0.08-0.75), but there was no significant effect when initiated after 4 wk (OR 0.76; 95% CI: 0.35-1.59; overall p-value 0.19). Compared to spiramycin alone, no treatment doubled the risk of intracranial lesions (OR 2.33; 95% CI: 1.04-5.50), but the risk did not differ with pyrimethamine-sulphonamide treatment (overall p-value 0.52). There was no consistent relationship between the type or timing of treatment and the risk of ocular lesions. Gestational age at maternal seroconversion was inversely associated with the risk of intracranial but not ocular lesions.

CONCLUSION

Only early versus no prenatal treatment for intracranial lesions showed a statistically significant benefit. A large randomized controlled trial and/or meta-analysis of individual patient data from cohort studies is required to confirm these findings.

Toxoplasma gondii prevents neuron degeneration by interferon-gamma-activated microglia in a mechanism involving inhibition of inducible nitric oxide synthase and transforming growth factor-beta1 production by infected microglia

Interferon (IFN)-gamma, the main cytokine responsible for immunological defense against Toxoplasma gondii, is essential in all infected tissues, including the central nervous system. However, IFN-gamma-activated microglia may cause tissue injury through production of toxic metabolites such as nitric oxide (NO), a potent inducer of central nervous system pathologies related to inflammatory neuronal disturbances. Despite potential NO toxicity, neurodegeneration is not commonly found during chronic T. gondii infection. In this study, we describe decreased NO production by IFN-gamma-activated microglial cells infected by T. gondii. This effect involved strong inhibition of iNOS expression in IFN-gamma-activated, infected microglia but not in uninfected neighboring cells. The inhibition of NO production and iNOS expression were parallel with recovery of neurite outgrowth when neurons were co-cultured with T. gondii-infected, IFN-gamma-activated microglia. In the presence of transforming growth factor (TGF)-beta1-neutralizing antibodies, the beneficial effect of the parasite on neurons was abrogated, and NO production reverted to levels similar to IFN-gamma-activated uninfected co-cultures. In addition, we observed Smad-2 nuclear translocation, a hallmark of TGF-beta1 downstream signaling, in infected microglial cultures, emphasizing an autocrine effect restricted to infected cells. Together, these data may explain a neuropreservation pattern observed during immunocompetent host infection that is dependent on T. gondii-triggered TGF-beta1 secretion by infected microglia.

CD11c- and CD11b-expressing mouse leukocytes transport single Toxoplasma gondii tachyzoites to the brain

The protozoan parasite Toxoplasma gondii enters hosts through the intestinal mucosa and colonizes distant tissues such as the brain, where its progeny persists for a lifetime. We investigated the role of CD11c- and CD11b-expressing leukocytes in T. gondii transport during the early step of parasitism from the mouse small intestine and during subsequent parasite localization in the brain. Following intragastric inoculation of cyst-containing parasites in mice, CD11c+ dendritic cells from the intestinal lamina propria, the Peyer patches, and the mesenteric lymph nodes were parasitized while in the blood, parasites were associated with the CD11c- CD11b+ monocytes. Using adoptive transfer experiments, we demonstrated that these parasitized cells triggered a parasitic process in the brain of naive recipient mice. Ex vivo analysis of parasitized leukocytes showed that single tachyzoites remained at the cell periphery, often surrounded by the host cell plasma membrane, but did not divide. Using either a dye that labels circulating leukocytes or an antibody known to prevent CD11b+ circulating leukocytes from leaving the microvascular bed lumen, and chimeric mice in which the hematopoietic cells expressed the green fluorescent protein, we established that T. gondii zoites hijacked CD11b+ leukocytes to reach the brain extravascular space.

