Immunogenicity and protective efficacy of Escherichia coli expressed Plasmodium falciparum merozoite surface protein-1(42) using human compatible adjuvants

Immunogenicity of PvCyRPA, PvCelTOS and Pvs25 chimeric recombinant protein of Plasmodium vivax in murine model

In the Americas, P. vivax is the predominant causative species of malaria, a debilitating and economically significant disease. Due to the complexity of the malaria parasite life cycle, a vaccine formulation with multiple antigens expressed in various parasite stages may represent an effective approach. Based on this, we previously designed and constructed a chimeric recombinant protein, PvRMC-1, composed by PvCyRPA, PvCelTOS, and Pvs25 epitopes. This chimeric protein was strongly recognized by naturally acquired antibodies from exposed population in the Brazilian Amazon. However, there was no investigation about the induced immune response of PvRMC-1. Therefore, in this work, we evaluated the immunogenicity of this chimeric antigen formulated in three distinct adjuvants: Stimune, AddaVax or Aluminum hydroxide (Al(OH)3) in BALB/c mice. Our results suggested that the chimeric protein PvRMC-1 were capable to generate humoral and cellular responses across all three formulations. Antibodies recognized full-length PvRMC-1 and linear B-cell epitopes from PvCyRPA, PvCelTOS, and Pvs25 individually. Moreover, mice's splenocytes were activated, producing IFN-γ in response to PvCelTOS and PvCyRPA peptide epitopes, affirming T-cell epitopes in the antigen. While aluminum hydroxide showed notable cellular response, Stimune and Addavax induced a more comprehensive immune response, encompassing both cellular and humoral components. Thus, our findings indicate that PvRMC-1 would be a promising multistage vaccine candidate that could advance to further preclinical studies.

Protective Immunity in Mice Immunized With P. vivax MSP119-Based Formulations and Challenged With P. berghei Expressing PvMSP119

The lack of continuous in vitro cultures has been an obstacle delaying pre-clinical testing of Plasmodium vivax vaccine formulations based on known antigens. In this study, we generated a model to test available formulations based on the P. vivax MSP1₁₉ antigen. The Plasmodium berghei strains ANKA and NK65 were modified to express PvMSP1₁₉ instead of the endogenous PbMSP1₁₉. The hybrid parasites were used to challenge C57BL/6 or BALB/c mice immunized with PvMSP1₁₉-based vaccine formulations. The PvMSP1₁₉ was correctly expressed in the P. berghei hybrid mutant lines as confirmed by immunofluorescence using anti-PvMSP1₁₉ monoclonal antibodies and by Western blot. Replacement of the PbMSP1₁₉ by the PvMSP1₁₉ had no impact on asexual growth in vivo. High titers of specific antibodies to PvMSP1₁₉ were not sufficient to control initial parasitemia in the immunized mice, but late parasitemia control and a balanced inflammatory process protected these mice from dying, suggesting that an established immune response to PvMSP1₁₉ in this model can help immunity mounted later during infection.

Plasmodium falciparum Clag9-Associated PfRhopH Complex Is Involved in Merozoite Binding to Human Erythrocytes

Cytoadherence-linked asexual gene 9 (Clag9), a conserved Plasmodium protein expressed during the asexual blood stages, is involved in the cytoadherence of infected red blood cells (RBCs) to the endothelial lining of blood vessels. Here, we show that Plasmodium falciparum Clag9 (PfClag9) is a component of the PfClag9-RhopH complex that is involved in merozoite binding to human erythrocytes. To characterize PfClag9, we expressed four fragments of PfClag9, encompassing the entire protein. Immunostaining analysis using anti-PfClag9 antibodies showed expression and localization of PfClag9 at the apical end of the merozoites. Mass spectrometric analysis of merozoite extracts after immunoprecipitation using anti-PfClag9 antibody identified P. falciparum rhoptry-associated protein 1 (PfRAP1), PfRAP2, PfRAP3, PfRhopH2, and PfRhopH3 as associated proteins. The identified rhoptry proteins were expressed, and their association with PfClag9 domains was assessed by using protein-protein interaction tools. We further showed that PfClag9 binds human RBCs by interacting with the glycophorin A-band 3 receptor-coreceptor complex. In agreement with its cellular localization, PfClag9 was strongly recognized by antibodies generated during natural infection. Mice immunized with the C-terminal domain of PfClag9 were partially protected against a subsequent challenge infection with Plasmodium berghei, further supporting a biological role of PfClag9 during natural infection. Taken together, these results provide direct evidence for the existence of a PfRhopH-Clag9 complex on the Plasmodium merozoite surface that binds to human RBCs.

Recombinant proteins of Plasmodium malariae merozoite surface protein 1 (PmMSP1): Testing immunogenicity in the BALB/c model and potential use as diagnostic tool

Background

Plasmodium malariae is the third most prevalent human malaria-causing species and has a patchy, but ample distribution in the world. Humans can host the parasite for years without presenting significant symptoms, turning its diagnosis and control into a difficult task. Here, we investigated the immunogenicity of recombinant proteins of P. malariae MSP1.

Methods

Five regions of PmMSP1 were expressed in Escherichia coli as GST-fusion proteins and immunized in BALB/c mice. The specificity, subtyping, and affinity of raised antibodies were evaluated by enzyme-linked immunosorbent assays. Cellular immune responses were analyzed by lymphoproliferation assays and cytokine levels produced by splenocytes were detected by cytometry.

Results

We found that N-terminal, central regions, and PmMSP1₁₉ are strongly immunogenic in mice. After three doses, the induced immune responses remained high for 70 days. While antibodies induced after immunization with N-terminal and central regions showed similar affinities to the target antigens, affinities of IgG against PmMSP1₁₉ were higher. All proteins induced similar antibody subclass patterns (predominantly IgG1, IgG2a, and IgG2b), characterizing a mixed Th1/Th2 response. Further, autologous stimulation of splenocytes from immunized mice led to the secretion of IL2 and IL4, independently of the antigen used. Importantly, IgG from P. malariae-exposed individuals reacted against PmMSP1 recombinant proteins with a high specificity. On the other hand, sera from P. vivax or P. falciparum-infected individuals did not react at all against recombinant PmMSP1 proteins.

