Intradermal infections of mice by low numbers of african trypanosomes are controlled by innate resistance but enhance susceptibility to reinfection

To the Skin and Beyond: The Immune Response to African Trypanosomes as They Enter and Exit the Vertebrate Host

African trypanosomes are single-celled extracellular protozoan parasites transmitted by tsetse fly vectors across sub-Saharan Africa, causing serious disease in both humans and animals. Mammalian infections begin when the tsetse fly penetrates the skin in order to take a blood meal, depositing trypanosomes into the dermal layer. Similarly, onward transmission occurs when differentiated and insect pre-adapted forms are ingested by the fly during a blood meal. Between these transmission steps, trypanosomes access the systemic circulation of the vertebrate host via the skin-draining lymph nodes, disseminating into multiple tissues and organs, and establishing chronic, and long-lasting infections. However, most studies of the immunobiology of African trypanosomes have been conducted under experimental conditions that bypass the skin as a route for systemic dissemination (typically via intraperitoneal or intravenous routes). Therefore, the importance of these initial interactions between trypanosomes and the skin at the site of initial infection, and the implications for these processes in infection establishment, have largely been overlooked. Recent studies have also demonstrated active and complex interactions between the mammalian host and trypanosomes in the skin during initial infection and revealed the skin as an overlooked anatomical reservoir for transmission. This highlights the importance of this organ when investigating the biology of trypanosome infections and the associated immune responses at the initial site of infection. Here, we review the mechanisms involved in establishing African trypanosome infections and potential of the skin as a reservoir, the role of innate immune cells in the skin during initial infection, and the subsequent immune interactions as the parasites migrate from the skin. We suggest that a thorough identification of the mechanisms involved in establishing African trypanosome infections in the skin and their progression through the host is essential for the development of novel approaches to interrupt disease transmission and control these important diseases.

Influence of the Draining Lymph Nodes and Organized Lymphoid Tissue Microarchitecture on Susceptibility to Intradermal Trypanosoma brucei Infection

Infection of the mammalian host with African trypanosomes begins when the tsetse fly vector injects the parasites into the skin dermis during blood feeding. After injection into the skin, trypanosomes first accumulate in the draining lymph node before disseminating systemically. Whether this early accumulation within the draining lymph node is important for the trypanosomes to establish infection was not known. Lymphotoxin-β-deficient mice (LTβ^-/- mice) lack most secondary lymphoid tissues, but retain the spleen and mesenteric lymph nodes. These mice were used to test the hypothesis that the establishment of infection after intradermal (ID) T. brucei infection would be impeded in the absence of the skin draining lymph nodes. However, LTβ^-/- mice revealed greater susceptibility to ID T. brucei infection than wild-type mice, indicating that the early accumulation of the trypanosomes in the draining lymph nodes was not essential to establish systemic infection. Although LTβ^-/- mice were able to control the first parasitemia wave as effectively as wild-type mice, they were unable to control subsequent parasitemia waves. LTβ^-/- mice also lack organized B cell follicles and germinal centers within their remaining secondary lymphoid tissues. As a consequence, LTβ^-/- mice have impaired immunoglobulin (Ig) isotype class-switching responses. When the disturbed microarchitecture of the B cell follicles in the spleens of LTβ^-/- mice was restored by reconstitution with wild-type bone marrow, their susceptibility to ID T. brucei infection was similar to that of wild-type control mice. This effect coincided with the ability to produce significant serum levels of Ig isotype class-switched parasite-specific antibodies. Thus, our data suggest that organized splenic microarchitecture and the production of parasite-specific Ig isotype class-switched antibodies are essential for the control of ID African trypanosome infections.

Inflammation following trypanosome infection and persistence in the skin

Human African trypanosomes rely for their transmission on tsetse flies (Glossina sp.) that inoculate parasites into the skin during blood feeding. The absence of a protective vaccine, limited knowledge about the infection immunology, and the existence of asymptomatic carriers sustaining transmission are major outstanding challenges towards elimination. All these relate to the skin where (i) parasites persist and transmit to tsetse flies and (ii) a successful vaccination strategy should ideally be effective. Host immune processes and parasite strategies that underlie early infection and skin tropism are essential aspects to comprehend the transmission-success of trypanosomes and the failure in vaccine development. Recent insights into the early infection establishment may pave the way to novel strategies aimed at blocking transmission.

