Secreted effectors in Toxoplasma gondii and related species: determinants of host range and pathogenesis?

GRA12 is a common virulence factor across Toxoplasma gondii strains and mouse subspecies

Toxoplasma gondii parasites exhibit extraordinary host promiscuity owing to over 250 putative secreted proteins that disrupt host cell functions, enabling parasite persistence. However, most of the known effector proteins are specific to Toxoplasma genotypes or hosts. To identify virulence factors that function across different parasite isolates and mouse strains that differ in susceptibility to infection, we performed systematic pooled in vivo CRISPR-Cas9 screens targeting the Toxoplasma secretome. We identified several proteins required for infection across parasite strains and mouse species, of which the dense granule protein 12 (GRA12) emerged as the most important effector protein during acute infection. GRA12 deletion in IFNγ-activated macrophages results in collapsed parasitophorous vacuoles and increased host cell necrosis, which is partially rescued by inhibiting early parasite egress. GRA12 orthologues from related coccidian parasites, including Neospora caninum and Hammondia hammondi, complement TgΔGRA12 in vitro, suggesting a common mechanism of protection from immune clearance by their hosts.

The trophoblast surface becomes refractory to adhesion by congenitally transmitted Toxoplasma gondii and Listeria monocytogenes during cytotrophoblast to syncytiotrophoblast development

The placenta is a critical barrier against viral, bacterial, and eukaryotic pathogens. For most teratogenic pathogens, the precise molecular mechanisms of placental resistance are still being unraveled. Given the importance of understanding these mechanisms and challenges in replicating trophoblast-pathogen interactions using in vitro models, we tested an existing stem-cell-derived model of trophoblast development for its relevance to infection with Toxoplasma gondii. We grew human trophoblast stem cells (TSCT) under conditions leading to either syncytiotrophoblast (TSSYN) or cytotrophoblast (TSCYT) and infected them with T. gondii. We evaluated T. gondii proliferation and invasion, cell ultrastructure, as well as for transcriptome changes after infection. TSSYNs cells showed similar ultrastructure compared to primary cells and villous explants when analyzed by transmission electron microscopy and scanning electron microscopy (SEM), a resistance to T. gondii adhesion could be visualized on the SEM level. Furthermore, TSSYNs were highly refractory to parasite adhesion and replication, while TSCYTs were not. RNA-seq data on mock-treated and infected cells identified differences between cell types as well as how they responded to T. gondii infection. We also evaluated if TSSC-derived SYNs and CYTs had distinct resistance profiles to another vertically transmitted facultative intracellular pathogen, Listeria monocytogenes. We demonstrate that TSSYNs are highly resistant to L. monocytogenes, while TSCYTs are not. Like T. gondii, TSSYN resistance to L. monocytogenes was at the level of bacterial adhesion. Altogether, our data indicate that stem-cell-derived trophoblasts recapitulate resistance profiles of primary cells to T. gondii and highlight the critical importance of the placental surface in cell-autonomous resistance to teratogens.IMPORTANCECongenital toxoplasmosis can cause a devastating consequence to the fetus. To reach the fetus's tissues, Toxoplasma gondii must cross the placenta barrier. However, how this parasite crosses the placenta and the precise molecular mechanisms of placental resistance to this parasite are still unknown. In this study, we aimed to characterize a new cellular model of human trophoblast stem cells to determine their resistance, susceptibility, and response to T. gondii. Syncytiotrophoblast derived from trophoblast stem cells recapitulate the resistance profile similarly to placenta cells. We also showed that these cells are highly resistant to Listeria monocytogenes, at the level of bacterial adhesion. Our results suggest that resisting pathogen adhesion/attachment may be a generalized mechanism of syncytiotrophoblast resistance, and trophoblast stem cells represent a promising model to investigate cell-intrinsic mechanisms of resistance to pathogen adhesion and replication.

