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.

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.

A preliminary analysis of Wuchereria bancrofti microfilarial antigens for potential use in diagnosis.

Several antigens from the microfilarial stage of Wuchereria bancrofti have been identified using immunoblots of microfilarial antigens and screening with immune sera and tropical pulmonary eosinophilia (TPE) sera. This analysis revealed an array of antigens with apparent molecular weights of 14kDa, 35kDa, 42kDa, 63kDa, 88kDa, 97kDa and 200kDa. Among these only the 14kDa and 42kDa antigens were consistently recognized by most of the immune sera. A 132kDa antigen was recognized only by TPE sera. Analysis of rabbit immune sera revealed that the 42kDa antigen was shared by two developmental stages of W. bancrofti, namely L3 and mF. This antigen could become a potential vaccine candidate. The 14kDa antigen seems specific for the microfilarial stage and therefore could be a diagnostic marker for active infection. The 132kDa antigen could aid in the diagnosis of TPE.

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.

Malaria blood-stage infection and its control by the immune system.

Malaria is caused by the protozoon Plasmodium, transmitted to humans by Anopheles mosquitoes. The most dangerous of the plasmodia infecting humans is Plasmodium falciparum. The disease is caused by those parasite stages which multiply asexually in red blood cells. In non-immune individuals, P. falciparum may cause severe and life-threatening disease. Another risk group is constituted by pregnant women, particularly during their first pregnancies. Immunity to malaria usually requires repeated exposure to the parasite to become long lasting. One reason for this is the capacity of the parasite to vary the antigens which are major targets for protective antibodies. Antibody-dependent protection is primarily mediated by cytophilic IgG antibodies activating cytotoxic and phagocytic effector functions of neutrophils and monocytes. Malaria infection also involves elevated production of IgE antibodies. However, IgE-containing immune complexes are pathogenic rather than protective by crosslinking IgE receptors (CD23) on monocytes, leading to local overproduction of TNF, a major pathogenic factor in this disease. T cells are essential for both acquisition and regulation of malaria immunity. The major T cells controlling blood stage infections are CD4+ of both the Thl and Th2 subsets. However, T cells carrying the gamma6 receptor also contribute to this control. The balance between the cytokines produced by different cell types is critical for the course of infection, with IFN-gamma having a key role in anti-malaria defence. Blood-stage infections are also under complex genetic control. Among the regulatory genes, those involved in elevated production of TNF are associated with increased risk of severe disease and death due to P. falciparum infection.

IgE deposition in brain microvessels and on parasitized erythrocytes from cerebral malaria patients.

Postmortem brain tissues of 21 cerebral malaria cases were obtained in Myanmar and Vietnam. The tissues were examined by light microscopy and by an immunohistochemical method. Brain microvessels (capillaries and venules) were examined for the presence of immunoglobulins IgE and IgG, Plasmodium falciparum antigen, and parasitized erythrocytes (PRBC). Deposition of IgE, IgG, and P. falciparum antigen was observed in the microvessels from all specimens examined. Sequestered PRBC in the microvessels were positive for IgG in all 21 cases and for IgE in six cases. In the latter cases, the percentage of microvessels with sequestered PRBC was > 50%, with the frequency of IgE-positive cells ranging from 42% to 52%. In contrast, in five cases that were only weakly positive for IgE, the percentage of microvessels with sequestered PRBC was remarkably low (< 1%). These data indicate that the degree of deposition of IgE in microvessels and on PRBC from cerebral malaria patients correlated with that of PRBC sequestration. As IgE-containing immune complexes are known to induce local overproduction of tumor necrosis factor-alpha (TNF-alpha), a major pathogenic factor in cerebral malaria, IgE may contribute to the pathogenesis of this severe disease.

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.

Inhibition of in vitro growth of plasmodium falciparum field isolates mediated by human antibodies to Pf155/RESA and Pf332.

The capacity of antibodies to interfere with Plasmodium falciparum growth in in vitro cultures is considered to reflect some of their potential protective effects in vivo. Almost all previous analyses of antibody mediated inhibition of parasite growth in vitro were performed with different laboratory strains of P. falciparum. This study was performed to investigate if the long-term culturing of parasites has any effect on their susceptibility to such growth inhibition. The growth inhibitory effects of human antibodies to the vaccine candidate antigens Pf155/RESA and Pf332 on fresh field isolates from children in Burkina Faso were analysed and compared with their effect on an established laboratory strain of the parasite. Although there was variation in the inhibition titres between different isolates tested against one antibody preparation, the differences in inhibition capacity for the three different antibodies were highly significant. No correlation was found between serum levels of anti-Pf155/RESA or -Pf332 antibodies and sensitivity of the corresponding parasite isolates to antibody mediated growth inhibition.

