Drissa Coulibaly

A MOLECULAR MARKER FOR CHLOROQUINE-RESISTANT FALCIPARUM MALARIA

BACKGROUND Chloroquine-resistant Plasmodium falciparum malaria is a major health problem, particularly in sub-Saharan Africa. Chloroquine resistance has been associated in vitro with point mutations in two genes, pfcrt and pfmdr 1, which encode the P. falciparum digestive-vacuole transmembrane proteins PfCRT and Pgh1, respectively. METHODS To assess the value of these mutations as markers for clinical chloroquine resistance, we measured the association between the mutations and the response to chloroquine treatment in patients with uncomplicated falciparum malaria in Mali. The frequencies of the mutations in patients before and after treatment were compared for evidence of selection of resistance factors as a result of exposure to chloroquine. RESULTS The pfcrt mutation resulting in the substitution of threonine (T76) for lysine at position 76 was present in all 60 samples from patients with chloroquine-resistant infections (those that persisted or recurred after treatment), as compared with a base-line prevalence of 41 percent in samples obtained before treatment from 116 randomly selected patients (P<0.001), indicating absolute selection for this mutation. The pfmdr 1 mutation resulting in the substitution of tyrosine for asparagine at position 86 was also selected for, since it was present in 48 of 56 post-treatment samples from patients with chloroquine-resistant infections (86 percent), as compared with a base-line prevalence of 50 percent in 115 samples obtained before treatment (P<0.001). The presence of pfcrt T76 was more strongly associated with the development of chloroquine resistance (odds ratio, 18.8; 95 percent confidence interval, 6.5 to 58.3) than was the presence of pfmdr 1 Y86 (odds ratio, 3.2; 95 percent confidence interval, 1.5 to 6.8) or the presence of both mutations (odds ratio, 9.8; 95 percent confidence interval, 4.4 to 22.1). CONCLUSIONS This study shows an association between the pfcrt T76 mutation in P. falciparum and the development of chloroquine resistance during the treatment of malaria. This mutation can be used as a marker in surveillance for chloroquine-resistant falciparum malaria.

A Field Trial to Assess a Blood-Stage Malaria Vaccine

BACKGROUND Blood-stage malaria vaccines are intended to prevent clinical disease. The malaria vaccine FMP2.1/AS02(A), a recombinant protein based on apical membrane antigen 1 (AMA1) from the 3D7 strain of Plasmodium falciparum, has previously been shown to have immunogenicity and acceptable safety in Malian adults and children. METHODS In a double-blind, randomized trial, we immunized 400 Malian children with either the malaria vaccine or a control (rabies) vaccine and followed them for 6 months. The primary end point was clinical malaria, defined as fever and at least 2500 parasites per cubic millimeter of blood. A secondary end point was clinical malaria caused by parasites with the AMA1 DNA sequence found in the vaccine strain. RESULTS The cumulative incidence of the primary end point was 48.4% in the malaria-vaccine group and 54.4% in the control group; efficacy against the primary end point was 17.4% (hazard ratio for the primary end point, 0.83; 95% confidence interval [CI], 0.63 to 1.09; P=0.18). Efficacy against the first and subsequent episodes of clinical malaria, as defined on the basis of various parasite-density thresholds, was approximately 20%. Efficacy against clinical malaria caused by parasites with AMA1 corresponding to that of the vaccine strain was 64.3% (hazard ratio, 0.36; 95% CI, 0.08 to 0.86; P=0.03). Local reactions and fever after vaccination were more frequent with the malaria vaccine. CONCLUSIONS On the basis of the primary end point, the malaria vaccine did not provide significant protection against clinical malaria, but on the basis of secondary results, it may have strain-specific efficacy. If this finding is confirmed, AMA1 might be useful in a multicomponent malaria vaccine. (Funded by the National Institute of Allergy and Infectious Diseases and others; ClinicalTrials.gov number, NCT00460525.).

Comparison of intermittent preventive treatment with chemoprophylaxis for the prevention of malaria during pregnancy in Mali.

