The Diverse Role of NK Cells in Immunity to Toxoplasma gondii Infection

PruΔcdpk2 Protects Pigs Against Acute Toxoplasmosis Depending on T-Lymphocyte Subsets and Natural Killer Cell Responses

Toxoplasma gondii is a widespread protozoan parasite approximately infecting one-third of the world population and can cause serious public health problems. In this study, we investigated the protective effect of the attenuated vaccine Pru:Δcdpk2 against acute toxoplasmosis and explored the underlying immune mechanisms of the protection in pigs. The systemic T-cell and natural killer (NK) cell responses were analyzed, including kinetics, phenotype, and multifunctionality (interferon [IFN]-γ, tumor necrosis factor [TNF]-α), and the IFN-γ levels were analyzed in PBMCs. Our results showed that T. gondii-specific antibodies were induced by Pru:Δcdpk2. After challenging with RH, the antibodies were able to respond quickly in the immunized group, and the expression level was significantly higher than that in the unimmunized group. The expression level of IFN-γ significantly increased after vaccination, and the CD3+ γδ-, NK, and CD3+ γδ+ cell subsets also significantly increased. At the same time, functional analysis indicated that these cells were polarized toward a Th1 phenotype, showing the ability to secrete IFN-γ and TNF-α. The CD4+CD8α-T cell population exhibited a higher frequency of IFN-γ+ producing cells compared with the CD4-CD8α+ and CD4+CD8α+ cell populations during the early days of vaccination. Our results indicated that the attenuated vaccine could induce the expression of NK, γδ, and CD3αβ cells in pigs, and IFN-γ and TNF-α secreted by these cells are important for resistance to T. gondii infection.

Expression of IL-1β in transgenic Eimeria necatrix enhances the immunogenicity of parasites and promotes mucosal immunity against coccidiosis

Anticoccidial vaccines comprising living oocysts of Eimeria tenella, Eimeria necatrix, Eimeria maxima, and Eimeria acervulina are used to control coccidiosis. This study explored the potential of IL-1β to act as a molecular adjuvant for enhancing the immunogenicity of Eimeria necatrix and mucosal immunity. We engineered E. necatrix to express a functional chIL-1β (EnIL-1β) and immunized chickens with oocysts of the wild type (EnWT) and tranegenic (EnIL-1β) strains, respectively. The chickens were then challenged with EnWT oocysts to examine the immunogenicity-enhancing potential of chIL-1β. As expected, the oocyst output of EnIL-1β-immunized chickens was significantly reduced compared to those immunized using EnWT. No difference in body weight gain and lesion scores of EnIL-1β and EnWT groups was observed. The parasite load in the small intestine and caeca showed that the invasion and replication of EnIL-1β was not affected. However, the markers of immunogenicity and mucosal barrier, Claudin-1 and avian β-defensin-1, were elevated in EnIL-1β-infected chickens. Ectopic expression of chIL-1β in E. necatrix thus appears to improve its immunogenicity and mucosal immunity, without increasing pathogenicity. Our findings support chIL-1β as a candidate for development of effective live-oocyst-based anticoccidial vaccines.

Transcriptomic analysis of reproductive organs of pregnant mice post toxoplasma gondii infection reveals the potential factors that contribute to poor prognosis

Toxoplasma gondii is an obligate intracellular parasite of phylum Apicomplexa that poses a huge threat to pregnant hosts, and induces tragic outcomes for pregnant hosts, fetuses and newborns. However, the molecular mechanism underlying the tragic consequences caused by T. gondii remains to be revealed. In the present study, we applied RNA-seq to study the transcriptomic landscape of the whole reproductive organ of pregnant mice post T. gondii infection, aiming to reveal the key altered biological characters of reproductive organs of pregnant mice that could contribute to the tragic outcomes caused by T. gondii infection. The results of the present study showed that the transcriptome of reproductive organs of pregnant mice was significantly altered by T. gondii infection. A total of 2,598 differentially expressed genes (DEGs) were identified, including 1,449 upregulated genes and 1,149 downregulated genes. Enrichment analysis of the DEGs showed that the significantly altered features of reproductive organs of pregnant mice were excessive inflammatory responses, downregulated metabolism processes, and congenital diseases. The chemotaxis of immune cells in the reproductive organs of infected pregnant mice could also be reshaped by 19 differentially expressed chemokines and 6 differentially expressed chemokine receptors that could contribute to the damages of reproductive organ in pregnant mice. Overall, the findings of present study may help to understand the pathogenic mechanism of the acute T. gondii infection in reproductive organs of pregnant mice, and it could also help to improve toxoplasmosis therapeutics for pregnant individuals.

