Protective immunity induced in Aotus monkeys by a recombinant SERA protein of Plasmodium falciparum: adjuvant effects on induction of protective immunity.

We report the results of vaccination trial 2 of Panamanian Aotus monkeys with a recombinant blood-stage antigen, SERA 1, of the malaria parasite Plasmodium falciparum. Monkeys were immunized with SERA 1, a 262-amino-acid fragment (amino acids 24 to 285) of the 989-amino-acid SERA protein produced by the Honduras 1 strain of the parasite. Immunization mixtures contained 100 micrograms of recombinant SERA 1 protein per dose mixed with one of five different adjuvants. The protein mixed with either Freund's adjuvant or MF75.2 adjuvant stimulated protective immunity. When other P. falciparum antigens were included in the SERA 1-Freund's adjuvant mixture, no protective immunity was observed, although high anti-SERA 1 antibody titers were produced. Three other adjuvants mixed with SERA 1 failed to induce a protective immune response. These results, their relationship to those reported previously in the first vaccination trial (trial 1), and their relationships to the quantitative measurement of anti-SERA 1 antibodies in enzyme-linked immunosorbent assays provided insights into the induction of a protective immune response in vaccinated monkeys.     

Protective immunity induced in Aotus monkeys by a recombinant SERA protein of Plasmodium falciparum: further studies using SERA 1 and MF75.2 adjuvant.

We describe the third of three vaccination trials of Panamanian Aotus monkeys with a recombinant blood-stage antigen derived from the malaria parasite Plasmodium falciparum. Immunization was performed with an N-terminal region of the SERA antigen (serine repeat antigen protein), SERA 1, that contains a 262-amino-acid fragment including amino acids 24 to 285 of the 989-amino-acid SERA protein. Vaccinations were carried out with the recombinant protein mixed with either Freund's, MF75.2, or MF59.2 adjuvant. A control group that did not receive SERA 1 but only MF75.2 adjuvant was included. Monkeys vaccinated with the antigen MF59.2 mixture produced low anti-SERA 1 titers and were not protected. Monkeys vaccinated with antigen and Freund's adjuvant had, in general, a higher average anti-SERA 1 titer (107,278) than did monkeys immunized with SERA 1 and MF75.2 (40, 143), yet monkeys in both groups were well protected. Monkeys that received only MF75.2 developed neither detectable anti-SERA 1 nor anti-P. falciparum antibodies prior to or 10 days after parasite challenge, yet were apparently protected against infection. Monkeys vaccinated with either SERA 1 and Freund's, SERA 1 and MF75.2, or MF75.2 alone and that had been challenged but did not develop a countable parasitemia were treated with a curative dose of mefloquine 100 days after parasite challenge and then rechallenged 40 days later. None of the five rechallenged monkeys that had originally received SERA 1 and Freund's developed a countable parasitemia. Only one of five rechallenged monkeys that originally received SERA 1 and MF75.2 developed a high countable parasitemia, while two animals developed a barely countable parasitemia. Four of the rechallenged monkeys that had originally received only MF75.2 developed a moderate to high countable parasitemia. The results indicate that vaccination with SERA 1 and either Freund's or MF75.2 adjuvant provides protection and vaccination with MF75.2 alone can provide a temporary protection unrelated to the induction of anti-SERA 1 or antimalarial antibodies.     

Immunogenicity of recombinant Plasmodium falciparum SERA proteins in rodents

We have expressed defined regions of the serine-repeat antigen (SERA) of the Honduras-1 strain of Plasmodium falciparum in the yeast Saccharomyces cerevisiae. Amino-terminal domains of the natural SERA protein have been shown previously to be targets for parasite-inhibitory murine monoclonal antibodies. Two recombinant SERA antigens were selected for purification and immunological analysis. The first (SERA 1), corresponding to amino acids 24-285 of the natural SERA precursor, was expressed by the ubiquitin fusion method. This allowed for in vivo cleavage by endogenous yeast ubiquitin hydrolase, and subsequent isolation of the mature polypeptide. The second, larger protein (SERA N), encompassing amino acids 24-506, was expressed at only low levels using this system, but could be isolated in high yields when fused to human gamma-interferon (gamma-IFN). Each purified protein was used to immunize mice with either Freund's adjuvant or a muramyl tripeptide adjuvant that has been used in humans. Sera from immunized mice were shown to be capable of in vitro inhibition of invasion of erythrocytes by the Honduras-1 strain of P. falciparum. The results suggest that a recombinant SERA antigen may be an effective component of a candidate malaria vaccine.     