Infections With Extracellular Trypanosomes Require Control by Efficient Innate Immune Mechanisms and Can Result in the Destruction of the Mammalian Humoral Immune System

Salivarian trypanosomes are extracellular parasites that affect humans, livestock, and game animals around the world. Through co-evolution with the mammalian immune system, trypanosomes have developed defense mechanisms that allow them to thrive in blood, lymphoid vessels, and tissue environments such as the brain, the fat tissue, and testes. Trypanosomes have developed ways to circumvent antibody-mediated killing and block the activation of the lytic arm of the complement pathway. Hence, this makes the innate immune control of the infection a crucial part of the host-parasite interaction, determining infection susceptibility, and parasitemia control. Indeed, trypanosomes use a combination of several independent mechanisms to avoid clearance by the humoral immune system. First, perpetuated antigenic variation of the surface coat allows to escape antibody-mediated elimination. Secondly, when antibodies bind to the coat, they are efficiently transported toward the endocytosis pathway, where they are removed from the coat proteins. Finally, trypanosomes engage in the active destruction of the mammalian humoral immune response. This provides them with a rescue solution in case antigenic variation does not confer total immunological invisibility. Both antigenic variation and B cell destruction pose significant hurdles for the development of anti-trypanosome vaccine strategies. However, developing total immune escape capacity and unlimited growth capabilities within a mammalian host is not beneficial for any parasite, as it will result in the accelerated death of the host itself. Hence, trypanosomes have acquired a system of quorum sensing that results in density-dependent population growth arrest in order to prevent overpopulating the host. The same system could possibly sense the infection-associated host tissue damage resulting from inflammatory innate immune responses, in which case the quorum sensing serves to prevent excessive immunopathology and as such also promotes host survival. In order to put these concepts together, this review summarizes current knowledge on the interaction between trypanosomes and the mammalian innate immune system, the mechanisms involved in population growth regulation, antigenic variation and the immuno-destructive effect of trypanosomes on the humoral immune system. Vaccine trials and a discussion on the role of innate immune modulation in these trials are discussed at the end.

Host Immune Responses and Immune Evasion Strategies in African Trypanosomiasis

Parasites, including African trypanosomes, utilize several immune evasion strategies to ensure their survival and completion of their life cycles within their hosts. The defense factors activated by the host to resolve inflammation and restore homeostasis during active infection could be exploited and/or manipulated by the parasites in an attempt to ensure their survival and propagation. This often results in the parasites evading the host immune responses as well as the host sustaining some self-inflicted collateral tissue damage. During infection with African trypanosomes, both effector and suppressor cells are activated and the balance between these opposing arms of immunity determines susceptibility or resistance of infected host to the parasites. Immune evasion by the parasites could be directly related to parasite factors, (e.g., antigenic variation), or indirectly through the induction of suppressor cells following infection. Several cell types, including suppressive macrophages, myeloid-derived suppressor cells (MDSCs), and regulatory T cells have been shown to contribute to immunosuppression in African trypanosomiasis. In this review, we discuss the key factors that contribute to immunity and immunosuppression during T. congolense infection, and how these factors could aid immune evasion by African trypanosomes. Understanding the regulatory mechanisms that influence resistance and/or susceptibility during African trypanosomiasis could be beneficial in designing effective vaccination and therapeutic strategies against the disease.

Route of inoculation influences Trypanosoma congolense and Trypanosoma brucei brucei virulence in Swiss white mice

Experiments on infections caused by trypanosomes are widely performed in Swiss white mice through various inoculation routes. To better understand the effect of route of trypanosome inoculation on disease outcomes in this model, we characterised the virulence of two isolates, Trypanosoma brucei KETRI 2710 and T. congolense KETRI 2765 in Swiss white mice. For each of the isolates, five routes of parasite inoculation, namely intraperitoneal (IP), subcutaneous (SC), intramuscular (IM) intradermal (ID) and intravenous (IV) were compared using groups (n = 6) of mice, with each mouse receiving 1x104 trypanosomes. We subsequently assessed impact of the routes on disease indices that included pre-patent period (PP), parasitaemia levels, Packed Cell Volume (PCV), bodyweight changes and survival time. Pre-patent period for IP inoculated mice was a mean ± SE of 3.8 ± 0.2 and 6.5 ± 0.0 for the T brucei and T. congolense isolates respectively; the PP for mice groups inoculated using other routes were not significantly different(p> 0.05) irrespective of route of inoculation and species of trypanosomes. With ID and IP routes, parasitaemia was significantly higher in T. brucei and significantly lower in T. congolense infected mice and the progression to peak parasitaemia routes showed no significant different between the routes of either species of trypanosome. The IM and ID routes in T. congolense inoculations, and IP and IV in T. b. brucei induced the fastest and slowest parasitaemia progressions respectively. There were significant differences in rates of reduction of PCV with time post infection in mice infected by the two species and which was more pronounced in sc and ip injected mice. No significant differences in mice body weight changes and survivorship was observed between the routes of inoculation. Inoculation route therefore appears to be a critical determinant of pathogenicity of Trypanosoma congolense and Trypanosoma brucei brucei in murine mouse model of African trypanosomiasis.

