Immunization with a Plasmodium falciparum merozoite surface antigen induces a partial immunity in monkeys

Multifunctional Fructose 1,6-Bisphosphate Aldolase as a Therapeutic Target

Fructose 1,6-bisphosphate aldolase is a ubiquitous cytosolic enzyme that catalyzes the fourth step of glycolysis. Aldolases are classified into three groups: Class-I, Class-IA, and Class-II; all classes share similar structural features but low amino acid identity. Apart from their conserved role in carbohydrate metabolism, aldolases have been reported to perform numerous non-enzymatic functions. Here we review the myriad "moonlighting" functions of this classical enzyme, many of which are centered on its ability to bind to an array of partner proteins that impact cellular scaffolding, signaling, transcription, and motility. In addition to the cytosolic location, aldolase has been found the extracellular surface of several pathogenic bacteria, fungi, protozoans, and metazoans. In the extracellular space, the enzyme has been reported to perform virulence-enhancing moonlighting functions e.g., plasminogen binding, host cell adhesion, and immunomodulation. Aldolase's importance has made it both a drug target and vaccine candidate. In this review, we note the several inhibitors that have been synthesized with high specificity for the aldolases of pathogens and cancer cells and have been shown to inhibit classical enzyme activity and moonlighting functions. We also review the many trials in which recombinant aldolases have been used as vaccine targets against a wide variety of pathogenic organisms including bacteria, fungi, and metazoan parasites. Most of such trials generated significant protection from challenge infection, correlated with antigen-specific cellular and humoral immune responses. We argue that refinement of aldolase antigen preparations and expansion of immunization trials should be encouraged to promote the advancement of promising, protective aldolase vaccines.

Genetic diversity and natural selection in the rhoptry-associated protein 1 (RAP-1) of recent Plasmodium knowlesi clinical isolates from Malaysia

BACKGROUND

The Plasmodium rhoptry-associated protein 1 (RAP-1) plays a role in the formation of the parasitophorous vacuole following the parasite's invasion of red blood cells. Although there is some evidence that the protein is recognized by the host's immune system, study of Plasmodium falciparum RAP-1 (PfRAP-1) suggests that it is not under immune pressure. A previous study on five old (1953-1962) P. knowlesi strains suggested that RAP-1 has limited genetic polymorphism and might be under negative selection. In the present study, 30 recent P. knowlesi isolates were studied to obtain a better insight into the polymorphism and natural selection of PkRAP-1.

METHODS

Blood samples from 30 knowlesi malaria patients were used. These samples were collected between 2010 and 2014. The PkRAP-1 gene, which contains two exons, was amplified by PCR, cloned into Escherichia coli and sequenced. Genetic diversity and phylogenetic analyses were performed using MEGA6 and DnaSP ver. 5.10.00 programs.

RESULTS

Thirty PkRAP-1 sequences were obtained. The nucleotide diversity (π) of exons 1, 2 and the total coding region (0.00915, 0.01353 and 0.01298, respectively) were higher than those of the old strains. Further analysis revealed a lower rate of non-synonymous (dN) than synonymous (dS) mutations, suggesting negative (purifying) selection of PkRAP-1. Tajima's D test and Fu and Li's D test values were not significant. At the amino acid level, 22 haplotypes were established with haplotype H7 having the highest frequency (7/34, 20.5 %). In the phylogenetic analysis, two distinct haplotype groups were observed. The first group contained the majority of the haplotypes, whereas the second had fewer haplotypes.

CONCLUSIONS

The present study found higher genetic polymorphism in the PkRAP-1 gene than the polymorphism level reported in a previous study. This observation may stem from the difference in sample size between the present (n = 30) and the previous (n = 5) study. Synonymous and non-synonymous mutation analysis indicated purifying (negative) selection of the gene. The separation of PkRAP-1haplotypes into two groups provides further evidence to the postulation of two distinct P. knowlesi types or lineages.

Antibodies to the Plasmodium falciparum rhoptry protein RAP-2/RSP-2 in relation to anaemia in Cameroonian children

Previous studies have implicated reactive antibodies to the low molecular weight rhoptry-associated proteins (RAP-1, RAP-2/RSP-2 and RAP-3) in erythroid cell destruction during Plasmodium falciparum infection. In this pilot study, the frequency, specificity and functional capacity of naturally acquired anti-RAP-2/RSP-2 antibodies were investigated in the sera of anaemic and nonanaemic malaria-infected Cameroonian children. All sera recognized RAP-2/RSP-2 by FACS, irrespective of the clinical status of the subjects. However, the anaemic children showed higher levels of IgG antibodies than the nonanaemic group, while both groups showed similar levels of IgM antibodies. Only few individuals had detectable levels of RAP-2/RSP-2-specific IgG1 and IgG3 subclass antibodies, while no IgG2 and IgG4 subclass antibodies were detected in these subjects. By ELISA, the anaemic group tended to show higher levels of antibodies to RAP-2/RSP-2 regarding all antibody classes tested, except for IgG4 and IgE. Unexpectedly, sera from the nonanaemic group activated complement to a greater extent than those from the anaemic group. These results need to be confirmed in extended studies but indicate that the effector functions of the RAP-2/RSP-2-reactive antibodies may be more important than their amounts. Such antibodies could play a role in both immunity and pathogenesis during P. falciparum infection.