Immune responses induced by gene gun or intramuscular injection of DNA vaccines that express immunogenic regions of the serine repeat antigen from Plasmodium falciparum.

The liver- and blood-stage-expressed serine repeat antigen (SERA) of Plasmodium falciparum is a candidate protein for a human malaria vaccine. We compared the immune responses induced in mice immunized with SERA-expressing plasmid DNA vaccines delivered by intramuscular (i.m.) injection or delivered intradermally by Gene Gun immunization. Mice were immunized with a pcdna3 plasmid encoding the entire 47-kDa domain of SERA (amino acids 17 to 382) or the N-terminal domain (amino acids 17 to 110) of SERA. Minimal antibody responses were detected following DNA vaccination with the N-terminal domain of SERA, suggesting that the N-terminal domain alone is not highly immunogenic by this route of vaccine delivery. Immunization of mice by Gene Gun delivery of the 47-kDa domain of SERA elicited a significantly higher serum antibody titer to the antigen than immunization of mice by i.m. injection with the same plasmid did. The predominant isotype subclass of the antibodies elicited to the SERA protein following i.m. and Gene Gun immunizations with SERA plasmid DNA was immunoglobulin G1. Coimmunization of mice with SERA plasmid DNA and a plasmid expressing the hepatitis B surface antigen (pCMV-s) by the i.m. route resulted in higher anti-SERA titers than those generated in mice immunized with the SERA DNA plasmid alone. Vaccination with DNA may provide a viable alternative or may be used in conjunction with protein-based subunit vaccines to maximize the efficacy of a human malaria vaccine that includes immunogenic regions of the SERA protein.     

Immunogenicity of the Plasmodium falciparum serine repeat antigen (p126) expressed by vaccinia virus.

cDNA encoding the serine repeat antigen (SERA) (also called p126) of Plasmodium falciparum has been isolated from the FCR3 strain and inserted into a recombinant vaccinia virus designated vP870. Expression analysis of vP870-infected Vero cells by immunoprecipitation has demonstrated several intracellular forms of SERA and a single secreted SERA peptide. Endoglycosidase digestion of these immunoprecipitated SERA peptides indicated that the intracellular SERA peptides contain simple, high-mannose N-linked oligosaccharides and that the secreted SERA peptide contains complex N-linked oligosaccharides. Pulse-chase experiments indicate that the multiple intracellular SERA peptides in infected Vero cells represent a trafficking pathway whereby the smallest SERA peptide is converted into larger peptides by co- and posttranslational modifications, including glycosylation, and eventually secreted from the cell with complex N-linked oligosaccharides. To study the immunogenicity of vaccinia virus-expressed SERA, rabbits were immunized with vP870 and their sera were analyzed for reactivity with authentic, parasite-derived SERA protein. The anti-vP870 rabbit sera reacted with P. falciparum-infected erythrocytes by immunofluorescence analysis, recognized authentic SERA from schizonts by both immunoprecipitation and Western blot (immunoblot) analyses, and recognized proteolytically processed fragments of SERA secreted into the culture medium by Western blot analysis. These results indicate that when expressed by vaccinia virus, SERA is glycosylated and secreted from infected cells and that in immunized rabbits, vaccinia virus-expressed SERA can stimulate a humoral immune response against SERA derived from blood-stage parasites.     

Ability of recombinant or native proteins to protect monkeys against heterologous challenge with Plasmodium falciparum.

