IgE elevation and IgE anti-malarial antibodies in Plasmodium falciparum malaria: association of high IgE levels with cerebral malaria.

In the course of studying immunoregulation in human Plasmodium falciparum malaria we have investigated IgE levels and IgE anti-plasmodial antibodies in children and adults from areas of high malaria endemicity in both Africa and Asia. On average, 85% of all donors had significantly elevated levels of total IgE. A fraction of the IgE had anti-plasmodial activity as revealed by ELISA with lysates of infected erythrocytes as antigen. Using synthetic peptides representing antigenic regions of two major plasmodial blood stage antigens, IgE antibody concentrations ranged from 5 to 15 ng/ml serum for each of the peptides. On average, the concentrations of the corresponding IgG antibodies were x 500-1000 higher. Immunoblotting of parasite lysates showed that most donors had IgE antibodies against one or several of a restricted number of plasmodial polypeptides, with antibodies against an antigen of mol.wt 45 kD already being present in all donors at an early age. Donors having IgE antibodies to particular antigens also frequently had corresponding IgG4 antibodies, reflecting underlying IL-4-dependent cellular mechanisms controlling formation of these isotypes. As infection with other parasites such as helminths is known to induce IgE elevation, the results do not prove that plasmodial infections were the primary cause of IgE induction. However, the importance of plasmodial infection for IgE elevation was supported by the finding of significantly higher levels of IgE, but not of IgG, in children with cerebral malaria compared with patients with uncomplicated disease.

Dissection of the human antibody response to the malaria antigen Pf155/RESA into epitope specific components.

The development of vaccines is presently receiving major attention in malaria research. As it is not possible to base malaria vaccines on the use of killed or attenuated organisms, the vaccines which are being developed are subunit vaccines in which the immunogens consist of defined parasite antigens or antigenic fragments. Since protective immunity to malaria involves both antibody-dependent and antibody-independent mechanisms, the immunogens in a subunit vaccine must have the capacity to induce relevant B- and T-cell responses in the majority of vaccinees. In turn, this requires good knowledge of these responses in humans who have acquired immunity through natural infection. In this paper we have summarized our recent work on the dissection into epitope-specific components of the human antibody response to the Plasmodium falciparum antigen Pf155/RESA, a recognized candidate for a vaccine against the asexual blood stages of this parasite. Epitope mapping of the antigen by means of short synthetic peptides led to the identification in several molecular regions of short amino acid sequences constituting linear and probably immunodominant B-cell epitopes. The antigenically most active region was located in the C-terminus of the molecule. This region, which consists of approximately 40 related, 4- or 8-amino acid long repeats, induced higher antibody concentrations in a larger number of malaria-immune donors than any of the other regions. A large fraction of these antibodies bound to short synthetic peptides representing the major repeat motifs of Pf155/RESA. Although these repeats are made up of closely related amino acid sequences, the antibody response to them was highly polyclonal, indicating the presence of several linear and probably also conformational epitopes which gave rise to a variety of cross-reacting as well as monospecific antibodies. Further analysis revealed that the levels of antibodies differing in specificity and/or avidity for different peptides varied independently of each other in individual donors. In an area (Liberia) where malaria transmission is holoendemic and perennial, these antibody profiles remained constant when individual donors were followed over several years. Since the C-terminal repeat region of Pf155/RESA is conserved in different P. falciparum strains, the results reflect differences in the genetic regulation of epitope-specific host responses rather than antigenic differences between infecting parasites. In donors living in an area with high but seasonal malaria transmission, antibody levels usually drop to lower levels when there is no transmission.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

High antibody responses in rabbits immunized with influenza virus ISCOMs containing a repeated sequence of the Plasmodium falciparum antigen Pf155/RESA.

Immunostimulating complexes (ISCOMs) are spherical structures where immunogens are presented as multimers in a matrix of the adjuvant Quil A. ISCOMs have been shown to enhance the immunogenicity of several antigens important to both human and veterinary vaccine development. We have coupled a fusion protein, designated ZZ-M2, comprising eight copies of the C-terminal repeat subunit EENV of the Plasmodium falciparum blood-stage antigen Pf155/RESA and two IgG-binding domains of staphylococcal protein A (SpA), to preformed influenza virus envelope protein ISCOMs. Rabbits immunized with the conjugated ISCOMs produced high titres of antibodies even after the first injection. These antibodies reacted with the EENV repeat sequence in ELISA and with Pf155/RESA in immunofluorescence on infected erythrocytes. The antibody response, which was sustained for more than 20 weeks, was efficiently boosted and superior or equal to that obtained after immunization with ZZ-M2 in Freund's complete adjuvant. In contrast, the antibody response induced in rabbits immunized with ZZ-M2 in Syntex Adjuvant Formulation-MF (SAF-MF) was weak and of short duration. The antibodies produced after immunization with ZZ-M2 coupled to influenza virus ISCOMs mainly recognized epitopes formed by two or more EENV subunits and were highly specific for Pf155/RESA. Furthermore, the antibodies efficiently inhibited merozoite reinvasion of erythrocytes in vitro, indicating that they recognized epitopes exposed on the native antigen. In addition, the ZZ-M2-conjugated ISCOMs also induced high titres of antibodies reacting with SpA or the influenza virus envelope protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytokine Profiles and Antibody Responses to Plasmodium Falciparum Malaria Infection in Individuals Living in Ibadan; Southwest Nigeria

Background: The ability of the host immune system to efficiently clear Plasmodium falciparum parasites during a malaria infection depends on the type of immune response mounted by the host. Study design: In a cross-sectional study; we investigated the cellular-and antibody responses in individuals with P. falciparum infection; in an attempt to identify immunological signs indicative of the development of natural immunity against malaria in Ibadan; Nigeria. Levels of IL-10; IL-12(p70); IFN-a; and IgM; IgG and IgG1-4 subclasses in the serum of 36 symptomatic children with microscopically confirmed malaria parasitaemia and 54 asymptomatic controls were analysed by ELISA. Results: IFN-a and IL-10 were significantly higher in the symptomatic children (p=0.009; p=0.025 respectively) than in the asymptomatic controls but no differences were seen for IL-12(p70). Estimated higher ratios of IFN-a/IL-10 and IFN-a/IL-12 were also observed in the symptomatic children while the asymptomatic controls had higher IL-12/IL-10 ratio. The mean concentration levels of anti-P. falciparum IgG1; IgG2; IgG3 antibodies were statistically significantly higher in the individuals 5 years of age than 5 years while anti-P. falciparum IgG3 antibodies were notably low in 5 years category. Children 5 years had higher IgM antibodies than IgG and the expression of IgG subclasses increased with age. Conclusion: Taken together; malaria infection is on a delicate balance of pro- and anti-inflammatory cytokines. The higher levels of IFN-a seen in the symptomatic children (6months) may be instrumental in immune-protection against malaria by limiting parasite replication. The observed variations in immunoglobulin subclass levels were age- dependent and exposure-related