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.     

Tricomponent immunopotentiating system as a novel molecular design strategy for malaria vaccine development

The creation of subunit vaccines to prevent malaria infection has been hampered by the intrinsically weak immunogenicity of the recombinant antigens. We have developed a novel strategy to increase immune responses by creating genetic fusion proteins to target specific antigen-presenting cells (APCs). The fusion complex was composed of three physically linked molecular entities: (i) a vaccine antigen, (ii) a multimeric α-helical coiled-coil core, and (iii) an APC-targeting ligand linked to the core via a flexible linker. The vaccine efficacy of the tricomponent complex was evaluated using an ookinete surface protein of Plasmodium vivax, Pvs25, and merozoite surface protein-1 of Plasmodium yoelii. Immunization of mice with the tricomponent complex induced a robust antibody response and conferred substantial levels of P. vivax transmission blockade as evaluated by a membrane feed assay, as well as protection from lethal P. yoelii infection. The observed effect was strongly dependent on the presence of all three components physically integrated as a fusion complex. This system, designated the tricomponent immunopotentiating system (TIPS), onto which any recombinant protein antigens or nonproteinaceous substances could be loaded, may be a promising strategy for devising subunit vaccines or adjuvants against various infectious diseases, including malaria.     

Plasmodium vivax malaria in the Brazilian Amazon: some aspects of its epidemiology, clinical spectrum and naturally induced immune responses

Malaria was a nationwide problem in Brazil in the 1940's. However during the late fifties a national and successful campaign gained strength in the country decreasing malaria to its lowest level by 1960, when only 36,9 thousand cases were registered. Although the Malaria Eradication Program of the Ministry of Health in Brazil succeeded by the late 60's in freeing the majority of the country from malaria transmission, it was unable to contain the rapid spread of the disease in the Amazon Basin. In the 1970's the Amazon region witnessed a huge transformation. Colonization programs sponsored by the government, mining exploration, massive migration and the construction of roads and dams brought a new reality for which the area was not prepared. The last data available show that in 2007, the Amazon registered around 450 thousand cases, 99.9% of the national cases. P. vivax has been reported as representing around 80% of all malaria cases. P. vivax is thought to cause little mortality but like P. falciparum, P. vivax accounts for vast amounts of morbidity and for huge burdens on the prosperity of endemic communities. However, in the last few years a pattern of unusual clinical complications with fatal cases associated with it have been reported in Brazil and is a matter of tremendous concern in the Brazilian community of malariologists. In addition, the emergence of P. vivax strains resistant to chloroquine and primaquine in some reports needs to be further investigated. In contrast, asymptomatic infections by P. falciparum and P. vivax were detected in epidemiological studies in the states of Rondonia and Amazonas. Seropidemiological studies investigating the type of immune responses elicited in naturally exposed populations to several malaria vaccine candidates in Brazilian populations have also been providing important information on whether immune responses specific to those antigens are generated in natural infections and their immunogenic potential as vaccine candidates. In fact ultimate test of a malaria vaccine will be determined in field trials under natural conditions of parasite exposure.     

间日疟原虫中国湖北分离株传播阻断疫苗候选抗原Pvs 28的基因多态性分析

从间日疟原虫感染患者血样制备的干滤纸片中提取寄生虫基因组DNA,以实验室Sall株的Pvs28基因序列设计合成特异性引物,以各分离株基因组DNA为模板,采用PCR技术扩增间日疟原虫传播阻断疫苗候选抗原Pvs28基因全长,其大小为696Bp的片段,将PCR产物直接测序.所获得的序列用Mac Vector软件分析并以Sall株为标准进行突变比较.结果表明,该Pvs28基因在核苷酸水平共存在两种突变形式,即3个点突变与5～7个不同数目的重复片段.点突变情况与我们以前的研究相同,但重复片段数目5和7与以前报道有差异.本文是对我国间日疟原虫单纯流行区湖北省疟原虫分离株传播阻断疫苗候选抗原Pvs28基因多态性的首次报导,研究结果为我国间日疟原虫传播阻断疫苗的实际开发与应用提供了必需的前提依据.     

