Perivascular Arrest of CD8+ T Cells Is a Signature of Experimental Cerebral Malaria

A quantitative brain map of experimental cerebral malaria pathology

The murine model of experimental cerebral malaria (ECM) has been utilised extensively in recent years to study the pathogenesis of human cerebral malaria (HCM). However, it has been proposed that the aetiologies of ECM and HCM are distinct, and, consequently, no useful mechanistic insights into the pathogenesis of HCM can be obtained from studying the ECM model. Therefore, in order to determine the similarities and differences in the pathology of ECM and HCM, we have performed the first spatial and quantitative histopathological assessment of the ECM syndrome. We demonstrate that the accumulation of parasitised red blood cells (pRBCs) in brain capillaries is a specific feature of ECM that is not observed during mild murine malaria infections. Critically, we show that individual pRBCs appear to occlude murine brain capillaries during ECM. As pRBC-mediated congestion of brain microvessels is a hallmark of HCM, this suggests that the impact of parasite accumulation on cerebral blood flow may ultimately be similar in mice and humans during ECM and HCM, respectively. Additionally, we demonstrate that cerebrovascular CD8+ T-cells appear to co-localise with accumulated pRBCs, an event that corresponds with development of widespread vascular leakage. As in HCM, we show that vascular leakage is not dependent on extensive vascular destruction. Instead, we show that vascular leakage is associated with alterations in transcellular and paracellular transport mechanisms. Finally, as in HCM, we observed axonal injury and demyelination in ECM adjacent to diverse vasculopathies. Collectively, our data therefore shows that, despite very different presentation, and apparently distinct mechanisms, of parasite accumulation, there appear to be a number of comparable features of cerebral pathology in mice and in humans during ECM and HCM, respectively. Thus, when used appropriately, the ECM model may be useful for studying specific pathological features of HCM.

Targeting Angiotensin II Type-1 Receptor (AT1R) Inhibits the Harmful Phenotype of Plasmodium-Specific CD8+ T Cells during Blood-Stage Malaria

CD8⁺ T-cell response is critical in the pathogenesis of cerebral malaria during blood-stage. Our group and other have been shown that angiotensin II (Ang II) and its receptor AT₁ (AT₁R), a key effector axis of renin-angiotensin system (RAS), have immune regulatory effects on T cells. Previously, we showed that inhibition of AT₁R signaling protects mice against the lethal disease induced by Plasmodium berghei ANKA infection However, most of the Ang II/AT₁R actions were characterized by using only pharmacological approaches, the effects of which may not always be due to a specific receptor blockade. In addition, the mechanisms of action of the AT₁R in inducing the pathogenic activity of Plasmodium-specific CD8⁺ T cells during blood-stage were not determined. Here, we examined how angiotensin II/AT₁R axis promotes the harmful response of Plasmodium-specific CD8⁺ T-cell during blood-stage by using genetic and pharmacological approaches. We evaluated the response of wild-type (WT) and AT₁R^−/−Plasmodium-specific CD8⁺ T cells in mice infected with a transgenic PbA lineage expressing ovalbumin; and in parallel infected mice receiving WT Plasmodium-specific CD8⁺ T cells were treated with losartan (AT₁R antagonist) or captopril (ACE inhibitor). Both, AT₁R^−/− OT-I cells and WT OT-I cells from losartan- or captopril-treated mice showed lower expansion, reduced IL-2 production and IL-2Rα expression, lower activation (lower expression of CD69, CD44 and CD160) and lower exhaustion profiles. AT₁R^−/− OT-I cells also exhibit lower expression of the integrin LFA-1 and the chemokine receptors CCR5 and CXCR3, known to play a key role in the development of cerebral malaria. Moreover, AT₁R^−/− OT-I cells produce lower amounts of IFN-γ and TNF-α and show lower degranulation upon restimulation. In conclusion, our results show the pivotal mechanisms of AT₁R-induced harmful phenotype of Plasmodium-specific CD8⁺ T cells during blood-stage malaria.

