Both IL-12 and IL-18 contribute to small intestinal Th1-type immunopathology following oral infection with Toxoplasma gondii, but IL-12 is dominant over IL-18 in parasite control

A novel mRNA vaccine, TGGT1_278620 mRNA-LNP, prolongs the survival time in BALB/c mice with acute toxoplasmosis

IMPORTANCE

Toxoplasma gondii, an obligate intracellular eukaryotic parasite, can infect about one-third of the world's population. One vaccine, Toxovax, has been developed and licensed commercially; however, it is only used in the sheep industry to reduce the losses caused by congenital toxoplasmosis. Various other vaccine approaches have been explored, including excretory secretion antigen vaccines, subunit vaccines, epitope vaccines, and DNA vaccines. However, current research has not yet developed a safe and effective vaccine for T. gondii. Here, we generated an mRNA vaccine candidate against T. gondii. We investigated the efficacy of vaccination with a novel identified candidate, TGGT1_278620, in a mouse infection model. We screened T. gondii-derived protective antigens at the genome-wide level, combined them with mRNA-lipid nanoparticle vaccine technology against T. gondii, and investigated immune-related factors and mechanisms. Our findings might contribute to developing vaccines for immunizing humans and animals against T. gondii.

IL-18BP mediates the balance between protective and pathological immune responses to Toxoplasma gondii

Interleukin-18 (IL-18) promotes natural killer (NK) and T cell production of interferon (IFN)-γ, a key factor in resistance to Toxoplasma gondii, but previous work has shown a limited role for endogenous IL-18 in control of this parasite. Although infection with T. gondii results in release of IL-18, the production of IFN-γ induces high levels of the IL-18 binding protein (IL-18BP). Antagonism of IL-18BP with a "decoy-to-the-decoy" (D2D) IL-18 construct that does not signal but rather binds IL-18BP results in enhanced innate lymphoid cell (ILC) and T cell responses and improved parasite control. In addition, the use of IL-18 resistant to IL-18BP ("decoy-resistant" IL-18 [DR-18]) is more effective than exogenous IL-18 at promoting innate resistance to infection. DR-18 enhances CD4+ T cell production of IFN-γ but results in CD4+ T cell-mediated pathology. Thus, endogenous IL-18BP restrains aberrant immune pathology, and this study highlights strategies that can be used to tune this regulatory pathway for optimal anti-pathogen responses.

IFN-γ production by brain-resident cells activates cerebral mRNA expression of a wide spectrum of molecules critical for both innate and T cell-mediated protective immunity to control reactivation of chronic infection with Toxoplasma gondii

We previously demonstrated that brain-resident cells produce IFN-γ in response to reactivation of cerebral infection with Toxoplasma gondii. To obtain an overall landscape view of the effects of IFN-γ from brain-resident cells on the cerebral protective immunity, in the present study we employed NanoString nCounter assay and quantified mRNA levels for 734 genes in myeloid immunity in the brains of T and B cell-deficient, bone marrow chimeric mice with and without IFN-γ production by brain-resident cells in response to reactivation of cerebral T. gondii infection. Our study revealed that IFN-γ produced by brain-resident cells amplified mRNA expression for the molecules to activate the protective innate immunity including 1) chemokines for recruitment of microglia and macrophages (CCL8 and CXCL12) and 2) the molecules for activating those phagocytes (IL-18, TLRs, NOD1, and CD40) for killing tachyzoites. Importantly, IFN-γ produced by brain-resident cells also upregulated cerebral expression of molecules for facilitating the protective T cell immunity, which include the molecules for 1) recruiting effector T cells (CXCL9, CXCL10, and CXCL11), 2) antigen processing (PA28αβ, LMP2, and LMP7), transporting the processed peptides (TAP1 and TAP2), assembling the transported peptides to the MHC class I molecules (Tapasin), and the MHC class I (H2-K1 and H2-D1) and Ib molecules (H2-Q1, H-2Q2, and H2-M3) for presenting antigens to activate the recruited CD8⁺ T cells, 3) MHC class II molecules (H2-Aa, H2-Ab1, H2-Eb1, H2-Ea-ps, H2-DMa, H2-Ob, and CD74) to present antigens for CD4⁺ T cell activation, 4) co-stimulatory molecules (ICOSL) for T cell activation, and 5) cytokines (IL-12, IL-15, and IL-18) facilitating IFN-γ production by NK and T cells. Notably, the present study also revealed that IFN-γ production by brain-resident cells also upregulates cerebral expressions of mRNA for the downregulatory molecules (IL-10, STAT3, SOCS1, CD274 [PD-L1], IL-27, and CD36), which can prevent overly stimulated IFN-γ-mediated pro-inflammatory responses and tissue damages. Thus, the present study uncovered the previously unrecognized the capability of IFN-γ production by brain-resident cells to upregulate expressions of a wide spectrum of molecules for coordinating both innate and T cell-mediated protective immunity with a fine-tuning regulation system to effectively control cerebral infection with T. gondii.

Immune response against toxoplasmosis-some recent updates RH: Toxoplasma gondii immune response

AIMS

Cytokines, soluble mediators of immunity, are key factors of the innate and adaptive immune system. They are secreted from and interact with various types of immune cells to manipulate host body's immune cell physiology for a counter-attack on the foreign body. A study was designed to explore the mechanism of Toxoplasma gondii (T. gondii) resistance from host immune response.

METHODS AND RESULTS

The published data on aspect of host (murine and human) immune response against T. gondii was taken from Google scholar and PubMed. Most relevant literature was included in this study. The basic mechanism of immune response starts from the interactions of antigens with host immune cells to trigger the production of cytokines (pro-inflammatory and anti-inflammatory) which then act by forming a cytokinome (network of cytokine). Their secretory equilibrium is essential for endowing resistance to the host against infectious diseases, particularly toxoplasmosis. A narrow balance lying between Th1, Th2, and Th17 cytokines (as demonstrated until now) is essential for the development of resistance against T. gondii as well as for the survival of host. Excessive production of pro-inflammatory cytokines leads to tissue damage resulting in the production of anti-inflammatory cytokines which enhances the proliferation of Toxoplasma. Stress and other infectious diseases (human immunodeficiency virus (HIV)) that weaken the host immunity particularly the cellular component, make the host susceptible to toxoplasmosis especially in pregnant women.

CONCLUSION

The current review findings state that in vitro harvesting of IL12 from DCs, Np and MΦ upon exposure with T. gondii might be a source for therapeutic use in toxoplasmosis. Current review also suggests that therapeutic interventions leading to up-regulation/supplementation of SOCS-3, IL12, and IFNγ to the infected host could be a solution to sterile immunity against T. gondii infection. This would be of interest particularly in patients passing through immunosuppression owing to any reason like the ones receiving anti-cancer therapy, the ones undergoing immunosuppressive therapy for graft/transplantation, the ones suffering from immunodeficiency virus (HIV) or having AIDS. Another imortant suggestion is to launch the efforts for a vaccine based on GRA6Nt or other similar antigens of T. gondii as a probable tool to destroy tissue cysts.

Innate Lymphoid Cells in Response to Intracellular Pathogens: Protection Versus Immunopathology

Innate lymphoid cells (ILCs) are a heterogeneous group of cytokine-producing lymphocytes which are predominantly located at mucosal barrier surfaces, such as skin, lungs, and gastrointestinal tract. ILCs contribute to tissue homeostasis, regulate microbiota-derived signals, and protect against mucosal pathogens. ILCs are classified into five major groups by their developmental origin and distinct cytokine production. A recently emerged intriguing feature of ILCs is their ability to alter their phenotype and function in response to changing local environmental cues such as pathogen invasion. Once the pathogen crosses host barriers, ILCs quickly activate cytokine production to limit the spread of the pathogen. However, the dysregulated ILC responses can lead to tissue inflammation and damage. Furthermore, the interplay between ILCs and other immune cell types shapes the outcome of the immune response. Recent studies highlighted the important role of ILCs for host defense against intracellular pathogens. Here, we review recent advances in understanding the mechanisms controlling protective and pathogenic ILC responses to intracellular pathogens. This knowledge can help develop new ILC-targeted strategies to control infectious diseases and immunopathology.