To circumvent problems associated with polymorphic vaccine candidates for Plasmodium falciparum malaria, we evaluated recombinant proteins representing sequences from relatively high conserved regions of the precursor to the major merozoite surface proteins, gp190, for their ability to protect Saimiri monkeys against malaria challenge. Recombinant proteins represented amino acid residues 147 to 321 (p190-1) or 147 to 321 and 1060 to 1195 (p190-3), and their efficacy was compared with that of native gp190 and its processed products. All antigens were derived from P. falciparum K1, a Thai isolate, while the challenge strain was Palo Alto (from Uganda, Africa), which contains, with the exception of the N-terminal 375 amino acids, which are almost identical to the K1 sequence, essentially the MAD-20 allelic form of gp190. By 12 days following challenge, each control monkey required drug treatment. Three monkeys injected with p190-3 required therapy, while one cleared the parasites without therapy. Two monkeys injected with p190-1 received therapy on day 14, while the remaining two cleared the parasites without therapy. Of four animals injected with native gp190, because of health reasons unrelated to malaria, one was not challenged with parasites and one was removed from the study 8 days after challenge when its parasitemia was 1.1% (parasitemias in control animals ranged from 4.3 to 9%); the remaining two cleared the parasites after maximum parasitemias of 0.45 and 0.53%. The highest levels of antiparasite antibody were produced by animals immunized with native gp190. There was a significant correlation between monkeys which did not require drug treatment and antiparasite antibody. These results may suggest that native gp190 and/or its processed products can provide excellent protection against heterologous challenge and that antibody is important for protection. The challenge for vaccine development is to identify the protective sequence(s).     

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

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

Primate antibody response to immunotoxin: serological and computer-aided analysis of epitopes on a truncated form of Pseudomonas exotoxin.

NLysPE38 is a 38-kDa derivative of Pseudomonas exotoxin (PE) in which domain Ia (amino acids 1 to 252) and part of domain Ib (365 to 380) are deleted and an 11-amino-acid N-terminal peptide is added. LMB-1 is an immunotoxin in which the monoclonal antibody B3 is coupled to NLysPE38 near its N terminus. LMB-7 is a single-chain immunotoxin in which the Fv fragment of B3 is fused to PE38. To identify the antigenic regions of PE38, 12 polyclonal serum samples from monkeys immunized with the immunotoxins LMB-1 (six monkeys) and LMB-7 (six monkeys) were tested for their reactivity to a panel of 120 synthetic, overlapping peptides representing the amino acid sequence of NLysPE38. The antibody responses to peptides were similar among the 12 serum specimens, identifying several major immunodominant B-cell epitopes. Predominant reactivity was seen in six locations: amino acids 272 to 287, 341 to 359, 504 to 516, 540 to 564, and 573 to 591 and the C-terminal amino acids 591 to 613. The sera did not react with approximately 75% of the peptides. Furthermore, a computer-aided analysis was done to predict the immunologically relevant areas and revealed the same antigenic regions defined by serum reactivity to peptides. Competition enzyme-linked immunosorbent assays and neutralization assays were performed with domain II, III, or III plus Ib of PE38 and confirmed the immunodominance of domain III. To analyze the role of specific amino acids in antibody binding, individual amino acids of PE38 with large accessible surface areas were altered by site-directed mutagenesis. These results also show that the predicted areas of immunogenicity agree with the reactivity of the anti-PE38 antibodies to peptides and to the mutants of PE.     

Comparison of protection induced by immunization with recombinant proteins from different regions of merozoite surface protein 1 of Plasmodium yoelii.

