Immunomodulatory Effects of the Neuropeptide Pituitary Adenylate Cyclase-Activating Polypeptide in Acute Toxoplasmosis

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) is an endogenous neuropeptide with distinct functions including the regulation of inflammatory processes. PACAP is able to modify the immune response by directly regulating macrophages and monocytes inhibiting the production of inflammatory cytokines, chemokines and free radicals. Here, we analyzed the effect of exogenous PACAP on peripheral immune cell subsets upon acute infection with the parasite Toxoplasma gondii (T. gondii). PACAP administration was followed by diminished innate immune cell recruitment to the peritoneal cavity of T. gondii-infected mice. PACAP did not directly interfere with parasite replication, instead, indirectly reduced parasite burden in mononuclear cell populations by enhancing their phagocytic capacity. Although proinflammatory cytokine levels were attenuated in the periphery upon PACAP treatment, interleukin (IL)-10 and Transforming growth factor beta (TGF-β) remained stable. While PACAP modulated VPAC1 and VPAC2 receptors in immune cells upon binding, it also increased their expression of brain-derived neurotrophic factor (BDNF). In addition, the expression of p75 neurotrophin receptor (p75^NTR) on Ly6C^hi inflammatory monocytes was diminished upon PACAP administration. Our findings highlight the immunomodulatory effect of PACAP on peripheral immune cell subsets during acute Toxoplasmosis, providing new insights about host-pathogen interaction and the effects of neuropeptides during inflammation.

p75NTR regulates brain mononuclear cell function and neuronal structure in Toxoplasma infection-induced neuroinflammation

Neurotrophins mediate neuronal growth, differentiation, and survival via tropomyosin receptor kinase (Trk) or p75 neurotrophin receptor (p75^NTR) signaling. The p75^NTR is not exclusively expressed by neurons but also by certain immune cells, implying a role for neurotrophin signaling in the immune system. In this study, we investigated the effect of p75^NTR on innate immune cell behavior and on neuronal morphology upon chronic Toxoplasma gondii (T. gondii) infection‐induced neuroinflammation. Characterization of the immune cells in the periphery and central nervous system (CNS) revealed that innate immune cell subsets in the brain upregulated p75^NTR upon infection in wild‐type mice. Although cell recruitment and phagocytic capacity of p75^NTRexonIV knockout (p75^−/−) mice were not impaired, the activation status of resident microglia and recruited myeloid cell subsets was altered. Importantly, recruited mononuclear cells in brains of infected p75^−/− mice upregulated the production of the cytokines interleukin (IL)‐10, IL‐6 as well as IL‐1α. Protein levels of proBDNF, known to negatively influence neuronal morphology by binding p75^NTR, were highly increased upon chronic infection in the brain of wild‐type and p75^−/− mice. Moreover, upon infection the activated immune cells contributed to the proBDNF release. Notably, the neuroinflammation‐induced changes in spine density were rescued in the p75^−/− mice. In conclusion, these findings indicate that neurotrophin signaling via the p75^NTR affects innate immune cell behavior, thus, influencing the structural plasticity of neurons under inflammatory conditions.

Comprehensive Kinetic Survey of Intestinal, Extra-Intestinal and Systemic Sequelae of Murine Ileitis Following Peroral Low-Dose Toxoplasma gondii Infection

We have recently shown that following peroral low-dose Toxoplasma gondii infection susceptible mice develop subacute ileitis within 10 days. Data regarding long-term intestinal and extra-intestinal sequelae of infection are scarce, however. We therefore challenged conventional C57BL/6 mice with one cyst of T. gondii ME49 strain by gavage and performed a comprehensive immunopathological survey 10, 36, and 57 days later. As early as 10 days post-infection, mice were suffering from subacute ileitis as indicated by mild-to-moderate histopathological changes of the ileal mucosa. Furthermore, numbers of apoptotic and proliferating/regenerating epithelial cells as well as of T and B lymphocytes in the mucosa and lamina propria of the ileum were highest at day 10 post-infection, but declined thereafter, and were accompanied by enhanced pro-inflammatory mediator secretion in ileum, colon and mesenteric lymph nodes that was most pronounced during the early phase of infection. In addition, subacute ileitis was accompanied by distinct shifts in the commensal gut microbiota composition in the small intestines. Remarkably, immunopathological sequelae of T. gondii infection were not restricted to the intestines, but could also be observed in extra-intestinal tissues including the liver, kidneys, lungs, heart and strikingly, in systemic compartments that were most prominent at day 10 post-infection. We conclude that the here provided long-term kinetic survey of immunopathological sequalae following peroral low-dose T. gondii infection provides valuable corner stones for a better understanding of the complex interactions within the triangle relationship of (parasitic) pathogens, the host immunity and the commensal gut microbiota during intestinal inflammation. The low-dose T. gondii infection model may be applied as valuable gut inflammation model in future pre-clinical studies in order to test potential treatment options for intestinal inflammatory conditions in humans.

