Differential regulation of IgG subclasses and IgE antimalarial antibody responses in complicated and uncomplicated Plasmodium falciparum malaria.

The aim of this study was to assess the immunoglobulin (Ig)-subclass distribution of antimalarial antibody responses in 110 and 169 Thai patients with complicated and uncomplicated Plasmodium falciparum malaria, respectively. Antimalarial plasma IgG subclasses and IgE antibody levels against a crude malaria blood stages, and antigen preparation were determined using enzyme-linked immunosorbent assay (ELISA). On admission, the levels of anti-P. falciparum IgG1, IgG2 and IgG3 were significantly lower in patients with complicated malaria than uncomplicated malaria (IgG1, P < 0.0001; IgG2, P < 0.0001; IgG3, P < 0.0001). The levels of antimalarial IgE were slightly lower, but not statistically significant (P = 0.389) in the complicated malaria. After adjusting all antibody levels and age, anti-P. falciparum IgG3 levels remained significantly associated with complicated malaria. None of the other antibody concentrations showed statistically significant associations with complicated malaria. The anti-P. falciparum IgG3 levels were related to the IgG1 as well as IgG2 levels. A correlation between anti-P. falciparum IgG2 and IgE was observed in the complicated malaria group, and this may indicate their roles in the severity of disease. Our data suggest that anti-P. falciparum IgG3 is associated with a reduced risk of complicated malaria and that antimalarial Ig-subclasses are differently regulated in patients with complicated and uncomplicated malaria.

Cytokine mRNA expression and iron status in children living in a malaria endemic area.

Iron deficiency has been reported to affect both malaria pathogenesis and cell-mediated immune responses; however, it is unclear whether the protection afforded by iron deficiency is mediated through direct effects on the parasite, through immune effector functions or through both. We have determined cytokine mRNA expression levels in 59 children living in a malaria endemic area on the coast of Kenya who we selected on the basis of their biochemical iron status. Real-time quantitative reverse transcriptase polymerase chain reaction analysis of cytokine mRNA levels of peripheral blood mononuclear cells (PBMC) obtained from these children showed an association between interleukin-4 (IL-4) mRNA levels and all the biochemical indices of iron that we measured. Furthermore, IL-10 mRNA was higher in parasite blood smear-positive children than in blood smear-negative children irrespective of their iron status. This study suggests that IL-4 expression by PBMC may be affected by iron status.

Allelic polymorphisms in the repeat and promoter regions of the interleukin-4 gene and malaria severity in Ghanaian children.

Immunoglobulin E has been associated with severe malaria suggesting a regulatory role for interleukin (IL)-4 and/or IgE in the pathogenesis of severe malaria. We have investigated possible associations between polymorphisms in the IL-4 repeat region (intron 3) and promoter regions (IL-4 +33CT and - 590CT) in Ghanaian children with severe malaria. There was a significantly higher frequency of IL-4 intron-3 B1B1 genotype in the cerebral malaria group [P < 0.0001, odds ratio (OR) = 8.7]. The genotype and allele frequencies of the IL-4 -590 and +33 polymorphisms did not differ between the four study groups. Carriers of IL-4 +33T/-590T with cerebral malaria had elevated total IgE compared to non-carriers (P = 0.03). Our data suggest that IL-4 and/or IgE play a regulatory role in the pathogenesis of severe or complicated malaria.

Immunoglobulin E (IgE) containing complexes induce IL-4 production in human basophils: effect on Th1-Th2 balance in malaria.

Human basophils are potent producers of IL-4 following cross-linking of the high affinity receptor for IgE (Fc epsilon R1). Elevated levels of both total- and malaria-specific IgE have been demonstrated in sera from people living in malaria-endemic regions. Whether or not these IgE antibodies are pathogenic is unclear. Serum containing high IgE levels obtained from malaria individuals was used to establish whether IgE-immune complexes could induce IL-4 production in human basophils. The basophils, obtained from healthy donors, were primed with 10 ng/ml of IL-3 before being transferred to wells containing goat anti-human IgE or human antimalarial IgE-immune complexes. IL-4 was induced upon stimulation of human basophils by plate bound IgE-containing immune complexes. Basophils treated similarly but with goat anti-IgG/human antimalarial- IgG-immune complexes did not secrete IL-4. Similarly mononuclear cells depleted of basophils in parallel culture did not secrete IL-4. Thus, human basophils may contribute to the polarization of T-helper type 2 in the (Th2) responses in malaria hosts via IgE-induced IL-4 production.

