Vivax Apical Membrane Antigen-1 Research Articles

Plasmodium vivax apical membrane antigen 1I-II from Nicaragua (2012-2013): genetic and antigenic polymorphism

Objective. Genetic and antigenic polymorphism of P. vivax apical membrane antigen-1 (pvama1I-II) from Nicaragua was examined. Materials and methods. Infected blood samples from patients were obtained during 2012-2013. A gene fragment comprising domains I-II was amplified and sequenced, and the genetic parameters, haplotype relationships, genetic structure, and amino acid variation in predicted B cell epitopes were analyzed. Results. 65 sequences of pvama1III had 19 nonsynonymous and five synonymous nucleotide changes. Nicaraguan parasites had low diversity, high linkage disequilibrium, and few recombination events. Neutrality tests indicate a positive and divergent selection, and three genetic clusters with loss of haplotypes were demonstrated. Amino acid variation predominated in predicted B cell epitopes and was closely related to that in Latin American parasites. Conclusions. Nicaraguan P. vivax is a moderately differentiated population under contraction and focalization processes, and the antigenic diversity resembles that of Latin American parasites. This information is relevant for vaccine development and epidemiological surveillance.

The direct binding of Plasmodium vivax AMA1 to erythrocytes defines a RON2-independent invasion pathway

We used a transgenic parasite in which Plasmodium falciparum parasites were genetically modified to express Plasmodium vivax apical membrane antigen 1 (PvAMA1) protein in place of PfAMA1 to study PvAMA1-mediated invasion. In P. falciparum, AMA1 interaction with rhoptry neck protein 2 (RON2) is known to be crucial for invasion, and PfRON2 peptides (PfRON2p) blocked the invasion of PfAMA1 wild-type parasites. However, PfRON2p has no effect on the invasion of transgenic parasites expressing PvAMA1 indicating that PfRON2 had no role in the invasion of PvAMA1 transgenic parasites. Interestingly, PvRON2p blocked the invasion of PvAMA1 transgenic parasites in a dose-dependent manner. We found that recombinant PvAMA1 domains 1 and 2 (rPvAMA1) bound to reticulocytes and normocytes indicating that PvAMA1 directly interacts with erythrocytes during the invasion, and invasion blocking of PvRON2p may result from it interfering with PvAMA1 binding to erythrocytes. It was previously shown that the peptide containing Loop1a of PvAMA1 (PvAMA1 Loop1a) is also bound to reticulocytes. We found that the Loop1a peptide blocked the binding of PvAMA1 to erythrocytes. PvAMA1 Loop1a has no polymorphisms in contrast to other PvAMA1 loops and may be an attractive vaccine target. We thus present the evidence that PvAMA1 binds to erythrocytes in addition to interacting with PvRON2 suggesting that the P. vivax merozoites may exploit complex pathways during the invasion process.

In silico structural modeling and quality assessment of Plasmodium knowlesi apical membrane antigen 1 using comparative protein models.

Plasmodium knowlesi is the most common zoonotic parasite associated with human malaria infection in Malaysia. Apical membrane antigen 1 (AMA1) protein in the parasite plays a critical role in parasite invasion into host cells. To date, there is no complete three-dimensional ectodomain structure of P. knowlesi AMA1 (PkAMA1) protein. The knowledge of a protein structure is important to understand the protein molecular functions. Three in silico servers with respective structure prediction methods were used in this study, i.e., SWISS-MODEL for homology modeling and Phyre2 for protein threading, which are template-based modeling, while I-TASSER for template-free ab initio modeling. Two query sequences were used in the study, i.e., native ectodomain of PkAMA1 strain H protein designated as PkAMA1-H and a modified PkAMA1 (mPkAMA1) protein sequence in adaptation for Pichia pastoris expression. The quality of each model was assessed by ProSA-web, QMEAN and SAVES v6.0 (ERRAT, Verify3D and Ramachandran plot) servers. Generated models were then superimposed with two models of Plasmodium AMA1 deposited in Protein Data Bank (PDB), i.e., PkAMA1 (4UV6.B) and Plasmodium vivax AMA1 (PvAMA1, 1W81) protein structures for similarity assessment, quantified by root-meansquare deviation (RMSD) value. SWISS-MODEL, Phyre2 and I-TASSER server generated two, one and five models, respectively. All models are of good quality according to ProSA-web assessment. Based on the average values of model quality assessment and superimposition, the models that recorded highest values for most parameters were selected as best predicted models, i.e., model 2 for both PkAMA1-H and mPkAMA1 from SWISS-MODEL as well as model 1 of PkAMA1-H and model 3 of mPkAMA1 from I-TASSER. Template-based method is useful if known template is available, but template-free method is more suitable if there is no known available template. Generated models can be used as guidance in further protein study that requires protein structural data, i.e., protein-protein interaction study.

