Determination of a Key Antigen for Immunological Intervention To Target the Latent Stage of Toxoplasma gondii

New advances in immune mechanism and treatment during ocular toxoplasmosis

Ocular toxoplasmosis (OT) is an intraocular infection caused by the parasite Toxoplasma gondii. OT is manifested as retinal choroiditis and is the most common infectious cause of posterior uveitis. Invasion of the retina by T. gondii leads to disruption of the blood-ocular barrier and promotes the migration of immune cells to the ocular tissues. Cytokines such as IFN-γ and IL-1β are effective for controlling parasite growth, but excessive inflammatory responses can cause damage to the host. In this review, we will discuss in detail the latest advances in the immunopathology and treatment of OT.

Immunization with the amino-terminus region of dense granule protein 6 (GRA6) of Toxoplasma gondii activates CD8+ cytotoxic T cells capable of removing tissue cysts of the parasite through antigen presentation by human HLA-A2.1

CD8+ T cells from HLA-A2.1-transgenic mice, but not wild-type mice, immunized with the amino-terminus region (aa 41-152) of dense granule protein 6 (GRA6Nt) of Toxoplasma gondii secreted large amounts of perforin and granzyme B in response to GRA6Nt through antigen presentation by HLA-A2.1 in vitro. When those CD8+ T cells were transferred into chronically infected HLA-A2.1-expressing NSG mice deficient in T cells, cerebral cyst burden of the recipients of HLA-A2.1-transgenic T cells, but not of WT T cells, became significantly less than that of control mice with no cell transfer. Furthermore, the significant reduction of the cyst burden by a transfer of the HLA-A2.1-transgenic CD8+ immune T cells required an expression of HLA-A2.1 in the recipient NSG mice. Thus, antigen presentation of GRA6Nt by human HLA-A2.1is able to activate anti-cyst CD8+ T cells that eliminate T. gondii cysts through antigen presentation by human HLA-A2.1.

Dense granule protein 3 of Toxoplasma gondii plays a crucial role in the capability of the tissue cysts of the parasite to persist in the presence of anti-cyst CD8+ T cells during the chronic stage of infection

Toxoplasma gondii establishes chronic infection by forming tissue cysts, and this chronic infection is one of the most common parasitic infections in humans. Our recent studies revealed that whereas CD8+ T cells of genetically resistant BALB/c mice have the capability to remove the tissue cysts of the parasite through their perforin-mediated activities, small portions of the cysts are capable of persisting in the presence of the anti-cyst CD8+ T cells. It is currently unknown how those small portions of the cysts resist or escape the T-cell immunity and persist in the hosts. In the present study, we discovered that the cysts, which persisted in the presence of the perforin-mediated CD8+ T-cell immunity, have significantly greater mRNA levels for four dense granule proteins, GRA1, GRA2, GRA3, and GRA7, and one rhoptry protein, ROP35, than the total population of the cysts present in the absence of the T cells. In addition, increased levels of mRNA for GRA1, GRA3, and ROP35 in the cysts significantly correlated with their successful persistence through the condition in which greater degrees of reduction of the cyst burden occurred through anti-cyst CD8+ T cells. In addition, GRA3-deficient T. gondii displayed significantly enhanced elimination of the cysts by anti-cyst CD8+ T cells when compared to the wild-type parasite. These results indicate that GRA3 is a key molecule that mediates in the capability of T. gondii cysts to persist by resisting or evading the anti-cyst activity of CD8+ T cells during the later stage of infection.

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.

