ICAM-1 is a key receptor mediating cytoadherence and pathology in the Plasmodium chabaudi malaria model

Investigation of Trypanosoma-induced vascular damage sheds insights into Trypanosoma vivax sequestration

Multiple blood-borne pathogens infecting mammals establish close interactions with the host vascular endothelium as part of their life cycles. In this work, we investigate differences in the interactions of three Trypanosoma species: T. brucei, T. congolense and T. vivax with the blood vasculature. Infection with these species results in vastly different pathologies, including different effects on vascular homeostasis, such as changes in vascular permeability and microhemorrhages. While all three species are extracellular parasites, T. congolense is strictly intravascular, while T. brucei is capable of surviving both extra- and intravascularly. Our knowledge regarding T. vivax tropism and its capacity of migration across the vascular endothelium is unknown. In this work, we show for the first time that T. vivax parasites sequester to the vascular endothelium of most organs, and that, like T. congolense, T. vivax Y486 is largely incapable of extravasation. Infection with this parasite species results in a unique effect on vascular endothelium receptors including general downregulation of ICAM1 and ESAM, and upregulation of VCAM1, CD36 and E-selectin. Our findings on the differences between the two sequestering species (T. congolense and T. vivax) and the non-sequestering, but extravasating, T. brucei raise important questions on the relevance of sequestration to the parasite's survival in the mammalian host, and the evolutionary relevance of both sequestration and extravasation.

Host-Derived Extracellular Vesicles in Blood and Tissue Human Protozoan Infections

Blood and tissue protozoan infections are responsible for an enormous burden in tropical and subtropical regions, even though they can also affect people living in high-income countries, mainly as a consequence of migration and travel. These pathologies are responsible for heavy socio-economic issues in endemic countries, where the lack of proper therapeutic interventions and effective vaccine strategies is still hampering their control. Moreover, the pathophysiological mechanisms associated with the establishment, progression and outcome of these infectious diseases are yet to be fully described. Among all the players, extracellular vesicles (EVs) have raised significant interest during the last decades due to their capacity to modulate inter-parasite and host-parasite interactions. In the present manuscript, we will review the state of the art of circulating host-derived EVs in clinical samples or in experimental models of human blood and tissue protozoan diseases (i.e., malaria, leishmaniasis, Chagas disease, human African trypanosomiasis and toxoplasmosis) to gain novel insights into the mechanisms of pathology underlying these conditions and to identify novel potential diagnostic markers.

Genetic variants of TLR4, including the novel variant, rs5030719, and related genes are associated with susceptibility to clinical malaria in African children

BACKGROUND

Malaria is a deadly disease caused by Plasmodium spp. Several blood phenotypes have been associated with malarial resistance, which suggests a genetic component to immune protection.

METHODS

One hundred and eighty-seven single nucleotide polymorphisms (SNPs) in 37 candidate genes were genotyped and investigated for associations with clinical malaria in a longitudinal cohort of 349 infants from Manhiça, Mozambique, in a randomized controlled clinical trial (RCT) (AgeMal, NCT00231452). Malaria candidate genes were selected according to involvement in known malarial haemoglobinopathies, immune, and pathogenesis pathways.

RESULTS

Statistically significant evidence was found for the association of TLR4 and related genes with the incidence of clinical malaria (p = 0.0005). These additional genes include ABO, CAT, CD14, CD36, CR1, G6PD, GCLM, HP, IFNG, IFNGR1, IL13, IL1A, IL1B, IL4R, IL4, IL6, IL13, MBL, MNSOD, and TLR2. Of specific interest, the previously identified TLR4 SNP rs4986790 and the novel finding of TRL4 SNP rs5030719 were associated with primary cases of clinical malaria.

CONCLUSIONS

These findings highlight a potential central role of TLR4 in clinical malarial pathogenesis. This supports the current literature and suggests that further research into the role of TLR4, as well as associated genes, in clinical malaria may provide insight into treatment and drug development.

CD4 T Cell-Dependent and -Independent Roles for IFN-γ in Blood-Stage Malaria

Production of IFN-γ by CD4 T cells is widely theorized to control Plasmodium parasite burden during blood-stage malaria. Surprisingly, the specific and crucial mechanisms through which this highly pleiotropic cytokine acts to confer protection against malarial disease remain largely untested in vivo. Here we used a CD4 T cell-restricted Cre-Lox IFN-γ excision mouse model to test whether and how CD4 T cell-derived IFN-γ controls blood-stage malaria. Although complete absence of IFN-γ compromised control of the acute and the chronic, recrudescent blood-stage infections with P. c. chabaudi, we identified a specific, albeit modest, role for CD4 T cell-derived IFN-γ in limiting parasite burden only during the chronic stages of P. c. chabaudi malaria. CD4 T cell IFN-γ promoted IgG Ab class switching to the IgG2c isotype during P. c. chabaudi malaria in C57BL/6 mice. Unexpectedly, our data do not support gross defects in phagocytic activity in IFN-γ-deficient hosts infected with blood-stage malaria. Together, our data confirm CD4 T cell-dependent roles for IFN-γ but suggest CD4 T cell-independent roles for IFN-γ in immune responses to blood-stage malaria.