Quantification of the dynamics of antibody response to malaria to inform sero-surveillance in pregnant women

BackgroundMalaria remains a major public health threat and tools sensitive to detect infections in low malaria transmission areas are needed to progress elimination efforts. Pregnant women are particularly vulnerable to malaria infections. Throughout pregnancy they access routine antenatal care, presenting a unique sentinel population to apply novel sero-surveillance tools to measure malaria transmission. The aim of this study was to quantify the dynamic antibody responses to multiple antigens during pregnancy so as to identify a single or multiple antibody response of exposure to malaria in pregnancy.MethodsThis study involved a secondary analysis of antibody responses to six parasite antigens [five commonly studied merozoite antigens and the variant surface antigen 2-chondroitin sulphate A (VAR2CSA), a pregnancy-specific erythrocytic antigen] measured by enzyme-linked immunosorbent assay (ELISA) over the gestation period until delivery (median of 7 measurements/woman) in 250 pregnant women who attended antenatal clinics located at the Thai-Myanmar border. A multivariate mixture linear mixed model was used to cluster the pregnant women into groups that have similar longitudinal antibody responses to all six antigens over the gestational period using a Bayesian approach. The variable-specific entropy was calculated to identify the antibody responses that have the highest influence on the classification of the women into clusters, and subsequent agreement with grouping of women based on exposure to malaria during pregnancy.ResultsOf the 250 pregnant women, 135 had a Plasmodium infection detected by light microscopy during pregnancy (39% Plasmodium falciparum only, 33% Plasmodium vivax only and 28% mixed/other species), defined as cases. The antibody responses to all six antigens accurately identified the women who did not have a malaria infection detected during pregnancy (93%, 107/115 controls). Antibody responses to P. falciparum merozoite surface protein 3 (PfMSP3) and P. vivax apical membrane antigen 1 (PvAMA1) were the least dynamic. Antibody responses to the antigens P. falciparum apical membrane antigen 1 (PfAMA1) and PfVAR2CSA were able to identify the majority of the cases more accurately (63%, 85/135).ConclusionThese findings suggest that the combination of antibodies, PfAMA1 and PfVAR2CSA, may be useful for sero-surveillance of malaria infections in pregnant women, particularly in low malaria transmission settings. Further investigation of other antibody markers is warranted considering these antibodies combined only detected 63% of the malaria infections during pregnancy.

Plasmodium vivax AMA1: Implications of distinct haplotypes for immune response.

In Brazil, Plasmodium vivax infection accounts for around 80% of malaria cases. This infection has a substantial impact on the productivity of the local population as the course of the disease is usually prolonged and the development of acquired immunity in endemic areas takes several years. The recent emergence of drug-resistant strains has intensified research on alternative control methods such as vaccines. There is currently no effective available vaccine against malaria; however, numerous candidates have been studied in the past several years. One of the leading candidates is apical membrane antigen 1 (AMA1). This protein is involved in the invasion of Apicomplexa parasites into host cells, participating in the formation of a moving junction. Understanding how the genetic diversity of an antigen influences the immune response is highly important for vaccine development. In this study, we analyzed the diversity of AMA1 from Brazilian P. vivax isolates and 19 haplotypes of P. vivax were found. Among those sequences, 33 nonsynonymous PvAMA1 amino acid sites were identified, whereas 20 of these sites were determined to be located in predicted B-cell epitopes. Nonsynonymous mutations were evaluated for their influence on the immune recognition of these antigens. Two distinct haplotypes, 5 and 16, were expressed and evaluated for reactivity in individuals from northern Brazil. Both PvAMA1 variants were reactive. Moreover, the IgG antibody response to these two PvAMA1 variants was analyzed in an exposed but noninfected population from a P. vivax endemic area. Interestingly, over 40% of this population had antibodies recognizing both variants. These results have implications for the design of a vaccine based on a polymorphic antigen.