Neuroimaging

Imaging of infection in the CNS has been handled using cross-sectional imaging for more than two decades now resulting in a large array of descriptive diagnostic criteria, capable, in most circumstances of narrowing the differential diagnosis, detecting life-threatening complications and establishing baseline for assessment of treatment response. Limitations however exist, and in many circumstances, both cross-sectional imaging and nonspecific molecular imaging, such as ¹⁸F-FDG, fail to establish a diagnosis. The availability of pathogen-specific imaging agents/ligands would have a great effect on the management of patients with CNS infection. Besides early diagnosis, avoidance of diagnostic brain biopsies can have significant effect on the mortality and morbidity of patients.

Transcriptomic profiling of microglia reveals signatures of cell activation and immune response, during experimental cerebral malaria

Cerebral malaria is a pathology involving inflammation in the brain. There are many immune cell types activated during this process, but there is little information on the response of microglia, in this severe complication. We examined microglia by genome wide transcriptomic analysis in a model of experimental cerebral malaria (ECM), in which C57BL/6 mice are infected with Plasmodium berghei ANKA. Thousands of transcripts were differentially expressed in microglia at two different time points during infection. Proliferation of microglia was a dominant feature before the onset of ECM, and supporting this, we observed an increase in numbers of these cells in the brain. When cerebral malaria symptoms were manifest, genes involved in immune responses and chemokine production were upregulated, which were possibly driven by Type I Interferon. Consistent with this hypothesis, in vitro culture of a microglial cell line with Interferon-β, but not infected red blood cells, resulted in production of several of the chemokines shown to be upregulated in the gene expression analysis. It appears that these responses are associated with ECM, as microglia from mice infected with a mutant P. berghei parasite (ΔDPAP3), which does not cause ECM, did not show the same level of activation or proliferation.

CD8+ T Cells Induce Fatal Brainstem Pathology during Cerebral Malaria via Luminal Antigen-Specific Engagement of Brain Vasculature

Cerebral malaria (CM) is a severe complication of Plasmodium falciparum infection that results in thousands of deaths each year, mostly in African children. The in vivo mechanisms underlying this fatal condition are not entirely understood. Using the animal model of experimental cerebral malaria (ECM), we sought mechanistic insights into the pathogenesis of CM. Fatal disease was associated with alterations in tight junction proteins, vascular breakdown in the meninges / parenchyma, edema, and ultimately neuronal cell death in the brainstem, which is consistent with cerebral herniation as a cause of death. At the peak of ECM, we revealed using intravital two-photon microscopy that myelomonocytic cells and parasite-specific CD8⁺ T cells associated primarily with the luminal surface of CNS blood vessels. Myelomonocytic cells participated in the removal of parasitized red blood cells (pRBCs) from cerebral blood vessels, but were not required for the disease. Interestingly, the majority of disease-inducing parasite-specific CD8⁺ T cells interacted with the lumen of brain vascular endothelial cells (ECs), where they were observed surveying, dividing, and arresting in a cognate peptide-MHC I dependent manner. These activities were critically dependent on IFN-γ, which was responsible for activating cerebrovascular ECs to upregulate adhesion and antigen-presenting molecules. Importantly, parasite-specific CD8⁺ T cell interactions with cerebral vessels were impaired in chimeric mice rendered unable to present EC antigens on MHC I, and these mice were in turn resistant to fatal brainstem pathology. Moreover, anti-adhesion molecule (LFA-1 / VLA-4) therapy prevented fatal disease by rapidly displacing luminal CD8⁺ T cells from cerebrovascular ECs without affecting extravascular T cells. These in vivo data demonstrate that parasite-specific CD8⁺ T cell-induced fatal vascular breakdown and subsequent neuronal death during ECM is associated with luminal, antigen-dependent interactions with cerebrovasculature.

Cerebral malaria (CM) is a severe and potentially fatal complication of malaria in humans that results in swelling and bleeding within the brain. The mechanisms that cause this fatal disease in humans are not completely understood. We studied an animal model known as experimental cerebral malaria to learn more about the factors that drive this disease process. Using a technique referred to as intravital microscopy, we captured movies of immune cells operating in the living brain as the disease developed. At the peak of disease, we observed evidence of immune cells interacting with and aggregating along blood vessels throughout the brain. These interactions were directly associated vascular leakage. This caused the brain to swell, which gave rise to an unsustainable pressure that ultimately killed neurons responsible for heart and lung function. The fatal swelling was induced by immune cells (referred to as T cells) interacting with bits of parasite presented by blood vessels in the brain. Removal of this parasite presentation protected the mice from fatal disease. We also evaluated a straightforward therapy that involved intravenous administration of antibodies that interfered with T cell sticking to blood vessels. Our movies revealed that this therapeutic approach rapidly displaced T cells from the blood vessels in the brain and prevented fatal disease.