Epidemiology, Pathophysiology, Diagnosis, and Management of Cerebral Toxoplasmosis

Toxoplasma gondii is known to infect a considerable number of mammalian and avian species and a substantial proportion of the world's human population. The parasite has an impressive ability to disseminate within the host's body and employs various tactics to overcome the highly regulatory blood-brain barrier and reside in the brain. In healthy individuals, T. gondii infection is largely tolerated without any obvious ill effects. However, primary infection in immunosuppressed patients can result in acute cerebral or systemic disease, and reactivation of latent tissue cysts can lead to a deadly outcome. It is imperative that treatment of life-threatening toxoplasmic encephalitis is timely and effective. Several therapeutic and prophylactic regimens have been used in clinical practice. Current approaches can control infection caused by the invasive and highly proliferative tachyzoites but cannot eliminate the dormant tissue cysts. Adverse events and other limitations are associated with the standard pyrimethamine-based therapy, and effective vaccines are unavailable. In this review, the epidemiology, economic impact, pathophysiology, diagnosis, and management of cerebral toxoplasmosis are discussed, and critical areas for future research are highlighted.

A pathway to cure chronic infection with Toxoplasma gondii through immunological intervention

Toxoplasma gondii, an obligate intracellular protozoan parasite, can establish a chronic infection in the brain by forming tissue cysts. This chronic infection is widespread in humans worldwide including developed countries, with up to one third of the population being estimated to be infected with this parasite. Diagnosis of this chronic infection is usually conducted by serological detection of IgG antibodies against this parasite. Since infected individuals remain positive for these antibodies for years, it has generally been considered that this infection is a lifelong infection. It is also often considered that this chronic infection is "latent" or "quiescent". However, recent discovery of the capability of perforin-dependent, CD8⁺ T cell-mediated immune responses to eliminate T. gondii cysts in collaboration with phagocytes illustrated dynamic interplays between T. gondii cysts and host immune system during this chronic infection. Importantly, the cytotoxic T cell-mediated protective immunity is able to remove mature cysts of the parasite. It is now clear that chronic T. gondii infection is not "latent" or "quiescent". Elucidating the mechanisms of the dynamic host-pathogen interactions between the anti-cyst protective immunity and T. gondii cysts and identifying the pathway to appropriately activate anti-cyst CD8⁺ cytotoxic T cells would be able to open a door for eradicating T. gondii cysts and curing chronic infection with this parasite.

The amino-terminal region of dense granule protein 6 of Toxoplasma gondii stimulates IFN-γ production by microglia

We examined activities of dense granule proteins (GRAs), which Toxoplasma gondii secretes within infected cells, to stimulate microglial IFN-γ production in vitro. We identified that the N-terminal region (amino acids 41-152) of GRA6 (GRA6Nt) stimulates IFN-γ production by both a microglia cell line and primary microglia purified from the brains of uninfected adult mice. In contrast, neither of GRA1, GRA2, GRA5Nt, nor the carboxyl-terminal (amino acids 174-224) of GRA6 stimulated microglial IFN-γ production. GRA6Nt appears to be a target molecule of the sentinel function of microglia to detect cerebral proliferation of T. gondii and activate their IFN-γ production for facilitating the protective immunity to control the pathogen.

The immune system utilizes two distinct effector mechanisms of T cells depending on two different life cycle stages of a single pathogen, Toxoplasma gondii, to control its cerebral infection

Toxoplasma gondii takes two different life cycle stages within intermediate hosts including humans. Tachyzoites proliferate during the acute stage, and they transform into cysts to establish a chronic infection preferentially in the brain. IFN-γ production by infiltrated CD4⁺ and CD8⁺ T cells is required for the prevention of cerebral tachyzoite growth. IFN-γ production by brain-resident cells, most likely microglia, plays a key first line defense role to facilitate both innate and T cell-mediated protective immunity to control the tachyzoite growth. IFN-γ produced by brain-resident cells activates cerebral expression of IFN-dependent effector molecules to suppress tachyzoite growth during the early stage of infection. Their IFN-γ production also induces an expression of CXCL9 and CXCL10 chemokines to recruit immune T cells into the brain, and upregulates cerebral expression of MHC class I and II molecules for antigen presentation to the recruited T cells to activate their IFN-γ production. CD8⁺ T cells also have the activity to remove T. gondii cysts from the brains of infected hosts. Of interest, the anti-cyst activity of CD8⁺ T cells does not require their IFN-γ but does require perforin. Notably, we discovered that CD8⁺ cytotoxic T cells penetrate in the cysts in a perforin-mediated manner, which induces morphological deterioration and destruction of the cysts and an accumulation of microglia and macrophages for their elimination. Thus, the immune system employs two distinct effector mechanisms mediated by IFN-γ or perforin depending on two different life cycle stages of a single pathogen, T. gondii, to control its cerebral infection.

