In vitro induction of nitric oxide synthase in astrocytes and microglia by Trypanosoma brucei brucei

Live imaging of microglia during sleeping sickness reveals early and heterogeneous inflammatory responses

Introduction

Invasion of the central nervous system (CNS) is the most serious consequence of Trypanosoma brucei infection, which causes sleeping sickness. Recent experimental data have revealed some more insights into the disease during the meningoencephalitic stage. However, detailed cellular processes befalling the CNS during the disease are poorly understood.

Methods

To further address this issue, we implanted a cranial window on the cortex of B6.129P2(Cg)-Cx3cr1tm1Litt/J mice, infected them with Trypanosoma brucei expressing RFP via intraperitoneal injection, and monitored microglial cells and parasites longitudinally over 30 days using in vivo 2-photon imaging. We correlated the observed changes with histological analyses to evaluate the recruitment of peripheral immune cells.

Results and discussion

We uncovered an early involvement of microglia that precedes invasion of the CNS by the parasite. We accomplished a detailed characterization of the progressive sequence of events that correlates with microglial morphological changes and microgliosis. Our findings unveiled a heterogeneous microglial response in places of initial homeostatic disruption near brain barriers and pointed out an exceptional capability of microglia to hamper parasite proliferation inside the brain. We also found early signs of inflammation in the meninges, which synchronize with the microglial response. Moreover, we observed a massive infiltration of peripheral immune cells into the parenchyma as a signature in the final disease stage. Overall, our study provides new insights into the host-pathogen immune interactions in the meningeal and parenchymal compartments of the neocortex.

Microglia in neuropathology caused by protozoan parasites

Involvement of the central nervous system (CNS) is the most severe consequence of some parasitic infections. Protozoal infections comprise a group of diseases that together affect billions of people worldwide and, according to the World Health Organization, are responsible for more than 500000 deaths annually. They include African and American trypanosomiasis, leishmaniasis, malaria, toxoplasmosis, and amoebiasis. Mechanisms underlying invasion of the brain parenchyma by protozoa are not well understood and may depend on parasite nature: a vascular invasion route is most common. Immunosuppression favors parasite invasion into the CNS and therefore the host immune response plays a pivotal role in the development of a neuropathology in these infectious diseases. In the brain, microglia are the resident immune cells active in defense against pathogens that target the CNS. Beside their direct role in innate immunity, they also play a principal role in coordinating the trafficking and recruitment of other immune cells from the periphery to the CNS. Despite their evident involvement in the neuropathology of protozoan infections, little attention has given to microglia-parasite interactions. This review describes the most prominent features of microglial cells and protozoan parasites and summarizes the most recent information regarding the reaction of microglial cells to parasitic infections. We highlight the involvement of the periphery-brain axis and emphasize possible scenarios for microglia-parasite interactions.

Morphological changes, nitric oxide production, and phagocytosis are triggered in vitro in microglia by bloodstream forms of Trypanosoma brucei

The flagellated parasite Trypanosoma brucei is the causative agent of Human African Trypanosomiasis (HAT). By a mechanism not well understood yet, trypanosomes enter the central nervous system (CNS), invade the brain parenchyma, and cause a fatal encephalopathy if is not treated. Trypanosomes are fast dividing organisms that, without any immune response, would kill the host in a short time. However, infected individuals survive either 6-12 months or more than 3 years for the acute and chronic forms, respectively. Thus, only when the brain defense collapses a lethal encephalopathy will occur. Here, we evaluated interactions between trypanosomes and microglial cells, which are the primary immune effector cells within the CNS. Using co-cultures of primary microglia and parasites, we found clear evidences of trypanosome phagocytosis by microglial cells. Microglia activation was also evident; analysis of its ultrastructure showed changes that have been reported in activated microglia undergoing oxidative stress caused by infections or degenerative diseases. Accordingly, an increase of the nitric oxide production was detected in supernatants of microglia/parasite co-cultures. Altogether, our results demonstrate that microglial cells respond to the presence of the parasite, leading to parasite's engulfment and elimination.

Michel Dumas and Pierre-Marie Preux: promoting tropical neurology silently--the gist of their contributions

This article presents the contribution by two senior French Neurologists over the past three decades in building, developing and promoting 'tropical neurology' in a number of neglected countries of Asia, Africa and Latin America. It talks about the 'human, dedicational and contributive value' of these two experts who do not come from an English-speaking world. It highlights meaningful changes that have been achieved in different tropical countries as a result of their direct contribution. This overview may likely be a cause for learning and motivation to others to really work in and for tropical countries, where a large proportion of global health burden is to be found.

Cerebral and peripheral changes occurring in nitric oxide (NO) synthesis in a rat model of sleeping sickness: identification of brain iNOS expressing cells

BACKGROUND

The implication of nitric oxide (NO) in the development of human African trypanosomiasis (HAT) using an animal model, was examined. The manner by which the trypanocidal activity of NO is impaired in the periphery and in the brain of rats infected with Trypanosoma brucei brucei (T. b. brucei) was analyzed through: (i) the changes occurring in NO concentration in both peripheral (blood) and cerebral compartments; (ii) the activity of nNOS and iNOS enzymes; (iii) identification of the brain cell types in which the NO-pathways are particularly active during the time-course of the infection.

