The membrane characteristics of Plasmodium falciparum-infected and -uninfected heterozygous alpha(0)thalassaemic erythrocytes

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.

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.

Adaptive immunity selects against malaria infection blocking mutations

The mutation responsible for Duffy negativity, which impedes Plasmodium vivax infection, has reached high frequencies in certain human populations. Conversely, mutations capable of blocking the more lethal P. falciparum have not succeeded in malarious zones. Here we present an evolutionary-epidemiological model of malaria which demonstrates that if adaptive immunity against the most virulent effects of malaria is gained rapidly by the host, mutations which prevent infection per se are unlikely to succeed. Our results (i) explain the rarity of strain-transcending P. falciparum infection blocking adaptations in humans; (ii) make the surprising prediction that mutations which block P. falciparum infection are most likely to be found in populations experiencing low or infrequent malaria transmission, and (iii) predict that immunity against some of the virulent effects of P. vivax malaria may be built up over the course of many infections.

Human genetics and malaria resistance

Malaria has been the pre-eminent cause of early mortality in many parts of the world throughout much of the last five thousand years and, as a result, it is the strongest force for selective pressure on the human genome yet described. Around one third of the variability in the risk of severe and complicated malaria is now explained by additive host genetic effects. Many individual variants have been identified that are associated with malaria protection, but the most important all relate to the structure or function of red blood cells. They include the classical polymorphisms that cause sickle cell trait, α-thalassaemia, G6PD deficiency, and the major red cell blood group variants. More recently however, with improving technology and experimental design, others have been identified that include the Dantu blood group variant, polymorphisms in the red cell membrane protein ATP2B4, and several variants related to the immune response. Characterising how these genes confer their effects could eventually inform novel therapeutic approaches to combat malaria. Nevertheless, all together, only a small proportion of the heritable component of malaria resistance can be explained by the variants described so far, underscoring its complex genetic architecture and the need for continued research.

The role of the red blood cell in host defence against falciparum malaria: an expanding repertoire of evolutionary alterations

The malaria parasite has co-evolved with its human host as each organism struggles for resources and survival. The scars of this war are carried in the human genome in the form of polymorphisms that confer innate resistance to malaria. Clinical, epidemiological and genome-wide association studies have identified multiple polymorphisms in red blood cell (RBC) proteins that attenuate malaria pathogenesis. These include well-known polymorphisms in haemoglobin, intracellular enzymes, RBC channels, RBC surface markers, and proteins impacting the RBC cytoskeleton and RBC morphology. A better understanding of how changes in RBC physiology impact malaria pathogenesis may uncover new strategies to combat the disease.

The Prevalence of α-Thalassemia and Its Relation to Plasmodium falciparum Infection in Patients Presenting to Clinics in Two Distinct Ecological Zones in Ghana

Thalassemia and sickle cell disease constitute the most monogenic hemoglobin (Hb) disorders worldwide. Clinical symptoms of α(+)-thalassemia (α(+)-thal) are related to inadequate Hb production and accumulation of β- and/or γ-globin subunits. The association of thalassemia with malaria remains contentious, though from its distribution it appears to have offered some protection against the disease. Data on the prevalence of thalassemia in Ghana and its link with malaria is scanty and restricted. It was an objective of this cross-sectional study to determine the prevalence of thalassemia in areas representing two of Ghana's distinct ecological zones. The relationship between thalassemia and Plasmodium falciparium (P. falciparum) infection was also ascertained. Overall, 277 patients presenting to health facilities in the study areas were recruited to participate. Tests were carried out to determine the presence of α(+)-thal, sickle cell and malaria parasites in the blood samples of participants. The outcome of this study showed an α(+)-thal frequency of 19.9% for heterozygotes (-α/αα) and 6.8% for homozygotes (-α/-α). Plasmodium falciparum was detected in 17.7% of the overall study population and 14.9% in those with α(+)-thal. No association was observed between those with α(+)-thal and the study sites (p > 0.05). A test of the Hardy-Weinberg law yielded no significant difference (p < 0.001). Findings from this study suggest a modest distribution of α(+)-thal in Ghana with no bias to the ecological zones. Although the prevalence and parasite density were relatively low in those with the disorder, no association was found between them.