Structure of Rhoptry Neck Protein 2 is essential for the interaction in vitro with Apical Membrane Antigen 1 in Plasmodium vivax

BackgroundIn several Apicomplexa, the formation of moving junctions (MJs) at the interface between the external membranes of the invading parasite and the host cell is essential for the process of parasite invasion. In Plasmodium falciparum and Toxoplasma gondii, the MJ is composed of the Apical Membrane Antigen 1 (AMA1) and Rhoptry Neck Proteins (RONs) complex; specifically, AMA1 interacts with RON2 during host cell invasion.MethodsRecombinant proteins based on Plasmodium vivax RON2 (A2033-P2100) and its synthetic peptide fragments, one cyclic and one linear, based on PvRON2 (D2035-T2074) were generated and used to evaluate the interaction with P. vivax AMA1 (PvAMA1) by the far western blot, surface plasmon resonance (SPR), and isothermal titration microcalorimetry (ITC) methods. The structural studies of peptides were performed by circular dichroism, and the structural analysis of the complex of PvAMA1 with peptides based on PvRON2 (D2035-T2074) was conducted with small-angle X-ray scattering (SAXS).ResultsSurface plasmon resonance (KD = 23.91 ± 2.078 μmol/L) and ITC (K = 3 × 105 mol/L) studies conclusively showed an interaction between the cyclic peptide based on PvRON2 and PvAMA1-His6. In contrast, the linear peptide and recombinant PvRON2 (GST fusion protein) did not show an interaction with PvAMA1. However, the interaction among recombinant proteins PvRON2.2 and PvAMA1-His6 was possible to show by far western blot.ConclusionsThe results show that the PvRON2 structure, particularly the S–S bond between C2051 and C2063, is determinant for the existence of the interaction between PvAMA1 and PvRON2.

Plasmodium vivax ligand-receptor interaction: PvAMA-1 domain I contains the minimal regions for specific interaction with CD71+ reticulocytes

The malarial parasite’s invasion is complex, active and coordinated, involving many low and high affinity interactions with receptors on target cell membrane. Proteomics analysis has described around 40 proteins in P. vivax which could be involved in reticulocyte invasion; few have been studied with the aim of elucidating how many of them establish specific interactions with their respective host cells. Given the importance of knowing which of the parasite’s protein regions are functionally important for invasion, minimum regions mediating specific interaction between Plasmodium vivax apical membrane antigen 1 (PvAMA-1) and its host cell were here elucidated. The region covering PvAMA-1 domains I and II (PvAMA-DI-II) specifically bound to the CD71+ red blood cell subpopulation. A 20 residue-long region (81EVENAKYRIPAGRCPVFGKG100) located in domain I was capable of inhibiting PvAMA-DI-II recombinant protein binding to young reticulocytes (CD71+CD45−) and rosette formation. This conserved peptide specifically interacted with high affinity with reticulocytes (CD71+) through a neuraminidase- and chymotrypsin-treatment sensitive receptor. Such results showed that, despite AMA-1 having universal functions during late Plasmodium invasion stages, PvAMA-1 had reticulocyte-preferring binding regions, suggesting that P. vivax target cell selection is not just restricted to initial interactions but maintained throughout the erythrocyte invasion cycle, having important implications for designing a specific anti-P. vivax vaccine.

Temporal genetic changes in Plasmodium vivax apical membrane antigen 1 over 19\xa0years of transmission in southern Mexico

BackgroundMexico advanced to the pre-elimination phase in 2009 due to a significant reduction in malaria cases, and since 2000, Plasmodium vivax is the only species transmitted. During the last two decades, malaria transmission has been mostly local and isolated to a few regions. It is important to gain further insights into the impact of control measures on the parasite population structure. Hence, the aim of the current study was to determine detailed changes in P. vivax genetic diversity and population structure based on analysing the gene that encodes the apical membrane antigen 1 (pvama1). This analysis covered from control to pre-elimination (1993–2011) in a hypo-endemic region in southern Mexico.ResultsThe 213 pvama1I-II sequences presently analysed were grouped into six periods of three years each. They showed low genetic diversity, with 15 haplotypes resolved. Among the DNA sequences, there was a gradual decrease in genetic diversity, the number of mixed genotype infections and the intensity of positive selection, in agreement with the parallel decline in malaria cases. At the same time, linkage disequilibrium (R2) increased. The three-dimensional haplotype network revealed that pvama1I-II haplotypes were separated by 1–11 mutational steps, and between one another by 0–3 unsampled haplotypes. In the temporal network, seven haplotypes were detected in at least two of the six-time layers, and only four distinct haplotypes were evidenced in the pre-elimination phase. Structure analysis indicated that three subpopulations fluctuated over time. Only 8.5% of the samples had mixed ancestry. In the pre-elimination phase, subpopulation P1 was drastically reduced, and the admixture was absent.ConclusionsThe results suggest that P. vivax in southern Mexico evolved based on local adaptation into three “pseudoclonal” subpopulations that diversified at the regional level and persisted over time, although with varying frequency. Control measures and climate events influenced the number of malaria cases and the genetic structure. The sharp decrease in parasite diversity and other related genetic parameters during the pre-elimination phase suggests that malaria elimination is possible in the near future. These results are useful for epidemiological surveillance.

