Human dendritic cells discriminate between viable and killed Toxoplasma gondii tachyzoites: dendritic cell activation after infection with viable parasites results in CD28 and CD40 ligand signaling that controls IL-12-dependent and -independent T cell production of IFN-gamma

Expression of cytokines and co-stimulatory molecules in the Toxoplasma gondii-infected dendritic cells of C57BL/6 and BALB/c mice

Toxoplasma gondii is an intracellular protozoan parasite which can infect most warm-blooded animals and humans. Among the different mouse models, C57BL/6 mice are more susceptible to T. gondii infection compared to BALB/c mice, and this increased susceptibility has been attributed to various factors, including T-cell responses. Dendritic cells (DCs) are the most prominent type of antigen-presenting cells and regulate the host immune response, including the response of T-cells. However, differences in the DC responses of these mouse strains to T. gondii infection have yet to be characterized. In this study, we cultured bone marrow-derived DCs (BMDCs) from BALB/c and C57BL/6 mice. These cells were infected with T. gondii. The activation of the BMDCs was assessed based on the expression of cell surface markers and cytokines. In the BMDCs of both mouse strains, we detected significant increases in the expression of cell surface T-cell co-stimulatory molecules (major histocompatibility complex (MHC) II, CD40, CD80, and CD86) and cytokines (tumor necrosis factor (TNF)-α, interferon (IFN)-γ, interleukin (IL)-12p40, IL-1β, and IL-10) from 3 h post-T. gondii infection. The expression of MHC II, CD40, CD80, CD86, IFN-γ, IL-12p40, and IL-1β was significantly higher in the T. gondii-infected BMDCs obtained from the C57BL/6 mice than in those from the BALB/c mice. These findings indicate that differences in the activation status of the BMDCs in the BALB/c and C57BL/6 mice may account for their differential susceptibility to T. gondii.

Toxoplasmosis vaccines: what we have and where to go?

Despite recent major advances in developing effective vaccines against toxoplasmosis, finding new protective vaccination strategies remains a challenging and elusive goal as it is critical to prevent the disease. Over the past few years, various experimental approaches have shown that developing an effective vaccine against T. gondii is achievable. However, more remains unknown due to its complicated life cycle, difficulties in clinical translation, and lack of a standardized platform. This minireview summarizes the recent advances in the development of T. gondii vaccines and the main obstacles to developing a safe, effective and durable T. gondii vaccine. The successes and failures in developing and testing vaccine candidates for the T. gondii vaccine are also discussed, which may facilitate the future development of T. gondii vaccines.

Key Limitations and New Insights Into the Toxoplasma gondii Parasite Stage Switching for Future Vaccine Development in Human, Livestock, and Cats

Toxoplasmosis is a parasitic disease affecting human, livestock and cat. Prophylactic strategies would be ideal to prevent infection. In a One Health vaccination approach, the objectives would be the prevention of congenital disease in both women and livestock, prevention/reduction of T. gondii tissue cysts in food-producing animals; and oocyst shedding in cats. Over the last few years, an explosion of strategies for vaccine development, especially due to the development of genetic-engineering technologies has emerged. The field of vaccinology has been exploring safer vaccines by the generation of recombinant immunogenic proteins, naked DNA vaccines, and viral/bacterial recombinants vectors. These strategies based on single- or few antigens, are less efficacious than recombinant live-attenuated, mostly tachyzoite T. gondii vaccine candidates. Reflections on the development of an anti-Toxoplasma vaccine must focus not only on the appropriate route of administration, capable of inducing efficient immune response, but also on the choice of the antigen (s) of interest and the associated delivery systems. To answer these questions, the choice of the animal model is essential. If mice helped in understanding the protection mechanisms, the data obtained cannot be directly transposed to humans, livestock and cats. Moreover, effectiveness vaccines should elicit strong and protective humoral and cellular immune responses at both local and systemic levels against the different stages of the parasite. Finally, challenge protocols should use the oral route, major natural route of infection, either by feeding tissue cysts or oocysts from different T. gondii strains. Effective Toxoplasma vaccines depend on our understanding of the (1) protective host immune response during T. gondii invasion and infection in the different hosts, (2) manipulation and modulation of host immune response to ensure survival of the parasites able to evade and subvert host immunity, (3) molecular mechanisms that define specific stage development. This review presents an overview of the key limitations for the development of an effective vaccine and highlights the contributions made by recent studies on the mechanisms behind stage switching to offer interesting perspectives for vaccine development.

Sustained Egr-1 Response via p38 MAP Kinase Signaling Modulates Early Immune Responses of Dendritic Cells Parasitized by Toxoplasma gondii

As a response to a diverse array of external stimuli, early growth response protein 1 (Egr-1) plays important roles in the transcriptional regulation of inflammation and the cellular immune response. However, a number of intracellular pathogens colonize immune cells and the implication of Egr-1 in the host-pathogen interplay has remained elusive. Here, we have characterized the Egr-1 responses of primary murine and human dendritic cells (DCs) upon challenge with the obligate intracellular parasite Toxoplasma gondii. We report that live intracellular parasites induce a sustained high expression of Egr-1 in DCs, different from the immediate-early Egr-1 response to parasite lysates, inactivated parasites or LPS. Moreover, a distinct nuclear localization of elevated amounts of Egr-1 protein was detected in infected DCs, but not in by-stander DCs. The ERK1/2 MAPK signaling pathway mediated the canonical immediate-early Egr-1 response to soluble antigens in a MyD88/TLR-dependent fashion. In contrast, a non-canonical extended Egr-1 response that relied primarily on p38 MAPK signaling was induced by intracellular parasites and was exhibited similarly by MyD88-deficient and wildtype DCs. The extended phase Egr-1 response was dramatically reduced upon challenge of DCs with T. gondii parasites deficient in GRA24, a secreted p38-interacting protein. Further, Egr-1-silenced primary DCs maintained their migratory responses upon T. gondii challenge. Importantly, Egr-1 silencing led to elevated expression of co-stimulatory molecules (CD40, CD80) in Toxoplasma-infected DCs and in LPS-challenged immature DCs, indicating that Egr-1 responses suppressed maturation of DCs. Moreover, the IL-12 and IL-2 responses of Toxoplasma-challenged DCs were modulated in a GRA24-dependent fashion. Jointly, the data show that the Egr-1 responses of DCs to microbial external stimuli and intracellular stimuli can be selectively mediated by ERK1/2 or p38 MAPK signaling, and that Egr-1 can act as an intrinsic negative modulator of maturation in primary DCs.

Toxoplasma and Dendritic Cells: An Intimate Relationship That Deserves Further Scrutiny

Toxoplasma gondii (Tg), an obligate intracellular parasite of the phylum Apicomplexa, infects a wide range of animals, including humans. A hallmark of Tg infection is the subversion of host responses, which is thought to favor parasite persistence and propagation to new hosts. Recently, a variety of parasite-secreted modulatory effectors have been uncovered in fibroblasts and macrophages, but the specific interplay between Tg and dendritic cells (DCs) is just beginning to emerge. In this review, we summarize the current knowledge on Tg-DC interactions, including innate recognition, cytokine production, and antigen presentation, and discuss open questions regarding how Tg-secreted effectors may shape DC functions to perturb innate and adaptive immunity.

