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

Apical membrane antigen 1: a malaria vaccine candidate in review

Apical membrane antigen 1 (AMA1) is a micronemal protein of apicomplexan parasites that appears to be essential during the invasion of host cells. Immune responses to Plasmodium AMA1 can have profound parasite-inhibitory effects, both as measured in vitro and in animal challenge models, suggesting AMA1 as a potential vaccine component. However, AMA1 is polymorphic, probably as a result of immune selection operating on an important target of naturally occurring immunity. The current understanding of AMA1 will be presented, particularly in relation to the vaccine potential of AMA1 and the approaches being taken towards clinical development.

Impact of a Plasmodium falciparum AMA1 vaccine on antibody responses in adult Malians

Background

Apical Membrane Antigen 1 (AMA1) of Plasmodium falciparum merozoites is a leading blood-stage malaria vaccine candidate. Protection of Aotus monkeys after vaccination with AMA1 correlates with antibody responses.

Study Design/Results

A randomized, controlled, double-blind phase 1 clinical trial was conducted in 54 healthy Malian adults living in an area of intense seasonal malaria transmission to assess the safety and immunogenicity of the AMA1-C1 malaria vaccine. AMA1-C1 contains an equal mixture of yeast-expressed recombinant proteins based on sequences from the FVO and 3D7 clones of P. falciparum, adsorbed on Alhydrogel. The control vaccine was the hepatitis B vaccine (Recombivax). Participants were enrolled into 1 of 3 dose cohorts (n = 18 per cohort) and randomized 2∶1 to receive either AMA1-C1 or Recombivax. Participants in the first, second, and third cohorts randomized to receive AMA1-C1 were vaccinated with 5, 20 and 80 µg of AMA1-C1, respectively. Vaccinations were administered on days 0, 28, and 360, and participants were followed until 6 months after the final vaccination. AMA1-C1 was well tolerated; no vaccine-related severe or serious adverse events were observed. AMA1 antibody responses to the 80 µg dose increased rapidly from baseline levels by days 14 and 28 after the first vaccination and continued to increase after the second vaccination. After a peak 14 days following the second vaccination, antibody levels decreased to baseline levels one year later at the time of the third vaccination that induced little or no increase in antibody levels.

Conclusions

Although the AMA1-C1 vaccine candidate was well-tolerated and induced antibody responses to both vaccine and non-vaccine alleles, the antibody response after a third dose given at one year was lower than the response to the initial vaccinations. Additionally, post-vaccination increases in anti-AMA1 antibody levels were not associated with significant changes in in vitro growth inhibition of P. falciparum.

Trial Registration

ClinicalTrials.gov NCT00343005

In immunization with Plasmodium falciparum apical membrane antigen 1, the specificity of antibodies depends on the species immunized

At least a million people, mainly African children under 5 years old, still die yearly from malaria, and the burden of disease and death has increased. Plasmodium falciparum apical membrane antigen 1 (PfAMA1) is one of the most promising blood-stage malarial vaccine candidates. However, the allelic polymorphism observed in this protein is a potential stumbling block for vaccine development. To overcome the polymorphism- and strain-specific growth inhibition in vitro, we previously showed in a rabbit model that vaccination with a mixture of two allelic forms of PfAMA1 induced parasite growth-inhibitory antisera against both strains of P. falciparum parasites in vitro. In the present study, we have established that, in contrast to a single-allele protein, the antigen mixture elicits primarily antibodies recognizing antigenic determinants common to the two antigens, as judged by an antigen reversal growth inhibition assay (GIA). We also show that a similar reactivity pattern occurs after immunization of mice. By contrast, sera from rhesus monkeys do not distinguish the two alleles when tested by an enzyme-linked immunosorbent assay or by GIA, regardless of whether the immunogen is a single AMA1 protein or the mixture. This is the first report that a malarial vaccine candidate induced different specificities of functional antibodies depending on the animal species immunized. These observations, as well as data available on human immune responses in areas of endemicity, suggest that polymorphism in the AMA1 protein may not be as formidable a problem for vaccine development as anticipated from studies with rabbits and mice.

Fine mapping of an epitope recognized by an invasion-inhibitory monoclonal antibody on the malaria vaccine candidate apical membrane antigen 1

Antibodies that inhibit red blood cell invasion by the Plasmodium merozoite block the erythrocytic cycle responsible for clinical malaria. The invasion-inhibitory monoclonal antibody (mAb) 4G2 recognizes a conserved epitope in the ectodomain of the essential Plasmodium falciparum microneme protein and vaccine candidate, apical membrane antigen 1 (PfAMA1). Here we demonstrate that purified Fab fragments of 4G2 inhibit invasion markedly more efficiently than the intact mAb, suggesting that the invasion-inhibitory activity of this mAb is not due solely to steric effects and that the epitope lies within a functionally critical region of the molecule. We have taken advantage of a synthetic gene encoding a modified form of PfAMA1, and existing x-ray crystal structure data, to fully characterize this epitope. We first validate the gene by demonstrating that it fully complements the function of the authentic gene in P. falciparum. We then use it to identify a group of residues within the previously described domain II loop of PfAMA1 that are critical for recognition by mAb 4G2 and demonstrate that the epitope lies exclusively within this loop with no contributions from residues in other domains of the molecule. This is the first complete characterization of a conserved invasion-inhibitory epitope on PfAMA1. Our results will aid in the design of subunit vaccines designed to generate a broadly effective, focused anti-PfAMA1 protective immune response and may help elucidate the function of PfAMA1.