Polymorphisms in interleukin-1beta and interleukin-1 receptor antagonist genes and malaria in Ghanaian children.

We have investigated the possible associations between polymorphisms in two interleukin-1 (IL-1) genes and severity of Plasmodium falciparum malaria in Ghanaian children with cerebral malaria, severe anaemia or uncomplicated malaria and controls. There was no significant difference in genotype and allele frequencies in IL-1beta exon 5 or interleukin-1 receptor antagonist (IL-1ra) polymorphisms between the studied groups, suggesting that the two polymorphisms may not be involved in the pathogenesis of severe malaria. When parasitaemias in uncomplicated malaria patients were evaluated, a significantly higher level of parasitaemia was observed among carriers of IL-1beta A2 allele as compared with noncarriers of this allele (P = 0.01). The mean parasitaemia in an age-matched asymptomatic group did not reveal such associations. These data suggest that IL-1beta exon 5 allele 2 may play a possible role in the clinical outcome of uncomplicated malaria.

Contrasting functions of IgG and IgE antimalarial antibodies in uncomplicated and severe Plasmodium falciparum malaria.

Plasmodial infection results in a significant elevation of the blood concentrations of immunoglobulins including IgE. Two well-characterized groups of adult Thai patients with either uncomplicated or severe Plasmodium falciparum malaria were studied over a period of four weeks. The mean parasitemias were approximately three-fold higher in patients with severe malaria than in those with uncomplicated disease. The mean concentrations of both total IgG and IgG antiplasmodial antibodies tended to be highest in the group with uncomplicated disease while total IgE and IgE antibodies were higher in the group with severe disease. The IgE antibodies detected in approximately 65% of the patients were positively correlated to parasitemia. These results suggest that antiplasmodial IgG antibodies are involved in reducing the severity of P. falciparum malaria, while IgE antibodies may contribute to the pathogenesis of this infection.

Human immune responses to the highly repetitive Plasmodium falciparum antigen Pf332.

The B and T cell responses to EB200, a repetitive part of the Plasmodium falciparum antigen Pf332, were examined in malaria-exposed Senegalese adults. Most donors had high levels of antibodies to recombinant EB200 and 17 overlapping peptides spanning EB200. Taking proliferation and/or cytokine (interferon-gamma and interleukin-4) production as a measure of T cell activation, eight of the EB200-derived peptides induced responses in > 40% of the donors tested. There was no general association between the different types of T cell responses measured, emphasizing the importance of including multiple parameters when analyzing T cell responses and suggesting that EB200 induces functionally distinct T cell responses. The most efficient peptide for induction of proliferative responses was one previously shown to induce T cell responses in five different H-2 congenic mouse strains primed with EB200, suggesting that this is a universal T cell epitope. The presence of multiple B and T cell epitopes in EB200, widely recognized by humans, is important since EB200 has been shown to elicit protective antibody responses in monkeys and may be considered for inclusion in malaria subunit vaccines.

IgE and tumor necrosis factor in malaria infection.

IgE, the immunoglobulin instrumental in atopic diseases is also elevated in many infections. This paper reports on the occurrence and possible pathogenic role of IgE in human Plasmodium falciparum malaria, one of the most widely spread and severe infectious diseases world wide. Plasmodial infections induce IgE elevation in the blood of the majority of people living in malaria endemic areas and up to 5% of this IgE constitutes anti-malaria antibodies. Production of IgE is controlled by T cells and elevated IgE concentrations in the blood of malaria patients are the result of an increased ratio of T-helper 2 (Th2) over T-helper 1 (Th1) cells. The underlying Th1 to Th2 switch is controlled by a variety of environmental and genetic factors. The importance of the latter is demonstrated by the IgE levels occurring in monozygotic or dizygotic twins originating from malarious areas of Africa. While these levels were indistinguishable within monozygotic twin pairs, they were different within the dizygotic pairs. Comparison of the levels of total IgE or IgE anti-malaria antibodies in patients with uncomplicated malaria with those in patients with the severe form of the disease (cerebral malaria or severe malaria without cerebral involvement) indicate that these levels are significantly higher in the cases with severe disease. This is the reverse with IgG and suggests that IgE plays a role in malaria pathogenesis. An important pathogenic mediator causing malaria fever and tissue lesions is tumor necrosis factor (TNF), generally believed to be induced by toxins released from the parasite. However, sera from malaria patients can also cause TNF release from monocytes in a reaction dependent on the presence of IgE containing immune complexes or aggregates. This results in induction and cross-linking of Fcepsilon receptor II (CD23) and by binding to and activating these cells, IgE will contribute to a local over-production of TNF in capillaries and post-capillary venules where P. falciparum parasites or their products accumulate in the severe forms of this disease.