Vaccination challenges and strategies against long-lived Toxoplasma gondii

Since the discovery of Toxoplasma gondii in 1908, it is estimated that one-third of the global population has been exposed to this ubiquitous intracellular protozoan. The complex life cycle of T. gondii has enabled itself to overcome stress and transmit easily within a broad host range thus achieving a high seroprevalence worldwide. To date, toxoplasmosis remains one of the most prevalent HIV-associated opportunistic central nervous system infections. This review presents a comprehensive overview of different vaccination approaches ranging from traditional inactivated whole-T. gondii vaccines to the popular DNA vaccines. Extensive discussions are made to highlight the challenges in constructing these vaccines, selecting adjuvants as well as delivery methods, immunisation approaches and developing study models. Herein we also deliberate over the latest and promising enhancement strategies that can address the limitations in developing an effective T. gondii prophylactic vaccine.

A one health approach to vaccines against Toxoplasma gondii

Toxoplasmosis is a serious disease with global impact, now recognised as one of the most important food borne diseases worldwide and a major cause of production loss in livestock. A one health approach to develop a vaccination programme to tackle toxoplasmosis is an attractive and realistic prospect. Knowledge of disease epidemiology, parasite transmission routes and main risk groups has helped to target key host species and outcomes for a vaccine programme and these would be to prevent/reduce congenital disease in women and sheep; prevent/reduce T. gondii tissue cysts in food animal species and to prevent/reduce T. gondii oocyst shedding in cats. Most animals, including humans, develop good protective immunity following infection, involving cell mediated immune responses, which may explain why live vaccines are generally more effective to protect against T. gondii. Recent advances in our knowledge of parasite genetics and gene manipulation, strain variation, key antigenic epitopes, delivery systems and induction of immune responses are all contributing to the prospects of developing new vaccines which may be more widely applicable. A key area in progressing vaccine development is to devise standard vaccine efficacy models in relevant animal hosts and this is where a one health approach bringing together researchers across different disciplines can be of major benefit. The tools and technologies are in place to make a real impact in tackling toxoplasmosis using vaccination and it just requires a collective will to make it happen.

The potential use of melatonin to treat protozoan parasitic infections: A review

Melatonin (N-acetyl-5-methoxytryptamine) is a circadian hormone produced in vertebrates by the pineal gland and other organs. Melatonin is believed to influence immune cells leading to modulation of the proliferative response of stimulated lymphocytes as well as cytokine production. Due to the antioxidant and immunomodulatory effects of melatonin, it is suggested that this molecule could be a therapeutic alternative agent to fight bacterial, viral, and parasitic infections by a variety of mechanisms. Herein, we review the effects of melatonin on the cell biology of protozoan parasites and host's immune response. In toxoplasmosis, African trypanosomiasis and Chagas' disease, melatonin enhances host's immune response against the parasite via regulating the secretion of inflammatory mediators. In amoebiasis, melatonin reduces the amoebic lesions as well as increasing the leukophagocytosis and the number of dead amoebae. In giardiasis, serum melatonin levels are elevated in these patients; this suggests a positive correlation between the level of melatonin and phagocytic activity in the G. duodenalis infected patients, possibly related to melatonin's immunomodulatory effect. In leishmaniasis, melatonin arrests parasite replication accompanied by releasing mitochondrial Ca²⁺ into the cytosol, increasing the level of mitochondrial nitrites as well as reducing superoxide dismutase (SOD) activity. In malaria, melatonin synchronizes the Plasmodium cell cycle via modulating cAMP-PKA and IP3-Ca²⁺ pathways. Thus, simultaneous administration of melatonin agonists or giving pharmacological doses of melatonin may be considered a novel approach for treatment of malarial infection.

Innate recognition of Toxoplasma gondii in humans involves a mechanism distinct from that utilized by rodents

Toxoplasma gondii is an intracellular protozoan parasite that infects rodents as part of its natural transmission cycle and induces disease in humans, an end-stage host. As one of the natural hosts of T. gondii, the mouse has been used extensively for elucidating the cellular and molecular basis of immunity to this pathogen while relatively few studies have focused on the response of humans. In our recent work, we identified CD16⁺ monocytes and DC1 dendritic cells as the major myeloid cell populations that respond to T. gondii in human peripheral blood. Interestingly, these myeloid subsets represent the opposite counterparts from those triggered by the parasite in mice. Moreover, whereas the innate cytokine response to T. gondii in the mouse involves stimulation of Toll-like receptors by a soluble parasite ligand, the response of human cells instead requires phagocytosis of the live pathogen. We speculate that these marked distinctions in the pathways utilized for innate recognition of toxoplasma in mouse and man reflect the differing roles of the two hosts in the biology of this parasite.

The IL-12 Response of Primary Human Dendritic Cells and Monocytes to Toxoplasma gondii Is Stimulated by Phagocytosis of Live Parasites Rather Than Host Cell Invasion

As a major natural host for Toxoplasma gondii, the mouse is widely used for the study of the immune response to this medically important protozoan parasite. However, murine innate recognition of toxoplasma depends on the interaction of parasite profilin with TLR11 and TLR12, two receptors that are functionally absent in humans. This raises the question of how human cells detect and respond to T. gondii. In this study, we show that primary monocytes and dendritic cells from peripheral blood of healthy donors produce IL-12 and other proinflammatory cytokines when exposed to toxoplasma tachyzoites. Cell fractionation studies determined that IL-12 and TNF-α secretion is limited to CD16(+) monocytes and the CD1c(+) subset of dendritic cells. In direct contrast to their murine counterparts, human myeloid cells fail to respond to soluble tachyzoite extracts and instead require contact with live parasites. Importantly, we found that tachyzoite phagocytosis, but not host cell invasion, is required for cytokine induction. Together these findings identify CD16(+) monocytes and CD1c(+) dendritic cells as the major myeloid subsets in human blood-producing innate cytokines in response to T. gondii and demonstrate an unappreciated requirement for phagocytosis of live parasites in that process. This form of pathogen sensing is distinct from that used by mice, possibly reflecting a direct involvement of rodents and not humans in the parasite life cycle.

Parasite Manipulation of the Invariant Chain and the Peptide Editor H2-DM Affects Major Histocompatibility Complex Class II Antigen Presentation during Toxoplasma gondii Infection

Toxoplasma gondii is an obligate intracellular protozoan parasite. This apicomplexan is the causative agent of toxoplasmosis, a leading cause of central nervous system disease in AIDS. It has long been known that T. gondii interferes with major histocompatibility complex class II (MHC-II) antigen presentation to attenuate CD4(+) T cell responses and establish persisting infections. Transcriptional downregulation of MHC-II genes by T. gondii was previously established, but the precise mechanisms inhibiting MHC-II function are currently unknown. Here, we show that, in addition to transcriptional regulation of MHC-II, the parasite modulates the expression of key components of the MHC-II antigen presentation pathway, namely, the MHC-II-associated invariant chain (Ii or CD74) and the peptide editor H2-DM, in professional antigen-presenting cells (pAPCs). Genetic deletion of CD74 restored the ability of infected dendritic cells to present a parasite antigen in the context of MHC-II in vitro. CD74 mRNA and protein levels were, surprisingly, elevated in infected cells, whereas MHC-II and H2-DM expression was inhibited. CD74 accumulated mainly in the endoplasmic reticulum (ER), and this phenotype required live parasites, but not active replication. Finally, we compared the impacts of genetic deletion of CD74 and H2-DM genes on parasite dissemination toward lymphoid organs in mice, as well as activation of CD4(+) T cells and interferon gamma (IFN-γ) levels during acute infection. Cyst burdens and survival during the chronic phase of infection were also evaluated in wild-type and knockout mice. These results highlight the fact that the infection is influenced by multiple levels of parasite manipulation of the MHC-II antigen presentation pathway.

