Phase 1 study of two merozoite surface protein 1 (MSP1(42)) vaccines for Plasmodium falciparum malaria

Objectives:

To assess the safety and immunogenicity of two vaccines, MSP1₄₂-FVO/Alhydrogel and MSP1₄₂-3D7/Alhydrogel, targeting blood-stage Plasmodium falciparum parasites.

Design:

A Phase 1 open-label, dose-escalating study.

Setting:

Quintiles Phase 1 Services, Lenexa, Kansas between July 2004 and November 2005.

Participants:

Sixty healthy malaria-naïve volunteers 18–48 y of age.

Interventions:

The C-terminal 42-kDa region of merozoite surface protein 1 (MSP1₄₂) corresponding to the two allelic forms present in FVO and 3D7 P. falciparum lines were expressed in Escherichia coli, refolded, purified, and formulated on Alhydrogel (aluminum hydroxide). For each vaccine, volunteers in each of three dose cohorts (5, 20, and 80 μg) were vaccinated at 0, 28, and 180 d. Volunteers were followed for 1 y.

Outcome Measures:

The safety of MSP1₄₂-FVO/Alhydrogel and MSP1₄₂-3D7/Alhydrogel was assessed. The antibody response to each vaccine was measured by reactivity to homologous and heterologous MSP1₄₂, MSP1₁₉, and MSP1₃₃ recombinant proteins and recognition of FVO and 3D7 parasites.

Results:

Anti-MSP1₄₂ antibodies were detected by ELISA in 20/27 (74%) and 22/27 (81%) volunteers receiving three vaccinations of MSP1₄₂-FVO/Alhydrogel or MSP1₄₂-3D7/Alhydrogel, respectively. Regardless of the vaccine, the antibodies were cross-reactive to both MSP1₄₂-FVO and MSP1₄₂-3D7 proteins. The majority of the antibody response targeted the C-terminal 19-kDa domain of MSP1₄₂, although low-level antibodies to the N-terminal 33-kDa domain of MSP1₄₂ were also detected. Immunofluorescence microscopy of sera from the volunteers demonstrated reactivity with both FVO and 3D7 P. falciparum schizonts and free merozoites. Minimal in vitro growth inhibition of FVO or 3D7 parasites by purified IgG from the sera of the vaccinees was observed.

Conclusions:

The MSP1₄₂/Alhydrogel vaccines were safe and well tolerated but not sufficiently immunogenic to generate a biologic effect in vitro. Addition of immunostimulants to the Alhydrogel formulation to elicit higher vaccine-induced responses in humans may be required for an effective vaccine.

Background: Generally, adults living in parts of the world where malaria is common develop protective immunity against the parasite. This means they may get infected but not become ill as a result. However, there are individuals, such as pregnant women and children under the age of five, who are more likely to develop symptoms of malaria due to no (or reduced) natural immunity. A successful malaria vaccine would stimulate an individual's immune system to respond to the malaria parasite and prevent serious clinical disease. Many different groups are currently developing potential vaccines. Several candidates are based on a protein called MSP1 (merozoite surface protein 1) which is found on the surface of the blood-stage form of the malaria parasite. However, in nature parasites carry different versions of the MSP1 protein, and ideally a successful vaccine would bring about immune responses against these different versions. The researchers carrying out this trial wanted to compare the safety and immune responses against candidate vaccines representing two different MSP1 proteins, which covered many different parasite lines. As a phase 1 trial, the study was carried out in healthy adult volunteers. Sixty individuals were assigned to receive an injection of the vaccines, either containing a recombinant protein analogous to the FVO parasite line (termed MSP1₄₂-FVO) or the 3D7 parasite line (termed MSP1₄₂-3D7) at three different dose levels. The trial's primary objective was to assess safety, which was done by collecting data on any abnormal signs or symptoms up to 14 d after each of three vaccinations. These outcomes were graded and then defined as related to the vaccine or not. The researchers also looked at antibody levels in participants' blood against different variants of the MSP1 protein, as well as using in vitro tests to see whether antibodies from vaccinated individuals could prevent malaria parasites from growing in lab culture.

What the trial shows: The safety outcomes of the trial showed that the most common type of side effect experienced by the volunteers was pain at the injection site. The vast majority of such events were graded as mild, although there was one single case of a severe event (high levels of pain experienced by one volunteer at the injection site). There was no significant association between the chance of side effects and the vaccine dosage that an individual received. Following vaccination, antibody levels against the protein on which the vaccine was based were detected, although these levels dropped over time. The researchers did not see a strong association between the vaccine dosage that individuals received and the level of antibody response. However, the two vaccines when compared seemed to be equally good at raising an immune response and both caused antibodies to be raised corresponding to different variants of the MSP1 protein. However, the antibodies raised did not seem to be particularly effective at preventing malaria parasites from growing in lab culture.