Are genetically modified protozoa eligible for ATMP status? Concerning the legal categorization of an oncolytic protozoan drug candidate

Neospora caninum is an obligate intracellular protozoan that affects several animal species. It is not pathogenic for humans, and its ability to infect and lyse a variety of cells and stimulate the immune system makes it an interesting drug candidate in oncology. The intrinsic oncolytic properties of N. caninum have been confirmed in several preclinical models. Moreover, it can be modified to improve its safety and/or efficacy against cancer cells. In this study, we propose the legal categorization of this new biological drug candidate and the impact of modifications, notably the integration of a suicide gene, the deletion of a gene allowing its multiplication in healthy cells, and/or the insertion of a gene coding for a therapeutic protein into its genome. When unmodified, N. caninum can be categorized as a biological medicinal product, whereas modifications aimed at increasing its safety classify it as a Somatic Cell Therapy Medicinal Product, and modifications aiming to increase its efficacy or both safety and efficacy make it as a Gene Therapy Medicinal Product. This categorization is fundamental because it determines the guidelines applicable for preclinical development. These guidelines being numerous and complex, we have focused on the key requirements necessary for the development of the future medicinal product.

Human trophoblast stem cells can be used to model placental susceptibility to Toxoplasma gondii and highlight the critical importance of the trophoblast cell surface in pathogen resistance

The placenta is a critical barrier against viral, bacterial, and eukaryotic pathogens. For most teratogenic pathogens, the precise molecular mechanisms of placental resistance are still being unraveled. Given the importance to understand these mechanisms and challenges in replicating trophoblast- pathogen interactions using in vitro models, we tested an existing stem-cell derived model of trophoblast development for its relevance to infection with Toxoplasma gondii . We grew human trophoblast stem cells (TS CT ) under conditions leading to either syncytiotrophoblast (TS SYN ) or cytotrophoblast (TS CYT ) and infected them with T. gondii . We evaluated T. gondii proliferation and invasion, cell ultrastructure, as well as for transcriptome changes after infection. TS SYNs cells showed similar ultrastructure compared to primary cells and villous explants when analyzed by TEM and SEM, a resistance to T. gondii adhesion could be visualized on the SEM level. Furthermore, TS SYNs were highly refractory to parasite adhesion and replication, while TS CYT were not. RNA-seq data on mock-treated and infected cells identified differences between cell types as well as how they responded to T. gondii infection. We also evaluated if TS SC -derived SYNs and CYTs had distinct resistance profiles to another vertically transmitted facultative intracellular pathogen, Listeria monocytogenes . We demonstrate that TS SYNs are highly resistant to L. monocytogenes , while TS CYTs are not. Like T. gondii , TS SYN resistance to L. monocytogenes was at the level of bacterial adhesion. Altogether, our data indicate that stem-cell derived trophoblasts recapitulate resistance profiles of primary cells to T. gondii and highlight the critical importance of the placental surface in cell-autonomous resistance to teratogens.

Toxoplasma gondii microneme protein MIC3 induces macrophage TNF-α production and Ly6C expression via TLR11/MyD88 pathway

Toxoplasma gondii is the most successful parasite worldwide. It is of great interest to understand how T. gondii induce different immune responses in different hosts. In this study, we found that a peptide of T. gondii microneme protein MIC3 induced TNF-α production, NF-κB phosphorylation, iNOS transcription and Ly6C expression in mouse macrophage RAW264.7 cells. MyD88 inhibition, small interfering RNA against Tlr11 and CRISPR/Cas9-mediated knock-out of Tlr11 all reduced MIC3-induced TNF-α production, NF-κB phosphorylation, iNOS transcription and Ly6C expression. Additionally, we determined the location of MIC3 peptide in mouse macrophages using immunofluorescence. MIC3 could both adhere to the cell membrane of mouse macrophages and enter the cells. These results suggest that MIC3 triggered the immune responses in mouse macrophages via TLR11/MyD88/NF-κB pathway. It is known that human macrophages lacking TLR11. We predicted that the immune responses induced by MIC3 in human macrophages were significantly different from those in mouse macrophages. As expected, MIC3 peptide failed to induce TNF-α expression, iNOS expression and NF-κB phosphorylation in human THP-1 derived macrophages. MIC3 induced macrophage immune responses via TLR11. Intriguingly, the amino acid sequence of MIC3 is completely different from the well-known TLR11 ligand profilin, which generates a potent IL-12p40, TNF-α and IL-6 response. In marked contrast to profilin, MIC3 could not induce IL-12p40 expression in both mouse RAW264.7 cells and human THP-1 derived macrophages. Furthermore, the simulated tertiary structure of MIC3 peptide shows poor similarity with the crystal structure of profilin, suggesting that MIC3 might be a different ligand from profilin. These findings about MIC3 and TLR11 will provide us with important insights into the pathogenesis of toxoplasmosis and coevolution during host-parasite interaction.