Toxoplasma gondii suppress human cord blood cell differentiation to the NK cell population

BACKGROUND

Toxoplasma gondii is an obligate intracellular protozoan parasite that can invade all mammalian cells. It is well established that natural killer (NK) cells have critical protective roles in innate immunity during infections by intracellular pathogens. In the current study, we conducted an in vitro experiment to evaluate NK cell differentiation and activation from human umbilical cord blood mononuclear cells (UCB-MNCs) after infection with T. gondii tachyzoites.

METHODS

UCB-MNCs were infected by fresh tachyzoites of type I (RH) or type II (PTG) strains of T. gondii pre-expanded in mesenchymal stem cells for 2 weeks in a medium enriched with stem cell factor, Flt3, IL-2, and IL-15. Flow cytometry analysis and western blot analysis were performed to measure the CD57+, CD56+, and Granzyme A (GZMA).

RESULTS

Data revealed that incubation of UCB-MNCs with NK cell differentiation medium increased the CD57+, CD56+, and GZMA. UCB-MNCs cocultured with PTG tachyzoites showed a significant reduction of CD56+ and GZMA, but nonsignificant changes, in the levels of CD56+ compared to the control UCB-MNCs (p > .05). Noteworthy, 2-week culture of UCB-MNCs with type I (RH) tachyzoites significantly suppressed CD57+, CD56+, and GZMA, showing reduction of NK cell differentiation from cord blood cells.

CONCLUSION

Our findings suggest that virulent T. gondii tachyzoites with cytopathic effects inhibit NK cell activation and eliminate innate immune responses during infection, and consequently enable the parasite to continue its survival in the host body.

Immune system roles in pathogenesis, prognosis, control, and treatment of Toxoplasma gondii infection

Toxoplasma gondii is the protozoan causative agent of toxoplasmosis in humans and warm-blooded animals. Recent studies have illustrated that the immune system plays a pivotal role in the pathogenesis of toxoplasmosis by triggering immune cytokines like IL-12, TNF-α, and IFN-γ and immune cells like DCs, Th1, and Th17. On the other hand, some immune components can serve as prognosis markers of toxoplasmosis. In healthy people, the disease is often asymptomatic, but immunocompromised people and newborns may suffer severe symptoms and complications. Therefore, the immune prognostic markers may provide tools to measure the disease progress and help patients to avoid further complications. Immunotherapies using monoclonal antibody, cytokines, immune cells, exosomes, novel vaccines, and anti-inflammatory molecules open new horizon for toxoplasmosis treatment. In this review article, we discussed the immunopathogenesis, prognosis, and immunotherapy of Toxoplasma gondii infection.

CD4+ T Cell Responses to Toxoplasma gondii Are a Double-Edged Sword

CD4+ T cells have been found to play critical roles in the control of both acute and chronic Toxoplasma infection. Previous studies identified a protective role for the Toxoplasma CD4+ T cell-eliciting peptide AS15 (AVEIHRPVPGTAPPS) in C57BL/6J mice. Herein, we found that immunizing mice with AS15 combined with GLA-SE, a TLR-4 agonist in emulsion adjuvant, can be either helpful in protecting male and female mice at early stages against Type I and Type II Toxoplasma parasites or harmful (lethal with intestinal, hepatic, and spleen pathology associated with a storm of IL6). Introducing the universal CD4+ T cell epitope PADRE abrogates the harmful phenotype of AS15. Our findings demonstrate quantitative and qualitative features of an effective Toxoplasma-specific CD4+ T cell response that should be considered in testing next-generation vaccines against toxoplasmosis. Our results also are cautionary that individual vaccine constituents can cause severe harm depending on the company they keep.

Influence of killer immunoglobulin-like receptors genes on the recurrence rate of ocular toxoplasmosis in Brazil

BACKGROUND

Recurrence is a hallmark of ocular toxoplasmosis (OT), and conditions that influence its occurrence remain a challenge. Natural killer cells (NK) are effectors cells whose primary is cytotoxic function against many parasites, including Toxoplasma gondii. Among the NK cell receptors, immunoglobulin-like receptors (KIR) deserve attention due to their high polymorphism.

OBJECTIVES

This study aimed to analyse the influence of KIR gene polymorphism in the course of OT infection and its association with recurrences after an active episode.

