A CD4(+) T-cell immune response to a conserved epitope in the circumsporozoite protein correlates with protection from natural Plasmodium falciparum infection and disease.

Many human T-cell responses specific for epitopes in Plasmodium falciparum have been described, but none has yet been shown to be predictive of protection against natural malaria infection. Here we report a peptide-specific T-cell assay that is strongly associated with protection of humans in The Gambia, West Africa, from both malaria infection and disease. The assay detects interferon-gamma-secreting CD4(+) T cells specific for a conserved sequence from the circumsporozoite protein, which binds to many human leukocyte antigen (HLA)-DR types. The correlation was observed using a cultured, rather than an ex vivo, ELISPOT assay that measures central memory-'type T cells rather than activated effector T cells. These findings provide direct evidence for a protective role for CD4(+) T cells in humans, and a precise target for the design of improved vaccines against P. falciparum.

Gambian children successfully treated with chloroquine can harbor and transmit Plasmodium falciparum gametocytes carrying resistance genes.

Polymorphisms in two genes of Plasmodium falciparum (P. falciparum multidrug resistance 1 [pfmdr1] and P. falciparum chloroquine [CQ] resistance transporter [pfcrt]) are associated with CQ treatment failure. We found significant linkage disequilibrium between these loci among isolates from symptomatic Gambian children (P = 0.026) and strong selection for the resistance-associated alleles pfmdr1-86Tyr and pfcrt-76Thr in children with persistent or re-emerging P. falciparum trophozoites during post-treatment follow-up (P = 1.9 x 10(-7)). Therefore, this genotype is characteristic of resistant infections among our study population. Since the long-term public health impact of parasites carrying such resistant genotypes depends upon their transmissibility, we examined the prevalence of pfmdr1-86Tyr and pfcrt-76Thr among Gambian children harboring sexual stage parasites during post-treatment follow-up. Gametocytes that emerged after successful treatment with CQ were significantly more likely to be of this genotype than were those emerging after other treatments (P = 4.83 x 10(-4)), and were infective to Anopheles mosquitoes. Therapeutic success may thus be accompanied by public health failure as cured children pass resistance genes on to mosquitoes at an enhanced rate.

Protective efficacy of the RTS,S/AS02 Plasmodium falciparum malaria vaccine is not strain specific.

RTS,S/AS02 is a recombinant protein malaria vaccine that contains a large portion of the C-terminal of the circumsporozoite protein (CSP) sequence of the NF54 isolate of Plasmodium falciparum fused to the hepatitis B virus surface antigen. It has been shown to induce significant protection to challenge infection with a homologous parasite strain in American volunteers. In a recently completed trial in semi-immune Gambian adults, vaccine efficacy against natural infection was 34% (95% confidence interval = 8-53%, P = 0.014) during the malaria season following vaccination. Breakthrough P. falciparum parasites sampled from vaccinated subjects and from controls were genotyped at two polymorphic regions of the csp gene encoding T cell epitopes (csp-th2r and csp-th3r) to determine if the vaccine conferred a strain-specific effect. The overall distribution of csp allelic variants was similar in infections occurring in vaccine and control groups. Also, the mean number of genotypes per infection in the RTS,S/AS02 group was not reduced compared with the controls.

Genetic complexity of Plasmodium falciparum gametocytes isolated from the peripheral blood of treated Gambian children.

The genetic complexity of Plasmodium falciparum gametocytes isolated from Gambian children participating in a controlled trial of anti-malarial therapy was investigated. RNA and DNA were prepared from gametocyte-positive blood, which was also used in transmission experiments with Anopheles gambiae mosquitoes. Amplification by a reverse transcriptase-polymerase chain reaction (RT-PCR) of transcripts from the genes for the ring-infected erythrocyte surface antigen and the 16-kD antigen, which exhibit asexual and sexual stage-specific expression, was used to identify 30 post-treatment gametocyte isolates in which trophozoites persisted below the threshold of detection by microscopy. These included isolates from children who received sulfadoxine/pyrimethamine plus artesunate. Twenty-nine gametocyte-positive isolates that were free of subpatent trophozoites were examined further by PCR amplification of polymorphic genomic loci. We estimate that an average minimum of 2.3 genotypes occurred in these gametocyte-only isolates, and many of these were shown to be infective to mosquitoes. Thus, meiotic recombination between different genotypes is predicted to be a common event in this study area.