Strengths and limitations: Strengths of this study include a comparison of three different dosage levels of the vaccines under study, as well as a comparison of two vaccines based on the same protein, representing different parasite lines. Limitations to the study include the small number of participants, which makes the trial underpowered to detect all but large differences in side effects between the groups being compared. A placebo arm was not included in the trial, so it is not possible to be sure that the numbers of side effects observed here can be attributed to the vaccines or not. Finally, the procedure for assigning individuals to the two different vaccines involved alternation, rather than true randomization, which could have minimized the risk of bias.

Contribution to the evidence: The trial reported here is an essential step in vaccine development. The results provide the first evidence relating to safety for these two vaccines, and do not raise any safety concerns at this stage. Although the vaccines raised an immune response, the antibodies raised did not seem to have much of an effect on malaria parasites in vitro. While these vaccines are safe, alternative MSP1 vaccine formulations anticipated to bring about a greater immune response will likely be studied before proceeding to field studies.

The essential mosquito-stage P25 and P28 proteins from Plasmodium form tile-like triangular prisms

P25 and P28 proteins are essential for Plasmodium parasites to infect mosquitoes and are leading candidates for a transmission-blocking malaria vaccine. The Plasmodium vivax P25 is a triangular prism that could tile the parasite surface. The residues forming the triangle are conserved in P25 and P28 from all Plasmodium species. A cocrystal structure shows that a transmission-blocking antibody uses only its heavy chain to bind Pvs25 at a vertex of the triangle.

Montanide® ISA 720 vaccines: quality control of emulsions, stability of formulated antigens, and comparative immunogenicity of vaccine formulations

Montanide ISA 720 is an experimental adjuvant, formulated as water-in-oil emulsions, that induces high antibody titers in several animal species. It has been used in human vaccine trials with malaria and HIV vaccines. The heightened response is likely due, in part, to the formation of a depot at the injection site. However, post-formulation modifications were seen with seven proteins tested during storage of ISA 720 formulations at 37 degrees C for 1 week and two proteins stored longer at 4 degrees C. Potency studies in mice, in which the stored vaccines were diluted into placebo emulsions for appropriate dosing, indicated that this instability could lead to loss of immunogenicity in the post-injection depot, limiting the allowable storage time of preformed vaccines. We describe point-of-injection formulation for ISA 720 vaccines that meets the requirement for in vitro stability. For preformed vaccines, addition of glycine or glycylglycine prevented antigen modification on storage at 37 degrees C, providing a potential way of stabilizing antigen/ISA 720 formulations for in vitro storage and the post-injection depot.

Effect of CpG oligodeoxynucleotides on the immunogenicity of Pfs25, a Plasmodium falciparum transmission-blocking vaccine antigen

Antibodies directed against Pfs25, a protein present on the surface of zygotes and ookinetes of Plasmodium falciparum, completely block pathogen transmission. We evaluated the immunomodulatory effect of CpG oligodeoxynucleotides (ODN) on the immunogenicity of recombinant Pfs25 (rPfs25) formulated in alum (Al). Immunization of mice with rPfs25 plus CpG ODN improved both the antibody titer (a 30-fold-higher antibody response than that with rPfs25-Al alone) and avidity. Coadministration of CpG ODN dramatically enhanced the titer of immunoglobulin G2A (IgG2a) compared to the titer of the IgG1-dominant response caused by rPfs25-Al alone, and the sera from the CpG ODN-coadministered group completely blocked the transmission of P. falciparum parasites to mosquitoes, as determined by membrane feeding assays. However, transmission-blocking experiments revealed that blocking efficacy was dependent on high-titer antibody levels, independent of isotypes. These results suggest that CpG ODN can be used as an adjuvant to enhance the immunogenicity of rPfs25 as a malaria transmission-blocking vaccine.

