What would PCR assessment change in the management of fevers in a malaria endemic area? A school-based study in Benin in children with and without fever

BACKGROUND

A recent school-based study in Benin showed that applying a policy of anti-malarial prescriptions restricted to parasitologically-confirmed cases on the management of fever is safe and feasible. Additional PCR data were analysed in order to touch patho-physiological issues, such as the usefulness of PCR in the management of malaria in an endemic area or the triggering of a malaria attack in children with submicroscopic malaria.

METHODS

PCR data were prospectively collected in the setting of an exposed (with fever)/non exposed (without fever) study design. All children had a negative malaria rapid diagnostic test (RDT) at baseline, were followed up to day 14 and did not receive drugs with anti-malarial activity. The index group was defined by children with fever at baseline and the control group by children without fever at baseline. Children with submicroscopic malaria in these two groups were defined by a positive PCR at baseline.

RESULTS

PCR was positive in 66 (27%) children of the index group and in 104 (44%) children of the control group respectively. The only significant factor positively related to PCR positivity at baseline was the clinical status (control group). When definition of malaria attacks included PCR results, no difference of malaria incidence was observed between the index and control groups, neither in the whole cohort, nor in children with submicroscopic malaria. The rate of undiagnosed malaria at baseline was estimated to 3.7% at baseline in the index group.

CONCLUSIONS

Treating all children with fever and a positive PCR would have led to a significant increase of anti-malarial consumption, with few benefits in terms of clinical events. Non malarial fevers do not or do not frequently trigger malaria attacks in children with submicroscopic malaria.

Plasmodium falciparum parasites causing cerebral malaria share variant surface antigens, but are they specific?

Background

Variant surface antigens (VSA) expressed on the surface of Plasmodium falciparum-infected red blood cells constitute a key for parasite sequestration and immune evasion. In distinct malaria pathologies, such as placental malaria, specific antibody response against VSA provides protection. This study investigated the antibody response specifically directed against VSA expressed by parasites isolated from individuals presenting a given type of clinical presentation.

Methods

Plasma and isolates were obtained from four groups of Beninese subjects: healthy adults, patients presenting uncomplicated malaria (UM), cerebral malaria (CM), or pregnancy-associated malaria (PAM). The reactivity of plasma samples from each clinical group was measured by flow cytometry against parasites isolated from individuals from each clinical group.

Results

Antibody responses against VSA_UM were predominant in CM, UM and HA plasmas. When analysed according to age in all plasma groups, anti-VSA_CM and -VSA_UM antibody levels were similar until six years of age. In older groups (6-18 and >19 years of age), VSA_UM antibody levels were higher than VSA_CM antibody levels (P = .01, P = .0008, respectively). Mean MFI values, measured in all plasmas groups except the PAM plasmas, remained low for anti-VSA_PAM antibodies and did not vary with age. One month after infection the level of anti-VSA antibodies able to recognize heterologous VSA_CM variants was increased in CM patients. In UM patients, antibody levels directed against heterologous VSA_UM were similar, both during the infection and one month later.

Conclusions

In conclusion, this study suggests the existence of serologically distinct VSA_CM and VSA_UM. CM isolates were shown to share common epitopes. Specific antibody response to VSA_UM was predominant, suggesting a relative low diversity of VSA_UM in the study area.

Towards the rational design of a candidate vaccine against pregnancy associated malaria: conserved sequences of the DBL6epsilon domain of VAR2CSA

BACKGROUND

Placental malaria is a disease linked to the sequestration of Plasmodium falciparum infected red blood cells (IRBC) in the placenta, leading to reduced materno-fetal exchanges and to local inflammation. One of the virulence factors of P. falciparum involved in cytoadherence to chondroitin sulfate A, its placental receptor, is the adhesive protein VAR2CSA. Its localisation on the surface of IRBC makes it accessible to the immune system. VAR2CSA contains six DBL domains. The DBL6epsilon domain is the most variable. High variability constitutes a means for the parasite to evade the host immune response. The DBL6epsilon domain could constitute a very attractive basis for a vaccine candidate but its reported variability necessitates, for antigenic characterisations, identifying and classifying commonalities across isolates.