Mechanistic Studies of the Negative Epistatic Malaria-protective Interaction Between Sickle Cell Trait and α+thalassemia

Background

Individually, the red blood cell (RBC) polymorphisms sickle cell trait (HbAS) and α⁺thalassemia protect against severe Plasmodium falciparum malaria. It has been shown through epidemiological studies that the co-inheritance of both conditions results in a loss of the protection afforded by each, but the biological mechanisms remain unknown.

Methods

We used RBCs from > 300 donors of various HbAS and α⁺thalassemia genotype combinations to study the individual and combinatorial effects of these polymorphisms on a range of putative P. falciparum virulence phenotypes in-vitro, using four well-characterized P. falciparum laboratory strains. We studied cytoadhesion of parasitized RBCs (pRBCs) to the endothelial receptors CD36 and ICAM1, rosetting of pRBCs with uninfected RBCs, and pRBC surface expression of the parasite-derived adhesion molecule P. falciparum erythrocyte membrane protein-1 (PfEMP1).

Findings

We confirmed previous reports that HbAS pRBCs show reduced cytoadhesion, rosetting and PfEMP1 expression levels compared to normal pRBC controls. Furthermore, we found that co-inheritance of HbAS with α⁺thalassemia consistently reversed these effects, such that pRBCs of mixed genotype showed levels of cytoadhesion, rosetting and PfEMP1 expression that were indistinguishable from those seen in normal pRBCs. However, pRBCs with α⁺thalassemia alone showed parasite strain-specific effects on adhesion, and no consistent reduction in PfEMP1 expression.

Interpretation

Our data support the hypothesis that the negative epistasis between HbAS and α⁺thalassemia observed in epidemiological studies might be explained by host genotype-specific changes in the pRBC-adhesion properties that contribute to parasite sequestration and disease pathogenesis in vivo. The mechanism by which α⁺thalassemia on its own protects against severe malaria remains unresolved.

An assessment of the impact of host polymorphisms on Plasmodium falciparum var gene expression patterns among Kenyan children

BACKGROUND

Host genotype accounts for a component of the variability in susceptibility to childhood Plasmodium falciparum malaria. However, despite numerous examples of host polymorphisms associated with tolerance or resistance to infection, direct evidence for an impact of host genetic polymorphisms on the in vivo parasite population is difficult to obtain. Parasite molecules whose expression is most likely to be associated with such adaptation are those that are directly involved in the host-parasite interaction. A prime candidate is the family of parasite var gene-encoded molecules on P. falciparum-infected erythrocytes, PfEMP1, which binds various host molecules and facilitates parasite sequestration in host tissues to avoid clearance by the spleen.

METHODS

To assess the impact of host genotype on the infecting parasite population we used a published parasite var gene sequence dataset to compare var gene expression patterns between parasites from children with polymorphisms in molecules thought to interact with or modulate display of PfEMP1 on the infected erythrocyte surface: ABO blood group, haemoglobin S, alpha-thalassaemia, the T188G polymorphism of CD36 and the K29M polymorphism of ICAM1.

RESULTS

Expression levels of 'group A-like' var genes, which encode a specific group of PfEMP1 variants previously associated with low host immunity and severe malaria, showed signs of elevation among children of blood group AB. No other host factor tested showed evidence for an association with var expression.

CONCLUSIONS

Our preliminary findings suggest that host ABO blood group may have a measurable impact on the infecting parasite population. This needs to be verified in larger studies.