Evidence for negative selection on the gene encoding rhoptry-associated protein 1 (RAP-1) in Plasmodium spp

Assessing how natural selection, negative or positive, operates on genes with low polymorphism is challenging. We investigated the genetic diversity of orthologous genes encoding the rhoptry-associated protein 1 (RAP-1), a low polymorphic protein of malarial parasites that is involved in erythrocyte invasion. We applied evolutionary genetic methods to study the polymorphism in RAP-1 from Plasmodium falciparum (n=32) and Plasmodium vivax (n=6), the two parasites responsible for most human malaria morbidity and mortality, as well as RAP-1 orthologous in closely related malarial species found in non-human primates (NHPs). Overall, genes encoding RAP-1 are highly conserved in all Plasmodium spp. included in this investigation. We found no evidence for natural selection, positive or negative, acting on the gene encoding RAP-1 in P. falciparum or P. vivax. However, we found evidence that the orthologous genes in non-human primate parasites (Plasmodium cynomolgi, Plasmodium inui, and Plasmodium knowlesi) are under purifying (negative) selection. We discuss the importance of considering negative selection while studying genes encoding proteins with low polymorphism and how selective pressures may differ among orthologous genes in closely related malarial parasites species.

Evaluation of recombinant fructose-1,6-bisphosphate aldolase ELISA test for the diagnosis of Schistosoma japonicum in water buffaloes

Fructose-1,6-bisphosphate aldolase (FBPA) is an ubiquitous enzyme essential for glycolysis, gluconeogenesis and the Calvin cycle. It has been demonstrated to induce immune responses and to be useful in the immunodiagnosis of malaria. In this study, FBPA was cloned from the adult worms of Schistosoma japonicum and tested as an antigen for the diagnosis of S. japonicum infection in water buffaloes. Enzyme-linked immunosorbent assay (ELISA) was performed on the sera from 32 infected water buffaloes and 20 negative controls using the recombinant FBPA protein or soluble worm antigen preparation (SWAP) as an antigen. The OD cut-off values were determined to be 0.57 with 100% specificity and 100% sensitivity for the FBPA ELISA and 1.13 with 93.8% specificity and 95.0% sensitivity for the SWAP ELISA. These findings indicate that the recombinant FBPA of S. japonicum should be an useful diagnostic tool for the detection of antibodies against S. japonicum.

Identification and characterisation of the Plasmodium vivax rhoptry-associated protein 2

Plasmodium vivax is currently the most widespread of the four parasite species causing malaria in humans around the world. It causes more than 75 million clinical episodes per year, mainly on the Asian and American continents. Identifying new antigens to be further tested as anti-P. vivax vaccine candidates has been greatly hampered by the difficulty of maintaining this parasite cultured in vitro. Taking into account that one of the most promising vaccine candidates against Plasmodium falciparum is the rhoptry-associated protein 2, we have identified the P. falciparum rhoptry-associated protein 2 homologue in P. vivax in the present study. This protein has 400 residues, having an N-terminal 21 amino-acid stretch compatible with a signal peptide and, as occurs with its falciparum homologue, it lacks repeat sequences. The protein is expressed in asexual stage P. vivax parasites and polyclonal sera raised against this protein recognised a 46 kDa band in parasite lysate in a Western blot assay.

Aldolase Forms a Bridge between Cell Surface Adhesins and the Actin Cytoskeleton in Apicomplexan Parasites

Host cell invasion by apicomplexan parasites requires coordinated interactions between cell surface adhesins and the parasite cytoskeleton. We have identified a complex of parasite proteins, including the actin binding protein aldolase, which specifically interacts with the C-terminal domains of several parasite adhesins belonging to the thrombospondin-related anonymous protein (TRAP) family. Binding of aldolase to the adhesin was disrupted by mutation of a critical tryptophan in the C domain, a residue that was previously shown to be essential for parasite motility. Our findings reveal a potential role for aldolase in connecting TRAP family adhesins with the cytoskeleton, and provide a model linking adhesion with motility in apicomplexan parasites.

Genetic and antigenic analysis of Babesia bigemina isolates from five geographical regions of Brazil

A molecular epidemiological study was performed with Babesia bigemina isolates from five geographical regions of Brazil. The genetic analysis was done with random amplification of polymorphic DNA (RAPD), repetitive extragenic palindromic elements-polymerase chain reaction (REP-PCR) and enterobacterial repetitive intergenic consensus sequences-polymerase chain reaction (ERIC-PCR) that showed genetic polymorphism between these isolates and generated fingerprinting. In RAPD, ILO872 and ILO876 primers were able to detect at least one fingerprinting for each B. bigemina isolate. The amplification of B. bigemina DNA fragments by REP-PCR and ERIC-PCR gave evidence for the presence in this haemoprotozoan of the sequences described previously in microorganisms of the bacterial kingdom. For the first time it was demonstrated that both techniques can be used for genetic analysis of a protozoan parasite, although the ERIC-PCR was more discriminatory than REP-PCR. The dendogram with similarity coefficient among isolates showed two clusters and one subcluster. The Northeastern and Mid-Western isolates showed the greatest genetic diversity, while the Southeastern and Southern isolates were the closest. The antigenic analysis was done through indirect fluorescent antibody technique and Western blotting using a panel of monoclonal antibodies directed against epitopes on the merozoite membrane surface, rhoptries and membrane of infected erythrocytes. As expected, the merozoite variable surface antigens, major surface antigen (MSA)-1 and MSA-2 showed antigenic diversity. However, B cell epitopes on rhoptries and infected erythrocytes were conserved among all isolates studied. In this study it was possible to identify variable and conserved antigens, which had already been described as potential immunogens. Considering that an attenuated Babesia clone used as immunogen selected populations capable of evading the immunity induced by this vaccine, it is necessary to evaluate more deeply the cross-protection conferred by genetically more distant Brazilian B. bigemina isolates and make an evaluation of the polymorphism degree of variable antigens such as MSA-1 and MSA-2.

