Cross-reactive antigenic determinants present on different Plasmodium falciparum blood-stage antigens

Antibodies to repeat-containing antigens in Plasmodium falciparum are exposure-dependent and short-lived in children in natural malaria infections

Protection against Plasmodium falciparum, which is primarily antibody-mediated, requires recurrent exposure to develop. The study of both naturally acquired limited immunity and vaccine induced protection against malaria remains critical for ongoing eradication efforts. Towards this goal, we deployed a customized P. falciparum PhIP-seq T7 phage display library containing 238,068 tiled 62-amino acid peptides, covering all known coding regions, including antigenic variants, to systematically profile antibody targets in 198 Ugandan children and adults from high and moderate transmission settings. Repeat elements - short amino acid sequences repeated within a protein - were significantly enriched in antibody targets. While breadth of responses to repeat-containing peptides was twofold higher in children living in the high versus moderate exposure setting, no such differences were observed for peptides without repeats, suggesting that antibody responses to repeat-containing regions may be more exposure dependent and/or less durable in children than responses to regions without repeats. Additionally, short motifs associated with seroreactivity were extensively shared among hundreds of antigens, potentially representing cross-reactive epitopes. PfEMP1 shared motifs with the greatest number of other antigens, partly driven by the diversity of PfEMP1 sequences. These data suggest that the large number of repeat elements and potential cross-reactive epitopes found within antigenic regions of P. falciparum could contribute to the inefficient nature of malaria immunity.

Characterization of the Duffy-Binding-Like Domain of Plasmodium falciparum Blood-Stage Antigen 332

Studies on Pf332, a major Plasmodium falciparum blood-stage antigen, have largely been hampered by the cross-reactive nature of antibodies generated against the molecule due to its high content of repeats, which are present in other malaria antigens. We previously reported the identification of a conserved domain in Pf332 with a high degree of similarity to the Duffy-binding-like (DBL) domains of the erythrocyte-binding-like (EBL) family. We here describe that antibodies towards Pf332-DBL are induced after repeated exposure to P. falciparum and that they are acquired early in life in areas of intense malaria transmission. Furthermore, a homology model of Pf332-DBL was found to be similar to the structure of the EBL-DBLs. Despite their similarities, antibodies towards Pf332-DBL did not display any cross-reactivity with EBL-proteins as demonstrated by immunofluorescence microscopy, Western blotting, and peptide microarray. Thus the DBL domain is an attractive region to use in further studies on the giant Pf332 molecule.

Specific antibody responses against membrane proteins of erythrocytes infected by Plasmodium falciparum of individuals briefly exposed to malaria

Background

Plasmodium falciparum infections could lead to severe malaria, principally in non-immune individuals as children and travellers from countries exempted of malaria. Severe malaria is often associated with the sequestration of P. falciparum-infected erythrocytes in deep micro-vascular beds via interactions between host endothelial receptors and parasite ligands expressed on the surface of the infected erythrocyte. Although, serological responses from individuals living in endemic areas against proteins expressed at surface of the infected erythrocyte have been largely studied, seldom data are available about the specific targets of antibody response from travellers.

Methods

In order to characterize antigens recognized by traveller sera, a comparison of IgG immune response against membrane protein extracts from uninfected and P. falciparum-infected red blood cells (iRBC), using immunoblots, was performed between non exposed individuals (n = 31) and briefly exposed individuals (BEI) (n = 38) to malaria transmission.

Results

Immune profile analysis indicated that eight protein bands from iRBC were significantly detected more frequently in the BEI group. Some of these antigenic proteins were identified by an original immuno-proteomic approach.

Conclusion

Collectively, these data may be useful to characterize the singular serological immune response against a primary malaria infection in individuals briefly exposed to transmission.

The malaria candidate vaccine liver stage antigen-3 is highly conserved in Plasmodium falciparum isolates from diverse geographical areas

Background

A high level of genetic stability has been formerly identified in segments of the gene coding for the liver stage antigen-3 (LSA-3), a subunit vaccine candidate against Plasmodium falciparum. The exploration of lsa-3 polymorphisms was extended to the whole sequence of this large antigen in 20 clinical isolates from four geographical areas; Senegal, Comoro islands, Brazil and Thailand.

Methods

The whole 4680 bp genomic sequence of lsa-3 was amplified by polymerase chain reaction and sequenced. The clinical isolate sequences were aligned on the sequence of the laboratory reference P. falciparum strain 3D7.

