A prospective study of the influence of alpha thalassaemia on morbidity from malaria and immune responses to defined Plasmodium falciparum antigens in Gambian children

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.

Lack of associations of α(+)-thalassemia with the risk of Plasmodium falciparum and Plasmodium vivax infection and disease in a cohort of children aged 3-21 months from Papua New Guinea

Despite consistent evidence of a protective effect of α(+)-thalassemia against severe Plasmodium falciparum disease, the mechanisms underlying this protection remain unknown. An increase in risk of Plasmodium vivax malaria in early childhood resulting in a cross-species protection against severe P. falciparum malaria has been proposed as a possible mechanism in Melanesian children. The association of α(+)-thalassemia genotypes with a risk of P. falciparum and P. vivax infection and uncomplicated illness was reassessed in a cohort of 1,112 Papua New Guinean children, followed from 3 to 21 months of age. Three hundred and eighty-nine (35.0%) children were homozygous for α(+)-thalassemia (-α/-α), 506 (45.5%) heterozygous (αα/-α) and 217 (19.5%) homozygous for the wild-type allele. No significant differences in the incidence of P. falciparum (Pf) or P. vivax (Pv) malaria were observed between α(+)-thalassemia homozygote (Pf: incidence rate ratio (IRR)=1.13, CI(95) (0.82, 1.56), P=0.45, Pv: IRR=1.15, CI(95) (0.88, 1.50), P=0.31), heterozygote (Pf: IRR=0.98, CI(95) (0.71, 1.34), P=0.93, Pv: IRR=1.14, CI(95) (0.88, 1.48), P=0.33) and wild-type children. The prevalence of infection with either species did not differ between α(+)-thalassemia genotypes, although densities of P. vivax (but not of P. falciparum) infections were significantly higher in α(+)-thalassemia homozygote and heterozygote children. An excessive risk of moderate-to-severe anemia (Hb<8 g/dl) was observed in α(+)-thalassemia homozygote children (IRR=1.54, CI(95) (1.12, 2.11), P=0.008). This study therefore failed to confirm an increased risk of P. vivax or P. falciparum malaria in very young, α(+)-thalassemic children without significant levels of acquired immunity. This confirms the lack of protection by α(+)-thalassemia against uncomplicated P. falciparum and challenges the hypothesis of immunological cross-protection between P. falciparum and P. vivax as a mechanism underlying α(+)-thalassemia protection against severe P. falciparum disease in Melanesian children.

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.

Haemoglobinopathies and the clinical epidemiology of malaria: a systematic review and meta-analysis

BACKGROUND

Haemoglobinopathies can reduce the risk of malaria syndromes. We aimed to quantify the relation between different haemoglobin mutations and malaria protection to strengthen the foundation for translational studies of malaria pathogenesis and immunity.

METHODS

We systematically searched the Medline and Embase databases for studies that estimated the risk of malaria in patients with and without haemoglobinopathies up to Sept 9, 2011, and identified additional studies from reference lists. We included studies that enrolled mainly children or pregnant women and had the following outcomes: Plasmodium falciparum severe malaria, uncomplicated malaria, asymptomatic parasitaemia, or pregnancy-associated malaria, and Plasmodium vivax malaria. Two reviewers identified studies independently, assessed quality of the studies, and extracted data. We produced odds ratios (ORs; 95% CIs) for case-control studies and incidence rate ratios (IRRs; 95% CIs) for prospective studies. We did the meta-analysis with a random-effects model when equivalent outcomes were reported in more than one study.

FINDINGS

Of 62 identified studies, 44 reported data for haemoglobin AS, 19 for haemoglobin AC and CC, and 18 for α-thalassaemia. Meta-analysis of case-control studies showed a decreased risk of severe P. falciparum malaria in individuals with haemoglobin AS (OR 0·09, 95% CI 0·06-0·12), haemoglobin CC (0·27, 0·11-0·63), haemoglobin AC (0·83, 0·67-0·96), homozygous α-thalassaemia (0·63, 0·48-0·83), and heterozygous α-thalassaemia (0·83, 0·74-0·92). In meta-analysis of prospective trials only haemoglobin AS was consistently associated with protection from uncomplicated malaria (IRR 0·69, 95% CI 0·61-0·79); no haemoglobinopathies led to consistent protection from asymptomatic parasitaemia. Few clinical studies have investigated β-thalassaemia, haemoglobin E, P. vivax malaria, or pregnancy-associated malaria.

