Analysis of behavior and trafficking of dendritic cells within the brain during toxoplasmic encephalitis

Not all (cells) who wander are lost: Upstream migration as a pervasive mode of amoeboid cell motility

Leukocytes possess the ability to migrate upstream-against the direction of flow-on surfaces of specific chemistry. Upstream migration was first characterized in vitro for T-cells on surfaces comprised of intracellular adhesion molecule-1 (ICAM-1). Upstream migration occurs when the integrin receptor αLβ2 (also known as lymphocyte function-associated antigen-1, or LFA-1) binds to ICAM-1. LFA-1/ICAM-1 interactions are ubiquitous and are widely found in leukocyte trafficking. Upstream migration would be employed after cells come to arrest on the apical surface of the endothelium and might confer an advantage for both trans-endothelial migration and tissue surveillance. It has now been shown that several other motile amoeboid cells which have the responsibility of trafficking from blood vessels into tissues, such as Marginal zone B cells, hematopoietic stem cells, and neutrophils (when macrophage-1 antigen, Mac-1, is blocked), can also migrate upstream on ICAM-1 surfaces. This review will summarize what is known about the basic mechanisms of upstream migration, which cells have displayed this phenomenon, and the possible role of upstream migration in physiology and tissue homeostasis.

Visualization of parasite dynamics in the host tissues: application of tissue transparency technology to parasite research

Many parasite species migrate to another site of infection after entering the host body. Such parasite dynamics are closely related to pathogenicity, but it is not easy to observe such parasite behavior deep within the organs. In recent years, technology that can make organs transparent has been developed that enables us to observe deep within organs ex vivo while maintaining their three-dimensional structure. This review describes a series of attempts to apply this technology to understand the behavior of Toxoplasma gondii in the host body. A series of studies has shown that T. gondii tachyzoites that infect leukocytes can reach target organs far from the site of invasion via the circulatory system. In addition, infected leukocytes in the bloodstream adhere more readily to vascular endothelial cells than uninfected leukocytes and are more likely to remain inside the target organs. When infected leukocytes adhere to the vascular endothelial cells of the target organ, the tachyzoites inside the cells immediately escape and infect the parenchyma of the organs. As described above, organ transparency technology is a powerful tool for understanding the internal dynamics of parasites.

Involvement of chemokine receptor CXCR3 in the defense mechanism against Neospora caninum infection in C57BL/6 mice

C-X-C motif chemokine receptor 3 (CXCR3) is an important receptor controlling the migration of leukocytes, although there is no report regarding its role in Neospora caninum infection. Herein, we investigated the relevance of CXCR3 in the resistance mechanism to N. caninum infection in mice. Wild-type (WT) C57BL/6 mice and CXCR3-knockout (CXCR3KO) mice were used in all experiments. WT mice displayed a high survival rate (100%), while 80% of CXCR3KO mice succumbed to N. caninum infection within 50 days. Compared with WT mice, CXCR3KO mice exhibited significantly lower body weights and higher clinical scores at the subacute stage of infection. Flow cytometric analysis revealed CXCR3KO mice as having significantly increased proportions and numbers of CD11c-positive cells compared with WT mice at 5 days post infection (dpi). However, levels of interleukin-6 and interferon-γ in serum and ascites were similar in all groups at 5 dpi. Furthermore, no differences in parasite load were detected in brain, spleen, lungs or liver tissue of CXCR3KO and WT mice at 5 and 21 dpi. mRNA analysis of brain tissue collected from infected mice at 30 dpi revealed no changes in expression levels of inflammatory response genes. Nevertheless, the brain tissue of infected CXCR3KO mice displayed significant necrosis and microglial activation compared with that of WT mice at 21 dpi. Interestingly, the brain tissue of CXCR3KO mice displayed significantly lower numbers of FoxP3⁺ cells compared with the brain tissue of WT mice at 30 dpi. Accordingly, our study suggests that the lack of active regulatory T cells in brain tissue of infected CXCR3KO mice is the main cause of these mice having severe necrosis and lower survival compared with WT mice. Thus, CXCR3⁺ regulatory T cells may play a crucial role in control of neosporosis.

Toxoplasma gondii Dissemination in the Brain Is Facilitated by Infiltrating Peripheral Immune Cells

Despite recent advances in our understanding of pathogenic access to the central nervous system (CNS), the mechanisms by which intracellular pathogens disseminate within the dense cellular network of neural tissue remain poorly understood. To address this issue, longitudinal analysis of Toxoplasma gondii dissemination in the brain was conducted using 2-photon imaging through a cranial window in living mice that transgenically express enhanced green fluorescent protein (eGFP)-claudin-5. Extracellular T. gondii parasites were observed migrating slowly (1.37 ± 1.28 μm/min) and with low displacement within the brain. In contrast, a population of highly motile infected cells transported vacuoles of T. gondii significantly faster (6.30 ± 3.09 μm/min) and with a higher displacement than free parasites. Detailed analysis of microglial dynamics using CX3CR1-GFP mice revealed that T. gondii-infected microglia remained stationary, and infection did not increase the extension/retraction of microglial processes. The role of infiltrating immune cells in shuttling T. gondii was examined by labeling of peripheral hematopoietic cells with anti-CD45 antibody. Infected CD45⁺ cells were found crawling along the CNS vessel walls and trafficked T. gondii within the brain parenchyma at significantly higher speeds (3.35 ± 1.70 μm/min) than extracellular tachyzoites. Collectively, these findings highlight a dual role for immune cells in neuroprotection and in facilitating parasite dissemination within the brain. IMPORTANCE T. gondii is a foodborne parasite that infects the brain and can cause fatal encephalitis in immunocompromised individuals. However, there is a limited understanding of how the parasites disseminate through the brain and evade immune clearance. We utilized intravital imaging to visualize extracellular T. gondii tachyzoites and infected cells migrating within the infected mouse brain during acute infection. The infection of motile immune cells infiltrating the brain from the periphery significantly increased the dissemination of T. gondii in the brain compared to that of free parasites migrating using their own motility: the speed and displacement of these infected cells would enable them to cover nearly 1 cm of distance per day! Among the infiltrating cells, T. gondii predominantly infected monocytes and CD8⁺ T cells, indicating that the parasite can hijack immune cells that are critical for controlling the infection in order to enhance their dissemination within the brain.

T cell surveillance of Toxoplasma gondii: Basic insights into how T cells operate in the central nervous system

The ability of T cells to operate in the central nervous system (CNS) is required for resistance to multiple pathogens that affect this tissue. The intracellular parasite Toxoplasma gondii has evolved to persist in the CNS and poses unique challenges to the immune system with the need to control parasite replication while balancing the adverse pathology associated with local inflammation. This article reviews the models used to study the response to T. gondii during toxoplasmic encephalitis and highlights some of the broader lessons that are relevant to understanding how T cells function in the CNS.

Microglia in CNS infections: insights from Toxoplasma gondii and other pathogens

Microglia, the resident immune cells of the central nervous system (CNS), are poised to respond to neuropathology. Microglia play multiple roles in maintaining homeostasis and promoting inflammation in numerous disease states. The study of microglial innate immune programs has largely focused on exploring neurodegenerative disease states with the use of genetic targeting approaches. Our understanding of how microglia participate in immune responses against pathogens is just beginning to take shape. Here, we review existing animal models of CNS infection, with a focus on how microglial physiology and inflammatory processes control protozoan and viral infections of the brain. We further discuss how microglial participation in over-exuberant immune responses can drive immunopathology that is detrimental to CNS health and homeostasis.