CpG oligodeoxynucleotide and Montanide ISA 51 adjuvant combination enhanced the protective efficacy of a subunit malaria vaccine

Unmethylated CpG dinucleotide motifs present in bacterial genomes or synthetic oligodeoxynucleotides (ODNs) serve as strong immunostimulatory agents in mice, monkeys and humans. We determined the adjuvant effect of murine CpG ODN 1826 on the immunogenicity and protective efficacy of the Saccharomyces cerevisiae-expressed 19-kDa C-terminal region of merozoite surface protein 1 (yMSP1(19)) of the murine malaria parasite Plasmodium yoelii. We found that in C57BL/6 mice, following sporozoite challenge, the degree of protective immunity against malaria induced by yMSP1(19) in a formulation of Montanide ISA 51 (ISA) plus CpG ODN 1826 was similar or superior to that conferred by yMSP1(19) emulsified in complete Freund's adjuvant (CFA/incomplete Freund's adjuvant). In total, among mice immunized with yMSP1(19), 22 of 32 (68.7%) with ISA plus CpG 1826, 0 of 4 (0%) with CFA/incomplete Freund's adjuvant, 0 of 4 (0%) with CpG 1826 mixed with ISA (no yMSP1(19)), and 0 of 11 (0%) with CpG 1826 alone were completely protected against development of erythrocytic stage infection after sporozoite challenge. The adjuvant effect of CpG ODN 1826 was manifested as both significantly improved complete protection from malaria (defined as the absence of detectable erythrocytic form parasites) (P = 0.007, chi square) and reduced parasite burden in infected mice. In vivo depletions of interleukin-12 and gamma interferon cytokines and CD4+ and CD8+ T cells in vaccinated mice had no significant effect on immunity. On the other hand, immunoglobulin G (IgG) isotype levels appeared to correlate with protection. Inclusion of CpG ODN 1826 in the yMSP1(19) plus ISA vaccine contributed towards the induction of higher levels of IgG2a and IgG2b (Th1 type) antibodies, suggesting that CpG ODN 1826 caused a shift towards a Th1 type of immune response that could be responsible for the higher degree of protective immunity. Our results indicate that this potent adjuvant formulation should be further evaluated for use in clinical trials of recombinant malarial vaccine candidates.

Biochemical and immunological characterization of bacterially expressed and refolded Plasmodium falciparum 42-kilodalton C-terminal merozoite surface protein 1

Protection against Plasmodium falciparum can be induced by vaccination in animal models with merozoite surface protein 1 (MSP1), which makes this protein an attractive vaccine candidate for malaria. In an attempt to produce a product that is easily scaleable and inexpensive, we expressed the C-terminal 42 kDa of MSP1 (MSP1(42)) in Escherichia coli, refolded the protein to its native form from insoluble inclusion bodies, and tested its ability to elicit antibodies with in vitro and in vivo activities. Biochemical, biophysical, and immunological characterization confirmed that refolded E. coli MSP1(42) was homogeneous and highly immunogenic. In a formulation suitable for human use, rabbit antibodies were raised against refolded E. coli MSP1(42) and tested in vitro in a P. falciparum growth invasion assay. The antibodies inhibited the growth of parasites expressing either homologous or heterologous forms of P. falciparum MSP1(42). However, the inhibitory activity was primarily a consequence of antibodies directed against the C- terminal 19 kDa of MSP1 (MSP1(19)). Vaccination of nonhuman primates with E. coli MSP1(42) in Freund's adjuvant protected six of seven Aotus monkeys from virulent infection with P. falciparum. The protection correlated with antibody-dependent mechanisms. Thus, this new construct, E. coli MSP1(42), is a viable candidate for human vaccine trials.

Blocking of transmission to mosquitoes by antibody to Plasmodium vivax malaria vaccine candidates Pvs25 and Pvs28 despite antigenic polymorphism in field isolates

We have previously demonstrated that mouse antisera against yeast-produced recombinant forms of the ookinete surface proteins of Plasmodium vivax (Pvs25 and Pvs28) blocks transmission of the homologous P. vivax (Sal I strain). In this study, we developed mouse and rabbit antisera against Pvs25 and Pvs28 and evaluated the efficacy of these vaccine candidates against natural isolates of P. vivax in Thailand. Although both Pvs25 and Pvs28 genes are polymorphic, sera from mice immunized using alum adjuvant completely inhibited oocyst development for most human isolates, whereas sera from rabbits immunized with either alum or Freund's adjuvant were partially inhibitory. All inhibition occurred in an antibody dose dependent fashion. Data from this study clearly demonstrates that antibodies raised against Sal I-based vaccines overcome the genetic polymorphism of Pvs25 and Pvs28 present in natural isolates of P. vivax, suggesting the wide range applicability of Sal I based vaccines.