Conclusion

Recombinant PmMSP1 proteins are very useful diagnostic markers of P. malariae in epidemiological studies or in the differential diagnosis of malaria caused by this species. Immunization with recombinant PmMSP1 proteins resulted in a significant humoral immune response, which may turn them potential component candidates for a vaccine against P. malariae.

Plasmodium falciparum MSP3 Exists in a Complex on the Merozoite Surface and Generates Antibody Response during Natural Infection

Plasmodium falciparum merozoite surface protein 3 (MSP3) is an abundantly expressed secreted merozoite surface protein and a leading malaria vaccine candidate antigen. However, it is unclear how MSP3 is retained on the surface of merozoites without a glycosylphosphatidylinositol (GPI) anchor or a transmembrane domain. In the present study, we identified an MSP3-associated network on the Plasmodium merozoite surface by immunoprecipitation of Plasmodium merozoite lysate using antibody to the N terminus of MSP3 (anti-MSP3N) followed by mass spectrometry analysis. The results suggested the association of MSP3 with other merozoite surface proteins: MSP1, MSP6, MSP7, RAP2, and SERA5. Protein-protein interaction studies by enzyme-linked immunosorbent assay (ELISA) and surface plasmon resonance (SPR) analysis showed that MSP3 complex consists of MSP1, MSP6, and MSP7 proteins. Immunological characterization of MSP3 revealed that MSP3N is strongly recognized by hyperimmune serum from African and Asian populations. Furthermore, we demonstrate that human antibodies, affinity purified against recombinant MSP3N (rMSP3N), promote opsonic phagocytosis of merozoites in cooperation with monocytes. At nonphysiological concentrations, anti-MSP3N antibodies inhibited the growth of P. falciparum in vitro Together, the data suggest that MSP3 and especially its N-terminal region containing known B/T cell epitopes are targets of naturally acquired immunity against malaria and also comprise an important candidate for a multisubunit malaria vaccine.

A Dual-Type L2 11-88 Peptide from HPV Types 16/18 Formulated in Montanide ISA 720 Induced Strong and Balanced Th1/Th2 Immune Responses, Associated with High Titers of Broad Spectrum Cross-Reactive Antibodies in Vaccinated Mice

E. coli-derived concatenated, multitype L2-conserved epitopes of human papillomavirus (HPV) L2 protein might represent a less expensive and pan-type vaccine alternative (compared to type-specific HPV L1 virus-like particles), if stable protein expression and strong immunogenicity features could be met. Herein, three dual-type- (DT-) HPV L2 fusion peptides comprising the three head-to-tail tandem repeats (multimers) of either HPV 16 epitope "17-36" or "69-81" or one copy (monomer) of 11-88 fused to the same residues of HPV 18 were constructed and expressed in E. coli. SDS-PAGE and Western blot analyses indicated the proper expression and stability of the E. coli-derived DT peptides. Mice immunized by formulation of the purified DT peptides and Freund's adjuvant (CFA/IFA) raised neutralizing antibodies (NAbs; the highest for DT: 11-88 peptide) which showed proper cross-reactivity to HPV types: 18, 16, 31, and 45 and efficiently neutralized HPV 18/16 pseudoviruses in vitro. Immunization studies in mice by formulation of the DT: 11-88 × 1 peptide with various adjuvants (alum, MF59, and Montanides ISA 720 and 50) indicated that Montanide adjuvants elicited the highest cross-reactive titers of NAbs and similar levels of IgG1 and IgG2a (switching towards balanced Th1/Th2 responses). The results implied development of low-cost E. coli-derived DT: 11-88 peptide formulated in human compatible ISA 720 adjuvant as a HPV vaccine.

The ears of the African elephant: unexpected high seroprevalence of Plasmodium ovale and Plasmodium malariae in healthy populations in Western Africa

Background

Malaria Is A Life-Threatening Pathology In Africa. Plasmodium Falciparum And Plasmodium Vivax Attract The Most Focus Because Of Their High Prevalence And Mortality. Knowledge About The Prevalence Of The Cryptic Pathogens Plasmodium Ovale And Plasmodium Malariae Is Limited. Thanks To Recombinant Tools, Their Seroprevalence Was Measured For The First Time, As Well As The Prevalence Of Mixed Infections In A Malaria-Asymptomatic Population In Benin, A Malaria-Endemic Country.

Methods

A Panel Of 1,235 Blood Donations Collected Over Ten Months In Benin Was Used For Validation Of The Recombinant Tools. Recombinant P. Falciparum, P. Malariae, P. Ovale MSP1, And P. Falciparum AMA1 Were Engineered And Validated On A Biobank With Malaria-Infected Patients (N = 144) Using A Species-Speific ELISA Test (Recelisa). Results Were Compared To An ELISA Using A Native P. Falciparum Antigen (NatELISA).

Results

Among Microscopically Negative African Blood Donors, 85% (1,050/1,235) Present Antibodies Directed To Native P. Falciparum, 94.4% (1,166/1,235) To rPfMSP1 And rPfAMA1, 56.8% (702/1,235) To rPoMSP1, 67.5% (834/1235) To rPmMSP1 And 45.3% Of The Malaria Seropositive Population Had Antibodies Recognizing The Three Species.

Conclusion

A High Rate Of Antibodies Against P. Ovale And P. Malariae Was Found In Asymptomatic Blood Donors. The Proportion Of Mixed Infections Involving Three Species Was Also Unexpected. These Data Suggest That Determining Seroprevalence For These Cryptic Species Is An Appropriate Tool To Estimate Their Incidence, At The Eve Of Upcoming Anti-P. Falciparum Vaccination Campaigns.