Results

The non-repeated sequence of lsa-3 was very well conserved with only a few allelic variations scattered along the sequence. Interestingly, a formerly identified immunodominant region, employed for the majority of pre-clinical vaccine development, was totally conserved at the genetic level. The most significant variations observed were in the number and organization of tetrapeptide repeated units, but not in their composition, resulting in different lengths of these repeated regions. The shorter repeated regions were from Brazilian origin. A correlation between the geographical distribution of the parasites with single nucleotide polymorphisms was not detected.

Conclusion

The lack of correlation between allelic polymorphisms with a specific transmission pressure suggests that LSA-3 is a structurally constrained molecule. The unusual characteristics of the lsa-3 gene make the molecule an interesting candidate for a subunit vaccine against malaria.

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.

Re-assessing the locations of components of the classical vesicle-mediated trafficking machinery in transfected Plasmodium falciparum

The malaria parasite, Plasmodium falciparum, exports proteins beyond the confines of its own plasma membrane, however there is debate regarding the machinery used for these trafficking events. We have generated transgenic parasites expressing chimeric proteins and used immunofluorescence studies to determine the locations of plasmodial homologues of the COPII component, Sar1p, and the Golgi-docking protein, Bet3p. The P. falciparum Sar1p (PfSar1p) chimeras bind to the endoplasmic reticulum surface and define a network of membranes wrapped around parasite nuclei. As the parasite matures, the endomembrane systems of individual merozoites remain interconnected until very late in schizogony. Antibodies raised against plasmodial Bet3p recognise two foci of reactivity in early parasite stages that increase in number as the parasite matures. Some of the P. falciparum Bet3p (PfBet3p) compartments are juxtaposed to compartments defined by the cis Golgi marker, PfGRASP, while others are distributed through the cytoplasm. The compartments defined by the trans Golgi marker, PfRab6, are separate, suggesting that the Golgi is dispersed. Bet3p-green fluorescent protein (GFP) is partly associated with punctate structures but a substantial population diffuses freely in the parasite cytoplasm. By contrast, yeast Bet3p is very tightly associated with immobile structures. This study challenges the view that the COPII complex and the Golgi apparatus are exported into the infected erythrocyte cytoplasm.

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.

A role for the Plasmodium falciparum RESA protein in resistance against heat shock demonstrated using gene disruption

During erythrocyte invasion, the Plasmodium falciparum Ring-infected erythrocyte surface antigen (RESA) establishes specific interactions with spectrin. Based on analysis of strains with a large chromosome 1 deletion, RESA has been assigned several functions, none of which is firmly established. Analysis of parasites with a disrupted resa1 gene and isogenic parental or resa3-disrupted controls confirmed the critical role of RESA in the surface reactivity of immune adult sera on glutaraldehyde-fixed ring stages. Absence of RESA did not influence merozoite invasion or erythrocyte membrane rigidity, was associated with a modest increase of cytoadhesion to CD36 under conditions of flow, but resulted in marked susceptibility to heat shock. resa1-KO-infected erythrocytes were prone to heat-induced vesiculation like uninfected erythrocytes, whereas parental or resa3-KO infected erythrocytes remained undamaged. Furthermore, a 6 h exposure of ring stages at 41 degrees C resulted in 33% culture inhibition of resa1-KO parasites while marginally impacting parental and resa3-KO parasite growth. This points to a role for RESA in protecting the infected erythrocyte cytoskeleton during febrile episodes. Infection patterns of resa1-KO and parental parasites in Saimiri sciureus indicated that RESA does not, at least on its own, modulate virulence in the squirrel monkey, as had been previously suggested.

Plasmodium falciparum merozoite surface protein 6 displays multiple targets for naturally occurring antibodies that mediate monocyte-dependent parasite killing

Plasmodium falciparum MSP6 is a merozoite surface antigen that shows organization and sequence homologies similar to those of MSP3. Within its C-terminus conserved region, it presents some epitopes that are cross-reactive with MSP3 and others that are not, both being targets of naturally occurring antibodies that block the P. falciparum erythrocytic cycle in cooperation with monocytes.

