RH5.1-CyRPA-Ripr antigen combination vaccine shows little improvement over RH5.1 in a preclinical setting

Background

RH5 is the leading vaccine candidate for the Plasmodium falciparum blood stage and has shown impact on parasite growth in the blood in a human clinical trial. RH5 binds to Ripr and CyRPA at the apical end of the invasive merozoite form, and this complex, designated RCR, is essential for entry into human erythrocytes. RH5 has advanced to human clinical trials, and the impact on parasite growth in the blood was encouraging but modest. This study assessed the potential of a protein-in-adjuvant blood stage malaria vaccine based on a combination of RH5, Ripr and CyRPA to provide improved neutralizing activity against P. falciparum in vitro.

Methods

Mice were immunized with the individual RCR antigens to down select the best performing adjuvant formulation and rats were immunized with the individual RCR antigens to select the correct antigen dose. A second cohort of rats were immunized with single, double and triple antigen combinations to assess immunogenicity and parasite neutralizing activity in growth inhibition assays.

Results

The DPX® platform was identified as the best performing formulation in potentiating P. falciparum inhibitory antibody responses to these antigens. The three antigens derived from RH5, Ripr and CyRPA proteins formulated with DPX induced highly inhibitory parasite neutralising antibodies. Notably, RH5 either as a single antigen or in combination with Ripr and/or CyRPA, induced inhibitory antibodies that outperformed CyRPA, Ripr.

Conclusion

An RCR combination vaccine may not induce substantially improved protective immunity as compared with RH5 as a single immunogen in a clinical setting and leaves the development pathway open for other antigens to be combined with RH5 as a next generation malaria vaccine.

Identification and Immune Assessment of T Cell Epitopes in Five Plasmodium falciparum Blood Stage Antigens to Facilitate Vaccine Candidate Selection and Optimization

The hurdles to effective blood stage malaria vaccine design include immune evasion tactics used by the parasite such as redundant invasion pathways and antigen variation among circulating parasite strains. While blood stage malaria vaccine development primarily focuses on eliciting optimal humoral responses capable of blocking erythrocyte invasion, clinically-tested Plasmodium falciparum (Pf) vaccines have not elicited sterile protection, in part due to the dramatically high levels of antibody needed. Recent development efforts with non-redundant, conserved blood stage antigens suggest both high antibody titer and rapid antibody binding kinetics are important efficacy factors. Based on the central role of helper CD4 T cells in development of strong, protective immune responses, we systematically analyzed the class II epitope content in five leading Pf blood stage antigens (RH5, CyRPA, RIPR, AMA1 and EBA175) using in silico, in vitro, and ex vivo methodologies. We employed in silico T cell epitope analysis to enable identification of 67 HLA-restricted class II epitope clusters predicted to bind a panel of nine HLA-DRB1 alleles. We assessed a subset of these for HLA-DRB1 allele binding in vitro, to verify the in silico predictions. All clusters assessed (40 clusters represented by 46 peptides) bound at least two HLA-DR alleles in vitro. The overall epitope prediction to in vitro HLA-DRB1 allele binding accuracy was 71%. Utilizing the set of RH5 class II epitope clusters (10 clusters represented by 12 peptides), we assessed stimulation of T cells collected from HLA-matched RH5 vaccinees using an IFN-γ T cell recall assay. All clusters demonstrated positive recall responses, with the highest responses – by percentage of responders and response magnitude – associated with clusters located in the N-terminal region of RH5. Finally, a statistically significant correlation between in silico epitope predictions and ex vivo IFN-γ recall response was found when accounting for HLA-DR matches between the epitope predictions and donor HLA phenotypes. This is the first comprehensive analysis of class II epitope content in RH5, CyRPA, RIPR, AMA1 and EBA175 accompanied by in vitro HLA binding validation for all five proteins and ex vivo T cell response confirmation for RH5.