IL-33 promotes innate lymphoid cell-dependent IFN-γ production required for innate immunity to Toxoplasma gondii

IL-33 is an alarmin required for resistance to the parasite Toxoplasma gondii, but its role in innate resistance to this organism is unclear. Infection with T. gondii promotes increased stromal cell expression of IL-33, and levels of parasite replication correlate with release of IL-33 in affected tissues. In response to infection, a subset of innate lymphoid cells (ILC) emerges composed of IL-33R+ NK cells and ILC1s. In Rag1-/-mice, where NK cells and ILC1 production of IFN-γ mediate innate resistance to T. gondii, the loss of the IL-33R resulted in reduced ILC responses and increased parasite replication. Furthermore, administration of IL-33 to Rag1-/- mice resulted in a marked decrease in parasite burden, increased production of IFN-γ, and the recruitment and expansion of inflammatory monocytes associated with parasite control. These protective effects of exogenous IL-33 were dependent on endogenous IL-12p40 and the ability of IL-33 to enhance ILC production of IFN-γ. These results highlight that IL-33 synergizes with IL-12 to promote ILC-mediated resistance to T. gondii.

RIPK3 Facilitates Host Resistance to Oral Toxoplasma gondii Infection

Toxoplasma gondii infection activates pattern recognition receptor (PRR) pathways that drive innate inflammatory responses to control infection. Necroptosis is a proinflammatory cell death pathway apart from the innate immune response that has evolved to control pathogenic infection. In this study, we further defined the role of Z-DNA binding protein 1 (ZBP1) as a PRR and assessed its contribution to necroptosis as a host protection mechanism to T. gondii infection. We found that ZBP1 does not induce proinflammatory necroptosis cell death, and ZBP1 null mice have reduced survival after oral T. gondii infection. In contrast, mice deleted in receptor-interacting serine/threonine-protein kinase 3 (RIPK3^-/-), a central mediator of necroptosis, have significantly improved survival after oral T. gondii infection without a reduction in parasite burden. The physiological consequences of RIPK3 activity did not show any differences in intestine villus immunopathology, but RIPK3^-/- mice showed higher immune cell infiltration and edema in the lamina propria. The contribution of necroptosis to host survival was clarified with mixed-lineage kinase domain-like pseudokinase null (MLKL^-/-) mice. We found MLKL^-/- mice succumbed to oral T. gondii infection the same as wild-type mice, indicating necroptosis-independent RIPK3 activity impacts host survival. These results provide new insights on the impacts of proinflammatory cell death pathways as a mechanism of host defense to oral T. gondii infection.

A Protective and Pathogenic Role for Complement During Acute Toxoplasma gondii Infection

The infection competence of the protozoan pathogen Toxoplasma gondii is critically dependent on the parasite’s ability to inactivate the host complement system. Toxoplasma actively resists complement-mediated killing in non-immune serum by recruiting host-derived complement regulatory proteins C4BP and Factor H (FH) to the parasite surface to inactivate surface-bound C3 and limit formation of the C5b-9 membrane attack complex (MAC). While decreased complement activation on the parasite surface certainly protects Toxoplasma from immediate lysis, the biological effector functions of C3 split products C3b and C3a are maintained, which includes opsonization of the parasite for phagocytosis and potent immunomodulatory effects that promote pro-inflammatory responses and alters mucosal defenses during infection, respectively. In this review, we discuss how complement regulation by Toxoplasma controls parasite burden systemically but drives exacerbated immune responses locally in the gut of genetically susceptible C57BL/6J mice. In effect, Toxoplasma has evolved to strike a balance with the complement system, by inactivating complement to protect the parasite from immediate serum killing, it generates sufficient C3 catabolites that signal through their cognate receptors to stimulate protective immunity. This regulation ultimately controls tachyzoite proliferation and promotes host survival, parasite persistence, and transmissibility to new hosts.

Beneficial effects of Strongyloides venezuelensis antigen extract in acute experimental toxoplasmosis

BACKGROUND

Toxoplasma gondii is a protozoan with worldwide distribution and triggers a strong Th1 immune response in infected susceptible hosts. On the contrary, most helminth infections are characterized by Th2 immune response and the use of helminth-derived antigens to regulate immune response in inflammatory disorders has been broadly investigated.

OBJECTIVES

The aim of this study was to investigate whether treatment with Strongyloides venezuelensis antigen extract (SvAg) would alter immune response against T gondii.

METHODS

C57BL/6 mice were orally infected with T gondii and treated with SvAg, and parasitological, histological and immunological parameters were investigated.

RESULTS

It was observed that SvAg treatment improved survival rates of T gondii-infected mice. At day 7 post-infection, the parasite load was lower in the lung and small intestine of infected SvAg-treated mice than untreated infected mice. Remarkably, SvAg-treated mice infected with T gondii presented reduced inflammatory lesions in the small intestine than infected untreated mice and decreased intestinal and systemic levels of IFN-γ, TNF-α and IL-6. In contrast, SvAg treatment increased T gondii-specific IgA serum levels in infected mice.

CONCLUSIONS

S venezuelensis antigen extract has anti-parasitic and anti-inflammatory properties during T gondii infection suggesting as a possible alternative to parasite and inflammation control.

Naïve CD8 T cell IFNγ responses to a vacuolar antigen are regulated by an inflammasome-independent NLRP3 pathway and Toxoplasma gondii ROP5

Host resistance to Toxoplasma gondii relies on CD8 T cell IFNγ responses, which if modulated by the host or parasite could influence chronic infection and parasite transmission between hosts. Since host-parasite interactions that govern this response are not fully elucidated, we investigated requirements for eliciting naïve CD8 T cell IFNγ responses to a vacuolar resident antigen of T. gondii, TGD057. Naïve TGD057 antigen-specific CD8 T cells (T57) were isolated from transnuclear mice and responded to parasite-infected bone marrow-derived macrophages (BMDMs) in an antigen-dependent manner, first by producing IL-2 and then IFNγ. T57 IFNγ responses to TGD057 were independent of the parasite’s protein export machinery ASP5 and MYR1. Instead, host immunity pathways downstream of the regulatory Immunity-Related GTPases (IRG), including partial dependence on Guanylate-Binding Proteins, are required. Multiple T. gondii ROP5 isoforms and allele types, including ‘avirulent’ ROP5A from clade A and D parasite strains, were able to suppress CD8 T cell IFNγ responses to parasite-infected BMDMs. Phenotypic variance between clades B, C, D, F, and A strains suggest T57 IFNγ differentiation occurs independently of parasite virulence or any known IRG-ROP5 interaction. Consistent with this, removal of ROP5 is not enough to elicit maximal CD8 T cell IFNγ production to parasite-infected cells. Instead, macrophage expression of the pathogen sensors, NLRP3 and to a large extent NLRP1, were absolute requirements. Other members of the conventional inflammasome cascade are only partially required, as revealed by decreased but not abrogated T57 IFNγ responses to parasite-infected ASC, caspase-1/11, and gasdermin D deficient cells. Moreover, IFNγ production was only partially reduced in the absence of IL-12, IL-18 or IL-1R signaling. In summary, T. gondii effectors and host machinery that modulate parasitophorous vacuolar membranes, as well as NLR-dependent but inflammasome-independent pathways, determine the full commitment of CD8 T cells IFNγ responses to a vacuolar antigen.

Parasites are excellent “students” of our immune system as they can deflect, antagonize and confuse the immune response making it difficult to vaccinate against these pathogens. In this report, we analyzed how a widespread parasite of mammals, Toxoplasma gondii, manipulates an immune cell needed for immunity to many intracellular pathogens, the CD8 T cell. Host pathways that govern CD8 T cell production of the immune protective cytokine, IFNγ, were also explored. We hypothesized the secreted T. gondii virulence factor, ROP5, work to inhibit the MHC 1 antigen presentation pathway therefore making it difficult for CD8 T cells to see T. gondii antigens sequestered inside a parasitophorous vacuole. However, manipulation through T. gondii ROP5 does not fully explain how CD8 T cells commit to making IFNγ in response to infection. Importantly, CD8 T cell IFNγ responses to T. gondii require the pathogen sensor NLRP3 to be expressed in the infected cell. Other proteins associated with NLRP3 activation, including members of the conventional inflammasome activation cascade pathway, are only partially involved. Our results identify a novel pathway by which NLRP3 regulates T cell function and underscore the need for NLRP3-activating adjuvants in vaccines aimed at inducing CD8 T cell IFNγ responses to parasites.

