Observing astrocyte polarization in brains from mouse chronically infected with Toxoplasma gondii

Toxoplasma gondii (T. gondii) is a protozoan parasite that infects approximately one-third of the global human population, often leading to chronic infection. While acute T. gondii infection can cause neural damage in the central nervous system and result in toxoplasmic encephalitis, the consequences of T. gondii chronic infection (TCI) are generally asymptomatic. However, emerging evidence suggests that TCI may be linked to behavioral changes or mental disorders in hosts. Astrocyte polarization, particularly the A1 subtype associated with neuronal apoptosis, has been identified in various neurodegenerative diseases. Nevertheless, the role of astrocyte polarization in TCI still needs to be better understood. This study aimed to establish a mouse model of chronic TCI and examine the transcription and expression levels of glial fibrillary acidic protein (GFAP), C3, C1q, IL-1α, and TNF-α in the brain tissues of the mice. Quantitative real-time PCR (qRT-PCR), enzyme-linked immunosorbent assay, and Western blotting were employed to assess these levels. Additionally, the expression level of the A1 astrocyte-specific marker C3 was evaluated using indirect fluorescent assay (IFA). In mice with TCI, the transcriptional and expression levels of the inflammatory factors C1q, IL-1α, and TNF-α followed an up-down-up pattern, although they remained elevated compared to the control group. These findings suggest a potential association between astrocyte polarization towards the A1 subtype and synchronized changes in these three inflammatory mediators. Furthermore, immunofluorescence assay (IFA) revealed a significant increase in the A1 astrocytes (GFAP+C3+) proportion in TCI mice. This study provides evidence that TCI can induce astrocyte polarization, a biological process that may be influenced by changes in the levels of three inflammatory factors: C1q, IL-1α, and TNF-α. Additionally, the release of neurotoxic substances by A1 astrocytes may be associated with the development of TCI.

The excretory/secretory products of fifth-stage larval Angiostrongylus cantonensis induces autophagy via the Sonic hedgehog pathway in mouse brain astrocytes

Angiostrongyliasis is induced by the nematode Angiostrongylus cantonensis and leads to eosinophilic meningitis and meningoencephalitis in humans. Excretory-secretory products (ESPs) are important investigation targets for studying the relationship between hosts and nematodes. These products assist worms in penetrating the blood-brain barrier and avoiding the host immune response. Autophagy is a catabolic process that is responsible for digesting cytoplasmic organelles, proteins, and lipids and removing them through lysosomes. This process is essential to cell survival and homeostasis during nutritional deficiency, cell injury and stress. In this study, we investigated autophagy induction upon treatment with the ESPs of the fifth-stage larvae (L5) of A. cantonensis and observed the relationship between autophagy and the Shh pathway. First, the results showed that A. cantonensis infection induced blood-brain barrier dysfunction and pathological changes in the brain. Moreover, A. cantonensis L5 ESPs stimulated autophagosome formation and the expression of autophagy molecules, such as LC3B, Beclin, and p62. The data showed that upon ESPs treatment, rapamycin elevated cell viability through the activation of the autophagy mechanism in astrocytes. Finally, we found that ESPs induced the activation of the Sonic hedgehog (Shh) signaling pathway and that the expression of autophagy molecules was increased through the Shh signaling pathway. Collectively, these results suggest that A. cantonensis L5 ESPs stimulate autophagy through the Shh signaling pathway and that autophagy has a protective effect in astrocytes.

In helminthes, Excretory-secretory products (ESPs) contains a wide range of molecules, including proteins, lipids, glycans, and nucleic acids, that assist in the penetration of host defensive barriers, reduction of oxidative stress, and avoid the host immune attack. It has been known as a key factor for parasite development, including feeding, invasion and molting. Therefore, ESPs is a valuable target for the investigation of the host-parasite relationships. However, only a few researches about the function of Angiostrongyliasis cantonensis ESPs have been verified to date. Angiostrongyliasis cantonensis, a blood-feeding nematode, and it is an important causative agent of eosinophilic meningitis and meningoencephalitis in human. Recent our studies have demonstrated that the A. cantonensis ESPs can induce oxidative stress, apoptosis, and immune response. In this study, we will use a mouse astrocytes as a model to investigate the signaling mechanisms of autophagy induction by ESPs treatment. First, the Microarray, Western blotting, and Transmission electron microscopy data demonstrated that A. cantonensis ESPs can induce autophagy generation in astrocytes. Next, ESPs-induced autophagy was activated via Sonic hedgehog (Shh) signaling, and it has a protective potential for astrocytes. These finding will provide new insights into the mechanisms and effects of the A. cantonensis ESPs.

Cerebral inducible nitric oxide synthase protein expression in microglia, astrocytes and neurons in Trypanosoma brucei brucei-infected rats

To study the anatomo-biochemical substrates of brain inflammatory processes, Wistar male rats were infected with Trypanosoma brucei brucei. With this reproducible animal model of human African trypanosomiasis, brain cells (astrocytes, microglial cells, neurons) expressing the inducible nitric oxide synthase (iNOS) enzyme were revealed. Immunohistochemistry was achieved for each control and infected animal through eight coronal brain sections taken along the caudorostral axis of the brain (brainstem, cerebellum, diencephalon and telencephalon). Specific markers of astrocytes (anti-glial fibrillary acidic protein), microglial cells (anti-integrin alpha M) or neurons (anti-Neuronal Nuclei) were employed. The iNOS staining was present in neurons, astrocytes and microglial cells, but not in oligodendrocytes. Stained astrocytes and microglial cells resided mainly near the third cavity in the rostral part of brainstem (periaqueductal gray), diencephalon (thalamus and hypothalamus) and basal telencephalon. Stained neurons were scarce in basal telencephalon, contrasting with numerous iNOS-positive neuroglial cells. Contrarily, in dorsal telencephalon (neocortex and hippocampus), iNOS-positive neurons were plentiful, contrasting with the marked paucity of labelled neuroglial (astrocytes and microglial) cells. The dual distribution between iNOS-labelled neuroglial cells and iNOS-labelled neurons is a feature that has never been described before. Functionalities attached to such a divergent distribution are discussed.

Apoptosis and necroptosis of mouse hippocampal and parenchymal astrocytes, microglia and neurons caused by Angiostrongylus cantonensis infection

BACKGROUND

Angiostrongylus cantonensis has been the only parasite among Angiostrongylidae to cause human central nervous system infection characterized by eosinophilic meningitis or meningoencephalitis. The mechanism of the extensive neurological impairments of hosts caused by A. cantonensis larvae remains unclear. The aim of the present study was to investigate apoptosis, necroptosis and autophagy in the brains of mice infected with A. cantonensis, which will be valuable for better understanding the pathogenesis of angiostrongyliasis cantonensis.

METHODS

Functional and histological neurological impairments of brain tissues from mice infected with A. cantonensis were measured by the Morris water maze test and haematoxylin and eosin (H&E) staining, respectively. The transcriptional and translational levels of apoptosis-, necroptosis- and autophagy-related genes were quantified by quantitative real-time polymerase chain reaction (RT-PCR), and assessed by western blot and immunohistochemistry (IHC) analysis. Apoptotic and necroptotic cells and their distributions in infected brain tissues were analysed by flow cytometry and transmission electron microscopy (TEM).