Malaria and the red blood cell membrane

Malaria is the most serious and widespread parasitic disease of humans and is arguably the commonest disease of red blood cells (RBCs). Malaria has exerted a powerful effect on human evolution and selection for resistance has led to the appearance and persistence of a number of inherited diseases. After parasite invasion, RBCs are progressively and dramatically modified. New structures appear inside the RBC and novel parasite proteins are exported to the erythrocyte cytoplasm and membrane skeleton. Radical biochemical, morphological, and rheological alterations manifest as increased membrane rigidity, reduced cell deformability, and greater adhesiveness for the vascular endothelium and other blood cells. Numerous protein-protein interactions between the malaria-parasite and the host RBC are important for many aspects of parasite biology and the pathogenesis of malaria. In addition, there are many other parasite proteins located within the infected red cell and at the membrane skeleton, for which no precise functional roles have yet been elucidated. Sequencing and annotation of the complete genome of Plasmodium falciparum, the production of proteomic and transcriptomic profiles of parasites, and the development of a transfection system for the asexual stage of the parasite are all recent achievements that should advance understanding of the molecular mechanisms that underlie the parasite-induced functional alterations in red cells.

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 malaria transmission-blocking vaccine candidate, y230.CA14 following crosslinking in the presence of tetanus toxoid

Proteolytically processed 310 kDa form of Plasmodium falciparum gamete surface antigen, Pfs230, is the target of malaria transmission-blocking monoclonal antibodies. To design a recombinant malaria transmission-blocking subunit vaccine, the amino terminus of the 310 kDa surface-exposed form of Pfs230 was mapped to amino acids (aa) 522 and 584 using a series of peptides and recombinant proteins encoding distinct regions of Pfs230. Antiserum generated against an Escherichia coli-produced recombinant protein, spanning the Pfs230 processing site and extending into the cysteine domains, r230/MBP.C (aa 443-1132), reduced parasite infectivity by 71.2-89.8%. To determine if the region spanning the cleavage site blocked malaria transmission when produced as a secreted protein by Saccharomyces cerevisiae, y230.CA14 (aa 467-584) was generated, purified, emulsified in adjuvant and used to vaccinate mice. In contrast to E. coli-produced r230/MBP.C, the immune response generated against y230. CA14 was very weak. To enhance the response, y230.CA14 was mixed with tetanus toxoid, chemically crosslinked, repurifed, and its immunogenicty compared with unconjugated y230.CA14. Conjugated-y230. CA14/TT required fewer booster injections to induce an immune response against Pfs230 and the antibodies generated reacted with the surface of intact gametes and immunoprecipitated radiolabelled Pfs230 extracted from 125I surface-labelled gametes to a greater extent. After seven injections, all y230.CA14 vaccinated mice developed anti-Pfs230 antibodies and the isotype profile was the same. In addition to enhancing the initial immune response generated against y230.CA14, conjugation focuses the immune response toward epitopes within the region of Pfs230 present on the surface of the gamete.

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.

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.

Plasmodium falciparum asparagine and aspartate rich protein 2 is an evolutionary conserved protein whose repeats identify a new family of parasite antigens

We describe here a new Plasmodium falciparum antigen, asparagine and aspartate rich protein 2 (PfAARP2) of 150 kDa, which is encoded by a unique gene on chromosome 1. PfAARP2 is first expressed 12 h post-invasion and accumulates in trophozoites and schizonts. Immunofluorescence studies indicate that PfAARP2 is translocated into the red blood cell cytoplasm. The central region of Pfaarp2 contains blocks of repetitions encoding asparagine and aspartate residues, which define a new family of related genes dispersed on different chromosomes, and two members of this family have also been identified. Interestingly, the non-repeated N- and C-termini of PfAARP2 display significant similarity to two yeast and human predicted proteins, and its possible function is discussed.

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.

A putative Plasmodium falciparum exported serine/threonine protein kinase

An 8kb gene coding for a putative serine/threonine protein kinase from Plasmodium falciparum has been cloned and sequenced. It is arranged in two exons: exon I is 2 kb and exon II is 5.6 kb. The gene codes for a large protein of 2510 amino acids. Antibodies raised against a fusion protein were used to localize the putative kinase. By immunofluorescence microscopy, it was found in the cytoplasm of infected red cells. By immunoelectron microscopy it was associated with membranous structures in the red cell and with the red cell membrane, particularly at parasite-induced knobs. This is the first putative protein kinase of P. falciparum to be exported from the parasite into its host cell.

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.

