Experimental Plasmodium falciparum cerebral malaria in the squirrel monkey Saimiri sciureus

Genetic mapping of determinants in drug resistance, virulence, disease susceptibility, and interaction of host-rodent malaria parasites

Genetic mapping has been widely employed to search for genes linked to phenotypes/traits of interest. Because of the ease of maintaining rodent malaria parasites in laboratory mice, many genetic crosses of rodent malaria parasites have been performed to map the parasite genes contributing to malaria parasite development, drug resistance, host immune response, and disease pathogenesis. Drs. Richard Carter, David Walliker, and colleagues at the University of Edinburgh, UK, were the pioneers in developing the systems for genetic mapping of malaria parasite traits, including characterization of genetic markers to follow the inheritance and recombination of parasite chromosomes and performing the first genetic cross using rodent malaria parasites. Additionally, many genetic crosses of inbred mice have been performed to link mouse chromosomal loci to the susceptibility to malaria parasite infections. In this chapter, we review and discuss past and recent advances in genetic marker development, performing genetic crosses, and genetic mapping of both parasite and host genes. Genetic mappings using models of rodent malaria parasites and inbred mice have contributed greatly to our understanding of malaria, including parasite development within their hosts, mechanism of drug resistance, and host-parasite interaction.

Establishment of a murine model of cerebral malaria in KunMing mice infected with Plasmodium berghei ANKA

Malaria remains one of the most devastating diseases. Cerebral malaria (CM) is a severe complication of Plasmodium falciparum infection resulting in high mortality and morbidity worldwide. Analysis of precise mechanisms of CM in humans is difficult for ethical reasons and animal models of CM have been employed to study malaria pathogenesis. Here, we describe a new experimental cerebral malaria (ECM) model with Plasmodium berghei ANKA infection in KunMing (KM) mice. KM mice developed ECM after blood-stage or sporozoites infection, and the development of ECM in KM mice has a dose-dependent relationship with sporozoites inoculums. Histopathological findings revealed important features associated with ECM, including accumulation of mononuclear cells and red blood cells in brain microvascular, and brain parenchymal haemorrhages. Blood-brain barrier (BBB) examination showed that BBB disruption was present in infected KM mice when displaying clinical signs of CM. In vivo bioluminescent imaging experiment indicated that parasitized red blood cells accumulated in most vital organs including heart, lung, spleen, kidney, liver and brain. The levels of inflammatory cytokines interferon-gamma, tumour necrosis factor-alpha, interleukin (IL)-17, IL-12, IL-6 and IL-10 were all remarkably increased in KM mice infected with P. berghei ANKA. This study indicates that P. berghei ANKA infection in KM mice can be used as ECM model to extend further research on genetic, pharmacological and vaccine studies of CM.

Identification of the Plasmodium berghei resistance locus 9 linked to survival on chromosome 9

Background

One of the main causes of mortality from severe malaria in Plasmodium falciparum infections is cerebral malaria (CM). An important host genetic component determines the susceptibility of an individual to develop CM or to clear the infection and become semi-immune. As such, the identification of genetic loci associated with susceptibility or resistance may serve to modulate disease severity.

Methodology

The Plasmodium berghei mouse model for experimental cerebral malaria (ECM) reproduces several disease symptoms seen in human CM, and two different phenotypes, a susceptible (FVB/NJ) and a resistant mouse strain (DBA/2J), were examined.

Results

FVB/NJ mice died from infection within ten days, whereas DBA/2J mice showed a gender bias: males survived on average nineteen days and females either died early with signs of ECM or survived for up to three weeks. A comparison of brain pathology between FVB/NJ and DBA/2J showed no major differences with regard to brain haemorrhages or the number of parasites and CD3+ cells in the microvasculature. However, significant differences were found in the peripheral blood of infected mice: For example resistant DBA/2J mice had significantly higher numbers of circulating basophils than did FVB/NJ mice on day seven. Analysis of the F2 offspring from a cross of DBA/2J and FVB/NJ mice mapped the genetic locus of the underlying survival trait to chromosome 9 with a Lod score of 4.9. This locus overlaps with two previously identified resistance loci (char1 and pymr) from a blood stage malaria model.