METHODOLOGY/PRINCIPAL FINDINGS

NO concentration (direct measures by voltammetry) was determined in central (brain) and peripheral (blood) compartments in healthy and infected animals at various days post-infection: D5, D10, D16 and D22. Opposite changes were observed in the two compartments. NO production increased in the brain (hypothalamus) from D10 (+32%) to D16 (+71%), but decreased in the blood from D10 (-22%) to D16 (-46%) and D22 (-60%). In parallel with NO measures, cerebral iNOS activity increased and peaked significantly at D16 (up to +700%). However, nNOS activity did not vary. Immunohistochemical staining confirmed iNOS activation in several brain regions, particularly in the hypothalamus. In peritoneal macrophages, iNOS activity decreased from D10 (-83%) to D16 (-65%) and D22 (-74%) similarly to circulating NO.

CONCLUSION/SIGNIFICANCE

The NO changes observed in our rat model were dependent on iNOS activity in both peripheral and central compartments. In the periphery, the NO production decrease may reflect an arginase-mediated synthesis of polyamines necessary to trypanosome growth. In the brain, the increased NO concentration may result from an enhanced activity of iNOS present in neurons and glial cells. It may be regarded as a marker of deleterious inflammatory reactions.

Endothelial cell activation in the presence of African trypanosomes

During human African trypanosomiasis, trypanosomes (Trypanosoma brucei gambiense or T. b. rhodesiense) invade the central nervous system (CNS). Mechanisms of blood-brain barrier and blood-cerebrospinal fluid barrier leakage remain unknown. To better understand the relationships between trypanosomes and endothelial cells, the principal cell population of those barriers, we cultured a human bone marrow endothelial cell (HBMEC) line in the presence or absence of T. b. gambiense, to study cell activation. As indicated by NF-kappaB translocation to the nucleus, cells were activated in the presence of trypanosomes. The expression of the adhesion molecules ICAM-1, E-selectin and VCAM-1 increased in co-culture. The parasites induced the synthesis of the pro-inflammatory cytokines TNF-alpha, IL-6 and IL-8, and of nitric oxide (NO) by HBMEC. Cells were also cultured in the presence of parasite variant surface glycoproteins (VSGs), and an increase in TNF-alpha, IL-6, IL-8, and NO synthesis was also observed. Soluble VSGs induced NF-kappaB translocation, and the expression of adhesion molecules, indicating that they could possibly be the molecular soluble factor responsible for endothelial cell activation. The permeability coefficient of HBMEC layer increased when cells were cultured in the presence of trypanosomes, parasite culture supernatant, or VSGs. Thus, T. b. gambiense can activate endothelial cells in vitro, through the release of soluble activating factors. Consequences of endothelial cell activation by parasite products may include a potentiation of the inflammatory reaction, leukocyte recruitment, passage of trypanosomes into the CNS, and barrier dysfunction observed during CNS involvement of HAT.

Role of microglia in central nervous system infections

The nature of microglia fascinated many prominent researchers in the 19th and early 20th centuries, and in a classic treatise in 1932, Pio del Rio-Hortega formulated a number of concepts regarding the function of these resident macrophages of the brain parenchyma that remain relevant to this day. However, a renaissance of interest in microglia occurred toward the end of the 20th century, fueled by the recognition of their role in neuropathogenesis of infectious agents, such as human immunodeficiency virus type 1, and by what appears to be their participation in other neurodegenerative and neuroinflammatory disorders. During the same period, insights into the physiological and pathological properties of microglia were gained from in vivo and in vitro studies of neurotropic viruses, bacteria, fungi, parasites, and prions, which are reviewed in this article. New concepts that have emerged from these studies include the importance of cytokines and chemokines produced by activated microglia in neurodegenerative and neuroprotective processes and the elegant but astonishingly complex interactions between microglia, astrocytes, lymphocytes, and neurons that underlie these processes. It is proposed that an enhanced understanding of microglia will yield improved therapies of central nervous system infections, since such therapies are, by and large, sorely needed.

Enhanced endothelial nitric oxide-synthase activity in mice infected with Trypanosoma brucei