Circulating inflammatory mediators as biomarkers of ocular toxoplasmosis in acute and in chronic infection

Toxoplasmosis is highly endemic worldwide. In Brazil, depending on the geographical region and socioeconomic status, 40-70% of individuals become seropositive at some point in their lives. A significant proportion of Toxoplasma gondii-chronically infected individuals who are otherwise immunocompetent develop recurrent ocular lesions. The inflammatory/immune mechanisms involved in development of ocular lesion are still unknown and, despite previous investigation, there are no reliable immune biomarkers to predict/follow disease outcome. To better understand the impact of the immune response on parasite control and immunopathology of ocular toxoplasmosis, and to provide insights on putative biomarkers for disease monitoring, we assessed the production of a large panel of circulating immune mediators in a longitudinal study of patients with postnatally acquired toxoplasmosis stratified by the presence of ocular involvement, both at the early acute stage and 6 months later during chronic infection, correlating them with presence of ocular involvement. We found that T. gondii-infected patients, especially during the acute stage of the disease, display high levels of chemokines, cytokines, and growth factors involved in the activation, proliferation, and migration of inflammatory cells to injured tissues. In particular, major increases were found in the IFN-induced chemokines CXCL9 and CXCL10 in T. gondii-infected patients regardless of disease stage or clinical manifestations. Moreover, a specific subgroup of circulating cytokines and chemokines including GM-CSF, CCL25, CCL11, CXCL12, CXCL13, and CCL2 was identified as potential biomarkers that accurately distinguish different stages of infection and predict the occurrence of ocular toxoplasmosis. In addition to serving as predictors of disease development, these host inflammatory molecules may offer promise as candidate targets for therapeutic intervention.

The role of macrophages in protective and pathological responses to Toxoplasma gondii

The ability of Toxoplasma gondii to cause clinical disease in immune-competent and immune-deficient hosts coupled with its ease of use in vitro and availability of murine models has led to its use as a model organism to study how the immune system controls an intracellular infection. This article reviews the studies that established the role of the cytokine IFN-γ in the activation of macrophages to control T gondii and the events that lead to the mobilization and expansion of macrophage populations and their ability to limit parasite replication. Macrophages also have pro-inflammatory functions that promote protective NK and T-cell activities as well as regulatory properties that facilitate the resolution of inflammation. Nevertheless, while macrophages are important in determining the outcome of infection, T gondii has evolved mechanisms to subvert macrophage activation and can utilize their migratory activities to promote dissemination and these two properties underlie the ability of this parasite to persist and cause disease.

TLR11-independent inflammasome activation is critical for CD4+ T cell-derived IFN-γ production and host resistance to Toxoplasma gondii

Innate recognition of invading intracellular pathogens is essential for regulating robust and rapid CD4+ T cell effector function, which is critical for host-mediated immunity. The intracellular apicomplexan parasite, Toxoplasma gondii, is capable of infecting almost any nucleated cell of warm-blooded animals, including humans, and establishing tissue cysts that persist throughout the lifetime of the host. Recognition of T. gondii by TLRs is essential for robust IL-12 and IFN-γ production, two major cytokines involved in host resistance to the parasite. In the murine model of infection, robust IL-12 and IFN-γ production have been largely attributed to T. gondii profilin recognition by the TLR11 and TLR12 heterodimer complex, resulting in Myd88-dependent IL-12 production. However, TLR11 or TLR12 deficiency failed to recapitulate the acute susceptibility to T. gondii infection seen in Myd88^-/- mice. T. gondii triggers inflammasome activation in a caspase-1-dependent manner resulting in cytokine release; however, it remains undetermined if parasite-mediated inflammasome activation impacts IFN-γ production and host resistance to the parasite. Using mice which lack different inflammasome components, we observed that the inflammasome played a limited role in host resistance when TLR11 remained functional. Strikingly, in the absence of TLR11, caspase-1 and -11 played a significant role for robust CD4+ T_H1-derived IFN-γ responses and host survival. Moreover, we demonstrated that in the absence of TLR11, production of the caspase-1-dependent cytokine IL-18 was sufficient and necessary for CD4+ T cell-derived IFN-γ responses. Mechanistically, we established that T. gondii-mediated activation of the inflammasome and IL-18 were critical to maintain robust CD4+ T_H1 IFN-γ responses during parasite infection in the absence of TLR11.

It is currently estimated that one third of the world’s population is seropositive for the parasite Toxoplasma gondii and this parasite can lead to serious illness and death in immunocompromised patients, and is one of the leading causes of foodborne-related deaths in the United States. Host immunity against the parasite has largely been attributed to recognition of the parasite-derived protein, profilin, by the innate Toll-like receptors (TLRs), TLR11 and TLR12. T. gondii also triggers inflammasome activation in a caspase-1-dependent manner resulting in cytokine release. However, how these innate recognition systems regulate T_H1 immunity and host resistance remains largely unknown. Therefore, using genetically modified mice, we investigated TLR11-dependent and -independent host immunity against the parasite. Our research establishes that in the absence of TLR11, inflammasome activation and subsequent production of the inflammasome-dependent molecule, IL-18 are critical for host immunity to the parasite. These data provide novel mechanistic insight into how TLR and inflammasomes cooperate in regulation of T_H1 immunity and host protection.

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.

Interleukin-18 in Health and Disease

Interleukin (IL)-18 was originally discovered as a factor that enhanced IFN-γ production from anti-CD3-stimulated Th1 cells, especially in the presence of IL-12. Upon stimulation with Ag plus IL-12, naïve T cells develop into IL-18 receptor (IL-18R) expressing Th1 cells, which increase IFN-γ production in response to IL-18 stimulation. Therefore, IL-12 is a commitment factor that induces the development of Th1 cells. In contrast, IL-18 is a proinflammatory cytokine that facilitates type 1 responses. However, IL-18 without IL-12 but with IL-2, stimulates NK cells, CD4⁺ NKT cells, and established Th1 cells, to produce IL-3, IL-9, and IL-13. Furthermore, together with IL-3, IL-18 stimulates mast cells and basophils to produce IL-4, IL-13, and chemical mediators such as histamine. Therefore, IL-18 is a cytokine that stimulates various cell types and has pleiotropic functions. IL-18 is a member of the IL-1 family of cytokines. IL-18 demonstrates a unique function by binding to a specific receptor expressed on various types of cells. In this review article, we will focus on the unique features of IL-18 in health and disease in experimental animals and humans.

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.

Group 1 innate lymphoid cells in Toxoplasma gondii infection

Innate lymphoid cells (ILCs) are a group of lymphocytes that carry out important functions in immunity to infections and in organ homeostasis at epithelial barrier surfaces. ILCs are innate immune cells that provide an early source of cytokines to initiate immune responses against pathogens. Cytotoxic ILCs (i.e. conventional (c)NK cells) and several subsets of helper-like ILCs are the major branches of the ILC family. Conventional NK cells and group 1 ILCs share several characteristics such as surface receptors and the ability to produce IFN-γ upon activation, but they differ in their developmental paths and in their dependence on specific transcription factors. Infection of mice with the intracellular parasite Toxoplasma gondii is followed by a strong Th1-mediated immune response. Previous studies indicate that NK1.1⁺ cells contribute to the production of IFN-γ and TNF and cytotoxicity during acute T. gondii infection. Upon oral infection, the parasite infects intestinal enterocytes, and within the lamina propria, innate immune responses lead to initial parasite control although the infection disseminates widely and persists long-term in immune privileged sites despite adaptive immunity. Upon parasite entry into the small intestine, during the acute stage, ILC1 produce high levels of IFN-γ and TNF protecting barrier surfaces, thus essentially contributing to early parasite control. We will discuss here the role of innate lymphocytes during T. gondii infection in the context of the only recently appreciated diversity of ILC subsets.

