Malaria Related Neurocognitive Deficits and Behavioral Alterations

Lymphatic network drainage resolves cerebral edema and facilitates recovery from experimental cerebral malaria

While brain swelling, associated with fluid accumulation, is a known feature of pediatric cerebral malaria (CM), how fluid and macromolecules are drained from the brain during recovery from CM is unknown. Using the experimental CM (ECM) model, we show that fluid accumulation in the brain during CM is driven by vasogenic edema and not by perivascular cerebrospinal fluid (CSF) influx. We identify that fluid and molecules are removed from the brain extremely quickly in mice with ECM to the deep cervical lymph nodes (dcLNs), predominantly through basal routes and across the cribriform plate and the nasal lymphatics. In agreement, we demonstrate that ligation of the afferent lymphatic vessels draining to the dcLNs significantly impairs fluid drainage from the brain and lowers anti-malarial drug recovery from the ECM syndrome. Collectively, our results provide insight into the pathways that coordinate recovery from CM.

Neurons upregulate PD-L1 via IFN/STAT1/IRF1 to alleviate damage by CD8+ T cells in cerebral malaria

BACKGROUND

Cerebral malaria (CM) is the most lethal complication of malaria, and survivors usually endure neurological sequelae. Notably, the cytotoxic effect of infiltrating Plasmodium-activated CD8+ T cells on cerebral microvasculature endothelial cells is a prominent feature of the experimental CM (ECM) model with blood-brain barrier disruption. However, the damage effect of CD8+ T cells infiltrating the brain parenchyma on neurons remains unclear. Based on the immunosuppressive effect of the PD-1/PD-L1 pathway on T cells, our previous study demonstrated that the systemic upregulation of PD-L1 to inhibit CD8+ T cell function could effectively alleviate the symptoms of ECM mice. However, it has not been reported whether neurons can suppress the pathogenic effect of CD8+ T cells through the PD-1/PD-L1 negative immunomodulatory pathway. As the important inflammatory factor of CM, interferons can induce the expression of PD-L1 via different molecular mechanisms according to the neuro-immune microenvironment. Therefore, this study aimed to investigate the direct interaction between CD8+ T cells and neurons, as well as the mechanism of neurons to alleviate the pathogenic effect of CD8+ T cells through up-regulating PD-L1 induced by IFNs.

METHODS

Using the ECM model of C57BL/6J mice infected with Plasmodium berghei ANKA (PbA), morphological observations were conducted in vivo by electron microscope and IF staining. The interaction between the ECM CD8+ T cells (immune magnetic bead sorting from spleen of ECM mice) and primary cultured cortical neurons in vitro was observed by IF staining and time-lapse photography. RNA-seq was performed to analyze the signaling pathway of PD-L1 upregulation in neurons induced by IFNβ or IFNγ, and verified through q-PCR, WB, IF staining, and flow cytometry both in vitro and in vivo using IFNAR or IFNGR gene knockout mice. The protective effect of adenovirus-mediated PD-L1 IgGFc fusion protein expression was verified in ECM mice with brain stereotaxic injection in vivo and in primary cultured neurons via viral infection in vitro.

RESULTS

In vivo, ECM mice showed infiltration of activated CD8+ T cells and neuronal injury in the brain parenchyma. In vitro, ECM CD8+ T cells were in direct contact with neurons and induced axonal damage, as an active behavior. The PD-L1 protein level was elevated in neurons of ECM mice and in primary cultured neurons induced by IFNβ, IFNγ, or ECM CD8+ T cells in vitro. Furthermore, the IFNβ or IFNγ induced neuronal expression of PD-L1 was mediated by increasing STAT1/IRF1 pathway via IFN receptors. The increase of PD-L1 expression in neurons during PbA infection was weakened after deleting the IFNAR or IFNGR. Increased PD-L1 expression by adenovirus partially protected neurons from CD8+ T cell-mediated damage both in vitro and in vivo.

