An engineered Plasmodium falciparum C-terminal 19-kilodalton merozoite surface protein 1 vaccine candidate induces high levels of interferon-gamma production associated with cellular immune responses to specific peptide sequences in Gambian adults naturally exposed to malaria

The 19-kDa C-terminal region of merozoite surface protein 1 (MSP1(19)), a major blood stage malaria vaccine candidate, is the target of cellular and humoral immune responses in humans naturally infected with Plasmodium falciparum. We have previously described engineered variants of this protein, designed to be better vaccine candidates, but the human immune response to these proteins has not been characterized fully. Here we have investigated the antigenicity of one such variant compared to wild-type MSP1(19)-derived protein and peptides. Gambian adults produced both high T helper type 1 (Th1) [interferon (IFN)-γ] and Th0/Th2 [interleukin (IL)-13 and sCD30] responses to the wild-type MSP1(19) and the modified protein as wells as to peptides derived from both forms. Response to the modified MSP1(19) (with three amino acid substitutions: Glu27Tyr, Leu31Arg and Glu43Leu) relative to the wild-type, included higher IFN-γ production. Interestingly, some peptides evoked different patterns of cytokine responses. Modified peptides induced higher IL-13 production than the wild-type, while the conserved peptides P16 and P19 induced the highest IFN-γ and IL-13 and/or sCD30 release, respectively. We identified P16 as the immunodominant peptide that was recognized by cells from 63% of the study population, and not restricted to any particular human leucocyte antigen D-related (HLA-DR) type. These findings provide new and very useful information for future vaccine development and formulation as well as potential Th1/Th2 immunmodulation using either wild-type or modified protein in combination with their peptides.

Total immunoglobulin G and IgG1 subclass levels specific for the MSP-1(19) of Plasmodium falciparum are different in individuals with either processing-inhibitory, blocking or neutral antibodies

BACKGROUND

Some MSP-1(19) specific antibodies that inhibit merozoite invasion also inhibit the secondary processing of MSP-1. However the binding of these inhibitory antibodies can be blocked by another group of antibodies, called blocking antibodies, which recognize adjacent or overlapping epitopes, but themselves have no effect on either MSP-1 processing or merozoite invasion. These antibodies have been reported to be present in individuals living in a malaria endemic area.

METHODS

Blood samples were obtained from children shown to have processing inhibitory, blocking, and neutral antibodies in a previous study. Enzyme linked immunosorbent assay (ELISA), was used to determine the total IgG, IgM and IgG subtypes.

RESULTS

There was a significant difference in anti-MSP-1(19) IgG, while there was no significant difference in the anti-MSP-1(19) IgM. Only anti MSP-1(19) IgG1, amongst the IgG subtypes was significantly different between the groups.

CONCLUSION

This study shows that antibodies against MSP-1 are different not only in specificity and function but also in the amount of total IgG and IgG subtype produced.

Signalling in malaria parasites. The MALSIG consortium

Depending on their developmental stage in the life cycle, malaria parasites develop within or outside host cells, and in extremely diverse contexts such as the vertebrate liver and blood circulation, or the insect midgut and hemocoel. Cellular and molecular mechanisms enabling the parasite to sense and respond to the intra- and the extra-cellular environments are therefore key elements for the proliferation and transmission of Plasmodium, and therefore are, from a public health perspective, strategic targets in the fight against this deadly disease. The MALSIG consortium, which was initiated in February 2009, was designed with the primary objective to integrate research ongoing in Europe and India on i) the properties of Plasmodium signalling molecules, and ii) developmental processes occurring at various points of the parasite life cycle. On one hand, functional studies of individual genes and their products in Plasmodium falciparum (and in the technically more manageable rodent model Plasmodium berghei) are providing information on parasite protein kinases and phosphatases, and of the molecules governing cyclic nucleotide metabolism and calcium signalling. On the other hand, cellular and molecular studies are elucidating key steps of parasite development such as merozoite invasion and egress in blood and liver parasite stages, control of DNA replication in asexual and sexual development, membrane dynamics and trafficking, production of gametocytes in the vertebrate host and further parasite development in the mosquito. This article, which synthetically reviews such signalling molecules and cellular processes, aims to provide a glimpse of the global frame in which the activities of the MALSIG consortium will develop over the next three years.

Plasmodium falciparum infection of the placenta impacts on the T helper type 1 (Th1)/Th2 balance of neonatal T cells through CD4(+)CD25(+) forkhead box P3(+) regulatory T cells and interleukin-10

Placental malaria infection affects the T helper type 1 (Th1)/Th2 balance in neonatal children. We investigated a potential role of regulatory T cells in this balance by comparing T cell responses of cord blood mononuclear cells (CBMC) from parasitized and non-parasitized placenta of Gambian women. CBMC were depleted of CD4(+)CD25(+) forkhead box P3 (FoxP3)(+) regulatory T cells and analysed in vitro for their ability to produce interferon (IFN)-gamma, sCD30 and interleukin (IL)-10 in response to phytohaemagglutinin (PHA), live Plasmodium falciparum, schizont extracts and the recombinant P. falciparum blood stage antigen merozoite surface protein 1 (MSP1(19)). As expected, lower IFN-gamma and higher sCD30 responses were observed for the cells from the parasitized group. In addition, higher IL-10 levels were produced by CBMC from the parasitized group. Depletion of regulatory T cells decreased IL-10 production, which resulted in a restoration of IFN-gamma expression in response to all stimuli. The Th2 marker sCD30 remained significantly higher in the parasitized group in response to malaria protein antigens while similar levels were recovered between both groups in response to live P. falciparum. Similar effects were observed by adding an antibody that blocks IL-10 function. These results suggest that the impact of P. falciparum infection on Th1 differentiation of neonatal T cells can be ascribed to regulatory T cells through production of IL-10.

Antibody specificities of children living in a malaria endemic area to inhibitory and blocking epitopes on MSP-1 19 of Plasmodium falciparum

Merozoite surface protein-1(19) (MSP-1(19)) specific antibodies which include processing inhibitory, blocking and neutral antibodies have been identified in individuals exposed to Plasmodium falciparum. Here we intend to look at the effect of single and multiple amino acid substitutions of MSP-1(19) on the recognition by polyclonal antibodies from children living in Igbo-Ora, Nigeria. This would provide us with information on the possibility of eliciting mainly processing inhibitory antibodies with a recombinant MSP-1(19) vaccine. Blood was collected from children in the rainy season and binding of anti-MSP-1(19) antibodies to modified mutants of MSP-1(19) was analysed by ELISA. The MSP-1(19) mutant proteins with single substitutions at positions 22 (Leu-->Arg), 43 (Glu-->Leu) and 53 (Asn-->Arg) and the MSP-1(19) mutant protein with multiple substitutions at positions 27+31+34+43 (Glu-->Tyr, Leu-->Arg, Tyr-->Ser, Glu-->Leu); which had inhibitory epitopes; had the highest recognition. Children recognised both sets of mutants with different age groups having different recognition levels. The percentage of malaria positive individuals (32-80%) with antibodies that bound to the mutants MSP-1(19) containing epitopes that recognise only processing inhibitory and not blocking antibodies, were significantly different from those with antibodies that did not bind to these mutants (21-28%). The amino acid substitutions that abolished the binding of blocking antibodies without affecting the binding of inhibitory antibodies are of particular interest in the design of MSP-1(19) based malaria vaccines. Although these MSP-1(19) mutants have not been found in natural population, their recognition by polyclonal antibodies from humans naturally infected with malaria is very promising for the future use of MSP-1(19) mutants in the design of a malaria vaccine.

