Plasmodium vivax Duffy binding protein: baseline antibody responses and parasite polymorphisms in a well-consolidated settlement of the Amazon Region

OBJECTIVE

To investigate risk factors associated with the acquisition of antibodies against Plasmodium vivax Duffy binding protein (PvDBP) - a leading malaria vaccine candidate - in a well-consolidated agricultural settlement of the Brazilian Amazon Region and to determine the sequence diversity of the PvDBP ligand domain (DBP(II)) within the local malaria parasite population.

METHODS

Demographic, epidemiological and clinical data were collected from 541 volunteers using a structured questionnaire. Malaria parasites were detected by conventional microscopy and PCR, and blood collection was used for antibody assays and molecular characterisation of DBP(II).

RESULTS

The frequency of malaria infection was 7% (6% for P. vivax and 1% for P. falciparum), with malaria cases clustered near mosquito breeding sites. Nearly 50% of settlers had anti-PvDBP IgG antibodies, as detected by enzyme-linked immunosorbent assay (ELISA) with subject's age being the only strong predictor of seropositivity to PvDBP. Unexpectedly, low levels of DBP(II) diversity were found within the local malaria parasites, suggesting the existence of low gene flow between P. vivax populations, probably due to the relative isolation of the studied settlement.

CONCLUSION

The recognition of PvDBP by a significant proportion of the community, associated with low levels of DBP(II) diversity among local P. vivax, reinforces the variety of malaria transmission patterns in communities from frontier settlements. Such studies should provide baseline information for antimalarial vaccines now in development.

An engineered vaccine of the Plasmodium vivax Duffy binding protein enhances induction of broadly neutralizing antibodies

Plasmodium vivax invasion into human reticulocytes is a complex process. The Duffy binding protein (DBP) dimerization with its cognate receptor is vital for junction formation in the invasion process. Due to its functional importance, DBP is considered a prime vaccine candidate, but variation in B-cell epitopes at the dimer interface of DBP leads to induction of strain-limited immunity. We believe that the polymorphic residues tend to divert immune responses away from functionally conserved epitopes important for receptor binding or DBP dimerization. As a proof of concept, we engineered the vaccine DEKnull to ablate the dominant Bc epitope to partially overcome strain-specific immune antibody responses. Additional surface engineering on the next generation immunogen, DEKnull-2, provides an immunogenicity breakthrough to conserved protective epitopes. DEKnull-2 elicits a stronger broadly neutralizing response and reactivity with long-term persistent antibody responses of acquired natural immunity. By using novel engineered DBP immunogens, we validate that the prime targets of protective immunity are conformational epitopes at the dimer interface. These successful results indicate a potential approach that can be used generally to improve efficacy of other malaria vaccine candidates.

Blood-stage Plasmodium vivax antibody dynamics in a low transmission setting: A nine year follow-up study in the Amazon region

Plasmodium vivax remains a global health problem and its ability to cause relapses and subpatent infections challenge control and elimination strategies. Even in low malaria transmission settings, such as the Amazon basin, where progress in malaria control has caused a remarkable reduction in case incidence, a recent increase in P. vivax transmission demonstrates the continued vulnerability of P.vivax-exposed populations. As part of a search for complementary approaches to P.vivax surveillance in areas in which adults are the majority of the exposed-population, here we evaluated the potential of serological markers covering a wide range of immunogenicity to estimate malaria transmission trends. For this, antibodies against leading P. vivax blood-stage vaccine candidates were assessed during a 9 year follow-up study among adults exposed to unstable malaria transmission in the Amazon rainforest. Circulating antibody levels against immunogenic P. vivax proteins, such as the Apical Membrane Antigen-1, were a sensitive measure of recent P. vivax exposure, while antibodies against less immunogenic proteins were indicative of naturally-acquired immunity, including the novel engineered Duffy binding protein II immunogen (DEKnull-2). Our results suggest that the robustness of serology to estimate trends in P.vivax malaria transmission will depend on the immunological background of the study population, and that for adult populations exposed to unstable P.vivax malaria transmission, the local heterogeneity of antibody responses should be considered when considering use of serological surveillance.