Epistasis between the haptoglobin common variant and α+thalassemia influences risk of severe malaria in Kenyan children

Haptoglobin (Hp) scavenges free hemoglobin following malaria-induced hemolysis. Few studies have investigated the relationship between the common Hp variants and the risk of severe malaria, and their results are inconclusive. We conducted a case-control study of 996 children with severe Plasmodium falciparum malaria and 1220 community controls and genotyped for Hp, hemoglobin (Hb) S heterozygotes, and α(+)thalassemia. Hb S heterozygotes and α(+)thalassemia homozygotes were protected from severe malaria (odds ratio [OR], 0.12; 95% confidence interval [CI], 0.07-0.18 and OR, 0.69; 95% CI, 0.53-0.91, respectively). The risk of severe malaria also varied by Hp genotype: Hp2-1 was associated with the greatest protection against severe malaria and Hp2-2 with the greatest risk. Meta-analysis of the current and published studies suggests that Hp2-2 is associated with increased risk of severe malaria compared with Hp2-1. We found a significant interaction between Hp genotype and α(+)thalassemia in predicting risk of severe malaria: Hp2-1 in combination with heterozygous or homozygous α(+)thalassemia was associated with protection from severe malaria (OR, 0.73; 95% CI, 0.54-0.99 and OR, 0.48; 95% CI, 0.32-0.73, respectively), but α(+)thalassemia in combination with Hp2-2 was not protective. This epistatic interaction together with varying frequencies of α(+)thalassemia across Africa may explain the inconsistent relationship between Hp genotype and malaria reported in previous studies.

Hemoglobinopathies: slicing the Gordian knot of Plasmodium falciparum malaria pathogenesis

Plasmodium falciparum malaria kills over 500,000 children every year and has been a scourge of humans for millennia. Owing to the co-evolution of humans and P. falciparum parasites, the human genome is imprinted with polymorphisms that not only confer innate resistance to falciparum malaria, but also cause hemoglobinopathies. These genetic traits—including hemoglobin S (HbS), hemoglobin C (HbC), and α-thalassemia—are the most common monogenic human disorders and can confer remarkable degrees of protection from severe, life-threatening falciparum malaria in African children: the risk is reduced 70% by homozygous HbC and 90% by heterozygous HbS (sickle-cell trait). Importantly, this protection is principally present for severe disease and largely absent for P. falciparum infection, suggesting that these hemoglobinopathies specifically neutralize the parasite's in vivo mechanisms of pathogenesis. These hemoglobin variants thus represent a “natural experiment” to identify the cellular and molecular mechanisms by which P. falciparum produces clinical morbidity, which remain partially obscured due to the complexity of interactions between this parasite and its human host. Multiple lines of evidence support a restriction of parasite growth by various hemoglobinopathies, and recent data suggest this phenomenon may result from host microRNA interference with parasite metabolism. Multiple hemoglobinopathies mitigate the pathogenic potential of parasites by interfering with the export of P. falciparum erythrocyte membrane protein 1 (PfEMP1) to the surface of the host red blood cell. Few studies have investigated their effects upon the activation of the innate and adaptive immune systems, although recent murine studies suggest a role for heme oxygenase-1 in protection. Ultimately, the identification of mechanisms of protection and pathogenesis can inform future therapeutics and preventive measures. Hemoglobinopathies slice the “Gordian knot” of host and parasite interactions to confer malaria protection, and offer a translational model to identify the most critical mechanisms of P. falciparum pathogenesis.

Evidence of recent natural selection on the Southeast Asian deletion (--(SEA)) causing α-thalassemia in South China

Background

The Southeast Asian deletion (--^SEA) is the most commonly observed mutation among diverse α-thalassemia alleles in Southeast Asia and South China. It is generally argued that mutation --^SEA, like other variants causing hemoglobin disorders, is associated with protection against malaria that is endemic in these regions. However, little evidence has been provided to support this claim.

Results

We first examined the genetic imprint of recent positive selection on the --^SEA allele and flanking sequences in the human α-globin cluster, covering a genomic region spanning ~410 kb, by genotyping 28 SNPs in a Chinese population consisting of 76 --^SEA heterozygotes and 138 normal individuals. The pattern of linkage disequilibrium (LD) and the long-range haplotype test revealed a signature of positive selection. The network of inferred haplotypes suggested a single origin of the --^SEA allele.

Conclusions

Thus, our data support the hypothesis that the --^SEA allele has been subjected to recent balancing selection, triggered by malaria.

