Definition of the epitope recognized by the Plasmodium falciparum-reactive human monoclonal antibody 33G2

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.

Antibody responses to a C-terminal fragment of the Plasmodium falciparum blood-stage antigen Pf332 in Senegalese individuals naturally primed to the parasite

Previous studies have shown that antibodies from humans exposed continuously to malaria recognize the Plasmodium falciparum asexual blood-stage antigen Pf332. Here we analysed the antibody responses to a C-terminal fragment of Pf332, designated C231, in individuals from Senegal, by measuring the serum levels of immunoglobulin M (IgM), IgG class and subclass and IgE antibodies. IgG antibody reactivity with crude P. falciparum antigen was detected in all the donors, while many of the children lacked or had low levels of such antibodies against C231. The antibody levels increased significantly with age for both crude P. falciparum antigen and C231, and in the older age groups most of the donors displayed antibodies to C231. This was also true for IgM, IgE and IgG subclass reactivity against C231. Moreover, the ratio of IgG1/IgG2 was considerably lower for C231 than for crude P. falciparum antigen, and in age groups 10-14 and 15-19 years the levels of IgG2 against C231 even exceeded that of IgG1. The IgG2/IgG3 ratios suggest that C231 gives similar levels of IgG2 and IgG3, except for children aged 4-9 years, where IgG3 was higher. Raw IgM, IgG class and subclass and IgE antibody levels to C231 tended to be higher in those who did not experience a malaria attack, but following linear multivariate analysis the trends were not significant.

A novel DBL-domain of the P. falciparum 332 molecule possibly involved in erythrocyte adhesion

Plasmodium falciparum malaria is brought about by the asexual stages of the parasite residing in human red blood cells (RBC). Contact between the erythrocyte surface and the merozoite is the first step for successful invasion and proliferation of the parasite. A number of different pathways utilised by the parasite to adhere and invade the host RBC have been characterized, but the complete biology of this process remains elusive. We here report the identification of an open reading frame (ORF) representing a hitherto unknown second exon of the Pf332 gene that encodes a cysteine-rich polypeptide with a high degree of similarity to the Duffy-binding-like (DBL) domain of the erythrocyte-binding-ligand (EBL) family. The sequence of this DBL-domain is conserved and expressed in all parasite clones/strains investigated. In addition, the expression level of Pf332 correlates with proliferation efficiency of the parasites in vitro. Antibodies raised against the DBL-domain are able to reduce the invasion efficiency of different parasite clones/strains. Analysis of the DBL-domain revealed its ability to bind to uninfected human RBC, and moreover demonstrated association with the iRBC surface. Thus, Pf332 is a molecule with a potential role to support merozoite invasion. Due to the high level of conservation in sequence, the novel DBL-domain of Pf332 is of possible importance for development of novel anti-malaria drugs and vaccines.

Differential antibody responses to Plasmodium falciparum-derived B-cell epitopes induced by diepitope multiple antigen peptides (MAP) containing different T-cell epitopes

Epitopes of universal character are needed when designing subunit vaccines against infectious diseases such as malaria. We have compared the immunogenicity of B-cell epitopes from the Plasmodium falciparum antigen repeats DPNANPNV (PfCS protein) and VTEEI (Pf332) when assembled with four different universal T-cell epitopes in diepitope multiple antigen peptides (MAP). T-epitopes employed were from P. falciparum antigens (CS.T3, [T(*)]4 and EBP3) or from the Clostridium tetani toxin (P2). In association with either of the T-epitopes, the genetic unresponsiveness to the B-epitopes was successfully bypassed. Our results show that the immunogenicity of a T-epitope alone does not necessarily predict the ability of the T-epitope to provide T-cell help when combined with other epitopes in an immunogen. Further, the nature of the immune responses in terms of total IgG antibodies and their subclass distribution, T-cell proliferation and IFN-gamma production, varied with the T-epitope and mouse strain, which may indicate the need for inclusion of a combination of different universal T-epitopes in a future malaria subunit vaccine.