Three multigene families in Plasmodium parasites: facts and questions

Multigene families optimise fitness by providing a set of related genes with possibly different temporal and/or topological expression patterns. We analyse here the structural organisation and sequence diversity of the rDNA, sera and var C Plasmodium falciparum families, and discuss their consequences for parasite biology. The low rDNA copy number, which reduces reshuffling, is probably the corollary of the need for functionally distinct rRNAs in the insect and in the vertebrate host. The unusual intra-genome and population rDNA sequence diversity results in cells equipped with mosaic ribosome sets. The functional constraints are such that ribosome compatibility could influence parasite fitness and contribute to population structuring. Unlike the dispersed rDNA units, the sera family is arranged as a tandem gene cluster, with seven contiguous similar genes, and one more distantly related paralog. We address the question of the inclusion criteria in family definition. We discuss the results concerning the SERA proteins expression and function in the context of the long overlooked multigene family. The var C module is shared by var genes, 'orphan' var C and var C pseudogenes. Analysis of 125 var C deduced protein sequences highlights a well-conserved framework, including putative phosphorylation sites, consistent with the proposed function of mediating interaction with cytoskeletal proteins. The 5' and 3' flanking sequences of the var C pseudogenes are heterogeneous. In contrast, the flanking sequences of the uninterrupted var C modules show remarkable conservation. This is interesting in view of the silencing activity of the var intronic sequence on var expression. The 5' flanking sequence dichotomy reported for internal and sub-telomeric var genes extends to the 3' flanking sequences. This has profound implications for transcription regulation and generation of diversity. The var C family suggests a role for pseudogenes as a diversity reservoir and in genome dynamics by promoting ectopic recombination.

Identification and disruption of the gene encoding the third member of the low-molecular-mass rhoptry complex in Plasmodium falciparum

The low-molecular-mass rhoptry complex of Plasmodium falciparum consists of three proteins, rhoptry-associated protein 1 (RAP1), RAP2, and RAP3. The genes encoding RAP1 and RAP2 are known; however, the RAP3 gene has not been identified. In this study we identify the RAP3 gene from the P. falciparum genome database and show that this protein is part of the low-molecular-mass rhoptry complex. Disruption of RAP3 demonstrated that it is not essential for merozoite invasion, probably because RAP2 can complement the loss of RAP3. RAP3 has homology with RAP2, and the genes are encoded on chromosome 5 in a head-to-tail fashion. Analysis of the genome databases has identified homologous genes in all Plasmodium spp., suggesting that this protein plays a role in merozoite invasion. The region surrounding the RAP3 homologue in the Plasmodium yoelii genome is syntenic with the same region in P. falciparum; however, there is a single gene. Phylogenetic comparison of the RAP2/3 protein family from Plasmodium spp. suggests that the RAP2/3 duplication occurred after divergence of these parasite species.

Functional analysis of Plasmodium falciparum merozoite antigens: implications for erythrocyte invasion and vaccine development

Malaria is a major human health problem and is responsible for over 2 million deaths per year. It is caused by a number of species of the genus Plasmodium, and Plasmodium falciparum is the causative agent of the most lethal form. Consequently, the development of a vaccine against this parasite is a priority. There are a number of stages of the parasite life cycle that are being targeted for the development of vaccines. Important candidate antigens include proteins on the surface of the asexual merozoite stage, the form that invades the host erythrocyte. The development of methods to manipulate the genome of Plasmodium species has enabled the construction of gain-of-function and loss-of-function mutants and provided new strategies to analyse the role of parasite proteins. This has provided new information on the role of merozoite antigens in erythrocyte invasion and also allows new approaches to address their potential as vaccine candidates.

Molecular insights into receptors used by malaria parasites for erythrocyte invasion

The invasion of erythrocytes by malaria parasites is a multi-step process that requires a series of highly specific molecular interactions. Here, the authors review what has been learned about receptor-ligand interactions that mediate erythrocyte invasion. Parasite proteins involved in these interactions are promising candidates for malaria vaccines. Clear understanding of these interactions is important for the rational design of vaccines that attempt to inhibit invasion and prevent malaria.

Functional analysis of proteins involved in Plasmodium falciparum merozoite invasion of red blood cells

Plasmodium falciparum causes the most lethal form of malaria in humans and is responsible for over two million deaths per year. The development of a vaccine against this parasite is an urgent priority and potential protein targets include those on the surface of the asexual merozoite stage, the form that invades the host erythrocyte. The development of methods to transfect P. falciparum has enabled the construction of gain-of-function and loss-of-function mutants and provided new strategies to analyse the role of parasite proteins. In this review, we describe the use of this technology to examine the role of merozoite antigens in erythrocyte invasion and to address their potential as vaccine candidates.

RAP1 controls rhoptry targeting of RAP2 in the malaria parasite Plasmodium falciparum

Rhoptry associated protein 1 (RAP1) and 2 (RAP2), together with a poorly described third protein RAP3, form the low molecular weight complex within the rhoptries of Plasmodium falciparum. These proteins are thought to play a role in erythrocyte invasion by the extracellular merozoite and are important vaccine candidates. We used gene-targeting technology in P.falciparum blood-stage parasites to disrupt the RAP1 gene, producing parasites that express severely truncated forms of RAP1. Immunoprecipitation experiments suggest that truncated RAP1 species did not complex with RAP2 and RAP3. Consistent with this were the distinct subcellular localizations of RAP1 and 2 in disrupted RAP1 parasites, where RAP2 does not traffic to the rhoptries but is instead located in a compartment that appears related to the lumen of the endoplasmic reticulum. These results suggest that RAP1 is required to localize RAP2 to the rhoptries, supporting the hypothesis that rhoptry biogenesis is dependent in part on the secretory pathway in the parasite. The observation that apparently host-protective merozoite antigens are not essential for efficient erythrocyte invasion has important implications for vaccine design.