Cerebral Malaria Model Applying Human Brain Organoids

Neural injuries in cerebral malaria patients are a significant cause of morbidity and mortality. Nevertheless, a comprehensive research approach to study this issue is lacking, so herein we propose an in vitro system to study human cerebral malaria using cellular approaches. Our first goal was to establish a cellular system to identify the molecular alterations in human brain vasculature cells that resemble the blood-brain barrier (BBB) in cerebral malaria (CM). Through transcriptomic analysis, we characterized specific gene expression profiles in human brain microvascular endothelial cells (HBMEC) activated by the Plasmodium falciparum parasites. We also suggest potential new genes related to parasitic activation. Then, we studied its impact at brain level after Plasmodium falciparum endothelial activation to gain a deeper understanding of the physiological mechanisms underlying CM. For that, the impact of HBMEC-P. falciparum-activated secretomes was evaluated in human brain organoids. Our results support the reliability of in vitro cellular models developed to mimic CM in several aspects. These systems can be of extreme importance to investigate the factors (parasitological and host) influencing CM, contributing to a molecular understanding of pathogenesis, brain injury, and dysfunction.

Trypanosoma cruzi dysregulates expression profile of piRNAs in primary human cardiac fibroblasts during early infection phase

Trypanosoma cruzi, the etiological agent of Chagas Disease, causes severe morbidity, mortality, and economic burden worldwide. Though originally endemic to Central and South America, globalization has led to increased parasite presence in most industrialized countries. About 40% of infected individuals will develop cardiovascular, neurological, and/or gastrointestinal pathologies. Accumulating evidence suggests that the parasite induces alterations in host gene expression profiles in order to facilitate infection and pathogenesis. The role of regulatory gene expression machinery during T. cruzi infection, particularly small noncoding RNAs, has yet to be elucidated. In this study, we aim to evaluate dysregulation of a class of sncRNAs called piRNAs during early phase of T. cruzi infection in primary human cardiac fibroblasts by RNA-Seq. We subsequently performed in silico analysis to predict piRNA-mRNA interactions. We validated the expression of these selected piRNAs and their targets during early parasite infection phase by stem loop qPCR and qPCR, respectively. We found about 26,496,863 clean reads (92.72%) which mapped to the human reference genome. During parasite challenge, 441 unique piRNAs were differentially expressed. Of these differentially expressed piRNAs, 29 were known and 412 were novel. In silico analysis showed several of these piRNAs were computationally predicted to target and potentially regulate expression of genes including SMAD2, EGR1, ICAM1, CX3CL1, and CXCR2, which have been implicated in parasite infection, pathogenesis, and various cardiomyopathies. Further evaluation of the function of these individual piRNAs in gene regulation and expression will enhance our understanding of early molecular mechanisms contributing to infection and pathogenesis. Our findings here suggest that piRNAs play important roles in infectious disease pathogenesis and can serve as potential biomarkers and therapeutic targets.

Intestinal inflammation and increased intestinal permeability in Plasmodium chabaudi AS infected mice

Background: Gastrointestinal symptoms are commonly associated with acute Plasmodium spp infection. Malaria-associated enteritis may provide an opportunity for enteric pathogens to breach the intestinal mucosa, resulting in life-threatening systemic infections. Methods: To investigate whether intestinal pathology also occurs during infection with a murine model of mild and resolving malaria, C57BL/6J mice were inoculated with recently mosquito-transmitted Plasmodium chabaudi AS. At schizogony, intestinal tissues were collected for quantification and localisation of immune mediators and malaria parasites, by PCR and immunohistochemistry. Inflammatory proteins were measured in plasma and faeces and intestinal permeability was assessed by FITC-dextran translocation after oral administration. Results: Parasitaemia peaked at approx. 1.5% at day 9 and resolved by day 14, with mice experiencing significant and transient anaemia but no weight loss. Plasma IFN-γ, TNF-α and IL10 were significantly elevated during peak infection and quantitative RT-PCR of the intestine revealed a significant increase in transcripts for ifng and cxcl10. Histological analysis revealed parasites within blood vessels of both the submucosa and intestinal villi and evidence of mild crypt hyperplasia. In faeces, concentrations of the inflammatory marker lactoferrin were significantly raised on days 9 and 11 and FITC-dextran was detected in plasma on days 7 to 14. At day 11, plasma FITC-dextran concentration was significantly positively correlated with peripheral parasitemia and faecal lactoferrin concentration. Conclusions: In summary, using a relevant, attenuated model of malaria, we have found that acute infection is associated with intestinal inflammation and increased intestinal permeability. This model can now be used to explore the mechanisms of parasite-induced intestinal inflammation and to assess the impact of increased intestinal permeability on translocation of enteropathogens.