METHODOLOGY/PRINCIPAL FINDINGS

Local alignment analysis of the DBL6epsilon domain had revealed that it is not as variable as previously described. Variability is concentrated in seven regions present on the surface of the DBL6epsilon domain. The main goal of our work is to classify and group variable sequences that will simplify further research to determine dominant epitopes. Firstly, variable sequences were grouped following their average percent pairwise identity (APPI). Groups comprising many variable sequences sharing low variability were found. Secondly, ELISA experiments following the IgG recognition of a recombinant DBL6epsilon domain, and of peptides mimicking its seven variable blocks, allowed to determine an APPI cut-off and to isolate groups represented by a single consensus sequence.

CONCLUSIONS/SIGNIFICANCE

A new sequence approach is used to compare variable regions in sequences that have extensive segmental gene relationship. Using this approach, the VAR2CSA DBL6 domain is composed of 7 variable blocks with limited polymorphism. Each variable block is composed of a limited number of consensus types. Based on peptide based ELISA, variable blocks with 85% or greater sequence identity are expected to be recognized equally well by antibody and can be considered the same consensus type. Therefore, the analysis of the antibody response against the classified small number of sequences should be helpful to determine epitopes.

Plasmodium falciparum population dynamics in a cohort of pregnant women in Senegal

BACKGROUND

Pregnant women acquire protective antibodies that cross-react with geographically diverse placental Plasmodium falciparum isolates, suggesting that surface molecules expressed on infected erythrocytes by pregnancy-associated malaria (PAM) parasites have conserved epitopes and, that designing a PAM vaccine may be envisaged. VAR2CSA is the main candidate for a pregnancy malaria vaccine, but vaccine development may be complicated by its sequence polymorphism.

METHODS

The dynamics of P. falciparum genotypes during pregnancy in 32 women in relation to VAR2CSA polymorphism and immunity was determined. The polymorphism of the msp2 gene and five microsatellites was analysed in consecutive parasite isolates, and the DBL5epsilon + Interdomain 5 (Id5) part of the var2csa gene of the corresponding samples was cloned and sequenced to measure variation.

RESULTS

In primigravidae, the multiplicity of infection in the placenta was associated with occurrence of low birth weight babies. Some parasite genotypes were able to persist over several weeks and, still be present in the placenta at delivery particularly when the host anti-VAR2CSA antibody level was low. Comparison of diversity among genotyping markers confirmed that some PAM parasites may harbour more than one var2csa gene copy in their genome.

CONCLUSIONS

Host immunity to VAR2CSA influences the parasite dynamics during pregnancy, suggesting that the acquisition of protective immunity requires pre-exposure to a limited number of parasite variants. Presence of highly conserved residues in surface-exposed areas of the VAR2CSA immunodominant DBL5epsilon domain, suggest its potential in inducing antibodies with broad reactivity.

Molecular aspects of Plasmodium falciparum Infection during pregnancy

Cytoadherence of Plasmodium-falciparum-parasitized red blood cells (PRBCs) to host receptors is the key phenomenon in the pathological process of the malaria disease. Some of these interactions can originate poor outcomes responsible for 1 to 3 million annual deaths mostly occurring among children in sub-Saharan Africa. Pregnancy-associated malaria (PAM) represents an important exception of the disease occurring at adulthood in malaria endemic settings. Consequences of this are shared between the mother (maternal anemia) and the baby (low birth weight and infant mortality). Demonstrating that parasites causing PAM express specific variant surface antigens (VSA(PAM)), including the P. falciparum erythrocyte membrane protein 1 (P f EMP1) variant VAR2CSA, that are targets for protective immunity has strengthened the possibility for the development of PAM-specific vaccine. In this paper, we review the molecular basis of malaria pathogenesis attributable to the erythrocyte stages of the parasites, and findings supporting potential anti-PAM vaccine components evidenced in PAM.