INTERPRETATION

Haemoglobin AS, CC, and AC genotypes and homozygous and heterozygous α-thalassaemia provide significant protection from severe malaria syndromes, but these haemoglobinopathies differ substantially in the degree of protection provided and confer mild or no protection against uncomplicated malaria and asymptomatic parasitaemia. Through attenuation of severity of malaria, haemoglobinopathies could serve as a model for investigation of the mechanisms of malaria pathogenesis and immunity.

FUNDING

US National Institute of Allergy and Infectious Diseases.

Effect of α(+)-thalassaemia on episodes of fever due to malaria and other causes: a community-based cohort study in Tanzania

Background

It is controversial to what degree α⁺-thalassaemia protects against episodes of uncomplicated malaria and febrile disease due to infections other than Plasmodium.

Methods

In Tanzania, in children aged 6-60 months and height-for-age z-score < -1.5 SD (n = 612), rates of fevers due to malaria and other causes were compared between those with heterozygous or homozygotes α⁺-thalassaemia and those with a normal genotype, using Cox regression models that accounted for multiple events per child.

Results

The overall incidence of malaria was 3.0/child-year (1, 572/526 child-years); no differences were found in malaria rates between genotypes (hazard ratios, 95% CI: 0.93, 0.82-1.06 and 0.91, 0.73-1.14 for heterozygotes and homozygotes respectively, adjusted for baseline factors that were predictive for outcome). However, this association strongly depended on age: among children aged 6-17 months, those with α⁺-thalassaemia experienced episodes more frequently than those with a normal genotype (1.30, 1.02-1.65 and 1.15, 0.80-1.65 for heterozygotes and homozygotes respectively), whereas among their peers aged 18-60 months, α⁺-thalassaemia protected against malaria (0.80, 0.68-0.95 and 0.78, 0.60-1.03; p-value for interaction 0.001 and 0.10 for hetero- and homozygotes respectively). No effect was observed on non-malarial febrile episodes.

Conclusions

In this population, the association between α⁺-thalassaemia and malaria depends on age. Our data suggest that protection by α⁺-thalassaemia is conferred by more efficient acquisition of malaria-specific immunity.

Alpha(+)-thalassaemia and malarial anaemia

The mechanisms by which alpha(+)-thalassaemia protects against severe malaria, and severe malarial anaemia in particular, are poorly understood. A recent report proposes that the increased count of microcytic and hypochromic erythrocytes in alpha(+)-thalassaemia reduces the haemoglobin decline during acute malaria and, thus, reduces the risk of anaemia. This mechanism might add to further alpha(+)-thalassaemic attributes that are involved in the attenuation of anaemia caused by both acute and chronic Plasmodium infections.

Haematological values from a Gambian cohort--possible reference range for a West African population

The objective of this study was to establish haematological reference ranges for the West African subregion using a Gambian cohort. We analysed full blood counts from 1279 subjects aged > or =1 year. Anthropometric and body composition measurements were performed. Haematological mean values, medians and 90% reference values were calculated and related to malnutrition in children and thinness and/or obesity in adults. Haemoglobin (Hb) and mean corpuscular volume (MCV) significantly increased with age (P < 0.00001). There were gender-related changes in Hb from 15 years of age (P = 0.001) and for MCV only in adults (P = 0.0002). Hb was significantly reduced in underweight and stunted children (P = 0.0001 and 0.0002, respectively) but was unaffected by thinness or obesity in adults. White blood cell (WBC) and platelet counts were highest under 5 years and declined significantly with age (P < 0.0001 and 0.0001). While, there were no gender-related differences with WBC, there were higher WBC counts in underweight (P = 0.0001) and stunted (P < 0.0001) children. Adult females had significantly higher mean platelet counts compared with males (P = 0.006). The mean and median values of haematological parameters in The Gambia are similar to other standards but the 90% reference range for each parameter encompasses lower values when compared with Western standards.