RESULTS

Inflammatory response in the central nervous system deteriorated as A. cantonensis infection evolved, as characterized by abundant inflammatory cell infiltration underneath the meninges, which peaked at 21 days post-infection (dpi). The learning and memory capacities of the mice were significantly decreased at 14 dpi, indicating prominent impairment of their cognitive functions. Compared with those of the control group, the mRNA levels of caspase-3, -4, -6, and RIP3 and the protein levels of caspase-4, cleaved caspase-3, cleaved caspase-6, RIP3, and pRIP3 were obviously elevated. However, no changes in the mRNA or protein levels of FADD, Beclin-1 or LC3B were evident, indicating that apoptosis and necroptosis, but not autophagy, occurred in the brain tissues of mice infected with A. cantonensis. The quantitative RT-PCR, western blot, IHC, flow cytometry and TEM results further revealed the apoptotic and necroptotic microglia, astrocytes and neurons in the parenchymal and hippocampal regions of infected mice.

CONCLUSIONS

To our knowledge, we showed for the first time that A. cantonensis infection causes the apoptosis and necroptosis of microglia and astrocytes in the parenchymal and hippocampal regions of host brain tissues, further demonstrating the pathogenesis of A. cantonensis infection and providing potential therapeutic targets for the management of angiostrongyliasis.

Cutting Edge: IFN-γ Produced by Brain-Resident Cells Is Crucial To Control Cerebral Infection with Toxoplasma gondii

In vitro studies demonstrated that microglia and astrocytes produce IFN-γ in response to various stimulations, including LPS. However, the physiological role of IFN-γ production by brain-resident cells, including glial cells, in resistance against cerebral infections remains unknown. We analyzed the role of IFN-γ production by brain-resident cells in resistance to reactivation of cerebral infection with Toxoplasma gondii using a murine model. Our study using bone marrow chimeric mice revealed that IFN-γ production by brain-resident cells is essential for upregulating IFN-γ-mediated protective innate immune responses to restrict cerebral T. gondii growth. Studies using a transgenic strain that expresses IFN-γ only in CD11b(+) cells suggested that IFN-γ production by microglia, which is the only CD11b(+) cell population among brain-resident cells, is able to suppress the parasite growth. Furthermore, IFN-γ produced by brain-resident cells is pivotal for recruiting T cells into the brain to control the infection. These results indicate that IFN-γ produced by brain-resident cells is crucial for facilitating both the protective innate and T cell-mediated immune responses to control cerebral infection with T. gondii.

Interferon-gamma promotes infection of astrocytes by Trypanosoma cruzi

The inflammatory cytokine interferon-gamma (IFNγ) is crucial for immunity against intracellular pathogens such as the protozoan parasite Trypanosoma cruzi, the causative agent of Chagas disease (CD). IFNγ is a pleiotropic cytokine which regulates activation of immune and non-immune cells; however, the effect of IFNγ in the central nervous system (CNS) and astrocytes during CD is unknown. Here we show that parasite persists in the CNS of C3H/He mice chronically infected with the Colombian T. cruzi strain despite the increased expression of IFNγ mRNA. Furthermore, most of the T. cruzi-bearing cells were astrocytes located near IFNγ+ cells. Surprisingly, in vitro experiments revealed that pretreatment with IFNγ promoted the infection of astrocytes by T. cruzi increasing uptake and proliferation of intracellular forms, despite inducing increased production of nitric oxide (NO). Importantly, the effect of IFNγ on T. cruzi uptake and growth is completely blocked by the anti-tumor necrosis factor (TNF) antibody Infliximab and partially blocked by the inhibitor of nitric oxide synthesis L-NAME. These data support that IFNγ fuels astrocyte infection by T. cruzi and critically implicate IFNγ-stimulated T. cruzi-infected astrocytes as sources of TNF and NO, which may contribute to parasite persistence and CNS pathology in CD.

Electron microscopic features of brain edema in rodent cerebral malaria in relation to glial fibrillary acidic protein expression

The mechanisms leading to cerebral malaria (CM) are not completely understood. Brain edema has been suggested as having an important role in experimental CM. In this study, CBA/CaH mice were infected with Plasmodium berghei ANKA blood-stage and when typical symptoms of CM developed on day 7, brain tissues were processed for electron-microscopic and immunohistochemical studies. The study demonstrated ultrastructural hallmarks of cerebral edema by perivascular edema and astroglial dilatation confirming existing evidence of vasogenic and cytogenic edema. This correlates closely with the clinical features of CM. An adaptive response of astrocytic activity, represented by increasing glial fibrillary acidic protein (GFAP) expression in the perivascular area and increasing numbers of large astrocyte clusters were predominately found in the CM mice. The presence of multivesicular and lamellar bodies indicates the severity of cerebral damage in experimental CM. Congestion of the microvessels with occluded white blood cells (WBCs), parasitized red blood cells (PRBCs) and platelets is also a crucial covariate role for CM pathogenesis.

In vitro study of the effects of Angiostrongylus cantonensis larvae extracts on apoptosis and dysfunction in the blood-brain barrier (BBB)

It has been hypothesized that blood-brain barrier (BBB) dysfunction in Angiostrongylus cantonensis infection might be due to the apoptosis of the hosts' BBB cells. Here, we evaluated this hypothesis through several methods, all based on an in vitro mouse BBB model consisting of primary culture brain microvascular endothelial cells (BMECs) and brain astrocytic cells (BACs). In the present study, a four-hour percolation and HRP permeability experiment showed that A. cantonensis larvae extracts can increase the permeability of the BBB. Apoptosis among BMECs and BACs after exposure to larvae extracts was monitored by TUNEL and annexin-V-FITC/PI double staining. A. cantonensis larvae extracts were found to induce apoptosis in both BMECs and BACs. For this reason, we concluded that the induction of apoptosis might participate in the BBB dysfunction observed during angiostrongyliasis. Improved fundamental understanding of how A. cantonensis induces apoptosis may lead to new approaches to the treatment or prevention of this parasitic disease.

Quantitation of brain edema and localisation of aquaporin 4 expression in relation to susceptibility to experimental cerebral malaria

The pathogenic mechanisms underlying the occurrence of cerebral malaria (CM) are still incompletely understood but, clearly, cerebral complications may result from concomitant microvessel obstruction and inflammation. The extent to which brain edema contributes to pathology has not been investigated. Using the model of P. berghei ANKA infection, we compared brain microvessel morphology of CM-susceptible and CM-resistant mice. By quantitative planimetry, we provide evidence that CM is characterized by enlarged perivascular spaces (PVS). We show a dramatic aquaporin 4 (AQP4) upregulation, selectively at the level of astrocytic foot processes, in both CM and non-CM disease, but significantly more pronounced in mice with malarial-induced neurological syndrome. This suggests that a threshold of AQP4 expression is needed to lead to neurovascular pathology, a view that is supported by significantly higher levels in mice with clinically overt CM. Numbers of intravascular leukocytes significantly correlated with both PVS enlargement and AQP4 overexpression. Thus, brain edema could be a contributing factor in CM pathogenesis and AQP4, specifically in its astrocytic location, a key molecule in this mechanism. Since experimental CM is associated with substantial brain edema, it models paediatric CM better than the adult syndrome and it is tempting to evaluate AQP4 in the former context. If AQP4 changes are confirmed in human CM, it may represent a novel target for therapeutic intervention.