Conclusions

Survival best distinguishes malaria infections between FVB/NJ and DBA/2J mice. The importance of char1 and pymr on chromosome 9 in malaria resistance to P. berghei was confirmed. In addition there was an association of basophil numbers with survival.

Imaging analysis of the brain in a primate model of cerebral malaria

This paper reviews our studies concerning imaging analysis of the brain in a primate model of cerebral malaria. To elucidate the clinical features of cerebral malaria, we performed positron emission tomography with (18)F-fluorodeoxyglucose (FDG-PET) scanning and magnetic resonance imaging (MRI) of the brain in Japanese macaques (Macaca fuscata) infected with Plasmodium coatneyi, a primate model of severe human malaria with cerebral involvement. On FDG-PET scanning, we observed diffuse and heterogeneous reduction of metabolism in the cerebral cortex in the acute phase of malaria infection. Although the monkey exhibited severe clinical signs, MR imaging did not reveal any significant changes during the course of infection. Histopathologic examination frequently revealed preferential sequestration of PRBCs in the cerebral and cerebellum capillaries, but neither parenchymal injury nor neuronal necrosis was found in the tissues. These results suggest that heterogeneous metabolic reduction and lack of abnormalities on MRI in the acute phase of CM may be due to any avoidance mechanisms from ischemia caused by sequestration. This may be one reason why more than half of CM patients have no neurological sequelae following recovery.

Cerebral malaria: why experimental murine models are required to understand the pathogenesis of disease

Cerebral malaria is a life-threatening complication of malaria infection. The pathogenesis of cerebral malaria is poorly defined and progress in understanding the condition is severely hampered by the inability to study in detail, ante-mortem, the parasitological and immunological events within the brain that lead to the onset of clinical symptoms. Experimental murine models have been used to investigate the sequence of events that lead to cerebral malaria, but there is significant debate on the merits of these models and whether their study is relevant to human disease. Here we review the current understanding of the parasitological and immunological events leading to human and experimental cerebral malaria, and explain why we believe that studies with experimental models of CM are crucial to define the pathogenesis of the condition.

Cerebral malaria

Malaria infection of the Central Nervous System (CNS) can cause a severe neurological syndrome termed Cerebral Malaria (CM). The central neuropathological feature of CM is the preferential sequestration of parasitised red blood cells (PRBC) in the cerebral microvasculature. The level of sequestration is related to the incidence of cerebral symptoms in severe malaria. Other neuropathological features of CM include petechial hemorrhages in the brain parenchyma, ring hemorrhages and Dürck's granuloma's. Immunohisto-chemical and electron microscopy studies have shown widespread cerebral endothelial cell activation and morphological changes occur in CM, as well as focal endothelial cell damage and necrosis. The immune cell response to intravascular sequestration appears to be limited, although activation of pigment-phagocytosing monocytes is a late feature. The mechanisms by which PRBC cause coma in malaria remain unclear. In vitro parasitised erythrocytes bind to endothelial cells by specific, receptor mediated interactions with host adhesion molecules such as ICAM-1, whose expression on cerebral endothelial cells is increased during CM as part of a systemic endothelial activation. Induction of local neuro-active mediators such as nitric oxide and systemic cytokines like TNF alpha may be responsible for the rapidly reversible symptoms of the coma of CM. The recent cloning of the parasite ligand PfEMP-1, thought to mediate binding to host sequestration receptors, promises further insight into the relationship between patterns of sequestration and the incidence and pathogenesis of coma in cerebral malaria.