Functional and immunological characterization of the var2CSA-DBL5epsilon domain of a placental Plasmodium falciparum isolate

Pregnancy-associated malaria (PAM) arises from sequestration of Plasmodium falciparum-parasitized erythrocytes (PE) in the placenta, leading to chronic symptoms in the expectant mother and serious consequences for fetal development. Placental sequestration has been linked to binding of chondroitin sulphate A (CSA) by the var2CSA variant of PfEMP1 expressed on the PE surface, and a substantial body of evidence shows that the immune response to var2CSA gives an effective protection against PAM. We have expressed the var2CSA-DBL5epsilon domain, derived from a placental isolate from Senegal, as soluble product in Escherichia coli and have shown using different criteria that the recombinant protein is obtained with the native conformation. Using surface plasmon resonance techniques, we have examined binding of DBL5epsilon to placental chondroitin sulphate proteoglycan and CSA; however, the recombinant protein also binds to other sulphated oligosaccharides, with higher affinity in some cases, indicating that the single domain lacks the specificity for CSA shown by the complete extra-cellular region of var2CSA and placental parasites. Recombinant DBL5epsilon was specifically recognized by sera from malaria-exposed Senegalese women in a parity-dependent manner but by sera not from children or males from the same endemic region. Conversely, DBL5epsilon induced antibodies in mice that recognized placental isolates from Benin but not isolates from children. The presence of universal epitopes thus supports DBL5epsilon as an interesting component of var2CSA to be considered for vaccine development.

Plasmodium falciparum exposure in utero, maternal age and parity influence the innate activation of foetal antigen presenting cells

Background

Malaria in pregnancy is associated with immunological abnormalities in the newborns, such as hampered T-helper 1 responses and increased T-regulatory responses, while the effect of maternal Plasmodium falciparum infection on foetal innate immunity is still controversial.

Materials and methods

The immunophenotype and cytokine release by dendritic cells (DC) and monocytes were evaluated in cord blood from 59 Beninese women with or without malaria infection by using flow cytometry.

Results

Accumulation of malaria pigment in placenta was associated with a partial maturation of cord blood myeloid and plasmacytoid DC, as reflected by an up-regulated expression of the major histocompatibility complex class II molecules, but not CD86 molecules. Cells of newborns of mothers with malaria pigment in their placenta also exhibited significantly increased cytokine responses upon TLR9 stimulation. In addition, maternal age and parity influenced the absolute numbers and activation status of cord blood antigen-presenting cells. Lastly, maternal age, but not parity, influenced TLR3, 4 and 9 responses in cord blood cells.

Discussion

Our findings support the view that placental parasitization, as indicated by the presence of malaria pigment in placental leukocytes, is significantly associated with partial maturation of different DC subsets and also to slightly increased responses to TLR9 ligand in cord blood. Additionally, other factors, such as maternal age and parity should be taken into consideration when analysing foetal/neonatal innate immune responses.

Conclusion

These data advocate a possible mechanism by which PAM may modulate foetal/neonatal innate immunity.

Comparison of sulfadoxine-pyrimethamine, unsupervised artemether-lumefantrine, and unsupervised artesunate-amodiaquine fixed-dose formulation for uncomplicated plasmodium falciparum malaria in Benin: a randomized effectiveness noninferiority trial

BACKGROUND

We compared sulfadoxine-pyrimethamine (SP) with unsupervised artemether-lumefantrine (AL) and unsupervised amodiaquine-artesunate (ASAQ) fixed-dose formulation for the treatment of uncomplicated malaria in children in Benin.

METHODS

This open-label, noninferiority comparative trial included children aged 6-60 months. The follow-up period was 6 weeks, and the primary objective was a comparison of polymerase chain reaction (PCR)-adjusted effectiveness rates at day 28.

RESULTS

The study included 240 children (48 received SP, and 96 each received AL and ASAQ). The intention-to-treat analysis showed effectiveness rates on day 28 of 20.8%, 78.1%, and 70.5% for SP, AL, and ASAQ, respectively. After adjustment for PCR results, these rates were 27.1%, 83.3%, and 87.4%, respectively. The per-protocol analysis (217 patients) showed effectiveness rates on day 28 of 21.7%, 88.0%, and 76.1% for SP, AL, and ASAQ, respectively. After adjustment for PCR results, these rates were 28.3%, 94.0%, and 93.2%, respectively. SP was less effective than the other drugs in the PCR-adjusted analysis, whereas AL and ASAQ were equally effective. The rate of new infection was higher among children treated with ASAQ than among those treated with AL.