Infection of humans with Trypanosoma brucei causes sleeping sickness, which is invariably fatal if left untreated. The course of infection is characterised, among others, by multiple organ damage including cardiovascular dysfunctions such as hypotension and breakdown of the blood-brain barrier. The latter eventually leads to the parasite invasion into central nervous system and ultimately to the death of the patient. Nitric oxide (NO) synthesised from L-arginine via endothelial NO-synthase (eNOS) is involved in the control of vascular tone and permeability. The present study explores the effect of T. brucei infection on the endothelium-dependent in vitro vasomotor response of isolated mouse aortas. Aorta rings were suspended in organ chambers for isometric tension recording. The endothelium-dependent NO-mediated relaxation in response to acetylcholine (10(-9) to 10(-5) M) was markedly enhanced in the infected mice compared to controls (P<0.05), whereas the endothelium-independent vasodilation to an exogenous NO-donor, sodium nitroprusside, was comparable in both groups. Norepinephrine-stimulated contraction was also comparable in the absence or presence of the NO-synthase inhibitor N(omega)-Nitro-L-arginine methyl ester (L-NAME; 10(-4)M) in both groups. The enhanced endothelium-dependent relaxation in the infected mice correlated well with a 3.5-fold increase in eNOS protein level in these aortas as compared to those of control mice (P=0.05). Thus, T. brucei infection enhances eNOS protein expression in the endothelium, causing a pronounced vasodilation. Overproduction of NO in trypanosomiasis may be involved in the observed generalised hypotension and in an increased vascular permeability that facilitates T. brucei invasion into surrounding tissues and its penetration into the central nervous system in later phases of infection.

In vitro induction of microglial and endothelial cell apoptosis by cerebrospinal fluids from patients with human African trypanosomiasis

In human African trypanosomiasis, trypanosomes first develop in the blood and lymph (Stage 1), then spread to the central nervous system (CNS) (Stage 2). Disruption of the blood-brain barrier of unknown mechanism occurs in Stage 2 disease. The hypothesis that cerebrospinal fluids (CSF) from African trypanosomiasis patients might contain factor(s) able to induce apoptosis in endothelial cells led us to evaluate this effect by two methods, the TdT-mediated dUTP nick end labelling (TUNEL) method and the measurement of soluble nucleosomes released by apoptotic cells in culture supernatant by ELISA. Apoptosis induction by CSF was also studied with microglial cells, the resident macrophages in the brain, which participate in the blood-brain barrier in the perivascular area. In contrast with control CSF, African trypanosomiasis patients' CSF induced apoptosis in both microglial and endothelial cells. The results obtained with the two methods correlated well, and showed that Stage 2 CSF induced apoptosis at higher levels in microglial cells, whereas the disease stage was not decisive for apoptosis induction in endothelial cells. We measured soluble Fas ligand (sFasL) and anti-Fas antibodies levels, two potent inducers of the Fas signalling pathway leading to apoptosis, in CSF from African trypanosomiasis patients and controls. CSF from African trypanosomiasis patients contained sFasL, and anti-Fas antibodies at higher levels than in controls. Stage 2 CSF contained more sFasL than Stage 1 CSF, and anti-Fas antibodies were detected only in Stage 2 CSF. Caspase-8 inhibitor effect and statistical data suggest that other pro-apoptotic factors may be involved in some CSF-induced apoptosis. Apoptosis induction may participate in the pathogenesis during African trypanosomiasis, and the presence of sFasL and anti-Fas antibodies may provide new tools for diagnosis and prognosis of the disease.

Disruption of circadian rhythms in synaptic activity of the suprachiasmatic nuclei by African trypanosomes and cytokines

Disturbances in biological rhythms pose a major disease problem, not the least in the aging population. Experimental sleeping sickness, caused by Trypanosoma brucei brucei, in rats constitutes a unique and robust chronic model for studying mechanisms of such disturbances. The spontaneous postsynaptic activity was recorded in slice preparations of the suprachiasmatic nuclei (SCN), which contain the master pacemaker for circadian rhythms in mammals, from trypanosome-infected rats. The excitatory synaptic events, which in normal rats show a daily variation, were reduced in frequency, while the inhibitory synaptic events did not significantly differ. This indicates selective disturbances in glutamate receptor-mediated neurotransmission in the SCN. Treatment with interferon-gamma in combination with lipopolysaccharide, which has synergistic actions with cytokines, and tumor necrosis factor-alpha similarly caused a reduction in excitatory synaptic SCN activity. We suggest that changes in the synaptic machinery of SCN neurons play an important pathogenetic role in sleeping sickness, and that proinflammatory cytokines can mimic these changes.

Microglia in neurodegeneration: molecular aspects

Inflammatory events in the CNS are associated with injuries as well as with well-known chronic degenerative diseases, such as Multiple Sclerosis, Parkinson's, or Alzheimer's disease. Compared to inflammation in peripheral tissues, inflammation in brain appears to follow distinct pathways and time-courses, which likely has to do with a relatively strong immunosuppression in that organ. For this reason, it is of great importance to get insights into the molecular mechanism governing immune reactions in brain tissue. This task is hard to achieve in vivo, but can be approached by studying the major cell type responsible for brain inflammation, the microglia, in culture. Since these cells are the only professional antigen-presenting cells resident in brain parenchyma, molecular mechanisms of antigen presentation are being discussed first. After covering the expression and regulation of anti- and proinflammatory cytokines, induction and regulation of two key enzymes and their products-COX-2 and iNOS-are summarized. Possibly, pivotal molecular targets for drug therapies of brain disorders will be discovered in intracellular signaling pathways leading to activation of transcription factors. Finally, the impact of growth factors, of neurotrophins in particular, is highlighted. It is concluded that the presently available data on the molecular level is far from being statisfying, but that only from better insights into molecular events will we obtain the information required for more specific therapies.