In vivo assessment of rodent Plasmodium parasitemia and merozoite invasion by flow cytometry

TLR7 modulates extramedullary splenic erythropoiesis in P. yoelii NSM-infected mice through the regulation of iron metabolism of macrophages with IFN-γ

Splenomegaly is a prominent clinical manifestation of malaria and the causes remain incompletely clear. Anemia is induced in malaria and extramedullary splenic erythropoiesis is compensation for the loss of erythrocytes. However, the regulation of extramedullary splenic erythropoiesis in malaria is unknown. An inflammatory response could facilitate extramedullary splenic erythropoiesis in the settings of infection and inflammation. Here, when mice were infected with rodent parasites, Plasmodium yoelii NSM, TLR7 expression in splenocytes was increased. To explore the roles of TLR7 in splenic erythropoiesis, we infected wild-type and TLR7 ^-/- C57BL/6 mice with P. yoelii NSM and found that the development of splenic erythroid progenitor cells was impeded in TLR7 ^-/- mice. Contrarily, the treatment of the TLR7 agonist, R848, promoted extramedullary splenic erythropoiesis in wild-type infected mice, which highlights the implication of TLR7 on splenic erythropoiesis. Then, we found that TLR7 promoted the production of IFN-γ that could enhance phagocytosis of infected erythrocytes by RAW264.7. After phagocytosis of infected erythrocytes, the iron metabolism of RAW264.7 was upregulated, evidenced by higher iron content and expression of Hmox1 and Slc40a1. Additionally, the neutralization of IFN-γ impeded the extramedullary splenic erythropoiesis modestly and reduced the iron accumulation in the spleen of infected mice. In conclusion, TLR7 promoted extramedullary splenic erythropoiesis in P. yoelii NSM-infected mice. TLR7 enhanced the production of IFN-γ, and IFN-γ promoted phagocytosis of infected erythrocytes and the iron metabolism of macrophages in vitro, which may be related to the regulation of extramedullary splenic erythropoiesis by TLR7.

KCC1 Activation protects Mice from the Development of Experimental Cerebral Malaria

Plasmodium falciparum malaria causes half a million deaths per year, with up to 9% of this mortality caused by cerebral malaria (CM). One of the major processes contributing to the development of CM is an excess of host inflammatory cytokines. Recently K+ signaling has emerged as an important mediator of the inflammatory response to infection; we therefore investigated whether mice carrying an ENU induced activation of the electroneutral K+ channel KCC1 had an altered response to Plasmodium berghei. Here we show that Kcc1^M935K/M935K mice are protected from the development of experimental cerebral malaria, and that this protection is associated with an increased CD4+ and TNFa response. This is the first description of a K+ channel affecting the development of experimental cerebral malaria.

Application of the automated haematology analyzer XN-30 in an experimental rodent model of malaria

Background

The erythrocytic stage, where malaria parasites proliferate in human blood, is clinically significant as this causes the symptoms and illness of malaria. Experimental rodent models of malaria at the erythrocytic stage are used for the development of anti-malarial drugs and for biological analysis. An automated haematology analyzer XN-30 was developed for detection of infected red blood cells (iRBCs) in human blood samples and measurement of their parasitaemia in approximately 1 min through flow cytometry analysis. Additionally, the analyzer simultaneously measured other haematological parameters in these samples. It is inferred that the analyzer would also allow easy and rapid measurement of parasitaemia in mice and provide important clues on the mouse haematological state during infection and treatment.

Results

The XN-30 analyzer is a simple and rapid tool to detect iRBCs in mouse blood samples infected with rodent malarial parasites, with three-dimensional analysis permitting the precise measurement of parasitaemia (referred herein as the ‘XN-30 system’). The XN-30 analyzer allowed not only the detection of iRBCs but also the monitoring of RBC, white blood cell, and platelet counts, as well as haematocrit, mean corpuscular volume and mean platelet volume values in the mouse blood sample. For anti-malarial drug development, aside from demonstrating possible efficacy in mouse models, XN-30 analyzer could provide a first glimpse of the safety profile of the drug.

