Neural Damage in Experimental Trypanosoma brucei gambiense Infection: Hypothalamic Peptidergic Sleep and Wake-Regulatory Neurons

Application of Light-Sheet Mesoscopy to Image Host-Pathogen Interactions in Intact Organs

Human African Trypanosomiasis (HAT) is a disease caused by the extracellular parasite Trypanosoma brucei that affects the central nervous system (CNS) during the chronic stage of the infection, inducing neuroinflammation, coma, and death if left untreated. However, little is known about the structural change happening in the brain as result of the infection. So far, infection-induced neuroinflammation has been observed with conventional methods, such as immunohistochemistry, electron microscopy, and 2-photon microscopy only in small portions of the brain, which may not be representative of the disease. In this paper, we have used a newly-developed light-sheet illuminator to image the level of neuroinflammation in chronically infected mice and compared it to naïve controls. This system was developed for imaging in combination with the Mesolens objective lens, providing fast sub-cellular resolution for tens of mm3-large imaging volumes. The mouse brain specimens were cleared using CUBIC+, followed by antibody staining to locate Glial Fibrillary Acid Protein (GFAP) expressing cells, primarily astrocytes and ependymocytes, used here as a proxy for cell reactivity and gliosis. The large capture volume allowed us to detect GFAP+ cells and spatially resolve the response to T. brucei infection. Based on morphometric analyses and spatial distribution of GFAP+ cells, our data demonstrates a significant increase in cell dendrite branching around the lateral ventricle, as well as dorsal and ventral third ventricles, that are negatively correlated with the branch extension in distal sites from the circumventricular spaces. To our knowledge, this is the first report highlighting the potential of light-sheet mesoscopy to characterise the inflammatory responses of the mouse brain to parasitic infection at the cellular level in intact cleared organs, opening new avenues for the development of new mesoscale imaging techniques for the study of host-pathogen interactions.

Plasma Neuron-Specific Enolase is not a reliable biomarker for staging Trypanosoma brucei rhodesiense sleeping sickness patients

Objective

Currently, the only available staging criterion for T. b. rhodesiense requires a lumber puncture to collect and later examine cerebrospinal fluid (CSF). This study examined the potential of plasma Neuron-Specific Enolase (NSE) in discriminating between early and late-stage patients.

Results

When median NSE levels were compared between early and late-stage patients, results showed a significant (P < 0.02) upregulation among late-stage patients (599.8 ng/mL). No significant differences (P > 0.9) in NSE levels were observed between early-stage patients (300 ng/mL) and controls (454 ng/mL). We used Receiver Operator Characteristic (ROC) curves to explore the likelihood of using plasma NSE as a potential stage biomarker in discriminating between early and late-stage HAT patients. Our results showed that NSE demonstrated an area under the curve (AUC) of 0.702 (95% CI 0.583–0.830). A high staging accuracy for NSE was obtained by using a cutoff of > 346.5 ng/mL with a sensitivity of 68.6% (95% CI 55–79.7%) and a specificity of 93.3% (95% CI 70.2–99.7%). Although our results demonstrate that plasma NSE is upregulated in T. b. rhodesiense sleeping sickness patients, its value in discriminating between late and early-stage patients is limited. However, future studies could consider improving its specificity by combining it with other identified plasma biomarkers.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13104-022-05981-w.

Role of T cells during the cerebral infection with Trypanosoma brucei

The infection by Trypanosoma brucei brucei (T.b.b.), a protozoan parasite, is characterized by an early-systemic stage followed by a late stage in which parasites invade the brain parenchyma in a T cell-dependent manner.

Here we found that early after infection effector-memory T cells were predominant among brain T cells, whereas, during the encephalitic stage T cells acquired a tissue resident memory phenotype (T_RM) and expressed PD1. Both CD4 and CD8 T cells were independently redundant for the penetration of T.b.b. and other leukocytes into the brain parenchyma. The role of lymphoid cells during the T.b.b. infection was studied by comparing T- and B-cell deficient rag1^-/- and WT mice. Early after infection, parasites located in circumventricular organs, brain structures with increased vascular permeability, particularly in the median eminence (ME), paced closed to the sleep-wake regulatory arcuate nucleus of the hypothalamus (Arc). Whereas parasite levels in the ME were higher in rag1^-/- than in WT mice, leukocytes were instead reduced. Rag1^-/- infected mice showed increased levels of meca32 mRNA coding for a blood /hypothalamus endothelial molecule absent in the blood-brain-barrier (BBB). Both immune and metabolic transcripts were elevated in the ME/Arc of WT and rag1^-/- mice early after infection, except for ifng mRNA, which levels were only increased in WT mice. Finally, using a non-invasive sleep-wake cycle assessment method we proposed a putative role of lymphocytes in mediating sleep alterations during the infection with T.b.b.

Thus, the majority of T cells in the brain during the early stage of T.b.b. infection expressed an effector-memory phenotype while T_RM cells developed in the late stage of infection. T cells and parasites invade the ME/Arc altering the metabolic and inflammatory responses during the early stage of infection and modulating sleep disturbances.

Trypanosoma brucei (T.b.) causes an early systemic and a late encephalitic infection characterized by sleep alterations. In rodent models, brain invasion by T.b. brucei (T.b.b.) is strictly dependent on T cells. However, an in-depth characterization of T cell functions and phenotypes in the outcome of T.b.b. infection is still lacking.