[In vitro co-cultivation of Toxoplasma gondii tachyzoites with rat brain astrocytes]

Purified astrocytes were cultured in plates. When astrocytes grew over 80% of the plate, tachyzoites of Toxoplasma gondii RH strain were added for co-culture. In the period of 0-72 h, change of the astrocytes and tachyzoites was observed after Giemsa staining. In 0-48 h, monodansylcadaverine (MDC) was used to study the action of autophagy in the process of tachyzoites invading astrocytes. At 1 h co-culture, tachyzoites had entered in astrocytes and the autophagosomes appeared. At 4 h, the autophagosomes increased pronouncedly. However, after 12 h, number of autophagosomes considerably decreased and damage of the cells occurred. 48 h later, autophagosomes disappeared and more astrocytes were destroyed. At 72 h most cells destroyed and tachyzoites were released. The result showed that autophagy is inhibited when the astrocytes were in vitro infected by tachyzoites.

The immune response in Taenia solium neurocysticercosis in pigs is associated with astrogliosis, axonal degeneration and altered blood-brain barrier permeability

Immunohistochemistry was used to examine the immuno-pathological changes and the extent of neuronal damage caused by either viable or dead Taenia solium cysticerci during porcine neurocysticercosis. Thirty pig brains with cerebral cysticercosis and 5 brains from T. solium free pigs were used in this study. Results revealed extensive astrogliosis, neuronal and mostly axonal damage in both early (grade I) and late (grades III and V) lesions as evidenced by an increased expression of glial fibrillary acidic protein (GFAP) and neurofilament protein (NFP). In many late lesions, astrocyte end-feet formed glial scars that surrounded the dead parasite. Rapid angiogenesis resulted in blood vessels lacking astrocyte end-feet suggesting loss of blood-brain barrier (BBB) hence allowing an influx of peripheral blood immune cells such as eosinophils, macrophages, CD3+ T cells, B lymphocytes and plasma cells into lesions. This study showed that porcine NCC was associated with severe nervous tissue damage, the host response of which is a collaborative effort between the local and peripheral immune responses comparable to that observed in human NCC. Results further implied that porcine NCC could be a useful model for understanding the course of NCC in human as well as provide useful information for therapeutic and/or immune strategies.

Effects of Toxoplasma gondii infection on the brain

Toxoplasma gondii, an intracellular protozoan parasite, can infect humans in 3 different ways: ingestion of tissue cysts, ingestion of oocysts, or congenital infection with tachyzoites. After proliferation of tachyzoites in various organs during the acute stage, the parasite forms cysts preferentially in the brain and establishes a chronic infection, which is a balance between host immunity and the parasite's evasion of the immune response. A variety of brain cells, including astrocytes and neurons, can be infected. In vitro studies using non-brain cells have demonstrated profound effects of the infection on gene expression of host cells, including molecules that promote the immune response and those involved in signal transduction pathways, suggesting that similar effects could occur in infected brain cells. Interferon-gamma is the essential mediator of the immune response to control T. gondii in the brain and to maintain the latency of chronic infection. Infection also induces the production of a variety of cytokines by microglia, astrocytes, and neurons, which promote or suppress inflammatory responses. The strain (genotype) of T. gondii, genetic factors of the host, and probably the route of infection and the stage (tachyzoite, cyst, or oocyst) of the parasite initiating infection all contribute to the establishment of a balance between the host and the parasite and affect the outcome of the infection.

Toxoplasma gondii in Human Astrocytes In Vitro: Interleukin (IL)-12 and IL-10 Do Not Influence Cystogenesis

Interleukin (IL)-12, IL-10, and interferon (IFN)-gamma are major cytokines involved in the immune response against Toxoplasma gondii. Nevertheless, the role of IL-12 and IL-10 in the control of parasite replication and cytogenesis is not known yet, whereas the importance of IFN-gamma is documented. Furthermore, it is of paramount importance to study the interaction between T. gondii and cells from the central nervous system, e.g., astrocytes. In this study, we report that IL-12 and IL-10 have no effect on penetration, replication, or cystogenesis of the T. gondii Prugniaud strain in human astrocytes in vitro and do not antagonize the role of IFN-gamma on cystogenesis.

Astroglial cells in primary culture: A valid model to study Neospora caninum infection in the CNS

The protozoan Neospora caninum has a veterinary importance because it causes abortion in cattle and neuromuscular alterations in dogs. We infected rat astrocytes, in vitro, with different concentrations of N. caninum. Astrocytes responded to infection by producing the pro-inflammatory cytokine TNF-alpha and the neurotoxic-free radical NO, 24 and 72 h post-infection. These data suggest that astrocytes, which are essential for brain function, are targets for the parasite and this represents a practical and valid model to study the effects of N. caninum on the CNS.

Neospora caninum: Infection induced IL-10 overexpression in rat astrocytes in vitro

The effect of Neospora caninum, a parasite that causes abortion and neuromuscular changes, has been investigated on a major population of neural cells, the astrocytes. Highly enriched astroglial primary cultures obtained from neonatal rats were infected after 21 days of culture. Astroglial reactivity, IL-10 and IFN-gamma expression, and cell viability (lactate dehydrogenase activity, metabolization of tetrazolium salt, and trypan blue exclusion assay) have been investigated after 24 and 72 h of infection. Astroglial hypertrophy, gliofilament reorganization, metabolic changes suggesting hypoxia and a strong IL-10 release have been observed in the infected cells. These results show that neural cells are targets for the parasite and that astrocytes may contribute to the CNS immune response to the parasite.

Disruption of Toxoplasma gondii parasitophorous vacuoles by the mouse p47-resistance GTPases

The p47 GTPases are essential for interferon-γ-induced cell-autonomous immunity against the protozoan parasite, Toxoplasma gondii, in mice, but the mechanism of resistance is poorly understood. We show that the p47 GTPases, including IIGP1, accumulate at vacuoles containing T. gondii. The accumulation is GTP-dependent and requires live parasites. Vacuolar IIGP1 accumulations undergo a maturation-like process accompanied by vesiculation of the parasitophorous vacuole membrane. This culminates in disruption of the parasitophorous vacuole and finally of the parasite itself. Over-expression of IIGP1 leads to accelerated vacuolar disruption whereas a dominant negative form of IIGP1 interferes with interferon-γ-mediated killing of intracellular parasites. Targeted deletion of the IIGP1 gene results in partial loss of the IFN-γ-mediated T. gondii growth restriction in mouse astrocytes.

Toxoplasma gondii is a small unicellular parasite infecting virtually every warm-blooded animal including humans. After infection, T. gondii does not stay in extracellular fluids such as the blood, but actively invades body cells. The parasite has developed elaborate mechanisms enabling it to form a so-called parasitophorous vacuole (PV) within the cell it invades. Within this vacuole the parasite multiplies until the host cell ruptures and the progeny are released into the extracellular space to infect further cells. Host cells have developed several special mechanisms to combat the parasite. In mice, these mechanisms include a protein family, the p47 GTPases, which are induced by immune-alert factors called interferons. This study begins to address how the mouse p47 GTPases function. The study shows that the p47 GTPases assemble on the PV very shortly after infection, apparently to form a “membrane attack complex.” Within an hour the PV membrane shows signs of damage, bulging into small out-foldings that separate from the membrane in small vesicles. Shortly afterward the PV membrane ruptures and the parasite deteriorates. The p47 GTPase have several properties in common with the dynamin GTPases, which deform cellular membranes, suggesting that the p47 GTPases function in a mechanistically similar manner.