Antibody responses to Plasmodium falciparum vaccine candidate antigens in three areas distinct with respect to altitude

Antibody levels against malaria antigens were measured among patients presenting with uncomplicated malaria at health centers from three locations in Zimbabwe (Bindura, Chiredzi and Kariba) that are distinct with regard to altitude and climatic conditions. Antibody levels were determined by ELISA using the antigens, apical membrane antigen 1 (AMA-1), erythrocyte binding antigen 175 (EBA-175), circumsporozoite surface protein (CSP), merozoite surface protein 1 (MSP-1) and Pfg27. For all the antigens tested, IgG and IgM levels were higher for Bindura (altitude 1100 m) compared to Kariba (<600 m, altitude) and Chiredzi (approximately 600 m, altitude) with the exception of IgG and IgM to AMA-1 and EBA-175 which were similar between Chiredzi and Bindura. Plasma samples were further analyzed for their functional activity by testing their ability to inhibit the growth of Plasmodium falciparum in culture. Our results, determined by microscopy and verified by the LDH assay revealed that plasma from the three locations had similar inhibitory activity against the growth of P. falciparum in vitro. Our data revealed that highest growth inhibition correlated with the highest levels of MSP-1 antibody values.

Mimotopes of apical membrane antigen 1: Structures of phage-derived peptides recognized by the inhibitory monoclonal antibody 4G2dc1 and design of a more active analogue

Apical membrane antigen 1 (AMA1) of the malaria parasite Plasmodium falciparum is an integral membrane protein that plays a key role in merozoite invasion of host erythrocytes. A monoclonal antibody, 4G2dc1, recognizes correctly folded AMA1 and blocks merozoite invasion. Phage display was used to identify peptides that bind to 4G2dc1 and mimic an important epitope of AMA1. Three of the highest-affinity binders--J1, J3, and J7--were chosen for antigenicity and immunogenicity studies. J1 and J7 were found to be true antigen mimics since both peptides generated inhibitory antibodies in rabbits (J. L. Casey et al., Infect. Immun. 72:1126-1134, 2004). In the present study, the solution structures of all three mimotopes were investigated by nuclear magnetic resonance spectroscopy. J1 adopted a well-defined region of structure, which can be attributed in part to the interactions of Trp11 with surrounding residues. In contrast, J3 and J7 did not adopt an ordered conformation over the majority of residues, although they share a region of local structure across their consensus sequence. Since J1 was the most structured of the peptides, it provided a template for the design of a constrained analogue, J1cc, which shares a structure similar to that of J1 and has a disulfide-stabilized conformation around the Trp11 region. J1cc binds with greater affinity to 4G2dc1 than does J1. These peptide structures provide the foundation for a better understanding of the complex conformational nature of inhibitory epitopes on AMA1. With its greater conformational stability and higher affinity for AMA1, J1cc may be a better in vitro correlate of immunity than the peptides identified by phage display.

Passive immunization with a multicomponent vaccine against conserved domains of apical membrane antigen 1 and 235-kilodalton rhoptry proteins protects mice against Plasmodium yoelii blood-stage challenge infection

During malaria parasite invasion of red blood cells, merozoite proteins bind receptors on the surface of the erythrocyte. Two candidate Plasmodium yoelii adhesion proteins are apical membrane antigen 1 (AMA1) and the 235-kDa rhoptry proteins (P235). Previously, we have demonstrated that passive immunization with monoclonal antibodies (MAbs) 45B1 and 25.77 against AMA1 and P235, respectively, protects against a lethal challenge infection with P. yoelii YM. We show that MAb 45B1 recognizes an epitope located on a conserved surface of PyAMA1, as determined by phage display and analysis of the three-dimensional structure of AMA1, in a region similar to that bound by the P. falciparum AMA1-specific inhibitory antibody 4G2. The epitope recognized by 25.77 could not be assigned. We report here that MAbs 45B1 and 25.77 also protect against challenge with the nonlethal parasite line 17X, in which PyAMA1 has a significantly different amino acid sequence from that in YM. When administered together, the two MAbs acted at least additively in providing protection against challenge with the virulent YM parasite. These results support the concept of developing a multicomponent blood-stage vaccine and the inclusion of polymorphic targets such as AMA1, which these results suggest contain conserved domains recognized by inhibitory antibodies.

Immune responses in mice induced by prime-boost schemes of the Plasmodium falciparum apical membrane antigen 1 (PfAMA1)-based DNA, protein and recombinant modified vaccinia Ankara vaccines

The apical membrane antigen 1 (AMA1) of malaria parasites is a leading vaccine candidate. Its expression in merozoites and sporozoites and its importance for erythrocyte and hepatocyte invasion underline the significance of both humoral and cellular immunities against this antigen in malaria protection. We have generated a DNA construct and a recombinant poxvirus (rMVA) for expressing the Plasmodium falciparum AMA1 ectodomain, produced recombinant AMA1 protein (rAMA1) and evaluated their antigenicity in mice using single and combinatory vaccine schemes. Our results showed that although vaccinations of mice by either DNA or rMVA alone did not yield high antibody responses, they had primed significant numbers of rAMA1-responsive splenocytes. Under heterologous prime-boost schemes, priming with DNA followed by boosting with rMVA or rAMA1 protein resulted in a significant increase in antibody titers. In addition, the antibody titers to AMA1 appeared to be correlated with the levels of inhibition of merozoite invasion of erythrocytes in vitro. Furthermore, different prime-boost schemes resulted in different AMA1-specific antibody isotype (IgG1/IgG2a) ratios, providing us with an indication about Th1 or Th2 responses the vaccination regimens have induced. This study has yielded useful information for further in vivo evaluation of the suitability and effectiveness of the heterologous prime-boost strategy in AMA1 vaccination.

