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

The persistence of naturally acquired antibodies and memory B cells specific to rhoptry proteins of Plasmodium vivax in patients from areas of low malaria transmission

Background

Rhoptries are the large, paired, secretory organelles located at the apical tip of the malaria merozoite that are considered important for parasite invasion processes. Plasmodium vivax rhoptry proteins have been shown to induce humoral immunity during natural infections. Therefore, these proteins may be potential novel vaccine candidates. However, there is a lack of data on the duration of antibody and memory B cell (MBC) responses. Here, the longitudinal analysis of antibody and MBC responses to the P. vivax rhoptry proteins PvRALP1-Ecto and PvRhopH2 were monitored and analysed in individuals to determine their persistence.

Methods

Thirty-nine samples from P. vivax-infected subjects (age 18–60 years) were recruited to explore the frequency and persistence of antibody and MBC responses against rhoptry proteins (PvRALP1-Ecto and PvRhopH2) using both cross-sectional and longitudinal cohort study designs. Antibody levels were determined by ELISA during clinical malaria, and at 3, 9 and 12 months post-infection. The frequency of MBC sub-sets and presence of rhoptry-specific MBCs in subjects 18 months after treatment were detected by flow cytometry and ELISPOT assay.

Results

The seroprevalence of antibodies against PvRALP1-Ecto and PvRhopH2 proteins was found to be high during acute infection, with IgG1, IgG2 and IgG3 sub-classes predominant. However, these anti-rhoptry responses were short-lived and significantly decreased at 9 months post-infection. To relate the durability of these antibody responses to MBC persistence at post-infection, 18-month post-infection peripheral blood mononuclear cells (PBMCs) samples were taken to detect rhoptry-specific MBCs and frequency of MBC sub-sets, and correlate with antibody responses. These late post-infection samples revealed that rhoptry-specific MBCs were present in about 70% of total subjects. However, the persistence of specific MBCs was not correlated with antibody responses as the majority of malaria subjects who were positive for PvRALP1-Ecto- or PvRhopH2-specific MBCs were seronegative for the rhoptry antigens. The frequencies of classical MBCs were increased after infection, whereas those of activated and atypical MBCs were decreased, indicating that MBC responses could switch from activated or atypical MBCs to classical MBCs after parasite clearance, and were maintained in blood circulating at post-infection.

Conclusion

The study showed that rhoptry antigens induced the development and persistence of MBC responses in P. vivax-infected subjects who lived in a region of low malaria transmission, which were not related to the longevity of antibody responses.

Chimeric peptide constructs comprising linear B-cell epitopes: application to the serodiagnosis of infectious diseases

Linear B-cell epitopes are ideal biomarkers for the serodiagnosis of infectious diseases. However, the long-predicted diagnostic value of epitopes has not been realized. Here, we demonstrated a method, diagnostic epitopes in four steps (DEIFS), that delivers a combination of epitopes for the serodiagnosis of infectious diseases with a high success rate. Using DEIFS for malaria, we identified 6 epitopes from 8 peptides and combined them into 3 chimeric peptide constructs. Along with 4 other peptides, we developed a rapid diagnostic test (RDT), which is able to differentiate Plasmodium falciparum (P. falciparum) from Plasmodium vivax (P. vivax) infections with 95.6% overall sensitivity and 99.1% overall specificity. In addition to applications in diagnosis, DEIFS could also be used in the diagnosis of virus and bacterium infections, discovery of vaccine candidates, evaluation of vaccine potency, and study of disease progression.