Phosphatidylcholine-specific phospholipase C but not gamma interferon regulate gene expression and secretion of CC Chemokine Ligand-2 (CCL-2) by human astrocytes during infection by Toxoplasma gondii

We have used human astrocytoma-derived cells to investigate the cellular responses of central nervous system cells to Toxoplasma gondii infection. At 24 h post inoculation, the secretion of CCL-2 (or Monocyte Chemotactic Protein-1) was augmented six-fold over the control. This secretion was down-regulated by D609, a specific inhibitor of phosphatidylcholine-dependent phospholipase C (PC-PLC), but not modulated by gamma interferon (IFN-gamma). Ribonuclease protection assay analyses showed significant down-regulation of CCL-2 mRNA production during infection by Toxoplasma gondii when cells were treated by D609. The mRNA levels of the seven other chemokines studied were not modified by D609. CCL-2 seems to contribute to the cell recruitment during human cerebral reactivation of Toxoplasma gondii. Cellular production of this CC chemokine during toxoplasmosis may be regulated by a PC-PLC-dependent pathway.

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.

Infection of human astrocytes and glioblastoma cells with Toxoplasma gondii: monocyte chemotactic protein-1 secretion and chemokine expression in vitro

In immunocompromised hosts, disruption of toxoplasmic cysts and conversion from bradyzoites to tachyzoites occur in brain. In these areas, infiltrates of mononuclear cells are observed. In the murine toxoplasmosis model, recent data suggest that chemokines may play a role in leukocyte recruitment in the central nervous system (CNS). This study analyzed the monocyte chemotactic protein-1 (MCP-1) secretion and chemokine expression after Toxoplasma gondii infection of human astrocytes, glioblastoma cells (U373) and fibroblasts (MRC5) in vitro. T. gondii infection of these CNS cells, astrocytes and glioblastoma cells significantly increased MCP-1 secretion, particularly for astrocytes. In our cellular models, the pattern of chemokine gene expression is dominated by MCP-1 expression. MCP-1 mRNAs were also quantified by real-time-PCR (LightCycler). The behavior of cells studied after T. gondii infection was different (invasion and growth) and the cell mechanisms of chemokine regulation could be dependent on the type of cells infected, while MCP-1 may contribute to the cell recruitment during human cerebral reactivation of T. gondii.

Enhancement of antimicrobial effects by glucocorticoids

In the past few years a body of evidence has accumulated showing that stimulation of human astrocytes and microvascular endothelial cells with IFN-gamma induces a potent antibacterial and anti-parasitic effect. We have found that the IFN-gamma-mediated activation of the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO) is, at least in part, responsible for this antimicrobial activity. Glucocorticoids are frequently used in inflammatory central nervous system diseases to reduce the inflammatory reaction and cerebral edema. Since in many inflammatory conditions infection is either a primary or secondary factor, steroids are administered, in these circumstances, during infection. We investigated whether steroids could affect the antimicrobial effect of IFN-gamma-induced IDO activation. We found that hydrocortisone and dexamethasone enhance IFN-gamma-mediated IDO activity in both human astrocytoma cells and native human astrocytes. Furthermore, we found that the amounts of IDO mRNA and of IDO protein are enhanced in cells treated with IFN-gamma and glucocorticoids. In addition, we were able to demonstrate that both steroids enhance the IFN-gamma-mediated antimicrobial activity against Toxoplasma gondii, Staphylococcus aureus and group B streptococci. The enhanced antimicrobial effect of IFN-gamma in the presence of glucocorticoids is due to the enhancement of the IDO-mediated tryptophan degradation, demonstrated by the complete abrogation of this antimicrobial effect by tryptophan resupplementation. These data show that glucocorticoids, which were often used to inhibit proinflammatory processes, do not decrease IDO-mediated antimicrobial effects. In contrast, high doses of steroids were able to enhance the IFN-gamma-induced antimicrobial activity.

In vitro multiplication of Toxoplasma gondii and Trypanosoma cruzi in mouse, rat, and hamster astrocytes

The infection and multiplication of Toxoplasma gondii and Trypanosoma cruzi were compared in primary cultures of white rat, mouse and hamster astrocytes. These cells were cultured on cover slides and infected with T. gondii tachyzoites or T. cruzi blood trypomastigotes. Results show that hamster astrocytes are more susceptible to the multiplication of both parasites than rat and mouse cells. There was no statistical difference between the T. gondii infection in rat and mouse astrocytes (p < 0.05), and this suggests an important role of other mechanisms or cells in the white rat natural resistance to this parasite. Because the hamster astrocytes are less resistant to these parasites multiplication and not necessarily to the invasion, any difference observed could be due to an intracellular effect: hamster brain astrocytes favor survival and multiplication of these parasites.

Soluble factors released by Toxoplasma gondii-infected astrocytes down-modulate nitric oxide production by gamma interferon-activated microglia and prevent neuronal degeneration

The maintenance of a benign chronic Toxoplasma gondii infection is mainly dependent on the persistent presence of gamma interferon (IFN-gamma) in the central nervous system (CNS). However, IFN-gamma-activated microglia are paradoxically involved in parasitism control and in tissue damage during a broad range of CNS pathologies. In this way, nitric oxide (NO), the main toxic metabolite produced by IFN-gamma-activated microglia, may cause neuronal injury during T. gondii infection. Despite the potential NO toxicity, neurodegeneration is not a common finding during chronic T. gondii infection. In this work, we describe a significant down-modulation of NO production by IFN-gamma-activated microglia in the presence of conditioned medium of T. gondii-infected astrocytes (CMi). The inhibition of NO production was paralleled with recovery of neurite outgrowth when neurons were cocultured with IFN-gamma-activated microglia in the presence of CMi. Moreover, the modulation of NO secretion and the neuroprotective effect were shown to be dependent on prostaglandin E(2) (PGE(2)) production by T. gondii-infected astrocytes and autocrine secretion of interleukin-10 (IL-10) by microglia. These events were partially eliminated when infected astrocytes were treated with aspirin and cocultures were treated with anti-IL-10 neutralizing antibodies and RP-8-Br cyclic AMP (cAMP), a protein kinase A inhibitor. Further, the modulatory effects of CMi were mimicked by the presence of exogenous PGE(2) and by forskolin, an adenylate cyclase activator. Altogether, these data point to a T. gondii-triggered regulatory mechanism involving PGE(2) secretion by astrocytes and cAMP-dependent IL-10 secretion by microglia. This may reduce host tissue inflammation, thus avoiding neuron damage during an established Th1 protective immune response.