The malaria-infected red blood cell: structural and functional changes

The asexual stage of malaria parasites of the genus Plasmodium invade red blood cells of various species including humans. After parasite invasion, red blood cells progressively acquire a new set of properties and are converted into more typical, although still simpler, eukaryotic cells by the appearance of new structures in the red blood cell cytoplasm, and new proteins at the red blood cell membrane skeleton. The red blood cell undergoes striking morphological alterations and its rheological properties are considerably altered, manifesting as red blood cells with increased membrane rigidity, reduced deformability and increased adhesiveness for a number of other cells including the vascular endothelium. Elucidation of the structural changes in the red blood cell induced by parasite invasion and maturation and an understanding of the accompanying functional alterations have the ability to considerably extend our knowledge of structure-function relationships in the normal red blood cell. Furthermore, interference with these interactions may lead to previously unsuspected means of reducing parasite virulence and may lead to the development of novel antimalarial therapeutics.

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.

Predominance of H-2d- and H-2k-restricted T-cell epitopes in the highly repetitive Plasmodium falciparum antigen Pf332

Genetic restriction of immune responses to malaria antigens is an important issue for a better comprehension of malaria immunity as well as for development of subunit vaccines. To experimentally define the major histocompatibility complex restriction of immune responses to the highly repetitive Plasmodium falciparum high-molecular-weight antigen Pf332, H-2-congenic mice were immunized with EB200, a recombinant fragment of Pf332 consisting of degenerate repeat motifs. Strong B- and T-cell responses were elicited in H-2d and H-2k mice whereas responses in H-2b, H-2q and H-2s mice were of lower magnitude. The T-cell specificity elicited by EB200 was defined by in vitro proliferative responses to a panel of overlapping peptides spanning EB200. Dominant epitopes were identified for H-2d and H-2k mice, respectively, and an additional epitope was recognized by all five mouse strains. Selected EB200-derived peptides were further investigated for their ability to elicit T-cell help when injected as multiple antigen peptides. Defined H-2d- and H-2k-restricted T-cell epitopes generated high antibody levels in the respective mouse strains, as did several peptides lacking defined epitopes indicating the presence of additional H-2d- and H-2k-restricted, cryptic or subdominant T-cell epitopes in EB200. The biased H-2 restriction pattern of T-cell epitopes in Pf332 and, as previously reported, in structurally related repeats in the malaria antigens Pf11.1 and Pf155/RESA may be explained by a shared motif for H-2d and H-2k class II-restricted T-cell epitopes, as revealed by alignment of these sequences.

Antibody reactivities to glutamate-rich peptides of Plasmodium falciparum parasites in humans from areas of different malaria endemicity

Synthetic P. falciparum peptides were evaluated as tools in epidemiological investigations of malaria. Plasma IgM and IgG antibody reactivities against synthetic peptides covering sequences of glutamate-rich protein (GLURP) and acidic-basic repeat antigen (ABRA) were measured by ELISA in individuals from malaria-endemic areas of Sudan, Indonesia and The Gambia to study antibody responses to these peptides in donors living in areas of different malaria endemicity. IgG and IgM reactivities to the peptides increased with malaria endemicity, although there were no differences in reactivities to the GLURP peptide between non-exposed donors and donors living in areas of low malaria endemicity. IgG reactivities to the GLURP peptide in Sudanese adults were high one month after treatment in all adults tested, while IgG reactivities to the ABRA peptide were infrequent. IgM responses to the peptides tested were shortlived in most patients. In Gambian children with malaria, IgM reactivities but not IgG antibody reactivities against the ABRA peptide were higher in those with mild malaria than in those with severe malaria. The peptides may be useful in future epidemiological studies, especially in areas of low malaria endemicity.

Immune responses in congenic mice to multiple antigen peptides based on defined epitopes from the malaria antigen Pf332