CONCLUSION

Our study demonstrates that both type I and type II IFNs can induce neurons to upregulate PD-L1 via the STAT1/IRF1 pathway mediated by IFN receptors to protect against activated CD8+ T cell-mediated damage, providing a targeted pathway to alleviate neuroinflammation during ECM.

Neurofilament Light Chain as a Biomarker of Neuronal Damage in Children With Malaria

Malaria can cause brain injury. Neurofilament light chain (NfL) is a biomarker of neuronal damage. Here we examined longitudinal plasma NfL levels in children aged 1-12 years with uncomplicated and severe malaria from Mozambique. NfL levels were similar in all malaria cases at hospital admission. However, levels increased over time and the increment was significantly higher in severe malaria cases with neurological manifestations (ie, coma, impaired consciousness, or repeated seizures). NfL may be useful to identify and quantify brain injury in malaria.

Human ApoE2 protects mice against Plasmodium berghei ANKA experimental cerebral malaria

Cerebral malaria (CM) is a severe neurological complication of Plasmodium falciparum infection with acute brain lesions. Genetic variations in both host and parasite have been associated with susceptibility to CM, but the underlying molecular mechanism remains unclear. Here, we demonstrate that variants of human apolipoprotein E (hApoE) impact the outcome of Plasmodium berghei ANKA (PbA)-induced experimental cerebral malaria (ECM). Mice carrying the hApoE2 isoform have fewer intracerebral hemorrhages and are more resistant to ECM than mice bearing the hApoE3, hApoE4, or endogenous murine ApoE (mApoE). hApoE2 mice infected with PbA showed increased splenomegaly and IFN-γ levels in serum but reduced cerebral cell apoptosis that correlated with the survival advantage against ECM. In addition, upregulated expression of genes associated with lipid metabolism and downregulated expression of genes linked to immune responses were observed in the brain tissue of hApoE2 mice relative to ECM-susceptible mice after PbA infection. Notably, serum cholesterol and the cholesterol content of brain-infiltrating CD8+ T cells are significantly higher in infected hApoE2 mice, which might contribute to a significant reduction in the sequestration of brain CD8+ T cells. Consistent with the finding that fewer brain lesions occurred in infected hApoE2 mice, fewer behavioral deficits were observed in the hApoE2 mice. Finally, a meta-analysis of publicly available data also showed an increased hApoE2 allele in the malaria-endemic African population, suggesting malaria selection. This study shows that hApoE2 protects mice from ECM through suppression of CD8+ T cell activation and migration to the brain and enhanced cholesterol metabolism.IMPORTANCECerebral malaria (CM) is the deadliest complication of malaria infection with an estimated 15%-25% mortality. Even with timely and effective treatment with antimalarial drugs such as quinine and artemisinin derivatives, survivors of CM may suffer long-term cognitive and neurological impairment. Here, we show that human apolipoprotein E variant 2 (hApoE2) protects mice from experimental CM (ECM) via suppression of CD8+ T cell activation and infiltration to the brain, enhanced cholesterol metabolism, and increased IFN-γ production, leading to reduced endothelial cell apoptosis, BBB disruption, and ECM symptoms. Our results suggest that hApoE can be an important factor for risk assessment and treatment of CM in humans.

Association of severe malaria with cognitive and behavioural outcomes in low- and middle-income countries: a meta-analysis and systematic review

BACKGROUND

Malaria affects 24 million children globally, resulting in nearly 500,000 child deaths annually in low- and middle-income countries (LMICs). Recent studies have provided evidence that severe malaria infection results in sustained impairment in cognition and behaviour among young children; however, a formal meta-analysis has not been published. The objective was to assess the association between severe malaria infection with cognitive and behavioural outcomes among children living in LMICs.