Variation in the relationship between anti-MSP-1(19) antibody response and age in children infected with Plasmodium falciparum during the dry and rainy seasons

Malaria remains a major parasitic disease in Africa, with 300-500 million new infections each year. There is therefore an urgent need for the development of new effective measures, including vaccines. Plasmodium falciparum merozoite surface protein-1(19) (MSP-1(19)) is a prime candidate for a blood-stage malaria vaccine. Blood samples were collected from children aged 10 days to 15 years in the months of January-March (N = 351) and October-November (N = 369) corresponding to the dry and rainy seasons, respectively. P. falciparum infection was determined by microscopy and enzyme linked immunosorbent assay (ELISA) was used to determine the total IgG and IgG subclasses. There was a significant increase in the mean anti-MSP-1(19) antibody titre in the dry season (p < 0.05), compared to the rainy season. A significantly positive correlation between the anti-MSP-1(19) antibody titre and parasite density (p < 0.01, r = 0.138) was observed. In the rainy season, unlike in the dry season, P. falciparum positive children had higher anti-MSP-1(19) antibody titres than P. falciparum negative children and this difference was significant (p < 0.05). When all individuals were grouped together, the anti-MSP-1(19) antibody titre increased with age in both seasons (r = 0.186 and 0.002), this increase was more apparent in the dry season. However, when the study population was divided into P. falciparum positive and negative groups, it was observed that in the rainy season, there was a negative correlation between anti-MSP-1(19) titre and age in P. falciparum positive individuals, while those who were P. falciparum negative had a positive correlation between anti-MSP-1(19) titre and age. Analysis of anti-MSP-1(19) IgG subclass showed that IgG1 and IgG3 mean titres were highest in both the dry and rainy seasons with an increase in the mean antibody titres for IgG1, IgG2 and IgG3 in the rainy season. In the dry season there was a positive correlation between IgG1, IgG2, and IgG3 titres with age, while IgG4 was negative, whereas in the rainy season there was a positive correlation between IgG2 and IgG4 (non-cytophilic antibodies) with age and a negative correlation for IgG1 and IgG3 (cytophilic antibodies) with age. Seasonal differences in the level of MSP-1(19) IgG subclass titres were observed for P. falciparum negative and positive individuals. Only samples, which were positive for IgG2 and IgG4, showed positive correlation between parasitemia and total IgG. The incidence of P. falciparum infection, which increases during the rainy season, might be an important determinant of anti-MSP-1(19) antibody levels in children living in Igbo-Ora and the results point to the fact that non-cytophilic antibodies to MSP-1(19) in children might be associated with an increase in total IgG and parasitemia.

What is the function of MSP-I on the malaria merozoite?

In the absence o f any clear enzymatic activity, attempts to define the role of merozoite surface protein-I have focused mainly on analysis of its structure, on its interaction with the immune system and on binding assays. But how does our knowledge of the structure o f this protein contribute to functional studies? Are there data to suggest a role in the evasion of effective host immune responses? Binding studies have used the intact protein or various fragments and peptides, but do such approaches provide a reliable indicator of function? In this article, Tony Holder and Mike Blackman review these areas.

Merozoite surface protein 4/5 provides protection against lethal challenge with a heterologous malaria parasite strain

Immunization with merozoite surface protein 4/5 (MSP4/5), the murine malaria homologue of Plasmodium falciparum MSP4 and MSP5, has been shown to protect mice against challenge by parasites expressing the homologous form of the protein. The gene encoding MSP4/5 was sequenced from a number of Plasmodium yoelii isolates in order to assess the level of polymorphism in the protein. The gene was found to be highly conserved among the 13 P. yoelii isolates sequenced, even though many of the same isolates showed pronounced variability in their MSP1(19) sequences. Nonsynonymous mutations were detected only for the isolates Plasmodium yoelii nigeriensis N67 and Plasmodium yoelii killicki 193L and 194ZZ. Immunization and challenge of BALB/c mice showed that the heterologous MSP4/5 proteins were able to confer a level of protection against lethal Plasmodium yoelii yoelii YM challenge infection similar to that induced by immunization with the homologous MSP4/5 protein. To explore the limits of heterologous protection, mice were immunized with recombinant MSP4/5 protein from Plasmodium berghei ANKA and Plasmodium chabaudi adami DS and challenged with P. y. yoelii YM. Interestingly, significant protection was afforded by P. berghei ANKA MSP4/5, which shows 81% sequence identity with P. y. yoelii YM MSP4/5, but it was abolished upon reduction and alkylation. Significant protection was not observed for mice immunized with recombinant P. c. adami DS MSP4/5, which shows 55.7% sequence identity with P. y. yoelii YM MSP4/5. This study demonstrates the robustness of MSP4/5 in conferring protection against variant forms of the protein in a murine challenge system, in contrast to the situation found for other asexual-stage proteins, such as MSP1(19) and AMA1.

Disulfide bonds in merozoite surface protein 1 of the malaria parasite impede efficient antigen processing and affect the in vivo antibody response

Vol. 34(3) 2004, DOI 10.1002/eji.200324514 Due to a technical error, the wrong affiliations were given for C. Moss and V. Lindo. These are correct as given above. See original article http://dx.doi.org/10.1002/eji.200324514.

Stage-specific transcription of distinct repertoires of a multigene family during Plasmodium life cycle

Members of a multigene family in the rodent malaria parasite Plasmodium yoelii yoelii code for 235-kilodalton proteins (Py235) that are located in the merozoite apical complex, are implicated in virulence, and may determine red blood cell specificity. We show that distinct subsets of py235 genes are expressed in sporozoites and hepatic and erythrocytic stages. Antibodies to Py235 inhibited sporozoite invasion of hepatocytes. The switch in expression profile occurred immediately after transition from one stage to another. The results suggest that this differential expression is driven by strong biological requirements and provide evidence that hepatic and erythrocytic merozoites differ.

The high molecular mass rhoptry protein, RhopH1, is encoded by members of the clag multigene family in Plasmodium falciparum and Plasmodium yoelii

Malarial merozoite rhoptries contain a high molecular mass protein complex called RhopH. RhopH is composed of three polypeptides, RhopH1, RhopH2, and RhopH3, encoded by distinct genes. Using monoclonal antibody-purified protein complex from both Plasmodium falciparum and Plasmodium yoelii, peptides were obtained by digestion of RhopH1 and their sequence determined either by mass spectrometry or Edman degradation. In both species the genes encoding RhopH1 were identified as members of the cytoadherence linked asexual gene (clag) family. In P. falciparum the family members on chromosome 3 were identified as encoding RhopH1. In P. yoelii two related genes were identified and sequenced. One of the genes, pyrhoph1a, was positively identified as encoding RhopH1 by the peptide analysis and the other gene, pyrhoph1a-p, was at least transcribed. Genes in the clag family present in both parasite species have a number of conserved features. The size and location of the P. yoelii protein complex in the rhoptries was confirmed. The first clag gene identified on chromosome 9 was implicated in cytoadherence, the binding of infected erythrocytes to host endothelial cells; this study shows that other members of the family encode merozoite rhoptry proteins, proteins that may be involved in merozoite-erythrocyte interactions. We propose that the family should be renamed as rhoph1/clag.