Human Intestinal Microbiota: Interaction Between Parasites and the Host Immune Response

The human gut is a highly complex ecosystem with an extensive microbial community, and the influence of the intestinal microbiota reaches the entire host organism. For example, the microbiome regulates fat storage, stimulates or renews epithelial cells, and influences the development and maturation of the brain and the immune system. Intestinal microbes can protect against infection by pathogenic bacteria, viruses, fungi and parasites. Hence, the maintenance of homeostasis between the gut microbiota and the rest of the body is crucial for health, with dysbiosis affecting disease. This review focuses on intestinal protozoa, especially those still representing a public health problem in Mexico, and their interactions with the microbiome and the host. The decrease in prevalence of intestinal helminthes in humans left a vacant ecological niche that was quickly occupied by protozoa. Although the mechanisms governing the interaction between intestinal microbiota and protozoa are poorly understood, it is known that the composition of the intestinal bacterial populations modulates the progression of protozoan infection and the outcome of parasitic disease. Most reports on the complex interactions between intestinal bacteria, protozoa and the immune system emphasize the protective role of the microbiota against protozoan infection. Insights into such protection may facilitate the manipulation of microbiota components to prevent and treat intestinal protozoan infections. Here we discuss recent findings about the immunoregulatory effect of intestinal microbiota with regards to intestinal colonization by protozoa, focusing on infections by Entamoeba histolytica, Blastocystis spp, Giardia duodenalis, Toxoplasma gondii and Cryptosporidium parvum. The possible consequences of the microbiota on parasitic, allergic and autoimmune disorders are also considered.

Crosstalk between intestinal epithelial cell and adaptive immune cell in intestinal mucosal immunity

Constantly challenged by luminal bacteria, intestinal epithelium forms both a physical and biochemical defense against pathogens. Besides, intestinal epithelium senses dynamic and continuous changes in luminal environment and transmits signals to subjacent immune cells accordingly. It has been long accepted that adaptive immune cells fulfill their roles partly by modulating function of intestinal epithelial cells. Recent studies have brought up the proposal that intestinal epithelial cells also actively participate in the regulation of adaptive immunity, especially CD4+ adaptive T cells, which indicates that there is reciprocal crosstalk between intestinal epithelial cells and adaptive immune cells, and the crosstalk may play important role in intestinal mucosal immunity. This Review makes a comprehensive summary about crosstalk between intestinal epithelial cells and CD4+ adaptive T cells in intestinal immunity. Special attention would be given to their implications in inflammatory bowel disease pathogenesis and potential therapeutic targets.

Intestinal, extra-intestinal and systemic sequelae of Toxoplasma gondii induced acute ileitis in mice harboring a human gut microbiota

Background

Within seven days following peroral high dose infection with Toxoplasma gondii susceptible conventionally colonized mice develop acute ileitis due to an underlying T helper cell (Th) -1 type immunopathology. We here addressed whether mice harboring a human intestinal microbiota developed intestinal, extra-intestinal and systemic sequelae upon ileitis induction.

Methodology/Principal findings

Secondary abiotic mice were generated by broad-spectrum antibiotic treatment and associated with a complex human intestinal microbiota following peroral fecal microbiota transplantation. Within three weeks the human microbiota had stably established in the murine intestinal tract as assessed by quantitative cultural and culture-independent (i.e. molecular 16S rRNA based) methods. At day 7 post infection (p.i.) with 50 cysts of T. gondii strain ME49 by gavage human microbiota associated (hma) mice displayed severe clinical, macroscopic and microscopic sequelae indicating acute ileitis. In diseased hma mice increased numbers of innate and adaptive immune cells within the ileal mucosa and lamina propria and elevated intestinal secretion of pro-inflammatory mediators including IFN-γ, IL-12 and nitric oxide could be observed at day 7 p.i. Ileitis development was accompanied by substantial shifts in intestinal microbiota composition of hma mice characterized by elevated total bacterial loads and increased numbers of intestinal Gram-negative commensals such as enterobacteria and Bacteroides / Prevotella species overgrowing the small and large intestinal lumen. Furthermore, viable bacteria translocated from the inflamed ileum to extra-intestinal including systemic compartments. Notably, pro-inflammatory immune responses were not restricted to the intestinal tract as indicated by increased pro-inflammatory cytokine secretion in extra-intestinal (i.e. liver and kidney) and systemic compartments including spleen and serum.

Conclusion/Significance

With respect to the intestinal microbiota composition “humanized” mice display acute ileitis following peroral high dose T. gondii infection. Thus, hma mice constitute a suitable model to further dissect the interactions between pathogens, human microbiota and vertebrate host immunity during acute intestinal inflammation.

Protozoan Parasites and Type I IFNs

For many years, the role of interferon (IFN)-I has been characterized primarily in the context of viral infections. However, regulatory functions mediated by IFN-I have also been described against bacterial infections and in tumor immunology. Only recently, the interest in understanding the immune functions mediated by IFN-I has dramatically increased in the field of protozoan infections. In this review, we discuss the discrete role of IFN-I in the immune response against major protozoan infections: Plasmodium, Leishmania, Trypanosoma, and Toxoplasma.

An NK Cell Perforin Response Elicited via IL-18 Controls Mucosal Inflammation Kinetics during Salmonella Gut Infection

Salmonella Typhimurium (S.Tm) is a common cause of self-limiting diarrhea. The mucosal inflammation is thought to arise from a standoff between the pathogen's virulence factors and the host's mucosal innate immune defenses, particularly the mucosal NAIP/NLRC4 inflammasome. However, it had remained unclear how this switches the gut from homeostasis to inflammation. This was studied using the streptomycin mouse model. S.Tm infections in knockout mice, cytokine inhibition and –injection experiments revealed that caspase-1 (not -11) dependent IL-18 is pivotal for inducing acute inflammation. IL-18 boosted NK cell chemoattractants and enhanced the NK cells' migratory capacity, thus promoting mucosal accumulation of mature, activated NK cells. NK cell depletion and Prf^-/- ablation (but not granulocyte-depletion or T-cell deficiency) delayed tissue inflammation. Our data suggest an NK cell perforin response as one limiting factor in mounting gut mucosal inflammation. Thus, IL-18-elicited NK cell perforin responses seem to be critical for coordinating mucosal inflammation during early infection, when S.Tm strongly relies on virulence factors detectable by the inflammasome. This may have broad relevance for mucosal defense against microbial pathogens.

Salmonella Typhimurium is a common cause of foodborne diarrhea. The disease symptoms arise already a few hours after infection. However, it had remained unclear how the immune system can mount the responses eliciting the disease symptoms so quickly. Earlier work in a mouse model had shown that the gut epithelium expresses a sensor, called NAIP/NLRC4/caspase-1 inflammasome that can detect the pathogen and mount a defense by 12-18h p.i. However, it has remained uncharacterized how inflammasome sensing drives the initial gut inflammation. Here, we found that the caspase-1 inflammasome triggers the production of IL-18, a pro-inflammatory cytokine that appears essential for the early onset of inflammation. IL-18 is driving the accumulation of NK cells into the infected mucosa, via the upregulation of NK cell chemoattractants and by the stimulation of their migratory capacity. Mature NK cells seem to induce mucosal inflammation via a perforin-mediated cytotoxic response. These data suggest that the inflammasome/IL-18/NK cell axis is a driver of early mucosal inflammation via a perforin-dependent cytotoxic NK cell response. Future work will have to address, if this mechanism is equally potent in the human gut and may contribute to ramping up the host's response during the first hours of infection. This may have implications for other gut infections and might provide leads for developing therapies.

Preventive Anti-CD2 Treatment does not Impair Parasite Control in a Murine Toxoplasmosis Model

Targeting human CD2 with the monoclonal antibody (mAb) CB.219 reduces intestinal inflammation in a colitis model where T cells carry human CD2. Here, we asked whether this mAb has adverse effects on infection control.

Mice expressing human CD2 on T cells (huCD2tg) were orally infected with Toxoplasma (T.) gondii and treated with the human CD2-specific mAb CB.219 in a preventive setting. The intestinal T. gondii loads in CB.219 treated mice did not differ from the control group. Histologically, huCD2tg mice showed moderate ileal inflammation that did not change with CB.219 treatment. In the ileum, CB.219 treatment reduced the protein levels of interferon-γ, transforming growth factor β and interleukin-6, whereas interleukin-18 mRNA was slightly increased. The infiltration of neutrophils, macrophages, and T cells into the ileum was unaffected by CB.219 treatment. However, CB.219 treatment decreased the numbers of forkhead box P3⁺ regulatory T cells (Treg) in ileum and liver of huCD2tg mice. This was confirmed in vitro using human peripheral blood mononuclear cells.

Taken together, targeting CD2⁺ T cells by the human CD2 mAb CB.219 does not prevent beneficial immune reactions necessary for pathogen control. Further experiments will address gut specificity, underlying mechanisms, and general applicability of CB.219 treatment.

