Evidence for finely-regulated asynchronous growth of Toxoplasma gondii cysts based on data-driven model selection

Strain-specific pre-existing immunity: A key to understanding the role of chronic Toxoplasma infection in cognition and Alzheimer's diseases?

Toxoplasma exposure can elicit cellular and humoral immune responses. In the case of chronic Toxoplasma infection, these immune responses are long-lasting. Some studies suggest that pre-existing immunity from Toxoplasma infection can shape immune responses and resistance to other pathogens and brain insults later in life. Much evidence has been generated suggesting Toxoplasma infection may contribute to cognitive impairment in the elderly. However, there have also been studies that disagree with the conclusion. Toxoplasma has many strain types, with virulence being the most notable difference. There is also considerable variation in the outcomes following Toxoplasma exposure ranging from resolved to persistent infection. Therefore, the brain microenvironment, particularly cellular constituents, differs based on the infecting strain (virulent versus hypovirulent) and infection stage (resolved versus persistent). Such difference might play a critical role in determining the outcome of the host on subsequent challengings to the brain. The ability of Toxoplasma strains to set up distinct stages for neurodegenerative pathology through varying degrees of virulence provides unique experimental tools for characterizing these pathways.

Soluble total antigen derived from Toxoplasma gondii tachyzoites increased the expression levels of NLRP1, NLRP3, NLRC4, AIM2, and the release of mature form of IL1β, but downregulated the expression of IL1β and IL18 genes in THP-1cell line

Toxoplasma gondii (T. gondii) is an intracellular parasitic protozoan infecting homoeothermic animals and about a third of the world's population. Inflammasomes are intracellular multi-protein complex, which are activated by many factors. Inflammasomes are activated during toxoplasmosis; however, there are a lot of obscure aspects. THP-1 monocyte cells were converted to M0 macrophages by PMA and treated by 100 μg/mL soluble total Ag (STAg) derived from T. gondii strain RH for two time points 3 h and 24 h. After total RNA extraction and cDNA synthesis, the expression pattern of NLRP1, NLRP3, NLRC4, AIM2, IL1β, and IL18 was evaluated by relative real-time PCR. In addition, the cytokine release of IL1β and TNFα was evaluated in the supernatant of each well. The results showed statistically significant time-dependent overexpression of inflammasomes. NLRP1 and NLRP3 showed the higher and lower expression, respectively, during 3 h and 24 h after exposure. Both IL1β and IL18 downregulated 3 h after exposure. IL18 presented statistically significant upregulation after 24 h, but IL1β showed statistically significant downregulation after 24 h. The release of IL1β increased after 3 h, but it slightly decreased during 24 h after exposure. The concentration of TNFα showed an insignificant decrease compared to control, while it increased during 24 h after exposure. Taken together, this study suggested that T. gondii STAg induces NLRP1 more than NLRP3, NLRC4, and AIM2. Our findings also proposed that T. gondii STAg downregulates the gene expression of IL1β, but increases the release of this cytokine. It seems that Toxoplasma STAg probably increase the release of IL1β via activating NLRPs and AIM2 to cleave pro-caspase 1 to caspase 1 that leads to conversion of pro IL1β to mature IL1β.

Burden and regional distribution of Toxoplasma gondii cysts in the brain of COBB 500 broiler chickens following chronic infection with 76K strain