Binding hot spot for invasion inhibitory molecules on Plasmodium falciparum apical membrane antigen 1

Apical membrane antigen 1 (AMA1) is expressed in schizont-stage malaria parasites and sporozoites and is thought to be involved in the invasion of host red blood cells. AMA1 is an important vaccine candidate, as immunization with this antigen induces a protective immune response in rodent and monkey models of human malaria. Additionally, anti-AMA1 polyclonal and monoclonal antibodies inhibit parasite invasion in vitro. We have isolated a 20-residue peptide (R1) from a random peptide library that binds to native AMA1 as expressed by Plasmodium falciparum parasites. Binding of R1 peptide is dependent on AMA1 having the proper conformation, is strain specific, and results in the inhibition of merozoite invasion of host erythrocytes. The solution structure of R1, as determined by nuclear magnetic resonance spectroscopy, contains two structured regions, both involving turns, but the first region, encompassing residues 5 to 10, is hydrophobic and the second, at residues 13 to 17, is more polar. Several lines of evidence reveal that R1 targets a "hot spot" on the AMA1 surface that is also recognized by other peptides and monoclonal antibodies that have previously been shown to inhibit merozoite invasion. The functional consequence of binding to this region by a variety of molecules is the inhibition of merozoite invasion into host erythrocytes. The interaction between these peptides and AMA1 may further our understanding of the molecular mechanisms of invasion by identifying critical functional regions of AMA1 and aid in the development of novel antimalarial strategies.

Structural comparison of apical membrane antigen 1 orthologues and paralogues in apicomplexan parasites

Apical membrane antigen 1 (AMA1) is a membrane protein present in Plasmodium species and is probably common to all apicomplexan parasites. The recent crystal structure of the complete ectoplasmic region of AMA1 from Plasmodium vivax has shown that it comprises three structural domains and that the first two domains are based on the PAN folding motif. Here, we discuss the consequences of this analysis for the three-dimensional structure of AMA1 from other Plasmodium species and other apicomplexan parasites, and for the Plasmodium paralogue MAEBL. Many polar and apolar interactions observed in the PvAMA1 crystal structure are made by residues that are invariant or highly conserved throughout all Plasmodium orthologues; a subgroup of these residues is also present in other apicomplexan orthologues and in MAEBL. These interactions presumably play a key role in defining the protein fold. Previous studies have shown that the ectoplasmic region of AMA1 must be cleaved from the parasite surface for host-cell invasion to proceed. The cleavage site in the crystal structure is not readily accessible to proteases and we discuss possible consequences of this observation. The three-dimensional distribution of polymorphic sites in PfAMA1 shows that these are all on the surface and that their positions are significantly biased to one side of the ectoplasmic region. Of particular note, a flexible segment in domain II, comprising about 40 residues and devoid of polymorphism, carries an epitope recognized by an invasion-inhibitory monoclonal antibody and a T-cell epitope implicated in the human immune response to AMA1.

Structure of AMA1 from Plasmodium falciparum reveals a clustering of polymorphisms that surround a conserved hydrophobic pocket

Apical membrane antigen 1 (AMA1) is a leading malaria vaccine candidate that possesses polymorphisms that may pose a problem for a vaccine based on this antigen. Knowledge of the distribution of the polymorphic sites on the surface of AMA1 is necessary to obtain a detailed understanding of their significance for vaccine development. For this reason we have sought to determine the three-dimensional structure of AMA1 using x-ray crystallography. The central two-thirds of AMA1 is relatively conserved among Plasmodium species as well as more distantly related apicomplexan parasites, and contains two clusters of disulfide-bonded cysteines termed domains I and II. The crystal structure of this fragment of AMA1 reported here reveals that domains I+II consists of two intimately associated PAN domains. PAN domain I contains many long loops that extend from the domain core and form a scaffold for numerous polymorphic residues. This extreme adaptation of a PAN domain reveals how malaria parasites have introduced significant flexibility and variation into AMA1 to evade protective human antibody responses. The polymorphisms on the AMA1 surface are exclusively located on one side of the molecule, presumably because this region of AMA1 is most accessible to antibodies reacting with the parasite surface. Moreover, the most highly polymorphic residues surround a conserved hydrophobic trough that is ringed by domain I and domain II loops. Precedents set by viral receptor proteins would suggest that this is likely to be the AMA1 receptor binding pocket.

posttranslational modification of recombinant Plasmodium falciparum apical membrane antigen 1: impact on functional immune responses to a malaria vaccine candidate

Recombinant apical membrane antigen 1 (AMA1) is a leading vaccine candidate for Plasmodium falciparum malaria, as antibodies against recombinant P. falciparum AMA1 (PfAMA1) interrupt merozoite invasion into erythrocytes. In order to investigate the role of posttranslational modification in modulating the functional immune response to recombinant AMA1, two separate alleles of PfAMA1 (FVO and 3D7), in which native N-glycosylation sites have been mutated, were produced using Escherichia coli and a Pichia pastoris expression system. Recombinant Pichia pastoris AMA1-FVO (PpAMA1-FVO) and PpAMA1-3D7 are O-linked glycosylated, and 45% of PpAMA1-3D7 is nicked, though all four recombinant molecules react with conformation-specific monoclonal antibodies. To address the immunological effect of O-linked glycosylation, we compared the immunogenicity of E. coli AMA1-FVO (EcAMA1-FVO) and PpAMA1-FVO antigens, since both molecules are intact. The effect of antigen nicking was then investigated by comparing the immunogenicity of EcAMA1-3D7 and PpAMA1-3D7. Our data demonstrate that there is no significant difference in the rabbit antibody titer elicited towards EcAMA1-FVO and PpAMA1-FVO or to EcAMA1-3D7 and PpAMA1-3D7. Furthermore, we have demonstrated that recombinant AMA1 (FVO or 3D7), whether expressed and refolded from E. coli or produced from the Pichia expression system, is equivalent and mimics the functionality of the native protein in in vitro growth inhibition assay experiments. We conclude that in the case of recombinant AMA1, the E. coli- and P. pastoris-derived antigens are immunologically and functionally equivalent and are unaffected by the posttranslational modification resulting from expression in these two systems.