Re-investigation of the circumsporozoite protein-based induction of sterile immunity against Plasmodium berghei infection

Although the circumsporozoite protein (CSP) of the malaria parasite is the most immunologically characterized protein, the goal of using this protein in an effective vaccine has not yet been realized. Monoclonal antibody against the repetitive immunodominant B-epitope of the CSP can protect mice from malaria, but vaccines that induce antibody against this epitope do not consistently induce protection. Toward developing a rationale for a CSP-based effective vaccine, we have re-investigated the ability of anti-CSP repeat antibodies, as induced by different CSP vaccine formulations with several adjuvants, to confer sterile immunity against sporozoite challenge. Using Plasmodium berghei rodent malaria model and several CSP subunit vaccine constructs, we found that a formulation consisting of the P. berghei CSP repetitive epitope, (DPPPPNPN)2 (CS), conjugated to BSA by carbodiimide, formulated in a block copolymer and detoxified lipopolysaccharide (RaLPS) adjuvant, was particularly promising. Mice were immunized and boosted with vaccines that contain varying malarial peptide-carrier ratios of 6:1 (CS6-BSA), 55:1 (CS55-BSA) and 170:1 (CS170-BSA). Following immunization, the animals were challenged with live sporozoites. Two types of effects were observed in vaccinated mice. First, sterile immunity was induced in 100%, 50% and 29% of mice that were immunized with the CS170-BSA, CS55-BSA, and CS6-BSA vaccine conjugates, respectively. The second effect of immunization was observed with the CS170-BSA conjugate vaccine primed mice; a boost in IFA titers followed sporozoite challenge. In addition, we observed that IgG1 isotype titer against the surface of the sporozoite, as measured by IFA, and antibody avidity parallel sterile immunity. These findings reiterate the potential of the CSP as a malaria vaccine candidate antigen, and suggest that the induction of sterile immune responses depends on inducing antibody of the appropriate isotype, avidity and specificity.

Mapping of two overlapping linear epitopes in Pfg27 recognized by Plasmodium falciparum transmission-blocking monoclonal antibodies

We have reported previously the production of Plasmodium falciparum transmission-blocking monoclonal antibodies (mAb) recognizing a reduction-insensitive cross-reacting epitope in the gametocyte antigen Pfg27 and the gamete surface antigens Pfs230 and Pfs48/45. In this study, the amino acid sequence of this epitope in Pfg27 was determined. First, the epitope was localized near the N terminus of the protein by probing recombinant overlapping fragments spanning Pfg27 with transmission-blocking mAb in immunoblot experiments. The amino acid sequence of the epitope was then determined by using overlapping synthetic peptides spanning the smallest immunoreactive recombinant fragment in an ELISA. The sequence KPLDKFGNIYDYHYEH (amino acids 10-25 in the Pfg27 sequence) was shown to contain two overlapping epitopes recognized by transmission-blocking mAb. Comparison of the sequence of the gene encoding Pfg27 in seven different P. falciparum strains demonstrated that these sequential epitopes are totally conserved. Immunization of mice with synthetic peptides derived from Pfg27, conjugated with keyhole limpet hemocyanin (KLH) and formulated in Freund's adjuvant or alum, resulted in the production of antibodies capable of recognizing the peptides as well as the native Pfg27.

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.

Towards a vaccine against asexual blood stage infection by Plasmodium falciparum

In this paper, we will summarize the progress obtained in the malaria vaccine project developed by the Institut Pasteur groups interacting through the International Network of Pasteur Institutes over the last fifteen years. While trying to follow the progress in scientific and technological concepts and methodologies, the basic approach was still essentially the same as that followed by Pasteur and his acolytes to try to artificially reproduce the natural processes that lead to the development of immunity to infection and disease. A longitudinal study of two villages from the Sine Saloum area of Senegal, Dielmo and N'Diop, conducted in recent years by teams of the Institut Pasteur of Dakar, Senegal, in collaboration with the local ORSTOM malaria unit has led to the detailed analysis of the natural acquisition of premunition against Plasmodium falciparum malaria in endemic areas. The Saimiri model developed at the Pasteur Institute in Cayenne, was an important step forward in terms of studies on the mechanisms of action of protective antibodies and on vaccinations assays. If we accept the conclusions of the Pasteur groups' research on the experimental primate model and on the development of natural immunity (premunition) in highly endemic areas, the main inhibitor of progress in vaccine development is our poor understanding of the regulation of the immune response. Therefore, the general approaches that were followed for vaccine development must now be further explored using the continually developing tools of immunology and molecular biology, to elucidate regulations of the immune responses to the parasite, and identify the molecular mechanisms used by the parasite to generate and change antigen specificities.