BACKGROUND Malaria during pregnancy contributes to maternal anemia and low birth weight. In East Africa, several studies have demonstrated that intermittent preventive treatment (IPT) with sulfadoxine-pyrimethamine (SP) is more efficacious than weekly chloroquine (CQ) chemoprophylaxis in preventing these adverse consequences. To our knowledge, there are no published trials evaluating IPT in West Africa. METHODS We undertook a randomized controlled trial of weekly CQ chemoprophylaxis, 2-dose IPT with CQ, and 2-dose IPT with SP; 1163 women were enrolled. RESULTS In multivariate analyses, when compared with weekly CQ, IPT/SP was associated with a reduction in third-trimester anemia (adjusted odds ratio [AOR], 0.49; P<.001), placental parasitemia (AOR, 0.69; P=.04), and low birth weight (<2500 g) (AOR, 0.69; P=.04). The prevalence of placental infection remained unexpectedly high, even in the IPT/SP group (24.5%), possibly because of the intensity of seasonal transmission. There were no significant differences in stillbirths, spontaneous abortions, or neonatal deaths among the 3 groups. CONCLUSIONS In Mali, IPT with SP appears more efficacious than weekly chloroquine chemoprophylaxis in preventing malaria during pregnancy. These data support World Health Organization recommendations to administer at least 2 doses of IPT during pregnancy. In intensely seasonal transmission settings in Mali, >2 doses may be required to prevent placental reinfection prior to delivery.

Impact of Trimethoprim-Sulfamethoxazole Prophylaxis on Falciparum Malaria Infection and Disease

BACKGROUND Trimethoprim-sulfamethoxazole (TS) prophylaxis is recommended for persons living with human immunodeficiency virus infection and acquired immunodeficiency syndrome in Africa. TS and the antimalarial combination sulfadoxine-pyrimethamine (SP) share mechanisms of action and resistance patterns, and concerns about the impact of TS resistance on SP efficacy have contributed to reluctance to implement TS prophylaxis in Africa. METHODS To determine whether TS prophylaxis impairs SP efficacy for treatment of uncomplicated falciparum malaria, we conducted a randomized, controlled, open-label study of TS prophylaxis. Two hundred and forty children 5-15 years old were randomized in a 2 : 1 fashion to receive either thrice-weekly TS for 12 weeks or no prophylaxis and were treated with SP for subsequent episodes of malaria. The incidence of malaria, SP efficacy, and the prevalence of parasite mutations that confer antifolate drug resistance were measured. RESULTS TS prophylaxis had a 99.5% protective efficacy against episodes of clinical malaria, with 97% efficacy against infection. Four SP treatment failures occurred in the control group, and none occurred in the TS group. No evidence was seen for selection by TS of antifolate resistance-conferring mutations in parasite dihydrofolate reductase or dihydropteroate synthase during subclinical infections. CONCLUSIONS In this setting of low antifolate resistance, TS was highly effective in preventing falciparum malaria infection and disease and did not appear to select for SP-resistant parasites.

Extreme Polymorphism in a Vaccine Antigen and Risk of Clinical Malaria: Implications for Vaccine Development

Children from Mali who are repeatedly infected with malaria are more likely to get sick if the parasite is highly variable, a possible obstacle to an effective vaccine. From the point of view of the parasite, malaria is a successful disease. Transmitted by the bite of infected mosquitoes, malaria sickens about 400 million people a year. Although drugs, insecticides, and bed nets can help, a vaccine would be the most effective way to fight malaria. But the malaria parasite is coated with ever-changing proteins, and it has proven difficult to develop an effective vaccine against these constantly moving targets. By examining the natural immune response to malaria infection in children from Mali, West Africa, Takala et al. have identified antibody targets in the parasite that can best protect children from infection, a boon in designing a vaccine to combat this quick-change artist. Vaccines against the malaria parasite, Plasmodium falciparum, are directed against proteins on its surface in order to prevent these proteins from initiating the invasion of host cells by the parasite. The main target protein for vaccines currently in development is the highly variable apical membrane antigen–1 (AMA-1). For 3 years, the authors of this study collected P. falciparum organisms that naturally infected 100 children, ages 3 months to 20 years, in a remote rural town. Because each child was repeatedly infected, the authors could determine which characteristics of the infecting parasite determined whether the child was protected from subsequent illness. Sequencing of the parasites’ AMA-1 genes revealed a startling amount of diversity; ~500 separate infections exhibited 214 unique combinations of altered nucleotides. The overall number of resulting amino acid changes in the AMA-1 protein from one infection to another determined whether the child would succumb to sickness with the second infection. That is, the child’s natural immunity from the first infection could only protect against illness in a subsequent infection if AMA-1 from the second infecting parasite was similar to that of the first. This seems like bad news for malaria vaccine development, but additional data suggest a new approach. The authors’ analysis points to a particular region of AMA-1 (domain I c1L) that is highly variable and is responsible for much of the parasite’s ability to escape control by the human immune system. Thus, a vaccine that elicits protective antibodies to the most common variations of this region might be effective. Understanding how natural immunity to P. falciparum can (and cannot) protect against malaria will help to harness our own immune systems for protection against this serious disease. Vaccines directed against the blood stages of Plasmodium falciparum malaria are intended to prevent the parasite from invading and replicating within host cells. No blood-stage malaria vaccine has shown clinical efficacy in humans. Most malaria vaccine antigens are parasite surface proteins that have evolved extensive genetic diversity, and this diversity could allow malaria parasites to escape vaccine-induced immunity. We examined the extent and within-host dynamics of genetic diversity in the blood-stage malaria vaccine antigen apical membrane antigen–1 in a longitudinal study in Mali. Two hundred and fourteen unique apical membrane antigen–1 haplotypes were identified among 506 human infections, and amino acid changes near a putative invasion machinery binding site were strongly associated with the development of clinical symptoms, suggesting that these residues may be important to consider in designing polyvalent apical membrane antigen–1 vaccines and in assessing vaccine efficacy in field trials. This extreme diversity may pose a serious obstacle to an effective polyvalent recombinant subunit apical membrane antigen–1 vaccine.

