Early Phagocytosis of Glucose-6-Phosphate Dehydrogenase (G6PD)-Deficient Erythrocytes Parasitized by Plasmodium falciparum May Explain Malaria Protection in G6PD Deficiency

An Optimized Dihydrodibenzothiazepine Lead Compound (SBI-0797750) as a Potent and Selective Inhibitor of Plasmodium falciparum and P. vivax Glucose 6-Phosphate Dehydrogenase 6-Phosphogluconolactonase

In Plasmodium, the first two and rate-limiting enzymes of the pentose phosphate pathway, glucose 6-phosphate dehydrogenase (G6PD) and the 6-phosphogluconolactonase, are bifunctionally fused to a unique enzyme named GluPho, differing structurally and mechanistically from the respective human orthologs. Consistent with the enzyme's essentiality for malaria parasite proliferation and propagation, human G6PD deficiency has immense impact on protection against severe malaria, making PfGluPho an attractive antimalarial drug target. Herein we report on the optimized lead compound N-(((2R,4S)-1-cyclobutyl-4-hydroxypyrrolidin-2-yl)methyl)-6-fluoro-4-methyl-11-oxo-10,11-dihydrodibenzo[b,f][1,4]thiazepine-8-carboxamide (SBI-0797750), a potent and fully selective PfGluPho inhibitor with robust nanomolar activity against recombinant PfGluPho, PvG6PD, and P. falciparum blood-stage parasites. Mode-of-action studies have confirmed that SBI-0797750 disturbs the cytosolic glutathione-dependent redox potential, as well as the cytosolic and mitochondrial H₂O₂ homeostasis of P. falciparum blood stages, at low nanomolar concentrations. Moreover, SBI-0797750 does not harm red blood cell (RBC) integrity and phagocytosis and thus does not promote anemia. SBI-0797750 is therefore a very promising antimalarial lead compound.

Vitamin C Inhibits Blood-Stage Plasmodium Parasites via Oxidative Stress

During the Plasmodium erythrocytic cycle, glucose is taken up by glucose transporters (GLUTs) in red blood cells (RBCs) and supplied to parasites via the Plasmodium hexose transporter. Here, we demonstrate that the glucose uptake pathway in infected RBCs (iRBCs) can be hijacked by vitamin C (Vc). GLUTs preferentially transport the oxidized form of Vc, which is subsequently reduced in the cytosol. Vc, which is expected to burden the intracellular reducing capacity, inhibits Plasmodium berghei and Plasmodium falciparum growth. Vc uptake is drastically increased in iRBCs, with a large proportion entering parasites. Increased absorption of Vc causes accumulation of reactive oxygen species, reduced ATP production, and elevated eryptosis in iRBCs and apoptosis in parasites. The level of oxidative stress induced by Vc is significantly higher in iRBCs than uninfected RBCs, not seen in chloroquine or artemisinin-treated iRBCs, and effective in inhibiting chloroquine or artemisinin-resistant parasites. These findings provide important insights into the drug sensitivity of Plasmodium.

Less Severe Cases of COVID-19 in Sub-Saharan Africa: Could Co-infection or a Recent History of Plasmodium falciparum Infection Be Protective?

Sub-Saharan Africa has generally experienced few cases and deaths of coronavirus disease 2019 (COVID-19). In addition to other potential explanations for the few cases and deaths of COVID-19 such as the population socio-demographics, early lockdown measures and the possibility of under reporting, we hypothesize in this mini review that individuals with a recent history of malaria infection may be protected against infection or severe form of COVID-19. Given that both the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and Plasmodium falciparum (P. falciparum) merozoites bind to the cluster of differentiation 147 (CD147) immunoglobulin, we hypothesize that the immunological memory against P. falciparum merozoites primes SARS-CoV-2 infected cells for early phagocytosis, hence protecting individuals with a recent P. falciparum infection against COVID-19 infection or severity. This mini review therefore discusses the potential biological link between P. falciparum infection and COVID-19 infection or severity and further highlights the importance of CD147 immunoglobulin as an entry point for both SARS-CoV-2 and P. falciparum into host cells.