Phase 1 clinical trial of apical membrane antigen 1: an asexual blood-stage vaccine for Plasmodium falciparum malaria

Apical membrane antigen 1 (AMA1), a polymorphic merozoite surface protein, is a leading blood-stage malaria vaccine candidate. A phase 1 trial was conducted with 30 malaria-naive volunteers to assess the safety and immunogenicity of the AMA1-C1 malaria vaccine. AMA1-C1 contains an equal mixture of recombinant proteins based on sequences from the FVO and 3D7 clones of Plasmodium falciparum. The proteins were expressed in Pichia pastoris and adsorbed on Alhydrogel. Ten volunteers in each of three dose groups (5 mug, 20 mug, and 80 mug) were vaccinated in an open-label study at 0, 28, and 180 days. The vaccine was well tolerated, with pain at the injection site being the most commonly observed reaction. Anti-AMA1 immunoglobulin G (IgG) was detected by enzyme-linked immunosorbent assay (ELISA) in 15/28 (54%) volunteers after the second immunization and in 23/25 (92%) after the third immunization, with equal reactivity to both AMA1-FVO and AMA1-3D7 vaccine components. A significant dose-response relationship between antigen dose and antibody response by ELISA was observed, and the antibodies were predominantly of the IgG1 isotype. Confocal microscopic evaluation of sera from vaccinated volunteers demonstrated reactivity with P. falciparum schizonts in a pattern similar to native parasite AMA1. Antigen-specific in vitro inhibition of both FVO and 3D7 parasites was achieved with IgG purified from sera of vaccinees, demonstrating biological activity of the antibodies. To our knowledge, this is the first AMA1 vaccine candidate to elicit functional immune responses in malaria-naive humans, and our results support the further development of this vaccine.

A human phase 1 vaccine clinical trial of the Plasmodium falciparum malaria vaccine candidate apical membrane antigen 1 in Montanide ISA720 adjuvant

A dose escalating, placebo-controlled phase 1 trial was conducted to test the safety and immunogenicity of a vaccine containing recombinant Plasmodium falciparum apical membrane antigen 1 (AMA1) formulated in Montanide ISA720. Three groups of volunteers were vaccinated intramuscularly with 5 microg, 20 microg or 80 microg of AMA1, respectively, in 0.5 mL of formulation at 0, 3 and 6 months. Anti-AMA1 antibody levels and T cell stimulation indices were measured before and after each vaccination. No vaccine-related serious adverse events were recorded. Most subjects generated a mild to moderate, transient local reaction after the first vaccination. Three subjects developed a local reaction approximately 10 days following vaccination. Six of the 29 subjects seroconverted. Only one of these developed a high antibody titre. However, the interpretation of this trial was compromised by a loss of potency of the formulated vaccine during the course of the study.

Crystal structure of the malaria vaccine candidate apical membrane antigen 1

Apical membrane antigen 1 from Plasmodium is a leading malaria vaccine candidate. The protein is essential for host-cell invasion, but its molecular function is unknown. The crystal structure of the three domains comprising the ectoplasmic region of the antigen from P. vivax, solved at 1.8 angstrom resolution, shows that domains I and II belong to the PAN motif, which defines a superfamily of protein folds implicated in receptor binding. We also mapped the epitope of an invasion-inhibitory monoclonal antibody specific for the P. falciparum ortholog and modeled this to the structure. The location of the epitope and current knowledge on structure-function correlations for PAN domains together suggest a receptor-binding role during invasion in which domain II plays a critical part. These results are likely to aid vaccine and drug design.

Allele specificity of naturally acquired antibody responses against Plasmodium falciparum apical membrane antigen 1

Antibody responses against proteins located on the surface or in the apical organelles of merozoites are presumed to be important components of naturally acquired protective immune responses against the malaria parasite Plasmodium falciparum. However, many merozoite antigens are highly polymorphic, and antibodies induced against one particular allelic form might not be effective in controlling growth of parasites expressing alternative forms. The apical membrane antigen 1 (AMA1) is a polymorphic merozoite protein that is a target of naturally acquired invasion-inhibitory antibodies and is a leading asexual-stage vaccine candidate. We characterized the antibody responses against AMA1 in 262 individuals from Papua New Guinea exposed to malaria by using different allelic forms of the full AMA1 ectodomain and some individual subdomains. The majority of individuals had very high levels of antibodies against AMA1. The prevalence and titer of these antibodies increased with age. Although antibodies against conserved regions of the molecule were predominant in the majority of individuals, most plasma samples also contained antibodies directed against polymorphic regions of the antigen. In a few individuals, predominantly from younger age groups, the majority of antibodies against AMA1 were directed against polymorphic epitopes. The D10 allelic form of AMA1 apparently contains most if not all of the epitopes present in the other allelic forms tested, which might argue for its inclusion in future AMA1-based vaccines to be tested. Some important epitopes in AMA1 involved residues located in domain II or III but depended on more than one domain.

Antibody response of naturally infected individuals to recombinant Plasmodium vivax apical membrane antigen-1

In the present study, we evaluate the naturally acquired antibody response to the Plasmodium vivax apical membrane antigen 1 (PvAMA-1), a leading vaccine candidate against malaria. The gene encoding the PvAMA-1 ectodomain region (amino acids 43-487) was cloned by PCR using genomic DNA from a Brazilian individual with patent P. vivax infection. The predicted amino acid sequence displayed a high degree of identity (97.3%) with a previously published sequence from the P. vivax Salvador strain. A recombinant protein representing the PvAMA-1 ectodomain was expressed in Escherichia coli and refolded. By ELISA, this recombinant protein reacted with 85 and 48.5% of the IgG or IgM antibodies, respectively, from Brazilian individuals with patent P. vivax malaria. IgG1 was the predominant subclass of IgG. The frequency of response increased according to the number of malaria episodes, reaching 100% in individuals in their fourth malaria episode. The high degree of recognition of PvAMA-1 by human antibodies was confirmed using a second recombinant protein expressed in Pichia pastoris (PV66/AMA-1). The observation that recognition of the bacterial recombinant PvAMA-1 was only slightly lower than that of the highly immunogenic 19kDa C-terminal domain of the P. vivax Merozoite Surface Protein-1 was also important. DNA sequencing of the PvAMA-1 variable domain from 20 Brazilian isolates confirmed the limited polymorphism of PvAMA-1 suggested by serological analysis. In conclusion, we provide evidence that PvAMA-1 is highly immunogenic during natural infection in humans and displays limited polymorphism in Brazil. Based on these observations, we conclude that PvAMA-1 merits further immunological studies as a vaccine candidate against P. vivax malaria.