Conclusions

The XN-30 system is a powerful tool that can be utilized for the in vivo screening, development, and evaluation of anti-malarial drugs as well as for pre-clinical pharmacology and/or toxicity tests in rodent models.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2313-6) contains supplementary material, which is available to authorized users.

cGAS-mediated control of blood-stage malaria promotes Plasmodium-specific germinal center responses

Sensing of pathogens by host pattern recognition receptors is essential for activating the immune response during infection. We used a nonlethal murine model of malaria (Plasmodium yoelii 17XNL) to assess the contribution of the pattern recognition receptor cyclic GMP-AMP synthase (cGAS) to the development of humoral immunity. Despite previous reports suggesting a critical, intrinsic role for cGAS in early B cell responses, cGAS-deficient (cGAS-/-) mice had no defect in the early expansion or differentiation of Plasmodium-specific B cells. As the infection proceeded, however, cGAS-/- mice exhibited higher parasite burdens and aberrant germinal center and memory B cell formation when compared with littermate controls. Antimalarial drugs were used to further demonstrate that the disrupted humoral response was not B cell intrinsic but instead was a secondary effect of a loss of parasite control. These findings therefore demonstrate that cGAS-mediated innate-sensing contributes to parasite control but is not intrinsically required for the development of humoral immunity. Our findings highlight the need to consider the indirect effects of pathogen burden in investigations examining how the innate immune system affects the adaptive immune response.

Erythrocyte β spectrin can be genetically targeted to protect mice from malaria

The malaria parasite hijacks host erythrocytes to shield itself from the immune system and proliferate. Red blood cell abnormalities can provide protection from malaria by impeding parasite invasion and growth within the cell or by compromising the ability of parasites to avoid host clearance. Here, we describe 2 N-ethyl-N-nitrosourea-induced mouse lines, Sptb^MRI26194 and Sptb^MRI53426 , containing single-point mutations in the erythrocyte membrane skeleton gene, β spectrin (Sptb), which exhibit microcytosis but retain a relatively normal ratio of erythrocyte surface area to volume and are highly resistant to rodent malaria. We propose the major factor responsible for malaria protection is the specific clearance of mutant erythrocytes, although an enhanced clearance of uninfected mutant erythrocytes was also observed (ie, the bystander effect). Using an in vivo erythrocyte tracking assay, we established that this phenomenon occurs irrespective of host environment, precluding the involvement of nonerythrocytic cells in the resistance mechanism. Furthermore, we recapitulated this phenotype by disrupting the interaction between ankyrin-1 and β spectrin in vivo using CRISPR/Cas9 genome editing technology, thereby genetically validating a potential antimalarial target. This study sheds new light on the role of β spectrin during Plasmodium infection and highlights how changes in the erythrocyte cytoskeleton can substantially influence malaria susceptibility with minimal adverse consequences for the host.

Ankyrin-1 Gene Exhibits Allelic Heterogeneity in Conferring Protection Against Malaria

Allelic heterogeneity is a common phenomenon where a gene exhibits a different phenotype depending on the nature of its genetic mutations. In the context of genes affecting malaria susceptibility, it allowed us to explore and understand the intricate host–parasite interactions during malaria infections. In this study, we described a gene encoding erythrocytic ankyrin-1 (Ank-1) which exhibits allelic-dependent heterogeneous phenotypes during malaria infections. We conducted an ENU mutagenesis screen on mice and identified two Ank-1 mutations, one resulting in an amino acid substitution (MRI95845), and the other a truncated Ank-1 protein (MRI96570). Both mutations caused hereditary spherocytosis-like phenotypes and confer differing protection against Plasmodium chabaudi infections. Upon further examination, the Ank-1^(MRI96570) mutation was found to inhibit intraerythrocytic parasite maturation, whereas Ank-1^(MRI95845) caused increased bystander erythrocyte clearance during infection. This is the first description of allelic heterogeneity in ankyrin-1 from the direct comparison between two Ank-1 mutations. Despite the lack of direct evidence from population studies, this data further supported the protective roles of ankyrin-1 mutations in conferring malaria protection. This study also emphasized the importance of such phenomena in achieving a better understanding of host–parasite interactions, which could be the basis of future studies.