NcGRA7 and NcROP40 Play a Role in the Virulence of Neospora caninum in a Pregnant Mouse Model

The intraspecific variability among Neospora caninum isolates in their in vitro behaviour and in vivo virulence has been widely studied. In particular, transcriptomic and proteomic analyses have shown a higher expression/abundance of specific genes/proteins in high-virulence isolates. Consequently, the dense granule protein NcGRA7 and the rhoptry protein NcROP40 were proposed as potential virulence factors. The objective of this study was to characterize the role of these proteins using CRISPR/Cas9 knockout (KO) parasites in a well-established pregnant BALB/c mouse model of N. caninum infection at midgestation. The deletion of NcGRA7 and NcROP40 was associated with a reduction of virulence, as infected dams displayed milder clinical signs, lower parasite burdens in the brain, and reduced mortality rates compared to those infected with the wild-type parasite (Nc-Spain7). Specifically, those infected with the NcGRA7 KO parasites displayed significantly milder clinical signs and a lower brain parasite burden. The median survival time of the pups from dams infected with the two KO parasites was significantly increased, but differences in neonatal mortality rates were not detected. Overall, the present study indicates that the disruption of NcGRA7 considerably impairs virulence in mice, while the impact of NcROP40 deletion was more modest. Further research is needed to understand the role of these virulence factors during N. caninum infection.

Infections at the maternal-fetal interface: an overview of pathogenesis and defence

Infections are a major threat to human reproductive health, and infections in pregnancy can cause prematurity or stillbirth, or can be vertically transmitted to the fetus leading to congenital infection and severe disease. The acronym 'TORCH' (Toxoplasma gondii, other, rubella virus, cytomegalovirus, herpes simplex virus) refers to pathogens directly associated with the development of congenital disease and includes diverse bacteria, viruses and parasites. The placenta restricts vertical transmission during pregnancy and has evolved robust mechanisms of microbial defence. However, microorganisms that cause congenital disease have likely evolved diverse mechanisms to bypass these defences. In this Review, we discuss how TORCH pathogens access the intra-amniotic space and overcome the placental defences that protect against microbial vertical transmission.

Toxoplasma gondii GRA28 Is Required for Placenta-Specific Induction of the Regulatory Chemokine CCL22 in Human and Mouse

Toxoplasma gondii is an intracellular protozoan pathogen of humans that can cross the placenta and result in adverse pregnancy outcomes and long-term birth defects. The mechanisms used by T. gondii to cross the placenta are unknown, but complex interactions with the host immune response are likely to play a role in dictating infection outcomes during pregnancy. Prior work showed that T. gondii infection dramatically and specifically increases the secretion of the immunomodulatory chemokine CCL22 in human placental cells during infection. Given the important role of this chemokine during pregnancy, we hypothesized that CCL22 induction was driven by a specific T. gondii-secreted effector. Using a combination of bioinformatics and molecular genetics, we have now identified T. gondii GRA28 as the gene product required for CCL22 induction. GRA28 is secreted into the host cell, where it localizes to the nucleus, and deletion of the GRA28 gene results in reduced CCL22 placental cells as well as a human monocyte cell line. The impact of GRA28 on CCL22 production is also conserved in mouse immune and placental cells both in vitro and in vivo. Moreover, parasites lacking GRA28 are impaired in their ability to disseminate throughout the animal, suggesting a link between CCL22 induction and the ability of the parasite to cause disease. Overall, these data demonstrate a clear function for GRA28 in altering the immunomodulatory landscape during infection of both placental and peripheral immune cells and show a clear impact of this immunomodulation on infection outcome. IMPORTANCE Toxoplasma gondii is a globally ubiquitous pathogen that can cause severe disease in HIV/AIDS patients and can also cross the placenta and infect the developing fetus. We have found that placental and immune cells infected with T. gondii secrete significant amounts of a chemokine (called CCL22) that is critical for immune tolerance during pregnancy. In order to better understand whether this is a response by the host or a process that is driven by the parasite, we have identified a T. gondii gene that is absolutely required to induce CCL22 production in human cells, indicating that CCL22 production is a process driven almost entirely by the parasite rather than the host. Consistent with its role in immune tolerance, we also found that T. gondii parasites lacking this gene are less able to proliferate and disseminate throughout the host. Taken together, these data illustrate a direct relationship between CCL22 levels in the infected host and a key parasite effector and provide an interesting example of how T. gondii can directly modulate host signaling pathways in order to facilitate its growth and dissemination.