Interaction of HLA and age on levels of antibody to Plasmodium falciparum rhoptry-associated proteins 1 and 2

The Plasmodium falciparum rhoptry-associated proteins 1 and 2 (RAP1 and RAP2) are candidate antigens for a subunit malaria vaccine. The design of the study, which looks at the acquisition of immunity to malaria from childhood to old age, has allowed us to document the interaction of HLA and age on levels of antibody to specific malarial antigens. Antibodies reach maximum levels to RAP1 after the age of 15 but to RAP2 only after the age of 30. The effect of HLA-DRB1 and -DQB1 and age on levels of antibody to rRAP1 and rRAP2 was analyzed with a multiple regression model in which all HLA alleles and age were independent variables. DQB1*0301 and -*03032 showed an age-dependent association with levels of antibody to rRAP1, being significant in children 5 to 15 years (P < 0.001) but not in individuals over 15 years of age. DRB1*03011 showed an age-dependent association with antibody levels to rRAP2; however, this association was in adults over the age of 30 years (P < 0.01) but not in individuals under the age of 30 years. No associations were detected between DRB1 alleles and RAP1 antibody levels or between DQB1 alleles and RAP2 antibody levels. Thus, not only the HLA allele but also the age at which an interaction is manifested varies for different malarial antigens. The interaction may influence either the rate of acquisition of antibody or the final level of antibody acquired by adults.

Assessment of the humoral immune response against Plasmodium falciparum rhoptry-associated proteins 1 and 2

Naturally occurring antibody responses to Plasmodium falciparum rhoptry-associated proteins 1 and 2 (RAP-1 and RAP-2) were measured with recombinant and parasite-derived forms of the antigens. For comparative purposes, responses to multiple forms of three other malarial antigens were also examined. The sera of 100 Papua New Guineans were screened for antibodies. Eighty-six and 82% of individuals over 30 years of age had antibodies that recognized parasite-derived RAP-1 and RAP-2, respectively. Importantly, we found that recombinant and native antigens share linear epitopes seen by the human immune system; thus, the recombinant proteins may be adequate human immunogens. However, antibodies affinity purified on recombinant RAP-1 reacted with other antigens in addition to parasite-derived RAP-1. Thus, the antigenicity of RAP-1 may have been overestimated previously. The recognition of RAP-1 and RAP-2 correlated with age and with the recognition of recombinant forms of the ring-infected erythrocyte surface antigen, merozoite surface protein 1, and merozoite surface antigen 2 (MSA2) antigens. Antibodies to these antigens appear to be generated in response to the total exposure to malaria of the host. Antibodies to conserved regions of MSA2 had stronger correlations with both age and the recognition of other antigens than did the full-length recombinant MSA2 molecule. In contrast to results with the other antigens, there was no significant difference in the ages of individuals with a certain antibody titer to the full-length recombinant or parasite-derived MSA2 molecule, but antibodies to these two antigens did correlate with parasitemia. For all antigens tested, antibody levels after two infections can approach the peak levels of antibodies obtained in immune individuals.

Immunogenicity of recombinant Plasmodium falciparum rhoptry associated proteins 1 and 2

Mice and rabbits immunized with recombinant forms of malaria vaccine candidate antigens rhoptry-associated proteins 1 and 2 (RAP-1, RAP-2 and rRAP-1, rRAP-2) produce antibodies at titres equivalent to monoclonal antibody ascites fluid raised against the native proteins. Sera from animals immunized with rRAP-1 contain antibodies which recognize the native protein by indirect immunofluorescence and immunoblotting, partially inhibit erythrocyte invasion in vitro and are long lasting. Epitope mapping shows these antibodies predominantly recognize epitopes in the N-terminal third of rRAP-1, some of which coincide with the targets of inhibitory monoclonal antibodies. By contrast, sera from animals immunized with rRAP-2 contain antibodies which recognize the recombinant but not the native protein.

Three matrix metalloproteinases form a non-covalent association with the rhoptry-associated protein-1 of Plasmodium falciparum

During the characterization of malaria vaccine candidate proteins, three metalloproteinases having a molecular mass of 220, 95 and 70 kDa were found to be co-isolated with the rhoptry-associated protein-1 (RAP-1) complex, but not with RAP-3 or gp195. These enzymes were also found in detergent extracts of saponin-lysed Plasmodium falciparum. Of nine proteinase inhibitors tested, only EDTA was found to abrogate activity. Dose-dependent curves were determined for several metal ions and cobalt was found to synergistically enhance enzyme activity. The gelatinases were immunoprecipitated with monospecific polyclonal antisera to macrophage and fibroblast gelatinase; however, these sera did not react with intracellular parasites by indirect immunofluorescence. These results indicate that the matrix metalloproteinases co-isolated with RAP-1 originate from human serum used to cultivate P. falciparum in vitro.

Inhibitory monoclonal antibodies recognise epitopes adjacent to a proteolytic cleavage site on the RAP-1 protein of Plasmodium falciparum

The low-molecular-weight rhoptry-associated protein (RAP) complex of Plasmodium falciparum consists of at least two gene products, RAP-1 and RAP-2, and has the ability to immunise Saimiri monkeys against experimental P. falciparum infection. Several monoclonal antibodies specifically recognise this complex and in this study we show that purified immunoglobulin derived from these monoclonals is capable of inhibiting parasite growth in vitro. It has previously been shown that RAP-1 initially appears as an 80-kDa protein (p80) in early schizogony and is processed to a 65-kDa protein (p65) in late schizogony. Several of the inhibitory monoclonals recognise both the 80- and 65-kDa proteins by Western blot analysis suggesting that they recognise linear epitopes on RAP-1. We have mapped these epitopes by testing the reactivity of the monoclonals against fragments of the rap-1 gene expressed as beta-galactosidase fusion proteins and subsequently against synthetic peptides. All of the epitopes map to a region 10-20 amino acids C-terminal to the proteolytic cleavage site for the processing of p80 to p65 at amino acid 190. We also show that the 65-kDa protein is not present in purified merozoites, suggesting that its generation is associated with merozoite release rather than erythrocyte invasion. These results are discussed with respect to possible inhibitory mechanisms for the monoclonals.

