Association between immune recognition of the malaria vaccine candidate antigen Pf155/RESA and resistance to clinical disease: a prospective study in a malaria-endemic region of west Africa

Target-agnostic identification of human antibodies to Plasmodium falciparum sexual forms reveals cross stage recognition of glutamate-rich repeats

Circulating sexual stages of Plasmodium falciparum (Pf) can be transmitted from humans to mosquitoes, thereby furthering the spread of malaria in the population. It is well established that antibodies (Abs) can efficiently block parasite transmission. In search for naturally acquired Ab targets on sexual stages, we established an efficient method for target-agnostic single B cell activation followed by high-throughput selection of human monoclonal antibodies (mAbs) reactive to sexual stages of Pf in the form of gamete and gametocyte extract. We isolated mAbs reactive against a range of Pf proteins including well-established targets Pfs48/45 and Pfs230. One mAb, B1E11K, was cross-reactive to various proteins containing glutamate-rich repetitive elements expressed at different stages of the parasite life cycle. A crystal structure of two B1E11K Fab domains in complex with its main antigen, RESA, expressed on asexual blood stages, showed binding of B1E11K to a repeating epitope motif in a head-to-head conformation engaging in affinity-matured homotypic interactions. Thus, this mode of recognition of Pf proteins, previously described only for PfCSP, extends to other repeats expressed across various stages. The findings augment our understanding of immune-pathogen interactions to repeating elements of the Plasmodium parasite proteome and underscore the potential of the novel mAb identification method used to provide new insights into the natural humoral immune response against Pf .

Impact Statement

A naturally acquired human monoclonal antibody recognizes proteins expressed at different stages of the Plasmodium falciparum lifecycle through affinity-matured homotypic interactions with glutamate-rich repeats.

Chemokine levels and parasite- and allergen-specific antibody responses in children and adults with severe or uncomplicated Plasmodium falciparum malaria

Chemokine and antibody response profiles were investigated in children and adults with severe or uncomplicated Plasmodium falciparum malaria; the aim was to reveal which profiles are associated with severe disease, as often seen in nonimmune children, or with mild and uncomplicated disease, as seen in semi-immune adults. Blood samples were obtained from children under 5 years of age as well as adults with falciparum malaria. Classification of malaria was performed according to parasite densities and hemoglobin concentrations. Plasma levels of chemokines (IL-8, IP-10, MCP-4, TARC, PARC, MIP-1δ, eotaxins) were quantified, and antibody responses (IgE, IgG1, and IgG4) to P. falciparum, Entamoeba histolytica-specific antigen, and mite allergen extracts were determined. In children with severe malaria proinflammatory, IL-8, IP10, MIP-1δ, and LARC were at highly elevated levels, suggesting an association with severe disease. In contrast, the Th2-type chemokines TARC, PARC, and eotaxin-2 attained in children the same levels as in adults suggesting the evolution of immune regulatory components. In children with severe malaria, an elevated IgG1 and IgE reactivity to mite allergens and intestinal protozoan parasites was observed. In conclusion, exacerbated proinflammatory chemokines together with IgE responses to mite allergens or E. histolytica-specific antigen extract were observed in children with severe falciparum malaria.

Interferon-γ--central mediator of protective immune responses against the pre-erythrocytic and blood stage of malaria

Immune responses against Plasmodium parasites, the causative organisms of malaria, are traditionally dichotomized into pre-erythrocytic and blood-stage components. Whereas the central role of cellular responses in pre-erythrocytic immunity is well established, protection against blood-stage parasites has generally been ascribed to humoral responses. A number of recent studies, however, have highlighted the existence of cellular immunity against blood-stage parasites, in particular, the prominence of IFN-γ production. Here, we have undertaken to chart the contribution of this prototypical cellular cytokine to immunity against pre-erythrocytic and blood-stage parasites. We summarize the various antiparasitic effector functions that IFN-γ serves to induce, review an array of data about its protective effects, and scrutinize evidence for any deleterious, immunopathological outcome in malaria patients. We discuss the activation and contribution of different cellular sources of IFN-γ production during malaria infection and its regulation in relation to exposure. We conclude that IFN-γ forms a central mediator of protective immune responses against pre-erythrocytic and blood-stage malaria parasites and identify a number of implications for rational malaria vaccine development.

The role of MHC- and non-MHC-associated genes in determining the human immune response to malaria antigens

Individual susceptibility to malaria infection, disease and death is influenced by host genotype, parasite virulence and a number of environmental factors including malaria-specific immunity. Immune responses are themselves determined by a combination of host genes and environmental effects. The extent to which host genotype limits the spectrum of possible immune responses may influence the outcome of infection and has consequences for vaccine design. Associations have been observed between human major histocompatibility complex (MHC) genotype and susceptibility to severe malaria, but no similar associations have been observed for mild malarial disease or for specific antibody responses to defined malaria antigens. Epidemiological studies have shown that, in practice, neither T helper cell nor antibody responses to malaria parasites are limited by host MHC genotype, but have revealed that genes lying outside the MHC may influence T cell proliferative responses. These genes have yet to be identified, but possible candidates include T cell receptor (TcR) genes, and genes involved in TcR gene rearrangements. More importantly, perhaps, longitudinal epidemiological studies have shown that the anti-malarial antibody repertoire is selective and becomes fixed in malaria-immune individuals, but is independent of host genotype. These findings suggest that the antibody repertoire may be determined, at least in part, by stochastic events. The first of these is the generation of the T and B cell repertoire, which results from random gene recombinations and somatic mutation and is thus partially independent of germline genes. Secondly, of the profusion of immunogenic peptides which are processed and presented by antigen presenting cells, a few will, by chance, interact with T and B cell surface antigen receptors of particularly high affinity. These T and B cell clones will be selected, will expand and may come to dominate the immune response, preventing the recognition of variant epitopes presented by subsequent infections - a process known as original antigenic sin or clonal imprinting. The immune response of an individual thus reflects the balance between genetic and stochastic effects. This may have important consequences for subunit vaccine development.

