Plasmodium falciparum Clearance Is Rapid and Pitting Independent in Immune Malian Children Treated With Artesunate for Malaria

Background. In Plasmodium falciparum-infected patients treated with artemisinins, parasitemia declines through so-called pitting, an innate splenic process that transforms infected red blood cells (iRBCs) into onceinfected RBCs (O-iRBCs). Methods. We measured pitting in 83 French travelers and 42 Malian children treated for malaria with artesunate. Results. In travelers, O-iRBCs peaked at 107.7% initial parasitemia. In Malian children aged 1.5-4 years, OiRBCs peaked at higher concentrations than in children aged 9-13 years (91.60% vs 31.95%; P = .0097). The parasite clearance time in older children was shorter than in younger children (P = .0001), and the decline in parasitemia in children aged 1.5-4 years often started 6 hours after treatment initiation, a lag phase generally absent in infants and older children. A 6-hour lag phase in artificial pitting of artesunate-exposed iRBCs was also observed in vitro. The proportion of iRBCs recognized by autologous immunoglobulin G (IgG) correlated with the parasite clearance time (r = −0.501; P = .0006) and peak O-iRBC concentration (r = −0.420; P = .0033). Conclusions. Antimalarial immunity correlates with fast artemisinin-induced parasite clearance and low pitting rates. In nonimmune populations, artemisinin-induced P. falciparum clearance is related to pitting and starts after a 6-hour lag phase. In immune populations, passively and naturally acquired immune mechanisms operating faster than pitting may exist. This mechanism may mitigate the emergence of artemisinin-resistant P. falciparum in Africa

Effect of red blood cell variants on childhood malaria in Mali: a prospective cohort study

Red blood cell (RBC) variants protect African children from severe Plasmodium falciparum malaria. Their individual and interactive impacts on mild disease and parasite density, and their modification by age-dependent immunity, are poorly understood

A Role for Fetal Hemoglobin and Maternal Immune IgG in Infant Resistance to Plasmodium falciparum Malaria

In Africa, infant susceptibility to Plasmodium falciparum malaria increases substantially as fetal hemoglobin (HbF) and maternal immune IgG disappear from circulation. During the first few months of life, however, resistance to malaria is evidenced by extremely low parasitemias, the absence of fever, and the almost complete lack of severe disease. This resistance has previously been attributed in part to poor parasite growth in HbF-containing red blood cells (RBCs). A specific role for maternal immune IgG in infant resistance to malaria has been hypothesized but not yet identified.We found that P. falciparum parasites invade and develop normally in fetal (cord blood, CB) RBCs, which contain up to 95% HbF. However, these parasitized CB RBCs are impaired in their binding to human microvascular endothelial cells (MVECs), monocytes, and nonparasitized RBCs--cytoadherence interactions that have been implicated in the development of high parasite densities and the symptoms of malaria. Abnormal display of the parasite's cytoadherence antigen P. falciparum erythrocyte membrane protein-1 (PfEMP-1) on CB RBCs accounts for these findings and is reminiscent of that on HbC and HbS RBCs. IgG purified from the plasma of immune Malian adults almost completely abolishes the adherence of parasitized CB RBCs to MVECs.Our data suggest a model of malaria protection in which HbF and maternal IgG act cooperatively to impair the cytoadherence of parasitized RBCs in the first few months of life. In highly malarious areas of Africa, an infant's contemporaneous expression of HbC or HbS and development of an immune IgG repertoire may effectively reconstitute the waning protective effects of HbF and maternal immune IgG, thereby extending the malaria resistance of infancy into early childhood

α-Thalassemia Impairs the Cytoadherence of Plasmodium falciparum-Infected Erythrocytes

α-Thalassemia results from decreased production of α-globin chains that make up part of hemoglobin tetramers (Hb; α(2)β(2)) and affects up to 50% of individuals in some regions of sub-Saharan Africa. Heterozygous (-α/αα) and homozygous (-α/-α) genotypes are associated with reduced risk of severe Plasmodium falciparum malaria, but the mechanism of this protection remains obscure. We hypothesized that α-thalassemia impairs the adherence of parasitized red blood cells (RBCs) to microvascular endothelial cells (MVECs) and monocytes--two interactions that are centrally involved in the pathogenesis of severe disease.We obtained P. falciparum isolates directly from Malian children with malaria and used them to infect αα/αα (normal), -α/αα and -α/-α RBCs. We also used laboratory-adapted P. falciparum clones to infect -/-α RBCs obtained from patients with HbH disease. Following a single cycle of parasite invasion and maturation to the trophozoite stage, we tested the ability of parasitized RBCs to bind MVECs and monocytes. Compared to parasitized αα/αα RBCs, we found that parasitized -α/αα, -α/-α and -/-α RBCs showed, respectively, 22%, 43% and 63% reductions in binding to MVECs and 13%, 33% and 63% reductions in binding to monocytes. α-Thalassemia was associated with abnormal display of P. falciparum erythrocyte membrane protein 1 (PfEMP1), the parasite's main cytoadherence ligand and virulence factor, on the surface of parasitized RBCs.Parasitized α-thalassemic RBCs show PfEMP1 display abnormalities that are reminiscent of those on the surface of parasitized sickle HbS and HbC RBCs. Our data suggest a model of malaria protection in which α-thalassemia ameliorates the pro-inflammatory effects of cytoadherence. Our findings also raise the possibility that other unstable hemoglobins such as HbE and unpaired α-globin chains (in the case of β-thalassemia) protect against life-threatening malaria by a similar mechanism

Relationship between malaria incidence and IgG levels to Plasmodium falciparum merozoite antigens in Malian children: impact of hemoglobins S and C.