Effects of cytokines, complement, and antibody on the neutrophil respiratory burst and phagocytic response to Plasmodium falciparum merozoites

The interaction between Plasmodium falciparum merozoites and human neutrophils, as well as the role of cytokines, complement, and antimalarial antibody on this interaction, was examined in vitro by measuring luminol-dependent chemiluminescence and phagocytosis. Merozoites, in the presence of heat-inactivated (56 degrees C/30 min) normal serum, had very little effect on the neutrophil chemiluminescence. This response was significantly enhanced by the addition of normal serum (containing normal complement activity). In the presence of serum or plasma containing anti-P. falciparum antibodies (IS) with no detectable complement activity, the merozoites induced a marked response characterized by an increase in initial peak rate of chemiluminescence and a sustained increased rate of chemiluminescence. However, this response was not further increased if IS containing complement activity was used. Pretreatment of neutrophils with either tumor necrosis factor alpha, lymphotoxin, or gamma interferon significantly increased the neutrophil response to IS-treated merozoites, reflected in an increased initial peak rate and sustained increased rate of chemiluminescence. The effects of cytokine treatment of neutrophils and IS opsonization of merozoites were synergistic. In association with the changes in the chemiluminescence responses, IS was shown to promote phagocytosis of merozoites by neutrophils, and this event was further increased by treating neutrophils with the cytokines. The results emphasize the importance of antibody and cytokines in neutrophil-mediated damage of P. falciparum merozoites.

Plasmodium falciparum: identification and purification of the phosphoglycerate kinase of the malaria parasite

Multiplication of the human malaria parasite Plasmodium falciparum within red blood cells is an energy-dependent process and glucose consumption increases dramatically in infected red blood cells (IRBC) versus normal red blood cells (NRBC). The major pathway for glucose metabolism in P. falciparum IRBC is anaerobic glycolysis. Phosphoglycerate kinase (PGK) is one of the key enzymes of this pathway as it generates ATP. We found that the PGK specific activity in P. falciparum IRBC is seven times higher than that in NRBC. The parasitic origin of the increase in PGK activity is confirmed by isoelectric focusing. Indeed, two P. falciparum isoenzymes with neutral isoelectric points were detected. P. falciparum PGK in purified form has a molecular mass of 48 kDa. Antiserum raised against purified P. falciparum PGK specifically recognizes the 48-kDa protein band in P. falciparum and also reacts with P. berghei and P. yoelii IRBC lysates but does not cross-react with PGK associated with NRBC.

A malaria vaccine strategy based on the induction of cellular immunity

In this article, Michael Good offers an alternative to the current emphasis on developing subunit, humoral-response-inducing malaria vaccines. He develops a three-pronged proposal comprising (1) a greater emphasis on cellular immunity, (2) analysis of the role of the spleen in protection, and (3) the development of an attenuated vaccine.

Strategies for the development of an antimalarial vaccine

Susceptible Aotus monkeys were immunized with Escherichia coli-derived fusion proteins containing partial sequences of the proteins MSAI, SERP, HRPII and with a group of three recombinant antigens isolated by screening with an antiserum raised against the protective 41 kDa protein band. HRPII, the combination of the fusion proteins of the 41 kDa group and a mixture of two sequences of SERP conferred significant protection against a challenge infection with Plasmodium falciparum blood stages. Based on the protective capacity of these recombinant antigens we have expressed two hybrid proteins (MS2/SERP/HRPII and SERP/MSAI/HRPII) in E. coli containing selected partial sequences. In two independent immunization trials it was shown that immunization of Aotus monkeys with either of the two hybrid proteins can protect the animals from an experimental P. falciparum infection.

The 42-kilodalton rhoptry-associated protein of Plasmodium falciparum

The gene coding for a 42-kDa rhoptry protein of Plasmodium falciparum has been cloned. On the basis of prior monkey vaccination studies, this protein is regarded as an important vaccine candidate, but its identity has been the subject of considerable uncertainty. Analysis of the cloned sequence shows that it is a basic, hydrophobic protein, without repetitive elements, unrelated to any of the previously postulated gene products and shows minimal sequence diversity. The availability of the corresponding recombinant protein will enable studies of its efficacy in human vaccine trials to be undertaken.

Antibody recognition and isoelectrofocusing of antigens of the malaria parasite Plasmodium yoelii

Inbred BALB/c mice were either immunized with Triton X-100-extracted antigens of blood-stage Plasmodium yoelii or infected with P. yoelii and cured in three successive schedules. Whereas the immunized BALB/c became only partially protected from subsequent challenge infection with blood-stage P. yoelii, the convalescent mice acquired total immunity. When total P. yoelii antigen extract was resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, transferred to nitrocellulose, and immunoblotted with anti-P. yoelii serum, five major protein bands of 150, 84, 40, 19, and 16 kDa were recognized by the sera of fully protected convalescent mice but not by the sera of partially protected mice. The utility of comparing reactivities of sera from fully protected and partially protected malaria hosts and the possibility that antigens uniquely recognized by the convalescent mouse sera may contribute to immunity against P. yoelii infection are discussed. Although previously reported to be an effective adjuvant for immunization against P. yoelii infection in (BALB/c x C57BL)F1 hybrid mice, saponin did not promote protection any better than did Freund adjuvant in BALB/c mice immunized with detergent-extracted P. yoelii antigen. Most of the P. yoelii proteins (14 to 250 kDa) found in Triton X-100 extracts of P. yoelii-parasitized erythrocytes isoelectrofocused as a single peak in the pH region 4.4 to 4.6, suggesting a rationale for previous findings that the most anti-P. yoelii protective and T-helper activities are induced by antigens isoelectrically focused in a fraction of similar pH.