Profiling the antibody immune response against blood stage malaria vaccine candidates

BACKGROUND

The complexity and diversity of the antibody immune response to the antigen repertoire of a pathogen has long been appreciated. Although it has been recognized that the detection of antibodies against multiple antigens dramatically improves the clinical sensitivity and specificity of diagnostic assays, the prognostic value of serum reactivity profiles against multiple microbial antigens in protection has not been investigated.

METHODS

Using malaria as a model we investigated whether antigen reactivity profiles in serum of children with different levels of clinical immunity to Plasmodium falciparum malaria correlated with protection. We developed a microarray immunoassay of 18 recombinant antigens derived from 4 leading blood-stage vaccine candidates for P. falciparum [merozoite surface protein 1 (MSP1), MSP2, MSP3, and apical membrane antigen (AMA)-1]. Associations between observed reactivity profiles and clinical status were sought using k-means clustering and phylogenetic networks.

RESULTS

The antibody immune response was unexpectedly complex, with different combinations of antigens recognized in different children. Serum reactivity to individual antigens did not correlate with immune status. By contrast, combined recognition of AMA-1 and allelic variants of MSP2 was significantly associated with protection against clinical malaria. This finding was confirmed independently by k-means clustering and phylogenetic networking.

CONCLUSIONS

The analysis of reactivity profiles provides a wealth of novel information about the immune response against microbial organisms that would pass unnoticed in analysis of reactivity to antigens individually. Extension of this approach to a large fraction of the proteome may expedite the identification of correlates of protection and vaccine development against microbial diseases.

Impact of red blood cell polymorphisms on the antibody response to Plasmodium falciparum in Senegal

The evidence of protection afforded by red blood cell polymorphisms against either clinical malaria or Plasmodium falciparum blood levels varies with the study site and the type of malaria transmission. Nevertheless, no clear implication of an antibody-related effect has yet been established in the protection related to red blood cell polymorphisms. We performed a prospective study, where plasma IgG and IgG subclasses directed to recombinant proteins from the merozoite surface protein 2 (MSP2/3D7 and MSP2/FC27) and the ring-infected erythrocyte surface antigen (RESA) were determined in a cohort of 413 Senegalese children before the annual malaria transmission season. The antibody response was dependent on age, and to a lesser extent, on the village of residence. IgG3 responders to all proteins, IgG responders to RESA and MSP2/3D7, as well as IgG2 to RESA and IgG1 responders to MSP2/3D7, presented enhanced mean values of parasite density, as evaluated during an 18-month follow-up. The levels of IgG and IgG3 to MSP2/3D7 were negatively associated with the risk of occurrence of a malaria attack during the following transmission season. Compared to normal children, sickle cell trait carriers presented lower levels of IgG to MSP2/3D7. Similarly, G6PD A- girls had lower levels of IgG and IgG3 to MSP2/FC27 than did G6PD normal girls. The impact of these particular genetic polymorphisms on the modulation of the antibody response is discussed.

Antibodies to the N-terminal block 2 of Plasmodium falciparum merozoite surface protein 1 are associated with protection against clinical malaria

This longitudinal prospective study shows that antibodies to the N-terminal block 2 region of the Plasmodium falciparum merozoite surface protein 1 (MSP-1) are associated with protection against clinical malaria in an area of stable but seasonal malaria transmission of Ghana. Antibodies to the block 2 region of MSP-1 were measured in a cohort of 280 children before the beginning of the major malaria transmission season. The cohort was then actively monitored for malaria, clinically and parasitologically, over a period of 17 months. Evidence is presented for an association between antibody responses to block 2 and a significantly reduced risk of subsequent clinical malaria. Furthermore, statistical survival analysis provides new information on the duration of the effect over time. The results support a conclusion that the block 2 region of MSP-1 is a target of protective immunity against P. falciparum and, thus, a promising new candidate for the development of a malaria vaccine.

Current status of malaria vaccine development

There is an urgent need to develop an effective vaccine against malaria--a disease that has approximately 10% of the world population at risk of infection at any given time. The economic burden this disease puts on the medico-social set-up of countries in Sub-Saharan Africa and South East Asia is phenomenal. Increasing drug resistance and failure of vector control strategies have necessitated the search for a suitable vaccine that could be integrated into the extended program of immunization for countries in the endemic regions. Malaria vaccine development has seen a surge of activity in the last decade or so owing largely to the advances made in the fields of genetic engineering and biotechnology. This revolution has brought sweeping changes in the understanding of the biology of the parasite and has helped formulate newer more effective strategies to combat the disease. Latest developments in the field of malaria vaccine development will be discussed in this chapter.