Generation, characterization and immunogenicity of a novel chimeric recombinant protein based on Plasmodium vivax AMA-1 and MSP119

Plasmodium vivax is the most widely distributed malaria species and the most prevalent species of malaria in America and Asia. Vaccine development against P. vivax is considered a priority in the global program for the eradication of malaria. Earlier studies have characterized the Apical Membrane Antigen 1 (AMA-1) ectodomain and the C-terminal region (19kDa) of the Merozoite Surface Protein 1 (MSP-1) of P. vivax as immunodominant antigens. Based on this characterization, we designed a chimeric recombinant protein containing both merozoite immunodominant domains (PvAMA166-MSP119). The recombinant PvAMA166-MSP119 was successfully expressed in Pichia pastoris and used to immunize two different mouse strains (BALB/c and C57BL/6) in the presence of the Poly (I:C) as an adjuvant. Immunization with the chimeric protein induced high antibody titers against both proteins in both strains of mice as detected by ELISA. Antisera also recognized the native proteins expressed on the merozoites of mature P. vivax schizonts. Moreover, this antigen was able to induce IFN-gamma-secreting cells in C57BL/6 mice. These findings indicate that this novel yeast recombinant protein containing PvAMA166 and PvMSP119 is advantageous, because of improved antibody titers and cellular immune response. Therefore, this formulation should be further developed for pre-clinical trials in non-human primates as a potential candidate for a P. vivax vaccine.

Analysis of Pvama1 genes from China-Myanmar border reveals little regional genetic differentiation of Plasmodium vivax populations.

BackgroundWith the premise of diminishing parasite genetic diversity following the reduction of malaria incidence, the analysis of polymorphic antigenic markers may provide important information about the impact of malaria control on local parasite populations. Here we evaluated the genetic diversity of Plasmodium vivax apical membrane antigen 1 (Pvama1) gene in a parasite population from the China-Myanmar border and compared it with global P. vivax populations.MethodsWe performed evolutionary analysis to examine the genetic diversity, natural selection, and population differentiation of 73 Pvama1 sequences acquired from the China-Myanmar border as well as 615 publically available Pvama1 sequences from seven global P. vivax populations.ResultsA total of 308 Pvama1 haplotypes were identified among the global P. vivax isolates. The overall nucleotide diversity of Pvama1 gene among the 73 China-Myanmar border parasite isolates was 0.008 with 41 haplotypes being identified (Hd = 0.958). Domain I (DI) harbored the majority (26/33) of the polymorphic sites. The McDonald Kreitman test showed a significant positive selection across the ectodomain and the DI of Pvama1. The fixation index (F ST) estimation between the China-Myanmar border, Thailand (0.01) and Myanmar (0.10) showed only slight geographical genetic differentiation. Notably, the Sal-I haplotype was not detected in any of the analyzed global isolates, whereas the Belem strain was restricted to the Thai population. The detected mutations are mapped outside the overlapped region of the predicted B-cell epitopes and intrinsically unstructured/disordered regions.ConclusionsThis study revealed high levels of genetic diversity of Pvama1 in the P. vivax parasite population from the China-Myanmar border with DI displaying stronger diversifying selection than other domains. There were low levels of population subdivision among parasite populations from the Greater Mekong Subregion.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-016-1899-1) contains supplementary material, which is available to authorized users.