Antigenic characterization of Anaplasma marginale isolates from different regions of Brazil

Antigenic characterization of A. marginale isolates has contributed to identifying the presence of common and restricts epitopes of major surface proteins (MSPs). The data may improve vaccine development to protect against A. marginale isolates from different regions. Brazilian A. marginale isolates were characterized antigenically by Western blot with monoclonal antibodies (MAbs) against MSPs and rabbit anti-MSP-4 from Florida strain. Six A. marginale isolates from MS, MG (AUFV1), SP, PR-L1, PR-HV, RS and Florida strain were tested with ANA22B1 to MSP-1a, AMR36A6 to MSP-1b, ANAF19E2 to MSP-2, AMG75C1 and AMG76B2 to MSP-3 and ANAF16C1 to MSP-5. ANA22B1 recognized MSP-1a epitope in all A. marginale isolates, and reacted with polypeptides of different size ranging 46-105kDa. MSP2 was not detected in MS and SP isolates by ANAF19E2, and only PR-L1 and MG (AUFV1) isolates reacted with MAbs which recognize MSP3 epitope. MSP4 and MSP5 were detected in all A. marginale isolates analyzed. The results revealed conservation of MSP-1a and MSP-5 epitopes among all Brazilian isolates, and showed antigenic variability to MSP-1b, MSP-2 and MSP-3 proteins, agreeing with recent data about the genetic diversity found in the polimorphic multigene family responsible for these proteins.

Duffy antigen receptor for chemokine (DARC) polymorphisms and its involvement in acquisition of inhibitory anti-duffy binding protein II (DBPII) immunity

The Plasmodium vivax Duffy binding protein (PvDBP) and its erythrocytic receptor, the Duffy antigen receptor for chemokines (DARC), are involved in the major P. vivax erythrocyte invasion pathway. An open cohort study to analyze DARC genotypes and their relationship to PvDBP immune responses was carried out in 620 volunteers in an agricultural settlement of the Brazilian Amazon. Three cross-sectional surveys were conducted at 6-month intervals, comprising 395, 410, and 407 subjects, respectively. The incidence rates of P. vivax infection was 2.32 malaria episodes per 100 person-months under survey (95% confidence interval [CI] of 1.92-2.80/100 person-month) and, of P. falciparum, 0.04 per 100 person-months (95% CI of 0.007-0.14/100 person-month). The distribution of DARC genotypes was consistent with the heterogeneous ethnic origins of the Amazon population, with a predominance of non-silent DARC alleles: FY*A > FY*B. The 12-month follow-up study demonstrated no association between DARC genotypes and total IgG antibodies as measured by ELISA targeting PvDBP (region II, DBPII or regions II-IV, DBPII-IV). The naturally acquired DBPII specific binding inhibitory antibodies (BIAbs) tended to be more frequent in heterozygous individuals carrying a DARC-silent allele (FY*BES). These results provide evidence that DARC polymorphisms may influence the naturally acquired inhibitory anti-Duffy binding protein II immunity.

Immune response of BALB/c mouse immunized with recombinant MSPs proteins of Anaplasma marginale binding to immunostimulant complex (ISCOM)

Anaplasmosis, caused by Anaplasma marginale, results in significant economic losses of cattle in tropical and subtropical regions worldwide. Six major surface proteins (MSPs) were well characterized and designated as MSP1, MSP2, MSP3, MSP4, and MSP5. The objective of this study was to evaluate the humoral immune response of BALB/c mice against the recombinant MSPs, incorporated into immunostimulating complex (ISCOM). The recombinant proteins purified by Ni-NTA columns were incorporated into ISCOM and ISCOMATRIX by the lipid film hydration method. BALB/c mice immunized with ISCOM/rMSPs and ISCOMATRIX/rMSPs vaccines produced whole IgG, IgG1, and IgG2a, in contrast to the negative groups (PBS and ISCOMATRIX adjuvant). All groups that received antigen responded specifically against the rMSPs by Western blotting, showing the rMSP1a (60-105kDa), rMSP1b (100kDa), rMSP4 (47kDa), and rMSP5 (29kDa). Additional studies will have to be performed in cattle to evaluate the humoral and cellular mechanisms of this subunit vaccine and their possible use as protective vaccines against homologous and heterologous strains of A. marginale.