Microglia overexpressing the macrophage colony-stimulating factor receptor are neuroprotective in a microglial-hippocampal organotypic coculture system

Microglia with increased expression of the macrophage colony-stimulating factor receptor (M-CSFR; c-fms) are found surrounding plaques in Alzheimer's disease (AD) and in mouse models for AD and after ischemic or traumatic brain injury. Increased expression of M-CSFR causes microglia to adopt an activated state that results in proliferation, release of cytokines, and enhanced phagocytosis. To determine whether M-CSFR-induced microglial activation affects neuronal survival, we assembled a coculture system consisting of BV-2 microglia transfected to overexpress the M-CSFR and hippocampal organotypic slices treated with NMDA. Twenty-four hours after assembly of the coculture, microglia overexpressing M-CSFR proliferated at a higher rate than nontransfected control cells and exhibited enhanced migration toward NMDA-injured hippocampal cultures. Surprisingly, coculture with c-fms-transfected microglia resulted in a dramatic reduction in NMDA-induced neurotoxicity. Similar results were observed when cocultures were treated with the teratogen cyclophosphamide. Biolistic overexpression of M-CSFR on microglia endogenous to the organotypic culture also rescued neurons from excitotoxicity. Furthermore, c-fms-transfected microglia increased neuronal expression of macrophage colony-stimulating factor (M-CSF), the M-CSFR, and neurotrophin receptors in the NMDA-treated slices, as determined with laser capture microdissection. In the coculture system, direct contact between the exogenous microglia and the slice was necessary for neuroprotection. Finally, blocking expression of the M-CSF ligand by exogenous c-fms-transfected microglia with a hammerhead ribozyme compromised their neuroprotective properties. These results demonstrate a protective role for microglia overexpressing M-CSFR in our coculture system and suggest under certain circumstances, activated microglia can help rather than harm neurons subjected to excitotoxic and teratogen-induced injury.

DIFFERENTIAL EFFECTS OF INTERFERON-γ AND TUMOR NECROSIS FACTOR-α ON TOXOPLASMA GONDII PROLIFERATION IN ORGANOTYPIC RAT BRAIN SLICE CULTURES

Organotypic slice culture explants of rat cortical tissue infected with Toxoplasma gondii tachyzoites were applied as an in vitro model to investigate host-pathogen interactions in cerebral toxoplasmosis. The kinetics of parasite proliferation and the effects of interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) in infected organotypic cultures were monitored by light microscopy, transmission electron microscopy (TEM), and quantitative polymerase chain reaction (PCR) assay. As assessed by the loss of the structural integrity of the glial fibrillary acidic protein-intermediate filament network, tachyzoites infected and proliferated mainly within astrocytes, whereas neurons and microglia remained largely unaffected. Toxoplasma gondii proliferation was severely inhibited by IFN-y. However, this inhibition was not linked to tachyzoite-to-bradyzoite stage conversion. In contrast, TNF-alpha treatment resulted in a dramatically enhanced proliferation rate of the parasite. The cellular integrity in IFN-gamma-treated organotypic slice cultures was severely impaired compared with untreated and TNF-alpha-treated cultures. Thus, on infection of organotypic neuronal cultures, IFN-gamma and TNF-alpha exhibit largely detrimental effects, which could contribute to either inhibition or acceleration of parasite proliferation during cerebral toxoplasmosis.

Reduction in adhesiveness to extracellular matrix components, modulation of adhesion molecules and in vivo migration of murine macrophages infected with Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite, able to disseminate into deep tissues and cross biological barriers, reaching immunoprivileged sites such as the brain and retina. In order to investigate whether the parasite uses leukocyte trafficking to disseminate throughout the host, the adhesive potential to extracellular matrix components, the expression of adhesion molecules and the in vivo migration of murine macrophages infected with RH strain of T. gondii were investigated. Cellular adhesion to fibronectin, laminin and collagen IV decreased after 24 h of T. gondii infection. However, the decrease in adhesion of infected macrophages observed at early infection was reversed after 48 h. Moreover, decreased adhesion was dependent on active penetration, since heat-killed parasites were unable to reproduce it. Expression of integrins alphaL, alpha4 and alpha5 chains was downmodulated early postinfection, but a progressive regain of expression was observed after 12 h of infection. Expression of beta2, alphav and alpha4 integrins by peritoneal macrophages at late infection was also gradually reestablished. The assessment of in vivo migration of infected macrophages labeled with the fluorescent dye 5-chloromethylfluorescein diacetate showed a 48-h delay in migration to cervical lymph nodes when compared to LPS pre-stimulated macrophages. Furthermore, cells that migrate to distal lymph nodes were loaded with live parasites. Taken together, these results provide insights about T. gondii escape from the host immune response, placing the macrophage as a "Trojan horse", contributing to parasite dissemination and access to immunoprivileged sites.