Meningeal lymphatic drainage promotes T cell responses against Toxoplasma gondii but is dispensable for parasite control in the brain

The discovery of meningeal lymphatic vessels that drain the CNS has prompted new insights into how immune responses develop in the brain. In this study, we examined how T cell responses against CNS-derived antigen develop in the context of infection. We found that meningeal lymphatic drainage promotes CD4⁺ and CD8⁺ T cell responses against the neurotropic parasite Toxoplasma gondii in mice, and we observed changes in the dendritic cell compartment of the dural meninges that may support this process. Indeed, we found that mice chronically, but not acutely, infected with T. gondii exhibited a significant expansion and activation of type 1 and type 2 conventional dendritic cells (cDC) in the dural meninges. cDC1s and cDC2s were both capable of sampling cerebrospinal fluid (CSF)-derived protein and were found to harbor processed CSF-derived protein in the draining deep cervical lymph nodes. Disrupting meningeal lymphatic drainage via ligation surgery led to a reduction in CD103⁺ cDC1 and cDC2 number in the deep cervical lymph nodes and caused an impairment in cDC1 and cDC2 maturation. Concomitantly, lymphatic vessel ligation impaired CD4⁺ and CD8⁺ T cell activation, proliferation, and IFN-γ production at this site. Surprisingly, however, parasite-specific T cell responses in the brain remained intact following ligation, which may be due to concurrent activation of T cells at non-CNS-draining sites during chronic infection. Collectively, our work reveals that CNS lymphatic drainage supports the development of peripheral T cell responses against T. gondii but remains dispensable for immune protection of the brain.

Blood-brain barrier-restricted translocation of Toxoplasma gondii from cortical capillaries

The cellular barriers of the central nervous system proficiently protect the brain parenchyma from infectious insults. Yet, the single-celled parasite Toxoplasma gondii commonly causes latent cerebral infection in humans and other vertebrates. Here, we addressed the role of the cerebral vasculature in the passage of T. gondii to the brain parenchyma. Shortly after inoculation in mice, parasites mainly localized to cortical capillaries, in preference over post-capillary venules, cortical arterioles or meningeal and choroidal vessels. Early invasion to the parenchyma (days 1-5) occurred in absence of a measurable increase in blood-brain barrier (BBB) permeability, perivascular leukocyte cuffs or hemorrhage. However, sparse focalized permeability elevations were detected adjacently to replicative parasite foci. Further, T. gondii triggered inflammatory responses in cortical microvessels and endothelium. Pro- and anti-inflammatory treatments of mice with LPS and hydrocortisone, respectively, impacted BBB permeability and parasite loads in the brain parenchyma. Finally, pharmacological inhibition or Cre/loxP conditional knockout of endothelial focal adhesion kinase (FAK), a BBB intercellular junction regulator, facilitated parasite translocation to the brain parenchyma. The data reveal that the initial passage of T. gondii to the central nervous system occurs principally across cortical capillaries. The integrity of the microvascular BBB restricts parasite transit, which conversely is exacerbated by the inflammatory response.

Toxoplasma gondii infection and its implications within the central nervous system

Toxoplasma gondii is a parasite that infects a wide range of animals and causes zoonotic infections in humans. Although it normally only results in mild illness in healthy individuals, toxoplasmosis is a common opportunistic infection with high mortality in individuals who are immunocompromised, most commonly due to reactivation of infection in the central nervous system. In the acute phase of infection, interferon-dependent immune responses control rapid parasite expansion and mitigate acute disease symptoms. However, after dissemination the parasite differentiates into semi-dormant cysts that form within muscle cells and neurons, where they persist for life in the infected host. Control of infection in the central nervous system, a compartment of immune privilege, relies on modified immune responses that aim to balance infection control while limiting potential damage due to inflammation. In response to the activation of interferon-mediated pathways, the parasite deploys an array of effector proteins to escape immune clearance and ensure latent survival. Although these pathways are best studied in the laboratory mouse, emerging evidence points to unique mechanisms of control in human toxoplasmosis. In this Review, we explore some of these recent findings that extend our understanding for proliferation, establishment and control of toxoplasmosis in humans.

GABAergic signaling by cells of the immune system: more the rule than the exception

Gamma-aminobutyric acid (GABA) is best known as an essential neurotransmitter in the evolved central nervous system (CNS) of vertebrates. However, GABA antedates the development of the CNS as a bioactive molecule in metabolism and stress-coupled responses of prokaryotes, invertebrates and plants. Here, we focus on the emerging findings of GABA signaling in the mammalian immune system. Recent reports show that mononuclear phagocytes and lymphocytes, for instance dendritic cells, microglia, T cells and NK cells, express a GABAergic signaling machinery. Mounting evidence shows that GABA receptor signaling impacts central immune functions, such as cell migration, cytokine secretion, immune cell activation and cytotoxic responses. Furthermore, the GABAergic signaling machinery of leukocytes is implicated in responses to microbial infection and is co-opted by protozoan parasites for colonization of the host. Peripheral GABA signaling is also implicated in inflammatory conditions and diseases, such as type 1 diabetes, rheumatoid arthritis and cancer cell metastasis. Adding to its role in neurotransmission, growing evidence shows that the non-proteinogenic amino acid GABA acts as an intercellular signaling molecule in the immune system and, as an interspecies signaling molecule in host–microbe interactions. Altogether, the data raise the assumption of conserved GABA signaling in a broad range of mammalian cells and diversification of function in the immune system.

Integrin-dependent migratory switches regulate the translocation of Toxoplasma-infected dendritic cells across brain endothelial monolayers

Multiple cellular processes, such as immune responses and cancer cell metastasis, crucially depend on interconvertible migration modes. However, knowledge is scarce on how infectious agents impact the processes of cell adhesion and migration at restrictive biological barriers. In extracellular matrix, dendritic cells (DCs) infected by the obligate intracellular protozoan Toxoplasma gondii undergo mesenchymal-to-amoeboid transition (MAT) for rapid integrin-independent migration. Here, in a cellular model of the blood–brain barrier, we report that parasitised DCs adhere to polarised endothelium and shift to integrin-dependent motility, accompanied by elevated transendothelial migration (TEM). Upon contact with endothelium, parasitised DCs dramatically reduced velocities and adhered under both static and shear stress conditions, thereby obliterating the infection-induced amoeboid motility displayed in collagen matrix. The motility of adherent parasitised DCs on endothelial monolayers was restored by blockade of β1 and β2 integrins or ICAM-1, which conversely reduced motility on collagen-coated surfaces. Moreover, parasitised DCs exhibited enhanced translocation across highly polarised primary murine brain endothelial cell monolayers. Blockade of β1, β2 integrins, ICAM-1 and PECAM-1 reduced TEM frequencies. Finally, gene silencing of the pan-integrin-cytoskeleton linker talin (Tln1) or of β1 integrin (Itgb1) in primary DCs resulted in increased motility on endothelium and decreased TEM. Adding to the paradigms of leukocyte diapedesis, the findings provide novel insights in how an intracellular pathogen impacts the migratory plasticity of leukocytes in response to the cellular environment, to promote infection-related dissemination.

Near-infrared fluorescence imaging in immunotherapy

Near-infrared (NIR) light possesses many suitable optophysical properties for medical imaging including low autofluorescence, deep tissue penetration, and minimal light scattering, which together allow for high-resolution imaging of biological tissue. NIR imaging has proven to be a noninvasive and effective real-time imaging methodology that provides a high signal-to-background ratio compared to other potential optical imaging modalities. In response to this, the use of NIR imaging has been extensively explored in the field of immunotherapy. To date, NIR fluorescence imaging has successfully offered reliable monitoring of the localization, dynamics, and function of immune responses, which are vital in assessing not only the efficacy but also the safety of treatments to design immunotherapies optimally. This review aims to provide an overview of the current research on NIR imaging of the immune response. We expect that the use of NIR imaging will expand further in response to the recent success in cancer immunotherapy. We will also offer our insights on how this technology will meet rapidly growing expectations in the future.

Influence of the Host and Parasite Strain on the Immune Response During Toxoplasma Infection

Toxoplasma gondii is an exceptionally successful parasite that infects a very broad host range, including humans, across the globe. The outcome of infection differs remarkably between hosts, ranging from acute death to sterile infection. These differential disease patterns are strongly influenced by both host- and parasite-specific genetic factors. In this review, we discuss how the clinical outcome of toxoplasmosis varies between hosts and the role of different immune genes and parasite virulence factors, with a special emphasis on Toxoplasma-induced ileitis and encephalitis.