Recent independent evolution of msp1 polymorphism in Plasmodium vivax and related simian malaria parasites

The Plasmodium MSP-1 is a promising malaria vaccine candidate. However, the highly polymorphic nature of the MSP-1 gene (msp1) presents a potential obstacle for effective vaccine development. To investigate the evolutionary history of msp1 polymorphism in P. vivax, we construct phylogenetic trees of msp1 from P. vivax and related monkey malaria parasite species. All P. vivax msp1 alleles cluster in the P. vivax lineage and are not distributed among other species. Similarly, all P. cynomolgi msp1 alleles cluster in the P. cynomolgi lineage. This suggests that, in contrast to presumed ancient origin of P. falciparum msp1 polymorphism, the origin of P. vivax msp1 polymorphism is relatively recent. We observed positive selection in the P. vivax lineage but not in P. cynomolgi. Also, positive selection acts on different regions of msp1 in P. vivax and P. falciparum. This study shows that the evolutionary history of msp1 differs greatly among parasite lineages.     

Antigenic diversity of Plasmodium falciparum and antibody-mediated parasite neutralization

The malaria parasite Plasmodium falciparum, causing the most severe form of the disease in humans, is characterized by a broad antigenic diversity between different strains and isolates of the parasite. The antigenic diversity reflects on the one hand polymorphisms in allelic gene products and, on the other hand, antigenic variation as a result of expression of alternative genes in multigene families. Using selected polymorphic regions in two merozoite surface antigens, a method for genotyping P. falciparum parasites has been developed. This has resulted in new information on the clonal multiplicity and dynamics of parasite populations. Observations from in vivo and in vitro studies have identified many potential parasite-neutralizing immune responses and several of the target antigens are being explored as vaccine candidates. Studies of antibody-mediated neutralization of parasites in P. falciparum in vitro cultures, with or without leukocytes as effector cells, have been instrumental in identifying potential target antigens for protective immunity and for elucidation of the effects of immune pressure on the dynamics of parasite populations and their antigenic plasticity.     

Novel antigen identification method for discovery of protective malaria antigens by rapid testing of DNA vaccines encoding exons from the parasite genome

We describe a novel approach for identifying target antigens for preerythrocytic malaria vaccines. Our strategy is to rapidly test hundreds of DNA vaccines encoding exons from the Plasmodium yoelii yoelii genomic sequence. In this antigen identification method, we measure reduction in parasite burden in the liver after sporozoite challenge in mice. Orthologs of protective P. y. yoelii genes can then be identified in the genomic databases of Plasmodium falciparum and Plasmodium vivax and investigated as candidate antigens for a human vaccine. A pilot study to develop the antigen identification method approach used 192 P. y. yoelii exons from genes expressed during the sporozoite stage of the life cycle. A total of 182 (94%) exons were successfully cloned into a DNA immunization vector with the Gateway cloning technology. To assess immunization strategies, mice were vaccinated with 19 of the new DNA plasmids in addition to the well-characterized protective plasmid encoding P. y. yoelii circumsporozoite protein. Single plasmid immunization by gene gun identified a novel vaccine target antigen which decreased liver parasite burden by 95% and which has orthologs in P. vivax and P. knowlesi but not P. falciparum. Intramuscular injection of DNA plasmids produced a different pattern of protective responses from those seen with gene gun immunization. Intramuscular immunization with plasmid pools could reduce liver parasite burden in mice despite the fact that none of the plasmids was protective when given individually. We conclude that high-throughput cloning of exons into DNA vaccines and their screening is feasible and can rapidly identify new malaria vaccine candidate antigens.     

Tricomponent Complex Loaded with a Mosquito-Stage Antigen of the Malaria Parasite Induces Potent Transmission-Blocking Immunity

The development of malaria vaccines is challenging, partly because the immunogenicity of recombinant malaria parasite antigens is low. We previously demonstrated that parasite antigens integrated into a tricomponent immunopotentiating complex increase antiparasitic immunity. In this study, the B domains of a group G Streptococcus (SpG) strain and Peptostreptococcus magnus (PpL) were used to evaluate whether vaccine efficacy is influenced by the type of immunoglobulin-binding domain (IBD) in the tricomponent complex. IBDs were fused to a pentameric cartilage oligomeric matrix protein (COMP) to increase the binding avidity of the complexes for their targets. The COMP-IBD fusion proteins generated (COMP-SpG and COMP-PpL and the previously constructed COMP-Z) bound a large fraction of splenic B lymphocytes but not T lymphocytes. These carrier molecules were then loaded with an ookinete surface protein of Plasmodium vivax, Pvs25, by chemical conjugation. The administration of the tricomponent complexes to mice induced more Pvs25-specific serum IgG than did the unloaded antigen. The PpL complex, which exhibited a broad Ig-binding spectrum, conferred higher vaccine efficacy than did the Z or SpG complexes when evaluated with a membrane feed assay. This study demonstrates that this tricomponent immunopotentiating system, incorporating IBDs as the B-lymphocyte-targeting ligands, is a promising technology for the delivery of malaria vaccines, particularly when combined with an aluminum salt adjuvant.     