Nonreplicating, cyst-defective type II Toxoplasma gondii vaccine strains stimulate protective immunity against acute and chronic infection

Live attenuated vaccine strains, such as type I nonreplicating uracil auxotroph mutants, are highly effective in eliciting lifelong immunity to virulent acute infection by Toxoplasma gondii. However, it is currently unknown whether vaccine-elicited immunity can provide protection against acute infection and also prevent chronic infection. To address this problem, we developed nonreverting, nonreplicating, live attenuated uracil auxotroph vaccine strains in the type II Δku80 genetic background by targeting the deletion of the orotidine 5'-monophosphate decarboxylase (OMPDC) and uridine phosphorylase (UP) genes. Deletion of OMPDC induced a severe uracil auxotrophy with loss of replication, loss of virulence in mice, and loss of the ability to develop cysts and chronic infection. Vaccination of mice using type II Δku80 Δompdc mutants stimulated a fully protective CD8(+) T cell-dependent immunity that prevented acute infection by type I and type II strains of T. gondii, and this vaccination also severely reduced or prevented cyst formation after type II challenge infection. Nonreverting, nonreplicating, and non-cyst-forming Δompdc mutants provide new tools to examine protective immune responses elicited by vaccination with a live attenuated type II vaccine.

Blockade of CD40-TRAF2,3 or CD40-TRAF6 is sufficient to inhibit pro-inflammatory responses in non-haematopoietic cells

Inhibition of the CD40-CD154 pathway controls inflammatory disorders. Unfortunately, administration of anti-CD154 monoclonal antibodies causes thromboembolism. Blockade of signalling downstream of CD40 may represent an approach to treat CD40-driven inflammatory disorders. Blocking tumour necrosis factor receptor-associated factor 6 (TRAF6) signalling downstream of CD40 in MHC II(+) cells diminishes inflammation. However, CD40-TRAF6 blockade may cause immunosuppression. We examined the role of CD40-TRAF2,3 and CD40-TRAF6 signalling in the development of pro-inflammatory responses in human non-haematopoietic and monocytic cells. Human aortic endothelial cells, aortic smooth muscle cells, renal proximal tubule epithelial cells, renal mesangial cells and monocytic cells were transduced with retroviral vectors that encode wild-type CD40, CD40 with a mutation that prevents TRAF2,3 recruitment (ΔT2,3), TRAF6 recruitment (ΔT6) or both TRAF2,3 plus TRAF6 recruitment (ΔT2,3,6). Non-haematopoietic cells that expressed CD40 ΔT2,3 exhibited marked inhibition in CD154-induced up-regulation of vascular cell adhesion molecule 1, intercellular adhesion molecule 1 (ICAM-1), monocyte chemotactic protein 1 (MCP-1), tissue factor and matrix metalloproteinase 9. Similar results were obtained with cells that expressed CD40 ΔT6. Although both mutations impaired ICAM-1 up-regulation in monocytic cells, only expression of CD40 ΔT6 reduced MCP-1 and tissue factor up-regulation in these cells. Treatment of endothelial and smooth muscle cells with cell-permeable peptides that block CD40-TRAF2,3 or CD40-TRAF6 signalling impaired pro-inflammatory responses. In contrast, while the CD40-TRAF2,3 blocking peptide did not reduce CD154-induced dendritic cell maturation, the CD40-TRAF6 blocking peptide impaired this response. Hence, preventing CD40-TRAF2,3 or CD40-TRAF6 interaction inhibits pro-inflammatory responses in human non-haematopoietic cells. In contrast to inhibition of CD40-TRAF6 signalling, inhibition of CD40-TRAF2,3 signalling did not impair dendritic cell maturation. Blocking CD40-TRAF2,3 signalling may control CD40-CD154-dependent inflammatory disorders.

Atg5 but not Atg7 in dendritic cells enhances IL-2 and IFN-γ production by Toxoplasma gondii-reactive CD4+ T cells

The autophagy proteins (Atg) modulate not only innate but also adaptive immunity against pathogens. We examined the role of dendritic cell Atg5 and Atg7 in the production of IL-2 and IFN-γ by Toxoplasma gondii-reactive CD4(+) T cells. T. gondii-reactive mouse CD4(+) T cells exhibited unimpaired production of IL-2 and IFN-γ when stimulated with Atg7-deficient mouse dendritic cells that were infected with T. gondii or pulsed with T. gondii lysate antigens. In marked contrast, dendritic cells deficient in Atg5 induced diminished CD4(+) T cell production of IL-2 and IFN-γ. This defect was not accompanied by changes in costimulatory ligand expression on dendritic cells or impaired production of IL-12 p70, IL-1β or TNF-α. Knockdown of Irg6a in dendritic cells did not affect CD4(+) T cell cytokine production. These results indicate that Atg5 and Atg7 in dendritic cells play differential roles in the modulation of IL-2 and IFN-γ production by T. gondii-reactive CD4(+) T cells.

CD40 promotes the development of early diabetic retinopathy in mice

AIMS/HYPOTHESIS

Microangiopathy is a leading complication of diabetes that commonly affects the retina. Degenerate capillaries are a central feature of diabetic retinopathy. An inflammatory process has been linked to the development of diabetic retinopathy but its regulation is incompletely understood. Cluster of differentiation (CD) 40 is a member of the TNF receptor superfamily that promotes the development of certain inflammatory disorders. The role of CD40 in diabetic microangiopathy is unknown.

METHODS

B6 and Cd40−/− mice were administered streptozotocin to induce diabetes. Leucostasis was assessed using fluorescein isothiocyanate-conjugated concanavalin A. Retinal Icam1 and Cd40 mRNA levels were examined using real-time PCR. Protein nitration was assessed by immunohistochemistry. Histopathology was examined in the retinal vasculature. CD40 expression was assessed by flow cytometry and immunohistochemistry. Intercellular adhesion molecule 1 (ICAM-1) and nitric oxide synthase 2 (NOS2) were examined by immunoblot and/or flow cytometry. Nitric oxide production was examined by immunoblot and Griess reaction.

RESULTS

In mouse models of diabetes, Cd40−/− mice exhibited reduced retinal leucostasis and did not develop capillary degeneration in comparison with B6 mice. Diabetic Cd40−/− mice had diminished ICAM-1 upregulation and decreased protein nitration. Cd40 mRNA levels were increased in the retinas of diabetic B6 mice compared with non-diabetic controls. CD40 expression increased in retinal Müller cells, endothelial cells and microglia of diabetic animals. CD40 stimulation upregulated ICAM-1 in retinal endothelial cells and Müller cells. CD40 ligation upregulated NOS2 and nitric oxide production by Müller cells.

CONCLUSIONS/INTERPRETATION

CD40-deficient mice were protected fromthe development of diabetic retinopathy. These mice exhibited diminished inflammatory responses linked to diabetic retinopathy. CD40 stimulation of retinal cells triggered these pro-inflammatory responses.