Antibodies to the Plasmodium falciparum rhoptry protein RAP-2/RSP-2 in relation to anaemia in Cameroonian children

Previous studies have implicated reactive antibodies to the low molecular weight rhoptry-associated proteins (RAP-1, RAP-2/RSP-2 and RAP-3) in erythroid cell destruction during Plasmodium falciparum infection. In this pilot study, the frequency, specificity and functional capacity of naturally acquired anti-RAP-2/RSP-2 antibodies were investigated in the sera of anaemic and nonanaemic malaria-infected Cameroonian children. All sera recognized RAP-2/RSP-2 by FACS, irrespective of the clinical status of the subjects. However, the anaemic children showed higher levels of IgG antibodies than the nonanaemic group, while both groups showed similar levels of IgM antibodies. Only few individuals had detectable levels of RAP-2/RSP-2-specific IgG1 and IgG3 subclass antibodies, while no IgG2 and IgG4 subclass antibodies were detected in these subjects. By ELISA, the anaemic group tended to show higher levels of antibodies to RAP-2/RSP-2 regarding all antibody classes tested, except for IgG4 and IgE. Unexpectedly, sera from the nonanaemic group activated complement to a greater extent than those from the anaemic group. These results need to be confirmed in extended studies but indicate that the effector functions of the RAP-2/RSP-2-reactive antibodies may be more important than their amounts. Such antibodies could play a role in both immunity and pathogenesis during P. falciparum infection.

Multiple antigen peptide vaccines against Plasmodium falciparum malaria

The multiple antigen peptide (MAP) approach is an effective method to chemically synthesize and deliver multiple T-cell and B-cell epitopes as the constituents of a single immunogen. Here we report on the design, chemical synthesis, and immunogenicity of three Plasmodium falciparum MAP vaccines that incorporated antigenic epitopes from the sporozoite, liver, and blood stages of the life cycle. Antibody and cellular responses were determined in three inbred (C57BL/6, BALB/c, and A/J) strains, one congenic (HLA-A2 on the C57BL/6 background) strain, and one outbred strain (CD1) of mice. All three MAPs were immunogenic and induced both antibody and cellular responses, albeit in a somewhat genetically restricted manner. Antibodies against MAP-1, MAP-2, and MAP-3 had an antiparasite effect that was also dependent on the mouse major histocompatibility complex background. Anti-MAP-1 (CSP-based) antibodies blocked the invasion of HepG2 liver cells by P. falciparum sporozoites (highest, 95.16% in HLA-A2 C57BL/6; lowest, 11.21% in BALB/c). Furthermore, antibodies generated following immunizations with the MAP-2 (PfCSP, PfLSA-1, PfMSP-1(42), and PfMSP-3b) and MAP-3 (PfRAP-1, PfRAP-2, PfSERA, and PfMSP-1(42)) vaccines were able to reduce the growth of blood stage parasites in erythrocyte cultures to various degrees. Thus, MAP-based vaccines remain a viable option to induce effective antibody and cellular responses. These results warrant further development and preclinical and clinical testing of the next generation of candidate MAP vaccines that are based on the conserved protective epitopes from Plasmodium antigens that are widely recognized by populations of divergent HLA types from around the world.

Erythrocyte binding protein PfRH5 polymorphisms determine species-specific pathways of Plasmodium falciparum invasion

Some human malaria Plasmodium falciparum parasites, but not others, also cause disease in Aotus monkeys. To identify the basis for this variation, we crossed two clones that differ in Aotus nancymaae virulence and mapped inherited traits of infectivity to erythrocyte invasion by linkage analysis. A major pathway of invasion was linked to polymorphisms in a putative erythrocyte binding protein, PfRH5, found in the apical region of merozoites. Polymorphisms of PfRH5 from the A. nancymaae-virulent parent transformed the nonvirulent parent to a virulent parasite. Conversely, replacements that removed these polymorphisms from PfRH5 converted a virulent progeny clone to a nonvirulent parasite. Further, a proteolytic fragment of PfRH5 from the infective parasites bound to A. nancymaae erythrocytes. Our results also suggest that PfRH5 is a parasite ligand for human infection, and that amino acid substitutions can cause its binding domain to recognize different human erythrocyte surface receptors.