α-Thalassemia impairs the cytoadherence of Plasmodium falciparum-infected erythrocytes

BACKGROUND

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.

METHODS AND FINDINGS

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.

CONCLUSIONS

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.

Genetics of susceptibility to Plasmodium falciparum: from classical malaria resistance genes towards genome-wide association studies

Plasmodium falciparum represents one of the strongest selective forces on the human genome. This stable and perennial pressure has contributed to the progressive accumulation in the exposed populations of genetic adaptations to malaria. Descriptive genetic epidemiology provides the initial step of a logical procedure of consequential phases spanning from the identification of genes involved in the resistance/susceptibility to diseases, to the determination of the underlying mechanisms and finally to the possible translation of the acquired knowledge in new control tools. In malaria, the rational development of this strategy is traditionally based on complementary interactions of heterogeneous disciplines going from epidemiology to vaccinology passing through genetics, pathogenesis and immunology. New tools including expression profile analysis and genome-wide association studies are recently available to explore the complex interactions of host-parasite co-evolution. Particularly, the combination of genome-wide association studies with large multi-centre initiatives can overcome the limits of previous results due to local population dynamics. Thus, we anticipate substantial advances in the interpretation and validation of the effects of genetic variation on malaria susceptibility, and thereby on molecular mechanisms of protective immune responses and pathogenesis.

Multiplicity of Plasmodium falciparum infection in asymptomatic children in Senegal: relation to transmission, age and erythrocyte variants

Background

Individuals living in malaria endemic areas generally harbour multiple parasite strains. Multiplicity of infection (MOI) can be an indicator of immune status. However, whether this is good or bad for the development of immunity to malaria, is still a matter of debate. This study aimed to examine the MOI in asymptomatic children between two and ten years of age and to relate it to erythrocyte variants, clinical attacks, transmission levels and other parasitological indexes.

Methods

Study took place in Niakhar area in Senegal, where malaria is mesoendemic and seasonal. Three hundred and seventy two asymptomatic children were included. Sickle-cell trait, G6PD deficiency (A- and Santamaria) and α⁺-thalassaemia (-α^3.7 type) were determined using PCR. Multiplicity of Plasmodium falciparum infection, i.e. number of concurrent clones, was defined by PCR-based genotyping of the merozoite surface protein-2 (msp2), before and at the end of the malaria transmission season. The χ²-test, ANOVA, multivariate linear regression and logistic regression statistical tests were used for data analysis.

Results

MOI was significantly higher at the end of transmission season. The majority of PCR positive subjects had multiple infections at both time points (64% before and 87% after the transmission season). MOI did not increase in α-thalassaemic and G6PD mutated children. The ABO system and HbAS did not affect MOI at any time points. No association between MOI and clinical attack was observed. MOI did not vary over age at any time points. There was a significant correlation between MOI and parasite density, as the higher parasite counts increases the probability of having multiple infections.

Conclusion

Taken together our data revealed that α-thalassaemia may have a role in protection against certain parasite strains. The protection against the increase in MOI after the transmission season conferred by G6PD deficiency is probably due to clearance of the malaria parasite at early stages of infection. The ABO system and HbAS are involved in the severity of the disease but do not affect asymptomatic infections. MOI was not age-dependent, in the range of two to ten years, but was correlated with parasite density. However some of these observations need to be confirmed including larger sample size with broader age range and using other msp2 genotyping method.

Cytoadherence between endothelial cells and P. falciparum infected and noninfected normal and thalassemic red blood cells

BACKGROUND

Cytoadhesion of P. falciparum infected red blood cells (RBCs) to endothelial cells (ECs) is an important phenomenon that causes cerebral malaria in man. Reduced adhesion especially in thalassemia and hemoglobinopathies may be related to a protective mechanism against malaria in such people.