Cytoadhesion of Plasmodium falciparum-infected erythrocytes and the infected placenta: a two-way pathway

Malaria is undoubtedly the world's most devastating parasitic disease, affecting 300 to 500 million people every year. Some cases of Plasmodium falciparum infection progress to the deadly forms of the disease responsible for 1 to 3 million deaths annually. P. falciparum-infected erythrocytes adhere to host receptors in the deep microvasculature of several organs. The cytoadhesion of infected erythrocytes to placental syncytiotrophoblast receptors leads to pregnancy-associated malaria (PAM). This specific maternal-fetal syndrome causes maternal anemia, low birth weight and the death of 62,000 to 363,000 infants per year in sub-Saharan Africa, and thus has a poor outcome for both mother and fetus. However, PAM and non-PAM parasites have been shown to differ antigenically and genetically. After multiple pregnancies, women from different geographical areas develop adhesion-blocking antibodies that protect against placental parasitemia and clinical symptoms of PAM. The recent description of a new parasite ligand encoded by the var2CSA gene as the only gene up-regulated in PAM parasites renders the development of an anti-PAM vaccine more feasible. The search for a vaccine to prevent P. falciparum sequestration in the placenta by eliciting adhesion-blocking antibodies and a cellular immune response, and the development of new methods for evaluating such antibodies should be key priorities in mother-child health programs in areas of endemic malaria. This review summarizes the main molecular, immunological and physiopathological aspects of PAM, including findings related to new targets in the P. falciparum var gene family. Finally, we focus on a new methodology for mimicking cytoadhesion under blood flow conditions in human placental tissue.

Pathophysiology of cerebral malaria: role of host cells in the modulation of cytoadhesion

Cerebral malaria (CM), one of the most serious complications of Plasmodium falciparum infection, is characterized by the sequestration of infected erythrocytes (IEs) in cerebral microvascular beds. The precise mechanisms involved in the onset of neuropathology remain unknown, but parasite sequestration in the brain, metabolic disturbances, and host immune responses all play a role. Studies in a murine model of CM showed a potential role for host cells, especially platelets, in the pathogenesis of CM. Indeed, urokinase plasminogen activator receptor (uPAR; CD87) deficiency attenuates the severity of CM, most likely by its important role in platelet kinetics and trapping. These results led us to evaluate whether platelets have a role in the human disease. By immunostaining of brain samples from Malawian patients, we determined that the surface of platelet accumulation and the proportion of vessels filled with platelets were significantly higher in patients who died of CM than in those who died of other causes. We then investigated the role of platelets in IE cytoadhesion in vitro, using CD36-binding IE (IECD36) and CD36-deficient (CD36DEF) brain microvascular endothelial cells (ECs). Coincubation studies indicated that platelets can induce strong IECD36 binding to CD36DEF ECs and, conversely, can hide constitutively expressed falciparum receptors such as chondroitin sulfate A. Thus, platelets may provide an adhesion receptor to microvascular beds originally devoid of it. This novel mechanism of cytoadhesion may reorient the sequestration of different parasite phenotypes and play an important role in the pathogenesis of severe malaria.

Plasmodium coatneyi-infected erythrocytes bind to C32 amelanotic melanoma cells under static and flow conditions

The ability of Plasmodium coatneyi-infected red blood cells (IRBCs) to bind to C32 amelanotic melanoma cells was examined under static and physiologic flow conditions in vitro. Six blood samples obtained from P. coatneyi-infected Japanese macaques (Macaca fuscata) with severe manifestations of disease were used in the static adhesion assay. All blood samples constantly exhibited binding of IRBCs to C32 cells under static conditions. Immunofluorescence staining with anti-CD36 mAb revealed a positive reaction at the surface of C32 cells with the infected erythrocytes, while the reaction with C32 cells without IRBCs was negative. To further examine the specificity of the interaction between P. coatneyi-infected erythrocytes and C32 cells, we carried out the binding assay under physiological flow conditions. In flow adhesion assay, three blood samples were used. Adhesion and rolling of IRBCs on C32 cells were detected at several rates of shear stress under flow conditions. At a shear stress of 1.0 dyne/cm(2), the number of IRBCs adherent to C32 cell averaged 5 to 6, and the number of IRBCs rolling on C32 cells averaged 6 to 11. The anti-CD36 mAb OKM5 inhibited 75-100% of IRBC adhesion and rolling, while the inhibitory effect of anti-ICAM-1 mAb 84H10 varied between 20-40%. The combination of anti-CD36 and anti-ICAM-1 mAb resulted in 83-100% inhibition of rolling and 100% inhibition of adhesion. These findings suggest that CD36 is one of the principal adhesion receptors of P. coatneyi-infected erythrocytes.