A novel ENU-induced ankyrin-1 mutation impairs parasite invasion and increases erythrocyte clearance during malaria infection in mice

Genetic defects in various red blood cell (RBC) cytoskeletal proteins have been long associated with changes in susceptibility towards malaria infection. In particular, while ankyrin (Ank-1) mutations account for approximately 50% of hereditary spherocytosis (HS) cases, an association with malaria is not well-established, and conflicting evidence has been reported. We describe a novel N-ethyl-N-nitrosourea (ENU)-induced ankyrin mutation MRI61689 that gives rise to two different ankyrin transcripts: one with an introduced splice acceptor site resulting a frameshift, the other with a skipped exon. Ank-1^(MRI61689/+) mice exhibit an HS-like phenotype including reduction in mean corpuscular volume (MCV), increased osmotic fragility and reduced RBC deformability. They were also found to be resistant to rodent malaria Plasmodium chabaudi infection. Parasites in Ank-1^(MRI61689/+) erythrocytes grew normally, but red cells showed resistance to merozoite invasion. Uninfected Ank-1^(MRI61689/+) erythrocytes were also more likely to be cleared from circulation during infection; the “bystander effect”. This increased clearance is a novel resistance mechanism which was not observed in previous ankyrin mouse models. We propose that this bystander effect is due to reduced deformability of Ank-1^(MRI61689/+) erythrocytes. This paper highlights the complex roles ankyrin plays in mediating malaria resistance.

Erythrocytic Iron Deficiency Enhances Susceptibility to Plasmodium chabaudi Infection in Mice Carrying a Missense Mutation in Transferrin Receptor 1

The treatment of iron deficiency in areas of high malaria transmission is complicated by evidence which suggests that iron deficiency anemia protects against malaria, while iron supplementation increases malaria risk. Iron deficiency anemia results in an array of pathologies, including reduced systemic iron bioavailability and abnormal erythrocyte physiology; however, the mechanisms by which these pathologies influence malaria infection are not well defined. In the present study, the response to malaria infection was examined in a mutant mouse line, Tfrc(MRI24910), identified during an N-ethyl-N-nitrosourea (ENU) screen. This line carries a missense mutation in the gene for transferrin receptor 1 (TFR1). Heterozygous mice exhibited reduced erythrocyte volume and density, a phenotype consistent with dietary iron deficiency anemia. However, unlike the case in dietary deficiency, the erythrocyte half-life, mean corpuscular hemoglobin concentration, and intraerythrocytic ferritin content were unchanged. Systemic iron bioavailability was also unchanged, indicating that this mutation results in erythrocytic iron deficiency without significantly altering overall iron homeostasis. When infected with the rodent malaria parasite Plasmodium chabaudi adami, mice displayed increased parasitemia and succumbed to infection more quickly than their wild-type littermates. Transfusion of fluorescently labeled erythrocytes into malaria parasite-infected mice demonstrated an erythrocyte-autonomous enhanced survival of parasites within mutant erythrocytes. Together, these results indicate that TFR1 deficiency alters erythrocyte physiology in a way that is similar to dietary iron deficiency anemia, albeit to a lesser degree, and that this promotes intraerythrocytic parasite survival and an increased susceptibility to malaria in mice. These findings may have implications for the management of iron deficiency in the context of malaria.

The influence of host genetics on erythrocytes and malaria infection: is there therapeutic potential?

As parasites, Plasmodium species depend upon their host for survival. During the blood stage of their life-cycle parasites invade and reside within erythrocytes, commandeering host proteins and resources towards their own ends, and dramatically transforming the host cell. Parasites aptly avoid immune detection by minimizing the exposure of parasite proteins and removing themselves from circulation through cytoadherence. Erythrocytic disorders brought on by host genetic mutations can interfere with one or more of these processes, thereby providing a measure of protection against malaria to the host. This review summarizes recent findings regarding the mechanistic aspects of this protection, as mediated through the parasites interaction with abnormal erythrocytes. These novel findings include the reliance of the parasite on the host enzyme ferrochelatase, and the discovery of basigin and CD55 as obligate erythrocyte receptors for parasite invasion. The elucidation of these naturally occurring malaria resistance mechanisms is increasing the understanding of the host-parasite interaction, and as discussed below, is providing new insights into the development of therapies to prevent this disease.