METHODS

Ninety-six patients from the Ophthalmologic Clinic of the National Institute of Infectology Evandro Chagas were followed for up to five years. After DNA extraction, genotyping of the patients was performed by polymerase chain reaction sequence-specific oligonucleotide (PCR-SSO) utilising Luminex equipment for reading. During follow-up, 60.4% had a recurrence.

FINDINGS

We identified 25 KIR genotypes and found a higher frequency of genotype 1 (31.7%) with worldwide distribution. We note that the KIR2DL2 inhibitor gene and the gene activator KIR2DS2 were more frequent in patients without recurrence. Additionally, we observed that individuals who carry these genes progressed recurrence episodes slowly compared to individuals who do not carry these genes.

MAIN CONCLUSIONS

The KIR2DL2 and KIR2DS2 are associated as possible protection markers against ocular toxoplasmosis recurrence (OTR).

Nanoparticles as a Delivery System of Antigens for the Development of an Effective Vaccine against Toxoplasma gondii

Nanoparticles include particles ranging in size from nanometers to micrometers, whose physicochemical characteristics are optimized to make them appropriate delivery vehicles for drugs or immunogens important in the fight and/or prevention of infectious diseases. There has been a rise in the use of nanoparticles in preventive vaccine formulations as immunostimulatory adjuvants, and as vehicles for immunogen delivery to target immune cells. Toxoplasma is important worldwide, and may cause human toxoplasmosis. In immunocompetent hosts, infection is usually asymptomatic, but in immunocompromised patients it can cause serious neurological and ocular consequences, such as encephalitis and retinochoroiditis. Primary infection during pregnancy may cause abortion or congenital toxoplasmosis. Currently, there is no effective human vaccine against this disease. Evidence has emerged from several experimental studies testing nanovaccines showing them to be promising tools in the prevention of experimental toxoplasmosis. For the present study, a literature review was carried out on articles published over the last 10 years through the PubMed database, pertaining to in vivo experimental models of T. gondii infection where nanovaccines were tested and protection and immune responses evaluated. This review aims to highlight the way forward in the search for an effective vaccine for toxoplasmosis.

Unraveling the role of natural killer cells in leishmaniasis

NK cells are known as frontline responders that are efficient in combating several maladies as well as leishmaniasis caused by Leishmania spp. As such they are being investigated to be used for adoptive transfer therapy and vaccine. In spite of the lack of antigen-specific receptors at their surface, NK cells can selectively recognize pathogens, accomplished by the activation of the receptors on the NK cell surface and also as the result of their effector functions. Activation of NK cells can occur through interaction between TLR-2 expressed on NK cells and. LPG of Leishmania parasites. In addition, NK cell activation can occur by cytokines (e.g., IFN-γ and IL-12) that also lead to producing cytokines and chemokines and lysis of target cells. This review summarizes several evidences that support NK cells activation for controlling leishmaniasis and the potentially lucrative roles of NK cells during leishmaniasis. Furthermore, we discuss strategies of Leishmania parasites in inhibiting NK cell functions. Leishmania LPG can utilizes TLR2 to evade host-immune responses. Also, Leishmania GP63 can directly binds to NK cells and modulates NK cell phenotype. Finally, this review analyzes the potentialities to harness NK cells effectiveness in therapy regimens and vaccinations.

LFA-1/ ICAM-1 promotes NK cell cytotoxicity associated with the pathogenesis of ocular toxoplasmosis in murine model

Ocular toxoplasmosis (OT) is one of the most common causes of posterior uveitis. However, the pathogenic mechanisms of OT have not been well elucidated. Here, we used C57BL/6 (B6) mice to establish OT by peroral infection with 20 cysts of the TgCtWh6 strain, and severe ocular damage was observed by histopathological analysis in the eyes of infected mice. RNA-sequencing results showed that infection with T. gondii increased the expression of the NK-mediated cytotoxicity gene pathway at Day 30 after ocular T. gondii infection. Both NK-cell and CD49a+ NK-cell subsets are increased in ocular tissues, and the expression levels of LFA-1 in NK cells and ICAM-1 in the OT murine model were upregulated upon infection. Furthermore, inhibition of the interaction between LFA-1 and ICAM-1 with lifitegrast, a novel small molecule integrin antagonist, inhibited the protein expression of LFA-1 and ICAM-1 in murine OT and NK cells, improved the pathology of murine OT and influenced the secretion of cytokines in the OT murine model. In conclusion, the interaction between LFA-1 and ICAM-1 plays a role in the early regulation of the CD49a+ NK-cell proportion in an OT murine model. LFA-1/ ICAM-1 may be a key molecule in the pathogenesis of OT, and may provide new insights for potential immunotherapy.