Interaction of the 140/130/110 kDa rhoptry protein complex of Plasmodium falciparum with the erythrocyte membrane and liposomes

During Plasmodium falciparum merozoite invasion into human and mouse erythrocytes, a 110-kDa rhoptry protein is secreted from the organelle into the erythrocyte membrane. In the present study our interest was to examine the interaction of rhoptry proteins of P. falciparum with the erythrocyte membrane. It was observed that the complex of rhoptry proteins of 140/130/110 kDa bind directly to a trypsin sensitive site on intact mouse erythrocytes, and not human, saimiri, or other erythrocytes. However, when erythrocytes were disrupted by hypotonic lysis, rhoptry proteins of 140/130/110 kDa were found to bind to membranes and inside-out vesicles prepared from human, mouse, saimiri, rhesus, rat, and rabbit erythrocytes. A binding site on the cytoplasmic face of the erythrocyte membrane suggests that the rhoptry proteins may be translocated across the lipid bilayer during merozoite invasion. Furthermore, pretreatment of human erythrocytes with a specific peptide derived from MSA-1, the major P. falciparum merozoite surface antigen of MW 190,000-200,000, induced binding of the 140/130/110-kDa complex. The rhoptry proteins bound equally to normal human erythrocytes and erythrocytes treated with neuraminidase, trypsin, and chymotrypsin indicating the binding site was independent of glycophorin and other major surface proteins. The rhoptry protein complex also bound specifically to liposomes prepared from different types of phospholipids. Liposomes containing PE effectively block binding of the rhoptry proteins to mouse cells, suggesting that there are two binding sites on the mouse membrane for the 140/130/110-kDa complex, one protein and a second, possibly lipid in nature. The results of this study suggest that the 140/130/110 kDa protein complex may interact directly with sites in the lipid bilayer of the erythrocyte membrane.

Failure of recombinant vaccinia viruses expressing Plasmodium falciparum antigens to protect Saimiri monkeys against malaria

Saimiri sciurus monkeys were immunized at multiple sites with recombinant vaccinia viruses expressing Plasmodium falciparum antigen genes and boosted 4 weeks later. Control monkeys were immunized with a thymidine kinase-negative vaccinia virus mutant. Two weeks later, all of the monkeys were challenged by intravenous inoculation of P. falciparum (Indochina strain) parasites. A group of unimmunized monkeys was challenged in parallel. All of the monkeys that received vaccinia virus recombinants or the control virus produced good anti-vaccinia virus antibody responses. However, those that received a single construct containing ring-infected erythrocyte surface antigen (RESA) given at eight sites did not produce significant antibody to any of the three major RESA repeat epitopes after immunization but were primed for an enhanced antibody response after challenge infection with P. falciparum. Most of the monkeys produced detectable antibodies to the RESA epitopes after challenge infection. One group of monkeys was immunized with four constructs (expressing RESA, two merozoite surface antigens [MSA-1 and MSA-2], and a rhoptry protein [AMA-1]), each given at two sites. While these monkeys failed to produce significant antibody against MSA-2 or AMA-1 after immunization, they produced enhanced responses against these antigens after challenge infection. Immunization involved an allelic form of MSA-2 different from that present in the parasite challenge strain, so that the enhanced responses seen after challenge infection indicated the presence of T-cell epitopes common to both allelic forms. No groups of monkeys showed any evidence of protection against challenge, as determined by examination of the resulting parasitemias.

Antibody responses to a synthetic peptide-based malaria vaccine candidate: influence of sequence variants of the peptide

A synthetic peptide (Asn-Ala-Asn-Pro)3, representing a protective sequence from the sporozoite stage of Plasmodium falciparum, conjugated to tetanus toxoid has undergone clinical testing. Although some protection was obtained, anti-parasite responses were generally low. In attempting to improve the anti-parasite protein antibody response, we evaluated the efficacy of tetanus toxoid conjugates containing seven sequence variants of the peptide. Most of the conjugates tested in both mice and monkeys elicited anti-peptide antibodies with fine specificity differences, although there was a broad degree of cross-reactivity. In general, each conjugate tested evoked similarly high anti-sporozoite antibody responses in both species and, thus, based on antibody titer no evidence for a superior vaccine candidate was obtained. The biological activity of one antiserum actually increased the penetration of sporozoites into human liver cells. In contrast, antisera against the other conjugates inhibited sporozoite penetration, but to a similar degree. Based on these two criteria, none of the other conjugates would appear to be better vaccine candidates than the original conjugate. The lack of concordance between anti-peptide or anti-sporozoite titers and inhibitory activity in the case of one antiserum indicates that these two measures of antibody cannot always be used for predicting anti-sporozoite activity.

Plasmodium falciparum: hetero-oligomeric complexes of rhoptry polypeptides

We have previously described several monoclonal antibodies (McAbs) which specifically recognize antigens in the rhoptries of Plasmodium falciparum and which immunoprecipitate polypeptides of 82, 70, 67, 39, and 37 kDa. We now show that only p82, p70, p67, and a 86-kDa precursor (Pr86) of p82 possessed epitopes for these McAbs. These four proteins were not synthesized until schizogony. These results and proteolysis experiments indicated that Pr86, p82, p70, and p67 were the products of the same gene, whereas the dissimilar digestion patterns of p39 and p37 suggested that p39 was encoded by a second gene and p37 by yet another. Complexes of these proteins (termed RI complexes) are maintained by noncovalent interactions since the ionic detergent SDS was sufficient to dissociate them into individual polypeptides. Sucrose gradient centrifugation demonstrated that RI complex formation was not dependent on the presence of antibody and that these complexes had higher sedimentation rates than the 185-kDa P. falciparum merozoite glycoprotein. Covalent crosslinking with the reversible, homobifunctional, primary amine-specific reagent 3,3'-dithiobis(sulfosuccinimidylpropionate) followed by RI McAb immunoprecipitation resulted in purification of intact complexes which were not dissociable by SDS alone. Immunodepletion experiments with a subtype of RI McAb which does not immunoprecipitate p37 suggested that the binding of p39 and p37 to the other RI proteins was mutually exclusive. Therefore, the minimal composition of the RI complexes is one molecule of Pr86, p82, p70, or p67 and one of p39 or p37. The epitopes of Pr86, p82, p70, and p67 for the RI McAbs were sensitive to disulfide bond reduction. Surprisingly, reduction increased their electrophoretic mobilities. This enhanced mobility could not be accounted for by post-translational glycosylation, phosphorylation, or acylation, or by covalent attachment via the sulfhydryl moiety of cysteine residues to additional parasite proteins. We suggest that, due to an asymmetric distribution of amino acids in the Pr86-class molecules, SDS binding results in a lower charge to mass ratio in the native folded polypeptides and a higher charge to mass ratio upon disulfide bond reduction and unfolding of the polypeptides.