Role of IDO activation in anti-microbial defense in human native astrocytes

The most serious complication of human toxoplasmosis is the development of toxoplasmic encephalitis. It is well established that in the brain Toxoplasma gondii is able to replicate in microglial cells, astrocytes and neurons, and that all three cell types can harbor toxoplasma cysts. The role of astrocytes in the defense against toxoplasma is not clear. The most prominent effector-mechanisms against toxoplasma are the induction of the inducible form of the nitric oxide synthase (iNOS), and the induction of indoleamine 2,3-dioxygenase (IDO). In this paper we show that interferon (IFN)-gamma-activated, native human astrocytes express IDO activity, as shown by the detection of IDO mRNA using RT-PCR, detection of enzyme expression with IDO-specific monoclonal antibodies in Western blots, as well as by direct measurement of enzyme activity in the activated cells. IFN-gamma-mediated IDO activity in human astrocytes inhibits the growth of Toxoplasma gondii and of group B streptococci. Furthermore, we show for the first time that IFN-gamma induced IDO activity is also effective in inhibiting the growth of Herpes Simplex Virus in astrocyte cultures. In addition, iNOS expression was detectable by RT-PCR in all batches of astrocytes tested when stimulated with a cytokine cocktail of IFN-gamma, TNF-alpha, IL-1 and LPS. Furthermore, we found that the amount of nitric oxide produced by astrocytes is not sufficient to inhibit either toxoplasmal or bacterial growth. Co-activation of iNOS and IDO on the other hand, results in an inhibition of IDO activity in astrocytes.

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.

The human nervous tissue in proximity to granulomatous lesions induced by Taenia solium metacestodes displays an active response

In neurocysticercosis, the nervous tissue surrounding the brain lesion is affected as a consequence of the local immune response induced by a Taenia solium metacestode. In this study, a histological and immunohistochemical analysis of five brain specimens from patients with neurocysticercosis revealed a proinflammatory activity reflected by an apparently altered blood-brain barrier permeability, secretion of pro-inflammatory cytokines, and up-regulation of molecules associated with antigen presentation. There were also anti-inflammatory cytokines, as well as an active wound-healing process reflected by angiogenesis, collagen deposition and glial scar formation. This immune response displayed by the nervous tissue adjacent to chronic neurocysticercosis lesions appeared to be contributing to the local tissue damage, and hence, may be fundamental in the pathology of NCC.

Chemokines are differentially expressed by astrocytes, microglia and inflammatory leukocytes in Toxoplasma encephalitis and critically regulated by interferon-gamma

The intracerebral formation of inflammatory infiltrates is a complex process, which may be regulated by chemokines. This study defines the kinetics and cellular sources of T cell- and macrophage-attracting chemokines in murine Toxoplasma encephalitis (TE) by ribonuclease protection assay, reverse transcription-PCR, in situ hybridization, and immunohistochemistry. Whereas astrocytes were the major source of interferon (IFN)-gamma-inducible protein-10 (CRG-2/IP-10) and monocyte chemoattractant protein (MCP)-1, microglia expressed RANTES, monokine induced by IFN-gamma (MuMIG) and occasionally CRG-2/IP-10 RNA. Despite being ubiquitously activated, only astrocytes and microglia confined to inflammatory infiltrates expressed chemokine genes. Intracerebral leukocytes transcribed RANTES, MuMIG, and occasionally CRG-2/IP-10 and MCP-1. IFN-gamma-deficient mice failed to produce CRG-2/IP-10, MuMIG, RANTES and expressed macrophage inflammatory protein (MIP-1)alpha, MIP-1 beta, and MCP-1 mRNA at reduced levels, functionally resulting in a strongly reduced recruitment of leukocytes across the blood-brain barrier and prevented their further invasion of the brain parenchyma. Since T cells are the single source of IFN-gamma in TE, these findings indicate that T cells pave the way of leukocytes to parenchymatous parasites via IFN-gamma.

Gamma interferon-induced inhibition of Toxoplasma gondii in astrocytes is mediated by IGTP

Toxoplasma gondii is an important pathogen in the central nervous system, causing a severe and often fatal encephalitis in patients with AIDS. Gamma interferon (IFN-gamma) is the main cytokine preventing reactivation of Toxoplasma encephalitis in the brain. Microglia are important IFN-gamma-activated effector cells controlling the growth of T. gondii in the brain via a nitric oxide (NO)-mediated mechanism. IFN-gamma can also activate astrocytes to inhibit the growth of T. gondii. Previous studies found that the mechanism in murine astrocytes is independent of NO and all other known anti-Toxoplasma mechanisms. In this study we investigated the role of IGTP, a recently identified IFN-gamma-regulated gene, in IFN-gamma inhibition of T. gondii in murine astrocytes. Primary astrocytes were cultivated from IGTP-deficient mice, treated with IFN-gamma, and then tested for anti-Toxoplasma activity. In wild-type astrocytes T. gondii growth was significantly inhibited by IFN-gamma, whereas in astrocytes from IGTP-deficient mice IFN-gamma did not cause a significant inhibition of growth. Immunoblot analysis confirmed that IFN-gamma induced significant levels of IGTP in wild-type murine astrocytes within 24 h. These results indicate that IGTP plays a central role in the IFN-gamma-induced inhibition of T. gondii in murine astrocytes.

Mouse model for Chagas disease: immunohistochemical distribution of different stages of Trypanosoma cruzi in tissues throughout infection

Different stages of Trypanosoma cruzi are seen during mammalian infection. Histologic sections of infected hearts have shown amastigotes and, when using immunohistochemistry (IHC), parasite antigens; however, demonstration of trypomastigotes in these tissues has proven elusive. Using a mouse strain that develops chagasic cardiomyopathy (histologically similar to human infection) 70 days after injecting T. cruzi-Brazil strain, we studied the distribution of parasite stages and the extent of inflammation. All organs had varying amounts of mononuclear inflammation by day 10, which peaked between day 20 and day 30, and decreased by day 50. Amastigotes were detected in myocytes, histiocytes, acinar pancreatic cells, astrocytes and ependymal cells by day 10, and the number of amastigotes peaked on day 30. Immunohistochemistry demonstrated trypomastigotes in sinusoids, vessels and interstitial tissues of several organs between day 15 and 50. Abundant parasite antigens (granular staining) were detected in connective tissues throughout the infection. The burden of amastigotes and trypomastigotes during the acute phase seems to correlate with the degree of inflammation and granular staining in the chronic stage.

Trypanosoma cruzi infection and the rat central nervous system: proliferation of parasites in astrocytes and the brain reaction to parasitism

Chagas' disease, caused by the protozoan Trypanosoma cruzi, is characterized by an acute phase in which parasites circulate in the blood and proliferate in several cell types, especially muscle cells. A life-long chronic phase follows the acute phase. In young patients, the acute phase is more severe, and meningoencephalitis frequently occurs in children before 2 years of age. Parasites have been rarely observed in neurons but their presence inside glial cells has been reported without characterization of the glial cell type. The cells involved in the brain reaction to the parasites and the time course of this reaction remain to be studied. Therefore, using suckling and juvenile rats and different T. cruzi populations, we aimed at determining the brain target for parasite proliferation and the cells involved in the brain reaction. Around the middle of the acute phase, histological and ultrastructural findings indicated that T. cruzi proliferates in astrocytes, forming nests devoid of enclosing membrane as described for non-glial cells. The brain nodular reaction comprised astrocytes, microglia, macrophages and neutrophils. Resting microglia was devoid of parasites in contrast to macrophages and neutrophils that probably participate in parasite removal. Suckling animals were significantly more affected, the numbers of nests and nodules varying with inoculum size. Histoquantitative analysis showed higher number of nests at the parasitemic peak (day 13) and drastic fall at day 20 post-inoculation. The highest number of nodules occurred at day 20 with drastic reduction at day 30. Recovery from histopathological alterations occurred even in surviving younger animals.