Production of the subdomains of the Plasmodium falciparum apical membrane antigen 1 ectodomain and analysis of the immune response

The apical membrane antigen 1 of Plasmodium falciparum is one of the leading candidate antigens being developed as a vaccine to prevent malaria. This merozoite transmembrane protein has an ectodomain that can be divided into three subdomains (D I, D II, and D III). We have previously expressed a major portion of this ectodomain and have shown that it can induce antibodies that prevent merozoite invasion into red blood cells in an in vitro growth and invasion assay. To analyze the antibody responses directed against the individual subdomains, we constructed six different genes that express each of the domains separately (D I, D II, or D III) or in combination with another domain (D I+II, D II+III, or D I+III). These proteins were purified and used to immunize rabbits to raise construct-specific antibodies. We demonstrated that D I+II induced a significant amount of the growth-inhibitory antibodies active in the growth and invasion assay.

Analysis of the Plasmodium and Anopheles transcriptomes during oocyst differentiation

Understanding the life cycle of the malaria parasite in its mosquito vector is essential for developing new strategies to combat this disease. Subtractive hybridization cDNA libraries were constructed that are enriched for Plasmodium berghei and Anopheles stephensi genes expressed during oocyst differentiation on the midgut. Sequencing of 1485 random clones led to the identification of 1137 unique expressed sequence tags. Of the 608 expressed sequence tags with data base hits, 320 (53%) had significant matches to the non-redundant protein data base, whereas 288 (47%) with matches only to genomic data bases represent novel Plasmodium and Anopheles genes. Transcription of six novel parasite genes and two previously identified asexual stage genes was up-regulated during oocyst differentiation. In addition, the mRNA for an Anopheles fibrinogen domain gene was induced on day 2 after an infectious blood meal, at the time of ookinete to oocyst differentiation. The subcellular distribution of MAEBL, a sporozoite surface protein, is developmentally regulated from presumed storage organelles in day 15 oocysts to uniform distribution on the surface in day 22 oocysts. This redistribution may reflect a sporozoite maturation program in preparation for salivary gland invasion. Furthermore, apical membrane antigen 1, another parasite surface molecule, is translationally regulated late in sporozoite development, suggesting a role during infection of the vertebrate host. The present results and those of an accompanying report (Abraham, E. G., Islam, S., Srinivasan, P., Ghosh, A. K., Valenzuela, J., Ribeiro, J. M., Kafatos, F. C., Dimopoulos, G., & Jacobs-Lorena, M. (2003) J. Biol. Chem. 279, 5573-5580) provide the foundation for studies seeking to understand at the molecular level Plasmodium development and its interactions with the mosquito.

Apical membrane antigen 1, a major malaria vaccine candidate, mediates the close attachment of invasive merozoites to host red blood cells

Apical membrane antigen 1 (AMA-1) of Plasmodium merozoites is established as a candidate molecule for inclusion in a human malaria vaccine and is strongly conserved in the genus. We have investigated its function in merozoite invasion by incubating Plasmodium knowlesi merozoites with red cells in the presence of a previously described rat monoclonal antibody (MAb R31C2) raised against an invasion-inhibitory epitope of P. knowlesi AMA-1 and then fixing the material for ultrastructural analysis. We have found that the random, initial, long-range (12 nm) contact between merozoites and red cells occurs normally in the presence of the antibody, showing that AMA-1 plays no part in this stage of attachment. Instead, inhibited merozoites fail to reorientate, so they do not bring their apices to bear on the red cell surface and do not make close junctional apical contact. We conclude that AMA-1 may be directly responsible for reorientation or that the molecule may initiate the junctional contact, which is then presumably dependent on Duffy binding proteins for its completion.

Plasmodium falciparum apical membrane antigen 1 (PfAMA-1) is translocated within micronemes along subpellicular microtubules during merozoite development

During the assembly of Plasmodium falciparum merozoites within the schizont stage, the parasite synthesizes and positions three sets of secretory vesicles (rhoptries, micronemes and dense granules) that are active during red cell invasion. There are up to 40 micronemes per merozoite, shaped like long-necked bottles, about 160 nm long and 65 nm at their widest diameter. On their external surfaces, they bear bristle-like filaments, each 3-4 nm thick and 25 nm long. Micronemes are translocated from a single Golgi-like cisterna near the nucleus along a band of two or three subpellicular microtubules to the merozoite apex, where they dock with the rhoptry tips. Dense granules are also formed around the periphery of the Golgi cisternae but their distribution is unrelated to microtubules. Three polyclonal antibodies raised against the recombinant PfAMA-1 ectodomain sequence recognizing both the 83 kDa and processed 66 kDa molecules label the peripheries of translocating and mature micronemes but do not label rhoptries significantly at any stage of merozoite development within schizonts. This result confirms that PfAMA-1 is a micronemal protein, and indicates that within the microneme it is located near or inserted into this organelle's boundary membrane.