Innate immune cell response to host-parasite interaction in a human intestinal tissue microphysiological system

Protozoan parasites that infect humans are widespread and lead to varied clinical manifestations, including life-threatening illnesses in immunocompromised individuals. Animal models have provided insight into innate immunity against parasitic infections; however, species-specific differences and complexity of innate immune responses make translation to humans challenging. Thus, there is a need for in vitro systems that can elucidate mechanisms of immune control and parasite dissemination. We have developed a human microphysiological system of intestinal tissue to evaluate parasite-immune-specific interactions during infection, which integrates primary intestinal epithelial cells and immune cells to investigate the role of innate immune cells during epithelial infection by the protozoan parasite, Toxoplasma gondii, which affects billions of people worldwide. Our data indicate that epithelial infection by parasites stimulates a broad range of effector functions in neutrophils and natural killer cell-mediated cytokine production that play immunomodulatory roles, demonstrating the potential of our system for advancing the study of human-parasite interactions.

GABAergic signaling in human and murine NK cells upon challenge with Toxoplasma gondii

Protective cytotoxic and proinflammatory cytokine responses by NK cells impact the outcome of infections by Toxoplasma gondii, a common parasite in humans and other vertebrates. However, T. gondii can also sequester within NK cells and downmodulate their effector functions. Recently, the implication of GABA signaling in infection and inflammation-related responses of mononuclear phagocytes and T cells has become evident. Yet, the role of GABAergic signaling in NK cells has remained unknown. Here, we report that human and murine NK cells synthesize and secrete GABA in response to infection challenge. Parasitized NK cells secreted GABA, whereas activation stimuli, such as IL-12/IL-18 or parasite lysates, failed to induce GABA secretion. GABA secretion by NK cells was associated to a transcriptional up-regulation of GABA synthesis enzymes (glutamate decarboxylases [GAD65/67]) and was abrogated by GAD inhibition. Further, NK cells expressed GABA-A receptor subunits and GABA signaling regulators, with transcriptional modulations taking place upon challenge with T. gondii. Exogenous GABA and GABA-containing supernatants from parasitized dendritic cells (DCs) impacted NK cell function by reducing the degranulation and cytotoxicity of NK cells. Conversely, GABA-containing supernatants from NK cells enhanced the migratory responses of parasitized DCs. This enhanced DC migration was abolished by GABA-A receptor antagonism or GAD inhibition and was reconstituted by exogenous GABA. Jointly, the data show that NK cells are GABAergic cells and that GABA hampers NK cell cytotoxicity in vitro. We hypothesize that GABA secreted by parasitized immune cells modulates the immune responses to T. gondii infection.

Mapping functional humoral correlates of protection against malaria challenge following RTS,S/AS01 vaccination

Vaccine development has the potential to be accelerated by coupling tools such as systems immunology analyses and controlled human infection models to define the protective efficacy of prospective immunogens without expensive and slow phase 2b/3 vaccine studies. Among human challenge models, controlled human malaria infection trials have long been used to evaluate candidate vaccines, and RTS,S/AS01 is the most advanced malaria vaccine candidate, reproducibly demonstrating 40 to 80% protection in human challenge studies in malaria-naïve individuals. Although antibodies are critical for protection after RTS,S/AS01 vaccination, antibody concentrations are inconsistently associated with protection across studies, and the precise mechanism(s) by which vaccine-induced antibodies provide protection remains enigmatic. Using a comprehensive systems serological profiling platform, the humoral correlates of protection against malaria were identified and validated across multiple challenge studies. Rather than antibody concentration, qualitative functional humoral features robustly predicted protection from infection across vaccine regimens. Despite the functional diversity of vaccine-induced immune responses across additional RTS,S/AS01 vaccine studies, the same antibody features, antibody-mediated phagocytosis and engagement of Fc gamma receptor 3A (FCGR3A), were able to predict protection across two additional human challenge studies. Functional validation using monoclonal antibodies confirmed the protective role of Fc-mediated antibody functions in restricting parasite infection both in vitro and in vivo, suggesting that these correlates may mechanistically contribute to parasite restriction and can be used to guide the rational design of an improved vaccine against malaria.