Repeat sequences from the Plasmodium falciparum blood stage antigen Pf332 frequently comprise the pentapeptide VTEEI, an epitope recognized by certain parasite neutralizing antibodies. This B-cell epitope was assembled in an octavalent multiple antigen peptide (MAP) system either as trimers (VTEEI)3 (MAP1) or as an integral part of a naturally occurring Pf332 undecamer repeat sequence SVTEEIAEEDK (MAP2). Characteristics of the immunogenicity of these subunit constructs were evaluated in H-2 congenic mice. MAP1 generated antibody responses in mice of the H-2d, H-2k and H-2q haplotypes, but not in H-2b or H-2s mice, whereas MAP2 only induced antibodies in mice of H-2k haplotype. When analysing T-cell responses induced by the MAP, lymph node cells from responder strains primed in vivo with MAP1 proliferated in response to restimulation with both MAP1 and the peptide (VTEEI)3. MAP2, however, did not induce a detectable T-cell proliferation. Additionally, the lack of antibody response to MAP1 in H-2b mice could be circumvented by combining the MAP1 peptide and a H-2b-restricted T-cell epitope in a diepitope MAP construct. Despite the fact that the motif VTEEI has not been identified in Pf332 sequences in the form of a trimer, MAP1 did induce Pf332 protein-reactive antibodies. Assembly of multimers of short defined epitopes in MAP constitutes an interesting approach for the design of polyvalent subunit immunogens.

Design of synthetic peptides for oral vaccination

The rational design of effective oral vaccines based on synthetic peptides is a very ambitious undertaking, and involves the solution of huge problems related to protection of the antigens against degradation in the alimentary tract, efficient uptake of the antigens by the relevant cells, and efficient induction of long lasting systemic immunity, local immunity, or both. This paper summarises the steps, necessary to develop such synthetic oral vaccines. These steps involve: (1) the definition of B-cell epitopes; (2) the definition of T-cell epitopes; (3) definition of the carrier or backbone molecule; (4) definition of an immunomodulating element; (5) definition of an adjuvant element; and (6) definition of a targeting element. Good progress is being made with respect to the first three steps, the other steps still provide major challenges, notably the definition of targeting elements. Nevertheless, the first synthetic oral vaccines may become reality in the near future, depending on the speed by which new technology in the area of molecular recognition will develop, i.e. the appropriate chemistry, organic chemistry, molecular modelling, resolution of the molecular interaction of key molecules in microbiology and immunology.

Immunogens containing sequences from antigen Pf332 induce Plasmodium falciparum-reactive antibodies which inhibit parasite growth but not cytoadherence

Immunogens based upon sequences from the P. falciparum asexual blood stage antigen Pf332 were assessed for their capacity to induce antibodies inhibiting parasite growth or cytoadherence of infected erythrocytes in vitro. Selection of the Pf332 sequences was based on their reactivity with the human monoclonal antibody (MoAb) 33G2 which inhibits parasite growth as well as cytoadherence in vitro. Octameric multiple antigen peptides (MAP) were assembled based upon either a trimer of the minimal epitope recognized by the MoAb, VTEEI, or a Pf332 sequence including that motif, SVTEEIAEEDK. A dimer of SVTEEIAEEDK was also expressed in Escherichia coli, genetically fused to ZZ, two IgG-binding domains of staphylococcal protein A. Rabbit antibodies elicited by the immunogens reacted with Pf332 in immunofluorescence and in ELISA with Pf332 peptides which were also recognized by MoAb 33G2. The MAP with branched (VTEEI)3 peptide induced the highest titres of P. falciparum-reactive antibodies. In contrast to MoAB 33G2, none of the polyclonal Pf332 reactive sera cross-reacted with repeat sequences of the malaria antigen Pf155/RESA. The polyclonal Pf332-reactive antibodies inhibited parasite growth efficiently but had no or very low inhibitory effect in a cytoadherence assay. Thus, while Pf332 may be an important target for parasite neutralizing antibodies its involvement in cytoadherence is unclear.

Synthesis of a diepitope multiple antigen peptide containing sequences from two malaria antigens using Fmoc chemistry

Multiple antigen peptides (MAP) consist of lysine residue cores with branching peptide arms and have been demonstrated to be efficient immunogens as well as useful antigens for ELISA. Synthesis of diepitope MAPs with two different branching peptides has previously been described using combined Boc and Fmoc chemistry. Here, the synthesis of a tetrameric diepitope MAP based on Fmoc chemistry is described. A lysine core was synthesized with N alpha- and N epsilon-amino groups othogonally protected by Fmoc and a recently described protection group, Dde, respectively. On the N alpha-amino groups, a sequence from the Plasmodium falciparum antigen Pf332 was synthesized with a capped N-terminus. After removal of Dde, a sequence from the P. falciparum circumsporozoite protein was synthesized on the core. Amino acid analysis of the MAP displayed equimolar amounts of the two peptide sequences, indicating the reliability of the protection group Dde. In ELISA, antibodies specific for either of the two malarial sequences reacted with the MAP. The major advantages of this approach for synthesis of diepitope MAPs are that only a panel of Fmoc-amino acid derivatives is required and that the more complicated cleavage procedure for Boc chemistry can be avoided.