Host immune responses to Toxoplasma gondii

Toxoplasma gondii can infect homoeothermic animals including humans and cause lethal toxoplasmosis in immunocompromised individuals. When hosts are infected with T. gondii, the cells induce immune responses against T. gondii. The pathogen infection is recognized by immune sensors that directly detect T. gondii structural components, leading to production of pro-inflammatory cytokines and chemokines. Antigen-presenting cells such as macrophages and dendritic cells strongly activate T cells and induce development of Th1 cells and antigen-specific killer CD8 T cells. These T cells and Group 1 innate lymphoid cells are main producers of IFN-γ, which robustly stimulates cell-autonomous immunity in cells infected with T. gondii. IFN-γ-inducible effectors such as IFN-inducible GTPases, inducible nitric oxide synthase and indoleamine-2,3-dioxygenase differentially play important roles in suppression of T. gondii growth and its direct killing in anti-T. gondii cell-autonomous immune responses. In this review, we will describe our current knowledge of innate, adaptive and IFN-γ-mediated cell-autonomous immunity against T. gondii infection.

Advances and Challenges in Understanding Cerebral Toxoplasmosis

Toxoplasma gondii is a widespread parasitic pathogen that infects over one third of the global human population. The parasite invades and chronically persists in the central nervous system (CNS) of the infected host. Parasite spread and persistence is intimately linked to an ensuing immune response, which does not only limit parasite-induced damage but also may facilitate dissemination and induce parasite-associated immunopathology. Here, we discuss various aspects of toxoplasmosis where knowledge is scarce or controversial and, the recent advances in the understanding of the delicate interplay of T. gondii with the immune system in experimental and clinical settings. This includes mechanisms for parasite passage from the circulation into the brain parenchyma across the blood-brain barrier during primary acute infection. Later, as chronic latent infection sets in with control of the parasite in the brain parenchyma, the roles of the inflammatory response and of immune cell responses in this phase of the disease are discussed. Additionally, the function of brain resident cell populations is delineated, i.e., how neurons, astrocytes and microglia serve both as target cells for the parasite but also actively contribute to the immune response. As the infection can reactivate in the CNS of immune-compromised individuals, we bring up the immunopathogenesis of reactivated toxoplasmosis, including the special case of congenital CNS manifestations. The relevance, advantages and limitations of rodent infection models for the understanding of human cerebral toxoplasmosis are discussed. Finally, this review pinpoints questions that may represent challenges to experimental and clinical science with respect to improved diagnostics, pharmacological treatments and immunotherapies.

Disruption of Purinergic Receptor P2X7 Signaling Increases Susceptibility to Cerebral Toxoplasmosis

Toxoplasmosis is a neglected disease that affects millions of individuals worldwide. Toxoplasma gondii infection is an asymptomatic disease, with lethal cases occurring mostly in HIV patients and organ transplant recipients. Nevertheless, atypical strains of T. gondii in endemic locations cause severe pathology in healthy individuals. Toxoplasmosis has no cure but it can be controlled by the proinflammatory immune response. The purinergic receptor P2X7 (P2X7) is involved in many inflammatory events and has been associated with genes that confer resistance against toxoplasmosis in humans. In vitro studies have reported parasite death after P2X7-receptor activation in various cell types. To understand the contribution of P2X7 during cerebral toxoplasmosis, wild-type and P2rx7 knockout mice were infected orally with T. gondii and their pathologic profiles were analyzed. We found that all P2rx7^-/- mice died 8 weeks after infection with an increased number of cysts and fewer inflammatory infiltrates in their brains. The cytokines interleukin-1β, interleukin-12, tumor necrosis factor-α, and reactive oxygen species were absent or reduced in P2rx7^-/- mice. Taken together, these data suggest that the P2X7 receptor promotes inflammatory infiltrates, proinflammatory cytokines, and reactive oxygen species production in the brain, and that P2X7 signaling mediates major events that confer resistance to cerebral toxoplasmosis.