Toxoplasma gondii- derived profilin triggers human toll-like receptor 5-dependent cytokine production

Up to a third of the world's population is infected with Toxoplasma gondii. Natural infection in humans can be life threatening during pregnancy and in immunocompromised individuals. Toll-like receptor (TLR) 11 is the mouse innate sensor that recognizes T. gondii profilin; however, in humans the TLR11 gene leads to transcription of no functional protein. Herein, by using a multiple sequence alignment phylogenetic analysis program between human and mouse species, we found that human TLR5 seems to be the evolutionarily closest member of the TLR gene family to mouse tlr11. We therefore asked whether human TLR5 could mediate IL-6, IL-8 and IL-12p70 production in response to the T. gondii profilin. We found that this was the case both in human cell lines as well as peripheral blood monocytes. Moreover, TLR5 neutralization and gene silencing mediated specific ablation of cytokine production after profilin exposure. Finally, peripheral blood monocytes carrying the TLR5 R392X mutation failed to produce cytokines in response to stimulation with profilin. Taken together, the results presented herein reveal a previously unappreciated cross-recognition of a relevant human pathogen-derived pathogen-associated molecular pattern.

Parasite fate and involvement of infected cells in the induction of CD4+ and CD8+ T cell responses to Toxoplasma gondii

During infection with the intracellular parasite Toxoplasma gondii, the presentation of parasite-derived antigens to CD4⁺ and CD8⁺ T cells is essential for long-term resistance to this pathogen. Fundamental questions remain regarding the roles of phagocytosis and active invasion in the events that lead to the processing and presentation of parasite antigens. To understand the most proximal events in this process, an attenuated non-replicating strain of T. gondii (the cpsII strain) was combined with a cytometry-based approach to distinguish active invasion from phagocytic uptake. In vivo studies revealed that T. gondii disproportionately infected dendritic cells and macrophages, and that infected dendritic cells and macrophages displayed an activated phenotype characterized by enhanced levels of CD86 compared to cells that had phagocytosed the parasite, thus suggesting a role for these cells in priming naïve T cells. Indeed, dendritic cells were required for optimal CD4⁺ and CD8⁺ T cell responses, and the phagocytosis of heat-killed or invasion-blocked parasites was not sufficient to induce T cell responses. Rather, the selective transfer of cpsII-infected dendritic cells or macrophages (but not those that had phagocytosed the parasite) to naïve mice potently induced CD4⁺ and CD8⁺ T cell responses, and conferred protection against challenge with virulent T. gondii. Collectively, these results point toward a critical role for actively infected host cells in initiating T. gondii-specific CD4⁺ and CD8⁺ T cell responses.

CD4⁺ and CD8⁺ T cells are critical for controlling many infections. To generate a T cell response during infection, T cells must encounter the microbial peptides that they recognize bound to MHC molecules on the surfaces of other cells, such as dendritic cells. It is currently unclear how dendritic cells acquire the antigens they present to T cells during infection with many intracellular pathogens. It is possible that these antigens are phagocytosed and processed by dendritic cells, or antigens may be presented by cells that are infected by pathogens such as Toxoplasma gondii, which invades host cells independently of phagocytosis. To differentiate these pathways, we developed a novel technique to track the fate of T. gondii in vivo that distinguishes actively infected cells from those that phagocytosed parasites. This technique was used to examine each of these cell populations. We also used pharmacological inhibitors of parasite invasion, and the transfer of sort-purified infected or uninfected dendritic cells and macrophages to determine what roles phagocytosis and active invasion have in the initiation of T cell responses. Our results demonstrate that phagocytosis of parasites is not sufficient to induce CD4⁺ or CD8⁺ T cell responses, whereas infected cells are critical for this process.

Kinetics of IL-23 and IL-12 secretion in response to Toxoplasma gondii antigens from THP-1 monocytic cells

IL-23 and IL-12 are structurally similar and critical for the generation of efficient cellular immune responses. Toxoplasma gondii induces a strong cell-mediated immune response. However, little is known about IL-23 secretion profiles in T. gondii-infected immune cells in connection with IL-12. We compared the patterns of IL-23 and IL-12 production by THP-1 human monocytic cells in response to stimulation with live or heat-killed T. gondii tachyzoites, or with equivalent quantities of either T. gondii excretory/secretory proteins (ESP) or soluble tachyzoite antigen (STAg). IL-23 and IL-12 were significantly increased from 6 hr after stimulation with T. gondii antigens, and their secretions were increased with parasite dose-dependent manner. IL-23 concentrations were significantly higher than those of IL-12 at the same multiplicity of infection. IL-23 secretion induced by live parasites was significantly higher than that by heat-killed parasites, ESP, or STAg, whereas IL-12 secretion by live parasite was similar to those of ESP or STAg. However, the lowest levels of both cytokines were at stimulation with heat-killed parasites. These data indicate that IL-23 secretion patterns by stimulation with various kinds of T. gondii antigens at THP-1 monocytic cells are similar to those of IL-12, even though the levels of IL-23 induction were significantly higher than those of IL-12. The detailed kinetics induced by each T. gondii antigen were different from each other.

Cryptosporidium parvum antigens induce mouse and human dendritic cells to generate Th1-enhancing cytokines

Cryptosporidium parvum is an opportunistic intracellular parasite that causes mild to severe diarrhoea, which can be life-threatening in an immunocompromised host. To increase our understanding of the mechanisms that play a role in host immune responses, we investigated the effects of C. parvum antigens on the phenotype of mouse and human dendritic cells (DCs). Cryptosporidium parvum antigens induced DC activation as indicated by upregulation of the maturation marker CD209, as well as by the production of the cytokines interleukin-12 p70, IL-2, IL-1beta, IL-6. In particular, significant increases in the expression of IL-12 p70 were observed from mouse DCs derived from bone marrow in response to solubilized sporozoite antigen and the recombinant cryptosporidial antigens, Cp40 and Cp23. We observed a small but significant increase in IL-18 expression following the exposure to Cp40. We found that the induction of Th1 cytokines was MyD88 dependent (MyD88 knockout mouse DCs were unresponsive). Additionally, both sporozoite preparations (solubilized and live) significantly induced IL-12 production by human monocytic dendritic cells (MoDCs). This finding indicates that solubilized as well as recombinant antigens can induce the maturation of DCs and subsequently initiate an innate immune response.

Use and abuse of dendritic cells by Toxoplasma gondii

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

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.

Toxoplasma gondii induces B7-2 expression through activation of JNK signal transduction

Toxoplasma gondii is a globally distributed parasite pathogen that infects virtually all warm-blooded animals. A hallmark of immunity to acute infection is the production of gamma interferon (IFN-γ) and interleukin-12 (IL-12), followed by a protective T cell response that is critical for parasite control. Naïve T cell activation requires both T-cell receptor (TCR) stimulation and the engagement of costimulatory receptors. Because of their important function in activating T cells, the expression of costimulatory ligands is believed to be under tight control. The molecular mechanisms governing their induction during microbial stimulation, however, are not well understood. We found that all three strains of T. gondii (types I, II, and III) upregulated the expression of B7-2, but not B7-1, on the surface of mouse bone marrow-derived macrophages. Additionally, intraperitoneal infection of mice with green fluorescent protein (GFP)-expressing parasites resulted in enhanced B7-2 levels specifically on infected, GFP(+) CD11b(+) cells. B7-2 induction occurred at the transcript level, required active parasite invasion, and was not dependent on MyD88 or TRIF. Functional assays demonstrated that T. gondii-infected macrophages stimulated naïve T cell proliferation in a B7-2-dependent manner. Genome-wide transcriptional analysis comparing infected and uninfected macrophages revealed the activation of mitogen-activated protein kinase (MAPK) signaling in infected cells. Using specific inhibitors against MAPKs, we determined that parasite-induced B7-2 is dependent on Jun N-terminal protein kinase (JNK) but not extracellular signal-regulated kinase (ERK) or p38 signaling. We also observed that T. gondii-induced B7-2 expression on human peripheral blood monocytes is dependent on JNK signaling, indicating that a common mechanism of B7-2 regulation by T. gondii may exist in both humans and mice.