Uptake of parasite-derived vesicles by astrocytes and microglial phagocytosis of infected erythrocytes may drive neuroinflammation in cerebral malaria

Astrocytes and microglia are activated during cerebral malaria (CM) and contribute to the production and release of several mediators during neuroinflammatory processes. Whether these changes are the consequence of a direct crosstalk between glial cells and the malarial parasite and how these cells participate in the pathogenesis of CM is not yet clear. We therefore examined the interaction of astrocytes and microglia with Plasmodium berghei ANKA-infected red blood cells using primary cell cultures derived from newborn C57BL/6 mice. We observed a dynamic transfer of vesicles from the parasite to astrocytes within minutes of contact, and the phagocytosis of infected red blood cells by microglia. Differential gene expression studies using the Affymetrix GeneChip^® microarray, and quantitative PCR analyses showed the increase in expression of the set of genes belonging to the immune response network in parasite activated astrocytes and microglia. Interestingly, expression of these genes was also significantly upregulated in brains of mice dying from CM compared with uninfected mice or infected mice that did not develop the neuropathology. Accumulation of parasite-derived vesicles within astrocytes, and the phagocytosis of infected red blood cells by microglia induced a subsequent increase in interferon gamma inducible protein 10 (IP10) in both the brain and plasma of infected mice at the onset of CM, confirming a role for this molecule in CM pathogenesis. Altogether, these observations point to a possible role for glial cells in the neuropathological processes leading to CM. GLIA 2016 GLIA 2017;65:75-92.

Neutrophil recruitment limited by high-affinity bent β2 integrin binding ligand in cis

Neutrophils are essential for innate immunity and inflammation and many neutrophil functions are β2 integrin-dependent. Integrins can extend (E(+)) and acquire a high-affinity conformation with an 'open' headpiece (H(+)). The canonical switchblade model of integrin activation proposes that the E(+) conformation precedes H(+), and the two are believed to be structurally linked. Here we show, using high-resolution quantitative dynamic footprinting (qDF) microscopy combined with a homogenous conformation-reporter binding assay in a microfluidic device, that a substantial fraction of β2 integrins on human neutrophils acquire an unexpected E(-)H(+) conformation. E(-)H(+) β2 integrins bind intercellular adhesion molecules (ICAMs) in cis, which inhibits leukocyte adhesion in vitro and in vivo. This endogenous anti-inflammatory mechanism inhibits neutrophil aggregation, accumulation and inflammation.

Recent advances in microscopic techniques for visualizing leukocytes in vivo

Leukocytes are inherently motile and interactive cells. Recent advances in intravital microscopy approaches have enabled a new vista of their behavior within intact tissues in real time. This brief review summarizes the developments enabling the tracking of immune responses in vivo.