Human immune recognition of recombinant proteins representing discrete domains of the Plasmodium falciparum gamete surface protein, Pfs230

The 230 kD gametocyte/gamete-specific surface protein of Plasmodium falciparum, Pfs230, is a target of antibodies which inhibit the development of the parasite inside the mosquito vector. A transmission blocking vaccine based on Pfs230 may be a powerful tool for malaria control. As a first step, Pfs230 has been expressed in E. coli as a series of recombinant proteins, fused to maltose binding protein. We have used the fusion proteins to assess cellular and humoral immune responses to Pfs230 in malaria-immune adult Gambian blood donors; responses to the fusion proteins have been compared with responses to native Pfs230. The tetrapeptide repeat region of the molecule appears to be immunodominant for both antibody-producing cells and peripheral blood T cells. We postulate that this may represent a mechanism for immune evasion since the N-terminal repeat region of the molecule is cleaved from the mature protein and shed into the plasma. Responses to fusion proteins representing the seven-cysteine motifs were correlated within individual donors, suggesting that cross-reactive epitopes occur within the motifs. Antibody responses to recombinant proteins were poorly correlated with responses to native Pfs230 suggesting that dominant epitopes of the native protein are not adequately represented in the recombinant proteins. Although prokaryotic expression products may be suitable for induction of cellular immune responses to Pfs230, alternative expression systems may be needed for creation of appropriate B cell epitopes.

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.

Immunogenicity and antigenicity of a Plasmodium falciparum protein fraction (90-110 kDa) able to protect squirrel monkeys against asexual blood stages

A monkey vaccination trial using a Plasmodium falciparum protein fraction containing antigens of 90-110 kDa is reported. The fraction was obtained by electroelution from preparative polyacrylamide gels. Three monkeys out of five resisted a heavy challenge dose of highly virulent parasites. Using specific antisera, several components of the fraction were identified, namely heat shock protein 90 (hsp90), Ag44/RhopH3, ABRA, 96tR/GBP130 and Pf96 protease. The fraction did not contain KAHRP, nor the SERP antigen. The antibody response of the monkeys was studied on these individual antigens purified by preparative immunoprecipitation. Surprisingly, hsp90 was found in the immunoprecipitates obtained with SERP antisera. Interestingly, the response to hsp90 correlated with protection, high antibody titres being found only in the protected monkeys. In contrast, no correlation with protection could be found for the response to the other antigens.

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)

The virulent Saimiri-adapted Palo Alto strain of Plasmodium falciparum does not express the ring-infected erythrocyte surface antigen

The Palo Alto strain of Plasmodium falciparum is highly virulent for the Saimiri sciureus monkey. We have observed that these parasites do not express the Ring-infected erythrocyte surface antigene (RESA) gene. Immunoblots indicated that the Pf155/RESA protein was absent. The RESA mRNA could not be detected. Polymerase chain reaction and Southern blot analysis demonstrated that this lack of expression is due to gene rearrangements. The majority of the Palo Alto parasites have a deletion of the entire RESA gene, whereas in a minor fraction the RESA sequences remain detectable, but the 5' miniexon 1 is inverted. These data show that the RESA protein is dispensable for in vivo parasite growth, at least in Saimiri monkeys.

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.

The red cell membrane and invasion by malarial parasites

The red cell membrane with its bilipid layer, integral membrane proteins (especially the GPs and band 3), and the red cell skeleton pose a formidable barrier for the malarial parasite to overcome during invasion. Invasion is an ordered and sequential process, indicating a highly complex and specific process involving numerous molecular interactions. For P. vivax and P. knowlesi infections the Duffy glycoprotein seems to be a specific requirement in invasion. For P. falciparum the GPs, and especially the N-acetyl neuraminic acid linked in an alpha 2-3 configuration on them, appear to act as specific ligands although some strains of P. falciparum may use alternate ligands for invasion. The parasite enters the red cells within an invagination continuous with the red cell bilipid layer, the parasitophorous vacuole membrane, and recent evidence would indicate that this membrane is largely of parasite origin. The numerous occasions in which the red cell needs to deform during invasion indicates that membrane deformability could be an important factor in determining invasion, but the dissociation of invasion and deformability as induced by a number of reagents would not support this contention. Instead it is suggested that reagents which modify invasion may be acting via alterations in red cell or parasite protein phosphorylation or dephosphorylation.