Safety and immunogenicity of an AMA1 malaria vaccine in Malian children: results of a phase 1 randomized controlled trial.

Background The objective was to evaluate the safety, reactogenicity and immunogenicity of the AMA-1-based blood-stage malaria vaccine FMP2.1/AS02A in adults exposed to seasonal malaria. Methodology/Principal Findings A phase 1 double blind randomized controlled dose escalation trial was conducted in Bandiagara, Mali, West Africa, a rural town with intense seasonal transmission of Plasmodium falciparum malaria. The malaria vaccine FMP2.1/AS02A is a recombinant protein (FMP2.1) based on apical membrane antigen-1 (AMA-1) from the 3D7 clone of P. falciparum, adjuvanted with AS02A. The comparator vaccine was a cell-culture rabies virus vaccine (RabAvert). Sixty healthy, malaria-experienced adults aged 18–55 y were recruited into 2 cohorts and randomized to receive either a half dose or full dose of the malaria vaccine (FMP2.1 25 µg/AS02A 0.25 mL or FMP2.1 50 µg/AS02A 0.5 mL) or rabies vaccine given in 3 doses at 0, 1 and 2 mo, and were followed for 1 y. Solicited symptoms were assessed for 7 d and unsolicited symptoms for 30 d after each vaccination. Serious adverse events were assessed throughout the study. Titers of anti-AMA-1 antibodies were measured by ELISA and P. falciparum growth inhibition assays were performed on sera collected at pre- and post-vaccination time points. Transient local pain and swelling were common and more frequent in both malaria vaccine dosage groups than in the comparator group. Anti-AMA-1 antibodies increased significantly in both malaria vaccine groups, peaking at nearly 5-fold and more than 6-fold higher than baseline in the half-dose and full-dose groups, respectively. Conclusion/Significance The FMP2.1/AS02A vaccine had a good safety profile, was well-tolerated, and was highly immunogenic in malaria-exposed adults. This malaria vaccine is being evaluated in Phase 1 and 2 trials in children at this site. Trial Registration ClinicalTrials.gov NCT00308061

Dynamics of polymorphism in a malaria vaccine antigen at a vaccine-testing site in Mali.