CD11a expression distinguishes infiltrating myeloid cells from plaque-associated microglia in Alzheimer's disease

Alzheimer's disease (AD) is the leading cause of age-related neurodegeneration and is characterized neuropathologically by the accumulation of insoluble beta-amyloid (Aβ) peptides. In AD brains, plaque-associated myeloid (PAM) cells cluster around Aβ plaques but fail to effectively clear Aβ by phagocytosis. PAM cells were originally thought to be brain-resident microglia. However, several studies have also suggested that Aβ-induced inflammation causes peripheral monocytes to enter the otherwise immune-privileged brain. The relationship between AD progression and inflammation in the brain remains ambiguous because microglia and monocyte-derived macrophages are extremely difficult to distinguish from one another in an inflamed brain. Whether PAM cells are microglia, peripheral macrophages, or a mixture of both remains unclear. CD11a is a component of the β2 integrin LFA1. We have determined that CD11a is highly expressed on peripheral immune cells, including macrophages, but is not expressed by mouse microglia. These expression patterns remain consistent in LPS-treated inflamed mice, as well as in two mouse models of AD. Thus, CD11a can be used as a marker to distinguish murine microglia from infiltrating peripheral immune cells. Using CD11a, we show that PAM cells in AD transgenic brains are comprised entirely of microglia. We also demonstrate a novel fluorescence-assisted quantification technique (FAQT), which reveals a significant increase in T lymphocytes, especially in the brains of female AD mice. Our findings support the notion that microglia are the lead myeloid players in AD and that rejuvenating their phagocytic potential may be an important therapeutic strategy.

Chronic Toxoplasma infection is associated with distinct alterations in the synaptic protein composition

Background

Chronic infection with the neurotropic parasite Toxoplasma gondii has been implicated in the risk for several neuropsychiatric disorders. The mechanisms, by which the parasite may alter neural function and behavior of the host, are not yet understood completely.

Methods

Here, a novel proteomic approach using mass spectrometry was employed to investigate the alterations in synaptic protein composition in a murine model of chronic toxoplasmosis. In a candidate-based strategy, immunoblot analysis and immunohistochemistry were applied to investigate the expression levels of key synaptic proteins in glutamatergic signaling.

Results

A comparison of the synaptosomal protein composition revealed distinct changes upon infection, with multiple proteins such as EAAT2, Shank3, AMPA receptor, and NMDA receptor subunits being downregulated, whereas inflammation-related proteins showed an upregulation. Treatment with the antiparasitic agent sulfadiazine strongly reduced tachyzoite levels and diminished neuroinflammatory mediators. However, in both conditions, a significant number of latent cysts persisted in the brain. Conversely, infection-related alterations of key synaptic protein levels could be partly reversed by the treatment.

Conclusion

These results provide evidence for profound changes especially in synaptic protein composition in T. gondii-infected mice with a downregulation of pivotal components of glutamatergic neurotransmission. Our results suggest that the detected synaptic alterations are a consequence of the distinct neuroinflammatory milieu caused by the neurotropic parasite.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1242-1) contains supplementary material, which is available to authorized users.

Transcriptional repression by ApiAP2 factors is central to chronic toxoplasmosis

Tachyzoite to bradyzoite development in Toxoplasma is marked by major changes in gene expression resulting in a parasite that expresses a new repertoire of surface antigens hidden inside a modified parasitophorous vacuole called the tissue cyst. The factors that control this important life cycle transition are not well understood. Here we describe an important transcriptional repressor mechanism controlling bradyzoite differentiation that operates in the tachyzoite stage. The ApiAP2 factor, AP2IV-4, is a nuclear factor dynamically expressed in late S phase through mitosis/cytokinesis of the tachyzoite cell cycle. Remarkably, deletion of the AP2IV-4 locus resulted in the expression of a subset of bradyzoite-specific proteins in replicating tachyzoites that included tissue cyst wall components BPK1, MCP4, CST1 and the surface antigen SRS9. In the murine animal model, the mis-timing of bradyzoite antigens in tachyzoites lacking AP2IV-4 caused a potent inflammatory monocyte immune response that effectively eliminated this parasite and prevented tissue cyst formation in mouse brain tissue. Altogether, these results indicate that suppression of bradyzoite antigens by AP2IV-4 during acute infection is required for Toxoplasma to successfully establish a chronic infection in the immune-competent host.

The Toxoplasma biology that underlies the establishment of a chronic infection is developmental conversion of the acute tachyzoite stage into the latent bradyzoite-tissue cyst stage. Despite the important clinical consequences of this developmental pathway, the molecular basis of the switch mechanisms that control formation of the tissue cyst is still poorly understood. A fundamental feature of tissue cyst formation is the expression of bradyzoite-specific genes. Here we show the transcription factor AP2IV-4 directly silences bradyzoite mRNA and protein expression in the acute tachyzoite stage demonstrating that developmental control of tissue cyst formation is as much about when not to express bradyzoite genes as it is about when to activate them. Losing the suppression of bradyzoite gene expression in the acute tachyzoite stage caused by deleting AP2IV-4 blocked the establishment of chronic disease in healthy animals via increased protective immunity suggesting a possible strategy for preventing chronic Toxoplasma infections.