Cell type- and species-specific host responses to Toxoplasma gondii and its near relatives

Toxoplasma gondii is remarkably unique in its ability to successfully infect vertebrate hosts from multiple phyla and can successfully infect most cells within these organisms. The infection outcome in each of these species is determined by the complex interaction between parasite and host genotype. As techniques to quantify global changes in cell function become more readily available and precise, new data are coming to light about how (i) different host cell types respond to parasitic infection and (ii) different parasite species impact the host. Here we focus on recent studies comparing the response to intracellular parasitism by different cell types and insights into understanding host-parasite interactions from comparative studies on T. gondii and its close extant relatives.

Immediate Interferon Gamma Induction Determines Murine Host Compatibility Differences between Toxoplasma gondii and Neospora caninum

Rodents are critical for the transmission of Toxoplasma gondii to the definitive feline host via predation, and this relationship has been extensively studied as a model for immune responses to parasites. Neospora caninum is a closely related coccidian parasite of ruminants and canines but is not naturally transmitted by rodents. We compared mouse innate immune responses to N. caninum and T. gondii and found marked differences in cytokine levels and parasite growth kinetics during the first 24 h postinfection (hpi). N. caninum-infected mice produced significantly higher levels of interleukin-12 (IL-12) and interferon gamma (IFN-γ) by as early as 4 hpi, but the level of IFN-γ was significantly lower or undetectable in T. gondii-infected mice during the first 24 hpi. "Immediate" IFN-γ and IL-12p40 production was not detected in MyD88-/- mice. However, unlike IL-12p40-/- and IFN-γ-/- mice, MyD88-/- mice survived N. caninum infections at the dose used in this study. Serial measures of parasite burden showed that MyD88-/- mice were more susceptible to N. caninum infections than wild-type (WT) mice, and control of parasite burdens correlated with a pulse of serum IFN-γ at 3 to 4 days postinfection in the absence of detectable IL-12. Immediate IFN-γ was partially dependent on the T. gondii mouse profilin receptor Toll-like receptor 11 (TLR11), but the ectopic expression of N. caninum profilin in T. gondii had no impact on early IFN-γ production or parasite proliferation. Our data indicate that T. gondii is capable of evading host detection during the first hours after infection, while N. caninum is not, and this is likely due to the early MyD88-dependent recognition of ligands other than profilin.

Genome Wide Identification of Mutational Hotspots in the Apicomplexan Parasite Neospora caninum and the Implications for Virulence

Neospora caninum is an apicomplexan parasite responsible for neosporosis, a disease causing hind limb paralysis in dogs and abortion in cattle, resulting in substantial economic losses to beef and dairy industries. Marked differences in pathogenicity exist between N. caninum strains suggesting that intrinsic genetic differences exist between them. These differences likely exist in genes expressed during the tachyzoite lifecycle stage which is responsible for the pathogenesis of neosporosis. An improved understanding of these genetic differences is essential to understanding N. caninum virulence, though such knowledge is scarce. Using a variant detection workflow we compared the tachyzoite transcriptomes of two N. caninum strains with different virulence properties: NC-Liverpool (virulent) and NC-Nowra (avirulent). This workflow identified 3130 SNPs and 6123 indels between the strains, and nine markers capturing 30 variants were Sanger sequenced for both strains. Sequencing of these loci was extended to an additional eight strains and subsequent phylogenetic analysis supported a genetic population structure comprised of two major clades with no geographical segregation. Sequence polymorphisms within coding regions of tachyzoite-associated genes were concentrated on chromosomes XI and XII, with 19 distinct tachyzoite-associated SNP hotspot regions identified within coding regions of the N. caninum nuclear genome. The variants were predominantly located in loci associated with protein binding, protein-protein interactions, transcription, and translation. Furthermore, 468 nonsynonymous SNPs identified within protein-coding genes were associated with protein kinase activity, protein binding, protein phosphorylation, and proteolysis. This work may implicate these processes and the specific proteins involved as novel effectors of N. caninum tachyzoite virulence.