The Plasmodium vivax homologues of merozoite surface proteins 4 and 5 from Plasmodium falciparum are expressed at different locations in the merozoite

Merozoite surface proteins of Plasmodium falciparum are one major group of antigens currently being investigated and tested as malaria vaccine candidates. Two recently described P. falciparum merozoite surface antigens, MSP4 and MSP5, are GPI-anchored proteins that each contain a single EGF-like domain and appear to have arisen by an ancient gene duplication event. The genes are found in tandem on chromosome 2 of P. falciparum and the syntenic region of the genome was identified in the rodent malarias P. chabaudi, P. yoelii and P. berghei. In these species, there is only a single gene, designated MSP4/5 encoding a single EGF-like domain similar to the EGF-like domain in both PfMSP4 and PfMSP5. Immunization of mice with PyMSP4/5 provides mice with high levels of protection against lethal challenge with blood stage P. yoelii. In this study, we show that in P. vivax, which is quite phylogenetically distant from P. falciparum, both MSP4 and MSP5 homologues can be found with their relative arrangements with respect to the surrounding genes mostly preserved. However, the gene for MSP2, found between MSP5 and adenylosuccinate lyase (ASL) in P. falciparum, is absent from P. vivax. The PvMSP4 and PvMSP5 genes have a two-exon structure and encode proteins with potential signal and GPI anchor sequences and a single EGF-like domain near the carboxyl-terminus. Rabbit antisera raised against purified recombinant proteins show that each of the antisera react with distinct proteins of 62 kDa for PvMSP4 and 86 kDa for PvMSP5 in parasite lysates. Indirect immunofluorescence assays (IFA) localized PvMSP4 over the entire surface of P. vivax merozoites, as expected, whereas, the MSP5 homologue was found to be associated with an apical organellar location consistent with micronemes or over the polar prominence.     

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.     

Genetic polymorphism in Plasmodium falciparum vaccine candidate antigens

Malaria is still a major public health problem in many tropical and subtropical countries. Malaria vaccine is highly desirable as an adjunct to existing malaria control measures. The polymorphisms in malaria vaccine candidates antigens might be a hurdle in developing an effective vaccine. The present article reviews the genetic polymorphism in several antigens expressed on the parasite surface, which are targets for immunological responses of the host and are good candidates for vaccine development against P. falciparum. Variable regions of most genes are generally dimorphic probably as a result of intragenic recombinations. Each allele in turn shows polymorphism resulting from point mutations, or other mechanisms. Several antigens like merozoite surface protein-1 and 2 (MSP-1 and MSP-2) and S antigen show high polymorphism while in others like circumsporozoite protein (CSP), apical membrane antigen-1 (AMA-1) and erythrocyte binding antigen-175 (EBA-175) functional constraints limit the degree of polymorphism. Polymorphism reported in these genes is discussed.     

IFA serological surveys of malaria in north, north-west and south-west parts of Iran

In the IFA serological surveys of malaria carried out in north, north-west and south-west parts of Iran during 1975-1982 altogether 9,132 subjects were studied for malaria antibodies and parasitaemia. Serological data indicated probable malaria transmission in a consolidation area where the autochthonous cases of malaria were reported a year after this serological study. Asymptomatic parasite carriers of P. malariae were found by IFA and parasite concentration techniques among the professional blood donors and the residents of a village without any recent malaria history. IFA results with P. falciparum antigen reflected the malaria histories in the studied areas of west Azerbaijan better than P. vivax antigen. The serological and parasitological findings in the nomads of Bakhtiary tribes showed that the nomads are more exposed to malaria infection in the winter quarters of Izeh area and they are also more under malaria control programme when they are living in this area. In comparison of IFA results of 438 paired plasma and dried blood samples tested with P. vivax and P. falciparum antigens, there was no significant difference between SPR in plasma and dried blood samples, however in the dried blood samples collected by malaria surveillance agents on filter paper SPR and GMRT were considerably low.     