Investigation into the mechanism of gamma interferon-mediated inhibition of Toxoplasma gondii in murine astrocytes

Toxoplasma gondii is an obligate intracellular parasite that is a common opportunistic pathogen of the central nervous system in AIDS patients. Gamma interferon (IFN-gamma) alone or in combination with interleukin-1 (IL-1), IL-6, or tumor necrosis factor alpha significantly inhibits the growth of T. gondii in murine astrocytes, suggesting these are important nonimmune effector cells in the brain. Inhibition was found to be independent of a nitric oxide-mediated or tryptophan starvation mechanism. Both reactive oxygen intermediates and iron deprivation are IFN-gamma-mediated mechanisms known to operate against intracellular parasites in other cell types. Astrocytes generated from mice genetically deficient in the production of reactive oxygen intermediates (phox(-/-) mice) were found to inhibit growth of T. gondii when stimulated with IFN-gamma alone or in combination with other cytokines. The reactive oxygen inhibitor catalase and the reactive oxygen scavengers mannitol and thiourea failed to reverse the IFN-gamma-induced inhibition of T. gondii in astrocytes. These data indicate that IFN-gamma-induced inhibition in astrocytes is independent of reactive oxygen intermediates. IFN-gamma-induced inhibition could not be reversed by the addition of iron salts, ferric citrate, ferric nitrate, or ferric transferrin. Pretreatment of astrocytes with desferrioxamine also did not induce the inhibition of T. gondii. These data indicate that the mechanism of IFN-gamma inhibition was not due to iron deprivation. IFN-gamma had no effect on T. gondii invasion of astrocytes, but inhibition of growth and loss of tachyzoite vacuoles were evident in IFN-gamma-treated astrocytes by 24 h after invasion. Overall, these data suggest that IFN-gamma-activated astrocytes inhibit T. gondii by an as-yet-unknown mechanism.

Differential development of Toxoplasma gondii in neural cells

In this article, Remi Fagard and colleagues discuss the properties of neurons that lead to their low infection by Toxoplasma gondii, and the role that cytokines such as tumour necrosis factor alpha (TNF-alpha) and interferon gamma (IFN-gamma) might play.

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

The central nervous system (CNS) of the intermediate host plays a central role in the lifelong persistence of Toxoplasma gondii as well as in the pathogenesis of congenital toxoplasmosis and reactivated infection in immunocompromised patients. In order to analyze the parasite-host interaction within the CNS, the host cell invasion, the intracellular replication, and the stage conversion from tachyzoites to bradyzoites was investigated in mixed cultures of dissociated CNS cells from cortices of Wistar rat embryos. Two days post infection (p.i.) with T. gondii tachyzoites, intracellular parasites were detected within neurons, astrocytes, and microglial cells as assessed by double immunofluorescence and confocal microscopy. Quantitative analyses revealed that approximately 10% of neurons and astrocytes were infected with T. gondii, while 30% of the microglial cells harbored intracellular parasites. However, the replication of T. gondii within microglial cells was considerably diminished, since 93% of the parasitophorous vacuoles (PV) contained only one to two parasites which often appeared degenerated. This toxoplasmacidal activity was not abrogated after treatment with NO synthase inhibitors or neutralization of IFN-gamma production. In contrast, 30% of the PV in neurons and astrocytes harbored clearly proliferating parasites with at least four to eight parasites per vacuole. Four days p.i. with tachyzoites of T. gondii, bradyzoites were detected within neurons, astrocytes, and microglial cells of untreated cell cultures. However, the majority of bradyzoite-containing vacuoles were located in neurons. Spontaneous differentiation to the bradyzoite stage was not inhibited after addition of NO synthase inhibitors or neutralization of IFN-gamma. In conclusion, our results indicate that intracerebral replication of T. gondii as well as spontaneous conversion from the tachyzoite to the bradyzoite stage is sustained predominantly by neurons and astrocytes, whereas microglial cells may effectively inhibit parasitic growth within the CNS.

Effector cells of both nonhemopoietic and hemopoietic origin are required for interferon (IFN)-gamma- and tumor necrosis factor (TNF)-alpha-dependent host resistance to the intracellular pathogen, Toxoplasma gondii

Although interferon (IFN)-gamma-activated, mononuclear phagocytes are considered to be the major effectors of resistance to intracellular pathogens, it is unclear how they control the growth of microorganisms that reside in nonhemopoietic cells. Pathogens within such cells may be killed by metabolites secreted by activated macrophages or, alternatively, directly controlled by cytokine-induced microbicidal mechanisms triggered within infected nonphagocytic cells. To distinguish between these two basic mechanisms of cell-mediated immunity, reciprocal bone marrow chimeras were constructed between wild-type and IFN-gamma receptor-deficient mice and their survival assessed following infection with Toxoplasma gondii, a protozoan parasite that invades both hemopoietic and nonhemopoietic cell lineages. Resistance to acute and persistent infection was displayed only by animals in which IFN-gamma receptors were expressed in both cellular compartments. Parallel chimera experiments performed with tumor necrosis factor (TNF) receptor-deficient mice also indicated a codependence on hemopoietic and nonhemopoietic lineages for optimal control of the parasite. In contrast, in mice chimeric for inducible nitric oxide synthase (iNOS), an enzyme associated with IFN-gamma-induced macrophage microbicidal activity, expression by cells of hemopoietic origin was sufficient for host resistance. Together, these findings suggest that, in concert with bone marrow-derived effectors, nonhemopoietic cells can directly mediate, in the absence of endogenous iNOS, IFN-gamma- and TNF-alpha-dependent host resistance to intracellular infection.

Effect of cytokines on growth of Toxoplasma gondii in murine astrocytes

Cytokines play a significant role in the regulation of Toxoplasma gondii in the central nervous system. Cytokine-activated microglia are important host defense cells in central nervous system infections. Recent evidence indicates that astrocytes can also be activated by cytokines to inhibit intracellular pathogens. In this study, we examined the effect of gamma interferon (IFN-gamma), tumor necrosis factor alpha (TNF-alpha), interleukin-6 (IL-6), and IL-1 on the growth of T. gondii in a primary murine astrocyte culture. Pretreatment of astrocytes with IFN-gamma resulted in 65% inhibition of T. gondii growth. Neither TNF-alpha, IL-1, nor IL-6 alone had any effect on T. gondii growth. IFN-gamma in combination with either TNF-alpha, IL-1, or IL-6 caused a 75 to 80% inhibition of growth. While nitric oxide was produced by astrocytes treated with these cytokines, inhibition of T. gondii growth was not reversed by the addition of the nitric oxide synthase inhibitor NG-monomethyl-L-arginine. Furthermore, IFN-gamma in combination with IL-1, IL-6, or TNF-alpha also induced inhibition in astrocytes derived from syngeneic mice deficient in the enzyme inducible nitric oxide synthase. This finding suggests that the mechanism of cytokine inhibition is not nitric oxide mediated. Similarly, the addition of tryptophan had no effect on inhibition, indicating that the mechanism was not mediated via induction of the enzyme indoleamine 2, 3-dioxygenase. The mechanism of inhibition remains to be elucidated. Results from this study demonstrate that cytokine-activated astrocytes are capable of significantly inhibiting the growth of T. gondii. These data indicate that astrocytes may be important host defense cells in controlling toxoplasmosis in the brain.