Differences in automated depolarization patterns of Plasmodium falciparum and P. vivax malaria infections defined by the Cell-Dyn CD4000 haematology analyser.

Of 1014 samples submitted for full blood count analysis and malaria screening, 854 were designated malaria-negative by blood film microscopy, 79 were unequivocally identified as Plasmodium vivax and 81 as P. falciparum. All samples were additionally analysed with the Abbott Cell-Dyn CD4000 haematology instrument, and leucocyte differential plots of 90 degrees polarized vs. 90 degrees depolarized (NEU-EOS plot) and 90 degrees depolarized vs. 0 degree light (EOS I plot) scatter were specifically examined for abnormal depolarization patterns. Depolarization pattern types were correlated with microscopy (species) results, and these correlations were consolidated by polymerase chain reaction analysis. All 854 microscopically-designated malaria-negative samples showed a type 1 (normal) CD4000 depolarization pattern. Abnormal pattern types 2, 3a and 3b were entirely restricted to one of the two malaria categories. Plasmodium falciparum malaria showed two CD4000 pattern types only; a 'normal' type 1 pattern was seen in 36/75 (48%) cases and the remaining 39 cases were all abnormal pattern type 3a. In contrast, most (79/85) P. vivax malaria cases showed a distinctive clustered EOS I population (types 2 and 3b patterns) that was not seen with P. falciparum. Automated depolarization analysis provides an effective means of detecting malaria-associated haemozoin, and the patterns of intracellular haemozoin further appear to provide species differentiation between P. falciparum and P. vivax.

A randomised, double-blind, placebo-controlled trial of atovaquone-proguanil vs. sulphadoxine-pyrimethamine in the treatment of malarial anaemia in Zambian children.

OBJECTIVE: To compare the efficacy of atovaquone-proguanil (AP) and sulphadoxine-pyrimethamine (SP) in the treatment of malarial anaemia in Zambian children. METHODS: An individually randomised, double-blind, controlled trial was undertaken in Zambian children with moderately severe anaemia and Plasmodium falciparum parasitaemia. The main trial endpoint was treatment failure defined as a need for blood transfusion or treatment with quinine, persistent anaemia or death within 14 days from the start of treatment. Secondary endpoints were parasitological and haematological findings 14 or 28 days after the start of treatment. RESULTS: A total of 128 children with a packed cell volume of <21% and >9% and P. falciparum parasitaemia received treatment with AP and 127 treatment with SP. Treatment failure occurred in 28 children (22%) who received SP and in 10 (8%) who received AP (OR: 3.34, 95% CI: 1.54, 7.21). Ten children required blood transfusion, all of whom were in the SP treatment group. Six children died, five of whom were in the AP group; none of the deaths were considered to be related directly to treatment. CONCLUSIONS: Atovaquone-proguanil proved more effective than SP in the treatment of malarial anaemia in an area with a modest level of SP resistance. AP is no longer available through the Malarone Donation Programme and is too expensive for routine use in Africa. However, this study has shown that in an area with a modest level of resistance to SP, use of a more effective antimalaria reduces the need for blood transfusion in children with malarial anaemia.

Chloroquine/sulphadoxine-pyrimethamine for gambian children with malaria: transmission to mosquitoes of multidrug-resistant Plasmodium falciparum.

OBJECTIVES: In the Gambia, chloroquine (CQ) plus sulphadoxine-pyrimethamine (SP) is the first-line antimalarial treatment. Plasmodium falciparum parasites carrying mutations associated with resistance to each of these drugs were present in 2001 but did not cause a significant loss of therapeutic efficacy among children receiving the combination CQ/SP. We measured their effect on parasite transmission to Anopheles gambiae mosquitoes. DESIGN: We conducted a single-blind, randomised, controlled trial with follow-up over 28 d. Mosquito feeding experiments were carried out 7, 10, or 14 d after treatment. SETTING: The study took place in the town of Farafenni and surrounding villages in the Gambia. PARTICIPANTS: Participants were 500 children aged 6 mo to 10 y with uncomplicated P. falciparum malaria. INTERVENTIONS: Children were randomised to receive CQ, SP, or CQ/SP. OUTCOME MEASURES: Outcomes related to transmission were determined, including posttreatment gametocyte prevalence and density. Infectiousness was assessed by membrane-feeding A. gambiae mosquitoes with blood from 70 gametocyte-positive patients. Mutations at seven loci in four genes associated with drug resistance were measured pre- and posttreatment and in the midguts of infected mosquitoes. RESULTS: After SP treatment, the infectiousness of gametocytes was delayed, compared to the other two treatment groups, despite comparable gametocyte densities. Among bloodmeal gametocytes and the midguts of infected mosquitoes, the presence of the four-locus multidrug-resistant haplotype TYRG (consisting of mutations pfcrt-76T, pfmdr1-86Y, pfdhfr-59R, and pfdhps-437G) was associated with significantly higher oocyst burdens after treatment with the combination CQ/SP. CONCLUSIONS: Parasites with a multidrug-resistant genotype had a substantial transmission advantage after CQ/SP treatment but did not have a significant impact on in vivo efficacy of this drug combination. Protocols that include measuring transmission endpoints as well as therapeutic outcomes may be a useful strategy when monitoring the evolution of drug resistance in malaria parasites in vivo.