Toxoplasmosis is a worldwide zoonosis caused by the obligate intracellular apicomplexan parasite Toxoplasma gondii (T. gondii). Chickens are ground-feeders and represent, especially if free-range, important intermediate hosts in the epidemiology of toxoplasmosis and are used as sentinels of environmental contamination with T. gondii oocysts. Until now, little is known about the burden and regional distribution of T. gondii cysts in the chicken brain. It was therefore the aim of this study to investigate the abundance and specific distribution of T. gondii cysts within the chicken brain following chronic infection with a type II strain (76 K) of T. gondii. A total of 29 chickens were included in the study and divided into control group (n = 9) and two different infection groups, a low dose (n = 10) and a high dose (n = 10) group, which were orally inoculated with 1500 or 150,000 T. gondii oocysts per animal, respectively. Seroconversion was detected in the majority of chickens of the high dose group, but not in the animals of the low dose and the control group. Moreover, T. gondii DNA was detected most frequently in the brain and more frequently in the heart than in liver, spleen, thigh and pectoral muscle using qPCR analysis. The number of T. gondii cysts, quantified in the chicken brain using histological analysis, seems to be considerably lower as compared to studies in rodents, which might explain why T. gondii infected chickens very rarely, if at all, develop neurological deficits. Similar to observations in mice, in which no lateralisation for T. gondii cysts was reported, T. gondii cysts were distributed nearly equally between the left and right chicken brain hemispheres. When different brain regions (fore-, mid- and hindbrain) were compared, all T. gondii cysts were located in the forebrain with the overwhelming majority of these cysts being present in the telencephalic pallium and subpallium. More studies including different strains and higher doses of T. gondii are needed in order to precisely evaluate its cyst burden and regional distribution in the chicken brain. Together, our findings provide insights into the course of T. gondii infection in chickens and are important to understand the differences of chronic T. gondii infection in the chicken and mammalian brain.

Differential expression of TgMIC1 in isolates of Chinese 1 Toxoplasma with different virulence

BACKGROUND

The predominant genotype of Toxoplasma in China is the Chinese 1 (ToxoDB#9) lineage. TgCtwh3 and TgCtwh6 are two representative strains of Chinese 1, exhibiting high and low virulence to mice, respectively. Little is known regarding the virulence mechanism of this non-classical genotype. Our previous RNA sequencing data revealed differential mRNA levels of TgMIC1 in TgCtwh3 and TgCtwh6. We aim to further confirm the differential expression of TgMIC1 and its significance in this atypical genotype.

METHODS

Quantitative real-time PCR was used to verify the RNA sequencing data; then, polyclonal antibodies against TgMIC1 were prepared and identified. Moreover, the invasion and proliferation of the parasite in HFF cells were observed after treatment with TgMIC1 polyclonal antibody or not.

RESULTS

The data showed that the protein level of TgMIC1 was significantly higher in high-virulence strain TgCtwh3 than in low-virulence strain TgCtwh6 and that the invasion and proliferation of TgCtwh3 were inhibited by TgMIC1 polyclonal antibody.

CONCLUSION

Differential expression of TgMIC1 in TgCtwh3 and TgCtwh6 may explain, at least partly, the virulence mechanism of this atypical genotype.

Computational image analysis reveals the structural complexity of Toxoplasma gondii tissue cysts

Toxoplasma gondii is an obligate intracellular parasite infecting up to one third of the human population. The central event in the pathogenesis of toxoplasmosis is the conversion of tachyzoites into encysted bradyzoites. A novel approach to analyze the structure of in vivo-derived tissue cysts may be the increasingly used computational image analysis. The objective of this study was to quantify the geometrical complexity of T. gondii cysts by morphological, particle, and fractal analysis, as well as to determine if it is impacted by parasite strain, cyst age, and host type. A total of 31 images of T. gondii brain cysts of four type-2 strains (Me49, and local isolates BGD1, BGD14, and BGD26) was analyzed using ImageJ software. The parameters of interest included diameter, circularity, packing density (PD), fractal dimension (FD), and lacunarity. Although cyst diameter varied widely, its negative correlation with PD was observed. Circularity was remarkably close to 1, indicating a perfectly round shape of the cysts. PD and FD did not vary among cysts of different strains, age, and derived from mice of different genetic background. Conversely, lacunarity, which is a measure of heterogeneity, was significantly lower for BGD1 strain vs. all other strains, and higher for Me49 vs. BGD14 and BGD26, but did not differ among Me49 cysts of different age, or those derived from genetically different mice. The results indicate a highly uniform structure and occupancy of the different T. gondii tissue cysts. This study furthers the use of image analysis in describing the structural complexity of T. gondii cyst morphology, and presents the first application of fractal analysis for this purpose. The presented results show that use of a freely available software is a cost-effective approach to advance automated image scoring for T. gondii cysts.