Host erythrocyte polymorphisms and exposure to Plasmodium falciparum in Papua New Guinea

Background

The protection afforded by human erythrocyte polymorphisms against the malaria parasite, Plasmodium falciparum, has been proposed to be due to reduced ability of the parasite to invade or develop in erythrocytes. If this were the case, variable levels of parasitaemia and rates of seroconversion to infected-erythrocyte variant surface antigens (VSA) should be seen in different host genotypes.

Methods

To test this hypothesis, P. falciparum parasitaemia and anti-VSA antibody levels were measured in a cohort of 555 asymptomatic children from an area of intense malaria transmission in Papua New Guinea. Linear mixed models were used to investigate the effect of α⁺-thalassaemia, complement receptor-1 and south-east Asian ovalocytosis, as well as glucose-6-phosphate dehydrogenase deficiency and ABO blood group on parasitaemia and age-specific seroconversion to VSA.

Results

No host polymorphism showed a significant association with both parasite prevalence/density and age-specific seroconversion to VSA.

Conclusion

Host erythrocyte polymorphisms commonly found in Papua New Guinea do not effect exposure to blood stage P. falciparum infection. This contrasts with data for sickle cell trait and highlights that the above-mentioned polymorphisms may confer protection against malaria via distinct mechanisms.

Impact of red blood cell polymorphisms on the antibody response to Plasmodium falciparum in Senegal

The evidence of protection afforded by red blood cell polymorphisms against either clinical malaria or Plasmodium falciparum blood levels varies with the study site and the type of malaria transmission. Nevertheless, no clear implication of an antibody-related effect has yet been established in the protection related to red blood cell polymorphisms. We performed a prospective study, where plasma IgG and IgG subclasses directed to recombinant proteins from the merozoite surface protein 2 (MSP2/3D7 and MSP2/FC27) and the ring-infected erythrocyte surface antigen (RESA) were determined in a cohort of 413 Senegalese children before the annual malaria transmission season. The antibody response was dependent on age, and to a lesser extent, on the village of residence. IgG3 responders to all proteins, IgG responders to RESA and MSP2/3D7, as well as IgG2 to RESA and IgG1 responders to MSP2/3D7, presented enhanced mean values of parasite density, as evaluated during an 18-month follow-up. The levels of IgG and IgG3 to MSP2/3D7 were negatively associated with the risk of occurrence of a malaria attack during the following transmission season. Compared to normal children, sickle cell trait carriers presented lower levels of IgG to MSP2/3D7. Similarly, G6PD A- girls had lower levels of IgG and IgG3 to MSP2/FC27 than did G6PD normal girls. The impact of these particular genetic polymorphisms on the modulation of the antibody response is discussed.

Negative epistasis between the malaria-protective effects of alpha+-thalassemia and the sickle cell trait

The hemoglobinopathies, disorders of hemoglobin structure and production, protect against death from malaria. In sub-Saharan Africa, two such conditions occur at particularly high frequencies: presence of the structural variant hemoglobin S and alpha(+)-thalassemia, a condition characterized by reduced production of the normal alpha-globin component of hemoglobin. Individually, each is protective against severe Plasmodium falciparum malaria, but little is known about their malaria-protective effects when inherited in combination. We investigated this question by studying a population on the coast of Kenya and found that the protection afforded by each condition inherited alone was lost when the two conditions were inherited together, to such a degree that the incidence of both uncomplicated and severe P. falciparum malaria was close to baseline in children heterozygous with respect to the mutation underlying the hemoglobin S variant and homozygous with respect to the mutation underlying alpha(+)-thalassemia. Negative epistasis could explain the failure of alpha(+)-thalassemia to reach fixation in any population in sub-Saharan Africa.

Alpha(+)-thalassemia protects African children from severe malaria

The high frequency of alpha(+)-thalassemia in malaria-endemic regions may reflect natural selection due to protection from potentially fatal severe malaria. In Africa, bearing 90% of global malaria morbidity and mortality, this has not yet been observed. We tested this hypothesis in an unmatched case-control study among 301 Ghanaian children with severe malaria and 2107 controls (62% parasitemic). In control children, alpha(+)-thalassemia affected neither prevalence nor density of Plasmodium falciparum. However, heterozygous alpha(+)-thalassemia was observed in 32.6% of controls but in only 26.2% of cases (odds ratio [OR], 0.74; 95% confidence interval [CI], 0.56-0.98). Protection against severe malaria was found to be pronounced comparing severe malaria patients with parasitemic controls (adjusted OR in children < 5 years of age, 0.52; 95% CI, 0.34-0.78) and to wane with age. No protective effect was discernible for homozygous children. Our findings provide evidence for natural selection of alpha(+)-thalassemia in Africa due to protection from severe malaria.