Antibodies to a non-repeat region of Plasmodium falciparum antigen Pf155/RESA in individuals from malaria-endemic areas.

Human antibodies to the repeat regions of the Plasmodium falciparum asexual blood stage antigen Pf155/RESA interfere with parasite growth in vitro, but the significance in this respect of antibodies to non-repetitive epitopes is less clear. In this study the levels of antibodies to a non-repetitive part of Pf155/RESA (residue 199-221) in malaria-exposed individuals were analysed, as was the parasite-inhibitory capacity of such antibodies. Residue 199-221 is of particular interest since it includes a sequence homologous to a cytoadherence-related motif from band 3. Sera from donors in Liberia and Tanzania were analysed for reactivity in ELISA with synthetic peptides together overlapping this part of Pf155/RESA. High antibody reactivity was observed in most of the sera with two peptides including residues 199-211 and 202-214, respectively. Specific antibodies were affinity-purified from selected sera using these peptide sequences and were shown to react with Pf155/RESA by immunofluorescence and Western blotting. The purified antibodies were furthermore shown to inhibit parasite growth in vitro. The results suggest that both repeat and non-repeat epitopes in Pf155/RESA elicit antibodies with potential to protect against malaria infection.

Characterization of antibody responses to a Plasmodium falciparum blood-stage antigen induced by a DNA prime/protein boost immunization protocol.

The humoral immune responses elicited by priming with a DNA plasmid and boosting with either the plasmid or the corresponding recombinant protein in alum adjuvant were compared. The plasmid DNA encoded a sequence (M3) derived from the Plasmodium falciparum antigen Pf155/RESA, and the recombinant protein consisted of the identical malarial sequence fused to an albumin-binding region (BB) of streptococcal protein G. Mice of different genetic backgrounds (CBA, Balb/c and C57Bl/6) were primed with plasmid DNA and boosted with either plasmid or recombinant protein. In all strains of mice, boosting with protein elicited higher anti-M3 antibody levels than obtained by boosting with plasmid, yet the kinetics and longevity of the secondary responses were comparable. Antiserum obtained after protein boosting displayed an immunoglobulin (Ig)G subclass profile skewed to the IgG1 isotype, regardless of the mouse strain. In contrast, mice receiving a second injection with plasmid responded with a more mixed IgG subclass profile. Inclusion of a P. falciparum circumsporozoite protein-derived T-helper epitope (CS.T3) in the immunization plasmid as well as in the fusion protein, did not significantly change the humoral responses to M3. The results show the potential of DNA vaccination for the purpose of priming an antibody response against the malarial blood-stage antigen Pf155/RESA. When combined with a protein boost, this DNA priming results in high-titred and long-lasting anamnestic responses.

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.

Antibodies to sequences in a non-repeat region of Plasmodium falciparum antigen Pf155/RESA inhibit either cytoadherence or parasite growth in vitro.

Antibodies to a non-repeat region of the Plasmodium falciparum antigen Pf155/RESA were investigated for their capacity to inhibit parasite cytoadherence to melanoma cells and parasite growth in vitro. The activities of these antibodies were studied since the target region in Pf155/RESA includes a cytoadherence-related motif also found in loop 3 and 7 of human erythrocyte band 3 protein. Overlapping multiple antigen peptides (MAPs) together spanning residues 199-220 of Pf155/RESA were used to raise antibodies in rabbits. Analysis of the fine specificity of these antibodies revealed that antibodies raised against largely overlapping sequences displayed highly different specificity patterns. Similarly, striking differences were seen when analysing the biological effect of antibodies to these MAPs. Antibodies to the cytoadherence-related motif of Pf155/RESA, as well as antibodies raised against a MAP based on a corresponding band 3 motif, inhibited cytoadherence but not parasite growth. In contrast, antibodies to sequences adjacent to the Pf155/RESA cytoadherence motif inhibited parasite growth in vitro but had no effect on cytoadherence.