Immune regulation of protection and pathogenesis in Plasmodium falciparum malaria.

The immune mechanisms whereby malaria parasites are eliminated by the human host or how they may avoid the immune response are poorly understood. Individuals living in malaria-endemic areas gradually acquire immunity. It is well established that this immunity involves both cell-mediated and humoral mechanisms and that T cells are the major regulators in both these events. The existence of functionally distinct P. falciparum-specific CD4+ T-cell subsets in humans has been shown in several studies. However, in contrast to what is the case in murine models there is no definitive link between the activation of various T cells and the course of human P. falciparum blood-stage infection. In the present paper we will review recent findings which illustrate how the balance between functionally different T-cell subsets affects the development of malaria immunity but also may contribute to its pathogenicity. An example of the latter is the deposition of IgE-containing immune complexes in small vessels, probably leading to local overproduction of tumor-necrosis factor (TNF), a pathogenic factor in malaria.

Humoral response to Plasmodium falciparum Pf155/ring-infected erythrocyte surface antigen and Pf332 in three sympatric ethnic groups of Burkina Faso.

The humoral immune response against synthetic peptides of two Plasmodium falciparum blood-stage antigens, Pf155/ring-infected erythrocyte surface antigen (RESA) (EENV)6 and Pf332 (SVTEEIAEEDK)2, in individuals belonging to three sympatric ethnic groups (Mossi, Rimaibe, and Fulani) living in the same conditions of hyperendemic transmission in a Sudan savanna area northeast of Ouagadougou, Burkina Faso were examined. The Mossi and Rimaibe are Sudanese Negroid populations with a long tradition of sedentary farming, while the Fulani are nomadic pastoralists partly settled and characterized by non-Negroid features of possible Caucasoid origin. A total of 764 subjects (311 Mossi, 273 Rimaibe, and 180 Fulani) were tested. A lower P. falciparum prevalence was observed in the Fulani of all age groups. The serologic results clearly indicate the existence of interethnic differences in the capacity to respond to these two P. falciparum antigens. The Mossi and Rimaibe showed similar responses, whereas the Fulani displayed consistently higher prevalences and levels of antibodies against both epitopes tested. The anti-(EENV)6 and anti-(SVTEEIAEEDK)2 seroprevalences were 29.9% and 38.9% in Mossi, 29.7% and 39.2% in Rimaibe, 86.1% and 76.1% in Fulani (all P values of Fulani-Mossi and Fulani-Rimaibe comparisons << 0.001). Anti-RESA and anti-Pf332 antibody levels were approximately 65% (P << 0.001) and 45% (P << 0.001), respectively, higher in seropositive Fulani than in seropositive Mossi and Rimaibe, who showed very similar values. The observed differences cannot be explained in terms of interethnic heterogeneity of malaria exposure since these communities have lived in the same area for more than 30 years and the P. falciparum inoculation rate, measured during two consecutive years, was substantially uniform for the three ethnic groups. The possibility of remarkable heterogeneities in the capacity to mount immune responses against P. falciparum antigens among populations with different genetic backgrounds must be taken into account in the development of anti-malaria vaccines.

Selected problems of malaria blood stage immunity.