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.

Evidence for a reduced effect of chloroquine against Plasmodium falciparum in alpha-thalassaemic children

Alpha-thalassaemia is common in malaria-endemic regions and is considered to confer protection from clinical disease due to infection with Plasmodium falciparum. In vitro, sensitivity to chloroquine (CQ) of P. falciparum infecting alpha-thalassaemic erythrocytes is reduced. We examined, in a cross-sectional study of 405 Nigerian children, associations between alpha-globin genotypes, blood concentrations of CQ, and P. falciparum parasitaemia. Of the children, 44% were alpha+-thalassaemic (36.8% heterozygous, 7.6% homozygous). CQ in blood and P. falciparum-infection were observed in 52 and 80%, respectively. CQ was more frequently found in homozygous alpha+-thalassaemic (71%) than in non-thalassaemic children (50%; odds ratio, 2.42; 95% confidence interval, 1.01-5.8). Among children with CQ in blood and despite similar drug concentrations, alpha+-thalassaemic individuals had fewer infections below the threshold of microscopy which were detectable by PCR only, and they had a higher prevalence of elevated parasitaemia than non-thalassaemic children. No such differences were discernible among drug-free children. CQ displays a lowered efficacy in the suppression of P. falciparum parasitaemia in alpha+-thalassaemic children; hence protection against malaria due to alpha+-thalassaemia may be obscured in areas of intense CQ usage. Moreover, alpha+-thalassaemia may contribute to the expansion of CQ resistance.

alpha+-Thalassemia protects children against disease caused by other infections as well as malaria

In the South West Pacific region, the striking geographical correlation between the frequency of alpha+-thalassemia and the endemicity of Plasmodium falciparum suggests that this hemoglobinopathy provides a selective advantage against malaria. In Vanuatu, paradoxically, alpha+-thalassemia increases the incidence of contracting mild malaria in the first 2 years of life, but severe disease was too uncommon to assess adequately. Therefore, we undertook a prospective case-control study of children with severe malaria on the north coast of Papua New Guinea, where malaria transmission is intense and alpha+-thalassemia affects more than 90% of the population. Compared with normal children, the risk of having severe malaria was 0.40 (95% confidence interval 0.22-0.74) in alpha+-thalassemia homozygotes and 0.66 (0.37-1.20) in heterozygotes. Unexpectedly, the risk of hospital admission with infections other than malaria also was reduced to a similar degree in homozygous (0. 36; 95% confidence interval 0.22-0.60) and heterozygous (0.63; 0. 38-1.07) children. This clinical study demonstrates that a malaria resistance gene protects against disease caused by infections other than malaria. The mechanism of the remarkable protective effect of alpha+-thalassemia against severe childhood disease remains unclear but must encompass the clear interaction between this hemoglobinopathy and both malarial and nonmalarial infections.

High incidence of malaria in alpha-thalassaemic children

The alpha+-thalassaemias are the commonest known human genetic disorders, affecting up to 80 per cent of some populations. Although there is good evidence from both epidemiological and clinical studies that these gene frequencies reflect selection by, and protection from, malaria, the mechanism is unknown. We have studied the epidemiology of malaria in childhood on the southwestern Pacific island of Espiritu Santo in Vanuatu and here we report that, paradoxically, both the incidence of uncomplicated malaria and the prevalence of splenomegaly, an index of malaria infection, are significantly higher in young children with alpha+-thalassaemia than in normal children. Furthermore, this effect is most marked in the youngest children and for the non-lethal parasite Plasmodium vivax. The alpha+-thalassaemias may have been selected for their ability beneficially to increase susceptibility to P. vivax, which, by acting as a natural vaccine in this community, induces limited cross-species protection against subsequent severe P. falciparum malaria.