Sparse serological evidence of Plasmodium vivax transmission in the Ouest and Sud-Est departments of Haiti

BackgroundPlasmodium vivax infections, while quite prevalent throughout South and Central America, are virtually non-existent in Haiti, where P. falciparum infections are detected in over 99% of malaria cases. Historically, few cases of P. vivax have been reported in Haiti; all of which were identified by microscopy and none were confirmed by molecular diagnostics. To further examine the transmission of P. vivax in Haiti, a cross-sectional seroepidemiological study was conducted. MethodsWhole blood was collected from 814 community members and school children ranging in age between 2 and 80 years-of-age from four locations in the Ouest and Sud-Est Departments of Haiti. After separation of serum, samples were screened for antibodies toward P. vivax apical membrane antigen (AMA-1) and merozoite surface protein-119 (MSP-1) using an indirect enzyme-linked immunosorbent assay (ELISA). ResultsOf all participants screened, 4.42% (36/814) were seropositive for AMA-1, 4.55% (37/814) were seropositive for MSP-1, 7.99% (65/814) were seropositive to either antigen, and only 0.98% (7/814) were seropositive for both antigens. Seroconversion rates (SCR) for AMA-1, MSP-1, either AMA-1 or MSP-1, and for both AMA-1 and MSP-1 estimated from the cross-sectional seroprevalence indicated rates of P. vivax transmission of less than 1% per year. ConclusionGiven the lack of historical evidence of P. vivax infections on the island of Hispaniola, the sparse serological evidence of antibodies toward P. vivax identified in the current study further support the notion that the transmission of P. vivax malaria might be extremely low or even completely absent in Haiti.

High-Level Expression, Purification and Characterization of A Recombinant Plasmodium vivax Apical Membrane Antigen 1: Implication for vivax Malaria Vaccine Development.

The apical membrane antigen-1 (AMA-1) is considered as a promising candidate for development of a malaria vaccine against Plasmodium parasites. The correct conformation of this protein appears to be necessary for the stimulation of parasite-inhibitory responses, and these responses, in turn, seem to be antibody-mediated. Therefore, in the present investigation, we expressed the Plasmodium vivax AMA-1 (PvAMA-1) ectodomain in Escherichia coli (E. coli), purified it using standard procedures and characterized it to determine its biological activities for it to be used as a potential target for developing a protective and safe vivax malaria vaccine. In this experimental investigation, the ectodomain of PvAMA-1 antigen (GenBank accession no. JX624741) was expressed in the E. coli M15pQE30 expression system and purified with immobilized-metal affinity chromatography. The correct conformation of the recombinant protein was evaluated by Western blotting and indirect immunofluorescence antibody (IFA) test. In addition, the immunogenic properties of PvAMA-1 were evaluated in BALB/c mice with the purified protein emulsified in Freund's adjuvant. In the present study, the PvAMA-1 ectodomain was expressed at a high-level (65 mg/L) using a bacterial system. Reduced and non-reduced sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) as well as Western blot analysis confirmed the appropriate conformation and folding of PvAMA-1. The evaluation of immunogenic properties of PvAMA-1 showed that both T helper-1 and 2 cells (Th1 and Th2) responses were present in mice after three immunizations and persisted up to one year after the first immunization. Moreover, the antibodies raised against the recombinant PvAMA-1 in injected mice could recognize the native protein localized on P. vivax parasites. We demonstrate that our recombinant protein had proper conformation and folding. Also, there were common epitopes in the recombinant forms corresponding to native proteins. These results; therefore, indicate that the expressed PvAMA-1 has the potential to be used as a vivax malaria vaccine.

Global Population Structure of the Genes Encoding the Malaria Vaccine Candidate, Plasmodium vivax Apical Membrane Antigen 1 (PvAMA1)

BackgroundThe Plasmodium vivax Apical Membrane Antigen 1 (PvAMA1) is a promising malaria vaccine candidate, however it remains unclear which regions are naturally targeted by host immunity and whether its high genetic diversity will preclude coverage by a monovalent vaccine. To assess its feasibility as a vaccine candidate, we investigated the global population structure of PvAMA1.Methodology and Principal FindingsNew sequences from Papua New Guinea (PNG, n = 102) were analysed together with published sequences from Thailand (n = 158), India (n = 8), Sri Lanka (n = 23), Venezuela (n = 74) and a collection of isolates from disparate geographic locations (n = 8). A total of 92 single nucleotide polymorphisms (SNPs) were identified including 22 synonymous SNPs and 70 non-synonymous (NS) SNPs. Polymorphisms and signatures of balancing (positive Tajima's D and low FST values) selection were predominantly clustered in domain I, suggesting it is a dominant target of protective immune responses. To estimate global antigenic diversity, haplotypes comprised of (i) non-singleton (n = 40) and (ii) common (≥10% minor allele frequency, n = 23) polymorphic amino acid sites were then analysed revealing a total of 219 and 210 distinct haplotypes, respectively. Although highly diverse, the 210 haplotypes comprised of only common polymorphisms were grouped into eleven clusters, however substantial geographic differentiation was observed, and this may have implications for the efficacy of PvAMA1 vaccines in different malaria-endemic areas. The PNG haplotypes form a distinct group of clusters not found in any other geographic region. Vaccine haplotypes were rare and geographically restricted, suggesting potentially poor efficacy of candidate PvAMA1 vaccines.ConclusionsIt may be possible to cover the existing global PvAMA1 diversity by selection of diverse alleles based on these analyses however it will be important to first define the relationships between the genetic and antigenic diversity of this molecule.

