Hemoglobinopathic erythrocytes affect the intraerythrocytic multiplication of Plasmodium falciparum in vitro

Application of optical tweezer technology reveals that PfEBA and PfRH ligands, not PfMSP1, play a central role in Plasmodium falciparum merozoite-erythrocyte attachment

Malaria pathogenesis and parasite multiplication depend on the ability of Plasmodium merozoites to invade human erythrocytes. Invasion is a complex multi-step process involving multiple parasite proteins which can differ between species and has been most extensively studied in P. falciparum. However, dissecting the precise role of individual proteins has to date been limited by the availability of quantifiable phenotypic assays. In this study, we apply a new approach to assigning function to invasion proteins by using optical tweezers to directly manipulate recently egressed P. falciparum merozoites and erythrocytes and quantify the strength of attachment between them, as well as the frequency with which such attachments occur. Using a range of inhibitors, antibodies, and genetically modified strains including some generated specifically for this work, we quantitated the contribution of individual P. falciparum proteins to these merozoite-erythrocyte attachment interactions. Conditional deletion of the major P. falciparum merozoite surface protein PfMSP1, long thought to play a central role in initial attachment, had no impact on the force needed to pull merozoites and erythrocytes apart, whereas interventions that disrupted the function of several members of the EBA-175 like Antigen (PfEBA) family and Reticulocyte Binding Protein Homologue (PfRH) invasion ligand families did have a significant negative impact on attachment. Deletion of individual PfEBA and PfRH ligands reinforced the known redundancy within these families, with the deletion of some ligands impacting detachment force while others did not. By comparing over 4000 individual merozoite-erythrocyte interactions in a range of conditions and strains, we establish that the PfEBA/PfRH families play a central role in P. falciparum merozoite attachment, not the major merozoite surface protein PfMSP1.

[Blood count abnormalities in the association of sickle cell disease and malaria in clinical hematology at the CNHU-HKM in Cotonou (Bénin)]

Introduction

Although a protective effect of hemoglobin S has been described, malaria has frequently been associated with increased morbidity and mortality in sickle cell disease patients in Africa. Various cytopenias are frequently found on the haemograms of these patients. In Benin, a malaria-endemic zone with a high prevalence of sickle cell disease, the aim of this study was to establish and compare the blood count profile according to hemoglobin type in the association of sickle cell disease and malaria.

Material and method

This was a prospective descriptive study. It covered a 24-month period from October 2020 to October 2022. It included all patients with major sickle cell syndrome seen in clinical haematology and with a positive thick drop/parasite density, whatever the parasitaemia value. For each patient, a blood count was performed on the Sysmex XT 4000i machine, supplemented by a smear study after staining with May-Grunwald Giemsa. Data were analyzed using R 3.6.1 software.

Results

Three hundred non-redundant cases with a positive thick smear were identified in sickle cell patients, including 208 SS homozygotes (69.3%) and 92 SC heterozygotes (30.7%). In contrast, there were 181 non-redundant cases with a negative thick smear, including 119 SS homozygotes (65.7%) and 62 SC heterozygotes (34.3%). Among subjects with a positive thick smear, the majority of patients (70%) exhibited clinical symptoms. Severe malaria was observed in 58% of the cases. The proportion of severe malaria was higher in SS homozygote patients than in double heterozygote SC patients (p < 0.0001). The mean parasite density was higher in SS individuals (4 320.7 ± 2 185 trophozoites/pL) compared to SC individuals (1 564.4 ± 1 221 trophozoites/pL; p < 0.0001). Plasmodium falciparum was the only species identified. The mean hemoglobin level in impaludated SS subjects was 6.1 g/dL, significantly lower than that in non-impaludated SS subjects (p < 0.0001). The average white blood cell count in impaludated SS subjects was 16.58 G/L, compared to 13.2 G/L in those with a negative thick smear (p < 0.0001). Twenty cases of thrombocytopenia were found in SS subjects with a positive thick smear, compared to 6 cases in those with a negative thick smear. As for SC subjects with a positive thick smear, the average hemoglobin levels and white blood cell counts were 9.8 g/dL and 10.63 G/L, respectively, compared to 11.27 g/dL and 7.3 G/L in SC subjects with a negative thick smear. Eighteen cases of thrombocytopenia were found in subjects with a positive thick smear, compared to 17 cases in those with a negative thick smear.