B- and T-cell responses in congenic mice to repeat sequences of the malaria antigen Pf332: effects of the number of repeats

The Plasmodium falciparum antigen Pf332 comprises degenerated 11-amino-acid repeats with regularly spaced pairs of glutamic acid. Epitopes formed by such repeats are recognized by polyclonal and monoclonal antibodies that interfere with the life cycle of the blood stages of the malaria parasite. In order to study the immunogenicity of one such Pf332 repeat sequence (SVTEEIAEEDK), fusion proteins containing ZZ (two IgG binding domains of staphylococcal protein A) and dimers, trimers or tetramers of the malarial sequence were injected into mice. To analyse possible major histocompatibility complex class II restrictions of the immune response, mice of different H-2 haplotypes were used. A significant antibody response was elicited by administration of all the three fusion proteins in mice expressing the I-Ak allele (B10.BR, B10.A(2R) and B10.A(4R)) whereas B10 and C57BL/6 (H-2b) mice were low responders. In comparison, B10.D2 (H-2d) mice were low responders to fusion proteins with 2 or 3 repeats but responded well to the protein containing 4 repeats. Lymph node cells from B10.BR (H-2k) mice, primed in vivo with ZZ-fusion proteins containing either 2 or 4 repeats, proliferated in vitro in response to repeat sequences fused to ZZ or to an unrelated fusion partner, as well as to a synthetic peptide containing less than two repeats. In contrast, a response of lymph node cells from B10.D2 (H-2d) mice was only obtained when a fusion protein containing 4 repeats was used both for in vivo priming and in vitro restimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

Analysis of a human monoclonal antibody reactive with multiple Plasmodium falciparum antigen repeat sequences using a solid phase affinity assay

A solid-phase affinity assay was set up for the determination of the affinity of the interaction between the human monoclonal antibody (mAb) 33G2 and peptides corresponding to repeated sequences in three blood stage antigens of the malaria parasite Plasmodium falciparum. The epitope of this mAb is of interest due to the parasite blocking capacity of the mAb. Previous studies with PEPSCAN have defined the minimal epitope for the mAb as the pentapeptide VTEEI, a sequence frequently found in antigen Pf332. In the previous study, epitopes responsible for the cross-reactivity of the mAb with antigens Pf155/RESA and Pf11.1 were also identified. In the affinity assay described herein, the mAb was coated on a solid phase and binding of a labelled peptide was displaced by homologous or heterologous peptides. The affinity of peptides corresponding to Pf332 increased with increasing length, and the highest affinity was displayed by a dimer (23 amino acids) of a Pf332 repeat (K = 1.9 x 10(8) M-1). Peptide length did not influence the binding of peptides corresponding to the Pf155/RESA and Pf11.1 repeats, which had lower affinities comparable to that of the shortest Pf332 octapeptide (K = 2.2 x 10(4) M-1). Only peptides containing binding sites as defined by PEPSCAN analysis showed a measurable binding. When using peptides as inhibitors in peptide ELISA, binding correlated with the affinity of the peptides, but only the high affinity peptides were inhibitory. In contrast, a poor correlation was found when peptides were used directly for coating in ELISA.(ABSTRACT TRUNCATED AT 250 WORDS)

Immune response in mouse and malaria-exposed humans to peptides derived from Pf11-1, a highly repetitive megadalton protein of Plasmodium falciparum