The 22 kDa component of the protein complex on the surface of Plasmodium falciparum merozoites is derived from a larger precursor, merozoite surface protein 7

The gene coding for merozoite surface protein 7 has been identified and sequenced in three lines of Plasmodium falciparum. The gene encodes a 351 amino acid polypeptide that is the precursor of a 22-kDa protein (MSP7(22)) on the merozoite surface and non-covalently associated with merozoite surface protein 1 (MSP1) complex shed from the surface at erythrocyte invasion. A second 19-kDa component of the complex (MSP7(19)) was shown to be derived from MSP7(22) and the complete primary structure of this polypeptide was confirmed by mass spectrometry. The protein sequence contains several predicted helical and two beta elements, but has no similarity with sequences outside the Plasmodium databases. Four sites of sequence variation were identified in MSP7, all within the MSP7(22) region. The MSP7 gene is expressed in mature schizonts, at the same time as other merozoite surface protein genes. It is proposed that MSP7(22) is the result of cleavage by a protease that may also cleave MSP1 and MSP6. A related gene was identified and cloned from the rodent malaria parasite, Plasmodium yoelii YM; at the amino acid level this sequence was 23% identical and 50% similar to that of P. falciparum MSP7.

Plasmodium falciparum homologue of the genes for Plasmodium vivax and Plasmodium yoelii adhesive proteins, which is transcribed but not translated

The 235-kDa family of rhoptry proteins in Plasmodium yoelii and the two reticulocyte binding proteins of P. vivax comprise a family of proteins involved in host cell selection and erythrocyte invasion. Here we described a member of the gene family found in P. falciparum (PfRH3) that is transcribed in its entirety, under stage-specific control, with correct splicing of the intron, but appears not to be translated, probably due to two reading frameshifts at the 5' end of the gene.

Inhibitory and blocking monoclonal antibody epitopes on merozoite surface protein 1 of the malaria parasite Plasmodium falciparum

Merozoite surface protein 1 (MSP-1) is a precursor to major antigens on the surface of Plasmodium spp. merozoites, which are involved in erythrocyte binding and invasion. MSP-1 is initially processed into smaller fragments; and at the time of erythrocyte invasion one of these of 42 kDa (MSP-1(42)) is subjected to a second processing, producing 33 kDa and 19 kDa fragments (MSP-1(33) and MSP-1(19)). Certain MSP-1-specific monoclonal antibodies (mAbs) react with conformational epitopes contained within the two epidermal growth factor domains that comprise MSP-1(19), and are classified as either inhibitory (inhibit processing of MSP-1(42) and erythrocyte invasion), blocking (block the binding and function of the inhibitory mAb), or neutral (neither inhibitory nor blocking). We have mapped the epitopes for inhibitory mAbs 12.8 and 12.10, and blocking mAbs such as 1E1 and 7.5 by using site-directed mutagenesis to change specific amino acid residues in MSP-1(19) and abolish antibody binding, and by using PEPSCAN to measure the reaction of the antibodies with every octapeptide within MSP-1(42). Twenty-six individual amino acid residue changes were made and the effect of each on the binding of mAbs was assessed by Western blotting and BIAcore analysis. Individual changes had either no effect, or reduced, or completely abolished the binding of individual mAbs. No two antibodies had an identical pattern of reactivity with the modified proteins. Using PEPSCAN each mAb reacted with a number of octapeptides, most of which were derived from within the first epidermal growth factor domain, although 1E1 also reacted with peptides spanning the processing site. When the single amino acid changes and the reactive peptides were mapped onto the three-dimensional structure of MSP-1(19), it was apparent that the epitopes for the mAbs could be defined more fully by using a combination of both mutagenesis and PEPSCAN than by either method alone, and differences in the fine specificity of binding for all the different antibodies could be distinguished. The incorporation of several specific amino acid changes enabled the design of proteins that bound inhibitory but not blocking antibodies. These may be suitable for the development of MSP-1-based vaccines against malaria.

Antibody recognition of rodent malaria parasite antigens exposed at the infected erythrocyte surface: specificity of immunity generated in hyperimmune mice

In regions where malaria is endemic, inhabitants remain susceptible to repeated reinfection as they develop and maintain clinical immunity. This immunity includes responses to surface-exposed antigens on Plasmodium sp.-infected erythrocytes. Some of these parasite-encoded antigens may be diverse and phenotypically variable, and the ability to respond to this diversity and variability is an important component of acquired immunity. Characterizing the relative specificities of antibody responses during the acquisition of immunity and in hyperimmune individuals is thus an important adjunct to vaccine research. This is logistically difficult to do in the field but is relatively easily carried out in animal models. Infections in inbred mice with rodent malaria parasite Plasmodium chabaudi chabaudi AS represent a good model for Plasmodium falciparum in humans. This model has been used in the present study in a comparative analysis of cross-reactive and specific immune responses in rodent malaria. CBA/Ca mice were rendered hyperimmune to P. chabaudi chabaudi (AS or CB lines) or Plasmodium berghei (KSP-11 line) by repeated infection with homologous parasites. Serum from P. chabaudi chabaudi AS hyperimmune mice reacted with antigens released from disrupted P. chabaudi chabaudi AS-infected erythrocytes, but P. chabaudi chabaudi CB and P. berghei KSP-11 hyperimmune serum also contained cross-reactive antibodies to these antigens. However, antibody activity directed against antigens exposed at the surfaces of intact P. chabaudi chabaudi-infected erythrocytes was mainly parasite species specific and, to a lesser extent, parasite line specific. Importantly, this response included opsonizing antibodies, which bound to infected erythrocytes, leading to their phagocytosis and destruction by macrophages. The results are discussed in the context of the role that antibodies to both variable and invariant antigens may play in protective immunity in the face of continuous susceptibility to reinfection.

Sequence diversity and antigenic polymorphism in the Plasmodium yoelii p235 high molecular mass rhoptry proteins and their genes

A gene family in Plasmodium yoelii YM encodes p235, a group of high molecular mass erythrocyte-binding rhoptry proteins. Sequence analysis of 6 cDNA clones from the 3' end of expressed p235 genes divided them into two groups corresponding to genes on chromosomes 1, and 5 and 6, respectively. Twelve partial p235 protein sequences, derived from cDNA sequences from the region with greatest protein sequence similarity to Plasmodium vivax RBP2, fell into three groups, together with one chimeric sequence. A comparison of these cDNA sequences with genomic DNA sequences from the same region suggested that only a subset of the gene repertoire is expressed. Three genomic DNA clones, derived from the 5' end of p235 genes designated E1, E2, and E5 and located on chromosome 5/6, were also obtained and aligned with sequences from the known E8 and E3 genes. In the region of overlap there was only approximately 27% protein sequence identity, indicating that the sequences in this p235 N-terminal region are more diverse than at the C-terminal end. This sequence variation in the expressed genes did not result in antigenically different rhoptry proteins as detected with a panel of p235-specific mAbs. Only one schizont out of 500 examined with mAb 25.86 appeared to be an antigenic variant, with all of the developing merozoites in this schizont being mAb 25.86 negative. No other antigenic variants were detected with the other antibodies, and therefore it is likely that these antibodies recognise conserved epitopes.