The mechanism and significance of deletion of parasite-specific CD4(+) T cells in malaria infection

It is thought that both helper and effector functions of CD4(+) T cells contribute to protective immunity to blood stage malaria infection. However, malaria infection does not induce long-term immunity and its mechanisms are not defined. In this study, we show that protective parasite-specific CD4(+) T cells were depleted after infection with both lethal and nonlethal species of rodent PLASMODIUM: It is further shown that the depletion is confined to parasite-specific T cells because (a) ovalbumin (OVA)-specific CD4(+) T cells are not depleted after either malaria infection or direct OVA antigen challenge, and (b) the depletion of parasite-specific T cells during infection does not kill bystander OVA-specific T cells. A significant consequence of the depletion of malaria parasite-specific CD4(+) T cells is impaired immunity, demonstrated in mice that were less able to control parasitemia after depletion of transferred parasite-specific T cells. Using tumor necrosis factor (TNF)-RI knockout- and Fas-deficient mice, we demonstrate that the depletion of parasite-specific CD4(+) T cells is not via TNF or Fas pathways. However, in vivo administration of anti-interferon (IFN)-gamma antibody blocks depletion, suggesting that IFN-gamma is involved in the process. Taken together, these data suggest that long-term immunity to malaria infection may be affected by an IFN-gamma-mediated depletion of parasite-specific CD4(+) T cells during infection. This study provides further insight into the nature of immunity to malaria and may have a significant impact on approaches taken to develop a malaria vaccine.

A model of development of acquired immunity to malaria in humans living under endemic conditions

Malaria remains a significant global health problem. Most morbidity and mortality in an endemic setting is in children less than 5 years old, and increasing resistance to infection and disease with age is thought to reflect a slow, gradual acquisition of protective immunity. It is not clear if the semi-immune status of adults, in which parasites are present at below clinical threshold, is the result of cumulative exposure to Plasmodium falciparum or reflects an underlying difference between adult and infant immunity. Immuno-epidemiological studies of people living in malaria-endemic areas have not produced consistent examples of surrogate markers of protection. This gulf in our understanding of immunity to malaria may be addressed by novel application of an established murine model of immune regulation of blood stage infection. This exploits two examples of loss of immunity, selective immunosuppression in pregnancy, and waning of maternally transferred protection in neonates, to distinguish the immunological determinants involved in the radical transition between susceptible and resistant immune status. It is suggested that application of this unique model should significantly advance knowledge of how acquired immunity to malaria develops and is highly relevant to the pathogenesis of malaria in human pregnancy and the design of antimalarial vaccines for use in children.

Do maternally acquired antibodies protect infants from malaria infection?

Neonates and infants are relatively protected from clinical malaria, but the mechanism of this protection is not well understood. Maternally derived antibodies are commonly believed to provide protection against many infectious diseases, including malaria, for periods of up to 6-9 months but several recent epidemiological studies have produced conflicting results regarding a protective role of passively acquired antimalarial antibodies. In this article, we review the epidemiological evidence for resistance of young infants to malaria, summarize the data on antimalarial antibody levels and specificity and their association with protection from malaria infection or clinical disease, and explore alternative explanations for resistance to malaria in infants.

Familial correlation of immunoglobulin G subclass responses to Plasmodium falciparum antigens in Burkina Faso

Host genes are thought to determine the immune response to malaria infection and the outcome. Cytophilic antibodies have been associated with protection, whereas noncytophilic antibodies against the same epitopes may block the protective activity of the protective ones. To assess the contribution of genetic factors to immunoglobulin G (IgG) subclass responses against conserved epitopes and Plasmodium falciparum blood-stage extracts, we analyzed the isotypic distribution of the IgG responses in 366 individuals living in two differently exposed areas in Burkina Faso. We used one-way analysis of variance and pairwise estimators to calculate sib-sib and parent-offspring correlation coefficients, respectively. Familial patterns of inheritance of IgG subclass responses to defined antigens and P. falciparum extracts appear to be similar in the two areas. We observed a sibling correlation for the IgG, IgG1, IgG2, IgG3, and IgG4 responses directed against ring-infected-erythrocyte surface antigen, merozoite surface protein 1 (MSP-1), MSP-2, and P. falciparum extract. Moreover, a parent-offspring correlation was found for several IgG subclass responses, including the IgG, IgG1, IgG2, IgG3, and IgG4 responses directed against conserved MSP-2 epitopes. Our results indicated that the IgG subclass responses against P. falciparum blood-stage antigens are partly influenced by host genetic factors. The localization and identification of these genes may have implications for immunoepidemiology and vaccine development.

Lack of association between maternal antibody and protection of African infants from malaria infection

Maternally derived antibodies are believed to protect infants against infection, but there is little direct evidence for a protective role of passively acquired antibodies against malaria. A longitudinal study of malaria infection in 143 infants was conducted in a region of southern Ghana where Plasmodium falciparum is endemic. Infants born in the high-transmission season were less likely to become infected in the first 20 weeks of life than children born in the low-transmission season. Plasma, obtained at birth, was tested for immunoglobulin G (IgG) and IgG subclasses to P. falciparum schizonts and recombinant circumsporozoite antigen, MSP-1(19), MSP-2, AMA-1, and Pf155 (also called ring-infected erythrocyte surface antigen). Antibody levels at birth were not associated with resistance to malaria infection. On the contrary, antibodies at birth were positively associated with infection, indicating that high levels of maternally derived antibodies represent a marker for intensity of exposure to malaria infection in infants. However, all five children who experienced high-density infections (>100 parasites/microl of blood) were seronegative for MSP-1(19) at the time of infection.