Immunogenicity of bacterial-expressed recombinant Plasmodium knowlesi merozoite surface protein-142 (MSP-142)

Background

Plasmodium knowlesi is the fifth Plasmodium species that can infect humans. The Plasmodium merozoite surface protein-1₄₂ (MSP-1₄₂) is a potential candidate for malaria vaccine. However, limited studies have focused on P. knowlesi MSP-1₄₂.

Methods

A ~42 kDa recombinant P. knowlesi MSP-1₄₂ (pkMSP-1₄₂) was expressed using an Escherichia coli system. The purified pkMSP-1₄₂ was evaluated with malaria and non-malaria human patient sera (n = 189) using Western blots and ELISA. The immunogenicity of pkMSP-1₄₂ was evaluated in mouse model.

Results

The purified pkMSP-1₄₂ had a sensitivity of 91.0% for detection of human malaria in both assays. Specificity was 97.5 and 92.6% in Western blots and ELISA, respectively. Levels of cytokine interferon-gamma, interleukin-2, interleukin-4, and interleukin-10 significantly increased in pkMSP-1₄₂-immunized mice as compared to the negative control mice. pkMSP-1₄₂-raised antibody had high endpoint titres, and the IgG isotype distribution was IgG1 > IgG2b > IgG3 > IgG2a.

Conclusions

pkMSP-1₄₂ was highly immunogenic and able to detect human malaria. Hence, pkMSP-1₄₂ would be a useful candidate for malaria vaccine development and seroprevalence studies.

Oral delivery of human biopharmaceuticals, autoantigens and vaccine antigens bioencapsulated in plant cells

Among 12billion injections administered annually, unsafe delivery leads to >20million infections and >100million reactions. In an emerging new concept, freeze-dried plant cells (lettuce) expressing vaccine antigens/biopharmaceuticals are protected in the stomach from acids/enzymes but are released to the immune or blood circulatory system when plant cell walls are digested by microbes that colonize the gut. Vaccine antigens bioencapsulated in plant cells upon oral delivery after priming, conferred both mucosal and systemic immunity and protection against bacterial, viral or protozoan pathogens or toxin challenge. Oral delivery of autoantigens was effective against complications of type 1 diabetes and hemophilia, by developing tolerance. Oral delivery of proinsulin or exendin-4 expressed in plant cells regulated blood glucose levels similar to injections. Therefore, this new platform offers a low cost alternative to deliver different therapeutic proteins to combat infectious or inherited diseases by eliminating inactivated pathogens, expensive purification, cold storage/transportation and sterile injections.

Malaria vaccine adjuvants: latest update and challenges in preclinical and clinical research

There is no malaria vaccine currently available, and the most advanced candidate has recently reported a modest 30% efficacy against clinical malaria. Although many efforts have been dedicated to achieve this goal, the research was mainly directed to identify antigenic targets. Nevertheless, the latest progresses on understanding how immune system works and the data recovered from vaccination studies have conferred to the vaccine formulation its deserved relevance. Additionally to the antigen nature, the manner in which it is presented (delivery adjuvants) as well as the immunostimulatory effect of the formulation components (immunostimulants) modulates the immune response elicited. Protective immunity against malaria requires the induction of humoral, antibody-dependent cellular inhibition (ADCI) and effector and memory cell responses. This review summarizes the status of adjuvants that have been or are being employed in the malaria vaccine development, focusing on the pharmaceutical and immunological aspects, as well as on their immunization outcomings at clinical and preclinical stages.

The utility of Plasmodium berghei as a rodent model for anti-merozoite malaria vaccine assessment

Rodent malaria species Plasmodium yoelii and P. chabaudi have been widely used to validate vaccine approaches targeting blood-stage merozoite antigens. However, increasing data suggest the P. berghei rodent malaria may be able to circumvent vaccine-induced anti-merozoite responses. Here we confirm a failure to protect against P. berghei, despite successful antibody induction against leading merozoite antigens using protein-in-adjuvant or viral vectored vaccine delivery. No subunit vaccine approach showed efficacy in mice following immunization and challenge with the wild-type P. berghei strains ANKA or NK65, or against a chimeric parasite line encoding a merozoite antigen from P. falciparum. Protection was not improved in knockout mice lacking the inhibitory Fc receptor CD32b, nor against a Δsmac P. berghei parasite line with a non-sequestering phenotype. An improved understanding of the mechanisms responsible for protection, or failure of protection, against P. berghei merozoites could guide the development of an efficacious vaccine against P. falciparum.

Immune responses elicited by co-immunization of Plasmodium vivax and P. falciparum MSP-1 using prime-boost immunization strategies

Carboxy-terminus of merozoite surface protein-1 (MSP-1(19) ) is the major protein on the surface of the plasmodial merozoite that acts as one of the most important blood-stage vaccine candidates. The present investigation was designed to evaluate the immune responses when either two recombinant antigens (rPvMSP-1(19) + rPfMSP-1(19)) or two plasmid constructs (pcDNA3.1 hygro-PvMSP-1(19) + pcDNA3.1 hygro-PfMSP-1(19)) were administered in combination at a single site in mice by using different immunization strategies (protein/protein, DNA/DNA and DNA/protein) at weeks 0, 5 and 8. All mice were monitored for the level of MSP-1(19) -specific antibody for up to 40 weeks. The inclusion of both recombinant antigens in a vaccine mixture could not inhibit induction of antibodies to the other antigen when the two recombinant antigens were combined in immunization formulation. Interestingly, antisera from immunized mice with either recombinant antigen failed to cross-react with heterologous antigen. Moreover, the results of this study showed that co-immunization with both antigens at a single site generated a substantial PvMSP-1(19) - and PfMSP-1(19) -specific antibody responses and also IFN-γ cytokine production (Th1 response) in DNA/protein prime-boost immunization strategies. The increased humoral response to PvMSP-1(19) and PfMSP-1(19) lasted nearly a year after immunization. Therefore, the results of this study are encouraging for the development of multi-species malaria vaccine based on MSP-1(19) antigen.