An outbreak of toxoplasmosis in squirrel monkeys (Saimiri sciureus) in South Korea

BACKGROUND

Toxoplasma gondii (T. gondii) is an intracellular protozoan parasite that can infect warm-blooded animals including humans. New World monkeys, such as squirrel monkeys, are more susceptible to T. gondii than Old World monkeys, often developing fatal disease.

METHODS

In this study, seven of thirteen dead squirrel monkeys at Seoul Grand Park were tested to find the cause of sudden death.

RESULTS

The main histopathological findings included interstitial pneumonia, necrotizing hepatitis, and splenitis. Periodic acid-Schiff staining of liver, spleen, and lung revealed cyst structures consistent with bradyzoites. Amplification of the B1 gene was detected in the liver or spleen of all monkeys. Additionally, a restriction fragment length polymorphism assay and phylogenetic analysis of the GRA6 amplicon revealed a consistent clustering with the type II strain of T. gondii.

CONCLUSIONS

This study is the first report of T. gondii infection of squirrel monkeys in Korea, and the first report of type II T. gondii based on GRA6 analysis in Korea.

Human Placental Syncytiotrophoblasts Restrict Toxoplasma gondii Attachment and Replication and Respond to Infection by Producing Immunomodulatory Chemokines

Toxoplasma gondii is a major source of congenital disease worldwide, but the cellular and molecular factors associated with its vertical transmission are largely unknown. In humans, the placenta forms the key interface between the maternal and fetal compartments and forms the primary barrier that restricts the hematogenous spread of microorganisms. Here, we utilized primary human trophoblast (PHT) cells isolated from full-term placentas and human midgestation chorionic villous explants to determine the mechanisms by which human trophoblasts restrict and respond to T. gondii infection. We show that placental syncytiotrophoblasts, multinucleated cells that are in direct contact with maternal blood, restrict T. gondii infection at two distinct stages of the parasite lytic cycle—at the time of attachment and also during intracellular replication. Utilizing comparative transcriptome sequencing (RNA-seq) transcriptional profiling, we also show that human placental trophoblasts from both the second and third trimesters respond uniquely to T. gondii infection compared to trophoblast cell lines, typified by the upregulation of several immunity-related genes. One of the most differentially induced genes was the chemokine CCL22, which relies on the secretion of a parasite effector(s) either during or after invasion for its induction. Collectively, our findings provide new insights into the mechanisms by which the human placenta restricts the vertical transmission of T. gondii at early and late stages of human pregnancy and demonstrate the existence of at least two interferon-independent pathways that restrict T. gondii access to the fetal compartment.

Toxoplasma gondii is a major source of congenital disease worldwide and must breach the placental barrier to be transmitted from maternal blood to the developing fetus. The events associated with the vertical transmission of T. gondii are largely unknown. Here, we show that primary human syncytiotrophoblasts, the fetus-derived cells that comprise the primary placental barrier, restrict T. gondii infection at two distinct stages of the parasite life cycle and respond to infection by inducing a unique immunomodulatory transcriptional profile. Collectively, our findings provide important insights into the mechanisms by which human syncytiotrophoblasts restrict T. gondii infection at early and late stages of human pregnancy, identify both permissive and resistant human placental cell types, and identify the placenta-enriched signaling pathways induced in response to infection.