A Recombinant Plasmodium vivax Apical Membrane Antigen-1 to Detect Human Infection in Iran

In Iran, Plasmodium vivax is responsible for more than 80% of the infected cases of malaria per year. Control interventions for vivax malaria in humans rely mainly on developed diagnostic methods. Recombinant P. vivax apical membrane antigen-1 (rPvAMA-1) has been reported to achieve designing rapid, sensitive, and specific molecular diagnosis. This study aimed to perform isolation and expression of a rPvAMA-1, derived from Iranian patients residing in an endemic area. Then, the diagnostic efficiency of the characterized Iranian PvAMA-1 was assessed using an indirect ELISA method. For this purpose, a partial region of AMA-1 gene was amplified, cloned, and expressed in pET32a plasmid. The recombinant His-tagged protein was purified and used to coat the ELISA plate. Antibody detection was assessed by indirect ELISA using rPvAMA-1. The validity of the ELISA method for detection of anti-P. vivax antibodies in the field was compared to light microscopy on 84 confirmed P. vivax patients and compared to 84 non-P. vivax infected individuals. The ELISA cut-off value was calculated as the mean+2SD of OD values of the people living in malaria endemic areas from a south part of Iran. We found a cut-off point of OD=0.311 that showed the best correlation between the sera confirmed with P. vivax infection and healthy control sera. A sensitivity of 81.0% and specificity of 84.5% were found at this cut off titer. A good degree of statistical agreement was found between ELISA using rPvAMA-1 and light microscopy (0.827) by Kappa analysis.

Evaluation of the genetic diversity of domain II of Plasmodium vivax Apical Membrane Antigen 1 (PvAMA-1) and the ensuing strain-specific immune responses in patients from Sri Lanka

Antigenic polymorphism displayed by malaria parasites is a skewed schema to escape the host immune system. The prevailing genetic diversity at domain II of the Plasmodium vivax Apical Membrane Antigen-1 (Pvama-1DII) was characterized in 64 single clone P. vivax isolates from Sri Lanka, where unstable malaria prevails with low intensity.In Sri Lanka, the Pvama-1DII gene showed meager meiotic recombination with the enclosure of single nucleotide polymorphisms (SNPs). Eleven amino acid (a.a.) variant positions defined 21 a.a. haplotypes with 9 unique to the island, where the predominant haplotype, H1, was identical to the reference Salvador I strain. A further 376 globally dispersed isolates defined 38 a.a. haplotypes (H22–H59), with 4 and 26 haplotypes exclusive to India and Thailand, respectively. The phylogenetic tree revealed no clustering, where most isolates had a very recent common origin.The polymorphism detected in PvAMA-1DII B and T cell epitopes evidenced an immune evasion mechanism exploited by the parasite. Majority of Sri Lankan patients developed antibody responses to both conformational and linear B cell epitopes.The ensuing strain-specific immunity due to extensive antigenic polymorphism was evaluated by aligning a.a. sequences of PvAMA-1DII with the homologous total (IgM+IgG) antibody responses assayed by in-house established indirect ELISAs against 7 PvAMA-1DII overlapping synthetic peptides, P01–P07. While the antibody responses to P01–P03, P06, P07 harbouring P. vivax clinical isolates with polymorphic a.a. haplotype to Sal I was clearly strain-transcending (cross-reactive), individuals with isolates identical to the Sal I strain observed varying antibody prevalence against the seven PvAMA-1DII Sal-I synthetic peptides, with the highest prevalence detected against P04.Synthetic peptide P04, spanning a.a. positions 302–324 of the PvAMA-1DII of the Sal I strain that included the epitope recognized by the invasion inhibitory 4G2 monoclonal antibody of PfAMA-1, was highly conserved in all 440 local and global P. vivax isolates examined. A functional role for this region is reinforced by the highly immunogenic nature of P04, and could point towards a presumably “protective” anti-P04 antibody response that elicited an isotype switch from IgM to IgG, with increasing exposure to malaria exclusively in endemic residents. Thus the conserved and seemingly “protective” nature of the domain II loop of PvAMA-1 makes it a putative contender to be included in a cocktail vaccine against P. vivax asexual erythrocytic stages in Sri Lanka.