The role of IL-12 in stimulating NK cells against Toxoplasma gondii infection: a mini-review

Toxoplasma gondii is an intracellular protozoan parasite that can remarkably infect, survive, and replicate in almost all mammalian cells and can cause severe neurological and ocular damage in immunocompromised individuals. It is known that Natural Killer cells (NK cells), as a type of cytotoxic lymphocyte, have critical protective roles in innate immunity during the T. gondii infection through releasing interferon gamma (IFN-γ). Interleukin 12 (IL-12) is a pivotal critical cytokine for the generation of IFN-γ-producing NK cells. Several studies have shown cytokines' impact on NK cell activation; and IL-2 has an important role with a potent stimulatory factor for NK cells. In this review, we summarized the mechanism of interleukin-12 production stimulation by T. gondii tachyzoites and discussed several factors affecting this mechanism.

Imbalance of uterine innate lymphoid cells is involved in the abnormal pregnancy induced by Toxoplasma gondii infection

Toxoplasma gondii (T. gondii) is a ubiquitous intracellular protozoan parasite that causes adverse pregnancy outcomes. Innate lymphoid cells (ILCs) are critical mediators of mucosal immunity, and have been reported to play an important role in uterine vascular adaptation for successful pregnancy. However, the specific role of ILCs in T. gondii-infection-induced adverse pregnancy outcomes remains elusive. In the present study, we found that T. gondii infection caused the imbalance of uterine ILC cells (uILCs). It was characterized by substantially lower expression of the transcription factor GATA-3 and RORγt and higher expression of T-bet in uILCs. Consistent with the transcription factor changes, uILCs from T. gondii-infected mice produced much less IL-5 and IL-17 and substantially more IFN-γ and TNF-α than did uILCs from uninfected mice. Notably, IL-12, IL-18, and their receptors were increased in the uterus of T. gondii-infected mice. In vitro experiments showed that IL-12 and IL-18 treatment reduced the percentages of uILC2 and uILC3 and increased the percentages of uILC1. Conclusion, our data suggest that alterations in uILC composition may disrupt the balance of immune microenvironment after T. gondii infection and contribute to the adverse pregnancy outcomes caused by T. gondii infection.

Effect of Imiquimod on Tachyzoites of Toxoplasma gondii and Infected Macrophages in vitro and in BALB/c Mice

Treatment for toxoplasmosis is not completely successful because of their unwanted side effects, and new treatments are needed. Imiquimod has ability to moderate immune response and used to treat a wide variety of infections and tumors. The aim of the present study was to evaluate the effect of imiquimod on the tachyzoites of T. gondii and infected macrophages in vitro and in BALB/c mice. The viability of T. gondii was assessed in the presence of various concentrations of imiquimod by direct counting after 6 and 24 h. The MTT assay was used to identify the viability of uninfected macrophages. The apoptotic effects were determined with flow cytometry on the tachyzoites and infected macrophages. For evaluation of parasite load in pre-treatment or post-treatment of macrophages Quantitative real time PCR (qPCR) was performed. For in vivo experiments, BALB/c mice received imiquimod before and after challenge with parasites. The mortality rate of mice, parasite numbers in spleen, and the INF-γ and IL-4 cytokine levels in spleen lymphocytes were evaluated. Imiquimod demonstrated anti-Toxoplasma effects by reducing the number of tachyzoites. The results of flow cytometry for drug-treated tachyzoites showed that apoptosis did not rise significantly relative to the control group (p < 0.05). Moreover, apoptosis was enhanced in infected macrophages as the concentration of imiquimod was reduced. The parasitic burden in imiquimod pretreated macrophages was significantly lower than those treated after infection (p < 0.01). A marked reduction was observed in survival rate, parasite load and INF-γ level in BALB/c mice that received imiquimod before parasitic challenge relative to those received drug after parasitic challenge (p < 0.01). Overall, imiquimod in the pretreated group had greater anti-Toxoplasma effects than imiquimod in posttreated group in vitro and in vivo. imiquimod may be considered as a candidate for use against Toxoplasmosis both therapeutically and prophylactically.

NK Cells Negatively Regulate CD8 T Cells to Promote Immune Exhaustion and Chronic Toxoplasma gondii Infection