Both antibody dependent and cell mediated mechanisms contribute to immunity in malaria. The parasites vary in sensitivity to antibody mediated inhibition due to underlying antigenic variation. When Plasmodium falciparum isolates are tested with antibodies from the donor originally harbouring the parasites or with those from other donors, growth inhibition is usually lowest in the autologous combinations. Parasites with decreased sensitivity are also generated in vitro by culturing them for prolonged periods in the presence of certain anti-plasmodial antibodies. When the antibodies are removed, a successive return of sensitivity develops. The decrease in sensitivity to inhibition may either be due to down-regulation of synthesis of the antigen or a selection of parasites with low antigen expression from the heterogeneous original populations. Both T lymphocytes carrying alpha/beta and gamma/delta antigen-receptors play a role in malaria immunity. However, although gamma/delta T cells may expand 40-fold or more in the peripheral immune system in acutely infected humans and also inhibit parasite growth in vitro and in vivo, their relative importance for protection or pathogenicity is presently unclear. Of the two major T cell subsets (CD4+, CD8+) carrying alpha/beta T cell receptors, the role of CD8+ T cells in blood stage infections appears to be limited. Instead, CD4+ T cells are of major importance. These cells comprise at least two functionally different subsets (Th1, Th2), distinguished on the basis of lymphokine secretion. In some rodent malaria models, Th1 cells producing primarily IL2 and IFN gamma give rise to protection in early infection while Th2 cells producing IL4 are essential for parasite clearance in late infection. In other mouse strains, the same parasites induce a strong Th2 response in early infection, resulting in a lethal course. CD4+ T cells of either Th1 or Th2 type also have regulatory functions in human P. falciparum malaria. Most humans living in areas of high endemicity have significantly elevated blood levels of IgE, reflecting a skewing of the underlying T helper cell ratio in favour of Th2, responsible for the switch in immunoglobulin isotypes. Less than 5% of the IgE in malaria represents antibodies to P. falciparum. IgE elevation is highest in patients with severe and particularly cerebral malaria and is frequently associated with an elevation of tumour necrosis factor alpha (TNF). The release of this cytokine from monocytes/macrophages may reflect crosslinking of their low affinity receptors for IgE (CD23) by IgE containing immune complexes from malarial sera. Local overproduction of TNF is considered a major pathogenic mechanism, responsible for fever and tissue lesions in severe malaria. Although TNF overproduction in malaria is generally assumed to be due to direct stimulation of effector cells by certain parasite derived toxins, the present results suggest that IgE elevation constitutes yet another mechanism contributing to the pathogenicity of P. falciparum in human malaria.

Seroreactivity to Pf 155/RESA in two malaria endemic ecological zones in Southern Ghana.

Seroepidemiological data are becoming important tools in determining transmission patterns of malaria. We have undertaken a study in 2 different ecological zones in southern Ghana during both the dry and wet seasons. Virtually all the subjects studied were positive to the crude parasite antigen (92%), with majority showing high titres in both areas. Whilst there was a statistical difference in Geometrical Mean Titre (GMT) between the two areas, seasonal variation was not seen. Antibody levels to the Pfl55/RESA and its repeat pepetide (EENV)6 increased progres notsively with age in endemic areas. These findings have relevance in determining malaria transmission in endemic areas.

Seroreactivity to Pf 155/RESA in Two Malaria Endemic Ecological Zones in Southern Ghana

Seroepidemiological data are becoming important tools in determining transmission patterns of malaria. We have undertaken a study in 2 different ecological zones in southern Ghana during both the dry and wet seasons. Virtually all the subjects studied were positive to the crude parasite antigen (92); with majority showing high titres in both areas. Whilst there was a statistical difference in Geometrical Mean Titre (GMT) between the two areas; seasonal variation was not seen. Antibody levels to the Pfl55/RESA and its repeat pepetide (EENV)6 increased progres notsively with age in endemic areas. These findings have relevance in determining malaria transmission in endemic areas

A malariometric survey in a rural community in the Muheza district, Tanzania: age profiles in the development of humoral immune responses.

A malariometric survey was carried out in a rural community situated in a malaria holoendemic endemic area of Tanzania. A random sample (n = 228) of different age groups was taken to elucidate the association between anti-Pf155/RESA and anti-Pf332 antibody responses and classical malaria indices. Parasitaemia, fever, splenomegaly, haematocrit and antimalarial consumption were assessed. Antibody responses against Pf155/RESA and Pf332 peptides were determined by ELISA. The age profiles of parasite density, splenomegaly, fever, haematocrit values and prevalence of antibody responses indicated intensive malaria exposure and the highest impact of malaria in small children. Forty-five percent of the study population had detectable chloroquine and desethyl-chloroquine blood levels, and the highest frequency and concentrations were recorded in the 12-23 months old. There was no significant association between the presence of drug and parasite density in the different age groups, although in the < 15 years old there was lower parasite prevalence among the children positive for drug in their blood (P < 0.05). High prevalence of antibody responses to all antigens was observed already at an early age, but the mean anti-Pf155/RESA and anti-Pf332 antibody levels increased significantly only in the adult group (P < 0.01). Significantly lower mean parasite densities were observed in high responders to Pf155/RESA and Pf332 peptides for the > or = 10 years old. For the 1-9 years, a similar difference was only observed in the high responders to Pf332. For the whole material, anti-Pf155/RESA and anti-Pf332 antibody levels correlated positively with age. When the effect of age was allowed for in analysing the relationship between parasite density and antibody level against the different antigens, a significant negative correlation was found only with regard to Pf332 in the > = 10 years age group. These results suggest that anti-Pf332 antibodies appear to be a better indicator for antiparasitic immunity, but both antigens are important for immune protection.