Reduced blood-stage malaria growth and immune correlates in humans following RH5 vaccination

BACKGROUND

Development of an effective vaccine against the pathogenic blood-stage infection of human malaria has proved challenging, and no candidate vaccine has affected blood-stage parasitemia following controlled human malaria infection (CHMI) with blood-stage Plasmodium falciparum.

METHODS

We undertook a phase I/IIa clinical trial in healthy adults in the United Kingdom of the RH5.1 recombinant protein vaccine, targeting the P. falciparum reticulocyte-binding protein homolog 5 (RH5), formulated in AS01B adjuvant. We assessed safety, immunogenicity, and efficacy against blood-stage CHMI. Trial registered at ClinicalTrials.gov, NCT02927145.

FINDINGS

The RH5.1/AS01B formulation was administered using a range of RH5.1 protein vaccine doses (2, 10, and 50 μg) and was found to be safe and well tolerated. A regimen using a delayed and fractional third dose, in contrast to three doses given at monthly intervals, led to significantly improved antibody response longevity over ∼2 years of follow-up. Following primary and secondary CHMI of vaccinees with blood-stage P. falciparum, a significant reduction in parasite growth rate was observed, defining a milestone for the blood-stage malaria vaccine field. We show that growth inhibition activity measured in vitro using purified immunoglobulin G (IgG) antibody strongly correlates with in vivo reduction of the parasite growth rate and also identify other antibody feature sets by systems serology, including the plasma anti-RH5 IgA1 response, that are associated with challenge outcome.

CONCLUSIONS

Our data provide a new framework to guide rational design and delivery of next-generation vaccines to protect against malaria disease.

FUNDING

This study was supported by USAID, UK MRC, Wellcome Trust, NIAID, and the NIHR Oxford-BRC.

Bridging Computational Vaccinology and Vaccine Development Through Systematic Identification, Characterization, and Downselection of Conserved and Variable Circumsporozoite Protein CD4 T Cell Epitopes From Diverse Plasmodium falciparum Strains

An effective malaria vaccine must prevent disease in a range of populations living in regions with vastly different transmission rates and protect against genetically-diverse Plasmodium falciparum (Pf) strains. The protective efficacy afforded by the currently licensed malaria vaccine, Mosquirix™, promotes strong humoral responses to Pf circumsporozoite protein (CSP) 3D7 but protection is limited in duration and by strain variation. Helper CD4 T cells are central to development of protective immune responses, playing roles in B cell activation and maturation processes, cytokine production, and stimulation of effector T cells. Therefore, we took advantage of recent in silico modeling advances to predict and analyze human leukocyte antigen (HLA)-restricted class II epitopes from PfCSP – across the entire PfCSP 3D7 sequence as well as in 539 PfCSP sequence variants – with the goal of improving PfCSP-based malaria vaccines. Specifically, we developed a systematic workflow to identify peptide sequences capable of binding HLA-DR in a context relevant to achieving broad human population coverage utilizing cognate T cell help and with limited T regulatory cell activation triggers. Through this workflow, we identified seven predicted class II epitope clusters in the N- and C-terminal regions of PfCSP 3D7 and an additional eight clusters through comparative analysis of 539 PfCSP sequence variants. A subset of these predicted class II epitope clusters was synthesized as peptides and assessed for HLA-DR binding in vitro. Further, we characterized the functional capacity of these peptides to prime and activate human peripheral blood mononuclear cells (PBMCs), by monitoring cytokine response profiles using MIMIC^® technology (Modular IMmune In vitro Construct). Utilizing this decision framework, we found sufficient differential cellular activation and cytokine profiles among HLA-DR-matched PBMC donors to downselect class II epitope clusters for inclusion in a vaccine targeting PfCSP. Importantly, the downselected clusters are not highly conserved across PfCSP variants but rather, they overlap a hypervariable region (TH2R) in the C-terminus of the protein. We recommend assessing these class II epitope clusters within the context of a PfCSP vaccine, employing a test system capable of measuring immunogenicity across a broad set of HLA-DR alleles.