Background Malaria vaccines based on the 19-kDa region of merozoite surface protein 1 (MSP-119) derived from the 3D7 strain of Plasmodium falciparum are being tested in clinical trials in Africa. Knowledge of the distribution and natural dynamics of vaccine antigen polymorphisms in populations in which malaria vaccines will be tested will guide vaccine design and permit distinction between natural fluctuations in genetic diversity and vaccine-induced selection. Methods and Findings Using pyrosequencing, six single-nucleotide polymorphisms in the nucleotide sequence encoding MSP-119 were genotyped from 1,363 malaria infections experienced by 100 children who participated in a prospective cohort study in Mali from 1999 to 2001. The frequencies of 14 MSP-119 haplotypes were compared over the course of the malaria transmission season for all three years, in three age groups, and in consecutive infections within individuals. While the frequency of individual MSP-119 haplotypes fluctuated, haplotypes corresponding to FVO and FUP strains of P. falciparum (MSP-119 haplotypes QKSNGL and EKSNGL, respectively) were most prevalent during three consecutive years and in all age groups with overall prevalences of 46% (95% confidence interval [CI] 44%–49%) and 36% (95% CI 34%–39%), respectively. The 3D7 haplotype had a lower overall prevalence of 16% (95% CI 14%–18%). Multiplicity of infection based on MSP-119 was higher at the beginning of the transmission season and in the oldest individuals (aged ≥11 y). Three MSP-119 haplotypes had a reduced frequency in symptomatic infections compared to asymptomatic infections. Analyses of the dynamics of MSP-119 polymorphisms in consecutive infections implicate three polymorphisms (at positions 1691, 1700, and 1701) as being particularly important in determining allele specificity of anti-MSP-119 immunity. Conclusions Parasites with MSP-119 haplotypes different from that of the leading vaccine strain were consistently the most prevalent at a vaccine trial site. If immunity elicited by an MSP-1-based vaccine is allele-specific, a vaccine based on either the FVO or FUP strain might have better initial efficacy at this site. This study, to our knowledge the largest of its kind to date, provides molecular information needed to interpret population responses to MSP-1-based vaccines and suggests that certain MSP-119 polymorphisms may be relevant to cross-protective immunity.

Epilepsy as a Consequence of Cerebral Malaria in Area in Which Malaria Is Endemic in Mali, West Africa

Summary:  Purpose: Cerebral malaria (CM) is suspected to be a potential cause of epilepsy in tropical areas, but little information is available. The purpose of this study was to evaluate the role of CM in epilepsy among children in Mali.

Molecular Basis of Allele-Specific Efficacy of a Blood-Stage Malaria Vaccine: Vaccine Development Implications

The disappointing efficacy of blood-stage malaria vaccines may be explained in part by allele-specific immune responses that are directed against polymorphic epitopes on blood-stage antigens. FMP2.1/AS02(A), a blood-stage candidate vaccine based on apical membrane antigen 1 (AMA1) from the 3D7 strain of Plasmodium falciparum, had allele-specific efficacy against clinical malaria in a phase II trial in Malian children. We assessed the cross-protective efficacy of the malaria vaccine and inferred which polymorphic amino acid positions in AMA1 were the targets of protective allele-specific immune responses. FMP2.1/AS02(A) had the highest efficacy against AMA1 alleles that were identical to the 3D7 vaccine-type allele at 8 highly polymorphic amino acid positions in the cluster 1 loop (c1L) but differed from 3D7 elsewhere in the molecule. Comparison of the incidence of vaccine-type alleles before and after vaccination in the malaria vaccine and control groups and examination of the patterns of allele change at polymorphic positions in consecutive malaria episodes suggest that the highly polymorphic amino acid position 197 in c1L was the most critical determinant of allele-specific efficacy. These results indicate that a multivalent AMA1 vaccine with broad efficacy could include only a limited set of key alleles of this extremely polymorphic antigen.

A high-throughput method for quantifying alleles and haplotypes of the malaria vaccine candidate Plasmodium falciparum merozoite surface protein-1 19 kDa

BackgroundMalaria vaccine efficacy may be compromised if the frequency of non-target alleles increases following vaccination with a genetically polymorphic target. Methods are needed to monitor genetic diversity in polymorphic vaccine antigens, but determining which genetic variants of such antigens are present in infected individuals is complicated by the frequent occurrence of mixed infections.MethodsPyrosequencing was used to determine allele frequencies at each of six single nucleotide polymorphisms in the Plasmodium falciparum blood-stage vaccine antigen merozoite surface protein 1 19 kDa (MSP-119) in field samples from a vaccine-testing site in Mali. Mixtures of MSP-119 clones were created to validate a haplotype-estimating algorithm that uses maximum likelihood methods to determine the most probable combination of haplotypes given the allele frequencies for an infection and the haplotypes known to be circulating in the population.ResultsFourteen unique MSP-119 haplotypes were identified among 351 genotyped infections. After adjustment to a standard curve, Pyrosequencing provided accurate and precise estimates of allele frequencies in mixed infections. The haplotype-estimating algorithm provided accurate estimates of haplotypes in mixed infections containing up to three haplotypes. Based on the MSP-119 locus, approximately 90% of the 351 infections contained two or fewer haplotypes.ConclusionPyrosequencing in conjunction with a haplotype-estimating algorithm provides accurate estimates of haplotypes present in infections with up to 3 haplotypes, and can be used to monitor genetic diversity in parasite populations prior to and following introduction of MSP-1-based malaria vaccines.