Anti-Inflammatory Properties of NAP in Acute Toxoplasma Gondii-Induced Ileitis in Mice

The octapeptide NAP has been shown to exert neuroprotective properties. Here, we investigated potential anti-inflammatory effects of NAP in an acute ileitis model. To address this, C57BL/6j mice were perorally infected with Toxoplasma gondii (day 0). Within 1 week postinfection (p.i.), placebo (PLC)-treated mice developed acute ileitis due to Th1-type immune responses. Mice that were subjected to intraperitoneal NAP treatment from day 1 until day 6 p.i., however, developed less distinct macroscopic and microscopic disease as indicated by less body weight loss, less distinct histopathological ileal changes, and lower ileal apoptotic, but higher proliferating cell numbers, less abundance of neutrophils, macrophages, monocytes, and T lymphocytes, but higher numbers of regulatory T cells in the ileal mucosa and lamina propria, and lower concentrations of pro-inflammatory mediators in the ilea as compared to PLC controls at day 7 p.i. Remarkably, NAP-mediated anti-inflammatory effects could also be observed in extra-intestinal compartments including liver and spleen. Strikingly, lower MCP-1, TNF, and IL-12p70 serum concentrations in NAP as compared to PLC-treated mice at day 7 p.i. indicate a pronounced systemic anti-inflammatory effect of NAP in acute ileitis. These findings provide first evidence for NAP as a potential novel treatment option in intestinal inflammation.

Lack of a Functioning P2X7 Receptor Leads to Increased Susceptibility to Toxoplasmic Ileitis

BACKGROUND

Oral infection of C57BL/6J mice with the protozoan parasite Toxoplasma gondii leads to a lethal inflammatory ileitis.

PRINCIPAL FINDINGS

Mice lacking the purinergic receptor P2X7R are acutely susceptible to toxoplasmic ileitis, losing significantly more weight than C57BL/6J mice and exhibiting much greater intestinal inflammatory pathology in response to infection with only 10 cysts of T. gondii. This susceptibility is not dependent on the ability of P2X7R-deficient mice to control the parasite, which they accomplish just as efficiently as C57BL/6J mice. Rather, susceptibility is associated with elevated ileal concentrations of pro-inflammatory cytokines, reactive nitrogen intermediates and altered regulation of elements of NFκB activation in P2X7R-deficient mice.

CONCLUSIONS

Our data support the thesis that P2X7R, a well-documented activator of pro-inflammatory cytokine production, also plays an important role in the regulation of intestinal inflammation.

Interleukin-22 induces interleukin-18 expression from epithelial cells during intestinal infection

T helper 1 (Th1) cell-associated immunity exacerbates ileitis induced by oral Toxoplasma gondii infection. We show here that attenuated ileitis observed in interleukin-22 (IL-22)-deficient mice was associated with reduced production of Th1-cell-promoting IL-18. IL-22 not only augmented the expression of Il18 mRNA and inactive precursor protein (proIL-18) in intestinal epithelial cells after T. gondii or Citrobacter rodentium infection, but also maintained the homeostatic amount of proIL-18 in the ileum. IL-22, however, did not induce the processing to active IL-18, suggesting a two-step regulation of IL-18 in these cells. Although IL-18 exerted pathogenic functions during ileitis triggered by T. gondii, it was required for host defense against C. rodentium. Conversely, IL-18 was required for the expression of IL-22 in innate lymphoid cells (ILCs) upon T. gondii infection. Our results define IL-18 as an IL-22 target gene in epithelial cells and describe a complex mutual regulation of both cytokines during intestinal infection.

Parasite distribution and associated immune response during the acute phase of Toxoplasma gondii infection in sheep

BACKGROUND

In many countries, Toxoplasma gondii (T. gondii) is a major cause of reproductive disorders and abortions in the sheep industry, and therefore responsible for important financial and economic losses. In addition, undercooked infected lamb is an important risk factor for human toxoplasmosis. In the present study, the initial phase of the infection was investigated: the parasite's entry site, the subsequent distribution of the parasite and the host-immune response.

RESULTS

Parasite DNA was already detected in the cranial small intestinal mucosa the first days after oral infection with T. gondii tissue cysts. Simultaneously, high IFN-gamma and IL-12 responses were induced mainly in the mesenteric lymph nodes. The emergence of IgG1 (at 8dpi), and IgG2 (at 11 dpi) was accompanied by a decrease or even disappearance of the IFN-gamma and IL-12 response in the Peyers' patches (PP), PBMC's and popliteal LN's. Meanwhile the parasite DNA could be recovered from most mucosal and systemic tissues to become undetectable in the small intestine, popliteal LN, PBMC and spleen 3 weeks pi.

CONCLUSIONS

Our results indicate that parasites enter the cranial small intestine the first days after infection and that after an increase the first two weeks after infection, the parasite DNA levels in the intestine drop below the detection limit three weeks after infection. This coincides with an increase in parastic-specific serum IgG1 and IgG2 and a decrease of the antigen-specific IFN-gamma response in PP, PBMC and popliteal LN. We suggest a role for IFN-gamma and IL-12 in controlling the infection.

Apicomplexan infections in the gut

Toxoplasma gondii and Cryptosporidium parvum are intracellular protozoan parasites that establish infection through the small intestinal bowel after the ingestion of contaminated food products. These Apicomplexan parasites have emerged as an important cause of chronic and fatal disease in immunodeficient individuals, in addition to being investigated as possible triggers of inflammatory bowel disease. T. gondii disseminates to the brain and other tissues after infection, whereas C. parvum remains localized to the intestine. In the following review, we will discuss the pathogenesis of these parasitic diseases in the small intestine, the site of initial invasion. Themes include the sequence of invasion, the structure of Th1 immunity provoked by these parasites and the contribution of intestinal microbiota to the development of the mucosal immune response.

Pituitary adenylate cyclase-activating polypeptide ameliorates experimental acute ileitis and extra-intestinal sequelae

Background

The neuropeptide Pituitary adenylate cyclase-activating polypeptide (PACAP) plays pivotal roles in immunity and inflammation. So far, potential immune-modulatory properties of PACAP have not been investigated in experimental ileitis.

Methodology/Principal Findings

Mice were perorally infected with Toxoplasma (T.) gondii to induce acute ileitis (day 0) and treated daily with synthetic PACAP38 from day 1 to 6 post infection (p.i.; prophylaxis) or from day 4 to 6 p.i. (therapy). Whereas placebo-treated control mice suffered from acute ileitis at day 7 p.i. and succumbed to infection, intestinal immunopathology was ameliorated following PACAP prophylaxis. PACAP-treated mice exhibited increased abundance of small intestinal FOXP3+ cells, but lower numbers of ileal T lymphocytes, neutrophils, monocytes and macrophages, which was accompanied by less ileal expression of pro-inflammatory cytokines such as IL-23p19, IL-22, IFN-γ, and MCP-1. Furthermore, PACAP-treated mice displayed higher anti-inflammatory IL-4 concentrations in mesenteric lymph nodes and liver and higher systemic anti-inflammatory IL-10 levels in spleen and serum as compared to control animals at day 7 p.i. Remarkably, PACAP-mediated anti-inflammatory effects could also be observed in extra-intestinal compartments as indicated by reduced pro-inflammatory mediator levels in spleen (TNF-α, nitric oxide) and liver (TNF-α, IFN-γ, MCP-1, IL-6) and less severe histopathological sequelae in lungs and kidneys following prophylactic PACAP treatment. Strikingly, PACAP prolonged survival of T. gondii infected mice in a time-of-treatment dependent manner.

Conclusion/Significance

Synthetic PACAP ameliorates acute small intestinal inflammation and extra-intestinal sequelae by down-regulating Th1-type immunopathology, reducing oxidative stress and up-regulating anti-inflammatory cytokine responses. These findings provide novel potential treatment options of inflammatory bowel diseases.

Role of Blimp-1 in programing Th effector cells into IL-10 producers

The transcriptional regulator Blimp-1 is absolutely required for IL-10 production in Th1 cells and limits inflammatory effector T cell responses downstream of IL-12 and IL-27.