Dendritic cell activation prevents MHC class II ubiquitination and promotes MHC class II survival regardless of the activation stimulus

The expression of MHC class II (MHC-II) on the surface of antigen-presenting cells, such as dendritic cells (DCs), is tightly regulated during cellular activation. Many cells, including DCs, are activated following stimulation of innate Toll-like receptors (TLRs) by products of microorganisms. In the resting (immature) state, MHC-II is ubiquitinated in immature DCs and is rapidly degraded; however, after activation of these cells with MyD88-dependent TLR ligands, MHC-II ubiquitination is blocked, and MHC-II survival is prolonged. We now show that DC activation using MyD88-dependent TLR ligands, MyD88-independent TLR ligands, and even infection with the intracellular parasite Toxoplasma gondii leads to identical changes in MHC-II expression, ubiquitination, and surface stability, revealing a conserved role for enhanced MHC-II stability after DC activation by different stimuli.

Avirulent uracil auxotrophs based on disruption of orotidine-5'-monophosphate decarboxylase elicit protective immunity to Toxoplasma gondii

The orotidine-5'-monophosphate decarboxylase (OMPDC) gene, encoding the final enzyme of the de novo pyrimidine biosynthesis pathway, was deleted using Toxoplasma gondii KU80 knockouts to develop an avirulent nonreverting pyrimidine auxotroph strain. Additionally, to functionally address the role of the pyrimidine salvage pathway, the uridine phosphorylase (UP) salvage activity was knocked out and a double knockout of UP and OMPDC was also constructed. The nonreverting DeltaOMPDC, DeltaUP, and DeltaOMPDC DeltaUP knockout strains were evaluated for pyrimidine auxotrophy, for attenuation of virulence, and for their ability to elicit potent immunity to reinfection. The DeltaUP knockout strain was replication competent and virulent. In contrast, the DeltaOMPDC and DeltaOMPDC DeltaUP strains were uracil auxotrophs that rapidly lost their viability during pyrimidine starvation. Replication of the DeltaOMPDC strain but not the DeltaOMPDC DeltaUP strain was also partially rescued in vitro with uridine or cytidine supplementation. Compared to their hypervirulent parental type I strain, the DeltaOMPDC and DeltaOMPDC DeltaUP knockout strains exhibited extreme attenuation in murine virulence (approximately 8 logs). Genetic complementation of the DeltaOMPDC strain using a functional OMPDC allele restored normal replication and type I parental strain virulence phenotypes. A single immunization of mice with either the live critically attenuated DeltaOMPDC strain or the DeltaOMPDC DeltaUP knockout strain effectively induced potent protective immunity to lethal challenge infection. The avirulent nonreverting DeltaOMPDC and DeltaOMPDC DeltaUP strains provide new tools for the dissection of the host response to infection and are promising candidates for safe and effective Th1 vaccine platforms that can be easily genetically engineered.

Disparity in circulating peripheral blood dendritic cell subsets and cytokine profile of pulmonary tuberculosis patients compared with healthy family contacts

Dendritic cell (DC) subsets, myeloid DCs (mDCs), and plasmacytoid DCs (pDCs) play a fundamental role in immune response to Mycobacterium tuberculosis (M. tuberculosis). Flow-cytometric estimation of DC subsets showed differences in the ratio of these subsets in untreated, smear-positive pulmonary tuberculosis patients compared with healthy family contacts (HFC, p < 0.05). The percentage of pDCs (0.14 +/- 0.01) was higher than mDCs (0.12 +/- 0.01) in patients, whereas in HFC, mDCs (0.15 +/- 0.01) was higher than pDCs (0.1 +/- 0.01). The percentage of mDCs (0.15 +/- 0.01) and pDCs (0.11 +/- 0.01) was restored in treated patients. Alteration in the DC subsets before and after chemotherapy was confirmed in the follow-up of acid-fast bacilli (AFB)-positive patients. This reversal in the percentage of mDC vs pDCs implicates the influence of active disease on circulating DC subsets. The cytokine bead array revealed an inverse relationship in the circulating levels of IL-12 and IFN-gamma. High IL-12 (37.9 +/- 15.2) and low IFN-gamma (11.09 +/- 3.6) was seen in HFCs derived serum samples compared with that of patients (p < 0.05). The higher percentage of mDCs and elevated IL-12 levels was found to be associated with high risk HFCs investigated. Furthermore CpG/LPS-stimulated whole-blood culture of untreated patients expressed high IFN-alpha in pDCs and less IL-12 in mDCs compared with those of treated patients.

CD40, autophagy and Toxoplasma gondii

Toxoplasma gondii represents a pathogen that survives within host cells by preventing the endosomal-lysosomal compartments from fusing with the parasitophorous vacuoles. The dogma had been that the non-fusogenic nature of these vacuoles is irreversible. Recent studies revealed that this dogma is not correct. Cell-mediated immunity through CD40 re-routes the parasitophorous vacuoles to the lysosomal compartment by a process called autophagy. Autophagosome formation around the parasitophorous vacuole results in killing of the T. gondii. CD40-induced autophagy likely contributes to resistance against T. gondii particularly in neural tissue.

Autophagy in immunity against Toxoplasma gondii

A decisive outcome during host-pathogen interaction is governed by whether pathogen-containing vacuoles fuse with lysosomes. Fusion with lysosomes typically kills microbes. Toxoplasma gondii represents a classical example of an intracellular pathogen that survives within host cells by preventing the endosomal-lysosomal compartments from fusing with the vacuoles that contain the pathogen. Thus, T. gondii provides an excellent model to determine if the immune system can target a pathogen for lysosomal degradation. CD40, a major regulator of cell-mediated immunity, activates macrophages to kill T. gondii through a process that requires recruitment of autophagosomes around the parasitophorous vacuole, leading to lysosomal degradation of the parasite. These studies demonstrate that cell-mediated immunity can activate autophagy to kill a pathogen. CD40-induced autophagy likely contributes to resistance against T. gondii, particularly in neural tissues, the main sites affected by this pathogen.

CD40 and the immune response to parasitic infections

The interaction between CD40 and CD154 regulates many aspects of cellular and humoral immunity. The CD40-CD154 pathway is important for resistance against a variety of parasites. Studies done with these pathogens have provided important insight into the various mechanisms by which this pathway enhances host protection, mechanisms by which pathogens subvert CD40 signaling, conditions in which the CD40-CD154 pathway promotes disease and on modulation of this pathway for immunotherapy.