Cross-Species Immune Recognition Between Plasmodium vivax Duffy Binding Protein Antibodies and the Plasmodium falciparum Surface Antigen VAR2CSA

Background

In pregnancy, Plasmodium falciparum parasites express the surface antigen VAR2CSA, which mediates adherence of red blood cells to chondroitin sulfate A (CSA) in the placenta. VAR2CSA antibodies are generally acquired during infection in pregnancy and are associated with protection from placental malaria. We observed previously that men and children in Colombia also had antibodies to VAR2CSA, but the origin of these antibodies was unknown. Here, we tested whether infection with Plasmodium vivax is an alternative mechanism of acquisition of VAR2CSA antibodies.

Methods

We analyzed sera from nonpregnant Colombians and Brazilians exposed to P. vivax and monoclonal antibodies raised against P. vivax Duffy binding protein (PvDBP). Cross-reactivity to VAR2CSA was characterized by enzyme-linked immunosorbent assay, immunofluorescence assay, and flow cytometry, and antibodies were tested for inhibition of parasite binding to CSA.

Results

Over 50% of individuals had antibodies that recognized VAR2CSA. Affinity-purified PvDBP human antibodies and a PvDBP monoclonal antibody recognized VAR2CSA, showing that PvDBP can give rise to cross-reactive antibodies. Importantly, the monoclonal antibody inhibited parasite binding to CSA, which is the primary in vitro correlate of protection from placental malaria.

Conclusions

These data suggest that PvDBP induces antibodies that functionally recognize VAR2CSA, revealing a novel mechanism of cross-species immune recognition to falciparum malaria.

Genetic variability in platelet integrin α2β1 density: possible contributor to Plasmodium vivax-induced severe thrombocytopenia

Understanding the pathogenesis of Plasmodium vivax malaria is challenging. We hypothesized that susceptibility to P. vivax-induced thrombocytopenia could be associated with polymorphisms on relevant platelet membrane integrins: integrin α2 (C807T), and integrin β3 (T1565C). Although β3 polymorphism was not related with P. vivax malaria, α2 807T carriers, which show high levels of integrin α2β1, had a higher probability for severe thrombocytopenia than wild-type carriers. This evidence of the association of integrin polymorphism and P. vivax morbidity was further demonstrated by a moderate but significant correlation between clinical disease and surface levels of the integrin α2β1.

Dynamics of IgM and IgG responses to the next generation of engineered Duffy binding protein II immunogen: Strain-specific and strain-transcending immune responses over a nine-year period

BACKGROUND

A low proportion of P. vivax-exposed individuals acquire protective strain-transcending neutralizing IgG antibodies that are able to block the interaction between the Duffy binding protein II (DBPII) and its erythrocyte-specific invasion receptor. In a recent study, a novel surface-engineered DBPII-based vaccine termed DEKnull-2, whose antibody response target conserved DBPII epitopes, was able to induce broadly binding-inhibitory IgG antibodies (BIAbs) that inhibit P. vivax reticulocyte invasion. Toward the development of DEKnull-2 as an effective P. vivax blood-stage vaccine, we investigate the relationship between naturally acquired DBPII-specific IgM response and the profile of IgG antibodies/BIAbs activity over time.

METHODOLOGY/PRINCIPAL FINDINGS

A nine-year follow-up study was carried-out among long-term P. vivax-exposed Amazonian individuals and included six cross-sectional surveys at periods of high and low malaria transmission. DBPII immune responses associated with either strain-specific (Sal1, natural DBPII variant circulating in the study area) or conserved epitopes (DEKnull-2) were monitored by conventional serology (ELISA-detected IgM and IgG antibodies), with IgG BIAbs activity evaluated by functional assays (in vitro inhibition of DBPII-erythrocyte binding). The results showed a tendency of IgM antibodies toward Sal1-specific response; the profile of Sal1 over DEKnull-2 was not associated with acute malaria and sustained throughout the observation period. The low malaria incidence in two consecutive years allowed us to demonstrate that variant-specific IgG (but not IgM) antibodies waned over time, which resulted in IgG skewed to the DEKnull-2 response. A persistent DBPII-specific IgM response was not associated with the presence (or absence) of broadly neutralizing IgG antibody response.

CONCLUSIONS/SIGNIFICANCE

The current study demonstrates that long-term exposure to low and unstable levels of P. vivax transmission led to a sustained DBPII-specific IgM response against variant-specific epitopes, while sustained IgG responses are skewed to conserved epitopes. Further studies should investigate on the role of a stable and persistent IgM antibody response in the immune response mediated by DBPII.