Secretion of the immunosuppressive cytokine interleukin (IL) 10 by effector T cells is an essential mechanism of self-limitation during infection. However, the transcriptional regulation of IL-10 expression in proinflammatory T helper (Th) 1 cells is insufficiently understood. We report a crucial role for the transcriptional regulator Blimp-1, induced by IL-12 in a STAT4-dependent manner, in controlling IL-10 expression in Th1 cells. Blimp-1 deficiency led to excessive inflammation during Toxoplasma gondii infection with increased mortality. IL-10 production from Th1 cells was strictly dependent on Blimp-1 but was further enhanced by the synergistic function of c-Maf, a transcriptional regulator of IL-10 induced by multiple factors, such as the Notch pathway. We found Blimp-1 expression, which was also broadly induced by IL-27 in effector T cells, to be antagonized by transforming growth factor (TGF) β. While effectively blocking IL-10 production from Th1 cells, TGF-β shifted IL-10 regulation from a Blimp-1–dependent to a Blimp-1–independent pathway in IL-27–induced Tr1 (T regulatory 1) cells. Our findings further illustrate how IL-10 regulation in Th cells relies on several transcriptional programs that integrate various signals from the environment to fine-tune expression of this critical immunosuppressive cytokine.

The impact of Toll-like-receptor-9 on intestinal microbiota composition and extra-intestinal sequelae in experimental Toxoplasma gondii induced ileitis

Background

Following peroral Toxoplasma (T.) gondii infection, susceptible mice develop acute ileitis due to a microbiota-dependent Th1 type immunopathology. Toll-like-receptor (TLR)-9 is known to recognize bacterial DNA and mediates intestinal inflammation, but its impact on intestinal microbiota composition and extra-intestinal sequelae following T. gondii infection has not yet been elucidated.

Methods and results

Seven days following peroral infection (p.i.) with 100 cysts of T. gondii ME49 strain, TLR-9^-/- and wildtype (WT) mice suffered from comparable ileitis, whereas ileal parasitic loads as well as IFN-γ and nitric oxide levels were higher in TLR-9^-/- compared to WT mice. Locally, TLR-9^-/- mice exhibited increased ileal CD3+, but not FOXP3+ cell numbers at day 7 p.i.; in mesenteric lymph nodes IFN-γ-producing CD4+ cell numbers and TNF-α and IFN-γ concentrations were also increased in TLR-9^-/- compared to WT mice. T. gondii DNA levels, however, did not differ in mice of either genotype. Differences in intestinal microbiota were rather subtle except for bifidobacteria that were virtually absent in both, naïve and T. gondii infected TLR-9^-/-, but not WT mice. Extra-intestinally, TLR-9^-/- mice displayed less distinct systemic immune responses as indicated by lower serum IL-6, and splenic TNF-α and IFN-γ levels as compared to WT mice despite higher translocation rates of intestinal bacteria to extra-intestinal compartments such as liver, spleen, kidney, and cardiac blood. Most importantly, brains were also affected in this inflammatory scenario as early as day 7 p.i. Remarkably, TLR-9^-/- mice exhibited more pronounced inflammatory infiltrates with higher numbers of F4/80+ macrophages and microglia in the cortex and meninges as compared to WT mice, whereas T. gondii DNA levels did not differ.

Conclusion

We here show that TLR-9 is not required for the development of T. gondii induced ileitis but mediates distinct inflammatory changes in intestinal and extra-intestinal compartments including the brain.

How is inflammation initiated? Individual influences of IL-1, IL-18 and HMGB1

Pro-inflammatory cytokines are crucial for fighting infection and establishing immunity. Recently, other proteins, such as danger-associated molecular patterns (DAMPs), have also been appreciated for their role in inflammation and immunity. Following the formation and activation of multiprotein complexes, termed inflammasomes, two cytokines, IL-1β and IL-18, along with the DAMP High Mobility Group Box 1 (HMGB1), are released from cells. Although these proteins all lack classical secretion signals and are released by inflammasome activation, they each lead to different downstream consequences. This review examines how various inflammasomes promote the release of IL-1β, IL-18 and HMGB1 to combat pathogenic situations. Each of these effector molecules plays distinct roles during sterile inflammation, responding to viral, bacterial and parasite infection, and tailoring the innate immune response to specific threats.

Natural killer cell intrinsic toll-like receptor MyD88 signaling contributes to IL-12-dependent IFN-γ production by mice during infection with Toxoplasma gondii

Myeloid differentiation factor 88 (MyD88)-dependent IL-12 secretion by dendritic cells is critical for natural killer cell-mediated IFN-γ production and innate resistance to Toxoplasma gondii. Although MyD88(-/-) mice challenged with T. gondii have defective IL-12 responses and succumb to infection, administration of IL-12 to MyD88(-/-) mice fails to prevent acute mortality, suggesting that MyD88 may mediate signals within natural killer cells important for IL-12-dependent IFN-γ production and innate resistance to this parasite. In this study, we found that T. gondii antigens and IL-12 could synergistically trigger IFN-γ secretion by natural killer cells, which was dependent on toll-like receptor-MyD88 signaling. Further analysis showed that p38 mitogen-activated protein kinase, extracellular signal-regulated kinase, c-Jun N-terminal kinase and NF-κB multiple pathways downstream of MyD88 contributed to IFN-γ production by natural killer cells. Moreover, the well-established toll-like receptor agonists, T. gondii profilin (Tgprofilin) and T. gondii heat shock protein 70 (TgHSP70) could evoke a similar IFN-γ secretory response in natural killer cells to that evoked by T. gondii antigens. In vivo adoptive transfer experiments showed that, upon challenge with T. gondii, NOD/SCID-β2 microglobulin null (NOD/SCID-β2m(-/-)) mice injected i.v. with MyD88(-/-) natural killer cells had reduced serum IFN-γ levels and increased splenic tachyzoite burdens compared with those injected i.v. with wild-type natural killer cells. Taken together, these findings demonstrate a critical role for natural killer cell intrinsic toll-like receptor-MyD88 signaling in IL-12-dependent early IFN-γ production and innate resistance to T. gondii.

Do you see what I see: Recognition of protozoan parasites by Toll-like receptors

Toll-like receptors (TLRs) are important for recognizing a variety of pathogens, including protozoan parasites, and initiating innate immune responses against them. TLRs are localized on the cell surface as well as in the endosome, and are implicated in innate sensing of these parasites. In this review, we will discuss recent findings on the identification of parasite-derived pathogen associated molecular patterns and the TLRs that bind them. The role of these TLRs in initiating the immune response against protozoan parasitic infections in vivo will be presented in the context of murine models of infection utilizing TLR-deficient mice. Additionally, we will explore evidence that TLRs and genetic variants of TLRs may impact the outcome of these parasitic infections in humans.

Role of IL-22 in microbial host defense

Interleukin (IL)-22 is a member of the IL-10 family of cytokines, which, besides IL-10, contains seven additional cytokines. Although the founding member IL-10 is an important immunoregulatory cytokine that represses both innate and adaptive immunity, the other family members preferentially target epithelial cells and enhance innate host defense mechanisms against various pathogens such as bacteria, yeast, and viruses. Based on their functions, the IL-10 family can be further divided into three subgroups, IL-10 itself, the IL-20 subfamily, and the IFNλ subfamily. IL-22 is the best-studied member of the IL-20 subfamily, and exemplifies the diverse biological effects of this subfamily. IL-22 elicits various innate immune responses from epithelial cells and is essential for host defense against several invading pathogens, including Citrobacter rodentium and Klebsiella pneumonia. IL-22 also protects tissue integrity and maintains the mucosal homeostasis. On the other hand, IL-22 is a proinflammatory cytokine with the capacity to amplify inflammatory responses, which might result in tissue damage, e.g., the IL-22-dependent necrosis of the small intestine during Toxoplasma gondii infection.

Regulation of immunopathogenesis during Plasmodium and Toxoplasma infections: more parallels than distinctions?

Toxoplasma and Plasmodium parasites exact a significant toll on public health. Host immunity required for efficient control of infection by these Apicomplexans involves the induction of potent T cell responses, which sometimes results in immunopathological damage. Thus, protective immune responses must be balanced by regulatory networks that limit immunopathology. We review several key cellular and molecular immunoregulatory networks operational during Toxoplasma and Plasmodium infections. Accumulating data show that despite differences in how the immune response controls these parasites, many host immunoregulatory pathways and cellular networks are common to both. Thus, understanding the cellular and molecular circuits that prevent or regulate immunopathological responses against one parasite is likely to inform our understanding of the host response to the other parasite.