Improvement of a dendritic cell-based therapeutic cancer vaccine with components of Toxoplasma gondii

The use of dendritic cells (DCs) as a cellular adjuvant is a promising approach to the immunotherapy of cancer. It has previously been demonstrated that DCs pulsed ex vivo with Toxoplasma gondii antigens trigger a systemic Th1-biased specific immune response and induce protective and specific antitoxoplasma immunity. In the present study, we demonstrate that tumor antigen-pulsed DCs matured in the presence of Toxoplasma gondii components induce a potent antitumor response in a mouse model of fibrosarcoma. Bone-marrow derived DCs (BMDCs) were cultured in the presence of granulocyte-macrophage colony-stimulating factor and interleukin-4. After 5 days, tumor lysates with or without the T. gondii lysate were added to the culture for another 2 days. The cytokine production in the BMDC culture and the coculture supernatants of DCs and splenic cells was evaluated. For immunization, 7 days after tumor challenge, different groups of BALB/c mice received different kinds of DCs subcutaneously around the tumor site. Tumor growth was monitored, and 2 weeks after DC immunotherapy, the cytotoxic activity and the infiltration of CD8(+) T cells were monitored in different groups. According to the findings, immunotherapy with T. gondii-matured DCs led to a significant increase in the activity of cytotoxic T cells and decreased the rate of growth of the tumor in immunized animals. Immature DCs did not cause any change in cytotoxic activity or the tumor growth rate compared to that in the healthy controls. The current study suggests that a specific antitumor immune response can be induced by DCs matured with T. gondii components and provide the basis for the use of T. gondii in DC-targeted clinical therapies.

Cell-mediated immunity to Toxoplasma gondii develops primarily by local Th1 host immune responses in the absence of parasite replication

A single inoculation of mice with the live, attenuated Toxoplasma gondii uracil auxotroph strain cps1-1 induces long-lasting immunity against lethal challenge with hypervirulent strain RH. The mechanism for this robust immunity in the absence of parasite replication has not been addressed. The mechanism of long-lasting immunity, the importance of route of immunization, cellular recruitment to the site of infection, and local and systemic inflammation were evaluated. Our results show that infection with cps1-1 elicits long-lasting CD8+ T cell- mediated immunity. We show that immunization with cps1-1-infected dendritic cells elicits long-lasting immunity. Intraperitoneal infection with cps1-1 induced a rapid influx of GR1+ neutrophils and two stages of GR1+CD68+ inflammatory monocyte infiltration into the site of inoculation. CD19+ B cells and CD3+ T cells steadily increase for 8 days after infection. CD8+ T cells were rapidly recruited to the site of infection and increased faster than CD4+ T cells. Surprisingly, cps1-1 infection induced high systemic levels of bioactive IL-12p70 and a very low level and transient systemic IFN-gamma. Furthermore, we show significant levels of these inflammatory cytokines were locally produced at the site of cps1-1 inoculation. These findings offer new insight into immunological mechanisms and local host responses to a non-replicating type I parasite infection associated with development of long-lasting immunity to Toxoplasma gondii.

The immunobiology of the innate response to Toxoplasma gondii

Toxoplasma gondii is a unique intracellular parasite. It can infect a variety of cells in virtually all warm-blooded animals. It has a worldwide distribution and, overall, around one-third of people are seropositive for the parasite, with essentially the entire human population being at risk of infection. For most people, T. gondii causes asymptomatic infection but the parasite can cause serious disease in the immunocompromised and, if contracted for the first time during pregnancy, can cause spontaneous abortion or congenital defects, which have a substantial emotional, social and economic impact. Toxoplasma gondii provokes one of the most potent innate, pro-inflammatory responses of all infectious disease agents. It is also a supreme manipulator of the immune response so that innate immunity to T. gondii is a delicate balance between the parasite and its host involving a coordinated series of cellular interactions involving enterocytes, neutrophils, dendritic cells, macrophages and natural killer cells. Underpinning these interactions is the regulation of complex molecular reactions involving Toll-like receptors, activation of signalling pathways, cytokine production and activation of anti-microbial effector mechanisms including generation of reactive nitrogen and oxygen intermediates.

Migration of Apicomplexa across biological barriers: the Toxoplasma and Plasmodium rides

The invasive stages of Apicomplexa parasites, called zoites, have been largely studied in in vitro systems, with a special emphasis on their unique gliding and host cell invasive capacities. In contrast, the means by which these parasites reach their destination in their hosts are still poorly understood. We summarize here our current understanding of the cellular basis of in vivo parasitism by two well-studied Apicomplexa zoites, the Toxoplasma tachyzoite and the Plasmodium sporozoite. Despite being close relatives, these two zoites use different strategies to reach their goal and establish infection.

Toxoplasma gondii triggers secretion of interleukin-12 but low level of interleukin-10 from the THP-1 human monocytic cell line

Previous studies using both in vitro and in vivo mouse models have demonstrated that a subtle balance between pro- and anti-inflammatory cytokines, among which interleukin-12 (IL-12) and interleukin-10 (IL-10), respectively is crucial to control Toxoplasma infection. However, the few studies performed with human cell lines highlighted important host-related differences in the immune response to Toxoplasma gondii. The goal of our work was thus to study the production of both IL-12 and IL-10 by the THP-1 human monocytic cell line in response to Toxoplasma. We demonstrated that infection by live parasites (RH strain) triggers secretion of IL-12, but low level of IL-10. IL-12 secretion appeared within 8 h, up to 48 h. We also showed that infection by live parasites is not mandatory since heat-killed parasites, crude tachyzoite lysate as well as excreted/secreted antigens induced significant, yet reduced production of IL-12.

Role of CD40-Dependent Down-Regulation of CD154 in Impaired Induction of CD154 in CD4+ T Cells from HIV-1-Infected Patients

CD40-CD154 interaction is pivotal for cell-mediated immunity. There are contradictory reports on whether HIV-1 infection impairs CD154 induction. The interaction between CD40 and CD154 is important not only because it results in activation of APCs but also because it controls CD154 by diminishing expression of this molecule. Compared with healthy controls, CD4(+) T cells from HIV-1(+) patients had impaired induction of CD154 when T cell activation was mediated by CD40(+) APCs. In contrast, T cell activation in the absence of these cells resulted in normal CD154 expression. CD154 induction in HIV-1(+) patients and controls were similar upon blockade of CD40-CD154 binding. Defective regulation of CD154 appeared to occur downstream of the control of mRNA levels because up-regulation of CD154 mRNA was not impaired by HIV-1 infection. This work identifies CD40 as a mediator of impaired CD154 induction in HIV-1 infection and explains why this defect was not detected by studies where T cell activation was triggered independently of CD40(+) APCs. In addition, dysregulation of CD154 in HIV-1 infection likely contributes to immunodeficiency because diminished expression of CD154 induced by CD40 is of functional relevance, resulting in decreased dendritic cell maturation.