Base excision repair plays an important role in the protection against nitric oxide- and in vivo-induced DNA damage in Trypanosoma brucei

Uracil-DNA glycosylase (UNG) initiates the base excision repair pathway by excising uracil from DNA. We have previously shown that Trypanosoma brucei cells defective in UNG exhibit reduced infectivity thus demonstrating the relevance of this glycosylase for survival within the mammalian host. In the early steps of the immune response, nitric oxide (NO) is released by phagocytes, which in combination with oxygen radicals produce reactive nitrogen species (RNS). These species can react with DNA generating strand breaks and base modifications including deaminations. Since deaminated cytosines are the main substrate for UNG, we hypothesized that the glycosylase might confer protection towards nitrosative stress. Our work establishes the occurrence of genotoxic damage in Trypanosoma brucei upon exposure to NO in vitro and shows that deficient base excision repair results in increased levels of damage in DNA and a hypermutator phenotype. We also evaluate the incidence of DNA damage during infection in vivo and show that parasites recovered from mice exhibit higher levels of DNA strand breaks, base deamination and repair foci compared to cells cultured in vitro. Notably, the absence of UNG leads to reduced infectivity and enhanced DNA damage also in animal infections. By analysing mRNA and protein levels, we found that surviving UNG-KO trypanosomes highly express tryparedoxin peroxidase involved in trypanothione/tryparedoxin metabolism. These observations suggest that the immune response developed by the host enhances the activation of genes required to counteract oxidative stress and emphasize the importance of DNA repair pathways in the protection to genotoxic and oxidative stress in trypanosomes.

Neutrophils enhance early Trypanosoma brucei infection onset

In this study, Trypanosoma brucei was naturally transmitted to mice through the bites of infected Glossina morsitans tsetse flies. Neutrophils were recruited rapidly to the bite site, whereas monocytes were attracted more gradually. Expression of inflammatory cytokines (il1b, il6), il10 and neutrophil chemokines (cxcl1, cxcl5) was transiently up-regulated at the site of parasite inoculation. Then, a second influx of neutrophils occurred that coincided with the previously described parasite retention and expansion in the ear dermis. Congenital and experimental neutropenia models, combined with bioluminescent imaging, indicate that neutrophils do not significantly contribute to dermal parasite control and elicit higher systemic parasitemia levels during the infection onset. Engulfment of parasites by neutrophils in the skin was rarely observed and was restricted to parasites with reduced motility/viability, whereas live parasites escaped phagocytosis. To our knowledge, this study represents the first description of a trypanosome infection promoting role of early innate immunological reactions following an infective tsetse fly bite. Our data indicate that the trypanosome is not hindered in its early development and benefits from the host innate responses with the neutrophils being important regulators of the early infection, as already demonstrated for the sand fly transmitted Leishmania parasite.

Adenylate Cyclases of Trypanosoma brucei, Environmental Sensors and Controllers of Host Innate Immune Response

Trypanosoma brucei, etiological agent of Sleeping Sickness in Africa, is the prototype of African trypanosomes, protozoan extracellular flagellate parasites transmitted by saliva (Salivaria). In these parasites the molecular controls of the cell cycle and environmental sensing are elaborate and concentrated at the flagellum. Genomic analyses suggest that these parasites appear to differ considerably from the host in signaling mechanisms, with the exception of receptor-type adenylate cyclases (AC) that are topologically similar to receptor-type guanylate cyclase (GC) of higher eukaryotes but control a new class of cAMP targets of unknown function, the cAMP response proteins (CARPs), rather than the classical protein kinase A cAMP effector (PKA). T. brucei possesses a large polymorphic family of ACs, mainly associated with the flagellar membrane, and these are involved in inhibition of the innate immune response of the host prior to the massive release of immunomodulatory factors at the first peak of parasitemia. Recent evidence suggests that in T. brucei several insect-specific AC isoforms are involved in social motility, whereas only a few AC isoforms are involved in cytokinesis control of bloodstream forms, attesting that a complex signaling pathway is required for environmental sensing. In this review, after a general update on cAMP signaling pathway and the multiple roles of cAMP, I summarize the existing knowledge of the mechanisms by which pathogenic microorganisms modulate cAMP levels to escape immune defense.