IgG Avidity Test in Congenital Toxoplasmosis Diagnoses in Newborns

The goal of this study was to investigate the importance of IgG avidity testing in newborns (NBs) diagnosed with early congenital toxoplasmosis. We collected samples from 88 puerperae infected by Toxoplasma gondii (T. gondii) and their NBs (48 acutely-infected puerperae (AIP) and 40 chronically-infected puerperae (CIP)), from two public maternity hospitals in Goiania city, Goias, Brazil, from 2010 to 2015. Specific anti-T. gondii IgM and IgG serum levels and IgG avidity tests were evaluated using chemiluminescence. Congenital toxoplasmosis was observed in 66.66% (n = 32) of NBs with AIP, 94.1% presenting low avidity (LA) and 51.61% presenting high avidity (HA) test results. The IgG and IgM levels of NBs with LA and their puerperae were higher in comparison with HA NBs and puerperae (p = 0.0001). The avidity tests showed 100% specificity and 50% sensitivity (p = 0.0001). NBs with LA had a 15-fold increased risk of developing congenital toxoplasmosis in comparison with HA NBs. The IgG avidity test could be used to assist in early congenital toxoplasmosis diagnoses in NBs and LA, identifying a greater probability of vertical transmission.

Vacuolar protein sorting mechanisms in apicomplexan parasites

The phylum Apicomplexa comprises more than 5000 species including pathogens of clinical and economical importance. These obligate intracellular parasites possess a highly complex endomembrane system to build amongst others three morphologically distinct secretory organelles: rhoptries, micronemes and dense granules. Proteins released by these organelles are essential for invasion and hijacking of the host cell. Due to the complexity of the internal organization of these parasites, a wide panoply of trafficking factors was expected to be required for the correct sorting of proteins towards the various organelles. However, Toxoplasma gondii and other apicomplexan parasites contain only a core set of these factors and several of the vacuolar protein sorting (VPS) homologues found in most eukaryotes have been lost in this phylum.

In this review, we will summarise our current knowledge about the role of trafficking complexes in T. gondii, highlighting recent studies focused on complexes formed by VPS proteins. We also present a novel, hypothetical model, suggesting the recycling of parasite membrane and micronemal proteins.

Recent advances in understanding apicomplexan parasites

Intracellular single-celled parasites belonging to the large phylum Apicomplexa are amongst the most prevalent and morbidity-causing pathogens worldwide. In this review, we highlight a few of the many recent advances in the field that helped to clarify some important aspects of their fascinating biology and interaction with their hosts. Plasmodium falciparum causes malaria, and thus the recent emergence of resistance against the currently used drug combinations based on artemisinin has been of major interest for the scientific community. It resulted in great advances in understanding the resistance mechanisms that can hopefully be translated into altered future drug regimens. Apicomplexa are also experts in host cell manipulation and immune evasion. Toxoplasma gondii and Theileria sp., besides Plasmodium sp., are species that secrete effector molecules into the host cell to reach this aim. The underlying molecular mechanisms for how these proteins are trafficked to the host cytosol ( T. gondii and Plasmodium) and how a secreted protein can immortalize the host cell ( Theileria sp.) have been illuminated recently. Moreover, how such secreted proteins affect the host innate immune responses against T. gondii and the liver stages of Plasmodium has also been unraveled at the genetic and molecular level, leading to unexpected insights.

Methodological advances in metabolomics and molecular biology have been instrumental to solving some fundamental puzzles of mitochondrial carbon metabolism in Apicomplexa. Also, for the first time, the generation of stably transfected Cryptosporidium parasites was achieved, which opens up a wide variety of experimental possibilities for this understudied, important apicomplexan pathogen.

Flagellar membrane proteins in kinetoplastid parasites

All kinetoplastid parasites, including protozoa such as Leishmania species, Trypanosoma brucei, and Trypanosoma cruzi that cause devastating diseases in humans and animals, are flagellated throughout their life cycles. Although flagella were originally thought of primarily as motility organelles, flagellar functions in other critical processes, especially in sensing and signal transduction, have become more fully appreciated in the recent past. The flagellar membrane is a highly specialized subdomain of the surface membrane, and flagellar membrane proteins are likely to be critical components for all the biologically important roles of flagella. In this review, we summarize recent discoveries relevant to flagellar membrane proteins in these parasites, including the identification of such proteins, investigation of their biological functions, and mechanisms of selective trafficking to the flagellar membrane. Prospects for future investigations and current unsolved problems are highlighted.