Experimental Cerebral Malaria Spreads along the Rostral Migratory Stream

It is poorly understood how progressive brain swelling in experimental cerebral malaria (ECM) evolves in space and over time, and whether mechanisms of inflammation or microvascular sequestration/obstruction dominate the underlying pathophysiology. We therefore monitored in the Plasmodium berghei ANKA-C57BL/6 murine ECM model, disease manifestation and progression clinically, assessed by the Rapid-Murine-Coma-and-Behavioral-Scale (RMCBS), and by high-resolution in vivo MRI, including sensitive assessment of early blood-brain-barrier-disruption (BBBD), brain edema and microvascular pathology. For histological correlation HE and immunohistochemical staining for microglia and neuroblasts were obtained. Our results demonstrate that BBBD and edema initiated in the olfactory bulb (OB) and spread along the rostral-migratory-stream (RMS) to the subventricular zone of the lateral ventricles, the dorsal-migratory-stream (DMS), and finally to the external capsule (EC) and brainstem (BS). Before clinical symptoms (mean RMCBS = 18.5±1) became evident, a slight, non-significant increase of quantitative T2 and ADC values was observed in OB+RMS. With clinical manifestation (mean RMCBS = 14.2±0.4), T2 and ADC values significantly increased along the OB+RMS (p = 0.049/p = 0.01). Severe ECM (mean RMCBS = 5±2.9) was defined by further spread into more posterior and deeper brain structures until reaching the BS (significant T2 elevation in DMS+EC+BS (p = 0.034)). Quantitative automated histological analyses confirmed microglial activation in areas of BBBD and edema. Activated microglia were closely associated with the RMS and neuroblasts within the RMS were severely misaligned with respect to their physiological linear migration pattern. Microvascular pathology and ischemic brain injury occurred only secondarily, after vasogenic edema formation and were both associated less with clinical severity and the temporal course of ECM. Altogether, we identified a distinct spatiotemporal pattern of microglial activation in ECM involving primarily the OB+RMS axis, a distinct pathway utilized by neuroblasts and immune cells. Our data suggest significant crosstalk between these two cell populations to be operative in deeper brain infiltration and further imply that the manifestation and progression of cerebral malaria may depend on brain areas otherwise serving neurogenesis.

Brain swelling is difficult to detect ex vivo and has recently been identified as a strong predictor of death not only in experimental cerebral malaria (ECM), but also in human cerebral malaria. As whole-brain in-vivo imaging methods have been widely underutilized in this disease model, little is known about the spatiotemporal evolution of brain swelling. To unravel this question, we monitored the evolution of ECM in vivo using high-field magnetic resonance imaging (MRI) with whole-brain coverage and have identified a distinct pattern of cerebral disease spread. Inflammatory disruption of the blood-brain-barrier and consecutive brain swelling initiates in the olfactory bulb and spreads from there along the rostral migratory stream—a neurogenic niche—deeper into the brain. When the brainstem is eventually reached, mice start to fall into a comatose state. Those findings correlate with previously published human MRI findings, which also show brain swelling of the brainstem in comatose children with cerebral malaria as well as early involvement of the striatum—recently recognized to serve neurogenesis in humans. Our study provides a novel link between neurogenic areas specifically permitting the spatiotemporal expansion of activated microglia, blood-brain-barrier disruption and consequent brain edema. Finally, the dominant role of the neurogenic axis in the transmission of inflammation may provide an explanation why children are more vulnerable to cerebral malaria.

Monocyte- and Neutrophil-Derived CXCL10 Impairs Efficient Control of Blood-Stage Malaria Infection and Promotes Severe Disease

CXCL10, or IFN-γ-inducible protein 10, is a biomarker associated with increased risk for Plasmodium falciparum-mediated cerebral malaria (CM). Consistent with this, we have previously shown that CXCL10 neutralization or genetic deletion alleviates brain intravascular inflammation and protects Plasmodium berghei ANKA-infected mice from CM. In addition to organ-specific effects, the absence of CXCL10 during infection was also found to reduce parasite biomass. To identify the cellular sources of CXCL10 responsible for these processes, we irradiated and reconstituted wild-type (WT) and CXCL10(-/-) mice with bone marrow from either WT or CXCL10(-/-) mice. Similar to CXCL10(-/-) mice, chimeras unable to express CXCL10 in hematopoietic-derived cells controlled infection more efficiently than WT controls. In contrast, expression of CXCL10 in knockout mice reconstituted with WT bone marrow resulted in high parasite biomass levels, higher brain parasite and leukocyte sequestration rates, and increased susceptibility to CM. Neutrophils and inflammatory monocytes were identified as the main cellular sources of CXCL10 responsible for the induction of these processes. The improved control of parasitemia observed in the absence of CXCL10-mediated trafficking was associated with a preferential accumulation of CXCR3(+)CD4(+) T follicular helper cells in the spleen and enhanced Ab responses to infection. These results are consistent with the notion that some inflammatory responses elicited in response to malaria infection contribute to the development of high parasite densities involved in the induction of severe disease in target organs.