Trypanosoma brucei growth control by TNF in mammalian host is independent of the soluble form of the cytokine

Infection of C57Bl/6 mice by pleomorphic African trypanosomes Trypanosoma brucei and T. congolense is characterized by parasitemia waves coupled with the production of systemic levels of TNF. This cytokine is known to control T. brucei growth, but also to contribute to tissue damage, shortening the survival time of infected mice. Using a dominant-negative version of TNF to discriminate between the effects of the membrane-form versus the soluble form of TNF, we show that the second form is involved in neither parasite control nor induction of liver injury. Therefore, soluble TNF is likely not a major contributor to disease outcome. We propose that membrane-bound TNF is responsible for both T. brucei control and host pathology.

Modeling Rabbit Responses to Single and Multiple Aerosol Exposures of Bacillus anthracis Spores

Survival models are developed to predict response and time-to-response for mortality in rabbits following exposures to single or multiple aerosol doses of Bacillus anthracis spores. Hazard function models were developed for a multiple-dose data set to predict the probability of death through specifying functions of dose response and the time between exposure and the time-to-death (TTD). Among the models developed, the best-fitting survival model (baseline model) is an exponential dose-response model with a Weibull TTD distribution. Alternative models assessed use different underlying dose-response functions and use the assumption that, in a multiple-dose scenario, earlier doses affect the hazard functions of each subsequent dose. In addition, published mechanistic models are analyzed and compared with models developed in this article. None of the alternative models that were assessed provided a statistically significant improvement in fit over the baseline model. The general approach utilizes simple empirical data analysis to develop parsimonious models with limited reliance on mechanistic assumptions. The baseline model predicts TTDs consistent with reported results from three independent high-dose rabbit data sets. More accurate survival models depend upon future development of dose-response data sets specifically designed to assess potential multiple-dose effects on response and time-to-response. The process used in this article to develop the best-fitting survival model for exposure of rabbits to multiple aerosol doses of B. anthracis spores should have broad applicability to other host-pathogen systems and dosing schedules because the empirical modeling approach is based upon pathogen-specific empirically-derived parameters.

Trypanosoma brucei Invasion and T-Cell Infiltration of the Brain Parenchyma in Experimental Sleeping Sickness: Timing and Correlation with Functional Changes

BACKGROUND

The timing of Trypanosoma brucei entry into the brain parenchyma to initiate the second, meningoencephalitic stage of human African trypanosomiasis or sleeping sickness is currently debated and even parasite invasion of the neuropil has been recently questioned. Furthermore, the relationship between neurological features and disease stage are unclear, despite the important diagnostic and therapeutic implications.

METHODOLOGY

Using a rat model of chronic Trypanosoma brucei brucei infection we determined the timing of parasite and T-cell neuropil infiltration and its correlation with functional changes. Parasite DNA was detected using trypanosome-specific PCR. Body weight and sleep structure alterations represented by sleep-onset rapid eye movement (SOREM) periods, reported in human and experimental African trypanosomiasis, were monitored. The presence of parasites, as well as CD4+ and CD8+ T-cells in the neuropil was assessed over time in the brain of the same animals by immunocytochemistry and quantitative analyses.

PRINCIPAL FINDINGS

Trypanosome DNA was present in the brain at day 6 post-infection and increased more than 15-fold by day 21. Parasites and T-cells were observed in the parenchyma from day 9 onwards. Parasites traversing blood vessel walls were observed in the hypothalamus and other brain regions. Body weight gain was reduced from day 7 onwards. SOREM episodes started in most cases early after infection, with an increase in number and duration after parasite neuroinvasion.

CONCLUSION

These findings demonstrate invasion of the neuropil over time, after an initial interval, by parasites and lymphocytes crossing the blood-brain barrier, and show that neurological features can precede this event. The data thus challenge the current clinical and cerebrospinal fluid criteria of disease staging.