The generation and evaluation of two panels of epitope-matched mouse IgG1, IgG2a, IgG2b and IgG3 antibodies specific for Plasmodium falciparum and Plasmodium yoelii merozoite surface protein 1-19 (MSP1(19))

Murine immunoglobulin G (IgG) plays an important role in mediating protective immune responses to malaria. We still know relatively little about which IgG subclasses protect against this disease in mouse models, although IgG2a and IgG2b are considered to be the most potent and dominate in successful passive transfer experiments in rodent malarias. To explore the mechanism(s) by which the different mouse IgG subclasses may mediate a protective effect, we generated mouse IgG1, IgG2a, IgG2b and IgG3 specific for the C-terminal 19-kDa region of Plasmodium falciparum merozoite surface protein 1 (PfMSP1₁₉), and to the homologous antigen from Plasmodium yoelii (P. yoelii), both major targets of protective immune responses. This panel of eight IgGs bound antigen with an affinity comparable to that seen for their epitope-matched parental monoclonal antibodies (mAbs) from which they were derived, although for reasons of yield, we were only able to explore the function of mouse IgG1 recognizing PfMSP1₁₉ in detail, both in vitro and in vivo. Murine IgG1 was as effective as the parental human IgG from which it was derived at inducing NADPH-mediated oxidative bursts and degranulation from neutrophils. Despite showing efficacy in in vitro functional assays with neutrophils, the mouse IgG1 failed to protect against parasite challenge in vivo. The lack of protection afforded by MSP1₁₉-specific IgG1 against parasite challenge in wild type mice suggests that this Ab class does not play a major role in the control of infection with mouse malaria in the Plasmodium berghei transgenic model.

Chimeric parasites as tools to study Plasmodium immunology and assess malaria vaccines

The study of pathogen immunity relies upon being able to track antigen specific immune responses and assess their protective capacity. To study immunity to Plasmodium antigens, chimeric rodent or human malaria parasites that express proteins from other Plasmodium species or unrelated species have been developed. Different types of chimeric parasites have been used to address a range of specific questions. Parasites expressing model T cell epitopes have been used to monitor cellular immune responses to the preerythrocytic and blood stages of malaria. Other parasites have been used to assess the functional significance of immune responses targeting particular proteins. Finally, a number of rodent malaria parasites that express vaccine-candidate antigens from P. falciparum and P. vivax have been used in functional assays of vaccine-induced antibody responses. Here, I review the experimental contributions that have been made using these parasites, and discuss the potential of these approaches to continue advancing our understanding of malaria immunology and vaccine research.

Immunogenicity and in vitro protective efficacy of recombinant Mycobacterium bovis bacille Calmette Guerin (rBCG) expressing the 19 kDa merozoite surface protein-1 (MSP-1(19)) antigen of Plasmodium falciparum

Vaccine development against the blood-stage malaria parasite is aimed at reducing the pathology of the disease. We constructed a recombinant Mycobacterium bovis bacille Calmette Guerin (rBCG) expressing the 19 kDa C-terminus of Plasmodium falciparum merozoite surface protein-1 (MSP-1(19)) to evaluate its protective ability against merozoite invasion of red blood cells in vitro. A mutated version of MSP-1(19), previously shown to induce the production of inhibitory but not blocking antibodies, was cloned into a suitable shuttle plasmid and transformed into BCG Japan (designated rBCG016). A native version of the molecule was also cloned into BCG (rBCG026). Recombinant BCG expressing the mutated version of MSP-1(19) (rBCG016) elicited enhanced specific immune response against the epitope in BALB/c mice as compared to rBCG expressing the native version of the epitope (rBCG026). Sera from rBCG016-immunized mice contained significant levels of specific IgG, especially of the IgG2a subclass, against MSP-1(19) as determined by enzyme-linked immunosorbent assay. The sera was reactive with fixed P. falciparum merozoites as demonstrated by indirect immunofluorescence assay (IFA) and inhibited merozoite invasion of erythrocytes in vitro. Furthermore, lymphocytes from rBCG016-immunized mice demonstrated higher proliferative response against the MSP-1(19) antigen as compared to those of rBCG026- and BCG-immunized animals. rBCG expressing the mutated version of MSP-1(19) of P. falciparum induced enhanced humoral and cellular responses against the parasites paving the way for the rational use of rBCG as a blood-stage malaria vaccine candidate.

Proteome analysis reveals a large merozoite surface protein-1 associated complex on the Plasmodium falciparum merozoite surface

Plasmodium merozoite surface protein-1 (MSP-1) is an essential antigen for the merozoite invasion of erythrocytes. A key challenge to the development of an effective malaria vaccine that can block the erythrocyte invasion is to establish the molecular interaction(s) among the parasite surface proteins as well as with the host cell encoded receptors. In the present study, we applied molecular interactions and proteome approaches to identify PfMSP-1 associated complex on the merozoite surface. Proteomic analysis identified a major malaria surface protein, PfRhopH3 interacting with PfMSP-1(42). Pull-down experiments with merozoite lysate using anti-PfMSP-1 or anti-PfRhopH3 antibodies showed 16 bands that when identified by tandem mass spectrometry corresponded to11 parasite proteins: PfMSP-3, PfMSP-6, PfMSP-7, PfMSP-9, PfRhopH3, PfRhopH1, PfRAP-1, PfRAP-2, and two RAP domain containing proteins. This MSP-1 associated complex was specifically seen at schizont/merozoite stages but not the next ring stage. We could also identify many of these proteins in culture supernatant, suggesting the shedding of the complex. Interestingly, the PfRhopH3 protein also showed binding to the human erythrocyte and anti-PfRhopH3 antibodies blocked the erythrocyte invasion of the merozoites. These results have potential implications in the development of PfMSP-1 based blood stage malaria vaccine.