Role of microglia in central nervous system infections

The nature of microglia fascinated many prominent researchers in the 19th and early 20th centuries, and in a classic treatise in 1932, Pio del Rio-Hortega formulated a number of concepts regarding the function of these resident macrophages of the brain parenchyma that remain relevant to this day. However, a renaissance of interest in microglia occurred toward the end of the 20th century, fueled by the recognition of their role in neuropathogenesis of infectious agents, such as human immunodeficiency virus type 1, and by what appears to be their participation in other neurodegenerative and neuroinflammatory disorders. During the same period, insights into the physiological and pathological properties of microglia were gained from in vivo and in vitro studies of neurotropic viruses, bacteria, fungi, parasites, and prions, which are reviewed in this article. New concepts that have emerged from these studies include the importance of cytokines and chemokines produced by activated microglia in neurodegenerative and neuroprotective processes and the elegant but astonishingly complex interactions between microglia, astrocytes, lymphocytes, and neurons that underlie these processes. It is proposed that an enhanced understanding of microglia will yield improved therapies of central nervous system infections, since such therapies are, by and large, sorely needed.

Differential Stimulation of Microglial Pro-Inflammatory Cytokines by Acanthamoeba culbertsoni versus Acanthamoeba castellanii

Acanthamoeba spp. are opportunistic pathogens that cause granulomatous amebic encephalitis. We compared the highly pathogenic species A. culbertsoni to the relatively less pathogenic species A. castellanii for its capacity to elicit from neonatal rat microglia the gene expression of pro-inflammatory cytokines. Acanthamoeba culbertsoni elicited a robust cytokine gene response by neonatal rat microglia in vitro as compared to A. castellanii. The preponderant cytokine elicited at the mRNA and protein levels was interleukin-1beta. In addition, transmission electron microscopy revealed that microglial cells were capable of phagocytozing A. castellanii. In contrast, A. culbertsoni destroyed microglia. Collectively, these results suggest that a combined action of pro-inflammatory cytokines and destruction of host cells by amebae contribute to the pathology caused by the more pathogenic species.

The role of astrocytes in the immunopathogenesis of toxoplasmic encephalitis

Challenge with the parasite Toxoplasma gondii eventually leads to persistent infection characterised by the presence of tissue cysts in the brain of the host. In immunocompetent individuals the parasite rarely leads to disease but in the immunocompromised host reactivation of these cysts can lead to toxoplasmic encephalitis. It is known that both CD4(+) and CD8(+) T cells are important in preventing reactivation of the parasite, however there is also evidence that astrocytes, a subset of glial cells dominant in the CNS, may be important in resistance to T. gondii. The aim of this paper is to review what is known about the immune functions of astrocytes, and the possible role they may play during toxoplasmic encephalitis.

Neuronal targeting and functional effects of infectious agents transmitted from animals to man

The nervous system is an «immune-privileged» site and can provide a reservoir to harbor as persistent or latent infections certain microbes that find their way to the brain. From an evolutionary standpoint, such infections are characterized at most times by low levels of the infectious agent in the systemic domain, except when multiplication has just taken place. Hence the ability for transmission of the pathogens from animals to Man will be determined by the availability of microbes to be transferred by a vector (e.g. in trypanosomiasis), or the amount of infective forms of the microbes shed into an environment (e.g. in toxoplasmosis). Using African trypanosomes, toxoplasma,Listeria and influenza A virus as examples, mechanisms by which microbes can spread and be targeted to and within the brain to cause various types of nervous system dysfunctions is reviewed. Newly revealed potentials of certain cytokines to stimulate neurons to control the growth, and even kill, microbes in their cell bodies is also described.

Antibiotics for toxoplasmic retinochoroiditis: an evidence-based systematic review

PURPOSE

To determine the effectiveness of systemic antibiotic treatment for toxoplasmic retinochoroiditis.

CLINICAL RELEVANCE

Toxoplasma retinochoroiditis is a significant cause of visual morbidity. Multiple different antibiotic regimens are used, but controversy about treatment effectiveness remains.