The Dermis as a Delivery Site of Trypanosoma brucei for Tsetse Flies

Tsetse flies are the sole vectors of Trypanosoma brucei parasites that cause sleeping sickness. Our knowledge on the early interface between the infective metacyclic forms and the mammalian host skin is currently highly limited. Glossina morsitans flies infected with fluorescently tagged T. brucei parasites were used in this study to initiate natural infections in mice. Metacyclic trypanosomes were found to be highly infectious through the intradermal route in sharp contrast with blood stream form trypanosomes. Parasite emigration from the dermal inoculation site resulted in detectable parasite levels in the draining lymph nodes within 18 hours and in the peripheral blood within 42 h. A subset of parasites remained and actively proliferated in the dermis. By initiating mixed infections with differentially labeled parasites, dermal parasites were unequivocally shown to arise from the initial inoculum and not from a re-invasion from the blood circulation. Scanning electron microscopy demonstrated intricate interactions of these skin-residing parasites with adipocytes in the connective tissue, entanglement by reticular fibers of the periadipocytic baskets and embedment between collagen bundles. Experimental transmission experiments combined with molecular parasite detection in blood fed flies provided evidence that dermal trypanosomes can be acquired from the inoculation site immediately after the initial transmission. High resolution thermographic imaging also revealed that intradermal parasite expansion induces elevated skin surface temperatures. Collectively, the dermis represents a delivery site of the highly infective metacyclic trypanosomes from which the host is systemically colonized and where a proliferative subpopulation remains that is physically constrained by intricate interactions with adipocytes and collagen fibrous structures.

Sleeping sickness is caused by trypanosomes that are transmitted by the blood feeding tsetse flies. The present study has established an experimental transmission model with fluorescently labeled parasites in mice that allows us to study their fate following natural transmission by a tsetse fly bite. Parasites that arise in the tsetse salivary glands were found to be highly infective following inoculation in the mammalian skin in contrast with previous observations made for trypanosomes purified from the blood stream. This study unveiled that a proportion of parasites is retained in the skin and actively proliferates close to the initial inoculation site resulting in significantly elevated skin temperatures. This retention was linked to interaction with fat cells and collagen fibrous structures. Experimental transmission experiments were able to demonstrate that parasites can be acquired from the inoculation site immediately after the initial transmission.

Nitric Oxide Protects against Infection-Induced Neuroinflammation by Preserving the Stability of the Blood-Brain Barrier

Nitric oxide (NO) generated by inducible NO synthase (iNOS) is critical for defense against intracellular pathogens but may mediate inflammatory tissue damage. To elucidate the role of iNOS in neuroinflammation, infections with encephalitogenic Trypanosoma brucei parasites were compared in inos^-/- and wild-type mice. Inos^-/- mice showed enhanced brain invasion by parasites and T cells, and elevated protein permeability of cerebral vessels, but similar parasitemia levels. Trypanosome infection stimulated T cell- and TNF-mediated iNOS expression in perivascular macrophages. NO nitrosylated and inactivated pro-inflammatory molecules such as NF-κΒp65, and reduced TNF expression and signalling. iNOS-derived NO hampered both TNF- and T cell-mediated parasite brain invasion. In inos^-/- mice, TNF stimulated MMP, including MMP9 activity that increased cerebral vessel permeability. Thus, iNOS-generated NO by perivascular macrophages, strategically located at sites of leukocyte brain penetration, can serve as a negative feed-back regulator that prevents unlimited influx of inflammatory cells by restoring the integrity of the blood-brain barrier.

Inflammatory responses can lead to harmful effects on the brain during many chronic parasitic infections, including those with African trypanosomes. T. brucei, the causative agent of African trypanosomiasis, that traverse the blood-brain barrier (BBB) to invade the brain, where, together with inflammatory infiltrates, they likely contribute to the neurologic disturbances of the disease. High levels of nitric oxide (NO) released by the inducible NO synthase (iNOS) are critical for defense against parasites, but also mediate inflammatory tissue damage. Using a mouse model of African trypanosomiasis, we uncovered an unexpected role of NO, preserving the integrity of the BBB and limiting the neuroinvasion of leukocytes and parasites, rather than mediating brain damage or killing of trypanosomes. iNOS-derived NO, nitrosylates molecules such as pro-inflammatory transcription factors. iNOS hampered both TNF- and T cell-mediated parasite and leukocyte brain invasion and passage of serum proteins across the BBB. In inos^-/- mice, exacerbated TNF secretion and signalling increased MMP9 activity that mediates cerebral vascular permeability. Thus, NO is crucial for maintenance of the integrity of the cerebral vessels and serves as a feed-back regulator by inhibiting leukocyte brain penetration during T. brucei infection. Consequently, therapies could target iNOS to reduce tissue damage during neuroinflammation.