Heritability and segregation analysis of immune responses to specific malaria antigens in Papua New Guinea

Familial patterns of inheritance of immune responses to specific Plasmodium falciparum antigens were studied in 214 adults in an area of Papua New Guinea highly endemic for malaria. Preliminary variance component analysis indicated familial aggregation in both humoral and cellular immune responses against the ring-infected erythrocyte surface antigen (RESA) and the FC27 allele of the Merozoite surface antigen 2 (MSA-2). Including a term for sharing houses in the models affected only the antibody response to RESA. Segregation analysis of the antibody responses against RESA indicated inheritance via a multifactorial model and analysis of the proliferation response suggested a possible recessive major gene. The best fitting models for the immune responses against MSA-2 (FC27) postulated dominant major gene inheritance. We found no significant associations between HLA class I or II alleles and these two antigens in this population. Although there was evidence of familial aggregation of antibody responses to MSA-2 (3D7), the segregation analysis failed to identify a mode of inheritance. There was little or no heritability of either humoral or cellular immune responses against the NANP repeats of the Circumsporozoite protein (NANP), the synthetic malaria vaccine SPf66, or a preparation of MSA-2 (3D7) from which the repetitive part was deleted (MSA-2 (d3D7)). Although it is often difficult to separate genetic effects from the effects of living in the same environment, it appears that some immune responses against certain malaria antigens may be partly influenced by genetic factors.

High immunoglobulin G2 (IgG2) and low IgG4 levels are associated with human resistance to Plasmodium falciparum malaria

There is accumulating evidence for a role of immunoglobulin G (IgG) in protection against malarial infection and disease. Only IgG1 and IgG3 are considered cytophilic and protective against P. falciparum, whereas IgG2 and IgG4 were thought to be neither and even to block protective mechanisms. However, no clear pattern of association between isotypes and protection has so far emerged. We analyzed the isotypic distribution of the IgG response to conserved epitopes and P. falciparum blood-stage extract in 283 malaria-exposed individuals whose occurrence of infection and malaria attack had been monitored for about 1 year. Logistic regression analyses showed that, at the end of the season of transmission, high levels of IgG2 to RESA and to MSP2 epitopes were associated with low risk of infection. Indeed, IgG2 is able to bind FcgammaRIIA in individuals possessing the H131 allele, and we showed that 70% of the study subjects had this allele. Also, high specific IgG4 levels were associated with an enhanced risk of infection and with a high risk of malaria attack. Moreover, specific IgG2 and IgG3 levels, as well as the IgG2/IgG4 and IgG3/IgG4 ratios, increased with the age of subjects, in parallel with the protection against infection and disease. IgG4 likely competes with cytophilic antibodies for antigen recognition and may therefore block cytotoxicity mediated by antibody-activated effector cells. In conclusion, these results favor a protective role of IgG3 and IgG2, which may activate effector cells through FcgammaRIIA, and provide evidence for a blocking role of IgG4 in malarial infection and disease.

A role for cytokines in potentiation of malaria vaccines through immunological modulation of blood stage infection

Malaria is the world's major parasitic disease, for which effective control measures are urgently needed. One of the difficulties hindering successful vaccine design against Plasmodium is an incomplete knowledge of antigens eliciting protective immunity, the precise types of immune response for which to aim, and how these can be induced. A greater appreciation of the mechanisms of protective immunity, on the one hand, and of immunopathology, on the other, should provide critical clues to how manipulation of the immune system may best be achieved. We are studying the regulation of the balance between T helper 1 (Th1) and T helper 2 (Th2) CD4+ T lymphocytes in immunity to asexual blood stages of malaria responsible for the pathogenicity of the disease. Protective immunity to the experimental murine malarias Plasmodium chabaudi and Plasmodium yoelii involves both Th1 and Th2 cells, which provide protection by different mechanisms at different times of infection characterised by higher and lower parasite densities, respectively. This model therefore facilitates a clearer understanding of the Th1/Th2 equilibrium that appears central to immunoregulation of all host/pathogen relationships. It also permits a detailed dissection in vivo of the mechanisms of antimalarial immunity. Here, we discuss the present state of malaria vaccine development and our current research to understand the factors involved in the modulation of vaccine-potentiated immunity.

Isolation of peptides that mimic epitopes on a malarial antigen from random peptide libraries displayed on phage

The ring-infected erythrocyte surface antigen (RESA) is a dense-granule protein of Plasmodium falciparum which binds to the cytoskeletal structure of the erythrocyte after parasite invasion. It is currently under trial as a vaccine candidate. In an effort to characterize further the antibody responses to this antigen, we have panned two independent libraries of random peptides expressed on the surface of filamentous phage with a monoclonal antibody (MAb 18/2) against RESA. One library consisted of a potentially constrained 17-mer peptide fused with the gpVIII phage coat protein, and the other displayed an unconstrained 15-mer as a fusion with the minor phage coat protein gpIII. Several rounds of biopanning resulted in enrichment from both libraries clones that interacted specifically with MAb 18/2 in protein-blotting and enzyme-linked immunosorbent assay experiments. Nucleotide sequencing of the random oligonucleotide insert revealed a common predominant motif: (S/T)AVDD. Several other clones had related but degenerate motifs. Thus, a monoclonal antibody against a malarial antigen can select common mimotopes from different random peptide libraries. We envisage many uses for this technology in malaria research.

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.