METHODS

The cytoadherence assay was performed using both conventional and floating conditions between ECs (ECV 304) and P. falciparum infected and noninfected RBCs from both normal and thalassemia subjects. In floating condition, RBC was fluorescently labeled with anti-glycophorin A antibody, whereas EC was identified by surface expression of PECAM-1, CD-36, ICAM-1, and E-selectin. The condition of floating EC was similar to the condition for subcultivation as they can adhere or bind to any surface. The phosphatidylserine (PS) exposure was also determined by using flow cytometer.

RESULTS

The adhesion of noninfected heterozygous thalassemic RBCs (all genotypes) to ECs was significantly increased as compared with normal RBCs (P < 0.02). Interestingly, after P. falciparum infection, the number of normal RBCs bound to ECs was significantly increased as compared with noninfected RBCs (P < 0.01), whereas heterozygous thalassemic RBCs infected by P. falciparum showed no significant difference compared with noninfected RBCs. In addition, we found a similar level of PS exposure in normal and thalassemic infected RBCs, which was related to the cytoadherence phenomenon.

CONCLUSION

The reduced adhesion between heterozygous thalassemic RBCs infected by P. falciparum to ECs provides an explanation for their protective mechanism against malaria, as increased adhesion is a high risk for cerebral malaria and nonbinding infected RBCs can be removed by the reticuloendothelial system and other mechanism(s) in vivo.

Mendelian and complex genetics of susceptibility and resistance to parasitic infections

Uncovering the complex genetic basis of susceptibility and resistance to parasitic infectious diseases is an enormous challenge. It probably involves many different host genes, interacting with multiple parasite genetic and environmental factors. Several genes of interest have been identified by family and association studies in humans and by using mouse models, but more robust epidemiological studies and functional data are needed to authenticate these findings. With new technologies and statistical tools for whole-genome association analysis, the next few years are likely to see acceleration in the rate of gene discovery, which has the potential to greatly assist drug and vaccine development.

The effect of alpha+-thalassaemia on the incidence of malaria and other diseases in children living on the coast of Kenya

BACKGROUND

The alpha-thalassaemias are the commonest genetic disorders of humans. It is generally believed that this high frequency reflects selection through a survival advantage against death from malaria; nevertheless, the epidemiological description of the relationships between alpha-thalassaemia, malaria, and other common causes of child mortality remains incomplete.

METHODS AND FINDINGS

We studied the alpha+-thalassaemia-specific incidence of malaria and other common childhood diseases in two cohorts of children living on the coast of Kenya. We found no associations between alpha+-thalassaemia and the prevalence of symptomless Plasmodium falciparum parasitaemia, the incidence of uncomplicated P. falciparum disease, or parasite densities during mild or severe malaria episodes. However, we found significant negative associations between alpha+-thalassaemia and the incidence rates of severe malaria and severe anaemia (haemoglobin concentration < 50 g/l). The strongest associations were for severe malaria anaemia (> 10,000 P. falciparum parasites/mul) and severe nonmalaria anaemia; the incidence rate ratios and 95% confidence intervals (CIs) for alpha+-thalassaemia heterozygotes and homozygotes combined compared to normal children were, for severe malaria anaemia, 0.33 (95% CI, 0.15,0.73; p = 0.006), and for severe nonmalaria anaemia, 0.26 (95% CI, 0.09,0.77; p = 0.015).

CONCLUSIONS

Our observations suggest, first that selection for alpha+-thalassaemia might be mediated by a specific effect against severe anaemia, an observation that may lead to fresh insights into the aetiology of this important condition. Second, although alpha+-thalassaemia is strongly protective against severe and fatal malaria, its effects are not detectable at the level of any other malaria outcome; this result provides a cautionary example for studies aimed at testing malaria interventions or identifying new malaria-protective genes.