Hypothermia and brain inflammation after cardiac arrest

The cessation (ischemia) and restoration (reperfusion) of cerebral blood flow after cardiac arrest (CA) induce inflammatory processes that can result in additional brain injury. Therapeutic hypothermia (TH) has been proven as a brain protective strategy after CA. In this article, the underlying pathophysiology of ischemia-reperfusion brain injury with emphasis on the role of inflammatory mechanisms is reviewed. Potential targets for immunomodulatory treatments and relevant effects of TH are also discussed. Further studies are needed to delineate the complex pathophysiology and interactions among different components of immune response after CA and identify appropriate targets for clinical investigations.

Peroral Low-Dose Toxoplasma gondii Infection of Human Microbiota-Associated Mice - A Subacute Ileitis Model to Unravel Pathogen-Host Interactions

Within 1 week following high-dose Toxoplasma gondii infection, mice develop lethal necrotizing ileitis. However, data from a subacute T. gondii-induced ileitis model are scarce. Therefore, mice harboring a human gut microbiota were perorally infected with one cyst of T. gondii. Within 9 days post-infection, the intestinal microbiota composition shifted towards higher loads of commensal enterobacteria and enterococci. Following T. gondii infection, mice were clinically only mildly affected, whereas ≈60% of mice displayed fecal blood and mild-to-moderate ileal histopathological changes. Intestinal inflammation was further characterized by increased apoptotic intestinal epithelial cells, which were accompanied by elevated proliferating gut epithelial cell numbers. As compared to naive controls, infected mice displayed elevated numbers of intestinal T lymphocytes and regulatory T-cells and increased pro-inflammatory mediator secretion. Remarkably, T. gondii-induced apoptotic and pro-inflammatory immune responses were not restricted to the gut, but could also be observed in extra-intestinal compartments including kidney, liver, and lung. Strikingly, low-dose T. gondii infection resulted in increased serum levels of pro- and anti-inflammatory cytokines. In conclusion, the here presented subacute ileitis model following peroral low-dose T. gondii infection of humanized mice allows for detailed investigations of the molecular mechanism underlying the “ménage à trois” of pathogens, human gut microbiota, and immunity.

Inducible nitric oxide synthase in innate immune cells is important for restricting cyst formation of Toxoplasma gondii in the brain but not required for the protective immune process to remove the cysts

Significantly larger numbers of Toxoplasma gondii cysts were detected in the brains of RAG1^-/-NOS2^-/- than RAG1^-/- mice following infection. In contrast, the cyst numbers markedly decreased in a same manner in both strains of mice after receiving CD8⁺ immune T cells. Thus, NOS2-mediated innate immunity is important for inhibiting formation of cysts in the brain but not required for the T cell-initiated cyst removal, which is associated with phagocyte accumulation. Treatment with chloroquine, an inhibitor of endolysosomal acidification, partially but significantly inhibited the T cell-mediated cyst removal, suggesting that phagosome-lysosome fusion could be involved in the T. gondii cyst elimination.

Effects of Acyclovir and IVIG on Behavioral Outcomes after HSV1 CNS Infection

Herpes simplex virus 1 (HSV) encephalitis (HSE) has serious neurological complications, involving behavioral and cognitive impairments that cause significant morbidity and a reduced quality of life. We showed that HSE results from dysregulated central nervous system (CNS) inflammatory responses. We hypothesized that CNS inflammation is casually involved in behavioral abnormalities after HSE and that treatment with ACV and pooled human immunoglobulin (IVIG), an immunomodulatory drug, would improve outcomes compared to mice treated with phosphate buffered saline (PBS) or ACV alone. Anxiety levels were high in HSV-infected PBS and ACV-treated mice compared to mice treated with ACV + IVIG, consistent with reports implicating inflammation in anxiety induced by lipopolysaccharide (LPS) or stress. Female, but not male, PBS-treated mice were cognitively impaired, and unexpectedly, ACV was protective, while the inclusion of IVIG surprisingly antagonized ACV's beneficial effects. Distinct serum proteomic profiles were observed for male and female mice, and the antagonistic effects of ACV and IVIG on behavior were paralleled by similar changes in the serum proteome of ACV- and ACV + IVIG-treated mice. We conclude that inflammation and other factors mediate HSV-induced behavioral impairments and that the effects of ACV and IVIG on behavior involve novel mechanisms.