LITERATURE REVIEWED

Searches were conducted of Cochrane Controlled Trials Register, Medline (1966 onward), Embase (1980 onward), Dissertation Abstracts (1861 onward), Lilacs (1982 onward), and Pascal (1984 onward). Pharmaceutical companies were contacted for unpublished data. Any randomized controlled trials that compared antibiotics versus placebo in immunocompetent patients with toxoplasmic retinochoroiditis were retrieved. Primary outcome measures were long-term visual acuity and risk of recurrent retinochoroiditis. Secondary outcomes included duration and severity of acute symptoms, size of the lesion at end of follow-up, and adverse effects of treatment.

RESULTS

Only 3 studies (total of 173 participants) were randomized controlled trials and hence met the inclusion criteria (level II). All 3 were methodologically poor, and 2 were carried out more than 35 years ago. None reported the effect on long-term visual outcome. We found no evidence for a beneficial effect on the duration and severity of signs of acute toxoplasmic retinochoroiditis (A,II). There was weak evidence for an effect of long-term treatment for chronic recurrent toxoplasmic retinochoroiditis on lesion recurrence. Treatment was associated with adverse effects.

CONCLUSIONS

There is a lack of evidence to support routine antibiotic treatment for acute toxoplasmic retinochoroiditis. Placebo-controlled randomized trials of antibiotic treatment in patients presenting with acute or chronic toxoplasmic retinochoroiditis arising in any part of the retina are required.

Toxoplasma gondii inhibits MHC class II expression in neural antigen-presenting cells by down-regulating the class II transactivator CIITA

Major histocompatibility complex (MHC) class II expression by microglia and astrocytes is critical for CD4+-mediated immune responses within the central nervous system. Here, we demonstrate that the obligate intracellular parasite, Toxoplasma gondii, down-regulates activation-induced MHC class II expression in human-derived glioblastoma cells as well as in primary astrocytes and microglia from cortices of rat fetuses. Down-regulation of MHC class II proteins was predominantly observed in parasite-positive, but not parasite-negative, host cells of T. gondii-infected cell cultures. MHC class II transcript levels induced by IFN-gamma alone or in combination with TNF-alpha were also clearly diminished after parasitic infection. Furthermore, T. gondii dose-dependently down-regulated the transcript levels of the class II transactivator CIITA. These results suggest that T. gondii partially evade CD4+-mediated intracerebral immune responses, a mechanism which may contribute to long-term persistence of the parasite within the CNS.

Distribution, markers, and functions of retinal microglia

Retinal microglia originate from hemopoietic cells and invade the retina from the retinal margin and the optic disc, most likely via the blood vessels of the ciliary body and iris, and the retinal vasculature, respectively. The microglial precursors that appear in the retina prior to vascularization are major histocompatibility complex (MHC) class I- and II-positive and express the CD45 marker, but lack specific macrophage markers. They differentiate into ramified parenchymal microglia in the adult retina. A second category of microglial precursors, which do express specific macrophage markers, migrate into the retina along with vascular precursors. They appear around blood vessels in the adult retina and are similar to macrophages or cells of the mononuclear phagocyte series (MPS). Microglia are distributed in the outer plexiform layer (OPL), outer nuclear layer (ONL), inner plexiform layer (IPL), ganglion cell layer (GCL), and nerve fiber layer (NFL) of the primate retina. The pattern of microglial distribution in the avascular retina of the quail indicates that blood vessels are not responsible for the final location of microglia in the retina. In the human retina, microglia express MHC class I, MHC class II, CD45, CD68, and S22 markers. In the rat and mouse retina, OX41, OX42, OX3, OX6, OX18, ED1, Mac-1, F4/80, 5D4 anti-keratan sulfate, and lectins are used to recognize microglia. Microglial cells play an important role in host defense against invading microorganisms, immunoregulation, and tissue repair. During neurodegeneration, activated microglial cells participate in the phagocytosis of debris and facilitate regenerative processes. In autoimmune disease, microglia have dual functions: initiating uveoretinitis, but also limiting subsequent inflammation. Retinal microglia may be associated with vitreoretinopathy, diabetic retinopathy, glaucoma, and age-related macular degeneration. The goal of this article was to review the present knowledge about retinal microglia and the function of retinal microglia in pathological conditions.