Vaccination with native full-length merozoite surface protein 1 (MSP1) or with recombinant C-terminal peptides protects mice against lethal challenge with virulent malaria parasites. To determine whether other regions of MSP1 can also induce protection, Plasmodium yoelii MSP1 was divided into four separate regions. Each was expressed in Escherichia coli as a fusion protein with glutathione S-transferase (GST). The N-terminal fragment began after the cleavage site for the signal sequence and ended in the region comparable to the cleavage site for the C terminus of the 82-kDa peptide of Plasmodium falciparum. This expressed protein was 30 kDa smaller than the predicted peptide. One peptide from the middle region was produced, and the C terminus consisted of a 42-kDa fragment corresponding to the analogous peptide of P. falciparum and a 19-kDa fragment that extended 37 amino acids in the amino-terminal direction beyond the probable cleavage site. To test protection of mice against lethal P. yoelii challenge, three mouse strains (CAF1, BALB/c, and A/J) were vaccinated with each of the four recombinant proteins of MSP1. Mice vaccinated with the C-terminal 19-kDa protein were highly protected (described previously), as were those vaccinated with the 42-kDa protein that contained the 19-kDa fragment. The N-terminally expressed fragment of P. yoelii was not full length because of proteolytic cleavage in E. coli. The GST-82-kDa partial fragments induced some immunity, but the surviving mice still had high parasitemias. Vaccination with the peptide from the middle region of MSP1 gave minimal to no protection. Therefore, in addition to the C-terminal 19- and 42-kDa proteins, the only other fragment to give protection was the 82-kDa protein. The protection induced by the truncated 82-kDa protein was minimal compared with that of the C-terminal fragments.     

Conservation of antigen components from two recombinant hybrid proteins protective against malaria.

Recently, we have shown that two hybrid proteins carrying partial sequences of the blood-stage antigens SERP, HRPII, and MSAI from Plasmodium falciparum confer protective immunity on Aotus monkeys against an experimental parasite infection (B. Knapp, E. Hundt, B. Enders, and H. A. Küpper, Infect. Immun. 60:2397-2401, 1992). The malarial components of the hybrid proteins consist of amino acid residues 630 to 892 of SERP, amino acid residues 146 to 260 of MSAI, and the 189 C-terminal residues of HRPII. We have studied the diversity of these protein regions in field isolates of P. falciparum. Genomic DNA was extracted from the blood of six donors from two different areas where malaria is endemic. The gene regions of SERP and MSAI coding for the corresponding sequences of the protective hybrid proteins and the exon II region of the HRPII gene were amplified by polymerase chain reaction and sequenced. All three regions were found to be highly conserved. In the 262-amino-acid fragment of SERP, one single conservative amino acid substitution was found. The exon II region of HRPII showed only a slight variability in number and arrangement of the repeat units. The 115-amino-acid fragment of MSAI which is located within an N-terminal region known to be conserved among different parasite strains was shown to be the most variable among the vaccine components: amino acid substitutions were found in 14 different positions of this MSAI region when both laboratory strains and field isolates were compared.     

A protective glycosylphosphatidylinositol-anchored membrane protein of Plasmodium yoelii trophozoites and merozoites contains two epidermal growth factor-like domains

Using sera from mice immunized and protected against Plasmodium yoelii malaria, we identified a novel blood-stage antigen gene, pypag-2. The 2.1-kb pypag-2 cDNA contains a single open reading frame that encodes a 409-amino-acid protein with a predicted molecular mass of 46.8 kDa. Unlike many characterized plasmodial antigens, blocks of tandemly repeated amino acids are lacking in the pypAg-2 protein sequence. Recombinant pypAg-2, comprising the full-length protein minus the predicted N-terminal signal and C-terminal anchor sequences, was produced and used to raise a high-titer polyclonal rabbit antiserum. This antiserum was used to identify and characterize the native protein through immunoblotting, immunoprecipitation and immunofluorescence assays. Consistent with the presence of a glycosylphosphatidylinositol anchor, pypAg-2 fractionated with the detergent phase of Triton X-114-solubilized proteins and could be metabolically labeled with [(3)H]palmitic acid. By immunofluorescence, pypAg-2 expression was localized to both the trophozoite and merozoite membranes. Similar to Plasmodium falciparum merozoite surface protein 1, pypAg-2 contains two C-terminal epidermal growth factor (EGF)-like domains. Most importantly, immunization with recombinant pypAg-2 protected mice against lethal P. yoelii malaria. Thus, pypAg-2 is a target of protective immune responses and represents a novel addition to the family of merozoite surface proteins that contain one or more EGF-like domains.     