We have investigated the immune response against the Plasmodium falciparum gametocyte-specific antigen Pf11-1. This megadalton parasite molecule has been implicated in the process of erythrocyte rupture during gametogenesis. The molecule is composed in great part of degenerated nonapeptide motifs which are tandemly repeated several hundred times. A computer algorithm searching for T sites predicted that the entire repeat region of the Pf11-1 represents potential T cell antigenic major histocompatibility complex class II-binding sites. To test this hypothesis, synthetic peptides corresponding to two nonamer subtype repeats, differing only at two amino acid positions, were used to immunize congenic mouse strains. Both peptides were shown to contain both B and T cell epitopes. The immune response is restricted to the H-2d and H-2k haplotypes. The T cell response against the peptides appeared to be highly specific, clearly discriminating between the two similar nonamer repeat sequences, whereas the humoral response produced cross-reacting antibodies. We also investigated the humoral and T cell reactivities of P. falciparum-primed individuals in West Africa against the synthetic Pf11-1 peptides. Among 51 individuals 35 had antibodies to at least one of the two peptides and a majority of them (28) had antibodies reacting with both peptides. The cellular response was analyzed by [3H]thymidine incorporation or interferon-gamma release. There was considerable variation in the response to the two peptides. Among the human samples 36% responded to one repeat subtype, while only 13% responded to the second subtype. Interestingly, in individual donors the T cell response to both peptides are associated, suggesting that, as shown for mice, the response is restricted by a genetic element. The data obtained on the two subtypes of the nonamer repeat region suggest that the entire Pf11-1 molecule might induce an unusually heterogenous B and T cell response during natural infection in man.

Epitope specificity and capacity to inhibit parasite growth in vitro of human antibodies to repeat sequences of the Plasmodium falciparum antigen Ag332

It has earlier been shown that the Plasmodium falciparum-reactive human monoclonal antibody 33G2 inhibits parasite growth in vitro as well as cytoadherence of infected red blood cells to melanoma cells in vitro. MoAb 33G2 recognizes an epitope of the P. falciparum antigen Ag332 and cross-reactive determinants in Pf155/RESA and Pf11.1 located in repetitive regions containing sequences of regularly spaced pairs of glutamic acid. To study whether antibodies of this specificity frequently occur in human immune sera and if they could be of importance for protective immunity, antibodies were affinity purified on MoAb 33G2 reactive Ag332 peptides. The epitope specificity of the affinity purified antibodies, determined by the Pepscan method, resembled that of MoAb 33G2, but showed differences in fine specificity. The antibodies cross-reacted to some extent with Pf11.1 and Pf155/RESA repeat peptides as detected by peptide ELISA and Pepscan. In indirect immunofluorescence all purified antibodies displayed a dotted pattern of staining of late stage infected red blood cells of two lines of the P. falciparum strain FCR3, including a Pf155/RESA deficient line. The in vitro growth of these two lines was efficiently inhibited by the affinity purified antibodies, indicating that their inhibitory effect was mainly due to reactivity with antigens other than Pf155/RESA. This, and the fact that Pf11.1 has been shown not to be expressed by the asexual stages suggests that Ag332 may be an important target for potentially protective antibodies in vivo and that Ag332 based immunogens are of interest for development of malaria subunit vaccines.

Pfl I-I and Pf332: Two giant proteins synthesized in erythrocytes infected with Plasmodium falciparum

Although the malaria parasite develops within erythrocytes, it has to modify the surrounding red blood cell membrane for its intracellular survival and maturation. These changes include the translocation of proteins across the parasite and the parasitophorous vacuole membranes to the host membrane. In this review, Denise Mattei, Katherine Hinterberg and Artur Scherf focus on two distinct giant parasite molecules of unprecedented size (approximately one MDa), called Pf332 and PflI-I, that are synthesized and exported into the cytoplasm of the host cell in the asexual and sexual blood stages of Plasmodium falciparum, respectively. The corresponding genes are located in genetically unstable subtelomeric chromosome regions.

Cross-reaction of antibodies to the nine-amino acid repeats of Plasmodium falciparum antigen 11.1 with human serum albumin

Mice immunized with the recombinant antigen 11.1 beta-galactosidase, consisting of 22 repeats of the nine-amino acid unit from Plasmodium falciparum antigen 11.1, produced antibodies reacting with human serum albumin. A positive reaction was observed in dot-blot assays, in enzyme-linked immunosorbent assay and on immunoblots of sodium dodecyl sulfate polyacrylamide gels as well as two-dimensional gels. Binding was specific for human albumin, as no reaction could be detected on bovine serum albumin, hen egg ovalbumin, rat serum albumin or another abundant human serum protein, the alpha 2-macroglobulin. In addition, rabbit antibodies raised to human serum albumin reacted with keyhole lympet hemocyanin coupled to synthetic dimers of the nine-amino acid repeats of the P. falciparum 11.1 antigen. These data indicate antigenic relationship between the 11.1 antigen and human albumin. The proteins have a short sequence of homology in a region where human serum albumin differs from the albumins of other species.