Host cell invasion by the opportunistic pathogen Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular protozoan that infects an astonishing variety of vertebrate hosts including humans. Classified in the phylum Apicomplexa, T. gondii causes an opportunistic disease, toxoplasmosis, in individuals with immune dysfunction and congenital disease in infected infants. Re-emergence of toxoplasmosis as a life-threatening disease in patients with AIDS is anticipated in the wake of emerging multi-drug resistant strains of HIV. In immunodeficient patients, the available evidence suggests that tissue pathology associated with T. gondii infection is due to parasite-directed lytic destruction of individual host cells. The Toxoplasma lytic cycle begins when the parasite actively invades a target cell. In association with invasion, T. gondii sequentially discharges three sets of secretory organelles beginning with the micronemes, which contain adhesive proteins involved in parasite attachment to a host cell. Deployed as protein complexes, several micronemal proteins possess vertebrate-derived adhesive sequences that function in binding receptors on the surface of a target cell. Each protein in these adhesive complexes fulfills a specific role in movement through the secretory pathway, targeting to the micronemes, or adhesion. It is anticipated that these adhesive complexes recognize a variety of host receptors, including some that are expressed on multiple cell types, and that this diversity in host cell receptors contributes to the remarkably broad tissue- and host-range of T. gondii.

Effect of prenatal treatment on the risk of intracranial and ocular lesions in children with congenital toxoplasmosis

BACKGROUND

Hydrocephalus, intracranial calcification and retinochoroiditis are the most common manifestations of tissue damage due to congenital toxoplasmosis, but the effect of prenatal treatment on these outcomes is unclear. We aimed to determine the effect of prenatal treatment for toxoplasmosis on the risk of intracranial and ocular lesions in congenitally infected children at 3 years of age.

METHODS

A cohort of mothers identified during pregnancy with toxoplasma infection and their 181 liveborn children with confirmed congenital toxoplasmosis was retrospectively analysed to determine the presence of intracranial and ocular lesions. As few women are not treated, we compared the effects of the treatment potency (pyrimethamine-sulfadiazine versus spiramycin or no treatment), and the timing of treatment, on the risks of intracranial lesions, time to detection of ocular lesions, and detection of any lesions (intracranial or ocular) by 3 years of age. Analyses took account of the gestation at maternal seroconversion.

RESULTS

There was no evidence for an effect of pyrimethamine-sulfadiazine on intracranial, ocular or any lesions by 3 years: odds ratio (OR) for any lesions 0.89 (95% CI : 0.41, 1.88). There was no evidence of an effect of delayed treatment on ocular lesions (hazard ratio = 0.69, 95% CI : 0.28, 1.68) or any lesions by 3 years of age (OR = 0.44, 95% CI : 0.16, 1.19).

CONCLUSIONS

Our study failed to detect a beneficial effect of early or more potent anti toxoplasma treatment on the risks of intracranial or ocular lesions in children with congenital toxoplasmosis. However, larger, prospective studies, which determine the effect of prenatal treatment on long-term developmental outcomes are required to justify changes in clinical practice.

Toxoplasma gondii infection of neurons induces neuronal cytokine and chemokine production, but gamma interferon- and tumor necrosis factor-stimulated neurons fail to inhibit the invasion and growth of T. gondii

The intracellular parasite Toxoplasma gondii has the capacity to persist in the brain within neurons. In this study we demonstrated that T. gondii infected murine cerebellar neurons in vitro and replicated within these cells. Stimulation with gamma interferon (IFN-gamma) and/or tumor necrosis factor (TNF) did not enable neurons to inhibit parasite invasion and replication. Cultured neurons constitutively produced interleukin 1 (IL-1), IL-6, macrophage inflammatory protein 1alpha (MIP-1alpha), and MIP-1beta but not transforming growth factor beta1 (TGF-beta1), IL-10, and granulocyte-macrophage colony-stimulating factor. Neuronal expression of some cytokines (IL-6, TGF-beta1) and chemokines (MIP-1beta) was regulated by infection and/or by IFN-gamma and TNF.