Identification of a Highly Antigenic Linear B Cell Epitope within Plasmodium vivax Apical Membrane Antigen 1 (AMA-1)

Apical membrane antigen 1 (AMA-1) is considered to be a major candidate antigen for a malaria vaccine. Previous immunoepidemiological studies of naturally acquired immunity to Plasmodium vivax AMA-1 (PvAMA-1) have shown a higher prevalence of specific antibodies to domain II (DII) of AMA-1. In the present study, we confirmed that specific antibody responses from naturally infected individuals were highly reactive to both full-length AMA-1 and DII. Also, we demonstrated a strong association between AMA-1 and DII IgG and IgG subclass responses. We analyzed the primary sequence of PvAMA-1 for B cell linear epitopes co-occurring with intrinsically unstructured/disordered regions (IURs). The B cell epitope comprising the amino acid sequence 290–307 of PvAMA-1 (SASDQPTQYEEEMTDYQK), with the highest prediction scores, was identified in domain II and further selected for chemical synthesis and immunological testing. The antigenicity of the synthetic peptide was identified by serological analysis using sera from P. vivax-infected individuals who were knowingly reactive to the PvAMA-1 ectodomain only, domain II only, or reactive to both antigens. Although the synthetic peptide was recognized by all serum samples specific to domain II, serum with reactivity only to the full-length protein presented 58.3% positivity. Moreover, IgG reactivity against PvAMA-1 and domain II after depletion of specific synthetic peptide antibodies was reduced by 18% and 33% (P = 0.0001 for both), respectively. These results suggest that the linear epitope SASDQPTQYEEEMTDYQK is highly antigenic during natural human infections and is an important antigenic region of the domain II of PvAMA-1, suggesting its possible future use in pre-clinical studies.

Identification and localization of minimal MHC-restricted CD8+ T cell epitopes within the Plasmodium falciparum AMA1 protein

BackgroundPlasmodium falciparum apical membrane antigen-1 (AMA1) is a leading malaria vaccine candidate antigen that is expressed by sporozoite, liver and blood stage parasites. Since CD8+ T cell responses have been implicated in protection against pre-erythrocytic stage malaria, this study was designed to identify MHC class I-restricted epitopes within AMA1.MethodsA recombinant adenovirus serotype 5 vector expressing P. falciparum AMA1 was highly immunogenic when administered to healthy, malaria-naive adult volunteers as determined by IFN-γ ELISpot responses to peptide pools containing overlapping 15-mer peptides spanning full-length AMA1. Computerized algorithms (NetMHC software) were used to predict minimal MHC-restricted 8-10-mer epitope sequences within AMA1 15-mer peptides active in ELISpot. A subset of epitopes was synthesized and tested for induction of CD8+ T cell IFN-γ responses by ELISpot depletion and ICS assays. A 3-dimensional model combining Domains I + II of P. falciparum AMA1 and Domain III of P. vivax AMA1 was used to map these epitopes.ResultsFourteen 8-10-mer epitopes were predicted to bind to HLA supertypes A01 (3 epitopes), A02 (4 epitopes), B08 (2 epitopes) and B44 (5 epitopes). Nine of the 14 predicted epitopes were recognized in ELISpot or ELISpot and ICS assays by one or more volunteers. Depletion of T cell subsets confirmed that these epitopes were CD8+ T cell-dependent. A mixture of the 14 minimal epitopes was capable of recalling CD8+ T cell IFN-γ responses from PBMC of immunized volunteers. Thirteen of the 14 predicted epitopes were polymorphic and the majority localized to the more conserved front surface of the AMA1 model structure.ConclusionsThis study predicted 14 and confirmed nine MHC class I-restricted CD8+ T cell epitopes on AMA1 recognized in the context of seven HLA alleles. These HLA alleles belong to four HLA supertypes that have a phenotypic frequency between 23% - 100% in different human populations.