Involvement of Pf155/RESA and cross-reactive antigens in Plasmodium falciparum merozoite invasion in vitro

Lines of Plasmodium falciparum FCR3 either expressing or not expressing the blood-stage antigen Pf155/RESA were used to analyze the possible involvement of this antigen in the merozoite invasion process in vitro. Antibodies from human sera, affinity purified on synthetic peptides corresponding to C-terminal repeated sequences in Pf155/RESA, were shown to inhibit merozoite invasion of both types of parasites with similar efficiency. Reversal of the invasion inhibition by fusion proteins containing repeated sequences of Pf155/RESA but not of the cross-reactive antigens Ag332 and Pf11.1 indicated that the inhibitory antibodies had similar target antigens in both Pf155/RESA+ and Pf155/RESA- parasites that involved cross-reacting epitopes present in Pf155/RESA. Rabbit antibodies specific for Pf155/RESA repeats inhibited merozoite invasion of Pf155/RESA expressing parasites efficiently but had no or very small effect on the invasion of Pf155/RESA-deficient parasites. In contrast, rabbit antibodies specific for Ag332 repeats as well as human antibodies affinity purified on synthetic Ag332 peptides inhibited merozoite invasion of both types of parasites with high efficiency. A similar inhibition pattern was seen with the human monoclonal antibody 33G2, which has specificity for Ag332 but also cross-reacts with Pf155/RESA and Pf11.1. Taken together, our data suggest that Pf155/RESA and related cross-reactive antigens as well as Ag332 are involved in the merozoite invasion process and may constitute targets for invasion inhibitory antibodies.

The Pf332 gene of Plasmodium falciparum codes for a giant protein that is translocated from the parasite to the membrane of infected erythrocytes

We studied the gene structure of the Plasmodium falciparum antigen 332 (Ag332). The gene size was estimated to be approx. 20 kb based on the large size of both the transcript found in mature asexual blood stage parasites and mung bean nuclease fragment generated from genomic DNA. Sequence analysis of genomic and cDNA clones representing different regions of the Pf332 locus showed that the gene product contains a large number of highly degenerated glutamic acid (Glu)-rich repeats (32% Glu). The gene shows dramatic restriction fragment length polymorphism in various P. falciparum isolates and was mapped to the subtelomeric region of chromosome 11. The recombinant 332 fusion protein reacts strongly with the human monoclonal antibody (mAb) 33G2, which is able to inhibit the cytoadherence of parasitized red blood cells on the melanoma cell line C32 and merozoite invasion in in vitro assays. The epitope recognized by this mAb is found frequently in the reported sequence. Ag332 monospecific antibodies were obtained by immunization of mice with a recombinant fusion protein. These antibodies react with a large parasite molecule with an apparent molecular size of 2500 kDa of trophozoite and schizont-infected erythrocytes on Western blot and by immunoprecipitation analysis. Immunofluorescence studies using a confocal microscope showed that Ag332 is exported from the parasite to the infected red blood cell membrane within large vesicle-like structures of about 1 micron diameter.

Plasmodium falciparum: an invasion inhibitory human monoclonal antibody is directed against a malarial glycolipid antigen

A Plasmodium falciparum malaria blood stage antigen was detected using a human monoclonal antibody (MAb A52A6) obtained from a clinically immune donor. Immunofluorescence analysis showed that the MAb reacted with the intracellular parasite throughout the asexual blood stage cycle as well as with gametocytes. The MAb also reacted with the surface of erythrocytes containing late stage P. falciparum parasites. The antigen seen by the MAb was species- but not strain- or isolate-specific. At rupture of the infected erythrocytes, antigenic material was deposited on the membrane of uninfected cells surrounding the parasite. At merozoite invasion MAb reactive material was present on the invaginating erythrocyte membrane, indicating an involvement of the antigen in the invasion process. This was also indicated by the high capacity of the MAb to inhibit merozoite invasion in vitro. The antigen appears to be a phosphoglycolipid, sensitive to phospholipase and present in lipid extracts of P. falciparum-infected erythrocytes.