NK cells regulate CD4+ and CD8+ T cells in acute viral infection, vaccination, and the tumor microenvironment. NK cells also become exhausted in chronic activation settings. The mechanisms causing these ILC responses and their impact on adaptive immunity are unclear. CD8+ T cell exhaustion develops during chronic Toxoplasma gondii (T. gondii) infection resulting in parasite reactivation and death. How chronic T. gondii infection impacts the NK cell compartment is not known. We demonstrate that NK cells do not exhibit hallmarks of exhaustion. Their numbers are stable and they do not express high PD1 or LAG3. NK cell depletion with anti-NK1.1 is therapeutic and rescues chronic T. gondii infected mice from CD8+ T cell exhaustion dependent death, increases survival after lethal secondary challenge and alters cyst burdens in brain. Anti-NK1.1 treatment increased polyfunctional CD8+ T cell responses in spleen and brain and reduced CD8+ T cell apoptosis in spleen. Chronic T. gondii infection promotes the development of a modified NK cell compartment, which does not exhibit normal NK cell characteristics. NK cells are Ly49 and TRAIL negative and are enriched for expression of CD94/NKG2A and KLRG1. These NK cells are found in both spleen and brain. They do not produce IFNγ, are IL-10 negative, do not increase PDL1 expression, but do increase CD107a on their surface. Based on the NK cell receptor phenotype we observed NKp46 and CD94-NKG2A cognate ligands were measured. Activating NKp46 (NCR1-ligand) ligand increased and NKG2A ligand Qa-1b expression was reduced on CD8+ T cells. Blockade of NKp46 rescued the chronically infected mice from death and reduced the number of NKG2A+ cells. Immunization with a single dose non-persistent 100% protective T. gondii vaccination did not induce this cell population in the spleen, suggesting persistent infection is essential for their development. We hypothesize chronic T. gondii infection induces an NKp46 dependent modified NK cell population that reduces functional CD8+ T cells to promote persistent parasite infection in the brain. NK cell targeted therapies could enhance immunity in people with chronic infections, chronic inflammation and cancer.

Effects of Toxoplasma gondii infection on the function and integrity of human cerebrovascular endothelial cells and the influence of verapamil treatment in vitro

Toxoplasma gondii can cause parasitic encephalitis, a life-threatening infection that predominately occurs in immunocompromised individuals. T. gondii has the ability to invade the brain, but the mechanisms by which this parasite crosses the blood-brain-barrier (BBB) remain incompletely understood. The present study reports the changes associated with infection and replication of T. gondii within human brain microvascular endothelial cells (BMECs) in vitro. Our results indicated that exposure to T. gondii had an adverse impact on the function and integrity of the BMECs - through induction of cell cycle arrest, disruption of the BMEC barrier integrity, reduction of cellular viability and vitality, depolarization of the mitochondrial membrane potential, increase of the DNA fragmentation, and alteration of the expression of immune response and tight junction genes. The calcium channel/P-glycoprotein transporter inhibitor verapamil was effective in inhibiting T. gondii crossing the BMECs in a dose-dependent manner. The present study showed that T. gondii can compromise several functions of BMECs and demonstrated the ability of verapamil to inhibit T. gondii crossing of the BMECs in vitro.

The role of innate lymphoid cells in response to microbes at mucosal surfaces

Innate lymphoid cells (ILCs) are a lymphocyte population that is mostly resident at mucosal surfaces. They help to induce an appropriate immune response to the microbiome at homeostasis. In healthy people, the mucosal immune system works symbiotically with organisms that make up the microbiota. ILCs play a critical role in orchestrating this balance, as they can both influence and in turn be influenced by the microbiome. ILCs also are important regulators of the early response to infections by diverse types of pathogenic microbes at mucosal barriers. Their rapid responses initiate inflammatory programs, production of antimicrobial products and repair processes. This review will focus on the role of ILCs in response to the microbiota and to microbial infections of the lung and intestine.

Detecting and analyzing murine innate lymphoid cells

During the last 10 years, different subsets of Innate lymphoid cells (ILCs) have been identified in murine models. ILCs play and important role in maintaining immune barriers, tissue homeostasis, and are able to regulate the immune response in several anatomic sites. They can be found in lymphoid and non-lymphoid organs of adult mice but are mainly tissue-resident cells that can expand locally under physiologic or pathologic conditions (Gasteiger, Fan, Dikiy, Lee, & Rudensky, 2015). Because ILCs need to be identified by a complex combination of several cell-surface and intracellular markers and by their production of specific sets of cytokines, multiparametric flow cytometry remains one of the most efficient methods to analyze and isolate the different ILC sub-populations. This chapter describes how ILCs can be identified in different murine organs and how ILC subsets can be isolated and functionally analyzed.