How malaria has affected the human genome and what human genetics can teach us about malaria

Malaria is a major killer of children worldwide and the strongest known force for evolutionary selection in the recent history of the human genome. The past decade has seen growing evidence of ethnic differences in susceptibility to malaria and of the diverse genetic adaptations to malaria that have arisen in different populations: epidemiological confirmation of the hypotheses that G6PD deficiency, alpha+ thalassemia, and hemoglobin C protect against malaria mortality; the application of novel haplotype-based techniques demonstrating that malaria-protective genes have been subject to recent positive selection; the first genetic linkage maps of resistance to malaria in experimental murine models; and a growing number of reported associations with resistance and susceptibility to human malaria, particularly in genes involved in immunity, inflammation, and cell adhesion. The challenge for the next decade is to build the global epidemiological infrastructure required for statistically robust genomewide association analysis, as a way of discovering novel mechanisms of protective immunity that can be used in the development of an effective malaria vaccine.

Erythrocyte variants and the nature of their malaria protective effect

The malaria threat to global health is exacerbated by widespread drug resistance in the Plasmodium parasite and its insect vector, and the lack of an efficacious vaccine. Infection with Plasmodium parasites can cause a wide spectrum of pathologies, from a transient mild form of anaemia to a severe and rapidly fatal cerebral disease. Epidemiological studies in humans and experiments in animal models have shown that genetic factors play a key role in the onset, progression, type of disease developed and ultimate outcome of malaria. The protective effect of polymorphic variants in erythrocyte-specific structural proteins or metabolic enzymes against the blood-stage of the disease is one of the clearest illustrations of this genetic modulation, and has suggested co-evolution of the Plasmodium parasite with its human host in areas of endemic disease. Here, we present a brief overview of erythrocyte polymorphisms with biological relevance to malaria pathogenesis, and current work on the mechanism(s) by which these mediate their protective effect. The recent addition of erythrocyte pyruvate kinase to this group of protective genes will also be discussed.

An immune basis for malaria protection by the sickle cell trait

BACKGROUND

Malaria resistance by the sickle cell trait (genotype HbAS) has served as the prime example of genetic selection for over half a century. Nevertheless, the mechanism of this resistance remains the subject of considerable debate. While it probably involves innate factors such as the reduced ability of Plasmodium falciparum parasites to grow and multiply in HbAS erythrocytes, recent observations suggest that it might also involve the accelerated acquisition of malaria-specific immunity.

METHODS AND FINDINGS

We studied the age-specific protection afforded by HbAS against clinical malaria in children living on the coast of Kenya. We found that protection increased with age from only 20% in the first 2 y of life to a maximum of 56% by the age of 10 y, returning thereafter to 30% in participants greater than 10 y old.

CONCLUSIONS

Our observations suggest that malaria protection by HbAS involves the enhancement of not only innate but also of acquired immunity to the parasite. A better understanding of the underlying mechanisms might yield important insights into both these processes.

Both heterozygous and homozygous alpha+ thalassemias protect against severe and fatal Plasmodium falciparum malaria on the coast of Kenya

Although the alpha+ thalassemias almost certainly confer protection against death from malaria, this has not been formally documented. We have conducted a study involving 655 case patients with rigorously defined severe malaria and 648 controls, frequency matched on area of residence and ethnic group. The prevalence of both heterozygous and homozygous alpha+ thalassemia was reduced in both case patients with severe malaria (adjusted odds ratios [ORs], 0.73 and 0.57; 95% confidence intervals [95% CIs], 0.57-0.94 and 0.40-0.81; P = .013 and P = .002, respectively, compared with controls) and among the subgroup of children who died after admission with severe malaria (OR, 0.60 and 0.37; 95% CI, 0.37-1.00 and 0.16-0.87; P = .05 and P = .02, respectively, compared with surviving case patients). The lowest ORs were seen for the forms of malaria associated with the highest mortality-coma and severe anemia complicated by deep, acidotic breathing. Our study supports the conclusion that both heterozygotes and homozygotes enjoy a selective advantage against death from Plasmodium falciparum malaria.