CONCLUSIONS

This was the first trial, to our knowledge, to compare unsupervised AL with unsupervised ASAQ fixed-dose formulation; both treatments provided high PCR-adjusted day 28 effectiveness rates. Efficacy rates for SP were surprisingly low. Clinical trials registration. NCT00460369.

PfHRP2 and PfLDH antigen detection for monitoring the efficacy of artemisinin-based combination therapy (ACT) in the treatment of uncomplicated falciparum malaria

BACKGROUND

An assessment of the accuracy of two malaria rapid diagnostic tests (RDT) for the detection of Plasmodium falciparum histidine-rich protein 2 (PfHRP2) or Pf lactate dehydrogenase (PfLDH) was undertaken in children aged between six and 59 months included in an anti-malarial efficacy study in Benin.

METHODS

In Allada (Benin), 205 children aged 6-59 months with falciparum malaria received either artesunate-amodiaquine (ASAQ), artemether-lumefantrine (AL), or sulphadoxine-pyrimethamine (SP). Children included in the study were simultaneously followed by both RDT and high-quality microscopy for up to 42 days.

RESULTS

At the time of inclusion, PfHRP2-based tests were positive in 203 children (99%) and PfLDH-based tests were positive in 204 (99.5%). During follow-up, independent of the treatment received, only 17.3% (28/162) of children effectively cured were negative with the PfHRP2 RDT at day 3, with a gradual increase in specificity until day 42. The specificity of antigen detection with the PfLDH test was 87% (141/162) on day 3, and between 92% and 100% on days 7 to 42. A statistical difference was observed between the persistence of PfHRP2 and PfLDH antigenaemia during follow-up in children treated with artemisinin-based combination therapy (ACT) but not with SP.

CONCLUSION

Although both RDTs are as sensitive as microscopy in detecting true malaria cases, the PfHRP2 RDT had very low specificity during follow-up until day 28. On the other hand, the PfLDH test could be used to detect failures and, therefore, to assess anti-malarial efficacy.

Towards a vaccine against pregnancy-associated malaria

The consequences of pregnancy-associated malaria on pregnant women (anaemia), their babies (birth weight reduction), and infants (increased morbidity and mortality) are well documented. Field observations during the last decade have underlined the key role of the interactions between P. falciparum variable surface antigens expressed on infected erythrocytes and a novel receptor: chondroitin sulfate A (CSA) for the placental sequestration of infected erythrocytes. Identification of a distinct P. folciparum erythrocyte membrane protein 1 (PfEMP1) variant, VAR2CSA, as the dominant variant surface antigen and as a clinically important target for protective immune response to pregnancyassociated malaria has raised hope for developing a new preventive strategy based on inducing these immune responses by vaccination. However, despite particular structure and interclonal conservation of VAR2CSA among other PfEMP1, significant challenges still exist concerning the development of a VAR2CSA-based vaccine with profound efficacy.

Plasmodium falciparum transcriptome analysis reveals pregnancy malaria associated gene expression

BACKGROUND

Pregnancy-associated malaria (PAM) causing maternal anemia and low birth weight is among the multiple manifestations of Plasmodium falciparum malaria. Infected erythrocytes (iEs) can acquire various adhesive properties that mediate the clinical severity of malaria. Recent advances on the molecular basis of virulence and immune evasion have helped identify var2csa as a PAM-specific var gene.

METHODOLOGY/PRINCIPAL FINDINGS

The present study presents a genome-wide microarray transcript analysis of 18 P. falciparum parasite isolates freshly collected from the placenta. The proportion of PAM over-expressed genes located in subtelomeric regions as well as that of PAM over-expressed genes predicted to be exported were higher than expected compared to the whole genome. The identification of novel parasite molecules with specificity to PAM and which are likely involved in host-pathogen interactions and placental tropism is described. One of these proteins, PFI1785w, was further characterized as the product of a two-exon PHIST gene, and was more often recognized by serum samples from P. falciparum-exposed women than from men.