Expression of malaria transmission-blocking vaccine antigen Pfs25 in Pichia pastoris for use in human clinical trials

In previously published studies, Saccharomyces cerevisiae recombinant protein expression systems have been employed to express the malaria parasite antigen Pfs25, a candidate transmission-blocking vaccine antigen against Plasmodium falciparum malaria. However, despite having been in two Phase 1 trials, the recombinant Pfs25 so produced (previously called TBV25H) exists as a mixture of two monomeric protein conformational forms, Pfs25H-A and Pfs25H-B. In this study, we optimized the expression and purification of the two Pfs25H conformers in S. cerevisiae, and characterized their biochemical and antigenic properties, immunogenicities, and transmission-blocking activities. Pfs25H-A is apparently homogeneous, and has the correct conformation as measured by monoclonal antibody recognition. It is, however, expressed at a low yield of only 0.19mg/l. By contrast, Pfs25H-B is produced as a heterogeneous population of molecules that do not seem to have the correct conformation. Nonetheless, both forms appear equally effective in their ability to produce transmission-blocking antibodies in mice. To address the low yield seen with S. cerevisiae, we also expressed Pfs25 in Pichia pastoris. P. pastoris is apparently superior to S. cerevisiae in producing higher yield, immunologically more potent, biologically more active Pfs25H-A.

Structure of the C-terminal domains of merozoite surface protein-1 from Plasmodium knowlesi reveals a novel histidine binding site

The protozoan parasite Plasmodium causes malaria, with hundreds of millions of cases recorded annually. Protection against malaria infection can be conferred by antibodies against merozoite surface protein (MSP)-1, making it an attractive vaccine candidate. Here we present the structure of the C-terminal domains of MSP-1 (known as MSP-1(19)) from Plasmodium knowlesi. The structure reveals two tightly packed epidermal growth factor-like domains oriented head to tail. In domain 1, the molecule displays a histidine binding site formed primarily by a highly conserved tryptophan. The protein carries a pronounced overall negative charge primarily due to the large number of acidic groups in domain 2. To map protein binding surfaces on MSP-1(19), we have analyzed the crystal contacts in five different crystal environments, revealing that domain 1 is highly preferred in protein-protein interactions. A comparison of MSP-1(19) structures from P. knowlesi, P. cynomolgi, and P. falciparum shows that, although the overall protein folds are similar, the molecules show significant differences in charge distribution. We propose the histidine binding site in domain 1 as a target for inhibitors of protein binding to MSP-1, which might prevent invasion of the merozoite into red blood cells.

Immunization with a combination of merozoite surface proteins 4/5 and 1 enhances protection against lethal challenge with Plasmodium yoelii

It is widely believed that subunit vaccines composed of multiple components will offer greater protection against challenge by malaria, and yet there is little experimental evidence to support this view. We set out to test this proposition in the Plasmodium yoelii challenge system in rodents by comparing the degree of protection conferred by immunization with a mixture of merozoite surface proteins to that conferred by single proteins. We therefore examined a defined protein mixture made of the epidermal growth factor-like domains of P. yoelli merozoite surface protein 1 (MSP1) and MSP4/5, the homologue of P. falciparum MSP4 and MSP5. In the present study we demonstrate that this combination of recombinant proteins dramatically enhances protection against lethal malaria challenge compared to either protein administered alone. Many mice immunized with the MSP4/5 plus MSP1(19) combination did not develop detectable parasitemia after challenge. Combined immunization with MSP1(19) and yMSP4/5, a product characterized by lower protective efficacy, also greatly enhanced protection by reducing peak parasitemias and increasing the numbers of survivors. In some combination trials, levels of antibodies to MSP1(19) were elevated compared to the MSP1(19) alone group; however, improved protection occurred regardless of whether boosting of the anti-MSP1(19) response was observed. Boosting of anti-MSP1(19) did not appear to be due to contaminating endotoxin in the EcMSP4/5 material since enhanced protection was observed in C3H/HeJ mice, which are endotoxin insensitive. Collectively, these experiments show that multiantigen combinations offer enhanced levels of protection against asexual stage infection and suggest that combinations of MSP1, MSP4, and MSP5 should be evaluated further for use in humans.

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.

Large-scale purification and characterization of malaria vaccine candidate antigen Pvs25H for use in clinical trials

The budding yeast Saccharomyces cerevisiae has been used to express the recombinant protein Pvs25H, currently the only candidate transmission-blocking vaccine against Plasmodium vivax malaria. This molecule contains four epidermal growth factor-like domains and is expressed as at least two stable monomeric forms with different physicochemical properties. Pvs25H-A is apparently homogeneous and seems to have a correct disulfide bond structure. By contrast, Pvs25H-B is produced as a heterogeneous population of molecules, some of which are associated with an as yet unidentified chromophore, and it contains both internal and N-terminal cleavages. We report here a procedure for successfully separating these two forms with a process suitable for clinical production of this antigen.