Intestinal inflammation and increased intestinal permeability in Plasmodium chabaudi AS infected mice

Background: Gastrointestinal symptoms are commonly associated with acute Plasmodium spp infection. Malaria-associated enteritis may provide an opportunity for enteric pathogens to breach the intestinal mucosa, resulting in life-threatening systemic infections. Methods: To investigate whether intestinal pathology also occurs during infection with a murine model of mild and resolving malaria, C57BL/6J mice were inoculated with recently mosquito-transmitted Plasmodium chabaudi AS. At schizogony, intestinal tissues were collected for quantification and localisation of immune mediators and malaria parasites, by PCR and immunohistochemistry. Inflammatory proteins were measured in plasma and faeces and intestinal permeability was assessed by FITC-dextran translocation after oral administration. Results: Parasitaemia peaked at approx. 1.5% at day 9 and resolved by day 14, with mice experiencing significant and transient anaemia but no weight loss. Plasma IFNγ, TNFα and IL10 were significantly elevated during peak infection and quantitative RT-PCR of the intestine revealed a significant increase in transcripts for ifng and cxcl10. Histological analysis revealed parasites within blood vessels of both the submucosa and intestinal villi and evidence of mild crypt hyperplasia. In faeces, concentrations of the inflammatory marker lactoferrin were significantly raised on days 9 and 11 and FITC-dextran was detected in plasma on days 7 to 14. At day 11, plasma FITC-dextran concentration was significantly positively correlated with peripheral parasitemia and faecal lactoferrin concentration. Conclusions: In summary, using a relevant, attenuated model of malaria, we have found that acute infection is associated with intestinal inflammation and increased intestinal permeability. This model can now be used to explore the mechanisms of parasite-induced intestinal inflammation and to assess the impact of increased intestinal permeability on translocation of enteropathogens.

Comparative proteomics reveals Cryptosporidium parvum manipulation of the host cell molecular expression and immune response

Cryptosporidium is a life-threating protozoan parasite belonging to the phylum Apicomplexa, which mainly causes gastroenteritis in a variety of vertebrate hosts. Currently, there is a re-emergence of Cryptosporidium infection; however, no fully effective drug or vaccine is available to treat Cryptosporidiosis. In the present study, to better understand the detailed interaction between the host and Cryptosporidium parvum, a large-scale label-free proteomics study was conducted to characterize the changes to the proteome induced by C. parvum infection. Among 4406 proteins identified, 121 proteins were identified as differentially abundant (> 1.5-fold cutoff, P < 0.05) in C. parvum infected HCT-8 cells compared with uninfected cells. Among them, 67 proteins were upregulated, and 54 proteins were downregulated at 36 h post infection. Analysis of the differentially abundant proteins revealed an interferon-centered immune response of the host cells against C. parvum infection and extensive inhibition of metabolism-related enzymes in the host cells caused by infection. Several proteins were further verified using quantitative real-time reverse transcription polymerase chain reaction and western blotting. This systematic analysis of the proteomics of C. parvum-infected HCT-8 cells identified a wide range of functional proteins that participate in host anti-parasite immunity or act as potential targets during infection, providing new insights into the molecular mechanism of C. parvum infection.

Cryptosporidium parvum is an emerging zoonotic pathogen transmitted via the fecal–oral route, and is considered a leading cause of moderate-to-severe diarrheal disease in young children in resource limited areas. After infection, C. parvum parasitizes intestinal epithelial cells and evokes an inflammatory immune response, leading to severe damage of the intestinal mucosa. The infection can be lethal to immunosuppressed individuals. However, no fully effective drug or vaccine is available for cryptosporidiosis, and the pathogenesis and immune mechanisms during C. parvum infection are obscure. Thus, an in-depth understanding of host-parasite interaction is needed. Hence, we established a C. parvum-infected HCT-8 cell model and performed comparative quantitative proteomic analyses to profile global host-parasite interactions and determine the molecular mechanisms that are activated during infection, aiming to offer new insights into the treatment of Cryptosporidium.