Association of host cell intermediate filaments with Toxoplasma gondii cysts in murine astrocytes in vitro

Toxoplasma gondii is an intracellular parasite that is a common opportunistic infection of AIDS patients where it causes a severe and often fatal encephalitis. Toxoplasmic encephalitis in AIDS patients results from a reactivation of the cyst stage of Toxoplasma gondii in the brain. A previous study found an association of host cell intermediate filaments with parasitophorous vacuoles and some studies have suggested the host cell cytoskeletal elements are incorporated into the cyst wall. In this study, the interaction of glial filaments with Toxoplasma gondii cysts was studied in cysts derived in vitro in mouse astrocytes and in cysts isolated from mouse brains. Glial filaments, detected by immunostaining of the glial fibrillary acidic protein, were found to accumulate around the perimeter of the cysts as they developed in mouse astrocytes. Transmission electron microscopy revealed a layer of glial filaments was wrapped around the cytoplasmic side of the cyst. The glial filaments were present in close apposition to the cyst wall and arranged around the cysts in a concentric layer, measuring 5-10 microns in thickness. The layer of glial filaments excluded host cell mitochondria and endoplasmic reticulum from the cytoplasmic surface of the cyst. Colocalisation of glial fibrillary acidic protein and the cyst wall via confocal and immunoelectron microscopy, confirmed that there was no glial fibrillary acidic protein present within the cyst wall. The cyst wall of cysts isolated from mouse brains were also found to be negative for glial fibrillary acidic protein. In conclusion, we found no evidence of structural integration of the host cell intermediate filaments in the cyst wall, but glial filaments were found to encase the cysts in the host cell during cyst development in host cells in vitro. The glial filaments wrapping of cysts may play a role in bradyzoite differentiation and/or cyst stabilisation in the host cell cytoplasm.

Gap junction disappearance in astrocytes and leptomeningeal cells as a consequence of protozoan infection

Trypanosoma cruzi and Toxoplasma gondii are protozoan parasites capable of causing infections of the nervous system. In order to determine effects of infection by these organisms on intercellular communication in the brain, dye coupling and connexin abundance and distribution were examined in leptomeningeal cells and astrocytes infected with T. cruzi or T. gondii. For both cell types infected with either type of protozoan parasite, intercellular diffusion of intracellularly injected Lucifer Yellow was dramatically reduced. Immunocytochemistry with antibodies specific for connexin43 (in astrocytes) or both connexin43 and connexin26 (for leptomeningeal cells) demonstrated that punctate gap junctional staining was much reduced in infected cells, although uninfected neighbors could display normal connexin abundance and distribution. Western blot analyses revealed that connexin43 abundance in both cell types infected with either parasite was similar to that in uninfected cells. Phosphorylation state of connexin43 (inferred from electrophoretic mobility of connexin43 isoforms) was not significantly affected by the infection process. Immunocytochemistry of whole brains from animals acutely infected with either parasite also showed a marked reduction in connexin43 expression. We conclude that infection of both types of brain cells with either protozoan parasite results in a loss of intercellular communication and organized gap junction plaques without affecting expression levels or posttranslational processing of gap junction proteins. Presumably, these changes in gap junction distribution result from altered targeting of the junctional protein to the plasma membrane, and/or from changes in assembly of subunits into functional channels.

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.

Cellular immune reactions directed against Toxoplasma gondii with special emphasis on the central nervous system

Toxoplasma gondii is an obligate intracellular parasite which, after primary infection of humans, is maintained in a dormant state by the host cellular immune system. In the event of an acquired immunosuppression, those parasites surviving as dormant cysts in the host may undergo a change in status, proliferate and cause a life-threatening toxoplasmic encephalitis. Over the last decade much knowledge has accumulated concerning the immune response against T. gondii. This review focuses attention particularly on the anti-parasitic effector mechanisms and the cellular immune reactions in the central nervous system during the course of reactivated toxoplasmic encephalitis.

Compromised blood-nerve barrier, astrogliosis, and myelin disruption in optic nerves during fatal murine cerebral malaria

We examined the optic nerve, as an analogous tissue to brain white matter, to assess possible relationships between changes in the blood-nerve barrier, axonal integrity, and astrocyte morphology in the central nervous system during fatal murine cerebral malaria (FMCM). In the FMCM model, namely, CBA mice infected with Plasmodium berghei ANKA, neurological symptoms begin around day 5 post-inoculation (p.i.) and mice become increasingly ill by day 7 p.i., at which time they lapse into coma and die. Using intravascular perfusion with horseradish peroxidase combined with light and electron microscopy, and GFAP immunohistochemistry, the optic nerves in malaria-infected mice were found to display i) breakdown of the blood-nerve barrier, detectable as early as day 3 p.i. (about 2 days before the onset of neurological symptoms) increasing to peak severity by day 7 p.i.; ii) monocytosis, vascular congestion, and monocyte adherence to the endothelium in the microvasculature during the later stages of the disease process; iii) an increased incidence of patchy axonal demyelination and degeneration, mostly associated with vascular changes and astrogliosis, beginning at day 5 p.i. and more evident by day 7 p.i.; and iv) an increased intensity of GFAP immunostaining, detectable from day 3 p.i. and peaking at day 7 p.i. These optic nerve changes were always seen in the infected individuals, though they varied in intensity. The temporal and anatomical coincidence between the compromised blood-nerve barrier, monocyte adherence to the vascular endothelium, astrocyte changes, neuronal degeneration, and demyelination in the optic nerve in FMCM suggests that these factors are mechanistically inter-related. These findings are consistent with the proposed immunopathological nature of FMCM and provide further evidence for the pivotal role of the CNS microvasculature in the disease process. This is the first investigation of involvement of the optic nerve in FMCM and the first demonstration, to our knowledge, of loss of axonal viability in this condition in any CNS tissue. The observed demyelination is consistent with reports by other workers on such changes in the brain in human cerebral malaria.

Growth and development of Toxoplasma gondii in human neurons and astrocytes

Toxoplasma gondii (T. gondii) is one of the most common opportunistic infections affecting the central nervous system (CNS) in AIDS patients. Disease results from a reactivation of a latent infection in the brain resulting in a severe and necrotizing encephalitis. In this study we infected a primary culture from human fetal brain with T. gondii and studied the behavior of both the active and latent stages in this culture system. We found that the active (tachyzoite) stage of T. gondii can infect both astrocytes and neurons. However, a higher percentage of astrocytes were infected than neurons. Additionally, astrocytes were found to support more replication of T. gondii than did neurons. Both astrocytes and neurons also supported the cyst stage, found in the latent infections. These data indicate that astrocytes are the host cells supporting most of the replication of T. gondii in the brain in reactivated infections, but both host cell types may be able to support the cyst stage in latent infections. However, evidence indicates that cysts formed in astrocytes may be distinct from neuronal cysts. These findings may have relevance to reactivation of latent T. gondii infections in AIDS patients.