Are extensive T cell epitope polymorphisms in the Plasmodium falciparum circumsporozoite antigen, a leading sporozoite vaccine candidate, selected by immune pressure?

Protective cellular immune responses depend on MHC presentation of pathogen-derived Ag fragments. MHC diversity renders this process sensitive to point mutations coding for altered amino acid sequence of the short target Ag-derived peptides epitopes. Thus, in a given host, a pathogen with an altered epitope sequence will be more likely to escape detection and elimination by the immune system. At a population level, selection by immune pressure will increase the likelihood of polymorphism in important pathogen antigenic epitopes. This mechanism of immune evasion is found in viruses and other pathogens. The detection of polymorphic hot spots in an Ag is often taken as a strong indication of its role in protective immunity. We provide evidence that polymorphisms in the T cell epitopes of a malaria vaccine candidate are unlikely to have been selected by immune pressure in the human host.

Combination therapy counteracts the enhanced transmission of drug-resistant malaria parasites to mosquitoes.

Malaria parasites carrying genes conferring resistance to antimalarials are thought to have a selective advantage which leads to higher rates of transmissibility from the drug-treated host. This is a likely mechanism for the increasing prevalence of parasites with resistance to chloroquine (CQ) and sulfadoxine-pyrimethamine in sub-Saharan Africa. Combination therapy is the key strategy being implemented to reduce the impact of resistance, but its effect on the transmission of genetically resistant parasites from treated patients to mosquito vectors has not been measured directly. In a trial comparing CQ monotherapy to the combination CQ plus artesunate (AS) in Gambian children with uncomplicated falciparum malaria, we measured transmissibility by feeding Anopheles gambiae mosquitoes with blood from 43 gametocyte-positive patients through a membrane. In the CQ-treated group, gametocytes from patients carrying parasites with the CQ resistance-associated allele pfcrt-76T prior to treatment produced infected mosquitoes with 38 times higher Plasmodium falciparum oocyst burdens than mosquitoes fed on gametocytes from patients infected with sensitive parasites (P < 0.001). Gametocytes from parasites carrying the resistance-associated allele pfmdr1-86Y produced 14-fold higher oocyst burdens than gametocytes from patients infected with sensitive parasites (P = 0.011). However, parasites carrying either of these resistance-associated alleles pretreatment were not associated with higher mosquito oocyst burdens in the CQ-AS-treated group. Thus, combination therapy overcomes the transmission advantage enjoyed by drug-resistant parasites.

Rare, highly pyrimethamine-resistant alleles of the Plasmodium falciparum dihydrofolate reductase gene from 5 African sites.

In eastern and southern Africa, there has been a rapid increase in the prevalence of alleles with mutations in the Plasmodium falciparum dihydrofolate reductase gene (dhfr) associated with increased risk of clinical failure of sulfadoxine-pyrimethamine (S/P). Molecular methods for surveillance of these mutations are now widespread, but the usual analysis detects only the most prevalent allele in a polyclonal sample. We used a yeast-expression system to identify rare, highly pyrimethamine-resistant alleles of dhfr in isolates from 5 African countries--Kenya, Tanzania, Malawi, Gabon, and Nigeria. Only the isolates from Nigeria yielded significant numbers of novel resistant alleles, and only 1 of the alleles from any location showed a >3-fold increase in resistance to S/P or to chlorproguanil-dapsone. Overall, these results suggest that dhfr alleles that confer high levels of resistance to antifolates are rare, even in eastern and southern Africa, where pyrimethamine has been intensively used.