Induction of parasite growth-inhibitory antibodies by a virosomal formulation of a peptidomimetic of loop I from domain III of Plasmodium falciparum apical membrane antigen 1

Apical membrane antigen 1 (AMA-1) of Plasmodium falciparum is a leading candidate antigen for inclusion in a malaria subunit vaccine. Its ectodomain can be divided into three subdomains, each with disulfide bond-stabilized structures. Since the majority of antibodies raised against the ectodomain appear to recognize strain-specific epitopes in domain I, we attempted to develop a vaccine formulation which directs the immune response to a region that contains more conserved epitopes. Here we demonstrate that a virosomal formulation of a peptide that mimics the semiconserved loop I of domain III elicits parasite growth-inhibitory antibodies. A synthetic peptide comprising residues 446 to 490 of AMA-1 (AMA-1(446-490)) was conjugated through the N terminus to a derivative of phosphatidylethanolamine and the phosphatidylethanolamine-peptide conjugate was incorporated into immunopotentiating reconstituted influenza virosomes as a human-compatible antigen delivery system. Both cyclized and linear versions of the peptide antigen elicited antibodies which specifically bound to parasite-expressed AMA-1 in Western blotting with parasite lysates as well as in immunofluorescence assays with blood stage parasites. All 11 peptidomimetic-specific monoclonal antibodies generated were cross-reactive with parasite-expressed AMA-1. Antigen binding assays with a library of overlapping cyclic peptides covering the target sequence revealed differences in the fine specificity of these monoclonal antibodies and provided evidence that at least some of them recognized discontinuous epitopes. The two immunodominant epitopes comprised the conserved linear sequences K(459)RIKLN(464) and D(467)DEGNKKII(475). A key feature of the synthetic vaccine formulation proposed here is the display of the peptide antigen in a native-like state on the surface of the virosome.

Geographical structure of diversity and differences between symptomatic and asymptomatic infections for Plasmodium falciparum vaccine candidate AMA1

Plasmodium falciparum apical membrane antigen 1 (AMA1) is a prime malaria vaccine candidate. Antigenic diversity within parasite populations is one of the main factors potentially limiting the efficacy of any asexual-stage vaccine, including one based on AMA1. The DNA coding for the most variable region of this antigen, domain I, was sequenced in 168 samples from the Wosera region of Papua New Guinea, including samples from symptomatic and asymptomatic infections. Neutrality tests applied to these sequences provided strong evidence of selective pressure operating on the sequence of ama1 domain I, consistent with AMA1 being a target of protective immunity. Similarly, a peculiar pattern of geographical diversity and the particular substitutions found were suggestive of strong constraints acting on the evolution of AMA1 at the population level, probably as a result of immune pressure. In addition, a strong imbalance between symptomatic and asymptomatic infections was detected in the frequency of particular residues at certain polymorphic positions, pointing to AMA1 as being one of the determinants of the morbidity associated with a particular strain. The information yielded by this study has implications for the design and assessment of AMA1-based vaccines and provides additional data supporting the importance of AMA1 as a malaria vaccine candidate.

Phage-displayed peptides bind to the malarial protein apical membrane antigen-1 and inhibit the merozoite invasion of host erythrocytes

Apical membrane antigen-1 (AMA1) is a transmembrane protein present on the surface of merozoites that is thought to be involved in the process of parasite invasion of host erythrocytes. Although it is the target of a natural immune response that can inhibit invasion, little is known about the molecular mechanisms by which AMA1 facilitates the invasion process. In an attempt to identify peptides that specifically interact with and block the function of AMA1, a random peptide library displayed on the surface of filamentous phage was panned on recombinant AMA1 from Plasmodium falciparum. Three peptides with affinity for AMA1 were isolated, and characterization of their fine binding specificities indicated that they bind to a similar region on the surface of AMA1. One of these peptides was found to be a potent inhibitor of the invasion of P. falciparum merozoites into human erythrocytes. We propose that this peptide blocks interaction between AMA1 and a ligand on the erythrocyte surface that is involved in a critical step in malarial invasion. The identification and characterization of these peptide inhibitors now permit an evaluation of the essential requirements that are necessary for efficient neutralization of merozoite invasion by blocking AMA1 function.

Gene expression analysis during liver stage development of Plasmodium

The complex life cycle of malaria parasites requires significant changes in gene expression as the parasites move from vector to host and back to the vector. Although recognised as an important vaccine and drug target, the liver stage parasite has remained difficult to study. One of the major impediments in identifying parasite gene expression at the liver stage has remained the large number of uninfected hepatocytes relative to the number of infected hepatocytes in the liver after sporozoite inoculation. This article describes several of the approaches that have been utilised to overcome this difficulty in rodent models of malaria. While significant progress has been made to identify genes that are expressed during liver stage parasite development, a great deal more work remains to be done.

In vitro studies with recombinant Plasmodium falciparum apical membrane antigen 1 (AMA1): production and activity of an AMA1 vaccine and generation of a multiallelic response

Apical membrane antigen 1 (AMA1) is regarded as a leading malaria blood-stage vaccine candidate. While the overall structure of AMA1 is conserved in Plasmodium spp., numerous AMA1 allelic variants of P. falciparum have been described. The effect of AMA1 allelic diversity on the ability of a recombinant AMA1 vaccine to protect against human infection by different P. falciparum strains is unknown. We characterize two allelic forms of AMA1 that were both produced in Pichia pastoris at a sufficient economy of scale to be usable for clinical vaccine studies. Both proteins were used to immunize rabbits, singly and in combination, in order to evaluate their immunogenicity and the ability of elicited antibodies to block the growth of different P. falciparum clones. Both antigens, when used alone, elicited high homologous anti-AMA1 titers, with reduced strain cross-reactivity. Similarly, sera from rabbits immunized with a single antigen were capable of blocking the growth of homologous parasite strains at levels theoretically sufficient to clear parasite infections. However, heterologous inhibition was significantly reduced, providing experimental evidence that AMA1 allelic diversity is a result of immune pressure. Encouragingly, rabbits immunized with a combination of both antigens exhibited titers and levels of parasite inhibition as good as those of the single-antigen-immunized rabbits for each of the homologous parasite lines, and consequently exhibited a broadening of allelic diversity coverage.