Influence of cytokines on Toxoplasma gondii growth in human astrocytoma-derived cells

The effects of interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha), interleukin 1-alpha (IL-1-alpha), and interleukin 6 (IL-6) on the growth of the Toxoplasma gondii RH strain were studied in vitro using a human astrocytoma-derived cell line. Cells were treated with cytokines at different concentrations at 24 h prior to infection with T. gondii tachyzoites. IFN-gamma did not induce any modification in T. gondii growth, whatever the concentration used. TNF-alpha induced a significant decrease in the total number of tachyzoites, whereas IL-1-alpha surprisingly induced an increase in the number of tachyzoites. Our results show that the effects of cytokines on T. gondii growth may be of great importance in the control of cerebral toxoplasmosis but that they can vary, depending on the cell type considered.

Production of prostaglandins D2 and E2 by mouse fibroblasts and astrocytes in culture caused by Trypanosoma brucei brucei products and endotoxin

A study was made to characterize the effects of living Trypanosoma brucei brucei and its products on prostaglandin D2 (PGD2) and PGE2 production by fibroblasts and astrocytes. Cultured fibroblasts were prepared from Microtus agrestis embryos and astrocyte cultures were prepared from neonatal rats. The cultures were maintained in low-endotoxin or defined media (i.e. endotoxin-free). The PG production was compared with and studied in combination with a defined lipopolysaccharide (LPS) from Escherichia coli. Living T. b. brucei were without effect on PG production. Preparations of T. b. brucei prepared by freeze-thawing and sonication produced dose- and time-dependent increases in PGD2 and PGE2 synthesis by both cell types. LPS caused a similar pattern of increases. The combination of parasite products with LPS caused synergistic production to levels higher than the maximal production by each mitogen alone. The findings have important implications for several pathological features that accompany trypanosomiasis.

Intracellular survival and multiplication of Toxoplasma gondii in astrocytes

Primary neonatal murine astrocyte cultures were used to investigate the role of these glial cells in host defense of the central nervous system (CNS) against Toxoplasma gondii. For comparison, neonatal murine microglial cells were also studied. Microscopic analyses revealed that uptake of T. gondii into astrocytes was parasite-driven and was followed by uniform intracellular survival and multiplication of tachyzoites. Treatment of astrocytes with interferon (IFN)-gamma and lipopolysaccharide (LPS) had no apparent effect on the survival or growth of T. gondii. Microglia, on the other hand, had both an intrinsic phagocytosis-associated antitoxoplasma activity and a nitric oxide-dependent inhibitory activity that was up-regulated by IFN-gamma and LPS. The results of this study suggest that in contrast to microglial cells, astrocytes may provide a safe harbor within the CNS for T. gondii.

Antitrichomonal action of emodin in mice

Emodin, an active component contained in the root and rhizome of Rheum palmatum L. (Polygonaceae), was found to have an inhibitory effect on the pathogenicity of Trichomonas vaginalis in mice. Emodin delayed the development of subcutaneous abscesses due to infection of this parasite. Also, it cures the intravaginal infection of trichomonads through oral administration. In cell cultures, it reduced the cytotoxic effect of this parasite towards mammalian cells. This inhibition was markedly reversed by the coexistence of free radical scavengers, indicating the possible mediation of free radicals.

Toxoplasma gondii: differential location of antigens secreted from encysted bradyzoites

Since we have previously demonstrated the protective role against infection played by Toxoplasma excreted-secreted antigens (Darcy et al. Parasite Immunology, 10, 553-567, 1988), the aim of the present work was an attempt to precisely define the location of GRA1, GRA2, and GRA5 in both the tachyzoite and the bradyzoite stages from distinct strains, in order to explore the mechanisms of secretion by Toxoplasma gondii. Three monoclonal antibodies (Charif et al. 1990) and colloidal immunogold labeling were used to localize the 27-, 28.5-, and 21-kDa target antigens to the matrix of the dense granules of tachyzoites and bradyzoites. They were, moreover, detected in the parasitophorous vacuole and in the cyst ground substance after host cell invasion. Our data suggest that a selective sorting mechanism for dense-granule contents exists at least in encysted bradyzoites. GRA2 was found preferentially associated with the ground substance of the cyst wall and the tubular elements of the network of the modified host cell phagosome, whereas GRA5 was located on the delimiting membrane of both the cyst wall and the parasitophorous vacuole. These observations reveal the selective targeting of dense-granule molecules, which could have different functions and fates when exocytosed into the parasite-containing vacuole.

Induction of toxoplasmostasis in a human glioblastoma by interferon gamma

In the course of human toxoplasmosis central nervous system involvement often occurs. As a model for toxoplasma growth within human brain cells the proliferation of Toxoplasma gondii strain BK within the human glioblastoma cell line 86HG39 was analysed. We found that 86HG39 cells support the growth of toxoplasma similar to human monocyte derived macrophages and in contrast to human monocytes. The growth of Toxoplasma gondii within interferon gamma (IFN gamma) treated 86HG39 cells is reduced due to toxoplasmostasis and not due to toxoplasmocide effects. The mechanism of IFN gamma induced toxoplasmostasis was also investigated. It was found that IFN gamma did not induce O2- production and/or nitrite oxide production, and inhibitors of O2- and NO2- did not influence IFN gamma induced toxoplasmostasis. In contrast, the supplementation of L-tryptophan to the culture medium completely abolished the IFN gamma effect. We therefore conclude that the induction of L-tryptophan degradation in 86HG39 cells by IFN gamma, possibly by activation of the indoleamine-2,3-dioxygenase, is responsible for the IFN gamma induced toxoplasmostasis within the glioblastoma cell line.

Neuropathology and host-parasite relationship of acute experimental toxoplasmosis of the blue fox (Alopex lagopus)

The neuropathology and host-parasite relationship of experimental infection with the RH-strain of Toxoplasma gondii were studied in 27 blue foxes (Alopex lagopus) aged 0 to 23 days, using light microscopy, including immunohistochemical staining, and transmission and scanning electron microscopy. All cases displayed multifocal necrotic lesions with numerous parasitic tachyzoites in the brain and spinal cord. The gray matter and the meninges were most seriously affected. Although a wide variety of cell types were parasitized, neurons and astrocytes seemed to be the main target cells. Individual parasitophorous vacuoles usually contained only a few tachyzoites, with rosette formations as a prominent feature. The present ultrastructural study supports the theory that the parasites actively invade the host cells by mechanisms that are different from those of phagocytosis. This is apparently the first report indicating that the formation of the network of tubular structures within the parasitophorous vacuole of T. gondii is associated with a transient, sack-like formation in the posterior end of the tachyzoites.

In vitro cultivation of Toxoplasma gondii cysts in astrocytes in the presence of gamma interferon

Long-term culturing of Toxoplasma gondii cysts was accomplished in vitro in association with murine astrocytes and intermittent additions of gamma interferon to the media. Phase-contrast microscopy was used to follow the stages of cyst development, and electron microscopy confirmed the presence of morphologic characteristics of T. gondii cysts. T. gondii cysts formed in vitro had a single trilaminar membrane during both intracellular and extracellular existence and contained amorphous electron-dense material either throughout the cyst or in a uniform layer under the trilaminar membrane. The bradyzoites were similar to previous descriptions of tachyzoites in vitro except that they were smaller and contained numerous electron-lucent vacuoles. Gamma interferon is not necessary for cyst formation, but it controls the division of tachyzoites and may allow cysts to remain for prolonged periods without rupturing. In vitro-cultivated T. gondii cysts will be useful for producing bradyzoite and cyst antigens and for measuring the effects of antimicrobial agents and immune modulators on the viability of intracystic T. gondii.