Heterozygous hemoglobin (Hb) AS (sickle-cell trait) and HbAC are hypothesized to protect against Plasmodium falciparum malaria in part by enhancing naturally-acquired immunity to this disease. To investigate this hypothesis, we compared antibody levels to four merozoite antigens from the P. falciparum 3D7 clone (apical membrane antigen 1, AMA1-3D7; merozoite surface protein 1, MSP1-3D7; 175 kDa erythrocyte-binding antigen, EBA175-3D7; and merozoite surface protein 2, MSP2-3D7) in a cohort of 103 HbAA, 73 HbAS and 30 HbAC children aged 3 to 11 years in a malaria-endemic area of Mali. In the 2009 transmission season we found that HbAS, but not HbAC, significantly reduced the risk of malaria compared to HbAA. IgG levels to MSP1 and MSP2 at the start of this transmission season inversely correlated with malaria incidence after adjusting for age and Hb type. However, HbAS children had significantly lower IgG levels to EBA175 and MSP2 compared to HbAA children. On the other hand, HbAC children had similar IgG levels to all four antigens. The parasite growth-inhibitory activity of purified IgG samples did not differ significantly by Hb type. Changes in antigen-specific IgG levels during the 2009 transmission and 2010 dry seasons also did not differ by Hb type, and none of these IgG levels dropped significantly during the dry season. These data suggest that sickle-cell trait does not reduce the risk of malaria by enhancing the acquisition of IgG responses to merozoite antigens

A potential role for plasma uric acid in the endothelial pathology of Plasmodium falciparum malaria.

BACKGROUND: Inflammatory cytokinemia and systemic activation of the microvascular endothelium are central to the pathogenesis of Plasmodium falciparum malaria. Recently, 'parasite-derived' uric acid (UA) was shown to activate human immune cells in vitro, and plasma UA levels were associated with inflammatory cytokine levels and disease severity in Malian children with malaria. Since UA is associated with endothelial inflammation in non-malaria diseases, we hypothesized that elevated UA levels contribute to the endothelial pathology of P. falciparum malaria. METHODOLOGY/PRINCIPAL FINDINGS: We measured levels of UA and soluble forms of intercellular adhesion molecule-1 (sICAM-1), vascular cell adhesion molecule-1 (sVCAM-1), E-selectin (sE-Selectin), thrombomodulin (sTM), tissue factor (sTF) and vascular endothelial growth factor (VEGF) in the plasma of Malian children aged 0.5-17 years with uncomplicated malaria (UM, n = 487) and non-cerebral severe malaria (NCSM, n = 68). In 69 of these children, we measured these same factors once when they experienced a malaria episode and twice when they were healthy (i.e., before and after the malaria transmission season). We found that levels of UA, sICAM-1, sVCAM-1, sE-Selectin and sTM increase during a malaria episode and return to basal levels at the end of the transmission season (p<0.0001). Plasma levels of UA and these four endothelial biomarkers correlate with parasite density and disease severity. In children with UM, UA levels correlate with parasite density (r = 0.092, p = 0.043), sICAM-1 (r = 0.255, p<0.0001) and sTM (r = 0.175, p = 0.0001) levels. After adjusting for parasite density, UA levels predict sTM levels. CONCLUSIONS/SIGNIFICANCE: Elevated UA levels may contribute to malaria pathogenesis by damaging endothelium and promoting a procoagulant state. The correlation between UA levels and parasite densities suggests that parasitized erythrocytes are one possible source of excess UA. UA-induced shedding of endothelial TM may represent a novel mechanism of malaria pathogenesis, in which activated thrombin induces fibrin deposition and platelet aggregation in microvessels. This protocol is registered at clinicaltrials.gov (NCT00669084)

Distribution and morphology of knobs on the surface of parasitized RBCs.

<p>Atomic force micrographs (AFMs) of parasitized −α/αα (HE) (<b>a,d</b>) and −α/−α (HO) (<b>b,e</b>) RBCs obtained from naturally-parasitized Malian children with malaria and −/−α (HH) (<b>c,f</b>) RBCs infected with a laboratory-adapted <i>P. falciparum</i> clone showing normal (<b>a,b</b>) or abnormal (<b>c–f</b>) knob distributions and morphologies. AFM images are representative of 32, 10 and 18 images of parasites in −α/αα, −/−αα and −/−α RBCs. Inlays show YOYO-1-stained parasites that correspond to those imaged by AFM. Comparison AFMs of parasitized HbA, HbC and HbS RBCs have been reported previously <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037214#pone.0037214-Arie1" target="_blank">[22]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037214#pone.0037214-Cholera1" target="_blank">[37]</a>.</p

Plasma uric acid levels correlate with inflammation and disease severity in Malian children with Plasmodium falciparum malaria.

