Reduced risk of Plasmodium vivax malaria in Papua New Guinean children with Southeast Asian ovalocytosis in two cohorts and a case-control study.

BACKGROUND: The erythrocyte polymorphism, Southeast Asian ovalocytosis (SAO) (which results from a 27-base pair deletion in the erythrocyte band 3 gene, SLC4A1Δ27) protects against cerebral malaria caused by Plasmodium falciparum; however, it is unknown whether this polymorphism also protects against P. vivax infection and disease. METHODS AND FINDINGS: The association between SAO and P. vivax infection was examined through genotyping of 1,975 children enrolled in three independent epidemiological studies conducted in the Madang area of Papua New Guinea. SAO was associated with a statistically significant 46% reduction in the incidence of clinical P. vivax episodes (adjusted incidence rate ratio [IRR] = 0.54, 95% CI 0.40-0.72, p<0.0001) in a cohort of infants aged 3-21 months and a significant 52% reduction in P. vivax (blood-stage) reinfection diagnosed by PCR (95% CI 22-71, p = 0.003) and 55% by light microscopy (95% CI 13-77, p = 0.014), respectively, in a cohort of children aged 5-14 years. SAO was also associated with a reduction in risk of P. vivax parasitaemia in children 3-21 months (1,111/µl versus 636/µl, p = 0.011) and prevalence of P. vivax infections in children 15-21 months (odds ratio [OR] = 0.39, 95% CI 0.23-0.67, p = 0.001). In a case-control study of children aged 0.5-10 years, no child with SAO was found among 27 cases with severe P. vivax or mixed P. falciparum/P. vivax malaria (OR = 0, 95% CI 0-1.56, p = 0.11). SAO was associated with protection against severe P. falciparum malaria (OR = 0.38, 95% CI 0.15-0.87, p = 0.014) but no effect was seen on either the risk of acquiring blood-stage infections or uncomplicated episodes with P. falciparum. Although Duffy antigen receptor expression and function were not affected on SAO erythrocytes compared to non-SAO children, high level (>90% binding inhibition) P. vivax Duffy binding protein-specific binding inhibitory antibodies were observed significantly more often in sera from SAO than non-SAO children (SAO, 22.2%; non-SAO, 6.7%; p = 0.008). CONCLUSIONS: In three independent studies, we observed strong associations between SAO and protection against P. vivax malaria by a mechanism that is independent of the Duffy antigen. P. vivax malaria may have contributed to shaping the unique host genetic adaptations to malaria in Asian and Oceanic populations. Please see later in the article for the Editors' Summary.

Minimal association of common red blood cell polymorphisms with Plasmodium falciparum infection and uncomplicated malaria in Papua New Guinean school children.

Southeast Asian ovalocytosis (SAO), α(+)-thalassemia, and low expression of complement receptor 1 (CR1) have been associated with protection against severe Plasmodium falciparum malaria. In a cohort of children 5-14 years of age the effect of α(+)-thalassemia, SAO (SLC4A1Δ27), CR1 polymorphisms, and Gerbich negativity (GYPCΔex3) on risk of P. falciparum infections and uncomplicated illness were evaluated. The risk of acquiring polymerase chain reaction (PCR)-diagnosed P. falciparum infections was significantly lower for α(+)-thalassemia heterozygotes (hazard ratio [HR]: 0.56) and homozygotes (HR: 0.51) than wild-type children. No such differences were seen in light of microscopy diagnosed infections (P = 0.71) or were α(+)-thalassemia genotypes associated with a reduced risk of uncomplicated P. falciparum malaria. No significant associations between the risk of P. falciparum infection or illness were observed for any of the other red blood cell polymorphisms (P > 0.2). This suggests that these polymorphisms are not associated with significant protection against P. falciparum blood-stage infection or uncomplicated malaria in school-aged children.

Association between naturally acquired antibodies to erythrocyte-binding antigens of Plasmodium falciparum and protection from malaria and high-density parasitemia.

BACKGROUND: Antibodies targeting blood stage antigens are important in protection against malaria, but the principle targets remain unclear. Erythrocyte-binding antigens (EBAs) are important erythrocyte invasion ligands used by merozoites and may be targets of protective immunity, but there are limited data examining their potential importance. METHODS: We examined antibodies among 206 Papua New Guinean children who were treated with antimalarials at enrollment and observed prospectively for 6 months for reinfection and malaria. Immunoglobulin (Ig) G, IgG subclasses, and IgM to different regions of EBA175, EBA140, and EBA181 expressed as recombinant proteins were assessed in comparison with several other merozoite antigens. RESULTS: High levels of IgG to each of the EBAs were strongly associated with protection from symptomatic malaria and high density parasitemia, but not with risk of reinfection per se. The predominant IgG subclasses were either IgG1 or IgG3, depending on the antigen. The predominance of IgG1 versus IgG3 reflected structural features of specific regions of the proteins. IgG3 was most strongly associated with protection, even for those antigens that had an IgG1 predominant response. CONCLUSIONS: The EBAs appear important targets of acquired protective immunity. These findings support their further development as vaccine candidates.

Dynamics of asymptomatic Plasmodium vivax infections and Duffy binding protein polymorphisms in relation to parasitemia levels in Papua New Guinean children.

The interaction between Plasmodium vivax Duffy binding protein II (PvDBPII) and human erythrocyte Duffy antigen is necessary for blood stage infections. However, PvDBPII is highly polymorphic. We recently observed that certain recombinant DBPII variants bind better to erythrocytes in vitro. To examine the hypothesis that haplotypes with enhanced binding have increased parasitemia levels, we followed 206 Papua New Guinean children biweekly for six months with a total of 713 P. vivax samples genotyped. Twenty-seven PvDBPII haplotypes were identified, and 3 haplotypes accounted for 57% of the infections. The relative frequencies of dominant haplotypes remained stable throughout the study. There was no significant association with PvDBPII alleles or haplotypes with P. vivax parasitemia. The dominant haplotype (26% of samples), however, corresponded to a high-binding haplotype. Thus, common haplotypes are not likely to have arisen from increased fitness as measured by greater parasitemia levels. The restricted number of common haplotypes increases the feasibility of a PvDBPII-based vaccine.

The risk of malarial infections and disease in Papua New Guinean children.

In a treatment re-infection study of 206 Papua New Guinean school children, we examined risk of reinfection and symptomatic malaria caused by different Plasmodium species. Although children acquired a similar number of polymerase chain reaction-detectable Plasmodium falciparum and P. vivax infections in six months of active follow-up (P. falciparum = 5.00, P. vivax = 5.28), they were 21 times more likely to develop symptomatic P. falciparum malaria (1.17/year) than P. vivax malaria (0.06/year). Children greater than nine years of age had a reduced risk of acquiring P. vivax infections of low-to-moderate (>150/microL) density (adjusted hazard rate [AHR] = 0.65 and 0.42), whereas similar reductions in risk with age of P. falciparum infection was only seen for parasitemias > 5,000/microL (AHR = 0.49) and symptomatic episodes (AHR = 0.51). Infection and symptomatic episodes with P. malariae and P. ovale were rare. By nine years of age, children have thus acquired almost complete clinical immunity to P. vivax characterized by a very tight control of parasite density, whereas the acquisition of immunity to symptomatic P. falciparum malaria remained incomplete. These observations suggest that different mechanisms of immunity may be important for protection from these malaria species.