Merozoite surface protein 3 and protection against malaria in Aotus nancymai monkeys

A blood-stage vaccine based on Plasmodium falciparum merozoite surface protein 3 (MSP3) was tested for efficacy in a primate model. Aotus nancymai monkeys were vaccinated with yeast-expressed MSP3 before a lethal challenge with Plasmodium falciparum parasites. Five of 7 control monkeys had acute infections and required treatment to control parasitemia. Only 1 of 7 monkeys vaccinated with MSP3 required this treatment. The efficacy of the MSP3 vaccination appeared to be comparable to that of MSP1(42), a leading asexual vaccine candidate, in response to which 2 monkeys experienced acute infections. In the MSP3-vaccinated group, protection correlated with prechallenge titers of antibody to MSP3. In the MSP1 and control groups, protection correlated with antibody to MSP3 raised by challenge infection.

Improved immunogenicity and efficacy of the recombinant 19-kilodalton merozoite surface protein 1 by the addition of oligodeoxynucleotide and aluminum hydroxide gel in a murine malaria vaccine model

Vaccination of mice with yeast-secreted Plasmodium yoelii-derived 19-kilodalton merozoite surface protein 1 (yMSP1(19)) has been shown to afford protection from challenge with a lethal strain of P. yoelii. Sterile immunity can be achieved when MSP1(19) is emulsified in Freund adjuvant but not when it is adsorbed to aluminum hydroxide gel (alum). Because complete Freund adjuvant is not an acceptable adjuvant for use in humans, alternative adjuvants must be identified for formulating MSP1(19) as a vaccine for use in humans. To determine whether oligodeoxynucleotides with CpG motifs (ODN), reported to be a powerful new class of adjuvants, could enhance the immunogenicity of yMSP1(19), C57BL/6 mice were vaccinated either with yMSP1(19) formulated with Freund adjuvant, with alum, or with ODN plus alum and challenged intravenously with P. yoelii 17XL asexual blood-stage parasites. Adsorption of immunogen and adjuvant to alum was optimized by adjusting buffer (phosphate versus acetate) and pH. We found that the adjuvant combination of ODN plus alum with yMSP1(19), injected intraperitoneally (i.p.), increased immunoglobulin G (IgG) yMSP1(19)-specific antibody production 12-fold over Freund adjuvant given i.p., 3-fold over Freund adjuvant given subcutaneously (s.c.), 300-fold over alum given i.p., and 48-fold over alum given s.c. The predominant antibody isotype in the group receiving alum-ODN-yMSP1(19) was IgG1. Increased antibody levels correlated to protection from a challenge with P. yoelii 17XL. Supernatant cytokine levels of gamma interferon in yMSP1(19)-stimulated splenocytes were dramatically elevated in the alum-ODN-yMSP1(19) group. Interleukin-10 (IL-10) levels were also elevated; however, no IL-5 was detected. The cytokine profile, as well as the predominant IgG1 antibody isotype, suggests the protective immune response was a mixed Th1/Th2 response.

A recombinant vaccine expressed in the milk of transgenic mice protects Aotus monkeys from a lethal challenge with Plasmodium falciparum

Two strains of transgenic mice have been generated that secrete into their milk a malaria vaccine candidate, the 42-kDa C-terminal portion of Plasmodium falciparum merozoite surface protein 1 (MSP1(42)). One strain secretes an MSP1(42) with an amino acid sequence homologous to that of the FVO parasite line, the other an MSP1(42) where two putative N-linked glycosylation sites in the FVO sequence have been removed. Both forms of MSP1(42) were purified from whole milk to greater than 91% homogeneity at high yields. Both proteins are recognized by a panel of monoclonal antibodies and have identical N termini, but are clearly distinguishable by some biochemical properties. These two antigens were each emulsified with Freund's adjuvant and used to vaccinate Aotus nancymai monkeys, before challenge with the homologous P. falciparum FVO parasite line. Vaccination with a positive control molecule, a glycosylated form of MSP1(42) produced in the baculovirus expression system, successfully protected five of six monkeys. By contrast, vaccination with the glycosylated version of milk-derived MSP1(42) conferred no protection compared with an adjuvant control. Vaccination with the nonglycosylated, milk-derived MSP1(42) successfully protected the monkeys, with 4/5 animals able to control an otherwise lethal infection with P. falciparum compared with 1/7 control animals. Analysis of the different vaccines used suggested that the differing nature of the glycosylation patterns may have played a critical role in determining efficacy. This study demonstrates the potential for producing efficacious malarial vaccines in transgenic animals.