CONCLUSIONS/SIGNIFICANCE

These findings suggest that other parasite proteins, such as PFI1785w, may contribute beside VAR2CSA to the pathogenesis of PAM. These data may be very valuable for future vaccine development.

Differential evolution of anti-VAR2CSA- IgG3 in primigravidae and multigravidae pregnant women infected by Plasmodium falciparum

BACKGROUND

Pregnant women develop protective anti-VSA IgG1 and IgG3 when infected by Plasmodium falciparum. The major target of IgG from serum of infected pregnant women is VAR2CSA.

METHODS

In this study, ELISA was used to compare the level of VAR2CSA DBL5epsilon- specific IgG subclasses at enrolment and at delivery in a cohort of pregnant women in Senegal. All antibody measures were analysed in relation to placental infection according to parity.

RESULTS

The results show an interaction between immune response to placental malaria and parity. A higher level of anti- DBL5epsilon- IgG3 at enrolment and a higher increase between enrolment and delivery were found in primigravidae who presented with uninfected placenta at delivery in comparison to those who presented with an infection of the placenta. However, high antibody level at delivery was associated with the infection of the placenta in multigravidae.

CONCLUSION

This high level of IgG3 in uninfected primigravidae suggests a protective role of these antibodies in this susceptible group, highlighting the importance of VAR2CSA in general and of some of its variants still to be defined, in the induction of protective immunity to pregnancy malaria.

Dramatically decreased therapeutic efficacy of chloroquine and sulfadoxine-pyrimethamine, but not mefloquine, in southern Benin

OBJECTIVE

To evaluate the in vivo therapeutic efficacy of chloroquine (CQ), sulfadoxine-pyrimethamine (SP) and mefloquine (MQ) in children presenting with uncomplicated malaria in Benin.

METHODS

Drug efficacy was tested according to the WHO in vivo 28-day protocol. For failures that occurred after 7 days of follow-up, paired pre- and post-treatment blood samples were genotyped at msp1 and msp2 loci to distinguish new infections and recrudescent strains. Children enrolled were randomly assigned to a therapeutic group (CQ, n=14; SP, n=42; MQ, n=44). The number of CQ treatment was intentionally restricted after 1 month, as its use was considered to constitute a danger for children.

RESULTS

Chloroquine and SP showed very high failure rates (85.7% and 50%, respectively), whereas MQ treatment was successful in 97.5%. The molecular tool allowed to re-evaluate two new infections previously considered as failures.

CONCLUSIONS

Chloroquine should no longer be used to treat children presenting with Plasmodium falciparum malaria in Benin.

IL-12 producing monocytes and IFN-γ and TNF-α producing T-lymphocytes are increased in placentas infected by Plasmodium falciparum

Placental Plasmodium falciparum sequestration is associated with dysregulated immune function. Placental inflammatory responses via IFN-gamma and TNF-alpha are implicated in functional damage. However, they are needed during placental infection to control asexual stage parasites. To test the hypothesis that placental immunomodulation associated with malaria disturbs cytokine secretion differently in monocytes and lymphocytes, we have determined the proportion of monocytes and/or lymphocytes secreting IFN-gamma, TNF-alpha, IL-10 and IL-12. Intervillous and peripheral blood monocyte (CD14+) and lymphocyte (CD3/CD4+; CD3/CD8+) cytokine production was compared between 17 P. falciparum-infected and 12 non-infected Senegalese women. After culture with phorbolmyristate acetate/ionomycin (PMA/iono), lipopolysaccharide (LPS) or P. falciparum-infected erythrocytes (IE), the intracellular expression of cytokines in lymphocytes (IFN-gamma, TNF-alpha) and monocytes (IL-10, IL-12, TNF-alpha), was detected. In response to IE, CD4+ and CD8+ T-cells produced IFN-gamma and TNF-alpha at similar rates in both compartments. In response to PMA/iono, the frequencies of CD4+ and CD8+ T-cells producing IFN-gamma and TNF-alpha were similar in both compartments, but increased in P. falciparum-infected placentas. In response to LPS or IE, IL-12 secreting monocytes were increased in infected women, while the frequency of TNF-alpha secreting monocytes was decreased compared to that in non-infected placenta. The monocyte IL-12 response is not impaired in infected women. IL-12 is an important factor for inducing IFN-gamma in T-cells. Thus, IL-12 and IFN-alpha responses may synergistically allow a protective immune response in placental malaria. TNF-alpha production by CD4+ and CD8+ T-cells is up-regulated in P. falciparum-infected placentas, suggesting that T-cells actively participate to inflammatory responses.