Discussion

Sickle cell disease and malaria represent two major public health problems. However, contrary to popular belief, sickle cell disease is not immune to malaria infestation. Malaria is recognized as one of the main causes of morbidity and mortality in sickle cell patients, particularly children. In Benin, its association with sickle cell emergencies has already been reported.Our study found that malaria was predominantly associated with the homozygous SS form (p < 0.00001). Severe malaria was the most common clinical form. All malaria infestations in our series were due to Plasmodium falciparum, and parasitaemia was significantly higher in SS patients (p < 0.0001).The hematological profile of the association of sickle cell disease and malaria in homozygous SS individuals in our series showed characteristics of a normocytic normochromic anemia with neutrophil-predominant leukocytosis. Compared to non-malaria-infected SS individuals, there was a significant worsening of anemia, neutrophil-predominant leukocytosis, and a decrease in the average platelet count. In SC individuals, there was rather a microcytic normochromic regenerative anemia associated with neutrophil-predominant leukocytosis. Compared to non-malaria-infected SC individuals, there was a significant decrease in the rate of anemia and neutrophil-predominant leukocytosis. Anemia is a constant feature in homozygous sickle cell disease, and the low values recorded illustrate the hemolytic nature of malaria, especially in SS individuals, and the better tolerance of SC individuals. Furthermore, the low baseline hemoglobin levels make SS individuals more vulnerable to malaria-induced anemia compared to SC individuals. The observed leukocytosis is generally accompanied by reticulocytosis in the case of major sickle cell syndrome, which must be taken into account for result validation. It is the expression of compensatory bone marrow reaction to anemia and inflammatory mechanisms resulting from malaria infestation. Finally, thrombocytopenia was significantly more common in SC patients, even though they were adults living in malaria-endemic areas. Malaria can frequently induce thrombocytopenia through platelet consumption during the "rosetting" phenomenon. In SS patients, the effects of "rosetting" could be compensated for by the bone marrow stimulation induced by anemia. In our series with adult subjects living in an endemic area, thrombocytopenia is not a frequent biological disturbance. In a clinicalbiological context combining a systemic inflammatory response syndrome with anemia and neutrophil-predominant leukocytosis in a SS or SC sickle cell patient, the clinician should be able to consider malaria and confirm or rule out this diagnosis.

Novel stem cell technologies are powerful tools to understand the impact of human factors on Plasmodium falciparum malaria

Plasmodium falciparum parasites have a complex life cycle, but the most clinically relevant stage of the disease is the invasion of erythrocytes and the proliferation of the parasite in the blood. The influence of human genetic traits on malaria has been known for a long time, however understanding the role of the proteins involved is hampered by the anuclear nature of erythrocytes that makes them inaccessible to genetic tools. Here we overcome this limitation using stem cells to generate erythroid cells with an in-vitro differentiation protocol and assess parasite invasion with an adaptation of flow cytometry to detect parasite hemozoin. We combine this strategy with reprogramming of patient cells to Induced Pluripotent Stem Cells and genome editing to understand the role of key genes and human traits in malaria infection. We show that deletion of basigin ablates invasion while deletion of ATP2B4 has a minor effect and that erythroid cells from reprogrammed patient-derived HbBart α-thalassemia samples poorly support infection. The possibility to obtain patient-secific and genetically modifed erythoid cells offers an unparalleled opportunity to study the role of human genes and polymorphisms in malaria allowing preservation of the genomic background to demonstrate their function and understand their mechanisms.

Cultivation of Asexual Intraerythrocytic Stages of Plasmodium falciparum

Successfully developed in 1976, the continuous in vitro culture of Plasmodium falciparum has many applications in the field of malaria research. It has become an important experimental model that directly uses a human pathogen responsible for a high prevalence of morbidity and mortality in many parts of the world and is a major source of biological material for immunological, biochemical, molecular, and pharmacological studies. Until present, the basic techniques described by Trager and Jensen and Haynes et al. remain unchanged in many malaria research laboratories. Nonetheless, different factors, including culture media, buffers, serum substitutes and supplements, sources of erythrocytes, and conditions of incubation (especially oxygen concentration), have been modified by different investigators to adapt the original technique in their laboratories or enhance the in vitro growth of the parasites. The possible effects and benefits of these modifications for the continuous cultivation of asexual intraerythrocytic stages of P. falciparum, as well as future challenges in developing a serum-free cultivation system and axenic cultures, are discussed.