Antibodies to Plasmodium vivax transmission-blocking vaccine candidate antigens Pvs25 and Pvs28 do not show synergism

Transmission-blocking vaccines against malaria parasites target molecules expressed by sexual stage parasites to elicit antibodies that prevent the infection of the mosquito vector. Pvs25 and Pvs28, expressed on the surface of ookinetes, are potential candidates for such a vaccine and induce antibodies that block the infectivity of Plasmodium vivax in immunized animals. To improve the ability to induce transmission-blocking antibodies, Pvs25 and Pvs28 were produced as a single fusion protein by the yeast Saccharomyces cerevisiae. Mice immunized with a low dose of the chimeric molecule (Pvs25-28) developed higher antibody responses compared with mice immunized with either Pvs25 or Pvs28. In membrane feeding assays, both anti-Pvs25-28 and anti-Pvs25 antisera had similarly potent transmission-blocking activities (and both were much greater than anti-Pvs28). Furthermore, serum from mice simultaneously immunized with both Pvs25 and Pvs28, or serum mixtures of anti-Pvs25 alone and anti-Pvs28 alone did not enhance the efficacy over anti-Pvs25 serum alone, demonstrating that there is no synergism in the ability to block transmission of P. vivax between anti-Pvs25 and anti-Pvs28 antibodies.

High-level production and purification of P30P2MSP1(19), an important vaccine antigen for malaria, expressed in the methylotropic yeast Pichia pastoris

P30P2MSP1(19) is a recombinant subunit vaccine derived from merozoite surface protein 1 (MSP1) of Plasmodium falciparum, the causative agent of malaria. P30P2MSP1(19) consists of two universal T-cell epitopes fused to the most C-terminal 19-kDa portion of MSP1, and this protein has previously shown promising potential as a vaccine for malaria. However, previous attempts at producing this molecule in Saccharomyces cerevisiae resulted in the production of a truncated form of the molecule missing most of the universal T-cell epitopes. Here, we report the production of full-length P30P2MSP1(19) in Pichia pastoris. As salt precipitation is a common problem during P. pastoris high-density fermentation, we utilized an alternative low-salt, fully defined medium that did not reduce growth rates or biomass yields to avoid precipitation. A total of 500 mg/L of secreted purified protein was produced in high cell density fermentation and the protein was purified in one step utilizing nickel-chelate chromatography. P30P2MSP1(19) produced in Pichia was reactive with monoclonal antibodies that recognize only conformational epitopes on correctly folded MSP1. Rabbits immunized with this molecule generated higher and more uniform antibody titers than rabbits immunized with the protein produced in Saccharomyces. P30P2MSP1(19) produced in Pichia may prove to be a more efficacious vaccine than that produced in Saccharomyces and Pichia would provide a system for the cost-effective production of such a vaccine.

Comparison of the protective efficacy of yeast-derived and Escherichia coli-derived recombinant merozoite surface protein 4/5 against lethal challenge by Plasmodium yoelii

The gene encoding the Plasmodium yoelii homologue of P. falciparum merozoite surface proteins 4 (MSP4) and 5 (MSP5) has been expressed in Escherichia coli and Saccharomyces cerevisiae. The protein contains a single epidermal growth factor (EGF)-like domain and is expressed in a form lacking the predicted N-terminal signal and glycosyl phosphatidylinositol (GPI) attachment sequences. The recombinant protein derived from E. coli (EcMSP4/5) was highly effective at protecting mice against lethal challenge with 10(5) parasites of the P. yoelii YM strain. In contrast, the protective efficacy of yeast-derived MSP4/5 (yMSP4/5) was considerably less. The antibody titres in both groups were significantly different with mice immunised with yeast-derived protein showing significantly lower pre-challenge antibody responses. There was a significant inverse correlation between antibody levels as measured by ELISA and peak parasitaemia. Mice immunised with EcMSP4/5 produced anti-PyMSP4/5 antibodies predominantly of the IgG2a and IgG2b isotypes, whereas, mice immunised with yMSP4/5 mainly produced antibodies of the IgG1 isotype. The differences in antibody titres and subtype distribution may account for the observed differences in protective efficacy of these protein preparations. Levels of protective efficacy of MSP4/5 were compared with that obtained using P. yoelii MSP1 produced in S. cerevisiae. Levels of protection induced by E. coli derived MSP4/5 were superior to those induced by MSP1 which in turn were better than those induced by yeast-derived MSP4/5.

Are trials in New World monkeys on the critical path for blood-stage malaria vaccine development?