Genetic Diversity of Human Host Genes Involved in Immune Response and the Binding of Malaria Parasite in Patients Residing along the Thai-Myanmar border

Polymorphisms of the genes encoding proteins involved in immune functions and the binding of malaria parasites to human host cells have been the focus of research in recent years, aiming to understand malaria pathogenesis and case severity and to exploit this knowledge to assert control over malaria. This study investigated the genetic diversity of the human host genes encoding proteins that are involved in immune functions and malaria parasite binding, i.e., MCP1 (−2518), TGFβ1 (−509), TNFα (−308), IL4 (VNTR), IL6 (−174), IL10 (−3575), TLR4 (299), CD36 (−188), and ICAM1 (469) in patients with mono-infection of Plasmodium falciparum and Plasmodium vivax infections in the multidrug-resistant areas along the Thai-Myanmar border. The association between gene polymorphisms and parasite density was also investigated. Genomic DNA (gDNA) of P. falciparum and P. vivax were extracted from whole blood and dried blood spot (DBS). Gene amplification and genotyping were performed by PCR and PCR-RFLP analysis, respectively. Of these samples, 178 and 209 samples were, respectively, mono-infection with P. falciparum and P. vivax. The ratio of P. falciparum: P. vivax was 46%:54%. Results showed marked variation in the frequency distribution and patterns of the genotypes and gene alleles of the nine immune response genes or human host genes. The SNPs of TGFβ1, IL10 and ICAM1, were significantly associated with P. falciparum, but not P. vivax parasite density. TGFβ1, IL10 and ICAM1, may play more significant roles in modulating P. falciparum than P. vivax parasitemia. The prevalence of the genotypes and gene alleles of these genes, including their association with parasite density, may vary depending on patient ethnicity and endemic areas. Information obtained from each endemic area is essential for treatment strategies and the development of vaccines for malaria prophylaxis in specific areas.

Molecular Characterization and Expression Analysis of Intercellular Adhesion Molecule-1 (ICAM-1) Genes in Rainbow Trout (Oncorhynchus mykiss) in Response to Viral, Bacterial and Parasitic Challenge

The intercellular adhesion molecule-1 (ICAM-1), known as CD54, is a transmembrane cell surface glycoprotein that interacts with two integrins (i.e., LFA-1 and Mac-l) important for trans-endothelial migration of leukocytes. The level of ICAM-1 expression is upregulated in response to some inflammatory stimulations, including pathogen infection and proinflammatory cytokines. Yet, to date, our knowledge regarding the functional role of ICAM-1 in teleost fish remains largely unknown. In this study, we cloned and characterized the sequence of ICAM-1 in rainbow trout (Oncorhynchus mykiss) for the first time, which exhibited that the molecular features of ICAM-1 in fishes were relatively conserved compared with human ICAM-1. The transcriptional level of ICAM-1 was detected in 12 different tissues, and we found high expression of this gene in the head kidney, spleen, gills, skin, nose, and pharynx. Moreover, upon stimulation with infectious hematopoietic necrosis virus (IHNV), Flavobacterium columnare G₄ (F. columnare), and Ichthyophthirius multifiliis (Ich) in rainbow trout, the morphological changes were observed in the skin and gills, and enhanced expression of ICAM-1 mRNA was detected both in the systemic and mucosal tissues. These results indicate that ICAM-1 may be implicated in the mucosal immune responses to viral, bacterial, and parasitic infections in teleost fish, meaning that ICAM-1 emerges as a master regulator of mucosal immune responses against pathogen infections in teleost fish.

G6pd-Deficient Mice Are Protected From Experimental Cerebral Malaria and Liver Injury by Suppressing Proinflammatory Response in the Early Stage of Plasmodium berghei Infection

Epidemiological studies provide compelling evidence that glucose-6-phosphate dehydrogenase (G6PD) deficiency individuals are relatively protected against Plasmodium parasite infection. However, the animal model studies on this subject are lacking. Plus, the underlying mechanism in vivo is poorly known. In this study, we used a G6pd-deficient mice infected with the rodent parasite Plasmodium berghei (P.berghei) to set up a malaria model in mice. We analyzed the pathological progression of experimental cerebral malaria (ECM) and acute liver injury in mice with different G6pd activity infected with P.berghei. We performed dual RNA-seq for host-parasite transcriptomics and validated the changes of proinflammatory response in the murine model. G6pd-deficient mice exhibited a survival advantage, less severe ECM and mild liver injury compared to the wild type mice. Analysis based on dual RNA-seq suggests that G6pd-deficient mice are protected from ECM and acute liver injury were related to proinflammatory responses. Th1 differentiation and dendritic cell maturation in the liver and spleen were inhibited in G6pd-deficient mice. The levels of proinflammatory cytokines were reduced, chemokines and vascular adhesion molecules in the brain were significantly down-regulated, these led to decreased cerebral microvascular obstruction in G6pd-deficient mice. We generated the result that G6pd-deficiency mediated protection against ECM and acute liver injury were driven by the regulatory proinflammatory responses. Furthermore, bioinformatics analyses showed that P.berghei might occur ribosome loss in G6pd-deficient mice. Our findings provide a novel perspective of the underlying mechanism of G6PD deficiency mediated protection against malaria in vivo.