Organ-specific ECM arrays for investigating Cell-ECM interactions during stem cell differentiation

Pluripotent stem cells are promising source of cells for tissue engineering, regenerative medicine and drug discovery applications. The process of stem cell differentiation is regulated by multi-parametric cues from the surrounding microenvironment, one of the critical one being cell interaction with extracellular matrix (ECM). The ECM is a complex tissue-specific structure which are important physiological regulators of stem cell function and fate. Recapitulating this native ECM microenvironment niche is best facilitated by decellularized tissue/ organ derived ECM, which can faithfully reproduce the physiological environment with high fidelity to in vivo condition and promote tissue-specific cellular development and maturation. Recognizing the need for organ specific ECM in a 3D culture environment in driving phenotypic differentiation and maturation of hPSCs, we fabricated an ECM array platform using native-mimicry ECM from decellularized organs (namely pancreas, liver and heart), which allows cell-ECM interactions in both 2D and 3D configuration. The ECM array was integrated with rapid quantitative imaging for a systematic investigation of matrix protein profiles and sensitive measurement of cell-ECM interaction during hPSC differentiation. We tested our platform by elucidating the role of the three different organ-specific ECM in supporting induced pancreatic differentiation of hPSCs. While the focus of this report is on pancreatic differentiation, the developed platform is versatile to be applied to characterize any lineage specific differentiation.

Robust Control of a Brain-Persisting Parasite through MHC I Presentation by Infected Neurons

Control of CNS pathogens by CD8 T cells is key to avoid fatal neuroinflammation. Yet, the modalities of MHC I presentation in the brain are poorly understood. Here, we analyze the antigen presentation mechanisms underlying CD8 T cell-mediated control of the Toxoplasma gondii parasite in the CNS. We show that MHC I presentation of an efficiently processed model antigen (GRA6-OVA), even when not expressed in the bradyzoite stage, reduces cyst burden and dampens encephalitis in C57BL/6 mice. Antigen presentation assays with infected primary neurons reveal a correlation between lower MHC I presentation of tachyzoite antigens by neurons and poor parasite control in vivo. Using conditional MHC I-deficient mice, we find that neuronal MHC I presentation is required for robust restriction of T. gondii in the CNS during chronic phase, showing the importance of MHC I presentation by CNS neurons in the control of a prevalent brain pathogen.

The Bradyzoite: A Key Developmental Stage for the Persistence and Pathogenesis of Toxoplasmosis

Toxoplasma gondii is a ubiquitous parasitic protist found in a wide variety of hosts, including a large proportion of the human population. Beyond an acute phase which is generally self-limited in immunocompetent individuals, the ability of the parasite to persist as a dormant stage, called bradyzoite, is an important aspect of toxoplasmosis. Not only is this stage not eliminated by current treatments, but it can also reactivate in immunocompromised hosts, leading to a potentially fatal outcome. Yet, despite its critical role in the pathology, the bradyzoite stage is relatively understudied. One main explanation is that it is a considerably challenging model, which essentially has to be derived from in vivo sources. However, recent progress on genetic manipulation and in vitro differentiation models now offers interesting perspectives for tackling key biological questions related to this particularly important developmental stage.

Dendritic Cell Accumulation in the Gut and Central Nervous System Is Differentially Dependent on α4 Integrins

Homing of pathogenic CD4⁺ T cells to the CNS is dependent on α4 integrins. However, it is uncertain whether α4 integrins are also required for the migration of dendritic cell (DC) subsets, which sample Ags from nonlymphoid tissues to present it to T cells. In this study, after genetic ablation of Itga4 in DCs and monocytes in mice via the promoters of Cd11c and Lyz2 (also known as LysM), respectively, the recruitment of α4 integrin-deficient conventional and plasmacytoid DCs to the CNS was unaffected, whereas α4 integrin-deficient, monocyte-derived DCs accumulated less efficiently in the CNS during experimental autoimmune encephalomyelitis in a competitive setting than their wild-type counterparts. In a noncompetitive setting, α4 integrin deficiency on monocyte-derived DCs was fully compensated. In contrast, in small intestine and colon, the fraction of α4 integrin-deficient CD11b⁺CD103⁺ DCs was selectively reduced in steady-state. Yet, T cell-mediated inflammation and host defense against Citrobacter rodentium were not impaired in the absence of α4 integrins on DCs. Thus, inflammatory conditions can promote an environment that is indifferent to α4 integrin expression by DCs.

CD40 in Endothelial Cells Restricts Neural Tissue Invasion by Toxoplasma gondii

Little is known about whether pathogen invasion of neural tissue is affected by immune-based mechanisms in endothelial cells. We examined the effects of endothelial cell CD40 on Toxoplasma gondii invasion of the retina and brain, organs seeded hematogenously. T. gondii circulates in the bloodstream within infected leukocytes (including monocytes and dendritic cells) and as extracellular tachyzoites. After T. gondii infection, mice that expressed CD40 restricted to endothelial cells exhibited diminished parasite loads and histopathology in the retina and brain. These mice also had lower parasite loads in the retina and brain after intravenous (i.v.) injection of infected monocytes or dendritic cells. The protective effect of endothelial cell CD40 was not explained by changes in cellular or humoral immunity, reduced transmigration of leukocytes into neural tissue, or reduced invasion by extracellular parasites. Circulating T. gondii-infected leukocytes (dendritic cells used as a model) led to infection of neural endothelial cells. The number of foci of infection in these cells were reduced if endothelial cells expressed CD40. Infected dendritic cells and macrophages expressed membrane-associated inducible Hsp70. Infected leukocytes triggered Hsp70-dependent autophagy in CD40⁺ endothelial cells and anti-T. gondii activity dependent on ULK1 and beclin 1. Reduced parasite load in the retina and brain not only required CD40 expression in endothelial cells but was also dependent on beclin 1 and the expression of inducible Hsp70 in dendritic cells. These studies suggest that during endothelial cell-leukocyte interaction, CD40 restricts T. gondii invasion of neural tissue through a mechanism that appears mediated by endothelial cell anti-parasitic activity stimulated by Hsp70.

Calling in the CaValry-Toxoplasma gondii Hijacks GABAergic Signaling and Voltage-Dependent Calcium Channel Signaling for Trojan horse-Mediated Dissemination

Dendritic cells (DCs) are regarded as the gatekeepers of the immune system but can also mediate systemic dissemination of the obligate intracellular parasite Toxoplasma gondii. Here, we review the current knowledge on how T. gondii hijacks the migratory machinery of DCs and microglia. Shortly after active invasion by the parasite, infected cells synthesize and secrete the neurotransmitter γ-aminobutyric acid (GABA) and activate GABA-A receptors, which sets on a hypermigratory phenotype in parasitized DCs in vitro and in vivo. The signaling molecule calcium plays a central role for this migratory activation as signal transduction following GABAergic activation is mediated via the L-type voltage-dependent calcium channel (L-VDCC) subtype Cav1.3. These studies have revealed that DCs possess a GABA/L-VDCC/Cav1.3 motogenic signaling axis that triggers migratory activation upon T. gondii infection. Moreover, GABAergic migration can cooperate with chemotactic responses. Additionally, the parasite-derived protein Tg14-3-3 has been associated with hypermigration of DCs and microglia. We discuss the interference of T. gondii infection with host cell signaling pathways that regulate migration. Altogether, T. gondii hijacks non-canonical signaling pathways in infected immune cells to modulate their migratory properties, and thereby promote its own dissemination.