Ly6Chi inflammatory monocytes promote susceptibility to Leishmania donovani infection

Ly6C^hi inflammatory monocytes (iMO) are critical for host defense against toxoplasmosis and malaria but their role in leishmaniasis is unclear. In this study, we report a detrimental role of Ly6C^hi iMOs in visceral leishmaniasis (VL) caused by Leishmania donovani. We demonstrate that Ly6C^hi iMOs are continuously recruited into the spleen and liver during L. donovani infection and they are preferential targets for the parasite. Using microarray-based gene expression profiling, we show that Ly6C^hi iMOs isolated from the infected liver and spleen have distinct phenotypic and activation profiles. Furthermore, we demonstrate that blocking the recruitment of Ly6C^hi iMOs into the liver and spleen during L. donovani infection using a CCR2 antagonist reduces the frequency of the pathogenic IFN-γ/IL10 dual producer CD4+ T cells in the spleen and leads to a significant reduction in parasite loads in the liver and spleen. Using STAT1−/− mice we show that STAT1 is critical for mediating the recruitment of Ly6C^hi iMOs into organs during L. donovani infection, and adaptive transfer of wild type Ly6C^hi iMOs into STAT1−/− recipients renders them susceptible to disease. Our findings reveal an unexpected pathogenic role for Ly6C^hi iMOs in promoting parasite survival in VL and open the possibility of targeting this population for host-directed therapy during VL.

Toxoplasma Co-infection Prevents Th2 Differentiation and Leads to a Helminth-Specific Th1 Response

Nematode infections, in particular gastrointestinal nematodes, are widespread and co-infections with other parasites and pathogens are frequently encountered in humans and animals. To decipher the immunological effects of a widespread protozoan infection on the anti-helminth immune response we studied a co-infection with the enteric nematode Heligmosomoides polygyrus in mice previously infected with Toxoplasma gondii. Protective immune responses against nematodes are dependent on parasite-specific Th2 responses associated with IL-4, IL-5, IL-13, IgE, and IgG1 antibodies. In contrast, Toxoplasma gondii infection elicits a strong and protective Th1 immune response characterized by IFN-γ, IL-12, and IgG2a antibodies. Co-infected animals displayed significantly higher worm fecundity although worm burden remained unchanged. In line with this, the Th2 response to H. polygyrus in co-infected animals showed a profound reduction of IL-4, IL-5, IL-13, and GATA-3 expressing T cells. Co-infection also resulted in the lack of eosinophilia and reduced expression of the Th2 effector molecule RELM-β in intestinal tissue. In contrast, the Th1 response to the protozoan parasite was not diminished and parasitemia of T. gondii was unaffected by concurrent helminth infection. Importantly, H. polygyrus specific restimulation of splenocytes revealed H. polygyrus-reactive CD4⁺ T cells that produce a significant amount of IFN-γ in co-infected animals. This was not observed in animals infected with the nematode alone. Increased levels of H. polygyrus-specific IgG2a antibodies in co-infected mice mirrored this finding. This study suggests that polarization rather than priming of naive CD4⁺ T cells is disturbed in mice previously infected with T. gondii. In conclusion, a previous T. gondii infection limits a helminth-specific Th2 immune response while promoting a shift toward a Th1-type immune response.

Enrichment of Ly6Chi monocytes by multiple GM-CSF injections with HBV vaccine contributes to viral clearance in a HBV mouse model

Adjuvants are considered a necessary component for HBV therapeutic vaccines but few are licensed in clinical practice due to concerns about safety or efficiency. In our recent study, we established that a combination protocol of 3-day pretreatments with GM-CSF before a vaccination (3 × GM-CSF+VACCINE) into the same injection site could break immune tolerance and cause over 90% reduction of HBsAg level in the HBsAg transgenic mouse model. Herein, we further investigated the therapeutic potential of the combination in AAV8–1.3HBV-infected mice. After 4 vaccinations, both serum HBeAg and HBsAg were cleared and there was a 95% reduction of HBV-positive hepatocytes, in addition to the presence of large number of infiltrating CD8⁺ T cells in the livers. Mechanistically, the HBV-specific T-cell responses were elicited via a 3 × GM-CSF+VACCINE-induced conversion of CCR2-dependent CD11b⁺ Ly6C^hi monocytes into CD11b⁺CD11c⁺ DCs. Experimental depletion of Ly6C^hi monocytes resulted in a defective HBV-specific immune response thereby abrogating HBV eradication. This vaccination strategy could lead to development of an effective therapeutic protocol against chronic HBV in infected patients.