The impact of IgG antibodies to recombinant Plasmodium falciparum 732var CIDR-1alpha domain in mothers and their newborn babies

Different domains of a novel full-length var gene (termed 732var) isolated from a placenta of a malaria-infected woman were expressed in Escherichia coli as recombinant proteins and analysed biochemically and immunologically. Two of these, the Duffy binding-like (DBL)-3gamma domain and the cysteine-rich interdomain region (CIDR)-1alpha were able to bind chondroitin sulfate A and CD36, respectively. The DBL-3gamma domain was investigated in a previous study and confirmed here to exhibit anti-disease characteristics related to pregnancy-associated malaria. Mothers with high anti-DBL-3gamma antibody levels were protected from placental infection. The novel finding in this study is that babies born to mothers carrying anti-CIDR-1alpha antibodies had a delayed time to the first infection.

Correlations between treatment outcome and both anti-MSP119 antibody response and erythrocyte-related genetic factors in Plasmodium falciparum malaria

Treatment efficacy is related to the interaction of three parameters: drug, parasites, and human factors. The role of human factors in treatment outcome has been poorly documented to date, although human genetic factors and specific immunity have been related to protection against malaria. This study aimed to evaluate a possible cooperation between drug efficacy and host factors in treatment success. The contribution of host factors to treatment efficacy was studied in Gabonese children with a non-severe malaria attack. Children (n=232) aged under 10 years were treated with either sulfadoxine-pyrimethamine or amodiaquine. The influence of erythrocyte-related genetic factors and humoral immune responses (IgG and subclasses) against MSP1(19) on anti-malarial treatment outcome during a 28-day follow-up was studied. Sickle-cell trait carriage and anti-MSP1(19) IgG3 levels were related to lower parasite densities at enrolment (multiple linear regression analysis, P<or=0.005). Strikingly, early failures after AQ or SP treatment were associated with decreased anti-MSP1(19) IgG, IgG1 and IgG3 levels at enrolment. However, this finding was achieved in a low number of children presenting with an early failure. Kinetics of anti-MSP1(19) IgG and subclasses between Days 0 and 28 were also related to treatment efficacy, as the most effective treatment (sulfadoxine-pyrimethamine) was characterised by a higher elevation of antibody titres by Day 28. No effect of erythrocyte-related genetic factors on treatment outcome was shown, although the protective role of sickle-cell trait against higher parasitaemias was confirmed at enrolment. Our data suggest that anti-MSP1(19) IgG1 may have a supportive role during the first days of treatment to prevent early failures. The interference between drug efficacy, immunity and human genetic factors needs further investigation to be elucidated.

Epitope mapping and topographic analysis of VAR2CSA DBL3X involved in P. falciparum placental sequestration

Pregnancy-associated malaria is a major health problem, which mainly affects primigravidae living in malaria endemic areas. The syndrome is precipitated by accumulation of infected erythrocytes in placental tissue through an interaction between chondroitin sulphate A on syncytiotrophoblasts and a parasite-encoded protein on the surface of infected erythrocytes, believed to be VAR2CSA. VAR2CSA is a polymorphic protein of approximately 3,000 amino acids forming six Duffy-binding-like (DBL) domains. For vaccine development it is important to define the antigenic targets for protective antibodies and to characterize the consequences of sequence variation. In this study, we used a combination of in silico tools, peptide arrays, and structural modeling to show that sequence variation mainly occurs in regions under strong diversifying selection, predicted to form flexible loops. These regions are the main targets of naturally acquired immunoglobulin gamma and accessible for antibodies reacting with native VAR2CSA on infected erythrocytes. Interestingly, surface reactive anti-VAR2CSA antibodies also target a conserved DBL3X region predicted to form an alpha-helix. Finally, we could identify DBL3X sequence motifs that were more likely to occur in parasites isolated from primi- and multigravidae, respectively. These findings strengthen the vaccine candidacy of VAR2CSA and will be important for choosing epitopes and variants of DBL3X to be included in a vaccine protecting women against pregnancy-associated malaria.

