Mechanisms of killing of Toxoplasma gondii by rat peritoneal macrophages

Patterns of Toxoplasma gondii cyst distribution in the forebrain associate with individual variation in predator odor avoidance and anxiety-related behavior in male Long-Evans rats

Toxoplasma gondii (T. gondii) is one of the world's most successful brain parasites. T. gondii engages in parasite manipulation of host behavior and infection has been epidemiologically linked to numerous psychiatric disorders. Mechanisms by which T. gondii alters host behavior are not well understood, but neuroanatomical cyst presence and the localized host immune response to cysts are potential candidates. The aim of these studies was to test the hypothesis that T. gondii manipulation of specific host behaviors is dependent on neuroanatomical location of cysts in a time-dependent function post-infection. We examined neuroanatomical cyst distribution (53 forebrain regions) in infected rats after predator odor aversion behavior and anxiety-related behavior in the elevated plus maze and open field arena, across a 6-week time course. In addition, we examined evidence for microglial response to the parasite across the time course. Our findings demonstrate that while cysts are randomly distributed throughout the forebrain, individual variation in cyst localization, beginning 3 weeks post-infection, can explain individual variation in the effects of T. gondii on behavior. Additionally, not all infected rats develop cysts in the forebrain, and attenuation of predator odor aversion and changes in anxiety-related behavior are linked with cyst presence in specific forebrain areas. Finally, the immune response to cysts is striking. These data provide the foundation for testing hypotheses about proximate mechanisms by which T. gondii alters behavior in specific brain regions, including consequences of establishment of a homeostasis between T. gondii and the host immune response.

Differences in iNOS and arginase expression and activity in the macrophages of rats are responsible for the resistance against T. gondii infection

Toxoplasma gondii infects humans and warm blooded animals causing devastating disease worldwide. It has long been a mystery as to why the peritoneal macrophages of rats are naturally resistant to T. gondii infection while those of mice are not. Here, we report that high expression levels and activity of inducible nitric oxide synthase (iNOS) and low levels of arginase-1 (Arg 1) activity in the peritoneal macrophages of rats are responsible for their resistance against T. gondii infection, due to high nitric oxide and low polyamines within these cells. The opposite situation was observed in the peritoneal macrophages of mice. This discovery of the opposing functions of iNOS and Arg 1 in rodent peritoneal macrophages may lead to a better understanding of the resistance mechanisms of mammals, particularly humans and livestock, against T. gondii and other intracellular pathogens.

Toxoplasma gondii strains defective in oral transmission are also defective in developmental stage differentiation

Toxoplasma gondii undergoes differentiation from rapidly growing tachyzoites to slowly growing bradyzoites during its life cycle in the intermediate host, and conversion can be induced in vitro by stress. Representative strains of the three clonal lineages showed equal capacity to differentiate into bradyzoites in vitro, as evidenced by induction of bradyzoite antigen 1, staining with Dolichos biflorus lectin (DBL), pepsin resistance, and oral infectivity in mice. We also examined several recently described exotic strains of T. gondii, which are genetically diverse and have a different ancestry from the clonal lineages. The exotic strain COUG was essentially like the clonal lineages and showed a high capacity to induce bradyzoites in vitro and in vivo, consistent with its ability to be efficiently transmitted by the oral route. In contrast, exotic strains MAS and FOU, which are defective in oral transmission, showed a decreased potential to develop into bradyzoites in vitro. This defect was evident from reduced staining with DBL and the cyst antigen CST1, failure to down-regulate tachyzoite antigens, such as tachyzoite surface antigens 1 and 2A, and decreased resistance to pepsin treatment. Despite normal in vitro differentiation, the exotic strains CAST and GPHT also showed decreased oral transmission, due to formation of smaller cysts and a lower tissue burden during chronic infection, traits also shared by MAS and FOU. Collectively, these findings reveal that the limited oral transmission in some strains of T. gondii is due to inefficient differentiation to the bradyzoite form, leading to defects in the formation of tissue cysts.