IL-10 and ICOS Differentially Regulate T Cell Responses in the Brain during Chronic Toxoplasma gondii Infection

Control of chronic CNS infection with the parasite Toxoplasma gondii requires ongoing T cell responses in the brain. Immunosuppressive cytokines are also important for preventing lethal immunopathology during chronic infection. To explore the loss of suppressive cytokines exclusively during the chronic phase of infection, we blocked IL-10R in chronically infected mice. Consistent with previous reports, IL-10R blockade led to severe, fatal tissue destruction associated with widespread changes in the inflammatory response, including increased APC activation, expansion of CD4⁺ T cells, and neutrophil recruitment to the brain. We then sought to identify regulatory mechanisms contributing to IL-10 production, focusing on ICOS, a molecule implicated in IL-10 production. Unexpectedly, ICOS ligand (ICOSL) blockade led to a local expansion of effector T cells in the brain without affecting IL-10 production or APC activation. Instead, we found that ICOSL blockade led to changes in T cells associated with their proliferation and survival. We observed increased expression of IL-2-associated signaling molecules CD25, STAT5 phosphorylation, Ki67, and Bcl-2 in T cells in the brain, along with decreased apoptosis. Interestingly, increases in CD25 and Bcl-2 were not observed following IL-10R blockade. Also, unlike IL-10R blockade, ICOSL blockade led to an expansion of both CD8⁺ and CD4⁺ T cells in the brain, with no expansion of peripheral T cells or neutrophil recruitment to the brain and no severe tissue destruction. Overall, these results suggest that IL-10 and ICOS differentially regulate T cell responses in the brain during chronic T. gondii infection.

Epidermal growth factor receptor promotes cerebral and retinal invasion by Toxoplasma gondii

Little is known about strategies used by pathogens to facilitate CNS invasion. Toxoplasma gondii reaches the CNS by circulating in blood within leukocytes or as extracellular tachyzoites. T. gondii induces EGFR signaling in vitro during invasion of mammalian cells. We examined the effects of endothelial cell EGFR on CNS invasion. Transgenic mice whose endothelial cells expressed a dominant negative (DN) EGFR (inhibits EGFR signaling) exhibited diminished parasite load and histopathology in the brain and retina after T. gondii infection. I.V. administration of infected leukocytes or extracellular tachyzoites led to reduced parasite loads in mice with DN EGFR. This was not explained by enhanced immunity or reduced leukocyte recruitment. Endothelial cell infection is key for CNS invasion. Parasite foci in brain endothelial cells were reduced by DN EGFR. DN EGFR in these cells led to recruitment of the autophagy protein LC3 around T. gondii and spontaneous parasite killing dependent on the autophagy protein ULK1 and lysosomal enzymes. The autophagy inhibitor 3-MA prevented DN EGFR mice from exhibiting reduced CNS invasion. Altogether, EGFR is a novel regulator of T. gondii invasion of neural tissue, enhancing invasion likely by promoting survival of the parasite within endothelial cells.

Voltage-dependent calcium channel signaling mediates GABAA receptor-induced migratory activation of dendritic cells infected by Toxoplasma gondii

The obligate intracellular parasite Toxoplasma gondii exploits cells of the immune system to disseminate. Upon T. gondii-infection, γ–aminobutyric acid (GABA)/GABA_A receptor signaling triggers a hypermigratory phenotype in dendritic cells (DCs) by unknown signal transduction pathways. Here, we demonstrate that calcium (Ca²⁺) signaling in DCs is indispensable for T. gondii-induced DC hypermotility and transmigration in vitro. We report that activation of GABA_A receptors by GABA induces transient Ca²⁺ entry in DCs. Murine bone marrow-derived DCs preferentially expressed the L-type voltage-dependent Ca²⁺ channel (VDCC) subtype Ca_v1.3. Silencing of Ca_v1.3 by short hairpin RNA or selective pharmacological antagonism of VDCCs abolished the Toxoplasma-induced hypermigratory phenotype. In a mouse model of toxoplasmosis, VDCC inhibition of adoptively transferred Toxoplasma-infected DCs delayed the appearance of cell-associated parasites in the blood circulation and reduced parasite dissemination to target organs. The present data establish that T. gondii-induced hypermigration of DCs requires signaling via VDCCs and that Ca²⁺ acts as a second messenger to GABAergic signaling via the VDCC Ca_v1.3. The findings define a novel motility-related signaling axis in DCs and unveil that interneurons and DCs share common GABAergic motogenic pathways. T. gondii employs GABAergic non-canonical pathways to induce host cell migration and facilitate dissemination.

Dendritic cells are considered the gatekeepers of the immune system but can, paradoxically, also function as ‘Trojan horses’ to mediate dissemination of the common intracellular parasite Toxoplasma gondii. Previous work has shown that Toxoplasma hijacks the migratory machinery of dendritic cells by inducing secretion of the neurotransmitter GABA and by activating GABAergic signaling pathways, thereby making infected dendritic cells hypermigratory in vitro and in vivo. Here, we show that the signaling molecule calcium plays a central role for this migratory activation and that signal transduction is preferentially mediated through a subtype of voltage-gated calcium channel (Ca_v1.3). This study functionally implicates Ca_v1.3 channels in a, hitherto uncharacterized, calcium signaling axis by which dendritic cells are induced to become migratory. The studies show how an obligate intracellular pathogen takes advantage of non-canonical signaling pathways in immune cells to modulate their migratory properties, and thereby facilitate the dissemination of the parasite.

Brains and Brawn: Toxoplasma Infections of the Central Nervous System and Skeletal Muscle

Toxoplasma gondii is a widespread parasitic pathogen that infects over a third of the world's population. Following an acute infection, the parasite can persist within its mammalian host as intraneuronal or intramuscular cysts. Cysts will occasionally reactivate, and - depending on the host's immune status and site of reactivation - encephalitis or myositis can develop. Because these diseases have high levels of morbidity and can be lethal, it is important to understand how Toxoplasma traffics to these tissues, how the immune response controls parasite burden and contributes to tissue damage, and what mechanisms underlie neurological and muscular pathologies that toxoplasmosis patients present with. This review aims to summarize recent important developments addressing these critical topics.

Behavior of Neutrophil Granulocytes during Toxoplasma gondii Infection in the Central Nervous System

Cerebral toxoplasmosis is characterized by activation of brain resident cells and recruitment of specific immune cell subsets from the periphery to the central nervous system (CNS). Our studies revealed that the rapidly invaded Ly6G⁺ neutrophil granulocytes are an early non-lymphoid source of interferon-gamma (IFN-γ), the cytokine known to be the major mediator of host resistance to Toxoplasma gondii (T. gondii). Upon selective depletion of Ly6G⁺ neutrophils, we detected reduced IFN-γ production and increased parasite burden in the CNS. Ablation of Ly6G⁺ cells resulted in diminished recruitment of Ly6C^hi monocytes into the CNS, indicating a pronounced interplay. Additionally, we identified infiltrated Ly6G⁺ neutrophils to be a heterogeneous population. The Ly6G⁺CD62-L^hiCXCR4⁺ subset released cathelicidin-related antimicrobial peptide (CRAMP), which can promote monocyte dynamics. On the other hand, the Ly6G⁺CD62-L^loCXCR4⁺ subset produced IFN-γ to establish early inflammatory response. Collectively, our findings revealed that the recruited Ly6G⁺CXCR4⁺ neutrophil granulocytes display a heterogeneity in the CNS with a repertoire of effector functions crucial in parasite control and immune regulation upon experimental cerebral toxoplasmosis.

Protective and Pathological Immunity during Central Nervous System Infections

The concept of immune privilege of the central nervous system (CNS) has dominated the study of inflammatory processes in the brain. However, clinically relevant models have highlighted that innate pathways limit pathogen invasion of the CNS and adaptive immunity mediates control of many neural infections. As protective responses can result in bystander damage, there are regulatory mechanisms that balance protective and pathological inflammation, but these mechanisms might also allow microbial persistence. The focus of this review is to consider the host-pathogen interactions that influence neurotropic infections and to highlight advances in our understanding of innate and adaptive mechanisms of resistance as key determinants of the outcome of CNS infection. Advances in these areas have broadened our comprehension of how the immune system functions in the brain and can readily overcome immune privilege.