Elevated Levels of the Endothelial Molecules ICAM-1, VEGF-A, and VEGFR2 in Microscopic Asymptomatic Malaria

Background

In malaria, clinical disease has been associated with increased levels of endothelial activation due to the sequestration of infected erythrocytes. However, the levels and impact of endothelial activation and pro-angiogenic molecules such as vascular endothelial growth factor (VEGF)–A and its receptor vascular endothelial growth factor receptor 2 (VEGFR2) in asymptomatic malaria have not been well characterized.

Methods

Blood samples were obtained from community children for malaria diagnosis using microscopy and polymerase chain reaction. A multiplex immunoassay was used to determine the levels of intracellular adhesion molecule (ICAM)–1, vascular endothelial growth factor (VEGF)–A, and VEGFR2 in the plasma of children with microscopic or submicroscopic asymptomatic parasitemia and compared with levels in uninfected controls.

Results

Levels of ICAM-1, VEGF-A, and VEGFR2 were significantly increased in children with microscopic asymptomatic parasitemia compared with uninfected controls. Also, levels of VEGF-A were found to be inversely associated with age. Additionally, a receiver operating characteristic analysis revealed that plasma levels of ICAM-1 (area under the curve [AUC], 0.72) showed a moderate potential in discriminating between children with microscopic malaria from uninfected controls when compared with VEGF-A (AUC, 0.67) and VEGFR2 (AUC, 0.69).

Conclusions

These data imply that endothelial activation and pro-angiogenic growth factors could be one of the early host responders during microscopic asymptomatic malaria and may play a significant role in disease pathogenesis.

Attenuated T Cell Responses Are Associated With the Blockade of Cerebral Malaria Development by YOP1-Deficient Plasmodium berghei ANKA

Reticulon and the REEP family of proteins stabilize the high curvature of endoplasmic reticulum tubules. The REEP5 homolog in Plasmodium, Plasmodium berghei YOP1 (PbYOP1), plays an important role in the erythrocytic cycle of the P. berghei ANKA and the pathogenesis of experimental cerebral malaria (ECM), but the mechanisms are largely unknown. Here, we show that protection from ECM in Pbyop1Δ-infected mice is associated with reduced intracerebral Th1 accumulation, decreased expression of pro-inflammatory cytokines and chemokines, and attenuated pathologies in the brainstem, though the total number of CD4⁺ and CD8⁺ T cells sequestered in the brain are not reduced. Expression of adhesive molecules on brain endothelial cells, including ICAM-1, VCAM-1, and CD36, are decreased, particularly in the brainstem, where fatal pathology is always induced during ECM. Subsequently, CD8⁺ T cell-mediated cell apoptosis in the brain is compromised. These findings suggest that Pbyop1Δ parasites can be a useful tool for mechanistic investigation of cerebral malaria pathogenesis.

Adipose tissue parasite sequestration drives leptin production in mice and correlates with human cerebral malaria

Circulating levels of the adipokine leptin are linked to neuropathology in experimental cerebral malaria (ECM), but its source and regulation mechanism remain unknown. Here, we show that sequestration of infected red blood cells (iRBCs) in white adipose tissue (WAT) microvasculature increased local vascular permeability and leptin production. Mice infected with parasite strains that fail to sequester in WAT displayed reduced leptin production and protection from ECM. WAT sequestration and leptin induction were lost in CD36KO mice; however, ECM susceptibility revealed sexual dimorphism. Adipocyte leptin was regulated by the mechanistic target of rapamycin complex 1 (mTORC1) and blocked by rapamycin. In humans, although Plasmodium falciparum infection did not increase circulating leptin levels, iRBC sequestration, tissue leptin production, and mTORC1 activity were positively correlated with CM in pediatric postmortem WAT. These data identify WAT sequestration as a trigger for leptin production with potential implications for pathogenesis of malaria infection, prognosis, and treatment.

An enhanced toolkit for the generation of knockout and marker-free fluorescent Plasmodium chabaudi

The rodent parasite Plasmodium chabaudi is an important in vivo model of malaria. The ability to produce chronic infections makes it particularly useful for investigating the development of anti- Plasmodium immunity, as well as features associated with parasite virulence during both the acute and chronic phases of infection. P. chabaudi also undergoes asexual maturation (schizogony) and erythrocyte invasion in culture, so offers an experimentally-amenable in vivo to in vitro model for studying gene function and drug activity during parasite replication. To extend the usefulness of this model, we have further optimised transfection protocols and plasmids for P. chabaudi and generated stable, fluorescent lines that are free from drug-selectable marker genes. These mother-lines show the same infection dynamics as wild-type parasites throughout the lifecycle in mice and mosquitoes; furthermore, their virulence can be increased by serial blood passage and reset by mosquito transmission. We have also adapted the large-insert, linear PlasmoGEM vectors that have revolutionised the scale of experimental genetics in another rodent malaria parasite and used these to generate barcoded P. chabaudi gene-deletion and –tagging vectors for transfection in our fluorescent P. chabaudi mother-lines. This produces a tool-kit of P. chabaudi lines, vectors and transfection approaches that will be of broad utility to the research community.