New candidate vaccines against blood-stage Plasmodium falciparum malaria: prime-boost immunization regimens incorporating human and simian adenoviral vectors and poxviral vectors expressing an optimized antigen based on merozoite surface protein 1

Although merozoite surface protein 1 (MSP-1) is a leading candidate vaccine antigen for blood-stage malaria, its efficacy in clinical trials has been limited in part by antigenic polymorphism and potentially by the inability of protein-in-adjuvant vaccines to induce strong cellular immunity. Here we report the design of novel vectored Plasmodium falciparum vaccines capable of overcoming such limitations. We optimized an antigenic insert comprising the four conserved blocks of MSP-1 fused to tandemly arranged sequences that represent both allelic forms of the dimorphic 42-kDa C-terminal region. Inserts were expressed by adenoviral and poxviral vectors and employed in heterologous prime-boost regimens. Simian adenoviral vectors were used in an effort to circumvent preexisting immunity to human adenoviruses. In preclinical studies these vaccines induced potent cellular immune responses and high-titer antibodies directed against MSP-1. The antibodies induced were found to have growth-inhibitory activity against dimorphic allelic families of P. falciparum. These vectored vaccines should allow assessment in humans of the safety and efficacy of inducing strong cellular as well as cross-strain humoral immunity to P. falciparum MSP-1.

Plasmodium vivax ookinete surface protein Pvs25 linked to cholera toxin B subunit induces potent transmission-blocking immunity by intranasal as well as subcutaneous immunization

The nontoxic cholera toxin B subunit (CTB) was evaluated as a potential delivery molecule for the Plasmodium vivax ookinete surface protein, Pvs25. Recombinant Pvs25 was expressed as a secreted protein in the yeast Pichia pastoris, as a mixture of isoforms including multimers and the A and B monomers. The A isoform with the presumed native protein fold was the most abundant, accounting for more than 40% of all expressed protein. The molecularly uniform A isoform was chemically conjugated to CTB via its primary amines, and the fusion protein, retaining GM1-ganglioside affinity, was administered to BALB/c mice by the subcutaneous (s.c.) or intranasal (i.n.) route. Immunization of mice with conjugated Pvs25 without supplemental adjuvant induced antisera that specifically recognized P. vivax ookinetes in vitro. Furthermore, the antisera, when mixed with parasitized blood isolated from P. vivax patients from Thailand, was found to reduce parasite transmission to mosquitoes, conferring a 93 to 98% (s.c.) or a 73 to 88% (i.n.) decrease in oocyst number. Unconjugated Pvs25 alone conferred only a 23 to 60% (s.c.) or a 0 to 6% (i.n.) decrease in oocyst number. Coadministration of extraneous adjuvants, however, further enhanced the vaccine efficacy up to complete blockade. Taken together, we conclude that a weakly immunogenic Pvs25 by itself, when linked to CTB, transforms into a potent transmission-blocking antigen in both i.n. and s.c. routes. In addition, the present study is, to the best of our knowledge, the first demonstration of the immune potentiating function of CTB for a vaccine antigen delivered by the s.c. route.

A novel Plasmodium falciparum erythrocyte binding protein associated with the merozoite surface, PfDBLMSP

Proteins on the surface of the merozoite, the invasive form of the malaria parasite Plasmodium falciparum,and those secreted from its apical secretory organelles are promising vaccine candidates against blood stage malaria. In the present study, we have identified a novel parasite protein (PfDBLMSP; Gene IDPF10_0348), that harbors a predicted signal sequence, a central Duffy binding-like (DBL) domain and a secreted polymorphic antigen associated with merozoites (SPAM) domain in its C-terminal half. Transcription and translation of pfdblmsp is up-regulated specifically in schizont stage parasites, similar to other well-chararacterized merozoite proteins involved in invasion of red blood cells (RBCs). PfDBLMSPwas localized on the merozoite surface with a GFP targeting approach using schizont-stage specific expression systems, and by immunofluorescence assays of the endogenous protein. PfDBLMSP expressed on the surface of mammalian cells (COS-7) showed binding with human RBCs and this binding was sensitive to trypsin and neuraminidase treatments. The recombinant proteins corresponding to the DBL and SPAM domains showed reactivity with immune sera from individuals residing in P. falciparum endemic areas. Polymorphism in PfDBLMSP sequences from different P. falciparum strains and field isolates suggested that its DBL domain is under natural immune pressure. Our data on localization and functional assays suggest a possible role of PfDBLMSP in binding of merozoites with erythrocytes during invasion.

Chloroplast-derived vaccine antigens confer dual immunity against cholera and malaria by oral or injectable delivery

Cholera and malaria are major diseases causing high mortality. The only licensed cholera vaccine is expensive; immunity is lost in children within 3 years and adults are not fully protected. No vaccine is yet available for malaria. Therefore, in this study, the cholera toxin-B subunit (CTB) of Vibrio cholerae fused to malarial vaccine antigens apical membrane antigen-1 (AMA1) and merozoite surface protein-1 (MSP1) was expressed in lettuce and tobacco chloroplasts. Southern blot analysis confirmed homoplasmy and stable integration of transgenes. CTB-AMA1 and CTB-MSP1 fusion proteins accumulated up to 13.17% and 10.11% (total soluble protein, TSP) in tobacco and up to 7.3% and 6.1% (TSP) in lettuce, respectively. Nine groups of mice (n = 10/group) were immunized subcutaneously (SQV) or orally (ORV) with purified antigens or transplastomic tobacco leaves. Significant levels of antigen-specific antibody titres of immunized mice completely inhibited proliferation of the malarial parasite and cross-reacted with the native parasite proteins in immunoblots and immunofluorescence studies. Protection against cholera toxin challenge in both ORV (100%) and SQV (89%) mice correlated with CTB-specific titres of intestinal, serum IgA and IgG1 in ORV and only IgG1 in SQV mice, but no other immunoglobulin. Increasing numbers of interleukin-10(+) T cell but not Foxp3(+) regulatory T cells, suppression of interferon-gamma and absence of interleukin-17 were observed in protected mice, suggesting that immunity is conferred via the Tr1/Th2 immune response. Dual immunity against two major infectious diseases provided by chloroplast-derived vaccine antigens for long-term (>300 days, 50% of mouse life span) offers a realistic platform for low cost vaccines and insight into mucosal and systemic immunity.