A rhoptry antigen of Plasmodium falciparum is protective in Saimiri monkeys

A non-polymorphic antigen associated with the rhoptry organelles of Plasmodium falciparum has been purified by immunoaffinity chromatography. The antigen, RAP-1 (rhoptry associated protein-1), which is defined by monoclonal antibodies which inhibit parasite growth in vitro, is a multi-component antigen consisting of four major proteins of 80, 65, 42 and 40 kDa and two minor proteins of 77 and 70 kDa. These proteins were electro-eluted from preparative sodium dodecyl sulphate polyacrylamide gels and protected Saimiri sciureus monkeys from a lethal blood-stage infection of P. falciparum malaria. Sera from the protected animals recognized only proteins of the RAP-1 antigen when used to probe a Western blot of total parasite protein extract, confirming that RAP-1 is responsible for eliciting the protective immune response.

The lower-molecular-weight protein complex (RI) of the Plasmodium falciparum rhoptries lacks the glycolytic enzyme aldolase

The gene for the Plasmodium falciparum glycolytic enzyme aldolase (PfA) has been cloned. Since polyclonal antibodies raised to an affinity-purified preparation of an approximately 41-kDa parasite rhoptry protein were used to isolate this clone, and its nucleotide sequence was verified using amino acid sequence information generated from the purified 41-kDa protein, the authors concluded that this rhoptry protein was PfA. In the present report, monoclonal antibodies which immunoprecipitate those rhoptry protein complexes containing 39-kDa (p39) and 37-kDa proteins (p37) were used along with PfA-specific polyclonal antibodies to further examine this conclusion. The electrophoretic mobilities of PfA and the rhoptry proteins in the presence of sodium dodecyl sulfate differed from each other in both reducing and nonreducing conditions after immunoprecipitation with these antibodies. Lysates of P. falciparum-infected erythrocytes contained abundant aldolase activity which was removed by anti-PfA antibodies. However, no aldolase activity was associated with affinity-purified rhoptry proteins. Controlled proteolysis with the endoproteinase V8 protease generated different digestion patterns for PfA, p39, and p37. While both PfA and the rhoptry proteins were components of multimeric protein complexes, as demonstrated by sucrose gradient centrifugation, their cellular localization patterns were quite different. These results demonstrate that PfA and the rhoptry-associated proteins p39 and p37 are different entities.

Expression, purification, biochemical characterization and inhibition of recombinant Plasmodium falciparum aldolase

The energy metabolism of the blood stage form of the human malaria parasite Plasmodium falciparum is adapted to the host cell. Like erythrocytes, P. falciparum merozoites lack a functional citric acid cycle. Generation of ATP depends therefore fully on the glycolytic pathway. Aldolase is a key enzyme of this pathway and a high degree of sequence diversity between parasite and host makes it a potential drug target. We have expressed the enzyme in its tetrameric form in Escherichia coli and the catalytic constants Vmax and Km of the recombinant enzyme correspond to the constants of parasite-derived aldolase. Rabbit antibodies against the recombinant P. falciparum aldolase inhibit the natural enzyme and no cross-reaction with human aldolase is detectable. Both the recombinant and the natural protein bind to the cytosolic domain of the band 3 membrane protein in vitro. A 19-residue synthetic peptide corresponding to the sequence of the binding domain of band 3 is an inhibitor when included in the binding assay. In addition, this peptide inhibits the catalytic activity of recombinant P. falciparum aldolase when assayed in a buffer system devoid of anions such as chloride or phosphate. The band 3-derived peptides compete with the aldolase substrate fructose-1,6-diphosphate for binding, suggesting that both reagents have a high affinity for the substrate pocket. A similar sequence motif exists in P. falciparum actin II. A 19-residue peptide corresponding to this sequence is also an inhibitor which could suggest that the P. falciparum aldolase can associate with the cytoskeleton of the parasite or of the host.

Characterisation and sequence of a protective rhoptry antigen from Plasmodium falciparum

We have recently demonstrated that a non-polymorphic rhoptry antigen, RAP-1 (rhoptry associated protein-1), which is recognised by human immune serum, can successfully protect Saimiri monkeys from a lethal infection of Plasmodium falciparum malaria. In this report we further characterise the antigen, which consists of four major proteins of 80, 65, 42 and 40 kDa and two minor proteins of 77 and 70 kDa, and present the antigen's gene sequence. Monoclonal antibody evidence, autocatalytic processing and immunological cross-reactivity suggest that all components of this antigen are derived from the same precursor protein. The antigen is lipophilic, and disulphide bonding plays an important role in its structure. We discuss the structure and function of RAP-1 in the light of its deduced amino acid sequence and consider the relationship of this antigen to other rhoptry antigens of similar subunit size and composition.