An enhanced toolkit for the generation of knockout and marker-free fluorescent Plasmodium chabaudi

The rodent parasite Plasmodium chabaudi is an important in vivo model of malaria. The ability to produce chronic infections makes it particularly useful for investigating the development of anti- Plasmodium immunity, as well as features associated with parasite virulence during both the acute and chronic phases of infection. P. chabaudi also undergoes asexual maturation (schizogony) and erythrocyte invasion in culture, so offers an experimentally-amenable in vivo to in vitro model for studying gene function and drug activity during parasite replication. To extend the usefulness of this model, we have further optimised transfection protocols and plasmids for P. chabaudi and generated stable, fluorescent lines that are free from drug-selectable marker genes. These mother-lines show the same infection dynamics as wild-type parasites throughout the lifecycle in mice and mosquitoes; furthermore, their virulence can be increased by serial blood passage and reset by mosquito transmission. We have also adapted the large-insert, linear PlasmoGEM vectors that have revolutionised the scale of experimental genetics in another rodent malaria parasite and used these to generate barcoded P. chabaudi gene-deletion and -tagging vectors for transfection in our fluorescent P. chabaudi mother-lines. This produces a tool-kit of P. chabaudi lines, vectors and transfection approaches that will be of broad utility to the research community.

Host-pathogen interaction in the tissue environment during Plasmodium blood-stage infection

Human malarial infection occurs after an infectious Anopheles mosquito bites. Following the initial liver-stage infection, parasites transform into merozoites, infecting red blood cells (RBCs). Repeated RBC infection then occurs during the blood-stage infection, while patients experience various malarial symptoms. Protective immune responses are elicited by this systemic infection, but excessive responses are sometimes harmful for hosts. As parasites infect only RBCs and their immediate precursors during this stage, direct parasite-host interactions occur primarily in the environment surrounded by endothelial lining of blood vessels. The spleen is the major organ where the immune system encounters infected RBCs, causing immunological responses. Its tissue structure is markedly altered during malarial infection in mice and humans. Plasmodium falciparum parasites inside RBCs express proteins, such as PfEMP-1 and RIFIN, transported to the RBC surfaces in order to evade immunological attack by sequestering themselves in the peripheral vasculature avoiding spleen or by direct immune cell inhibition through inhibitory receptors. Host cell production of regulatory cytokines IL-10 and IL-27 limits excessive immune responses, avoiding tissue damage. The regulation of the protective and inhibitory immune responses through host-parasite interactions allows chronic Plasmodium infection. In this review, we discuss underlying interaction mechanisms relevant for developing effective strategies against malaria.

The malaria parasite has an intrinsic clock

Malarial rhythmic fevers are the consequence of the synchronous bursting of red blood cells (RBCs) on completion of the malaria parasite asexual cell cycle. Here, we hypothesized that an intrinsic clock in the parasite Plasmodium chabaudi underlies the 24-hour-based rhythms of RBC bursting in mice. We show that parasite rhythms are flexible and lengthen to match the rhythms of hosts with long circadian periods. We also show that malaria rhythms persist even when host food intake is evenly spread across 24 hours, suggesting that host feeding cues are not required for synchrony. Moreover, we find that the parasite population remains synchronous and rhythmic even in an arrhythmic clock mutant host. Thus, we propose that parasite rhythms are generated by the parasite, possibly to anticipate its circadian environment.

A novel murine model for assessing fetal and birth outcomes following transgestational maternal malaria infection

Plasmodium falciparum infection during pregnancy is a major cause of severe maternal illness and neonatal mortality. Mouse models are important for the study of gestational malaria pathogenesis. When infected with Plasmodium chabaudi chabaudi AS in early gestation, several inbred mouse strains abort at midgestation. We report here that outbred Swiss Webster mice infected with P. chabaudi chabaudi AS in early gestation carry their pregnancies to term despite high parasite burden and malarial hemozoin accumulation in the placenta at midgestation, with the latter associated with induction of heme oxygenase 1 expression. Infection yields reduced fetal weight and viability at term and a reduction in pup number at weaning, but does not influence postnatal growth prior to weaning. This novel model allows for the exploration of malaria infection throughout pregnancy, modeling chronic infections observed in pregnant women prior to the birth of underweight infants and enabling the production of progeny exposed to malaria in utero, which is critical for understanding the postnatal repercussions of gestational malaria. The use of outbred mice allows for the exploration of gestational malaria in a genetically diverse model system, better recapitulating the diversity of infection responses observed in human populations.