Identification, Selection and Immune Assessment of Liver Stage CD8 T Cell Epitopes From Plasmodium falciparum

Immunization with radiation-attenuated sporozoites (RAS) has been shown to protect against malaria infection, primarily through CD8 T cell responses, but protection is limited based on parasite strain. Therefore, while CD8 T cells are an ideal effector population target for liver stage malaria vaccine development strategies, such strategies must incorporate conserved epitopes that cover a large range of class I human leukocyte antigen (HLA) supertypes to elicit cross-strain immunity across the target population. This approach requires identifying and characterizing a wide range of CD8 T cell epitopes for incorporation into a vaccine such that coverage across a large range of class I HLA alleles is attained. Accordingly, we devised an experimental framework to identify CD8 T cell epitopes from novel and minimally characterized antigens found at the pre-erythrocytic stage of parasite development. Through in silico analysis we selected conserved P. falciparum proteins, using P. vivax orthologues to establish stringent conservation parameters, predicted to have a high number of T cell epitopes across a set of six class I HLA alleles representative of major supertypes. Using the decision framework, five proteins were selected based on the density and number of predicted epitopes. Selected epitopes were synthesized as peptides and evaluated for binding to the class I HLA alleles in vitro to verify in silico binding predictions, and subsequently for stimulation of human T cells using the Modular IMmune In-vitro Construct (MIMIC®) technology to verify immunogenicity. By combining the in silico tools with the ex vivo high throughput MIMIC platform, we identified 15 novel CD8 T cell epitopes capable of stimulating an immune response in alleles across the class I HLA panel. We recommend these epitopes should be evaluated in appropriate in vivo humanized immune system models to determine their protective efficacy for potential inclusion in future vaccines.

A Peptide-Based PD1 Antagonist Enhances T-Cell Priming and Efficacy of a Prophylactic Malaria Vaccine and Promotes Survival in a Lethal Malaria Model

The blockade of programmed cell death-1 (PD1) and its ligand PDL1 has been proven to be a successful immunotherapy against several cancers. Similar to cancer, PD1 contributes to the establishment of several chronic infectious diseases, including malaria. While monoclonal antibodies (mAbs) targeting checkpoint receptors are revolutionary in cancer treatment, the immune-related adverse events (irAEs) may prevent their utilization in prophylactic and therapeutic treatments of infectious diseases. The irAEs are, in part, due to the prolonged half-life of mAbs resulting in prolonged activation of the immune system. As an alternative modality to mAbs, peptides represent a viable option because they possess a shorter pharmacokinetic half-life and offer more formulation and delivery options. Here, we report on a 22-amino acid immunomodulatory peptide, LD01, derived from a Bacillus bacteria. When combined prophylactically with an adenovirus-based or irradiated sporozoite-based malaria vaccine, LD01 significantly enhanced antigen-specific CD8⁺ T cell expansion. Therapeutically, LD01 treatment of mice infected with a lethal malaria strain resulted in survival that was associated with lower numbers of FOXP3⁺Tbet⁺CD4⁺ regulatory T cells. Taken together, our results demonstrate that LD01 is a potent immunomodulator that acts upon the adaptive immune system to stimulate T cell responses both prophylactically and therapeutically.

Better Epitope Discovery, Precision Immune Engineering, and Accelerated Vaccine Design Using Immunoinformatics Tools