Quantification of the relative contribution of major histocompatibility complex (MHC) and non-MHC genes to human immune responses to foreign antigens

Understanding the extent to which genetic factors influence the immune response is important in the development of subunit vaccines. Associations with HLA gene polymorphisms appear insufficient to explain the range of variation in immune responses to vaccines and to infections by major pathogens. In this study of Gambian twins we report that regulation of the immune response to a variety of antigens from Plasmodium falciparum and Mycobacterium tuberculosis is controlled by factors which are encoded by genes that lie both within and outside the major histocompatibility complex (MHC). We define the relative contribution of these genes, which varies for different antigens. The cumulative genetic contribution of non-MHC genes to the total phenotypic variance exceeds that of the MHC-encoded genes.

Plasmodium chabaudi: immunogenicity of a highly antigenic glutamate-rich protein

The immunogenicity of a 93-kDa Plasmodium chabaudi protein that contains glutamate-rich tandem repeats was investigated in this study. Immunoblotting with various monoclonal antibodies indicates that this 93-kDa protein is equivalent to a potential P. chabaudi RESA analogue. However, the sequence of the P. chabaudi protein does not exhibit any significant homology to Pf155/RESA. Antibodies against the 93-kDa protein appear early during P. chabaudi infection and reach high titers. The highest antibody titers are found when the parasitemia is descending, suggesting that this protein may play some role in immunity. Immunization of mice with the recombinant protein also results in high antibody titers, indicating that the protein is quite immunogenic. However, mice immunized with recombinant protein and challenged with P. chabaudi do not exhibit a delayed appearance of parasitemia, a reduced parasitemia, or a shortened duration of parasitemia. Glutamate-rich P. falciparum proteins such as Pf155/RESA, are being considered as vaccine candidates. The studies with P. chabaudi suggest that interpretation of serological data using glutamate-rich proteins should proceed with caution. The glutamate-rich repeats, although highly immunogenic, may not be important in host immunity against malaria. However, antibodies that appear late in the P. chabaudi infection do appear to play a role in anti-malarial immunity.

Different response to Plasmodium falciparum malaria in west African sympatric ethnic groups

The comparison of malaria indicators among populations that have different genetic backgrounds and are uniformly exposed to the same parasite strains is one approach to the study of human heterogeneties in the response to the infection. We report the results of comparative surveys on three sympatric West African ethnic groups, Fulani, Mossi, and Rimaibé, living in the same conditions of hyperendemic transmission in a Sudan savanna area northeast of Ouagadougou, Burkina Faso. The Mossi and Rimaibé 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. Parasitological, clinical, and immunological investigations showed consistent interethnic differences in Plasmodium falciparum infection rates, malaria morbidity, and prevalence and levels of antibodies to various P. falciparum antigens. The data point to a remarkably similar response to malaria in the Mossi and Rimaibé, while the Fulani are clearly less parasitized, less affected by the disease, and more responsive to all antigens tested. No difference in the use of malaria protective measures was demonstrated that could account for these findings, and sociocultural or environmental factors do not seem to be involved. Known genetic factors of resistance to malaria did not show higher frequencies in the Fulani. The differences in the immune response were not explained by the entomological observations, which indicated substantially uniform exposure to infective bites. The available data support the existence of unknown genetic factors, possibly related to humoral immune responses, determining interethnic differences in the susceptibility to malaria.

Assessment of the role of the humoral response to Plasmodium falciparum MSP2 compared to RESA and SPf66 in protecting Papua New Guinean children from clinical malaria

The prevalence and concentration of naturally acquired humoral response (IgG) to merozoite surface protein 2 (MSP2), RESA, SPf66 and crude schizont extract were measured in a population living in a malaria highly endemic area of Papua New Guinea. A prospective longitudinal study in 0.5-15 year old children was conducted for one year in order to examine the relationship between the humoral response to these antigens and subsequent susceptibility to clinical malaria using a series of clinical definitions. The prevalence and concentration of antibodies to all antigens increased with age. Such correlation with age was most marked for MSP2 recombinant proteins. When age and previous exposure were controlled for, only antibody levels to MSP2 recombinant proteins (3D7 and d3D7) and to RESA predicted a reduction in incidence rate of episodes of clinical malaria. Our results support the inclusion of the recombinant proteins of the 3D7 allelic family of merozoite surface antigen 2 and RESA into a subunit vaccine against malaria.

A longitudinal study of seroreactivities to Plasmodium falciparum antigens in Nigerian infants during their first year of life

The kinetics of passively transferred maternal antibodies to antigens of Plasmodium falciparum and the dynamics of acquisition of these antibodies during the first year of life was investigated in infants born in a malaria endemic area of south-western Nigeria. Blood samples were collected from the infants at bi-monthly follow-up visits for the analysis of total serum immunoglobulin G, IgM, IgA and antibodies to the antigen Pf155/RESA and against synthetic peptides representing antigenic sequences of the blood stage antigen Pf155/RESA and Ag332 or the circumsporozoite protein (CSP). IgG levels fell from birth till 4 months and a steady rise was observed thereafter till ten months of life. On the contrary mean IgM and IgA levels increased throughout the first year of life. Generally the number of infants positive for antibodies to the antigens under investigation fell from birth and between 4-6 months of age was either low or absent. None of the infants were positive for antibodies to the peptide representing Ag332 during the first year of life. The earliest seroconversion was detected at 6 months of age involving the Pf155/RESA and (NANP)6 antigens. The results indicate a high level of exposure in this study area to malaria infection early in life. The finding of an active antibody response to malarial antigens in infancy encourages the hope that a malaria vaccine administered early in life may accelerate the development of naturally acquired immunity and thus protect the population most at risk.