The merozoite surface protein 6 gene codes for a 36 kDa protein associated with the Plasmodium falciparum merozoite surface protein-1 complex

A complex of non-covalently bound polypeptides is located on the surface of the merozoite form of the human malaria parasite Plasmodium falciparum. Four of these polypeptides are derived by proteolytic processing of the merozoite surface protein 1 (MSP-1) precursor. Two components, a 22 and a 36 kDa polypeptide are not derived from MSP-1. The N-terminal sequence of the 36 kDa polypeptide has been determined, the corresponding gene cloned, and the protein characterised. The 36 kDa protein consists of 211 amino acids and is derived from a larger precursor of 371 amino acids. The precursor merozoite surface protein 6 (MSP-6) has been designated, and the 36 kDa protein, MSP-6(36). Mass spectrometric analysis of peptides released from the polypeptide by tryptic digestion confirmed that the gene identified codes for MSP-6(36). Antibodies were produced to a recombinant protein containing the C-terminal 45 amino acid residues of MSP-6(36). In immunofluorescence studies these antibodies bound to antigen at the parasite surface or in the parasitophorous vacuole within schizonts, with a pattern indistinguishable from that of antibodies to MSP-1. MSP-6(36) was present in the MSP-1 complex immunoprecipitated from the supernatant of in vitro parasite cultures, but was also immunoprecipitated from this supernatant in a form not bound to MSP-1. Examination of the MSP-6 gene in three parasite lines detected no sequence variation. The sequence of MSP-6(36) is related to that of the previously described merozoite surface protein 3 (MSP-3). The MSP-6(36) amino acid sequence has 50% identity and 85% similarity with the C-terminal region of MSP-3. The proteins share a specific sequence pattern (ILGWEFGGG-[AV]-P) and a glutamic acid-rich region. The remainder of MSP-6 and MSP-3 are unrelated, except at the N-terminus. Both MSP-6(36) and MSP-3 are partially associated with the parasite surface and partially released as soluble proteins on merozoite release. MSP-6(36) is a hydrophilic negatively charged polypeptide, but there are two clusters of hydrophobic amino acids at the C-terminus, located in two amphipathic helical structures identified from secondary structure predictions. It was suggested that this 35 residue C-terminal region may be involved in MSP-6(36) binding to MSP-1 or other molecules; alternatively, based on the secondary structure and coil formation predictions, the region may form an intramolecular anti-parallel coiled-coil structure.

Construction and immunogenicity in mice of attenuated Salmonella typhi expressing Plasmodium falciparum merozoite surface protein 1 (MSP-1) fused to tetanus toxin fragment C

One strategy to develop a multi-antigen malaria vaccine is to employ live vectors to carry putative protective Plasmodium falciparum antigens to the immune system. The 19 kDa carboxyl terminus of P. falciparum merozoite surface protein 1 (MSP-1), which is essential for erythrocyte invasion and is a leading antigen for inclusion in a multivalent malaria vaccine, was genetically fused to fragment C of tetanus toxin and expressed within attenuated Salmonella typhi CVD 908. Under conditions in the bacterial cytoplasm, the fragment C-MSP-1 fusion did not form the epidermal growth factor (EGF)-like domains of MSP-1; monoclonal antibodies failed to recognize these conformational domains in immunoblots of non-denatured protein extracted from live vector sonicates. The MSP-1 was nevertheless immunogenic. One month following intranasal immunization of BALB/c mice with the live vector construct, four out of five mice exhibited > or =four-fold rises in anti-MSP-1 by ELISA (GMT=211); a single intranasal booster raised titers further (GMT=1280). Post-immunization sera recognized native MSP-1 on merozoites as determined by indirect immunofluorescence. These data encourage efforts to optimize MSP-1 expression in S. typhi (e.g. as a secreted protein), so that the EGF-like epitopes, presumably necessary for stimulating protective antibodies, can form.

Plasmodium chabaudi-infected erythrocytes adhere to CD36 and bind to microvascular endothelial cells in an organ-specific way

Adherence of erythrocytes infected with Plasmodium falciparum to microvascular endothelial cells (sequestration) is considered to play an important role in parasite virulence and pathogenesis. However, the real importance of sequestration for infection and disease has never been fully assessed. The absence of an appropriate in vivo model for sequestration has been a major barrier. We have examined the rodent malaria parasite Plasmodium chabaudi chabaudi AS in mice as a potential model. Erythrocytes infected with this parasite adhere in vitro to purified CD36, a critical endothelium receptor for binding P. falciparum-infected erythrocytes. P. c. chabaudi-infected erythrocytes adhere in vitro to endothelial cells in a gamma interferon-dependent manner, suggesting the involvement of additional adhesion molecules in the binding process, as is also the case with P. falciparum-infected cells. Furthermore, plasma or sera from infected and hyperimmune mice, respectively, have the ability to block binding of infected erythrocytes to endothelial cells. In vivo, erythrocytes containing mature P. c. chabaudi parasites are sequestered from the peripheral circulation. Sequestration is organ specific, occurring primarily in the liver, although intimate contact between infected erythrocytes and endothelial cells is also observed in the spleen and brain. The results are discussed in the context of the use of this model to study (i) the relationship between endothelial cell activation and the level of sequestration and (ii) the primary function of sequestration in malaria infection.

Plasmodium yoelii: effects of red blood cell modification and antibodies on the binding characteristics of the 235-kDa rhoptry protein

The 235-kDa rhoptry protein of the rodent malaria parasite Plasmodium yoelii yoelii was shown to bind to the surface of mouse red blood cells in a calcium-independent process, using a erythrocyte-binding assay. This binding is affected by modification of the surface of the red blood cells by enzymatic treatment. Chymotrypsin and trypsin but not neuraminidase treatment of the erythrocytes significantly reduced the binding of the 235-kDa proteins. The binding of an unrelated 135-kDa protein was abolished by treatment with chymotrypsin. Although the 235-kDa proteins bind to both reticulocytes and mature red blood cells, the binding to mature cells was more pronounced. In the presence of hyperimmune infection serum or specific polyclonal antibodies to the 235-kDa protein its binding to erythrocytes was reduced, further demonstrating the specificity of this ligand-receptor interaction.

Characterization of N-myristoyltransferase from Plasmodium falciparum

The gene coding for myristoyl-CoA:protein N-myristoyltransferase (NMT) has been cloned from the malaria parasite Plasmodium falciparum. The gene appears to be single copy and mRNA is expressed in asexual blood-stage forms. Comparison of cDNA and genomic sequences identified three small introns. The open reading frame codes for a 410-amino-acid protein and no evidence of forms with an extended N-terminal coding sequence was obtained. Residues important in substrate binding and in the catalytic mechanism in other species are conserved. The protein was expressed from a plasmid in Escherichia coli, partially purified and shown to have enzymic activity using a synthetic peptide substrate. Comparison of the malaria parasite protein with that derived from the human gene showed a different pattern of inhibition by chemical modification. Human NMT activity was inhibited by diethylpyrocarbonate and partially inhibited by iodacetamide, whereas P. falciparum NMT activity was not inhibited by either pre-treatment. Since the enzyme in infectious fungi is a target for potential chemotherapeutic drugs, it should also be investigated in the context of parasitic infections such as that responsible for malaria.

Immunization with parasite-derived apical membrane antigen 1 or passive immunization with a specific monoclonal antibody protects BALB/c mice against lethal Plasmodium yoelii yoelii YM blood-stage infection

We have purified apical merozoite antigen 1 (AMA-1) from extracts of red blood cells infected with the rodent malaria parasite Plasmodium yoelii yoelii YM. When used to immunize mice, the protein induced a strong protective response against a challenge with the parasite. Monoclonal antibodies specific for P. yoelii yoelii AMA-1 were prepared, and one was very effective against the parasite on passive immunization. A second protein that appears to be located in the apical rhoptry organelles and associated with AMA-1 was identified.