Differential induction of immunoglobulin G subclasses by immunization with DNA vectors containing or lacking a signal sequence.

The route and method used to immunize mice with antigen-expressing DNA plasmids have an impact on the resulting T-helper cell response and IgG subclass distribution. Previous findings further indicate that the intracellular targeting of expressed antigens influences the differentiation of naive T-cells into either a Th1 or a Th2 type of response. In the present study, we analyzed the levels of IgG1 and IgG2a antibodies, as correlates of Th2 and Th1 responses, respectively, after intramuscular injection of mice with plasmids encoding a chimeric protein containing a Plasmodium falciparum blood stage antigen expressed in two different forms. One plasmid expresses the antigen in a secreted form as it is preceded by a signal sequence while expression from the other plasmid, lacking this sequence, results in cytoplasmic localization of the antigen. Mice immunized with the plasmid encoding secreted antigen responded with predominantly IgG1 antibodies. In contrast, sera from mice immunized with the plasmid expressing cytosolic protein displayed a mixed IgG1/IgG2a profile. In line with previous findings, our results suggest that the intracellular targeting of proteins expressed by DNA plasmids is an important factor for the differentiation of Th cells and the resulting subclass pattern of IgG responses.

Antibodies to nonrepeat sequences of antigen Pf155/RESA of Plasmodium falciparum inhibit parasite growth in vitro.

Immune responses to the repeat regions of the Plasmodium falciparum antigen Pf155/RESA have been extensively studied, and antibodies to the repeats are known to interfere with parasite growth both in vitro and in vivo. Less is known with regard to the effect on parasites of antibodies to the nonrepeat regions of the antigen. In the present study, rabbits were immunized with synthetic peptides corresponding to three different nonrepeated sequences of antigen Pf155/RESA. The reactivity of the antibodies with the particular peptides was analyzed by enzyme-linked immunosorbent assay (ELISA) and that with the parasite antigen, by immunoblotting and immunofluorescence. Although all antisera reacted strongly with the corresponding synthetic peptides, they reacted only weakly with full-length Pf155/RESA in either of the methods used. The specificity of the antibodies for Pf155/RESA was confirmed by their failure to stain Pf155/RESA-deficient parasites in erythrocyte membrane immunofluorescence, a method mainly detecting this antigen. Antibodies to the nonrepeat sequences also efficiently inhibited the merozoite invasion in vitro of Pf155/RESA+ parasites. However, these antibodies also inhibited Pf155/RESA-deficient parasites, indicating the presence of an antigen exhibiting a high degree of homology with Pf155/RESA. The results indicate that nonrepeat sequences of Pf155/RESA are immunogenic and may serve as targets for parasite-neutralizing antibodies, and, thus, the potential of the antigen as a vaccine candidate is emphasized.

Immunogenicity of chimeric multiple antigen peptides based on Plasmodium falciparum antigens: impact of epitope orientation.

Assembly of B and T epitopes in multiple antigen peptides (MAP) can bypass genetically predisposed unresponsiveness to B epitopes. Although the underlying mechanisms are unknown, B-cell responses to such diepitope MAP are influenced by intramolecular epitope orientation. In this study, MAP constructs were synthesized, encompassing two epitopes derived from the Plasmodium falciparum antigens circumsporozoite protein (CS) and Pf332. In addition to B epitopes, the sequences comprised T epitopes restricted to mouse H-2b (CS) or to H-2d and H-2k (Pf332) haplotypes. Congenic H-2b, H-2d and H-2k Balb mice were immunized with MAP in which the two epitopes were arranged either tandemly or in parallel. Tandemly arranged (B-T)4 MAP, in which the relevant T epitope was positioned adjacent to the lysine core [(Pf332-CS)4-core for H-2b mice and (CS-Pf332)4-core for H-2d and H-2k mice], elicited the most potent antibody responses in terms of reactivity to both epitopes. Additionally, the (B-T)4 constructs were generally most efficient in recalling proliferative T-cell responses in vitro, irrespective of the MAP used for in vivo priming. As high antibody titers were generated to both epitopes, the position of B epitopes in the constructs does not appear to be critical for an efficient B-cell response. Rather, the association of strong B- and T-cell responses to the (B-T)4 MAP constructs suggests that the intramolecular position of the relevant T epitope determines the magnitude of specific antibody production.

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.

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.