Humoral and cell-mediated immunity to the Plasmodium falciparum ring-infected erythrocyte surface antigen in an adult population exposed to highly endemic malaria

A parasitological and immunological survey was carried out in an area in Papua New Guinea highly endemic for malaria. Two hundred fourteen adult individuals were selected for studies to assess their immune responses against the malaria vaccine candidate ring-infected erythrocyte surface antigen (RESA). Total immunoglobulin G (IgG) antibodies directed against RESA as well as specific IgG1, IgG2, and IgG3 antibodies were determined. Humoral responses directed against RESA were frequent in all IgG subclasses. Only IgG3 responses were found to be age dependent. Total anti-RESA IgG antibodies were not correlated with protection against malaria as measured by parasite prevalence, parasite density, or health center attendance. In contrast, cytophilic antibodies (IgG1 and IgG3) were associated with reduced Plasmodium falciparum prevalence and reduced health center attendance. T-cell proliferation in general was low and very infrequent. No correlation between humoral and cellular immune responses could be found. Parasite density, parasite prevalence, and health center visits tended to be reduced in individuals with good humoral and cell-mediated immune responses.

Antibodies to ring-infected erythrocyte surface antigen (Pf155/RESA) protect against P. falciparum parasitemia in highly exposed multigravidas women in Malawi

To determine whether antibodies to defined B-cell epitopes of Plasmodium falciparum antigens were related to protection against parasitemic attacks in highly exposed pregnant women, two samples of 235 with no initial P. falciparum parasitemia (NP) and 89 multigravidas who presented initial P. falciparum parasitemia (IP) were enrolled in an antimalarial prophylaxis trial in the Mangochi District in Malawi. Sera were collected under effective prophylaxis and tested for antibody measurement using FAST-ELISA. Mean antibody titers to synthetic peptides reproducing the 3 major B-cell epitopes of the ring-infected erythrocyte surface antigen (Pf155/RESA), as (EENV)4, (EENVEHDA)4 and (DDEHVEEPTVA)3, were higher in the NP than in the IP multigravidas, and this remained consistent within the season of malaria transmission (all p < 0.05). All antibodies to Pf155/RESA were positively intercorrelated within each group. Mean antibody titers to peptide (PNAN)5 reproducing the major B-cell epitope of the circumsporozoite protein (CS protein) were similar between NP and IP multigravidas in both dry and rainy season. Antibodies to Pf155/RESA epitopes may contribute to immune protection against blood-stage parasite multiplication in these highly malaria-exposed pregnant women.

Seasonal variation of anti-RESA/Pf155 Plasmodium falciparum antibodies in three localities from the state of Amapá, Brazil

Anti-RESA/Pf155 antibodies were assayed in sera of individuals from three localities (Laranjal do Jari, Vila Padaria and Vila Paraíso) in the State of Amapá, Brazil, during the long-rains and short-rains seasons. All of these had negative blood smears for malaria. Most of the sera collected were positive in Indirect Fluorescent Antibody (IFA) with P. falciparum parasites, with no seasonal variation. A high percentage of these sera (62% to 100%) was RESA positive by Modified Indirect Fluorescent Antibody (MIFA), with a significant (p < 0.05) increase of geometric mean titers during the short-rains season, when the transmission of the disease is highest. ELISA with three repetitive RESA peptides (EENV)3 (4 x 3), (EENVEHDA)2 (8 x 2) and (DDEHVEEPTVA)2 (11 x 2) did not reveal statistically significant seasonal variations, although a small enhancement of positivity was observed in V. Padaria (15.3 to 38.8%) in the short-rains season with the 8 x 2 peptides, and with 4 x 3 and 8 x 2 peptides in V. Paraíso, with a decrease in 11 x 2. MIFA titers appeared to be correlated mainly to the peptide 4 x 3 and it was the immunodominant in the three localities.

Development of vaccines against malaria

Substantial progress towards development of a vaccine against Plasmodium falciparum malaria has been accomplished. A number of vaccine candidate antigens have been identified and in some of these antigens immunodominant B-cell and T-cell epitopes have been defined. New adjuvants and delivery systems suitable for human vaccines are available. The selection of suitable epitopes and their assembly in multi-antigenic constructs are research tasks for the future.

The multifactorial and multistage character of protective immunity to Plasmodium falciparum, naturally acquired by an indigenous population in Burkina Faso

In malaria-endemic areas, protective immunity is acquired gradually. Some authors have proposed that different stages can be distinguished during development. To test this hypothesis, several in vitro assays of the host immune response to P. falciparum were performed in three groups of individuals: 'unprotected' children with clinical attacks, 'semi-immune' children, without clinical attacks but with transient high parasitaemias during the transmission period, and 'protected' adults with low residual parasitaemias. By comparison of immune responses in these groups and multifactorial analyses, discriminant factors and potential protective mechanisms were identified. Anti-RESA antibody levels were lower in 'unprotected' than in 'semi-immune' children, while specific cellular responses, TNF levels and percentage of activated T lymphocytes were higher. Low humoral immunity and high cellular activation in children were followed by high humoral immunity and low cellular activation in adults. Therefore, protective immunity seems to pass through different stages and to result from the association of different immune mechanisms according to the level and duration of the individual experience of malaria.