METHODS

Six online bibliographic databases were searched and reviewed in November 2022. Studies included involved children < 18 years of age living in LMICs with active or past severe malaria infection and measured cognitive and/or behaviour outcomes. The quality of studies was assessed. Definitions of severe malaria included cerebral malaria, severe malarial anaemia, and author-defined severe malaria. Results from all studies were qualitatively summarized. For studies with relevant data on attention, learning, memory, language, internalizing behaviour and externalizing behaviour, results were pooled and a meta-analysis was performed. A random-effects model was used across included cohorts, yielding a standardized mean difference between the severe malaria group and control group.

RESULTS

Out of 3,803 initial records meeting the search criteria, 24 studies were included in the review, with data from 14 studies eligible for meta-analysis inclusion. Studies across sub-Saharan Africa assessed 11 cohorts of children from pre-school to school age. Of all the studies, composite measures of cognition were the most affected areas of development. Overall, attention, memory, and behavioural problems were domains most commonly found to have lower scores in children with severe malaria. Meta-analysis revealed that children with severe malaria had worse scores compared to children without malaria in attention (standardized mean difference (SMD) -0.68, 95% CI -1.26 to -0.10), memory (SMD -0.52, 95% CI -0.99 to -0.06), and externalizing behavioural problems (SMD 0.45, 95% CI 0.13-0.78).

CONCLUSION

Severe malaria is associated with worse neuropsychological outcomes for children living in LMICs, specifically in attention, memory, and externalizing behaviours. More research is needed to identify the long-term implications of these findings. Further interventions are needed to prevent cognitive and behavioural problems after severe malaria infection.

TRIAL REGISTRATION

This systematic review was registered under PROSPERO: CRD42020154777.

Increased Neutrophil Percentage and Neutrophil-T Cell Ratio Precedes Clinical Onset of Experimental Cerebral Malaria

Newly emerging data suggest that several neutrophil defense mechanisms may play a role in both aggravating and protecting against malaria. These exciting findings suggest that the balance of these cells in the host body may have an impact on the pathogenesis of malaria. To fully understand the role of neutrophils in severe forms of malaria, such as cerebral malaria (CM), it is critical to gain a comprehensive understanding of their behavior and functions. This study investigated the dynamics of neutrophil and T cell responses in C57BL/6 and BALB/c mice infected with Plasmodium berghei ANKA, murine models of experimental cerebral malaria (ECM) and non-cerebral experimental malaria, respectively. The results demonstrated an increase in neutrophil percentage and neutrophil-T cell ratios in the spleen and blood before the development of clinical signs of ECM, which is a phenomenon not observed in the non-susceptible model of cerebral malaria. Furthermore, despite the development of distinct forms of malaria in the two strains of infected animals, parasitemia levels showed equivalent increases throughout the infection period evaluated. These findings suggest that the neutrophil percentage and neutrophil-T cell ratios may be valuable predictive tools for assessing the dynamics and composition of immune responses involved in the determinism of ECM development, thus contributing to the advancing of our understanding of its pathogenesis.

Interplay Between the Immune and Nervous Cognitive Systems in Homeostasis and in Malaria

The immune and nervous systems can be thought of as cognitive and plastic systems, since they are both involved in cognition/recognition processes and can be architecturally and functionally modified by experience, and such changes can influence each other's functioning. The immune system can affect nervous system function depending on the nature of the immune stimuli and the pro/anti-inflammatory responses they generate. Here we consider interactions between the immune and nervous systems in homeostasis and disease, including the beneficial and deleterious effects of immune stimuli on brain function and the impact of severe and non-severe malaria parasite infections on neurocognitive and behavioral parameters in human and experimental murine malaria. We also discuss the effect of immunization on the reversal of cognitive deficits associated with experimental non-severe malaria in a model susceptible to the development of the cerebral form of the illness. Finally, we consider the possibility of using human vaccines, largely exploited as immune-prophylactics for infectious diseases, as therapeutic tools to prevent or mitigate the expression of cognitive deficits in infectious and chronic degenerative diseases.