Molecular cloning, genomic structure and localization in a blood stage antigen of Plasmodium falciparum characterized by a serine stretch

Two short DNA segments were isolated by screening of a lambda gt11 library from Plasmodium falciparum schizont cDNA with an antiserum against the 140 kDa protein, which confers protective immunity to monkeys. The segments were used to identify a genomic fragment which carries the entire coding sequence for a protein of 113 kDa characterized by a stretch of serine residues (SERP I). We present the complete nucleotide and deduced amino acid sequence as well as the structure of the SERP I gene. The gene consists of four exons interrupted by three short introns located at the amino-terminal half. Exon 1 and the first part of exon 2 code for hydrophobic amino acids of a putative signal sequence. Exon 2 contains two repetitive segments, the first encoding six glycine rich octapeptides and a second region coding for 37 consecutive serine residues. Southern blot analysis demonstrated the conservation of the SERP I gene in four different parasite strains. SERP I could be localized in the parasitophorous vacuole and in the surrounding membranes. We discuss the relationship of this protein to the recently described P126 polypeptide and the possible role of this antigen as a vaccine candidate.     

The clinical-grade 42-kilodalton fragment of merozoite surface protein 1 of Plasmodium falciparum strain FVO expressed in Escherichia coli protects Aotus nancymai against challenge with homologous erythrocytic-stage parasites

A 42-kDa fragment from the C terminus of major merozoite surface protein 1 (MSP1) is among the leading malaria vaccine candidates that target infection by asexual erythrocytic-stage malaria parasites. The MSP1(42) gene fragment from the Vietnam-Oak Knoll (FVO) strain of Plasmodium falciparum was expressed as a soluble protein in Escherichia coli and purified according to good manufacturing practices. This clinical-grade recombinant protein retained some important elements of correct structure, as it was reactive with several functional, conformation-dependent monoclonal antibodies raised against P. falciparum malaria parasites, it induced antibodies (Abs) that were reactive to parasites in immunofluorescent Ab tests, and it induced strong growth and invasion inhibitory antisera in New Zealand White rabbits. The antigen quality was further evaluated by vaccinating Aotus nancymai monkeys and challenging them with homologous P. falciparum FVO erythrocytic-stage malaria parasites. The trial included two control groups, one vaccinated with the sexual-stage-specific antigen of Plasmodium vivax, Pvs25, as a negative control, and the other vaccinated with baculovirus-expressed MSP1(42) (FVO) as a positive control. Enzyme-linked immunosorbent assay (ELISA) Ab titers induced by E. coli MSP1(42) were significantly higher than those induced by the baculovirus-expressed antigen. None of the six monkeys that were vaccinated with the E. coli MSP1(42) antigen required treatment for uncontrolled parasitemia, but two required treatment for anemia. Protective immunity in these monkeys correlated with the ELISA Ab titer against the p19 fragment and the epidermal growth factor (EGF)-like domain 2 fragment of MSP1(42), but not the MSP1(42) protein itself or the EGF-like domain 1 fragment. Soluble MSP1(42) (FVO) expressed in E. coli offers excellent promise as a component of a vaccine against erythrocytic-stage falciparum malaria.     

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

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

Protection of rhesus macaques against lethal Plasmodium knowlesi malaria by a heterologous DNA priming and poxvirus boosting immunization regimen