The Current Status of Malaria Vaccines

A vaccine against Plasmodium falciparum malaria is needed now more than ever due the resurgence of the parasite and the increase in drug resistance. However, success in developing an effective malaria vaccine has been elusive. Among pre-erythrocytic antigens, the major antigen coating the surface of the sporozoite, the circumsporozoite protein (CS), has been, and continues to be, the major target for vaccine development. Despite initial limited success with CS-based vaccines, the use of new adjuvant formulations has led to the development of a promising candidate (the RTS,S vaccine) which has shown significant efficacy in a preliminary trial. In addition to CS, many other malaria antigens have been identified that play an important role in the parasite life cycle which are being considered for, or are currently undergoing, clinical trials. Among the blood stage antigens, the merozoite surface protein 1 (MSP-1) is the most promising vaccine candidate. New approaches to immunisation against malaria being considered include the use of multistage, multicomponent vaccines in attenuated viral vectors (NYVAC-Pf7), or in a combination DNA vaccine. While there is reason to be optimistic about the prospects for an effective vaccine, many challenges lie ahead that still have to be overcome. Among these are the antigenic polymorphism exhibited by wild parasite strains and the genetic restriction of immune responses.     

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.     

Immunologic cross-reactivity between structural proteins of human T-cell lymphotropic virus type I and the blood stage of Plasmodium falciparum.

To determine the serologic cross-reactivity between human T-cell lymphotropic virus type I (HTLV-I) and parasite antigens, we measured antibody responses against HTLV-I, Plasmodium falciparum, Plasmodium vivax, and Brugia malayi in serum specimens obtained from regions where malaria (n = 482) and filariasis (n = 101) are endemic. Analysis of immune reactivity to HTLV-I antigens showed that specimens from regions where malaria is endemic had significantly higher rates of enzyme immunoassay (EIA) reactivity (76 of 482 [15.8%] than those from regions where filariasis is endemic (0 of 101 [0%]). Western blot (immunoblot) analysis of the HTLV-I EIA-reactive specimens demonstrated predominant Gag reactivity (HTLV-Iind). Only two specimens each from Indonesia and Brazil and four specimens from Papua New Guinea had Env reactivity by radioimmunoprecipitation analysis. Furthermore, a positive correlation between HTLV-EIA and titers of antibody to the blood stage of P. falciparum (rs = 0.24, P < 0.005) was discerned; no correlation was observed between antibodies to the blood stage or the circumsporozoite protein of P. vivax and the circumsporozoite protein of P. falciparum. In addition, P. falciparum-infected erythrocyte lysate specifically abrogated binding of Gag-specific antibodies in HTLV-Iind specimens from regions where malaria is endemic without affecting binding in HTLV-I-seropositive specimens, suggesting that the immunologic cross-reactivity between HTLV Gag proteins and malaria parasites is restricted to the blood-stage antigens of plasmodia in specimens from regions where malaria is endemic. However, HTLV-seroindeterminate specimens from the United States did not demonstrate serologic cross-reactivity, suggesting that antigenic mimicry of HTLV proteins extends to other nonplasmodial antigens as well.     

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.     

Baculovirus-Vectored Multistage Plasmodium vivax Vaccine Induces Both Protective and Transmission-Blocking Immunities against Transgenic Rodent Malaria Parasites

A multistage malaria vaccine targeting the pre-erythrocytic and sexual stages of Plasmodium could effectively protect individuals against infection from mosquito bites and provide transmission-blocking (TB) activity against the sexual stages of the parasite, respectively. This strategy could help prevent malaria infections in individuals and, on a larger scale, prevent malaria transmission in communities of endemicity. Here, we describe the development of a multistage Plasmodium vivax vaccine which simultaneously expresses P. vivax circumsporozoite protein (PvCSP) and P25 (Pvs25) protein of this species as a fusion protein, thereby acting as a pre-erythrocytic vaccine and a TB vaccine, respectively. A new-concept vaccine platform based on the baculovirus dual-expression system (BDES) was evaluated. The BDES-Pvs25-PvCSP vaccine displayed correct folding of the Pvs25-PvCSP fusion protein on the viral envelope and was highly expressed upon transduction of mammalian cells in vitro. This vaccine induced high levels of antibodies to Pvs25 and PvCSP and elicited protective (43%) and TB (82%) efficacies against transgenic P. berghei parasites expressing the corresponding P. vivax antigens in mice. Our data indicate that our BDES, which functions as both a subunit and DNA vaccine, can offer a promising multistage vaccine capable of delivering a potent antimalarial pre-erythrocytic and TB response via a single immunization regimen.     