T- and B-cell responses of malaria immune individuals to synthetic peptides corresponding to non-repeat sequences in the N-terminal region of the Plasmodium falciparum antigen Pf155/RESA.

While the C-terminal repeat region of Pf155/RESA, a Plasmodium falciparum vaccine candidate has been extensively studied for B- and T-cell reactivities, little is so far known about the non-repeat region in this respect. The present study aimed at investigating the non-repeat sequence 171-227 of Pf155/RESA for T- and B-cell epitopes. Eight overlapping peptides were synthesised and assayed for their ability to stimulate peripheral blood mononuclear cells obtained from P. falciparum-immune donors to proliferate and to induce secretion of interferon-gamma (IFN-gamma) and/or interleukin 4 (IL-4) using the ELISPOT assay. The plasmas of the corresponding donors were tested for antibody reactivity with the same peptides in ELISA. The individual cellular responses to the different peptides varied and in general they were not correlated, emphasising the importance of including several parameters for T-cell activation. The most frequent T-cell responses (proliferation, IFN-gamma and/or IL-4) were seen with two partially overlapping peptides corresponding to the sequences 171-185 and 181-195 that induced responses in 71 and 62% of the donors, respectively. Although, the frequency of responders was high, the magnitude of the responses was generally low. Two overlapping peptides corresponding to the sequence 186-206 bound antibodies from a large number of plasma samples. IL-4 producing cells were frequently found in donors whose sera contained antibodies to the corresponding peptide. However, there was no absolute correlation and many donors having anti-peptide antibodies could also be induced to produce IFN-gamma. In conclusion, the non-repeat region of Pf155/RESA contains several epitopes inducing functionally distinct T-cell responses. The sequence 171-206 was found to contain both B- and T-cell epitopes recognised by almost all individuals naturally primed to malaria. Thus, this sequence should be a useful tool in future immuno-epidemiological studies and/or for inclusion into a subunit vaccine against the asexual blood stages of the P. falciparum parasite.

Elevated plasma levels of IgE in Plasmodium falciparum-primed individuals reflect an increased ratio of IL-4 to interferon-gamma (IFN-gamma)-producing cells.

People living in Plasmodium falciparum-endemic areas frequently have elevated levels of total as well as P. falciparum-specific serum IgE. This study aimed at investigating whether the elevated serum IgE levels reflect a shift in the balance between CD4+ T helper 1 (Th1) and T helper 2 (Th2) cells in individuals naturally exposed to the P. falciparum parasite. To investigate the role of Th1 and Th2 cells in the human P. falciparum system we used the ELISPOT assay to determine the ratio of IFN-gamma- and IL-4-producing cells after specific antigen or mitogen activation in vitro. The donors were individuals who had acquired immunity through natural exposure to the parasite. In response to the specific malaria antigens, very few IL-4-producing cells were seen. However, in the response of individual donors to the polyclonal T cell activator, leucoagglutinin (La), the anti-malarial IgE levels in plasma were correlated with an increased ratio of IL-4/IFN-gamma producing cells. Thus, donors with ratios of IL-4/IFN-gamma > 1 exhibited mean plasma anti-malarial IgE levels significantly greater than those with ratios < 1. In individuals not living in P. falciparum-endemic areas the ratio of IL-4/IFN-gamma was always < 1. Taken together, our data suggest a shift in the balance between Th1 and Th2 cells in naturally P. falciparum-primed individuals, associated with elevated anti-P. falciparum plasma IgE levels. The role and biological significance of IgE (Th2-type immune response) for protection against P. falciparum and/or pathogenesis of malaria require further study.

Wild isolates of Plasmodium falciparum malaria show decreased sensitivity to in vitro inhibition of parasite growth mediated by autologous host antibodies.