[Cloning, expression, molecular characterization of the MSP5 protein from PR1 strain of Anaplasma marginale and its application in a competitive enzyme-linked immunosorbent test]

A competitive enzyme-linked immunosorbent test using the PR1 recombinant major surface protein 5 (rMSP5-PR1-ELISA) of Anaplasma marginale was standardized and validated using sera from anaplasmosis free and endemic regions. The sequencing of the msp5 gene of PR1 isolate showed 98% of identity with the Florida and Saint Maries isolates, 97% with Brazil (Pernambuco) and Havana isolates; and 91% with A. centrale. The cELISA-PR1 test was compared to IFI and cELISA-USA. For the standardization and validation of the cELISA-PR1, 380 bovine sera were used, whereas 245 truly positives and 135 truly negatives sera tested by the cELISA-USA. In the standardization of the cELISA-PR1 135 negative and 148 positive bovine sera were used. The cELISA-PR1 and IFI tests showed 100 and 99.3% specificity, 100 and 98%, sensibility, and a kappa coefficient of 0.993 and 0.978, respectively. For test validation, 245 bovine sera from an anaplasmosis endemic area were analyzed by the cELISA-PR1 and IFI, which showed 96.7 and 69.1% specificity, 98.9 e 96.3% sensibility and kappa coefficient of 0.956 and 0.699, respectively. These results indicate that the cELISA-PR1, likewise the cELISA-USA, could sensitively and specifically detect cattle naturally infected with A. marginale and would be recommended for epidemiological studies, eradications program, and regulation of international cattle movement, while IFI, which presented lower specificity should not be used in situations that demand more specific diagnosis.

Natural genetic diversity of the DBL domain of a novel member of the Plasmodium vivax erythrocyte binding-like proteins (EBP2) in the Amazon rainforest

In malaria parasites, the erythrocyte binding-like proteins (EBL) are a family of invasion proteins that are attractive vaccine targets. In the case of Plasmodium vivax, the widespread malaria parasite, blood-stage vaccines have been largely focused on a single EBL candidate, the Duffy binding-like domain (DBL) of the Duffy binding protein (DBPII), due to its well-characterized role in the reticulocyte invasion. A novel P. vivax EBL family member, the Erythrocyte binding protein (EBP2, also named EBP or DBP2), binds preferentially to reticulocytes and may mediate an alternative P. vivax invasion pathway. To gain insight into the natural genetic diversity of the DBL domain of EBP2 (region II; EBP2-II), we analyzed ebp2-II gene sequences of 71 P. vivax isolates collected in different endemic settings of the Brazilian Amazon rainforest, where P. vivax is the predominant malaria-associated species. Although most of the substitutions in the ebp2-II gene were non-synonymous and suggested positive selection, the results showed that the DBL domain of the EBP2 was much less polymorphic than that of DBPII. The predominant EBP2 haplotype in the Amazon region corresponded to the C127 reference sequence first described in Cambodia (25% C127-like haplotype). An overview of ebp2-II gene sequences available at GenBank (n = 352) from seven countries (Cambodia, Madagascar, Myanmar, PNG, South Korea, Thailand, Vietnam) confirmed the C127-like haplotype as highly prevalent worldwide. Two out of 43 haplotypes (5 to 20 inferred per country) showed a global frequency of 60%. The results presented here open new avenues of research pursuit while suggesting that a vaccine based on the DBL domain of EBP2 should target a few haplotypes for broad coverage.

Antibody response to a new member of the DBL family (EBP2) after a brief Plasmodium vivax exposure