Toxoplasma gondii: Comparison of human CD34+ and monocyte-derived dendritic cells after parasite infection

Human dendritic cells (DC) obtained in vitro from CD34(+) progenitors (CD34-DC) or blood monocytes (mo-DC) are different DC which may be used in a model of T. gondii infection. We compared the survival, infection rate and cell surface receptor expression of both DC types after living T. gondii tachyzoite infection. CD34-DC appeared less resistant to the parasite than mo-DC. At 48h post-infection, chemokine receptors responsible for DC homing and migration were absent in mo-DC, while down regulation of CCR6 and up regulation of CCR7 was observed in CD34-DC. This result, suggesting migration ability of CD34-DC, was confirmed by in vitro migration experiments against different chemokines. Tachyzoite supernatant, used as chemokine, attracted immature CD34-DC as observed by MIP3alpha, while MIP3beta, as expected, attracted mature CD34-DC. Under similar conditions, no significant difference was noticed between mature or immature mo-DC. These data indicated that CD34-DC represent an alternative model that allows migration assay of infected DC by T. gondii.

Primary immunodeficiencies and susceptibility to parasitic infections

Primary immunodeficiencies are important disorders because they typically cause severe illness in affected patients. In addition, these diseases provide a unique glimpse at the underpinnings of the immune system in humans. Susceptibility to infections, including those caused by parasites, is a hallmark of these immune defects. Understanding the association between primary immunodeficiencies and parasitic infections will likely improve our grasp on the mechanisms of defense against these pathogens.

Toxoplasma gondii in Human Astrocytes In Vitro: Interleukin (IL)-12 and IL-10 Do Not Influence Cystogenesis

Interleukin (IL)-12, IL-10, and interferon (IFN)-gamma are major cytokines involved in the immune response against Toxoplasma gondii. Nevertheless, the role of IL-12 and IL-10 in the control of parasite replication and cytogenesis is not known yet, whereas the importance of IFN-gamma is documented. Furthermore, it is of paramount importance to study the interaction between T. gondii and cells from the central nervous system, e.g., astrocytes. In this study, we report that IL-12 and IL-10 have no effect on penetration, replication, or cystogenesis of the T. gondii Prugniaud strain in human astrocytes in vitro and do not antagonize the role of IFN-gamma on cystogenesis.

Critical role for serum opsonins and complement receptors CR3 (CD11b/CD18) and CR4 (CD11c/CD18) in phagocytosis of Francisella tularensis by human dendritic cells (DC): uptake of Francisella leads to activation of immature DC and intracellular survival of the bacteria

Francisella tularensis is one of the most infectious human pathogens known. Although much has been learned about the immune response of mice using an attenuated live vaccine strain (LVS) derived from F. tularensis subspecies holarctica (Type B), little is known about the responses of human monocyte-derived immature dendritic cells (DC). Here, we show that optimal phagocytosis of LVS by DC is dependent on serum opsonization. We demonstrate that complement factor C3-derived opsonins and the major complement receptors expressed by DC, the integrins CR3 (CD11b/CD18) and CR4 (CD11c/CD18), play a critical role in this adhesion-mediated phagocytosis. LVS induced proinflammatory cytokine production and up-regulation of costimulatory surface proteins (CD40, CD86, and MHC Class II) on DC but resisted killing. Once taken up, LVS grew intracellularly, resulting in DC death. DC maturation and cytokine production were induced by direct contact/phagocytosis of LVS or interaction with soluble products of the bacteria, and enhanced activation was seen when LVS was pretreated with serum. Sonicated LVS and supernatants from LVS cultures were potent activators of DC, but LVS LPS failed to activate DC maturation or cytokine production. Serum-treated LVS rapidly induced (within 6 h) a number of cytokines including IL-10, a potent suppressor of macrophage functions and down-regulator of Th1-like responses and the Th1 response inducer IL-12. These results suggest that the simultaneous production of an activating (IL-12, IL-1beta, and TNF-alpha) and a suppressing (IL-10) cytokine profile could contribute to the immunopathogenesis of tularemia.

Toxoplasma gondii-derived heat shock protein 70 stimulates maturation of murine bone marrow-derived dendritic cells via Toll-like receptor 4

Toxoplasma gondii-derived heat shock protein 70 (T.g.HSP70) induced maturation of bone marrow-derived dendritic cells (DCs) of wild-type (WT) C57BL/6 mice as evidenced by an increase in surface expression of MHC class I and II molecules and costimulatory molecules such as CD40, CD80, and CD86. Functionally, decreased phagocytic ability and increased alloreactive T cell stimulatory ability were observed in T.g.HSP70-stimulated DCs. These phenotypic and functional changes of T.g.HSP70-stimulated DCs were demonstrated in Toll-like receptor (TLR) 2- and myeloid differentiation factor 88 (MyD88)-deficient but not TLR4-deficient C57BL/6 mice. DCs from WT and TLR2-deficient but not TLR4-deficient mice produced IL-12 after T.g.HSP70 stimulation. T.g.HSP70-stimulated DCs from WT, TLR2-deficient, and MyD88-deficient, but not TLR4-deficient mice expressed IFN-beta mRNA. Thus, T.g.HSP70 stimulates murine DC maturation via TLR4 through the MyD88-independent signal transduction cascade.

CD11c- and CD11b-expressing mouse leukocytes transport single Toxoplasma gondii tachyzoites to the brain

The protozoan parasite Toxoplasma gondii enters hosts through the intestinal mucosa and colonizes distant tissues such as the brain, where its progeny persists for a lifetime. We investigated the role of CD11c- and CD11b-expressing leukocytes in T. gondii transport during the early step of parasitism from the mouse small intestine and during subsequent parasite localization in the brain. Following intragastric inoculation of cyst-containing parasites in mice, CD11c+ dendritic cells from the intestinal lamina propria, the Peyer patches, and the mesenteric lymph nodes were parasitized while in the blood, parasites were associated with the CD11c- CD11b+ monocytes. Using adoptive transfer experiments, we demonstrated that these parasitized cells triggered a parasitic process in the brain of naive recipient mice. Ex vivo analysis of parasitized leukocytes showed that single tachyzoites remained at the cell periphery, often surrounded by the host cell plasma membrane, but did not divide. Using either a dye that labels circulating leukocytes or an antibody known to prevent CD11b+ circulating leukocytes from leaving the microvascular bed lumen, and chimeric mice in which the hematopoietic cells expressed the green fluorescent protein, we established that T. gondii zoites hijacked CD11b+ leukocytes to reach the brain extravascular space.

Manipulation of mitogen-activated protein kinase/nuclear factor-kappaB-signaling cascades during intracellular Toxoplasma gondii infection

The intracellular protozoan Toxoplasma gondii exerts profound effects on nuclear factor-kappaB (NF-kappaB)- and mitogen-activated protein kinase (MAPK)-signaling cascades in macrophages. During early infection, nuclear translocation of NF-kappaB is blocked, and later, the cells display defects in lipopolysaccharide (LPS)-induced MAPK phosphorylation after undergoing initial activation in response to Toxoplasma itself. Infected macrophages that are subjected to triggering through Toll-like receptor 4 (TLR4) with LPS display defective production of tumor necrosis factor-alpha and IL-12 (IL-12) that likely reflects interference with NF-kappaB- and MAPK-signaling cascades. Nevertheless, T. gondii possesses molecules that themselves induce eventual proinflammatory cytokine synthesis. For interleukin-12, this occurs through both myeloid differentiation factor 88-dependent and chemokine receptor CCR5-dependent pathways. The balance between activation and interference with proinflammatory signaling is likely to reflect the need to achieve an appropriate level of immunity that allows the host and parasite to maintain a stable interaction.