Computational vaccinology includes epitope mapping, antigen selection, and immunogen design using computational tools. Tools that facilitate the in silico prediction of immune response to biothreats, emerging infectious diseases, and cancers can accelerate the design of novel and next generation vaccines and their delivery to the clinic. Over the past 20 years, vaccinologists, bioinformatics experts, and advanced programmers based in Providence, Rhode Island, USA have advanced the development of an integrated toolkit for vaccine design called iVAX, that is secure and user-accessible by internet. This integrated set of immunoinformatic tools comprises algorithms for scoring and triaging candidate antigens, selecting immunogenic and conserved T cell epitopes, re-engineering or eliminating regulatory T cell epitopes, and re-designing antigens to induce immunogenicity and protection against disease for humans and livestock. Commercial and academic applications of iVAX have included identifying immunogenic T cell epitopes in the development of a T-cell based human multi-epitope Q fever vaccine, designing novel influenza vaccines, identifying cross-conserved T cell epitopes for a malaria vaccine, and analyzing immune responses in clinical vaccine studies. Animal vaccine applications to date have included viral infections of pigs such as swine influenza A, PCV2, and African Swine Fever. “Rapid-Fire” applications for biodefense have included a demonstration project for Lassa Fever and Q fever. As recent infectious disease outbreaks underscore the significance of vaccine-driven preparedness, the integrated set of tools available on the iVAX toolkit stand ready to help vaccine developers deliver genome-derived, epitope-driven vaccines.

Novel Peptide-Based PD1 Immunomodulators Demonstrate Efficacy in Infectious Disease Vaccines and Therapeutics

Many pathogens use the same immune evasion mechanisms as cancer cells. Patients with chronic infections have elevated levels of checkpoint receptors (e.g., programed cell death 1, PD1) on T cells. Monoclonal antibody (mAb)-based inhibitors to checkpoint receptors have also been shown to enhance T-cell responses in models of chronic infection. Therefore, inhibitors have the potential to act as a vaccine “adjuvant” by facilitating the expansion of vaccine antigen-specific T-cell repertoires. Here, we report the discovery and characterization of a peptide-based class of PD1 checkpoint inhibitors, which have a potent adaptive immunity adjuvant capability for vaccines against infectious diseases. Briefly, after identifying peptides that bind to the recombinant human PD1, we screened for in vitro efficacy in reporter assays and human peripheral blood mononuclear cells (PBMC) readouts. We first found the baseline in vivo performance of the peptides in a standard mouse oncology model that demonstrated equivalent efficacy compared to mAbs against the PD1 checkpoint. Subsequently, two strategies were used to demonstrate the utility of our peptides in infectious disease indications: (1) as a therapeutic in a bacteria-induced lethal sepsis model in which our peptides were found to increase survival with enhanced bacterial clearance and increased macrophage function; and (2) as an adjuvant in combination with a prophylactic malaria vaccine in which our peptides increased T-cell immunogenicity and the protective efficacy of the vaccine. Therefore, our peptides are promising as both a therapeutic agent and a vaccine adjuvant for infectious disease with a potentially safer and more cost-effective target product profile compared to mAbs. These findings are essential for deploying a new immunomodulatory regimen in infectious disease primary and clinical care settings.

Semicontinuous Bioreactor Production of Recombinant Butyrylcholinesterase in Transgenic Rice Cell Suspension Cultures

An active and tetrameric form of recombinant butyrylcholinesterase (BChE), a large and complex human enzyme, was produced via semicontinuous operation in a transgenic rice cell suspension culture. After transformation of rice callus and screening of transformants, the cultures were scaled up from culture flask to a lab scale bioreactor. The bioreactor was operated through two phases each of growth and expression. The cells were able to produce BChE during both expression phases, with a maximum yield of 1.6 mg BChE/L of culture during the second expression phase. Cells successfully regrew during a 5-day growth phase. A combination of activity assays and Western blot analysis indicated production of an active and fully assembled tetramer of BChE.