We tested a cytokine-enhanced, multiantigen, DNA priming and poxvirus boosting vaccine regimen for prevention of malaria in the Plasmodium knowlesi-rhesus macaque model system. Animals were primed with a mixture of DNA plasmids encoding two preerythrocytic-stage proteins and two erythrocytic-stage proteins from P. knowlesi and combinations of the cytokines granulocyte-macrophage colony-stimulating factor, interleukin-4, and tumor necrosis factor alpha and were boosted with a mixture of four recombinant, attenuated vaccinia virus strains encoding the four P. knowlesi antigens. Two weeks after boosting, the geometric mean immunofluorescence titers in the immunized groups against sporozoites and infected erythrocytes ranged from 160 to 8,096 and from 1,810 to 5,120, respectively. The geometric mean anti-P. knowlesi circumsporozoite protein (PkCSP) titers ranged from 1,761 to 24,242. Peripheral blood mononuclear cells (PBMC) from the immunized monkeys produced gamma interferon (IFN-gamma) in response to incubation with pooled peptides from the PkCSP at frequencies of 10 to 571 spot-forming cells/10(6) PBMC. Following challenge with 100 infectious P. knowlesi sporozoites, 2 of 11 immunized monkeys were sterilely protected, and 7 of the 9 infected monkeys resolved their parasitemias spontaneously. In contrast, all four controls became infected and required treatment for overwhelming parasitemia. Early protection was strongly associated with IFN-gamma responses against a pool of peptides from the preerythrocytic-stage antigen, PkCSP. These findings demonstrate that a multistage, multiantigen, DNA priming and poxvirus boosting vaccine regimen can protect nonhuman primates from an otherwise lethal malaria sporozoite challenge.     

A Plasmodium falciparum malaria vaccine candidate which contains epitopes from the circumsporozoite protein and a blood stage antigen, 5.1.

The previously described Plasmodium falciparum blood stage antigen, 5.1 (also referred to as exp-1) was expressed at a high level in Escherichia coli. Saimiri monkeys immunised with purified recombinant antigen 5.1 were partially protected from P. falciparum blood stage parasite challenge. The gene coding for 5.1 was combined with DNA coding for an (Asn-Ala-Asn-Pro)19 sequence (abbreviated (NANP)19 in the one-letter amino acid code). To facilitate purification of the recombinant protein, DNA coding for a hexahistidine (His6) sequence was introduced at the 5' end of the gene (proteins containing His6 have high affinity for Ni(2+)-chelate columns even in the presence of 6 M guanidine HCl). The recombinant protein, His6-5.1-(NANP)19 with an apparent molecular size of 40 kDa could be highly purified by a combination of 4 steps: (1) release and solubilization of the recombinant fusion protein from E. coli in the presence of 6 M guanidine-HCl; (2) precipitation of over 60% of the bacterial proteins by the addition of ammonium sulphate to 50% saturation; (3) affinity chromatography on a Ni(2+)-chelate column in the presence of 6 M guanidine-HCl; (4) adsorption onto a cation exchange resin in the presence of 6 M urea, and elution with an increasing NaCl gradient. Compared with the previously tested tetanus toxoid-(NANP)3 malaria vaccine, this protein elicits an anti-(NANP)n response which more closely resembles that evoked by native sporozoites. The recombinant vaccine also induces the production of antibodies against the blood stages of the malaria parasite.     

A recombinant baculovirus 42-kilodalton C-terminal fragment of Plasmodium falciparum merozoite surface protein 1 protects Aotus monkeys against malaria.

The immunogenicity and protective efficacy of baculovirus recombinant polypeptide based on the Plasmodium falciparum merozoite surface protein 1 (MSP-1) has been evaluated in Aotus lemurinus griseimembra monkeys. The MSP-1-based polypeptide, BVp42, corresponds to the 42-kDa C-terminal processing fragment of the precursor molecule. Immunization of Aotus monkeys with BVp42 in complete Freund's adjuvant resulted in high antibody titers against the immunogen as well as parasite MSP-1. Fine specificity studies indicated that major epitopes recognized by these antibodies localize to conserved determinants of the 19-kDa C-terminal fragment derived from cleavage of the 42-kDa processing fragment. Effective priming of MSP-1-specific T cells was also demonstrated in lymphocyte proliferation assays. All three Aotus monkeys immunized with BVp42 in complete Freund's adjuvant showed evidence of protection of protection against blood-stage challenge with P. falciparum. Two animals were completely protected, with only one parasite being detected in thick blood films on a single days after injection. The third animal had a modified course of infection, controlling its parasite infection to levels below detection by thick blood smears for an extended period in comparison with adjuvant control animals. All vaccinated, protected Aotus monkeys produced antibodies which inhibited in vitro parasite growth, indicating that this assay may be a useful correlate of protective immunity and that immunity induced by BVp42 immunization is mediated, at least in part, by a direct effect of antibodies against the MSP-1 C-terminal region. The high level of protection obtained in these studies supports further development of BVp42 as a candidate malaria vaccine.     