Redox and antioxidant systems of the malaria parasite Plasmodium falciparum

The malaria parasite Plasmodium falciparum is highly adapted to cope with the oxidative stress to which it is exposed during the erythrocytic stages of its life cycle. This includes the defence against oxidative insults arising from the parasite's metabolism of haemoglobin which results in the formation of reactive oxygen species and the release of toxic ferriprotoporphyrin IX. Central to the parasite's defences are superoxide dismutases and thioredoxin-dependent peroxidases; however, they lack catalase and glutathione peroxidases. The vital importance of the thioredoxin redox cycle (comprising NADPH, thioredoxin reductase and thioredoxin) is emphasized by the confirmation that thioredoxin reductase is essential for the survival of intraerythrocytic P. falciparum. The parasites also contain a fully functional glutathione redox system and the low-molecular-weight thiol glutathione is not only an important intracellular thiol redox buffer but also a cofactor for several redox active enzymes such as glutathione S-transferase and glutaredoxin. Recent findings have shown that in addition to these cytosolic redox systems the parasite also has an important mitochondrial antioxidant defence system and it is suggested that lipoic acid plays a pivotal part in defending the organelle from oxidative damage.

Haemoglobinopathies and resistance to malaria

The haemoglobinopathies have a celebrated role in the study of human genetics as the first examples of balanced polymorphisms described in human populations. Over the last 50 years, considerable evidence has been provided to show that these traits do confer protection from malaria. More recently, the underlying mechanisms of protection have been examined. This short review summarizes these studies and where possible shows how the putative mechanisms of protection may be linked to redox processes.

Limited influence of haemoglobin variants on Plasmodium falciparum msp1 and msp2 alleles in symptomatic malaria

Haemoglobin (Hb) S, HbC, and alpha(+)-thalassaemia confer protection from malaria. Accordingly, these traits may influence the multiplicity of infection (MOI) of Plasmodium falciparum and the presence of distinct parasite genotypes. In 840 febrile children in northern Ghana, we typed the P. falciparum merozoite surface protein genes (msp1, msp2) and examined effects of the Hb variants on MOI and parasite diversity. HbAC, HbAS, heterozygous, and homozygous alpha(+)-thalassaemia occurred in 21, 5, 29 and 4% of the children, respectively. Plasmodium falciparum was detected in 95%. The haemoglobinopathies did not influence MOI, nor did the Hb type bias the distribution of the msp allelic families. However, IC type parasites were most common among patients with homozygous alpha(+)-thalassaemia (93%), less frequent in heterozygotes (89%), and least frequent in alpha-globin normal children (84%, P(chi2 trend) = 0.03). The opposite was seen for Mad20 type parasites (34%, 47%, 53%, P(chi2 trend) = 0.02). Only a few of the 72 individual msp alleles were selected by the haemoglobinopathies. HbC and alpha(+)-thalassaemia are frequent in northern Ghana. In symptomatic children, the effect of Hb variants on parasite multiplicity and diversity appears to be limited. This may reflect an actual lack of influence or indicate abrogation in symptomatic malaria.

Reevaluation of flow cytometry for investigating antibody binding to the surface of Plasmodium falciparum trophozoite-infected red blood cells

BACKGROUND

The acquisition of antibodies directed toward variant surface antigens (VSAs) expressed on the surface of the trophozoite-infected red blood cell is an important determinant of natural immunity to Plasmodium falciparum malaria. In recent years, flow cytometry has been used increasingly to investigate these responses, but few systematic assessments of this method are available in the published literature.

METHODS

We developed a highly standardized experimental protocol and used parasites of the A4 laboratory clone, a monoclonal antibody to the VSA expressed by this clone (monoclonal antibody BC6), and a single pool of hyperimmune plasma to explore the parameters responsible for variations in VSA antibody responses measured by flow cytometry.

RESULTS

Despite strenuous efforts to standardize our flow cytometric assay, we found marked variability in our assay readout, even between repeat experiments using identical antibody and parasite combinations. We found no remediable cause for much of this variability. However, we identified three major factors that we considered important contributors: antibody concentration, nonspecific antibody binding to uninfected red blood cells, and parasite agglutination.

CONCLUSIONS

A number of potential pitfalls should be considered when designing and interpreting studies using this technique. In particular, we suggest that comparisons between assays conducted on different occasions can be made only through reference to carefully selected standards. We anticipate that a better appreciation of the factors that lead to assay variation will assist the design of improved experimental protocols.