Geographical and temporal conservation of antibody recognition of Plasmodium falciparum variant surface antigens

The slow acquisition of protection against Plasmodium falciparum malaria probably reflects the extensive diversity of important antigens. The variant surface antigens (VSA) that mediate parasite adhesion to a range of host molecules are regarded as important targets of acquired protective immunity, but their diversity makes them questionable vaccine candidates. We determined levels of VSA-specific immunoglobulin G (IgG) in human plasma collected at four geographically distant and epidemiologically distinct localities with specificity for VSA expressed by P. falciparum isolates from three African countries. Plasma levels of VSA-specific IgG recognizing individual parasite isolates depended on the transmission intensity at the site of plasma collection but were largely independent of the geographical origin of the parasites. The total repertoire of immunologically distinct VSA thus appears to be finite and geographically conserved, most likely due to functional constraints. Furthermore, plasma samples frequently had high IgG reactivity to VSA expressed by parasites isolated more than 10 years later, showing that the repertoire is also temporally stable. Parasites from patients with severe malaria expressed VSA (VSASM) that were better recognized by plasma IgG than VSA expressed by other parasites, but importantly, VSASM-type antigens also appeared to show substantial antigenic homogeneity. Our finding that the repertoire of immunologically distinct VSA in general, and in particular that of VSASM, is geographically and temporally conserved raises hopes for the feasibility of developing VSA-based vaccines specifically designed to accelerate naturally acquired immunity, thereby enhancing protection against severe and life-threatening P. falciparum malaria.     

Naturally acquired and vaccine-elicited antibodies block erythrocyte cytoadherence of the Plasmodium vivax Duffy binding protein

Malaria merozoites require the presence of specific surface receptors on the red blood cell for invasion. Plasmodium vivax, requires the Duffy blood group antigen as an obligate receptor for invasion. The parasite Duffy binding protein (DBP) is the ligand involved in this process, making the DBP a potential vaccine candidate. A preliminary objective was to study whether people exposed to vivax malaria acquire antibodies that have the ability to block erythrocyte cytoadherence to the PvDBP. In comparison, we studied the immunogenicity of various recombinant DBP vaccines and investigated their potential to induct antifunctional antibodies. In order to do so, recombinant proteins to different regions of the putative ectodomain of the DBP and a DNA vaccine were used to immunize laboratory animals. An in vitro cytoadherence assay was used to investigate the presence of antifunctional antibodies in plasmas from people naturally exposed to vivax malaria, as well as in antisera obtained by animal vaccination. Our results showed that human plasma from populations naturally exposed to vivax malaria, as well as antisera obtained by vaccination using recombinant proteins, a DNA vaccine, and a synthetic peptide to DBP, inhibited in vitro binding of human erythrocytes to the DBP ligand domain (DBP(II)) in correlation to their previously measured antibody titer. Our results provide further evidence for the vaccine potential of this essential parasite adhesion molecule.     

Immunogenicity and antigenicity of Plasmodium vivax merozoite surface protein 10

Among the proteins involved in the invasion by merozoite, the glycosylphosphatidylinositol-anchored proteins (GPI-APs) are suggested as potential vaccine candidates because of their localization to apical organelles and the surface; these candidates are predicted to play essential roles during invasion. As a GPI-AP, Plasmodium vivax merozoite surface protein 10 (PvMSP-10) induces high antibody titers. However, such high antibody titers have shown no protective efficacy for animals challenged with P. vivax parasites in a previous study. To adequately evaluate the immunogenicity and further characterize PvMSP-10 in order to understand its vaccine potential, we assessed its immunogenicity by immunizing BALB/c mice with cell-free expressed recombinant PvMSP-10 protein. The antigenicity of MSP-10 was analyzed, and we found 42 % sensitivity and 95 % specificity using serum samples from P. vivax-infected Korean patients. The IgG1 and IgG3 were the predominant immunoreactive antibodies against PvMSP-10 in vivax patient sera, and IgG1 and IgG3 and Th1-type cytokines were predominantly secreted in PvMSP-10-immunized mice. We conclude that the immunogenicity and antigenicity of MSP-10 may serve as a potential vaccine against vivax malaria.