Plasmodium chabaudi-infected erythrocytes adhere to CD36 and bind to microvascular endothelial cells in an organ-specific way

Adherence of erythrocytes infected with Plasmodium falciparum to microvascular endothelial cells (sequestration) is considered to play an important role in parasite virulence and pathogenesis. However, the real importance of sequestration for infection and disease has never been fully assessed. The absence of an appropriate in vivo model for sequestration has been a major barrier. We have examined the rodent malaria parasite Plasmodium chabaudi chabaudi AS in mice as a potential model. Erythrocytes infected with this parasite adhere in vitro to purified CD36, a critical endothelium receptor for binding P. falciparum-infected erythrocytes. P. c. chabaudi-infected erythrocytes adhere in vitro to endothelial cells in a gamma interferon-dependent manner, suggesting the involvement of additional adhesion molecules in the binding process, as is also the case with P. falciparum-infected cells. Furthermore, plasma or sera from infected and hyperimmune mice, respectively, have the ability to block binding of infected erythrocytes to endothelial cells. In vivo, erythrocytes containing mature P. c. chabaudi parasites are sequestered from the peripheral circulation. Sequestration is organ specific, occurring primarily in the liver, although intimate contact between infected erythrocytes and endothelial cells is also observed in the spleen and brain. The results are discussed in the context of the use of this model to study (i) the relationship between endothelial cell activation and the level of sequestration and (ii) the primary function of sequestration in malaria infection.

Plasmodium falciparumin the squirrel monkey (Saimiri sciureus): infection of non-splenectomised animals as a model for exploring clinical manifestations of malaria

Plasmodium falciparum infection in humans leads to a variety of symptoms ranging from an influenza-like syndrome to life-threatening complications. Animal models are useful tools for the detailed analysis of the interaction between both parasite and host factors leading to these various clinical manifestations. In this review, examining the different clinical, parasitological and haematological parameters associated with P. falciparum infection in spleen-intact monkeys, we propose this model as a good alternative for exploring some aspects of the host-parasite relationship in malaria.

Characterisation of the chondroitin sulphate of Saimiri brain microvascular endothelial cells involved in Plasmodium falciparum cytoadhesion

Cytoadhesion of Plasmodium falciparum-infected erythrocytes (IRBC) to chondroitin-4-sulphate (CSA) is inhibited by soluble CSA in vitro on Saimiri brain microvascular endothelial cells (SBEC) and in vivo in P. falciparum-infected Saimiri monkeys. We tested whether the SBEC model was appropriate for studying CSA-binding IRBC using four cell lines. All SBEC expressed a chondroitin sulphate (CS), with a composition of CSA. The mean sizes of these CSA were 20.5, 22, 23, 32.5 and 36 kDa for SBEC 3A and C2, CHO, SBEC 1D and 17, respectively. We found that cytoadhesion of the Palo-Alto (FUP)1 CSA-binding phenotype, selected by panning on SBEC 17, was specifically inhibited in a dose-dependent manner by all the purified CSA. The extent of inhibition depended on the cellular origin of the tested CSA. SBEC 17 CSA was 33 times more efficient than CHO-CSA and 21 times more efficient than the 50 kDa commercial bovine trachaea CSA. Dynabeads coated with a total extract of SBEC 1D CS-proteoglycans interacted with CSA- but not with CD36- or ICAM-1-binding IRBC. These Dynabeads also interacted specifically with the PfEMP1 DBL-3 domain, on the surface of CHO transfectants, but not with the CIDR-1 domain. Thrombomodulin was involved in IRBC adhesion to all SBEC whereas CD44 was only expressed by SBEC 1D and 17. These two CSA-proteoglycans have also been detected at the surface of human endothelial cells. Thus, the two homologous models, SBEC/Saimiri sciureus, are useful and reliable tools for the evaluation of new anti-CSA adhesion treatments and anti-disease vaccines for pregnant women.