Enzyme-linked staining method for light microscopic detection of antibodies to parasite antigens on the membrane of Plasmodium falciparum infected erythrocytes

Indirect erythrocyte membrane immunofluorescence (EMIF), a standard method for detection of antibodies to the P. falciparum blood stage vaccine candidate antigen Pf155/RESA, has been adapted to light microscopy by enzyme-linked immunostaining of the erythrocyte membrane, using alkaline phosphatase and chromogenic substrate. This method gives a dark blue staining of the membranes of ring infected erythrocytes. Results obtained with 70 African sera in EMIF and in enzyme immunostaining correlated well although the enzyme based method sometimes resulted in higher antibody titers and appeared to be slightly more sensitive. Similar results were obtained when comparing immunofluorescence with enzyme immunostaining for detection of antibodies to intraerythrocytic parasite antigens. The enzyme linked immunostaining described is simple and fast and does not require expensive equipment and should, thus, be well suited for use in laboratories with limited resources or under field conditions.

Seroreactivity with the Plasmodium falciparum blood stage antigen Pf332 in adults and children from malaria-endemic regions

It has earlier been reported that the human monoclonal antibody (MoAb 33G2) and polyclonal antibodies reactive with Pf332 may interfere in vitro with the erythrocytic cycle of Plasmodium falciparum at two potential target sites for protective antibodies, indicating that the antigen may constitute an important target for immune responses during malaria infections. MoAb 33G2 shows its highest reactivity with repeated sequences in the antigen Pf332 and also cross-reacts with determinants in Pf155/RESA. This study was conducted in order to assess the prevalence of seroreactivity against Pf332 in individuals residing in areas of different malaria endemicity, and in children with different degrees of disease severity. We now report that individuals resident in malaria-endemic regions show a high prevalence of seroreactivity to antigen Pf332 repeat sequences. The mean antibody concentrations were significantly higher in donors from Liberia, Madagascar and Gambia compared with Thai and Colombian donors, probably reflecting the higher degree of exposure in the African regions. Although the levels of such antibodies in individual sera correlated well with the levels of antibodies to one Pf155/RESA repeat peptide, only a minor part of the peptide-reactive antibodies were cross-reactive between the two antigens. In Gambian children, the mean concentrations of antibodies reactive with Pf332 or Pf155/RESA peptides were significantly higher in children with severe than with mild malaria. Further longitudinal studies are needed to evaluate the capacity of Pf332 to induce potentially protective or harmful antibody responses.

Isotypic analysis of Plasmodium falciparum-specific antibodies and their relation to protection in Madagascar

We previously reported that antibodies to the central repeat (DDEHVEEPTVA) of Pf155/RESA, a major Plasmodium falciparum antigen, were negatively related to antimalarial protection. We measured levels of isotype antibodies to this epitope and to P. falciparum in 76 Madagascan subjects. Immunoglobulin G (IgG) and isotype antibodies specific for P. falciparum had similar levels in individuals who were considered protected and those who were not (as determined during a longitudinal follow-up). The levels of IgG1 antibodies to (DDEHVEEPTVA)3 were higher in nonprotected subjects. The levels of total IgG and of other isotype antibodies had a similar tendency to be higher in nonprotected than in protected individuals.

A prospective study of the influence of alpha thalassaemia on morbidity from malaria and immune responses to defined Plasmodium falciparum antigens in Gambian children

The protective effect of alpha thalassaemia (-alpha/alpha alpha) against morbidity from falciparum malaria was assessed in a prospective study of rural Gambian children. The gene frequency for single alpha-globin gene deletions was 0.12. Malariometric indices measured during cross-sectional surveys and morbidity from malaria determined by weekly surveillance were similar in children with alpha thalassaemia and in those with a normal alpha-globin genotype. However, the small number of children who carried both alpha thalassaemia and the sickle cell trait had fewer clinical episodes of malaria than children with the sickle cell trait alone. Specific antibody responses and cell-mediated immune responses in vitro to defined Plasmodium falciparum antigens were measured in children participating in the study. In general, there was no evidence of an increased prevalence or intensity of humoral or cell-mediated immune responses to the malaria antigens studied in children heterozygous for alpha thalassaemia compared with children with a normal alpha-globin genotype.

Seasonal changes in human immune responses to malaria

Cellular as well as humorol immune responses to malaria antigens fluctuate in time in individuals living in molono-endemic areas, particularly where malaria transmission is seasonal. The most pronounced changes are seen in association with clinical attacks, but osymptomatic infection can also lead to apparent immune depression. However, recent data have shown that seasonal variation in cellular immune responses may occur even in the absence of detectable porositaemia. Here, Lars Hviid and Thor G. Theonder review the seasonal variation in human immune responses to malaria, and discuss its possible causes and implications.