Vaccination of monkeys with recombinant Plasmodium falciparum apical membrane antigen 1 confers protection against blood-stage malaria

A major challenge facing malaria vaccine development programs is identifying efficacious combinations of antigens. To date, merozoite surface protein 1 (MSP1) is regarded as the leading asexual vaccine candidate. Apical membrane antigen 1 (AMA1) has been identified as another leading candidate for an asexual malaria vaccine, but without any direct in vivo evidence that a recombinant form of Plasmodium falciparum AMA1 would have efficacy. We evaluated the efficacy of a form of P. falciparum AMA1, produced in Pichia pastoris, by vaccinating Aotus vociferans monkeys and then challenging them with P. falciparum parasites. Significant protection from this otherwise lethal challenge with P. falciparum was observed. Five of six animals had delayed patency; two of these remained subpatent for the course of the infection, and two controlled parasite growth at <0.75% of red blood cells parasitized. The protection induced by AMA1 was superior to that obtained with a form of MSP1 used in the same trial. The protection induced by a combination vaccine of AMA1 and MSP1 was not superior to the protection obtained with AMA1 alone, although the immunity generated appeared to operate against both vaccine components.

Malaria vaccine: candidate antigens, mechanisms, constraints and prospects

More than 30 years after the first report of successful vaccination against malaria using radiation-attenuated sporozoites, an effective malaria vaccine is not yet available. However, field and experimental data indicate that it can be developed. An astonishing amount of data has accumulated concerning parasite biology, host-parasite interactions, immunity and escape mechanisms, targets and modulators of immune responses. Nevertheless, so far this knowledge has not been enough to make us understand how to properly manipulate the whole system to build an effective vaccine. In this article, we describe candidate antigens, mechanisms, targets and trials performed with potential malaria vaccines and discuss the approaches, in vivo and in vitro models, constraints and how technologies such as DNA vaccination, genomics/proteomics and reverse immunogenetics are providing exciting results and opening new doors to make malaria vaccine a reality.

Identification of antigenically active tryptic fragments of apical membrane antigen-1 (AMA1) of Plasmodium chabaudi malaria: strategies for assembly of immunologically active peptides

Apical membrane antigen-1 (AMA1) is a prime vaccine candidate for inclusion in a vaccine against malaria. It is known that the disulphide bond stabilised conformation of this antigen is important for eliciting a protective antibody response, however little is known about the epitopes within this molecule that are targeted by the immune response. We have used a peptide approach for the identification and characterisation of such regions. In this study, the in vitro refolded, recombinant ectodomain of AMA1 from the D strain of Plasmodium chabaudi adami, was digested with trypsin and individual peptide fragments examined for antigenic activity. We found that a tryptic fragment, which was derived from a loop-like structure within the putative domain I of the intact AMA1 molecule, was highly reactive with antibodies from the sera of hyperimmune mice. Two different synthetic peptide constructs incorporating this antigenically active fragment were assembled. The first consisted of two separate peptide chains which were linked through a disulphide bond formed using chemo-selective chemistry. A larger 45-mer loop peptide, generated by the oxidation of two cysteine residues close to the N- and C-termini of the 45-mer, represented the complete loop structure and incorporated the tryptic fragment. Each peptide construct was also able to elicit production of high titres of antibodies in mice and furthermore, the 45-residue loop peptide elicited antibodies capable of binding to AMA1 with titres comparable to those present in a mouse which had recovered from multiple exposures to P. chabaudi adami parasites. Passive immunisation with anti-loop antibodies did not suppress the development of parasitaemia in mice challenged with P. chabaudi adami suggesting that although highly immunogenic, the peptides represented inadequate or inappropriate epitopes for vaccination purposes.

Immunity to asexual blood stage malaria and vaccine approaches

The development of a malaria vaccine seems to be a definite possibility despite the fact that even individuals with a life time of endemic exposure do not develop sterile immunity. An effective malaria vaccine would be invaluable in preventing malaria-associated deaths in endemic areas, especially amongst children less than 5 years of age and pregnant women. This review discusses our current understanding of immunity against the asexual blood stage of malaria - the stage that is responsible for the symptoms of the disease - and approaches to the design of an asexual blood stage vaccine.

Malaria: current status of control, diagnosis, treatment, and a proposed agenda for research and development

Rolling back malaria is possible. Tools are available but they are not used. Several countries deploy, as their national malaria control treatment policy, drugs that are no longer effective. New and innovative methods of vector control, diagnosis, and treatment should be developed, and work towards development of new drugs and a vaccine should receive much greater support. But the pressing need, in the face of increasing global mortality and general lack of progress in malaria control, is research into the best methods of deploying and using existing approaches, particularly insecticide-treated mosquito nets, rapid methods of diagnosis, and artemisinin-based combination treatments. Evidence on these approaches should provide national governments and international donors with the cost-benefit information that would justify much-needed increases in global support for appropriate and effective malaria control.

High-level expression of the malaria blood-stage vaccine candidate Plasmodium falciparum apical membrane antigen 1 and induction of antibodies that inhibit erythrocyte invasion

Apical membrane antigen 1 (AMA-1) is a highly promising malaria blood-stage vaccine candidate that has induced protection in rodent and nonhuman primate models of malaria. Authentic conformation of the protein appears to be essential for the induction of parasite-inhibitory antibody responses. Here we have developed a synthetic gene with adapted codon usage to allow expression of Plasmodium falciparum FVO strain AMA-1 (PfAMA-1) in Pichia pastoris. In addition, potential N-glycosylation sites were changed, exploiting the lack of conservation of these sites in Plasmodium, to obtain high-level secretion of a homogeneous product, suitable for scale-up according to current good manufacturing procedures. Purified PfAMA-1 displayed authentic antigenic properties, indicating that the amino acid changes had no deleterious effect on the conformation of the protein. High-titer antibodies, raised in rabbits, reacted strongly with homologous and heterologous P. falciparum by immunofluorescence. In addition, purified immunoglobulin G from immunized animals strongly inhibited invasion of red blood cells by homologous and, to a somewhat lesser extent, heterologous P. falciparum.