Toxoplasma gondii Modulates the Host Cell Responses: An Overview of Apoptosis Pathways

Infection with Toxoplasma gondii has a major implication in public health. Toxoplasma gondii is an obligate intracellular protozoan parasite that can infect all nucleated cells belonging to a wide range of host species. One of the particularities of this parasite is its invasion and persistence in host cells of immunocompetent people. This infection is usually asymptomatic. In immunocompromised patients, the infection is severe and symptomatic. The mechanisms by which T. gondii persists are poorly studied in humans. In mouse models, many aspects of the interaction between the parasite and the host cells are being studied. Apoptosis is one of these mechanisms that could be modulated by Toxoplasma to persist in host cells. Indeed, Toxoplasma has often been implicated in the regulation of apoptosis and viability mechanisms in both human and murine infection models. Several of these studies centered on the regulation of apoptosis pathways have revealed interference of this parasite with host cell immunity, cell signalling, and invasion mechanisms. This review provides an overview of recent studies concerning the effect of Toxoplasma on different apoptotic pathways in infected host cells.

Reexamining Chronic Toxoplasma gondii Infection: Surprising Activity for a "Dormant" Parasite

PURPOSE OF REVIEW

Despite over a third of the world's population being chronically infected with Toxoplasma gondii, little is known about this largely asymptomatic phase of infection. This stage is mediated in vivo by bradyzoites within tissue cysts. The absence of overt symptoms has been attributed to the dormancy of bradyzoites. In this review, we reexamine the conventional view of chronic toxoplasmosis in light of emerging evidence challenging both the nature of dormancy and the consequences of infection in the CNS.

RECENT FINDINGS

New and emerging data reveal a previously unrecognized level of physiological and replicative capacity of bradyzoites within tissue cysts. These findings have emerged in the context of a reexamination of the chronic infection in the brain that correlates with changes in neuronal architecture, neurochemistry, and behavior that suggest that the chronic infection is not without consequence.

SUMMARY

The emerging data driven by the development of new approaches to study the progression of chronic toxoplasma infection reveals significant physiological and replicative capacity for what has been viewed as a dormant state. The emergence of bradyzoite and tissue cyst biology from what was viewed as a physiological "black box" offers exciting new areas for investigation with direct implications on the approaches to drug development targeting this drug-refractory state. In addition, new insights from studies on the neurobiology on chronic infection reveal a complex and dynamic interplay between the parasite, brain microenvironment, and the immune response that results in the detente that promotes the life-long persistence of the parasite in the host.

Experimental Toxoplasmosis in Rats Induced Orally with Eleven Strains of Toxoplasma gondii of Seven Genotypes: Tissue Tropism, Tissue Cyst Size, Neural Lesions, Tissue Cyst Rupture without Reactivation, and Ocular Lesions

BACKGROUND

The protozoan parasite Toxoplasma gondii is one of the most widely distributed and successful parasites. Toxoplasma gondii alters rodent behavior such that infected rodents reverse their fear of cat odor, and indeed are attracted rather than repelled by feline urine. The location of the parasite encysted in the brain may influence this behavior. However, most studies are based on the highly susceptible rodent, the mouse.

METHODOLOGY/PRINCIPAL FINDINGS

Latent toxoplasmosis was induced in rats (10 rats per T. gondii strains) of the same age, strain, and sex, after oral inoculation with oocysts (natural route and natural stage of infection) of 11 T. gondii strains of seven genotypes. Rats were euthanized at two months post inoculation (p.i.) to investigate whether the parasite genotype affects the distribution, location, tissue cyst size, or lesions. Tissue cysts were enumerated in different regions of the brains, both in histological sections as well in saline homogenates. Tissue cysts were found in all regions of the brain. The tissue cyst density in different brain regions varied extensively between rats with many regions highly infected in some animals. Overall, the colliculus was most highly infected although there was a large amount of variability. The cerebral cortex, thalamus, and cerebellum had higher tissue cyst densities and two strains exhibited tropism for the colliculus and olfactory bulb. Histologically, lesions were confined to the brain and eyes. Tissue cyst rupture was frequent with no clear evidence for reactivation of tachyzoites. Ocular lesions were found in 23 (25%) of 92 rat eyes at two months p.i. The predominant lesion was focal inflammation in the retina. Tissue cysts were seen in the sclera of one and in the optic nerve of two rats. The choroid was not affected. Only tissue cysts, not active tachyzoite infections, were detected. Tissue cysts were seen in histological sections of tongue of 20 rats but not in myocardium and leg muscle.