T cell-derived interferon-γ is required for host defense to Toxoplasma gondii

Interferon-γ (IFN-γ) is important for host defense against various intracellular organisms including a protozoan pathogen Toxoplasma gondii. Various immune cells are recently shown to produce IFN-γ in T. gondii infection, however, it remains elusive which cell types are important for anti-T. gondii host defense so far. Here we generate a new IFN-γ reporter "GREVEN" mouse line in which a fusion protein of Venus and NanoLuc to analyze IFN-γ producing cells during T. gondii infection and find that CD4⁺, CD8⁺, γδ T cells and natural killer cells express Venus in a time dependent manner. Furthermore, Lck-Cre/Ifng^fl/fl mice are highly susceptible to T. gondii infection. Taken together, our results demonstrate that T cell-derived IFN-γ plays an important role in anti-T. gondii host defense.

Echinococcus multilocularis inoculation induces NK cell functional decrease through high expression of NKG2A in C57BL/6 mice

BACKGROUND

Alveolar echinococcosis (AE) is caused by the larval stage of Echinococcus multilocularis (E. multilocularis), and considered as public health issue. Parasite-host immune interaction is pivotal during infection. As a subset of innate lymphoid cells, NK cells are known to play an important role during virus, bacteria, intra/extracellular parasitic infections and tumor progression. However, the possible role of NK cells in E. multilocularis infection in both human and murine is little known. Herein, the functional alteration of hepatic NK cells and their related molecules in E. multilocularis infected mice were studied.

METHODS

2000 protoscoleces (PSCs) were injected to C57BL/6 mice via the portal vein to establish secondary E. multilocularis infection. NK cells population and their related molecules (CD69, Ly49D, Ly49G2, Ly49H, Ly49I, NKG2A, NKG2D, granzyme B, IFN-γ, TNF-α) were assessed by using fluorescence-activated cell sorter (FACS) techniques and qRT-PCR. NK cell depletion was performed for further understanding the possible function of NK cells during infection.

RESULTS

The total frequencies of NK cells and NK-derived IFN-γ production were significantly reduced at designated time points (2, 4, 12 weeks). The liver resident (CD49a+DX5-) NK cells are decreased at 4 weeks after inoculation and which is significantly lower than in control mice. Moreover, in vivo antibody-mediated NK cell depletion increased parasitic load and decreased peri-parasitic fibrosis. Expression of the inhibitory receptor NKG2A was negatively related to NK- derived IFN-γ secretion.

CONCLUSIONS

Our study showed down regulates of NK cells and upper regulates of NKG2A expression on NK cells during E. multilocularis infection. Reduction of NK cell frequencies and increased NKG2A might result in low cytotoxic activity through decreased IFN-γ secretion in E. multilocularis infection. This result might be helpful to restore NK cell related immunity against E. multilocularis infection to treat alveolar echinococcosis.

Innate Lymphoid Cells in Protection, Pathology, and Adaptive Immunity During Apicomplexan Infection

Apicomplexans are a diverse and complex group of protozoan pathogens including Toxoplasma gondii, Plasmodium spp., Cryptosporidium spp., Eimeria spp., and Babesia spp. They infect a wide variety of hosts and are a major health threat to humans and other animals. Innate immunity provides early control and also regulates the development of adaptive immune responses important for controlling these pathogens. Innate immune responses also contribute to immunopathology associated with these infections. Natural killer (NK) cells have been for a long time known to be potent first line effector cells in helping control protozoan infection. They provide control by producing IL-12 dependent IFNγ and killing infected cells and parasites via their cytotoxic response. Results from more recent studies indicate that NK cells could provide additional effector functions such as IL-10 and IL-17 and might have diverse roles in immunity to these pathogens. These early studies based their conclusions on the identification of NK cells to be CD3–, CD49b+, NK1.1+, and/or NKp46+ and the common accepted paradigm at that time that NK cells were one of the only lymphoid derived innate immune cells present. New discoveries have lead to major advances in understanding that NK cells are only one of several populations of innate immune cells of lymphoid origin. Common lymphoid progenitor derived innate immune cells are now known as innate lymphoid cells (ILC) and comprise three different groups, group 1, group 2, and group 3 ILC. They are a functionally heterogeneous and plastic cell population and are important effector cells in disease and tissue homeostasis. Very little is known about each of these different types of ILCs in parasitic infection. Therefore, we will review what is known about NK cells in innate immune responses during different protozoan infections. We will discuss what immune responses attributed to NK cells might be reconsidered as ILC1, 2, or 3 population responses. We will then discuss how different ILCs may impact immunopathology and adaptive immune responses to these parasites.

iTRAQ-based phosphoproteomic analysis reveals host cell's specific responses to Toxoplasma gondii at the phases of invasion and prior to egress