Trichinella spiralis: immune response and protective immunity elicited by recombinant paramyosin formulated with different adjuvants

Our previous studies showed that immunization with recombinant paramyosin from Trichinella spiralis (rTs-Pmy) formulated with Freund's adjuvant significantly reduced larval burden in mice after T. spiralis larval challenge. Since Freund's adjuvant is toxic and not a suitable adjuvant for clinical vaccine trials, we evaluated the ability of the adjuvants Montanide ISA206 and ISA720 to stimulate immune responses during rTs-Pmy immunization and to enhance protective immunity. The results revealed that immunization of BALB/c mice with rTs-Pmy formulated with either ISA206 or ISA720 triggered Th1 and Th2 immune responses similar to those produced by the conventional Freund's adjuvant formulation and also provided a similar level of protection against T. spiralis larval challenge. This indicates that the recombinant Ts-Pmy formulated with Montanide ISA206 or ISA720 may be an effective and safety vaccine strategy for trichinellosis.

Plasmodium falciparum merozoite surface protein 1 (MSP-1)-MSP-3 chimeric protein: immunogenicity determined with human-compatible adjuvants and induction of protective immune response

A chimeric gene, MSP-Fu(24), was constructed by genetically coupling immunodominant, conserved regions of the two leading malaria vaccine candidates, Plasmodium falciparum merozoite surface protein 1 (C-terminal 19-kDa region [PfMSP-1(19)]) and merozoite surface protein 3 (11-kDa conserved region [PfMSP-3(11)]). The recombinant MSP-Fu(24) protein was produced in Escherichia coli cells and purified to homogeneity by a two-step purification process with a yield of approximately 30 mg/liter. Analyses of conformational properties of MSP-Fu(24) using PfMSP-1(19)-specific monoclonal antibody showed that the conformational epitopes of PfMSP-1(19) that may be critical for the generation of the antiparasitic immune response remained intact in the fusion protein. Recombinant MSP-Fu(24) was highly immunogenic in mice and in rabbits when formulated with two different human-compatible adjuvants and induced an immune response against both PfMSP-1(19) and PfMSP-3(11). Purified anti-MSP-Fu(24) antibodies showed invasion inhibition of P. falciparum 3D7 and FCR parasites, and this effect was found to be dependent on antibodies specific for the PfMSP-1(19) component. The protective potential of MSP-Fu(24) was demonstrated by in vitro parasite growth inhibition using an antibody-dependent cell inhibition (ADCI) assay with anti-MSP-Fu(24) antibodies. Overall, the antiparasitic activity was mediated by a combination of growth-inhibitory antibodies generated by both the PfMSP-1(19) and PfMSP-3(11) components of the MSP-Fu(24) protein. The antiparasitic activities elicited by anti-MSP-Fu(24) antibodies were comparable to those elicited by antibodies generated with immunization with a physical mixture of two component antigens, PfMSP-1(19) and PfMSP-3(11). The fusion protein induces a protective immune response with human-compatible adjuvants and may form a part of a multicomponent malaria vaccine.

Identification of a vaccine candidate antigen, PfMAg-1, from Plasmodium falciparum with monoclonal antibody M26-32

Monoclonal antibody M26-32 has been shown to strongly inhibit the growth of Plasmodium falciparum in vitro. To identify the target antigen of M26-32, a P. falciparum Dd2 asexual stage cDNA expression library was screened with this antibody, and a full open reading frame cDNA was obtained. This gene, named pfmag-1, encodes a polypeptide of 589 amino acids. The protein PfMAg-1 was characterized as a membrane-associated protein that expressed on the surface of merozoite during erythrocytic stage. Remarkably, at the C terminus of PfMAg-1, there are 14 copies of a deca-peptide sequence of QTDEIKND (H/N) I. This tandem repeat domain was identified to harbor the epitope of the protective M26-32 monoclonal antibody, and was also recognized by sera of patients infected with P. falciparum. Rabbit antibody elicited against this deca-peptide repeat domain effectively inhibited P. falciparum invasion in vitro. Our work suggests that PfMAg-1 is a promising malaria vaccine candidate.

Construction of transgenic Plasmodium berghei as a model for evaluation of blood-stage vaccine candidate of Plasmodium falciparum chimeric protein 2.9

Background

The function of the 19 kDa C-terminal region of the merozoite surface protein 1 (MSP1-19) expressed by Plasmodium has been demonstrated to be conserved across distantly related Plasmodium species. The green fluorescent protein (GFP) is a reporter protein that has been widely used because it can be easily detected in living organisms by fluorescence microscopy and flow cytometry.

Methodology and Results

In this study, we used gene targeting to generate transgenic P. berghei (Pb) parasites (designated as PfMSP1-19Pb) that express the MSP1-19 of P. falciparum (Pf) and the GFP reporter protein simultaneously. The replacement of the PbMSP1-19 locus by PfMSP1-19 was verified by PCR and Southern analysis. The expression of the chimeric PbfMSP-1 and the GFP was verified by Western blot and fluorescence microscopy, respectively. Moreover, GFP-expressing transgenic parasites in blood stages can be readily differentiated from other blood cells using flow cytometry. A comparion of growth rates between wild-type and the PfMSP1-19Pb transgenic parasite indicated that the replacement of the MSP1-19 region and the expression of the GFP protein were not deleterious to the transgenic parasites. We used this transgenic mouse parasite as a murine model to evaluate the protective efficacy in vivo of specific IgG elicited by a PfCP-2.9 malaria vaccine that contains the PfMSP1-19. The BALB/c mice passively transferred with purified rabbit IgG to the PfCP-2.9 survived a lethal challenge of the PfMSP1-19Pb transgenic murine parasites, but not the wild-type P. berghei whereas the control mice passively transferred with purified IgG obtained from adjuvant only-immunized rabbits were vulnerable to both transgenic and wild-type infections.