CD11c-Expressing Cells Affect Regulatory T Cell Behavior in the Meninges during Central Nervous System Infection

Regulatory T cells (Tregs) play an important role in the CNS during multiple infections, as well as autoimmune inflammation, but the behavior of this cell type in the CNS has not been explored. In mice, infection with Toxoplasma gondii leads to a Th1-polarized parasite-specific effector T cell response in the brain. Similarly, Tregs in the CNS during T. gondii infection are Th1 polarized, as exemplified by their T-bet, CXCR3, and IFN-γ expression. Unlike effector CD4⁺ T cells, an MHC class II tetramer reagent specific for T. gondii did not recognize Tregs isolated from the CNS. Likewise, TCR sequencing revealed minimal overlap in TCR sequence between effector T cells and Tregs in the CNS. Whereas effector T cells are found in the brain parenchyma where parasites are present, Tregs were restricted to the meninges and perivascular spaces. The use of intravital imaging revealed that activated CD4⁺ T cells within the meninges were highly migratory, whereas Tregs moved more slowly and were found in close association with CD11c⁺ cells. To test whether the behavior of Tregs in the meninges is influenced by interactions with CD11c⁺ cells, mice were treated with anti-LFA-1 Abs to reduce the number of CD11c⁺ cells in this space. The anti-LFA-1 treatment led to fewer contacts between Tregs and the remaining CD11c⁺ cells and increased the speed of Treg migration. These data suggest that Tregs are anatomically restricted within the CNS, and their interaction with CD11c⁺ populations regulates their local behavior during T. gondii infection.

Splenic differentiation and emergence of CCR5+CXCL9+CXCL10+ monocyte-derived dendritic cells in the brain during cerebral malaria

Dendritic cells have an important role in immune surveillance. After being exposed to microbial components, they migrate to secondary lymphoid organs and activate T lymphocytes. Here we show that during mouse malaria, splenic inflammatory monocytes differentiate into monocyte-derived dendritic cells (MO-DCs), which are CD11b⁺F4/80⁺CD11c⁺MHCII^highDC-SIGN^highLy6c⁺ and express high levels of CCR5, CXCL9 and CXCL10 (CCR5⁺CXCL9/10⁺ MO-DCs). We propose that malaria-induced splenic MO-DCs take a reverse migratory route. After differentiation in the spleen, CCR5⁺CXCL9/10⁺ MO-DCs traffic to the brain in a CCR2-independent, CCR5-dependent manner, where they amplify the influx of CD8⁺ T lymphocytes, leading to a lethal neuropathological syndrome.

Cerebral malaria is an often fatal complication of Plasmodium infection involving accumulation of inflammatory leukocytes in the central nervous system. Here the authors map the development and trafficking of CCR5⁺ monocyte-derived dendritic cells from the spleen to the brains of Plasmodium berghei ANKA infected mice.

Transcriptional profile of SH-SY5Y human neuroblastoma cells transfected by Toxoplasma rhoptry protein 16

Toxoplasma rhoptry protein 16 (ROP16) is crucial in the host-pathogen interaction by acting as a virulent factor during invasion. To improve understanding of the molecular function underlying the effect of ROP16 on host cells, the present study analyzed the transcriptional profile of genes in the ROP16-transfected SH-SY5Y human neuroblastoma cell line. The transcriptional profile of the SH-SY5Y human neuroblastoma cell line overexpressing ROP16 were determined by microarray analysis in order to determine the host neural cell response to the virulent factor. Functional analysis was performed using the Protein Analysis Through Evolutionary Relationships classification system. The ToppGene Suite was used to select candidate genes from the differentially expressed genes. A protein-protein interaction network was constructed using Cytoscape software according to the interaction associations determined using the Search Tool for the Retrieval of Interacting Genes/Proteins. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) analysis of the selected genes confirmed the results of the microarray. The results showed that 383 genes were differentially expressed in response to ROP16 transfection, of which 138 genes were upregulated and 245 genes were downregulated. Functional analysis indicated that the differentially expressed genes (DEGs) were involved in several biological processes, including developmental process, biological regulation and apoptotic process. A total of 15 candidate genes from the DEGs were screened using the ToppGene Suite. No significant differences in expression were observed between the RT-qPCR data and the microarray data. Transfection with ROP16 resulted in alterations of several biological processes, including nervous system development, apoptosis and transcriptional regulation. Several genes, including CXCL12, BAI1, ZIC2, RBMX, RASSF6, MAGE-A6 and HOX, were identified as significant DEGs. Taken together, these results may contribute to understanding the mechanisms underlying the functions of ROP16 and provide scope for further investigation of the pathogenesis of Toxoplasma gondii.

Infection by Toxoplasma gondii Induces Amoeboid-Like Migration of Dendritic Cells in a Three-Dimensional Collagen Matrix

Toxoplasma gondii, an obligate intracellular parasite of humans and other warm-blooded vertebrates, invades a variety of cell types in the organism, including immune cells. Notably, dendritic cells (DCs) infected by T. gondii acquire a hypermigratory phenotype that potentiates parasite dissemination by a ‘Trojan horse’ type of mechanism in mice. Previous studies have demonstrated that, shortly after parasite invasion, infected DCs exhibit hypermotility in 2-dimensional confinements in vitro and enhanced transmigration in transwell systems. However, interstitial migration in vivo involves interactions with the extracellular matrix in a 3-dimensional (3D) space. We have developed a collagen matrix-based assay in a 96-well plate format that allows quantitative locomotion analyses of infected DCs in a 3D confinement over time. We report that active invasion of DCs by T. gondii tachyzoites induces enhanced migration of infected DCs in the collagen matrix. Parasites of genotype II induced superior DC migratory distances than type I parasites. Moreover, Toxoplasma-induced hypermigration of DCs was further potentiated in the presence of the CCR7 chemotactic cue CCL19. Blocking antibodies to integrins (CD11a, CD11b, CD18, CD29, CD49b) insignificantly affected migration of infected DCs in the 3D matrix, contrasting with their inhibitory effects on adhesion in 2D assays. Morphological analyses of infected DCs in the matrix were consistent with the acquisition of an amoeboid-like migratory phenotype. Altogether, the present data show that the Toxoplasma-induced hypermigratory phenotype in a 3D matrix is consistent with integrin-independent amoeboid DC migration with maintained responsiveness to chemotactic and chemokinetic cues. The data support the hypothesis that induction of amoeboid hypermigration and chemotaxis/chemokinesis in infected DCs potentiates the dissemination of T. gondii.

Flt3 Ligand Is Essential for Survival and Protective Immune Responses during Toxoplasmosis

Dendritic cells (DCs) are critical for resistance to Toxoplasma gondii, and infection with this pathogen leads to increased numbers of DCs at local sites of parasite replication and in secondary lymphoid organs, but the factors that regulate this expansion are poorly understood. The cytokine Flt3 ligand (Flt3L) is critical for the generation and maintenance of DCs, and Flt3L(-/-) mice were found to be highly susceptible to acute toxoplasmosis. This phenotype correlated with decreased production of IL-12 and IFN-γ, as well as impaired NK cell responses. Surprisingly, despite low basal numbers of DCs, Flt3L(-/-) mice infected with T. gondii displayed an expansion of CD8α(+) and CD11b(lo)CD8α(-) DCs. Infection also induced an expansion of parasite-specific CD4(+) and CD8(+) T cells in Flt3L(-/-) mice; however, these cells were reduced in number and displayed impaired ability to produce IFN-γ relative to wild-type controls. Exogenous IL-12 treatment partially restored NK and T cell responses in Flt3L(-/-) mice, as well as acute resistance; however, these mice eventually succumbed to toxoplasmic encephalitis, despite the presence of large numbers of DCs and T cells in the brain. These results highlight the importance of Flt3L for resistance to toxoplasmosis and demonstrate the existence of Flt3L-independent pathways that can mediate infection-induced expansion of DCs and T cell priming.

Therapeutic antiviral T cells noncytopathically clear persistently infected microglia after conversion into antigen-presenting cells

Clearance of neurotropic infections is challenging because the CNS is relatively intolerant of immunopathological reactions. Herz et al. use a model of persistent viral infection in mice to demonstrate therapeutic antiviral T cells can purge the CNS infection without causing tissue damage resulting from limited recruitment of inflammatory innate immune cells and conversion of microglia into APCs.