Linkage of exogenous T-cell epitopes to the 19-kilodalton region of Plasmodium yoelii merozoite surface protein 1 (MSP1(19)) can enhance protective immunity against malaria and modulate the immunoglobulin subclass response to MSP1(19)

The degree of protection against Plasmodium yoelii asexual blood stages induced by immunization of mice with the 19-kDa region of merozoite surface protein 1 (MSP1(19)) is H-2 dependent. As a strategy to improve the protection, mouse strains with disparate H-2 haplotypes were immunized with glutathione S-transferase (GST)-MSP1(19) proteins including either a universal T-cell epitope from tetanus toxin (P2) or an I-A(k)-restricted T-cell epitope (P8) from Plasmodium falciparum Pf332. In H-2(k) mice which are poorly protected following immunization with GST-MSP1(19), GST-P2-MSP1(19) significantly improved the protection. In mice partially (H-2(k/b)) or well protected by GST-MSP1(19) (H-2(d) and H-2(b)), P2 did not further increase the protection. However, the protection of H-2(k/b) mice and to some extent H-2(k) mice was improved by immunization with GST-P8-MSP1(19). The magnitudes of immunoglobulin G1 (IgG1) and IgG2a responses in mice immunized with the GST-MSP1(19) variants correlated with low peak parasitemia, indicating a protective capacity of these IgG subclasses. In H-2(k) mice immunized with GST-P2-MSP1(19), both IgG1 and IgG2a responses were significantly enhanced. The epitope P2 appeared to have a general ability to modulate the IgG subclass response since all four mouse strains displayed elevated IgG2a and/or IgG2b levels after immunization with GST-P2-MSP1(19). In contrast, GST-P8-MSP1(19) induced a slight enhancement of IgG responses in H-2(k/b) and H-2(k) mice without any major shift in IgG subclass patterns. The ability to improve the protective immunity elicited by P. yoelii MSP1(19) may have implications for improvement of human vaccines based on P. falciparum MSP1(19).

Processing and localisation of a GPI-anchored Plasmodium falciparum surface protein expressed by the baculovirus system

We describe the expression, in insect cells using the baculovirus system, of two protein fragments derived from the C-terminus of merozoite surface protein 1(MSP-1) of the human malaria parasite Plasmodium falciparum, and their glycosylation and intracellular location. The transport and intracellular localisation of the intact C-terminal MSP-1 fragment, modified by addition of a signal sequence for secretion, was compared with that of a similar control protein in which translation of the GPI-cleavage/attachment site was abolished by insertion of a stop codon into the DNA sequence. Both proteins could only be detected intracellularly, most likely in the endoplasmic reticulum. This lack of transport to the cell surface or beyond, was confirmed for both proteins by immunofluorescence with a specific antibody and characterisation of their N-glycans. The N-glycans had not been processed by enzymes localised in post-endoplasmic reticulum compartments. In contrast to MSP-1, the surface antigen SAG-1 of Toxoplasma gondii was efficiently transported out of the endoplasmic reticulum of insect cells and was located, at least in part, on the cell surface. No GPI-anchor could be detected for either of the MSP-1 constructs or SAG-1, showing that the difference in transport is a property of the individual proteins and cannot be attributed to the lack of a GPI-anchor. The different intracellular location and post-translational modification of recombinant proteins expressed in insect cells, as compared to the native proteins expressed in parasites, and the possible implications for vaccine development are discussed.

Antigenic and sequence diversity at the C-terminus of the merozoite surface protein-1 from rodent malaria isolates, and the binding of protective monoclonal antibodies

Merozoite surface protein-1 (MSP-1) is a major candidate in the development of a vaccine against malaria. Immunisation with a recombinant fusion protein containing the two Plasmodium yoelii MSP-1 C-terminal epidermal growth factor-like domains (MSP-1(19)) can protect mice against homologous but not heterologous challenge, and therefore, antigenic differences resulting from sequence diversity in MSP-1(19) may be crucial in determining the potential of this protein as a vaccine. Representative sequence variants from a number of distinct P. yoelii isolates were expressed in Escherichia coli and the resulting recombinant proteins were screened for binding to a panel of monoclonal antibodies (Mabs) capable of suppressing a P. yoelii YM challenge infection in passive immunisation experiments. The sequence polymorphisms affected the binding of the antibodies to the recombinant proteins. None of the Mabs recognised MSP-1(19) of P. yoelii yoelii 2CL or 33X or P. yoelii nigeriensis N67. The epitopes recognised by the Mabs were further distinguished by their reactivity with the other fusion proteins. The extent of sequence variation in MSP-1(19) among the isolates was extensive, with differences detected at 35 out of the 96 positions compared. Using the 3-dimensional structure of the Plasmodium falciparum MSP-1(19) as a model, the locations of the amino acid substitutions that may affect Mab binding were identified. The DNA sequence of MSP-1(19) from two Plasmodium vinckei isolates was also cloned and the deduced amino acid sequence compared with that in other species.

Merozoite surface protein 1, immune evasion, and vaccines against asexual blood stage malaria

There is an urgent need for a vaccine against malaria and proteins on the surface of the merozoite are good targets for development as vaccine candidates because they are exposed to antibody. However, it is possible that the parasite has evolved mechanisms to evade a protective immune response to these proteins. Merozoite surface protein 1 (MSP-1) is a candidate for vaccine development and its C-terminal sequence is the target of protective antibody. MSP-1 is cleaved by proteases in two processing steps, the second step releases the bulk of the protein from the surface and goes to completion during successful red blood cell invasion. Antibodies binding to the C-terminus of Plasmodium falciparum MSP-1 can inhibit both the processing and erythrocyte invasion. Other antibodies that bind to either the C-terminal sequence or elsewhere in the molecule are 'blocking' antibodies, which on binding prevent the binding of the inhibitory antibodies. Blocking antibodies are a mechanism of immune evasion, which may be based on antigenic conservation rather than diversity. This mechanism has a number of implications for the study of protective immunity and the development of malaria vaccines, emphasising the need for appropriate functional assays and careful design of the antigen.

Solution structure of an EGF module pair from the Plasmodium falciparum merozoite surface protein 1

The solution structure of the 96-residue C-terminal fragment of the merozoite surface protein 1 (MSP-1) from Plasmodium falciparum has been determined using nuclear magnetic resonance (NMR) spectroscopic measurements on uniformly13C/15N-labelled protein, efficiently expressed in the methylotrophic yeast Komagataella (Pichia) pastoris. The structure has two domains with epidermal growth factor (EGF)-like folds with a novel domain interface for the EGF domain pair interactions, formed from a cluster of hydrophobic residues. This gives the protein a U-shaped overall structure with the N-terminal proteolytic processing site close to the C-terminal glycosyl phosphatidyl inositol (GPI) membrane anchor site, which is consistent with the involvement of a membrane-bound proteinase in the processing of MSP-1 during erythrocyte invasion. This structure, which is the first protozoan EGF example to be determined, contrasts with the elongated structures seen for EGF-module pairs having shared Ca2+-ligation sites at their interface, as found, for example, in fibrillin-1. Recognition surfaces for antibodies that inhibit processing and invasion, and antibodies that block the binding of these inhibitory antibodies, have been mapped on the three-dimensional structure by considering specific MSP-1 mutants.

Chromosomal organisation of a gene family encoding rhoptry proteins in Plasmodium yoelii

The genomic organisation of the genes coding for a group of high molecular mass rhoptry proteins of the rodent malaria parasite Plasmodium yoelii YM was investigated using blotting, two dimensional gel electrophoresis and restriction fragment length analysis. The genes were found on chromosomes 1, 5, 6 and 10, with the possibility that related genes were also present on chromosomes 3 and 4. On chromosome 1 the genes were located close to one end, whereas they were present at both ends of chromosome 5, 6 and 10. Two genes, e3 and e8, that had been partially characterised previously were present on chromosomes 5 and 1, respectively. Based on an analysis of the 3' end of the genes, three subfamilies present on chromosomes 1, 5 and 6, and 10, respectively, were identified.