Functions of Antibodies

Antibodies can impact pathogens in the presence or in the absence of effector cells or effector molecules such as complement, and experiments can often sort out with precision the mechanisms by which an antibody inhibits a pathogen in vitro . In addition, in vivo models, particularly those engineered to knock in or knock out effector cells or effector molecules, are excellent tools for understanding antibody functions. However, it is highly likely that multiple antibody functions occur simultaneously or sequentially in the presence of an infecting organism in vivo . The most critical incentive for measuring antibody functions is to provide a basis for vaccine development and for the development of therapeutic antibodies. In this respect, some functions, such as virus neutralization, serve to inhibit the acquisition of a pathogen or limit its pathogenesis. However, antibodies can also enhance replication or contribute to pathogenesis. This review emphasizes those antibody functions that are potentially beneficial to the host. In addition, this review will focus on the effects of antibodies on organisms themselves, rather than on the toxins the organisms may produce.

Ion transport pathology in the mechanism of sickle cell dehydration

Polymers of deoxyhemoglobin S deform sickle cell anemia red blood cells into sickle shapes, leading to the formation of dense, dehydrated red blood cells with a markedly shortened life-span. Nearly four decades of intense research in many laboratories has led to a mechanistic understanding of the complex events leading from sickling-induced permeabilization of the red cell membrane to small cations, to the generation of the heterogeneity of age and hydration condition of circulating sickle cells. This review follows chronologically the major experimental findings and the evolution of guiding ideas for research in this field. Predictions derived from mathematical models of red cell and reticulocyte homeostasis led to the formulation of an alternative to prevailing gradualist views: a multitrack dehydration model based on interactive influences between the red cell anion exchanger and two K(+) transporters, the Gardos channel (hSK4, hIK1) and the K-Cl cotransporter (KCC), with differential effects dependent on red cell age and variability of KCC expression among reticulocytes. The experimental tests of the model predictions and the amply supportive results are discussed. The review concludes with a brief survey of the therapeutic strategies aimed at preventing sickle cell dehydration and with an analysis of the main open questions in the field.

Nonopsonic monocyte/macrophage phagocytosis of Plasmodium falciparum–parasitized erythrocytes: a role for CD36 in malarial clearance

Plasmodium falciparum is the most lethal form of malaria and is increasing both in incidence and in its resistance to antimalarial agents. An improved understanding of the mechanisms of malarial clearance may facilitate the development of new therapeutic interventions. We postulated that the scavenger receptor CD36, an important factor in cytoadherence of P falciparum–parasitized erythrocytes (PEs), might also play a role in monocyte- and macrophage-mediated malarial clearance. Exposure of nonopsonized PEs to Fc receptor–blocked monocytes resulted in significant PE phagocytosis, accompanied by intense clustering of CD36 around the PEs. Phagocytosis was blocked 60% to 70% by monocyte pretreatment with monoclonal anti-CD36 antibodies but not by antibodies to αvβ3, thrombospondin, intercellular adhesion molecule-1, or platelet/endothelial cell adhesion molecule-1. Antibody-induced CD36 cross-linking did result in the early increase of surface CD11b expression, but there was no increase in, or priming for, tumor necrosis factor (TNF)-α secretion following either CD36 cross-linking or PE phagocytosis. CD36 clustering does support intracellular signaling: Antibody-induced cross-linking initiated intracellular tyrosine phosphorylation as well as extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK) phosphorylation. Both broad-spectrum tyrosine kinase inhibition (genistein) and selective ERK and p38 MAPK inhibition (PD98059 and SB203580, respectively) reduced PE uptake to almost the same extent as CD36 blockade. Thus, CD36-dependent binding and signaling appears to be crucial for the nonopsonic clearance of PEs and does not appear to contribute to the increase in TNF-α that is prognostic of poor outcome in clinical malaria.