Several viruses can infect the mammalian nervous system and induce neurological dysfunction. Adoptive immunotherapy is an approach that involves administration of antiviral T cells and has shown promise in clinical studies for the treatment of peripheral virus infections in humans such as cytomegalovirus (CMV), Epstein-Barr virus (EBV), and adenovirus, among others. In contrast, clearance of neurotropic infections is particularly challenging because the central nervous system (CNS) is relatively intolerant of immunopathological reactions. Therefore, it is essential to develop and mechanistically understand therapies that noncytopathically eradicate pathogens from the CNS. Here, we used mice persistently infected from birth with lymphocytic choriomeningitis virus (LCMV) to demonstrate that therapeutic antiviral T cells can completely purge the persistently infected brain without causing blood–brain barrier breakdown or tissue damage. Mechanistically, this is accomplished through a tailored release of chemoattractants that recruit antiviral T cells, but few pathogenic innate immune cells such as neutrophils and inflammatory monocytes. Upon arrival, T cells enlisted the support of nearly all brain-resident myeloid cells (microglia) by inducing proliferation and converting them into CD11c⁺ antigen-presenting cells (APCs). Two-photon imaging experiments revealed that antiviral CD8⁺ and CD4⁺ T cells interacted directly with CD11c⁺ microglia and induced STAT1 signaling but did not initiate programmed cell death. We propose that noncytopathic CNS viral clearance can be achieved by therapeutic antiviral T cells reliant on restricted chemoattractant production and interactions with apoptosis-resistant microglia.

Dynamic two-photon imaging of the immune response to Toxoplasma gondii infection

Toxoplasma gondii is a highly successful parasite that can manipulate host immune responses to optimize its persistence and spread. As a result, a highly complex relationship exists between T. gondii and the immune system of the host. Advances in imaging techniques, and in particular, the application of two-photon microscopy to mouse infection models, have made it possible to directly visualize interactions between parasites and the host immune system as they occur in living tissues. Here, we will discuss how dynamic imaging techniques have provided unexpected new insight into (i) how immune responses are dynamically regulated by cells and structures in the local tissue environment, (ii) how protective responses to T. gondii are generated and (iii) how the parasite exploits the immune system for its own benefit.

Persistence of Toxoplasma gondii in the central nervous system: a fine-tuned balance between the parasite, the brain and the immune system

Upon infection of humans and animals with Toxoplasma gondii, the parasites persist as intraneuronal cysts that are controlled, but not eliminated by the immune system. In particular, intracerebral T cells are crucial in the control of T. gondii infection and are supported by essential functions from other leukocyte populations. Additionally, brain-resident cells including astrocytes, microglia and neurons contribute to the intracerebral immune response by the production of cytokines, chemokines and expression of immunoregulatory cell surface molecules, such as major histocompatibility (MHC) antigens. However, the in vivo behaviour of these individual cell populations, specifically their interaction during cerebral toxoplasmosis, remains to be elucidated. We discuss here what is known about the function of T cells, recruited myeloid cells and brain-resident cells, with particular emphasis on the potential cross-regulation of these cell populations, in governing cerebral toxoplasmosis.

Characteristics and critical function of CD8+ T cells in the Toxoplasma-infected brain

The rise of the AIDS epidemic made the requirement for T cells in our continuous protection from pathogens critically apparent. The striking frequency with which AIDS patients exhibited profound neurological pathologies brought attention to many chronic infections that are latent within the immune-privileged CNS. One of the most common lethal opportunistic infections of these patients was with the protozoan parasite, Toxoplasma gondii. Reactivation of Toxoplasma cysts within the brain causes massive tissue destruction evidenced as multiple ring-enhancing lesions on MRI and is called toxoplasmic encephalitis (TE). TE is not limited to AIDS patients, but rather is a risk for all severely immunocompromised patients, including recipients of chemotherapy or transplant recipients. The lessons learned from these patient populations are supported by T cell depletion studies in mice. Such experiments have demonstrated that CD4+ and CD8+ T cells are required for protection against TE. Although it is clear that these T cell subsets work synergistically to fight infection, much evidence has been generated that suggests CD8+ T cells play a dominant role in protection during chronic toxoplasmosis. In other models of CNS inflammation, such as intracerebral infection with LCMV and experimental autoimmune encephalomyelitis (EAE), infiltration of T cells into the brain is harmful and even fatal. In the brain of the immunocompetent host, the well-regulated T cell response to T. gondii is therefore an ideal model to understand a controlled inflammatory response to CNS infection. This review will examine our current understanding of CD8+ T cells in the CNS during T. gondii infection in regards to the (1) mechanisms governing entry into the brain, (2) cues that dictate behavior within the brain, and (3) the functional and phenotypic properties exhibited by these cells.

Dectin-1-CD37 association regulates IL-6 expression during Toxoplasma gondii infection

Toxoplasma gondii can establish chronic infection and is characterized by the formation of tissue cysts in the brain. Although T. gondii can infect any kind of nucleated cells, macrophages and related mononuclear phagocytes are its preferred targets in vivo. Microglial cells are the resident macrophages in the central nervous system. It has been reported that CD37, a tetraspanin molecule, is expressed exclusively in the immune system; Dectin-1, an important pattern-recognition receptor, is expressed on the surface of murine primary microglia. The Dectin-1-CD37 association can affect Dectin-1-mediated IL-6 secretion. However, there is no report concerning the relationship among the expressions of Dectin-1, IL-6, and CD37 during T. gondii infection. In the present study, Kunming outbred mice were infected with Prugniaud (Pru), a type II strain of T. gondii by oral gavage, and BV-2 murine microglial cells were cocultured with RH tachyzoites of T. gondii. By H&E and immunohistochemical staining, the results showed that marked inflammation and a significantly increased activation of Iba1-positive microglial cells were observed in the brain tissues of mice infected with T. gondii Pru strain at 5 weeks postinfection (p.i.) in comparison of uninfected controls. Using quantitative real-time PCR detection, Dectin-1 messenger RNA (mRNA) expressions were significantly upregulated in both brains at 3 (P < 0.01), 5 (P < 0.01), 7 (P < 0.01), and 9 (P < 0.05) weeks p.i. and spleens at 3, 5, 7, and 9 weeks p.i. (P < 0.01). IL-6 expressions showed similar dynamic tendency as that of Dectin-1 in both the brains and spleens at the same times in comparison of uninfected controls; CD37 expressions were significantly increased in the brain tissues at all the times (P < 0.01) and no significant differences in the spleens at 3 weeks p.i. but significantly downregulated in the spleens at 5, 7, and 9 weeks p.i. (P < 0.01). In vitro study showed that compared with uninfected controls, the mRNA expressions of Dectin-1 at 2, 4, 8, and 10 h (P < 0.01); IL-6 at 8 and 10 h (P < 0.01); and CD37 at 4 (P < 0.05), 8 (P < 0.01), and 10 h (P < 0.01) were significantly upregulated in BV-2 murine microglial cells stimulated with RH tachyzoites of T. gondii. Our data suggested that the expression of Dectin-1 was positively correlated with that of IL-6 in toxoplasmic encephalitis (TE) mouse model; Dectin-1 interaction with tetraspanin CD37 regulated IL-6 expression in both the brain tissues of TE mouse model and in the T. gongdii-infected BV-2 murine microglial cells.

Spinal cord pathology in chronic experimental Toxoplasma gondii infection

Infection with the protozoan Toxoplasma (T.) gondii causes chronic infection of the central nervous system and can lead to life-threatening encephalomyelitis in immunocompromised patients. While infection with T. gondii has long time been considered asymptomatic in immunocompetent hosts, this view is challenged by recent reports describing links between seropositivity and behavioral alterations. However, past and current researches are mainly focused on the brain during Toxoplasma encephalitis, neglecting the spinal cord as a key structure conveying brain signals into motion. Therefore, our study aimed to fill the gap and describes the spinal cord pathology in an experimental murine model of toxoplasmosis. In the spinal cord, we found distinct histopathological changes, inflammatory foci and T. gondii cysts similar to the brain. Furthermore, the recruitment of immune cells from the periphery was detected. Moreover, resident microglia as well as recruited monocytes displayed an increased MHC classes I and II expression. Additionally, the expression of pro- and anti-inflammatory cytokines was enhanced in the brain as well as in the spinal cord. In summary, the pathology observed in the spinal cord was similar to the previously described changes in the brain during the infection. This study provides the first detailed description of histopathological and immunological alterations due to experimental T. gondii induced myelitis in mice. Thus, our comparison raises awareness of the importance of the spinal cord in chronic T. gondii infection.