Immunoglobulin M and G antibody responses to Plasmodium falciparum glutamate-rich protein: correlation with clinical immunity in Gambian children

The aims of the present study were to describe the age-related immunoglobulin M (IgM) and IgG response to part of a 220-kDa glutamate-rich protein (GLURP) from Plasmodium falciparum and to determine possible correlations of possession of these antibodies with malaria morbidity. IgM and IgG levels were measured with a recombinant fusion protein consisting of the carboxy-terminal 783 amino acids of the GLURP. Samples for the study were obtained during a longitudinal malaria morbidity survey performed in The Gambia; cross-sectional surveys were performed at the beginning of the transmission season in May and in October. Seropositivity rates increased with age to a maximum of 77% for IgM and 95% for IgG in adults. High prevalences of seropositivity were associated with certain human leukocyte antigen class II alleles (DRw8, DR9, DR7, DR4, DQw7, and DQw2) or haplotypes. The relationship between anti-GLURP489-1271 antibodies and clinical immunity is not clear; asymptomatically infected children aged 5 to 8 years had significantly higher levels of IgG than clinically ill children of the same age, suggesting that antibodies to the carboxy-terminal part of the GLURP may contribute to immunity to P. falciparum. However, this was not significant for younger children.

A prospective study of the association between the human humoral immune response to Plasmodium falciparum blood stage antigen gp190 and control of malarial infections

The human humoral immune response to the Plasmodium falciparum merozoite surface antigen gp190 was analyzed to determine the rate of reinfection by the parasite and the ability to control parasite density. The prospective study was carried out in a West African village where malaria is hyperendemic. No correlation between the antibody titers and protection against infection was observed within the group of children. Positive and negative associations of antibody specificities with protection against and/or control of parasitemia were, however, found for adolescents and adults, respectively. Thus, in adolescents, the presence of antibodies to gp190 fragment M6 correlates with a 50% reduced risk of P. falciparum infection and an increased ability to control parasitemia, whereas in adults, the humoral response to some of the polymorphic regions of gp190 associates with an increased risk of infection.

In vitro lymphoproliferative responses to malaria antigens: a prospective study of residents of a holoendemic area with perennial malaria transmission

A longitudinal, prospective study to examine the relationship between the outcome of infection with Plasmodium falciparum parasites and in vitro T-cell proliferative responses to a P. falciparum schizont extract (PfSE) was conducted in a village in south-eastern Gabon, an area where malaria is holoendemic and transmission is intense and perennial. The donor's age was found to have a strong independent influence on all malariometric indices. At the community level, the in vitro lymphoproliferative response to PfSE was bimodal with 30% of the villagers studied showing persistently low responses. The frequency of low or high responders within the study population did not show any consistent relationship with the community parasite rates or the number of either patent parasitaemic episodes or clinical malarial attacks per individual. At the individual donor level, the response was negatively correlated with P. falciparum parasite density in those donors who were parasitaemic at the time of sampling. High in vitro lymphoproliferative responses to PfSE were predictive of resistance to clinical malaria. The PfSE-induced in vitro lymphoproliferative response was dependent on antigen presenting cells, CD4+ T-cells and UCHL-1+ cells.

Is Immunity to malaria really short-lived?

Protection against Plasmodium falciparum malaria is usually considered to be the cumulative product of repeated exposure to parasites, and thus a function of age, in endemic areas. The recent outbreak of malaria in the central highlands of Madagascar gave Philippe Deloron and Claire Chougnet the opportunity to compare the incidence of malaria in children and young adults exposed to malaria for the first time, with that in older adults who spent their childhood in the study area before malaria control was introduced. Protection, as well as immune responses to two major P. falciparum antigens, was not related to age. Individuals older than 40 years were more protected than were younger adults. This increased protection was probably due to immunological memory.

Morbidity from malaria and immune responses to defined Plasmodium falciparum antigens in children with sickle cell trait in The Gambia

Morbidity from Plasmodium falciparum malaria and humoral and in vitro cellular immune responses to defined malaria antigens were measured in rural Gambian children with haemoglobin phenotype AS (HbAS) and in those with a normal haemoglobin (HbAA). In a survey undertaken during the dry season, HbAS children had a higher parasite rate than HbAA children but a lower prevalence of parasitaemia at a level of 500/microliters or greater. Malariometric indices measured during a rainy season survey were similar in the 2 groups of children. During the rainy season, the incidence of infection with P. falciparum did not vary with haemoglobin phenotype. However, in children aged 6 years or less, a significantly smaller proportion of HbAS children who acquired infection developed clinical symptoms than did HbAA children. During both the dry season and rainy season surveys, humoral and in vitro cellular immune responses to defined antigens from the sporozoite and merozoite stages of P. falciparum were similar in the 2 groups of children. Thus, despite the differences in parasite indices and morbidity from malaria between the 2 groups of children, we found no evidence of an enhanced immune response to malaria infection amongst HbAS children compared with normal children.

Genetic regulation of human anti-malarial antibodies in twins

Immune responses to defined antigens may differ between individuals in a population as the reflection of differences in genetic regulation. In experimental animals, variation in responsiveness to a given epitope may be due to major histocompatibility complex (HLA, in humans) class II restrictions, implying serious limitations for the development of subunit vaccines. For human populations, knowledge of the relative importance of genetic as opposed to environmental factors affecting the immune response is scarce. We have compared antibody levels after immunization through repeated infections to a major malarial antigen (Pf155/RESA) in monozygotic twins with those in dizygotic twins, siblings, or unrelated controls. Antibody responses to the intact antigen and to some of its immunodominant epitopes were found to be more concordant within monozygotic twin pairs than in dizygotic pairs or age- and sex-matched siblings living under similar environmental conditions. The results support the conclusion that the antibody responses were genetically regulated. When the responses were assessed for possible associations with different HLA class II DRB, DQA, and DQB alleles had haplotypes, no associations were found. This suggests that the regulation of the Pf155/RESA antibody responses seen in this study reflects the impact of factors encoded by genes outside the HLA class II regions.