The role of monocytes in models of infection by protozoan parasites

The confirmation of developmental differences between tissue macrophages and peripheral monocytes has changed our view of the functions and dynamics of these two important components of the innate immune system. It has been demonstrated conclusively that homeostasis of tissue resident macrophages is maintained by a low proliferative turn over. During an inflammatory response, bone marrow derived monocytes enter the tissue in large numbers and take part in the defense against the pathogens. After the destruction of invading pathogens, these cells disappear and tissue resident macrophages can be detected again. This new appreciation of the innate immune response has not only answered many outstanding questions regarding the role of the different myeloid cell types in inflammation, but also opened up new areas of research relating to the tissue- and pathogen-specific fate of the inflammatory macrophages or dendritic cells (DCs), and the transfer of this knowledge from mouse models to the human immune system. Nevertheless, there is still confusion in infection models, and especially in studies of human infections, as to what extent these recent observations and findings influence previous interpretations of data. This review will focus on insights from mouse models, summarize the literature on the ontogeny of macrophages and monocytes, explain the role of frequently used monocyte markers and effector molecules, and finally, discuss the role of inflammatory monocytes/macrophages/DCs in two experimental parasitic diseases.

Dissecting Amyloid Beta Deposition Using Distinct Strains of the Neurotropic Parasite Toxoplasma gondii as a Novel Tool

Genetic and pathologic data suggest that amyloid beta (Aβ), produced by processing of the amyloid precursor protein, is a major initiator of Alzheimer's disease (AD). To gain new insights into Aβ modulation, we sought to harness the power of the coevolution between the neurotropic parasite Toxoplasma gondii and the mammalian brain. Two prior studies attributed Toxoplasma-associated protection against Aβ to increases in anti-inflammatory cytokines (TGF-β and IL-10) and infiltrating phagocytic monocytes. These studies only used one Toxoplasma strain making it difficult to determine if the noted changes were associated with Aβ protection or simply infection. To address this limitation, we infected a third human amyloid precursor protein AD mouse model (J20) with each of the genetically distinct, canonical strains of Toxoplasma (Type I, Type II, or Type III). We then evaluated the central nervous system (CNS) for Aβ deposition, immune cell responses, global cytokine environment, and parasite burden. We found that only Type II infection was protective against Aβ deposition despite both Type II and Type III strains establishing a chronic CNS infection and inflammatory response. Compared with uninfected and Type I-infected mice, both Type II- and Type III-infected mice showed increased numbers of CNS T cells and microglia and elevated pro-inflammatory cytokines, but neither group showed a >2-fold elevation of TGF-β or IL-10. These data suggest that we can now use our identification of protective (Type II) and nonprotective (Type III) Toxoplasma strains to determine what parasite and host factors are linked to decreased Aβ burden rather than simply with infection.

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.

Inflammatory early events associated to the role of P2X7 receptor in acute murine toxoplasmosis

Activation of the purinergic P2X7 receptor by extracellular ATP (eATP) potentiates proinflammatory responses during infections by intracellular pathogens. Extracellular ATP triggers an antimicrobial response in macrophages infected with Toxoplasma gondii in vitro, suggesting that purinergic signaling may stimulate host defense mechanisms against toxoplasmosis. Here, we provide in vivo evidence in support of this hypothesis, by showing that P2X7^-/- mice are more susceptible than P2X7^+/+ mice to acute infection by the RH strain of T. gondii, and that this phenomenon is associated with a deficient proinflammatory response. Four days post-infection, peritoneal washes from infected P2X7^-/- mice had no or little increase in the levels of the proinflammatory cytokines IL-12, IL-1β, IFN-γ, and TNF-α, whose levels increased markedly in samples from infected P2X7^+/+ mice. Infected P2X7^-/- mice displayed an increase in organ weight and histological alterations in some of the 'shock organs' in toxoplasmosis - the liver, spleen and mesenteric lymph nodes. The liver of infected P2X7^-/- mice had smaller granulomas, but increased parasite load/granuloma. Our results confirm that the P2X7 receptor is involved in containing T. gondii spread in vivo, by stimulating inflammation.

Ly6C(hi) Monocytes Provide a Link between Antibiotic-Induced Changes in Gut Microbiota and Adult Hippocampal Neurogenesis

Antibiotics, though remarkably useful, can also cause certain adverse effects. We detected that treatment of adult mice with antibiotics decreases hippocampal neurogenesis and memory retention. Reconstitution with normal gut flora (SPF) did not completely reverse the deficits in neurogenesis unless the mice also had access to a running wheel or received probiotics. In parallel to an increase in neurogenesis and memory retention, both SPF-reconstituted mice that ran and mice supplemented with probiotics exhibited higher numbers of Ly6C(hi) monocytes in the brain than antibiotic-treated mice. Elimination of Ly6C(hi) monocytes by antibody depletion or the use of knockout mice resulted in decreased neurogenesis, whereas adoptive transfer of Ly6C(hi) monocytes rescued neurogenesis after antibiotic treatment. We propose that the rescue of neurogenesis and behavior deficits in antibiotic-treated mice by exercise and probiotics is partially mediated by Ly6C(hi) monocytes.