Regulation of the development of the hepatic B cell compartment during Schistosoma mansoni infection

During infection with the helminth parasite Schistosoma mansoni, Ab regulates hepatic inflammation, and local production of Ig in the liver appears to play a role in this process. Exploring the development of the B cell response during infection, we found that parasite-specific IgG1-secreting plasma cells appeared first in the hepatic and mesenteric lymph nodes (LNs) and then at later times in the spleen, liver, and bone marrow. The LN B cell population peaked between weeks 10 and 12 of infection, and then contracted at a time that coincided with the expansion of the hepatic IgG1(+) B cell compartment, suggesting that B cells migrate from LNs to liver. CXCL9 and -16 expression in the liver increased during the time frame of B cell recruitment. Expression of the CXCL16 receptor CXCR6 was increased on B cells within the hepatic LNs, but not the mesenteric LNs. CXCR3, the receptor for CXCL9, was broadly expressed on IgG1(+) B cells in LNs and liver during infection. Increased hepatic expression of CXCL9 and -16 failed to occur if the IL-10R was blocked in vivo, an intervention associated with decreased liver B cell infiltration and the development of severe disease. Hepatic LN IgG1(+) cells migrated toward CXCL9 and -16 in vitro and to the liver in a pertussis toxin-sensitive fashion. Our data suggest that the coordinated expression of CXCL9 and -16 in the liver and of CXCR6 and CXCR3 on responding B cells within the hepatic LNs underpins establishment of the hepatic B cell infiltrate during chronic schistosomiasis.

Rapid cytoskeleton remodelling in dendritic cells following invasion by Toxoplasma gondii coincides with the onset of a hypermigratory phenotype

Host cell manipulation is an important feature of the obligate intracellular parasite Toxoplasma gondii. Recent reports have shown that the tachyzoite stages subvert dendritic cells (DC) as a conduit for dissemination (Trojan horse) during acute infection. To examine the cellular basis of these processes, we performed a detailed analysis of the early events following tachyzoite invasion of human monocyte-derived DC. We demonstrate that within minutes after tachyzoite penetration, profound morphological changes take place in DC that coincide with a migratory activation. Active parasite invasion of DC led to cytoskeletal actin redistribution with loss of adhesive podosome structures and redistribution of integrins (CD18 and CD11c), that concurred with the onset of DC hypermotility in vitro. Inhibition of parasite rhoptry secretion and invasion, but not inhibition of parasite or host cell protein synthesis, abrogated the onset of morphological changes and hypermotility in DC dose-dependently. Also, infected DC, but not by-stander DC, exhibited upregulation of C-C chemokine receptor 7 (CCR7). Yet, the onset of parasite-induced DC hypermotility preceded chemotactic migratory responsesin vitro. Collectively, present data reveal that invasion of DC by T. gondii initiates a series of regulated events, including rapid cytoskeleton rearrangements, hypermotility and chemotaxis, that promote the migratory activation of DC.

Toxoplasma gondii modulates the dynamics of human monocyte adhesion to vascular endothelium under fluidic shear stress

Toxoplasma gondii actively infects circulating immune cells, including monocytes and DCs, and is thought to use these cells as Trojan horses for parasite dissemination. To investigate the interactions of T. gondii-infected human monocytes with vascular endothelium under conditions of shear stress, we developed a fluidic and time-lapse fluorescence microscopy system. Both uninfected and infected monocytes rolled, decelerated, and firmly adhered on TNF-α-activated endothelium. Interestingly, T. gondii-infected primary human monocytes and THP-1 cells exhibited altered adhesion dynamics compared with uninfected monocytes: infected cells rolled at significantly higher velocities (2.5- to 4.6-fold) and over greater distances (2.6- to 4.8-fold) than uninfected monocytes, before firmly adhering. During monocyte searching, 29-36% of infected monocytes compared with 0-11% of uninfected monocytes migrated >10 μm from the point where they initiated searching, and these "wandering" searches were predominantly in the direction of flow. As infected monocytes appeared delayed in their transition to firm adhesion, we examined the effects of infection on integrin expression and function. T. gondii did not affect the expression of LFA-1, VLA-4, or MAC-1 or the ability of Mn(2+) to activate these integrins. However, T. gondii infection impaired LFA-1 and VLA-4 clustering and pseudopod extension in response to integrin ligands. Surprisingly, a single intracellular parasite was sufficient to mediate these effects. This research has established a system for studying pathogen modulation of human leukocyte adhesion under conditions of physiological shear stress and has revealed a previously unappreciated effect of T. gondii infection on ligand-dependent integrin clustering.

Visualizing leukocyte trafficking in the living brain with 2-photon intravital microscopy

Intravital imaging of the superficial brain tissue in mice represents a powerful tool for the dissection of the cellular and molecular cues underlying inflammatory and infectious central nervous system (CNS) diseases. We present here a step-by-step protocol that will enable a non-specialist to set up a two-photon brain-imaging model. The protocol offers a two-part approach that is specifically optimized for imaging leukocytes but can be easily adapted to answer varied CNS-related biological questions. The protocol enables simultaneous visualization of fluorescently labeled immune cells, the pial microvasculature and extracellular structures such as collagen fibers at high spatial and temporal resolution. Intracranial structures are exposed through a cranial window, and physiologic conditions are maintained during extended imaging sessions via continuous superfusion of the brain surface with artificial cerebrospinal fluid (aCSF). Experiments typically require 1-2 h of preparation, which is followed by variable periods of immune cell tracking. Our methodology converges the experience of two laboratories over the past 10 years in diseased animal models such as cerebral ischemia, lupus, cerebral malaria, and toxoplasmosis. We exemplify the utility of this protocol by tracking leukocytes in transgenic mice in the pial vessels under steady-state conditions.

Comparing mechanisms of host manipulation across host and parasite taxa

Parasites affect host behavior in several ways. They can alter activity, microhabitats or both. For trophically transmitted parasites (the focus of our study), decreased activity might impair the ability of hosts to respond to final-host predators, and increased activity and altered microhabitat choice might increase contact rates between hosts and final-host predators. In an analysis of trophically transmitted parasites, more parasite groups altered activity than altered microhabitat choice. Parasites that infected vertebrates were more likely to impair the host’s reaction to predators, whereas parasites that infected invertebrates were more likely to increase the host’s contact with predators. The site of infection might affect how parasites manipulate their hosts. For instance, parasites in the central nervous system seem particularly suited to manipulating host behavior. Manipulative parasites commonly occupy the body cavity, muscles and central nervous systems of their hosts. Acanthocephalans in the data set differed from other taxa in that they occurred exclusively in the body cavity of invertebrates. In addition, they were more likely to alter microhabitat choice than activity. Parasites in the body cavity (across parasite types) were more likely to be associated with increased host contact with predators. Parasites can manipulate the host through energetic drain, but most parasites use more sophisticated means. For instance, parasites target four physiological systems that shape behavior in both invertebrates and vertebrates: neural, endocrine, neuromodulatory and immunomodulatory. The interconnections between these systems make it difficult to isolate specific mechanisms of host behavioral manipulation.

GABAergic signaling is linked to a hypermigratory phenotype in dendritic cells infected by Toxoplasma gondii

During acute infection in human and animal hosts, the obligate intracellular protozoan Toxoplasma gondii infects a variety of cell types, including leukocytes. Poised to respond to invading pathogens, dendritic cells (DC) may also be exploited by T. gondii for spread in the infected host. Here, we report that human and mouse myeloid DC possess functional γ-aminobutyric acid (GABA) receptors and the machinery for GABA biosynthesis and secretion. Shortly after T. gondii infection (genotypes I, II and III), DC responded with enhanced GABA secretion in vitro. We demonstrate that GABA activates GABA_A receptor-mediated currents in T. gondii-infected DC, which exhibit a hypermigratory phenotype. Inhibition of GABA synthesis, transportation or GABA_A receptor blockade in T. gondii-infected DC resulted in impaired transmigration capacity, motility and chemotactic response to CCL19 in vitro. Moreover, exogenous GABA or supernatant from infected DC restored the migration of infected DC in vitro. In a mouse model of toxoplasmosis, adoptive transfer of infected DC pre-treated with GABAergic inhibitors reduced parasite dissemination and parasite loads in target organs, e.g. the central nervous system. Altogether, we provide evidence that GABAergic signaling modulates the migratory properties of DC and that T. gondii likely makes use of this pathway for dissemination. The findings unveil that GABA, the principal inhibitory neurotransmitter in the brain, has activation functions in the immune system that may be hijacked by intracellular pathogens.