Studies of Plasmodium falciparum rhoptry-associated membrane antigen (RAMA) protein peptides specifically binding to human RBC

Plasmodium falciparum rhoptry-associated membrane antigen (RAMA) peptides used in normal red blood cell (RBC) binding assays revealed that peptides 33426 (79NINILSSVHRKGRILYDSF97) and 33460 (777HKKREKSISPHSYQKVSTKVQ797) bound with high activity, presenting nanomolar affinity constants. Such high binding activity peptides (HABPs) displayed helicoid and random coil structures as determined by circular dichroism. HABPs inhibited P. falciparumin vitro invasion of normal RBC by up to 61% (depending on concentration), suggesting that some RAMA protein regions could be involved in P. falciparum invasion of RBC. The nature and localisation of receptors on RBC surface responsible for HABP binding were studied using enzyme-treated erythrocytes and structural analysis.

Characterization of a Membrane-associated Rhoptry Protein of Plasmodium falciparum

Invasive forms of apicomplexan parasites contain secretory organelles called rhoptries that are essential for entry into host cells. We present a detailed characterization of an unusual rhoptry protein of the human malaria parasite Plasmodium falciparum, the rhoptry-associated membrane antigen (RAMA) that appears to have roles in both rhoptry biogenesis and host cell invasion. RAMA is synthesized as a 170-kDa protein in early trophozoites, several hours before rhoptry formation and is transiently localized within the endoplasmic reticulum and Golgi within lipid-rich microdomains. Regions of the Golgi membrane containing RAMA bud to form vesicles that later mature into rhoptries in a process that is inhibitable by brefeldin A. Other rhoptry proteins such as RhopH3 and RAP1 are found in close apposition with RAMA suggesting direct protein-protein interactions. We suggest that RAMA is involved in trafficking of these proteins into rhoptries. In rhoptries, RAMA is proteolytically processed to give a 60-kDa form that is anchored in the inner face of the rhoptry membrane by means of the glycosylphosphatidylinositol anchor. The p60 RAMA form is discharged from the rhoptries of free merozoites and binds to the red blood cell membrane by its most C-terminal region. In early ring stages RAMA is found in association with the parasitophorous vacuole.

Plasmodium falciparum: red blood cell binding studies using peptides derived from rhoptry-associated protein 2 (RAP2)

Plasmodium falciparum rhoptry-associated proteins 1 (RAP1) and 2 (RAP2) are antigens presenting themselves as candidates for a subunit malaria vaccine. RAP2 protein, non-overlapping, consecutive peptides were synthesised and tested in red blood cell (RBC) binding assays to identify their receptor-ligand interaction in recognising RAP2 regions involved in the in vitro merozoite invasion process. Four high activity binding peptides (HABPs) were identified in the resulting 20 peptides. Peptides 26220 ((61)NHFSSADELIKYLEKTNINT(80)), 26225 ((161)IKKNPFLRVLNKASTTTHAT(180)) and 26229 ((241)RSVNNVISKNKTLGLRKRSS(260)) were located in the amino terminal and central part of the protein and HABP 26235 ((361)FLAEDFVELFDVTMDCYSRQ(380)) was located at the carboxy terminal. All these HABPs showed saturable binding and presented dissociation constants between 500 and 950 nM; the number of binding sites per RBC ranged from 48,000 to 160,000. High binding peptides' critical amino acids involved in RBC binding were determined by competition binding assays; their amino acids appear in bold in the sequences shown above. SDS-PAGE results showed that peptides 26220, 26225 and 26229 had at least two different sets of 62 and 42 kDa HABP receptors on RBCs and that peptide 26235 had at least two different sets of 77 and 62 kDa. HABPs inhibited in vitro merozoite invasion by between 54% and 94% at 200 microM, suggesting that these RAP2 peptides are involved in the in vitro P. falciparum invasion process.