Purification, characterization, and immunogenicity of the refolded ectodomain of the Plasmodium falciparum apical membrane antigen 1 expressed in Escherichia coli

The apical membrane antigen 1 (AMA1) has emerged as a promising vaccine candidate against malaria. Advanced evaluation of its protective efficacy in humans requires the production of highly purified and correctly folded protein. We describe here a process for the expression, fermentation, refolding, and purification of the recombinant ectodomain of AMA1 (amino acids 83(Gly) to 531(Glu)) of Plasmodium falciparum (3D7) produced in Escherichia coli. A synthetic gene containing an E. coli codon bias was cloned into a modified pET32 plasmid, and the recombinant protein was produced by using a redox-modified E. coli strain, Origami (DE3). A purification process was developed that included Sarkosyl extraction followed by affinity purification on a Ni-nitrilotriacetic acid column. The recombinant AMA1 was refolded in the presence of reduced and oxidized glutathione and further purified by using two ion-exchange chromatographic steps. The final product, designated AMA1/E, was homogeneous, monomeric, and >99% pure and had low endotoxin content and low host cell contamination. Analysis of AMA1/E showed that it had the predicted primary sequence, and tertiary structure analysis confirmed its compact disulfide-bonded nature. Rabbit antibodies made to the protein recognized the native parasite AMA1 and inhibited the growth of the P. falciparum homologous 3D7 clone in an in vitro assay. Reduction-sensitive epitopes on AMA1/E were shown to be necessary for the production of inhibitory anti-AMA1 antibodies. AMA1/E was recognized by a conformation-dependent, growth-inhibitory monoclonal antibody, 4G2dc1. The process described here was successfully scaled up to produce AMA1/E protein under GMP conditions, and the product was found to induce highly inhibitory antibodies in rabbits.

A novel Plasmodium falciparum erythrocyte binding protein-2 (EBP2/BAEBL) involved in erythrocyte receptor binding

The 175-kDa erythrocyte binding protein (EBA-175) of Plasmodium falciparum and Duffy antigen binding proteins of P. vivax and P. knowlesi are members of a protein family. The features of this protein family include a cysteine-rich motif present in the erythrocyte receptor-binding domain. We identify here a novel 140-kDa P. falciparum erythrocyte binding protein (EBP2/BAEBL) containing the signature cysteine-rich motif by comparative analysis of gene sequence information. Polyclonal antibodies generated by immunization with an EBP2/BAEBL DNA vaccine immunoprecipitated a 140-kDa protein from P. falciparum schizont-infected erythrocyte lysates. Similar to EBA-175, the binding of EBP2/BAEBL to human erythrocytes was dependent on sialic acids because neuraminidase treatment of those erythrocytes rendered them incapable of binding, but differed from EBA-175 in that trypsin treatment decreased EBP2/BAEBL binding by only twofold compared to a 10-fold reduction in EBA-175 binding. Antibodies raised against the putative erythrocyte-binding domain of EBP2/BAEBL effectively blocked the binding of native EBP2/BAEBL to erythrocytes. These functional antibodies localize EBP2/BAEBL to the invasive apical end of the merozoite. We identify EBP2/BAEBL as a paralogue of EBA-175 and as a novel P. falciparum vaccine candidate.

Erythrocyte-binding activity of Plasmodium yoelii apical membrane antigen-1 expressed on the surface of transfected COS-7 cells

Malaria merozoite surface and apical organellar molecules facilitate invasion into the host erythrocyte. The underlying molecular mechanisms of invasion are poorly understood, and there are few data to delineate roles for individual merozoite proteins. Apical membrane antigen-1 (AMA-1) is a conserved apicomplexan protein present in the apical organelle complex and at times on the surface of Plasmodium and Toxoplasma zoites. AMA-1 domains 1/2 are conserved between Plasmodium and Toxoplasma and have similarity to the defined ligand domains of MAEBL, an erythrocyte-binding protein identified from Plasmodium yoelii. We expressed selected portions of the AMA-1 extracellular domain on the surface of COS-7 cells to assay for erythrocyte-binding activity. The P. yoelii AMA-1 domains 1/2 mediated adhesion to mouse and rat erythrocytes, but not to human erythrocytes. Adhesion to rodent erythrocytes was sensitive to trypsin and chymotrypsin, but not to neuraminidase. Other parts of the AMA-1 ectodomain, including the full-length extracellular domain, mediated significantly less erythrocyte adhesion activity than the contiguous domains 1/2. The results support the role of AMA-1 as an adhesion molecule during merozoite invasion of erythrocytes and identify highly conserved domains 1/2 as the principal ligand of the Plasmodium AMA-1 and possibly the Toxoplasma AMA-1. Identification of the AMA-1 ligand domains involved in interaction between the parasite and host cell should help target the development of new therapies to block growth of the blood-stage malaria parasites.

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.

Molecular characterisation of Plasmodium reichenowi apical membrane antigen-1 (AMA-1), comparison with P. falciparum AMA-1, and antibody-mediated inhibition of red cell invasion

Apical membrane antigen 1 is a candidate vaccine component for malaria. It is encoded by a single copy gene and has been characterised in a number of malaria species as either an 83-kDa de novo product (Plasmodium falciparum; Pf AMA-1) or a 66-kDa product (all other species). All members of the AMA-1 family are expressed during merozoite formation in maturing schizonts and are initially routed to the rhoptries. Processed forms may subsequently be associated with the merozoite surface. Because of the unique occurrence of the 83-kDa form in P. falciparum we were interested to determine whether the phylogenetically closely related chimpanzee malaria Plasmodium reichenowi shared characteristics with Pf AMA-1. Here we show that the molecular structure, the localisation and processing are similar to that of Pf AMA-1 and that in vitro growth inhibitory mAbs reactive with Pf AMA-1 also inhibit P. reichenowi growth in an in vitro assay. Polymorphism in the 83-kDa AMA-1 family was analysed through comparison of Pr ama-1 with Pf ama-1 alleles, which showed the most significant evidence for selection maintaining polymorphism in Domains I-III of AMA-1 in P. falciparum. The most substantial divergence between Pr AMA-1 and Pf AMA-1 sequences was in the N-terminal region unique to the 83-kDa form of AMA-1. It was confirmed that the specific Pr ama-1-type allele was not present among P. falciparum parasites in an African population, and an allele coding for lysine at amino acid 187 was uniquely associated with field isolates in this population.