Distinct Contributions of CD4+ and CD8+ T Cells to Pathogenesis of Trypanosoma brucei Infection in the Context of Gamma Interferon and Interleukin-10

Although gamma interferon (IFN-γ) and interleukin-10 (IL-10) have been shown to be critically involved in the pathogenesis of African trypanosomiasis, the contributions to this disease of CD4(+) and CD8(+) T cells, the major potential producers of the two cytokines, are incompletely understood. Here we show that, in contrast to previous findings, IFN-γ was produced by CD4(+), but not CD8(+), T cells in mice infected with Trypanosoma brucei. Without any impairment in the secretion of IFN-γ, infected CD8(-/-) mice survived significantly longer than infected wild-type mice, suggesting that CD8(+) T cells mediated mortality in an IFN-γ-independent manner. The increased survival of infected CD8(-/-) mice was significantly reduced in the absence of IL-10 signaling. Interestingly, IL-10 was also secreted mainly by CD4(+) T cells. Strikingly, depletion of CD4(+) T cells abrogated the prolonged survival of infected CD8(-/-) mice, demonstrating that CD4(+) T cells mediated protection. Infected wild-type mice and CD8(-/-) mice depleted of CD4(+) T cells had equal survival times, suggesting that the protection mediated by CD4(+) T cells was counteracted by the detrimental effects of CD8(+) T cells in infected wild-type mice. Interestingly, CD4(+) T cells also mediated the mortality of infected mice in the absence of IL-10 signaling, probably via excessive secretion of IFN-γ. Finally, CD4(+), but not CD8(+), T cells were critically involved in the synthesis of IgG antibodies during T. brucei infections. Collectively, these results highlight distinct roles of CD4(+) and CD8(+) T cells in the context of IFN-γ and IL-10 during T. brucei infections.

Chronic Trypanosoma congolense infections in mice cause a sustained disruption of the B-cell homeostasis in the bone marrow and spleen

Trypanosoma congolense is one of the main species responsible for Animal African Trypanosomosis (AAT). As preventive vaccination strategies for AAT have been unsuccessful so far, investigating the mechanisms underlying vaccine failure has to be prioritized. In T. brucei and T. vivax infections, recent studies revealed a rapid onset of destruction of the host B-cell compartment, resulting in the loss of memory recall capacity. To assess such effect in experimental T. congolense trypanosomosis, we performed infections with both the cloned Tc13 parasite, which is considered as a standard model system for T. congolense rodent infections and the noncloned TRT55 field isolate. These infections differ in their virulence level in the C57BL/6 mouse model for trypanosomosis. We show that early on, an irreversible depletion of all developmental B cells stages occur. Subsequently, in the spleen, a detrimental decrease in immature B cells is followed by a significant and permanent depletion of Marginal zone B cells and Follicular B cells. The severity of these events later on in infection correlated with the virulence level of the parasite stock. In line with this, it was observed that later-stage infection-induced IgGs were largely nonspecific, in particular in the more virulent TRT55 infection model.

Low-dose intradermal infection with trypanosoma congolense leads to expansion of regulatory T cells and enhanced susceptibility to reinfection

BALB/c mice are highly susceptible to experimental intraperitoneal Trypanosoma congolense infection. However, a recent report showed that these mice are relatively resistant to primary intradermal low-dose infection. Paradoxically, repeated low-dose intradermal infections predispose mice to enhanced susceptibility to an otherwise noninfectious dose challenge. Here, we explored the mechanisms responsible for this low-dose-induced susceptibility to subsequent low-dose challenge infection. We found that akin to intraperitoneal infection, low-dose intradermal infection led to production of interleukin-10 (IL-10), IL-6, IL-12, tumor necrosis factor alpha (TNF-α), transforming growth factor β (TGF-β), and gamma interferon (IFN-γ) by spleen and draining lymph node cells. Interestingly, despite the absence of parasitemia, low-dose intradermal infection led to expansion of CD4+ CD25+ Foxp3+ cells (T regulatory cells [Tregs]) in both the spleens and lymph nodes draining the infection site. Depletion of Tregs by anti-CD25 monoclonal antibody (MAb) treatment during primary infection or before challenge infection following repeated low-dose infection completely abolished the low-dose-induced enhanced susceptibility. In addition, Treg depletion was associated with dramatic reduction in serum levels of TGF-β and IL-10. Collectively, these findings show that low-dose intradermal infection leads to rapid expansion of Tregs, and these cells mediate enhanced susceptibility to subsequent infection.