Human immune system mice immunized with Plasmodium falciparum circumsporozoite protein induce protective human humoral immunity against malaria

In this study, we developed human immune system (HIS) mice that possess functional human CD4+ T cells and B cells, named HIS-CD4/B mice. HIS-CD4/B mice were generated by first introducing HLA class II genes, including DR1 and DR4, along with genes encoding various human cytokines and human B cell activation factor (BAFF) to NSG mice by adeno-associated virus serotype 9 (AAV9) vectors, followed by engrafting human hematopoietic stem cells (HSCs). HIS-CD4/B mice, in which the reconstitution of human CD4+ T and B cells resembles to that of humans, produced a significant level of human IgG against Plasmodium falciparum circumsporozoite (PfCS) protein upon immunization. CD4+ T cells in HIS-CD4/B mice, which possess central and effector memory phenotypes like those in humans, are functional, since PfCS protein-specific human CD4+ T cells secreting IFN-γ and IL-2 were detected in immunized HIS-CD4/B mice. Lastly, PfCS protein-immunized HIS-CD4/B mice were protected from in vivo challenge with transgenic P. berghei sporozoites expressing the PfCS protein. The immune sera collected from protected HIS-CD4/B mice reacted against transgenic P. berghei sporozoites expressing the PfCS protein and also inhibited the parasite invasion into hepatocytes in vitro. Taken together, these studies show that our HIS-CD4/B mice could mount protective human anti-malaria immunity, consisting of human IgG and human CD4+ T cell responses both specific for a human malaria antigen.

Proteolytic Cleavage of the Plasmodium falciparum Circumsporozoite Protein Is a Target of Protective Antibodies

Studies in animals and human volunteers demonstrate that antibodies against the repeat-region of the Plasmodium circumsporozoite protein (CSP) abrogate sporozoite infection. However, the realization that the N- and C- terminal regions flanking the repeats play essential roles in parasite infectivity raised the possibility that they could be targeted by protective antibodies. We characterized a monoclonal antibody (mAb5D5) specific for the N-terminus of the P. falciparum CSP, which inhibits the proteolytic cleavage of the CSP, a key requirement for parasite infection of hepatocytes. Adoptive transfer of mAb5D5 strongly inhibits the in vivo infection of sporozoites expressing the N-terminus of P. falciparum CSP, and this protection is greatly enhanced when combined with antirepeat antibodies. Our results show that antibodies interfering with molecular processes required for parasite infectivity can exert a strong in vivo protective activity and indicate that pre-erythrocytic vaccines against Plasmodium should include the CSP N-terminal region.

A full-length Plasmodium falciparum recombinant circumsporozoite protein expressed by Pseudomonas fluorescens platform as a malaria vaccine candidate

The circumsporozoite protein (CSP) of Plasmodium falciparum is a major surface protein, which forms a dense coat on the sporozoite's surface. Preclinical research on CSP and clinical evaluation of a CSP fragment-based RTS, S/AS01 vaccine have demonstrated a modest degree of protection against P. falciparum, mediated in part by humoral immunity and in part by cell-mediated immunity. Given the partial protective efficacy of the RTS, S/AS01 vaccine in a recent Phase 3 trial, further improvement of CSP-based vaccines is crucial. In this report, we describe the preclinical development of a full-length, recombinant CSP (rCSP)-based vaccine candidate against P. falciparum malaria suitable for current Good Manufacturing Practice (cGMP) production. Utilizing a novel high-throughput Pseudomonas fluorescens expression platform, we demonstrated greater efficacy of full-length rCSP as compared to N-terminally truncated versions, rapidly down-selected a promising lead vaccine candidate, and developed a high-yield purification process to express immunologically active, intact antigen for clinical trial material production. The rCSP, when formulated with various adjuvants, induced antigen-specific antibody responses as measured by enzyme-linked immunosorbent assay (ELISA) and immunofluorescence assay (IFA), as well as CD4+ T-cell responses as determined by ELISpot. The adjuvanted rCSP vaccine conferred protection in mice when challenged with transgenic P. berghei sporozoites containing the P. falciparum repeat region of CSP. Furthermore, heterologous prime/boost regimens with adjuvanted rCSP and an adenovirus type 35-vectored CSP (Ad35CS) showed modest improvements in eliciting CSP-specific T-cell responses and anti-malarial protection, depending on the order of vaccine delivery. Collectively, these data support the importance of further clinical development of adjuvanted rCSP, either as a stand-alone product or as one of the components in a heterologous prime/boost strategy, ultimately acting as an effective vaccine candidate for the mitigation of P. falciparum-induced malaria.