Dynamics and immunomodulation of cognitive deficits and behavioral changes in non-severe experimental malaria

Data recently reported by our group indicate that stimulation with a pool of immunogens capable of eliciting type 2 immune responses can restore the cognitive and behavioral dysfunctions recorded after a single episode of non-severe rodent malaria caused by Plasmodium berghei ANKA. Here we explored the hypothesis that isolated immunization with one of the type 2 immune response-inducing immunogens, the human diphtheria-tetanus (dT) vaccine, may revert damages associated with malaria. To investigate this possibility, we studied the dynamics of cognitive deficits and anxiety-like phenotype following non-severe experimental malaria and evaluated the effects of immunization with both dT and of a pool of type 2 immune stimuli in reversing these impairments. Locomotor activity and long-term memory deficits were assessed through the open field test (OFT) and novel object recognition task (NORT), while the anxiety-like phenotype was assessed by OFT and light/dark task (LDT). Our results indicate that poor performance in cognitive-behavioral tests can be detected as early as the 12^th day after the end of antimalarial treatment with chloroquine and may persist for up to 155 days post infection. The single immunization strategy with the human dT vaccine showed promise in reversal of long-term memory deficits in NORT, and anxiety-like behavior in OFT and LDT.

Characterization of Lymphocyte Subsets in Lymph Node and Spleen Sections in Fatal Pediatric Malaria

Secondary lymphoid tissues play a major role in the human immune response to P. falciparum infection. Previous studies have shown that acute falciparum malaria is associated with marked perturbations of the cellular immune system characterized by lowered frequency and absolute number of circulating T cell subsets. A temporary relocation of T cells, possibly by infiltration to secondary lymphoid tissue, or their permanent loss through apoptosis, are two proposed explanations for this observation. We conducted the present study to determine the phenotype of lymphocyte subsets that accumulate in the lymph node and spleen during acute stages of falciparum malaria infection in Malawian children, and to test the hypothesis that lymphocytes are relocated to lymphoid tissues during acute infection. We stained tissue sections from children who had died of the two common clinical forms of severe malaria in Malawi, namely severe malarial anemia (SMA, n = 1) and cerebral malaria (CM, n = 3), and used tissue sections from pediatric patients who had died of non-malaria sepsis (n = 2) as controls. Both lymph node and spleen tissue (red pulp) sections from CM patients had higher percentages of T cells (CD4+ and CD8+) compared to the SMA patient. In the latter, we observed a higher percentage of CD20+ B cells in the lymph nodes compared to CM patients, whereas the opposite was observed in the spleen. Both lymph node and spleen sections from CM patients had increased percentages of CD69+ and CD45RO+ cells compared to tissue sections from the SMA patient. These results support the hypothesis that the relocation of lymphocytes to spleen and lymph node may contribute to the pan-lymphopenia observed in acute CM.

Treatment Reducing Endothelial Activation Protects against Experimental Cerebral Malaria

Cerebral malaria (CM) is the most severe neurological complication of malaria caused by Plasmodium falciparum infection. The available antimalarial drugs are effective at clearing the parasite, but the mortality rate remains as high as 20% of CM cases. At the vascular level, CM is characterized by endothelial activation and dysfunction. Several biomarkers of endothelial activation have been associated with CM severity and mortality, making the brain vascular endothelium a potential target for adjunctive therapies. Statins and Angiotensin II Receptor Blockers (ARBs) are drugs used to treat hypercholesterolemia and hypertension, respectively, that have shown endothelial protective activity in other diseases. Here, we used a combination of a statin (atorvastatin) and an ARB (irbesartan) as adjunctive therapy to conventional antimalarial drugs in a mouse experimental model of CM. We observed that administration of atorvastatin–irbesartan combination decreased the levels of biomarkers of endothelial activation, such as the von Willebrand factor and angiopoietin-1. After mice developed neurological signs of CM, treatment with the combination plus conventional antimalarial drugs increased survival rates of animals 3–4 times compared to treatment with antimalarial drugs alone, with animals presenting lower numbers and smaller hemorrhages in the brain. Taken together, our results support the hypothesis that inhibiting endothelial activation would greatly reduce the CM-associated pathology and mortality.