Plasmodium schizogony, a chronology of the parasite's cell cycle in the blood stage

Malaria remains a significant threat to global health, and despite concerted efforts to curb the disease, malaria-related morbidity and mortality increased in recent years. Malaria is caused by unicellular eukaryotes of the genus Plasmodium, and all clinical manifestations occur during asexual proliferation of the parasite inside host erythrocytes. In the blood stage, Plasmodium proliferates through an unusual cell cycle mode called schizogony. Contrary to most studied eukaryotes, which divide by binary fission, the parasite undergoes several rounds of DNA replication and nuclear division that are not directly followed by cytokinesis, resulting in multinucleated cells. Moreover, despite sharing a common cytoplasm, these nuclei multiply asynchronously. Schizogony challenges our current models of cell cycle regulation and, at the same time, offers targets for therapeutic interventions. Over the recent years, the adaptation of advanced molecular and cell biological techniques have given us deeper insight how DNA replication, nuclear division, and cytokinesis are coordinated. Here, we review our current understanding of the chronological events that characterize the unusual cell division cycle of P. falciparum in the clinically relevant blood stage of infection.

Red Blood Cells Oligosaccharides as Targets for Plasmodium Invasion

The key element in developing a successful malaria treatment is a good understanding of molecular mechanisms engaged in human host infection. It is assumed that oligosaccharides play a significant role in Plasmodium parasites binding to RBCs at different steps of host infection. The formation of a tight junction between EBL merozoite ligands and glycophorin receptors is the crucial interaction in ensuring merozoite entry into RBCs. It was proposed that sialic acid residues of O/N-linked glycans form clusters on a human glycophorins polypeptide chain, which facilitates the binding. Therefore, specific carbohydrate drugs have been suggested as possible malaria treatments. It was shown that the sugar moieties of N-acetylneuraminyl-N-acetate-lactosamine and 2,3-didehydro-2-deoxy-N-acetylneuraminic acid (DANA), which is its structural analog, can inhibit P. falciparum EBA-175-GPA interaction. Moreover, heparin-like molecules might be used as antimalarial drugs with some modifications to overcome their anticoagulant properties. Assuming that the principal interactions of Plasmodium merozoites and host cells are mediated by carbohydrates or glycan moieties, glycobiology-based approaches may lead to new malaria therapeutic targets.

Elucidating parasite and host-cell factors enabling Babesia infection in sickle red cells under hypoxic/hyperoxic conditions

Sickle red blood cells (RBCs) represent a naturally existing host-cell resistance mechanism to hemoparasite infections. We investigate the basis of this resistance using Babesia divergens grown in sickle (SS) and sickle trait (AS) cells. We found that oxygenation and its corresponding effect on RBC sickling, frequency of fetal hemoglobin positive (HbF+) cells, cellular redox environment, and parasite proliferation dynamics, all played a role in supporting or inhibiting Babesia proliferation. To identify cellular determinants that supported infection, an image flow cytometric tool was developed that could identify sickled cells and constituent Hb. We showed that hypoxic conditions impaired parasite growth in both SS and AS cells. Furthermore, cell sickling was alleviated by oxygenation (hyperoxic conditions), which decreased inhibition of parasite proliferation in SS cells. Interestingly, our tool identified HbF+-SS as host-cells of choice under both hypoxic and hyperoxic conditions, which was confirmed using cord RBCs containing high amounts of HbF+ cells. Uninfected SS cells showed a higher reactive oxygen species-containing environment, than AA or AS cells, which was further perturbed on infection. In hostile SS cells we found that Babesia alters its subpopulation structure, with 1N dominance under hypoxic conditions yielding to equivalent ratios of all parasite forms at hyperoxic conditions, favorable for growth. Multiple factors, including oxygenation and its impact on cell shape, HbF positivity, redox status, and parasite pleiotropy allow Babesia propagation in sickle RBCs. Our studies provide a cellular and molecular basis of natural resistance to Babesia, which will aid in defining novel therapies against human babesiosis.