Spatial and temporal dynamics of the secretory pathway during differentiation of the Plasmodium yoelii schizont

A specialized complex of apical organelles facilitates Plasmodium merozoite invasion into the erythrocyte. Even though the apical organelles are crucial to the invasion process, relatively little is known about how they function or their biosynthesis during asexual replication. MAEBL is an erythrocyte binding protein located in the rhoptries and on the surface of mature merozoites and is expressed at the beginning of schizogony before the first nuclear division. Therefore, we have characterized MAEBL as a marker for the biosynthetic pathway of the rhoptry apical organelle during the final phase of intraerythrocytic development and as a marker for the nascent rhoptry vesicle in the immature schizont. An extensive proliferation of the endoplasmic reticulum occurred at the onset of schizogony and was seen as a complex but transient tubule array near the parasite surface. Both the rhoptry protein MAEBL and surface protein MSP-1 appeared to be present in this tubular reticular network together with endoplasmic reticulum markers. MAEBL then transits through Golgi bodies positioned near the parasite plasma membrane, directly adjacent to the network. Rhoptry organelle precursors are seen at the three to four nuclei stage of schizont development, remaining near the plasma membrane throughout schizogony. These studies constitute the first direct evidence that proteins of the rhoptry organelles transit through compartments of the 'classical' secretory pathway.

Plasmodium yoelii YM MAEBL protein is coexpressed and colocalizes with rhoptry proteins

We have previously cloned genes from multiple rodent malaria species exhibiting characteristics of the genes encoding Duffy binding like-erythrocyte binding proteins (DBL-EBP). Homology is seen in the intron/exon structure of the genes and in the carboxyl terminal region (including the deduced carboxyl cysteine-rich domain) of the proteins they encode. However, the amino termini of these proteins are not homologous to the DBL-EBP but contain tandem cysteine-rich regions that are similar to the cysteine-rich region of AMA-1 (apical membrane antigen-1), a rhoptry protein. This new family of proteins has been termed MAEBL and these are paralogues of both AMA-1 and the DBL-EBP. Serum against the carboxyl cysteine-rich region of the Plasmodium yoelii YM MAEBL reacted to parasites with a punctate fluorescence pattern characteristic of apical organelle proteins and also localized MAEBL to the surface of merozoites within schizonts. This antiserum immunoprecipitated a protein doublet (120/128 kDa) that was unexpectedly insoluble when compared to members of the DBL-EBP. Characterization of MAEBL was extended through colocalization studies comparing the P. yoelii YM MAEBL to other parasite proteins. This protein appeared to be located in the rhoptry organelles as it colocalized with both AMA-1 and the P. yoelii 235 kDa rhoptry proteins within parasites. In addition, MAEBL is expressed relatively early in schizont development and appears on the merozoite surface after segmentation. Both the pattern and time of expression of the P. yoelii YM MAEBL are consistent with a rhoptry rather than a microneme protein.