Antigenic diversity in field populations of Plasmodium falciparum parasites may delay the acquisition of protective immunity to malaria, the development of which may thus require repeated exposure to infection over a prolonged period of time. In this study we show that P. falciparum parasites may vary in their sensitivity to antibody-mediated invasion/growth inhibition in vitro. Wild isolates of P. falciparum from children living in an endemic area of Burkina Faso were tested for their sensitivity to the growth inhibitory effects of antibodies originating from the same (autologous) and from other donors (heterologous). A significantly lower invasion inhibition activity was obtained when the isolates and antibodies were tested in autologous compared with heterologous combinations. The lower sensitivity to growth inhibition by autologous antibodies may be due to immune pressure in vivo, selecting from a heterogeneous parasite population those with a low expression of the antigens recognized by the host's antibodies. Alternatively, the parasites cultured from each child might represent expanding parasite populations, mainly constituting strains not earlier seen by the immune system of that specific host. The results reinforce the concern about Plasmodium antigenic diversity as a major obstacle towards the development of an effective malaria vaccine.

Immunoglobulin E, a pathogenic factor in Plasmodium falciparum malaria.

Most children and adults living in areas where the endemicity of Plasmodium falciparum malaria is high have significantly elevated levels of both total immunoglobulin E (IgE) and IgE antimalarial antibodies in blood. This elevation is highest in patients with cerebral malaria, suggesting a pathogenic role for this immunoglobulin isotype. In this study, we show that IgE elevation may also be seen in severe malaria without cerebral involvement and parallels an elevation of tumor necrosis factor alpha (TNF). IgE-containing serum from malaria immune donors was added to tissue culture plates coated with rabbit anti-human IgE antibodies or with P. falciparum antigen. IgE-anti-IgE complexes as well as antigen-binding IgE antibodies induced TNF release from peripheral blood mononuclear cells (PBMC). Nonmalaria control sera with no IgE elevation induced significantly less of this cytokine, and the TNF-inducing capacity of malaria sera was also strongly reduced by passing them over anti-IgE Sepharose columns. The cells giving rise to TNF were adherent PBMC. The release of this cytokine probably reflects cross-linking of their low-affinity receptors for IgE (CD23) by IgE-containing immune complexes known to give rise to monocyte activation via the NO transduction pathway. In line with this, adherent monocytic cells exposed to IgE complexes displayed increased expression of CD23. As the malaria sera contained IgG anti-IgE antibodies, such complexes probably also play a role in the induction of TNF in vivo. Overproduction of TNF is considered a major pathogenic mechanism responsible for fever and tissue lesions in P. falciparum malaria. This overproduction is generally assumed to reflect a direct stimulation of effector cells by certain parasite-derived toxins. Our results suggest that IgE elevation constitutes yet another important mechanism involved in excessive TNF induction in this disease.

Lack of association between levels of transplacentally acquired Plasmodium falciparum-specific antibodies and age of onset of clinical malaria in infants in a malaria endemic area of Nigeria.

A cohort of 117 newborns was followed longitudinally for 12 months to determine the age of onset of clinical malaria and the subsequent episodes of malaria, and to investigate the possible existence of a correlation between level of transplacentally acquired Plasmodium falciparum-specific antibodies and age of onset of malaria in the infant. The mean age of onset of malaria in 49 infants was 4.48 +/- 1.54 months. Mean (+/- S.D.) age of onset of clinical malaria in haemoglobin AA infants (4.38 +/- 1.14) was significantly (P < 0.05) lower compared with haemoglobin AS (5.58 +/- 2.43) infants. No correlation was obtained between the age of onset of malaria and the level of cord serum total IgG, IgM and antibodies to P. falciparum antigens. Cord blood seropositivity for antibodies to the blood stage antigen Pf155/RESA and its C-terminal repeat sequence (EENV)6 or to the (NANP)6 peptide representing repeats of the circumsporozoite protein (CSP) did not influence the age of onset of clinical malaria. However, infants with haemoglobin AS whose cord blood was seropositive for antibodies to the (EENV)6 or (NANP)6 peptide showed delayed onset (P < 0.001) of malaria compared with AA seropositive infants. Although our results indicate that transplacentally acquired antibodies to the studied antigens alone offer no significant protection against malaria during the first few months of life, antibodies in concert with other factors such as haemoglobin genotype may contribute to the protection of the newborn.