Protein phosphorylation plays a key role in host cell-T. gondii interaction. However, the phosphoproteome data of host cell at various phases of T. gondii infection has not been thoroughly described. In this study, we assessed the host phosphoproteome data with isobaric tags for relative and absolute quantification (iTRAQ) method during the phases of T. gondii invasion (30 min post infection, PI) and prior to egress (28 h PI). Our iTRAQ analysis revealed a total of 665 phosphoproteins, among which the significantly regulated phosphoproteins in different between-group comparisons were further analyzed. Functional analysis of these significantly regulated phosphoproteins suggested that T. gondii modulated host cell processes through phosphorylation including cell cycle regulation, inducing apoptosis, blocking the synthesis of some inflammatory factors, mediating metabolism to support its proliferation at the infection phase prior to egress, and utilizing membrane and energy from host cell, reorganizing cytoskeleton to favor its invasion and PV formation at the phase of invasion. The phosphorylation level of Smad2, CTNNA1, and HSPB1 identified with western blot revealed a consistent trend of change with iTRAQ result. These newly identified and significantly regulated phosphoproteins from our phosphoproteome data may provide new clues to unravel the host cell's complex reaction against T. gondii infection and the interaction between the host cell and T. gondii.

Parasitized Natural Killer cells do not facilitate the spread of Toxoplasma gondii to the brain

Toxoplasma gondii is a protozoan parasite capable of invading immune cells and co‐opting their migratory pathways to disseminate through the host. Natural Killer (NK) cells can be directly invaded by the parasite and this invasion alters NK cell migration, producing a hypermotile phenotype. However, the consequences of this hypermotile phenotype for the dissemination of T. gondii to the brain remain unknown. To address this, C57BL6/J mice were infected with freshly egressed tachyzoites (type IIPrugniaud strain) or with parasitized NK cells. Under both conditions, parasite loads in the brain were comparable, indicating that parasitized NK cells were not able to facilitate spread of T. gondii to the brain. Consistent with this, we found no evidence for the recruitment of endogenous NK cells to the brain at early time points post‐infection, nor any changes in the expression of α4β1 integrin, involved in recruitment of NK cells to the brain. We therefore found no evidence for a role for hypermotile NK cells in delivery of parasites to the brain during acute infection with T. gondii.

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.

Downmodulation of Effector Functions in NK Cells upon Toxoplasma gondii Infection

The obligate intracellular parasite Toxoplasma gondii can actively infect any nucleated cell type, including cells from the immune system. The rapid transfer of T. gondii from infected dendritic cells to effector natural killer (NK) cells may contribute to the parasite's sequestration and shielding from immune recognition shortly after infection. However, subversion of NK cell functions, such as cytotoxicity or production of proinflammatory cytokines, such as gamma interferon (IFN-γ), upon parasite infection might also be beneficial to the parasite. In the present study, we investigated the effects of T. gondii infection on NK cells. In vitro, infected NK cells were found to be poor at killing target cells and had reduced levels of IFN-γ production. This could be attributed in part to the inability of infected cells to form conjugates with their target cells. However, even upon NK1.1 cross-linking of NK cells, the infected NK cells also exhibited poor degranulation and IFN-γ production. Similarly, NK cells infected in vivo were also poor at killing target cells and producing IFN-γ. Increased levels of transforming growth factor β production, as well as increased levels of expression of SHP-1 in the cytosol of infected NK cells upon infection, were observed in infected NK cells. However, the phosphorylation of STAT4 was not altered in infected NK cells, suggesting that transcriptional regulation mediates the reduced IFN-γ production, which was confirmed by quantitative PCR. These data suggest that infection of NK cells by T. gondii impairs NK cell recognition of target cells and cytokine release, two mechanisms that independently could enhance T. gondii survival.

Neurophysiological Changes Induced by Chronic Toxoplasma gondii Infection

Although the parasite Toxoplasma gondii is one of the most pervasive neurotropic pathogens in the world, the host-parasite interactions during CNS infection and the consequences of neurological infection are just beginning to be unraveled. The chronic stages of infection have been considered dormant, although several studies have found correlations of infection with an array of host behavioral changes. These may facilitate parasite transmission and impact neurological diseases. During infection, in addition to the presence of the parasites within neurons, host-mediated neuroimmune and hormonal responses to infection are also present. T. gondii induces numerous changes to host neurons during infection and globally alters host neurological signaling pathways, as discussed in this review. Understanding the neurophysiological changes in the host brain is imperative to understanding the parasitic mechanisms and to delineate the effects of this single-celled parasite on health and its contribution to neurological disease.