Endothelial Protein C Receptor Could Contribute to Experimental Malaria-Associated Acute Respiratory Distress Syndrome

The severity of Plasmodium falciparum malaria is associated with parasite cytoadherence, but there is limited knowledge about the effect of parasite cytoadherence in malaria-associated acute respiratory distress syndrome (ARDS). Our objective was to evaluate the cytoadherence of infected red blood cells (iRBCs) in a murine model of ARDS and to appraise a potential function of endothelial protein C receptor (EPCR) in ARDS pathogenesis. DBA/2 mice infected with P. berghei ANKA were classified as ARDS- or hyperparasitemia- (HP-) developing mice according to respiratory parameters and parasitemia. Lungs, blood, and bronchoalveolar lavage were collected for gene expression or protein analyses. Primary cultures of microvascular lung endothelial cells from DBA/2 mice were analyzed for iRBC interactions. Lungs from ARDS-developing mice showed evidence of iRBC accumulation along with an increase in EPCR and TNF concentrations. Furthermore, TNF increased iRBC adherence in vitro. Dexamethasone-treated infected mice showed low levels of TNF and EPCR mRNA expression and, finally, decreased vascular permeability, thus protecting mice from ARDS. In conclusion, we identified that increased iRBC cytoadherence in the lungs underlies malaria-associated ARDS in DBA/2-infected mice and that inflammation increased cytoadherence capacity, suggesting a participation of EPCR and a conceivable target for drug development.

Neutrophil extracellular traps drive inflammatory pathogenesis in malaria

Neutrophils are essential innate immune cells that extrude chromatin in the form of neutrophil extracellular traps (NETs) when they die. This form of cell death has potent immunostimulatory activity. We show that heme-induced NETs are essential for malaria pathogenesis. Using patient samples and a mouse model, we define two mechanisms of NET-mediated inflammation of the vasculature: activation of emergency granulopoiesis via granulocyte colony-stimulating factor production and induction of the endothelial cytoadhesion receptor intercellular adhesion molecule-1. Soluble NET components facilitate parasite sequestration and mediate tissue destruction. We demonstrate that neutrophils have a key role in malaria immunopathology and propose inhibition of NETs as a treatment strategy in vascular infections.

Rodent malaria models: insights into human disease and parasite biology

The use of rodents as model organisms to study human disease is based on the genetic and physiological similarities between the species. Successful molecular methods to generate transgenic reporter or humanized rodents has rendered rodents as powerful tools for understanding biological processes and host-pathogen interactions relevant to humans. In malaria research, rodent models have been pivotal for the study of liver stages, syndromes arising from blood stages of infection, and malaria transmission to and from the mammalian host. Importantly, many in vivo findings are comparable to pathology observed in humans only when adequate combinations of rodent strains and Plasmodium parasites are used.

The Glycosylphosphatidylinositol Transamidase Complex Subunit PbGPI16 of Plasmodium berghei Is Important for Inducing Experimental Cerebral Malaria

In animal models of experimental cerebral malaria (ECM), the glycosylphosphatidylinositols (GPIs) and GPI anchors are the major factors that induce nuclear factor kappa B (NF-κB) activation and proinflammatory responses, which contribute to malaria pathogenesis. GPIs and GPI anchors are transported to the cell surface via a process called GPI transamidation, which involves the GPI transamidase (GPI-T) complex. In this study, we showed that GPI16, one of the GPI-T subunits, is highly conserved among Plasmodium species. Genetic knockout of pbgpi16 (Δpbgpi16) in the rodent malaria parasite Plasmodium berghei strain ANKA led to a significant reduction of the amounts of GPIs in the membranes of merozoites, as well as surface display of several GPI-anchored merozoite surface proteins. Compared with the wild-type parasites, Δpbgpi16 parasites in C57BL/6 mice caused much less NF-κB activation and elicited a substantially attenuated T helper type 1 response. As a result, Δpbgpi16 mutant-infected mice displayed much less severe brain pathology, and considerably fewer Δpbgpi16 mutant-infected mice died from ECM. This study corroborated the GPI toxin as a significant inducer of ECM and further suggested that vaccines against parasite GPIs may be a promising strategy to limit the severity of malaria.

Host immune evasion strategies of malaria blood stage parasite

Host immune evasion is a key strategy for the continual survival of many microbial pathogens including Apicomplexan protozoan: Plasmodium spp., the causative agent of Malaria. The malaria parasite has evolved a variety of mechanisms to evade the host immune responses within its two hosts: the female Anopheles mosquito vector and vertebrate host. In this review, we will focus on the molecular mechanisms of the immune evasion strategies used by the Plasmodium parasite at the blood stage which is responsible for the clinical manifestations of human malaria. We also aim to provide some insights on the potential targets for malaria interventions through the recent advancement in understanding the molecular biology of the parasite.