The erythrocyte membrane properties of beta thalassaemia heterozygotes and their consequences for Plasmodium falciparum invasion

Malaria parasites such as Plasmodium falciparum have exerted formidable selective pressures on the human genome. Of the human genetic variants associated with malaria protection, beta thalassaemia (a haemoglobinopathy) was the earliest to be associated with malaria prevalence. However, the malaria protective properties of beta thalassaemic erythrocytes remain unclear. Here we studied the mechanics and surface protein expression of beta thalassaemia heterozygous erythrocytes, measured their susceptibility to P. falciparum invasion, and calculated the energy required for merozoites to invade them. We found invasion-relevant differences in beta thalassaemic cells versus matched controls, specifically: elevated membrane tension, reduced bending modulus, and higher levels of expression of the major invasion receptor basigin. However, these differences acted in opposition to each other with respect to their likely impact on invasion, and overall we did not observe beta thalassaemic cells to have lower P. falciparum invasion efficiency for any of the strains tested.

Deconstructing the parasite multiplication rate of Plasmodium falciparum

Epidemiological indicators describing population-level malaria transmission dynamics are widely used to guide policy recommendations. However, the determinants of malaria outcomes within individuals are still poorly understood. This conceptual gap partly reflects the fact that there are few indicators that robustly predict the trajectory of individual infections or clinical outcomes. The parasite multiplication rate (PMR) is a widely used indicator for the Plasmodium intraerythrocytic development cycle (IDC), for example, but its relationship to clinical outcomes is complex. Here, we review its calculation and use in P. falciparum malaria research, as well as the parasite and host factors that impact it. We also provide examples of metrics that can help to link within-host dynamics to malaria clinical outcomes when used alongside the PMR.

From marginal to essential: the golden thread between nutrient sensing, medium composition and Plasmodium vivax maturation in in vitro culture

Historically neglected, due to its biological peculiarities, the absence of a continuous long-term in vitro blood stage culture system and a propensity towards high morbidity rather than mortality, Plasmodium vivax was put back on the agenda during the last decade by the paradigm shift in the fight against malaria from malaria control to malaria eradication. While the incidence of the deadliest form of malaria, Plasmodium falciparum malaria, has declined since this paradigm shift took hold, the prospects of eradication are now threatened by the increase in the incidence of other human malaria parasite species. Plasmodium vivax is geographically the most widely distributed human malaria parasite, characterized by millions of clinical cases every year and responsible for a massive economic burden. The urgent need to tackle the unique biological challenges posed by this parasite led to renewed efforts aimed at establishing a continuous, long-term in vitro P. vivax blood stage culture. Based on recent discoveries on the role of nutrient sensing in Plasmodium’s pathophysiology, this review article critically assesses the extensive body of literature concerning Plasmodium culture conditions with a specific focus on culture media used in attempts to culture different Plasmodium spp. Hereby, the effect of specific media components on the parasite’s in vitro fitness and the maturation of the parasite’s host cell, the reticulocyte, is analysed. Challenging the wide-held belief that it is sufficient to find the right parasite isolate and give it the right type of cells to invade for P. vivax to grow in vitro, this review contends that a healthy side-by-side maturation of both the parasite and its host cell, the reticulocyte, is necessary in the adaptation of P. vivax to in vitro growth and argues that culture conditions and the media in particular play an essential role in this maturation process.

Validity of simple clinical and biological parameters as screening tool for sickle cell anemia for referral to tertiary center in highly resource constraints

BACKGROUND

In the Democratic Republic of Congo, the incidence of sickle cell anemia (SCA) is estimated around 40 000 neonates per year. However, it is notoriously difficult to perform conventional electrophoresis in all hospitals and laboratories, especially at peripheral levels and rural area. A panel of multiple clinical and laboratory features that would enhance sickle cell disease were assessed for the detection of the disease in highly resource-scarce settings.

METHODS

A prospective study was conducted in Kinshasa. Venous blood samples were drawn from each study participant in order to determine the hematologic parameters, the peripheral smears, and the hemoglobin electrophoresis. We used Cohen's κ statistic to examine the agreement of each variable and diagnosis of sickle cell disease.

RESULTS

A total of 807 patients were screened for sickle cell disease. Among these 807 children, 36 (4.5%) were homozygous for Hb S disease. The presence of at least 8% erythroblasts (PPV: 91%, NPV: 99%, sensitivity: 83.3%, specificity: 99.6%, κ value: .86) and sickle cells (PPV:100%, NPV: 98%, sensitivity: 50%, specificity: 100%, κ value: .66) in the peripheral blood smear had an acceptable agreement for sickle cell disease.

