Measuring Plasmodium falciparum Erythrocyte Invasion Phenotypes Using Flow Cytometry

A Potent Kalihinol Analogue Disrupts Apicoplast Function and Vesicular Trafficking in P. falciparum Malaria

Here we report the discovery of MED6-189, a new analogue of the kalihinol family of isocyanoterpene (ICT) natural products. MED6-189 is effective against drug-sensitive and -resistant P. falciparum strains blocking both intraerythrocytic asexual replication and sexual differentiation. This compound was also effective against P. knowlesi and P. cynomolgi. In vivo efficacy studies using a humanized mouse model of malaria confirms strong efficacy of the compound in animals with no apparent hemolytic activity or apparent toxicity. Complementary chemical biology, molecular biology, genomics and cell biological analyses revealed that MED6-189 primarily targets the parasite apicoplast and acts by inhibiting lipid biogenesis and cellular trafficking. Genetic analyses in P. falciparum revealed that a mutation in PfSec13, which encodes a component of the parasite secretory machinery, reduced susceptibility to the drug. The high potency of MED6-189 in vitro and in vivo, its broad range of efficacy, excellent therapeutic profile, and unique mode of action make it an excellent addition to the antimalarial drug pipeline.

F-erythrocytes promote Plasmodium falciparum proliferation in sickle cell disease

BACKGROUND

Sickle cell disease (SCD) remains prevalent because heterozygous carriers (HbAS) are partially resistant to Plasmodium falciparum malaria. Sickle hemoglobin (HbS) polymerization in low and intermediate oxygen (O2 ) conditions is the main driver of HbAS-driven resistance to P. falciparum malaria. However, epidemiological studies have reported mixed malaria morbidity and mortality outcomes in individuals with sickle cell disease (SCD). While maximum-tolerated dose hydroxyurea has been shown to lower malaria incidence, fetal hemoglobin (HbF), an inhibitor of HbS polymerization that is variably packaged in F-erythrocytes, might provide hemoglobin that is accessible to the parasite for feeding.

METHODS

To explore that risk, we examined the effect of variable mean corpuscular fetal hemoglobin (MCHF) on P. falciparum proliferation, invasion, and development in HbSS RBCs.

RESULTS

We found that greater MCHF in HbSS red blood cells (RBCs) is associated with increased P. falciparum proliferation in O2 environments comparable with the microcirculation. Moreover, both parasite invasion and intracellular growth, the major components of proliferation, occur predominantly in F-erythrocytes and are augmented with increasing MCHF.

CONCLUSIONS

HbF modifies P. falciparum infection in HbSS RBCs, further highlighting the complexity of the molecular interactions between these two diseases. Other inhibitors of HbS polymerization that do not increase HbF or F-erythrocytes should be independently assessed for their effects on P. falciparum malaria proliferation in HbSS RBCs.

A modified two-color flow cytometry assay to quantify in-vitro reinvasion and determine invasion phenotypes at low Plasmodium falciparum parasitemia

Invasion of human red blood cells (RBCs) by Plasmodium parasites is a crucial yet poorly characterised phenotype. Two-color flow cytometry (2cFCM) promises to be a very sensitive and high throughput method for phenotyping parasite invasion. However, current protocols require high (~1.0%) parasitemia for assay set-up and need to be adapted for low parasitemia samples, which are becoming increasingly common in low transmission settings. Background fluorescence from nuclei-containing uninfected RBCs and high autologous reinvasion rates (merozoite invasion of donor uninfected RBCs present at 50% assay volume) are some of the limitations to the method's sensitivity to enumerate low parasitemia (<0.5%) with nucleic acid-based stains. Here, we describe modifications for plating unlabeled donor to labeled target RBCs per assay well and for gating parasitemia, that produces accurate quantifications of low reinvasion parasitemia. Plasmodium falciparum 3D7, Dd2 and field isolates at various low and high parasitemia (0.05%-2.0%) were used to set-up SyBr Green 1-based 2cFCM invasion assays. Target RBCs were labeled with CTFR proliferation dye. We show that this dye combination allowed for efficient parasite invasion into target RBCs and that a 1:3 ratio of unlabeled to labeled RBCs per assay greatly skewed autologous reinvasion (p < 0.001). Accuracy of quantifying reinvasion was limited to an assay parasitemia of 0.02% with minimal background interference. Invasion inhibition by enzymatic treatments increased averagely by 10% (p<0.05) across the entire parasitemia range. The effect was greater for samples with <0.5% parasitemia. Overall, a more sensitive method for phenotyping invasion of low P. falciparum parasitemia is described.

Preliminary Evaluation of the Safety and Immunogenicity of an Antimalarial Vaccine Candidate Modified Peptide (IMPIPS) Mixture in a Murine Model

Malaria continues being a high-impact disease regarding public health worldwide; the WHO report for malaria in 2018 estimated that ~219 million cases occurred in 2017, mostly caused by the parasite Plasmodium falciparum. The disease cost the lives of more than 400,000 people, mainly in Africa. In spite of great efforts aimed at developing better prevention (i.e., a highly effective vaccine), diagnosis, and treatment methods for malaria, no efficient solution to this disease has been advanced to date. The Fundación Instituto de Inmunología de Colombia (FIDIC) has been developing studies aimed at furthering the search for vaccine candidates for controlling P. falciparum malaria. However, vaccine development involves safety and immunogenicity studies regarding their formulation in animal models before proceeding to clinical studies. The present work has thus been aimed at evaluating the safety and immunogenicity of a mixture of 23 chemically synthesised, modified peptides (immune protection-inducing protein structure (IMPIPS)) derived from different P. falciparum proteins. Single and repeat dose assays were thus used with male and female BALB/c mice which were immunised with the IMPIPS mixture. It was found that single and repeat dose immunisation with the IMPIPS mixture was safe, both locally and systemically. It was observed that the antibodies so stimulated recognised the parasite's native proteins and inhibited merozoite invasion of red blood cells in vitro when evaluating the humoral immune response induced by the IMPIPS mixture. Such results suggested that the IMPIPS peptide mixture could be a safe candidate to be tested during the next stage involved in developing an antimalarial vaccine, evaluating local safety, immunogenicity, and protection in a nonhuman primate model.