Elimination of intravascular thrombi prevents early mortality and reduces gliosis in hyper-inflammatory experimental cerebral malaria

BACKGROUND

Cerebral malaria (CM) is the most lethal outcome of Plasmodium infection. There are clear correlations between expression of inflammatory cytokines, severe coagulopathies, and mortality in human CM. However, the mechanisms intertwining the coagulation and inflammation pathways, and their roles in CM, are only beginning to be understood. In mice with T cells deficient in the regulatory cytokine IL-10 (IL-10 KO), infection with Plasmodium chabaudi leads to a hyper-inflammatory response and lethal outcome that can be prevented by anti-TNF treatment. However, inflammatory T cells are adherent within the vasculature and not present in the brain parenchyma, suggesting a novel form of cerebral inflammation. We have previously documented behavioral dysfunction and microglial activation in infected IL-10 KO animals suggestive of neurological involvement driven by inflammation. In order to understand the relationship of intravascular inflammation to parenchymal dysfunction, we studied the congestion of vessels with leukocytes and fibrin(ogen) and the relationship of glial cell activation to congested vessels in the brains of P. chabaudi-infected IL-10 KO mice.

METHODS

Using immunofluorescence microscopy, we describe severe thrombotic congestion in these animals. We stained for immune cell surface markers (CD45, CD11b, CD4), fibrin(ogen), microglia (Iba-1), and astrocytes (GFAP) in the brain at the peak of behavioral symptoms. Finally, we investigated the roles of inflammatory cytokine tumor necrosis factor (TNF) and coagulation on the pathology observed using neutralizing antibodies and low-molecular weight heparin to inhibit both inflammation and coagulation, respectively.

RESULTS

Many blood vessels in the brain were congested with thrombi containing adherent leukocytes, including CD4 T cells and monocytes. Despite containment of the pathogen and leukocytes within the vasculature, activated microglia and astrocytes were prevalent in the parenchyma, particularly clustered near vessels with thrombi. Neutralization of TNF, or the coagulation cascade, significantly reduced both thrombus formation and gliosis in P. chabaudi-infected IL-10 KO mice.

CONCLUSIONS

These findings support the contribution of cytokines, coagulation, and leukocytes within the brain vasculature to neuropathology in malaria infection. Strikingly, localization of inflammatory leukocytes within intravascular clots suggests a mechanism for interaction between the two cascades by which cytokines drive local inflammation without considerable cellular infiltration into the brain parenchyma.

Host genetics in malaria: lessons from mouse studies

Malaria remains a deadly parasitic disease caused by Plasmodium, claiming almost half a million lives every year. While parasite genetics and biology are often the major targets in many studies, it is becoming more evident that host genetics plays a crucial role in the outcome of the infection. Similarly, Plasmodium infections in mice also rely heavily on the genetic background of the mice, and often correlate with observations in human studies, due to their high genetic homology with humans. As such, murine models of malaria are a useful tool for understanding host responses during Plasmodium infections, as well as dissecting host-parasite interactions through various genetic manipulation techniques. Reverse genetic approach such as quantitative trait loci studies and random mutagenesis screens have been employed to discover novel host genes that affect malaria susceptibility in mouse models, while other targeted studies utilize mouse models to validate observation from human studies. Herein, we review the findings from the past and present studies on murine models of hepatic and erythrocytic stages of malaria and speculate on how the current mouse models benefit from the recent development in CRISPR/Cas9 gene editing technology.

When Epidemiology Is the Clue to a Positive Outcome: A Case of Malaria During Pregnancy

BACKGROUND Malaria infection during pregnancy is associated with increased perinatal and maternal morbidity and mortality. CASE REPORT A 29-year-old primigravida at 37 weeks of gestation, with no significant medical history, presented complaining of fever, chills, and generalized body aches. She had been living in Malawi for 1 year and was on atovaquone/proguanil prophylaxis until she was found to be pregnant. Prophylaxis was changed to mefloquine and discontinued upon her return to the US. Six weeks prior to presentation, she traveled to Malawi for 1 month when she was off prophylaxis. On admission, vital signs and physical exam results were normal. Given epidemiologic findings, a malaria smear was performed and showed 4% parasitemia. She was treated with mefloquine and discharged. Two days after discharge, she again presented with fever, chills, and body aches. A malaria smear showed <0.01% parasitemia, with 2 ring forms. Serologies for dengue, chikungunya, leptospira, and blood cultures were negative. These symptoms were deemed secondary to early recrudescence. The species was later identified as P. falciparum. The patient was treated with quinine sulfate and clindamycin. She delivered at full term without complication. CONCLUSIONS Pregnant women are more susceptible to severe forms of malaria, such as P. falciparum. A high index of suspicion and early identification of malaria are vital to prevent deleterious outcomes.