Organization, transcription, and expression of rhoptry associated protein genes in the Babesia bigemina rap-1 locus

The Babesia bigemina rap-1 gene locus contains five tandemly arranged copies of rap-1a genes. However, the size of the locus, as defined by conserved, unrelated orfs at the 5' and 3' ends, suggests that additional genes may be present. In this study, we identified all additional genes in the locus and characterized their pattern of expression in merozoites. The rap-1a genes are separated by 3.38-kbp intergenic (IG) regions, each of which contains an identical copy of a related gene designated rap-1b. One additional copy of rap-1b and one copy of another related gene designated rap-1c is present in the 3' end of the locus. Common sequence features that define the Babesia rap-1 family are present in rap-1b and rap-1c, but otherwise these genes average only 27% identity to rap-1a. Homologues of the rap-1b and rap-1c genes identified in diverse B. bigemina strains have a high degree of predicted amino acid sequence conservation (averaging >90%), with the largest number of changes in the carboxyl end of RAP-1c. We tested whether all rap-1 genes in the locus are co-transcribed in merozoites using RT-PCR, Northern blots, and quantitative real-time PCR. Rap-1a genes produce the most abundant transcripts of the family, while rap-1b transcripts are the least abundant despite the large number of gene copies. Similar patterns of transcription were observed whether merozoites were obtained from in vitro cultures or in vivo infection. Immunoblot analysis of merozoites revealed the expected RAP-1a expression but failed to detect expressed RAP-1b and RAP-1c, indicating that expression of the rap-1 genes is regulated both at the transcriptional and translational levels.

Identification and characterization of a conserved, stage-specific gene product of Plasmodium falciparum recognized by parasite growth inhibitory antibodies

We have identified a novel conserved protein of Plasmodium falciparum, designated D13, that is stage-specifically expressed in asexual blood stages of the parasite. The predicted open reading frame (ORF) D13 contains 863 amino acids with a calculated molecular mass of 99.7 kDa and displays a repeat region composed of pentapeptide motives. Northern blot analysis with lysates of synchronized blood stage parasites showed that D13 is highly expressed at the mRNA level during schizogony. The first N'-terminal 138 amino acids of D13 were expressed in Escherichia coli and the purified protein was used to generate anti-D13 monoclonal antibodies (MAbs). Using total lysates of blood stage parasites and Western blot analysis, these MAbs stained one single band of approximately 100 kDa, corresponding to the predicted molecular mass of ORF D13. Western blot analysis demonstrated further that D13 is expressed during schizogony, declines rapidly in early ring stages and is undetectable in trophozoites. D13 protein is localized in individual merozoites in a distinct area, as demonstrated by indirect immunofluorescence analysis. After subcellular fractionation, D13 was confined to the pelleted fraction of the parasite lysate and its extraction by alkaline carbonate buffer treatment indicated that D13 is not a membrane-integral protein. Inclusion of certain anti-D13 MAbs into in vitro cultures of blood stage parasites resulted in considerable reduction in parasite growth. The N'-terminal domain encompassing 158 amino acids is 94 and 95%, respectively, identical at the amino acid level between Plasmodium knowlesi, Plasmodium yoelii, and P. falciparum. In summary, we describe a novel stage-specifically expressed, highly conserved gene product of P. falciparum that is recognized by parasite growth inhibitory antibodies.

Rhoptry-associated protein 1-binding monoclonal antibody raised against a heterologous peptide sequence inhibits Plasmodium falciparum growth in vitro