Different Plasmodium falciparum recombinant MSP1(19) antigens differ in their capacities to stimulate in vitro peripheral blood T lymphocytes in individuals from various endemic areas

This study reports on T-cell proliferative responses to the 19-kDa C-terminal domain of the Plasmodium falciparum merozoite surface protein (MSP1(19)). Three different recombinant proteins were used: an Escherichia coli product expressing the first EGF-like domain and Saccharomyces cerevisiae and baculovirus/insect-cell-produced proteins containing both EGF-like domains, the latter protein being produced with or without N-glycosylation. Cell donors were P. falciparum-immune adults with no recent history of clinical malaria and recruited from three Senegalese settings with different epidemiological parasite transmission. Each mononuclear-blood-cell preparation was stimulated with a range of concentrations of the three proteins. Most subjects' mononuclear cells were reactive to at least one protein, but significant differences in lymphoproliferation were seen between the settings and within individual cultures depending on the protein source and concentration. Importantly, lymphoproliferation indices correlated inversely with the intensity of P. falciparum malaria transmission. When purified T lymphocytes were cultured in the presence of MSP1(19) plus autologous monocytes, B lymphocytes or a proposed CD1+ dendritic-cell population as costimulatory cells, significant differences were observed depending on the individual's previous exposure to parasites. This study shows that the stimulation of lymphocyte proliferation in vitro with MSP1(19) depends on several factors, including epidemiological conditions and protein preparations.

Human antibodies to the 19kDa C-terminal fragment of Plasmodium falciparum merozoite surface protein 1 inhibit parasite growth in vitro

The 19kDa, C-terminal fragment of the major surface protein of Plasmodium falciparum (PfMSP1(19)) is a candidate for inclusion in a subunit malaria vaccine. In this study, we show that PfMSP1(19)-specific antibodies, affinity purified from malaria-immune human serum, can: (i) compete with invasion-inhibitory monoclonal antibodies for binding to PfMSP1(19) and (ii) mediate inhibition of parasite growth in vitro, in the absence of complement and mononuclear cells, at physiological antibody concentrations (100 micrograms/ml). Parasites expressing either the Kl or 3D7 allele of PfMSP1(19) were equally susceptible to inhibition of merozoite invasion, indicating that the target epitopes of inhibitory antibodies are conserved or cross-reactive. These studies suggest that vaccines designed to induce antibodies to PfMSP1(19) may protect against the high levels of malaria parasitaemia which are associated with clinical disease.

Acute Plasmodium chabaudi chabaudi malaria infection induces antibodies which bind to the surfaces of parasitized erythrocytes and promote their phagocytosis by macrophages in vitro

CBA/Ca mice infected with 5 x 10(4) Plasmodium chabaudi chabaudi AS-parasitized erythrocytes experience acute but self-limiting infections of relatively short duration. Parasitemia peaks ( approximately 40% infected erythrocytes) on day 10 or 11 and is then partially resolved over the ensuing 5 to 6 days, a period referred to as crisis. How humoral and cellular immune mechanisms contribute to parasite killing and/or clearance during crisis is controversial. Humoral immunity might be parasite variant, line, or species specific, while cellular immune responses would be relatively less specific. For P. c. chabaudi AS, parasite clearance is largely species and line specific during this time, which suggests a primary role for antibody activity. Accordingly, acute-phase plasma (APP; taken from P. c. chabaudi AS-infected mice at day 11 or 12 postinfection) was examined for the presence of parasite-specific antibody activity by enzyme-linked immunosorbent assay. Antibody binding to the surface of intact, live parasitized erythrocytes, particularly those containing mature (trophozoite and schizont) parasites, was demonstrated by immunofluorescence in APP and the immunoglobulin G (IgG)-containing fraction thereof. Unfractionated APP (from P. c. chabaudi AS-infected mice), as well as its IgG fraction, specifically mediated the opsonization and internalization of P. c. chabaudi AS-parasitized erythrocytes by macrophages in vitro. APP from another parasite line (P. c. chabaudi CB) did not mediate the same effect against P. c. chabaudi AS-parasitized erythrocytes. These results, which may represent one mechanism of parasite removal during crisis, are discussed in relation to the parasite variant, line, and species specificity of parasite clearance during this time.

Passive immunization with antibodies against three distinct epitopes on Plasmodium yoelii merozoite surface protein 1 suppresses parasitemia

We have produced monoclonal antibodies against Plasmodium yoelii merozoite surface protein 1 (MSP-1) and have assessed their ability to suppress blood stage parasitemia by passive immunization. Six immunoglobulin G antibodies were characterized in detail: three (B6, D3, and F5) were effective in suppressing a lethal blood stage challenge infection, two (B10 and G3) were partially effective, and one (B4) was ineffective. MSP-1 is the precursor to a complex of polypeptides on the merozoite surface; all of the antibodies bound to this precursor and to an approximately 42-kDa fragment (MSP-142) that is derived from the C terminus of MSP-1. MSP-142 is further cleaved to an N-terminal approximately 33-kDa polypeptide (MSP-133) and a C-terminal approximately 19-kDa polypeptide (MSP-119) comprised of two epidermal growth factor (EGF)-like modules. D3 reacted with MSP-142 but not with either of the constituents MSP-133 and MSP-119, B4 recognized an epitope within the N terminus of MSP-133, and B6, B10, F5, and G3 bound to MSP-119. B10 and G3 bound to epitopes that required both C-terminal EGF-like modules for their formation, whereas B6 and F5 bound to epitopes in the first EGF-like module. These results indicate that at least three distinct epitopes on P. yoelii MSP-1 are recognized by antibodies that suppress parasitemia in vivo.

A Plasmodium chabaudi protein contains a repetitive region with a predicted spectrin-like structure

cDNA and genomic DNA clones covering the entire open reading frame (ORF) for a Plasmodium chabaudi 96V protein were isolated. From the first ATG codon the intronless gene codes for a 229-kDa protein. Antisera raised against recombinant polypeptides coded by two different regions of the gene reacted with a 240/225-kDa doublet on Western blots of parasite extracts. In immunofluorescence studies the same sera detected the antigen at the apical end of the merozoite, possibly in rhoptry organelles. In Western blotting experiments the recombinant polypeptides were recognised by antibodies induced by natural infection. A 364-amino acid residue repetitive region, based on 32 11-mer repeats divided by two 6-mer repeats into three blocks, is located in the centre of the protein. Analysis of this repetitive region led us to propose a model in which each of the three units forms an alpha-helical coiled-coil triple-helix containing a possible leucine-histidine zipper. Each unit resembles in structure the units present in spectrin. The repeat region is flanked by predicted heptad based alpha-helical coiled-coil regions, and we propose that the protein forms a dimer. The 229-kDa protein has the overall character of a cytoskeletal protein. We have named the 229-kDa protein repetitive organellar protein (ROPE) and suggest that ROPE may be involved in the process of invasion, possibly by interacting with the erythrocyte cytoskeleton, and that the leucine histidine-zipper may be involved in molecular mimicry of spectrin.