Receptor-binding studies of the DBLgamma domain of Plasmodium falciparum erythrocyte membrane protein 1 from a placental isolate

We have previously identified a number of DBLgamma domains in Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) transcripts obtained from placental parasite isolates, showing that they bind specifically to chondroitin sulfate A (CSA) (Khattab A, Kun J, Deloron P, Kremsner PG, Klinkert MQ. Variants of Plasmodium falciparum erythrocyte membrane protein 1 expressed by different placental parasites are closely related and adhere to chondroitin sulfate A. J Infect Dis 2001;183:1165-9). Here we give a more detailed physico-chemical and binding characterisation of the soluble, recombinant DBLgamma domain derived from one of these isolates. Results from circular dichroism and limited proteolysis experiments are consistent with the recombinant domain being expressed with the native fold. Specific binding of DBLgamma to placental cryosections was demonstrated by labeling with antibodies raised against the recombinant domain; binding was diminished after treatment of the cryosections with chondroitinase or by blocking with anti-CSA antibody, showing that CSA mediates the interaction. Binding of the DBLgamma domain to purified placental chondroitin sulfate proteoglycan (CSPG) was also studied using surface plasmon resonance techniques, with DBLgamma as analyte and CSPG immobilised on the sensor chip; these quantitative measurements gave an affinity constant in the mu-molar range under the conditions used. The native conformation of the DBLgamma domain is essential for CSPG recognition since binding to the sensor chip is abolished when the protein is irreversibly reduced. As with the placental cryosections, association was significantly reduced after treating the immobilised CSPG with chondroitinase. Together, these results demonstrate specific interaction between the DBLgamma domain and the placental receptor.

Prediction of Plasmodium falciparum placental infection according to the time of infection during pregnancy

Malarial infection during pregnancy leads to placental infection, a known risk factor for low birth weight. Whether the stage of pregnancy at infection has a differential influence on these effects is not clearly known, but may be of importance for prevention strategies, including intermittent preventive treatment of pregnant women. Malaria infection during early (before 20 weeks), middle (20-28 weeks), or late (after 28 weeks) pregnancy was evaluated by logistic regression and receiver operating characteristics analysis in relation to placental infection in pregnant Senegalese women. Plasmodium falciparum infections during late pregnancy are strongly related to placental infection, as well as those that occur in middle pregnancy. Knowledge of parasitological events over the entire duration of pregnancy permits a highly accurate prediction of placental infection. Not only malaria infections during late pregnancy increase the likelihood of placental infection. The current policy of intermittent preventive treatment of pregnant women, which implies an initial antimalarial cure after 20 weeks of pregnancy, will not avoid early infections. An earlier initiation of malaria prevention might improve its efficacy.

Development of cellular immune responses to Plasmodium falciparum blood stage antigens from birth to 36 months of age in Cameroon

Naturally acquired immunity to Plasmodium falciparum is related to immune system that changes during normal development and ageing. The effects of repeated infections during the early life on the maturation of the immune system are still unknown. Elucidation of these effects is of considerable interest given that malaria originates high mortality, especially during the first years of life. We conducted a cohort study to identify naturally acquired immune responses to P. falciparum. Cellular responses of Cameroonian neonates from birth to 36 months of age were evaluated every 6 months by cell proliferation and cytokines (IFN-gamma, IL-2 and IL-4) production after in vitro culture in the presence of schizont extract and Pf155/RESA peptides. Data were analyzed by a multiple correspondence analysis (MCA) exhibiting three main findings. Firstly, the lack of time-dependant evolution of specific immune pathways recruitment in the response to a given antigen, no antigen inducing a specific mode of response at a given time-point. Secondly, most of the data variability was expressed by IFN-gamma and IL-4 productions, and the major variation of the immune response with age involved this change in IFN-gamma production. Thirdly, the age-related immune response evolution is characterized by the acquisition of the capacity to mount a IFN-gamma response, a transient phase during which children produce a high IL-4 response, and the fast vanishing of the dominance of the IL-2 response. These results suggest that P. falciparum specific immune responses are first oriented towards a Th2-type of response, and later switch to Th1-type of response.