The rat Toxo1 locus directs toxoplasmosis outcome and controls parasite proliferation and spreading by macrophage-dependent mechanisms

Toxoplasmosis is a healthcare problem in pregnant women and immunocompromised patients. Like humans, rats usually develop a subclinical chronic infection. LEW rats exhibit total resistance to Toxoplasma gondii infection, which is expressed in a dominant mode. A genome-wide search carried out in a cohort of F(2) progeny of susceptible BN and resistant LEW rats led to identify on chromosome 10 a major locus of control, which we called Toxo1. Using reciprocal BN and LEW lines congenic for chromosome 10 genomic regions from the other strain, Toxo1 was found to govern the issue of T. gondii infection whatever the remaining genome. Analyzes of rats characterized by genomic recombination within Toxo1, reduced the interval down to a 1.7-cM region syntenic to human 17p13. In vitro studies showed that the Toxo1-mediated refractoriness to T. gondii infection is associated with the ability of the macrophage to impede the proliferation of the parasite within the parasitophorous vacuole. In contrast, proliferation was observed in fibroblasts whatever the genomic origin of Toxo1. Furthermore, ex vivo studies indicate that macrophage controls parasitic infection spreading by a Toxo1-mediated mechanism. This forward genetics approach should ultimately unravel a major pathway of innate resistance to toxoplasmosis and possibly to other apicomplexan parasitic diseases.

Toxoplasma gondii: selective killing of extracellular parasites by oxidation using pyrrolidine dithiocarbamate

Extracellular Toxoplasma parasites are sensitive to pyrrolidine dithiocarbamate (PDTC) at low micromolar concentrations. Loss of parasite viability following PDTC treatment is shown to be mediated by oxidation, which is reminiscent of PDTC killing in mammalian cells. Intracellular parasites, by contrast, are resistant to PDTC killing, although treatment does cause reversible growth arrest. In addition to the possible implications relative to the biology of the parasite, these observations suggest that PDTC could be of use in eliminating undesired extracellular parasites during assays and selections in vitro.

The effect of cytokines on the replication of T. gondii within rat retinal vascular endothelial cells

Toxoplasma gondii infection of the eye can result in a recurrent necrotising retinochoroiditis (TR) which may lead to a permanent loss of visual acuity. The mechanisms responsible for the control of TR within the retina are unknown. The aim of this study was to examine the effects of cytokines on the replication of T. gondii RH strain tachyzoites within rat retinal vascular endothelial (rRVE) cells. Pretreatment of rRVE with IFNgamma, TNF or IL-1beta resulted in a significant decrease in T. gondii replication from day 2 onwards. There was no significant difference in nitric oxide (NO) production by IFNgamma, TNF or IL-1beta treated rRVE as compared to controls at any time point. By comparison, the addition of L-tryptophan to IFNgamma treated cultures significantly restored T. gondii replication from 48 h post inoculation. Thus, IFNgamma, TNF and IL-1beta can significantly inhibit the replication of T. gondii within rRVE. However, this inhibition appears to be independent of NO production. L-tryptophan catabolism may have a role in IFNgamma mediated inhibition of T. gondii replication in rRVE cells.

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.

Functional immaturity of rat alveolar macrophages during postnatal development

Alveolar macrophages (AM) are important in the regulation of immune responses in the lung, through their role as scavenger cells and through the production of many bioactive factors. Because in early infancy pulmonary infections are a recurrent problem, we studied the postnatal functional maturation of AM in a rat model. AM were isolated from rat lungs by bronchoalveolar lavage at several time intervals after birth and tested for their ability to ingest Escherichia coli in the presence of surfactant protein A (SP-A). Furthermore, their capacity to produce nitric oxide (NO) and interleukin-1 beta (IL-1 beta) after in vitro lipopolysaccharide (LPS) stimulation was analysed, as well as their capacity to downregulate proliferation of T cells from both mature and neonatal rats. SP-A-mediated phagocytosis of E. coli by AM was reduced in 14-day-old neonatal rats, as compared with mature rats (P < or = 0.05). Also the IL-1 beta production by rat AM after LPS stimulation was impaired at 14 days of age, as compared with IL-1 beta production by AM from mature rats (P < or = 0.05). In contrast, the LPS-induced NO production by rat AM as well as the capacity to inhibit T-cell proliferation were well developed at all ages tested. In conclusion, during postnatal development the rat AM is functionally immature, with respect to phagocytosis and secretion of inflammatory mediators. These differences may underly the enhanced susceptibility to pulmonary infections as found in human neonates.