Plasmodium vivax blood-stage invasion into reticulocyte is critical for parasite development. Thus, validation of novel parasite invasion ligands is essential for malaria vaccine development. Recently, we demonstrated that EBP2, a Duffy binding protein (DBP) paralog, is antigenically distinct from DBP and could not be functionally inhibited by anti-DBP antibodies. Here, we took advantage of a small outbreak of P.vivax malaria, located in a non-malarious area of Brazil, to investigate for the first time IgM/IgG antibodies against EBP2 and DEKnull-2 (an engineering DBPII vaccine) among individuals who had their first and brief exposure to P.vivax (16 cases and 22 non-cases). Our experimental approach included 4 cross sectional surveys at 3-month interval (12-month follow-up). The results demonstrated that while a brief initial P.vivax infection was not efficient to induce IgM/ IgG antibodies to either EBP2 or DEKnull-2, IgG antibodies against DEKnull-2 (but not EBP2) were boosted by recurrent blood-stage infections following treatment. Of interest, in most recurrent P. vivax infections (4 out of 6 patients) DEKnull-2 IgG antibodies were sustained for 6 to 12 months. Polymorphisms in the ebp2 gene does not seem to explain EBP2 low immunogenicity as the ebp2 allele associated with the P.vivax outbreak presented high identity to the original EBP2 isolate used as recombinant protein. Although EBP2 antibodies were barely detectable after a primary episode of P.vivax infection, EBP2 was highly recognized by serum IgG from long-term malaria-exposed Amazonians (range from 35 to 92% according to previous malaria episodes). Taken together, the results showed that individuals with a single and brief exposure to P.vivax infection develop very low anti-EBP2 antibodies, which tend to increase after long-term malaria exposure. Finally, the findings highlighted the potential of DEKnull-2 as a vaccine candidate, as in non-immune individuals anti-DEKnull-2 IgG antibodies were boosted even after a brief exposure to P.vivax blood stages.

Antibody response to Plasmodium vivax in the context of Epstein-Barr virus (EBV) co-infection: A 14-year follow-up study in the Amazon rainforest

BACKGROUND

To develop an effective vaccine against Plasmodium vivax, the most widely dispersed human malaria parasite, it is critical to understand how coinfections with other pathogens could impact malaria-specific immune response. A recent conceptual study proposed that Epstein-Barr virus (EBV), a highly prevalent human herpesvirus that establishes lifelong persistent infection, may influence P. vivax antibody responses. Here, it was investigated whether EBV could impact the longevity of humoral immune response to P. vivax.

METHODOLOGY/PRINCIPAL FINDINGS

A 14-year follow-up study was carried out among long-term P. vivax-exposed Amazonian individuals (272, median age 35 years), and included 9 cross-sectional surveys at periods of high and low malaria transmission. The experimental approach focused on monitoring antibodies to the major blood-stage P. vivax vaccine candidate, the Duffy binding protein region II (DBPII-Sal1), including a novel engineered DBPII-based vaccine targeting conserved epitopes (DEKnull-2). In parallel, the status of EBV infection was determined over time by the detection of circulating EBV DNA (EBV-DNAemia) and EBV-specific antibodies to lytic (VCAp18) or latent (EBNA1) antigens. Regardless of the malaria transmission period, the results demonstrated that one or multiple episodes of EBV-DNAemia did not influence the longevity of DBPII immune responses to both strain-specific (Sal-1) or strain-transcending (DEKnull-2) antibodies. Also, the average time in which DBPII-responders lost their antibodies was unrelated to the EBV serostatus. Considering all malaria cases detected during the study, there was a predominance of P. vivax mono-infection (76%), with a positive correlation between malaria infection and EBV-DNAemia.

CONCLUSIONS/SIGNIFICANCE

In an immunocompetent P. vivax-exposed adult population neither sporadic episodes of EBV-DNAemia nor antibody responses to lytic/latent EBV antigens influence the longevity of both strain-specific and strain-transcending DBPII immune responses. Further studies should investigate the role of acute P. vivax infection in the activation of EBV replication cycle.

Multiplexed Microsphere-Based Flow Cytometric Assay to Assess Strain Transcending Antibodies to Plasmodium vivax Duffy Binding Protein II Reveals an Efficient Tool to Identify Binding-Inhibitory Antibody Responders