Rhoptry and Dense Granule Secreted Effectors Regulate CD8+ T Cell Recognition of Toxoplasma gondii Infected Host Cells

Toxoplasma gondii secretes rhoptry (ROP) and dense granule (GRA) effector proteins to evade host immune clearance mediated by interferon gamma (IFN-γ), immunity-related GTPase (IRG) effectors, and CD8⁺ T cells. Here, we investigated the role of parasite-secreted effectors in regulating host access to parasitophorous vacuole (PV) localized parasite antigens and their presentation to CD8⁺ T cells by the major histocompatibility class I (MHC-I) pathway. Antigen presentation of PV localized parasite antigens by MHC-I was significantly increased in macrophages and/or dendritic cells infected with mutant parasites that lacked expression of secreted GRA (GRA2, GRA3, GRA4, GRA5, GRA7, GRA12) or ROP (ROP5, ROP18) effectors. The ability of various secreted GRA or ROP effectors to suppress antigen presentation by MHC-I was dependent on cell type, expression of IFN-γ, or host IRG effectors. The suppression of antigen presentation by ROP5, ROP18, and GRA7 correlated with a role for these molecules in preventing PV disruption by IFN-γ-activated host IRG effectors. However, GRA2 mediated suppression of antigen presentation was not correlated with PV disruption. In addition, the GRA2 antigen presentation phenotypes were strictly co-dependent on the expression of the GRA6 protein. These results show that MHC-I antigen presentation of PV localized parasite antigens was controlled by mechanisms that were dependent or independent of IRG effector mediated PV disruption. Our findings suggest that the GRA6 protein underpins an important mechanism that enhances CD8⁺ T cell recognition of parasite-infected cells with damaged or ruptured PV membranes. However, in intact PVs, parasite secreted effector proteins that associate with the PV membrane or the intravacuolar network membranes play important roles to actively suppress antigen presentation by MHC-I to reduce CD8⁺ T cell recognition and clearance of Toxoplasma gondii infected host cells.

Penetration of CD8+ Cytotoxic T Cells into Large Target, Tissue Cysts of Toxoplasma gondii, Leads to Its Elimination

CD8⁺ cytotoxic T cells kill target cells through direct cell-cell contact. However, it remains unclear how these T cells eliminate a target of large mass. We investigated how CD8⁺ T cells remove tissue cysts of Toxoplasma gondii, which can grow to the size of >50 μm in diameter within infected cells. Notably, immunohistologic analyses in the brains of infected mice visualized the presence of numbers of CD8⁺ immune T cells that had migrated halfway through the cyst wall as well as T cells located fully within the cysts. Perforin was required for their invasion and cyst elimination. Cysts invaded by the T cells displayed morphologic deterioration and destruction. Within these deteriorated cysts, granular structures intensely positive for granzyme B were detected in association with T. gondii bradyzoites. Furthermore, the bradyzoites within the destroyed cysts were located within accumulated ionized calcium binding adaptor molecule 1 (Iba1)-positive microglia and Ly6C⁺ macrophages, suggesting that these phagocytes had phagocytosed those organisms for their eradication. The present study uncovered a previously unappreciated capability of CD8⁺ cytotoxic T cells to penetrate into a large target, T. gondii cysts, for their elimination. This invasive capability of CD8⁺ cytotoxic T cells in collaboration with phagocytes appears to be a powerful effector mechanism that functions against not only T. gondii cysts but also other large targets, including solid cancers.