Serum concentration of the growth medium markedly affects monocyte-derived dendritic cells' phenotype, cytokine production profile and capacities to stimulate in MLR

We have investigated how the maturation of monocyte-derived dendritic cells (Mo-DC) is affected by the serum concentration of the culture medium. Day 6 DC cultured in 1% human serum were a heterogeneous population of CD1a(-) and CD1a(+) DC that were separated by flow sorting. In contrast, Mo-DC generated in 10% human serum formed a homogenous population of CD1a(-) cells. Other phenotypically immature characteristics also varied, and three subsets were still distinguishable upon maturation in LPS. Furthermore, CD1a(-) DC and CD1a(+) DC from 1% culture conditions were excellent stimulators in MLR, while DC cultured in 10% serum were poor stimulators. Similarly, different cytokine profiles of the three subsets were identified. DC cultured in 1% serum had low expression of interleukin-12 (IL-12) p40 and IL-10 mRNA at day 6. Upon maturation, expression of IL-12 p40 mRNA was upregulated in CD1a(+) DC, whereas the level remained relatively low in CD1a(-) DC. In contrast, DC cultured in 10% had high levels of IL-10 mRNA at day 6 that was downregulated upon maturation. We conclude that the differentiation of monocytes into DC is significantly influenced by the serum concentration of the growth medium with effects on phenotype, cytokine profile and stimulatory activity.

Hyper immunoglobulin M syndrome due to CD40 deficiency: clinical, molecular, and immunological features

CD40 is a member of the tumor necrosis factor receptor family, which is expressed by a variety of cells including B cells, macrophages, dendritic cells, and other nonimmune cell types. CD40 activation is critical for B-cell proliferation, immunoglobulin (Ig)-isotype switching, and germinal center formation. In physiological conditions, the activation of CD40 occurs by binding to its natural ligand, CD154, which is expressed on activated T cells. The in vivo critical role of CD40-CD154 interaction on B-cell differentiation and isotype switching is provided by the discovery that mutations in either CD40 or CD154 gene cause the hyper IgM syndrome, termed HIGM3 or HIGM1, respectively, characterized by very low levels of serum IgG, IgA, and IgE, with normal or elevated IgM, associated with a defective germinal center formation. Originally considered humoral primary immunodeficiencies, the clinical features and the defect of T-cell priming, resulting from a defective T-B cell or dendritic cell interaction, is now considered as combined immunodeficiencies. In this article, we present a comprehensive overview of the clinical, genetic, and immunological features of patients with hyper IgM syndrome due to CD40 mutations.

Migration and maturation of human dendritic cells infected with Toxoplasma gondii depend on parasite strain type

Migration and maturation of human dendritic cells derived from CD34+ progenitor cells (DC) infected by Toxoplasma gondii were studied in an in vitro model. We demonstrated that infection with virulent type I strains RH and ENT or type II low virulent strains PRU and CAL induced DC migration towards MIP-3beta. However, type II strains induced a higher percentage of migrating cells than that induced by type I strains or positive controls (chemical allergen or lipopolysaccharides). Type II strains produced soluble factors responsible of the high migration whereas heat killed tachyzoites did not induced a migration higher than positive controls. We also demonstrated that infection by virulent strains and not by type II stains or heat killed tachyzoites triggers DC maturation. A soluble factor released by type II strains was responsible of the absence of DC maturation. Taken together, these results demonstrated that the interference of T. gondii in the behaviour of DC functions is related to the strain types and can be supported by secretion of soluble factors by the parasite.

Inhibition of the Growth ofToxoplasma gondiiin Immature Human Dendritic Cells Is Dependent on the Expression of TNF-α Receptor 2

An effective immunity to Toxoplasma gondii in humans is dependent on the cellular immune response. Toxoplasma can infect and replicate in almost all nucleated cells, and the most important cytokine regulating the growth in humans is IFN-gamma; however, the role of TNF-alpha has to date been largely described to be synergistic. We show that, compared with mature human dendritic cells (mDC), immature human DC (iDC) demonstrate a reduced parasite proliferation when infected with Toxoplasma. This toxoplasmostasis was only present in iDC after 11 days of culture and was not present in DC that had been matured ex vivo using a cytokine mixture (mDC). Spontaneous toxoplasmostatic activity has previously only been described in fresh human monocytes, and the mechanism involved is as yet unclear. We show that, in comparison with an absence of expression in mDC, TNF-R2 is expressed in both iDC and monocytes infected with Toxoplasma, and furthermore, that blocking the TNF-R2 with Abs abrogates the toxoplasmostasis in the iDC. These findings demonstrate a functional role for TNF-R2 in the newly described spontaneous toxoplasmostasis of iDC.

Neutrophils, dendritic cells and Toxoplasma

Toxoplasma gondii rapidly elicits strong Type 1 cytokine-based immunity. The necessity for this response is well illustrated by the example of IFN-gamma and IL-12 gene knockout mice that rapidly succumb to the effects of acute infection. The parasite itself is skilled at sparking complex interactions in the innate immune system that lead to protective immunity. Neutrophils are one of the first cell types to arrive at the site of infection, and the cells release several proinflammatory cytokines and chemokines in response to Toxoplasma. Dendritic cells are an important source of IL-12 during infection with T. gondii and other microbial pathogens, and they are also specialized for high-level antigen presentation to T lymphocytes. Tachyzoites express at least two types of molecules that trigger innate immune cell cytokine production. One of these involves Toll-like receptor/MyD88 pathways common to many microbial pathogens. The second pathway is less conventional and involves molecular mimicry between a parasite cyclophilin and host CC chemokine receptor 5-binding ligands. Neutrophils, dendritic cells and Toxoplasma work together to elicit the immune response required for host survival. Cytokine and chemokine cross-talk between parasite-triggered neutrophils and dendritic cells results in recruitment, maturation and activation of the latter. Neutrophil-empowered dendritic cells possess properties expected of highly potent antigen presenting cells that drive T helper 1 generation.

The polysaccharide capsule of Cryptococcus neoformans interferes with human dendritic cell maturation and activation

The ability of encapsulated and acapsular strains of Cryptococcus neoformans to activate dendritic cells (DC) derived from monocytes stimulated with granulocyte macrophage-colony stimulating factor and interleukin-4 was evaluated. Profound differences in DC response to encapsulated and acapsular C. neoformans strains were observed. In particular, (i) the acapsular strain was easily phagocytosed by immature DC, and the process induced several molecular markers, such as major histocompatibility complex (MHC) class I and class II, CD40, and CD83, which are characteristic of mature DC; (ii) the encapsulated strain did not up-regulate MHC class I and class II and CD83 molecules; (iii) the soluble capsular polysaccharide glucuronoxylomannan (GXM) is unable to regulate MHC class I and class II molecules; (iv) the addition of monoclonal antibody to GXM (anti-GXM) to the encapsulated strain facilitated antigen-presenting cell maturation by promoting ingestion of C. neoformans via Fc receptor for immunoglobulin G (FcgammaR)II (CD32) and FcgammaRIII (CD16); (v) pertubation of FcRgammaII or FcgammaRIII was insufficient to promote DC maturation; and (vi) optimal DC maturation permitted efficient T cell activation and differentiation, as documented by the enhancement of lymphoproliferation and interferon-gamma production. These results indicate that the C. neoformans capsule interferes with DC activation and maturation, indicating a new pathway by which the fungus may avoid an efficient T cell response.