Here we found that during the early stage of infection of mice, most brain T cells differentiated into memory cells, and acquired a tissue-resident memory phenotype during the encephalitic stage. CD4 and CD8 T cells were redundant for the invasion of other T cells and parasites into the brain. Early after infection T.b.b. and leukocytes invade different circumventricular organs (brain areas that lack a blood-brain barrier) including the median eminence (ME) located close to sleep-regulating arcuate nucleus (Arc). T.b.b. infection induced the expression of immune and metabolic molecules in this area. Lymphocytes modulated 1) the levels of invading parasites and leukocytes in the ME; 2) the structure of the blood/ hypothalamus interphase and 3) the expression of IFN-γ in the ME/Arc early after infection. Lymphocytes may also be involved in the regulation of sleep alterations observed in African trypanosomiasis.

Neuropathogenesis caused by Trypanosoma brucei, still an enigma to be unveiled

Trypanosoma brucei is one of the protozoa parasites that can enter the brain and cause injury associated with toxic effects of parasite-derived molecules or with immune responses against infection. Other protozoa parasites with brain tropism include Toxoplasma, Plasmodium, Amoeba, and, eventually, other Trypanosomatids such as T. cruzi and Leishmania. Together, these parasites affect billions of people worldwide and are responsible for more than 500.000 deaths annually. Factors determining brain tropism, mechanisms of invasion as well as processes ongoing inside the brain are not well understood. But, they depend on the parasite involved. The pathogenesis caused by T. brucei initiates locally in the area of parasite inoculation, soon trypanosomes rich the blood, and the disease enters in the so-called early stage. The pathomechanisms in this phase have been described, even molecules used to combat the disease are effective during this period. Later, the disease evolves towards a late-stage, characterized by the presence of parasites in the central nervous system (CNS), the so-called meningo-encephalitic stage. This phase of the disease has not been sufficiently examined and remains a matter of investigation. Here, I stress the importance of delve into the study of the neuropathogenesis caused by T. brucei, which will enable the identification of pathways that may be targeted to overcome parasites that reached the CNS. Finally, I highlight the impact that the application of tools developed in the last years in the field of neuroscience will have on the study of neglected tropical diseases.

Sleeping Sickness Disrupts the Sleep-Regulating Adenosine System

Patients with sleeping sickness, caused by the parasite Trypanosoma brucei, have disruptions in both sleep timing and sleep architecture. However, the underlying cause of these sleep disturbances is not well understood. Here, we assessed the sleep architecture of male mice infected with T. brucei and found that infected mice had drastically altered sleep patterns. Interestingly, T. brucei-infected mice also had a reduced homeostatic sleep response to sleep deprivation, a response modulated by the adenosine system. We found that infected mice had a reduced electrophysiological response to an adenosine receptor antagonist and increased adenosine receptor gene expression. Although the mechanism by which T. brucei infection causes these changes remains to be determined, our findings suggest that the symptoms of sleeping sickness may be because of alterations in homeostatic adenosine signaling.SIGNIFICANCE STATEMENT Sleeping sickness is a fatal disease that disrupts the circadian clock, causes disordered temperature regulation, and induces sleep disturbance. To examine the neurologic effects of infection in the absence of other symptoms, in this study, we used a mouse model of sleeping sickness in which the acute infection was treated but brain infection remained. Using this model, we evaluated the effects of the sleeping sickness parasite, Trypanosoma brucei, on sleep patterns in mice, under both normal and sleep-deprived conditions. Our findings suggest that signaling of adenosine, a neuromodulator involved in mediating homeostatic sleep drive, may be reduced in infected mice.

Sleep and brain infections

Sleep is frequently altered in systemic infections as a component of sickness behavior in response to inflammation. Sleepiness in sickness behavior has been extensively investigated. Much less attention has instead been devoted to sleep and wake alterations in brain infections. Most of these, as other neuroinfections, are prevalent in sub-Saharan Africa. The present overview highlights the importance of this topic from both the clinical and pathogenetic points of view. Vigilance states and their regulation are first summarized, emphasizing that key nodes in this distributed brain system can be targeted by neuroinflammatory signaling. Sleep-wake changes in the parasitic disease human African trypanosomiasis (HAT) and its animal models are then reviewed and discussed. Experimental data have revealed that the suprachiasmatic nucleus, the master circadian pacemaker, and peptidergic cell populations of the lateral hypothalamus (the wake-promoting orexin neurons and the sleep-promoting melanin-concentrating hormone neurons) are targeted by African trypanosome infection. It is then discussed how prominent and disturbing are sleep changes in HIV/AIDS, also when the infection is cured with antiretroviral therapy. This recalls attention on the bidirectional interactions between sleep and immune system, including the specialized brain immune response of which microglial cells are protagonists. Sleep changes in an ancient viral disease, rabies, and in the emerging infection due to Zika virus which causes a congenital syndrome, are also dealt with. Altogether the findings indicate that sleep-wake regulation is targeted by brain infections caused by different pathogens and, although the relevant pathogenetic mechanisms largely remain to be clarified, these alterations differ from hypersomnia occurring in sickness behavior. Thus, brain infections point to the vulnerability of the neural network of sleep-wake regulation as a highly relevant clinical and basic science challenge.