Toxoplasma gondii: CD8 T Cells Cry for CD4 Help

Toxoplasma gondii, an apicomplexan parasite, is a pathogenic protozoan that can infect the central nervous system. In pregnant women, infection can result in congenital problems of the fetus, while in immunocompromised individual it can lead to severe neurological consequences. Although CD8 T cells play an important effector role in controlling the chronic infection, their maintenance is dependent on the critical help provided by CD4 T cells. In a recent study, we demonstrated that reactivation of the infection in chronically infected host is a consequence of CD8 T dysfunction caused by CD4 T cell exhaustion. Furthermore, treatment of chronically infected host with antigen-specific non-exhausted CD4 T cells can restore CD8 T cell functionality and prevent reactivation of the latent infection. The exhaustion status of CD4 T cells is mediated by the increased expression of the transcription factor BLIMP-1, and deletion of this molecule led to the restoration of CD4 T cell function, reversal of CD8 exhaustion and prevention of reactivation of the latent infection. In a recent study from our laboratory, we also observed an increased expression of miR146a levels by CD4 T cells from the chronically infected animals. Recent reports have demonstrated that microRNAs (especially miR146a) has a strong impact on the immune system of T. gondii infected host. Whether these molecules have any role in the BLIMP-1 up-regulation and dysfunctionality of these cells needs to be investigated.

Presence of Toxoplasma gondii infection in brain as a potential cause of risky behavior: a report of 102 autopsy cases

Toxoplasmosis was linked to impairment in brain function, encompassing a wide range of behavioral and neuropsychiatric changes. Currently, the precise localization of Toxoplasma gondii in the human brain is limited and the parasite DNA was not found in population-based screening of autopsy cases. The aim of proposed study was to identify the presence of parasite DNA within the brain and its association with risky behavior and alcohol consumption in postmortem examination. Preliminarily, 102 cases with certain circumstances of death at time of forensic autopsy was included. Due to high risk of bias, the females were excluded from the analysis and final study group consists 97 cases divided into three groups: risky behavior, inconclusively risky behavior, and control group. The obtained tissue samples for Nested PCR covered four regions of the brain: symmetric left/right and anterior/posterior horns of lateral ventricles comprising lining ependyma and hippocampus. The second type of material comprised blood evaluated for antibodies prevalence using ELISA and alcohol concentration using HS-GC-FID. Analysis demonstrated 16.5% prevalence concerning the parasite DNA presence in examined brain tissue samples without specific distribution and association with age at death or days after death until an autopsy was performed. Results have shown correlation between occurrence of risky behavior leading to death and higher proportions of positive parasite DNA presence within the brain. Correlation was not observed between parasite DNA presence and excessive alcohol consumption. Conducted screening demonstrated correlation between parasite DNA presence in the brain with risky behavior and provided new information on possible effects of latent toxoplasmosis.

Toxoplasma gondii: isolation, biological and molecular characterisation of samples from free-range Gallus gallus domesticus from countryside Southeast Brazil

Toxoplasma gondii presents a high prevalence worldwide, infecting several animals. Felines are considered the definitive hosts and among the intermediate hosts we highlight mammals and birds. The man can become infected by ingesting tissue cysts present in birds and mammals. Biological and molecular aspects of T. gondii allows a better understanding of the epidemiology of toxoplasmosis. This work is a serologic screening of 58 chickens grown (Gallus gallus domesticus) for human consumption in Espírito Santo State, by means of indirect haemagglutination assay (IHA). Thirteen chickens tested positive for anti-T. gondii antibodies. The heart and brain of five positive chickens were harvested, treated with pepsin and inoculated separately, in two Swiss mice, intraperitoneally. Tachyzoites were observed in the peritoneum of all the animals, between seven and 10 days after the inoculum. Ten isolates were obtained and biologically characterised in BALB/c mice inoculated with 101 to 104 tachyzoites. All isolates were classified as virulent or intermediately virulent. Isolates were genotyped by means of polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis, revealing three different genotypes. None of the isolates exhibited the clonal type I, II or III genotype. No genotypic differences were observed between the isolates from the brain or heart from the same bird.