In vitro phagocytosis inhibition assay for the screening of potential candidate antigens for sub-unit vaccines against the asexual blood stage of Plasmodium falciparum

We have previously established a direct correlation between immune protection against the asexual blood stage Plasmodium falciparum infection and the presence of opsonizing antibodies promoting phagocytosis of parasitized red blood cells. In the present communication we describe an in vitro assay for measuring phagocytosis inhibition (PIA) specific for P. falciparum-infected erythrocytes. The phagocytosis inhibition assay is a simple procedure for screening potential candidates for sub-unit vaccines against P. falciparum based on the correlation between opsonizing antibodies and immunoprotection. The assay was used to analyse 18 recombinant molecules, corresponding to 11 distinct antigens of P. falciparum. Pre-incubation and selective antibody depletion experiments demonstrate the antigen-antibody specificity of the PIA. The presence of epitopes participating as targets of opsonic antibodies were demonstrated in six distinct polypeptide antigens.

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.

Plasmodium falciparum: Characterization of gene R45 encoding a trophozoite antigen containing a central block of six amino acid repeats

We describe here an antigen, called R45, expressed by the young trophozoites of Plasmodium falciparum. This antigen contains a block of tandem repeats of six amino acids which are recognized by sera from humans living in endemic areas. The R45 gene is located on chromosome 3. It is present in all strains examined and shows limited size polymorphism. The C-terminal unique region of the protein shows a strong homology with the catalytic domain of the serine protein kinases. Interestingly, the central repeats contain a large number of putative phosphorylation sites. The implications of these features are discussed.

Crossreactive antigens between life cycle stages ofplasmodium faiciparum

The high mortality and morbidity induced by falciparum malaria has motivated research to find an efficient antimalarial vaccine. The parasite has a complex life cycle, both in the mosquito and human hosts, and presents a number of potential targets for vaccine-induced immune attack. Here, Inge Moelans and John Schoenmakers discuss how the search for protective antigens has been complicated by the discovery of multiple crossreactivities between different parasite proteins.

Transcription of the gene for the merozoite surface antigen MSA2 of the human malaria parasite Plasmodium falciparum during the asexual cycle

The transcription of the Plasmodium falciparum gene for the MSA2 antigen has been studied throughout the parasite's asexual growth cycle. For this purpose poly(A)+ RNA from different times of the cycle was tested for the presence of the mRNA encoding MSA2 by in vitro translation and subsequent analysis of the translation products by immunoprecipitation with an antibody against MSA2. The results revealed that this mRNA is present in trophozoites, reaches the highest concentration during the transition from the trophozoite into the schizont stage, and persists until the cycle end. Minute amounts of this mRNA were also detected in rings. In addition, the data confirmed that the primary translation product is not proteolytically processed at any time of the cycle.

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.

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.

The major cysteine proteinase (cruzipain) from Trypanosoma cruzi is antigenic in human infections

Antibodies against the major cysteine proteinase (cruzipain) from Trypanosoma cruzi were detected in human sera obtained from patients with chronic Chagas' disease. Not only the intact 60-kDa enzyme but also its 25-kDa self-proteolysis fragment are antigenic in vivo. Although T. cruzi antigens 13 and 36 also reacted with the apparently monospecific antiproteinase serum, the antigenicity of cruzipain to human patients is genuine, since its reactivity was not modified by the adsorption of human sera with the recombinant proteins 13 and 36.

A study of the genomic diversity of Plasmodium falciparum in Senegal. 1. Typing by Southern blot analysis

The genomic polymorphism of Plasmodium falciparum was investigated in a series of samples collected in Senegal during one transmission season. Restriction site polymorphism was studied by Southern blot analysis using six different probes. The patterns of the ribosomal RNA genes and of the gene coding for antigen 2L indicated a limited genomic polymorphism. Sequences hybridizing to the repeats of the Palo Alto/Wellcome serotype of S-antigen were found in one out of twelve isolates examined. This strain was shown to express the Palo Alto serotype. Restriction fragment length polymorphism was observed for the 332 gene and the 11.1 locus. The hybridization patterns showed that each sample had a distinct 11.1 locus. A comparison of three probes (332, 11.1 and rep20) detecting fragment length polymorphism indicated that maximum sensitivity was obtained using the subtelomeric repeats rep20; less sensitive patterns were observed using the 11.1 27 bp repeat probe. By using these three probes it was found that all samples were genetically distinct.