Antibodies Reactive with the N-Terminal Domain of Plasmodium falciparum Serine Repeat Antigen Inhibit Cell Proliferation by Agglutinating Merozoites and Schizonts

The serine repeat antigen (SERA) is a vaccine candidate antigen of Plasmodium falciparum. Immunization of mice with Escherichia coli-produced recombinant protein of the SERA N-terminal domain (SE47') induced an antiserum that was inhibitory to parasite growth in vitro. Affinity-purified mouse antibodies specific to the recombinant protein inhibited parasite growth between the schizont and ring stages but not between the ring and schizont stages. When Percoll-purified schizonts were cultured with the affinity-purified SE47'-specific antibodies, schizonts and merozoites were agglutinated. Indirect-immunofluorescence assays with unfixed parasite cells showed that SE47'-specific immunoglobulin G (IgG) bound to SERA molecules on rupturing schizonts and merozoites but the IgG did not react with the schizont-infected erythrocytes (RBC). Furthermore, double-fluorescence staining against SE47'-specific IgG and anti-human RBC membrane IgG showed that the RBC membrane disappeared from SE47'-specific-IgG-bound schizonts after cultivation. These observations suggest that the SE47'-specific antibodies inhibit parasite growth by cross-linking SERA molecules that are associated with merozoites in rupturing schizonts with partly broken RBC and parasitophorous vacuole membranes, blocking merozoite release.     

Structure and expression of the knob-associated histidine-rich protein of Plasmodium falciparum

cDNA clones encoding 473 amino acids of the knob-associated histidine-rich protein (PfHRPI) of Plasmodium falciparum clone FCR-3/A2 (Gambia) have been isolated and sequenced. Although a short region close to the amino terminus of the predicted sequence contains three blocks of six, seven or nine consecutive histidine residues, the most abundant amino acid is lysine. The predicted sequence contains a putative amino-terminal signal sequence and two potential asparagine glycosylation sites. A 1284 bp Sau3A cDNA fragment was expressed in Escherichia coli as a fusion protein that was recognized by an anti-PfHRPI monoclonal antibody. Pulsed field gradient electrophoresis indicated that the PfHRPI gene is located on chromosome 2. The PfHRPI gene was present, apparently intact, in knobless parasites derived from one uncloned P. falciparum isolate (St. Lucia). In a knobless derivative of another uncloned isolate (Malayan Camp) and in a cloned knobless line (FCR-3/D4) of a third isolate (Gambian), that part of the gene covered by the cDNA clone has been deleted. Loss of PfHRPI expression may therefore arise via several different mechanisms of gene alteration.     

A novel protein antigen of the malaria parasite Plasmodium falciparum, located on the surface of gametes and sporozoites

A Plasmodium falciparum cDNA clone was isolated of which the insert is transcribed at high rates as a 1.4-kb mRNA in the sexual stages of the malaria parasite. The cDNA clone contains a copy of a non-interrupted gene which codes for a protein of 157 amino acids (Mr = 16607). This 16-kDa protein does not contain repetitive sequences and is characterised by a putative N-terminal signal sequence, a hydrophobic membrane anchor sequence and a highly hydrophilic C-terminal region suggesting that it is an integral membrane protein. Rabbit antisera raised against a synthetic peptide covering amino acids 31-47 of the 16-kDa protein and against recombinant fusion proteins recognised the 16-kDa antigen in protein extracts of gametocytes, macrogamete/zygotes and sporozoites by Western blot analysis. The rabbit antisera also reacted with gametes, gametocytes and sporozoites in a standard immunofluorescence assay. By immunoelectron microscopy using the protein A-gold method the 16-kDa protein could be clearly visualised on the surface of macrogametes and sporozoites, whereas the antigen was not detectable in the asexual erythrocytic stages of the parasite. The 16-kDa antigen of P. falciparum therefore might have the potential to elicit a dual protective immune response against the sporozoite and sexual stage parasites.