Monocyte-mediated defense against bacteria, fungi, and parasites

Circulating blood monocytes are a heterogeneous leukocyte population that contributes critical antimicrobial and regulatory functions during systemic and tissue-specific infections. These include patrolling vascular tissue for evidence of microbial invasion, infiltrating peripheral tissues and directly killing microbial invaders, conditioning the inflammatory milieu at sites of microbial tissue invasion, and orchestrating the activation of innate and adaptive immune effector cells. The central focus of this review is the in vivo mechanisms by which monocytes and their derivative cells promote microbial clearance and immune regulation. We include an overview of murine models to examine monocyte functions during microbial challenges and review our understanding of the functional roles of monocytes and their derivative cells in host defense against bacteria, fungi, and parasites.

Chronic Toxoplasma gondii infection enhances β-amyloid phagocytosis and clearance by recruited monocytes

INTRODUCTION

Alzheimer's disease (AD) is associated with the accumulation of β-amyloid (Aβ) as senile plaques in the brain, thus leading to neurodegeneration and cognitive impairment. Plaque formation depends not merely on the amount of generated Aβ peptides, but more importantly on their effective removal. Chronic infections with neurotropic pathogens, most prominently the parasite Toxoplasma (T.) gondii, are frequent in the elderly, and it has been suggested that the resulting neuroinflammation may influence the course of AD. In the present study, we investigated how chronic T. gondii infection and resulting neuroinflammation affect plaque deposition and removal in a mouse model of AD.

RESULTS

Chronic infection with T. gondii was associated with reduced Aβ and plaque load in 5xFAD mice. Upon infection, myeloid-derived CCR2(hi) Ly6C(hi) monocytes, CCR2(+) Ly6C(int), and CCR2(+) Ly6C(low) mononuclear cells were recruited to the brain of mice. Compared to microglia, these recruited mononuclear cells showed highly increased phagocytic capacity of Aβ ex vivo. The F4/80(+) Ly6C(low) macrophages expressed high levels of Triggering Receptor Expressed on Myeloid cells 2 (TREM2), CD36, and Scavenger Receptor A1 (SCARA1), indicating phagocytic activity. Importantly, selective ablation of CCR2(+) Ly6C(hi) monocytes resulted in an increased amount of Aβ in infected mice. Elevated insulin-degrading enzyme (IDE), matrix metalloproteinase 9 (MMP9), as well as immunoproteasome subunits β1i/LMP2, β2i/MECL-1, and β5i/LMP7 mRNA levels in the infected brains indicated increased proteolytic Aβ degradation. Particularly, LMP7 was highly expressed by the recruited mononuclear cells in the brain, suggesting a novel mechanism of Aβ clearance.

CONCLUSIONS

Our results indicate that chronic Toxoplasma infection ameliorates β-amyloidosis in a murine model of AD by activation of the immune system, specifically by recruitment of Ly6C(hi) monocytes and by enhancement of phagocytosis and degradation of soluble Aβ. Our findings provide evidence for a modulatory role of inflammation-induced Aβ phagocytosis and degradation by newly recruited peripheral immune cells in the pathophysiology of AD.

MIF Promotes Classical Activation and Conversion of Inflammatory Ly6C(high) Monocytes into TipDCs during Murine Toxoplasmosis

Macrophage migration inhibitory factor (MIF) mediates immunity against Toxoplasma gondii infection by inducing inflammatory cytokines required to control the parasite replication. However, the role of this inflammatory mediator in the cell-mediated immune response against this infection is still poorly understood. Here, we used T. gondii-infected WT and Mif^−/− mice to analyze the role of MIF in the maturation of CD11b⁺ and CD8α⁺ dendritic cells (DCs). We found that MIF promotes maturation of CD11b⁺ but not CD8α⁺ DCs, by inducing IL-12p70 production and CD86 expression. Infected Mif^−/− mice showed significantly lower numbers of TNF and inducible nitric oxide synthase- (iNOS-) producing DCs (TipDCs) compared to infected WT mice. The adoptive transfer of Ly6C^high monocytes into infected WT or Mif^−/− mice demonstrated that MIF participates in the differentiation of Ly6C^high monocytes into TipDCs. In addition, infected Mif^−/− mice display a lower percentage of IFN-γ-producing natural killer (NK) cells compared to WT mice, which is associated with reducing numbers of TipDCs in Mif^−/− mice. Furthermore, administration of recombinant MIF (rMIF) into T. gondii-infected Mif^−/− mice restored the numbers of TipDCs and reversed the susceptible phenotype of Mif^−/− mice. Collectively, these results demonstrate an important role for MIF inducing cell-mediated immunity to T. gondii infection.