Toxoplasma gondii soluble tachyzoite antigen triggers protective mechanisms against fatal intestinal pathology in oral infection of C57BL/6 mice

Toxoplasma gondii induces a potent IL-12 response early in infection that results in IFN-γ-dependent control of parasite growth. It was previously shown that T. gondii soluble tachyzoite antigen (STAg) injected 48 hr before intraperitoneal infection reduces lipoxin A₄ and 5-lipoxygenase (5-LO)-dependent systemic IL-12 and IFN-γ production as well as hepatic immunopathology. This study investigated the ability of STAg-pretreatment to control the fatal intestinal pathology that develops in C57BL/6 mice orally infected with 100 T. gondii cysts. STAg-pretreatment prolonged the animals’ survival by decreasing tissue parasitism and pathology, mainly in the ilea. Protection was associated with decreases in the systemic IFN-γ levels and IFN-γ and TNF message levels in the ilea and with increased TGF-β production in this tissue, but protection was independent of 5-LO and IL-4. STAg-pretreatment decreased CD4⁺ T cell, NK cell, CD11b⁺ monocyte and CD11b⁺CD11c⁺ dendritic cell numbers in the lamina propria and increased CD8⁺ T cells in the intestinal epithelial compartment. In parallel, decreases were observed in iNOS and IL-17 expression in this organ. These results demonstrate that pretreatment with STAg can induce the recruitment of protective CD8⁺ T cells to the intraepithelial compartment and decrease proinflammatory immune mechanisms that promote intestinal pathology in T. gondii infection.

Pro-inflammatory potential of Escherichia coli strains K12 and Nissle 1917 in a murine model of acute ileitis

Non-pathogenic Escherichia coli (Ec) strains K12 (EcK12) and Nissle 1917 (EcN) are used for gene technology and probiotic treatment of intestinal inflammation, respectively. We investigated intestinal colonization and potential pro-inflammatory properties of EcK12, EcN, and commensal E. coli (EcCo) strains in Toxoplasma (T.) gondii-induced acute ileitis. Whereas gnotobiotic animals generated by quintuple antibiotic treatment were protected from ileitis, mice replenished with conventional microbiota suffered from small intestinal necrosis 7 days post-T. gondii infection (p.i.). Irrespective of the Ec strain, recolonized mice revealed mild to moderate histopathological changes in their ileal mucosa. Upon stable recolonization with EcK12, EcN, or EcCo, development of inflammation was accompanied by pro-inflammatory responses at day 7 p.i., including increased ileal T lymphocyte and apoptotic cell numbers compared to T. gondii-infected gnotobiotic controls. Strikingly, either Ec strain was capable to translocate to extra-intestinal locations, such as MLN, spleen, and liver. Taken together, Ec strains used in gene technology and probiotic treatment are able to exert inflammatory responses in a murine model of small intestinal inflammation. In conclusion, the therapeutic use of Ec strains in patients with broad-spectrum antibiotic treatment and/or intestinal inflammation should be considered with caution.

Regulation and migratory role of P-selectin ligands during intestinal inflammation

Dendritic cells from mesenteric lymph nodes (MLN) can convert retinal to retinoic acid (RA), which promotes induction of the gut-specific homing receptor α4β7. In contrast, priming within peripheral lymph nodes leads to upregulation of E- and P-selectin ligands (E- and P-lig). Apart from its α4β7 promoting effect, RA was shown to suppress E- and P-lig induction in vitro. However, enhanced frequencies of P-lig(+) CD4(+) T cells were reported during intestinal inflammation. To understand this contradiction, we first determined whether location of intestinal inflammation, that is, ileitis or colitis, affects P-lig induction. Both conditions promoted P-lig expression on CD4(+) T cells; however, P-lig expressed on T cells facilitated Th1 cell recruitment only into the inflamed colon but not into inflamed small intestine induced by oral Toxoplasma gondii infection. A majority of P-lig(+)CD4(+) T cells found within MLN during intestinal inflammation co-expressed α4β7 confirming their activation in the presence of RA. Mesenteric P-lig(+)CD4(+) cells co-expressed the 130 kDa isoform of CD43 which requires activity of core 2 (beta)1,6-N-acetyl-glycosaminyltransferase-I (C2GlcNAcT-I) suggesting that C2GlcNAcT-I contributes to P-lig expression under these conditions. To test whether inflammatory mediators can indeed overrule the inhibitory effect of RA on P-lig expression we stimulated CD4(+) T cells either polyclonal in the presence of IL-12 and IFNγ or by LPS-activated MLN-derived dendritic cells. Both conditions promoted P-lig induction even in the presence of RA. While RA impeded the induction of fucosyltransferase-VII it did not affect IL-12-dependent C2GlcNAcT-I induction suggesting that C2GlcNAcT-I can support P-lig expression even if fucosyltransferase-VII mRNA upregulation is dampened.

Intraepithelial lymphocytes, goblet cells and VIP-IR submucosal neurons of jejunum rats infected with Toxoplasma gondii

Toxoplasma gondii (T. gondii) crosses the intestinal barrier in oral infections and can lead to changes in different cell types, including the neurons located there. In the gastrointestinal system, the autonomous nervous system component that regulate blood flow and mucous secretion is the submucosal plexus. The aim of this study was to examine the effects of T. gondii infection on intraepithelial lymphocytes (IELs), goblet cells and submucosal neurons that are immunoreactive to vasoactive intestinal peptide (VIP-IR) of rat jejunum. Twenty male rats distributed as a control group (CG) and an infected group (IG), which received a suspension with 500 parasite oocysts (strain ME-49, genotype II) orally, were assessed. Routine histological sections were used to quantify IELs and to detect mucins secreted by goblet cells. Whole mounts including the submucosal layer were examined using immunofluorescence to detect the VIP neurotransmitter. Quantitative alterations in IELs were not observed. However, the reduction (P < 0.05) in the number of goblet cells that produce neutral mucins (PAS+) and sulphomucins (AB pH 1.0) and the maintenance of sialomucin-secreting cells (AB pH 2.5) resulting in a more fluid mucous were observed. Concerning the VIP-IR submucosal neurons, an increase in fluorescence on IG animals was observed. There was a reduction (P < 0.05) in the number of VIP-IR submucosal neurons and atrophy of their cell bodies in IG rats. Infection with T. gondii caused alterations in the chemical composition of the intestinal mucous and reduction in the neuron number and atrophy of the remaining neurons in this cell subpopulation.

Treatment with interleukin-18 binding protein ameliorates Toxoplasma gondii-induced small intestinal pathology that is induced by bone marrow cell-derived interleukin-18

Peroral infection with Toxoplasma gondii results in a Th1-type immunopathology characterized by small intestinal necrosis and is dependent on IL-18. In the present study, we investigated whether treatment with IL-18 binding protein (IL-18bp) prevents ileal pathology. We observed increased expression of IL-18bp in intestinal biopsies of mice following infection. Whereas small intestines of control mice showed severe necrosis with complete destruction of the small intestinal architecture, mice treated with IL-18bp daily displayed only mild inflammatory changes including flattening of villi and edema in the space between the epithelium and lamina propria. Small intestinal parasite loads and concentrations of pro-inflammatory cytokines did not differ in control and IL-18bp-treated mice. Binding of IL-18 to immobilized IL-18bp revealed a remarkably slow dissociation rate, indicating high affinity. Using chimeric mice we observed that bone marrow-derived rather than stromal cells were the primary source of IL-18 that resulted in small intestinal pathology following peroral infection with T. gondii. In conclusion, the results presented here suggest that IL-18bp may be an effective and safe treatment for small intestinal inflammation. Antigen-presenting rather than epithelial cells appear to be the main source of IL-18 in T. gondii-induced small intestinal inflammation.

Immune response and immunopathology during toxoplasmosis

Toxoplasma gondii is a protozoan parasite of medical and veterinary significance that is able to infect any warm-blooded vertebrate host. In addition to its importance to public health, several inherent features of the biology of T. gondii have made it an important model organism to study host-pathogen interactions. One factor is the genetic tractability of the parasite, which allows studies on the microbial factors that affect virulence and allows the development of tools that facilitate immune studies. Additionally, mice are natural hosts for T. gondii, and the availability of numerous reagents to study the murine immune system makes this an ideal experimental system to understand the functions of cytokines and effector mechanisms involved in immunity to intracellular microorganisms. In this article, we will review current knowledge of the innate and adaptive immune responses required for resistance to toxoplasmosis, the events that lead to the development of immunopathology, and the natural regulatory mechanisms that limit excessive inflammation during this infection.