The development of malaria blood-stage vaccines is gathering momentum: there are several new funding initiatives, one multiantigen formulation is currently being tested and at least one other blood-stage vaccine is expected to begin trials in 2001. However, there is no consensus over the best way to select which form of an antigen to take into clinical testing. There is thus a danger that less-effective vaccines might be tested in the field in the order of their availability, rather than merit. Here, we argue that first proving efficacy in the New World monkey challenge model would accelerate development.

Efficacy of two alternate vaccines based on Plasmodium falciparum merozoite surface protein 1 in an Aotus challenge trial

In an attempt to produce a more defined, clinical-grade version of a vaccine based on Plasmodium falciparum merozoite surface protein 1 (MSP1), we evaluated the efficacy of two recombinant forms of MSP1 in an Aotus nancymai challenge model system. One recombinant vaccine, bvMSP1(42), based on the 42-kDa C-terminal portion of MSP1, was expressed as a secreted protein in baculovirus-infected insect cells. A highly pure baculovirus product could be reproducibly expressed and purified at yields in excess of 8 mg of pure protein per liter of culture. This protein, when tested for efficacy in the Aotus challenge model, gave significant protection, with only one of seven monkeys requiring treatment for uncontrolled parasitemia after challenge with P. falciparum. The second recombinant protein, P30P2MSP1(19), has been used in previous studies and is based on the smaller, C-terminal 19-kDa portion of MSP1 expressed in Saccharomyces cerevisiae. Substantial changes were made in its production process to optimize expression. The optimum form of this vaccine antigen (as judged by in vitro and in vivo indicators) was then evaluated, along with bvMSP1(42), for efficacy in the A. nancymai system. The new formulation of P30P3MSP1(19) performed significantly worse than bvMSP1(42) and appeared to be less efficacious than we have found in the past, with four of seven monkeys in the vaccinated group requiring treatment for uncontrolled parasitemia. With both antigens, protection was seen only when high antibody levels were obtained by formulation of the vaccines in Freund's adjuvant. Vaccine formulation in an alternate adjuvant, MF59, resulted in significantly lower antibody titers and no protection.

Structural conformers produced during malaria vaccine production in yeast

A recombinant protein expression system based on Saccharomyces cerevisiae has been used to express malarial vaccine candidate antigens. The antigens so produced have been used in three Phase 1 clinical trials and numerous preclinical non-human primate trials. Further Phase I trials are planned using these candidate vaccine antigens. These molecules were identified as attractive candidates for antimalarial vaccines, as they are all surface-exposed at some stage in the parasite's life cycle. They all share an unusual structural feature: epidermal growth factor (EGF)-like motifs. When these proteins are expressed in our S. cerevisiae expression system, they are produced as a series of stable structural conformers, each with a different disulphide bonding pattern. This leads to both biochemical and, more importantly, antigenic differences between the conformers (e.g. presence or absence of an antibody B cell epitope). These findings have important ramifications for other EGF-domain-containing proteins expressed in S. cerevisiae, or for proteins which contain other cysteine-folding motifs not normally expressed by this organism, both for vaccine production or for research/reagent purposes.

Antibodies to malaria vaccine candidates Pvs25 and Pvs28 completely block the ability of Plasmodium vivax to infect mosquitoes

Transmission-blocking vaccines are one strategy for controlling malaria, whereby sexual-stage parasites are inhibited from infecting mosquitoes by human antibodies. To evaluate whether the recently cloned Plasmodium vivax proteins Pvs25 and Pvs28 are candidates for a transmission-blocking vaccine, the molecules were expressed in yeast as secreted recombinant proteins. Mice vaccinated with these proteins adsorbed to aluminum hydroxide developed strong antibody responses against the immunogens, although for Pvs28, this response was genetically restricted. Antisera against both recombinant Pvs25 and Pvs28 recognized the corresponding molecules expressed by cultured sexual-stage parasites isolated from patients with P. vivax malaria. The development of malaria parasites in mosquitoes was completely inhibited when these antisera were ingested with the infected blood meal. Pvs25 and Pvs28, expressed in Saccharomyces cerevisiae, are as yet the only fully characterized transmission-blocking vaccine candidates against P. vivax that induce such a potent antiparasite response.