Monoclonal antibodies (MAbs) specific for Plasmodium falciparum rhoptry-associated protein 1 (RAP-1) were generated and tested for inhibition of parasite growth in vitro. The majority of indirect immunofluorescence assay (IFA)-positive MAbs raised against recombinant RAP-1 positions 23 to 711 (rRAP-1(23-711)) recognized epitopes located in the immunodominant N-terminal third of RAP-1. MAbs specific for the building block 35.1 of the synthetic peptide malaria vaccine SPf66 also yielded an IFA staining pattern characteristic for rhoptry-associated proteins and reacted specifically with rRAP-1 and parasite-derived RAP-1 molecules p67 and p82. Cross-reactivity with RAP-1 was blocked by the 35.1 peptide. Epitope mapping with truncated rRAP-1 molecules and overlapping peptides identified the linear RAP-1 sequence Y218KYSL222 as a target of the anti-35.1 MAbs. This sequence lacks primary sequence similarity with the 35.1 peptide (YGGPANKKNAG). Cross-reactivity of the anti-35.1 MAbs thus appears to be associated with conformational rather than sequence homology. While the anti-35.1 MAb SP8.18 exhibited parasite growth-inhibitory activity, none of the tested anti-rRAP-1(23-711) MAbs inhibited parasite growth, independently of their fine specificity for the RAP-1 sequences at positions 33 to 42, 213 to 222, 243 to 247, 280 to 287, or 405 to 446. The growth-inhibitory activity of MAb SP8.18 was, however, accelerated by noninhibitory anti-RAP-1 MAbs. Results demonstrate that in addition to fine specificity, other binding parameters are also crucial for the inhibitory potential of an antibody.

Interaction of HLA and age on levels of antibody to Plasmodium falciparum rhoptry-associated proteins 1 and 2

The Plasmodium falciparum rhoptry-associated proteins 1 and 2 (RAP1 and RAP2) are candidate antigens for a subunit malaria vaccine. The design of the study, which looks at the acquisition of immunity to malaria from childhood to old age, has allowed us to document the interaction of HLA and age on levels of antibody to specific malarial antigens. Antibodies reach maximum levels to RAP1 after the age of 15 but to RAP2 only after the age of 30. The effect of HLA-DRB1 and -DQB1 and age on levels of antibody to rRAP1 and rRAP2 was analyzed with a multiple regression model in which all HLA alleles and age were independent variables. DQB1*0301 and -*03032 showed an age-dependent association with levels of antibody to rRAP1, being significant in children 5 to 15 years (P < 0.001) but not in individuals over 15 years of age. DRB1*03011 showed an age-dependent association with antibody levels to rRAP2; however, this association was in adults over the age of 30 years (P < 0.01) but not in individuals under the age of 30 years. No associations were detected between DRB1 alleles and RAP1 antibody levels or between DQB1 alleles and RAP2 antibody levels. Thus, not only the HLA allele but also the age at which an interaction is manifested varies for different malarial antigens. The interaction may influence either the rate of acquisition of antibody or the final level of antibody acquired by adults.

Assessment of the humoral immune response against Plasmodium falciparum rhoptry-associated proteins 1 and 2

Naturally occurring antibody responses to Plasmodium falciparum rhoptry-associated proteins 1 and 2 (RAP-1 and RAP-2) were measured with recombinant and parasite-derived forms of the antigens. For comparative purposes, responses to multiple forms of three other malarial antigens were also examined. The sera of 100 Papua New Guineans were screened for antibodies. Eighty-six and 82% of individuals over 30 years of age had antibodies that recognized parasite-derived RAP-1 and RAP-2, respectively. Importantly, we found that recombinant and native antigens share linear epitopes seen by the human immune system; thus, the recombinant proteins may be adequate human immunogens. However, antibodies affinity purified on recombinant RAP-1 reacted with other antigens in addition to parasite-derived RAP-1. Thus, the antigenicity of RAP-1 may have been overestimated previously. The recognition of RAP-1 and RAP-2 correlated with age and with the recognition of recombinant forms of the ring-infected erythrocyte surface antigen, merozoite surface protein 1, and merozoite surface antigen 2 (MSA2) antigens. Antibodies to these antigens appear to be generated in response to the total exposure to malaria of the host. Antibodies to conserved regions of MSA2 had stronger correlations with both age and the recognition of other antigens than did the full-length recombinant MSA2 molecule. In contrast to results with the other antigens, there was no significant difference in the ages of individuals with a certain antibody titer to the full-length recombinant or parasite-derived MSA2 molecule, but antibodies to these two antigens did correlate with parasitemia. For all antigens tested, antibody levels after two infections can approach the peak levels of antibodies obtained in immune individuals.