Phosphatidylthreonine and Lipid-Mediated Control of Parasite Virulence

The major membrane phospholipid classes, described thus far, include phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn), phosphatidylserine (PtdSer), and phosphatidylinositol (PtdIns). Here, we demonstrate the natural occurrence and genetic origin of an exclusive and rather abundant lipid, phosphatidylthreonine (PtdThr), in a common eukaryotic model parasite, Toxoplasma gondii. The parasite expresses a novel enzyme PtdThr synthase (TgPTS) to produce this lipid in its endoplasmic reticulum. Genetic disruption of TgPTS abrogates de novo synthesis of PtdThr and impairs the lytic cycle and virulence of T. gondii. The observed phenotype is caused by a reduced gliding motility, which blights the parasite egress and ensuing host cell invasion. Notably, the PTS mutant can prevent acute as well as yet-incurable chronic toxoplasmosis in a mouse model, which endorses its potential clinical utility as a metabolically attenuated vaccine. Together, the work also illustrates the functional speciation of two evolutionarily related membrane phospholipids, i.e., PtdThr and PtdSer.

An exclusive membrane lipid, phosphatidylthreonine, is revealed to be naturally abundant in the widespread protist parasite Toxoplasma gondii, where it has evolved adaptively and is essential for parasite virulence.

Lipids are essential constituents of biological membranes, and most organisms across the tree of life use a relatively limited repertoire of lipids in their membranes. This work reveals the natural and abundant presence of an exclusive lipid phosphatidylthreonine (PtdThr) in Toxoplasma gondii, a ubiquitous protozoan parasite of humans and animals. PtdThr is made by a novel parasite-specific enzyme, PtdThr synthase, which has evolved from the widespread enzyme phosphatidylserine synthase. The study shows that PtdThr is required for asexual reproduction and virulence of the parasite in vivo, and a metabolically attenuated mutant strain of Toxoplasma lacking PtdThr can protect vaccinated mice against acute and currently incurable chronic infection. This discovery demonstrates adaptive “speciation” of PtdThr from an otherwise near-universal membrane lipid phosphatidylserine and reveals de novo PtdThr synthesis in T. gondii as a potential drug target.

TFF Peptides Play a Role in the Immune Response Following Oral Infection of Mice with Toxoplasma Gondii

The peptide trefoil factor family 3 (TFF3) is a major constituent of the intestinal mucus, playing an important role in the repair of epithelial surfaces. To further understand the role of TFF3 in the protection of intestinal epithelium, we tested the influence of TFF3 in a murine Toxoplasma gondii-induced ileitis model. Surprisingly, TFF3^KO mice showed a reduced immune response in the ileum when compared to wild-type animals. Interleukin-12 and interferon-γ expression levels as well as the number of CD4⁺ lymphocytes were reduced in the infected TFF3^KO mice. These effects were in line with the trend of elevated parasite levels in the ileum. Moreover, TFF1 expression was upregulated in the spleen of infected mice. These initial results indicate that TFF3 is involved in the immune pathology of T. gondii infection-induced intestinal inflammation. Thus far, the mechanisms of how TFF3 influences the immune response are not fully understood. Further studies should identify if TFF3 affects mucus sensing of dendritic cells and how TFF3 is involved in regulating the immune response as an intrinsic secretory peptide of immune cells.

Specific depletion of Ly6C(hi) inflammatory monocytes prevents immunopathology in experimental cerebral malaria

Plasmodium berghei ANKA (PbA) infection of C57BL/6 mice leads to experimental cerebral malaria (ECM) that is commonly associated with serious T cell mediated damage. In other parasitic infection models, inflammatory monocytes have been shown to regulate Th1 responses but their role in ECM remains poorly defined, whereas neutrophils are reported to contribute to ECM immune pathology. Making use of the recent development of specific monoclonal antibodies (mAb), we depleted in vivo Ly6C(hi) inflammatory monocytes (by anti-CCR2), Ly6G+ neutrophils (by anti-Ly6G) or both cell types (by anti-Gr1) during infection with Ovalbumin-transgenic PbA parasites (PbTg). Notably, the application of anti-Gr1 or anti-CCR2 but not anti-Ly6G antibodies into PbTg-infected mice prevented ECM development. In addition, depletion of Ly6C(hi) inflammatory monocytes but not neutrophils led to decreased IFNγ levels and IFNγ+CD8+ T effector cells in the brain. Importantly, anti-CCR2 mAb injection did not prevent the generation of PbTg-specific T cell responses in the periphery, whereas anti-Gr1 mAb injection strongly diminished T cell frequencies and CTL responses. In conclusion, the specific depletion of Ly6C(hi) inflammatory monocytes attenuated brain inflammation and immune cell recruitment to the CNS, which prevented ECM following Plasmodium infection, pointing out a substantial role of Ly6C+ monocytes in ECM inflammatory processes.