Conclusions

We generated a transgenic P. berghei line that expresses PfMSP1-19 and the GFP reporter gene simultaneously. The availability of this parasite line provides a murine model to evaluate the protective efficacy in vivo of anti-MSP1-19 antibodies, including, potentially, those elicited by the PfCP-2.9 malaria vaccine in human volunteers.

Protection induced by Plasmodium falciparum MSP1(42) is strain-specific, antigen and adjuvant dependent, and correlates with antibody responses

Vaccination with Plasmodium falciparum MSP1₄₂/complete Freund's adjuvant (FA) followed by MSP1₄₂/incomplete FA is the only known regimen that protects Aotus nancymaae monkeys against infection by erythrocytic stage malaria parasites. The role of adjuvant is not defined; however complete FA cannot be used in humans. In rodent models, immunity is strain-specific. We vaccinated Aotus monkeys with the FVO or 3D7 alleles of MSP1₄₂ expressed in Escherichia coli or with the FVO allele expressed in baculovirus (bv) combined with complete and incomplete FA, Montanide ISA-720 (ISA-720) or AS02A. Challenge with FVO strain P. falciparum showed that suppression of cumulative day 11 parasitemia was strain-specific and could be induced by E. coli expressed MSP1₄₂ in combination with FA or ISA-720 but not with AS02A. The coli42-FVO antigen induced a stronger protective effect than the bv42-FVO antigen, and FA induced a stronger protective effect than ISA-720. ELISA antibody (Ab) responses at day of challenge (DOC) were strain-specific and correlated inversely with c-day 11 parasitemia (r = −0.843). ELISA Ab levels at DOC meeting a titer of at least 115,000 ELISA Ab units identified the vaccinees not requiring treatment (noTx) with a true positive rate of 83.3% and false positive rate of 14.3 %. Correlation between functional growth inhibitory Ab levels (GIA) and cumulative day 11 parasitemia was weaker (r = −0.511), and was not as predictive for a response of noTx. The lowest false positive rate for GIA was 30% when requiring a true positive rate of 83.3%. These inhibition results along with those showing that antigen/FA combinations induced a stronger protective immunity than antigen/ISA-720 or antigen/AS02 combinations are consistent with protection as ascribed to MSP1-specific cytophilic antibodies. Development of an effective MSP1₄₂ vaccine against erythrocytic stage P. falciparum infection will depend not only on antigen quality, but also upon the selection of an optimal adjuvant component.

Identification and characterization of a novel Plasmodium falciparum merozoite apical protein involved in erythrocyte binding and invasion

Proteins that coat Plasmodium falciparum merozoite surface and those secreted from its apical secretory organelles are considered promising candidates for the vaccine against malaria. In the present study, we have identified an asparagine rich parasite protein (PfAARP; Gene ID PFD1105w), that harbors a predicted signal sequence, a C-terminal transmembrane region and whose transcription and translation patterns are similar to some well characterized merozoite surface/apical proteins. PfAARP was localized to the apical end of the merozoites by GFP-targeting approach using an inducible, schizont-stage expression system, by immunofluorescence assays using anti-PfAARP antibodies. Immuno-electron microsopic studies showed that PfAARP is localized in the apical ends of the rhoptries in the merozoites. RBC binding assays with PfAARP expressed on COS cells surface showed that it binds to RBCs through its N-terminal region with a receptor on the RBC surface that is sensitive to trypsin and neuraminidase treatments. Sequencing of PfAARP from different P. falciparum strains as well as field isolates showed that the N-terminal region is highly conserved. Recombinant protein corresponding to the N-terminal region of PfAARP (PfAARP-N) was produced in its functional form in E. coli. PfAARP-N showed reactivity with immune sera from individuals residing in P. falciparum endemic area. The anti-PfAARP-N rabbit antibodies significantly inhibited parasite invasion in vitro. Our data on localization, functional assays and invasion inhibition, suggest a role of PfAARP in erythrocyte binding and invasion by the merozoite.

Plasmodium berghei merozoite surface protein-9: immunogenicity and protective efficacy using a homologous challenge model

Merozoite surface protein-9 (MSP-9) from Plasmodium is considered a promising vaccine candidate due to its location and possible role in erythrocyte invasion. We report the identification and characterization of Plasmodium berghei MSP-9 (PbMSP-9) and its properties as an immunogen using a recombinant PbMSP-9 fragment to immunize BALB/c mice. PbMSP-9 was found to harbor erythrocyte binding and serine protease activity. PbMSP-9 formulation in alum was highly immunogenic in BALB/c mice. To evaluate the protective efficacy, immunized mice were submitted to homologous challenge with P. berghei NK65 blood-stage parasites. Protection against the parasite challenge was observed in BALB/c mice immunized with the PbMSP-9 formulation. These results suggest for the first time that MSP-9 based immunogens may constitute part of an effective malaria vaccine.

Polymorphism and epitope sharing between the alleles of merozoite surface protein-1 of Plasmodium falciparum among Indian isolates

Background

The C-terminal region of merozoite surface protein-1 (MSP-1) is one of the leading candidates for vaccination against the erythrocytic stages of malaria. However, a major concern in the development of MSP-1 based malaria vaccine is the polymorphism observed in different geographical Plasmodium falciparum isolates. To explore whether the sequence heterogeneity of PfMSP-1 leads to variation in naturally acquired anti-MSP-119 antibodies, the present study was undertaken to study PfMSP-119 sequence polymorphism in malaria-endemic villages in eastern India and also carried out a competition enzyme-linked immunosorbent assay using three PfMSP-119 variant forms.