Malaria remains a major public health problem worldwide, and Plasmodium vivax is the most widely distributed malaria parasite. Naturally acquired binding inhibitory antibodies (BIAbs) to region II of the Duffy binding protein (DBPII), a P. vivax ligand that is critical for reticulocyte invasion, are associated with a reduced risk of clinical malaria. Owing to methodological issues in evaluating antibodies that inhibit the DBPII-DARC interaction, a limited number of studies have investigated DBPII BIAbs in P. vivax-exposed populations. Based on the assumption that individuals with a consistent BIAb response are characterized by strain-transcending immune responses, we hypothesized that detecting broadly reactive DBPII antibodies would indicate the presence of BIAb response. By taking advantage of an engineered DBPII immunogen targeting conserved DBPII neutralizing epitopes (DEKnull-2), we standardized a multiplex flow cytometry-based serological assay to detect broadly neutralizing IgG antibodies. For this study, a standard in vitro cytoadherence assay with COS-7 cells expressing DBPII was used to test for DBPII BIAb response in long-term P. vivax-exposed Amazonian individuals. Taken together, the results demonstrate that this DBPII-based multiplex assay facilitates identifying DBPII BIAb carriers. Of relevance, the ability of the multiplex assay to identify BIAb responders was highly accurate when the positivity for all antigens was considered. In conclusion, the standardized DBPII-based flow cytometric assay confirmed that DBPII-BIAb activity was associated with the breadth rather than the magnitude of anti-DBPII antibodies. Altogether, our results suggest that multiplex detection of broadly DBPII-reactive antibodies facilitates preliminary screening of BIAb responders.

Impact of Epstein-Barr virus co-infection on natural acquired Plasmodium vivax antibody response

Background

The simultaneous infection of Plasmodium falciparum and Epstein-Barr virus (EBV) could promote the development of the aggressive endemic Burkitt’s Lymphoma (eBL) in children living in P. falciparum holoendemic areas. While it is well-established that eBL is not related to other human malaria parasites, the impact of EBV infection on the generation of human malaria immunity remains largely unexplored. Considering that this highly prevalent herpesvirus establishes a lifelong persistent infection on B-cells with possible influence on malaria immunity, we hypothesized that EBV co-infection could have impact on the naturally acquired antibody responses to P. vivax, the most widespread human malaria parasite.

Methodology/Principal findings

The study design involved three cross-sectional surveys at six-month intervals (baseline, 6 and 12 months) among long-term P. vivax exposed individuals living in the Amazon rainforest. The approach focused on a group of malaria-exposed individuals whose EBV-DNA (amplification of balf-5 gene) was persistently detected in the peripheral blood (PersVDNA, n = 27), and an age-matched malaria-exposed group whose EBV-DNA could never be detected during the follow-up (NegVDNA, n = 29). During the follow-up period, the serological detection of EBV antibodies to lytic/ latent viral antigens showed that IgG antibodies to viral capsid antigen (VCA-p18) were significantly different between groups (PersVDNA > NegVDNA). A panel of blood-stage P. vivax antigens covering a wide range of immunogenicity confirmed that in general PersVDNA group showed low levels of antibodies as compared with NegVDNA. Interestingly, more significant differences were observed to a novel DBPII immunogen, named DEKnull-2, which has been associated with long-term neutralizing antibody response. Differences between groups were less pronounced with blood-stage antigens (such as MSP1-19) whose levels can fluctuate according to malaria transmission.

Conclusions/Significance

In a proof-of-concept study we provide evidence that a persistent detection of EBV-DNA in peripheral blood of adults in a P. vivax semi-immune population may impact the long-term immune response to major malaria vaccine candidates.

Cloning, sequencing, expression, and antigenic characterization of rMSP4 from Anaplasma marginale isolated from Paraná State, Brazil

Anaplasmosis is a bovine intraerythrocytic disease caused by the bacterium Anaplasma marginale; it causes significant economic losses in tropical and subtropical regions, worldwide. The msp4 gene of an A. marginale strain isolated in Paran , Brazil, was amplified by PCR and sequenced; its cloning into the pET102/D-TOPO vector produced an msp4-6xHis-V5-HP thioredoxin fusion gene construct. This recombinant clone was over-expressed in Escherichia coli BL21(DE-3); the expressed fusion protein was found almost entirely in the insoluble form (inclusion bodies) in the cell lysate. The inclusion bodies were solubilized with urea and the recombinant protein was purified by Ni-NTA column and dialyzed. This method produced a relatively high yield of rMSP4, which was used to immunize rabbits. The deduced amino acid sequence encoded by MSP4 showed 99% homology to A. marginale isolates from Florida, USA, and from Minas Gerais, Brazil. Both rMSP4 and native MSP4 were recognized by post-immunization rabbit serum, showing that rMSP4 has conserved epitopes. As antigenicity was preserved, rMSP4 might be useful for the development of vaccine against anaplasmosis.