CONCLUSION/SIGNIFICANCE

This study reevaluated in depth the rat model of toxoplasmosis visualizing cyst rupture and clarified many aspects of the biology of the parasite useful for future investigations.

Novel Approaches Reveal that Toxoplasma gondii Bradyzoites within Tissue Cysts Are Dynamic and Replicating Entities In Vivo

Despite their critical role in chronic toxoplasmosis, the biology of Toxoplasma gondii bradyzoites is poorly understood. In an attempt to address this gap, we optimized approaches to purify tissue cysts and analyzed the replicative potential of bradyzoites within these cysts. In order to quantify individual bradyzoites within tissue cysts, we have developed imaging software, BradyCount 1.0, that allows the rapid establishment of bradyzoite burdens within imaged optical sections of purified tissue cysts. While in general larger tissue cysts contain more bradyzoites, their relative “occupancy” was typically lower than that of smaller cysts, resulting in a lower packing density. The packing density permits a direct measure of how bradyzoites develop within cysts, allowing for comparisons across progression of the chronic phase. In order to capture bradyzoite endodyogeny, we exploited the differential intensity of TgIMC3, an inner membrane complex protein that intensely labels newly formed/forming daughters within bradyzoites and decays over time in the absence of further division. To our surprise, we were able to capture not only sporadic and asynchronous division but also synchronous replication of all bradyzoites within mature tissue cysts. Furthermore, the time-dependent decay of TgIMC3 intensity was exploited to gain insights into the temporal patterns of bradyzoite replication in vivo. Despite the fact that bradyzoites are considered replicatively dormant, we find evidence for cyclical, episodic bradyzoite growth within tissue cysts in vivo. These findings directly challenge the prevailing notion of bradyzoites as dormant nonreplicative entities in chronic toxoplasmosis and have implications on our understanding of this enigmatic and clinically important life cycle stage.

The protozoan Toxoplasma gondii establishes a lifelong chronic infection mediated by the bradyzoite form of the parasite within tissue cysts. Technical challenges have limited even the most basic studies on bradyzoites and the tissue cysts in vivo. Bradyzoites, which are viewed as dormant, poorly replicating or nonreplicating entities, were found to be surprisingly active, exhibiting not only the capacity for growth but also previously unrecognized patterns of replication that point to their being considerably more dynamic than previously imagined. These newly revealed properties force us to reexamine the most basic questions regarding bradyzoite biology and the progression of the chronic phase of toxoplasmosis. By developing new tools and approaches to study the chronic phase at the level of bradyzoites, we expose new avenues to tackle both drug development and a better understanding of events that may lead to reactivated symptomatic disease.

Image-based systems biology of infection

The successful treatment of infectious diseases requires interdisciplinary studies of all aspects of infection processes. The overarching combination of experimental research and theoretical analysis in a systems biology approach can unravel mechanisms of complex interactions between pathogens and the human immune system. Taking into account spatial information is especially important in the context of infection, since the migratory behavior and spatial interactions of cells are often decisive for the outcome of the immune response. Spatial information is provided by image and video data that are acquired in microscopy experiments and that are at the heart of an image-based systems biology approach. This review demonstrates how image-based systems biology improves our understanding of infection processes. We discuss the three main steps of this approach--imaging, quantitative characterization, and modeling--and consider the application of these steps in the context of studying infection processes. After summarizing the most relevant microscopy and image analysis approaches, we discuss ways to quantify infection processes, and address a number of modeling techniques that exploit image-derived data to simulate host-pathogen interactions in silico.