A longitudinal study of type-specific antibody responses to Plasmodium falciparum merozoite surface protein-1 in an area of unstable malaria in Sudan

Merozoite surface protein-1 (MSP-1) of Plasmodium falciparum is a malaria vaccine candidate Ag. Immunity to MSP-1 has been implicated in protection against infection in animal models. However, MSP-1 is a polymorphic protein and its immune recognition by humans following infection is not well understood. We have compared the immunogenicity of conserved and polymorphic regions of MSP-1, the specificity of Ab responses to a polymorphic region of the Ag, and the duration of these responses in Sudanese villagers intermittently exposed to P. falciparum infections. Recombinant Ags representing the conserved N terminus (Block 1), the conserved C terminus, and the three main types of the major polymorphic region (Block 2) of MSP-1 were used to determine the specificity and longitudinal patterns of IgG Ab responses to MSP-1 in individuals. Abs from 52 donors were assessed before, during, and after malaria transmission seasons for 4 yr. Ags from the Block 1 region were rarely recognized by any donor. Responses to the C-terminal Ag occurred in the majority of acutely infected individuals and thus were a reliable indicator of recent clinical infection. Ags from the polymorphic Block 2 region of MSP-1 were recognized by many, although not all individuals after clinical malaria infections. Responses to Block 2 were type specific and correlated with PCR typing of parasites present at the time of infection. Responses to all of these Ags declined within a few months of drug treatment and parasite clearance, indicating that naturally induced human Ab responses to MSP-1 are short lived.

Definition of T cell epitopes within the 19 kDa carboxylterminal fragment of Plasmodium yoelii merozoite surface protein 1 (MSP1(19)) and their role in immunity to malaria

MSP1(19) is one of the leading malaria vaccine candidates. However, the mechanism of protection is not clear. To determine whether MSP1(19)-specific effector T cells can control parasitaemia, we analysed the specificity of T cells induced following immunization with recombinant forms of P. yoelii MSP1(19) and asked whether they could protect mice. There was no evidence that effector T cells were capable of protecting since: (1) immunization of mice with yMSP1(19), but not defined epitopes, was able to induce protection; and (2) long term MSP1(19)-specific CD4+ T cell lines were incapable of adoptively transferring protection. In contrast, priming mice with the T cell epitopes resulted in a rapid anamnestic antibody response to MSP1(19) after either challenge with MSP1(19) or parasite. Thus, MSP1(19) contains multiple T cell epitopes but such epitopes are the targets of helper T cells for antibody response but not of identified effector T cells capable of controlling parasitaemia.

Glycosylphosphatidylinositols of Plasmodium chabaudi chabaudi: a basis for the study of malarial glycolipid toxins in a rodent model

Free and protein-bound glycosylphosphatidylinositols (GPIs) of the blood stages of the rodent malarial parasite Plasmodium chabaudi chabaudi AS were identified and characterized. TLC analysis of material extracted by organic solvents from metabolically labelled parasites revealed a distinct set of glycolipids. These glycolipids were identified as GPIs by specific chemical and enzymic treatments and by structural analysis of their glycan and hydrophobic parts. These analyses revealed that P.c.chabaudi AS synthesizes a set of GPI-biosynthesis intermediates and two potential GPI-anchor precursors exhibiting the following structures: ethanolamine-phosphate [(alpha1-2)mannose]mannose (alpha 1-2) mannose (alpha 1-6) mannose (alpha 1-4) glucosamine - (acyl) inositol-phosphate-diacylglycerol (P.ch. alpha) and ethanolamine-phosphate - mannose (alpha 1-2) mannose (alpha 1-6) mannose (alpha 1-4) glucosamine-(acyl)inositol-phosphate-diacylglycerol (P.ch. beta). One of these GPI-anchor precursors (P.ch. alpha) possesses the same carbohydrate structure as the GPI membrane anchor of merozoite surface protein-1 from P.c.chabaudi AS.

Antibodies that inhibit malaria merozoite surface protein-1 processing and erythrocyte invasion are blocked by naturally acquired human antibodies

Merozoite surface protein-1 (MSP-1) of the human malaria parasite Plasmodium falciparum undergoes at least two endoproteolytic cleavage events during merozoite maturation and release, and erythrocyte invasion. We have previously demonstrated that mAbs which inhibit erythrocyte invasion and are specific for epitopes within a membrane-proximal, COOH-terminal domain of MSP-1 (MSP-119) prevent the critical secondary processing step which occurs on the surface of the extracellular merozoite at around the time of erythrocyte invasion. Certain other anti-MSP-119 mAbs, which themselves inhibit neither erythrocyte invasion nor MSP-1 secondary processing, block the processing-inhibitory activity of the first group of antibodies and are termed blocking antibodies. We have now directly quantitated antibody-mediated inhibition of MSP-1 secondary processing and invasion, and the effects on this of blocking antibodies. We show that blocking antibodies function by competing with the binding of processing-inhibitory antibodies to their epitopes on the merozoite. Polyclonal rabbit antibodies specific for certain MSP-1 sequences outside of MSP-119 also act as blocking antibodies. Most significantly, affinity-purified, naturally acquired human antibodies specific for epitopes within the NH2-terminal 83-kD domain of MSP-1 very effectively block the processing-inhibitory activity of the anti-MSP-119 mAb 12.8. The presence of these blocking antibodies also completely abrogates the inhibitory effect of mAb 12.8 on erythrocyte invasion by the parasite in vitro. Blocking antibodies therefore (a) are part of the human response to malarial infection; (b) can be induced by MSP-1 structures unrelated to the MSP-119 target of processing-inhibitory antibodies; and (c) have the potential to abolish protection mediated by anti-MSP-119 antibodies. Our results suggest that an effective MSP-119-based falciparum malaria vaccine should aim to induce an antibody response that prevents MSP-1 processing on the merozoite surface.

Immunization with a recombinant C-terminal fragment of Plasmodium yoelii merozoite surface protein 1 protects mice against homologous but not heterologous P. yoelii sporozoite challenge

It has been reported previously that immunization with recombinant protein containing the two epidermal growth factor (EGF)-like modules from merozoite surface protein 1 (MSP-1) of Plasmodium yoelii (strain YM) protects mice against a lethal blood-stage challenge with the same parasite strain. Since MSP-1 is expressed in both liver- and blood-stage schizonts and on the surface of merozoites, we evaluated the effectiveness of immunization with recombinant proteins containing either the individual or the two combined EGF-like modules in producing a protective response against a sporozoite challenge. The recombinant protein expressing the combined EGF-like modules of the YM strain protected mice against a homologous sporozoite challenge, and sterile protection, as defined by the absence of detectable blood-stage parasites, was observed in the majority of the mice. In contrast, mice immunized with recombinant P. yoelii YM MSP-1 were not protected against a heterologous challenge with sporozoites from strain 265 BY of P. yoelii. The lack of protection may be explained by differences identified in the amino acid sequences of MSP-1 for the two strains. A recombinant protein containing the two EGF-like modules of MSP-1 from P. yoelii 265 BY was produced and used to immunize mice. These mice were protected against a homologous challenge with sporozoites of P. yoelii 265 BY. The results suggest that a recombinant MSP-1 has potential as a vaccine against malaria, but its efficacy may be limited by sequence polymorphism and selection of variants.

Immunization against the murine malaria parasite Plasmodium yoelii using a recombinant protein with adjuvants developed for clinical use

Mice vaccinated with a recombinant protein containing the two EGF-like modules of Plasmodium yoelii merozoite surface protein-1 in liposomes or combined with the formulations SBAS2.1 and SBAS2, were protected against a lethal malaria infection. The protection achieved with these adjuvants developed for clinical use was as good as or better than that achieved with Freund's adjuvant. A parasite-specific response was needed for protection. Analysis of the immunoglobulin sub-class response showed that MSP-1-specific IgG1, and to a lesser extent IgG2a and IgG2b, were induced, suggesting that these antibodies were important for protection. Mice passively immunized with serum or purified IgG from vaccinated mice had delayed onset of parasitemia and were able to control the infection.