Toxoplasma gondii is an obligate intracellular protozoan parasite and an important food- and water-borne human and veterinary pathogen. Toxoplasmosis is normally self-limiting but severe manifestations occur upon congenital transmission to the developing fetus or during infection in immune-compromised individuals. Toxoplasma invades a variety of cell types and mounting evidence shows that certain white blood cells, e.g. dendritic cells, can shuttle parasites in the infected host by a Trojan horse type of mechanism. Dendritic cells are considered the gatekeepers of the immune system but can, paradoxically, also mediate dissemination of the parasite. Previous work has shown that Toxoplasma induces a hypermigratory state in dendritic cells when they become infected. Here, we show that, shortly after infection by the parasite, dendritic cells start secreting γ-aminobutyric acid (GABA), also known as the major inhibitory neurotransmitter in the brain. We show that dendritic cells express GABA receptors, as well as the machinery to synthesize and transport GABA. When GABA synthesis, transport or receptor function was inhibited, the migration of infected dendritic cells was impaired. In a mouse model of toxoplasmosis, treatment of infected dendritic cells with GABA inhibitors resulted in reduced propagation of the parasite. This study establishes that GABAergic signaling modulates the migratory properties of dendritic cells and that the intracellular pathogen Toxoplasma gondii sequesters the GABAergic signaling of dendritic cells to assure propagation.

Chitinase dependent control of protozoan cyst burden in the brain

Chronic infections represent a continuous battle between the host's immune system and pathogen replication. Many protozoan parasites have evolved a cyst lifecycle stage that provides it with increased protection from environmental degradation as well as endogenous host mechanisms of attack. In the case of Toxoplasma gondii, these cysts are predominantly found in the immune protected brain making clearance of the parasite more difficult and resulting in a lifelong infection. Currently, little is known about the nature of the immune response stimulated by the presence of these cysts or how they are able to propagate. Here we establish a novel chitinase-dependent mechanism of cyst control in the infected brain. Despite a dominant Th1 immune response during Toxoplasma infection there exists a population of alternatively activated macrophages (AAMØ) in the infected CNS. These cells are capable of cyst lysis via the production of AMCase as revealed by live imaging, and this chitinase is necessary for protective immunity within the CNS. These data demonstrate chitinase activity in the brain in response to a protozoan pathogen and provide a novel mechanism to facilitate cyst clearance during chronic infections.

Described here is a novel mechanism of protozoan cyst clearance in the CNS during chronic infection. These data show the presence of a population of alternatively activated macrophages in the brain that secrete the active chitinase, AMCase, in response to chitin in the cyst wall. Using both chemical and genetic inhibition in vitro, it is revealed that this enzyme is required for efficient degradation and destruction of the cyst. The necessity for AMCase is demonstrated in vivo, as the absence of the enzyme resulted in a significant increase in cyst burden and decrease in survival during chronic infection. Together, these data identify an important mechanism of parasite control and cyst clearance in the CNS. Currently, no therapies exist that lead to the total clearance of this parasite from the brain. Therefore, developing an understanding of the natural mechanisms of cyst clearance has the potential to lead to new and effective therapies for this and other chronic infections.

Use and abuse of dendritic cells by Toxoplasma gondii

The ubiquitous apicomplexan parasite Toxoplasma gondii stimulates its host’s immune response to achieve quiescent chronic infection. Central to this goal are host dendritic cells. The parasite exploits dendritic cells to disseminate through the body, produce pro-inflammatory cytokines, present its antigens to the immune system and yet at the same time subvert their signaling pathways in order to evade detection. This carefully struck balance by Toxoplasma makes it the most successful parasite on this planet. Recent progress has highlighted specific parasite and host molecules that mediate some of these processes particularly in dendritic cells and in other cells of the innate immune system. Critically, there are several important factors that need to be taken into consideration when concluding how the dendritic cells and the immune system deal with a Toxoplasma infection, including the route of administration, parasite strain and host genotype.

Brain dendritic cells: biology and pathology

Dendritic cells (DC) are the professional antigen-presenting cells of the immune system. In their quiescent and mature form, the presentation of self-antigens by DC leads to tolerance; whereas, antigen presentation by mature DC, after stimulation by pathogen-associated molecular patterns, leads to the onset of antigen-specific immunity. DC have been found in many of the major organs in mammals (e.g. skin, heart, lungs, intestines and spleen); while the brain has long been considered devoid of DC in the absence of neuroinflammation. Consequently, microglia, the resident immune cell of the brain, have been charged with many functional attributes commonly ascribed to DC. Recent evidence has challenged the notion that DC are either absent or minimal players in brain immune surveillance. This review will discuss the recent literature examining DC involvement within both the young and aged steady-state brain. We will also examine DC contributions during various forms of neuroinflammation resulting from neurodegenerative autoimmune disease, injury, and CNS infections. This review also touches upon DC trafficking between the central nervous system and peripheral immune compartments during viral infections, the new molecular technologies that could be employed to enhance our current understanding of brain DC ontogeny, and some potential therapeutic uses of DC within the CNS.

Migration of toxoplasma gondii-infected dendritic cells across human retinal vascular endothelium

PURPOSE

Toxoplasma gondii, the parasite responsible for ocular toxoplasmosis, accesses the retina from the bloodstream. We investigated the dendritic cell as a potential taxi for T. gondii tachyzoites moving across the human retinal endothelium, and examined the participation of adhesion molecules and chemokines in this process.

METHODS

CD14-positive monocytes were isolated from human peripheral blood by antibody-mediated cell enrichment, and cultured in granulocyte-macrophage colony-stimulating factor and interleukin-4 to generate dendritic cells. Transmigration assays were performed over 18 hours in transwells seeded with human retinal endothelial cells and using dendritic cells exposed to laboratory or natural strains of T. gondii tachyzoites. Parasites were tagged with yellow fluorescent protein to verify infection. In some experiments, endothelial monolayers were preincubated with antibody directed against adhesion molecules, or chemokine was added to lower chambers of transwells.

RESULTS

Human monocyte-derived dendritic cell preparations infected with laboratory or natural strain T. gondii tachyzoites transmigrated in larger numbers across simulated human retinal endothelium than uninfected dendritic cells (P ≤ 0.0004 in 5 of 6 experiments). Antibody blockade of intercellular adhesion molecule (ICAM)-1, vascular cell adhesion molecule (VCAM)-1, and activated leukocyte cell adhesion molecule (ALCAM) inhibited transmigration (P ≤ 0.007), and CCL21 or CXCL10 increased transmigration (P ≤ 0.031).

CONCLUSIONS

Transmigration of human dendritic cells across retinal endothelium is increased following infection with T. gondii. Movement may be impacted by locally produced chemokines and is mediated in part by ICAM-1, VCAM-1, and ALCAM. These findings have implications for development of novel therapeutics aimed at preventing retinal infection by T. gondii.

Toxoplasma gondii and the blood-brain barrier

Infection with the protozoan parasite Toxoplasma gondii is characterized by asymptomatic latent infection in the central nervous system and skeletal muscle tissue in the majority of immunocompentent individuals. Life-threatening reactivation of the infection in immunocompromized patients originates from rupture of Toxoplasma cysts in the brain. While major progress has been made in our understanding of the immunopathogenesis of infection the mechanism(s) of neuroinvasion of the parasite remains poorly understood. The present review presents the current understanding of blood-brain barrier (patho)physiology and the interaction of Toxoplasma gondii with cells of the blood-brain barrier.