In vivo sequestration of Plasmodium falciparum-infected human erythrocytes: a severe combined immunodeficiency mouse model for cerebral malaria

Cerebral malaria is a fatal complication of infection by Plasmodium falciparum in man. The neurological symptoms that characterize this form of malarial disease are accompanied by the adhesion of infected erythrocytes to the vasculature of the brain. To study this phenomenon in vivo, an acute phase severe combined immunodeficiency (SCID) mouse model was developed in which sequestration of P. falciparum-infected human erythrocytes took place. During acute cerebral malaria in humans, the expression of intercellular adhesion molecule-1 (ICAM-1) is induced in vascular endothelium by inflammatory reactions. Acute phase ICAM-1 expression can also be obtained in SCID mice. The endothelium of the midbrain region was the most responsive to such inflammatory stimulus. It is noteworthy that the reticular formation in the midbrain controls the level of consciousness, and loss of consciousness is a symptom of cerebral malaria. We found that infected human erythrocytes were retained 24 times more than normal erythrocytes in ICAM-1-positive mouse brain. Sequestration to the brain was reduced by anti-ICAM-1 antibodies. These in vivo results were confirmed by the binding of P. falciparum-infected erythrocytes to the ICAM-1-positive endothelium in tissue sections of mouse brain. We conclude that the SCID mouse serves as a versatile in vivo model that allows the study of P. falciparum-infected erythrocyte adhesion as it occurs in human cerebral malaria. Upregulation of ICAM-1 expression in the region of the midbrain correlates with increased retention of malaria-infected erythrocytes and with the symptoms of cerebral malaria.

Isolation and characterization of brain microvascular endothelial cells from Saimiri monkeys. An in vitro model for sequestration of Plasmodium falciparum-infected erythrocytes

The adhesion of parasitized red blood cells (PRBC) to the endothelium (sequestration) may contribute to the pathogenic events in severe human malaria caused by P. falciparum. However, the factors involved in the pathophysiology, especially cerebral malaria are poorly understood. Previously, we have shown that the squirrel monkey Saimiri sciureus is a potential model for human cerebral malaria. In this paper we describe five stable clones of endothelial cell lines isolated immediately postmortem from different regions of the brain of Saimiri monkeys. The endothelial cell characteristics of these clones were confirmed by analyzing their ultrastructural aspects by transmission electron microscopy and by immunodetection of various endothelial cell markers. The Saimiri brain endothelial cell clones (SBEC) varied in their expression of different surface molecules. For example, various combinations of receptors involved in P. falciparum PRBC adherence such as CD36, ICAM-1 and E-selectin, were expressed at baseline values and could be up-regulated by human srTNF-alpha and human srIFN-gamma. One of the SBEC clones showed a strong cytoadherence for various laboratory strains of P. falciparum despite the absence of surface expression of any of the known endothelial receptors implicated in PRBC adherence. This finding suggests the existence of a new and uncharacterized PRBC binding receptor. The use of target organ specific endothelial cell lines expressing a number of different potential P. falciparum PRBC cytoadherence receptors, will be a useful in vitro system for the evaluation of strategies for the development of vaccine and antimalarial drugs to prevent human cerebral malaria.