Plasmodium falciparum elicits host inflammatory responses that cause the symptoms and severe manifestations of malaria. One proposed mechanism involves formation of immunostimulatory uric acid (UA) precipitates, which are released from sequestered schizonts into microvessels. Another involves hypoxanthine and xanthine, which accumulate in parasitized red blood cells (RBCs) and may be converted by plasma xanthine oxidase to UA at schizont rupture. These two forms of 'parasite-derived' UA stimulate immune cells to produce inflammatory cytokines in vitro.We measured plasma levels of soluble UA and inflammatory cytokines and chemokines (IL-6, IL-10, sTNFRII, MCP-1, IL-8, TNFα, IP-10, IFNγ, GM-CSF, IL-1β) in 470 Malian children presenting with uncomplicated malaria (UM), non-cerebral severe malaria (NCSM) or cerebral malaria (CM). UA levels were elevated in children with NCSM (median 5.74 mg/dl, 1.21-fold increase, 95% CI 1.09-1.35, n = 23, p = 0.0007) and CM (median 5.69 mg/dl, 1.19-fold increase, 95% CI 0.97-1.41, n = 9, p = 0.0890) compared to those with UM (median 4.60 mg/dl, n = 438). In children with UM, parasite density and plasma creatinine levels correlated with UA levels. These UA levels correlated with the levels of seven cytokines [IL-6 (r = 0.259, p<0.00001), IL-10 (r = 0.242, p<0.00001), sTNFRII (r = 0.221, p<0.00001), MCP-1 (r = 0.220, p<0.00001), IL-8 (r = 0.147, p = 0.002), TNFα (r = 0.132, p = 0.006) and IP-10 (r = 0.120, p = 0.012)]. In 39 children, UA levels were 1.49-fold (95% CI 1.34-1.65; p<0.0001) higher during their malaria episode [geometric mean titer (GMT) 4.67 mg/dl] than when they were previously healthy and aparasitemic (GMT 3.14 mg/dl).Elevated UA levels may contribute to the pathogenesis of P. falciparum malaria by activating immune cells to produce inflammatory cytokines. While this study cannot identify the cause of elevated UA levels, their association with parasite density and creatinine levels suggest that parasite-derived UA and renal function may be involved. Defining pathogenic roles for parasite-derived UA precipitates, which we have not directly studied here, requires further investigation.ClinicalTrials.gov NCT00669084

Relative cytoadherence and surface PfEMP1 levels of parasitized RBCs. a,

<p>Adherence of parasitized RBCs to MVECs. The numbers of parasitized −α/αα (HE), −α/−α (HO) and −/−α (HH) RBCs adhering to MVECs were normalized to those of parasitized αα/αα RBCs tested in parallel. The mean (± SEM) number of parasitized αα/αα RBCs per 100 MVECs was 260±40, <i>N</i> = 19. Results were obtained from 19 naturally-circulating parasite isolates and 2 laboratory-adapted parasite clones (A4tres and FCR-3), multiple blood donors (5 αα/αα, 2−α/αα, 2 −α/−α and 2−/−α), and 4 MVEC donors (not all combinations tested). This resulted in −α/αα, −α/−α and −/−α samples being compared to αα/αα samples 12, 5 and 4 times. <b>b,</b> Adherence of parasitized RBCs to monocytes. The numbers of parasitized −α/αα, −α/−α and −/−α RBCs adhering to monocytes were normalized to those of αα/αα RBCs tested in parallel. The mean (± SEM) number of parasitized αα/αα RBCs per 100 monocytes was 136±10, <i>N</i> = 12. Results were obtained from 3 naturally-circulating parasite isolates and 3 laboratory-adapted parasite clones (3D7, A4tres and FCR-3), multiple blood donors (5 αα/αα, 3 −α/αα, 2 −α/−α and 2−/−α) and 4 monocyte donors (not all combinations tested). This resulted in −α/αα, −α/−α and −/−α samples being compared to αα/αα samples 20, 3 and 4 times. The αα/αα and −α/αα RBCs were different from those used in endothelial cell adherence assays. <b>c,</b> PfEMP1 expression levels (median fluorescence intensities, MFI) on the surface of parasitized RBCs. The mean (± SEM) MFI of parasitized αα/αα RBCs was 556±153, <i>N</i> = 6. Results were obtained from 2 laboratory-adapted parasite clones (A4tres, FVO and FCR3^CSA), multiple blood donors (4 αα/αα, 6 −α/αα and 2−/−α), and various concentrations of 2 antisera (not all combinations tested). This resulted in −α/αα, and −/−α samples being compared to αα/αα samples 10 and 6 times. The αα/αα and −α/αα RBCs were different from those used in endothelial cell and monocyte adherence assays.</p