CONCLUSION

These two biological markers may guide the clinician in the decision-making to initiate the management of the children as a sickle cell patient, pending confirmation of the disease by electrophoresis techniques.

Oxidative insult can induce malaria-protective trait of sickle and fetal erythrocytes

Plasmodium falciparum infections can cause severe malaria, but not every infected person develops life-threatening complications. In particular, carriers of the structural haemoglobinopathies S and C and infants are protected from severe disease. Protection is associated with impaired parasite-induced host actin reorganization, required for vesicular trafficking of parasite-encoded adhesins, and reduced cytoadherence of parasitized erythrocytes in the microvasculature. Here we show that aberrant host actin remodelling and the ensuing reduced cytoadherence result from a redox imbalance inherent to haemoglobinopathic and fetal erythrocytes. We further show that a transient oxidative insult to wild-type erythrocytes before infection with P. falciparum induces the phenotypic features associated with the protective trait of haemoglobinopathic and fetal erythrocytes. Moreover, pretreatment of mice with the pro-oxidative nutritional supplement menadione mitigate the development of experimental cerebral malaria. Our results identify redox imbalance as a causative principle of protection from severe malaria, which might inspire host-directed intervention strategies.

Ticket to ride: export of proteins to the Plasmodium falciparum-infected erythrocyte

The malaria parasite Plasmodium falciparum exports numerous proteins to its chosen host cell, the mature human erythrocyte. Many of these proteins are important for parasite survival. To reach the host cell, parasites must cross multiple membrane barriers and then furthermore be targeted to their correct sub-cellular localisation. This novel transport pathway has received much research attention in the past decades, especially as many of the mechanisms are expected to be parasite-specific and thus potential targets for drug development. In this article we summarize some of the most recent advances in this field, and highlight areas in which further research is needed.

Hemoglobin E Prevalence among Ethnic Groups Residing in Malaria-Endemic Areas of Northern Thailand and Its Lack of Association with Plasmodium falciparum Invasion In Vitro

Hemoglobin E (HbE) is one of the most common hemoglobin variants caused by a mutation in the β-globin gene, and found at high frequencies in various Southeast Asian groups. We surveyed HbE prevalence among 8 ethnic groups residing in 5 villages selected for their high period malaria endemicity, and 5 for low endemicity in northern Thailand, in order to uncover factors which may affect genetic persistence of HbE in these groups. We found the overall HbE prevalence 6.7%, with differing frequencies from 0% in the Pwo Karen, the Lawa, and the Skaw Karen to 24% in the Mon. All HbE genes were heterozygous (AE). Differences in HbE prevalence among the studied ethnic groups indirectly documents that ancestries and evolutionary forces, such as drift and admixture, are the important factors in the persistence of HbE distribution in northern Thailand. Furthermore, the presence of HbE in groups of northern Thailand had no effect on the in vitro infectivity and proliferation of Plasmodium falciparum, nor the production of hemozoin, a heme crystal produced by malaria parasites, when compared to normal red-blood-cell controls. Our data may contribute to a better understanding on the persistence of HbE among ethnic groups and its association with malaria.

Hemoglobin S and C affect protein export in Plasmodium falciparum-infected erythrocytes

Malaria is a potentially deadly disease. However, not every infected person develops severe symptoms. Some people are protected by naturally occurring mechanisms that frequently involve inheritable modifications in their hemoglobin. The best studied protective hemoglobins are the sickle cell hemoglobin (HbS) and hemoglobin C (HbC) which both result from a single amino acid substitution in β-globin: glutamic acid at position 6 is replaced by valine or lysine, respectively. How these hemoglobinopathies protect from severe malaria is only partly understood. Models currently proposed in the literature include reduced disease-mediating cytoadherence of parasitized hemoglobinopathic erythrocytes, impaired intraerythrocytic development of the parasite, dampened inflammatory responses, or a combination thereof. Using a conditional protein export system and tightly synchronized Plasmodium falciparum cultures, we now show that export of parasite-encoded proteins across the parasitophorous vacuolar membrane is delayed, slower, and reduced in amount in hemoglobinopathic erythrocytes as compared to parasitized wild type red blood cells. Impaired protein export affects proteins targeted to the host cell cytoplasm, Maurer's clefts, and the host cell plasma membrane. Impaired protein export into the host cell compartment provides a mechanistic explanation for the reduced cytoadherence phenotype associated with parasitized hemoglobinopathic erythrocytes.