Isolation, cloning, and pathologic analysis of Trypanosoma evansi field isolates

In recent years, the emergence of highly pathogenic Trypanosoma evansi strains in the Philippines has resulted in substantial losses in livestock production. In this study, we isolated T. evansi from infected-water buffaloes in the Philippines and analyzed their virulence using mice and cattle. A total of 10 strains of T. evansi were isolated. Evaluation of the virulence of each strain using mice depicted significant differences among the strains in the prepatent period, the level of parasitemia, and the survival time of the infected animals. In mice infected with the highly pathogenic T. evansi, signs of excessive inflammation such as marked splenomegaly and increase more than 6-fold in the number of leukocytes were observed at 8 days post-infection. To study the virulence of the parasite strains in cattle (which are the common T. evansi hosts in Philippines), cattle were infected with the T. evansi isolates that showed high and low virulence in mice. The rate of parasite growth and the length of the prepatent periods were found to be similar to those observed in mice for the respective strains. The cattle infected with the highly pathogenic strain developed anemia and a marked decrease in leukocyte counts. To determine the cause of the pathological changes, we analyzed the expression levels of inflammatory cytokines and observed up-regulation of tumor necrosis factor-α in anemic infected cattle. Our findings suggest that the epidemic of T. evansi in the Philippines is characterized by T. evansi strains with varying virulences from low to very high pathogenicity in cattle.

Non-invasive in vivo study of the Trypanosoma vivax infectious process consolidates the brain commitment in late infections

Trypanosoma vivax, one of the leading parasites responsible for Animal African Trypanosomosis (Nagana), is generally cyclically transmitted by Glossina spp. but in areas devoid of the tsetse flies in Africa or in Latin American countries is mechanically transmitted across vertebrate hosts by other haematophagous insects, including tabanids. We followed on from our recent studies on the maintenance of this parasite in vivo and in vitro, and its genetic manipulation, by constructing a West African IL1392 T. vivax strain that stably expresses firefly luciferase and is fully virulent for immunocompetent mice. We report here on a study where murine infection with this strain was monitored in vivo using a non-invasive method. Study findings fully support the use of this strain in the assessment of parasite dynamics in vivo since a strong correlation was found between whole body light emission measured over the course of the infection and parasitemia determined microscopically. In addition, parasitemia and survival rates were very similar for mice infected by the intraperitoneal and sub-cutaneous routes, except for a longer prepatent period following sub-cutaneous inoculation with the parasite. Our results clearly show that when administered by the subcutaneous route, the parasite is retained few days in the skin close to the inoculation site where it multiplies before passing into the bloodstream. Ex vivo bioluminescence analyses of organs isolated from infected mice corroborated our previous histopathological observations with parasite infiltration into spleen, liver and lungs. Finally, our study reinforces previous observations on the presence of the parasite in the central nervous system and consequently the brain commitment in the very late phases of the experimental infection.

Regulatory T cells enhance susceptibility to experimental Trypanosoma congolense infection independent of mouse genetic background

Background

BALB/c mice are highly susceptible while C57BL/6 are relatively resistant to experimental Trypanosoma congolense infection. Although regulatory T cells (Tregs) have been shown to regulate the pathogenesis of experimental T. congolense infection, their exact role remains controversial. We wished to determine whether Tregs contribute to distinct phenotypic outcomes in BALB/c and C57BL/6 mice and if so how they operate with respect to control of parasitemia and production of disease-exacerbating proinflammatory cytokines.

Methodology/Findings

BALB/c and C57BL/6 mice were infected intraperitoneally (i.p) with 10³T. congolense clone TC13 and both the kinetics of Tregs expansion and intracellular cytokine profiles in the spleens and livers were monitored directly ex vivo by flow cytometry. In some experiments, mice were injected with anti-CD25 mAb prior or post T. congolense infection or adoptively (by intravenous route) given highly enriched naïve CD25⁺ T lymphocytes prior to T. congolense infection and the inflammatory cytokine/chemokine levels and survival were monitored. In contrast to a transient and non significant increase in the percentages and absolute numbers of CD4⁺CD25⁺Foxp3⁺ T cells (Tregs) in C57BL/6 mouse spleens and livers, a significant increase in the percentage and absolute numbers of Tregs was observed in spleens of infected BALB/c mice. Ablation or increasing the number of CD25⁺ cells in the relatively resistant C57BL/6 mice by anti-CD25 mAb treatment or by adoptive transfer of CD25⁺ T cells, respectively, ameliorates or exacerbates parasitemia and production of proinflammatory cytokines.