Plasmodium falciparum genotype population dynamics in asymptomatic children from Senegal

In areas where malaria is endemic, infected individuals generally harbor a mixture of genetically distinct Plasmodium falciparum parasite populations. For the first time, we studied temporal variations of blood parasite densities and circulating genotypes in asymptomatic Senegalese children, at time intervals as short as 4-12 h. Twenty-one Senegalese children, presenting with an asymptomatic P. falciparum infection, were sampled eight times within three days. Parasite density was assessed by thick blood smears, and all infecting genotypes were quantified by the fragment-analysis method. Parasite densities showed dramatic fluctuations up to a 1 to 1,000 ratio, with at least one peak of parasite density. Polyclonal infections were detected in all children, with a multiplicity of infection of 5.2-6.8 genotypes per child. A single sample never reflected the full complexity of the parasite populations hosted by a given individual. Genotypes with different behaviors were detected in all children, some genotypes undergoing major fluctuations, while others were highly stable during the follow-up. A single peripheral blood sampling does not reflect the total parasite load. Repeated sampling is needed to have a more detailed scheme of parasite population dynamics and a better knowledge of the true complexity of an infection.

Acquisition of antibodies to variant antigens on the surface of Plasmodium falciparum-infected erythrocytes during pregnancy

Pregnancy-associated malaria is characterized by Plasmodium falciparum adherence to chondroitin sulfate A (CSA) in placenta, through a particular variant surface antigen (VSA). VSA(CSA)-specific IgG are involved in protection against placental malaria. In order to assess the relationship between VSA(CSA)-specific antibody responses and parity as well as protection against placental malaria, the occurrence of P. falciparum infection was assessed in 306 pregnant women from a low malaria transmission area of Senegal. Anti-VSA(CSA) antibodies against three placental parasite isolates were measured by flow cytometry, at enrollment and delivery. Placental infection prevalence rates were highest in primigravidae, but no clear decreasing trend was observed from the second pregnancy onwards. Anti-VSA(CSA) antibody prevalence rates increased with parity. Both anti-VSA(CSA) antibody prevalence rates and levels increased during pregnancy only in women infected with P. falciparum. Although a single or a very limited number of P. falciparum infections were able to induce an anti-VSA(CSA) antibody response, the level or the quality of this response did not appear to confer protection against placental malaria infection.

Analysis of human antibodies to erythrocyte binding antigen 175 peptide 4 of Plasmodium falciparum

BACKGROUND

The IgG1 and IgG3 antibodies are considered cytophilic and protective against Plasmodium falciparum, whereas IgG2 and IgG4 are thought to block protective mechanisms.

OBJECTIVES

The main objective was to measure antibodies directed against erythrocyte binding antigen-175 (EBA-175) peptide 4 and analyze the relationship between such antibodies and clinical malaria attack.

METHODS

Using an enzyme-linked immunosorbent assay, a retrospective analysis of naturally acquired antibodies to synthetic peptide from EBA-175 peptide 4 has been carried out in 158 school children from the village of Dienga in Gabon.

RESULTS

The overall prevalence rates of antibodies to EBA-175 peptide 4 were 85.2%, 66.8%, 52.6%, 71.6% and 64.0% for total IgG, IgG1, IgG2, IgG3 and IgG4, respectively. Protection from clinical malaria, determined after a prospective 1-year study, was associated with the levels of IgG and IgG1 antibodies that increased with age.

CONCLUSION

Together, these data suggest that age/exposure-related acquisition of anti-EBA-175 antibodies may contribute to the development of clinically protective immunity and could be taken into account in malaria control strategies when they are confirmed.