Chondroitin-4-sulphate (proteoglycan), a receptor for Plasmodium falciparum-infected erythrocyte adherence on brain microvascular endothelial cells

Adherence of Plasmodium falciparum parasitized erythrocytes to the microvascular endothelium is mediated by different receptors expressed by endothelial cells. The study of the adherence of P. falciparum-infected erythrocytes to Saimiri monkey brain microvascular endothelial cells revealed the presence of an additional receptor, which was identified and further characterized. This receptor was also found on the surface of primary human lung endothelial cells (HLEC). We developed two mAbs to this receptor which very efficiently blocked the adherence of parasite strains to Saimiri brain endothelial cells (SBEC). The ability of these mAb to bind to SBEC was partially blocked by chondroitin-4-sulphate (CSA). Competitive inhibition assays on adherence of parasitized red blood cells (PRBC) showed that CSA, but not hyaluronic acid, chondroitin-6-sulphate, dermatan sulphate, keratane sulphate, heparan sulphate or chondroitin-4S-disaccharide, was able to almost completely inhibit PRBC adherence. The same effect was obtained with chondroitinase ABC and AC, but not B, hyaluronidase or heparinase. These results strongly suggest that a member of the chondroitin-glycosaminoglycan family, CSA, represents an additional receptor used by P. falciparum PRBC to cytoadhere to microvascular endothelial cells.

Karyotype analysis of virulent Plasmodium falciparum strains propagated in Saimiri sciureus: strain adaptation leads to deletion of the RESA gene

The squirrel monkey, Saimiri sciureus, is an important experimental model for the study of the human malaria parasite Plasmodium falciparum. A detailed karyotype analysis of four different P. falciparum strains propagated in S. sciureus was done using various subtelomeric antigen gene probes. We observed deletion of the complete RESA gene from chromosome 1 in all four strains. Interestingly, a loss of RESA was observed immediately upon adaptation to the squirrel monkey, suggesting that this DNA rearrangement is fundamental for the P. falciparum infection of S. sciureus erythrocytes. However, a RESA-specific gene probe hybridized with chromosome 1 of wild isolates from 28 different patients, indicating that this gene is maintained during infection of humans.

Immune pressure selects for Plasmodium falciparum parasites presenting distinct red blood cell surface antigens and inducing strain-specific protection in Saimiri sciureus monkeys

The passive transfer of specific antibodies to a naive splenectomized Saimiri sciureus monkey infected with the Palo Alto FUP/SP strain of Plasmodium falciparum resulted in the emergence of parasites resistant to the transferred antibodies. Molecular typing indicated that the original and resistant parasites were isogenic. Saimiri monkeys primed with original parasites were fully susceptible to a challenge by the resistant ones, and vice versa. This absence of crossprotection indicates that strain-specific determinants would be the major targets of protective immunity developed in these monkeys. Phenotypic analysis showed that the surface of the infected red blood cells differed in both lines. Original parasites formed rosettes, autoagglutinated, presented characteristic knobs at the surface of the infected red blood cell, and did not agglutinate in the presence of a pool of human immune sera. In contrast, the resistant parasites did not form rosettes, did not spontaneously autoagglutinate, presented abnormal flattened knobs, and formed large aggregates in the presence of a pool of human immune sera. The presence of strain-specific determinants at the surface of the resistant parasites was confirmed by surface immunofluorescence and agglutination using homologous Saimiri serum. Neither the original nor the resistant parasites cytoadhered to an amelanotic melanoma cell line, suggesting that cytoadherence and agglutination can be dissociated. These results indicate that parasites that differ by the antigens exposed at the surface of the red blood cell induce strain-specific immunity. Furthermore they show that rosetting and nonrosetting parasites differ in their antigenic properties and do not crossprotect.

Rosetting

Why do some individuals get severe falciparum malaria while others don't? Rosetting (the binding of uninfected erythrocytes to Plasmodium falciparum-infected erythrocytes), together with endothelial cytoadherence, has been shown to play a crucial role in the obstruction of the microvosculoture in P. falciparum malaria. Here, Mats Wahlgren, Victor Fernandez, Carin Scholonder and Johan Carlson review the literature surrounding rosetting.