A family of chimeric erythrocyte binding proteins of malaria parasites

Proteins sequestered within organelles of the apical complex of malaria merozoites are involved in erythrocyte invasion, but few of these proteins and their interaction with the host erythrocyte have been characterized. In this report we describe MAEBL, a family of erythrocyte binding proteins identified in the rodent malaria parasites Plasmodium yoelii yoelii and Plasmodium berghei. MAEBL has a chimeric character, uniting domains from two distinct apical organelle protein families within one protein. MAEBL has a molecular structure homologous to the Duffy binding-like family of erythrocyte binding proteins located in the micronemes of merozoites. However, the amino cysteine-rich domain of MAEBL has no similarity to the consensus Duffy binding-like amino cysteine-rich ligand domain, but instead is similar to the 44-kDa ectodomain fragment of the apical membrane antigen 1 (AMA-1) rhoptry protein family. MAEBL has a tandem duplication of this AMA-1-like domain, and both of these cysteine-rich domains bound erythrocytes when expressed in vitro. Differential transcription and splicing of the maebl locus occurred in the YM clone of P. yoelii yoelii. The apical distribution of MAEBL suggested localization within the rhoptry organelles of the apical complex. We propose that MAEBL is a member of a highly conserved family of erythrocyte binding proteins of Plasmodium involved in host cell invasion.

Erythrocyte binding protein homologues of rodent malaria parasites

Erythrocyte invasion by malaria parasites requires specific molecular interactions between the merozoite and erythrocyte surface receptors. A well-conserved, functionally important family of erythrocyte binding proteins is the EBP family. The EBP family includes the Plasmodium vivax, P. knowlesi Duffy binding protein (DBP) family and the P. falciparum erythrocyte binding antigen-175 (EBA-175). The EBP are transmembrane proteins, characterized by two conserved cysteine-rich domains, expressed in the micronemes of invasive merozoites. Oligonucleotide primers matching the region encoding the carboxyl cysteine-rich domain of the EBA-175 were used in a polymerase chain reaction to identify homologous genes in P. berghei and P. yoelii yoelii, leading to the isolation of a P. berghei partial genomic clone. This clone contained a 323 bp region that had high deduced amino acid sequence similarity to the amino acid sequences of the carboxyl cysteine-rich domains of the DBP family and EBA-175. The P. berghei carboxyl cysteine-rich domain was followed by a putative transmembrane domain and a cytoplasmic domain, demonstrating an exon-intron structure at the 3' end homologous to P. vivax dbp and P. falciparum eba-175. The carboxyl cysteine-rich domain is also highly conserved among P. berghei, P. y. yoelii, P. chabaudi and P. vinckei and is encoded by a single copy gene. Antisera prepared against the carboxyl cysteine-rich domain of the rodent malaria EBP homologues reacted with a 120 and 128 kDa protein doublet on Western blots of P. berghei parasite antigen and showed an apical localization pattern within merozoites by indirect immunofluorescence assays.

Expression and serologic activity of a soluble recombinant Plasmodium vivax Duffy binding protein

Plasmodium vivax Duffy binding protein (DBP) is a conserved functionally important protein. P. vivax DBP is an asexual blood-stage malaria vaccine candidate because adhesion of P. vivax DBP to its erythrocyte receptor is essential for the parasite to continue development in human blood. We developed a soluble recombinant protein of P. vivax DBP (rDBP) and examined serologic activity to it in residents of a region of high endemicity. This soluble rDBP product contained the cysteine-rich ligand domain and most of the contiguous proline-rich hydrophilic region. rDBP was expressed as a glutathione S-transferase (GST) fusion protein and was isolated from GST by thrombin treatment of the purified fusion protein bound on glutathione agarose beads. P. vivax rDBP was immunogenic in rabbits and induced antibodies that reacted with P. vivax and Plasmodium knowlesi merozoites. Human sera from adult residents of a region of Papua New Guinea where malaria is highly endemic or P. vivax-infected North American residents reacted with rDBP in an immunoblot and an enzyme-linked immunosorbent assay. The reactivity to reduced, denatured P. vivax rDBP and the cross-reactivity with P. knowlesi indicated the presence of immunogenic conserved linear B-cell epitopes. A more extensive serologic survey of Papua New Guinea residents showed that antibody response to P. vivax DBP is common and increases with age, suggesting a possible boosting of the antibody response in some by repeated exposure to P. vivax. A positive humoral response to P. vivax DBP correlated with a significantly higher response to P. vivax MSP-1(19). The natural immunogenicity of this DBP should strengthen its usefulness as a vaccine.