Methods

The sequence variations in the C-terminal region of PfMSP-119 were determined in a malaria endemic region. Three PfMSP-119 variants were produced in Escherichia coli (PfMSP119QKNG-L, PfMSP119EKNG-L and PfMSP119ETSR-F) and an immunodepletion assay was carried out using the corresponding patients' sera.

Results

Results revealed predominance of PfMAD20 allele among Indian field isolates. Seven PfMSP-119 variant forms were isolated in a singe geographical location. Three of PfMSP-119 variant forms when expressed in E. coli showed presence of cross-reaction as well as variant specific antibodies in malaria infected patient sera.

Conclusion

The present study demonstrates the existence of allele specific antibodies in P. falciparum-infected patient sera, however their role in protection requires further investigation. These results thereby, suggest the importance of a multi-allelic PfMSP-119 based vaccine for an effective malaria control.

HCV core protein immunization with Montanide/CpG elicits strong Th1/Th2 and long-lived CTL responses

An efficient vaccine against Hepatitis C virus (HCV) infection requires induction of strong humoral and cellular responses against viral proteins. We evaluated the immunogenicity of HCV core protein (HCVcp), a prime vaccine candidate, formulated in various human compatible adjuvants. An Escherichia coli-expressed HCVcp, purified in native conditions was used for murine immunization in separate groups of: free HCVcp (Ag), Ag+C/IFA (Freunds), Ag+CpG, Ag+M720 (Montanide ISA 720), Ag+F127 (Pluronic acid) and cocktails of Ag+F127+CpG and Ag+M720+CpG. Mice immunized with M720(+CpG) developed the highest HCVcp-specific titers of total IgG, IgG1, 2a, 2b, and that of IFN-gamma and IL-4 cytokines compared to all other groups. HCVcp-specific-CTLs against relevant MHC class I peptides were detected only for Ag+M720+CpG, Ag+M720, and Ag+CpG groups and could be blocked by antimouse-CD8 antibodies. While CTLs were stable, only F127 formulated groups demonstrated detectable IgG antibodies one year post-immunization. Hence, HCVcp formulated in M720 (with a synergistic effect by inclusion of CpG) could induce balanced and strong Th1/Th2 responses with long-lived CD4(-)CD8(+) CTLs.

An edible vaccine for malaria using transgenic tomatoes of varying sizes, shapes and colors to carry different antigens

Malaria, a disease caused by protozoan parasites of genus Plasmodium, is one of the world's biggest scourges. Over two billion individuals reside in the malaria endemic areas and the disease affects 300-500 million people annually. As a result of malarial-infection, an estimated three million lives are lost annually, among them over one million children (majority under 5 years of age). The mortality due to malaria has increased because of the spread of drug-resistant strains of the parasite, the breakdown of health services in many affected areas, the interaction of the disease with human immunodeficiency virus (HIV) infection, and possibly the effects of climate change. Infants and young children with malaria often die from severe anemia, cerebral involvement,or prostration caused by overwhelming infection; many new borns die from complications of low birth weight caused by maternal malaria during pregnancy. The scarce economic resources and lack of communication, infrastructure and adequate means of travel in the endemic areas make it extremely difficult to implement traditional infection control measures (i.e., mosquito control, preventive anti-malarial drugs and nets). To make the matter worse, both malarial parasites and its insect vectors are increasingly becoming resistant to anti-malarial agents (chloroquine) and insecticides (both DDT and melathione and related chemicals), respectively. By conventional wisdom, the immune mechanisms responsible for protection against malaria will require a multiple of 10-15 antigen targets for proper protection against various stages of malarial infection. By standard vaccination protocols, such a large number of targets would not be appropriate to be used for vaccination as a single dose due to antigenic competition. It would be almost impossible to immunize over two billion individuals who live in malaria susceptible areas with several carefully crafted immunization schedules delivered 4-6 weeks apart in the form of two different antigens as a single dose. Besides, if immunization schedules could be arranged, the stability of vaccines carrying different malarial antigens, their transport, and the logistics of vaccination would be an almost impossible task to achieve under the current fiscal constraints. We are proposing a unique way to circumvent these logistical difficulties to deliver the malaria vaccines to every susceptible home at a small fraction of a cost. We hypothesize that the anti-malaria edible vaccines in transgenic tomato plants where different transgenic plants expressing different antigenic type(s). Immunizing individuals against 2-3 antigens and against each stage of the life cycle of the multistage parasites would be an efficient, inexpensive and safe way of vaccination. Tomatoes with varying sizes, shapes and colors carrying different antigens would make the vaccines easily identifiable by lay individuals.

Plasmodium falciparum: immunization with MSP1-42 induced non-inhibitory antibodies that have no blocking activities but enhanced the potency of inhibitory anti-MSP1-42 antibodies

Hyperimmunization with Plasmodium falciparum MSP1-42 could induce antibodies that have little or no parasite growth inhibitory activities. These antisera had no blocking activities as determined by their ability to interfere with the in vitro activities of growth inhibitory anti-MSP1-42 sera. Equally important, they enhanced the potency of growth inhibitory anti-MSP1-42 sera.

A review of human vaccine research and development: malaria

The last several years have seen significant progress in the development of vaccines against malaria. Most recently, proof-of-concept of vaccine-induced protection from malaria infection and disease was demonstrated in African children. Pursued by various groups and on many fronts, several other candidate vaccines are in early clinical trials. Yet, despite the optimism and promise, an effective malaria vaccine is not yet available, in part because of the lack of understanding of the types of immune responses needed for protection, added to the difficulty of identifying, selecting and producing the appropriate protective antigens from a parasite with a genome of well over five thousand genes and to the frequent need to enhance the immunogenicity of purified antigens through the use of novel adjuvants or delivery systems. Insufficient clinical trial capacity and normative research functions such as local ethical committee reviews also contribute to slow down the development process. This article attempts to summarize the state of the art of malaria vaccine development.