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

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

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

Malaria parasite development in mosquitoes

Mosquitoes of the genus Anopheles transmit malaria parasites to humans. Anopheles mosquito species vary in their vector potential because of environmental conditions and factors affecting their abundance, blood-feeding behavior, survival, and ability to support malaria parasite development. In the complex life cycle of the parasite in female mosquitoes, a process termed sporogony, mosquitoes acquire gametocyte-stage parasites from blood-feeding on an infected host. The parasites carry out fertilization in the midgut, transform to ookinetes, then oocysts, which produce sporozoites. Sporozoites invade the salivary glands and are transmitted when the mosquito feeds on another host. Most individual mosquitoes that ingest gametocytes do not support development to the sporozoite stage. Bottle-necks occur at every stage of the cycle in the mosquito. Powerful new techniques and approaches exist for evaluating malaria parasite development and for identifying mechanisms regulating malaria parasite-vector interactions. This review focuses on those interactions that are important for the development of new approaches for evaluating and blocking transmission in nature.

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.

Seasonal variation of anti-Plasmodium falciparum antibodies directed against a repetitive peptide of gametocyte antigen pfs2400 in inhabitants in the State of Amapá, Brazil

Antibodies to the Pfs2400 gametocyte antigen have been shown to inhibit the development of Plasmodium falciparum in anophelines and therefore this antigen is a candidate for a transmission-blocking vaccine. To test seasonal variation of these antibodies under field conditions, sera from 72 individuals were collected twice, first during the long-rains season with low malaria transmission and then, 6 months later, during the short-rains season, when transmission is high. This study was conducted in several localities in the State of Amapá, Brazil. All but three individuals had a positive indirect fluorescent antibody test (IFAT) with asexual forms of P. falciparum. Most of them did not report malaria attacks during the period between the first and second sampling. Their sera were tested by IFAT with P. falciparum gametocytes. The overall positivity of this test did not vary between seasons, and was 47.2 (34/72) and 48.6% (35/72), respectively. The sera were also tested by ELISA with the Pfs2400 repeat peptide. The positivity rate dropped from 29.2 (21/72) to 15.3% (11/72) and the mean absorbancies from 0.623 to 0.354, when we compared the results of the long-rains and short-rains seasons. Fifteen out of the 21 ELISA positive sera turned negative, with no change of geometric mean of titres (GMT) of asexual IFAT, while five negatives became ELISA positive on second sampling, with increase of GMT. Soon after the second sampling a malaria outbreak was reported in one of the localities. These results point toward a relatively short persistence of anti-Pfs2400 repeat peptide antibodies, under natural field conditions. A gametocyte antigen booster before a high transmission period might contribute towards lowering malaria incidence by eliciting a partially effective antibody response.

The biology of malarial parasite in the mosquito--a review

The purpose of this review is to summarize the biology of Plasmodium in the mosquito including recent data to contribute to better understanding of the developmental interaction between mosquito and malarial parasite. The entire sporogonic cycle is discussed taking into consideration different parasite/vector interactions and factors affecting parasite development to the mosquito.

Detection of anti-Plasmodium falciparum antibodies directed against a repetitive peptide of the gametocyte antigen Pfs2400 in malaria patients in Brazil

Sera collected from 164 individuals who had clinical Plasmodium falciparum malaria and came from several areas of Brazil where malaria is endemic were tested for the presence of anti-gametocyte antibodies. Antibodies directed against P. falciparum gametocytes were detected, by IFAT, in the sera of 67.1% of these patients. The prevalence of these antibodies was significantly higher in patients who had undergone multiple attacks of malaria than in those who were experiencing their first attack at the time of serum collection. Although circulating gametocytes were detected in 22% of the patients at this time, there was no difference in the percentages of IFAT positivity between apparent gametocyte 'carriers' and 'non-carriers'. All sera were also tested by ELISA, using a dimer of the nonamer peptide [PEE(L/V)VEEV(I/V)]2, which represents a tandem consensus repeat of the P. falciparum gametocyte antigen, Pfs2400, a target of transmission-blocking antibodies. ELISA demonstrated that 32.9% of the patients had antibodies that reacted with this peptide. Positive ELISA reactions were significantly more frequent amongst the sera of patients who had had multiple malaria attacks than in those undergoing their first malaria episode; positivity was lower in the gametocyte 'carriers' than in their 'non-carriers'. These results demonstrate that anti-gametocyte antibodies, which have already been shown to have potential transmission-blocking activity, are naturally elicited in Brazilian patients, the highest rates of seropositivity occurring after multiple malaria attacks.

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)

Cloning and characterization of a novel Plasmodium falciparum sporozoite surface antigen, STARP

A novel Plasmodium falciparum sporozoite antigen, STARP (Sporozoite Threonine and Asparagine-Rich Protein), detected consistently on the surface of sporozoites obtained from laboratory strains and field isolates, has been identified and cloned, following a systematic approach aimed at isolating novel non-CS sporozoite surface antigens. The 2.0-kb STARP gene has a 5' miniexon/large central exon structure and contains a complex repetitive region encoding multiple dispersed motifs and tandem 45- and 10-amino acid repeats. In sporozoites, transcription of the STARP gene has been conclusively demonstrated by reverse PCR and Northern blot hybridisation and the 78-kDa protein has been localized by immunofluorescence and immunoelectron microscopy to the sporozoite surface. STARP is also expressed in liver stages, as revealed by immunofluorescence assays using antisera raised either to the central repetitive region or the C-terminal non-repetitive region. Expression is also detected in early ring stages, though not in mature erythrocytic or sexual stages. Identification and elucidation of this novel antigen is a step forward in current efforts aimed at developing an effective preerythrocytic-stage malaria vaccine.