Immunoglobulin GM 3 23 5,13,14 phenotype is strongly associated with IgG1 antibody responses to Plasmodium vivax vaccine candidate antigens PvMSP1-19 and PvAMA-1

BackgroundHumoral immune responses play a key role in the development of immunity to malaria, but the host genetic factors that contribute to the naturally occurring immune responses to malarial antigens are not completely understood. The aim of the present investigation was to determine whether, in subjects exposed to malaria, GM and KM allotypes--genetic markers of immunoglobulin γ and κ-type light chains, respectively--contribute to the magnitude of natural antibody responses to target antigens that are leading vaccine candidates for protection against Plasmodium vivax.MethodsSera from 210 adults, who had been exposed to malaria transmission in the Brazilian Amazon endemic area, were allotyped for several GM and KM determinants by a standard hemagglutination-inhibition method. IgG subclass antibodies to P. vivax apical membrane antigen 1 (PvAMA-1) and merozoite surface protein 1 (PvMSP1-19) were determined by an enzyme-linked immunosorbent assay. Multiple linear regression models and the non-parametric Mann-Whitney test were used for data analyses.ResultsIgG1 antibody levels to both PvMSP1-19 and PvAMA-1 antigens were significantly higher (P = 0.004, P = 0.002, respectively) in subjects with the GM 3 23 5,13,14 phenotype than in those who lacked this phenotype.ConclusionsResults presented here show that immunoglobulin GM allotypes contribute to the natural antibody responses to P. vivax malaria antigens. These findings have important implications for the effectiveness of vaccines containing PvAMA-1 or PvMSP1-19 antigens. They also shed light on the possible role of malaria as one of the evolutionary selective forces that may have contributed to the maintenance of the extensive polymorphism at the GM loci.

A recombinant vaccine based on domain II of Plasmodium vivax Apical Membrane Antigen 1 induces high antibody titres in mice

The Apical Membrane Antigen 1 (AMA-1) is considered a promising candidate for development of a malaria vaccine against asexual stages of Plasmodium. We recently identified domain II (DII) of Plasmodium vivax AMA-1 (PvAMA-1) as a highly immunogenic region recognised by IgG antibodies present in many individuals during patent infection with P. vivax. The present study was designed to evaluate the immunogenic properties of a bacterial recombinant protein containing PvAMA-1 DII. To accomplish this, the recombinant protein was administered to mice in the presence of each of the following six adjuvants: Complete/Incomplete Freund's Adjuvant (CFA/IFA), aluminium hydroxide (Alum), Quil A, QS21 saponin, CpG-ODN 1826 and TiterMax. We found that recombinant DII was highly immunogenic in BALB/c mice when administered in the presence of any of the tested adjuvants. Importantly, we show that DII-specific antibodies recognised the native AMA-1 protein expressed on the surface of P. vivax merozoites isolated from the blood of infected patients. These results demonstrate that a recombinant protein containing PvAMA-1 DII is immunogenic when administered in different adjuvant formulations, and indicate that this region of the AMA-1 protein should continue to be evaluated as part of a subunit vaccine against vivax malaria.

Structural Characterization of Apical Membrane Antigen 1 (AMA1) from Toxoplasma gondii

Apical membrane antigen 1 (AMA1) is an essential component of the moving junction complex used by Apicomplexan parasites to invade host cells. We report the 2.0 A resolution x-ray crystal structure of the full ectodomain (domains I, II, and III) of AMA1 from the pervasive protozoan parasite Toxoplasma gondii. The structure of T. gondii AMA1 (TgAMA1) is the most complete of any AMA1 structure to date, with more than 97.5% of the ectodomain unambiguously modeled. Comparative sequence analysis reveals discrete segments of divergence in TgAMA1 that map to areas of established functional importance in AMA1 from Plasmodium vivax (PvAMA1) and Plasmodium falciparum (PfAMA1). Inspection of the TgAMA1 structure reveals a network of apical surface loops, reorganized in both size and chemistry relative to PvAMA1/PfAMA1, that appear to serve as structural filters restricting access to a central hydrophobic groove. The terminal portion of this groove is formed by an extended loop from DII that is 14 residues shorter in TgAMA1. A pair of tryptophan residues (Trp(353) and Trp(354)) anchor the DII loop in the hydrophobic groove and frame a conserved tyrosine (Tyr(230)), forming a contiguous surface that may be critical for moving junction assembly. The minimalist DIII structure folds into a cystine knot that probably stabilizes and orients the bulk of the ectodmain without providing excess surface area to which invasion-inhibitory antibodies can be generated. The detailed structural characterization of TgAMA1 provides valuable insight into the mechanism of host cell invasion by T. gondii.