Conclusion

Collectively, our results show that regulatory T cells contribute to susceptibility in experimental murine trypanosomiasis in both the highly susceptible BALB/c and relatively resistant C57BL/6 mice.

BALB/c mice are highly susceptible while C57BL/6 is relatively resistant to experimental Trypanosoma congolense infection. Acute death observed in infected BALB/c mice is usually associated with the excessive production of pro-inflammatory cytokines. Regulatory T cells (Tregs) have been shown to play a significant role in the pathogenesis of many diseases including those caused by parasites. However, the role of Tregs in the pathogenesis of T. congolense infection remains unclear. We were interested in addressing the following questions: Do Tregs contribute to the distinct phenotypic outcomes observed in T. congolense-infected BALB/c and C57BL/6 mice? If so, where and how do they operate with respect to parasitemia and cytokine response? By selectively altering the numbers of these cells either by targeted depletion with monoclonal antibody or adoptive transfer of highly enriched naïve CD25⁺ cells prior to infection, we show that Tregs impairs efficient parasite control and impacts on production of disease-exacerbating proinflammatory cytokines. Collectively, our findings suggest that Tregs contribute to enhanced susceptibility to experimental T. congolense infection in mice.

Adenylate cyclases of Trypanosoma brucei inhibit the innate immune response of the host

The parasite Trypanosoma brucei possesses a large family of transmembrane receptor-like adenylate cyclases. Activation of these enzymes requires the dimerization of the catalytic domain and typically occurs under stress. Using a dominant-negative strategy, we found that reducing adenylate cyclase activity by about 50% allowed trypanosome growth but reduced the parasite's ability to control the early innate immune defense of the host. Specifically, activation of trypanosome adenylate cyclase resulting from parasite phagocytosis by liver myeloid cells inhibited the synthesis of the trypanosome-controlling cytokine tumor necrosis factor-α through activation of protein kinase A in these cells. Thus, adenylate cyclase activity of lyzed trypanosomes favors early host colonization by live parasites. The role of adenylate cyclases at the host-parasite interface could explain the expansion and polymorphism of this gene family.

Distinct Toll-like receptor signals regulate cerebral parasite load and interferon α/β and tumor necrosis factor α-dependent T-cell infiltration in the brains of Trypanosoma brucei-infected mice

BACKGROUND

The penetration of T cells and trypanosomes into the brain parenchyma is a major pathogenetic event in African trypanosomiasis.

METHODS

The role of innate immune responses in the penetration of T cells and Trypanosoma brucei brucei into the brain was studied in knockout mice by using double immunofluorescent staining and real-time polymerase chain reaction.

RESULTS

We demonstrate that Toll-like receptor (TLR)-MyD88-mediated signaling is required for T-cell and parasite penetration into the brain and microglial activation, besides controlling parasitemia and antigen-specific T-cell activation. Among different TLR-deficient mice studied, TLR9 mediated parasitemia control and T-cell penetration into the brain. TLR-MyD88 signals increased levels of interferon (IFN) β and tumor necrosis factor (TNF) α transcripts in the brains of infected mice and both TNF-α and IFN-α/β, receptors promoted T-cell and trypanosoma infiltration into the brain parenchyma. Both resident and infiltrating inflammatory cells in the brain controlled parasite densities in a TLR2- and TLR9-MyD88-mediated manner. However, neither IFN-α/β nor TNF-α contributed to parasite control in the brain.

CONCLUSIONS

Our data indicate that innate immune TLR signals stimulate the expression of TNF-α and IFN-α/β that initiate brain invasion of T cells and trypanosomes, and control T. brucei brucei load in the brain by molecules distinct from these.

Mouse models for pathogenic African trypanosomes: unravelling the immunology of host-parasite-vector interactions

African trypanosomiasis is a parasitic disease that affects a variety of mammals, including humans, on the sub-Saharan African continent. To understand the diverse parameters that govern the host-parasite-vector interactions, mouse models for the disease have proven to be a cornerstone. Despite the fact that most trypanosomes cannot be considered natural pathogens for rodents, experimental infections in mice have shed a tremendous amount of light on the general biology of these parasites and their interaction with and evasion of the mammalian immune system. Different aspects including inflammation, vaccine failure, antigenic variation, resistance/sensitivity to normal human serum and the influence of tsetse compounds on parasite transmission have all been addressed using mouse models. In more recent years, the introduction of various 'knock-out' mouse strains has allowed to analyse the implication of various cytokines, particularly TNF, IFNγ and IL-10, in the regulation of parasitaemia and induction of pathological conditions during infection.