Immunization of Aotus monkeys with Plasmodium falciparum blood-stage recombinant proteins

The current spread of multidrug-resistant malaria demands rapid vaccine development against the major pathogen Plasmodium falciparum. The high quantities of protein required for a worldwide vaccination campaign select recombinant DNA technology as a practical approach for large-scale antigen production. We describe the vaccination of Aotus monkeys with two recombinant blood-stage antigens (recombinant p41 and 190N) that were considered as vaccine candidates because parasite-derived antigen preparations could protect susceptible monkeys from an otherwise lethal malaria infection. In contrast to the natural antigen, recombinant p41 protein (P. falciparum aldolase) could not protect monkeys, although all animals seroconverted. 190N antigen, a recombinant protein containing conserved sequences of the major merozoite surface antigen p190, protected two of five monkeys from critical levels of infection with the highly virulent FVO isolate of P. falciparum. However, the B- and T-cell responses to 190N antigen were similar in protected and unprotected animals so that other unknown factors may contribute to protection. Higher purity or lack of protective epitopes or different structure of protective epitopes in the recombinant proteins might explain the better performance of parasite-derived antigens in vaccination trials. The partial protection obtained with 190N antigen suggests that this molecule could contribute to a vaccine mixture against P. falciparum.

Eimeria acervulina: cloning of a cDNA encoding an immunogenic region of several related merozoite surface and rhoptry proteins

A cDNA encoding a recombinant Eimeria acervulina antigen, designated EAMZp30-47, that contains an epitope shared among several surface and rhoptry proteins of merozoites was characterized. The respective parasite proteins are between 30 and 47 kDa as revealed by immunostaining of nitrocellulose membrane containing extracts of 125I-labeled merozoites. As indicated by immunofluorescence and immunoelectron microscopic staining, the reactive epitope was localized to both the surface membrane and the internal rhoptries of this asexual stage of the parasite. The recombinant beta-galactosidase fusion protein EAMZp30-47 is 130 kDa, thus representing 15 kDa or 30-50% of the respective parasite protein. Purified EAMZp30-47 stimulates T cells from E. acervulina-immune inbred chickens, but is not recognized by immune chicken serum, suggesting that T cell and not B cell epitopes recognized by the host immune system during a natural infection are present on the recombinant protein. Northern and Southern blot hybridization experiments indicated that expression of EAMZp30-47 is restricted to the merozoite stage of the parasite and the gene occurs as a single copy sequence within the genome.

Plasmodium falciparum aldolase: gene structure and localization

A genomic clone was isolated which codes for the fructose bisphosphate aldolase of Plasmodium falciparum. The aldolase gene is interrupted by one intron which divides the coding region into two exons. The first one codes for one amino acid only, the initiation methionine, while the second one encodes the residual 368 amino acids of the protein. The gene, which is represented only once in the genome, is transcribed at high rates as a 2.4-kb mRNA in the P. falciparum blood stage. The aldolase gene encodes a protein of 40,105 Da, which is 61-68% homologous to known eukaryotic aldolases. The protein was expressed in Escherichia coli cells in an unfused and enzymatically active form. Antisera raised against amino acids 9-96 recognize a 41-kDa protein band previously shown to protect monkeys against a P. falciparum infection. These antisera cross-react with aldolases of different species, which confirms the strong conservation of this enzyme during evolution. The aldolase could be localized in the cytoplasm of the parasite as an active and soluble form. An inactive form was found to be associated with the membrane fraction. Digestion data with phospholipase C suggest a membrane association of this polypeptide via a glycosylphosphatidylinositol anchor.

Plasmodium falciparum: two antigens of similar size are located in different compartments of the rhoptry

Two previously described antigens, AMA-1 and QF3, which are located in the rhoptries of Plasmodium falciparum merozoites have polypeptides of similar relative molecular masses. On immunoblots, antibodies to both antigens recognized polypeptides of relative molecular mass 80,000 and 62,000 in all isolates tested. Two-dimensional electrophoresis showed that the isoelectric points of the two antigens were different. QF3 being more basic than AMA-1. AMA-1 was soluble in Triton X-114 whereas QF3 partitioned into the aqueous phase after temperature-dependent phase separation. In immunoelectron microscopic studies. QF3 was found in the body of the rhoptry whereas AMA-1 was consistently found in the neck of the rhoptry. Both antigens gave a punctate double-dot pattern in mature schizonts and merozoites when visualized by fluorescence microscopy, but AMA-1 antibodies also appeared to label the merozoite surface. QF3 was also detected in ring-infected erythrocytes whereas AMA-1 was not. Synthesis of both antigens was first observed in mature trophozoites and immature schizonts. Pulse-chase experiments showed that the Mr 80,000 polypeptide of the AMA-1 gene was subject to immediate processing to the Mr 62,000 product. This cleavage pattern was not stage specific. The Mr 80,000 polypeptide of QF3 was derived from a short-lived Mr 84,000 precursor polypeptide. Processing of the Mr 80,000 polypeptide to an Mr 62,000 polypeptide was restricted to the period of merozoite maturation and reinvasion. Hence AMA-1 and QF3 are different antigens with polypeptides of similar size but located in different compartments of the merozoite rhoptries.

Evidence for surface-coat localisation of a monoclonal antibody-isolated merozoite antigen of Babesia divergens

The localisation of monoclonal antibody-derived merozoite antigens of Babesia divergens was examined using immunogold electron microscopy and immunoprecipitation of the monoclonal antibody with both biosynthetically and surface-labelled parasites. Immunogold labelling provided evidence that the antigens are components of the surface coat of the merozoite. Immunoprecipitation with biosynthetically labelled parasites showed the antigens to be of parasite origin, whereas surface labelling confirmed that the antigens form part of the surface coat.

Integral membrane protein located in the apical complex of Plasmodium falciparum

We describe the cloning of a novel antigen of Plasmodium falciparum which contains a hydrophobic domain typical of an integral membrane protein. This antigen is designated apical membrane antigen 1 because it appears to be located in the apical complex. Apical membrane antigen 1 appears to be transported to the merozoite surface near the time of schizont rupture.

Integral membrane protein located in the apical complex of Plasmodium falciparum

We describe the cloning of a novel antigen of Plasmodium falciparum which contains a hydrophobic domain typical of an integral membrane protein. This antigen is designated apical membrane antigen 1 because it appears to be located in the apical complex. Apical membrane antigen 1 appears to be transported to the merozoite surface near the time of schizont rupture.