Toxoplasmosis of rats: a review, with considerations of their value as an animal model and their possible role in epidemiology

We critically review and summarize information on the prevalence of Toxoplasma gondii infections in rats, mainly Rattus norvegicus, and their possible role as a source of infection for larger carnivores and omnivores. We also review information on immunology and natural resistance, contributing to the model value of rats in the analysis of human infection. Rats can be successfully infected with oocysts (sporozoites), tissue cysts (bradyzoites), and tachyzoites. Even adult rats, that are resistant to clinical toxoplasmosis, can be infected orally with a few oocysts or tissue cysts. Infections with tachyzoites of the RH strain are highly variable. Congenital transmission of T. gondii occurs at a high rate when rats are infected during pregnancy. Congenitally infected rats can harbor viable T. gondii in the absence of detectable antibodies to T. gondii and rats with low antibody titers may harbor few or no organisms. The isolation of viable T. gondii by bioassay is the only reliable means to determine persistence of chronic T. gondii infection in feral rats. No evidence was found for maintenance of T. gondii in rats by vertical transmission in the absence of cats.

Brain-tissue cysts in rats infected with the RH strain of Toxoplasma gondii

Virulent strains of the coccidian parasite Toxoplasma gondii become attenuated so as to survive and complete their life cycle; however, it is not known whether the attenuation process is attributable to an innate cystogenic capacity of the parasite or to host-induced mechanisms. This report presents direct evidence of RH cystogenesis in non-immunised Fischer rats and subsequent attenuation of RH pathogenicity in non-immunised mice following a single passage through rats. Taken together, these preliminary observations tend to suggest that at least one mechanism of T. gondii involves intermediate host attenuation.

Neutrophil chemotactic factors secreted from Toxoplasma gondii

Neutrophil chemotactic activity was detected in the fluid of Toxoplasma gondii cultures by the agarose plate and the Boyden chamber methods. Toxoplasma culture fluid was obtained by incubating the tachyzoites at 37 degrees C in a 5% CO2 atmosphere for 6 h in Dulbecco's modified Eagle's minimum essential medium containing 10% heat-inactivated foetal calf serum. Soluble extracts from tachyzoites had negligible activity, indicating that the chemotactic factors were metabolites secreted from tachyzoites. The chemotactic activity in Toxoplasma culture fluid depended on the number of tachyzoites and the period of incubation. The substances responsible for neutrophil chemotaxis were two heat-labile peptides with estimated Mr 4.5 and 14 kDa with N-terminal groups free.

Cytotoxic activity of monocytes against Toxoplasma gondii in acute, chronic and reactivated murine toxoplasmosis

The phagocytic activity and cytotoxicity of peripheral blood monocytes (against toxoplasma tachyzoites) was studied in acute, chronic and reactivated toxoplasma infected Swiss albino mice. During acute infection, a low phagocytic activity was observed on the 4th day post infection (dpi) (P less than 0.01) and a low monocyte cytotoxicity was noticed after the 2nd dpi (P less than 0.01) which further decreased till the 8th dpi. In contrast, both the parameters were significantly increased during chronic infection. Increase in monocytic cytotoxicity was manifested on the 3rd dpi (P less than 0.001) whereas phagocytosis showed an increase on the 12th dpi (P less than 0.05). The reactivated group showed no change in both the parameters when compared with the control immunosuppressed group (P greater than 0.05).

Nonoxidative microbicidal activity in normal human alveolar and peritoneal macrophages

Although Toxoplasma gondii multiplies within normal murine alveolar and peritoneal macrophages, it is killed by normal rat alveolar and peritoneal macrophages. The killing by rat macrophages is by a nonoxidative mechanism. Studies on normal human alveolar macrophages have reported disparate results in regard to their ability to inhibit or kill T. gondii. We considered it of interest to explore further the effect of normal human alveolar and peritoneal macrophages on T. gondii. Unstimulated alveolar macrophages from each of seven individuals demonstrated a marked ability to kill or inhibit multiplication of T. gondii in vitro (e.g., the number of parasites per 100 alveolar macrophages was 31 at time zero and 2 at 18 h, whereas this value increased from 37 at time zero to 183 at 18 h in murine macrophages assayed in parallel). In quantitative assays of superoxide, alveolar macrophages released a substantial amount of superoxide when exposed to phorbol myristate acetate or to candidae. In contrast, alveolar macrophages incubated with T. gondii released no more superoxide than when in medium alone. Scavengers of superoxide anions, hydrogen peroxide, singlet oxygen, and hydroxyl radicals failed to inhibit killing of T. gondii by alveolar macrophages. Peritoneal macrophages from each of six normal women undergoing laparoscopy killed T. gondii in vitro; results of quantitative superoxide assays and scavenger experiments demonstrated that no oxidative burst was triggered in these macrophages by exposure to T. gondii. These data indicate that normal human alveolar and peritoneal macrophages can kill an intracellular parasite by nonoxidative mechanisms and suggest that these mechanisms are important in inhibition or killing of other opportunistic intracellular pathogens.