A region of Plasmodium falciparum antigen Pfs25 that is the target of highly potent transmission-blocking antibodies

Each of the four epidermal growth factor (EGF)-like domains of the Plasmodium falciparum sexual-stage antigen Pfs25 has been individually expressed as a yeast-secreted recombinant protein (yEGF1 through yEGF4). All four are recognized by the immune sera of animals and humans vaccinated with TBV25H (the corresponding yeast-secreted full-length recombinant form of Pfs25), with antibody titers to yEGF1 and yEGF2 weakly correlating with the ability of the sera to block the transmission of parasites to the mosquito host. All four proteins are poorly immunogenic in mice vaccinated with aluminum hydroxide-absorbed formulations. However, all four successfully primed the mice to mount an effective secondary antibody response after a single boost with TBV25H. Sera from mice vaccinated with yEGF2-TBV25H completely block the development of oocysts in mosquito midguts in membrane-feeding assays. Further, of the four proteins, only the depletion of antibodies to yEGF2 from the sera of rabbits vaccinated with TBV25H consistently abolished the ability of those sera to block oocyst development. Thus, antibodies to the second EGF-like domain of Pfs25 appear to mediate a very potent blocking activity, even at low titers. Vaccination strategies that target antibody response towards this domain may improve the efficacy of future transmission-blocking vaccines.

A peptide derived from a B cell epitope of Plasmodium falciparum rhoptry associated protein 2 specifically raises antibodies to rhoptry associated protein 1

Mice immunised with a recombinant form of malarial antigen rhoptry-associated protein 2 (RAP2) produce antisera which recognise the native protein by indirect immunofluorescence and immunoblotting. Purified IgG components of the antisera partially inhibit erythrocyte invasion in vitro. This response was obtained only if the recombinant immunogen was presented to the mice in the presence of reducing and denaturing agents. An 8-mer epitope in RAP2 was recognised by antibodies in three of the antisera: E25TEFSKLY32. Immunisation with this octapeptide raised antibodies which strongly recognised reduced RAP2 in seven out of eight mice. However, this antisera either failed to recognise (five out of eight mice), or only weakly (three out of eight mice) recognised nonreduced RAP2. Examination of disulphide bonds in native RAP2 showed that the 4 cysteines of RAP2 form two disulphide bridges: Cys24-Cys88, and Cys277-Cys376. One of these (Cys24-Cys88) is adjacent to the octapeptide in the native protein. Surprisingly, seven out of eight mice immunised with the octapeptide also raised antibodies against native rhoptry-associated protein 1 (RAP1). The raising of antibodies which recognise RAP1 was induced specifically by the RAP2 octapeptide rather than the carrier protein used for immunisation. The epitope in RAP1 recognised by the antibodies was identified and shown not to be the result of any shared contiguous homologous sequence between the two proteins, but to shared homologous amino acids in critical positions within the epitope. Purified IgG components from the antisera of mice immunised with the octapeptide gave partial inhibition of erythrocyte invasion in vitro.

Preparative scale purification of recombinant proteins to clinical grade by isotachophoresis

An electrophoretic procedure based on isotachophoresis has been developed for protein purification on a preparative scale in the 10 to 500 mg range. The system is simple, uses well understood physical properties, does not need ampholyte spacers and is able to produce sterile products of clinical grade. We demonstrate the applicability of this apparatus for the purification of denatured recombinant proteins and complex mixtures of proteins. The system may also be used for both cationic and anionic purification of proteins in their native form. The system is scalable from analytical to preparative protein loads at consistently high protein yields and purity levels. Total protein loads may vary as much as 1000 fold with the use of interchangeable columns of varying diameter and constant length. At both preparative and analytical scales concentration of products at greater than 20 mg/ml are obtainable. Toxicological considerations are addressed with assays for endotoxin, acrylamide and SDS concentrations, as well as the prevention of covalent protein modification.

Plasmodium falciparum genetic diversity can be characterised using the polymorphic merozoite surface antigen 2 (MSA-2) gene as a single locus marker

The genetic diversity of Solomon Island Plasmodium falciparum isolates was examined using MSA-2 as a single locus marker. Amplification of MSA-2 gene fragments showed size polymorphism and the presence of mixed infections. Sequence analysis indicated a global representation of MSA-2 alleles with representatives of 3D7/CAMP allelic subfamilies and the FCQ-27 allelic family being identified. A simplified method of characterisation, utilising PCR-RFLPs of MSA-2 gene fragments, was developed. The RFLPs allowed identification of allelic families and further distinction within the 3D7/CAMP family. The amplification of MSA-2 gene fragments from culture derived lines revealed a loss of diversity for a number of Solomon Island isolates. Genomic diversity was confirmed for Solomon Island lines, along with Papua New Guinean and Thai lines, by the generation of 7H8/6 fingerprints. All lines were distinct and band sharing frequencies and Wagner tree construction failed to identify any geographic clustering.