Insights into inflammatory bowel disease using Toxoplasma gondii as an infectious trigger

Oral infection of certain inbred mouse strains with the protozoan Toxoplasma gondii triggers inflammatory pathology resembling lesions seen during human inflammatory bowel disease, in particular Crohn's disease (CD). Damage triggered by the parasite is largely localized to the distal portion of the small intestine, and as such is one of only a few models for ileal inflammation. This is important because ileal involvement is a characteristic of CD in over two-thirds of patients. The disease induced by Toxoplasma is mediated by Th1 cells and the cytokines tumor necrosis factor-α and interferon-γ. Inflammation is dependent upon IL-23, also identified by genome-wide association studies as a risk factor in CD. Development of lesions is concomitant with emergence of E. coli that display enhanced adhesion to the intestinal epithelium and subepithelial translocation. Furthermore, depletion of gut flora renders mice resistant to Toxoplasma-triggered ileitis. Recent findings suggest complex CCR2-dependent interactions between lamina propria T cells and intraepithelial lymphocytes in fueling proinflammatory pathology in the intestine. The advantage of the Toxoplasma model is that disease develops rapidly (within 7-10 days of infection) and can be induced in immunodeficient mice by adoptive transfer of mucosal T cells from infected donors. We propose that Toxoplasma acts as a trigger setting into motion a series of events culminating in loss of tolerance in the intestine and emergence of pathogenic T cell effectors. The Toxoplasma trigger model is providing new leaps in our understanding of immunity in the intestine.

Toxoplasma gondii effectors are master regulators of the inflammatory response

Toxoplasma is a highly successful parasite that establishes a life-long chronic infection. To do this, it must carefully regulate immune activation and host cell effector mechanisms. Here we review the latest developments in our understanding of how Toxoplasma counteracts the immune response of the host, and in some cases provokes it, through the use of specific parasite effector proteins. An emerging theme from these discoveries is that Toxoplasma effectors are master regulators of the pro-inflammatory response, which elicits many of the toxoplasmacidal mechanisms of the host. We speculate that combinations of these effectors present in certain Toxoplasma strains work to maintain an optimal parasite burden in different hosts to ensure parasite transmission.

IL-23 in infections, inflammation, autoimmunity and cancer: possible role in HIV-1 and AIDS

The growing family of interleukin (IL)-12-like cytokines produced by activated macrophages and dendritic cells became the important players in the control of infections, development of inflammation, autoimmunity and cancer. However, the role of one of them-heterodimer IL-23, which consists of IL12p40 and the unique p19 subunit in HIV-1 infection pathogenesis and progression to AIDS, represent special interest. We overviewed findings of IL-23 involvement in control of peripheral bacterial pathogens and opportunistic infection, central nervous system (CNS) viral infections and autoimmune disorders, and tumorogenesis, which potentially could be applicable to HIV-1 and AIDS.

Pathogenesis of chagas' disease: parasite persistence and autoimmunity

Acute Trypanosoma cruzi infections can be asymptomatic, but chronically infected individuals can die of Chagas' disease. The transfer of the parasite mitochondrial kinetoplast DNA (kDNA) minicircle to the genome of chagasic patients can explain the pathogenesis of the disease; in cases of Chagas' disease with evident cardiomyopathy, the kDNA minicircles integrate mainly into retrotransposons at several chromosomes, but the minicircles are also detected in coding regions of genes that regulate cell growth, differentiation, and immune responses. An accurate evaluation of the role played by the genotype alterations in the autoimmune rejection of self-tissues in Chagas' disease is achieved with the cross-kingdom chicken model system, which is refractory to T. cruzi infections. The inoculation of T. cruzi into embryonated eggs prior to incubation generates parasite-free chicks, which retain the kDNA minicircle sequence mainly in the macrochromosome coding genes. Crossbreeding transfers the kDNA mutations to the chicken progeny. The kDNA-mutated chickens develop severe cardiomyopathy in adult life and die of heart failure. The phenotyping of the lesions revealed that cytotoxic CD45, CD8(+) γδ, and CD8α(+) T lymphocytes carry out the rejection of the chicken heart. These results suggest that the inflammatory cardiomyopathy of Chagas' disease is a genetically driven autoimmune disease.

Immunology of Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite. Following oral infection the parasite crosses the intestinal epithelial barrier to disseminate throughout the body and establish latent infection in central nervous tissues. The clinical presentation ranges from asymptomatic to severe neurological disorders in immunocompromised individuals. Since the clinical presentation is diverse and depends, among other factors, on the immune status of the host, in the present review, we introduce parasitological, epidemiological, clinical, and molecular biological aspects of infection with T. gondii to set the stage for an in-depth discussion of host immune responses. Since immune responses in humans have not been investigated in detail the present review is exclusively referring to immune responses in experimental models of infection. Systemic and local immune responses in different models of infection are discussed, and a separate chapter introduces commonly used animal models of infection.

Anti-inflammatory effects of resveratrol, curcumin and simvastatin in acute small intestinal inflammation

Background

The health beneficial effects of Resveratrol, Curcumin and Simvastatin have been demonstrated in various experimental models of inflammation. We investigated the potential anti-inflammatory and immunomodulatory mechanisms of the above mentioned compounds in a murine model of hyper-acute Th1-type ileitis following peroral infection with Toxoplasma gondii.

Methodology/Principal Findings

Here we show that after peroral administration of Resveratrol, Curcumin or Simvastatin, mice were protected from ileitis development and survived the acute phase of inflammation whereas all Placebo treated controls died. In particular, Resveratrol treatment resulted in longer-term survival. Resveratrol, Curcumin or Simvastatin treated animals displayed significantly increased numbers of regulatory T cells and augmented intestinal epithelial cell proliferation/regeneration in the ileum mucosa compared to placebo control animals. In contrast, mucosal T lymphocyte and neutrophilic granulocyte numbers in treated mice were reduced. In addition, levels of the anti-inflammatory cytokine IL-10 in ileum, mesenteric lymph nodes and spleen were increased whereas pro-inflammatory cytokine expression (IL-23p19, IFN-γ, TNF-α, IL-6, MCP-1) was found to be significantly lower in the ileum of treated animals as compared to Placebo controls. Furthermore, treated animals displayed not only fewer pro-inflammatory enterobacteria and enterococci but also higher anti-inflammatory lactobacilli and bifidobacteria loads. Most importantly, treatment with all three compounds preserved intestinal barrier functions as indicated by reduced bacterial translocation rates into spleen, liver, kidney and blood.

Conclusion/Significance

Oral treatment with Resveratrol, Curcumin or Simvastatin ameliorates acute small intestinal inflammation by down-regulating Th1-type immune responses and prevents bacterial translocation by maintaining gut barrier function. These findings provide novel and potential prophylaxis and treatment options of patients with inflammatory bowel diseases.

Small intestinal inflammation following oral infection with Toxoplasma gondii does not occur exclusively in C57BL/6 mice: review of 70 reports from the literature

Small intestinal immunopathology following oral infection with tissue cysts of Toxoplasma gondii has been described in C57BL/6 mice. Seven days after infection, mice develop severe small intestinal necrosis and succumb to infection. The immunopathology is mediated by local overproduction of Th1-type cytokines, a so-called 'cytokine storm'. The immunopathogenesis of this pathology resembles that of inflammatory bowel disease in humans, i.e., Crohn's disease. In this review, we show that the development of intestinal pathology following oral ingestion of T. gondii is not limited to C57BL/6 mice, but frequently occurs in nature. Using a Pubmed search, we identified 70 publications that report the development of gastrointestinal inflammation following infection with T. gondii in 63 animal species. Of these publications, 53 reports are on accidental ingestion of T. gondii in 49 different animal species and 17 reports are on experimental infections in 19 different animal species. Thus, oral infection with T. gondii appears to cause immunopathology in a large number of animal species in addition to mice. This manuscript reviews the common features of small intestinal immunopathology in the animal kingdom and speculates on consequences of this immunopathology for humankind.