The histones of Plasmodium falciparum: identification, purification and a possible role in the pathology of malaria

A quick and simple method of purifying the histones from Plasmodium falciparum culture supernatant or from infected erythrocytes is described. The proteins were present only in preparations rich in P. falciparum nuclear material and were soluble at acid pH and in strongly anionic detergents. Four proteins, of 14-18 kDa were identified as the P. falciparum core histones. N-terminal sequence analysis of the 16 and 18 kDa proteins and of a tryptic fragment of the protein mixture revealed strong homologies with deduced cDNA or protein sequences of histones H2A, H2B, and H3 of P. falciparum and other species. Antibodies raised against the proteins cross-reacted weakly with histones of other species and, on immunofluorescence, localized the proteins to schizont nuclei. Anti-P. falciparum histone antibodies were detected in sera of semi-immune human adults but these antibodies did not react with human histones on a Western blot. The large quantities of P. falciparum histone released and the chronic nature of malarial infection, together with the unusually high avidity of histones for ligands found in renal and vascular basement membrane, raise the question of a role for histones in the pathogenesis of malarial infection. We suggest that histones or histone-antibody complexes may contribute to disease pathology.

Growth and storage of YAC clones in Hogness Freezing Medium

To date, frozen storage of YAC libraries have relied on the administration of glycerol to the medium subsequent to cell growth. By adding Hogness Freezing Medium prior to inoculation, cultures can be frozen directly after cell growth, with no adverse effect on the stability of the YAC DNA or on the viability of the cells even after repeated freezing and defrosting. Although a relatively simple modification, the procedure notably improves the handling of YAC libraries and significantly reduces the risk of contamination, especially when dealing with large numbers of clones.

Glycosyl-phosphatidylinositols in protozoa structure, biosynthesis and intracellular localisation

We are investigating the structure and biosynthesis of glycosyl-phosphatidylinositols (GPI) in the protozoa Toxoplasma gondii, Plasmodium falciparum, Plasmodium yoelii and Paramecium primaurelia. This comparison of structural and biosynthesis data should lead us to common and individual features of the GPI-biosynthesis and transport in different organisms.

Isolation, expression and characterization of the gene for an ADP-ribosylation factor from the human malaria parasite, Plasmodium falciparum

We have isolated an ADP-ribosylation factor (ARF) gene from the human malarial parasite, Plasmodium falciparum. The gene (P. falciparum arf1) has four introns and the exons encode a protein of 181 amino acids with high similarity to the mammalian class I ARF proteins 1-3 (> or = 74% amino acid identity). Southern hybridization suggests there is at least one additional arf in the P. falciparum genome. Northern analysis identified a single P. falciparum arf1 mRNA of 1.8 kb in the asexual blood stage form of the parasite. The P. falciparum arf1 mRNA levels are developmentally regulated, reaching a maximum during nuclear division towards the end of the intraerythrocytic cycle. P. falciparum arf1 cDNA was isolated by reverse-transcriptase polymerase chain reaction and used to express a recombinant protein in Escherichia coli. Recombinant P. falciparum ARF1 protein was purified with stoichiometric amounts of bound GDP, although intrinsic guanose triphosphatase activity of the protein could not be detected. The protein stimulated cholera-toxin-catalyzed ADP-ribosyltransferase activity in a reaction that was dependent upon the addition of both dimyristoylglycerophosphocholine and cholate. The protein bound GTP with first-order kinetics with an apparent rate constant, k', of 0.0145 (+/- 0.0019) min-1. These results suggest that P. falciparum ARF1 is a member of the class 1 ARF family and provide additional evidence for the existence of a classical secretory pathway in P. falciparum.

A 22 kDa protein associated with the Plasmodium falciparum merozoite surface protein-1 complex

The Plasmodium falciparum merozoite surface protein-1 (MSP-1) is synthesized as a precursor of approximately 195 kDa and is processed to form a complex of polypeptides on the surface of free merozoites. As a result of a second processing event, the entire MSP-1 complex is shed from the surface, apart from a C-terminal fragment that remains anchored to the merozoite membrane. We have identified a 22 kDa protein (p22) on the surface of merozoites by cell surface radioiodination and indirect immunofluorescence assay on unfixed free merozoites. p22 is also a component of the shed MSP-1 complex where it is present in part as a 19 kDa form (p22(19)) as shown by immunochemical and peptide mapping analyses. The soluble complex contains MSP-1-derived polypeptides and p22 in approximately stoichiometrically equal amounts. N-terminal amino acid sequence analyses of p22/p22(19) showed that the protein is not derived from the MSP-1 precursor.

Immunization of Aotus nancymai with recombinant C terminus of Plasmodium falciparum merozoite surface protein 1 in liposomes and alum adjuvant does not induce protection against a challenge infection

Merozoite surface protein 1 (MSP-1) of Plasmodium falciparum is an antimalarial vaccine candidate. The highly conserved 19-kDa C-terminal processing fragment of MSP-1 (MSP-1(19)) is of particular interest since it contains epitopes recognized by monoclonal antibodies which inhibit the invasion of erythrocytes in vitro. The presence of naturally acquired anti-MSP-1(19) antibodies in individuals exposed to malaria has been correlated with reduced morbidity, and immunization with an equivalent recombinant P. yoelii antigen induces substantial protection against this parasite in mice. We have expressed P. falciparum MSP-1(19) in Escherichia coli as a correctly folded protein and immunized Aotus nancymai monkeys by using the protein incorporated into liposomes and adsorbed to alum. After vaccination, the sera from these animals contained anti-MSP-1(19) antibodies, some of which competed for binding to MSP-1(19) with monoclonal antibodies that inhibit parasite invasion of erythrocytes in vitro. However, after challenge with either a homologous or a heterologous strain of parasite, all animals became parasitemic and required treatment. The immunization did not induce protection in this animal model.

Cytokines and antibody subclass associated with protective immunity against blood-stage malaria in mice vaccinated with the C terminus of merozoite surface protein 1 plus a novel adjuvant

A blood-stage malaria antigen comprising the C terminus of merozoite surface protein 1 fused to glutathione S-transferase, combined with an adjuvant formulation containing squalane, Tween 80, and pluronic L121 (AF), administered subcutaneously protected mice against death from a lethal Plasmodium yoelii infection. The protection induced by this antigen-adjuvant combination was compared with that induced by the antigen plus saponin in terms of survival from the lethal infection and clearance of parasitemia. The levels of gamma interferon and interleukin-4 in spleens were measured as indicators of Th1 and Th2 cell activation, and antibody classes and subclasses were determined by immunofluorescence. With a 10-micrograms dose of antigen and AF as adjuvant, all mice recovered, but with saponin as the adjuvant, there were only a few survivors. With 30 micrograms of antigen plus AF, the peak parasitemias were 10-fold lower than those with 10 micrograms; with saponin, survival was slightly improved. The levels of both gamma interferon and interleukin-4 rose more rapidly and to higher levels with AF as the adjuvant than with saponin, and the same was true for immunoglobulin G1 (IgG1), IgG2a, and IgG2b subclasses. Thus, in terms of both cytokine production and antibody levels, AF is a more potent adjuvant for a malaria vaccine than is saponin.